N Hammi

S Chen

F Dumeignil

S Royer

A El Kadib
email: a.elkadib@ueuromed.org

Chitosan as a Sustainable Precursor for Nitrogen-Containing Carbon Nanomaterials: Synthesis and Uses

Keywords: Chitosan, N-doped carbon, Metal-N-doped carbon, N-doped graphene, N-doped carbon

Carbon materials are on the rise owing to their outstanding properties compared to their metal-based homologs. This interest triggered intensive research toward the preparation of carbon-based nanostructures including those doped with heteroatoms, among which nitrogencontaining carbon materials are the most ubiquitous. Biomass and biowastes stand as sustainable and cost-effective precursors to access tunable carbon-based nanomaterials.

Excitingly, chitosan -an aminopolysaccharide marine waste -displays additional advantages of distinctively featuring nitrogen in the biopolymer skeleton, thereby avoiding the tedious step of nitrogen enrichment of the carbonaceous framework that often necessitates the use of exogenous chemicals. This review sheds light on the opportunities offered by transforming chitosan into nitrogen-containing carbon framework, including porous carbon, graphene and

Introduction

Carbon materials have captured intensive interest due to their chemical stability, low density, tuneable textural properties, electric and thermal conductivity, low cost and wide availability [START_REF] Deng | Biomass-derived carbon: synthesis and applications in energy storage and conversion[END_REF][2][START_REF] Hu | Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass[END_REF]. The discoveries of fullerenes, carbon nanotubes and graphene make the related areas of advanced carbon materials particularly attractive. The presence of heteroatoms (e.g., N, B, P and S) in the carbon matrix has opened new possibilities for functional carbon-based materials. The synthesis and use of nitrogen-containing carbon nanomaterials is one of the most emerging topics [START_REF] Inagaki | Nitrogen-doped carbon materials[END_REF]. The introduction of nitrogen atoms into carbon lattices promotes their physicochemical properties and provides abundant, highly-active sites due to the creation of different types of nitrogen functional groups, that is, pyridinic, pyrrolic, graphitic (quaternary) and oxidized nitrogen species (Figure 1) [START_REF] Antolini | Nitrogen-doped carbons by sustainable N-and C-containing natural resources as nonprecious catalysts and catalyst supports for low temperature fuel cells[END_REF][START_REF] Zhang | Engineering selfassembled N-doped graphene-carbon nanotube composites towards efficient oxygen reduction electrocatalysts[END_REF]. The strong electron donating ability of nitrogen species imparts the resulting carbon materials with substantial basicity, metal-ligation ability and modified electronic properties [START_REF] Zhou | Enhancement of Pt and Pt-alloy fuel cell catalyst activity and durability via nitrogen-modified carbon supports[END_REF]. [START_REF] Deng | Review on recent advances in nitrogen-doped carbons: preparations and applications in supercapacitors[END_REF]. Copyright 2016 The Royal Society of Chemistry.

Hitherto, methods for manufacturing nitrogen-containing carbon materials have included: (i) carbonization of nitrogen-containing synthetic materials (e.g., polyacrylonitrile, polyamide, polyimide, polyvinylpyridine, polyaniline, acetonitrile, polypyrrole, melamine-formaldehyde and urea-formaldehyde resins) or natural raw material (i.e., biomass-derived carbon sources);

(ii) post nitrogen enrichment of the as-prepared carbon materials by gas-phase treatment or liquid-phase impregnation treatment. Unfortunately, the use of synthetic precursors and the post nitrogen enrichment of carbon materials suffer from high production cost, low conversion efficiency, harsh and energy intensive synthetic conditions (e.g., electric-arc discharge, chemical vapour deposition, and laser ablation), mitigated environmental issues (i.e., generation of considerable amount of wastes and release of hazardous chemicals), and large dependence on unsustainable fossil fuel resources [START_REF] Deng | Biomass-derived carbon: synthesis and applications in energy storage and conversion[END_REF][START_REF] Ilnicka | Marine and Freshwater Feedstocks as a Precursor for Nitrogen-Containing Carbons: A Review[END_REF].

These issues triggered the shift in producing carbon nanomaterials towards the exploitation and selective transformation of renewable biomass-derived precursors [START_REF] Chen | How Catalysts and Experimental Conditions Determine the Selective Hydroconversion of Furfural and 5-Hydroxymethylfurfural[END_REF]. Beyond its benign and eco-friendly nature, this approach offers many benefits, including the abundance and availability of the starting material precursors, and provides additional advantages in terms of morphological and structural diversities, the presence of multiple elements including nitrogen and carbon, among other species [START_REF] Yaman | Pyrolysis of biomass to produce fuels and chemical feedstocks[END_REF]. Biomass resources encompass all of the biologically produced materials on earth such as energy crops, agricultural crops and their residues, wood and wood wastes, municipal wastes and animal wastes, aquatic plants and algae, which are recyclable and highly available [2]. Transformation of biomass into value-added chemicals and biofuels has been a hot topic during the last decades [START_REF] Yaman | Pyrolysis of biomass to produce fuels and chemical feedstocks[END_REF][START_REF] Deng | Review on recent advances in nitrogen-doped carbons: preparations and applications in supercapacitors[END_REF]. Meanwhile, as carbon-based precursor, biomass possesses hierarchical structures and periodic patterns that replicate in the resulting carbon materials at different scales including the macro, micro and the nanosized dimension. This allowed their use in a wide range of applications such as sensors, supercapacitors, batteries, adsorbents, catalysts and catalysts' supports in electro-and photocatalysis as well as in fine chemical synthesis. Consequently, many carbon materials were produced from biomass precursors, including graphene, carbon dots, and nanostructured porous carbon, in which polysaccharides stand out as a bridge connecting biomass resources and the production of carbon materials.

Chitin is a natural, non-synthetic aminocarbohydrate that originates from various biomass sources, such as shellfish wastes, insects (scorpion, cockroach, spider) and microorganisms (green algae, ascomycetes, yeast, brown algae), which all exist in large quantities in nature.

Chitosan is a linear copolymer featuring repeating units of D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit), linked via β(1-4) glycosidic bonds (Figure 2).

It is generally prepared by deacetylation of chitin, which is the second most abundant polysaccharide after cellulose. Chitin is extracted from shrimp shell and other crustaceans using alkaline reagents (e.g., 30 wt.% to 60 wt.% NaOH, 80-160 °C) [START_REF] Kumar | Chitosan Chemistry and Pharmaceutical Perspectives[END_REF][START_REF] Suginta | Electrochemical Biosensor Applications of Polysaccharides Chitin and Chitosan[END_REF][START_REF] Jayakumar | Biomedical Applications of Chitin and Chitosan Based Nanomaterials-A Short Review[END_REF]. Enzymatic hydrolysis in the presence of a chitin deacetylase could also lead to different molecular weight chitosans, including also water-soluble oligo-chitosan [START_REF] Jayakumar | Biomedical Applications of Chitin and Chitosan Based Nanomaterials-A Short Review[END_REF]. Shellfish wastes primarily produced in massive quantity from shellfish industry then serve as the main source of chitin and consequently of chitosan. Then, chitosan as a cheap, abundant, and renewable biomass resource rich in N content (∼7 wt. %) has vast potential for the preparation of N-doped carbon materials. The properties of chitosan relate to its multiple functional groups, including hydroxyl, amino and acetamido groups, and to its polyelectrolytic and sugar-based polymeric properties [START_REF] Jayakumar | Biomedical Applications of Chitin and Chitosan Based Nanomaterials-A Short Review[END_REF]. Depending on the marine source and the applied chemical treatment, chitosan with different molecular weight, crystallinity and deacetylation degrees can be obtained [START_REF] Kadib | Recent Progress in Chitosan Bio-Based Soft Nanomaterials[END_REF].

The properties of the biopolymer are largely affected by these three parameters [START_REF] Kadib | Recent Progress in Chitosan Bio-Based Soft Nanomaterials[END_REF]. The presence of the aforementioned functional groups are at the basis of the formation of intermolecular hydrogen bonds, and brings strong chelating and complexing properties as well as interesting film-forming and gelation ability [START_REF] Kadib | Green and Functional Aerogels by Macromolecular and Textural Engineering of Chitosan Microspheres[END_REF][START_REF] Kadib | Chitosan Bio-Based Organic-Inorganic Hybrid Aerogel Microspheres[END_REF]. Hitherto, chitosan derivatives were used in different areas including flocculating agents for water filtration and waste water treatment, adsorbents to remove heavy metals from aqueous solutions, coating to improve dyeing characteristics of glass fibres, wet strength additives for paper, adhesives, photographic and printing applications, thickeners, food preservation and many others [START_REF] Kadib | Recent Progress in Chitosan Bio-Based Soft Nanomaterials[END_REF][START_REF] Kadib | Chitosan as a Sustainable Organocatalyst: A Concise Overview[END_REF]. Recently, transformation of chitosan into functional nanomaterials emerged as a sustainable approach for furthering more innovation in chemistry and material science, and has thus garnered tremendous interest. Presently, a variety of carbon materials utility of biomass transformation and its great impact on chemistry and material science (Figure 3). Given the thermally-induced evolution of chitosan skeleton during the carbonization process, the precise control of the experimental parameters is a crucial matter. In most cases, inert gas atmosphere is required and the choice of the temperature range and its variation is also pivotal depending on the nature of the target carbon materials. In some cases, activation treatment of the resulting carbon materials is necessary for improving their textural properties including primarily the opening porosity. As a matter of fact, the carbonization of chitosan is a complex process, which involves a decrease of oxygen content and rearrangement of the structure through sequential reactions including hydrolysis, dehydration, decarboxylation, aromatization chronologically caused by elimination of oxygenated groups, ring opening, recondensation and cyclization [START_REF] Hu | Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass[END_REF]. The key to selective transformation lies in a good understanding of the carbonization mechanism, based on which a proper tuning of the carbonization parameters could lead to carbon materials with well-defined molecular composition and high framework quality.

The purpose of the present review is to go beyond a phenomenological description of the processes of fabrication of carbon materials from chitosan. We aim here at singling out key factors influencing the synthesis of different carbon materials from chitosan, and at building on information gathered in order to improve the yield and the structural properties of a given product: surface functionality, texture and porosity, and inherent properties of the corresponding carbon materials, with a special emphasis on the mechanism involved in the transformation of chitosan to the given carbon material. The first part of the review deals with nitrogen-containing porous carbon materials, for which general strategies to tune the textural porosity are summarized. The second part is devoted to the thermal conversion of chitosan to graphene, which is still a less explored and more complex domain, but in which promising results have been obtained over the last years. The next part focuses on the synthesis of carbon dots from chitosan, a comparatively very recent field for which general developments linking product yield and quantum yield can now be established. A concluding section is provided at the end to summarize the key points for the transformation of chitosan into different carbon materials, to outline the limitations encountered and to highlight recent trends and further developments that are expected in the near future.

Synthesis of nitrogen-doped porous carbon and metal-N-doped carbon composites

N-doped porous carbon

Recently, nitrogen-doped porous carbons (N-PCs) have attracted tremendous attention due to their excellent performance in applications such as CO2 sequestration [START_REF] Plaza | CO2 capture by adsorption with nitrogen enriched carbons[END_REF], removal of contaminants from gas and liquid phases [START_REF] Stavropoulos | Effect of activated carbons modification on porosity, surface structure and phenol adsorption[END_REF][START_REF] Jia | Adsorption of Metal Ions on Nitrogen Surface Functional Groups in Activated Carbons[END_REF], catalysts and catalytic supports for thermal-/electro-catalysis [START_REF] Shao | Nitrogen-doped carbon nanostructures and their composites as catalytic materials for proton exchange membrane fuel cell[END_REF] and supercapacitors [START_REF] Hulicova | Electrochemical Performance of Nitrogen-Enriched Carbons in Aqueous and Non-Aqueous Supercapacitors[END_REF]. Conventional way to synthesize N-PCs relies on the post-treatment of carbon with ammonia [START_REF] Rivera-Utrilla | Activated carbon modifications to enhance its water treatment applications. An overview[END_REF] or on the templating route combined with activation of nitrogen-rich carbon precursors [START_REF] Liu | Adjusting the texture and nitrogen content of ordered mesoporous nitrogen-doped carbon materials prepared using SBA-15 silica as a template[END_REF][START_REF] Sakintuna | Templated Porous Carbons: A Review Article[END_REF]. High cost, decreases in surface area and pore volume, as well as instability of surface nitrogen species are common setbacks of these methods. In contrast, the synthesis of N-PCs materials from biomass-derived chitosan allows for tackling these drawbacks and displays other advantages including its greenest nature and possible recyclability [START_REF] Zhao | Sustainable nitrogen-doped carbonaceous materials from biomass derivatives[END_REF]. This section focuses on the synthesis of porous N-PCs from chitosan, detailing the preparation methods, surface functionalization including co-doping with other heteroatoms (i.e., B, P and S), its textural engineering and the plausible mechanism involved in their preparation.

The transformation of chitosan to N-PCs generally involves hydrothermal carbonization and/or pyrolysis treatment. The hydrothermal treatment is usually performed in aqueous medium at moderate temperature (100 to 350 °C) under autogenous pressure (Table 1). The formation of carbon results from dehydration, condensation or polymerization, and aromatization reactions [START_REF] Sevilla | Chemical and Structural Properties of Carbonaceous Products Obtained by Hydrothermal Carbonization of Saccharides[END_REF]. This process is distinguished by several features, among them [START_REF] Wang | Monodispersed hard carbon spherules with uniform nanopores[END_REF]: (i) the easily generated porosity; (ii) the possible combination with other components (e.g., metal species) to form composites with special physicochemical and/or catalytic properties; (iii) the formation of surface-reactive functional groups which facilitate postsynthetic treatment. Notably, the temperature of hydrothermal treatment has a crucial influence on the properties of the ensuing carbon materials. For example, hydrothermal treatment of chitosan at 180 °C provides porous nitrogen-doped carbon with high chemical stability, good dispersibility in acid solutions and high Cr(VI) adsorption capacity. A detrimental effect was however observed above this temperature with a decrease in the oxygen and nitrogen content as well as in the adsorption capacity of the resultant carbon [START_REF] Shen | Chitosan-derived carbonaceous material for highly efficient adsorption of chromium (VI) from aqueous solution[END_REF].

In contrast to hydrothermal treatment under relatively low temperature conditions, pyrolysis of chitosan is commonly performed at high temperatures (600-1000 °C) (Table 1). Inert atmosphere of nitrogen or argon is usually required to prevent oxidation. The pyrolysis temperature has a significant effect on the textural properties of the resulting carbon material [START_REF] Hao | Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high performance supercapacitors[END_REF]. In some cases, this process could lead to porous graphitic carbon materials [START_REF] Xu | A facile approach to suppress the sulfation in lead acid batteries using N-doped carbon derived from chitosan[END_REF]. As previously noticed for porous metal oxides, a good balance is necessary for the reconciliation of the two antagonistic phenomena, as crystallization occurs often at the expense of the open porosity content [START_REF] Brahmi | Low temperature synthesis of ordered mesoporous stable anatase nanocrystals: The phosphorus dendrimer approach[END_REF]. However, one may also notice that transforming amorphous carbon to crystalline graphene sheet does not induce an expansion of the crystal size, while it is commonly observed in metal oxide chemistry [START_REF] Kadib | Organophosphonate Bridged Anatase Mesocrystals: Low Temperature Crystallization, Thermal Growth and Hydrogen Photo-Evolution[END_REF]. The textural properties (i.e., high surface area and hierarchical porous structure) are among the most concerned properties of the chitosan derived N-PCs for their uses in different applications (e.g. metal-free catalysts and catalyst' supports in electro-and photo-catalysis). Direct thermal treatment of chitosan often leads to limited surface area. For example, Verma et al. observed a surface area of 92.83 m 2 g - 1 when the N-PC is prepared through the carbonization of chitosan at 300 °C under nitrogen atmosphere [START_REF] Verma | Porous nitrogen-enriched carbonaceous material from marine waste: chitosan-derived carbon nitride catalyst for aerial oxidation of 5hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid[END_REF]. In contrast, the activation of the ensuing carbon materials and/or the use of special additives, as discussed below, can benefit the forming of rich porosity of N-PCs.

Chemical activation is still commonly used to optimize the pore structure of N-PCs.

Activation of N-PCs is generally performed through two different methods: (i) the first one involves chitosan thermal treatment (450-700 °C), and the resulting carbonaceous materials is latter mixed with an activating agent (e.g., KOH, NaOH, ZnCl2, KHCO3, K2FeO4) and subjected to high temperature treatment (700-900 °C); (ii) the second approach consists rather on mixing chitosan first with the activator followed by high temperature treatment (600-950 °C) [START_REF] Jin | Chitosan-derived three-dimensional porous carbon for fast removal of methylene blue from wastewater[END_REF][START_REF] Liu | Activated carbon derived from chitosan as air cathode catalyst for high performance in microbial fuel cells[END_REF][START_REF] Marrakchi | Mesoporousactivated carbon prepared from chitosan flakes via single-step sodium hydroxide activation for the adsorption of methylene blue[END_REF]. The second method is however less reported in literature. Examples could be found in the work by Marrakchi et al., where NaOH was used as activator and led to N-PCs with a surface area of 318 m 2 g -1 [START_REF] Marrakchi | Mesoporousactivated carbon prepared from chitosan flakes via single-step sodium hydroxide activation for the adsorption of methylene blue[END_REF]. During the activation process, the reaction between the activator and the amorphous regions of the carbon framework leads to the removal of carbon species and the formation of soluble salts which could be degraded and/or washed away, leaving expanded empty pores [START_REF] Sevilla | Chemical and Structural Properties of Carbonaceous Products Obtained by Hydrothermal Carbonization of Saccharides[END_REF][START_REF] Hao | Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high performance supercapacitors[END_REF]. The effectiveness of this approach allows to broaden the range of potential applications of N-PCs to adsorption of dyes, supercapacitors and air cathode catalysts for high performance in microbial fuel cells, etc. [START_REF] Zhao | Sustainable nitrogen-doped carbonaceous materials from biomass derivatives[END_REF][START_REF] Wang | Monodispersed hard carbon spherules with uniform nanopores[END_REF][START_REF] Jin | Chitosan-derived three-dimensional porous carbon for fast removal of methylene blue from wastewater[END_REF][START_REF] Wang | Porous N-doped carbon material derived from prolific chitosan biomass as a high-performance electrode for energy storage[END_REF].

Notably, the activator loading and the temperature treatment play a pivotal role. The presence of a chemical activator under high temperature could enlarge the total surface area by creating more micropores through the depletion of carbon molecules from the char network. This, however, induces a decrease in the yield of the N-PCs product. Meanwhile, an excess amount of activator (e.g., KOH) and over-high temperature might reduce the total surface area due to the combination of two factors: (i) the adjacent micropores collapse and the formation of more mesopores; and (ii) the presence of K2CO3 and metallic potassium entrapped inside the carbon framework, which cannot be leached out, thereby inducing pore clogging [START_REF] Verma | Porous nitrogen-enriched carbonaceous material from marine waste: chitosan-derived carbon nitride catalyst for aerial oxidation of 5hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid[END_REF][START_REF] Liu | Activated carbon derived from chitosan as air cathode catalyst for high performance in microbial fuel cells[END_REF]. However, a detrimental effect was observed on nitrogen content that substantially decreased because of the favourable evaporation of nitrogen species such as ammonia (NH3) [START_REF] Lieder | N-doped mesoporous carbon nanosheets obtained by pyrolysis of a chitosan-melamine mixture for the oxygen reduction reaction in alkaline media[END_REF]. This discrepancy renders necessary the optimisation of the activation conditions. Liu and co-workers prepared a wide range of N-PCs by using three different KOH/char weight ratios (1, 2 and 3) at different temperatures (600-950 °C). N-PCs activated at 850 °C with a KOH/char weight ratio of 2 displayed the high surface area of 2334 m 2 g -1 as well as high pyridinic sites-N doping (33.4%). When used as a cathode catalyst, the obtained material exhibited a high power density of ~ 1435 mWm -2 , higher than that of the pristine activated carbon (967 mWm -2 ) [START_REF] Liu | Activated carbon derived from chitosan as air cathode catalyst for high performance in microbial fuel cells[END_REF]. In addition, a careful heat treatment during the activation process is necessary to create more porosity in N-PCs. After the pre-carbonization treatment at 450 °C for 4 h, a stepwise heating process (450 °C, 0.5 h; 600 °C, 0.5 h; 900 °C, 1.5 h) during the activation provided N-PCs with an impressive surface area up to 3496 m 2 g -1 [START_REF] Jin | Chitosan-derived three-dimensional porous carbon for fast removal of methylene blue from wastewater[END_REF].

The use of additives (e.g., urea, hexaketocyclohexane, acetic acid, polyethylene glycol and ionic liquids) during the hydrothermal treatment of chitosan imparts many advantages, among them: (i) a straightforward generation of an open porous structure with high surface area; (ii) an increase in nitrogen content and surface functional groups quantity; (iii) the avoidance of additional functionalisation steps; (iv) enhanced reagents accessibility; and (v) improved framework graphitization.

Jordan et al. reported an activation-free strategy for the preparation of highly porous N-PCs materials [START_REF] Jordan | Porous nitrogen-doped carbon monoliths derived from biopolymer-structured liquid precursors[END_REF]. Their approach consisted in crosslinking chitosan network in the presence of urea and hexaketocyclohexane, leading to a rigid monolithic-like polymer network (Figure 4). After the hydrothermal treatment (100 °C, 12 h), solvent exchange (to prevent significant shrinkage), CO2 supercritical drying (to preserve the porous structure) and carbonization (500 °C, 1 h), mesoporous materials featuring a surface area of 348 m 2 g -1 , a pore volume of 0.71 mLg -1 and a high N content of 12 wt.% were obtained. These materials demonstrated their efficiencies for carbon dioxide capture and dye removal through adsorption.

Alternatively, Ren et al. prepared carbon materials by pyrolysis of chitosan and urea mixture [START_REF] Ren | Selective and metal-free oxidation of biomass-derived 5-hydroxymethylfurfural to 2,5-diformylfuran over nitrogen-doped carbon materials[END_REF]. Urea decomposed to NH3 and then transferred nitrogen into the carbon framework during pyrolysis. The nature of nitrogen groups and the surface area were controlled by tuning the pyrolysis temperature. These experimental conditions affected the catalytic activity of the resulting material for the oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran.

The highest activity and catalyst stability were observed over the material pyrolyzed at 950 °C. With an impressive surface area of 2446 m 2 g -1 and a nitrogen content of 4.3 at.%, this material provided a full 5-hydroxymethylfurfural conversion with a diformylfuran yield of 95% after 14 hours at 100 °C under 10 bar [START_REF] Ren | Selective and metal-free oxidation of biomass-derived 5-hydroxymethylfurfural to 2,5-diformylfuran over nitrogen-doped carbon materials[END_REF]. The simultaneous use of additives and the activation process resulted also in ultrahigh surface area materials. As reported by Huang et al. [START_REF] Huang | Ultrahigh-surface-area hierarchical porous carbon from chitosan: acetic acid mediated efficient synthesis and its application in superior supercapacitors[END_REF], the presence of acetic acid favoured the construction of a conducive environment for accessibility of the activation agents (i.e., KOH), and the formation of a rigid semi-carbonized framework, leading to N-PCs with an ultrahigh surface area of 3532 m 2 g -1 and with a hierarchically porous carbon structure (Figure 5). The ensuing N-PCs material displayed abundant electrochemically active sites and benefited the rapid ion diffusion, leading to excellent capacitive behaviour. The use of polyethylene glycol as an additive and of KOH as an activator also afforded N-PCs with a high surface area of ~2269 m 2 g -1 [START_REF] Ba | Nitrogen-doped hierarchical porous carbon derived from a chitosan/polyethylene glycol blend for high performance supercapacitors[END_REF]. The PEG content affected the textural properties and should be controlled since an excess of PEG may result in large pores that can collapse during the heat treatment.

Owing to their particularism, ionic liquids operate as dispersant, structure-directing agent and nitrogen source when used as additives during the conversion of chitosan marine wastes into carbon materials [START_REF] Wu | Chitosan-based layered carbon materials prepared via ionic-liquid-assisted hydrothermal carbonization and their performance study[END_REF]. Cheng et al. prepared N-PCs from a polyelectrolyte derived from chitosan and betaine hydrochloride, where ionic interactions occur between the carboxylic acid groups in betaine hydrochloride and the amino groups in chitosan (Figure 6) [START_REF] Cheng | Ultrahigh-surface-area nitrogen-doped hierarchically porous carbon materials derived from chitosan and betaine hydrochloride sustainable precursors for high-performance supercapacitors[END_REF]. After a pre-carbonization step (400 °C for 1 h) under argon atmosphere followed by activation with potassium hydroxide (800 °C for 2 h), hierarchical N-PCs with an ultrahigh surface area (3300 m 2 g -1 ) and a large pore volume exceeding 2.24 cm 3 g -1 were obtained. It was claimed that the betaine hydrochloride improved both the graphitisation of the carbon framework and the creation of hierarchically porous structure (reflected in the high pore volume). The activation process was also found to be important for creating porous structure, but at the same time, it had a detrimental effect because of the destruction of the graphite crystals. The ensuing N-PCs exhibited good capacitive performance giving a high electrochemical capacitance (367 Fg -1 at a current density of 0.1 Ag -1 ), good retention capability during cycling stability (94% capacitance retention at current densities up to 4 Ag -1 after 20000 cycles), large power capability (high energy density of 12.7 Whkg -1 at a power density of 25 Wkg -1 ) [START_REF] Huang | Ultrahigh-surface-area hierarchical porous carbon from chitosan: acetic acid mediated efficient synthesis and its application in superior supercapacitors[END_REF]. Besides the activation treatment and the use of exogenous promoters, the introduction of a second heteroatom (i.e., P, B or S) also shows significant effects on the properties of the resulting carbon materials. This further enriches the surface defects and functionalities of the carbon network and induces synergies between nitrogen and other heteroatoms. The incorporation of heteroatoms has been reported to promote properties for various applications of chitosan-derived porous carbon materials, including catalysis for oxygen reduction reaction (ORR) [START_REF] Xu | A facile approach to suppress the sulfation in lead acid batteries using N-doped carbon derived from chitosan[END_REF][START_REF] Brahmi | Low temperature synthesis of ordered mesoporous stable anatase nanocrystals: The phosphorus dendrimer approach[END_REF], supercapacitance [START_REF] Lin | Facile synthesis of chitosan-based carbon with rich porous structure for supercapacitor with enhanced electrochemical performance[END_REF], and CO2 capture [START_REF] Jordan | Porous nitrogen-doped carbon monoliths derived from biopolymer-structured liquid precursors[END_REF].

For example, nitrogen and phosphorus co-doped porous carbon was synthesized in two steps by post-impregnation of chitosan-derived char with phosphoric acid followed by pyrolysis under N2 [START_REF] Liang | Nitrogen and phosphorus dual-doped carbon derived from chitosan: An excellent cathode catalyst in microbial fuel cell[END_REF]. The as-obtained N,P co-doped carbon displayed excellent specific surface area (982 m 2 g -1 ) at optimised pyrolysis temperature (850 °C, without activation treatment).

Interestingly, high power density of 1604 mWm -2 was recorded when used as a catalyst for air cathode in a microbial fuel cell. Nitrogen and boron co-doped activated carbon were also prepared through hydrothermal treatment of KOH-activated N-PCs with boric acid (160 °C, 5 h) [START_REF] Lin | Facile synthesis of chitosan-based carbon with rich porous structure for supercapacitor with enhanced electrochemical performance[END_REF]. KOH was added to the hydrothermal solution in order to expand the pore size and to further increase the specific surface area, and thus facilitated the incorporation of heteroatoms. The resulting N,B co-doped carbon material was used as electrode and exhibited preeminent specific capacitance performance (316 Fg -1 at 0.2 Ag -1 in a three-electrodes system), high energy density, low internal resistance and excellent cycle stability due to the high surface area (1129 m 2 g -1 ), rich active sites and ion channels [START_REF] Lin | Facile synthesis of chitosan-based carbon with rich porous structure for supercapacitor with enhanced electrochemical performance[END_REF]. Sun et al. synthesized porous graphitic carbon material bearing isolated boron and nitrogen sites and a high surface area of 1567 m 2 g -1 through hydrothermal coordination/ZnCl2-activation process from chitosan in the presence of a Fe catalyst. In this case, Fe 3+ ions were first coordinated with NH2 group of chitosan to form a chitosan-Fe complex, which prevented the formation of neutral Lewis acid-Lewis base B-N bond when boric acid was introduced. The incorporation of Fe 3+ ions promoted graphitization and also favoured nitrogen fixation. The resulting B and N co-doped N-PCs material showed a high capacitance of 313 Fg -1 at 1 Ag -1 and long-term durability and columbic efficiency of over 99.5% after 5000 cycles [START_REF] Sun | Isolated Boron and Nitrogen Sites on Porous Graphitic Carbon Synthesized from Nitrogen-Containing Chitosan for Supercapacitors[END_REF]. Hydrothermal treatment of chitosan and D-methionine mixture (200 °C, 12 h) followed by pyrolysis at 750 °C for 3 h led to N,S co-doped porous carbon material [START_REF] Li | Nitrogen/Sulfur-Codoped Carbon Materials from Chitosan for Supercapacitors[END_REF]. The as-prepared carbon material exhibited enhanced wettability and improved electrochemical performance and cycling stability. A specific capacitance of 135 Fg -1 at 1 Ag -1 , and capacitance retention of 97.2% after 5000 cycles were reached.

Incorporation of dopants and heteroatoms in N-containing carbon structures: synthesis and uses

The incorporation of clay [START_REF] Zhou | One-step synthesis of aminofunctionalized attapulgite clay nanoparticles adsorbent by hydrothermal carbonization of chitosan for removal of methylene blue from wastewater[END_REF], gelatine [START_REF] Yuan | Chitosan-derived hybrid porous carbon with the novel tangerine pith-like surface as supercapacitor electrode[END_REF], metal and non-metal species (Se, Si) [START_REF] Prasanna | Chitosan complements entrapment of silicon inside nitrogen doped carbon to improve and stabilize the capacity of Li-ion batteries[END_REF], in N-PCs could further broaden the applications of the corresponding chitosan-based materials.

Metal species, including precious and non-precious ones, incorporated into N-PCs are stabilized in the carbon framework, owing to the excellent chelating properties of the glucosamine and acetyl glucosamine units in chitosan that contain plenty of nitrogen atoms [START_REF] Kadib | Recent Progress in Chitosan Bio-Based Soft Nanomaterials[END_REF]. Chitosan plays indeed an important role not only as a nitrogen-containing precursor and a soft-templating agent, but also in controlling the hierarchical porous structure of the asderived metal-carbon composites. The resulting hierarchically porous nitrogen-doped carbonmetal composites are, at present, increasingly appealing for applications as electrode materials, heterogeneous catalysts, adsorbents to remove pollutants from water and so on. In the followings, we summarize the synthesis of chitosan-based porous nitrogen-doped carbonmetal composites and their applications in these fields.

Metal doped compositions-based electrode materials

Chitosan-based metal-carbon composites with different physicochemical properties have been designed according to specific target applications. The dispersion of the metal species, the textural features of the carbon network and the synergistic effect between the metal and the nitrogen species are crucial for energy storage capacity. In order to overcome the swelling of tin-based alloys that causes poor cycle life when applied as anodes in advanced Li-ion batteries, Shi et al. proposed a facile inorganic-organic nanostructured methodology based on hydrogel for uniform immobilization of ultrafine Sn-M (M = Fe, Co, Ni, and Cu) alloys in hierarchical carbon frameworks [START_REF] Shi | Double-Network Nanostructured Hydrogel-Derived Ultrafine Sn-Fe Alloy in Three-Dimensional Carbon Framework for Enhanced Lithium Storage[END_REF]. In their method, the freeze-drying and pyrolysis treatments of a double-network nanostructured gel, consisting of 3D intertwined cyanobridged Sn(IV)-Fe(II) inorganic gel and chitosan-glutaraldehyde organic polymer gel, led to ultrafine Sn-Fe alloy particles (average size of 2.7 nm) uniformly embedded in hierarchical 1D to 3D carbon framework (Figure 7). The ensuing Sn-Fe@C framework electrodes exhibited long cycle life (516 mAhg -1 after 500 cycles at 0.1 Ag -1 ) and high rate capability (491 and 270 mAhg -1 at 1 and 10 Ag -1 , respectively). Duan et al. synthesized CuP2@C through three steps involving CuCl2 adsorption on chitosan, fabrication of monolithic materials via freeze drying, thermal reduction, and phosphorylation (Figure 8) [START_REF] Duan | Chitosan Derived Carbon Matrix Encapsulated CuP2 Nanoparticles for Sodium-Ion Storage[END_REF]. In the resulting composite, crystalline CuP2 nanoparticles (~ 4 nm) were uniformly embedded in the carbon matrix. The resulting CuP2@C composite delivered a high reversible capacity of 630 mAhg -1 at a current density of 100 mAg -1 and a capacity retention ratio of 91% after 200 cycles, benefiting from the synergistic effect of small particle size and conductive carbon matrix [START_REF] Duan | Chitosan Derived Carbon Matrix Encapsulated CuP2 Nanoparticles for Sodium-Ion Storage[END_REF]. MoS2@N-doped carbon micro/nanocomposite with a continuous hierarchical flower ridge-like structure was prepared through a chitosan-induced one-pot hydrothermal and post-sintering method [START_REF] Wang | Chitosan-Induced Synthesis of Hierarchical Flower Ridge-like MoS2/N-Doped Carbon Composites with Enhanced Lithium Storage[END_REF]. The interconnected nano-flowers were composed of fewlayer MoS2 (less than 3 layers) nano-flakes with an expanded interlayer distance vertically growing on 2D N-doped carbon nanosheets in MoS2@N-C composite (Figure 9). This material showed high electrochemical performance, giving a reversible capacity of 905 mAhg -1 at 200 Ag -1 after 100 cycles, an outstanding cycle stability at high rates (742, 686, 534 mAhg -1 at 500, 1000, 2000 mAg -1 after 400 cycles, respectively) and superior rate performance. The hierarchical structure and the nitrogen content of the metal-carbon composites could be modulated by introducing zeolite imidazole frameworks (ZIFs) into the chitosan cryogels [START_REF] Li | Carbon materials derived from chitosan/cellulose cryogel-supported zeolite imidazole frameworks for potential supercapacitor application[END_REF]. After carbonization, the resulting ZIF-7@chitosan cryogel with microscopical sandwich and coral structure exhibited a suitable micropore content of 30%, low internal resistance less than 2 Ω, and high specific capacitance of 173.1 Fg -1 , which could serve as a potential supercapacitor electrode material [START_REF] Li | Carbon materials derived from chitosan/cellulose cryogel-supported zeolite imidazole frameworks for potential supercapacitor application[END_REF]. Guo et al. reported the preparation and use of Mn3O4@nitrogen-doped porous carbon nanocomposite as supercapacitance [START_REF] Guo | Anchoring Mn3O4 nanoparticles onto nitrogen-doped porous carbon spheres derived from carboxymethyl chitosan as superior anodes for lithium-ion batteries[END_REF]. The material was prepared via a solvothermal treatment of chitosan in the presence of manganese acetate tetrahydrate. The as-obtained nanocomposite featured crystalline Mn3O4 nanoparticles with a size of 10-20 nm. When used as anode materials for lithium-ion batteries, the nanocomposite displayed a high specific capacity of 1629 mAhg -1 at 100 mAg -1 after 100 cycles, higher than the values reached by pure Mn3O4 and nitrogen-doped porous carbon spheres [START_REF] Guo | Anchoring Mn3O4 nanoparticles onto nitrogen-doped porous carbon spheres derived from carboxymethyl chitosan as superior anodes for lithium-ion batteries[END_REF]. Copied with permission from Ref. [START_REF] Advani | Stabilization of palladium nanoparticles on chitosan derived N-doped carbon for hydrogenation of various functional groups[END_REF]. Copyright 2019 Elsevier B.V.

Metal doped compositions in conventional heterogeneous catalysis and electrocatalysis

Precious metal-doped compositions in conventional heterogeneous catalysis

The catalytic performance of chitosan-derived metal-carbon nanocomposites are generally related to their textural properties, the presence of nitrogen species and the uniform dispersion of metal nanoparticles. For instance, Xu et al. prepared Pd supported on N-doped hierarchically porous carbon (Pd@N-C) using pluronic P123 as a porogen, PdCl2 as a Pd source and chitosan as a nitrogen and carbon source in a one-pot carbonization process [START_REF] Xu | One-pot carbonization of chitosan/P123/PdCl2 blend hydrogel membranes to N-doped carbon supported Pd catalytic composites for Ullmann reactions[END_REF].

A moderate carbonization temperature (750 °C) was necessary to obtain both relatively high surface area (159 m 2 g -1 ) and metal dispersion (Pd particle size of 5-20 nm). The resulting Pd@N-C catalyst showed high activity in the Ullmann homo-coupling reactions but unfortunately suffered from deactivation. Advani et al. improved the Pd nanoparticle dispersion and stability [START_REF] Advani | Stabilization of palladium nanoparticles on chitosan derived N-doped carbon for hydrogenation of various functional groups[END_REF]. The one-pot carbonization of Pd-chitosan complex at 800 °C under N2 atmosphere led to a uniform distribution of Pd nanoparticles with an average size of 2-6 nm (Figure 10). The as-prepared Pd@N-C catalyst exhibited high catalytic performances in the hydrogenation of various unsaturated or reducible groups including C=C, C≡C, -NO2, -

CHO and epoxides at room temperature (30 °C) and moderate H2 pressure (5 bar). Nearly complete conversion, high selectivity and recyclability for five runs were observed. The excellent catalytic performance was attributed to the well-dispersed and stabilized Pd nanoparticles and the outer electronic environment stemming from nitrogen. (Ag@N-C) nanocomposite through hydrothermal treatment of chitosan hydrogel followed by pyrolysis (Figure 11) [START_REF] Alhokbany | AgNPs embedded N-doped highly porous carbon derived from chitosan based hydrogel as catalysts for the reduction of 4-nitrophenol[END_REF]. The as-synthesized Ag@N-C nanocomposite exhibited high specific surface area (875 m 2 g -1 ), uniformly dispersed AgNPs (mean size of 13 nm), and excellent adsorption properties. From a catalytic point of view, the Ag@N-C catalyst could reduce 98% of 4-nitrophenol to 4-aminophenol under visible light within 40 min, and could be used for seven successive cycles without obvious loss of activity.

The incorporation of a second heteroatom in N-doped carbon framework could effectively modulate the electronic structure of active metals and thus help boosting the catalytic properties. Zhang et al. reported a one-pot synthesis of PdP alloy nanoparticles immobilized on N-doped graphitic carbon nanosheets (Pd-P@N-C) by using inorganic-organic hybrid cyanogel as a precursor [START_REF] Zhang | Cyanogelderived N-doped C nanosheets immobilizing Pd-P nanoparticles: One-pot synthesis and enhanced hydrogenation catalytic performance[END_REF]. The unique 2D sheet-like nanostructure of the resulting material accounted for its high catalytic activity in hydrogenation of 4-nitrophenol to 4-aminophenol.

The fabrication of chitosan-based bimetallic-carbon composites for different catalysis applications has also been reported. Verma et al. synthesized a carbon nitride support via direct pyrolysis of chitosan and used it as a support to prepare a magnetically separable bimetallic AgPd/Fe@CN catalyst by stepwise impregnation and chemical reduction method [START_REF] Verma | Visible light-mediated and water-assisted selective hydrodeoxygenation of lignin-derived guaiacol to cyclohexanol[END_REF]. This catalyst featured monophasic silver, palladium and ferrites, with particle sizes ranging from 10 to 15 nm and high activity for the dissociation of formic acid to hydrogen [START_REF] Verma | Visible light-mediated and water-assisted selective hydrodeoxygenation of lignin-derived guaiacol to cyclohexanol[END_REF].

Non-precious metal doped compositions in conventional heterogeneous catalysis and electrocatalysis

Compared to noble metal@carbon composites, non-precious metal-carbon composites are also highly desirable owing to the abundance and low cost of the starting metal precursors.

These composites have shown high potential when applied in thermal catalysis and, especially, in electrocatalysis.

The control of the chemical valence states of the metal species in the non-precious metal carbon composites significantly affects their catalytic performances. In the pioneering work by Beller and co-workers, different Co/N-C catalysts were applied for different hydrogenation transformations. In 2017, they prepared a cobalt carbon composite by complexing Co(OAc)2 with chitosan (70 °C, 20 h, in ethanol) followed by pyrolysis under optimal conditions (700 °C, 2 h, in Ar). The as-synthesized material (Co-Co3O4@Chitosan-700) was composed of three different types of cobalt species: metallic cobalt, cobalt oxides and cobalt particles with a metallic core and an oxide shell, all of which were embedded in the carbon matrix. This catalyst was successfully employed for the first time in hydrodehalogenation of a wide range of alkyl and aryl halides and presented a high activity at 120-140 °C under 30-50 bar H2 (e.g., 4-bromo-biphenyl to the corresponding non-bromo compound with a 95% yield) [START_REF] Sahoo | A Biomass-Derived Non-Noble Cobalt Catalyst for Selective Hydrodehalogenation of Alkyl and (Hetero)Aryl Halides[END_REF], and also succeeded in the hydrogenation of nitroarenes (e.g., 2,4,6-tri-tert-butylaniline obtained in a 90% yield from the corresponding nitrosubstrate at 110 °C under 40 bar H2) [START_REF] Sahoo | Biomass-Derived Catalysts for Selective Hydrogenation of Nitroarenes[END_REF]. This catalyst could be recycled for four runs without obvious loss of activity.

In the following year, the same group reported a similar catalyst -Co@Chitosan-700 but without any crystalline cobalt oxides (Co3O4 or CoO) [START_REF] Scharnagl | Hydrogenation of terminal and internal olefins using a biowaste-derived heterogeneous cobalt catalyst[END_REF]. This catalyst was highly stable in air and water for months in storage. This system was active without putting any other additives in the selective hydrogenation of diisobutane in the presence of several solvents: high yields of 2,2,4-trimethylpentane (over 88%) were obtained in heptane, methanol, and water at 40 °C under 10 bar H2. In water, elevated temperature (150 °C) and H2 pressure (60 bar) could lead to a 2,2,4-trimethylpentane yield of 95% after 5 hours. Recycling studies demonstrated that the catalyst stayed stable for 6 consecutive runs. In order to gain insight into the effects of different supports on the catalytic performances of this Co-containing material, the same team fabricated different supported cobalt materials by impregnating commercial supports (i.e., carbon, titania, silica, ceria, alumina) with cobalt (II) acetate in presence of different N-containing organic ligands like chitosan and phenonthroline (phen) [START_REF] Hertrich | Supported Cobalt Nanoparticles for Hydroformylation Reactions[END_REF]. High target products yields from n-butyl acrylate were obtained with Co/phen@TiO2 (82%) and Co/phen@C (83%) while low yields were obtained with Co/chitosan@TiO2 (32%) due to in situ formation of a metal carbonyl species.

Non-precious metal carbon composites are widely used in electrocatalysis. In fact, nitrogendoped carbon catalysts alone have proven to be active for ORR as illustrated by the work of Liu et al. (nitrogen-doped carbon catalyst prepared from chitosan and urea) [START_REF] Liu | Direct Synthesis of Nitrogen-Doped Carbon Nanosheets with High Surface Area and Excellent Oxygen Reduction Performance[END_REF] and the work of Wang et al. (nitrogen-doped carbon catalyst fabricated from phenolic resin and chitosan) [START_REF] Wang | Phenolic resin/chitosan composite derived nitrogen-doped carbon as highly durable and anti-poisoning electrocatalyst for oxygen reduction reaction[END_REF]. However, the electrocatalytic performance of metal-free doped carbon catalysts is still unsatisfactory (e.g., their limited stability because of problems such as microporous flooding, protonation of active sites and oxidation of carbon). Due to good metal dispersion and the presence of active nitrogen species, non-precious metal-carbon nanocomposites showed higher potential in electrocatalysis, namely, oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and other electrocatalysis.

Notably, metal dispersion is largely affected by the pyrolysis temperature necessary to convert the carbohydrate skeleton to a carbon framework. Increasing the temperature treatment during the pyrolysis of chitosan-Co 2+ from 700 °C, 800 °C to 900 °C resulted in a Co@N-C composite with an increase of Co particle size from 24.9 nm, 34.4 nm to 78 nm [START_REF] Zhang | Cobalt nanoparticles anchoring on nitrogen doped carbon with excellent performances for transfer hydrogenation of nitrocompounds to primary amines and N-substituted formamides with formic acid[END_REF]. As demonstrated by Xie et al., the uniform dispersion of metal species and high content of active nitrogen sites were mandatory for ORR activity [START_REF] Xie | Chitosan Waste-Derived Co and N Co-doped Carbon Electrocatalyst for Efficient Oxygen Reduction Reaction[END_REF]. The authors designed a cobalt and nitrogen co-doped carbon catalyst (Co@N-C) by pyrolysis of chitosan-cobalt mixture (Figure 12). The resulting Co@N-C was composed of randomly stacked graphite-like planes, with uniform dispersion of Co particles. Different types of N species, pyridinic N, pyrrolic N, and graphitic N were present in the obtained catalyst. The high contents of the pyridinic N and the graphitic N in Co@N-C led to better ORR performance (current density of 5.36 mAcm -2 and a half-wave potential of -0.105 V) than that of the commercial Pt/C catalyst (5.04 mAcm -2 and 0.120 V) with higher methanol tolerance and better long-term stability [START_REF] Xie | Chitosan Waste-Derived Co and N Co-doped Carbon Electrocatalyst for Efficient Oxygen Reduction Reaction[END_REF]. with Pt-like over potential, in N-PCs [START_REF] Hu | Alveolate porous carbon aerogels supported Co9S8 derived from a novel hybrid hydrogel for bifunctional oxygen electrocatalysis[END_REF]. For example, Hu et al. developed a honeycomblike Co9S8-doped carbon aerogel via sol-gel polymerization of chitosan, sodium tripolyphosphate and polyhexamethyleneguanidine phosphate [START_REF] Hu | Alveolate porous carbon aerogels supported Co9S8 derived from a novel hybrid hydrogel for bifunctional oxygen electrocatalysis[END_REF]. The repeating units of the biopolymer contain binding sites (-NH2) for Co 2+ ions, leading to a uniform anchorage of Co9S8 particles within the carbon aerogels after pyrolysis (Figure 13). The resulting catalyst revealed positive onset potential (0.89 V) and half-wave potential (0.78 V) for the ORR, and a low over-potential (363 mV) at 10 mAcm -2 for the OER. When other precursors are involved (carbon or heteroatom sources, chelating ligands, stabilizers), the order of addition was shown to affect the material composition, its texture and final properties [START_REF] Shijina | Chitosan Intercalated Metal Organic Gel as a Green Precursor of Fe Entrenched and Fe Distributed N-Doped Mesoporous Graphitic Carbon for Oxygen Reduction Reaction[END_REF]. Indeed, the chelating of iron chloride with trimesic acid first followed by chitosan complexation and pyrolysis led to iron-carbon complex (CHI-TMA-FeCW-M1) with high surface area (565 m 2 g -1 ), high nitrogen content (1.6 at.%), significant graphitic nitrogen amount and low iron content (0.01 at.%). In turn, chelating of iron chloride with chitosan first followed by the addition of trimesic acid led to iron-carbon complex (CHI-TMA-Fe-CW) with relatively low surface area (381 m 2 g -1 ), low nitrogen content (0.2 at.%), low graphitic nitrogen and high iron content (0.06 at.%) (Figure 14). This discrepancy was attributed to the stronger gel network formed during the synthesis of CHI-TMA-FeCW-M1.

The increased N content, especially the graphitic N content, in CHI-TMA-Fe-CW-M1 played a pivotal role in achieving much higher ORR activity than that of CHI-TMA-Fe-CW.

Besides, the entrapment of bimetallic species can also bring high catalytic activity. For example, Dong et al. showed that Zr and Fe co-doping in carbon provided a remarkable enhancement of the ORR activity in both acidic and alkaline media, giving a current density doped carbon catalyst -at 0.9 V, which was 1.7 times higher than that of Fe 2 of 0.64 mAcm [START_REF] Dong | Enhancement of Oxygen Reduction Performance of Biomass-Derived Carbon through Co-Doping with Early Transition Metal[END_REF]. The improvement of the catalytic activity was assigned to the benefits brought by Zr--Fe - backbone through electrostatic interactions of the carboxylic acid groups with cationic NH and then coating the mixture over nano-sized silica spheres, etching-off silica template and performing pyrolysis (Figure 15) [START_REF] Ma | Bio-inspired chitosan-hemevitamin B12-derived Fe-Co bimetallic-doped mesoporous carbons for efficiently electroactivating oxygen[END_REF]. The as-obtained Fe-Co@N-C composites showed an Chitosan-based metal carbide@carbon nanocomposites work also well in HER and other electrocatalysis applications. For example, Wei et al. reported a Mo2C-based electrocatalyst prepared by spray-drying method, and followed by annealing. The electrostatic interaction between the Mo7O2 4-and the NH3 + groups of chitosan provided the driving force for mixing (NH4)6Mo7O24 used as source of Mo with chitosan before pyrolyzing the mixture at about 750 °C [START_REF] Wei | Molybdenum Carbide Nanoparticles Coated into the Graphene Wrapping N-Doped Porous Carbon Microspheres for Highly Efficient Electrocatalytic Hydrogen Evolution Both in Acidic and Alkaline Media[END_REF]. The as-prepared electrocatalyst (Mo2C@G-NCS) displayed ultrafine Mo2C nanoparticles, graphene wrapping (ensuring electrical conductivity), porous structure, and Ndopant modifying the electronic structures of adjacent Mo and C atoms. As a result, Mo2C@G-NCS demonstrated superior HER activity with a very low over-potential of 70 or 66 mV to achieve a current density of 10 mAcm -2 , and a small Tafel slope of 39 or 37 mVdec -1 , respectively in acidic and alkaline media, together with high stability. Wang et al.

reported the use of tungsten carbide@graphitic nanocarbon composite-supported Pt catalyst in the electro-oxidation of methanol, where WC@GC was fabricated by incorporation of Ni 2+ and [PW12O40] 3-in chitosan to form a complex thanks to the strong coordinating ability of PW12 with amino groups. Ni 2+ was used as the catalyst source during the carbonization of the as-obtained complex to form graphitic nanocarbon (GC). The resulting catalyst was able to afford high catalytic activity and stability [START_REF] Wang | Chitosan: a green carbon source for the synthesis of graphitic nanocarbon, tungsten carbide and graphitic nanocarbon/tungsten carbide composites[END_REF].

Metal doped compositions in adsorption applications

Chitosan and related systems have been proposed as green adsorbents for the removal of heavy metals [START_REF] Shen | Chitosan-derived carbonaceous material for highly efficient adsorption of chromium (VI) from aqueous solution[END_REF], organic dyes [START_REF] Jin | Chitosan-derived three-dimensional porous carbon for fast removal of methylene blue from wastewater[END_REF][START_REF] Marrakchi | Mesoporousactivated carbon prepared from chitosan flakes via single-step sodium hydroxide activation for the adsorption of methylene blue[END_REF], aromatic [START_REF] Stavropoulos | Effect of activated carbons modification on porosity, surface structure and phenol adsorption[END_REF] and linear hydrocarbons [START_REF] Nisticò | Chitosan-Derived Iron Oxide Systems for Magnetically Guided and Efficient Water Purification Processes from Polycyclic Aromatic Hydrocarbons[END_REF] from waste water. For example, Zhou et al. reported a kind of amino-functionalized attapulgite clay nanoparticle adsorbent (ATP@CCS) fabricated by hydrothermal carbonization of chitosan at a mild temperature [START_REF] Zhou | One-step synthesis of aminofunctionalized attapulgite clay nanoparticles adsorbent by hydrothermal carbonization of chitosan for removal of methylene blue from wastewater[END_REF]. The resulting composite pre-treated in acidic solution exhibited large adsorption capacity for the removal of methylene blue from wastewater.

Nistico ̀ et al. fabricated a magnetic iron oxide@carbon composite via a one-step coprecipitation method using iron chloride, iron sulfate and chitosan as precursors and aqueous ammoniac as precipitating agent, followed by pyrolysis [START_REF] Nisticò | Chitosan-Derived Iron Oxide Systems for Magnetically Guided and Efficient Water Purification Processes from Polycyclic Aromatic Hydrocarbons[END_REF]. The resulting composite combined excellent adsorption properties attributed to chitosan and magnetic properties owing to the presence of iron oxide, allowing its easy recovery from the medium.

Wang et al. reported a nitrogen doped mesoporous carbon matrix-supported tungsten oxide composite material (WO3-x@N-C) prepared from chitosan and polyoxometalates as precursors through calcination. The as-obtained material featured small tungsten oxide nanoparticles (around 5-8 nm) uniformly dispersed in the carbon matrix and displayed excellent activity in the removal of organic dyes, such as Rhodamine B and methyl orange, where oxygen vacancies from WO2 and the good conductivity of nitrogen doped carbon matrix seemed to be the key for its enhanced reactivity [START_REF] Wang | WO3-x based composite material with chitosan derived nitrogen doped mesoporous carbon as matrix for oxygen vacancy induced organic pollutants catalytic reduction and IR driven H2 production[END_REF].

As a conclusion, N-PCs materials are generally synthesized via either hydrothermal and/or pyrolysis treatment. The use of exogenous activators (e.g., KOH, NaOH, ZnCl2, KHCO3, K2FeO4) and/or additives (e.g., urea, hexaketocyclohexane, acetic acid, polyethylene glycol) during the preparation procedure provides N-PCs with high specific surface area (classically reported above 1000 m 2 g -1 ), increased nitrogen content (up to 12 wt.%) and surface functional groups (OH, NH/NH2, COOH), and improved framework graphitization. The amount of the activator (per mass) and the temperature treatment are the most important parameters for the preparation of porous N-PCs materials. Without any doubt, the preparation of chitosan-based N-PCs materials with attractive characteristics, such as specific morphologies, dimensionality and configurations that derive from chitosan templating and supramolecular chemistry, is key to broaden their potential applications. There is however still an urgent need for further innovative preparation methods to access N-PCs with high nitrogen contents and to better control the opening porosity. To this respect, several issues should be tackled, including: (i) efficient synthesis approach for ordered porous structures of N-PCs, (ii) accurate control of the selective formation of nitrogen species in N-PCs to tune the electronic/chemical properties and help unveiling the exact mechanism, and lastly (iii) scalable method for large scale production towards commercialization.

The co-doping of N-PCs with a second heteroatom (i.e., P, B and S) increases surface defects and supplies additional reactivity with respect to nitrogen. In spite of a huge progress, a fundamental understanding of the doping effects and structural diversity of such N-PCs materials is still largely lacking, especially regarding the molecular structures of active centers (e.g., dopant type, location, distribution) in the catalysis processes, for example, and the specific doping effects that control electrocatalytic reactions.

The chelating property of chitosan makes it a highly suitable precursor for the immobilization of metal species in the carbon matrix. The chitosan derived metal-carbon composites show beneficial physicochemical properties, namely, large specific surface area, uniform metal dispersion, existence of active nitrogen species, which facilitate their applications in thermal/electro-catalysis and adsorptions. However, it is still challenging so far to prepare highly dispersed metal-carbon composites, especially those with average particle size less than 10 nm. To do so, the exploration of suitable chelating additives and further optimisation

Synthesis of N-doped graphene and metal-N doped graphene composites

While graphite is one of the earliest known carbon allotrope, its recent pilling off that provides graphene (G) has revolutionized the realm of carbon materials. Graphene is built from tiny sheets of one-atom-thick layers of sp 2 carbon atoms assembled in hexagonal arrangement, constituting the physical limit for 2D materials [START_REF] Zhu | New solutions to a new problem[END_REF]. Since its discovery, graphene has received considerable attention owing to its high mechanical and chemical resistances, light weight, charge mobility, and electrical conductivity [START_REF] Xu | What is the choice for supercapacitors: graphene or graphene oxide?[END_REF]. In addition, graphene doped with heteroatoms results in new materials with different electrical, physicochemical and structural properties [START_REF] Chernyak | Mechanism and kinetics of decomposition of Ncontaining functional groups in few-layer graphene nanoflakes[END_REF][START_REF] Chernyak | N-doping and oxidation of carbon nanotubes and jellyfish-like graphene nanoflakes through the prism of Raman spectroscopy[END_REF], and thus could serve as an advantageous alternative to the current materials used in supercapacitors [START_REF] Xu | What is the choice for supercapacitors: graphene or graphene oxide?[END_REF], dye-sensitized solar cells [START_REF] Li | Boron Doping of Graphene for Graphene-Silicon p-n Junction Solar Cells[END_REF], electrochemical and electronic sensors [START_REF] Kwon | Flexible FET-Type VEGF Aptasensor Based on Nitrogen-Doped Graphene Converted from Conducting Polymer[END_REF] and gas storage [START_REF] Parambhath | Effect of Nitrogen Doping on Hydrogen Storage Capacity of Palladium Decorated Graphene[END_REF]. Common route to access G-based materials consists in the Hummers oxidation of graphite using potassium permanganate in a mixture of sulfuric and nitric acids, and subsequent H2O2 addition [START_REF] Hummers | Preparation of Graphitic Oxide[END_REF].

However, this procedure is limited by several drawbacks: (i) the generation of a large amount of wastes containing high concentration of transition metals at strongly acidic pH values; (ii) the low efficiency of the direct exfoliation of the starting stacked graphite layers; (iii) in the case of viscous solvents, tedious separation of graphene from its medium [START_REF] Primo | High-yield production of N-doped graphitic platelets by aqueous exfoliation of pyrolyzed chitosan[END_REF]. Besides, Ndoped graphene ((N)G) is often prepared through wet chemical methods [START_REF] Wu | Enhancing the electrochemical reduction of hydrogen peroxide based on nitrogen-doped graphene for measurement of its releasing process from living cells[END_REF], and thermal annealing of graphene oxides (GO) with exogenous heteroatom-containing precursors [START_REF] Jun | Enhanced conduction and charge-selectivity by N-doped graphene flakes in the active layer of bulkheterojunction organic solar cells[END_REF].

However, most of these methods require complex procedures and/or harsh conditions, and suffer from low yield and high cost [START_REF] Wang | Heteroatom-doped graphene materials: syntheses, properties and applications[END_REF]. In contrast, the synthesis of (N)G directly from natural polysaccharides (i.e., chitosan), allows for circumventing these setbacks and afford additional benefits of low costs and wide availability. This section summarizes the preparation methods, formation mechanism and the corresponding physicochemical properties of G-based materials derived from chitosan.

Pyrolysis of metal-free chitosan, shaped as nano powders, films, hydrogels or aerogels, leads to the formation of high quality (N)G. A breakthrough in the field was pioneered by Primo and Garcia. In 2012, they succeeded in preparing layer-structured graphene with a thickness of a few nanometers through pyrolysis of spin-coated chitosan films [START_REF] Primo | From biomass wastes to large-area, high-quality, N-doped graphene: catalyst-free carbonization of chitosan coatings on arbitrary substrates[END_REF]. The obtained (N)G

showed high catalytic activity in the glucose oxidation to succinic acid owing to the presence of quaternary (graphitic) nitrogen atoms, which are among the most active sites on graphene to activate O2 [START_REF] Rizescu | N-Doped graphene as a metal-free catalyst for glucose oxidation to succinic acid[END_REF]. The same authors also succeeded in preparing N-doped graphene from chitosan aerogels, taking advantage of the use of (NH4)2SO4 and KOH as the carbonization promoter and activation agent, respectively [START_REF] Primo | From Biomass Wastes to Highly Efficient CO2 Adsorbents: Graphitisation of Chitosan and Alginate Biopolymers[END_REF]. Because of the solubility of chitosan in acids, the use of H2SO4 to promote the carbonization of the aerogels was discarded. For this reason, H2SO4 was formed from in situ decomposition of (NH4)2SO4, which was mixed with chitosan. KOH served to promote the creation of pores that arise from corrosion induced by the strong base as described in the previous section. The as-prepared graphene displayed high CO2 adsorption capacity (11.5 mmolcm -3 , after adsorption of 0.4 bar of CO2 by the carbon, the bed was submitted to 650 mA and 0.3 bar of CO2 desorption was observed in 5 min) owing to its graphitic carbon structure, high nitrogen content of 11.5 wt.%, high surface area of 970 m 2 g -1 and ultra-microporosity, spherical micrometric morphology of the beads.

Materials displayed high density and electrical conductivity (0.21 Ωm -1 ) [START_REF] Primo | From Biomass Wastes to Highly Efficient CO2 Adsorbents: Graphitisation of Chitosan and Alginate Biopolymers[END_REF]. Notably, the facile exfoliation of pyrolyzed chitosan has been substantiated in different solvents (e.g., water, ethanol, acetonitride, tetrahydrofuran, dichloromethane) to afford Ndoped graphitic platelet suspension [START_REF] Primo | High-yield production of N-doped graphitic platelets by aqueous exfoliation of pyrolyzed chitosan[END_REF]. Insight was further gained on acidic/basic properties of N-doped graphene (obtained from chitosan) by thermo-programmed desorption (TPD) with CO2 and NH3, respectively, where the concentration of basic sites was about 22 times higher than that of the acid sites. Comparison of the total amount of basic sites determined by CO2 adsorption with the N content showed that only 6.8% N atoms present in (N)G exhibited measurable basicity against CO2. These materials, when synthesized according to the procedure described in Figure 16, revealed also high catalytic activity in C-C coupling such as Michael and Henry additions [START_REF] Candu | Nitrogen-doped graphene as metal free basic catalyst for coupling reactions[END_REF]. A comparative study revealed the role of nitrogen in these materials: as compared to borate and phosphate esters of alginic acid-derived borondoped and phosphorus-doped graphene, chitosan-derived (N)G showed higher catalytic activity in CO2 hydrogenation to methane, which was assigned to the basicity introduced in the (N)G by pyridinic N atoms, as well as significant stability at temperatures below 500 °C [START_REF] Jurca | N-Doped Defective Graphene from Biomass as Catalyst for CO2 Hydrogenation to Methane[END_REF].

Notably, the pyrolysis temperature plays a key role because it affects directly the physicochemical properties of (N)G by influencing the graphene quality. Higher pyrolysis temperature (i.e., 700-1000 °C) leads to lower content of amorphous carbon residue in graphene and thus higher quality of graphene product [START_REF] Hao | Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high performance supercapacitors[END_REF]. The temperature affects also the density of the surface defects, which are related to the content of nitrogen on graphene.

Increasing the temperature from 900 to 1200 °C results in the decrease of N content in graphene [START_REF] Latorre-Sánchez | p-n Heterojunction of Doped Graphene Films Obtained by Pyrolysis of Biomass Precursors[END_REF]. Graphitisation at 900 °C under argon atmosphere is mostly applied as a result of balancing the graphene quality and N content to give graphene with a high quality and at the same time adequate surface defect density. Besides, the use of carbonization promotors like (NH4)2SO4 improves chitosan graphitization. Lastly, the activation agent, generally KOH, favours the formation of micropores and affords high surface area with hierarchical porous structure [START_REF] Guo | Role of Hydrogen Peroxide Preoxidizing on CO2 Adsorption of Nitrogen-Doped Carbons Produced from Coconut Shell[END_REF][START_REF] Shi | Activated Carbons Derived from Hydrothermally Carbonized Sucrose: Remarkable Adsorbents for Adsorptive Desulfurization[END_REF]. The effect of KOH activation on graphene structure under different temperatures was then reported [START_REF] Hao | Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high performance supercapacitors[END_REF]. When the pyrolysis temperature was over 800 °C, the exothermic reaction between amorphous carbon and KOH led to the gradual etching-away of the amorphous carbon and to a decreased quantity of carbon particles attached to the graphene sheet. At 1000 °C, all the amorphous carbon could be removed and resulted in a pure graphene framework.

The combination of urea and KOH during chitosan carbonisation was also investigated [START_REF] Lee | Straightforward Synthesis of Hierarchically Porous Nitrogen-Doped Carbon via Pyrolysis of Chitosan/Urea/KOH Mixtures and Its Application as a Support for Formic Acid Dehydrogenation Catalysts[END_REF].

As shown in Figure 17, above 500 °C urea was transformed into g-C3N4 sheets, between which chitosan-derived carbon was infiltrated. At the same time, KOH initiated carbon the formation of boron-nitrogen (B, N) co-doped graphene. Modification of aqueous chitosan solution with borate followed by pyrolysis of the corresponding boron-containing chitosan at 900 °C and final exfoliation gave (B, N) co-doped graphene ((B, N)G). The presence of these two heteroatoms in the graphene sheets was ascertained by means of XPS analyses. The asprepared (B, N)G was used as a metal-free carbon oxidative catalyst of aromatic hydrocarbons and cyclooctane to the corresponding alcohol/ketone mixture. (B, N)G promoted the formation of hydroperoxides which could act as radical initiators in the oxidation mechanism [START_REF] Dhakshinamoorthy | Doped Graphene as a Metal-Free Carbocatalyst for the Selective Aerobic Oxidation of Benzylic Hydrocarbons, Cyclooctane and Styrene[END_REF]. Apart from (B, N)G, other dopants including phosphorus/sulphur, nitrogen (P/S, N) doped graphene are also appealing functional graphene materials [START_REF] Ananthanarayanan | Nitrogen and phosphorus co-doped graphene quantum dots: synthesis from adenosine triphosphate, optical properties, and cellular imaging[END_REF][START_REF] Zhang | Nitrogen, Phosphorus, and Fluorine Tri-doped Graphene as a Multifunctional Catalyst for Self-Powered Electrochemical Water Splitting[END_REF][START_REF] Ito | High Catalytic Activity of Nitrogen and Sulfur Co-Doped Nanoporous Graphene in the Hydrogen Evolution Reaction[END_REF][START_REF] Liang | Sulfur and Nitrogen Dual-Doped Mesoporous Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance[END_REF]. However, based on our literature survey, the synthesis of these materials using chitosan as N/C source has not been reported so far.

Alternative approach of doping consists in the combination of N-graphene with boron-doped graphene, boron nitride flacks and urea-derived carbon nitride, where different electronic/catalytic properties could be obtained. For example, a p-n heterojunction was built up between chitosan derived (N)G film and borate ester of alginate derived B-doped graphene ((B)G) film by stepwise coating of a quartz plate using a mask [START_REF] Candu | Nitrogen-doped graphene as metal free basic catalyst for coupling reactions[END_REF]. The heterojunction was created by the partial overlapping of the (B)G-(N)G films. Outstandingly, Pt nanoparticles grew preferentially on (N)G due to higher population of electrons while MnO2 nanoparticles seed grew on (B)G due to the excessive positive holes. The (B)G-(N)G conjunction benefited the preferential electron migration from (B)G to (N)G, increasing the photocurrent and the photo voltage by at least a factor of 3 and 2, respectively [START_REF] Latorre-Sánchez | p-n Heterojunction of Doped Graphene Films Obtained by Pyrolysis of Biomass Precursors[END_REF]. Two years later, few-layer (N)G/boron nitride (h-BN) sheets were accessed through pyrolysis at ammonium borate salt as a precursor (Figure 18). The existence of a smooth graphene film covering the cracked boron nitride was confirmed by a set of characterisations. The thickness of the N(G) and h-BN layers was controlled by varying the precursor concentrations and the spin coating rate, and was typically below 5 nm. The resulting film exhibited high electrical resistance but with no improvement in electrical conductivity with respect to the conductivity values of N-doped graphene [START_REF] Esteve-Adell | Catalyst-free one step synthesis of large area vertically stacked N-doped graphene-boron nitride heterostructures from biomass source[END_REF]. In their attempt to rationalize the thermally-induced structural evolution of the related carbon materials, Hao et al. described the preparation of (N)G from chitosan aerogel precursor, as illustrated in Figure 19 [START_REF] Hao | Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high performance supercapacitors[END_REF]. In acetic acid aqueous solution, a planar network consisting of chitosan and acetate was formed through hydrogen bonding between chitosan chains, and between chitosan and acetate molecules. Quick-freezing and lyophilisation led to the arrangement of the secondary structure of macromolecular chitosan into film with 3-D amorphous carbon and graphene nanosheets. Nitrogen atoms emanating from the decomposition of NH2 groups remained in the material framework to form N-doped carbon aerogel. During the high temperature activation, amorphous carbon was gradually etched away by reaction with potassium hydroxide. When the activation temperature was high enough (e.g., at 1000 ºC), a pure graphene framework could be obtained. Apart from being potential metal-free catalysts in different applications as described above, it is also interesting to use the corresponding (N)Gs as heterogeneous supports of metal NPs catalysts due to their unique properties: (i) large specific surface area totally accessible; (ii)

the extended and n orbitals on (N)G interact strongly with the empty d orbitals of transition metal atoms at the interface; (iii) (N)G can also adsorb substrates and reagents through π-π stacking and hydrophobic interactions, bringing them at the proximity of the active metal NPs for activation and conversion [START_REF] Blandez | Copper Nanoparticles Supported on Doped Graphenes as Catalyst for the Dehydrogenative Coupling of Silanes and Alcohols[END_REF]. It has been observed that the activity of the metal NPs on (N)G can be higher than on other supports because of the combination of the above effects.

Distinctively, Primo et al. showed that the pyrolysis of chitosan films containing adsorbed HAuCl4 under inert atmosphere at 900 °C provided (1 1 1) oriented Au nanoplatelets (20 nm lateral size, 3-4 nm height) on a few layers of N-doped graphene (Au/fl-G) (Figure 20) [START_REF] Primo | One-Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets Supported on Graphene and Six Orders of Magnitude Enhancement of the Resulting Catalytic Activity[END_REF].

The key enablers for this approach are: (i) the ability of chitosan film to adsorb metal ions by strong interactions between the positive amino groups of the glucosamine units of the polymer fibrils and the metal anions (i.e., AuCl4 -); (ii) high thermodynamic stability of the Au [START_REF] Kadib | Metal-Polysaccharide Interplay: Beyond Metal Immobilization, Graphenization-Induced-Anisotropic Growth[END_REF] facet; (iii) the templating effect of N-doped graphene for the growth of Au nanoplatelets. Au/fl-G exhibited six orders of magnitude enhancement for Ullmann-like homocoupling, for C-N cross coupling and for the oxidative coupling reactions of benzene to benzoic acid as compared to isotropic Au NPs supported on graphene [START_REF] Primo | One-Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets Supported on Graphene and Six Orders of Magnitude Enhancement of the Resulting Catalytic Activity[END_REF]. This strategy has been expanded for the fabrication of oriented copper and platinum nanoclusters on graphene [START_REF] Esteve-Adell | Oriented Pt Nanoparticles Supported on Few-Layers Graphene as Highly Active Catalyst for Aqueous-Phase Reforming of Ethylene Glycol[END_REF][START_REF] Kadib | Metal-Polysaccharide Interplay: Beyond Metal Immobilization, Graphenization-Induced-Anisotropic Growth[END_REF]. Oriented platinum clusters grown on graphene films exhibited at least 2 orders of magnitude higher catalytic activity in aqueous-phase reforming of ethylene glycol to H2 and

51

CO2 as compared to analogous samples of randomly oriented Pt NPs supported on un-doped graphene. A similar trend was also observed during the comparison of isotropic and oriented copper nanoparticles grown on graphene, which conceptually opened new avenue in metal nanoparticles supported on graphene [START_REF] Frindy | Isotropic and Oriented Copper Nanoparticles Supported on Graphene as Aniline Guanylation Catalysts[END_REF]. Notably, in some cases N(G) as a support may exhibit adverse effects for catalytic phenomena. For example, the use of N-doped graphene (derived from the pyrolysis of chitosan containing Cu 2+ at 900 °C) as a support of Cu NPs led to a remarkable decrease in catalytic activity in the dehydrogenative coupling of silane and alcohols as compared to Cu@G and to Cu@B-G, while the reason for such adverse effect remains elusive [START_REF] Blandez | Copper Nanoparticles Supported on Doped Graphenes as Catalyst for the Dehydrogenative Coupling of Silanes and Alcohols[END_REF].

Another work revealed that Cu NPs supported on graphene derived from the pyrolysis of alginate, exhibited high activity in A3 coupling (aldehyde-alkyne-amine reaction) of benzaldehydes [START_REF] Frindy | Copper nanoparticles supported on graphene as an efficient catalyst for A3 coupling of benzaldehydes[END_REF]. In turn, B-doped graphene as a support (derived from pyrolysis of boric acid-modified alginate) resulted in a slight decrease in catalytic activity while the presence of both N and B on graphene (derived from pyrolysis of boric acid modified chitosan) caused total inactivation of Cu NPs. The negative influence of B and N dopants on the catalyst activity was assigned to the interaction of the doping elements with supported Cu NPs, hindering the metal participation in catalysis [START_REF] Frindy | Copper nanoparticles supported on graphene as an efficient catalyst for A3 coupling of benzaldehydes[END_REF].

In addition to their use as metal-free catalysts and catalyst supports, N-doped graphene can be implemented in other areas related to energy. For example, Xu et al. reported that N-doped graphite carbon prepared from chitosan via freeze drying and annealing at 600-800 °C could improve the cathode performance of lithium ion batteries (LIBs) [START_REF] Xu | A facile synthetic route of nitrogendoped graphite derived from chitosan for modifying LiFePO4 cathode[END_REF].

As a conclusion, the synthesis of (N)G from biomass-derived chitosan by pyrolysis represents a new and environment-friendly strategy for the production of graphene materials. Pyrolysis temperature affects both the graphitization degree and the N content of (N)G. Activation agents (i.e., KOH) play an important role in fabricating hierarchical structures in (N)G with improved physicochemical properties. Co-dopants like boron can be easily introduced into (N)G by modification of chitosan with the corresponding boron precursors. Important applications of (N)G include metal-free catalysts, catalyst supports and battery cathode materials. Strong metal-support interaction in metal NPs/(N)G catalysts facilitates the catalytic activity of some metals (e.g., Au and Pt) but may also have adverse effect on catalyst activity (e.g., Cu). Subsequent research efforts are expected to further elucidate the mechanism of conversion from chitosan to (N)G, to produce other elements-doped graphene (e.g., with P and S), and to improve the metal dispersion and the stability of (N)G-supported catalysts. Some important features have been identified in N(G) materials that are crucial for their activity in different applications: (i) specific surface area and pore volume, which can be controlled through carbonization temperature and process; (ii) degree of graphitization that affects the activity of the catalyst by influencing the electron transfer resistance; and

(iii) contents of nitrogen and carbon and their electronic structures, which are critical for charge separation efficiency and visible light absorption. However, deeper understanding on the structures of doping-induced active sites and doping-property diversity of chitosan based N (G) is still needed. To sum up, there is still enough space to be explored to setup innovative, active, reproducible, more selective and economically viable carbon nanomaterials from chitosan carbonisation route.

Synthesis of N-doped carbon dots and metal-N doped carbon dots composites

Carbon dots (CDs) are a relatively new class of low-cost nanomaterials. The term is generally used to describe materials composed of quasi-spherical carbon nanoparticles (with a diameter ranging from 1 to 10 nm), and consisting in a nanoscale carbon core with abundant surface groups such as carboxylic acids, alcohols, and amine moieties. CDs are hydrophilic, which makes them highly compatible with aqueous chemistry applications [START_REF] Briscoe | Biomass-Derived Carbon Quantum Dot Sensitizers for Solid-State Nanostructured Solar Cells[END_REF][START_REF] Cayuela | Semiconductor and carbon-based fluorescent nanodots: the need for consistency[END_REF][START_REF] Liu | Carbon dots: synthesis, formation mechanism, fluorescence origin and sensing applications[END_REF]. Since the isolation of CDs issued from arc-discharge soot for the first time in 2004, this kind of carbon materials has attracted increasing attention due to their photoluminescence properties [START_REF] Xu | Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments[END_REF].

Compared to inorganic quantum dots, widely explored particularly as semiconductors and in biological applications, CDs display additional advantages including chemical inertness, versatile surface chemistry, potentially low cytotoxicity, low cost and environmental friendliness [START_REF] Baker | Luminescent Carbon Nanodots: Emergent Nanolights[END_REF][START_REF] Li | Photoluminescent Carbon Dots as Biocompatible Nanoprobes for Targeting Cancer Cells in Vitro[END_REF]. CDs have been applied as optical nanoprobes for fluorescence sensing both in the liquid and the solid phases, and for biological labelling [START_REF] Lin | An inner filter effect based sensor of tetracycline hydrochloride as developed by loading photoluminescent carbon nanodots in the electrospun nanofibers[END_REF]. CDs can be prepared by different methods like laser ablation, hydrothermal treatment [START_REF] Pan | Cutting sp2clusters in graphene sheets into colloidal graphene quantum dots with strong green fluorescence[END_REF],

electrochemical oxidation [START_REF] Lu | Hydroxyl-rich C-dots synthesized by a one-pot method and their application in the preparation of noble metal nanoparticles[END_REF], hot injection, microwave irradiation or pyrolysis [START_REF] Yamani | Enhanced arsenic removal using mixed metal oxide impregnated chitosan beads[END_REF], and can be prepared starting from non-toxic carbon precursors such as organisms, waste materials and protein products [START_REF] Liu | Carbon dots: synthesis, formation mechanism, fluorescence origin and sensing applications[END_REF][START_REF] Moradi | Easy synthesis, characterization and cell cytotoxicity of green nano carbon dots using hydrothermal carbonization of Gum Tragacanth and chitosan bio-polymers for bioimaging[END_REF]. The structure and composition of CDs can vary significantly based on the preparation method and the starting materials, and thus leads to enormous influence on their physical and chemical properties [START_REF] Hutton | Carbon dots as photosensitisers for solar-driven catalysis[END_REF]. The presence of heteroatoms such as nitrogen in the starting precursors affords N-doped CDs that display enhanced fluorescence efficiency [START_REF] Chernyak | Synthesis and modification of pristine and nitrogen-doped carbon dots by combining template pyrolysis and oxidation[END_REF]. This section focuses on the latest progress in the preparation of N-doped CDs from chitosan, summarizing the different synthesis methods, the strategies to promote CDs and quantum yields, as well as the preparation of metal-or chitosan-CDs composites.

Currently, hydrothermal/solvothermal treatment and microwave irradiation are widely used for the synthesis of CDs from chitosan. Hydrothermal/solvothermal treatment involves the direct carbonization of chitosan in water (hydrothermal) or other organic solvents (solvolthermal) at a temperature ranging from 180 to 220 °C for 3 h to 24 h (Table 3). This method is simple and cost-effective but usually time-consuming. Microwave irradiation usually shortens the treatment time, to few minutes' irradiation [START_REF] Xiao | Microwave-assisted one-step green synthesis of amino-functionalized fluorescent carbon nitride dots from chitosan[END_REF]. Some issues are however associated with these procedures including particles' aggregation during carbonization, poor uniformity and difficulties for controlling the size of the growing particles [START_REF] Namdari | Synthesis, properties and biomedical applications of carbon-based quantum dots: An updated review[END_REF].

Since the first report on the synthesis of CDs by hydrothermal treatment of chitosan, which or acrylamide derived CDs alone. Alternatively, alkaline-peroxide treatment favours higher yield of CDs. Akaline-peroxide treatment of hydrochar derived from the hydrothermal treatment of chitosan with diluted NaOH/H2O2 solution led to a CDs yield of 28.1% (Figure 22) [START_REF] Jing | Facile and High-Yield Synthesis of Carbon Quantum Dots from Biomass-Derived Carbons at Mild Condition[END_REF], which was much higher than that of the non-treated material (CDs yield of 10%). The abundant conjugated double bonds, carbonyl groups, and quinoid groups in the hydrochar were oxidized during the alkaline-peroxide treatment, which was related to the concentration of HOO -released from the reaction between H2O2 and NaOH. The functions present on the surface of N-doped carbon dots could be regulated by adjusting the oxidation condition (NaOH/H2O2 concentration), and thus their fluorescence efficiency can be significantly improved. Notably, 57 this treatment method led to higher CDs yields when using glucose, cellulose and hemicellulose as carbon sources (glucose giving an unusually high yield of 76.9%). Starting from chitosan, the as-prepared N-doped carbon dots showed high potential in Pb 2+ detection, along with good linear correlation ranging from 1.3 to 106.7 μM [START_REF] Jing | Facile and High-Yield Synthesis of Carbon Quantum Dots from Biomass-Derived Carbons at Mild Condition[END_REF]. Recently, high CDs yield was achieved using ethanol-precipitation-assisted hydrothermal treatment method [START_REF] Zhan | Ethanol-Precipitation-Assisted Highly Efficient Synthesis of Nitrogen-Doped Carbon Quantum Dots from Chitosan[END_REF]. The hydrothermal carbonization of chitosan and citric acid followed by precipitation using ethanol resulted in 85.3% yield of N-doped CDs, with a size ranging the interactions between the positive charges in the protonated amino groups of chitosan and the negative charges in the carboxylate groups of citric acid led to the formation of a chitosan-citrate complex, which was carbonized to form N-doped carbon dots with carboxyl, hydroxyl, and amino groups stabilized on the surface. When ethanol was added to the CDs' solution, the intermolecular hydrogen bonding between CDs and water was dismantled while the -COOH and -NH2 groups on the surface formed an extended hydrogen bonding network.

These features were beneficial for the precipitation and purification and consequently improved the yield of the resulting CDs; the later displayed a quantum yield of 6.6% and were promising for ion detection. The photoluminescence properties are of great importance for CDs materials. The first work on chitosan derived CDs showed that the resulting CDs were able to achieve high quantum yield (43%) without surface passivation or other further treatments. This was attributed to abundant functional groups, among carboxyl acids and amines derived from chitosan which could introduce different defects on CDs surface acting as excitation energy traps. This quantum yield of 43% is the highest reported in the literature so far [START_REF] Yang | One-step synthesis of amino-functionalized fluorescent carbon nanoparticles by hydrothermal carbonization of chitosan[END_REF]. Nevertheless, possible strategies to improve the photoluminescence of CDs through surface functionalisation, are still appealing. In general, the photoluminescence properties are dependent on the size, surface ligands and defects of CDs [START_REF] Hola | Photoluminescence effects of graphitic core size and surface functional groups in carbon dots: COOinduced red-shift emission[END_REF]. Given the limitations of the size control (which is highly dependent on the carbon source and the treatment parameters (e.g., the growth time, the pH environment, etc.), control of the surface functionalization and defects is quite promising [START_REF] Hu | Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass[END_REF][START_REF] Li | Hydrosoluble, UV-crosslinkable and injectable chitosan for patterned cell-laden microgel and rapid transdermal curing hydrogel in vivo[END_REF].

The control of surface functionalization and defects of CDs can be realized by applying multi-component carbon precursors. In the case of chitosan-derived CDs, ethylenediamine is usually used as a second component. For instance, direct hydrothermal carbonization of a mixture of chitosan, ethylenediamine and mercaptosuccinic acid provided a novel class of multifunctional fluorescent carbon dots [START_REF] Hua | Carbon quantum dots with intrinsic mitochondrial targeting ability for mitochondria-based theranostics[END_REF]. In this process, several chemical groups were formed due to the dehydration and subsequent carbonization between the carboxyl, amine and hydroxyl groups from chitosan, ethylenediamine and mercaptosuccinic acid, which resulted in good fluorescence properties. These carbon dots were used as mitochondrial trackers, where the formation of delocalized lipophilic cations (DLCs) in the carbon dots was confirmed, being responsible for efficient mitochondria-targeting. Interestingly, CDs synthesised by a one-step solvothermal carbonization of chitosan and ethylenediamine exhibited higher N content and a higher quantum yields up to 10.4%, higher than that of CDs from chitosan (quantum yield 5.7%) [START_REF] Zhang | Photoelectrochemical response of carbon dots (CDs) derived from chitosan and their use in electrochemical imaging[END_REF]. The resulting CDs were covalently linked to an indium tin oxide (ITO) surface through a 3-glycidoxypropyldimethoxymethylsilane (Gsilane) monolayer. The ITO-silane-CDs with CDs giving a 10.4% quantum yield showed higher photocurrents (~ 175 nA at 1.0 V) under the same binding and measurement conditions than those with CDs giving 5.7% quantum yield (~ 25 nA at 1.0 V), indicating that N doping played an important role in the photoelectrochemical response of CDs. Gum tragacanth and chitosan were also simultaneously used as a source of carbon and nitrogen [START_REF] Moradi | Easy synthesis, characterization and cell cytotoxicity of green nano carbon dots using hydrothermal carbonization of Gum Tragacanth and chitosan bio-polymers for bioimaging[END_REF]. Owing to the presence of a large number of functionalized surface groups, the obtained carbon dots (150 nm in diameter) displayed favourable luminescent properties. He et al.

synthesized N-doped carbon dots using hydrothermal treatment of 3,6-di(2-thienyl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and chitosan for photodynamic therapy [START_REF] He | Diketopyrrolopyrrole-based carbon dots for photodynamic therapy[END_REF].

Highly N-doped CDs were also synthesized by microwave-assisted pyrolysis of chitosan, acetic acid and ethylenediamine as the carbon source, condensation agent and N-dopant, respectively [START_REF] Gong | Facile synthesis of nitrogen-doped carbon dots for Fe 3+ sensing and cellular imaging[END_REF]. It was found that the addition of ethylenediamine favoured the formation of N-doped CDs with stronger emissions and enhanced quantum yield (from 7% to 20%).

The resulting N-doped CDs exhibited long-term colloidal stability (6 months) and good sensitivity and selectivity to Fe 3+ , and could serve as an effective probe for photoluminescence detection of Fe 3+ .

The precise tuning of the metal-carbon interface also enables modification of the physico A one-step approach for the growth of Ag NPs on CDs under a mild reducing condition (5 min at 50 °C) was also reported [START_REF] Shen | Growth and Stabilization of Silver Nanoparticles on Carbon Dots and Sensing Application[END_REF]. CDs acted as an electron donor ensuring the reduction of Ag + to Ag 0 on CDs surface, as well as a capping agent to maintain fine Ag NPs (3.1 nm) stable for 45 days in aqueous medium at room temperature. The Ag NPs caused obvious resonance light scattering, which correlated closely with the concentration of silver cation.

This facilitated quantitative detection of silver in aqueous medium. Heterodimeric material (various nanoparticles with unique hetero-structures) was also obtained by irradiation with UV light of CDs in the presence of AgNO3 as a source of Ag NPs [START_REF] Choi | Interfacecontrolled synthesis of heterodimeric silver-carbon nanoparticles derived from polysaccharides[END_REF]. CDs with quantum yield of 5% and a size of 7.9 nm were prepared by microwave pyrolysis of chitosan and ethylenediamine. The growth mechanism occurred through several steps, starting with the adsorption of Ag + ions onto the surface of CDs via electrostatic interactions with amine and other functional groups. After the free electrons from the photo-excited CDs reached the Ag + ions at the surface, the interfacial junction allowed electrons to pass through the conductive Ag nucleation site and to recruit more Ag + ions, leading to heterodimeric Ag-CD NPs.

Notably, only a single Ag NP was grown on each carbon nanodot, and the presence of Ag NPs did not completely quench the photoluminescence of the CDs due to the control of the heterodimeric nanostructure. In 2018, Song et al. synthesized CDs/AuNPs composites by 62 reduction of HAuCl4 with CDs that acted by themselves as a reducing agent (Figure 24) [START_REF] Song | Carbon Quantum Dots Prepared with Chitosan for Synthesis of CQDs/AuNPs for Iodine Ions Detection[END_REF]. The resulting CDs@AuNPs had a particle size of about 20-30 nm and was used to detect iodine ions by the colorimetric method. Good recoveries between 98.4% and 107.4% were obtained. respectively [START_REF] Song | Carbon Quantum Dots Prepared with Chitosan for Synthesis of CQDs/AuNPs for Iodine Ions Detection[END_REF].

CDs have also been recognized as a promising support for the construction of optical sensors for quantitative assay of biomolecules and environmental pollutants [START_REF] Shi | A Tunable Ratiometric pH Sensor Based on Carbon Nanodots for the Quantitative Measurement of the Intracellular pH of Whole Cells[END_REF][START_REF] Wei | Sensing metal ions with ion selectivity of a crown ether and fluorescence resonance energy transfer between carbon dots and graphene[END_REF], as well as for the preparation of catalysts. For example, Zhang et al. synthesized Rh/CDs by the reaction of CDsprepared by microwave treatmentwith RhCl3 aqueous solution at 120 °C for 1 h [START_REF] Zhang | The synthesis of rhodium/carbon dots nanoparticles and its hydrogenation application[END_REF]. The electron-rich surface groups of CDs coordinated with Rh 3+ and stabilized Rh NPs.

The resulting Rh NPs had an average size of about 2.8 nm, and the corresponding Rh@CDs the carbon dots functionalisation to the performance of the resulting solar cells [START_REF] Marinovic | Carbon-Nanodot Solar Cells from Renewable Precursors[END_REF].

Glucose (hydroxyl groups), chitosan (hydroxyl groups, amines, amides), chitin (hydroxyl groups, amides), L-arginine (carboxylic groups, amines) and L-cysteine (carboxylic groups, hydroxyl groups, thiol) were applied to prepare CDs via hydrothermal treatment. The assynthesized CDs gave quantum yields of 2.2%, 35.3%, 9.7%, 43.3%, 9.1%, respectively, and contained different functional groups, which originated from the starting precursors. These functional groups were responsible for the photoluminescence properties and the solar-cell performance of the resulting CDs sensitized TiO2 to visible light.

The preparation of chitosan-CDs composites based on combining the properties of chitosan and CDs could bring further applications (e.g., electrodeposition). For instance, Wang et al.

constructed a CDs/chitosan composite using a one-step microwave treatment of chitosan solution (700 W, 250 s) [START_REF] Wang | Electrodeposition of a carbon dots/chitosan composite produced by a simple in situ method and electrically controlled release of carbon dots[END_REF]. The obtained CDs/chitosan composite retained the properties of both CDs (e.g., multicolour fluorescence) and chitosan (e.g., pH-responsive and filmforming properties), which made this composite promising for applications in photoluminescence coatings and patterns, fluorescent labelling and controlled release in the drug delivery system. Reproduced with permission from Ref. [START_REF] Su | Homogeneously Synchronous Degradation of Chitin into Carbon Dots and Organic Acids in Aqueous Solution[END_REF]. Copyright 2019 American Chemical Society.

There is still a lack of systematic mechanism study for the formation of CDs from chitosan.

During hydrothermal treatment, it is generally described that chitosan undergoes structural rearrangement through hydrolysis, dehydration, decarboxylation, aromatization (caused by elimination of hydroxyl groups and carboxyl groups) and re-condensation, which leads to the formation of fluorescent carbon NPs [START_REF] Funke | Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering[END_REF][START_REF] Sun | Colloidal Carbon Spheres and Their Core/Shell Structures with Noble-Metal Nanoparticles[END_REF][START_REF] Libra | Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis[END_REF]. When using hydrosoluble chitosan (2-amino-2-deoxy-β-D-glycopyranose) as a carbon source of CDs, Wang et al. proposed a multistep arrangement consisting in: (i) hydrolysis, dehydration and decomposition of the hydrosoluble chitosan which resulted in soluble small products (e.g., furfural, pyrrole, etc.); (ii) polymerization and carbonization of the resulting products; (iii) nucleation, growth and oxidation of carbon substances [START_REF] Wang | High-yield synthesis of strong photoluminescent N-doped carbon nanodots derived from hydrosoluble chitosan for mercury ion sensing via smartphone[END_REF]. Oppositely, when using chitin as carbon source, Su et al. postulated that chitin chains were first decomposed to glucosamine (N-acetyl-glucose) which was then degraded during carbonization to give CDs via the HMF route and to give various organic acids via the glyceraldehyde or dihydroxyacetone routes (Figure 25) [START_REF] Su | Homogeneously Synchronous Degradation of Chitin into Carbon Dots and Organic Acids in Aqueous Solution[END_REF].

The multistep evolution of chitosan under microwave irradiation has not been investigated so far.

To summarize, N-doped CDs can be readily constructed from chitosan by hydrothermal/solvothermal or microwave assisted treatment. The production is still limited by low yields of CDs and quantum yield. The yield of CDs could be enhanced using hydrosoluble chitosan, adding diamines (e.g., ethylenediamine), performing alkaline-peroxide treatment and ethanol precipitation. In spite of these attempts, advancement in enhancement of quantum yield is still scant at present. Quantitative prediction of the physicochemical properties of CDs remains a significant challenge because of their complex chemical structure. Besides, elucidating the factors contributing to the physiochemical behaviour of CDs as well as the mechanism of CDs formation from chitosan is still in lack of due attention.

a : quantum yield.

Conclusion and perspectives

Carbon-based materials are of great interest owing to the combination of a set of unusual physical and chemical properties, including electrical conductivity, optical properties, hydrophobicity, anisotropy and rigidity. This review highlights various approaches towards the use of chitosan as precursor of nanostructured materials ranging from hierarchically porous N-doped carbon and N-doped graphene to N-doped carbon dots. A number of approaches have been discussed with a particular focus on the hydrothermal and heat treatment synthetic strategies. The remarkable and arguably unique properties of chitosan allow as consequence the obtaining of carbon materials with highly functional, high surfacearea and pore-volume, chemically rich surfaces, and interesting textures and morphologies.

The main factors which make chitosan so attractive for the aforementioned materials are related to: availability, high carbon yield after carbonization (∼50 wt.%), the possibility for carbonisation and graphitisation under metal-free conditions, and the presence of the amine group in its structure, resulting in nitrogen self-doping of the derived carbon materials.

Distinctively, carbonisation route is the sole sustainable route to Haber-independent nitrogencontaining carbon substructures. The nature of nitrogen species can be modulated and tuned depending on the synthesis conditions, chitosan conjugation and the presence of dopant species. This flexibility further broadens the library of as-designed carbon materials will undoubtedly induce further numerous works in the field of (electro)catalysis, where the nature of nitrogen species plays a pivotal role.

The heteroatom doping achieved by using chitosan as precursor is also presented and co-dopant like B, P or S can be facilely introduced by modification of chitosan with the corresponding precursors. Co-doping graphitic carbons with different heteroatoms can create a variety of coexisting active sites for multiple functionalities in the final materials. A precise control of the dopant location is still in its infancy, and more researches are requested to better understand the structure-activity relationship. Such understanding will be decisive for the rational-design of more sophisticated carbon materials.

Furthermore, the structure directing role of chitosan in making porous carbons with high surface area and tunable porosity is discussed by drawing attention to the role of the activation agent (i.e., KOH) which plays an important role in fabricating hierarchical structure with improved physicochemical properties. The application of these carbon materials lies in a range of important fields of renewable energy and environmental applications.

A significant challenge in the field at the moment is the synthesis mechanism, which is still not fully understood and also the control of the surface functionality and properties is still far away.

One of the most notorious applications in which nitrogen-doped carbon materials are requested is the field of catalysis. However, the identification of the atomic location and the chemical nature of the catalytic active centers, which enables understanding the reaction mechanisms, are still lacking. Continued research efforts and approaches need to be developed to provide opportunities for precisely controlling and guiding catalytic activities through active center modification and interfacial functionalization.

Figure 1 .

 1 Figure 1. Different nitrogen species on N-containing carbon materials. Reproduced with permission from Ref. [11]. Copyright 2016 The Royal Society of Chemistry.

Figure 2 .

 2 Figure 2. Chemical production route of chitosan. Reproduced with permission from Ref. [14]. Copyright 2010 Elsevier Ltd.

Figure 3 .

 3 Figure 3. Conversion routes of chitosan to porous carbon, graphene and carbon dots materials

Figure 4 .

 4 Figure 4. Schematic illustration of preparation of carbon monolith. (a) Chitosan dissolved in 0.1 M acetic acid; (b) Chitosan gel after addition of the urea/hexaketocyclohexane complex precursor; (c) Gel monolith after hydrothermal treatment; (d) Monolith after solvent exchange; (e) Monolith after supercritical drying; (f) Carbon monolith after carbonization at 500 °C. Reproduced with permission from Ref. [41]. Copyright 2017 Elsevier Inc.

Figure 5 .

 5 Figure 5. Schematic illustration of synthesis of chitosan-derived hierarchical porous carbon with acetic acid as additive and the control sample synthesized without acetic acid. Copied with permission from Ref. [43]. Copyright 2017 The Royal Society of Chemistry.

Figure 6 .

 6 Figure 6. Preparation of chitosan-betaine hydrochloride polyelectrolyte in water. Reproduced

Figure 7 .

 7 Figure 7. Schematic procedure of synthesis of the Sn-Fe@carbon composites. Reproduced with permission from Ref. [54]. Copyright 2018 American Chemical Society.

Figure 8 .

 8 Figure 8. Schematic synthesis process of CuP2@C composite. Reproduced with permission from Ref. [55]. Copyright 2019 American Chemical Society.

Figure 9 .

 9 Figure 9. Schematic illustration of the construction process of flower ridge-like MoS2@N-C micro/nanostructure. Reproduced with permission from Ref. [56]. Copyright 2018 American Chemical Society.

Figure 10 .

 10 Figure 10. Schematic illustration of the construction process of Pd@N-C nanocatalysts.

Figure 11 .

 11 Figure 11. Schematic illustration of the formation of Ag/N-C nanocomposites. Copied with permission from Ref. [61]. Copyright 2019 Elsevier Ltd.

Figure 12 .

 12 Figure 12. Fabrication of Co@NC catalysts and schematized view of the ORR catalyzed by Co@NC. The small particles demonstrat the special bonds and the balls in the matrix stand for the different elements: carbon (blue), nitrogen (green), and cobalt (purple). Reproduced with permission from Ref. [71]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 13 .

 13 Figure 13. Illustration of the synthesis of Co9S8@N-C composite. Reproduced with permission of Ref. [72]. Copyright 2018 Elsevier Ltd.

Figure 14 .

 14 Figure 14. Schematic illustration of the formation of (a) CHI-TMA-Fe-CW (b) CHI-TMA-FeCW-M1. Reproduced with permission from Ref. [73]. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA.

  by pyrolysis at 500 °C[START_REF] Chhetri | Chitosan-Derived NiO-Mn2O3/C Nanocomposites as Non-Precious Catalysts for Enhanced Oxygen Reduction Reaction[END_REF]. Their results indicated that the presence of NiO triggered changes in the oxidation state of Mn species, resulting in good catalytic activity for ORR. As vs. @C nanocomposites showed efficient ORR activity at 0cu showed reduced electrocatalytic activity. Multistep preparation of Fe-and Co-doped carbon catalyst (Fe-Co@N-C) was also studied. Synthesis consisted in grafting an anionic heme with

  abundant graphitization grains and Fe external sphere active sites. The materials displayed efficient ORR activity and stability in both alkaline and acidic conditions[START_REF] Hertrich | Supported Cobalt Nanoparticles for Hydroformylation Reactions[END_REF].

Figure 15 .

 15 Figure 15. Illustration of the synthesis of chitosan-heme-vitamin B12-derived Fe-Co@N-C composite. Reproduced with permission of Ref. [76]. Copyright 2019 The Royal Society of Chemistry.

Figure 16 .

 16 Figure 16. Preparation procedure for N(G) from aerogels. (i) Precipitation in basic solution, (ii) Ethanol by water exchange, (iii) Supercritical CO2 drying, (iv) Pyrolysis of chitosan aerogel and dispersion of the resulting carbon residue. Reproduced with permission of Ref. [96]. Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 18 .

 18 Figure 18. Preparation procedure for N(G)/h-BN film. (i) Spin coating; (ii) Pyrolysis at 900 °C under Ar atmosphere. Reproduced with permission of Ref. [107]. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 19 .

 19 Figure 19. Proposed mechanism of structure evolution during the preparation process of

Figure 20 .

 20 Figure 20. Preparation procedure for Au on few layers of graphene. (i) Spin coating of an aqueous chitosan solution on quartz (2 × 2 cm 2 ); (ii) Adsorption of AuCl 4-on chitosan film before pyrolysis under inert atmosphere at 900 °C. Copied with permission from Ref. [109]. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 21 .

 21 Figure 21. Preparation of CDs from chitosan and acrylamide via microwave-hydrothermal carbonization. Adapted with permission from Ref. [133]. Copyright 2019 Elsevier B.V.

Figure 22 .

 22 Figure 22. Schematic process of preparation of CDs from chitosan with alkaline-peroxide treatment of the hydrochar. Adapted with permission from Ref. [134]. Copyright 2019 American Chemical Society.

Figure 23 .

 23 Figure 23. Schematic process of the preparation of CDs from citric acid modified chitosan using an ethanol-precipitation-assisted hydrothermal treatment method. Reproduced from Ref. [135]. Copyright 2019 American Chemical Society. ACS Author Choice. Doi: 10.1021/acsomega.9b03318.

  chemical properties of CDs. Chowdhury et al. prepared N-doped carbon dots with a size ranging from 0.6 to 8.7 nm by direct hydrothermal carbonization method and performed in situ (I) and ex situ (E) incorporation of Ag or Au NPs into the gel matrix to obtain Ag-I/CDs, Ag-E/CDs, Au-I/CDs and Au-E/CDs composites with large sizes ranging from 0.5 to 1.6 µm, compared to CDs [141]. The as-synthesized composites (Ag-I/CDs and Au-I/CDs) showed an enhancement of the photoluminescence due to the shorter distance between the CDs and NPs (Ag or Au) in the obtained composite by in situ incorporation (Ag-I/CDs or Au-I/CDs) than that in those obtained by ex situ Ag-E/CDs or Au-E/CDs.

Figure 24 .

 24 Figure 24. Schematic illustration of the preparation CQDs and CQDs/AuNPs, and illustration of the detection of I -based on CQDs/AuNPs. The yellow and blue liquid in centrifuge tube is a photograph of CQDs illuminated by natural light and UV lamp with 365 nm light,

Figure 25 .

 25 Figure 25. Possible reaction route for the synthesis of the N-doped carbon dots from chitin.

Table 1 .

 1 Overview of chitosan derived porous N-doped carbon: synthesis and properties.

	Polymer	Additive	Activator Synthesis method	Synthesis conditions	SBET (m 2 /g)	Average pore diameter (nm)	N content (%)	Application	Ref.
	Chitosan	D-methionine	-	Hydrothermal & pyrolysis	200 °C/12 h & 750 °C/3 h, 2 °C/min, under Ar flow	-	-	5.7	Supercapacitors	[50]
	Chitosan	-	-	Hydrothermal	180 °C/10 h	-	-	-	Adsorption of Cr(IV)	[30]
	Chitosan	Acetic acid	KOH	Hydrothermal & pyrolysis	200 °C/2 h & 800 °C/2 h, 5 °C/min, under N2 flow	3532	1.3	1	Supercapacitors	[43]
	Chitosan	Hexaketocyclohexa -ne, urea	-	Hydrothermal & pyrolysis	100 °C/12 h & 500 °C/1 h, under N2 flow	348	8.2	12	-	[41]
	Chitosan	1-Butyl-3 methylinidazolium	-	Hydrothermal & Pyrolysis	210 °C/10 h & 900 °C/2 h, 7 °C/min, under N2 flow	301	2.3	4.8	Supercapacitors	[45]
	Chitosan	-	ZnCl2	Pyrolysis	700 °C/3 h, 5 °C/min, under N2 flow	1785	-	4.2	Electrode for energy storage	[39]
	Chitosan	Urea	-	Pyrolysis	950 °C/5 h, 3 °C/min, under N2 flow	1103	0.5	9.7	Oxidation of 5-HMF to 2,5-diformylfuran	[42]

Table 1 .

 1 Continued.

	Polymer	Additive	Activator	Synthesis method	Synthesis conditions	SBET (m 2 /g)	Average pore diameter (nm)	N content (%)	Application	Ref.
	Chitosan	-	NaOH	Pyrolysis	800 °C/1.5 h, 10 °C/min, under N2 flow	318	3.2	-	Adsorption of methylene blue	[38]
					350 °C/3 h,	600 °C/2 h	1683		4.5	Air cathode catalyst
	Chitosan	-	KOH	Pre-treatment & pyrolysis	5 °C/min, under N2 flow	700 °C/2 h 850 °C/2 h 950 °C/2 h	2219 2334 1081	-	2.7 2.5 0.4	for high performance in microbial fuel cells	[37]
	Chitosan	Boric acid	KOH	Pyrolysis	700 °C/1.5 h, 5 °C/min, under N2 flow	1129	4.2	4.7	Supercapacitors	[47]
						800 °C/3 h	140		-
						850 °C/3 h	982		3.0
	Chitosan	-	Phosphoric acid	Pre-treatment & pyrolysis	350 °C/3 h, 5 °C/min, under N2	900 °C/3 h 950 °C/3 h	891 889	-	--	Cathode catalyst in microbial fuel cell	[48]
			Phosphoric acid with KOH		flow	850 °C/3 h	2295		-
			None			850 °C/3 h	12		4.2

Table 2 .

 2 Overview of chitosan derived metal-carbon composites: synthesis and properties

	N-doped carbon precursor	Additive	Method	Synthesis conditions	Particle size (nm)	Metal content (%)	Heteroatom content (%)	SBET (m 2 /g)	Application	Ref.
		K2PdCl
	Chitosan	EDTMPA	Thermal annealing 600 °C/2 h, under Ar	15.2	-	-	101	Hydrogenation of 4-NP	[62]
		K3Co(CN)6
	Chitosan	PdCl2 P123	Thermal annealing (4 h, under N2)	750 °C 900 °C	5-20 50-100	3.2	(N) 3.7	159 293	Ullmann reaction	[59]
	Chitosan	Co(NO3)2	Thermal annealing 900 °C/3 h, under N2	-	-	-	-	ORR Catalyst	[71]
	Chitosan	Na2MoO4,2H2O (NH2)2CS	Hydrothermal & thermal annealing	200 °C/24 h & 700 °C/3 h, under Ar	150-200	-	(N) 2.3	76	Lithium ion batteries anode	[56]
	Chitosan	FeCl3 ZnCl2, H3BO3	Hydrothermal & thermal annealing	180 °C/12 h & 900 °C/2 h, under N2	-	-	(N) 4.9 (B) 3.4	1567	Electrode material for energy storage	[49]
	Chitosan	Fe(Cl)3 Trimesic acid	Thermal annealing 900 °C, under N2	15-20	-	(N) 1.5	565	ORR Catalyst	[73]
	Chitosan	ZIF-7	Thermal annealing 950 °C, under Ar	-	-	(N) 5.6	285	Supercapacitor	[57]
		Polyhexamethylene
	Chitosan	Guanidine phosphate Co(NO3)2⸱6H2O	Thermal annealing 600 °C/3 h, under N2	-	-	(N) 5.8 (P) 2.4	143	Bifunctional oxygen electrocatalysis	[72]
		Na2S⸱9H2O

Table 3 .

 3 Overview of preparation of CDs from chitosan synthesis and properties

	Polymer	Additive	Preparation method	Treatment conditions	CDs size (nm)	QY a (%)	Yield (%)	Application	Ref.
	Chitosan	-	Hydrothermal	180 °C/12 h	5	43	7.8	Biological labelling	[130]
	Chitosan	-	Hydrothermal	220 °C/12 h	3.8	31.8	38.4	Mercury ion sensors	[132]
	Chitosan	-	Solvothermal	200 °C/2 h	4	5.7	-	Electrochemical imaging	[138]
	Chitin	-	Hydrothermal	240 °C/24 h	4.5	6.1	-	-	[148]
	Chitin				14.1	1.6
	Chitosan	-	Solvothermal	200 °C/6 h	8.1	13.4	-	ZnO nanorods sensitizer	[115]
	Glucose				2.5	1.4
	Glucose				5.1	2.2
	L-arginine	-	Hydrothermal	200 °C/6 h	15	43.3	-	Sensitizers for TiO2-based nanostructured solar cells	[149]
	L-cysteine				11.4	9.1
	Chitosan	-	Hydrothermal	200 °C/24 h	2.4	8.3	76.9	Pb 2+ detection	[134]
			Microwave-assisted
	Chitosan	Acrylamide	hydrothermal	220 °C/0.5-1 h		12.7	45.9	Selective detection of ferrous ions	[133]
			carbonization
		Ethylenediamine,
	Chitosan	mercaptosuccinic	Hydrothermal	180 °C/24 h	2.1	11.8	-	Mitochondria-based theranostics	[137]
		acid

Acknowledgements

PHC Toubkal n° TBK 18/70 (CNRST-CNRS) and UEMF are acknowledged for financial support and scholarship, respectively. Chevreul institute (FR 2638), Ministère de l'Enseignement Supérieur, de la Recherche, et de l'Innovation, Région Nord -Pas de Calais and FEDER are acknowledged for supporting and funding partially this work.

activation producing K2CO3 and K. When the carbonization temperature was above 700 °C, carbon activation continued with a reaction between carbon and CO2 (produced from K2CO3), and K intercalation among the carbon structure. At the same time, the infiltrated amorphous carbon was transformed into graphene structure between g-C3N4 layers.

Graphene-like carbon nanoflakes with in-plane micropore structure were then formed.

Further increase of the temperature up to 900 °C led to N-doping into graphene through g-C3N4 decomposition. The obtained N-doped graphene was used to support palladium, and the resulting Pd@N-doped graphene catalyst exhibited high activity in formic acid dehydrogenation (conversion of 81.2%), which was 5-16% higher than that of the benchmark Pd/N-charcoal catalyst under comparable conditions [START_REF] Lee | Straightforward Synthesis of Hierarchically Porous Nitrogen-Doped Carbon via Pyrolysis of Chitosan/Urea/KOH Mixtures and Its Application as a Support for Formic Acid Dehydrogenation Catalysts[END_REF]. The chemical diversity of these carbon materials can be enriched by adding other dopants either by post-grafting or during carbonisation. Pyrolysis of chitosan with boric acid leads to led to a relatively low yield of CDs (7.8%) [START_REF] Yang | One-step synthesis of amino-functionalized fluorescent carbon nanoparticles by hydrothermal carbonization of chitosan[END_REF], various strategies were undertaken to improve the yield of CDs from chitosan, including: (i) the use of hydrosoluble chitosan as carbon source [START_REF] Li | Hydrosoluble, UV-crosslinkable and injectable chitosan for patterned cell-laden microgel and rapid transdermal curing hydrogel in vivo[END_REF]; (ii) adding amide additives such as ethylenediamine and urea [START_REF] Jing | Facile and High-Yield Synthesis of Carbon Quantum Dots from Biomass-Derived Carbons at Mild Condition[END_REF]; (iii) alkaline-peroxide treatment [START_REF] Jing | Facile and High-Yield Synthesis of Carbon Quantum Dots from Biomass-Derived Carbons at Mild Condition[END_REF]; (iv) using ethanol precipitation [START_REF] Zhan | Ethanol-Precipitation-Assisted Highly Efficient Synthesis of Nitrogen-Doped Carbon Quantum Dots from Chitosan[END_REF]. Some of these investigations' results are summarized below.

The use of hydrosoluble chitosan as carbon source was found to be fruitful for enhancing the CDs yield. Hydrosoluble chitosan can be synthesized by single-step chemoselective Nacylation between chitosan and methacrylic anhydride [START_REF] Li | Hydrosoluble, UV-crosslinkable and injectable chitosan for patterned cell-laden microgel and rapid transdermal curing hydrogel in vivo[END_REF]. After hydrothermal treatment of hydrosoluble chitosan, CDs with 38.4% yield could be obtained [START_REF] Wang | High-yield synthesis of strong photoluminescent N-doped carbon nanodots derived from hydrosoluble chitosan for mercury ion sensing via smartphone[END_REF]. Nitrogen belonging to CDs framework existed in the form of N-C3 (10%), pyridinic-like N (67%) and NH2 (23%) respectively, which contributed to the quantum yield of 31.8%. The obtained CDs hold great promise for multicolor bioimaging, portable Hg 2+ detection by laptop/smartphone due to their high quantum yield.

Adding polymerizable amide additives is another efficient strategy to promote CDs yield. Liu et al. synthesized N-doped CDs starting from chitosan and acrylamide as carbon source, according to the procedure described in Figure 21, with the aim of increasing the yield of CDs [START_REF] Liu | Rapid and high yield synthesis of carbon dots with chelating ability derived from acrylamide/chitosan for selective detection of ferrous ions[END_REF]. Because chitosan possesses many pendent -NH2 and -OH groups, it supplied abundant sites for grafting. Under microwave irradiation, the grafting reaction between acrylamide and chitosan was initiated due to the generation of free radicals, and then, the acrylamide polymerized to form polyacrylamide. The grafting polymers served as intermediate product for the synthesis of N-doped CDs. The yield of the as-synthesized Ndoped carbon dots was 45.9%, which was higher than the one of chitosan derived carbon dots NPs exhibited high catalytic activity in the hydrogenation of hydroxyl-terminated polybutadiene and hydroxyl-terminated butadiene-acrylonitrile.

As aforementioned, the physical and chemical properties of CDs are directly related to the starting materials. Following the trend of using bio-sourced carbon precursors, other renewables such as chitin, alginic acid, starch, D-glucose and amino acids (i.e., L-arginine and L-cysteine) were also used for the synthesis of CDs.

Su et al. used chitin for the synthesis of N-doped CDs [START_REF] Su | Homogeneously Synchronous Degradation of Chitin into Carbon Dots and Organic Acids in Aqueous Solution[END_REF]. Their procedure consisted in first dissolution of chitin in KOH/urea aqueous solution at low temperature (-30 °C), followed by a hydrothermal treatment at 320 °C for 12 h. The resulting CDs had an average diameter of 4.5 nm. However, the CDs yield and the quantum yield were relatively low as compared to the literature (Table 3), being of 6.14 % and 6%, respectively. Briscoe et al.

compared three different types of CDs derived from chitin, chitosan and glucose [START_REF] Briscoe | Biomass-Derived Carbon Quantum Dot Sensitizers for Solid-State Nanostructured Solar Cells[END_REF]. The solvothermal treatment at 200 °C applied by the authors resulted in self-passivated N-doped CDs with an average diameter of 14.1 nm, 8.1 nm and 2.57 nm, and quantum yield of 1.6%, 13.4% and 1.4%, respectively. The highest photoluminescence performance exhibited by the chitosan-derived CDs was assigned to its surface functional groups (i.e., amine groups and carboxylic acid groups) maintained from chitosan initial precursor. The resulting CDs were used as sensitizers for ZnO nanorod-based solid-state nanostructured solar cells. The performance of the devices relied on the functional groups of CDs (e.g., carboxylic groups, hydroxyl groups, amines, amides). A layer combining chitosan-and chitin-derived CDs produced devices with highest efficiency (0.077%) compared to the chitin-derived CDs device (0.032%) and chitosan derived CDs device (0.061%). Later, the same group correlated Consequently, more efforts have to be given to the development of nitrogen doped carbon materials by taking advantages from the properties of chitosan. Nitrogen doped carbon research will continue to thrive because of the wide opportunities provided by the flexibility in synthesis approached (activating agent use, heteroatom doping and additives incorporation). This review aims to furnish useful clues for developing new and controllable synthesis methods and a better understanding of the properties of nitrogen doped carbon materials. We also hope that it will inspire more exciting applications of this growing family of nanomaterials.