Cascade synthesis and optoelectronic applications of intermediate bandgap Cu ₃VSe ₄ nanosheets

Abstract

Two-dimensional (2D) ternary materials recently generated interest in optoelectronics and energy-related applications, alongside their binary counterparts. To date, only a few naturally occurring layered 2D ternary materials have been explored. The plethora of benefits owed to reduced dimensionality prompted exploration of expanding non-layered ternary chalcogenides into the 2D realm. This work presents a templating method that uses 2D transition metal dichalcogenides as initiators to be converted into the corresponding ternary chalcogenide upon addition of copper, via a solution-phase synthesis, conducted in high boiling point solvents. The process starts with preparation of VSe ₂ nanosheets, which are next converted into Cu ₃VSe ₄ sulvanite nanosheets (NSs) which retain the 2D geometry while presenting an X-ray diffraction pattern identical with the one for the bulk Cu ₃VSe ₄. Both the scanning electron microscopy and transmission microscopy electron microscopy show the presence of quasi-2D morphology. Recent studies of the sulfur-containing sulvanite Cu ₃VS ₄ highlight the presence of an intermediate bandgap, associated with enhanced photovoltaic (PV) performance. The Cu ₃VSe ₄ nanosheets reported herein exhibit multiple UV–Vis absorption peaks, related to the intermediate bandgaps similar to Cu ₃VS ₄ and Cu ₃VSe ₄ nanocrystals. To test the potential of Cu ₃VSe ₄ NSs as an absorber for solar photovoltaic devices, Cu ₃VSe ₄ NSs thin-films deposited on FTO were subjected to photoelectrochemical testing, showing p-type behavior and stable photocurrents of up to ~ 0.036 mA/cm ². The photocurrent shows a ninefold increase in comparison to reported performance of Cu ₃VSe ₄ nanocrystals. This proves that quasi-2D sulvanite nanosheets are amenable to thin-film deposition and could show superior PV performance in comparison to nanocrystal thin-films. The obtained electrical impedance spectroscopy signal of the Cu ₃VSe ₄ NSs-FTO based electrochemical cell fits an equivalent circuit with the circuit elements of solution resistance (R _s), charge-transfer resistance (R _ct), double-layer capacitance (C _dl), and Warburg impedance (W). The estimated charge transfer resistance value of 300 Ω cm ² obtained from the Nyquist plot provides an insight into the rate of charge transfer on the electrode/electrolyte interface.