Ruqaya Buhaibeh

Tiphaine Richard

Régis M Gauvin

Mathieu Sauthier

Clément Dumont

Catalytic hydroesterification of lignin: A versatile and efficient entry into fully biobased tunable materials

Introduction

Non-renewable fossil resources continue to play a major role in the chemical industry, materials manufacturing, and energy production. Their consumption releases increasingly more CO2 into the atmosphere, playing a negative role in climate change and depleting the planet's resources. For a more sustainable economy, the use of renewable carbon for the production of materials and chemicals is particularly attractive as it allows recycling and storage of CO2. Lignocellulosic biomass is one of the major alternative sources to oil, is not an edible resource and is available in large quantities. Lignin represents about 20 % of lignocellulosic biomass and 100 million tons of lignin are currently industrially produced as side product of the paper industry and within secondgeneration bioethanol production plants. Lignin is a bio-based polymer with high aromatics content and is clearly very attractive for plastic industry. Still, the compound is essentially used as low-value fuel and is currently 98 % burned to generate electricity and heat. 1 The valorization of lignins for the production of materials is indeed largely impeded by weak mechanical properties and poor compatibility with synthetic polymers. To overcome these limitations, some research fields involve the functionalization of phenolic and aliphatic hydroxyl groups of lignin to improve its properties and thus offer access to new valorization routes. 2 Among the chemical transformations of lignin, esterification reactions are particularly used in the development of new material solutions. The esterification of hydroxyl groups present in lignins has been for example used to improve the compatibility of lignin in blends, 3 to use lignin as a macromonomer, 4 or to graft polymerization initiators. 5 Other fields of application also use esters such as adhesives, 6 lubricants, 7 anti-UV films 8 or energy production. 9 However, if aliphatic alcohols can be readily involved in esterification reactions with carboxylic acids, the phenol functions of lignins show a much lower reactivity. [START_REF] Shelkov | Difference of reactivity between aliphatic alcohol and phenol in esterification reaction through carbodiimide coupling[END_REF] Thus, these transformations currently involve "activated carboxylic acids" through the use of the Steglich reaction 11 (with DCC/DMAP), acid halide 12 or anhydride 13 . Nevertheless, these reactions have the disadvantage of using harmful reagents generating salts or deleterious coproducts in stoichiometric amounts. Consequently, this significantly increases the environmental impact of these transformations. As an environmentally friendly pathway, ester functionalization methodologies using homogeneous catalysis such as the carbonylative telomerization reaction 14 have shown high efficiency for efficient lignin functionalization (Scheme 1).

Scheme 1 Overall scheme for Pd-catalyzed hydroesterification on lignin

The present article proposes to open a new lignin functionalization pathway via the implementation of ester synthesis through the hydroesterification reaction of olefins. This catalytic transformation affords esters that are thus readily obtained from the combination of carbon monoxide, an alkene and a nucleophilic alcohol. This reaction is atom-economical and avoids formation of stoichiometric amounts of salts. 15 Different kinds of catalysts have been studied for this reaction, including base metals such as cobalt 16 and nickel 17 as developed by W. Reppe in the 1960s. However, these catalysts require drastic conditions of temperature and pressure, which significantly counterbalance the interest in their use. On the other hand, palladium catalysis proceeds under softer reaction conditions, is more tolerant to functionalities and thus appears as highly suitable to convert a functionalized derivative such as lignin without prior purification. 18 Interestingly, the practicality of this industrially relevant reaction opens the way to efficient and atom-economical syntheses of esterified lignins from olefins, either as bulk products from large scale petroleum-based process, or as bio-based terpenes and unsaturated fatty acids or esters.

Results and Discussion

The study aiming at the synthesis of esterified lignins from the hydroesterification reaction was initiated with guaiacol and benzyl alcohol as substrates to check the efficiency of the reaction towards both aliphatic and phenolic hydroxyl groups. Such functionalities are found in lignin (Scheme 2) and the use of such simple molecules is highly suitable to collect spectroscopic references for the assessment of lignin grafting by 1 H NMR (see below). For this purpose, a simple and industrially relevant olefin, 1-hexene, was used as co-reagent. Standard reaction conditions classically used for the hydroesterification of olefins with simple aliphatic alcohols were initially applied for the transformation of the substrates. 18 The catalyst is prepared in situ by combining 1 mol% of Pd(OAc)2 with 0.25 equivalents in respect to alcohol of both PPh3 and paratoluenesulfonic acid (PTSA). With these reaction conditions, a very high conversion (99 %) of phenol was obtained and the ester was isolated with 93 % yield. 1 H NMR of the isolated product evidences the presence of two isomers with linear (L) 1a and branched (B) 1b structures with a measured L/B ratio of 96/4. The yield of the ester 2 derived from benzyl alcohol was 96 % and a slightly higher proportion of branched isomer was obtained (L/B = 87/13). These molecules validate the possibility of carrying out the hydroesterification reaction of olefins with hydroxyl groups in THF which is known as suitable solvent for lignin. Interestingly, both types of alcohols present within the lignin structure are amenable to efficient conversion into esters. The same reaction parameters were then used for the hydroesterification of Kraft lignins with 1-hexene. This industrially available material was used as received for all the catalytic tests. As the polymeric structure of the industrial grade Kraft lignin is very different from the pure simple compounds used, a first optimization study was initiated in order to maximize the conversion of the alcohols (Table 1). The impact of the reaction temperature on the conversion of the OH groups in lignin was first studied. At 90 °C, 28 % of the total alcohols were converted (entry 1). Increasing the reaction temperature improves the substitution degree from 51 % (100 °C) to 61 % (120 °C) (entries 2 and 3). Although the reaction is rather efficient in terms of substitution degrees, it is noteworthy that large amounts of precipitated lignin were observed at the end of the catalytic runs. For example, at 120 °C, half of the initial lignin is lost as insoluble material. This precipitate proved to be also insoluble in commonly used organic solvents or water. The formation of this material during the reaction may be due to well reported condensation reactions in an acidic environment. 19 This insoluble fraction is not drastically influenced by the reaction temperature, as 32 % to 52 % of material is obtained between 90 °C and 120 °C. A reduced amount of acid leads to decreased substitution degree that ranges from 61 % with 0.25 equivalents of PTSA /OH to 28 % with 0.08 (entries 3 to 5). As expected in the case of the hydroesterification reaction, a high acid concentration is clearly needed to promote the formation of catalytically-relevant palladium hydrides and, hence, reach higher substitution degrees. The amount of PTSA doesn't drastically impact the formation of the insoluble lignin and rather similar amounts of functionalized lignin are obtained in most cases (for example, compare entries 3 and 5). A control test at 120 °C with lignin without PTSA in THF also generate insoluble compounds thus indicating that the observed insolubilities doesn't come from the acid used at this temperature (entry 6). A kinetic study performed with reaction conditions of entry 3 indicates that 6 hours of reaction are necessary to reach 61 % of substitution degree (entries 7-9 and entry 3) without improving the mass of recoverable lignin. Reducing the amount of olefin decreases the conversion of hydroxyl groups from 61 % to 39 % and 22 % with respectively 8 eq, 3 eq and 1.5 eq of 1-hexene. On the other hand, the decrease of olefin equivalents seems to play a role in the formation of the insoluble lignin fraction, as the %weight of functionalized lignin increased from 52 % to 72 % by reducing the amount of olefin from 8 eq to 1.5 eq (entries 3 and 10-11). It is notorious that lignin badly tolerates lipophilic chains and tends to precipitate in the presence of alkanes. It can thus be hypothesized that the starting lignin precipitated upon addition of 1-hexene in the reactor and further cured within the precipitate under the reaction conditions used. Other solvents were thus explored in order to find a better solubilizing medium for all the reagents. The reaction was first carried out in DMSO but no conversion was observed due to the fact that 1-hexene is poorly soluble in this solvent (biphasic liquid/liquid system formed), thus limiting the contact between all the reagents and catalyst. It should be noted that with DMSO, no lignin was insoluble at the end of the reaction (entry 12). The use of dioxane allowed an excellent conversion of 70 % and a slight improvement in the mass of lignin recovered (compare entries 3 and 13). With this solvent, a further decrease of the number of equivalents of olefin allowed a higher recovery of functionalized lignin (entries 14 and 15). No insoluble lignin was observed after reaction and the recovered masses of 105 % and 113 % are in agreement with the increase of mass due to the grafting of the aliphatic ester chains on the lignin.

Other parameters such as the pressure of CO, the catalytic charge in Pd and the amount of solvent were studied without significant improvement of the substitution degrees nor the lignin insolubility (see S.I. Table SI 1). Detailed characterization by 1 H NMR of the product obtained in entry 15 of Table 1 evidences the presence of alkyl chains with characteristic signals at 0.7-1.1 ppm corresponding to the terminal CH3, 1.1-1.6 ppm for the CH2 groups of the aliphatic chain and at 1.6-1.95 ppm for the CH2 in the β position of the ester (Figure 1). These spectroscopic data correlate well with the data recorded for hydroesterified guaiacol and benzyl alcohol (Figures SI 1 and SI 3). Most particularly, the presence of the peak at 2.15-2.85 ppm corresponding to the CH2 in α position to the ester indicates the presence of esters grafted onto the phenolic groups of the lignin. The α-CH2 of esters from aliphatic alcohols are less deshielded and give signals at 2.10-2.45 ppm as confirmed by the 1 H NMR data recorded for the hydroesterified benzyl alcohol. All the 1 H NMR spectra of the compounds obtained during the optimization of the reaction have this same pattern. Only the intensity of the alkyl peaks with respect to the methoxy and aromatic groups of lignin differ according to the degree of substitution.

The comparative study of the infrared spectra of native and modified lignin highlights a band at 1728 cm -1 characteristic of the C=O elongations corresponding to the new ester functions grafted on the lignin (Figure 2). The decrease in intensity of the broad band at 3360 cm -1 (O-H stretching) evidences the conversion of the hydroxyl groups. Also note the decrease after reaction of the bands at 1511 cm -1 and 1270 cm -1 corresponding respectively to the O-H deformations of phenolic alcohols and aliphatic alcohols. This suggests that the reaction occurs on both phenols and aliphatic alcohols. New intense characteristic bands also appear at 2928, 1121 and 679 cm -1 corresponding to the C-H and C-C elongations of the aliphatic chains grafted on lignin. The esterification of the hydroxyl groups is also confirmed by the 31 P NMR analysis performed after the derivatization of the reaction product with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane. 20 The comparison with the analytical data recorded for the technical lignins The optimized protocol with 1-hexene was applied to other commercially available lignins. Substitution degrees of 84 % and 85 % were respectively obtained with Organosolv and Soda lignins (Table 2 entry 2 and3). This demonstrates the broad applicability of this methodology to other types of lignin. The linear to branched ratio of esters (L/B in table 2) was quantified using quantitative 13 C NMR and showed similar results to the simple molecules with less than 10 % of branched isomer. The palladium content in the lignins was measured by ICP in order to evaluate the amount of metal leached in the final material. The ICP titrations indicate that only 25 % of the catalyst remains in the lignin. Thus, most of the catalyst is extracted from the lignin thanks to the purification steps. A major line of future development of our process is isolation and recovery of the spent metal catalyst. From Kraft lignin, other olefins were grafted to probe the impact of the chain size on the thermal properties of the new materials (Table 3). The reaction with 1-octene leads to conversions of 87 % of total lignin alcohols (100 % of aliphatics and 81 % of phenolics -entry 2). 1-decene gives slightly lower results with 70 % conversion while 1-dodecene gives the best results with 91 % alcohol functionalization (entries 3 and 4).

Regarding thermal properties, Kraft lignins functionalized with esters carrying a 6-carbon chain issuing from 1-hexene, the glass transition temperature (Tg) of the compounds drops from 140 °C (technical Kraft lignin) to 102 °C (entry 1). With 8 and 12 carbon chains, the Tg is further lowered to 80 °C and 72 °C respectively (entry 2 and 4). Concerning the 10-carbons chain from 1-decene, the glass transition temperature of the sample is the highest of the series (110 °C) and this can be explained by a lower substitution degree (entry 3). 0.190 Conditions: 0.5 g of lignin, 10 mL of Dioxane, Pd(OAc)2 1 mol% in respect to OH groups, 25 eq/Pd of PPh3 and PTSA, 3 eq/OH 1-hexene, 30 bar of CO a) Conversion of total, aliphatic, and phenolic OH groups, respectively, and of acid groups, determined by 31 P NMR with a correction factor considering modified lignin content. see correction factor calculation after table SI2. b) determined by 13 C NMR, see SI15. SEC measurements were run on the isolated materials to determine the number average molecular weight Mn and the dispersity Đ (see S.I. table SI 4). SEC analysis of used technical Kraft lignin after acetylation revealed a number average molecular weight Mn of 1800 g.mol -1 with a dispersity Đ of 3.1. After hydroesterification with 1-hexene, higher molecular weight (Mn= 2600 g.mol -1 ) corresponding to the large amount of grafted chains on the macromolecule is observed. For similar conversion rates, the more carbons in the grafted chain, the higher the molecular weight, as expected. SEC analysis indicate Mn values of 2900 g.mol -1 with the material obtained from the hydroesterification of 1-octene and of 3100 g.mol -1 with 1-dodecene (Table 3 entry 2 and4).

This new synthesis procedure opens up opportunities for bio-sourced fatty acid grafting through the transformation of the unsaturation while the carboxylic acid remains pending. The most abundant fatty acid, oleic acid, has been reacted by using the optimized reaction conditions. With this internal olefin, 26 % of the hydroxyl groups were converted (see Table 4 entry 1). A deeper analytic study showed that only the aliphatic hydroxyl groups are substituted. In addition, 1 H NMR indicates the presence of untouched unsaturations (double bonds) within the product. Thus, the hydroesterification reaction has not occurred, and the grafting was essentially obtained from a classical esterification of aliphatic alcohols with oleic acid in the presence of PTSA. The reaction with methyl oleate results in a slightly improved alcohol conversion that reaches 29 %. With this starting material, 12 % of the hydroxyl groups have been esterified thanks to the carbonylation reaction (entry 2). As internal olefins proved to be less reactive, we turned our attention towards 10-undecenoic acid, derived from renewable ricinoleic acid via thermolytic pathway. Indeed, this terminal olefin exhibits a higher reactivity than oleic acid in carbonylative esterification. A conversion of 36 % of the total alcohols was obtained, including 22 % of the phenols, exclusively via hydroesterification reactions (entry 3). The highest conversions were obtained with the ester derivative methyl 10-undecenoate with 90 % conversion of hydroxyl groups including 90 % of phenolic groups (entry 4). As a further example, estragole as a terminal

Kraft Lignin Kraft Lignin

olefin bearing an aromatic group leads to 48 % overall conversion of hydroxyl groups, including 36 % substitution of phenol groups (entry 5). Remarkably, in the present study, this enhanced reactivity pattern of terminal olefins favours the grafting of the hydroxyl sites through the hydroesterification reaction rather than classical esterification or transesterification processes.

The 1 H NMR analysis of lignin modified with 10-undecenoate shows the presence of methyl esters groups with a strong, sharp peak at 3.6 ppm (Figure 3, label 6). The assignment is further evidenced by the sharper linewidth of this signal compared to others from the material, as expected by the higher chain mobility from the grafted ester chain ends. The protons of the carbon atom in alpha position relative to the carbonyl groups are also observed in the range 2.2-2.7 ppm as two signals (Figure 3, labels 1 and 5). The influence from the addition of this long aliphatic tail into the lignin is also evidenced by DSC with a glass transition temperature of 55 °C.

The practicality of our approach to lignin materials functionalization with methyl 10-undecenoate was probed further through a scale-up of the process. It proved to be amenable to a 100 grams scale: the resulting fully biosourced lignin-vegetable oil-based resin could then be readily shaped on hot-plates (Figure 4). This functionalized lignin is particularly interesting due to the improved compatibility provided by the long fatty chains, combined with the availability of the reactive ester functions which can be used in the formulation of blends or as a polyester macromonomer. 4 entry 4 Grafting of other polymerizable functionalities is also attractive for the valorization of lignin as a material. More specifically, unsaturations have aroused a recent interest. 21 Indeed, these alkenes, in addition to being directly polymerizable by metathesis, thiol-ene or radical polymerization, can also be used as a precursor for epoxy resins. 22 In this regard, the hydroesterification reaction can be a clean solution for grafting unsaturations onto lignin using butadiene as a reagent. Butadiene has several advantages. As a gas under ambient conditions, the removal of excess butadiene is straightforward and the reagent is simply released at the reactor opening, easing up purification procedures. Furthermore, although butadiene is nowadays mostly petroleum-based, it can also be obtained from bioethanol. 23 The methoxycarbonylation of 1,3-butadiene described by M. Beller 24 shows the feasibility of using this gas as a reagent with aliphatic alcohols. As part of our study, this reaction was first performed with guaiacol as a phenol model to ensure the reactivity of phenols as nucleophile. In contrast to the hydroesterification reaction of non-conjugated olefins (vide supra), the hydroesterification of butadiene requires a different catalytic system and softer reaction conditions. A combination of Pd(OAc)2 with dppb (1,4-bis(diphenylphosphino)butane) is used to generate the catalyst, benzoic acid is employed as co-catalyst. The reaction with guaiacol was evaluated with 1 mol% of Pd(OAc)2, 6 equivalents per Pd of dppb at 120°C for 24 h under 25 bar of carbon monoxide. At the end of the reaction, 90 % conversion of the starting guaiacol was observed, with formation of two isomers in a 6:4 ratio. Their purification can be performed by silica gel column chromatography, though similar properties hinder the efficiency of the separation. The isomers differ in the position of the double bond on the aliphatic chain, mostly as the trans isomers from 1 H and 13 C NMR (See SI). The β-δ isomer was isolated with 52 % yield and the α-β isomer with 11 % yield (Scheme 3). It should be noted that no additional isomers were observed during the study, the low isolated yields are due to the arduous separation of these two products as analytically pure compounds. In addition, the same procedure was carried out with benzylic alcohol as substrate. As in the case of gaiacol, a mixture of two isomers was obtained in 94 % yield with ratio of 60 % β-δ isomer and 40 % α-β isomer (see S.I. Figure SI 9).

Scheme 3 Hydroesterification of butadiene with guaiacol -Reaction conditions: Guaiacol 9 mmol, benzoic acid 0.3 eq, butadiene 7 eq, Pd(OAc)2 1 mol%, dppb 6 eq/Pd, 120 °C, 24 h, 25 bar CO, THF.

The implementation of this procedure on Kraft lignin leads to conversion of 43 % of all types of alcohols, including 40 % of phenols and 47 % of aliphatic alcohols (Table 5 entry 1). As in the case of the simple molecules, two isomers are observed as shown by 1 H NMR analysis of the product (see below for details). The integration of proton signals evidences a selectivity of 63 % for β-δ and 37 % for α-β isomers. Interestingly, increasing the reaction time does not improve the conversion, but does increase the ratio of α-β isomer (entry 2). The catalyst thus appears to initially functionalize the alcohols with the unsaturation in the β-δ position as the kinetic product of the reaction, and then subsequently isomerizes the double bond to the α-β position, thus forming the thermodynamic product. This assumption is confirmed as a reduced catalyst loading of 0.5 % leads to a lower selectivity toward the isomer α-β which accounts for 14 % of the mixture (entry 3). In order to maximize the functionalization of lignin alcohols, a higher catalytic loading up to 1.6 mol% of Pd was also investigated and allowed an increase of the total conversion to 67 %. Under these conditions, 52 % of the phenols and almost all of the aliphatic alcohols were converted (entry 4). Finally, such as in the case of olefin hydroesterification described above, the use of dioxane significantly increases the efficiency of the reaction. With this solvent, the reaction reaches 95 % conversion of the alcohols (93 % of phenols -entry 5). Other optimization parameters such as the reaction time or temperature have also been studied and showed a limited impact on the conversion of lignin's hydroxyl groups (see S.I. Table SI 3) Interestingly, DSC analysis of this product gives a glass transition temperature of 59 °C, similar to that observed with methyl 10undecenoate (Figure 5 C). Although the grafted chains are shorter, the presence of unsaturation and a slightly higher conversion seem to significantly impact the glass transition temperature. This modified lignin thus presents new interesting properties from a thermal point of view, with visibly improved compatibility with solvents. This material is for example soluble in toluene. Furthermore, the C-C unsaturations grafted via our catalytic functionalization protocol allow the modified lignin to be further chemically modified, for instance as macromonomer for innovative green polymer materials.

Conclusions

A new method of lignin functionalization through the palladium-catalyzed hydroesterification reaction has been developed. The reaction readily converts hydroxyl groups of lignin into esters from a reaction with CO and an olefin without generating salts in stoichiometric amounts. Importantly, the method is effective for all types of alcohol functions, whether aliphatic or phenolic. The characterization of the esterified lignins was carried out using a combination of spectroscopic methods (NMR and IR). Thanks to a comparative study using simple molecules which provides a robust tool for the assessment of selectivity towards all alcohol types, the lignin grafting with ester functions was fully determined. Under suitable reaction conditions and broadly available catalysts, up to about 90 % of the hydroxyl groups in Kraft lignin have been readily converted. The process also proved to be suitable with other types of lignins such as Organosolv and Soda. Interestingly, the chain length of the grafted esters can be tuned by varying the nature of the starting olefin. DSC and SEC analyses demonstrated the modulation of the physicochemical properties of lignin depending on the length of the grafted chain. An ICP study also indicates that only limited amount of palladium is contained in the final product thanks to its removal during the purification phase. Reagents such as butadiene or bio-based fatty esters have allowed the introduction of new reactive functions within lignin while significantly lowering glass transition temperatures. Furthermore, a scale-up to 100 g scale was successfully carried out, resulting in a moldable bio-sourced material. Thanks to the versatility of this approach, new lignin-based shapable resins can thus be produced and these can be functionalized with cross-linkable functions such as double bonds or esters. Thus, our versatile catalytic process offers an efficient and selective entry towards new biosourced thermosetting polyester materials based on lignin with highly tunable properties.

Materials and methods

General considerations

Kraft lignin was provided by UPM Biochemicals, Organosolv by ChemicalPoint and Soda by GreenValue Enterprises LLC. Lignins were dried at reduced pressure at 60 °C for 48 h before use. Chemicals were purchased from Aldrich, Alfa Aesar, Acros and Strem. Butadiene was purchased from Linde Gas France. Tetrahydrofuran was purified using a MBraun SPS-800 solvent purification system. DSC measurements were carried out using a Perkin-Elmer DSC 4000 differential scanning calorimeter. The measurements were done with a heating rate of 10 K/min in the temperature range from 243 K to 453 K, under nitrogen flow (20 ml/min

Titration of the hydroxyl groups in the technical lignins and modified lignins

31 P NMR experiments were carried out following a previously reported method 25 : Lignin (30 mg) was dissolved in a CDCl3/pyridine mixture (1:1.6 v/v, 0,5 mL). The phosphitylation reagent, 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (150 μL), and the internal standard N-hydroxy-6-norbornene-2,3-dicarboximide (100 μL of 0.1 M solution in 1:1.6 v/v CDCl3/pyridine mixture) were added successively. Chromium(III) acetylacetonate (100 μL of 0.014M solution) in the same CDCl3/pyridine mixture was added to the solution in order to homogenize and accelerate phosphorus relaxation. Spectra were recorded with a 15 sec relaxation time and an average number of 512 scans. Chemical shifts are relative to the signal of the phospholane hydrolysis product at 132.2 ppm. The integrated value of the internal standard was used for calculation of the absolute amount of each functional group.

Comparison between the amount of technical lignin functional group and the amount of hydroesterified lignin functional groups gave the substitution ratio of hydroxyl groups. This method includes a correction factor for integrations of modified lignins considering the reduced lignin content (see SI).

General procedure for olefins/fatty esters hydroesterification on lignin

An autoclave was charged with lignin (500 mg), palladium acetate (6 mg, 0.027 mmol), triphenylphosphine (175 mg, 0.67 mmol), and paratoluenesulfonic acid (115 mg, 0.67 mmol). After performing three vacuum/N2 cycles, degassed dioxane (10 mL) and degassed olefin (9.15 mmol) were added into the autoclave. The reactor was pressurized under 30 bar of carbon monoxide and then heated to 120 °C for 22 h under magnetic stirring with a cross magnet. After cooling, the excess gas was evacuated, the solvent was evaporated, and the crude was solubilized in 3 mL THF and precipitated with petroleum ether (30 mL). The product was washed 3 times with petroleum ether (10 mL) The resulting brown powder was dried at 60 °C under reduced pressure for 24 h.

General procedure for butadiene hydroesterification on kraft lignin

Kraft lignin (500 mg), palladium acetate (6 mg, 0.026 mmol), 1,4-bis(diphenylphosphino)butane (70 mg, 0.16 mmol), and benzoic acid (100 mg, 0.82 mmol) were introduced into an autoclave. After performing three vacuum/N2 cycles, distilled and degassed THF (10 mL) is added to the reaction medium and the autoclave is cooled to -40 °C. Butadiene (2 mL, 1.2 g, 22 mmol) is condensed in a Schlenk tube kept cold by an acetone/dry ice mixture and then introduced into the reactor by cannula. The reactor is pressurized under 50 bar of carbon monoxide and then heated at 120 °C for 22 h under magnetic stirring with a cross-shaped magnet. After the reaction, the reactor is cooled to room temperature and the excess gas is evacuated. The solvent is extracted with a rotary evaporator. The crude is solubilized in 3 mL THF and the modified lignin is precipitated in 30 mL petroleum ether. After settling for one hour, the lignin is recovered and dried under vacuum at 80 °C overnight.

Figure 1

 1 Figure1Comparison of the 1 H NMR spectra of the hydroesterified lignin (red spectrum) with those of esters of guaïacol (green) and benzyl alcohol (blue).

Figure 2

 2 Figure 2 Comparison of IR spectra of technical (blue) and modified kraft lignin (red)

  of Kraft lignin, 10 mL of Dioxane, Pd(OAc)2 1 mol% in respect to the OH groups, 25eq/Pd of PPh3 and PTSA, 3 eq/OH of olefin, 30 bar of CO; a) Conversion of total, aliphatic, and phenolic OH groups, respectively, and of acid groups, determined by 31 P NMR with correction factor (see SI). b) Comparison between direct esterification and carbonylation by hydroesterification -Determined by 1 H NMR considering the remaining unsaturations on the lignin (5.2-5.5 ppm)

Figure 3 .

 3 Figure 3. 1 H NMR and DSC analysis of the product of hydroesterification of methyl 10-undecenoate with Kraft lignin (Table 4, entry 4)

Figure 4

 4 Figure 4 Lignin shaping on hot plates after hydroesterification of methyl 10-undecenoate -upscaling of the experiment from Table4 entry 4

Figure 5 .

 5 Figure 5. 1 H NMR (A), 31 P NMR with intensities calibrated on the internal standard (B) and DSC (C) analysis of the product of hydroesterification of butadiene with Kraft lignin (Table5, entry 5).

Table 1 -

 1 Optimization of the palladium catalyzed hydroesterification of 1-hexene with lignin

	Kraft				Kraft
	Lignin				Lignin
	Entry	Hexene (eq/OH)	PTSA (eq/OH)	T (°C)	Solvent	t (h)	Conv [a] OH total	%W [b]
	1	8	0.25	90	THF	22h	28 %	32 %
	2	8	0.25	100	THF	22h	51 %	33 %
	3	8	0.25	120	THF	22h	61 %	52 %
	4	8	0.15	120	THF	22h	42 %	24 %
	5	8	0.08	120	THF	22h	28 %	42 %
	6	8	0	120	THF	22h	0 %	54 %

Table 2

 2 Scope of the hydroesterification of 1-hexene on lignins from different extraction processes

Table 3

 3 Impact of olefin length in hydroesterification on kraft lignin properties Conditions: 0.5 g of Kraft lignin, 10 mL of Dioxane, Pd(OAc)2 1 mol% in respect to OH groups, 25eq/Pd of PPh3 and PTSA, 3 eq/OH olefin, 30 bar of CO a) Conversion of total, aliphatic, and phenolic OH groups, respectively, and of acid groups, determined by 31 P NMR with correction factor (see SI).

				Conversion [%] [a]		DSC		SEC
	E	Olefin	OHtot	OHal	OHph	COOH	Tg (°C)	Mn	Mw	Đ
		1-Hexene
	1		75	91	68	30	102	2600	4400	1.7
		1-Octene
	2		87	100	81	44	80	2900	5100	1.7
		1-Decene
	3		70	82	64	40	110	2500	4000	1.6
		1-Dodecene
	4		91	90	91	58	72	3100	6700	2.2

Table 4

 4 Hydroesterification

of fatty acids, fatty ester and estragole on kraft lignin Conversion [%]

[a]

E Olefin OHtot OHal OHph COOH Carbonylation ratio [b]

	1	26	85	0	60	1 %
	2	29	92	3	87	12 %
	3	36	68	22	31	100 %

Table 4

 4

, entry 4)

  ). NMR spectra were recorded in CDCl3 (99.8 % atom D), purchased from Aldrich. 1 H NMR spectra were obtained at either 300 and 400 MHz,13 C were recorded at 75 MHz with 1 H decoupling and 101 MHz with both 1 H and 31 P decouplings, and 31 P spectra were acquired at 121 and 162 MHz with 1 H decoupling. Chemical shifts ( 1 H and 13 C) were referenced in parts of million relative to TMS (δ=0.00 ppm) and were referenced to the residual solvent peak ( 1 H, CDCl3 7.26 ppm). Diffuse reflectance infrared spectra were collected with a Harrick cell on a Nicolet Avatar spectrometer fitted with a MCT detector. Typically, 64 scans were accumulated for each spectrum (resolution of 4 cm -1 ). The number-average (Mn), the weight-average (Mw) molecular weights and the dispersity (Đ) of the product were determined by Size Exclusion Chromatography (SEC) in tetrahydrofuran at 40 °C at a flow rate of 1 mL.min -1 . Sample concentration was of 0.50 w%. Mn, Mw, and Đ were determined from the Refractive Index (RI) signal using a calibration curve based on polystyrene (PS) standards from Polymer Standards Service, on a Waters apparatus equipped with Waters Styragel columns HR2, HR3, HR5 and HR5E. Nickel (231,604 nm) and palladium (340,458 nm) concentrations were determined using an Agilent's 5110 Vertical Dual View ICP-OES equipped with a OneNeb nebulizer, a quartz double pass spray chamber and a quartz torch (Agilent). Quantifications were carried out by an external calibration approach.

Acknowledgements

We acknowledge the Ministère de l′Enseignement Supérieur, de la Recherche et de l′Innovation, the CNRS, the Hauts-de-France Region and Icam for their financial support. We acknowledge C. Delabre for HR-MS analysis. ICP analyses were performed in the « Spectrométrie par torche à plasma » platform of the Research Federation Michel-Eugène Chevreul hosted by the LASIRE laboratory.

Notes and references

 [a]Isomers E Pd(OAc)2 (eq/OH) t (h) OHtot OHal OHph Ratio β-δ [b] Ratio α-β [b] 1 5 [c] 1 % 15 95 99 93 95 % 5 % Conditions: 0.5 g of Kraft lignin, 6 eq/Pd of dppb, 7 eq/OH of butadiene, 10 mL of degassed THF, 50 bar CO and 30 eq/Pd of benzoic acid, 110 °C; a) Conversion of total OH groups: comparison of the amount of chains grafted onto the lignin with the quantity of available hydroxyl groups; determined by 1 H NMR with pentafluorobenzaldehyde as internal standard: b) Ratio calculated by 1 H NMR from the integrals of signals between 2.17-2.4 and 2,7-3,45 ppm. c) Reaction performed with dioxane instead of THF Detailed analysis of the 1 H NMR spectrum recorded for the material obtained from entry 5 of Table 5 shows extensive grafting of chains containing unsaturation into the lignin. The spectrum fits very well with the spectra recorded for the products obtained from simple compounds guaiacol and benzyl alcohol ( . Speculatively associated to internal esterification reactions as seen above, the reaction being performed under acidic conditions. The SEC analysis reveals a molecular weight of 5100 g.mol -1 and a polydispersity index of 2.3. The Mn value after modification with butadiene is significantly higher compared to those obtained above with other olefins most probably due to the presence of polybutadiene, whose shoulder is clearly observed in SEC analysis at short retention times (see S.I figure SI 40).

Kraft Lignin Kraft Lignin

Kraft Lignin

Conflicts of interest

There are no conflicts to declare.


João Pedro Norenberg

Roberto Luo

Vinicius Gonçalves Lopes

João Victor Ligier

Lopes Peterson

Americo Cunha

Vinicius Gonçalves

Jo Ão

Pedro Norenberg

Vinicius Lopes

Victor Peterson

Nonlinear analysis of compensated asymmetric energy harvester

Keywords: vibration energy harvesting, nonlinear dynamics, bistable oscillator, asymmetries, basins of attraction

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Energy harvesting can be an essential technique to provide greater autonomy to electronic mobile devices. It consists of converting available energy sources in the environment such as vibration, heat, and wind into electricity. This technology proved to be promising in low-energy demand applications, e.g., sensors, actuators, pacemakers, and IoT devices. Several recent works have been developed in different applications.

Among commonly available energy sources present in the environment, the vibration source from environmental and structural noise has been explored for harvesting energy. Linear vibration energy harvesting devices, despite their dynamic simplicity, typically provide reduced output power in regions away from the resonance. To overcome this limitation, [START_REF] Cottone | Nonlinear Energy Harvesting[END_REF] and [START_REF] Erturk | A piezomagnetoelastic structure for broadband vibration energy harvesting[END_REF] proposed nonlinear systems to potentially increase the frequency bandwidth.

Numerous efforts have been dedicated to bistable energy harvesters proposed by [START_REF] Erturk | A piezomagnetoelastic structure for broadband vibration energy harvesting[END_REF]. They built a piezo-magnetic-elastic system. Several works suppose a perfectly symmetric system, but it is arduous to fabricate it mainly because, in most cases, the asymmetries are unavoidable, for instance, from the geometry, physical properties, mechanical loads, and magnetic fields. Recently, Cao et al. (2015) and Wang et al. (2018a) assessed the asymmetry effect of a bistable energy harvester. They verified that asymmetries are prejudicial to harvesting performance, decreasing the energy output compared to a symmetrical system. They also visualized that the dynamic becomes more complex because one more nonlinearity appeared. This work investigates the dynamic behavior of asymmetric bistable energy harvesting systems. We calculate the bifurcation diagrams to study the multiple solutions sweeping the amplitude and frequency of excitation. The basins of attraction based on Test 0-1 for chaos proposed by Norenberg et al. (2022b) are also calculated to examine the incidence of chaotic behavior concerning the initial conditions. Using these classical nonlinear tools, we conduct a comprehensive study of asymmetric bistable energy harvesters and compare them to the symmetric model. A particular condition of the asymmetric model is established and studied to compensate the asymmetric effect.

ASYMMETRIC PIEZO-MAGNETO-ELASTIC ENERGY HARVESTER

Figure 1 illustrates the asymmetric bistable piezo-magneto-elastic energy harvester proposed by Norenberg et al. (2022a). This system consists of a vertical clamped-free ferromagnetic beam. The beam is fixed at a rigid base where a pair of magnets is placed on the lower part. Two piezoelectric layers are placed on the beam's highest part, the region of higher deformation. The rigid base is periodically excited by an external source. When this motion deforms the piezoelectric layers, the kinetic energy of vibration is converted into electricity and is dissipated in the resistor. In order to introduce the asymmetric features, the harvester is attached to a plane with a sloping angle φ , arising an external force due to gravitational effect, and we also assume that the magnets are not identical, inducing asymmetric potential energy. Taking into account these features, we obtain a sophisticated model. The system is perfectly symmetric when we neglect these features.

The following dimensionless initial value problem describes the governing lumped-parameter equation of motion of the asymmetric piezo-magneto-elastic energy harvester

ẍ + 2 ξ ẋ - 1 2 x (1 + 2δ x -x 2 ) -χ v = f cos (Ω t) + p sin (φ ), (1)
v + λ v + κ ẋ = 0, (2)
x(0) = x 0 , ẋ(0) = ẋ0 , v(0) = v 0 , (3)
where t denotes time; x is the beam tip amplitude of oscillation; v is the voltage in the resistor; ξ is the damping ratio; f is the rigid base oscillation amplitude; Ω is the external excitation frequency; λ is a reciprocal time constant; the piezoelectric coupling terms are represented by χ, in the mechanical equation, and by κ in the electrical one; δ is a coefficient of the quadratic nonlinearity (asymmetry of not identical magnets), p is the equivalent gravity of ferromagnetic beam, and φ is the sloping angle; x 0 represents the beam edge initial position; ẋ0 is the beam edge initial velocity; and v 0 denotes the initial voltage over the resistor. The upper dot is an abbreviation for time derivative. These parameters are all dimensionless.

Compensation of the asymmetric effects

In order to compensate the asymmetry configurations and obtain a symmetric model, the sum of the nonlinear restoring force for the equilibrium point needs to be zero (Wang et al., 2018b), i.e., F r (x

1 ) + F r (x 2 ) = -1 2 x 1 (1 + 2δ x 1 -x 2 1 ) - 1 2 x 2 (1+2δ x 2 -x 2
2 )-2p sin (φ ) = 0, where x 1 and x 1 are the stable equilibrium points. When this condition is established, the asymmetries compensate themselves, and the system has a symmetric configuration. Therefore, we can calculate the value of the sloping angle (φ opt ) that cancels the asymmetry of the quadratic coefficient as

φ opt = arcsin 8δ 3 + 9δ 27p . (4)

RESULTS AND DISCUSSION

Numerical simulations are carried out for ξ = 0.01, χ = 0.05, λ = 0.05, κ = 0.5, and δ = 0.15. The amplitude and frequency of excitation are varied. Three values of the slope angle φ are evaluated: (i) φ = 0 • , for symmetric condition (in this situation, δ = 0); (ii) φ = 35 • , for strong asymmetric condition; (iii) φ = -5 • , for the condition of asymmetries compensation (value obtained from Eq. ( 4) when δ = 0.15). The codes are available on [START_REF] Norenberg | STONEHENGE Suite for nonlinear analysis of energy harvesting systems[END_REF].

Bifurcation diagrams

Bifurcation diagrams were plotted, in Fig. 2, sweeping up and down the excitation amplitude and frequency. Sampling analysis intervals for forcing parameters are set as 0.1 ≤ Ω ≤ 1.4 for the frequency and 0.01 ≤ f ≤ 0.3 for the amplitude.

The symmetric condition, Fig. 2a, provides a rich behavior regards the chaos and period multiplicity occurrence. The bifurcation diagrams sweeping the amplitude for the asymmetric model when φ = 35 • are shown in Fig. 2b. The chaotic regions disappeared, and the dynamic behavior is periodic for all excitation values with low power energy. For sweeping the frequency of excitation, there are periodic conditions (one or more periods), but the chaos disappeared. The bifurcation diagram for the asymmetric compensation model is shown in Fig. 2c. The dynamic behavior of this condition presented similar to the symmetric case in that the chaos and multiple periodic responses appear in the same forcing conditions.

Basins of attractions

Basins of attraction numerically study the dynamic response for different initial conditions. It indicates where the different attractors can coexist with the underlying dynamical system. This study integrates the dynamics for different sets of initial conditions and maps the possible attractors in which the phase space trajectory can accumulate. The test 0-1 for chaos is used to characterize the dynamics, determining if the system evolves to the chaotic or regular response [START_REF] Gottwald | The 0-1 Test for Chaos: A Review[END_REF]. When the test indicates an irregular response, we scheduled to catalog the node with a grey color. When it indicates the regular response, we establish one step to identify the attractors and distinguish the different periodic orbits. The green color is a pattern color to indicate the regular high-energy orbit (cycle limit behavior) that is the best scenario for the harvesting process. The red and blue colors indicate regular low-energy orbits, not appropriate for harvesting energy because the system is monostable. Other colors, e.g., magenta and cyan, indicate regular behaviors but are different energy from red and blue orbits.

Figure 3 displays the basins of attractions when the amplitude of excitation is fixed at f = 0.019 and its frequency is fixed at Ω = 0.8. While Fig. 4 displays the basins of attractions for the f = 0.211 and Ω = 0.8. The three configurations are presented in each figure. The initial displacement × initial velocity plane is defined in the region (x 0 , ẋ0 ) ∈ [-3 ≤ x 0 ≤ 3] × [-3 ≤ ẋ0 ≤ 3] at the restriction plane to v 0 = 0.

For the symmetrical and compensated asymmetrical at Fig. 3a andc, respectively, we observed four regular attractors with pulverized regions that express the sensitivity to the initial conditions. However, for the asymmetric model (Fig. 3b), the red basins, low energy attractor, is prevalent throughout the plane, demonstrating the worst scenario for harvesting performance. When the amplitude of excitation increases, f = 0.211 (Fig. 4), for the symmetric and compensated asymmetric, the basin is entirely covered by a high energy orbit, demonstrating the desired situation for the energy harvesting process. For the asymmetric model, there are two solutions, green and red basins, but the low-energy solution is dominant. In this situation, we can also visualize that the basins are also asymmetric, such as the well of the right stable equilibrium point is deeper than the left stable equilibrium point.

It is worth mentioning that the basins are symmetric for the compensated asymmetric model. The basins of attractions of the symmetric are similar to the asymmetric, and the solutions presented the same behaviors. Therefore, despite the asymmetric terms added in the model, the asymmetries were compensated, and the system became symmetric.

CONCLUSION

This work numerically investigates the dynamic behavior of the asymmetric bistable energy harvester. Forward and backward bifurcations diagrams of the amplitude and frequency of excitation were plotted to indicate multiple solutions. Basins of attraction based on Test 0-1 were proposed to investigate the attractor projections for different initial conditions. We performed this examination for a strong asymmetric and compensated asymmetric configuration and compared them to the symmetric one. The predominance of low-energy orbits of the asymmetric model was visualized. However, when the external gravitational force canceled the asymmetry of magnets, the system has similar dynamic behavior to the

Figure 1 :

 1 Figure 1: Illustration of the asymmetric piezo-magneto-elastic energy harvester.

  Figure 2: Power bifurcation diagram sweeping forward (blue scale) and backward (red scale) amplitude of excitation (top) and the frequency of excitation (bottom).

  asymmetric, φ = 35 • (c) asymmetric, φ = -5 •

Figure 3 :

 3 Figure 3: Basins of attraction to the region {(x 0 , ẋ0 ) | -3 ≤ x 0 ≤ 3 and -3 ≤ ẋ0 ≤ 3} for f = 0.019 and Ω = 0.8.

  asymmetric, φ = 35 • (c) asymmetric, φ = -5 •

Figure 4 :

 4 Figure 4: Basins of attraction to the region {(x 0 , ẋ0 ) | -3 ≤ x 0 ≤ 3 and -3 ≤ ẋ0 ≤ 3} for f = 0.211 and Ω = 0.8.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support from CAPES, CNPq, and FAPERJ.

RESPONSIBILITY NOTICE

The authors are the only responsible for the printed material included in this paper.


R Mahfouz

F J Cadete

Santos Aires

A Brenier

B Jacquier

J C Bertolini

Synthesis and physico-chemical characteristics of nanosized particles produced by laser ablation of a nickel target in water

Keywords: Ni, NiO, Nanoparticles, Laser ablation in liquid phase, HRTEM, XPS photoemission

Introduction

In recent years, many researches have been done on noble metal nanoparticles because of their unique character differing from those in bulk state. Metal colloids are one of the important nanosize materials that will open significant opportunities in chemistry and material science [START_REF]Cluster and Colloids[END_REF]. Metal colloid nanoparticles have a great interest because of the size dependence of their optical properties, magnetic properties, catalytic activities, etc. [START_REF] Takagi | [END_REF]3]. Indeed, metal nanoparticles have magnetic properties different of bulk materials of the same composition due to the coupling of magnetism with structure, surface effects, and thermally activated processes [START_REF] Ichinose | Superfine Particle Technology[END_REF][START_REF] Sanchez | [END_REF]. Gold nanoparticles smaller than 5 nm in diameter have an intense photoluminescence [6] and those with a diameter smaller than 10 nm show significant catalytic activities in low temperature CO oxidation [3,6,7]. Besides, laser ablated suspended metal nanoparticles prepared in liquid phase can be used directly for catalysis purposes. For instance, it was shown that (bare or stabilized by a surfactant) palladium nanoparticles can be efficient for several target hydrogenation reactions [8] and that nickel particles exhibit a catalytic effect for the preparation of 3-aryl propanenitrile from benzylchloride and bromoacetonitrile under mild conditions without any activation, whereas Ni particles larger than submicrometer size have not shown this activity [9]. Thus, metal nanoparticles may have specific physical and chemical properties depending upon their size and/or morphology. It exists many ways to produce nanoparticles. Among them, ablation of metals by laser vaporization from a bulk target in liquid environment is a rather simple method which has attracted recently some attention [8][9][10][11][12][13][14][15][16][17][18][19][20]. But, the characteristics of the metal particles formed and the ablation efficiency strongly depend upon many parameters such as the wavelength of the laser impinging the metallic target [10,13,16], the duration of the laser pulses (in the femto-, pico-and nanosecond regime) both in liquid and gas phase [11,21], the laser fluence [10,16,18,19], the ablation time duration [16] and the effective liquid medium, with or without the presence of surfactants [19]. The laser fluence is one of the most important parameters. Indeed, the ejection of metal particles from the target requires a minimum power (or fluence) [11,19]. The mean size of the nanoparticles has been found generally to increase with increasing laser fluence and is generally smallest for fluences not too far above the laser breakdown threshold [10]. But, for platinum it has been shown that at low laser fluence compounds with platinum in a non-zero oxidation state can be produced, probably due to the presence of a large percentage of metal atoms, together with small clusters, which could be able to react with the liquid medium [18]. Besides the laser fluence, the number of laser shots (i.e. the time spent during laser vaporization) must influence the concentration and the morphology of metal particles released in a liquid. For longer times under the laser beam the metal particle concentration is expected to increase, but it can saturate due to light absorption in the colloid highly concentrated in metal particles [19]. Moreover, nanoparticles can be modified in size and shape due to their further interaction with the laser light passing through [13,22]. But, the interaction between a laser pulse and a solid target in a confining liquid is not obvious and its mechanism is not well understood (see for instance the review of paper of Yang [23]). In any case, it is a way to synthesize nanocrystals and nanostructures ( [23] and references therein).

In the present study, suspensions of nanoparticles were prepared by laser ablation in water from a pure nickel target, at various laser interaction times (number of laser shots). Morphology and structure of the nanoparticles obtained in the colloidal solutions were characterized mainly by transmission electron microscopy (TEM and HRTEM). Measurements of the weight loss of the target after laser ablation and chemical analysis (ICP) were used to determine the amount of metal released in the aqueous solution. In addition, UV-vis spectrometry was tentatively employed to characterize further the properties of the particles in suspension. XPS photoelectron spectra of the impacted region of the sample were also measured in order to gain complementary information.

Experimental

The laser ablation method was employed to fabricate particle suspensions.

The nickel samples used as metal sources were in shape of discs of about 10 mm diameter and 1.5 mm thickness. They were obtained by cutting a high purity (>99.95%) nickel single crystal bar by spark erosion; they are further mechanically polished (down to 1 mm) and cleaned by ultrasonication into dried acetone and then ethanol in sequence, before use as metallic sources. The Ni sample was fixed onto a Teflon support plugged onto a glass vessel containing approximately 5 cm 3 of distilled water and closed by a welded glass (or quartz) window (Fig. 1). The nickel target is completely immersed which secures a liquid/solid interface free of gas. A Nd:YAG laser operating at the second harmonic (l = 532 nm, pulse duration = 8 ns, 10 Hz) was focused ($2 mm in diameter) on the target. The laser power wasfixedat40mJ/pulse.Theablationtimewasvariedfrom2 to 15 min.

The amount released in the aqueous solution by the laser ablation was measured from the weight loss of the metal plate after the laser ablation. The precision of the (Sartorius) balance was AE1 mg.

Induced coupled plasma (ICP) chemical analysis was used to determine the metal contents. The colloidal solutions of nanoparticles were evaporated in a quartz container, and then the nickel nanoparticles were dissolved in an acid solution (H 2 SO 4 + HNO 3 ). The solutions were filtered before analysis by optical ICP in a SPECTRO monochromatic spectrophotometer (Ni wavelength: 231,604 nm).

The optical absorption spectra of colloids were measured by UV-vis spectrometry with using a Perkin-Elmer Lambda 35 spectrometer working in the 190-1000 nm range and equipped with a photodiode detector. Standard quartz optical cells of QStype (with 10 mm light path) were employed; the scanning velocity used was 120 nm/min with an interval of 2 nm.

The morphology and structure of the particles obtained by laser ablation in water were studied by transmission electron microscopy (TEM). We used a JEOL JEM 2010 microscope operated at 200 kV, equipped with a LaB 6 tip, a high resolution pole-piece (0.196 nm point resolution and 0.14 nm limit information) and a Pentafet-LinK ISIS energy dispersive Xray (EDX) spectrometer (Oxford Instruments). The samples were prepared by simply putting a drop of the solution containing the particles on a holey-carbon thin film supported on a microscopy copper grid (3.05 mm, 200 mesh).

The particle size was determined from the size histograms (performed over more than 400 particles) measured from TEM images. High-resolution imaging gave insight on the structure of the nanoparticles and allowed the determination of the structural phases and local orientations. Interplanar distances (d) and angles (a) were measured either directly on the HRTEM images or by performing 2D-FFTs on the resolved regions and/ or particles on those images. Since the regions and/or nanoparticles used to perform 2D-FFT are quite small (a few nm), only very few interplanar distances contribute to the measurement yielding small variations on the measured interplanar distances and angles: typically AE0.01 nm for d and AE18 for a.

EDX analysis was performed in order to check the presence (or not) of impurities.

XPS experiments were carried out in a KRATOS AXIS Ultra DLD spectrometer using a hemispherical analyzer and working at a vacuum better than 10 À9 mbar. All the data were acquired using a monochromatic Al Ka X-ray source (1486.6 eV, 150 W), pass energy of 20 eV and a hybrid lens mode. Measurements were performed on different regions of the sample surface, with an analyzed area fixed at 700 mm  300 mm. In addition, 4 keV Ar + ion bombardment was used to clean the Ni surface in order to obtain a reference spectrum for clean nickel. Quantitative analysis were performed on the basis of Ni 2p 3/2 and O 1s characteristic spectra, after subtraction of a Shirley-type background.

Results and discussion

The weight loss of the metal plate after the laser ablation versus time is shown in Fig. 2 and Table 1. The black line in Fig. 2 characterizes the mass expected for a process where the ablation efficiency is constant in time. The mass of the nickel nanoparticles increases as time of ablation increases (Fig. 2) but the efficiency of ablation decreases with time of ablation since the mass of ablated nickel increases slower than expected (black line in Fig. 2). Such effect was also observed by Mafune ét al. for Pt in water [19]. This can be related either to the abundance of the nanoparticles in liquid which could interact with the laser beam and thus could lower the laser power on the metal plate or to the modification of the surface properties of the metal plate by the irradiation of the pulsed laser which could diminish the ablation efficiency.

The amount of nickel deduced from ICP show the same tendency as the weight loss of the target measured by the Sartorius balance, but the values are not strictly proportional (Table 1). This can be due to some difficulties to collect properly the whole metal in the solution or/and to an incomplete dissolution of the metal in the acid solution before ICP analysis.

Optical absorption spectra of nickel nanoparticles in water exhibit structureless broad bands whose intensity decreases continuously above 190 nm (Fig. 3). Therefore, one cannot get information about the size/morphology of the metal particles. Nevertheless, one sees that the UV light absorption increases with the metal abundance in the solution.

Morphologic and structural characterization

The particles synthesized by laser ablation in solution were characterized by transmission electron microscopy in order to determine their size distribution, morphology and structure (HRTEM). EDX local analysis (by varying the size of the electron probe) showed that there are no impurities associated with particles. The quantification of oxygen within the nanoparticles is not very precise since there is always some inhomogeneously spread oxygen associated with the amorphous carbon of the supporting film and so we could not determine the stoichiometry of the particles by this method.

Fig. 4 shows the particles obtained for different times of ablation with the corresponding size distributions. The mean diameters and the standard deviations are summarised in Table 1. Nickel nanoparticles having a mean diameter in the range 3-5.3 nm are actually obtained. One can see that the mean size and the percentage of large particles decrease with ablation time. This can be explained by a redistribution of the size of the particles through their interaction with the laser beam.

We can distinguish two types of particles:

(i) The smaller ones (up to around 8 nm) are single crystalline, mostly pseudo-round-shaped and some times pseudoparallelipipedic-shaped (Figs. 5 and6).

(ii) The larger ones are either polycrystalline, containing internal structural defects or exhibiting core-shell contrast with typical Moire ´patterns (Fig. 7).

The former sort of particles always exhibit interplanar distances and angles typical of fcc NiO (a = b = c = 0.4177 nm). This is true even for the smaller particles as the one represented in Fig. 5 which is only 1.3 nm. Basically we observe two different 6b, d andg) and [À1, 1, 0] (Fig. 6e) types. We can indeed identify type (1 0 0) distances with 908 angles between them in the former orientation whereas we identify type (1 1 1) and type (1 0 0) plane which exhibit 70.58 angles (between two (1 1 1) type planes) and 54.78 (between (1 1 1) and (1 0 0) type planes).

The latter type of particles (>8 nm) are very interesting especially since they can give us some hints on the formation of our nanoparticles. This is particularly the case of those that exhibit core-shell contrasts with Moire ´patterns. The Moire ṕatterns arise from the interference of two different lattices and may be due to the presence of two crystalline phases surrounding or superposing each other. Indeed, in our case, we could determine that these particles are formed by a Ni central core surrounded by a NiO shell.

In Fig. 8, a nanoparticle (9 nm) exhibits an asymmetric coreshell structure as it is evidenced by the presence of the off-centred Moire ´pattern on its right side (the NiO shell is much larger on the left side). The 2D-FFTof the shell (Fig. 8b) shows the presence of two interplanar distances S1 (0.241 nm) and S2 (0.209 nm) separated by an angle of 54.78 corresponding respectively to d (1 1 1) and d (2 0 0) in NiO. The 2D-FFT of the region with the Moire ´pattern (Fig. 8c) shows the presence of six main spots S1, S2, C1, C2, M1 and M2. S1 and S2 correspond to the previously referred spots in Fig. 8b and can thus be attributed to the shell. A new set of values, C1 (0.203 nm) and C2 (0.176 nm) separated also by an angle of 54.78, can be attributed to Ni which has also an fcc structure (a = b = c = 0.3524 nm). C1 and C2, respectively correspond thus to d (1 1 1) and d (2 0 0) in Ni. Furthermore C1 is perfectly aligned with S1 and C2 with S2 giving rise to two parallel Moire ´spots M1 (1.30 nm) and M2 (1.15 nm), respectively. The relation between the Moire ´pattern distance and the two different lattice distance at its origin is given by D = d 1 d 2 /jd 1 À d 2 j [START_REF] Hirsh | Electron Microscopy of Thin Crystals[END_REF] which in our case gives the relations M1 = C1S1/jC1 À S1j = 1.29 nm and M2 = C2S2/jC2 À S2j = 1.12 nm. These values are very close to the measured ones and the low discrepancy is acceptable due to the poor precision of the measurement of large distances (corresponding to short distances -close to the central spot -in reciprocal space) in the 2D-FFT.

A peculiar example is the one treated in Fig. 9 where the surrounding NiO shell is uncomplete and there is still a Ni region of the particle at the periphery (Fig. 9b). Furthermore, the shell is polycrystalline with different regions with similar orientation but rotated with respect to each other (Fig. 9c-e). The Moire ´region corresponding to where a NiO shell surrounds a Ni core presents similar characteristics to the previous example. In this case, two sets of spots can be attributed to d This should give rise to the observation of three parallel Moire ´s; however only two are observed in the 2D-FFT (Fig. 9f). M1 0 is not observed certainly due to the low periodicity in the corresponding direction that will give a very low signal in the FFT; however one can see three distances in the Moire ´pattern in the micrograph of Fig. 9a. The two other Moire ´distances corresponding to M1 (1.38 nm) and M2 (1.14 nm) are detected and, as for the previous example, correspond well to the expected ones within the measurement error.

Such core-shell effects were also observed by Liu et al. [START_REF] Liu | [END_REF] during laser ablation of Ni targets in the presence of a partial pressure (40 Pa) of oxygen under vacuum conditions. In this case the core-shell structure seems to be related to a surface oxidation of the synthesized particles due to the presence of oxygen. Our observations lead us to think that the early stages of particle formation yield small nickel metal particles that can grow by accretion of several of those particles. These larger particles can undergo surface oxidation, due to partial decomposition of water under laser interaction, yielding the core-shell morphology and the polycrystalline particles that we observe. Smaller particles (<8 nm) are mainly single crystalline NiO. This can either be attributed to total oxidation of the Ni particles formed in the early stages or to laser ablation of an oxidised or hydroxilated Ni target during laser interaction process. A surface analysis study of the target surface before and after ablation presented hereafter shows that this latter possibility is confirmed.

Surface analysis of the target surface

It is a priori surprising that laser ablation from a pure Ni target yield NiO nanoparticles. But, this can be the con-sequence of chemical reactions which could take place at the interface between the liquid (water) and the laser induced plasma (containing Ni atoms and ions) due to the high temperature and high pressure in front of the laser induced plasma [23]. However, one can also suppose that some oxidation of the surface of the Ni target could occur during the laser ablation process. For this purpose, XPS photoemission measurements were performed on various regions of the target after laser production of the nanoparticles: (i) the zone impacted (10 min) by the laser beam, (ii) one region out of the laser impact zone and (iii) clean Ni obtained by Ar + ion bombardment of the ''out of the laser impact zone'' region directly in the XPS spectrometer. XPS measurements were made on two different areas for each zone leading to quite similar results.

The XPS spectra for the Ni 2p and O 1s regions are reported in Figs. 10 and11. In order to obtain complementary information on the chemical state of the species present at the surface of the samples, characteristic LMM Auger peaks of Ni are also reported as complement of the XPS peaks (Fig. 12).

After laser ablation, the spectra obtained both for the ''laser impact zone'' and ''out of the laser impact zone'' show additional structures in the Ni 2p 3/2 region with respect to clean Ni (Fig. 10); one can clearly see additional peaks located nearby 854 and 856 eV. They are very probably due to nickel oxides. This is coherent with the observation of a loss peak at $861 eV which is actually present on various nickel oxides The terms EL, MS and ME correspond respectively to energy loss peak, multiplet splitting and charge transfer multielectron excitation (shake up) [START_REF] Briggs | Practical Surface Analysis[END_REF].

(NiO, Ni 2 O 3 , Ni(OH) 2 , NiOOH) [26]. A safe interpretation of the Ni 2p spectra is not trivial. Indeed, the resulting spectra of nickel oxides are envelops of intra-atomic multiplets with many satellite structures [26].

The presence of different states of nickel oxides is confirmed by the spectra in the O 1s region, which show the appearance of (at least) three states of oxygen. Indeed, the spectra can be decomposed in three peaks located at 529.7, 531.5 and 532.8 eV. By taking into account previously published data for XPS spectra of Ni and Ni oxides [START_REF] Moulder | Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation[END_REF][START_REF] Briggs | Practical Surface Analysis[END_REF][START_REF] Kim | [END_REF][30] and reported in the Table 2, one can clearly identify the presence of NiO (Ni 2p 3/2 at $854 eV together with O 1s at 529.7 eV). Nevertheless, the Ni 2p 3/2 spectrum for NiO presents a quite different fine structure with an energy loss peak located near 856 eV less intense than the main peak near 854 eV [26,[START_REF] Moulder | Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation[END_REF][START_REF] Kim | [END_REF]. It exists therefore other kinds of Ni oxides. The presence of O 1s peaks located near 531.5 and 532.9 eV, in addition to the peak at 529.7 eV characteristic of NiO, could attest in favour of this statement. With respect to the reference data reported in Table 2, we can propose that the O 1s peak at 531.5 eV is associated to the presence of Ni 2 O 3 and/or Ni(OH) 2 . However, the presence of NiOOH species cannot be ruled out. Attribution of the high energy structure (near 532.8 eV) remains questionable. It has to be noticed that such a peak has been seen for highly oxidised nickel surface.

We can try to get additional information by looking at the LMM Ni Auger spectra (Fig. 12). The maximum located near 846.5 eV is coherent with the presence of metallic Ni both on pure Ni and in the region ''out of the laser impact zone''; it is at 846.1-846.3 eV for pure Ni [START_REF] Moulder | Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation[END_REF][START_REF] Briggs | Practical Surface Analysis[END_REF][START_REF] Kim | [END_REF]. The maximum is expected at a lower kinetic energy (KE) for NiO and Ni(OH) 2 (respectively at 843.7 and 842.3 eV), but at a higher KE (854.4 eV) for Ni 2 O 3 [START_REF] Kim | [END_REF]. Therefore, the LMM Auger spectrum measured on the ''laser impact zone'' let us think that the presence of Ni(OH) 2 is more probable than that of Ni 2 O 3 . Looking at Fig. 10 it is clear that the ''out of the laser impact zone'' area contains more Ni 0 and less Ni oxides than the ''laser impact zone''. This is confirmed when considering the O/Ni atomic ratio deduced from the O 1s and Ni 2p 3/2 XPS spectra; it is respectively equal to 2.6 and 1.3 for the ''laser impact zone'' and the ''out of the laser impact zone''. The amounts of oxygen seem to be quite high with respect to the stoichiometry of the nickel oxides previously discussed, but the presence of some oxygen pollution at the surface cannot be ruled out in such an analysis process.

In summary, nickel oxides do exist both on the ''laser impact zone'' and on the ''out of the laser impact zone''. Their presence is a priori not obvious on the ''out of the laser impact zone''; but, it is known that besides nanoparticles production surface coating of the areas located out of the impacted zone can exist during laser ablation of solids in liquids [23].

In conclusion, the production of NiO particles in water can be due to the ablation of species containing both Ni and O directly from the oxidised nickel surface impacted by the laser beam. However, the formation of Ni nanoparticles could exist for the early stages of laser ablation, followed by further accretion and total or partial oxidation leading to small single crystal NiO particles and larger core-shell (Ni-NiO) particles.

Summary and conclusion

Ablation of Ni targets in water by laser impact (532 nm, 40 mJ/pulse, 10 Hz and 8 ns duration) focused on massive samples ($2 mm diameter) generates colloids with fine nanoparticles.

The weight loss of the metal target, and therefore the metal released in the solution increases with ablation time, but slower than expected for a process with constant ablation efficiency. This is probably related to the abundance of the nanoparticles in liquid which could interact with the laser beam and thus lower the laser power on the metal sample surface.

Actually, pulsed laser ablation of the nickel target in aqueous solutions produces nanoparticles in the range 3-5.3 nm in diameter, the mean diameter and the size distribution decreasing with increasing the ablation time. This can be explained by a redistribution of the size of the particles through their interaction with the laser beam. The small particles (<8 nm) show a crystalline phase of nickel oxide, but some large particles (>8 nm) show a core-shell contrast with Moire ṕatterns which is formed by a Ni central core surrounded by a NiO shell.

XPS photoemission measurements evidenced the presence of different nickel oxide species on the crater caused by the laser shots on the nickel target surface that can lead to the formation of NiO nanoparticles.

Fig. 1 .

 1 Fig. 1. Experimental setup for laser ablation.

Fig. 2 .

 2 Fig. 2. Mass of nickel (mg) released in pure water measured as a function of time of ablation (min), the black line shows the linear estimated evolution of nickel concentration in pure water as a function of time (see text for further details).

Fig. 3 .

 3 Fig. 3. UV-vis absorption spectra of Ni colloids obtained in water for 2, 4, 8, 10 and 15 min of laser ablation.

Fig. 4 .

 4 Fig. 4. TEM images and size histograms for nickel nanoparticles obtained after 2 min (a), 4 min (b), 8 min (c), 10 min (d) and 15 min (e) of laser ablation.

  (1 1 1) type distances in NiO shell (S1: 0.241 nm; S1 0 : 0.241 nm) which are aligned with d (1 1 1) distances in Ni core (C1: 0.203 nm; C1 0 : 0.203 nm); the third set of distances is attributed to d (2 0 0) type distance in NiO shell (S2: 0.209 nm) aligned with d (2 0 0) type distance in Ni core (C2: 0.176 nm).

Fig. 4 .Fig. 5 .

 45 Fig. 4. (Continued ).

Fig. 6 .

 6 Fig. 6. HRTEM images (a, c and f) and corresponding 2D-FFTs (b, d, e, and g) showing that the particles formed have the cfc structure of NiO (see text for details).

Fig. 7 .Fig. 8 .

 78 Fig. 7. HRTEM images of large particles: (a) polycrystalline particle, (b) particle with multiple defects and (c) core-shell particle exhibiting a central Moire´.

Fig. 9 .

 9 Fig. 9. HRTEM of a core-shell particle with an uncompleted polycrystalline NiO shell (a) with a Ni region on the periphery (b), three different regions of the NiO shell (c-e) and the Moire ´region (f) revealed by the 2D-FFTs taken on the respective regions. S1, S1 0 , S2, C1, C1 0 C2, M1 and M2 are defined in the text.

Fig. 10 .

 10 Fig. 10. Ni 2p XPS spectra obtained after impact (10 min) of the Ni target: (a) zone impacted by the laser beam, (b) out of the zone impacted by the laser beam and (c) after argon ion bombardment (clean Ni).

Fig. 11 .

 11 Fig. 11. O 1s XPS spectra obtained after impact (10 min) of the Ni target: (a) zone impacted by the laser beam and (b) out of the zone impacted by the laser beam.

Fig. 12 .

 12 Fig. 12. LMM Ni Auger spectra obtained after impact (10 min) of the Ni target: (a) zone impacted by the laser beam, (b) out of the zone impacted by the laser beam and (c) after argon ion bombardment (clean Ni).

Table 1

 1 Characteristics of the Ni nanoparticles

	Sample Ni (ablation time in min)	Weight (mg)	ICP (mg)	Mean diameter (nm)	Standard deviation
	2	49	34	5.3	AE4.2
	4	82	71	3.6	AE2.4
	8	149	87	3.7	AE1.5
	10	150	96.5	3.4	AE1.7
	15	210	154	3	AE1.6

Table 2 X

 2 -ray photoelectron binding energies (eV) of the nickel-oxygen systems

	System	Ni 2p 3/2	O 1s	Reference
	Pure Ni	852.7	-	[26,27]
		852.9 + 858.8 (EL)	-	[28]
		852.5	-	[29]
		852.6 (+856.3) + 858.6	-	[25]
		852.6 + $858	-	This work
	NiO	853.8 + EL	529.6	[26]
		854.4	529.5	[27]
		854.5 + 856.3 (MS) + 861.7 (ME)	529.9	[28]
		854	529.6	[29]
		853.7 + 855.8 + 861		[25]
	Ni 2 O 3	855.8	531.8	[26]
		856		[27]
		855.8 + 861.4 (ME)	531.7	[28]
	Ni(OH) 2	855.6		[26]
		855.9	531.2	[25]
		855.5 + $861		[27]
		856.6 + 862.4 (ME)	531.7	[28]
		855.6	531.2	[29]
	NiOOH	856.3 + $861		[25]

Acknowledgements

The authors thank Laurence Massin for her expert assistance with the XPS measurements and for continuing discussions on XPS spectra. Rami Mahfouz is grateful to the National Council for Scientific Research (Lebanon) for providing a Ph.D. grant.