Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating

Nano Energy (]]]]) ], ]]]–]]]

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Available online at www.sciencedirect.com

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RAPID COMMUNICATION

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Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating

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Zhiming Baia, Xiaoqin Yana, Zhuo Kanga, Yaping Huc, Xiaohui Zhanga, Yue Zhanga,b,n a

State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 10083, PR China b Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 10083, PR China c School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, PR China

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KEYWORDS

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Photoelectrochemical; Water splitting; ZnO; ZnIn2S4; Graphene

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Abstract Developing photoanodes with high light-harvesting efficiency and great electronic transmission capacity remains a key challenge in photoelectrochemical (PEC) water splitting. In this paper, we reported an effective approach to enhance the PEC performance of ZnO nanowire arrays (NAs) photoanodes via overcoating ZnIn2S4 nanosheets onto the ZnO surfaces. The ZnIn2S4 electrocatalyst nanosheets were grown on the reduced graphene oxide (RGO) substrates by solvothermal synthesis and then grafted onto ZnO NAs, forming ZnO NAs/RGO/ZnIn2S4 heterojunctions. The ZnIn2S4 shells acted as visible light sensitizers, and the type-II band alignment between the ZnIn2S4 shells and the ZnO cores contributed to charge separation and transport. Meanwhile, the introduction of RGO nanosheets largely increased the surface area and accelerated the PEC process by reducing the energy barrier of interfacial electrochemical reaction. As a result, over 200% enhancement of photo-to-hydrogen conversion efficiency was achieved from the ZnO NAs/RGO/ZnIn2S4 heterojunctions compared to bare ZnO NAs. The results demonstrate that the RGO-based core/shell heterojunction arrays can provide a facile and compatible configuration for the potential applications in solar water splitting. & 2014 Elsevier Ltd. All rights reserved.

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Introduction

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Corresponding author at: State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 10083, PR China. Tel.: +86 10 62334725; fax: +86 10 62333113. E-mail address: [email protected] (Y. Zhang).

Solar to chemical energy conversion is the ultimate aim of researchers in the field of energy generation [1]. Through photosynthesis, plants use sunlight to convert carbon dioxide and water into carbohydrate and oxygen in nature. Motivated

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http://dx.doi.org/10.1016/j.nanoen.2014.09.005 2211-2855/& 2014 Elsevier Ltd. All rights reserved.

Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

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by this idea, hydrogen generation from solar water splitting, so-called artificial photosynthesis, has attracted considerable attention in scientific community [2–4]. Photoelectrochemical (PEC) cells emulate photosynthesis using man-made materials, such as semiconductors, to store light energy in hydrogen through water splitting reaction [5]. This process uses hugely abundant water as the precursor and is powered by the renewable solar energy [6], producing no greenhouse gases. In order to achieve efficient splitting of water, it is crucial to optimize the semiconductor photoanodes with long-term photochemical stability in aqueous solution, a wide spectrum of absorption, matching energy band to H2/O2 evolution potential and good crystallinity for efficient charge transfer [5,7,8]. Due to appropriate band gap, high carrier mobilities, good corrosion resistance and relatively low cost [9–11], semiconducting metal oxides such as TiO2 [12,13], Fe2O3 [14,15], WO3 [16,17] and ZnO [18,19] with various morphologies have been extensively investigated for application in PEC water splitting. As a typical photoanode material, ZnO is environmentally friendly and inexpensive, and has strong photogenerated hole oxidation ability [20]. In addition, the electron mobility of ZnO is 10–100 times higher than that of TiO2 [21], resulting in improved electron transfer efficiency and reduction of electron–hole recombination. One-dimensional ZnO nanomaterials, such as nanowire arrays, offer ideal geometrical architectures for solar water splitting apllications, because they have larger specific surface area, direct electrical pathways for rapid charge transport, improved optical absorption due to light scattering and trapping, as well as shortened photoinduced holes collection pathways perpendicular to the charge collecting substrate [9,22]. However, the ZnO photoelectrode is not photocatalytic in the visible region, due to the wide band gap (3.37 eV), which hinders its large-scale industrial applications. To solve these problems, considerable efforts have been made, such as doping [10,23,24], deposition of quantum dots [25,26] and construction of heterojunctions with other semiconductors [27,28]. Construction of a heterojunction between ZnO and other semiconductors with a suitable band gap is an effective method to extend the light absorption spectrum and accelerate photogenerated electron-hole separation, thus enhancing the solar-to-hydrogen conversion efficiency [29,30]. Zinc indium sulfide (ZnIn2S4) is a ternary chalcogenide semiconductor with a narrow band gap well corresponding to the visible spectrum and considerable photostability in aqueous solution under light irradiation [31,32]. As ZnIn2S4 is non-toxic and pollution-free, and not susceptible to damage from photocorrosion, it is a potential substitute for CdS. Various nanostructures of ZnIn2S4 such as nanosheet, nanowire and microsphere have been synthesized and investigated for solar water splitting [33–35]. Therefore, it is highly desirable to tailor the ZnO nanowire/ZnIn2S4 heterojunction arrays for improving the efficiency of energy conversion. Graphene, a monolayer of a two-dimensional carbon atomic sheet, has been widely reported to be an efficient cocatalyst for photocatalytic water splitting due to its extremely high electron mobility and very large specific surface area [35,36]. It can act as an electron mediator to inhibit photoinduced charge carriers recombination [37]. The in situ growth of semiconductor on reduced graphene oxide

(RGO) can provide a stronger interfacial contact, which facilitates electron transfer and the charge separation. Also, the large surface area of graphene enhances charge transport across the solid–liquid interfaces, allowing water redox reactions to occur at relatively low current densities [9]. The ZnO nanowire/RGO/ZnIn2S4 heterojunction arrays offer an optimal configuration for photoanodes of PEC cells, as it incorporates the merits of each composition. In this work, we present the design, synthesis and characterization of ZnO/RGO/ZnIn2S4 heterojunction arrays, with ZnO nanowire cores, RGO medium interlayers, and ZnIn2S4 shells that work as visible light sensitizers. Compared with ZnO NAs and ZnO NAs/RGO, the ZnO NAs/RGO/ZnIn2S4 heterojunctions exhibit superior PEC performance under light illumination. A built-in electric field on the interface between ZnO and ZnIn2S4 could significantly enhance the separation efficiency of the photoexcited electron–hole pairs, thereby increasing the photocurrent density. The introduction of RGO interlayer can not only benefit the charge transport between ZnO and ZnIn2S4, but also extend the surface area of the photoanodes, boosting photochemical reaction rates at the photoanode/electrolyte interface. The mechanism of the improved PEC performance of the ZnO/RGO/ZnIn2S4 heterostructure was systematically investigated.

Experimental section Preparation of ZnIn2S4/RGO and RGO GO sheets were purchased from Pula Nano-tech Ltd., Tianjin, China. ZnIn2S4/RGO nanocomposites were synthesized via a solvothermal reaction between Zn(NO3)2, InCl3 and thioacetamide (TAA) in the presence of GO. First, 5 mg GO was dispersed into a mixed solution that contained 15 mL ethanol and 5 mL glycerol under vigorous stirring for 1 h and then ultrasonication for 1 h. Then, 0.2 mmol Zn(NO3)2, 0.4 mmol InCl3 and 0.8 mmol TAA was added into the GO solution and ultrasonicated for 30 min. The resultant solution was transferred to a 50 mL Teflon liner, sealed in the stainless steel autoclave and heated at 180 1C for 6 h. After the autoclave was cooled to the room temperature, the product was collected by centrifugal separation and washed with ethanol three times. 10 mL ethanol was added to the collected product to get the ZnIn2S4/RGO suspension liquid. For comparison, bare RGO was prepared under the same experimental conditions.

Preparation of ZnO NAs/RGO/ZnIn2S4 and ZnO NAs/RGO The ZnO NAs were synthesized on the fluorine doped tin oxide (FTO) glass using a hydrothermal method detailed in a previous report [38]. The ZnIn2S4/RGO composites were deposited on the ZnO NAs by a dip-coating method. To strengthen adhesion between RGO sheets and ZnO nanowires, the ZnO NAs/RGO/ZnIn2S4 was baked at 400 1C for 30 min in an argon atmosphere. Then, the samples were immersed in anhydrous alcohol and irradiated by a UV lamp (254 nm, 0.5 mW/cm2) for 2 h to further reduce the RGO. The ZnO NAs/RGO samples were fabricated in the same way.

Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

Photoelectrochemical performance enhancement of ZnO photoanodes 1

Characterization

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Morphology analyses were conducted on a field emission scanning electron microscope (FESEM, FEI QUANTA 3D FESEM). The nanostructure and composition of the products were characterized using a transmission electron microscope (TEM, FEI Tecnai F30 S-TWIN) equipped with an energy dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) patterns were obtained on a Rigaku DMAX-RB using Cu Kα X-ray radiation source to determine the crystal phase of the obtained samples. Raman spectra were obtained using a Jobin-Yvon LabRAM HR800 with a 514 nm Ar ion laser. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra DLD with a monochromatic Al Kα source (1486.6 eV). The UV–vis absorption spectra were recorded on a UV––vis–NIR spectrophotometer (Varian Cary 5000).

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PEC measurements

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PEC characterizations were carried out using an electrochemical workstation (Solartron SI1287/SI 1260) under AM 1.5 G illumination provided by a solar simulator (Oriel, 91159 A, 100 mW/cm2). The as-prepared ZnO NAs based samples were made into working electrodes via placing copper wires onto a bare part of the FTO substrates and

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fixed with high purity silver paste. The electrodes were sealed on all edges with modified acrylate glue leaving an active area of
0.5 cm2. 1 M Na2SO4 aqueous solution purged with N2 was used as the electrolyte. An Ag/AgCl reference electrode (+ 0.19 V versus NHE) and a Pt wire counter electrode were employed during the measurement. The electrochemical impedance spectra (EIS) were recorded with a sinusoidal perturbation with 10 mV amplitude and frequencies ranging from 100 kHz to 0.1 Hz.

Results and discussion Figure 1(a) and the inset show the top- and cross-section view FESEM images of the as-grown ZnO NAs. Large-scale ZnO nanowires are vertically aligned on a FTO substrate. The average diameter of the nanowires is about 300 nm, and the mean length is about 3 μm. The nanorods with smooth surface have a hexagonal cross section. It can be seen from Figure 1(b) that the crumpled silk-like RGO sheets are attached to the top of the nanowires. Each piece of RGO is several micrometers in size, and almost completely transparent, implying that the RGO sheet is ultrathin [39]. Compared to Figure 1(b), we can see from Figure 1(c) that the ZnIn2S4 curled sheets evenly grew on the RGO layer. The RGO interlayer can expedite the charge carrier transfer

27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 Figure 1 SEM images of (a) ZnO NAs, (b) ZnO NAs/RGO, and (c) ZnO NAs/RGO/ZnIn2S4. Inset is the cross-sectional view of ZnO NAs. Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

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between ZnO nanowires and ZnIn2S4, resulting in suppression of charge recombination. TEM analysis was employed to further reveal the microstructure and composition of ZnIn2S4/RGO nanosheets. The ZnIn2S4 sheets with thickness of several nanometers tightly adhere to the RGO substrates (Figure 2(a)). The corresponding HRTEM image in Figure 2(b) clearly depicts the lattice fringes with a spacing of about 0.408 nm calculated from the line profile (inset of Figure 2(b)), which can be assigned to the (006) plane of the hexagonal ZnIn2S4 phase. The EDS analysis of the ZnIn2S4/RGO composite reveals the existence of the elements C, Zn, In, S and Cu (the signals of Cu originate from the underlying support). The phase and crystal structure of the obtained samples were examined by XRD. As shown in Figure 2(d), the XRD patterns reveal a typical XRD spectra of ZnO NAs on the FTO substrates with characteristic diffraction peaks [40]. The ultrahigh spike at 34.521, corresponding to ZnO (002) planes, indicates the ZnO NAs are highly c-axis oriented. After combination with ZnIn2S4/RGO, additional peaks appear at 21.51 and 27.81, which could be indexed to planes (006) and (102) of hexagonal ZnIn2S4 (JCPDS card no.72-0773). There are no typical diffraction peaks of RGO on the pattern due to its low amount and relatively low diffraction intensity [41].

As shown in the Raman spectra (Figure 3(a)), the G band corresponding to the sp2 hybridized carbon and the D band originating from defects in the hexagonal graphitic layers are observed for all samples [42]. The intensity ratios of G-to-D band are 1.12, 1.21 and 1.14 for ZnO NAs/GO, ZnO NAs/RGO and ZnO NAs/RGO/ZnIn2S4, respectively. In contrast to ZnO NAs/GO, the enhanced ratio for ZnO NAs/GO composites indicates that the GO has been reduced, to some extent, after solvothermal and photochemical processing. In addition, the G band shifts from 1597.5 cm  1 to 1590.9 cm  1 in ZnO NAs/RGO, as a result of the chemical interaction between the ZnO nanowires and the RGO sheets [37]. For the ZnO NAs/ RGO/ZnIn2S4, the intensity ratio becomes lower than that of ZnO NAs/RGO, which is regarded as a reduction in the size of sp2 carbon domains and the formation of chemical bond between ZnIn2S4 nanosheets and RGO substrates [43]. The results provide evidence that the charge can efficiently transfer between ZnO and ZnIn2S4 via the RGO medium layer. The valence states of the as-prepared products were further characterized by XPS measurement. For both ZnO NAs/GO and ZnO NAs/RGO/ZnIn2S4, the characteristic peaks of Zn2p, O1s and C1s have been observed in Figure 3(b). Additional XPS signals appears at 161.4 eV (S2p), 444.3 eV (In3d5/2), 451.4 eV (In3d3/2) and 665.4 eV (In3p3/2), which can be

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Figure 2 TEM (a) and HRTEM (b) images, and EDX spectrum (c) of ZnIn2S4/RGO. (d) XRD pattern of ZnO NAs and ZnO NAs/RGO/ ZnIn2S4. The inset of (b) is the corresponding intensity profile for the line scan across the lattice fringes. Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

Photoelectrochemical performance enhancement of ZnO photoanodes

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Figure 3 (a) Raman spectra of ZnO NAs/GO, ZnO NAs/RGO, and ZnO NAs/RGO/ZnIn2S4. (b) Survey spectra of ZnO NAs/GO and ZnO NAs/RGO/ZnIn2S4. C1s XPS spectra of (c) ZnO NAs/GO, and (d) ZnO NAs/RGO/ZnIn2S4.

assigned to S2 and In3 + . High-resolution XPS spectra of C1s region are presented in Figure 3(c) and (d). The binding energy of 284.8 eV is consistent with the sp2 carbon band (C–C, CQC and C–H). The deconvoluted peaks centered at the binding energies of 286.5, 288 and 289.4 eV are assigned to the epoxy/hydroxyls (C–O), carbonyls (CQO) and carboxyl (OQC–OH) functional groups, respectively [43]. After reduction process, the peaks for the oxygencontaining functional groups are much lower than these in GO; the corresponding oxygen content decrease from the original of 55–27% in ZnO NAs/RGO/ZnIn2S4. The results reveal that the graphene in the prepared composite has a very high degree of reduction, which is in good agreement with the Raman analysis. The substantially deoxygenation will increase the conductivity of RGO interlayers, thereby enhancing the separation efficiency of photoinduced charges during the photocatalytic process. The optical properties of the prepared samples were analyzed by the UV–vis diffuse reflectance spectra (Figure 4). For the pure ZnO NAs, a steep UV absorption edge occurs at
388 nm, consistent with the band gap of ZnO (3.37 eV). When ZnO nanowires are coated with RGO sheets, the optical absorption intensity increases in both UV and visible light regions. Also, there is a redshift of absorption edge in UV range, as a result of the formation of

Figure 4 UV–vis diffuse reflectance spectra of (a) ZnO NAs, (b) ZnO NAs/RGO, and (c) ZnO NAs/RGO/ZnIn2S4.

the Zn–O–C bond between ZnO and RGO [44], which is similar to the case of carbon-doping ZnO nanotetrapods [2]. It can be seen that the ZnO/RGO/ZnIn2S4 heterojunction arrays show a broad absorption band centered at
500 nm, corresponding well with the band gap of ZnIn2S4 (
2.5 eV) [33]. The results exhibit that the ZnO NAs

Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

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coupling with RGO interlayers and ZnIn2S4 nanosheets, could have a better sunlight harvesting capacity, and then an enhanced PEC water splitting performance. Electrochemical measurements were carried out to evaluate the PEC performance of the fabricated samples. The photocurrent densities versus applied voltage of the asprepared photocatalysts were recorded in the dark and with illumination of 100 W/cm2 (Figure 5(a)). Dark scan linear sweep voltammagrams from  0.5 to+1.5 V shows a small current density in the range of 10  6 A/cm2. Upon illumination the pure ZnO NAs exhibited an enhanced photocurrent density over the entire potential range and reached the maximum value of 1.09 mA/cm2 at +1.5 V versus Ag/AgCl. In contrast, the ZnO NAs/RGO showed an increase in photoresponse with a photocurrent density of 1.43 mA/cm2 at the same applied potential. The improvement in the photocurrent density can be attributed to the fact that the introduction of RGO extends the surface area of the photoanodes, and then promotes the water oxidation reaction at the ZnO/electrolyte interfaces due to its high special surface area [45]. In addition, because of its extraordinary electron transport property, the RGO sheets effectively improve the transport of electrons from ZnO nanowires to the FTO substrate so that the electron–hole recombination rate is greatly reduced [30]. ZnO NAs loaded with RGO/ZnIn2S4 composites show the highest photocurrent 2 density of 2.25 mA/cm , which is 2-fold and 1.6-fold that of

ZnO NAs and ZnO NAs/RGO, respectively. Higher solar light harvesting efficiency of ZnIn2S4 nanosheets is responsible for such enhancement in the PEC performance. Interestingly, no saturation of photocurrent was observed in the whole potential scan range, indicating efficient charge separation in the ZnO NAs/RGO/ZnIn2S4 heterojunction. The solar-to-hydrogen conversion efficiency (η) for a PEC cell can be calculated by the equation [46]
Jp 1:23 V app η¼ P light where Jp is the photocurrent density, Vapp is the applied potential versus RHE, and Plight is the power density of the illumination. The maximum efficiency of 0.15% was observed for the pristine ZnO NAs at +0.25 V versus Ag/AgCl (Figure 5(b)). Under the same conditions, the ZnO NAs/RGO sample has a maximum efficiency of 0.22% at +0.24 V versus Ag/ACl. For the ZnO NAs/RGO/ZnIn2S4 heterojunction, the highest efficiency (0.46% at +0.18 V versus Ag/AgCl) is about 200% and 109% higher than that of the ZnO NAs and ZnO NAs/RGO, respectively. The photoconversion efficiency for the heterojunction in this study is also larger than the previously reported values for undoped [47] and Al-doped ZnO nanowires [48]. To examine the photoresponse of the photoanodes over time, the transient photocurrents were recorded at a fixed potential of +0.5 V versus Ag/AgCl with light on/off cycles at

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Figure 5 (a) Variation of current density versus applied potential, (b) photoconversion efficiency as a function of applied potential, and (c) transient current densities versus time of ZnO NAs, ZnO NAs/RGO and ZnO NAs/RGO/ZnIn2S4 in the dark and under AM 1.5 G illumination (100 mW/cm2). Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

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100 mW/cm2 (see Figure 5(c)). Under illumination, there is a spike in photocurrent for all photoanodes, because of the transient effect in power irradiation, and then the photocurrent quickly returns to a steady state, due to the formation of a diffusion layer at the solution and electrode interface [10,49]. The rise and reset time are both less than 400 ms, which are much faster than the response time of the ZnO nanowires based photodetectors, suggesting that oxygen-related photoconductive response mechanism stops working [50]. The ZnO NAs/ RGO/ZnIn2S4 exhibited a photocurrent density of 1.41 mA/cm2, which was higher than those of ZnO NAs/RGO (0.85 mA/cm2) and ZnO NAs/RGO/ZnIn2S4 (0.60 mA/cm2). EIS measurements were performed to characterize the intrinsic electronic and charge transport properties of the ZnO NAs based photoelectrodes without and with light illumination. The arc radii of the EIS spectra in both Figure 6(a) and (b) are decreased step-by-step from ZnO NAs, ZnO NAs/RGO to ZnO NAs/RGO/ZnIn2S4. Besides, due to the enhancement in the electron conductivity of the photoeletrodes, the arc radii under irradiation are two orders of magnitude less power than those in the dark. After loading RGO, the arc radius was much reduced compared with that of ZnO NAs, indicating that the RGO sheets significantly improved the interfacial charge transfer velocity and lowered down the interfacial reaction resistance [20]. This phenomenon may be attributed to the

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formation of the space electric field at RGO/ZnO interfaces, which speeds up the photogenerated electron–hole pair separation. On the other hand, the RGO loading can improve the electron transfer at interfaces, and reduce the potential barrier of interfacial electrochemical reaction, owing to its excellent electrical conductivity and huge ratio surface. The ZnO NAs/RGO/ZnIn2S4 heterostructure has the smallest radius of arc, suggesting a higher electron-hole pair separation efficiency and faster interfacial charge transport. Based on the data obtained above, the mechanism of the enhancement in the PEC properties of the ZnO NAs/RGO/ ZnIn2S4 was proposed as displayed in Figure 7. Upon irradiation, a large number of free electrons and holes are generated in the conduction band (CB) and the valence band (VB) of ZnO and ZnIn2S4. The type-II band alignment of the heterostructure is beneficial for charge separation and transport [51]. Furthermore, the photogenerated electrons in the CB of ZnIn2S4 can more easily transfer to the CB of ZnO via the RGO media layer, minimizing charge recombination. Then, the electrons finally migrate to the Pt counter electrode and participate in the reduction of water to form H2 under the applied electric field. Meanwhile, the holes left on the VB of ZnO would flow to the VB of ZnIn2S4 through the RGO sheets, and be eliminated by oxidizing water. So, the synergistic effects of the ZnO/ZnIn2S4 type-II band alignment and the introduction of RGO sheets conduce to the highly efficient PEC water splitting.

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Figure 6 EIS Nyquist plots of ZnO NAs, ZnO NAs/RGO and ZnO NAs/RGO/ZnIn2S4: (a) in the dark, and (b) under AM 1.5 G illumination (100 mW/cm2).

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Figure 7 Schematic of the energy band structure of the ZnO NAs/RGO/ZnIn2S4 heterojunction and proposed mechanism for the improvement of the PEC performance. Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

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Conclusion

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In summary, we have successfully synthesized a novel ZnO NAs/RGO/ZnIn2S4 heterojunction for PEC water splitting. ZnO nanowires work as core materials, RGO sheets serve as the charge transfer interlayer, and ZnIn2S4 acts as a visible light sensitizer. In contrast to the pure ZnO NAs and the ZnO NAs/RGO, the ZnO NAs/RGO/ZnIn2S4 heterojunction reaches a maximum photoconversion efficiency of 0.46%. The ZnO/ ZnIn2S4 type-II band alignment benefits the photogenerated electron–hole pair separation and visible light adsorption. Also, the RGO sheets provide quick and effective passageways for photogenerated charges, effectively reducing the charge recombination rate. This method has great advantages for preparing PEC cells, and may be extended for the preparation of other photovoltaic devices in the future.

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This work was supported by the National Major Research Program of China (2013CB932602), the Major Project of International Cooperation and Exchanges (2012DFA50990), the ProQ4 gram of Introducing Talents of Discipline to Universities, the NSFC (51232001, 51172022, 51372023, 51372020), the Research Fund of Co-construction Program from Beijing Municipal Commission of Education, the Fundamental Research Funds for the Central Universities, the Program for Changjiang Scholars and Innovative Research Team in University.

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Zhiming Bai is a Ph.D. candidate under the supervision of Prof. Yue Zhang at the School of Materials Science and Engineering in University of Science and Technology Beijing. His research focuses on fabrication of onedimensional nanomaterials for photoelectrochemical water splitting and self-powered photodetectors.

Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005

Photoelectrochemical performance enhancement of ZnO photoanodes 1 3 5 7 9

Xiaoqin Yan received his Ph.D. degree in Physics from Institute of Physics of the Chinese Academy of Sciences in 2004. She was funded by JSPS and did postdoctoral research in Tohoku University in 2004–2007. She is currently a professor in Department of Materials Physics and Chemistry of University of Science and Technology Beijing. Her research interests are focused design and synthesis of semiconducting nanomaterials and photoelectric devices based on one-dimensional nanomaterials.

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Zhuo Kang is a Ph.D. candidate from Prof. Yue Zhang's group in School of Material Science and Engineering at University of Science and Technology Beijing. His research interest mainly focus on nanomaterial based enzyme biosensors and photoelectrochemical biosensors. He received his B.Sc. in Material Physics from University of Science and Technology Beijing in 2011.

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Yaping Hu is a master student in Guilin University of Technology majored in materials science and engineering. She currently does research works in University of Science and Technology Beijing as a co-cultivate postgraduate. Her research interests mainly focus on the synthesis of highly ordered ZnO nanomaterials for photoelectrochemical water splitting.

9 Xiaohui Zhang received her B.Sc. and M.Sc. degree from University of Science and Technology Beijing (USTB), in 2006 and 2009. She is currently a Ph.D. candidate under the supervision of Prof. Yue Zhang at USTB. Her scientific interests focus on design of biosensors based on nanomaterials and research on the interaction between biomolecules and nanomaterials.

Yue Zhang, Ph.D., professor, vice-president of University of Science and Technology Beijing. He has been awarded the financial support for outstanding young scientist foundation of China. His most recent research focuses on ZnO nanowires and nanobelts, in-situ techniques for nanoscale measurements, self-assembly nanostructures, nanodevices and nanodamages. He published about 200 papers in peer reviewed scientific journals, and more than 1000 times cited by others. He coauthorized 6 books, applied over 30 patents, and has been approved over 15 patents.

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Please cite this article as: Z. Bai, et al., Photoelectrochemical performance enhancement of ZnO photoanodes from ZnIn2S4 nanosheets coating, Nano Energy (2014), http://dx.doi.org/10.1016/j.nanoen.2014.09.005