Photoelectrochemical CO2 reduction By Using Different Buffer Layers (In2S3, CdS) -Decorated Cu2ZnSnS4
Authors: Sasaki Yosuke , Kamimura Sunao , Ohno Teruhisa 
Published: February 2020

Introduction
Since atmospheric CO2 levels have been on the rise, global warming has become a serious problem. Recently, photoelectrochemical (PEC) CO2 reduction has attracted considerable attention as a potential means of converting solar energy into chemical energy in the form of usable organic fuels such as carbon monoxide, hydrocarbon, alcohol, and so on. To achieve the PEC CO2 reduction, the development of photocathode which possess a high energy-conversion efficiency and a high chemical stability, are strongly demanded.
Cu2ZnSnS4 (CZTS) has attracted much attention as a thin film photocathode due to their specific characteristics: p-type semiconductor, high stability, ability to promote H2 evolution in response to visible light irradiation. In the present study, we fabricated CZTS photocathode by sol-gel method, and evaluated its PEC CO2 reduction properties under visible light irradiation. Moreover, we introduce n-type semiconductors (CdS, In2S3, etc.) as suitable buffer layer of the CZTS photocathode to improve the photocurrent response and selectivity of CO2 reduction.

Experimental
The CZTS photocathode was fabricated by sol-gel/spin-coated method on a molybdenum coated soda-lime glass (Mo/SLG) substrate. The precursor sol was prepared by dissolving raw materials (Cu2(CH3COO)2, SnCl2, Zn(CH3COO)2 and CH4N2S) into 2-methoxyethanol. Moreover, monoethanolamine was added into the prepared solution to avoid cracks during spin coating, resulting that the precursor sol was obtained. The precursor sol was spin coated on Mo/SLG substrate and annealed at 200 ℃ for 5 min on a hotplate. After this process had been repeated for a maximum of 5 times, the electrode was calcined in sulfur/N2 atmosphere. Then the CZTS photocathode was obtained.
Surface modification of the CZTS photocathode was performed by chemical bath deposition (CBD) method. The CZTS photocathode was soaked in the bath solution containing raw materials for producing n-type semiconductors, and then annealed in air. Then surface modified CZTS photocathode could be prepared.
The crystalline phase of CZTS was characterized by using a powder X−ray diffraction (XRD) instrument, and Raman spectroscopy. The PEC properties of the CZTS photocathode was investigated in a closed three-electrode configuration using a Ag/AgCl reference electrode and a Pt coil counter electrode. The electrolyte was 0.1 M NaHCO3 solution. The electrolyte was stirred and purged with CO2 gas for 40 min before measurement. The PEC CO2 reduction was investigated by using Xe lamp (420 < λ < 800 nm, 100 mW cm-2). The CO2 reduction products were evaluated by using gas-chromatography and ion chromatography.
Results and Discussion
The XRD pattern of the CZTS photocathode on Mo/SLG substrate composed of three peaks located at 28.1ﾟ, 47.0ﾟ, and 55.8ﾟ,which corresponds to (112), (220), (312) phase of kesterite structure of CZTS. For the raman spectrum, main sharp peak was observed at 336 cm-1 which ascribed to the kesterite structure of CZTS. These results indicated that the CZTS thin film could be prepared by sol-gel/spin-coating method. The morphology of CZTS thin film and surface-modified CZTS thin films were investigated by cross sectional SEM: flat and uniform films were observed. UV-vis spectrum has an absorption edge at around 830 nm, which corresponding to the optical absorption edge of the CZTS.
The photocurrent of CZTS photocathode was dramatically enhanced by surface modification of n-type semiconductor layers due to efficient separation of the charge carriers at n-type/CZTS semiconductor interface (p-n heterojunction). The highest photocurrent was obtained from CdS/CZTS electrode because conduction band offset between CZTS and CdS was smaller than the other n-type layers. As a result of CO2 reduction product analysis, CO and HCOOH were obtained. Moreover, selectivity for CO2 reduction was improved by surface modification of CZTS photocathode: when CdS was deposited on the CZTS, the CO generation was increased. On the other hands, In2S3 was deposited, the HCOOH generation was increased. The detailed information will be discussed in poster session.