Morphological, optical and photoelectrochemical properties of ZnS/CZTS QDs modified α-Fe2O3 Thin Films

Abstract

Quantum Dots (QDs) have tremendous potential for utilizing solar energy due to their high extinction coefficient and tunable bandgap on account of quantum confinement effect. Till date, many QDs like PbS, CdS, CdSe, CdTe have been widely studied in photoelectrochemical (PEC) generation of hydrogen [1]. All these QDs are highly toxic in nature which demand a need of alternative green QDs. In this view, Cu₂ZnSnS₄ (CZTS) QDs has come up with its low cost and environment-friendly nature. They are widely known to be a good photo-absorber as they possess large absorption coefficient (up to 10⁵ cm⁻¹) and its bandgap lies in visible region of solar spectrum which makes them useful with other metal oxide for harnessing light effectively [2]. In PEC, metal oxides are old gems of the field and widely explored which require modification as they lack with some of the desired properties. The main considerations for the design of a PEC material are band structure, quantum efficiency, and resistance to both photocorrosion and corrosion in an aqueous environment. In order to achieve highly efficient PEC system, α-Fe₂O₃ has been sensitized with CZTS QDs. Since, hydrothermal synthesis of CZTS QDs produces defect in it [3] so this system was coated with ZnS layer as a defect passivation layer. The band edges of this system are favorable for easy transfer of the carriers. Hence, this investigation for the first time explores the possibility of using CZTS QDs to improve the properties of hematite (α-Fe₂O₃) thin film photoelectrode for PEC water splitting.

The photoelectrode preparation includes the synthesis of CZTS QDs, α-Fe₂O₃, and ZnS. CZTS QDs were prepared by hydrothermal method using Cu(CH₃COO)₂, Zn(NO3)₂, SnCl₂ and thiourea as precursors [4]. α-Fe₂O₃ thin films were prepared by sol-gel spin coating method using Fe(NO₃)₃ [5] and sensitized by CZTS QDs by chemical bath deposition method. These films were then subjected to ZnS coating by successive ionic layer adsorption and reaction (SILAR) method [6]. These films were implemented as photoanode in photoelectrochemical cell for photoresponse measurements. XRD, TEM, SEM and UV- visible spectroscopy techniques were used to characterize these thin films for deeper analysis. The size of synthesized CZTS QDs was well within the quantum confinement regime as examined by TEM image. This confinement was also verified by examining the UV-Vis spectra of CZTS QDs as they showed more value of bandgap compared to bulk counterpart. The UV-Vis spectra of modified Fe₂O₃ thin films showed the increased value of absorption over visible region. It was also analysed that the value of resistance was smaller for modified sample as compared to pristine. The photoelectrochemical studies were carried out in PEC cell in 1M NaOH under Xe lamp. Highest photocurrent density of 1.98 mA/cm² at 0.75 V/SCE was observed for ZnS/CZTS QDs modified α- Fe₂O₃ film. This photocurrent is 9.9 times than that of pristine. An increase in photocurrent could be attributed to better light absorption ability and reduced resistance after modification. Applied bias to photon conversion efficiency was also calculated using open circuit potential value and found higher for modified sample.