Cascade synthesis and optoelectronic applications of intermediate bandgap CuVSe nanosheets.

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 nanosheets, which are next converted into CuVSe sulvanite nanosheets (NSs) which retain the 2D geometry while presenting an X-ray diffraction pattern identical with the one for the bulk CuVSe. Both the scanning electron microscopy and transmission microscopy electron microscopy show the presence of quasi-2D morphology. Recent studies of the sulfur-containing sulvanite CuVS highlight the presence of an intermediate bandgap, associated with enhanced photovoltaic (PV) performance. The CuVSe nanosheets reported herein exhibit multiple UV-Vis absorption peaks, related to the intermediate bandgaps similar to CuVS and CuVSe nanocrystals. To test the potential of CuVSe NSs as an absorber for solar photovoltaic devices, CuVSe 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. The photocurrent shows a ninefold increase in comparison to reported performance of CuVSe 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 CuVSeNSs-FTO based electrochemical cell fits an equivalent circuit with the circuit elements of solution resistance (R), charge-transfer resistance (R), double-layer capacitance (C), and Warburg impedance (W). The estimated charge transfer resistance value of 300 Ω cm obtained from the Nyquist plot provides an insight into the rate of charge transfer on the electrode/electrolyte interface.