Sulfur Vacancy and Ti3C2T x Cocatalyst Synergistically Boosting Interfacial Charge Transfer in 2D/2D Ti3C2T x /ZnIn2S4 Heterostructure for Enhanced Photocatalytic Hydrogen Evolution

Abstract Constructing an efficient photoelectron transfer channel to promote the charge carrier separation is a great challenge for enhancing photocatalytic hydrogen evolution from water. In this work, an ultrathin 2D/2D Ti3C2T x /ZnIn2S4 heterostructure is rationally designed by coupling the ultrathin ZnIn2S4 with few‐layered Ti3C2T x via the electrostatic self‐assembly strategy. The 2D/2D Ti3C2T x /ZnIn2S4 heterostructure possesses larger contact area and strong electronic interaction to promote the charge carrier transfer at the interface, and the sulfur vacancy on the ZnIn2S4 acting as the electron trap further enhances the separation of the photoinduced electrons and holes. As a consequence, the optimal 2D/2D Ti3C2T x /ZnIn2S4 composite exhibits a high photocatalytic hydrogen evolution rate of 148.4 µmol h−1, which is 3.6 times and 9.2 times higher than that of ZnIn2S4 nanosheet and flower‐like ZnIn2S4, respectively. Moreover, the stability of the ZnIn2S4 is significantly improved after coupling with the few‐layered Ti3C2T x . The characterizations and density functional theory calculation demonstrate that the synergistic effect of the sulfur vacancy and Ti3C2T x cocatalyst can greatly promote the electrons transfer from ZnIn2S4 to Ti3C2T x and the separation of photogenerated charge carriers, thus enhancing the photocatalytic hydrogen evolution from water.


Photodeposition of Pt
Typically, H 2 PtCl 6 solution (0.1 mg mL -1 ) was directly added in an aqueous triethanolamine solution containing a photocatalyst, and the amount of H 2 PtCl 6 depending on the mass ratio of Pt to the photocatalyst. The reactor was purged with ultra-pure Ar gas for 30 min. A 300 W Xenon lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd.) equipped with a 400 nm cutoff filter was used as the light source, and Pt species were reduced under light irradiation for 30 min while stirring constantly. Subsequently, the photocatalytic hydrogen evolution reaction was carried out after the reactor was purged with high purity Ar gas for 30 min.

Synthesis of x-TC/I-ZIS by in situ growth method
Different qualities of Ti 3 C 2 T x (4.20 mg, 8.46 mg, 16.90 mg) were homodispersed into an aqueous solution of glycerin (50 mL, 20%, pH = 2.5). Then, 1.0 mmol InCl 3 •4H 2 O, 0.5 mmol ZnCl 2 and 3.0 mmol thioacetamide were added into the above solution under stirring. The reaction mixture was then stirred in an oil bath at 80 C for 2 h. After cooling to room temperature, the samples were collected by centrifugation and washed with deionized water and ethanol for 3 times respectively, and the photocatalyst powder was obtained by freeze drying. The synthesized composites were labeled as x-TC/I-ZIS (x = 2 wt.%, 4 wt.%, 8 wt.%), and the sample without Ti 3 C 2 T x was labeled as I-ZnIn 2 S 4 .

Apparent quantum efficiency (AQE) measurement
The apparent quantum efficiency was measured in a quartz reactor.
Where r is the yield of hydrogen, N A is Avogadro constant, h is Planck constant, c is the speed of light, S is the illumination area, I is the light intensity, t is the illumination time, and λ is the wavelength of the light source.

Characterization
The zeta potential value of the sample was measured on a Nano-ZS90X at 25 °C.
Tapping-mode AFM measurement was performed on a 5100N (HITACHI, Japan) atomic force microscope. SEM images were recorded on a field emission scanning electron microscope (HITACHI SU8220, 10 kV). HRTEM images were recorded on a FEI Talos F200X high resolution transmission electron microscope with an acceleration voltage of 300 kV. The X-ray diffraction (XRD) patterns were collected on a SMARTLAB3KW powder diffractometer equipped with a Cu Kα radiation source. Raman spectra were collected on a HORIBA Scientific XploRA Plus spectrometer with laser excitation at 633 nm. Fourier transform infrared (FT-IR) spectra was obtained on a BRUKER TENSOR II infrared spectrometer. The N 2 adsorption and desorption curve, the Brunauer-Emmett-Teller (BET) specific surface area and pore size of the samples were obtained on a TriStar II system. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific K-Alpha spectrometer with Al Kα radiation excitation source. Ultraviolet photoelectron spectroscopy (UPS) spectra were collected on a Thermo Fisher Nexsa spectrometer and calibrated with Ag standards. The steady-state fluorescence spectra were recorded using a Thermo Scientific Lumina fluorescence spectrometer. Time-resolved fluorescence spectra was carried out on an Edinburgh FLS 1000 spectrometer equipped with a laser (λ = 365 nm) at room temperature.
The UV-Vis diffuse reflectance spectra (UV-Vis DRS) was collected on a TU-19 UV-Vis spectrophotometer. Surface photovoltage spectra (SPV) were recorded on an CEL-SPS 1000 system (Beijing China Education Au-light Co., Ltd.).

Photoelectrochemical measurements
Photoelectrochemical measurements were performed on a CHI760E electrochemical workstation using a three-electrode system with an Ag/AgCl electrode as reference electrode, Pt mesh as the counter electrode, and the photocatalyst as the working electrode. A 0.5 M Na 2 SO 4 solution was used as the electrolyte. The working electrodes were prepared as follows: 20 mg catalyst and 20 μL Nafion ® solutions were added into 400 μL anhydrous ethanol, and then treated by ultrasonic wave for 90 min, the obtained suspension was uniformly coated on an FTO substrate (1 cm × 1 cm) and dried at room temperature to obtain the working electrode. The transient photocurrent response (TPC) was measured with a 300 W Xenon lamp equipped with a 400 nm cut-off filter as the light source, and the light source was 15 cm away from the electrode surface and the bias voltage was 0.2 V. Electrochemical impedance spectroscopy (EIS) was measured in the dark with an alternating amplitude of 5 mV and a frequency range of 0.01 kHz to 1000 kHz. The Mott-Schottky curves were measured under the same conditions with an amplitude of 10 mV and frequency of 0.5 kHz, 1 kHz, and 1.5 kHz.

S vacancy capture and Electron paramagnetic resonance (EPR)
All measurements were performed on the Bruker EMX Plus spectrometer. At 77 K, S vacancy capture was performed in the dark. For ·O 2-, 5 mg of catalyst was added to 1 mL methanol, then 45 μL DMPO was added to the above suspension solution, and sonicated for 10 min. For ·OH, 5 mg catalyst was added to 1 mL ultrapure water, followed by adding of 45 μL DMPO and ultrasound for 10 min. A 500 W Xenon lamp with a 400 nm cutoff filter was used as the light source for free radical capture measurement. The first data point was collected in the dark, and the signal was collected at 10 min of illumination.