Constructing Direct Z‐Scheme Heterostructure by Enwrapping ZnIn2S4 on CdS Hollow Cube for Efficient Photocatalytic H2 Generation

Abstract Rational design hybrid nanostructure photocatalysts with efficient charge separation and transfer, and good solar light harvesting ability have critical significance for achieving high solar‐to‐chemical conversion efficiency. Here a highly active and stable composite photocatalyst is reported by integrating ultrathin ZnIn2S4 nanosheets on surface of hollow CdS cube to form the cube‐in‐cube structure. Experimental results combined with density functional theory calculations confirm that the Z‐scheme ZnIn2S4/CdS heterojunction is formed, which highly boosts the charge separation and transfer under the local‐electric‐field at semiconductor/semiconductor interface, and thus prolongs their lifetimes. Moreover, such a structure affords the highly enhanced light‐harvesting property. The optimized ZnIn2S4/CdS nanohybrids exhibit superior H2 generation rate under visible‐light irradiation (λ ≥ 420 nm) with excellent photochemical stability during 20 h continuous operation.


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and high-resolution TEM (HRTEM, FEI TalosF200S). X-ray diffraction (XRD) measurements were performed with a Rigaku D/Max-2550 diffractometer using Cu Kα radiation (λ = 1.54056 Å) at 50 kV and 200 mA in the 2θ range of 20-80° at a scanning rate of 4° min −1 . Electron paramagnetic resonance (EPR) spectra were performed on a Bruker A300 spectrometer to detect the hydroxyl radical (•OH) with the assistance of 5,5-Dimethyl-l-pyrroline N-oxide (DMPO) under visible-light irradiation at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed on an AXIS Supra spectrometer using monochromatized Al Kα excitation. Physisorption analyzer (Micromeritics ASAP 2420) was used to measure the surface area and the pore volume of the catalysts at −196°C. Before each measurement, the samples were degassed at 150°C for 3 h to remove the physically adsorbed moisture. The UV-vis diffuse reflectance spectra (UV-vis DRS) were measured on a UV-vis-NIR spectrophotometer (Shimadzu UV-3600) equipped with an integrating sphere assembly, using BaSO 4 as the reflectance sample. The steady-state photoluminescence (PL) emission spectra with an excitation wavelength of 350 nm were measured on a spectrofluorometer HORIBA,France). A spectrofluorometer (FLS980, Edinburgh, England) was employed to record time-resolved PL (TRPL) spectra. The average lifetime () of charge carriers can be estimated according to the following Equation S1: where τ 1 is the short carrier lifetime attributed to quasi-free excitons, τ 2 is the long component due to localized exciton recombination, A 1 and A 2 are the percentages of the short and long component in the total lifetime, respectively.

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The lock-in-based surface photovoltage (SPV) spectrum was measured in a photovoltaic cell with a fluorine-doped tin oxide (FTO)-sample-FTO sandwich structure.

Electrochemical and photoelectrochemical measurements
The electrochemical and photoelectrochemical (PEC) measurements were performed on an electrochemical workstation (RST5000, Shiruisi Instrument Technology Co., Ltd., Zhengzhou) using a three-electrode system at room temperature with 0.5 M Na 2 SO 4 aqueous solution as the electrolyte.
The as-prepared electrode film, Pt wire and Ag/AgCl (saturated KCl) were served as the working electrode, counter electrode and reference electrode, respectively. For PEC test, the simulated solar light was supported by a 300 W Xe lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd) equipped with an AM 1.5G optical filter. Electrochemical impedance spectroscopy (EIS) was conducted under light irradiation under open circuit voltage with an amplitude of 5 mV over the S5 frequency range from 100 kHz to 0.01 Hz. Open circuit potential (OCP) decay curves were employed to analyze the charge decay behaviors, which were performed after turning off the light irradiation.
The carrier recombination rate can be fitted to the following Equation S2: where V, V light and V dark are the OCP values (V) at any time, under irradiation and in dark, respectively. k is the pseudo-first order recombination rate constant (s −1 ). The potential dependent carrier lifetime determined from OCP decay of the accumulated electrons can be measured in qualitative using the following Equation S3: where k B is the Boltzmann constant (1.38 × 10 −23 J K −1 ), T is the absolute temperature (298 K), e is the charge of electron (1.602 × 10 −19 C), and dV/dt is the derivative of the OCP transient decay.
Current-time (i-t) curves were conducted with a light on/off cycle for every 40 s at 0 V vs. Ag/AgCl.
Mott-Schottky (M-S) plots were performed and recorded over an AC frequency of 1000, 2000 and 3000 Hz in dark condition to obtain the flat band potentials (E fb ) according to the following Equation

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: where C is the space charge capacitance (F cm −2 ),  r is the relative dielectric constant of CdS or ZnIn 2 S 4 ,  is the vacuum permittivity (8.85 × 10 −12 F m −1 ), N D is the charge donor density (cm −3 ), E is the electrode applied potential (V),  is the Boltzmann's constant (1.38 × 10 −23 J K −1 ), T is the absolute temperature (K) which is generally small and could be neglected, and e is the electron S6 charge (1.60 × 10 −19 C).

Photocatalytic H 2 generation
The photocatalytic water splitting experiments were performed in a quartz glass reaction cell connected to an automatic on-line trace gas analysis system (Labsolar 6A, Beijing Perfectlight). In a typical experiment, 25 mg of photocatalyst was dispersed in a 0.35 M Na 2 S + 0.25 M Na 2 SO 3 aqueous solution (100 mL). After the system was degassed completely, the reaction solution was irradiated by a 300 W Xe lamp (PLS-SXE300D, Beijing Perfectlight) equipped with a 420 nm cut-off filter. The photoreactor was equipped with a cooling circulating water flow to maintain a constant temperature (5°C). H 2 gas generation was analyzed using an online gas chromatograph (GC-2014C, Shimadzu Co., Japan, with Ar as the carrier gas) equipped with an MS-5A column and a thermal conductivity detector (TCD). The stability tests were performed for 4 cycles of 20 h. After every cycle was done, the system needs to be fully degassed until vacuumed.
The apparent quantum efficiency (AQE) was measured under monochromatic light irradiation with the band-pass filters of 420, 450 and 500 nm, respectively. In detail, 25 mg of photocatalyst was dispersed in 100 mL of 0.35 M Na 2 S + 0.25 M Na 2 SO 3 aqueous solution. After the system was degassed completely under stirring, the solution was irradiated by a 300 W Xe lamp equipped with a band-pass filter. The AQE was calculated from the following formula: where N1 Figure S8 shows the photocatalytic activities of ZIS, CS and ZIS/CS in pure water under visible light irradiation (≥ 420 nm). No O 2 generation was detected during the whole reaction process. To estimate whether there are H 2 O 2 generated and its amount evaluated, the redox titration with KMnO 4 was performed ( 2MnO 4 -+ 5H 2 O 2 + 6H + → 2Mn 2+ + 5O 2 ↑ +8H 2 O ). Figure  vs. NHE). [S1] These results indicate that the VB-holes of CdS could oxide H 2 O to H 2 O 2 , thus proving the Z-scheme charge transfer pathway in ZIS/CS heterojunction.  Figure S11 displays the partial density of states (PDOSs) of Zn, In, S and Cd elements and total density of states (TDOS) of ZIS/CS. From Figure S11, the following two key informations can be obtained: i) the orbital contribution of each element and ii) the conduction band maximum (CBM) and valence band maximum (VBM) constitutions. More specifically, Cd and S are at the VBM while the In and S are at the CBM, indicating the VB and CB of ZIS/CS composite are CdS and ZnIn 2 S 4 , respectively. In this way, the photocatalytic H 2 generation reaction was confirmed from the CB side of ZIS and the heterojunction interface realized the Z-scheme charge transfer path. S16