Enhanced Charge Separation and Hydrogen Evolution from Water over ZnTe-CdTe Solid Solution Based Photocathode with a Composition Gradient
Authors: Veiga Lionel Sebastian Sebastian , Sugiyama Masakazu , Minegishi Tsutomu 
Published: December 2024

Introduction

The evident climate change associated with the greenhouse emissions from fossil fuels urgently requires the transition towards sustainable and reliable carbon-neutral resources. Photoelectrochemical (PEC) generation of hydrogen through sunlight driven water splitting is a promising alternative to be used as an energy source and as a raw material for chemical feedstock. Achieving efficient PEC cells requires the fabrication of high-quality photocatalytic materials capable of absorbing sunlight efficiently while minimizing carrier recombination. Cadmium telluride (CdTe) has shown promise as photocathode because of its narrow band gap of 1.5 eV with an expected high photocurrent density of 29 mAcm-2. Nevertheless, challenges persist, particularly concerning the limited onset potential (OP) of 0.6 VRHE for cathodic photocurrent, crucial for hydrogen evolution from water.1

In the present study, the introduction of a multilayer structure with copper (Cu) and zinc telluride (ZnTe) as back contact onto the CdTe based photocathode is investigated2. Enhancement on photocurrent at higher potential was attributed to the interdiffusion during film preparation by close space sublimation (CSS) method, and the spontaneously formed functional structure with composition gradient. Consequently, the novel band alignment of the ZnxCd1-xTe (0≤x≤1), generates a gradient on the conduction band, enhancing charge separation and harnessing of electrons outside the depletion layer while reducing electron-hole recombination.

Experimental

Prior to CdTe and ZnTe deposition, a 3x3 cm glass slide coated with indium tin oxide (ITO) was coated with 1 nm of Cu deposited through physical vacuum evaporation. 3 µm-thick ZnTe and 3 µm-thick CdTe were deposited using a CSS method, with a source temperature of 790ºC, and 700ºC for 3.5, and 2.5 min, respectively. During the deposition process, substrate temperature, Ar flow rate and pressure were maintained to be 500ºC, 50 sccm, and 6.0 Pa, respectively. Then a rapid thermal processing (RTP) was performed in the samples at 400°C for 2 minutes. The surface was modified with a cadmium sulfide layer (CdS), deposited over treated samples through the chemical bath deposition (CBD) method using aqueous chemical bath containing cadmium acetate, thiourea, and ammonia of 25 mM, 375 mM, and 14 wt%, respectively. Finally, nominally 2 nm thick platinum (Pt) was deposited using vacuum evaporation as a cocatalyst. PEC properties of the photocathodes were characterized by using a gas tight glass cell purged with Argon with three electrode configurations. Sample, Ag/AgCl in saturated aqueous KCl solution, and Pt wire were connected to a potentiostat as the working, reference, and counter electrodes, respectively. 1 M KPi with pH 6.5 was employed as an electrolyte. As a light source, AM 1.5G light from a solar simulator was employed.

Results and discussions

Figure 1 presents the current-potential curves under chopped simulated sunlight for CdS and Pt modified photocathodes prepared using CdTe monolayer (ITO/CdTe/CdS/Pt), CdTe layer with ZnTe back contact layer (ITO/ZnTe/CdTe/CdS/Pt), ITO/ZnTe/CdTe/CdS/Pt with Cu layer (ITO/Cu/ZnTe/CdTe/CdS/Pt), and ITO/Cu/ZnTe/CdTe/CdS/Pt with RTP. While the ITO/CdTe/CdS/Pt showed a typical photocurrent of -8 mA cm-2 at 0 VRHE, with an OP of 0.6 VRHE, the introduction of ZnTe severely decreased the photocurrent to -1.5 mA cm-2 at 0 VRHE. SEM structural analysis on cross-sectional reveals the presence of small ZnTe crystals and grain boundaries between the ZnTe and CdTe layers, favoring charge recombination. On the other hand, ITO/Cu/ZnTe/CdTe/CdS/Pt showed significantly enhanced photocurrent of -15 mA cm-2 at 0 VRHE with onset potential of >0.7 VRHE. Electron backscatter diffraction reveals the epitaxial growth columnar grains composed CdTe layer on the ZnTe layer on ITO/Cu/ZnTe/CdTe/CdS/Pt, while SEM-EDS analysis suggests the formation of a solid solution ZnxCd1-xTe (0≤x≤1) with composition gradient. It can be concluded that introduction of Cu enhances PEC hydrogen evolution from water through improvement of structural properties, larger grains, band alignment with composition gradient, and decrease of series resistance for the photocathodes3. Finally, RTP on ITO/Cu/ZnTe/CdTe/CdS/Pt was accessed, increasing the photocurrent at all potential ≥ 0 VRHE. In particular, the photocurrent was enhanced from −1.2 to −4 mA cm-2 at 0.5 VRHE which is the highest among reported CdTe-based photocathodes, and, consequently, a half-cell solar-to-hydrogen conversion efficiency (HC-STH) reached 3.7% at 0.4 VRHE.
1- Su, T. Minegishi, and K. Domen, J. Mater. Chem. A.
5, 13154 (2017).
2- Veiga, H. Kumagai, M. Sugiyama, T. Minegishi. Sustain. Energy Fuels (2024) DOI: (10.1039/D4SE00067F)
3- Minegishi, S. Yamaguchi, and M. Sugiyama, Appl. Phys. Lett..
119, 123905 (2021).

Figure 1