Photocatalytic reduction of graphene oxide with cuprous oxide film under UV-vis irradiation

We have grown Cu2O films by different routes including self-oxidation and metal-organic deposition (MOD). The reduction efficiency of Cu2Ofilms on graphene oxide (GO) synthesizedbymodifiedHummer’smethodhas been studied. Surface morphology and chemical state of as-prepared Cu2O film and GO sheets reduced at different conditions have also been investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). Results show that self-oxidation Cu2O film is more effective on phtocatalytic reduction of GO thanMOD-Cu2O film.Moreover, reduction effect of self-oxidation Cu2Ofilm to GO is comparable to that of environmental-friendly reducing agent of vitamin C. The present results offer a potentially eco-friendly and low-cost approach for the manufacture of reduced graphene oxide (RGO) byphotocatalytic reduction.


Introduction
Graphene as a two dimensional carbon material has gathered much more attention with its application prospect in various fields, such as energy storage, radiator, sensors, aviation and space, anti-bacterial, due to its high electric conductivity, good thermal conductivity, large specific surface area, and high mechanical strength, antibacterial activity [1][2][3][4][5][6]. Because of this, the synthesis method of graphene has become a hot topic. Various methods have been developed to fabricate graphene, including mechanical exfoliation, microwave-assistant exfoliation, chemical vapor deposition (CVD), epitaxial growth, electrochemical reduction and chemical oxidation-reduction method [6][7][8][9][10]. Among these synthesis ways for graphene, CVD is mainly used in producing graphene film due to its application potential in flexible display field [11,12]. However, modified Hummer's method, a chemical oxidationreduction method, offers lower processing costs and more simple operation than other synthesis methods. And this chemical synthesis method is promising for mass production of GO and RGO because it is easy for scale-up [10,13,14]. It should be pointed out that hydrazine hydrate explosively used to be as a reductant is highly toxic and it is not suitable to be in large-scale implementation [15]. Therefore, it becomes very significant to explore an efficient and environmentally friendly technique to prepare reduced graphene oxide. At present, several new reductants including vitamin C, hydrohalic acids, Sodium borohydride, grape seed extract, and new reduction routes, such as hydrothermal, photocatalytic reduction, photothermally reduced, flash reduction have been developed [15][16][17][18]. Among all the strategies developed, photocatalytic reduction has attracted more attention because of its many advantages, such as safety, eco-friendly, easy-operation and high efficiency. In addition, a lot of photocatalysts, which used in the reduction of GO, have been developed at present. Choobtashani and Akhavan [18] found out that tungsten oxide film synthesized by various routes showed significant differences in photocatalytic reduction efficiency of the GO platelets especially after exposure to visible light of the environment [18]. In addition, he thought that the ZnO nanoparticles in the ZnO/GO composite could be applied in a gradual chemical reduction and consequently tuning the electrical conductivity of the GO platelets by variation of UV irradiation time in a photocatalytic process [19]. G. Williams fount out that the direct interaction between TiO 2 particles and graphene sheets hinders the collapse of exfoliated sheets of graphene while GO undergoes reduction by accepting electrons from UVirradiated TiO 2 [20].
In fact, transition metal oxides have been extensively applied in many fields, such as catalysts, supercapacitors, and superconductors due to their special physical properties and good stability [21][22][23][24][25][26]. To explore efficient and environmentally friendly technique to prepare reduced graphene oxide, we choose transition metal oxide Cu 2 O as a model material. Compared with other common photocatalysts, Cu 2 O has a low band gap energy (2.2 eV) [27][28][29][30]. Thus Cu 2 O can absorb visible light at a wavelength of about 400-800 nanometers along with the excitation of electron-hole pair. And it has been widely investigated for photocatalyst reaction under visible light irradiation [21][22][23]31]. Thus Cu 2 O is considered to be a promising visible light active photocatalyst to produce graphene [23]. In addition, photocatalysis reduction efficiency of Cu 2 O is reported to be related to its morphology [32,33]. In this paper, GO has been reported to be fabricated by modified Hummer's method. Then we have explored the photocatalytic reduction effect of Cu 2 O film prepared by different methods on GO sheets. And we have also discussed the influence of visible light in the environment on the photocatalytic reduction of GO. In addition, a comparative study of the potocatalytic reduction efficiency and eco-friendly reducing agent vitamin C to GO has been done.

Synthesis of GO
GO was synthesized using natural flake graphite powder (from Qingdao Tianheda graphite Co., LTD) by a modified Hummers' method. The pre-oxidation of graphite was initially carried out by reacting powered flake graphite with K 2 S 2 O 8 and P 2 O 5 in sulfuric acid with continuous stirring in an oil heating bath at 80 ∘ C. Then the suspension was filtered, followed by washing and vacuum drying treatment resulting in dark grey powder. Graphite oxide was synthesized by stirring pre-oxided graphite and KMnO 4 in sulfuric acid. The reaction system went through three steps including the ice-bath, room-temperature reaction and hightemperature reaction process. Then the sticky suspension was purified by dialysis for 2 weeks to remove other impurities. The graphite oxide suspension with high concentration was dewatered using anhydrous ethanol and then was dispersed in ethanol by bath sonication of graphite oxide suspension at a power of 300 W for 30 min to obtain the GO ethanol dispersion solution.

Fabrication of Cu 2 O film by MOD
The metal-organic deposition (MOD) method is widely used for the preparation of oxide film because of cost effective and high deposition rate [34]. Here, the MOD pre-cursor solution was prepared in the ambient atmosphere. Copper (II) acetate monohydrate was dissolved in propionic acid. After heating with continuous stirring, a stable bottle green-colored precursor solution was obtained. The yttria stabilized zirconia (YSZ) single crystal substrate was cleaned by sonication in acetone for 20 min. And the precursor was spin-coated onto YSZ single crystal substrate, followed by heat-treatment at 1000 ∘ C for 30 min in a reducing forming gas atmosphere of Ar-4%H 2 . At the end of the heat-treatment cycles, finally, the sample was quenched to room temperature in the same atmosphere.

Preparation of Cu 2 O film by self-oxidation reaction
Just like other metals and alloys, copper is widely used in many fields due to its series of advantages, such as high strength, high corrosion resistance, and easy processing [35][36][37]. On the other hand, thermal oxidation techniques are often used to improve some significant performance of metal, without special requirements for substrate geomeries [38]. Thus, Cu was chosen as the substrate for self-oxidation of Cu 2 O film. Cu substrate was obtained by rolling and then annealing in vacuum at 800 ∘ C for 3 h. In order to improve the surface quality of the metallic substrate, the electropolishing modification was carried out. The Cu substrate was cleaned by sonication in ethanol and acetone for 30 min, respectively. The sample was introduced into a furnace and heat-treated at 900 ∘ C for 30 min in a flowing gas atmosphere of Ar. After heat-treatment cycles, the sample was quenched to room temperature with the same atmosphere.

Photocatalytic reduction of GO on Cu 2 O film
The as-prepared GO sheets were deposited on the as-grown Cu 2 O films through drop-casting the GO ethanol dispersion solution onto the films fabricated on the different substrates. Then the coating GO/Cu 2 O samples were dried at room temperature for 24 h in darkness. For comparison, the same method was also used to deposite GO sheets on YSZ substrate. To investigate the photocatalytic reduction of the GO sheets deposited on the Cu 2 O films grown on the different substrates using different deposition methods, the samples were irradiated by an ultraviolet lamp for different times at room temperature. Some of the samples were also exposed to the visible light of the environment to realize photocatalytic reduction of GO.

Characterization
The thickness of GO sheets and the microstructure of Cu 2 O films prepared by different methods were performed using a digital instruments nanoscope SPI3800-SPA-400 atomic force microscopy (AFM) in tapping mode. Cu 2 O films were characterized by using X-ray diffraction (XRD), which was performed to carry out θ-2θ scan by using CuKα radiation at 40 kV and 50 mA, for phase purity, crystallinity and texture. X-ray photoelectron spectroscopy (XPS) was performed to identify the phase constitution of Cu 2 O films and GO sheets using an X-ray photoelectron spectrometer equipped with a monochromatic X-ray source of AlKa. Cu2p, O1s and C1s core level XPS spectra were measured. Figure 1 shows the AFM 2D image and the high profile diagram of GO sheets deposited on Si single crystal substrate by drop-casting. Irregularly shaped individual GO sheet, with about two hundred nanometers in size and one nanometer in height is observed. This height is corresponding to the typical thickness of exfoliated singlelayer GO sheet, which is greater than that of pristine singlelayer graphene sheet due to the insertion of the oxygencontaining groups. XRD diffraction patterns for Cu 2 O films deposited by different deposition methods were collected at room temperature to find the phase purity and growth orientation of crystalline grains, as shown in Figure 2. It can be found that all the as-fabricated Cu 2 O films have completely crystallized by different synthesis methods. And the Cu 2 O films prepared by two methods are not essentially single preferred oriented because both the Cu 2 O (111) and (200) diffraction peaks appear in the patterns. However, the intensity ratio of the (111) to (200) peaks for self-oxidation-Cu 2 O film is significantly greater than that for MOD-Cu 2 O film. There is a big difference in photocatalytic activity between different crystallographic planes in Cu 2 O. In addition, Cu 2 O (111) crystal plane has a better photocatalytic activity and stability than its (l00) plane [39][40][41]. Therefore, the as-prepared self-oxidation-Cu 2 O film may have better photocatalytic efficiency compared with the MOD-Cu 2 O film.   Figure 4 shows Cu 2p core-level spectra of cuprous oxide films grown on YSZ and Cu substrates by different deposition methods. Cu2p peak has significantly split spinorbit components of 2p3/2 and 2p1/2 with energy splitting of 19.9 eV. The appearance of asymmetric shape with a high binding energy tail indicates that little quantity of CuO is formed in films. It is believed that the oxidation of a very small amount of Cu 2 O to CuO occurred during the calcination process as reported in other studies [23]. Nevertheless, the over-oxidation was very minimal because no obvious CuO diffraction peaks were observed in the XRD pattern disscussed above. Moreover, Cu(I) is predominant in the film due to the presence of a very weak satellite at~934 eV, which is associated with the existence of Cu(II) oxide. After the deconvolution of the each spectrum into two doublets, which corresponds to the chemical states of Cu(I) (at 932.5 and 952.4 eV, respectively) and Cu(II) (at 934.0 and 953.9 eV, respectively), as shown in Figure 4(a). Thus the Cu(I) percent of the film can be calculated accordingly. It is clear that a percent of Cu(I) is about 79.99% for the MOD-Cu 2 O film, whereas Cu(I) percent is around 99.99% for selfoxidation Cu 2 O film. It indicates that Cu 2 O film with high purity is favorable to be synthesized by self-oxidation reaction compared using MOD methods due to the difference of the driving force from the interface between the substrate and the top oxide film.

Results and discussion
XPS is also employed to analyze the as-deposited GO and the degree of photocatalytic reduction of exfoliated GO sheets on Cu 2 O film fabricated by different methods. The high-resolution C1s spectra of the as-prepared GO/YSZ and GO/Cu 2 O/YSZ as well as GO/Cu 2 O/Cu exposed to UV light irradiation for different times are shown in Figure 5. For GO sheets deposited on YSZ substrate dried in dark, an asymmetric line shape indicates that there are different types of carbon existing on the surface of GO/YSZ, as seen in Figure 5(a). The assumption being that the C1s spec- trum is attributed to the non-oxygenated ring C including C-C and C=C bonds (at~284.8 eV), carbonyl C in C-OH bond (at~286.5 eV), ether C in C-O-C bond (at~287 eV), ketone C in C=O (at~287.9 eV), carboxyl C in O=C-OH (at 289 eV) [18,19]. After irradiated in environment for 24 h, the intensities of the oxygen functionality peaks on the surface of GO/Cu 2 O/YSZ sample decrease obviously compared with those of as-deposited GO sheets, as show in Figure 5(b). However, the intensities of C1s peaks with oxygencontaining functional group decrease dramatically for the GO/Cu 2 O/YSZ sample after 6 h UV-vis light irradiation. And further 18 h UV-vis light irradiation shows a very weak reduction for GO deposited on Cu 2 O/YSZ substrate. Nevertheless, a small amount of oxygen-containing functional groups still exists in GO sheets deposited on the surface of Cu 2 O/YSZ when 24 h UV-vis light irradiation is adopted. It is noteworthy that no obvious deconvoluted peak corresponding to oxygen-containing functional groups is observed on the surface of GO/Cu 2 O/Cu sample after 24 h UV-vis light irradiation. It indicates that the self-oxidation Cu 2 O film shows a more effective photocatalytic reduction for the GO sheets compared with the MOD-Cu 2 O film.  To comparative study the reduction effect with light irradiation on oxygen-containing functional groups in GO sheets at different conditions, the peak area ratios of C-OH, C-O-C, C=O, O=C-OH bonds to the non-oxygenated ring CC bonds was calculated and given in Table 1. GO sheets deposited on MOD-Cu 2 O film are slightly reduced after visible light irradiation for 24 h in the environment. And its reduction degree of GO is comparable to that of GO deposited on MOD-Cu 2 O film after 6 h UV-vis light irradiation. However, GO deposited on MOD-Cu 2 O film is not thoroughly reduced although further reduction happens with prolonging the UV-vis irradiation time. It illustrates that the UV part in UV-vis light irradiation does not play a leading role in the photocatalytic reduction of GO sheets deposited on Cu 2 O film. The self-oxidation Cu 2 O film, by contrast, shows an effective photocatalytic reduction to GO sheets, which can be even compared with the reduction effect of vitamin C to GO. Moreover, better reduction efficiency of self-oxidation Cu 2 O is even observed on C-O-C bond in GO compared with that of vitamin C.
It is well known that photocatalytic activity mainly involves light absorption, generation and transport of electron-hole pair. The concentration, surface characteristics and contact angle with the GO and cuprous oxide photocatalyst are very important parameters for photocatalytic efficiency [5][6][7]. The higher content of Cu 2 O in selfoxidation film promotes its excitation on the absorption of light, resulting in its better photocatalytic efficiency. In addition, small surface roughness of self-oxidation Cu 2 O film enhances its contact area with GO sheets, which favors electron transport and further improves photocatalytic reduction of GO under UV-vis light irradiation. In particular, Cu 2 O (111) crystal plane shows better stability and photocatalytic activity than (l00) plane. Thus, self-oxidation Cu 2 O film with larger ratio of (111)/(200) diffraction peaks show better photocatalytic performance than MOD-Cu 2 O film.

Conclusion
In summary, we have demonstrated an environmentallyfriendly and high-efficiency method for reduction of graphene oxide with cuprous oxide thin film. GO sheets coated on Cu 2 O thin films, which are prepared by different methods, have been reduced by UV-visible light irradiation. Self-oxidation Cu 2 O shows a better photocatalytic reduction efficiency on GO sheets than MOD-Cu 2 O, which is related to their different morphologies and Cu(I) percentage in as-prepared Cu 2 O films. Most importantly, the intensity ratio of (111) to (200) diffraction peaks of the selfoxidation-Cu 2 O film is much larger than that of the MOD film. A better photocatalytic activity of the (111) crystal plane leads to higher photocatalytic efficiency of the former. Moreover, the photocatalytic reduction rate of selfoxidation Cu 2 O on GO is even comparable to that of vitamin C. Such self-oxidation Cu 2 O film can be used as a suitable photocatalytic agent for the reduction of GO sheets.