Wen Luo

Spyridon Zafeiratos

K S Novoselov

A K Geim

S V Morozov

D Jiang

Y Zhang

A brief review of synthesis and catalytic applications of graphene-coated oxides

Keywords: oxides, graphene, coat, catalyst, [1]

The potential applications of graphene-based materials in catalysis have been intensively studied due to the unique 2D structure of graphene and its superior physicochemical properties. Recently, graphene-coated oxide composites have been fabricated and tested as catalysts or catalytic supports, demonstrating a significant improvement on the catalytic performance. This article briefly reviews the recent progress in the design and fabrication of graphene-coated oxides including wet transfer, chemical vapor deposition and self-assembly methods. The applications of these materials in catalysis as well as the role of graphene in the modification of oxide properties and the enhancement of catalytic activity are also discussed.

Introduction

Graphene is a single layer of carbon atoms tightly packed in a hexagonal lattice. Since its discovery in 2004 via "Scotch tape" method [1] , graphene has attracted enormous attention due to its unique properties, such as high charge carrier mobility (~2 x 10 5 cm 2 V -1 S -1 ) [2,3] , high thermal conductivity (5000 Wm -1 K -1 ) [4] , extremely high specific surface area (~2600 m 2 g -1 for a single layer) [5] and increased transparency [6] . All these superior properties make graphene an attractive catalytic material and in the last few years, the number of publications exploiting graphene as photo-or electro-catalyst has been exponentially increased. Notably, heteroatom-doped graphene (nitrogen [7] , sulfur [8] , boron [9] etc.) have shown much better electrocatalytic activity and long-term operation stability for oxygen reduction reaction. When used as catalysts' support, the high specific surface area of graphene can improve the metal dispersion and therefore increase the catalyst activity and stability [10] . However, the intrinsic characteristics of graphene are unfavorable for some specific catalytic applications. For example, graphene is a zero bandgap material and therefore cannot be used directly as a photocatalyst. Furthermore, the free-standing single layer graphene is often corrugated and wrinkled, or even recombines to form few-layer graphene, therefore loses many of its unique properties mentioned above. Oxides are long-established materials in catalytic applications and they possess some certain advantages which cannot be easily attained by graphene. These advantages comprise low preparation cost, variable band gap, good mechanical properties and facility to be prepared in different shapes and forms. However, oxides have also some major shortcomings when they are used in catalysis. For example, SiO2 and Al2O3 have been widely applied as catalytic supports in Fischer-Tropsch reaction since they can sufficiently disperse the active metals (Co, Fe etc.). However the formation of unreducible mixed oxides between metal catalyst and support during the preparation and the reaction process has been blamed for irreversible deactivation of the catalyst [11,12] . Similarly, while ZnO and CeO2 are considered to have potentially promising applications in ethanol steam reforming reaction, the oxidation of active metal sites (such as Co) by the support decreases the catalytic activity [13,14] . In the case of photocatalytic reactions, TiO2 is the most widely studied material [15,16] , but still engineering of its band-gap to absorb more in the visible electromagnetic radiation region remains a difficult problem. In recent years, hybrid materials consisting of graphene-coated metal oxides have gained increasing attention with the hope to find a nanocomposite material which overcomes the shortcomings of both graphene and oxides and promotes the catalytic performance. For instance, graphene has the potential to be an ideal buffer layer to protect metal catalysts from deactivation induced by the interfacial reaction with oxide supports, since it is one atomic layer thick, but at the same time rigid and impermeable to support constituents accountable for oxidation [17] . In addition, coating semiconductor oxides (such as TiO2 and ZnO) with graphene can effectively decrease their bandgap leading to higher absorption yields in the visible light region [18,19] . Close contact of graphene with coated semiconductor oxides can also enhance the charge transfer efficiency, thus increases the catalytic activity. Studies of graphene either as a catalytic support or as a metalfree catalyst, have been summarized in some comprehensive reviews [20][21][22][23][24][25] . However, an overview emphasizing on the synthesis and catalytic applications of graphene-coated metal oxides is lacking, and given that the research on these hybrid materials is progressing fast, such a report is greatly needed. This review briefly describes the recent progress in the design and fabrication of graphene-coated metal oxides and discusses the specific advantages of these materials in photo-and heterogeneous catalysis (Fig. 1). Some major challenges and future opportunities for further development are also discussed.

Preparation strategies for graphene-coated metal oxide supports

Metal oxide supports with planar morphology

As a single carbon layer, graphene is a perfect candidate to produce planar, highly-controlled model catalysts, for detailed studies of the interaction between carbon and metals, as well as carbon with gas molecules. The most facile method to prepare graphene-coated oxide model samples is by transferring preformed graphene layers onto planar oxide substrates using the so called wet transfer method. A schematic diagram of the transfer process is shown in Fig. 2 [26] . Initially, large-area graphene layers are produced on metal substrates, through chemical vapor deposition (CVD) method. Next, the graphene layer is coated with a thin poly-methyl methacrylate (PMMA) film (about 500 nm thicknesses) and consequently the underlying metal substrate is etched away in an appropriate solution, leaving a free-standing PMMA/graphene film. After rinsing with distilled water to remove the etching solvent, the PMMA/graphene layer is transferred easily, usually in liquid medium, onto any planar oxide substrate. Once the transfer is completed, the protective PMMA layer is removed by dissolution in acetone, leaving a graphene-coated oxide substrate. Since after the dissolving process traces of PMMA might remain on the graphene, it is suggested to anneal several hours under vacuum at temperatures around 300 o C for graphene surfaces with minimum contaminants. Through this method, graphene-coated SiO2 [27][28][29] and ZnO [17,30,31] samples among others, have been prepared and studied. .

Although the wet transfer method is relatively simple and easily reproducible, it has some considerable drawbacks. During the transfer the graphene layer often forms wrinkles and defects, while the residues of the protective PMMA layer are sometimes difficult to be completely removed. Thus, alternative methods which allow direct growth of graphene layer on oxide surfaces have been explored. For example, Peng et al. [32] directly grew bilayer graphene on SiO2 substrates by carbon diffusion through a nickel layer, sandwiched between the oxide substrate and a polymer film. Annealing the sample (polymer film/400 nm Ni/300 nm SiO2/Si) at 1000 o C under low pressure Ar/H2 conditions decomposes the polymer film and the remaining carbon dissolves into the nickel layer. After cooling down, graphene is formed on both sides of Ni from the precipitated carbon. Consequent dissolution and removal of the Ni layer produces a bi-layer graphene film directly deposited on the SiO2 substrate. In following works, this Ni-assisted carbon diffusion method was applied using other carbon sources such as CH4 [33] , C2H4 [34] , nitrogen containing polymer [35] and amorphous carbon [36,37] . Depending on the carbon source graphene was grown in different scale and layer thickness on SiO2, Al2O3 etc. Graphene has been also prepared through CVD method directly on various oxides surfaces, such as MgO [38][39][40][41] , SiO2 [42,43] , Al2O3 [44] , ZrO2 [40] and HfO2 [45] , however in most of the cases this direct method produces graphene films with relatively poor quality.

Metal oxide supports with complex morphology

Graphene-coated oxides can be prepared through a simple electrostatic self-assembly strategy, using graphene oxide (GO) as a precursor [19,46,47] . Generally the fabrication process involves three steps, which are as follows: initially the surface of the metal oxide support material is modified/functionalized with cationic surfactants (which later act as molecular linkers), such as 3-aminopropyl triethoxysilane (APTES), Poly(allylamine hydrochloride) (PAH) etc.. In a second step the metal oxide is coated with GO layers through electrostatic attraction forces between the negatively charged GO and the positively charged metal oxide. Finally, the GO shell is reduced to form graphene. The reduction of GO layers can be performed either through a hydrothermal process [19] or in a hydrazine (N2H4) solution [46] . Because of its simplicity and efficiency, this method has been widely applied to coat metal oxides with various structures, such as SnO2 nanowires [46] , α-Fe2O3 nanoparticles [46] , hollow TiO2 nanospheres [48] , and SiO2 nanoparticles [47] . Following a similar approach, Wang et al. [49] fabricated GO wrapped Fe2O3 nanocrystals with the assistance of chitosan. In this case hydrogen bonding and electrostatic interactions between the chitosan chains and the GO sheets were the driving force to assemble the GO with the metal oxide. , (b)α-Fe2O3 particles [50] , (c) hollow TiO2 spheres [48] , (d) α-Fe2O3 particles [46] , (e) Fe2O3 particles [49] and (f) SiO2 particles [47] .

A drawback of the above described fabrication method is that residuals of the molecular linkers, which are used to bind GO on the oxide supports, might remain after GO reduction and potentially influence the catalytic properties of the hybrid material. To overcome these problems Wei et al. [50,51] proposed a facile room-temperature approach to prepare GO-wrapped metal oxides (e.g. Fe2O3) using metal cations instead of molecular linkers, as GO anchor sites. Generally, when the metal oxide nanoparticles (e.g. Fe2O3) are dispersed into a solution which contains the same metal cations (i.e. Fe 3+ ), the metal cation will be absorbed on the surface of the metal oxide according to the Fajans rule to form positively-charged metal oxide particles [52] . When these positively-charged particles are mixed with a negatively-charged GO solution they will be covered by GO due to electrostatic interactions. This procedure is followed by a final reduction step of GO to form graphenecovered metal oxide particles. A selection of TEM images of various oxides coated with graphene using the electrostatic selfassembly fabrication method is given in Fig. 3. As shown in the TEM images, during synthesis the graphene oxide will follow the morphology of the oxide substrate by wrapping around its surface. This is a significant advantage of the electrostatic selfassembly method, since the morphology of the graphene-coated oxides can be well controlled through the designed structure of the oxide precursor. Instead of coating GO onto metal oxide nanostructures through the two-steps solution mixing approach, Son et al. [53] demonstrated a direct, single-step, technique for the synthesis of ZnO-graphene quasi-core-shell quantum dots. As shown in the schematic of the chemical synthesis process presented in Fig. 4, initially zinc acetate dihydrate and GO are mixed together in dimethyl formamide (DMF) solution and the temperature is increased to 95 o C, where the ZnO quantum dots are formed. Subsequently, Zn 2+ ions in the solution are used as binders between ZnO quantum dots and GO layers, through a simple chemisorption process and/or by formation of Zn-O-C bonds.

During this process, graphene can be peeled off layer-by-layer from the GO layers through the chemical force, and cover the ZnO quantum dots to form graphene-coated ZnO particles. A fundamental difference in this "soft" integration method as compared to the electrostatic self-assembly approach mentioned above, is that in the former the oxide particles are grown in situ and are coated by graphene oxides in the solution phase.

Therefore in this case it is not the oxide support, but the graphene oxide, which plays the "structure-directing" role that defines the final composite material. [19,54] Besides using GO precursor, the graphene coating layer on metal oxides can be also generated starting from other carbon sources. For example, Mendes et al. [55] used oleic acid as a carbon source to form a few layer graphene coating on iron oxide nanoparticles (NPs). Two graphitization approaches were demonstrated in that work, either using electron beam irradiation or thermal annealing. The electron beam irradiation method was carried out in situ in a transmission electron microscope using low voltage electron beam (80 keV). Alternatively, thermal annealing at 500 o C under high vacuum conditions (~ 10 -6 mbar) was sufficient to graphitize the oleic acid layer and form few layer graphene on iron oxide support. However, a side effect of thermal annealing is the partial reduction of iron oxide and the aggregation of the oxide nanoparticles, which might cause deactivation problems in catalytic applications.

Graphene-coated metal oxides in catalytic applications

Graphene-coated metal oxides as photocatalysts

The high charge carrier mobility and the transparency of graphene make it an excellent candidate material for photocatalytic applications. On the other hand, graphene displays a semi-metallic (or zero-gap semiconductor) character, which excludes its use as a stand-alone material in such applications. Combination of graphene with a semiconductor (composite material) can significantly enhance the lifetime of photo-generated electro-hole pairs and increase the charge transfer rate of electrons toward surface-adsorbed molecules during a photocatalytic process [18,[56][57][58] . Besides of its role as an electron reservoir, graphene can also enhance the adsorption capacity [59,60] , extend the light absorption range [48,61] and act as a macromolecular photosensitizer [62][63][64] . Therefore, in recent years, numerous attempts have been made to apply the graphene-semiconductor composite materials in the area of photocatalysis, some excellent reviews on this topic have been written recently [25,[65][66][67][68] . However, a review that focuses on graphene-coated oxides needs to overview the role of graphene coating layers in the photocatalytic performance. Certainly the structure of graphene-based composites photocatalysts affects their behavior as has been demonstrated in several studies [60,64,66,[69][70][71] . In the following part some typical examples are selected to highlight the superior performance of graphene coated oxides on photocatalytic reactions. .

TiO2 is the most widely studied semiconductor photocatalyst due to its low cost, high chemical stability, non-toxicity, and resistance to photo-corrosion. However, due to an intrinsic large band gap (3.2 eV for anatase and 3.0 eV for rutile phase), the photocatalytic activity of TiO2 is mainly restricted in the ultraviolet region. Several strategies have been proposed to extend the yield of visible light absorption of TiO2, such as doping with metals [72,73] and non-metals [74,75] , as well as its combination with electron acceptor materials [76] . In the last few years, graphene has been explored as an electron acceptor material to prepare graphene-TiO2 hybrids with reduced band gap so as to enhance the photocatalytic performance. Various methods have been applied to prepare TiO2-graphene composite materials with different structures [16,76,77] . However, a "hard" integration of reduced graphene oxides with TiO2 nanoparticles did not show significant difference in the photocatalytic performance as compared to its carbon allotrope (such as carbon nanotubes) [60] . The enhanced photocatalytic activity of graphene-TiO2 composite materials can be attributed to their intimate interfacial contact, as in the case of graphene-coated TiO2 [15,48,78] .

In order to study the effect of contact area between graphene and TiO2, Kim et al. [79] prepared nanosized graphene oxidecoated TiO2 nanoparticles (r-NGOT) and compared their photocatalytic hydrogen generation performance with TiO2 nanoparticles supported on large size graphene oxide sheets (r-LGOT). It was observed that the rates of H2 production and photocurrent generation in the case of r-NGOT were higher as compared to r-LGOT. This was attributed to the higher contact area between graphene and TiO2 in r-NGOT composites which facilitates the interfacial electron transfer. It is worth noting that, although more functional groups and defects in the NGOs precursor should reduce the conductivity, they can also serve as nuclear sites to combine with TiO2 nanoparticles forming core/shell structures. This ensures an intimate interfacial contact between graphene and oxide and enhances the photoelectrochemical performance. In addition to the manipulation of the structure of graphene precursors, the properties of the TiO2 precursors also play a key role that affects the contact between TiO2 and graphene and consequently the photocatalytic performance. Recently, Lee et al. [18] synthesized graphene-coated anatase TiO2 nanoparticles by a one-step GO reduction and amorphous TiO2 crystallization process via hydrothermal treatment and tested their photocatalytic performance in the degradation of methylene blue (MB). As compared to the bare anatase TiO2 NPs and graphene coated crystalline anatase TiO2 NPs prepared by two-step hydrothermal method, the one step graphene-TiO2 NPs showed a significantly reduced bandgap (2.8 eV instead of 3.2 eV) (see Fig. 5A), which was attributed to the direct interaction between carbon (from graphene) and Ti atoms on the TiO2 NP surface. In this case, strong photocurrent response was recorded for graphene-wrapped TiO2 under visible irradiation for each switchon/off event, while no photocurrent was observed for both bare anatase TiO2 NPs and graphene-TiO2 NPs (two-step hydrothermal) (see Fig. 5B). The authors suggested a mechanism to explain the photocatalytic degradation of MB over graphene-TiO2 NPs under visible light irradiation (see Fig. 5C). They proposed that electrons from excited MB (MB*) can flow to the conduction band of TiO2 NPs via graphene, while at the same time, the valence electrons of TiO2 can be also excited to the conduction band state by absorbing visible light. These excited electrons convert the oxygen molecules to singlet oxygen which will oxidatively decompose MB through demethylation process. Meanwhile, the accelerated electron transfer from excited MB to TiO2 conduction band by graphene can hinder the recombination of excited electrons and improve the self-decomposition of MB. Apart from TiO2 nanoparticles, the morphology of TiO2 can be also engineered to fabricate graphene-TiO2 composite with coreshell structure. Zhang et al. [48] investigated the photocatalytic activity of graphene-encapsulated hollow TiO2 nanospheres for degradation of rhodamine B (RhB) under UV light irradiation and found excellent photocatalytic activity. This was attributed to the effect of graphene to promote the electron transfer from the inner hollow TiO2 to the outer graphene shells, and substantially suppress the recombination of photo-excited electrons and holes. Very recently, Lee et al. [80] prepared graphene and palladium nanoparticles (Pd NPs) encapsulated titanium dioxide (TiO2) composite nanowires and tested their photocatalytic activities. Under visible light irradiation, this material converted nearly 100 % of 4-nitrophenol. The UV-vis spectrophotometry study showed that coated TiO2 nanowires resulted in a lowered band gap (2.0 eV) as compared to that of pristine TiO2 nanowires (2.9 eV) which induced much stronger visible light absorption. Besides TiO2, another oxide which has been extensively studied as photocatalyst is ZnO. In recent years, graphene-coated ZnO materials have been reported as efficient photocatalysts for decomposition of pollutants in water. For example, Bu et al. [81] prepared graphene-ZnO quasi-shell-core composite materials by following the method described of Son et al. [53] and investigated their photocatalytic efficiency for degradation of RhB and their photoelectrochemical performance. They showed that the amount of graphene in the graphene-ZnO composite is critical in both photocatalytic and photoelectrochemical processes. Furthermore, they proposed a mechanism for the improved catalytic performance of the graphene-ZnO quasishell-core composite material. Similar to graphene-TiO2 hybrids, the graphene coating can effectively reduce the transfer resistance of the photo-generated electrons and at the same time inhibits their recombination with holes. Moreover, the close contact of graphene with ZnO will positively shift the Fermi level of the latter, forming an interfacial electric field at the graphene-ZnO interface. This electric field will further improve the separation efficiency of photo-induced electron-hole pairs resulting to an enhanced photocatalytic activity. In addition to the role of a photoelectron mediator, graphene coating on ZnO also exhibit an enhanced dye-adsorption [59,82] and light absorption [19] capability which in turn improves the photo degradation performance. Apart from the above mentioned activity studies on graphene coated oxides, the photostability of these catalysts is another important issue that has been investigated. ZnO is widely accepted to suffer from the photo-corrosion under UV light, which significantly decreases its photocatalytic activity and limits its application in photocatalysis [19,83] . The photostability of ZnO can be greatly enhanced by coating a graphene layer on its surface, as shown by successive recycling tests of RhB degradation [19] . This is also in agreement with reports from photoelectrochemical RhB degradation experiments where graphene and ZnO formed a quasi-shell-core structure [81] . The high stability of graphene coated ZnO can be attributed to two main reasons, the first being the close contact between graphene and ZnO which works as a protection layer for the ZnO surface preventing the photocorrosion of ZnO. [19] The second reason is the fast consumption of photogenerated holes by the surface adsorbed RhB molecules, which inhibits the corrosion of ZnO [19,81] . The degradation of graphene caused by the oxidation of •OH radicals, which are formed by the reaction of photogenerated holes and the surrounding water/hydroxyl species, has been observed on graphene-oxide composites photocatalysts [66,84] . On the contrary, no degradation of graphene has been reported in the graphene (reduced) coated oxide systems after recycling tests. [19,81] Although further and systematic studies on the stability of graphene are still needed, the stability of graphene in graphene-coated oxide composites as compared to grapheneoxide ones, can be attributed to the appropriate interfacial interaction between the graphene layer and the oxide support, as well as the effective consumption of •OH radicals by the reactants. [66] , Reproduced from ref. [18] .

As such, we can see that the graphene-semiconductor oxide composites with core-shell structures can be fabricated with various synthesis methods and different types of precursors which are able to sufficiently strengthen the interfacial interaction between graphene and oxides. The significantly improved photocatalytic activity was attributed to the important role of graphene, which not only performs as a conductive medium for accepting and transporting photoelectrons, but also tunes the light absorption range and enhances the reactant adsorption capacity. Moreover, both the graphene coating layer and the semiconductor oxides show high photostability. Although, up to now, the most widely used precursors are GO, TiO2 and ZnO, shell-core composites of graphene with other semiconductor oxides may be a promising route to build a new class of catalysts with high photocatalytic performance.

Graphene-coated metal oxides as other heterogeneous catalysts

Graphene (assuming no defect sites) is a chemically inert material therefore it can be used as a coating to increase the resistance of metal oxides in harsh chemical environments. Very recently, Wei et al. [85] prepared porous nitrogen-doped graphene-coated cobalt oxide catalysts (CoOx@CN) and tested them in the hydrogenation of quinoline compounds. Their results indicated an excellent catalytic activity and good stability of CoOx@CN in this reaction. This was attributed to the effect of the graphene coating layer, which effectively reduces the extent of leaching of cobalt from CoOx nanoparticles while at the same time it protects them from agglomeration. In addition, the large surface area and the porous structure of the carbon layer ensure the facile diffusion of reactants and products in order to reach the catalytic sites. Besides, the graphene coating weakens the strong coordination between the N-heterocycle and active metal nanoparticles and prolongs the catalyst lifetime. Another case of catalytic performance improvement was found for nitrogen-doped graphene coating over Fe2O3. The catalysts were prepared by pyrolysis of iron-phenanthroline complexes on carbon and tested for hydrogenation of nitrobenzene in a watertetrahydrofuran (THF) solvent mixture [86] . Catalysts pyrolyzed at 800 °C (Fe-phen/C-800) showed maximum activity and selectivity towards the desired product (aniline), as compared to catalysts pyrolized at lower (400 °C) or higher (1000 °C) temperatures. HR-TEM characterization of the Fe-phen/C-800 showed that the active catalyst consists of iron oxide particles surrounded by a shell of 3 to 5 nitrogen-doped graphene layers (Fig. 6a). When the pyrolysis temperature is raised to 1000 °C, the less active iron oxide particles are coated by a much thicker (10-to 40 nm) graphite layer (Fig. 6a). X-ray photoelectron spectroscopy (XPS) analysis was used to differentiate the various nitrogen species on the surface of the catalysts. As shown in Fig. 6b, the N 1s spectrum of the most active Fephen/C-800 catalyst indicate 3 types of nitrogen species which are assigned to pyridinic nitrogen, Fe-N centers and nitrogen in a graphite-like structure. The N 1s peak originating from the Fe-N species disappears in the less active sample (after pyrolysis at 1000 °C) and is replaced by graphitic-N, which made the authors to propose that the high catalytic activity is mainly related to the number of the Fe-N centers. [86] [86] .

Graphene coated oxides composites have also been utilized as electrocatalysts. For example, Lin et al. [87] prepared graphene wrapped Fe3O4 with the electrostatic self-assembly method and decorated them with Pt nanoparticles for the electrochemical oxidation of methanol. The catalysts showed high catalytic activity and stability compared with graphene supported Pt. This was attributed both to their structure, which improved the dispersion of Pt nanoparticles by homogeneously exposing them on the surface of graphene, and to the high conductivity of the graphene coated Fe3O4. More recently, Zhao et al. [88] have prepared porous graphene (PGE, Fig. 7a) from GO and designed porous graphene wrapped CoO nanoparticles catalysts (PEG-CoO, Fig. 7b) for oxygen evolution reaction (OER). The activity tests showed that PEG-CoO displayed much higher OER activity than both its constituents (CoO and PGE) and of non-porous graphene-CoO (GE-CoO). Electrochemical capacitance measurements and electrical impedance spectroscopy results indicated that the PEG-CoO catalyst exhibited larger active surface area and lower charge transfer resistance, which were attributed to its superior electrocatalytic performance. Compared with GE-CoO, the PGE-CoO hybrid showed excellent durability in a 0.1M alkaline solution, as shown in Fig. 7e. This was ascribed to the advantage of the wrapped structure, where CoO nanoparticles are well confined by porous graphene, which prevents CoO from aggregation and corrosion. Moreover, in such a structure, the strong interaction and synergistic effect between CoO and porous graphene also play an important role on the high OER of PGE-CoO. Adapt from reference [88] .

With the selected examples, we can see that, besides photocatalysis, graphene coated oxides find applications also in several heterogeneous catalytic reactions. In the latter case, the nature of the oxide support can be varied and is not limited to semiconductors, as in the case of photocatalysts. This opens several new routes not only for the applications but also in the diversity of the preparation methods. However, one should keep in mind that the synergistic effect between graphene and oxides plays an important role in the activity and stability performance, independent of the choice of the oxide support.

Graphene-coated oxides as catalytic supports

Heterogeneous catalysts typically consist of one or more active metals dispersed on a support. The choice of the most appropriate catalytic support is based on diverse criteria, such as low cost, good mechanical properties and high surface area. Oxides like SiO2, Al2O3, TiO2 and ZnO, are the most commonly used support materials in heterogeneous catalysis. Their primary function is to improve the dispersion of the active metal sites, enhance their stability (chemical and structural), but in some cases they also participate in the reaction. However, some oxide supports, especially easily reducible oxides notably TiO2 or ZnO, may also cause the deactivation of the catalyst under certain conditions. This can take place through strong interactions with metals, for example, by encapsulation of the active centers or oxidation of the metals and formation of inert mixed-oxide compounds [89,90] . Recently the idea to cover oxides with carbon in order to improve their stability, and consequently their performance, has attracted a lot of attention. In the graphitic form, carbon has been used to increase the stability of Al2O3 in biomass conversion reactions [91] , while more stable catalytic particles are formed over carbon-coated silica as compared to the uncovered one [92,93] . In spite of the increased interest on carbon-coated oxides, graphene coating on oxide supports has been only explored in model planar catalytic systems, which will be summarized below.

In one of these studies single-layer graphene was applied over single crystal ZnO (0001) planar surfaces and its effect on cobalt NPs, grown by thermal evaporation in vacuum, was studied [17] .

Interestingly it was shown that when a graphene layer was inserted between cobalt particles and the oxide substrate, the morphology and the oxidation/reduction properties of cobalt was modified. As shown by the Co 2p3/2 XPS spectra in Fig. 8, after annealing under UHV conditions cobalt supported on graphenecoated ZnO could maintain the metallic state till 350 o C (Fig. 8b). Contrarily, without the graphene layer Co was readily oxidized to CoO (Fig. 8a) presumably through a solid state interface interaction with the ZnO support, as indicated later by angleresolved XPS spectra [31] . In a more recent study, it was shown that graphene layer also weakens the interactions between Co and SiO2, even if SiO2 is considered as a relatively inert substrate as compared to ZnO [30,31] . Moreover, the interaction between Co and graphene dictates not only the oxidation state but also the particles size of cobalt. In particular at low temperature graphene improves the dispersion of cobalt as compared to oxide substrates, while upon annealing it provokes particle agglomeration leading to bigger cobalt particle size.

Apart from the metal-support interface interaction, graphene can also influence the oxidation/reduction properties of cobalt nanoparticles supported on coated oxides (ZnO and SiO2) under low (10 -7 mbar) and medium (7 mbar) pressure O2/H2 environment [30,31] . In particular, under low pressure O2 the oxidation of Co supported on graphene-coated ZnO and SiO2 substrates was limited [31] , as compared to cobalt directly attached on the oxide support. However, when the O2 pressure increases to the mbar range the differences between cobalt supported on bare and graphene-coated oxides fade. This observation was explained by the differences induced by graphene on the cobalt particles size. Specifically, in mild oxidation conditions, large cobalt particles (in the range of 20 nm) are preferentially oxidized on their surface leaving the particle core in the metallic state due to oxygen diffusion limitations.

When the particles are considerably smaller, or more flat, they are totally oxidized (Fig. 8c). Adapted from references [19,30] .

The reduction of cobalt oxide particles under low and medium pressure H2 conditions was dramatically promoted by the graphene layer, leading to cobalt reduction at significantly lower temperatures. Angle resolved XPS measurements indicate that this is mainly a consequence of the restriction of Co NPs interaction with the oxide supports [31] . Moreover, at high temperature graphene protects Co from the formation of mixed CoxZn1-xO2 compounds [30] , which are more difficult to reduce than cobalt oxides (CoO or Co3O4). It is interesting to note that the graphene-supported cobalt particles show differences in morphology as compared to graphite supported ones. This suggests that the cobalt-graphite interaction is weaker than that of graphene, therefore cobalt surface diffusion and agglomeration to form bigger particles is favored on graphite.

Based on this finding it is suggested that graphene and graphitecoated oxides might influence the supported particles in a different way and therefore it is of particular interest to control the number of graphene layers over the oxide. [31]

Stability of oxide-supported single layer graphene in reactive environments

A crucial issue for practical applications is definitely the stability of graphene-coated oxides under harsh catalytic reaction environments. Raman studies can give a relatively accurate report about the number and the nature of the defects induced on the graphene layer under reactive environments. Fig. 9a shows Raman spectra of graphene-covered SiO2 supports before and after cobalt deposition as well as after an oxidation/reduction cycle in 7 mbar O2 and H2 at temperatures up to 400 °C [30] . The G band at 1580 cm -1 and the 2D band at ~2700 cm -1 are due to the in-plane vibrational (E2g) mode and the two phonon intervalley double resonance scattering of graphene [94,95] , respectively. The narrow symmetric 2D band and the relatively low G/2D bands intensity ratio can be used as safe indicators of single-layer graphene [94] . After cobalt deposition and prior to any treatment, two additional features at ~1350 cm -1 and 1625 cm -1 appear in the Raman spectrum (Fig. 9a). The peak at 1350 cm -1 is assigned to the D band and is activated due to a single-phonon intervalley process caused by defects in the graphene lattice (edges, vacancies etc.). The other one, known as D' band, is activated by an intravalley scattering process which also requires defects. In addition to the appearance of the D and D' bands, after Co deposition both the G and 2D bands become slightly broader due to graphene disorder [96] . It is therefore evident that vacuum deposition of cobalt (0. Adapted from [30] .

By combining Raman and XPS results, a mechanism of graphene quality deterioration over cobalt covered areas was described (Fig. 9c). XPS results gave evidence of a chemical interaction at the Co-graphene interface through a carbon dissolution-precipitation mechanism. This interaction is likely accountable for the formation of local defects on graphene in contact with cobalt and can be possibly enhanced with temperature [97] . The carbon atoms of graphene lattice which are locally diluted in cobalt particles are vulnerable to oxidation in O2 atmosphere. When the gas atmosphere switch to H2, oxidized carbon species transform to volatile C-O and/or C-O-H compounds, leaving behind defective graphene layer in the vicinity of cobalt particles. This mechanism is consistent with the observation of highly defective graphene only in cobalt coved areas of graphene (Co/G/SiO2) but not on areas without Co (G/SiO2).

Conclusion and perspectives

In summary, graphene-coated metal oxides have attracted great interest in catalysis research, mainly due to their promising applications as photo-and heterogeneous catalysts. Moreover, studies on model systems proposed that graphene has a great potential as support modifier in heterogeneous catalytic reactions, especially when the interaction between metal and the support has to be moderated. In recent years, various methods to produce graphene with high quality and/or large scale have been proposed including mechanical exfoliation, chemical exfoliation, chemical vapor deposition, epitaxial growth etc. In addition, a variety of synthetic strategies such as wet transfer, electrostatic self-assembly synthesis and chemical vapor deposition, have been developed for fabricating graphenecoated oxides both for models and realistic catalytic systems. Scale up production of these strategies is important to promote industrial applications of graphene-coated metal oxides. Many studies have shown that graphene coating significantly modifies the metal/oxide synergy and consequently the catalytic performance. Although graphene can be an efficient mediator of the metal-support interactions, it has a minimal footprint in the macroscopic metal oxide characteristics. In photocatalysis, graphene-coated semiconductor oxides, in particular TiO2 and ZnO, have shown their great potential in environmental and energy-related applications. Nevertheless, homogeneous coating with controllable graphene thickness remains a challenge that has to be faced to enable the full potential of these materials. Since graphene properties are strongly dependent on the number of layers, development of new methods to prepare high quality graphene-coated oxides is required.

The stability of graphene might be an issue, since under the catalytic reaction conditions graphene quality deteriorates and defective graphene sites are formed. However, defects on graphene can also act as anchor sites for metal particles or for dopants and even as active sites for reactions. Therefore, defect formation should be further studied considering that modification of graphene and formation of defects might be deliberately performed for some applications. These findings might help to design supported metal catalysts with enhanced stability by coating a layer of graphene on the oxide support. To achieve high performance graphene-based catalysts, it is essential to control in the highest possible degree both graphene properties and structures. This should be taken into account in the recent effort to develop better heterogeneous catalysts and move from the typically used amorphous or graphitic multilayer carbon oxide coatings to single or few layer graphene.

Entry for the Table of Contents

Figure 1 .

 1 Figure 1. Schematic illustration of the multi-functionality of graphene coated oxides in heterogeneous catalysis.

Figure 2 .

 2 Figure 2. Schematic illustration of the graphene wet transfer process. From[26]

Figure 3 .

 3 Figure 3. TEM images of oxides with various morphologies coated by graphene through electrostatic self-assembly strategy: (a) SnO2 nano wires

Figure 3 .

 3 Figure 3. Chemical synthesis process for the ZnO-graphene quasi-quantum dots and TEM image of the as prepared ZnO-graphene quantum dots. From[53]

Figure 5 .

 5 Figure 5. UV-visible spectra (A) and photocurrent response (B) of bare anatase TiO2 NPs, grapheneTiO2 NPs, and graphene-TiO2 NPs (two-step hydrothermal). (C) Illustration of the photocatalytic mechanism of graphenewrapped anatase TiO2 NPs for the degradation of MB under visible light.Reproduced from ref.[18] .

Figure 6 .

 6 Figure 6. (A) TEM images of Fe-phen/C-800 (top) and Fe-phen/C-1000 (bottom) catalysts. (B) N1s XPS peaks of the Fe-phen/C-1000 (top) and Fephen/C-800 (bottom) catalysts. Adapted from reference[86] .

Figure 7 .

 7 Figure 7. (a) SEM image of PGE, (b) TEM images of PGE-CoO, (c) polarization curves for PGE-CoO, GE-CoO, CoO and PGE in 0.1 M KOH, (d) Tafel curves of PGE-CoO, GE-CoO and CoO in 0.1 M KOH, and (e) The stability of PGE-CoO and GE-CoO modified electrodes before and after linear sweep voltammetry testing for 1000 cycles. Adapt from reference[88] .

Figure 8 .

 8 Figure 8. XPS spectra of (a) Co/ZnO and (b) Co/G-ZnO upon annealing at different temperatures under UHV conditions. (c) Schematic representation of the particle size and the oxidation state of cobalt particles upon annealing in O2 and H2 environments. Adapted from references[19,30] .

  8 nm) at room temperature forms defects on the graphene layer. This observation suggests that Co plays a catalytic role on the introduction of defects in graphene. The Raman spectra of Co/G/SiO2 after redox treatments show more intense D and D' bands indicating highly defective graphene. Spectra from different sample regions, notably bilayer-graphene island (abbreviated as BL-Co/G/SiO2) and graphene areas at the corner of the sample, which were protected from cobalt deposition by the mounting clip (abbreviated as G/SiO2), are included in the same figure. The different regions are indicated in the optical images of the samples shown in Fig.9b. After analysis of the Raman spectra, it was found that the defect density of graphene after the redox treatment increases considerably on single layer areas, while the bilayer graphene areas or areas without cobalt, were proved to be much more resistant with lower defect sites.

Figure 9 .

 9 Figure 9.(a) Raman spectra recorded over graphene-covered SiO2 before and after 0.8 nm cobalt deposition as well as after 7 mbar O2 and H2 treatments up to 400 °C. In the latter case Raman spectra were recorded at different sample regions as shown in (b) the optical image. (c) Schematic representation of the proposed mechanism of the formation of graphene defects in contact with cobalt during the redox treatment. Adapted from[30] .

(

  Please choose one layout)Layout 1: MINIREVIEW Graphene coated oxides have been an emerging platform for both foundmental studies and applycations in catalysis. The latest advances in the synthesis and applications of graphene-coated oxides are reviewed. The chanllenges in the future studies are also discussed.


A Chapelle

F Oudrhiri-Hassani

L Presmanes

A Barnabé
email: barnabe@chimie.ups-tlse.fr

Ph Tailhades

CO 2 sensing properties of semiconducting copper oxide and spinel ferrite nanocomposite thin film

Keywords:

A new active layer for CO 2 sensing based on semiconducting CuO-Cu x Fe 3-x O 4 (with 0 ≤ x ≤ 1) nanocomposite was prepared by radiofrequency sputtering from a delafossite CuFeO 2 target using a specific in situ reduction method followed by post annealing treatment in air. The tenorite-spinel ferrite nanocomposite layer was deposited on a simplified test device and the response in a carbon dioxide atmosphere was measured by varying the concentration up to 5000 ppm, at different working temperatures (130-475 • C) and frequencies (0.5-250 kHz). The results showed a high response of 50% (R air /R CO 2 = 1.9) at 250 • C and 700 Hz for a CO 2 concentration of 5000 ppm.

Introduction

Carbon dioxide detection is currently in demand in key fields such as indoor air quality monitoring [START_REF] Kaneyasu | A carbon dioxide gas sensor based on solid electrolyte for air quality control[END_REF][START_REF] Frank | Sensor system for indoor air monitoring using semiconduction metal oxide and IR-absorption[END_REF]. Most of the currently available carbon dioxide sensing devices are based on optical principles but they are usually bulky and expensive. This has fuelled the research of several groups in the development of an alternative sensor for CO 2 monitoring which is cheap and efficient. In this context, semiconductors are always a competitive low-cost choice because they are efficient [START_REF] Herrán | Solid state gas sensor for fast carbon dioxide detection[END_REF], cheap and easy to integrate into electronic and control devices. Concretely for carbon dioxide detection, BaTiO 3 -CuO mixed oxide composites have been studied by several research groups [START_REF] Ishihara | Improved response of CuO-BaTiO3 capacitive-type CO2 sensor by additives[END_REF][START_REF] Haeusler | A novel thick film conductive type CO2 sensor[END_REF][6][START_REF] Herrán | On the structural characterization of BaTiO3-CuO as CO2 sensing material[END_REF] ever since Ishihara et al. [START_REF] Ishihara | Application of mixed oxide capacitor to the selective carbon dioxide sensor[END_REF] reported that the resulting capacitance of this composite depends strongly on the CO 2 concentration in air. A theory based on the behaviour of the p-n heterojunction between n-type BaTiO 3 and p-type CuO has been developed by Herran et al. [START_REF] Herrán | Physical behaviour of BaTiO3-CuO thin-film under carbon dioxide atmospheres[END_REF] to explain how AC electrical properties change with CO 2 concentration in this system. Other oxides have also been reported [START_REF] Steiner | Ca-and Pt-catalysed thinfilm SnO2 gas sensors for CO and CO2 detection[END_REF][START_REF] Michel | An alternative gas sensor material: synthesis and electrical characterization of SmCoO3[END_REF][START_REF] Michel | Carbon dioxide gas sensing behavior of nanostructured GdCoO3 prepared by a solution-polymerization method[END_REF][START_REF] Michel | Improvement of the gas sensing behavior in nanostructured Gd0.9Sr0.1CoO3 by addition of silver[END_REF][START_REF] Prim | A novel mesoporous CaO-loaded In2O3 material for CO2 sensing[END_REF][START_REF] Jo | Gas sensing properties of WO3 doped rutile TiO2 thick film at high operating temperature[END_REF], but research work focused on semiconducting oxides for CO 2 sensing is still limited and only a few systems have been tested.

On one hand, spinel oxides have never been studied for CO 2 detection to date, while for the detection of other gases such as Cl 2 , H 2 and NH 3 , these oxides, i.e. NiFe 2 O 4 , CoFe 2 O 4 , ZnFe 2 O 4 [START_REF] Reddy | Preparation and characterization of ferrites as gas sensor materials[END_REF][START_REF] Xiangfeng | Ethanol gas sensor based on CoFe2O4 nano-crystallines prepared by hydrothermal method[END_REF] and CuFe 2 O 4 [START_REF] Reddy | Preparation and characterization of ferrites as gas sensor materials[END_REF][START_REF] Tao | Preparation and gas-sensing properties of CuFe2O4 at reduced temperature[END_REF][START_REF] Sun | Simple synthesis of CuFe2O4 nanoparticles as gas-sensing materials[END_REF] have been widely studied in the form of thick and thin films, and pellets prepared by various methods. On the other hand, even though some spinel structures like Cu x Fe 3-x O 4 [START_REF] Tao | Preparation and gas-sensing properties of CuFe2O4 at reduced temperature[END_REF][START_REF] Patil | Conduction mechanism in CuxFe3-xO4[END_REF] exhibit n-type semiconducting properties, they have never been reported for use in p-n heterojunction in cooperation with p-type CuO [START_REF] Muhibbullah | Studies on Seebeck effect in spray deposited CuO thin film on glass substrate[END_REF][START_REF] Jeong | Nonstoichiometry and electrical conduction of CuO[END_REF].

Furthermore, thick film technology and sintering are commonly used to synthesize materials for such applications, while thin film technology is used only by a few groups [START_REF] Herrán | Solid state gas sensor for fast carbon dioxide detection[END_REF][START_REF] Steiner | Ca-and Pt-catalysed thinfilm SnO2 gas sensors for CO and CO2 detection[END_REF][START_REF] Mandayo | BaTiO3-CuO sputtered thin film for carbon dioxide detection[END_REF] even though it allows mass production of devices at low cost with high control and repeatability. Moreover, due to its high intrinsic surface-tovolume ratio, thin film technology exhibits high capabilities for miniaturization and very short response. Among the different processes to prepare thin films, radiofrequency (RF) sputtering ensures relatively high deposition rates and high versatility, allowing the deposition of different materials and the possibility of preparing out-of-equilibrium phases [START_REF] Mugnier | Thin films preparation by RF-sputtering of copper/iron ceramic targets with Cu/Fe = 1: from nanocomposites to delafossite compounds[END_REF][START_REF] Mauvernay | Nanocomposite Fe1-xO/Fe3O4, Fe/Fe1-xO thin films prepared by RF sputtering and revealed by magnetic coupling effects[END_REF][START_REF] Presmanes | Magnetic and semi-conducting nano-composite films of spinel ferrite and cubic zinc oxide[END_REF].

In this study, thin films of copper oxide and spinel ferrite have been deposited by RF sputtering with the aim to achieve CO 2 sensing. The size, content, distribution and microstructure of this nanocomposite has been varied by using specific in situ reduction method during the deposition [START_REF] Mugnier | Thin films preparation by RF-sputtering of copper/iron ceramic targets with Cu/Fe = 1: from nanocomposites to delafossite compounds[END_REF]. The gas-sensing characteristics of the Cu x Fe 3-x O 4 and CuO nanocomposites have been tested for carbon dioxide monitoring. doi:10.1016/j.apsusc.2010.02.079

Experimental

The thin films were deposited with an Alcatel A450 apparatus using a sintered ceramic target of pure CuFeO 2 with a relative density of 70%. The RF power was fixed at 4 W/cm 2 and the pressure inside the deposition chamber was 5 × 10 -6 mbar or lower before deposition is commenced. During the deposition of the films, the argon pressure and the target-to-substrate distance were kept at 0.5 Pa and 5 cm respectively. Thin films of 300 nm were deposited on glass substrate for structural characterizations. The film thickness was fixed to a value of 50 nm when making the sensor device for CO 2 sensing tests.

Thickness calibrations were performed with a DEKTAK 3030ST profilometer. The structural properties were determined by Grazing incidence (˛= 1 • ) X-ray Diffraction (GXRD) using a Siemens D5000 diffractometer with the Cu K␣ radiation ( = 1.5418 Å).

Microscopic studies were carried out with a Veeco Dimension 3000 Atomic Force Microscope (AFM) equipped with a super sharp TESP-SS Nanoworld©tip (nominal resonance frequency 320 kHz, nominal radius curvature 2 nm), and a JEOL 2100 Transmission Electron Microscope (TEM) operating at 200 kV equipped with a X-ray Energy Dispersive Spectroscopy (X-EDS) system.

Our simplified CO 2 sensor consists of a substrate, two interdigitated electrodes and the sensitive oxide layer. The substrate is a piece of 17 mm × 17 mm silicon wafer electrically insulated with a 1 m thick SiO 2 blanket-layer. Two gold interdigitated electrodes were screen-printed onto the surface of the silicon dioxide layer. The drying and densification of the gold paste was made by thermal annealing at 700 • C. This device is presented in Fig. 1. The sensing nanocomposite layer was then deposited on the top of the electrode by RF-sputtering with the electrical contacts at each sides of the electrodes left uncovered.

The electrical response of the sensing layer was investigated by registering the variations in impedance with a RCL-meter (FLUKE PM6306) as a function of temperature, time, frequency and CO 2 concentration. The test device was placed in a furnace operating under controlled atmosphere with the temperature range of 20-475 • C. The total gas flow was fixed at 100 cm 3 /min. Two gas cylinders were used to set the CO 2 concentrations between 0 and 5000 ppm with the use of mass flow regulators.

Impedance measurements were done under two atmospheres: synthetic air and a mixture of air + CO 2 (5000 ppm), which will be referred to as CO 2 henceforth. During these measurements, an alternating current of 1 V was applied to avoid polarization effects. High frequencies are not effective for this study because depletion sites located at grain interfaces are not mobilized as the electri-cal oscillation is too fast [START_REF] Herrán | Physical behaviour of BaTiO3-CuO thin-film under carbon dioxide atmospheres[END_REF]. Therefore, a low frequency was used to avoid the relaxation phenomena and to acquire signals due to spatial-charge phenomena. An arbitrary value of 700 Hz was chosen.

The response S is defined by Eq. ( 1) as follows:

S(%) = 100 × Z CO 2 -Z air Z air (1)
where Z air and Z CO 2 are the electrical impedances of the sensor measured in air and in CO 2 respectively. The electrical responses were recorded using dynamic tests whereby the sensor was exposed to alternating air and CO 2 pulses at a constant temperature.

In this report, tests have only been made in dry atmospheres. In our future work, we plan to investigate the effect of humidity by testing the sensing layer in an atmosphere with controlled water partial pressures. This nanocomposite thin film can only be used in a practical device if the CO 2 response is reproducible even in varying humidity atmospheres. The oxide phase cannot be unambiguously identified using GXRD in the as-deposited films due to the nanocrystallinity of the material [START_REF] Mugnier | Thin films preparation by RF-sputtering of copper/iron ceramic targets with Cu/Fe = 1: from nanocomposites to delafossite compounds[END_REF]. TEM image of a as-deposited sample of 50 nm thickness (Fig. 3a) shows a homogenous distribution of particles of about 50 nm which are attributed to metallic copper according to X-EDS measurements. The grainy area between the copper particles belongs to the mixed oxide phase. These are in agreement with the GXRD results. AFM done on the same sample shows a regular surface morphology as shown in Fig. 3b.

Results and discussion

Structural characterization

Thermal annealing at 450 • C under vacuum increases the crystallite size of each phase and magnifies the diffraction peaks related to copper, delafossite and cuprospinel phases as shown in Fig. 2b. This allows clearer identification of the thin film composition. It has been previously demonstrated [START_REF] Mugnier | Nanocomposites of metallic copper and spinel ferrite films: growth and selfassembly of copper particles[END_REF] that deposition by RF sput- tering from a delafossite CuFeO 2 target leads to the presence of reduced species in the film. Metallic copper and spinel oxide can then be obtained at a temperature close to ambient. This reduction is favored by several factors, including low oxygen pressure during the deposition, and the bombardment of the target and the substrate by energetic species during the deposition. The formation of these two reduced phases is expected from Cu-Fe-O phase diagram [START_REF] Katkov | Spinel phase relations in the Fe3O4-CuFe2O4 system[END_REF] at high temperature and low oxygen partial pressures, according to Eq. ( 2).

CuFeO 2 → (1/(3 -x))Cu x Fe 3-x O 4 + ((3-2x)/(3 -x))Cu + ((1 -x)/(3 -x))O 2 (2)
However, sputtering deposition is an out-of-equilibrium process and during the thin film deposition, Eq. ( 2) is not complete. In addition to spinel and metallic copper, the delafossite phase is also present in the final deposited compound. The as-deposited film is then made of nanometric copper metal particles homogeneously distributed in a copper and iron oxide matrix consisting of spinel and delafossite phases.

The second step in the preparation of CuO-Cu x Fe 3-x O 4 composite consists of heat treatment of the as-deposited film in air at 450 • C. This treatment leads to the oxidation of metallic copper into CuO and the decomposition of nano-crystallized delafossite CuFeO 2 into CuO and Cu x Fe 3-x O 4 . As a result, the film is only constituted of CuO and spinel oxide, in line with the GXRD pattern (Fig. 2c) and according to the phase diagram [START_REF] Zalazinskii | Oxygen pressure-composition" diagram in a copper-iron-oxygen system at 1000 • C[END_REF]. This in situ reduction of the target, followed by ex situ oxidation of the thin film, lead to a homogeneous and nanosized composite thin film comprising of CuO and Cu x Fe 3-x O 4 . Due to their individual n and p-type semiconducting properties already reported in the literature [START_REF] Tao | Preparation and gas-sensing properties of CuFe2O4 at reduced temperature[END_REF][START_REF] Patil | Conduction mechanism in CuxFe3-xO4[END_REF][START_REF] Muhibbullah | Studies on Seebeck effect in spray deposited CuO thin film on glass substrate[END_REF][START_REF] Jeong | Nonstoichiometry and electrical conduction of CuO[END_REF], this material can be described as a thin layer made of nanometric p-n heterojunctions.

CO 2 gas-sensing properties

The electrical properties depend on several parameters such as for example temperature, frequency and gas concentration. Temperature plays a crucial role on the electrical response of semiconductor-based gas sensors. In fact, the adsorption of gases is directly related to the temperature at the surface of the sensing layer.

Fig. 4 shows the response of the layer operating at different temperatures with alternating air and CO 2 pulses. The measurements were performed at 50 • C intervals between 150 • C and 400 • C. The carbon dioxide concentration was fixed at 5000 ppm in order to obtain the maximum signal. The curve shows a classical evolution of the response with respect to temperature, with a maximum at around 250 • C. Below 250 • C, the increase of the response with the temperature, is due to the increase of the depletion layer width [START_REF] Biswas | Studies on the gas sensing behaviour of nanosized CuNb2O6 towards ammonia, hydrogen and liquefied petroleum gas[END_REF] and the desorption of contaminating species adsorbed at the material surface, both of which are promoted by higher temperature. The desorption of contaminants also make more sites available for the adsorption of sensory species. At higher temperature, the decrease of the response results from the surface desorption of the sensory species which are responsible for changing the response factor. The best response was found to be -48% at 250 • C, which corresponds to a Z air /Z CO 2 ratio of 1.9 approximately.A few hypothesis on the sensing reaction of CuO-Cu x Fe 3-x O 4 can be introduced on the basis of the results reported on the CuO-BaTiO 3 system: in this system, the authors claimed that the capacitance changes of the sensing layer were caused by the carbonation of either the copper oxide [START_REF] Ishihara | Application of mixed oxide capacitor to the selective carbon dioxide sensor[END_REF] or barium oxides [START_REF] Herrán | Physical behaviour of BaTiO3-CuO thin-film under carbon dioxide atmospheres[END_REF][START_REF] Baraton | Determination oft he gas sensing potentiality of nanosized powders by FTIR spectrometry[END_REF]. However, this carbonation effect could not lead to capacitance or impedance variations without the coupling of the two oxides [START_REF] Liao | Study on CuO-BaTiO3 semiconductor CO2 sensor[END_REF]. Grain boundary effect of the semiconductor must also be taken into account in order to explain the sensitivity of the material [START_REF] Liao | Study on CuO-BaTiO3 semiconductor CO2 sensor[END_REF]. In our system, we assume a sensing mechanism based on the carbonation of the oxides that affects the potential barrier at the boundary between p-type CuO and n-type Cu x Fe 3-x O 4 .Fig. 5 shows the impedance transient of the nanocomposite layer recorded at the optimum temperature of 250 • C and a CO 2 concentration of 5000 ppm. The decrease in impedance when CO 2 gas is injected reveals that Z CO 2 is lower than Z air . CO 2 is an oxidizing gas which leads to a decrease in impedance in our case; it behaves like in the presence of a p-type semiconductor. The sig- nal is totally reversible but the stabilization under CO 2 gas requires very long time. The response time (time to obtain 90% of the total response) is 9.5 h. This value is too high to be considered for application in a sensory device.

A sensitive layer is usually modeled by a porous layer whose constituents are grains of oxides which are in contact at the grain boundaries. Because of its morphology, this layer may be the site of many phenomena when exposed to a gas: competitive adsorption on the surface of the grains, reaction with the molecules that are pre-adsorbed and diffusion between the grains through the pores and grain boundaries. The magnitude and kinetics of each of these processes are different and strongly influenced by the morphology of the film, i.e. by the deposition conditions. In our case, the long response time observed is probably the result of a mechanism limited by diffusion between the grains. A modification of the deposition conditions to obtain more inter-granular porosity and decrease of the layer thickness may be a suitable method to improve the response time.

The aim of this work is to report the unprecedented use of n-type spinel Cu x Fe 3-x O 4 coupled with a p-type CuO to form a p-n heterojunction which can be used as CO 2 sensing layer. The two-step process to fabricate an active thin film of nanometric CuO-Cu x Fe 3-x O 4 heterojunction is both simple and convenient. Parameters for the active layer, for example, deposition conditions, film thickness, and composition and doping, can be modified in order to improve the response time. Similar, there exists plenty of room for optimization of the device in terms of the nature of the electrodes, stacking design, and the use of micro-heater platform to perform temperature modulated measurements, etc. These works to optimize the overall performance of the sensor are currently underway and will be reported in another publication at a later time. The influence of frequency on the electrical responses of the CuO-Cu x Fe 3-x O 4 semiconducting nanocomposite thin films was also verified. Impedance was measured between 700 Hz and 235 kHz, in air and subsequently under 5000 ppm CO 2 . The response was then calculated at each frequency. At 250 • C, the best results were obtained at low frequencies (Fig. 6). When the frequency increases, the absolute value of the response decreases abruptly until it reaches a plateau at around 5% for frequencies over 100 kHz. As mentioned previously, this variation of the response with the frequency can be attributed to the relaxation phenomena present in the layer as explained in the literature [START_REF] Herrán | Physical behaviour of BaTiO3-CuO thin-film under carbon dioxide atmospheres[END_REF]. At high frequencies, the charge transfer at the depletion region appears to be constant because the oscillation of the electrical signal is too fast to detect this physical phenomenon.

The response of the sensing layer as a function of CO 2 concentration ranging from 1000 to 5000 ppm was studied at the optimum temperature of 250 • C (Fig. 7). An increase of the sensitivity is evident when the concentration of CO 2 increases. This is in accordance with the increase in the number of molecules arriving at the sites of adsorption. The signal remains significant even for the lowest CO 2 concentrations (< 2000 ppm).

Conclusion

Novel thin films of semiconducting nanocomposite CuO-Cu x Fe 3-x O 4 were tested for CO 2 gas sensing. These films were deposited by radiofrequency sputtering from a delafossite CuFeO 2 target. Si/SiO 2 substrates covered with gold interdigitated electrodes were used for gas-sensing measurement. Impedance variations were recorded under two different atmospheres: air then a mixture of air and 5000 ppm of CO 2 . The optimal response was obtained at a temperature of 250 • C with a value of 48% (Z air /Z CO 2 ∼1.9). Best results were achieved at low frequencies (<1000 Hz), and the layer remained still sensitive even for CO 2 concentration lower than 2000 ppm.

In comparison with the bibliographical data, these results obtained with a simplified test device are very promising due to the low operating temperature and the high sensitivity of the material. However, the transient impedance showed a very long response time of about 9.5 h. This time is too long to allow the use of this sensitive layer in a CO 2 gas sensor for the moment. Ongoing work is now focused on the optimization of the sensing layer and the design of the test device to improve the response time.

Fig. 1 .

 1 Fig. 1. Optical micrograph of the test device before deposition of nanocomposite sensing layer. The bright grids correspond to the gold electrode.

Fig

  Fig. 2a shows the GXRD pattern of the thin film deposited on glass substrate, which constitutes the first step of the deposition process. The GXRD data clearly demonstrates the presence of metallic copper with the characteristic (1 1 1) and (2 0 0) Bragg's peaks located at 2Â = 43.3 • and 2Â = 50.43 • respectively. The broad peak located at 2Â = 35.5 • , i.e. with d h k l ≈ 2.5 Å, can be attributed to the various mixed oxide phases, such as Cu 2 O (d 1 1 1 = 2.46 Å), CuO (d 1 1 1 = 2.52 Å ), CuFeO 2 (d 0 1 2 = 2.51 Å), and/or CuFe 2 O 4 (d 2 1 1 = 2.50 Å).The oxide phase cannot be unambiguously identified using GXRD in the as-deposited films due to the nanocrystallinity of the material[START_REF] Mugnier | Thin films preparation by RF-sputtering of copper/iron ceramic targets with Cu/Fe = 1: from nanocomposites to delafossite compounds[END_REF]. TEM image of a as-deposited sample of 50 nm thickness (Fig.3a) shows a homogenous distribution of particles of about 50 nm which are attributed to metallic copper according to X-EDS measurements. The grainy area between the copper particles belongs to the mixed oxide phase. These are in agreement with the GXRD results. AFM done on the same sample shows a regular surface morphology as shown in Fig.3b.Thermal annealing at 450 • C under vacuum increases the crystallite size of each phase and magnifies the diffraction peaks related to copper, delafossite and cuprospinel phases as shown in Fig.2b. This allows clearer identification of the thin film composition. It has been previously demonstrated[START_REF] Mugnier | Nanocomposites of metallic copper and spinel ferrite films: growth and selfassembly of copper particles[END_REF] that deposition by RF sput-

Fig. 2 .

 2 Fig. 2. Grazing angle X-ray diffraction patterns of the deposited film on glass substrate from a CuFeO2 target, (a) as-deposited film, (b) after annealing film under vacuum at 450 • C and (c) after annealing in air at 450 • C for 21 h. Thickness = 300 nm.

Fig. 3 .

 3 Fig. 3. TEM (a) and AFM (b) images of a as-deposited sample of 50 nm thickness.

Fig. 4 .

 4 Fig. 4. Response of the sensing layer as a function of temperature, f = 700 Hz, thickness = 50 nm.

Fig. 5 .

 5 Fig. 5. Impedance transient (at 700 Hz) for response and recovery of a 50 nm CuO/CuxFe3-xO4 sensing layer.

Fig. 6 .

 6 Fig. 6. Variation of the response of CuO-CuxFe3-xO4 film as a function of the frequency. T = 250 • C, CO2 concentration = 5000 ppm, thickness = 50 nm.

Fig. 7 .

 7 Fig. 7. Response of CuO-CuxFe3-xO4 layer as a function of CO2 concentration. T = 250 • C, f = 700 Hz, thickness = 50 nm.

Acknowledgement

The authors thank Xavier Hochart from Temex Ceramics Company for his help in preparation of gold coated Si/SiO 2 substrates.