Hierarchically CuInS 2 Nanosheet‐Constructed Nanowire ...yfzhang.sjtu.edu.cn/en/publications/2016/8.pdf · 2 Nanosheet-Constructed Nanowire Arrays for Photoelectrochemical Water

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Hierarchically CuInS2 Nanosheet-Constructed Nanowire Arrays for Photoelectrochemical Water Splitting

Ming Li, Renjie Zhao, Yanjie Su,* Jing Hu, Zhi Yang, and Yafei Zhang*

of a photocathode and a photoanode that are responsible for water reduction and oxidation, respectively.[3,4] Although much attention has been devoted to n-type photo-anode materials such as TiO2,[5,6] ZnO,[7,8] Fe2O3,[9,10] and WO3,[11,12] the development of PEC tandem cell is limited by the iden-tification of efficient p-type photocathode materials.[13,14] The key point of highly effi-cient photocathodes lies on the design and development of the affordable semicon-ductors with high light absorbance, short diffusion distance of minority carriers, as well as large surface area for charge sepa-ration and interfacial redox reactions.[15,16] It is thus highly desirable to develop a photocathode that meets all the above requirements.

Copper chalcopyrite-based p-type semiconductors such as CuInSe2,[17,18] CuGaSe2,[19,20] CuInS2,[21,22] CuGaS2,[23,24] and their mixed crystals are very attractive candidates for solar-driven water splitting due to their high absorption coefficients (10−4–10−5), relatively high carrier mobility,

tunable band gap values (1.0–2.4 eV), and suitable band align-ment for water reduction.[25,26] Especially, CuInS2 (direct band gap, ≈1.5 eV) provides large photocarrier density under illu-mination and does not require a highly toxic Se source.[27,28] Recently, various routes have been proposed to prepare CuInS2 photocathodes for efficient PEC water reduction. Among them, nonvacuum methods have attracted much attention owing to their low-cost fabrication processes. Until now, there are mainly two nonvacuum methods for the fabrication of CuInS2 photocathodes: direct electrochemical film deposition and film formation by drop-casting the presynthesized nanoparticles or nanoinks.[25,28] However, the PEC efficiency of the CuInS2 pho-tocathodes prepared by the drop-casting deposition methods still lags far behind the vacuum evaporation methods because of the charge recombination at the boundaries of the nanocrys-tals.[29] Though numerous electrochemical deposition methods have been investigated to fabricate efficient CuInS2 photocath-odes, it is challenging to control the stoichiometry and compo-sition of the films.[30]

To realize highly efficient PEC photoelectrodes, 1D nanoarray architectures such as nanowire arrays (NWAs),[31,32] nanorod arrays,[33,34] and nanotube arrays[35,36] are especially attractive as they have intrinsic advantages of enhanced light harvesting,

Dr. M. Li, R. Zhao, Prof. Y. Su, Dr. J. Hu, Prof. Z. Yang, Prof. Y. ZhangKey Laboratory for Thin Film and Microfabrication of the Ministry of EducationDepartment of Micro/Nano ElectronicsSchool of ElectronicsInformation and Electrical EngineeringShanghai Jiao Tong UniversityShanghai 200240, P. R. ChinaEmail: [email protected]; [email protected]

DOI: 10.1002/admi.201600494

This paper reports a facile self-templated method to prepare hierarchically CuInS2 nanosheet-constructed nanowire arrays (NCNAs) using Cu2S nano-wires arrays (NWAs) as the template. The as-synthesized CuInS2 nanosheets show ultrathin thickness of ≈1.2 nm, corresponding to the thickness of 4 atomically thick CuInS2 slab along the [221] direction. The CuInS2 nanosheet-constructed nanowires exhibit diameters of several hundred nanometers and lengths of several micrometers. The novel exchange-peeling growth mecha-nism suggests that the In3+ insertion proceeds preferentially along the (−204) facets of pristine Cu2S nanowires, and the distortions and strains sourced from lattice mismatch cause the longitudinal expansion along the c-axis and the splitting of S−S bond during the formation of 3D CuInS2 NCNAs. It is also found that relative higher In3+ concentration is beneficial to this process. Compared to 0.15 mA cm−2 of the pristine Cu2S NWAs, the CuInS2 photocath-odes show an enhanced photocurrent of 0.49 mA cm−2 at −0.1 V versus the reversible hydrogen electrode, and the photocurrent can be further increased to 1.14 mA cm−2 via decoration with CdS quantum dots. The density func-tional theory calculation results confirm that the ultrathin CuInS2 nanosheets favor for higher carrier mobility, thus ensure promoted photoelectrochemical efficiency.

1. Introduction

As one of the most attractive approaches to store solar energy by producing hydrogen in an eco-friendly manner without carbon emission, solar-driven photoelectrochemical (PEC) water splitting has aroused significant interest in the recent years.[1,2] A tandem water splitting system has emerged as the most viable solution for practical applications, which consists

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decoupling light absorption and charge carrier collection, shortening minority carrier diffusion, and increased electrode/electrolyte interface for charge separation compared to bulk structures.[37,38] Furthermore, a 3D branched nanoarray struc-ture can be used to further increase the light absorption and contact surface area, thus enhance the PEC performance accord-ingly.[39,40] On the other hand, 2D nanosheets have recently attracted considerable attention in the areas of PEC water split-ting owing to their unique electronic and optical properties, as well as their extremely large surface areas.[41] Moreover, the lay-ered junction can also shorten the charge transport time and distance, thereby promoting the separation of photocarriers and then improving the PEC performance.[42,43] Therefore, it is rea-sonable to predict that the PEC performance would be further improved by applying a photoelectrode with 3D hierarchically nanosheet-constructed NWAs (NCNAs) because it can integrate the advantages of both 1D nanoarrays and 2D nanosheets. Nev-ertheless, to the best of our knowledge, there is so far no report on the synthesis of CuInS2 NCNAs.

Herein, we demonstrate the solvothermal synthesis of hierarchically CuInS2 NCNAs by a novel self-sacrificial tem-plate-directed method using Cu2S NWAs as the template.

The formation process of hierarchically CuInS2 NCNAs was investigated in detail, and a novel exchange-peeling growth mechanism was proposed to illustrate the formation of CuInS2 NCNAs from Cu2S NWAs. The PEC performance of the CuInS2 NCNAs photocathodes was characterized compared to the pristine Cu2S NWAs, which can be further improved by decorating CdS quantum dots (QDs). Additionally, the density functional theory (DFT) calculations were carried out to study the electronic and band structures of the CuInS2 nanosheets.

2. Results and Discussion

The Cu2S nanowires as templates were grown on Cu foils by a facile gas–solid reaction method,[44] which show diameters of ≈200 nm and lengths of several micrometers (Figure 1b). To prepare CuInS2 NCNAs, the Cu2S NWAs were solvothermally treated in an autoclave containing InCl3 and thioacetamide in ethylene glycol solution, and the formation process of CuInS2 NCNAs is illustrated in Figure 1a. As shown in Figure 1c, the average diameter of the nanowires is increased to ≈400 nm after only 0.5 h reaction, indicating the reaction between Cu2S


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Figure 1. a) Schematic illustration for the formation process of CuInS2 NCNAs. b–g) SEM images of the samples obtained with various reaction times. h) XRD patterns of the samples obtained with various reaction times. i) Relationship of the reaction time with the average nanowire diameter for the samples. j) Relationship of the reaction time with the atomic ratio for the samples.

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and In3+. When the reaction time is prolonged to 1 h, the nanowires start to exhibit a flocky morphology (Figure 1d). Fur-ther increasing the reaction time to more than 2 h, 3D NCNAs can be observed (Figure 1e–g). The top view scanning electron microscopy (SEM) image of a broken nanosheet-constructed nanowire (NCN) demonstrates that the nanowire is entirely constructed by interconnected nanosheets (Figure S1, Sup-porting Information).

X-ray diffraction (XRD) was used to investigate the phase transformation from Cu2S to CuInS2, as shown in Figure 1h. After 0.5 h reaction, the diffraction peaks of Cu2S (JCPDS card No. 33–0490) become very weak,[44] while three dominant peaks emerge at 28°, 47°, and 55° corresponding to the (112), (220), and (132) planes of the tetragonal structured CuInS2 (JCPDS card No. 47–1372), respectively.[29] Meanwhile, two impurity phases for In2S3 can be observed at 27° and 30.2° (JCPDS card No. 33–0624). As the reaction time is prolonged to more than 2 h, all the peaks of Cu2S disappear, and the intensity of the In2S3 impurity peaks can also be ignored compared to that of the diffraction peak for the (112) plane of CuInS2. The results indicate that the reaction time of 2 h is enough for the com-pletely transformation of Cu2S into CuInS2. Furthermore, Raman spectroscopy was used to obtain further insight into phase identification of the CuInS2 nanostructures (Figure S2, Supporting Information). A strong peak at 294 cm−1 and a relatively weak peak at 340 cm−1 for CuInS2 can be observed, which is consistent with the reported Raman study for CuInS2

nanostructures.[45,46] Additionally, a weak peak at 327 cm−1 can be assigned to the In2S3 impurity, and no other Raman peaks related to CuxS can be observed. The existence of In2S3 impu-rities may result from the formation of In2S3 microparticles in the reaction solution (Figure S3, Supporting Information), which can easily adhere to the CuInS2 NCNAs in the solvo-thermal reaction process. Fortunately, In2S3 was reported to be an efficient n-type surface modifier for improving the PEC properties of CuInS2-based photocathodes.[27,47]

As shown in Figure 1i, the average diameter of the nano wires firstly increases with reaction time and then reach a plateau at the reaction time of 4 h, which can attribute to the increase of CuInS2 content and formation of the loosened 3D NCNAs mor-phology with prolonging the reaction time. The lengths of the CuInS2 NCNs (several micrometers, see Figure S4, Supporting Information) are consistent with that of the pristine Cu2S nanowires. As the electrical properties of the Cu–In–S strongly depend on the stoichiometry,[48] the energy-dispersive X-ray spectroscopy (EDX) was used to analyze the composition of the as-synthesized samples, and the results were summarized in Table S1 of the Supporting Information. The EDX results sug-gest that the slightly Cu-rich CuInS2 NCNAs should behave as a p-type semiconductor because the expression of (Cu+3In)/2S is smaller than unity (Figure 1j).[49]

Transmission electron microscopy (TEM) was used to fur-ther observe the microstructures of the CuInS2 NCNs. As shown in Figure 2a–c, the nanowires are entirely composed of


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Figure 2. a–c) Typical TEM images of CuInS2 NCNs at different magnifications. d) Representative SAED pattern acquired from the CuInS2 NCN in (c). e,f) TEM images of the CuInS2 nanosheets at different magnifications. g) HRTEM image acquired from the circular region of the CuInS2 nanosheet in (f), inset: the corresponding FFT pattern. h–j) Element mapping of Cu, In, and S, respectively, for a CuInS2 NCN by EDX.

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CuInS2 nanosheets. The representative selected area electron diffraction (SAED) pattern (Figure 2d) reveals their polycrys-talline nature. The TEM image of typical CuInS2 nanosheets (Figure 2e) demonstrate that they have thickness as thin as ≈1.2 nm, corresponding to the 1.28 nm thickness of 4 atomi-cally thick CuInS2 slab along the [221] direction. The high-mag-nification TEM image (Figure 2f) further proves the high crys-tallinity of the CuInS2 nanosheets. Figure 2g shows the top view high-resolution transmission electron microscopy (HRTEM) image of a typical CuInS2 nanosheet, in which the continuous lattice fringes of 0.20 nm and the angle of 60° consist well with those of the (2–20) and (20–4) facets, respectively. The corre-sponding quasi-hexagonal-shaped fast Fourier transforma-tion (FFT) pattern (inset, Figure 2g) is a typical feature of the reciprocal lattice projected along the [221] zone axis, revealing that the CuInS2 nanosheets are highly oriented. The fringe spacing of 0.32 nm in the side view HRTEM image of a CuInS2 nanosheet (Figure S5, Supporting Information) matches well with the interplanar spacing of the {112} planes. It can be con-cluded that the ultrathin CuInS2 nanosheets exhibit preferen-tially exposed (112) facets, which is consistent with the XRD results (Figure 1h). Furthermore, EDX mapping of a CuInS2 NCN (Figure 2h–j) show that the Cu, In, and S element sig-nals are distributed homogeneously through the NCN, further proving the complete transformation of Cu2S into CuInS2.

X-ray photoelectron spectroscopy (XPS) was used to fur-ther confirm the composition and valence states of the CuInS2 NCNAs. The survey XPS spectrum (Figure 3a) proves the presence of Cu, In, and S elements in the surfaces of CuInS2 NCNs, and the Cu 2p, In 3d, and S2p core levels were also

studied in detail, respectively. The banding energies of Cu 2p1/2 and Cu 2p3/2 centered at 951.6 and 931.8 eV can be unambigu-ously assigned to the Cu+ sate, and the absence of satellite peak between Cu 2p1/2 and Cu 2p3/2 rules out the possibility of Cu2+ (Figure 3b).[50] The In 3d core peaks occurring at 3d3/2 (452.1 eV) and 3d5/2 (444.6 eV) are consistent with a valence state of +3 in CuInS2 (Figure 3c).[51] The S 2p core splits into two obvious peaks at 162.6 and 161.3 eV, which can be assigned to the S 2p1/2 and S 2p3/2 due to the different binding with In and Cu, respectively (Figure 3d).[52] According to the XPS meas-urements, the observed binding energies of Cu 2p, In 3d, and S 2p core levels of CuInS2 are in good accordance with those reported.[52,53]

To investigate the growth procedure of the CuInS2 NCNAs, TEM was used to characterize the nanostructures of the as-synthesized sample obtained with 0.5 h reaction (Figure 4). It can be clearly seen that the nanowire has been partially eroded during the reaction process, and exhibits an irregular, screw-like morphology. Moreover, the screw direction is parallel to the growth direction of the pristine Cu2S nanowires, i.e., the c axis of the monoclinic Cu2S, indicating that the reaction proceeds preferentially along the (−204) facets of Cu2S nanowires.

Based on the above results, an exchange-peeling growth mechanism can be proposed to illustrate the transfer proce-dure from Cu2S NWAs to CuInS2 NCNAs, as shown Figure 1a. During the solvothermal treatment, H2S will be released via the reaction between thioacetamide and water in the heated solu-tion (CH3CSNH2 + H2O → CH3CONH2+H2S), which then decomposes and provides H+ and S2− ions (H2S → 2H+ + S2−). As the screw direction of the as-synthesized nanowire (Figure 4)



Figure 3. XPS spectra of CuInS2 NCNAs: a) Survey, b) Cu 2p, c) In 3d, and d) S 2p.

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is parallel to the c-axis of the pristine Cu2S nanowires, it can be inferred that Cu2S nanowires are firstly eroded by the H+ ions preferentially along the (−204) facets of Cu2S,[29] which can be ascribed to the alternated Cu and S layers structure along c-axis of Cu2S nanowires.[54] Meanwhile, the Cu+ ions diffusing outward exchange with the In3+ ions diffusing inward form CuInS2 in situ.[25] Benefiting from the unique layer structure of Cu2S nanowires, the distortions and strains sourced from lattice mismatch cause the longitudinal expansion along the c-axis, and then lead to the splitting of SS bond, ultimately resulting in the peeling of CuInS2 nanosheets and the forma-tion of 3D NCN morphology.[55] Additionally, the released Cu+ ions can also react with In3+ and S2− in the solution, leading to the growth of CuInS2 nanosheets (Cu+ + In3+ + 2S2− → CuInS2). As the interplanar spacing of the {−204} planes of Cu2S nanowire (0.34 nm) has a small discrepancy (≈5.9%) with that of the {112} planes of CuInS2 (0.32 nm),[44] the (−204) facets of Cu2S are beneficial for the formation of CuInS2 nanosheets with preferentially exposed (112) facets.

It has been found that the In3+ concentration in the reac-tion solution has a great influence on the formation of CuInS2 NCNAs. Figure 5 shows the SEM images of the samples obtained with different In3+ concentrations. It can be seen that at the In3+ concentration of 0.05 m orderly protruding nanoplates with thickness of ≈50 nm vertically grow on the sur-faces of nanowires (Figure 5a), further proving that the (−204)

facets of Cu2S nanowires are the preferential planes for the growth of CuInS2. When the concentration is increased to 0.15 m, CuInS2 nanoflakes with thickness of ≈10 nm can be observed on the nanowires (Figure 5b), and the impurity phase of In2S3 is negligible (Figure S6, Supporting Information). Further increasing the concentration to 0.25 m, the nanowires are entirely composed of ultrathin CuInS2 nanosheets (Figure 5c) with relatively pure phase (Figure S6, Supporting Informa-tion). However, the NWAs morphology will be destroyed when the concentration of In3+ is increased to as high as 0.5 m (Figure S7, Supporting Information). The above results suggest that relative higher In3+ concentra-tion can enhance the reaction rate and is ben-eficial for the insertion of In3+ to erode more (−204) facets of Cu2S nanowire for the forma-tion of CuInS2 NCNs.

The CuInS2 NCNAs have been further dec-orated with CdS QDs using a facile successive ionic layer adsorption and reaction (SILAR) method to accelerate the charge separation at the electrode and electrolyte interface in the PEC water splitting process. The deposition cycle time was used to control the amounts of CdS QDs, and the CuInS2/CdS-x NCNAs were obtained, where x represents the depo-sition cycle of CdS. Figure 6a–c shows SEM images of the CuInS2/CdS-10 NCNAs at different magnifications. It can be observed that the surfaces of NCNs become obviously

coarse after 10 cycles deposition, suggesting the successful decoration of CdS QDs, which has been confirmed by the EDX analysis (Figure S8, Supporting Information). The TEM charac-terization of CuInS2/CdS-10 nanosheets (Figure 6e,f) demon-strates the uniform distribution of CdS QDs on their surfaces, as illustrated in Figure 6d. The corresponding HRTEM image (Figure 6g) shows that the CdS QDs exhibit sizes of several nanometers, and the lattice fringes enclosed within the circles correspond to the (111) planes of CdS.[56] The corresponding SAED pattern (Figure 6h) also reveals the polycrystalline nature of the CdS layer. Additionally, the EDX mapping of a CuInS2/CdS-10 NCN (Figure 6i–l) show that the Cu, In, Cd, and S element signals are distributed homogeneously through the nanowire, further testifying the uniform distribution of CdS QDs on the whole surfaces of CuInS2 NCNs.

The PEC properties were investigated using a three-electrode configuration with Ag/AgCl as the reference electrode and a Pt mesh as the counter electrode in an aqueous solution containing 1.0 m KCl (pH = 5.97).[14] Figure 7a shows the linear sweep vol-tammetry (LSV) curves under chopped white light illumination (AM 1.5G, 100 mW cm−2) for the CuInS2 NCNAs and the pris-tine Cu2S NWAs. It is obvious that the CuInS2 photo cathode exhibits greatly enhanced photocurrent compared to the pris-tine Cu2S NWAs. The absolute photo current of CuInS2 NCNAs at −0.1 V versus the reversible hydrogen electrode (RHE) has reached 0.49 mA cm−2, which is about 3.3 times larger than



Figure 4. Different TEM images of the sample obtained with 0.5 h solvothermal reaction time: a) Low magnification image for the top of a nanowire, b,c) High magnification image for the different edges of the nanowire show in (a), and d) the corresponding HRTEM image.

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that of the pristine Cu2S NWAs (0.15 mA cm−2). Furthermore, the photocurrent of the CuInS2 NCNAs decreases with the pos-itive scanning of the applied bias, indicating a standard p-type feature of CuInS2, which is consistent with the previous EDX analysis (Figure 1j). Figure 7b shows a close comparison of the

transient current density at 0 V versus RHE under chopped illumination for the corresponding photocathodes. It can be seen that the CuInS2 NCNAs demonstrate not only enhanced photocurrent but also improved stability. Additionally, it can also be observed that the photocurrent of the CuInS2 NCNAs



Figure 6. a–c) SEM images of the CuInS2/CdS10 NCNAs at different magnifications. d) Schematic illustration for a CuInS2 NCN decorated with CdS QDs. e,f) TEM images of the CuInS2/CdS10 nanosheets at different magnifications. g) HRTEM image acquired from the circular region of the CuInS2/CdS10 nanosheet in (f). h) SAED pattern acquired from the CuInS2/CdS10 nanosheet in (f). i–l) Element mapping of Cu, In, Cd, and S, respectively, for a CuInS2/CdS10 NCN by EDX.

Figure 5. SEM images of the samples obtained with different In3+ concentrations: a) 0.05 m, (b) 0.15 m, and c) 0.25 m.

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photocathode exhibits a prompt rise under each illumination, and quickly drops when the light is switched off, implying that the charge transport of the photoelectrode is faster than the chopper.

To investigate the reason for the enhanced PEC performance, the light absorbance of both CuInS2 NCNAs and Cu2S NWAs were characterized (Figure S9, Supporting Information). It can be observed that the CuInS2 NCNAs exhibit enhanced light trapping ability compared to the pristine Cu2S NWAs over the wavelength range of 350–850 nm, especially in the long wave-length region. For instance, the CuInS2 NCNAs and the Cu2S NWAs show absorbance of 97.5% and 94.8% at 600 nm, respec-tively. The band gap is estimated to be ≈1.25 and ≈1.53 eV for Cu2S NWAs and CuInS2 NCNAs, respectively (Figure S10, Sup-porting Information). Both the optimized band gap and the hierarchically 3D morphology for the CuInS2 NCNAs are in favor of the improved light absorption for the CuInS2 NCNAs,

which then results in enhanced PEC performance. Further-more, the transient current density for the CuInS2 nanostruc-tures obtained with different reaction times at 0 V versus RHE under chopped illumination was also characterized. The photo-current density of the samples increases with reaction time and then reaches a plateau at 2 h (Figure S11, Supporting Infor-mation). The results further suggest that the 3D NCNAs mor-phology can be beneficial for improving the PEC ability because the ultrathin thickness of CuInS2 nanosheets can provide short diffusion distance for excellent charge transport, and greatly increased contact area for fast interfacial photocarrier separa-tion and PEC reactions.[41–43]

As shown in Figure 7c, the PEC performance of CuInS2 NCNAs has been further enhanced by decorating CdS QDs, which can be mainly ascribed to the enhanced charge sepa-ration between CdS and CuInS2 due to the formation of p–n junction because only slightly absorption improvement



Figure 7. a) LSV curves of the pristine Cu2S NWAs and CuInS2 NCNAs under chopped AM 1.5G (100 mW cm−2) simulated solar illumination. b) Amperometric I–t curves of the pristine Cu2S NWAs and the CuInS2 NCNAs at 0 V versus RHE under chopped illumination. c) LSV curves of the CuInS2/CdSx NCNAs under chopped illumination. d) Amperometric I–t curves of the CuInS2/CdSx NCNAs at 0 V versus RHE under chopped illumination. e) Nyquist plots of the CuInS2 and CuInS2/CdS10 NCNAs under illumination. f) Roomtemperature PL spectra of CuInS2 and CuInS2/CdS10 NCNAs, excited at 587 nm.

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can be observed in the short wavelength region (Figure S12, Supporting Information).[29] The photocurrent of CuInS2/CdS-x NCNAs at −0.1 V versus RHE (Figure S13, Supporting Information) first increases along with the increase of the deposition cycle of CdS and reaches a maximum value of 1.14 mA cm−2 for the CuInS2/CdS-10 NCNAs. However, the photocurrent of CuInS2/CdS NCNAs decreases with the fur-ther elongation of deposition cycle as charge transfer will be hindered by a dense packing of CdS QDs.[7] The above results suggest that the decoration of CdS QDs onto CuInS2 NCNAs at a suitable level can effectively enhance the PEC activity. As shown in Figure 7d, the I–t curves of the CuInS2/CdS-x NCNAs at 0 V versus RHE under chopped illumination also demonstrate the enhanced PEC activity as well as good switching behavior of the samples.

The electrochemical impedance spectra (EIS) of the CuInS2-based photocathodes under illumination were performed to investigate the reason for the enhanced PEC performance (Figure 7e). It can be observed that all Nyquist plots display a semicircle at high frequencies whose diameter represents the charge transfer resistance (Rct), which controls the electron transfer kinetics of the redox probe at the electrode interface.[31] The corresponding equivalent circuit is depicted in the inset of Figure 7e, where Rs denotes the bulk resistance, originating from the electrolyte and the electrode, CPE is the constant phase element that models capacitance of the double layer, and W stands for the Warburg impedance originated from the dif-fusion process at the electrode surface. It is obvious that the Rct for the CuInS2/CdS-10 NCNAs under illumination is much smaller than that of the bare CuInS2 NCNAs. The results indicate that the decoration of CdS QDs can facilitate electron transfer from CuInS2 to electrolyte due to the formation of p–n junction.[29]

The recombination and transfer behaviour of the photogen-erated carriers in the heterojunction is further verified by pho-toluminescence (PL) as shown in Figure 7f. The spectrum of bare CuInS2 NCNAs exhibits a broad emission at 767 nm corre-sponding to peak energy of 1.62 eV, which is attributed to near-band-edge emission. The emission peak of CuInS2 NCNAs is close to that previously reported for the CuInS2 nanoparticles (1.68 eV).[57] After the modification of CdS QDs, the PL inten-sity of CuInS2 NWAs is significantly deceased. This quenching behavior confirms the excited-state interaction between the two semiconductors and demonstrates deactivation of the excited CuInS2 nanosheets via electron transfer to the CdS layers. That is to say, the photo-generated electrons and holes can be sepa-rated more efficiently in the CuInS2/CdS heterostructure. This result matches well with the EIS analysis, and also proves the enhanced PEC performance by CdS QDs decoration.

To further evaluate the PEC performance of the CuInS2/CdS NCNAs-based photocathodes, the incident photon to current efficiency (IPCE) measurement was carried out at 0 V versus RHE. As shown in Figure 8a, the CuInS2/CdS-10 NCNAs show relatively higher IPCE in the short wavelength region, and exhibit an optimal IPCE of 11.0% at 450 nm. Fur-thermore, the stability of the CuInS2/CdS-10 NWAs photocath-odes was evaluated with chronoamperometric measurements at 0 V versus RHE under continuous AM 1.5G irradiation over 30 min (Figure 8b). When the light is turned on (inset,

Figure 8b), a transient photocurrent density of 0.34 mA cm−2 can be observed, which then dramatically decreases to a rela-tively stable value of 0.22 mA cm−2 in 20 s. The photocathode current overshoots in the beginning of switching light on can be ascribed to the accumulation of photogenerated carriers at the interfaces between NCNs and electrolyte.[39] During the continuous irradiation over 30 min, fluctuating current can be clearly seen. Additionally, as the light is turned off, the photo-current is reduced to 0.12 mA cm−2. The above phenomena can be attributed to the photocorrosion of the photocathodes (Figure S14, Supporting Information) as both CuInS2 and CdS are not good photocorrosion-resistant material. Luo et al.[29] reported that the modification of aluminum-doped zinc oxide and titanium dioxide protecting layers on the CuInS2/CdS hetero junction nanostructured films by atomic layer deposition method can greatly improve the photocurrent as well as the sta-bility of the photocathodes. It is believed that both the photo-current and stability of the CuInS2/CdS NCNAs photocathodes can be further improved by the modification of these oxide pro-tecting layers.

Finally, the DFT calculations were carried out to study the electronic and band structures of the CuInS2 nanosheets with 4 atomic thicknesses (Figure 9a, Supporting Information). As shown in Figure 9b, the calculated density of states (DOS) by the Perdew–Burke–Ernzerhof (PBE) method reveal that the atomically thin CuInS2 nanosheets possess greatly increased DOS at the edge of valence band with respect to bulk coun-terpart.[58–60] It is noticeable that the increased DOS near the



Figure 8. a) IPCE spectrum of the CuInS2/CdS10 NCNAs at 0 V versus RHE. b) Photocurrent density decay at 0 V versus RHE for the CuInS2/CdS10 NCNAs photocathode under continuous AM 1.5G irradiation, inset: the magnification of the change in photocurrent for this electrode between 580 and 630 s.

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Fermi level for the atomically thin nanosheets can lead to increased carrier mobility, which implies that the ultrathin CuInS2 nanosheets with 4 atomic thicknesses favor for higher carrier mobility, thus ensure higher PEC efficiency. The calcu-lated band gap for CuInS2 (≈1.0 eV) is lower than the experi-mental value of ≈1.53 eV, as the band gap energy is often underestimated by the PBE method. However, the discussion of DOS is not affected by this underestimation as only changes of DOS between ultrathin and bulk CuInS2 are compared, sim-ilar to previous reports.[12,32]

3. Conclusions

In conclusion, we have demonstrated the synthesis of hier-archically CuInS2 NCNAs via a self-templated method using Cu2S NWAs as the template. Experimental results suggest that the unique alternated Cu and S layers structure along the c-axis of Cu2S nanowires and the relatively high In3+ concentra-tion in the reaction are the two key factors for the growth of CuInS2 NCNAs. A novel exchange-peeling growth mechanism is proposed to illustrate the formation of CuInS2 NCNAs. The CuInS2 NCNAs photocathodes have shown greatly enhanced PEC activity compared to the pristine Cu2S NWAs, and the PEC performance has been further improved by decorating CdS QDs. The DFT calculation results indicate that the ultrathin CuInS2 nanosheets exhibit higher carrier mobility compared to the bulk counterpart. Though here only the transformation of Cu2S NWAs into CuInS2 NCNAs is demonstrated, there is no barrier to generalize the method to other copper chalcopy-rite-based NCNAs, which may show numerous applications in photo voltaic and optoelectronic devices.

4. Experimental Section

Transformation of Cu2S NWAs into CuInS2 NCNAs: The pristine Cu2S NWAs were grown on Cu foils at room temperature in an airtight stainless steel reactor via a gas–solid reaction method.[44] The Cu2S NWAs were transformed into CuInS2 NCNAs by solvothermal treatment in 0.25 m InCl3 and 0.5 m thioacetamide ethylene glycol solution. Typically, a Cu2S NWAs substrate (2 cm × 2 cm) was put into a Teflon autoclave angled against the vessel wall, facing down, and the solution was poured into the autoclave to immerse the sample. The autoclave was then put into an oven heating at 200 °C for a certain reaction time (2–8 h) and cooled down to room temperature naturally.

Decoration of CdS QDs onto CuInS2 NCNAs: The CdS QDs were decorated onto the CuInS2 NCNAs by a successive ionic layer adsorption and reaction (SILAR) method. Typically, the CuInS2 NCNAs were immersed into the following solutions in sequence: 0.1 m Cd(NO3)2 ethanol solution for 20 s, deionized water for 10 s, 0.1 m Na2S aqueous solution for 20 s, and then ethanol for 10 s. The fourstep process is called one SILAR cycle, and the desired amount of CdS QDs was obtained by controlling the deposition cycle.

Characterization: The morphologies were characterized by a field emission scanning electron microscopy (FESEM, Carl Zeiss Ultra 55, Germany) operating at 20 kV. TEM (JEM2100, JEOL, Japan) operating at 200 kV was used to analyse the microstructures. The crystalline structure was characterized by XRD using an advanced Xray diffractometer (D8 ADVANCE, Bruker, Germany) in the diffraction angle range 2θ = 20°–80°, with Cu Kα radiation (λ = 0.154056 nm) at voltage of 40 kV and a current of 40 mA. The Raman spectra were acquired using a dispersive Raman microscope (Senterra R200L, Bruker Optics, Germany), operated with a 532 nm laser. The XPS was performed using a Japan Kratos Axis UltraDLD spectrometer with a monochromatic Al Kα source (1486.6 eV). The UV–visNIR absorption was carried out using an UV–visNIR spectrophotometer (Lambda 950, PerkinElmer, USA). The photoluminescence (PL) spectra were obtained with a steadystate and timeresolved fluorescence spectrofluorometer (QM/TM/IM, PTI, USA).

PEC Measurements: A PEC test system composed of a electrochemical station (CHI 650E) and a solar simulator (CHFXM500, Beijing Perfectlight) using a 500 W Xenon lamp and equipped with AM 1.5G filter was used to carry out the PEC characterizations. For easy measurement, the samples were fixed on glass slides by epoxy resin. As shown in Figure S15 of the Supporting Information, a copper wire was connected to the side part of the Cu substrate using conductive silver paint. To prevent photocurrent leakage, the uncoated parts of the electrode were isolated with epoxy resin, and the exposed area was ≈1.0 cm2. The sample as working electrode, Pt mesh as counter electrode, and Ag/AgCl (saturated KCl) as reference electrode were the three electrodes. A 1.0 m KCl aqueous solution (pH = 5.97) was used as an electrolyte. The EIS were carried out with a sinusoidal perturbation with 5 mV amplitude and frequencies ranging from 100 kHz to 0.1 Hz. The IPCE was measured under monochromatic irradiation from the Xenon lamp equipped with bandpass filters. The IPCE is expressed as IPCE = (1240I)/(λPlight), where I is the photocurrent density (mA cm−2), λ is the incident light wavelength (nm), and Plight (mW cm−2) is the power density of monochromatic light at a specific wavelength.

Density Functional Theory Calculation: The DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) code.[58–60] Generalized gradient approximation in the parametrization of PBE was chosen.[59] The energy cutoff was set to be 550 eV. Geometry structures are fully relaxed until the convergence criteria of energy and force were less than 10−5 eV and 0.01 eV Å−1, respectively. A MonkhorstPack mesh of 7 × 7 × 1 kpoints was used in the Brillouin zone for geometry optimizations and electronic structure calculations. The [221]oriented tetragonal CuInS2 quadruple layer was simulated by periodically repeating the four atomic layers along the [221] direction of the unit cell. We also used a large vacuum space to calculate the absolute position of the band dispersion.



Figure 9. a) Structural models of CuInS2. b) Calculated DOS by the PEB function for the ultrathin CuInS2 nanosheets with four atomic thicknesses and the bulk counterpart.

FULL

PAPER


Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the National Natural Science Foundation of China (Nos. 51402190 and 61574091). The authors thank Dr. Yunhui Wang from Nanjing University of Science and Technology for helping the theory calculation. The authors also acknowledge the analysis support from the Center for Advanced Electronic Materials and Devices and the Instrumental Analysis Center of SJTU.

Received: May 31, 2016Revised: July 7, 2016

Published online:

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