Graphene Photocathode for Efficient

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P-Type Cu-Doped Zn0.3Cd0.7S/Graphene Photocathode for Efficient Water Splitting in Photoelectrochemical Tandem Cell Yijie Wu, Zongkuan Yue, Aijuan Liu, Ping Yang, and Mingshan Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01795 • Publication Date (Web): 06 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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ACS Sustainable Chemistry & Engineering

P-Type Cu-Doped Zn0.3Cd0.7S/Graphene Photocathode for Efficient Water Splitting in Photoelectrochemical Tandem Cell Yijie Wu †§, Zongkuan Yue †§, Aijuan Liu,† Ping Yang*,†, and Mingshan Zhu*,‡ †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China ‡

Department Chemistry, University of Toronto, Toronto M5S 3H6, Canada

ABSTRACT: By doping Cu(I) ions in Zn0.3Cd0.7S, a novel p-type Cu doped Zn0.3Cd0.7S modified graphene (Zn0.3Cd0.7S (Cu)/GR) film photocathode was prepared. The as-prepared ptype Zn0.3Cd0.7S (Cu)/GR photocathode and an n-type WO3/graphene (WO3/GR) photoanode were used to assemble a photoelectrochemical tandem cell. Through examination of the optoelectronic and photoelectrochemical properties of Zn0.3Cd0.7S (Cu)/GR and WO3/GR photoelectrode, we evaluate the feasibility of the tandem cell for overall water splitting under UV-vis light irradiation. The optimal Cu doping in Zn0.3Cd0.7S photocathode concentration was found to be 6%. The rates of hydrogen and oxygen evolved from this tandem cell with the optimal electrodes were 65.6 and 12.3 µmol g-1 h-1 (80.5 and 15.1 µmol cm-2 h-1), respectively. This study suggests a promising method for constructing an efficient photoelectrochemical tandem device for overall water splitting. KEYWORDS: Cu-doped Zn0.3Cd0.7S; photoelectrochemical tandem cell; overall water splitting

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INTRODUCTION Since water splitting system based on TiO2 electrode was first discovered by Fujishima et. al in 1972,1 semiconductor-based photocatalytic systems have gained popularity.2,3 Many efforts have been directed to the development of new types powdered photocatalysts for water splitting.4-6 However, gas separation is necessary considering the issue of practical application and safety hazard. The photoelectrochemical (PEC) system, of which a photoanode and a photocathode are connected in tandem via external circuit, has been applied widely for photocatalytic water splitting to produce hydrogen and oxygen, photocatalytic reduction carbon dioxide to generate hydrocarbon fuels, et al.4-9 Usually, the photoanode needs n-type semiconductor, on which the photoexcited holes accumulate and oxidation reactions occur, while the photocathode needs p-type semiconductor, on which the photoexcited electrons accumulate and reduction reactions take place for hydrogen evolution. To date, various n-type semiconductor photoanodes, such as TiO2, WO3, etc.10-14, have been investigated for overall water splitting. Among various n-type catalysts, WO3 is regarded as an active photocatalyst for oxygen evolution under visible light irradiation, though its band gap (2.6~2.8 eV) is still too large to cover the whole visible region.11-14 On the other hand, p-type semiconductors, such as GaP, WS2, WSe2, Cu2O, InP and GaInP2, used as photocathode have also been intensively investigated.15-17 ZnxCd1-xS solid solution has been regarded as an efficient hydrogen-generation photocatalyst since it was first reported as a photocatalyst.18-21 Although the intrinsic property of ZnxCd1-xS is a n-type semiconductor,22-24 the conductivity type of semiconductors can be transformed when other elements are introduced in the body of the semiconductors.25-29 For example, doping n-type Fe2O3 with Zn can obtain p-type Fe2O324 and ntype CdS turns to be a p-type semiconductor when Cu element is doped in29.


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Herein, Cu(I) doped Zn0.3Cd0.7S semiconductor was prepared by a facile hydrothermal Cu(I)-doping method. We discover that traditional n-type ZnxCd1-xS turns to be a p-type semiconductor when Cu(I) ions are doped in. Based on this point, we report a PEC tandem cell composed of a p-type Zn0.3Cd0.7S electrode as photocathode and an n-type WO3 electrode as photoanode for overall water splitting. H2 and O2 generated separately from each electrode under UV-vis irradiation demonstrates the efficiency of the as-assembled PEC cell for photocatalytic water splitting. With optimal doping quantity of Cu(I), the rates of hydrogen evolved from Zn0.3Cd0.7S (6, Cu) electrode and oxygen from WO3 electrode were 19.5 and 2.9 µmol g-1 h-1 (24 and 3.5 µmol cm-2 h-1), respectively. Moreover, by hybridization of graphene, the activities of the PEC cell were further enhanced. This investigation might provide a simple paradigm for realizing photocatalytic overall water splitting to produce H2 and O2 separately. EXPERIMENTAL SECTION Materials. Graphene was purchased from XFNANO Materials Company, and all other chemicals were purchased from the J&K Company and were used without further purification. Synthesis of WO3, Cu(I) doped Zn0.3Cd0.7S, and corresponding graphene hybrid nanocomposites. The WO3 sample used for preparing photoanode was synthesized by precipitation technique from aqueous solutions of ammonium tungstate parapentahydrate ((NH4)10W12O41·5H2O) and nitric acid (HNO3).30 A pre-determined amount of the tungstate salt was dissolved in deionized water and the resulting solution was heated to 80 °C under vigorous stirring. And then pre-warmed concentrated nitric acid was added dropwise into the above solution. The mixture solution was kept at 80 °C for 30-60 min. After that, the mixture was kept at room temperature for one day to let the formed solid settle. After filtration, the obtained solid



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was washed by deionized water several times, dried at 100 °C overnight and then calcined in air at 400 °C for 4 h. The p-type Cu doped Zn0.3Cd0.7S was synthesized by a hydrothermal treatment. In a typical experiment, 0.67 g (2.5×10-3 mol) cadmium acetate and 0.23 g (1.07×10-3 mol) zinc acetate were dissolved in 40 mL of deionized water with 5 mL triethanolamine (TEA, 3.5 M) and 5 mL NH4OH (14.4 M), then 5 mL thiourea ([(NH2)2CS], 1 M) solution were added dropwise into the above solution under magnetic stirring. After that various quantities of 5 mM CuCl suspension were added into the above solution under magnetic stirring. The mixture was transferred into a 100 mL Teflon-lined stainless autoclave and then heated at 180 °C for 10 h. The obtained solid was washed with deionized water several times, followed by drying in air at 60 °C. The asprepared sample is designated as Zn0.3Cd0.7S (x, Cu), where x stands for percent atomic ratio of Cu to Cd+Zn. The hybridization of WO3 or Zn0.3Cd0.7S (Cu) with graphene (GR) was carried out according to a self-assembly process. Typically, 100 mg of WO3 or Zn0.3Cd0.7S (Cu) powders and 2 mL of graphene colloidal suspension (1 mg mL-1) were added into 48 mL of deionized water under ultrasonic agitation for 24 h. After that, the solid was collected by centrifugation, washed with deionized water several times, and dried at 50 °C in a vacuum oven. The as-prepared sample is designated as WO3/GR or Zn0.3Cd0.7S (Cu)/GR. Photoelectrode preparation. The photoelectrodes were prepared by spreading a portion of viscous suspension of WO3/GR or Zn0.3Cd0.7S (Cu)/GR onto the silver substrates about 7 cm in diameter (Φ=7 cm) with Teflon coating on the back. While for photoelectrochemical analyses, including the linear sweep voltammetry, Mott-Schottky plots, electrochemical impedance spectroscopy (EIS), the photoelectrodes were prepared by spreading a portion of viscous


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suspension of WO3/GR or Zn0.3Cd0.7S (Cu)/GR onto an indium tin oxide (ITO) conductive glass. The suspension was made by grinding the photocatalyst powders (10 mg) with 2 mL of trichloromethane - ethylene glycol - ethanol - PVP (10 mL : 15 mL :75 mL : 30 mg) solution in a mortar for 10 min. The amount of the material deposited the electrode was optimized, which was ca. 8 mg and 6 mg in an area of ca. 6 cm2 for photoanode and photocathode, respectively. The electrode with deposited layer was dried on a heating plate at 40 °C for 30 min and then calcined at 400 °C in Ar atmosphere for 2 h. Characterization. The X-ray diffraction (XRD) patterns were measured on a Philips diffractometer equipped with Ni-filtered Cu Kα radiation. Scanning electron microscopy (SEM) measurements were performed on SEM FEI Quanta 250 FEG microscope. X-ray photoelectron spectroscopy (XPS) measurements were taken by an AXIS Ultra DLD system (Kratos Analytical Inc.) using monochromatic Al Kα radiation. Raman measurements were performed on a JobinYvon HR-800 spectrometer (λ = 633 nm, spectral resolution < 0.4 cm-1). The UV-vis diffuse reflectance spectrometry was accomplished on a Hitachi UV-3010 spectrophotometer using BaSO4 as a reference. The linear sweep voltammetry, Mott-Schottky plots, photocurrent responses, Nyquist plots of the samples were carried out on a CHI660D potentiostat/galvanostat electrochemical analyzer in a three-electrode system consisting of an ITO conductive glass covered with one of the as-prepared photocatalyst as the working electrode, a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The photocurrent-time test under chopped light (light on/off cycles: 30 s) was carried out at a fixed potential of 0 V (vs. RHE). The electrodes were immersed in 0.1 M Na2SO4 aqueous solution (pH ~ 6) and the working electrode was irradiated with a 150 W xenon lamp during the measurement. The electrochemical impedance spectra (EIS) was measured on the same system



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in the frequency range of 1 to 105 Hz under a perturbation signal of 5 mV in 0.1 M Na2SO4 solution with 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] mixture as a redox probe. The Mott-Schottky (MS) plots were measured with a frequency of 1000 Hz and amplitude of 5 mV. The flat band potential of the semiconductors was calculated by the Mott-Schottky relation:31 n-type:

1 2 = 2 C eεε 0 N d

kT   ×  E − E fb −  e  

(1)

p-type:

1 2 = 2 C eεε 0 N a

kT   ×  − E + E fb −  e  

(2)

where C is the capacitance of the electrode, e is the elementary charge, ε is the dielectric constant of the electrode material, ε0 is permittivity of vacuum, E is the applied potential, k is Boltzmann constant and T is the temperature. The potential of flat band (Efb) is determined by fitting the linear of the plot to the intercept of the potential axis. And the donor density Nd and the acceptor Na can be determined from the slope of the linear region. The width of the space charge layer (W) for the p-type semiconductor can be estimated from eqn. 3: 1

 2εε 0 ( E − E fb )  2 W = −  e Nd  

(3)

For converting the measured potential vs. SCE to the potential vs. reversible hydrogen electrode (RHE), the following Nernst equation was used:32 o ERHE = ESCE + 0.059 pH + ESCE

(4)

where ERHE is the potential vs. RHE, ESCE is the measured potential vs. SCE, and E°SCE = 0.2415 V at 25 °C.

Photocatalytic reaction. The photocatalytic reaction was carried out in a self-made threeelectrode PEC device (Scheme 1). The silver substrates covered with p-type Zn0.3Cd0.7S (Cu)/GR


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or n-type WO3/GR film was used as photocathode or photoanode, respectively, and a SCE as a reference electrode. The electrodes are connected via a potentiostat under short circuit. The system was deaerated by bubbling argon into the 0.1 M Na2SO4 solution for 1 h before the reaction. The electrodes were irradiated by GY-10 xenon lamps (150 W) at 25 °C and atmospheric pressure. The distance between the flat optical window of the flask and the lamp was 10 cm. The gases evolved during the reaction were analyzed by an online gas chromatograph (GC1690) equipped with a thermal conductivity detector and 5 Å molecular sieve columns using argon as a carrier gas. The external quantum yield (QY) was measured by using GY-10 xenon lamp (150 W) with a 365 nm band-pass filter as light sources. The focused light intensity on the reactor was measured by using a UV radiometer (Model: UV-A, Photoelectric Instrument Factory of Beijing Normal University, China). The incident light intensity at 365 nm and active area on the electrodes was ca. 7.0 mW cm-2 and 1 cm2, respectively. The external quantum yield (QY) of hydrogen is defined by the eqn. 5:

Number of reaction electrons Number of inicient photons 2 × nH 2 = × 100% I0 × t

Quantum yield [QY , %] =


(5)


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Scheme 1. The schematic diagram of the PEC device composed of p-type Zn0.3Cd0.7S (Cu)/GR and n-type WO3/GR nanostructures electrodes.

RESULTS AND DISCUSSION The crystal structure and the phase purity of the as-prepared samples are analyzed by X-ray diffraction and the results are shown in Figure 1. For the WO3 sample (Figure 1, curve a), all of its diffraction peaks can be indexed to the standard spectrum (JCPDS card: NO.43-1035),30 indicating that WO3 is well prepared. For GR modified WO3, no apparent diffraction peaks representing GR can be identified (Figure 1, curve b), which may be due to the low content GR in the composite.33 The main strong peaks in Zn0.3Cd0.7S (Figure 1, curve c) are also match with those reported in the literature,23,24 confirming that the Zn0.3Cd0.7S solid solution was formed. However, for the Cu and GR modified samples (Figure 1, curve d and e), no apparent diffraction peaks corresponding to Cu and GR can be indexed, which may also attributed to the low content and high dispersion of Cu and GR in the composite.


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Figure 1. XRD patterns of (a) WO3, (b) WO3/GR, (c) Zn0.3Cd0.7S, (d) Zn0.3Cd0.7S (6, Cu), (e) Zn0.3Cd0.7S (6, Cu)/GR and the XRD standard data of Cu2S (f) for comparison. The surface structure and surface morphology of the films on the surface of the Ag substrate are investigated by SEM and the results are shown in Figure 2. It can be seen that both the films of Zn0.3Cd0.7S (6, Cu) and WO3 are composed of irregular particles. The particle sizes are in the range of 1 ~ 3 µm for Zn0.3Cd0.7S (Cu) and of 0.8 ~ 1.5 µm for WO3, respectively. Moreover, it also can be observed that the Zn0.3Cd0.7S (6, Cu) or WO3 have homogeneously anchored on the GR sheets, and the two-dimensional structures of GR sheets with obvious wrinkles were still retained after the ultrasonic treatment. The TEM and AFM images of graphene used for fabricating the composites are displayed in Figure S1. The thickness of the carbon material is around 2.3 nm, corresponding to a ca. 3 to 5 layer graphene flake. In order to investigate whether or not the Cu dopant was uniformly distributed throughout the material, more than three different areas of the Zn0.3Cd0.7S (6, Cu) sample were examined by SEM-EDX microscopy. Figure S2 presents the SEM-EDX image of a random selected area and elemental distribution mapping of Cu of the Zn0.3Cd0.7S (6, Cu) sample. The concentration 9 Environment ACS Paragon Plus


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mapping of Cu demonstrated that the Cu element was uniformly distributed throughout the material.

Figure 2. SEM images of (A) Zn0.3Cd0.7S (6, Cu)/GR and (B) WO3/GR films.


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Figure 3. (A) XPS survey spectra of Zn0.3Cd0.7S (6, Cu). (B) XPS spectrum of Cu in Zn0.3Cd0.7S (Cu). XPS spectra of Zn (C) and Cd (D) in (a) Zn0.3Cd0.7S and (b) Zn0.3Cd0.7S (6, Cu). XPS technology was applied to reveal the chemical composition of Zn0.3Cd0.7S (6, Cu) nanostructures. The survey spectra (Figure 3A) of Zn0.3Cd0.7S (6, Cu) show the presence of Zn, Cd, S and Cu, while some small peaks attributing to C and O from CO2 absorbed on the surface of the sample before characterization and adventitious hydrocarbon of the XPS instrument. The binding energy values of the Cu 2p3/2 and Cu 2p1/2 are 931.3 eV and 951.1 eV with a peak separation of 19.8 eV (Figure 3B). These values are quite agreeable to the ones of pure Cu2S (931.5 and 951.3 eV, Figure S3 D), suggest that the Cu element may be present in the composite as the Cu+ state.34 However, since the binding energy of Cu (0) is at 932-933 eV, the metal Cu in Zn0.3Cd0.7S (6, Cu) could not be excluded. Besides, the Zn 2p (Figure 3C, curve a) peaks in undoped Zn0.3Cd0.7S centered at 1021.9 eV (Zn 2p3/2) and 1044.9 eV (Zn 2p1/2) are matching well with Zn2+. Cd 3d (Figure 3D, curve a) peaks centered at 405.0 eV (Cd 3d5/2) and 411.8 eV (Cd 3d3/2) with a peak separation of 6.8 eV are characteristics of Cd2+ in Zn0.3Cd0.7S.16 It is noteworthy to note that Cu doping remarkably shifts the binding energies of both Zn and Cd (curve b of Figure 3C and 3D), which may be attributed to the interaction of Cu, Zn and Cd in Zn0.3Cd0.7S (6, Cu). The Cu doping may also induce a shift of the Fermi level of Zn0.3Cd0.7S to more negative potentials, which might improve the energetics of the composite system and enhance the efficiency of interfacial charge transfer process.35 The similar observation was also reported for p-type Ga doped SnO2 with strong interactions between the two components.21 For the graphene incorporation samples, XPS spectra demonstrate that the positions of the peaks corresponding to Cd and Zn display 0.3 eV shift to higher binding energy compared with those of the Zn0.3Cd0.7S (6, Cu) nanoparticles; while the positions of the peaks corresponding to W



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have 0.2 eV shift to higher binding energy compared with those of WO3 (Figure S3). These facts may suggest that the semiconductors and graphene in the composite have formed some kind of chemical bonds (M-O-C).36,37

Figure 4. Raman spectra of (a) GR, (b) Zn0.3Cd0.7S (6, Cu)/GR and (c) WO3/GR nanostructures. Insert: the Raman spectra extended to 3200 cm-1 of GR. The interaction between Zn0.3Cd0.7S (6, Cu) (or WO3) and graphene was also confirmed by the Raman spectral analysis (Figure 4). Graphene displays two prominent peaks centered at 1340 cm-1 (D-band) and 1585 cm-1 (G-band), which are attributed to the defect-induced vibration of CC bonds and the vibration of sp2 C-C bonds, respectively.38 The 2D band of as-prepared graphene-including sample centered at 2679 cm-1 may further confirm that the carbon-based material in the composite is graphene (Inset of Figure 4). Moreover, the positions of the G-band


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of Zn0.3Cd0.7S (6, Cu)/GR (curve b) and WO3/GR (curve c) exhibit slight blue shift, which further suggests an interaction between semiconductors and graphene in the composites.39

Figure 5. UV-vis diffuse reflectance spectra and photos of (a) WO3, (b) Zn0.3Cd0.7S, (c) Zn0.3Cd0.7S (6, Cu), (d) WO3/GR and (e) Zn0.3Cd0.7S (6, Cu)/GR. Inset: the images of Zn0.3Cd0.7S and Zn0.3Cd0.7S (6, Cu). The UV-vis diffuse reflectance spectra (DRS) of samples are shown in Figure 5. The WO3 (curve a) demonstrates a strong absorption in the UV-vis range up to 460 nm and Zn0.3Cd0.7S shows an absorption from ca. 300 nm to 530 nm (curve b). It can be seen that the absorption band-edge of Zn0.3Cd0.7S (Cu) (curve c) red shifts in visible light region. The color of the asprepared samples changes from yellow (Zn0.3Cd0.7S) to brown (Zn0.3Cd0.7S (6, Cu)) (inset in Figure 5). The reduction of the band gap of the hybrid can be attribute to forming a new energy level in the band structure of Zn0.3Cd0.7S since chemical bonds between Zn0.3Cd0.7S and Cu(I) may have created.40 The analogous phenomenon has been observed in Cu doped ZnO system.41 Moreover, graphene included composites (curve d and e) show the enhanced absorption in whole



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region due to the introduction of graphene extends the broad background absorption into the visible-light region, indicating more efficient utilization of the incident light. The similar phenomenon also reported in other graphene composite studies.42-44

Figure 6. (A) The linear sweep voltammetry of Zn0.3Cd0.7S (6, Cu). (B) The Mott-Schottky plot of Zn0.3Cd0.7S (6, Cu). The linear scan voltammogram was performed to investigate the photoelectrochemical properties of the as-prepared samples. For simplicity, only the results of Cu doped Zn0.3Cd0.7S are shown in Figure 6A and the results of the other samples are demonstrated in supplementary information (Figure S4). Compared to the dark current, the Cu doped Zn0.3Cd0.7S electrode demonstrates enhanced cathodic current under illumination, which is the characteristic of a ptype semiconductor.26 Figure S4 demonstrates the cyclic voltammograms (CVs) of Zn0.3Cd0.7S (6, Cu)/GR, WO3 and WO3/GR which show clear anodic and cathodic peaks for each sample. The current density of the samples electrodes follows the order: Zn0.3Cd0.7S (6, Cu)/GR > Zn0.3Cd0.7S (6, Cu) and WO3/GR > WO3, demonstrating that GR could improve electron transfer. The negative slope of Zn0.3Cd0.7S (6, Cu) in the Mott-Schottky curves (Figure 6B) further confirmed that p-type Zn0.3Cd0.7S (6, Cu) had been successfully prepared.45 The flat band potential of Zn0.3Cd0.7S (6, Cu) is 2.23 V vs. RHE, which can be approximately considered as valence band


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edge of p-type semiconductors.46 Based on the assumption that both doped and undoped samples (Figure S5) have a same dielectric constant (ε) 8.7 F m-1,47 the acceptor density Na for Zn0.3Cd0.7S (6, Cu) calculated from eqn. 2 is 1.5×1019 cm-3, and the donor density Nd for undoped Zn0.3Cd0.7S is 2.9×1018 cm-3. The width of the space charge layer (W) for the p-type Zn0.3Cd0.7S (6, Cu) calculated from eqn. 3 is 88 nm. These results indicate that the Cu doping created a sufficient space charge layer to enhance the photoexcited charge carriers separation.48 The color of the surface of the Zn0.3Cd0.7S-electrode changed a little bit after the electrochemical measurement, indicating the electrode was not stable to high positive potential. The pH of the solution did not change obviously since the measurement was carried out in a relative large quantity solution of Na2SO4 and lasted only a short time (ca. 2.6 min).

Figure 7. (A) Photocurrent responses of (a) WO3, (b) Zn0.3Cd0.7S (6, Cu), (c) WO3/GR and (d) Zn0.3Cd0.7S (6, Cu)/GR. The electrolyte was 0.1 M Na2SO4 aqueous solution (pH~6). Photocurrent response was under light irradiation and recorded at 0 V vs. SCE. (B) Nyquist plots of (a) WO3, (b) Zn0.3Cd0.7S (6, Cu), (c) WO3/GR and (d) Zn0.3Cd0.7S (6, Cu)/GR. The measurements were carried out in a 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M Na2SO4 solution at an electrode potential of 0.2 V. Inset: the equivalent circuit used for fitting EIS curves.



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The photocurrent and electrochemical impedance spectra (EIS) of the electrodes are measured to investigate the charger transmission and the results are shown in Figure 7. Firstly, the electrodes showed a prompt response, indicating effective carrier transfer in the samples. A relatively weak photocurrent response was observed on both WO3 (0.8 µA cm-2, Figure 7A, curve a), Zn0.3Cd0.7S (6, Cu) (-1.1 µA cm-2, Figure 7A, curve b) and Zn0.3Cd0.7S (0.8 µA cm-2, Figure S6) electrodes. While for the graphene included composites, obviously enhanced photocurrent responses occurred, the WO3/GR electrode reached to 4.9 µA cm-2 and the Zn0.3Cd0.7S (6, Cu)/GR electrode to -8.2 µA cm-2. The enhancement can be owing to high light absorption, efficient electron transfer and charge separation, and higher surface area of the graphene-including nanocomposites. The result indicates that graphene as an electron acceptor can effectively promote charge transfer and decrease the recombination of photoinduced electron-holes.44,49 The fact that the photocurrent of the Zn0.3Cd0.7S (6, Cu)/GR electrode gradually reached to the saturated value may be attributed to that the photogenerated electrons transferred from the semiconductor to the graphene were stored on the graphene sheets, rather than directly transferred to the external circuit, which results in the gradually increase of the current under illumination.50 To further confirm the charge transport behavior in the Zn0.3Cd0.7S (6, Cu), WO3 and their corresponding graphene composites, the EIS of the samples are shown in Figure 7B. The electrical equivalent circuit used for fitting EIS curves is shown in the inset of Figure 7B, where Rs, Rct and Wo represent electrolyte resistance, bulk electrode resistance, Warburg impedance, respectively, and CPE is capacitance corresponding to the electrolyte/electrode double layer. The separation efficiency of the photogenerated electron-hole pairs is reflected by the arc sizes of the Nyquist plots.51 Obviously, the Zn0.3Cd0.7S (Cu)/GR electrode shows a smaller radius of Nyquist


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curve than the Zn0.3Cd0.7S (Cu) electrode. Rct calculated from the electrical equivalent circuit is 56 and 26 Ω for the Zn0.3Cd0.7S (6, Cu) electrode and the Zn0.3Cd0.7S (6, Cu)/GR electrode, respectively, indicating an enhanced charge transfer ability for the Zn0.3Cd0.7S (6, Cu)/GR electrode. Similarly, Nyquist curve radius for the WO3/GR electrode is smaller than that of the WO3 electrode. The calculated Rct is 101 and 41 Ω for the WO3 and WO3/GR electrode, respectively. These results have proved that the graphene can accelerate the separation of photogenerated electron hole pairs and improve the charge transfer efficiency.52

Figure 8. (A) Amount of hydrogen evolved from the PEC device with different Cu concentrations in Zn0.3Cd0.7S (Cu) under 7 h UV-vis irradiation. (B) Amount of hydrogen and oxygen evolved from the PEC device with (a) Zn0.3Cd0.7S (6, Cu) and (b) Zn0.3Cd0.7S (6, Cu)/GR as the photocathode, and (a′) WO3 and (b′) WO3/GR as the photoanode, respectively. The broken line shows ideal hydrogen evolution assuming a Faradic efficiency of 100%. Reaction conditions: electrolyte: 0.1 M Na2SO4 aqueous solution (pH~6), applied bias: 0 V, light source: 150 W Xe lamp. In order to investigate the photoelectrochemical tandem cell composed of the as-prepared electrodes for overall water splitting, the photocatalytic performances of the device were



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investigated under light irradiation. Firstly, the effect of Cu doping concentration in the Zn0.3Cd0.7S (Cu) composite on the photocatalysis was analyzed, as shown in Figure 8A. It can be seen that the amounts of H2 and O2 evolved from the PEC device with 3% Cu doping in 7 h UVvis irradiation are 81.7 and 12.3 µmol g-1, respectively, while the amounts of H2 and O2 evolved in 7 h UV-vis irradiation increase to 136.8 and 20.2 µmol g-1 when the concentration of Cu is 6%. The enhancement of photocatalytic activity is attributed to effective doping of the Zn0.3Cd0.7S (Cu) electrode, on which an electric field developed in the semiconductor/solution interface that directs photogenerated electrons to move into the solution. However, the rate of gas evolution started to decrease when the content of Cu in the composite was further increased. The decrease of the gas evolved mainly caused by the counteraction effect of Cu doping, which deplete the photogenerated electrons.34 Moreover, when the graphene was hybridized with the corresponding electrode, the great enhancement of the photocatalytic activity was obtained. Figure 8B shows time courses of gas evolution from the PEC device with graphene modified electrodes. The photocatalytic result showed that when 2% graphene was introduced into the two photocatalysts, the amounts of H2 and O2 evolved in 7 h UV-vis irradiation are up to 459.0 and 86.2 µmol g-1, respectively, which is ca. 3.4 and 4 times as high as those evolved from bare Zn0.3Cd0.7S (6, Cu) and WO3 electrode. The amount of H2 evolved is less than half of the electrons passing through the outer circuit (e-/2, shown as a broken line in Figure 8B). The calculated Faraday efficiencies for the H2 and O2 production from Zn0.3Cd0.7S (6, Cu)/GR and WO3/GR electrodes were 92% and 27%, respectively. The fact that the molar ratio between evolved hydrogen and oxygen is not in stoichiometric ratio may be interpreted as the formation and accumulation of peroxo species on the surface of WO3 electrode, which means that a significant portion of the photon-generated


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holes were used to form peroxo species since the formation of peroxo species was kinetically competitive with O2 production.53 In order to calculate quantum yield at 365 nm, the band-pass filter was put between the window of the Zn0.3Cd0.7S (6, Cu) electrode and light source. The amount of H2 evolved from Zn0.3Cd0.7S (6, Cu)/GR is 1.03 µmol in 1 h irradiation. The external quantum yield at 365 nm calculated according to eqn. 5 is 2.3 %.

Figure 9. The recycling experiment of the PEC device with Zn0.3Cd0.7S (6, Cu)/GR as the photocathode.

The long-term stability is an important factor for the practical application. Accordingly, the stability of the electrodes was evaluated by performing the recycle experiments and the results are shown in Figure 9. Under 7 h UV-vis irradiation, about 458.1 µmol g-1 of H2 and 81.9 µmol g-1 of O2 evolved in the first run (the average rates were 65.4 and 11.7 µmol g-1 h-1, respectively). The amount of gas production only decreased slightly in the next two runs (393.4, 367.2 µmol g-1



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(average rate: 56.2, 52.4 µmol g-1 h-1) for H2 and 70.8, 69.8 µmol g-1 (average rate: 10.1, 9.9 µmol g-1 h-1 for O2).21,54

Scheme 2. Schematic illustration of the tandem photoelectrochemical device. The proposed water-splitting mechanism for the tandem photoelectrochemical device is illustrated in Scheme 2. Firstly, under light irradiation, the photogenerated electrons are excited from the valence band to the conduction band of Zn0.3Cd0.7S (Cu)/GR and WO3/GR of two electrodes, respectively. The photogenerated electrons in photoanode transfer to the Zn0.3Cd0.7S (Cu)/GR electrode through external circuit, combining with the holes remained in the valence band of Zn0.3Cd0.7S (Cu)/GR. At the same time, the photogenerated electrons transfer from the conduction band of Zn0.3Cd0.7S (Cu) to graphene reducing H+ to form H2 at photocathode and the holes on the valence band of WO3 react with water to produce O2 and protons at photoanode. The Nafion film allows protons exchange between anode and cathode compartments to sustain charge balance during the reaction.


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CONCLUSIONS In summary, we successfully prepared photoelectrochemical tandem cell containing a ptype Zn0.3Cd0.7S (Cu)/GR electrode and an n-type WO3/GR electrode. H2 and O2 gas were produced from the tandem photoelectrochemical device, separately, under light illumination. By doping Cu, the type of Zn0.3Cd0.7S semiconductor transfers to p-type one as an efficient photocathode in the tandem cell. In addition, the Cu dopants also enhance the photogenerated electrons transfer greatly during photocatalytic process. Graphene as a superior supporting matrix assists the charge transfer and further enhances the photocatalytic activity. This study indicates that Zn0.3Cd0.7S (Cu)/GR is a promising p-type semiconductor for fabricating photocathode at the same time, overall water splitting cell.

ASSOCIATED CONTENT Supporting Information The TEM and AFM image of GR; The SEM-EDX element mapping of Cu element distribution; XPS spectra of Zn, Cd, W and Cu; CV curves of Zn0.3Cd0.7S(6, Cu) and WO3-based samples; The Mott-Schottky plot of Zn0.3Cd0.7S and Photocurrent responses of Zn0.3Cd0.7S. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

Tel./Fax:

+86-512-65880089.

E-mail:

[email protected]

[email protected] (M. Zhu)


(P.

Yang);


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Author Contributions §

These authors contributed equally to this work. The manuscript was written through

contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes: The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful for the financial support to this research by the National Natural Science Foundation of China (21373143), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Outstanding Talent Training Plan of Soochow University (5832000213).

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For Table of Contents Use Only

P-Type Cu-Doped Zn0.3Cd0.7S/Graphene Photocathode for Efficient Water Splitting in Photoelectrochemical Tandem Cell Yijie Wu, Zongkuan Yue, Aijuan Liu, Ping Yang, and Mingshan Zhu

By doping Cu ions in Zn0.3Cd0.7S, p-type Zn0.3Cd0.7S(Cu)/graphene photocathode was synthesized for constructing p-n type photoelectrochemical tandem cell for overall water splitting.


Graphene Photocathode for Efficient

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