The large-area preparation and photoelectrochemical properties of graphene/ZnO nanorod composite film

We proposed a new method for preparing a single-crystalline (002)-oriented ZnO nanorod (ZnONR) array and graphene/ZnO nanorod (GZN) composite film on quartz plates using double glow plasma surface alloying (DGPSA) technology. The effects of graphene film on the photoelectrochemical properties of ZnO thin film were investigated. Ultraviolet (UV)-visible spectroscopy showed that the total transmittance of the GZN composite film was maintained at nearly 75% and the addition of graphene caused an increase in the absorption intensity at wavelengths greater than 375 nm. The contact angle (51°) of the GZN film after UV irradiation was lower than that (82.4°) of the film before UV irradiation. A comparison of the photoelectrochemical properties of graphene, ZnO and GZN films indicated that the synergistic effects of graphene and the ZnONR array enhanced charge transfer on the surface and reduced impedance in the solid state interface layer. In other words, the introduction of graphene has an obvious influence on the photoelectrochemical properties of GZN composite film. Therefore, our study suggests for the first time that double-glow plasma surface treatment can be considered as a new potential technology for the large-area preparation of graphene and GZN composite film.


Background
ZnO, with a direct wide bandgap ($3.4 eV) and large exciton binding energy (60 meV), has been widely used in optoelectronic devices, e.g. UV nanolasers, 1 photodetectors, 2 eld-effect transistors, 3 and photovoltaic devices. [4][5][6] However, single ZnO lm has low interfacial charge-transfer efficiency and a high recombination rate of photo-generated electrons and holes. Recently, ZnO-based hybrid nanostructures with 2D nanomaterials have been attracting more attention, since 2D carbon nanostructures, as conductive carbon mats, anchor metal oxide materials to form new nanocomposite hybrid materials with potential applications in optoelectronics and energy conversion devices. Graphene, as a single layer of 2D-honeycomb carbon atoms, is one of the most hotly studied materials. [7][8][9] Owing to its fascinating physical properties, e.g. high electrical conductivity, ultrahigh mobility and high transparency, 10-14 graphene has been successfully modied with various ZnO nanostructures for high-efficiency photovoltaic devices, [15][16][17] highly sensitive exible sensors, 18,19 solar water splitting 20 and supercapacitors. 21,22 Furthermore, research has demonstrated that the incorporation of graphene into ZnO nanostructures will provide synergistic effects in light absorption and electron transport. 23 It has been reported that graphene/ZnO composite lms have potential applications in dye sensitized solar cells (DSSCs) and UV photoelectrochemical detection devices. 24,25 In view of application to DSSCs, the combination of graphene and semiconductors (i.e., ZnO and TiO 2 ) can provide a large receptor interface, which is benecial for electron injection into semiconductors, improving the conversion efficiency of solar cells. Moreover, graphene can also enhance the adsorption of dye molecules on the photoanode. For UV detection, the incorporation of graphene into ZnO nanostructures enhances charge transfer and the separation rate of photoinduced charges, which can improve the sensitivity for UV photoelectrochemical detection. For example, Kang Z. et al. showed a self-powered, UV-activated photoelectrochemical biosensor based on a vertical ZnO nanorod (NR) array directly synthesized on graphene through hydrothermal methods; the rGO/ZnONR array provided a robust approach for the UV photoelectrochemical detection of biomolecules. 26 In recent years, many studies have reported the fabrication of ZnONR lms via sol-gel or magnetron sputtering methods. Simultaneously, the growth of graphene was mainly performed via chemical deposition, epitaxial growth or self-assembly techniques and Hummers' method. 27,28 In view of conventional synthesis methods for ZnO, Nie 29 proposed that poly(methyl methacrylate) (PMMA)supported monolayer graphene lm could be transferred onto the surface of a ZnONR array and the PMMA could then be removed with acetone solution. The major obstacle in these methods is that graphene directly transferred onto the surface of ZnO lm is not suitable for the large-area fabrication of composite lms, and there is poor adhesion between the graphene and the ZnO lm. A comparative study of ZnO lm, graphene lm and ZnO/graphene composite lm is provided in Table 1.
However, few studies involve the large-area preparation of graphene/ZnO nanorod (GZN) composite lm, using plasma surface technology, for application in various photodetector devices. Comparing with other glow discharge techniques, the double-glow plasma surface alloying (DGPSA) technique is a novel surface modication method, which involves a doubleglow plasma discharge and can provide the ability to enhance the sputtering yield. One of the major advantages of DGPSA technology is that strong adhesion between the coating and the substrate can be obtained.
Herein, we rstly propose a novel method for preparing transparent GZN composite lm on a quartz substrate through a simple two-step process for ZnONRs and graphene lm, using the DGPSA technique. The composition, morphology and microstructure were characterized using Raman spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). UV-induced photosensitivity measurements were collected using UV-visible spectroscopy, and water contact angles and electrochemical impedance spectra (EIS) were collected for samples exposed and unexposed to UV irradiation. The research aims to conrm that GZN composite lm prepared using DGPSA shows effective transmission of photogenerated electrons, and can act as an alternative photoanode material in solar cells and UV photoelectrochemical devices.

Synthesis of GZN composite lm
Experiments were performed in a double glow plasma surface metallurgy device. A schematic diagram of the device used in this study is shown in Fig. 1(a). There was double glow discharge in the vacuum chamber. Three electrodes were xed in the chamber, one anode and two negatively charged members. There was one glow discharge as the work-piece electrode heated the substrate to a high temperature and a second glow discharge as the source electrode provided the desired deposition material. The hollow cathode effect was used to improve the sputtering density of the double-glow plasma.
In this experiment, a zinc target (100 mm Â 100 mm Â 3 mm, 99.99%) and graphene paper (Jiangnan Graphene Research Institute, China) were used as the sputtering targets for ZnO and graphene lm, respectively. A quartz plate (30 mm Â 30 mm Â 3 mm) was used as the cathode (workpiece electrode). High purity argon (99.999%) and oxygen (99.999%) were  used as the working gases. The processing parameters are given in Table 2. Fig. 1(b) illustrates the procedure for fabricating the GZF composite lm. In the rst step, a ZnONR array was fabricated on the quartz substrate, and then graphene nanosheets were sputtered to deposit them onto the surface of the ZnONR array.

Characterization
The surface and cross-sectional morphologies of the GZN composite lm were investigated via atomic force microscopy (AFM, Veeco Instrument NanoScope D3100, USA) and scanning electron microscopy with an energy dispersive spectrometer (SEM, Carl Zeiss Auria-39-35, Germany), respectively. For AFM measurements, the scanning area was xed at 4 Â 4 mm 2 and measurements were in the near central region of the sample, via tapping mode. The surface topology derived from the AFM measurements represents the surface roughness of the GZN composite lm. The crystal structure of the GZN composite lm was determined via Xray diffraction (XRD) using Cu Ka radiation (XRD, Bruker D8). Raman spectra were collected using a Renishaw inVia Raman microscope, with excitation at room temperature using an excitation laser wavelength of 514 nm. The chemical states of the elements in graphene were analyzed using an X-ray photoelectron spectrometer (XPS, Thermo Fisher Scientic, K-Alpha), which was equipped with a monochromatic Mg Ka X-ray source (hn ¼ 1253.6 eV), with the anode operating at 14.0 kV and current emission of 18 mA. Data analysis was carried out using XPS Peak4.1 (the Chinese University of Hong Kong, China) with a variation of Gaussian (90-80%) AE Lorentzian (10-20%) proles. In order to understand the optical properties of GZN composite lm, an ultraviolet (UV)-visible spectrophotometer (TU-1901, China) was employed to record the total transmittance and absorption spectra from 250 to 800 nm.
Water contact angle investigations were performed using a contact angle system (XG-CAMC1, Shanghai Xuanyichuangxi Industrial Equipment Co., Ltd. China). Distilled water was used as the testing liquid. A 30 W UV lamp was used to irradiate the surfaces of graphene, ZnONRs and GZN composite lm. Contact angles were measured using 5 min time intervals and the data were recorded with a 60 s step.
The charge transfer resistance was determined using electrochemical impedance spectra (EIS), which were collected at the open circuit potential in the dark and under UV irradiation with a CHI660E (ChenHua, China). The amplitude of the sinusoidal wave was 5 mV and the frequency range examined was 100 kHz to 0.1 Hz. Electric resistance was measured using a resistance instrument for samples unexposed and exposed to 30 W UV irradiation.

Results and discussion
Morphological and microstructural characterization AFM images of the surface morphologies of the ZnONR array and GZN composite lm are shown in Fig. 2(a) and (b),  respectively. The surface roughness of GZN composite lm (9.9 nm) is lower than that of the ZnONR array (24.8 nm), as graphene nanosheets with two-dimensional (2D) structure bonded with the ZnONR array. Moreover, the ZnONR array reduced the high aspect ratio of the graphene nanosheets with 2D structure, which causes the formation of a crumpled surface structure in the graphene. The crumpled graphene could induce an arealdensity increase to obtain higher optical absorption per unit area. As reported in the literature, [30][31][32][33][34][35] graphene, as an acceptor and bridge for the ZnO semiconductor, can facilitate electron transfer and slow down the recombination of charge carriers; therefore, interactions between graphene and ZnO can greatly improve the photoresponsivity. Fig. 2(c) shows a top-view SEM image of GZN composite lm prepared via DGPSA methods.
The results indicate that ZnONRs with a rod diameter of around 140 nm grow on the surface of the quartz substrate. Simultaneously, it can been seen from the top-view image that the GZN composite lm is compact and uniform due to the high sputtering yield provided by the double-glow plasma discharge. In Fig. 2(d), an area-scanning energy spectrum of the GZN lm reveals the occurrence of carbon, zinc and oxygen peaks, which implies the existence of graphene and ZnO in the GZN lm. A typical cross-sectional SEM image of GZN composite lm is shown in Fig. 2(e). Well-aligned ZnONRs, with a length of 1.5 mm, are observed and the high-magnication images of the interface exhibit the good adhesion between the GZN lm and the quartz substrate. The ZnONRs can shorten the path of charge transfer and enhance the scattering of photons effectively. The cross-sectional element distribution in the GZN lm is analyzed using the line-scanning energy spectrum in Fig. 2(f). The distribution of carbon, zinc and oxygen elements shows similar trends to the substrate along the surface, from which the existence of graphene bonded into ZnONR arrays can be deduced. The XRD pattern of GZN composite lm is shown in Fig. 3(a). A sharp diffraction peak from crystalline ZnO, which is preferably grown along the (002) direction, at 2q ¼ 34.98 can be observed and the half width is only 0.236 , due to the high crystallinity of ZnO. This indicates that the ZnONRs are welloriented in the direction of the c-axis, which is perpendicular to the substrate surface. Such an orientation of ZnO is an acceptable point, since the (002) plane of ZnO has the lowest surface energy. 36,37 The enlarged image between 10 and 30 shows a broad diffraction peak at 22.50 ; this is interrelated with short-range order in the stacked graphene nanosheets. 38 Raman spectroscopy is employed to identify the crystallographic structures of GZN composite lm, especially to distinguish ordered and disordered carbon related structures, as shown in Fig. 3(b). The peak at 438 cm À1 is due to the E 2 vibrational mode, which is characteristic of wurtzite phase ZnO. 39 Simultaneously, the peaks at 1350 cm À1 and 1590 cm À1 correspond to the D and G bands of graphene. The G peak results from the presence of sp 2 carbon, whereas the D peak results from the presence of disorder in the graphene nanosheet structure. 40 XPS was performed to conrm the presence of graphene in the GZN composite lm. The XPS survey scan of GZN in Fig. 3(c) conrms the existence of carbon, oxygen and zinc elements, without other impurity elements. Fig. 3(d) presents the original and tted Zn 2p XPS spectra, obtained from the GZN composite lm. There are two typical strong peaks at 1021.5 eV and 1044.5 eV, corresponding to the doublet of Zn 2p 3/2 and Zn 2p 1/ 2 , respectively, which can be attributed to the existence of Zn-O bonds. In Fig. 3(e), we present C 1s spectra of GZN composite lm. The spectra are deconvoluted into three lines corresponding to C]C/C-C bonds centered at a binding energy of 284.8 eV, oxygen-containing functional groups (C-O/C-O-C) situated at 286.2 eV, and C]O/COOH groups at 288.9 eV. 41 Among these, the C]C/C-C bonds correspond to the graphitelike (sp 2 ) structure of the conjugated honeycomb lattice. Graphene is deposited onto the ZnONRs via electrostatic interaction energy, which can improve charge transfer at the interface. 42 The binding energies and relative area percentages for GZN composite lm are shown in Table 3. The XPS data results indicate that the intensity of the peaks corresponding to the oxygen functional groups conrms the presence of partial graphene oxide in the composite lm.  Table 3 XPS data for C 1s, including binding energies (eV) and relative area percentages (%) Effects of the introduction of graphene on the photoelectrochemical properties of GZN composite lm To investigate the optical response properties of GZN composite lm, UV-visible spectroscopy was employed. As can be seen from Fig. 4(a), the average optical transmittance of the ZnONR array with 1.5 mm thickness is nearly 95% in the visible-light region before the deposition of graphene nanosheets. Aer the deposition of graphene nanosheets, the total transmittance was maintained at nearly 75%, which conrms the average number of graphene nanosheets to be more than ve. 43 Compared with the ZnONR array, the decrease in transmittance in the visible-light region for the GZN composite lm is mainly a result of reection and absorption at the interface between the graphene nanosheets and ZnONRs. It can be seen from Fig. 5(b) that the absorption edge of the ZnONRs is at about 375 nm; no apparent absorption is observed in the visible-light region. Aer the introduction of graphene, the absorption edge of the GZN composite lm undergoes a red shi, and the absorption intensity increases signicantly at wavelengths greater than 375 nm, which might be attributed to the enhancement in the charge transport properties of the GZN composite lm. 44 UV-induced contact angles were used to analyze the effects of the optical response on the surface hydrophilic-hydrophobic properties of GZN composite lm. The relationships between the change in contact angle and the UV irradiation time for the ZnONR array, graphene lm and GZN composite lm are shown in Fig. 5. It can be seen that the contact angles for all three lms decrease upon prolonged UV irradiation. The obvious reduction in contact angle for GZN composite lm occurs from 82.4 to 51 , at a rate of 6.6 min À1 , as compared with that for the ZnONR array (2 min À1 ) and graphene lm (0.86 min À1 ). Under the conditions of being exposed to UV irradiation, ZnO generates pairs of electrons and holes, and the photogenerated electrons react with oxygen (O 2 ) to produce superoxide radical anions (O 2À ). Subsequently, the photogenerated holes react with water to produce hydroxyl (OH) radicals. In the process, oxygen atoms are ejected, creating oxygen vacancies. 45 The decrease in the contact angle under UV irradiation was attributed to the dissociative adsorption of water molecules on the photogenerated surface defect oxygen vacancy sites. 42 In the case of GZN composite lm, aer quickly moving through the graphene/ZnO interface, the photogenerated electrons transfer to the crumpled graphene surface and result in photochemical reactions. The rapid production of oxygen vacancies leads to the contact angle decreasing at a rapid rate. In other words, the introduction of crumpled graphene to GZN composite lm could provide an effective transmission channel for photogenerated electrons and weaken the recombination of charge carriers.
As has been reported, graphene is an excellent electron acceptor and transporter due to its 2D conjugation structure. 46 Graphene/ZnO interfacial charge transfer is a vital factor for its photoelectrical activity, which can be investigated via typical electrochemical impedance spectra (EIS). EIS of the ZnONR array, graphene lm and GZN composite lm in as-prepared photoanodes under ultraviolet irradiation are shown in Fig. 6(a). Semicircles in the curves in the EIS spectra reect interfacial impedance occurring at the surface of the electrode, and smaller semicircle curves reveal lower electrochemical impedance, which can help charge transfer easily. 26,47 It is observed that the arc radius for the GZN composite lm is much smaller than that for the ZnONR array and slightly smaller than that for the graphene lm, indicating that the synergistic effects of these two component units can reduce impedance in the   solid state interfacial layer and enhance charge transfer at the surface. Fortunately, the presence of graphene nanosheets can be benecial for the transport of photo-induced electrons, as a result of the excellent conductivity of graphene, as seen when comparing the plots of electrodes with and without graphene. Fig. 6(b)-(d) show the electric resistance of three lms both unexposed and exposed to UV irradiation. It can be noted that the electric resistance of the ZnONR array and graphene lm remains in the range of GU, while that of GZN composite lm greatly decreases to the range of kU. The decrease in the electric resistance of GZN composite lm depends on the formation of electrical interactions and interelectron transfer at the interface between the graphene nanosheets and ZnONR array. Furthermore, the electric resistances of all three types of lm are reduced aer ultraviolet irradiation, due to the fact that electron-hole pairs were generated and the density of charge carriers increased under UV excitation. 48

Conclusions
In summary, a novel method was developed to prepare largearea graphene/ZnO nanorod (GZN) composite lm using a simple two-step DGPSA process. The GZN composite lm exhibited obvious UV photoresponsive behavior; in particular, the introduction of graphene to ZnO nanorod lm has an obvious inuence on the photoelectrochemical properties of GZN composite lm. The photoelectrochemical properties of GZN composite lm under UV irradiation indicated that the graphene/ZnO interface can provide an effective transmission channel for photogenerated electrons and weaken the recombination of charge carriers. The synergistic effects of the ZnONR array and graphene nanosheets can reduce the interfacial impedance and enhance charge transfer. Therefore, GZN as a photoanode material could have potential applications in UV photoelectrochemical detection or solar cells. Furthermore, this work can be considered as a competitive candidate for providing methods for the large-area fabrication of graphene lm and GZN composite lms.

Conflicts of interest
There are no conicts to declare.