ZnO Nanowire Arrays with

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Synthesis of CuInS2 Quantum Dots/In2S3/ZnO Nanowire Arrays with High Photoelectrochemical Activity Ying-Chu Chen,† Hao-Hsuan Chang,‡ and Yu-Kuei Hsu*,‡ †

Karlsruhe Institute of Technology (KIT), Institutfür Anorganische Chemie, Engesserstraße 15, D-76131 Karlsruhe, Germany Department of Optoelectronic Engineering, National Dong Hwa University, No. 1, Sec. 2, Da Hsueh Road, Shoufeng, Hualien, 97401 Taiwan

‡

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S Supporting Information *

ABSTRACT: Decoration of CuInS2 (CIS) quantum dots (QDs) on ZnO nanowires (NWs) with an interlayer of In2S3 as photoelectrode has been successfully fabricated on FTO via the simple solution routes for photoelectrochemical (PEC) application. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction are utilized to systematically analyze the morphology and structure of the CIS QD/ In2S3/ZnO NWs heterostructure. The composition of this multilayer heterostructure and the removal of QD ligands by a thermal process are confirmed by X-ray photoelectron spectra. In comparison with CIS QDs/ZnO NWs, the CIS QD/In2S3/ ZnO heterostructural photoelectrode displays an efficient charge separation and carrier transport path for photocurrent up to 2.4 mA·cm−2 that is competitive with other Cd- and Pb-free QD-based materials. In addition, Mott−Schottky analysis demonstrates the negative shift of the flat band in the CIS QD/In2S3/ZnO, which benefits the early onset potential. Significantly, this hierarchical photoelectrode shows the improvement the absorption and conversion of solar light in the visible region obtained using a pristine ZnO structure. Our research paves the way for exploring lead-free and lead-free sulfide materials in the new category of solar applications. KEYWORDS: Copper(I) indium sulfide, Indium sulfide, Heterojunction, Photoelectrochemistry

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INTRODUCTION Since sunlight is a sustainable, clean, and abundant renewable energy, converting solar energy into a usable fuel has attracted considerable attention in the past few ten years. Among all techniques for the solar conversion, a photoelectrochemical (PEC) process that generates hydrogen fuel by direct water decomposition through a semiconductor photoelectrode has become an attractive solution. Although in recent years, various forms and materials of semiconductor electrodes have exerted intense efforts on PEC performance, the satisfaction of all the efficiency, stability, and cost requirements for commercial products has not been achieved by a single material.1,2 The development of the heterostructural photoelectrode for highly efficient PEC hydrogen generation is an important route. In particular, the decoration of quantum dots (QDs) on one-dimensional (1D) metal oxide nanostructures, such as ZnO and TiO2, has been studied as efficient photoelectrodes owing to the unique characteristics of QDs, such as tunable band gap, large absorption coefficient, multiple exciton generation, as well as simple and low-cost synthesis.3−6 However, the most reported QD-based photoelectrodes include toxic cadmium and lead levels, which may limit their commercial use, taking into account environmental and health issues. Therefore, the development of highly efficient, nonheavy metal QDs as light absorbers is essential for the practical application of QD-type photoelectrodes. © XXXX American Chemical Society

Nontoxic CuInS2 (CIS) QDs are attracting more attention due to their high absorption coefficient (∼105 cm−1) and tunable band gap energy.7−11 The synthesis of CIS QDs usually contained the capping agent with long-chain ligand (e.g., 1-dodecanethiol) on the QD surface, and the removal of capping agent by ligand exchange is highly required. But when the typical ligand exchange process involved toxic chemicals, the alternative route to remove the capping ligand should be concerned.12 In addition, K. Yong et al. found that direct deposition of CIS nanoparticles on ZnO nanowires (NW) has the problem of large band mismatch, but the further insert of CdS whose bandgap in between CIS and ZnO could facilitate the carriers transfer at the interface.13 As concerns the green process and nontoxic materials, we propose the CIS QD/ In2S3/ZnO NW array as efficient photoanode, because the In2S3 has similar bandwidth to CdS. In this study, the colloidal CIS QDs was synthesized by the chemical solution route, and then the deposition of CIS QDs on the surface of ZnO NWs utilized the dip-coating process while the ZnO NW decorated the In2S3 shell by a successive ionic layer adsorption and reaction (SILAR) method before use. Finally the Received: May 10, 2018 Revised: June 10, 2018

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DOI: 10.1021/acssuschemeng.8b02154 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

CIS QD/In2S3/ZnO NW was thermally treated to remove the capping ligands. Significantly, this novel CIS QD/In2S3/ZnO NW photoelectrode illustrates excellent PEC activity in comparison with CIS QD/ZnO NW and other Cd- and Pb-free QDs photoelectrodes. To our knowledge, this hierarchical CIS QD/In2S3/ZnO NW photoelectrode has not been reported yet. Moreover, these Cd- and Pb-free QDs not only function as photoelectrodes but also provide a new opportunity for the development of electronic and solar energy equipment based on three-dimensional core−shell structures.

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EXPERIMENTS

Copper iodide, indium acetate, dodecanethiol (DDT), zinc acetate, zinc nitrate, hexamethylenetetramine, ethanol, and polyethylenimine were purchased from Sigma-Aldrich. Ammonium hydroxide, indium chloride (InCl3), sodium sulfide (Na2S), and sodium sulfate (Na2SO4) were purchased from Aladdin. The above reagents are analytical reagents and can be used without any further purification. In the CIS QDs synthesis, copper iodide (1 mmol) and indium acetate (1 mmol) were used as precursors, and 5 mL of DDT solvent was employed as capping ligand and S donor. Under vacuum, the solution was heated for 30 min at 100 °C, and then the temperature was increased to 230 °C. The temperature was kept at 230 °C for 30 min to complete QDs synthesis. After synthesis, the temperature was naturally cool down 25 °C, and the CIS QD thus synthesized is stored in N2 purged toluene until use. In the synthesis of ZnO NW, the ZnO nanoparticles with 10−15 nm in diameter were deposited on a FTO as seed layer by dip-coating in 5 mM zinc acetate solution and then heat treatment of 350 °C for 5 min in air. The substrate with ZnO seed layer was placed in a solution containing 5 mM zinc nitrate, 5 mM hexamethylenetetramine, 5 mM polyethylenimine, and 0.25 M ammonium hydroxide at 90 °C for 5 h to grow the ZnO NWs. Subsequently, for the SILAR, 0.1 M InCl3 and 75 mM Na2S function as the In and S precursor solutions. In each cycle, ZnO NW samples

Figure 1. (a) TEM image of CIS QDs colloid; (b) absorption and emission spectra of a sample of CIS QDs.

Figure 2. FESEM images of (a) pristine ZnO NWs, (b) In2S3/ZnO NWs, and (c) CIS QDs/In2S3/ZnO NWs samples. B


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Figure 3. HRTEM image of (a) the ZnO core of the NW within orange frame, (b) the internal shell of the NW within red frame, (c) the external shell of the NW within green frame. (d) HAADF-STEM image of multishell NW and the quantification of the EDXS line scan. (e) SAED pattern of CIS QDs/In2S3/ZnO NW.

Figure 4. (a) HAADF-STEM image and EDXS maps of (b) Zn−O, (c) Zn−S, (d) Zn−In, and (e) Zn−Cu distributions. C


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Figure 5. (a) In 3d and (b) S 2p XPS spectra of In2S3/ZnO NW sample; (c) Cu 2p, (d) In 3d, and (e) S 2p XPS spectra of CIS QDs/In2S3/ZnO NW sample; (f) S 2p XPS spectra of CIS QDs/In2S3/ZnO NW sample without thermal treatment. were immersed in the solution in the order of InCl3 60 s, deionized water 30 s, Na2S 240 s, deionized water 30 s, InCl3 60 s, and DI 30 s; this cycle was repeated five times to synthesize the In2S3 shell on the ZnO NW surface. The five cycles for the depositing In2S3 layer were selected based on the PEC measurement, as shown in Figure S1. Then, the In2S3/ZnO NW was immersed into CIS QDs colloid for 3 s and rinsed with acetone. The short immersion time was selected due to the acidity of CIS QDs colloid. The as-prepared sample was then thermally treated at 350 °C for 1 h in vacuum. The morphology of hierarchical CIS QD/In2S3/ZnO NW arrays was examined with a scanning electron microscope (SEM, JEM4000EX). Their crystalline structure was examined with a X-ray diffractometer (XRD, Cu Kα radiation, λ = 0.1506 nm, Bruker D8 Advance). The chemical analysis of the elements was examined by X-ray photoelectron spectra (XPS, PerkinElmer model PHI 1600). The chemical composition and microscopic structure of the CIS QD/ In2S3/ZnO NW were studied using high-resolution scanning transmission electron microscopes (HR-STEM, JEM2200FS, and JEM2010F operating at 200 kV, JEOL). Absorption spectra of CIS QD/In2S3/ ZnO NW samples were measured using a Varian Cary 100 spectrometer and further analyzed by means of Kubelka−Munk transformation. The PEC characteristic of the samples was examined in 0.35 M

Na2SO3 and 0.1 M Na2S electrolyte by a potentiostat/galvanostat (CHI 6273D). A three-electrode system including of the CIS QD/ In2S3/ZnO as working electrode, an Ag/AgCl reference electrode in 3 M KCl solution, and a square platinum sheet was implemented. All potentials reported herein are relative to Ag/AgCl electrode. A 150 W Xe lamp was utilized with an A.M. 1.5 filter, and the illumination intensity at the sample position was determined to be 100 mW·cm−2. The monochromator equipped with a Xe lamp provides monochromatic excitation light to determine the incident photons-to-electrons conversion efficiency (IPCE). The incident light passes through a quartz window, and the electrolyte and illuminates on the CIS QD/ In2S3/ZnO NW sample. Electrochemical impedance spectra (EIS) were measured with this apparatus at a frequency of 100 kHz at varied potential with 10 mV amplitude.

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RESULTS AND DISCUSSION Characterization of Structure and Composition. The morphology and size of CIS QDs were determined by TEM, as shown in Figure 1a. The shapes of the QDs are irregular, and the size distribution of nanocrystals is determined by measuring their average edge length. From TEM images, about D


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SAED (Figure 3e). The SAED pattern of CIS QD/In2S3/ZnO NW shows a few Deby−Scherrer rings with orange, green, and red colors, which can be attributed to the hexagonal ZnO, tetragonal CIS, and In2S3 crystallites, respectively. In addition, EDXS elemental maps are further utilized to study the distribution of Cu, In, S, Zn, and O in CIS QD/In2S3/ZnO multishell NW. Figure 4 illustrates the HAADF-STEM image and EDXS maps of Zn−O, Zn−S, Zn−In, and Zn−Cu distributions that demonstrate the formation of a rod with ZnO core and shell containing Cu, In, and S. These results are also consist with the finding from HRTEM. The chemical compositions and binding energy (BE) of CIS QD/In2S3/ZnO and In2S3/ZnO NW were investigated by XPS, respectively. The In 3d and S 2p core level peaks of In2S3/ZnO NW were detected in the XPS spectra (Figure 5a and 5b). The positions of the In 3d5/2 peak at BE of 444.6 eV and the S 2p peak at BE of 161.5 eV are in good agreement with that reported for SILAR-deposited In2S3.13 Figure 5c−e display the Cu 2p3/2 peak at a BE of 931.6 eV, In 3d5/2 at a BE of around 444.3 eV, and S 2p at a BE of 161 eV from CIS QD/ In2S3/ZnO NW, indicating that the valence states of S, In, and Cu are −2, + 3, and +1, respectively. The three main core levels of S 2p, In 3d, and Cu 2p are consistent with those reported in the literature.15 Notably, the S 2p core level peaks of CIS QD/In2S3/ZnO NW before thermal treatment were also presented in Figure 5f for comparison. The appearance of thiol (R-SH) BE at 164 eV illustrates the DDT capping layer

200 particles were sampled to obtain the average size of the QD, which is around 7.5 ± 0.5 nm. In addition, the PL and absorption spectra of CIS QDs are illustrated in Figure 1b. The absorption edge of QDs colloid displays that the nanocrystal has a larger bandgap than the bulk CIS, representing a quantum confinement effect of CIS QDs. From the PL spectrum, CIS QDs exhibit wide emission from visible to near-infrared wavelengths. Notably, a corresponding Stokes shift (24 nm) that is the spectral separation between absorption onset and emission was observed probably due to the strong self-reabsorption or/and the trap sites in intragap during the transition process.14 The small Stokes shift may result from the relatively larger size of nanocrystals. The morphologic evolution of CIS QD/In2S3/ZnO NWs was determined by SEM images. Figure 2a displays the surface morphologies of ZnO NW arrays grown by a hydrothermal method. A dense and straight NW structure uniformly and tightly covers a large area of the FTO substrate, while the NWs have diameter of 90−150 nm. After coating a In2S3 layer on ZnO NW by the SILAR method, their surface morphologies were displayed in Figure 2b. The NW density still remains, besides the slight aggregation of NWs owing to the evaporation of solvent during the SILAR process. Notably, the diameter and surface roughness of the NWs substantially increase, which implies the successful deposition of the In2S3 shell on ZnO NWs. After immersing in QDs colloid and thermally treating at 350 °C in vacuum, the sample still shows the dense 1D shape, indicating that the QDs colloid solution would not etch the In2S3/ZnO NW within the short immersion time. Interestingly, the surface layers become compact probably owing to the thermal treatment process, but the QDs on In2S3/ZnO NW cannot be clearly observed at this magnification. To further analyze the heterostructural CIS QDs/In2S3/ ZnO NWs, the HRTEM was performed. Figure 3a shows the core of the NW that is composed of single ZnO monocrystals with a hexagonal structure, as shown by the very good agreement between its 2-dimensional Fourier transform and the calculation of the diffraction pattern in the hexagonal ZnO (space group P63/mc, space group number 186) with a = 3.250 Å and c = 5.207 Å in the [762]-zone axis. The structure elucidation of the In2S3 inner shell of the NW between the external CIS shell and the ZnO core of the NW was carried out, as illustrated in Figure 3b. The average distance of d = 2.7 ± 0.1 Å in the lattice fringes that correspond well to the latticeplane distance d(220) = 2.68 Å in bulk tetragonal In2S3 can be observed within the red frame. This indicates that the whole red frame of the rod is a single tetragonal In2S3 monocrystal (space group I41/amd, space group number 141). The CIS QDs crystallize on the external shell of the NW within the green frame as displayed in Figure 3c. The whole particle is a single CIS monocrystal with a tetragonal structure, as indicated by the very good consistency between its 2-dimensional Fourier transform and the calculation of the diffraction pattern of bulk tetragonal CIS (space group I4̅2d, space group number 122) with a = 5.523 Å and c = 11.140 Å in the [110]-zone axis. The Cu-, In-, S-, Zn-, and O-compositions within single CIS QD/ In2S3/ZnO multishell NW are also measured from EDXS line profiles, which are recorded along a line that passes through a NW and is vertical to the NW axis (Figure 3d). Obviously, the core material is ZnO and the shell layer with dozens of nanometers in thickness contains the sulfide compounds. The crystalline structure of CIS QD/In2S3/ZnO NW was investigated by

Figure 6. (a) Absorption spectra and (b) diffusion reflection of uncoated ZnO, In2S3/ZnO, and CIS QDs/In2S3/ZnO NW samples in UV−vis test. E


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Figure 7. Linear sweep voltammetry of (a) CIS QDs/ZnO NW and (b) CIS QDs/In2S3/ZnO NW samples under illumination. (c) A schematic diagram of the electronic structure of QDs/In2S3/ZnO NW. (d) Nyquist plots of CIS QDs photoelectrode with and without In2S3 inner layer.

electrolyte of 0.35 M Na2SO3 and 0.1 M Na2S for analysis. The photocurrent−potential curves of uncoated ZnO, CIS QD/ ZnO, and CIS QD/In2S3/ZnO NW arrays were recorded via linear sweep voltammetry (LSV) under continuous and chopped illumination, as shown in Figure 7. The uncoated ZnO NW shows low photocurrent of approximately 0.5 mA·cm−2 owing to the narrow absorption region of UV light. After the deposition of CIS QD on the ZnO sample, the dark current was dramatically increased owing to the oxidation reaction of electrolyte; meanwhile, the weak photocurrent response of a few ten μA·cm−2 was observed (in Figure 7a). This low photocurrent response could be ascribed to the large band mismatch of CIS QD and ZnO, which resulted in the obstruction of photoinduced electron−hole pairs transport.13 Notably, the photocurrents were significantly enhanced and the dark current decreased after the deposition of the In2S3 layer in between the CIS QD and ZnO NWs. This indicates that the In2S3 inner shell could compensate the band mismatch and facilitate the photoinduced electron transport to ZnO. In addition, the decrease of the dark current may be attributed to the extension of the width of the depletion layer at the electrode−liquid interface and reduction of the energy band bending between CIS and ZnO in the presence of the In2S3 layer. Otherwise, the sharp narrow-band bending of the CIS QDs/ZnO electrode in the absence of the In2S3 inner shell will lead to a significant oxidation reaction with the electrolyte through the tunneling effect, as shown in Figure 7a. Furthermore, a diagrammatic

on the CIS QDs, but the thiol signal disappeared after thermal treatment, indicating the successful removal of capping materials.16 The absorption spectra of the CIS QD/In2S3/ZnO, In2S3/ ZnO, and uncoated ZnO NW arrays are displayed in Figure 6. The uncoated ZnO illustrates the absorption at 3.12 eV, and the absorbance of In2S3/ZnO NWs is enhanced in the visible region with an absorption edge of approximately 2.8 eV. Compared with bare ZnO and In2S3/ZnO NW samples, the prepared CIS QD/In2S3/ZnO NW array not only has excellent optical absorption properties in the ultraviolet region but also in the visible region. The above results confirm that CIS QD can effectively enhance the light absorption characteristics of ZnO NWs. Besides, the absorption band edge of CIS QD/In2S3/ZnO NW is about 1.7 eV due to the significant blue shift relative to bulk CIS (ca. 1.5 eV), showing that the size of the CIS QDs after thermal treatment is still in the region of quantum confinement. In addition, Figure 6b shows the diffusion reflection of the pristine ZnO, In2S3/ ZnO, and CIS QDs/In2S3/ZnO samples in UV−vis test, and these data show results consistent with the absorption spectra results. Thus, compared with pure ZnO, CIS QD/ In2S3/ZnO NW samples are promising photoelectrodes under visible light. PEC Performance of CIS QD/In2S3/ZnO NW Photoelectrode. To probe the PEC activities of the CIS QD/In2S3/ ZnO NW sample, a three-electrode system was used in the F


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transfer resistance in the CIS QD/In2S3/ZnO NW sample is smaller and the interface charges are transferred faster. Therefore, those results illustrate that the CIS QD/In2S3/ ZnO NW photoelectrode illustrates excellent PEC activity and enhanced electron-transfer reaction. The photocurrent action spectra of uncoated ZnO, In2S3/ ZnO, and CIS QD/In2S3/ZnO NW arrays (Figure 8a) show the IPCE value as a function of monochromatic wavelength at a potential of 0.4 V. The In2S3/ZnO samples showed similar IPCE behavior as the uncoated ZnO probably due to the absence of further annealing after the SILAR deposited In2S3 shell samples. It is worth noting that the photoresponses of CIS QD/In2S3/ZnO NWs and uncoated ZnO NWs, respectively, show an initial wavelength of approximately 730 and 390 nm photocurrent generation in the electrolyte. According to this comparison, the presence of CIS QD takes responsibility for converting the visible light in accordance with the results of absorption spectroscopy and also indicates that the main contribution to photocurrent is CIS. In addition, Mott−Schottky analysis of uncoated ZnO, In2S3/ZnO, and CIS QD/In2S3/ZnO NWs was performed in darkness, as displayed in Figure 8b, to probe the information on the energyband position of a heterojunction. The flat-band potential could be extracted by utilizing the Mott−Schottky equation from the abscissal intercept, whose flat-band potentials for uncoated ZnO, In2S3/ZnO, and CIS QD/In2S3/ZnO NWs are −0.85, −0.88, and −0.93 V, respectively. This negative shift of flat band can be attributed to the heterojunction structure of CIS QD/In2S3/ ZnO, where contained the p-type nature of CIS, and this phenomenon can be similarly found from other CIS/TiO2 photoelectrodes.21 Moreover, this characteristic will further facilitate the effective separation of the charge of the early initiation potential and promote the hydrogen reduction reaction of photogenerated electrons, resulting in the enhancement of photoresponse in the CIS QD/In2S3/ZnO NWs photoelectrode.

Table 1. Comparison with Literature Regarding CIS-Based Photoelectrode with Cadmium- and Lead-free Photoactive material CuInS2 QDs/ZnO NR GO/Tremella-like CuInS2 /Graphene sheet CuInS2 NPs/Ag NPs/ZnO NWs CuInS2 NPs/ZnO NRs CuInS2 NPs/TiO2 NTs CuInS2 QDs/In2S3/ZnO NWs

Electrolytes

Photocurrent (mA cm−2)

ref

0.5 M Na2SO4 1 M Na2S

1.2 2.5

17 18

0.28 M Na2SO3 + 1 M Na2S 0.35 M Na2SO3 + 0.25 M Na2S 0.02 M Na2SO3 + 0.1 M Na2S 0.1 M Na2SO3 + 0.35 M Na2S

0.073

19

1.5

20

0.5

21

2.4

this study

sketch of the electronic structure of the CIS QD/In2S3/ZnO NW array electrode with the rough band positions is shown in Figure 7c, which could clearly support the function of the In2S3 inner layer.13 The high photocurrent of 2.4 mA·cm−2 can be achieved, which is 2 orders of magnitude higher than the CIS QD/ZnO NW sample and also higher than that of many other CIS-based photoelectrodes with Cd- and Pb-free, as shown in Table 1. Besides, the Nyquist diagrams of EIS experiments under illumination were further performed with a potential of −0.3 V to investigate the interface charge transfer processes. In Figure 7d, Nyquist diagrams for the CIS QD photoelectrodes with and without the In2S3 inner shell illustrate that CIS QD/In2S3/ZnO NW has a smaller impedance arc radius than the other one. This factor indicates that the charge

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CONCLUSION Hierarchical heterojunction of CIS QD/In2S3/ZnO NWs arrays was successfully synthesized through a dip-coating process in CIS QD colloid while the ZnO NW decorated the In2S3 layer by the SILAR method. The morphologic and structural characteristics of CIS QD/In2S3/ZnO NWs were demonstrated by SEM and TEM. XPS analysis demonstrated the removal of capping material on CIS QDs by thermal treatment. The absorption spectra data could clearly identify the energy bandgap of ZnO, In2S3, and CIS QD. Significantly, the CIS QD/In2S3/ZnO NWs photoelectrode exhibits excellent photocurrent of 2.4 mA·cm−2 that is 2 orders of magnitude larger than that of uncoated ZnO NWs. The photocurrent action spectrum exhibits the near-infrared photoactivity related to the bandgap of CIS QD. Furthermore, this novel heterojunction photoelectrode with high photoactivity can be ascribed to the effective charge separation and charge transfer path of the In2S3 inner shell in the heterostructural NWs. Therefore, the proposed quantum dot photoelectrode based on the green light process not only can demonstrate the great prospect of solar power generation but also provides a blueprint for the design of the photocatalyst in the future.

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Figure 8. (a) IPCE as a function of excitation wavelength at a potential of 0.4 V from uncoated ZnO and CIS QDs/In2S3/ZnO NW samples; (b) Mott−Schottky plot of uncoated ZnO and CIS QDs/ In2S3/ZnO NW samples.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02154. G


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(12) Zhao, C.; Bai, Z.; Liu, X.; Zhang, Y.; Zou, B.; Zhong, H. Small GSH-Capped CuInS2 Quantum Dots: MPA-Assisted Aqueous Phase Transfer and Bioimaging Applications. ACS Appl. Mater. Interfaces 2015, 7, 17623−17629. (13) Choi, Y.; Beak, M.; Yong, K. Solar-driven hydrogen evolution using a CuInS2/CdS/ZnO heterostructure nanowire array as an efficient photoanode. Nanoscale 2014, 6, 8914−8918. (14) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (15) Gunawan; Septina, W.; Harada, T.; Nose, Y.; Ikeda, S. Investigation of the Electric Structures of Heterointerfaces in Pt- and In2S3-Modified CuInS2 Photocathodes Used for Sunlight-Induced Hydrogen Evolution. ACS Appl. Mater. Interfaces 2015, 7, 16086− 16092. (16) So, D.; Konstantatos, G. Thiol-Free Synthesized Copper Indium Sulfide Nanocrystals as Optoelectronic Quantum Dot Solids. Chem. Mater. 2015, 27, 8424. (17) Yang, Y.; Que, W.; Zhang, X.; Xing, Y.; Yin, X.; Du, Y. Facile synthesis of ZnO/CuInS2 nanorod arrays for photocatalytic pollutants degradation. J. Hazard. Mater. 2016, 317, 430−439. (18) Bo, W.; Liu, Z.; Hong, T.; Han, J.; Guo, K.; Zhang, X.; Chen, D. Trilaminar graphene/ tremella-like CuInS2/graphene oxide nanofilms and the enhanced activity for photoelectron- chemical water splitting. J. Nanopart. Res. 2015, 17, 1−8. (19) Cheng, Z.; Zhan, X.; Wang, F.; Wang, Q.; Xu, K.; Liu, Q.; He, J. Construction of CuInS2/Ag sensitized ZnO nanowire arrays for efficient hydrogen generation. RSC Adv. 2015, 5, 81723−81727. (20) Tang, Y.; Yun, J. H.; Wang, L.; Amal, R.; Ng, Y. H. Complete surface coverage of ZnO nanorod arrays by pulsed electrodeposited CuInS2 for visible light energy conversion. Dalton Trans. 2015, 44, 7127−7130. (21) Yun, J. H.; Ng, Y. H.; Huang, S.; Conibeer, G.; Amal, R. Wrapping the walls of n-TiO2 nanotubes with p-CuInS2 nanoparticles using pulsed-electrodeposition for improved heterojunction photoelectrodes. Chem. Commun. 2011, 47, 11288−11290.

Photocurrent−potential curves of CIS QD/In2S3/ZnO NW arrays with different cycle times for deposition of In2S3; HAADF-STEM image and EDXS maps of Zn (Zn Kα1 line-blue), O (O K line- red), S (S K line-green), In (In Lα1 line-orange), and Cu (Cu Kα1 line-lavender) distributions; Variation of the square root of IPCE (η) times hν with photon energy for ZnO and CIS QDs/ In2S3/ZnO NW samples; Stability test of CIS QDs/ In2S3/ZnO NW sample at a potential of 0.4 V vs Ag/ AgCl for 2000 s (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +886-3-863-3196; Fax: +886-3-863-3180; E-mail address: [email protected]. ORCID

Yu-Kuei Hsu: 0000-0003-1963-5172 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS National Dong Hwa University and National Science Council supported this study under the contracts MOST 105-2221-E259-024-MY3 and MOST 105-2221-E-259-026, respectively.

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REFERENCES

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