Cascade synthesis and optoelectronic applications of intermediate bandgap Cu3VSe4 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 VSe2 nanosheets, which are next converted into Cu3VSe4 sulvanite nanosheets (NSs) which retain the 2D geometry while presenting an X-ray diffraction pattern identical with the one for the bulk Cu3VSe4. Both the scanning electron microscopy and transmission microscopy electron microscopy show the presence of quasi-2D morphology. Recent studies of the sulfur-containing sulvanite Cu3VS4 highlight the presence of an intermediate bandgap, associated with enhanced photovoltaic (PV) performance. The Cu3VSe4 nanosheets reported herein exhibit multiple UV–Vis absorption peaks, related to the intermediate bandgaps similar to Cu3VS4 and Cu3VSe4 nanocrystals. To test the potential of Cu3VSe4 NSs as an absorber for solar photovoltaic devices, Cu3VSe4 NSs thin-films deposited on FTO were subjected to photoelectrochemical testing, showing p-type behavior and stable photocurrents of up to ~ 0.036 mA/cm2. The photocurrent shows a ninefold increase in comparison to reported performance of Cu3VSe4 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 Cu3VSe4 NSs-FTO based electrochemical cell fits an equivalent circuit with the circuit elements of solution resistance (Rs), charge-transfer resistance (Rct), double-layer capacitance (Cdl), and Warburg impedance (W). The estimated charge transfer resistance value of 300 Ω cm2 obtained from the Nyquist plot provides an insight into the rate of charge transfer on the electrode/electrolyte interface.

Sulvanites: The Promise at the Nanoscale

“…This study also showed that increasing the excitation wavelength caused a redshift in the peak position, which was attributed to the distribution of both size and different emissive sites of the nanocrystals. The same group also investigated Cu 3 VSe 4 in nanosheet form with comparable results regarding the photoluminescence and absorption bands [8].…”

“…However, the synthesized product did not have good crystallinity, and a post-annealing process was utilized to improve the crystallinity of the Cu 3 VS 4 nanoparticles. Later, the syntheses of cubic Cu 3 VS 4 nanocubes [12], Cu 3 VSe 4 nanocubes [9], and Cu 3 VSe 4 nanosheets [8] were reported. Extending the reaction time leads to the facile preparation of high purity Cu 3 VS 4 and Cu 3 VSe 4 without the postannealing treatment.…”

“…However, when using vanadium sulfide as a precursor, a very interesting result was observed [8]. Herein, Liu et al used a cascade synthesis strategy where vanadium and selenium precursors were allowed to react first, forming VSe 2 nanosheets, as VSe 2 is a 2D transition metal dichalcogenide (TMD).…”

“…This is especially true for Cu 3 MTe 4 , which has seen the least amount of published information on the sulvanite compounds, primarily due to the necessity for low-temperature solid-state methods [38] instead of the commonly used high-temperature annealing process or complications with reactivity and solubility in solution-phase methods [52]. However, the research of these compounds beyond polycrystalline powders has recently gained interest, particularly in colloidal nanocrystals [8,9,12,24]. The following section is a summary of the recent developments of these materials.…”

“…In particular, the class of compounds Cu 3 MX 4 (M = V, Nb, Ta; X = S, Se, Te), known as sulvanites or sulvanite-type compounds, are p-type intermediate band gap (IB) semiconductors that have shown promise in optoelectronics due to their large ionic mobility [1,2], high absorption in the visible and UV range [3,4], low hole effective mass [5], and optical band gaps suitable for their use as an absorber layer in thin film solar photovoltaics [6]. Such characteristics have driven several studies into the fabrication of the sulvanite-type compounds as thin-film photovoltaics (PV) [4,[7][8][9][10] and photocatalysts for H 2 production [11]. The fabrication of sulvanite nanocrystals with tunable size and properties [9,12] became an important endeavor, toward enabling the fabrication of devices on flexible substrates.…”