Cascade synthesis and optoelectronic applications of intermediate bandgap Cu3VSe4 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 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.

Introduction of a heteroatom into a ultrathin 2D TMDC nanosheets to construct a 2D ternary metal chalcogenide nanosheet is a compelling way to obtain unprecedented morphologies of compounds known only in bulk 10 . Ternary 2D nanosheets are of recent interest in important applications, including energy storage (e.g. TiNb 2 O 7 -due to its high theoretical specific capacity) 11,12 ; high-performance photodetectors (e.g. CuInSe 2 ), and highly-sensitive/selective fluorescence DNA sensors (e.g. Ta 2 NiS 5 ) 13 .
Various synthetic strategies have been explored to produce 2D ternary materials with well-defined ternary 2D crystal structures, comprising top-down exfoliation, bottom-up chemical vapor deposition (CVD), and solution synthesis methods 14 ; often, a combination of two different methods, e.g., chemical vapor transport (CVT) and mechanical exfoliation, is required for preparation of the final product 9 . Several challenges, including harsh preparation conditions, and subsequently safety of the process, along with reproducibility and preparation time 9,10,13,15 are encountered in all of the aforementioned approaches. To advance the field of 2D ternary materials and accommodate an increasing demand for a number of applications, versatile synthetic methods that are robust, simple, safe and time-effective, became a practical necessity. Solution-based methods have several significant advantages over solid-state methods to synthesize nanomaterials including: (1) low reaction temperatures; (2) size-selective growth; and (3) morphological control 16 .
The ability to generate such materials in 2D could enable fabrication of ultra-thin-films that would revolutionize a variety of applications. However, typical one-pot solution-based synthesis utilized for synthesizing copper chalcogenides for many of these materials result in nanoparticles 23,24 .
The sulvanite family is known for decades, but it was only recently explored at the nanoscale. Solution-phase synthesized Cu 3 VS 4 nanocrystals 23,25 show an interesting UV-Vis absorption, with three absorption peaks in the visible range, attributed to the presence of the intermediate band gap (IB) in their electronic structure 25 . We recently showed that Cu 3 VSe 4 nanocrystals, synthesized through a solution process, showcase a similar UV-Vis absorption pattern, suggesting the presence of the IB in the nanoscale vanadium-sulvanites 26 .
Intermediate band semiconductors recently raised special attention for their potential to exceed the Shockley-Queisser limits in thin-film solar photovoltaics. Semiconductors with an intermediate band can absorb energies below the bandgap energy through two optical transitions from the valence to the intermediate band and from the intermediate to the conduction band, resulting in enhanced conversion efficiency [27][28][29] . Besides, the three optical transitions caused by the intermediate band can also lower the energy losses relying on thermal relaxation of optically excited carriers 30 . Theoretical predictions for a solar cell using an intermediate band semiconductor as absorber, could reach an efficiency of 63.1% in a single junction, exceeding the Shockley-Queisser barrier 31 .
We recently demonstrated that kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) could be prepared in a quasi-2D morphology through a cascade reaction starting from a binary chalcogenide SnSe 2 with nanosheet morphology. Upon addition of copper and subsequently zinc, quasi-2D CZTSSe with inherited nanosheet morphology formed 32 . Our recent report on synthesizing Cu 3 VSe 4 cubic nanocrystals using a hot-injection method, paved the road toward obtaining nanoscale Cu 3 VSe 4 . The method involves temperatures up to 260 °C, and leads to a high purity material in matter of hours 26 , in comparison with the six weeks required by the solid state synthesis conducted at 600 °C 33 .
Our hypothesis was that Cu 3 VSe 4 could be synthesized in nanosheet morphology, and that the approach is amenable to other multinary chalcogenides, including other sulvanites. In this work, we demonstrate a facile synthesis of Cu 3 VSe 4 nanosheets (NSs) using the same template approach that allows a seamless morphing of the nanosheet from the binary compound to the ternary one. The solution-phase process involves synthesis of VSe 2 nanosheets followed by insertion of Cu(II) cations in the same reaction vessel, leading to Cu 3 VSe 4 nanosheets. VSe 2 nanosheets were prepared at 250 ℃ by hot injection synthesis and characterized by X-ray diffraction and transmission electron microscopy (TEM), demonstrating the 2D morphology. Upon introducing Cu cations into VSe 2 nanosheets, the quasi-2D Cu 3 VSe 4 formed, exhibiting large surface area (micron size) nanosheets, thus retaining the template morphology. The Cu 3 VSe 4 formation mechanism was investigated by X-ray diffraction, indicating that the insertion and new compound formation happens within minutes. It is remarkable that the Cu 3 VSe 4 nanosheets maintain the crystal structure of bulk sulvanites (cubic), as identified by XRD. The 2D morphology was confirmed by TEM and SEM. Three distinct absorption bands similar to Cu 3 VS 4 NCs were investigated by UV-Vis-NIR measurements. Additionally, the photocurrent generated by a Cu 3 VSe 4 NSs thin film on FTO evidences the potential of Cu 3 VSe 4 NSs in solar photovoltaic applications.

Results and discussion
VSe 2 nanosheets characterization. The VSe 2 nanosheets, obtained through a modified literature procedure 34 , show high crystallinity; the X-ray diffraction pattern of the synthesized VSe 2 corresponds to the 1 T-VSe 2 polytype 35 belonging to the P ͞ 3m1 space group (PDF# 40723) (Fig. 1a). The Raman spectrum of the synthesized VSe 2 (Fig. 1b) exhibits two primary peaks (254.3 cm −1 and 282.3 cm −1 ) where the primary peak (around 251.3 cm −1 ) corresponds to E 2g in-plane vibration mode while the peak at 286 cm −1 is consistent with A 1g out-of-plane vibrational mode. The slight redshift (5-10 cm −1 ) when compared with literature reported data could be attributed to the layered structure 34,36 . The TEM image in Fig. 1c shows micrometer-size lateral dimen-Scientific Reports | (2020) 10:21679 | https://doi.org/10.1038/s41598-020-78649-9 www.nature.com/scientificreports/ sion VSe 2 NSs. The SEM image of the synthesized VSe 2 nanosheets (Fig. 1d), further confirms the nanosheet morphology. The powder subjected to SEM was not subjected to exfoliation and shows an aggregate of VSe 2 nanosheets.
Cu 3 VSe 4 nanosheets characterization. The preparation of Cu 3 VSe 4 NSs presented herein demonstrates the ability to maintain the morphology and the large dimension of the template upon copper introduction into VSe 2 NSs. By contrast, a similar copper insertion, done at 25 °C, was reported for In 2 Se 3 ; however, the experiment rendered wrinkled CuInSe 2 nanosheets which did not resemble the template. Further, the same reaction conducted at elevated temperatures results in nanodisks 37 . The X-ray diffraction pattern in Fig. 2a, agrees with the bulk XRD, showing cubic Cu 3 VSe 4 structure (PDF# 40125, a = b = c = 5.572 Å) in the P ͞ 43m space group. Raman spectra of as-synthesized Cu 3 VSe 4 in Fig. 2b presents five peaks (134.3 cm −1 , 158.3 cm −1 , 185.6 cm −1 , 219.6 cm −1 , and 343.3 cm −1 ), consistent with the Cu 3 VSe 4 nanocrystals Raman 26 . The intensity ratio of the peaks of Cu 3 VSe 4 NSs is different from the ones attributed to the nanocrystals could be attributed to the morphology change 34,38 .
To investigate the thermal stability of the Cu 3 VSe 4 nanosheets, thermogravimetric analysis (TGA) was performed in the temperature interval 25-800 ℃ at a ramping rate of 20 ℃ min −1 , under argon atmosphere.   www.nature.com/scientificreports/ displays the TGA curve of the synthesized Cu 3 VSe 4 nanosheets, showing a rapid weight loss that starts at 362 ℃, indicating the decomposition onset. As expected, Cu 3 VSe 4 with nanosheet morphology have much lower thermal stability than Cu 3 VSe 4 nanocrystals. The slight weight loss between 200 and 350 ℃ is characteristic to the decomposition of the organic ligand residues originated from the synthesis. The morphology of the Cu 3 VSe 4 NSs was evaluated by TEM and SEM. At low magnification, TEM imaging ( Fig. 3a) reveal a nanosheet morphology with Cu 3 VSe 4 NSs possessing a large lateral dimension (several hundred nanometers). At higher magnification (Fig. 3b) the images suggest a belt-shape concatenation of small a squareshaped nanocrystal, which self-assemble to render the quasi-2D nanosheets. The d-spacing of ~ 5.32 nm and 3.21 nm, determined by electron diffraction (Fig. 3c,d) correspond to the (100) and (111) facets of cubic Cu 3 VSe 4 . The nanosheet morphology is further supported by SEM imaging (Figs. 4a and 5); the nanosheets self-assembly in an arrangement reminiscent of flower petals. The uniform distributions of Cu, V, Se is reflected in the elemental maps collected by SEM-EDS ( Fig. 4b-d, respectively). Importantly, the thickness of the nanosheets subjected to AFM is in the vicinity of 10 nm, as illustrated in Figure S1 ("Supporting Information"). As shown in Fig. 5, using VSe 2 NSs as template, we obtained Cu 3 VSe 4 NSs, demonstrating the ability of the synthetic method to maintain the morphology and the large dimension of the starting materials. To assess the chemical and electronic structures of the synthesized Cu 3 VSe 4 nanosheets, XPS was performed as shown in Supplementary Figure S2, indicating that the oxidation states of Cu and Se in Cu 3 VSe 4 NSs are + 1 and − 2, respectively, as the same in the Cu 3 VSe 4 nanocrystals, and the vanadium is in presence of V 5+ but with a negative shift of 2.5 eV in the binding energy of V 5+ (The detailed discussion can be found in "Supporting Information") 26 .

Study of the Cu 3 VSe 4 formation mechanism.
To get insight into the conversion mechanism of VSe 2 NSs to Cu 3 VSe 4 NSs, we conducted a time study, involving a series of syntheses with all parameters kept constant, excepting the reaction times. The corresponding XRD patterns of the products are showed in Fig. 6. The study indicated that the primary crystalline phase in all products is cubic Cu 3 VSe 4 with representative 2θ peaks of 15.9°, 27.7°, and 46.03°. Interestingly, Cu 3 VSe 4 NSs forms within 1 min after the Cu 2+ injection. A minor amount of Cu 2 Se and trace amounts of VSe 2 were present in the products for the reactions conducted for 1 min, 5 min, and 15 min. When the reaction time exceeded an hour, VSe 2 templating material and Cu 2 Se impurities are no longer present and the sole product was Cu 3 VSe 4 NSs. Likewise, when extending the reaction time to two hours, the product shows the same Cu 3 VSe 4 phase with a slightly higher crystallinity. It is important to note that no decomposition occurs.
We posit that the 2D layering nature of the VSe 2 template provides a short diffusion distance for the injected Cu 2+ which, along with the large reaction surface area, enables a short reaction time. Provided that the reaction is conducted in a cascade, involving VSe 2 formation and further addition of Cu(II), unreacted Se from VSe 2 formation could react with Cu(II) resulting in Cu 2 Se impurities. It is conceivable that as the reaction proceeds,   Figure 7b shows the photoluminescence (PL) spectrum of as-synthesized Cu 3 VSe 4 NSs, with a significant emission peak at 690 nm when using 460 nm as the excitation  www.nature.com/scientificreports/ wavelength. Interestingly, the variation of the excitation wavelength from 430 to 520 nm at 10 nm intervals leads to PL emission peak steadily redshift from 650 nm (1.9 eV) to 780 nm (1.59 eV) (Supporting Information Figure S7). The excitation-dependent emission feature could be associated with the thickness distribution of Cu 3 VSe 4 NSs and the effect of different surface functionalization on the NSs 40-43 . However, the emission peak at 690 nm has the highest intensity, suggesting that for most of the Cu 3 VSe 4 NSs product, 460 nm is the optimal excitation wavelength, corresponding to a band gap of 1.80 eV.
To interrogate the effect of surface functionalization on the PL spectra of Cu 3 VSe 4 NSs by conducting PL measurements on the Cu 3 VSe 4 NSs that were subjected to ligand exchange. Although the same distribution of emission wavelengths remained, the main peak remained at 690 nm for the 460 nm excitation wavelength (Supporting Information Figure S3). The impact of thickness dependence of PL emission was documented for other materials, including layered 2D MoS 2 44 and WS 2 45 . The FTIR spectra in Figure S4 ("Supporting Information") demonstrated the complete removal of the organic ligands after the ligand exchange.
Theoretical predictions place sulvanites, including Cu 3 VSe 4 , in the optically active materials category, holding promise in applications including photodetectors and photovoltaics 19,21 . A photocurrent evaluation was carried out using Cu 3    Electrolyte pH affects the photoelectrochemical behavior of the semiconductor, impacting the rate of photoreduction/photo-oxidation as well as the rate of electron-hole recombination in the system 46,47 . The current density-voltage (J-V) curves of Cu 3 VSe 4 NSs were therefore explored, using a 0.6 M KCl aqueous solution with varying pH as electrolyte (Fig. 8a). The Cu 3 VSe 4 NSs-FTO thin film exhibits a cathodic photocurrent response with highest photocurrent obtained in the electrolyte with pH 4, indicating that Cu 3 VSe 4 is a p-type semiconductor. The cathodic photocurrent generation could be attributed to the H + reduction 26 ; Cu 3 VSe 4 NSs illumination drives generation of electron/hole pairs, with the photogenerated electrons reaching the electrode/ electrolyte interface reducing H + to H 2 32 . The effect of pH can be understood in terms of surface reaction on the Cu 3 VSe 4 NSs-FTO thin film. At low pH, namely using acid KCl aqueous solution as the electrolyte, the proposed H + reduction reaction at cathode (Cu 3 VSe 4 NSs-FTO thin film) is 2H + + 2e − → H 2 , which consumes hydrogen ions; whereas, at high pH (pH = 10), the reaction at cathode consumes water rather than H + , being 2H 2 O + 2e − → H 2 + 2OH −48,49 . The consumption of water takes place via an extra water dissociation H 2 O ↔ H + + OH − , thus, the process of photocatalytic reduction is naturally slowed at higher pHs 50 , in turn, the larger amount of H + present at acid pH facilitates the H 2 production. Besides, at pH of 4, the surface of Cu 3 VSe 4 NSs-FTO thin film is positively charged by adsorbing H + , facilitating the reduction reaction, whereas, at higher pH (pH of 7 and 10) the electrode/electrolyte interface is neutral or negatively charged, which eliminates the electrostatic interaction, or even creates a strong electrostatic repulsion towards Cl − that results in a lower hole capture rate and in a low production of hydrogen 51 . The changing trend of photocurrent response is consistent with their photocatalytic H 2 evolution. Therefore, the increase of photocurrent response at pH of 4 can be attributed to the larger capture of holes which reduces charge recombination, as well as the large amount of H + which facilitating the H + reduction reaction at the cathode/electrolyte interface. A chronoamperometry experiment using the same photoelectrochemical cell setup and the KCl aqueous solution with pH 4 as electrolyte was carried out at − 425 mV through several 10 s light on-off cycles (inset, Fig. 8b); a stable photocurrent of ~ 0.036 mA cm −2 was observed. According to literature, the bare FTO substrate sometimes presents photocurrent response during the electrochemical measurements [52][53][54] . A control experiment was performed with bare FTO as working electrode and the chronoamperometry experiment being conducted under the same condition with the measurement of Cu 3 VSe 4 NSs-FTO thin film. The photocurrent response of bare FTO substrate is shown in Supplementary Fig S6 (red line), where no significant photocurrent is observed under the same conditions, suggesting that the photocurrent response of Cu 3 VSe 4 NSs-FTO thin film is substantially resulted from the Cu 3 VSe 4 NSs thin film. To investigate the stability of the photocurrent response of Cu 3 VSe 4 NSs, a chronoamperometry measurement was conducted on the Cu 3 VSe 4 NSs-FTO thin film which was kept in air for 6 months after the photoelectrochemical measurement. As shown in Figure S6 ("Supporting Information") blue line, the Cu 3 VSe 4 NSs-FTO thin film could still display active photoelectrochemical behaviour after 6 months. Furthermore, Cu 3 VSe 4 NSs showed good stability in the electrochemical measurements, as the XRD pattern and TEM (Supporting Information Figure S7) of Cu 3 VSe 4 NSs after the photoelectrochemical test indicate that the Cu 3 VSe 4 preserved the nanosheet morphology and cubic crystals.
A comparative test was conducted using a thin film fabricated with Cu 3 VSe 4 nanocrystals (NCs) on FTO as the photoelectrode (Supporting Information Figure S6). The observed photocurrent generated by Cu 3 VSe 4 NSs-FTO thin film is around nine fold higher than the photocurrent produced by the reported Cu 3 VSe 4 NCs-FTO thin film 26 . The photocurrent enhancement in Cu 3 VSe 4 NSs could be ascribed to the higher surface area provided by the 2D geometry of the nanosheets.
The electrochemical cell fabricated with the Cu 3 VSe 4 NSs-FTO thin film was further subjected to an Electrochemical Impedance Spectroscopy (EIS) measurement to study the interfacial charge transfer characteristics at www.nature.com/scientificreports/ the semiconductor/electrolyte interface. Figure 9 shows the Nyquist plot obtained at the open-circuit potential of 0.215 V vs Ag/AgCl in the frequency range of 1 Hz to 100 KHz. An electrical equivalent circuit, as shown in the inset in Fig. 9, was designed to rationalize the charge-transfer and transport phenomena in the electrochemical cell, where R s , R ct , C dl , and W represents solution resistance, charge-transfer resistance, double-layer capacitance, and Warburg impedance, respectively. The first small semicircle at high frequency region is related to the solution resistance R s , whereas the large semicircle at low frequency region could be assigned to the charge-transfer resistance R ct and the double-layer capacitance C dl at the electrode/electrolyte interface 55,56 . Generally, the diameter of semicircle extrapolated in the Nyquist plot represents resistance R, in turn, reflects the reaction rate 57,58 . Herein, the charge transfer resistance (R CT ) of Cu 3 VSe 4 NSs-FTO thin film in 0.6 M KCl aqueous electrolyte was found to be 300 Ω cm 2 from the second semicircle of the Nyquist plot. The linear part located at low frequency could be ascribed to the Warburg impedance W that corresponds to the diffusion processes.

Conclusion
Quasi-2D Cu 3 VSe 4 nanosheets with cubic crystal structure have been prepared through a templating method using binary VSe 2 NSs as a solid-state precursor in a solution-phase approach, which enabled the preservation of the 2D morphology. Microscopy analyses (TEM, SEM, and AFM) highlighted the large lateral size and thin thickness of the synthesized Cu 3 VSe 4 nanosheets. Three strong absorption bands at 382 nm, 552 nm, and 664 nm were observed for Cu 3 VSe 4 nanosheets, indicating the presence of an intermediate bandgap. The emission peak redshift in the PL spectra, from 650 to 780 nm, when varying the excitation wavelength from 430 to 520 nm, is attributed to the size/thickness distribution and has been observed in other layered materials, including 2D MoS 2 44 and WSe 2 45 . The mechanism study suggested that the formation of Cu 3 VSe 4 nanosheets involves primarily the Cu 3 VSe 4 nanosheets formation as a result of reaction of Cu(II) cations with VSe 2 NSs. At the same time, unreacted Se present in the reaction reacts with Cu cations to form a minor Cu 2 Se is consumed during the reaction by reacting with VSe 2 NSs and leads to Cu 3 VSe 4 nanosheets.
To evaluate the conduction type of Cu 3 VSe 4 NS, thin films were made by depositing the nanosheets (after ligand exchange) on FTO substrates. The fabricated Cu 3 VSe 4 NSs thin film exhibited a p-type semiconductor behavior. A photocurrent of ~ 0.036 mA cm −2 was measured in an electrochemical setting, in a KCl aqueous solution at pH 4.0. The electrical impedance spectroscopy was measured assuming that the Cu3VSe4 NSs-FTO based electrochemical cell fits an equivalent circuit with the circuit elements of solution resistance (Rs), chargetransfer resistance (Rct), double-layer capacitance (Cdl), 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 at the electrode/electrolyte interface.
The cascade synthesis method reported herein for the Cu 3 VSe 4 ternary chalcogenides could be expanded to other significant chalcogenides that could benefit from 2D morphology. The lightweight nanostructures could be useful in a plethora of optoelectronic devices that support contemporary applications, including wearable electronics, biosensors and the Internet-of-Things (IoT).

Preparation of VSe 2 nanosheets (NSs).
In the preparation of VSe 2 NSs synthesis, Vanadium (IV) oxide acetylacetonate was used as the source of V 4+ cations, whereas elemental Se served as the source of Se 2− anions. Vanadium (IV) oxide acetylacetonate (133 mg, 0.5 mmol) and 15 mL OLA were loaded in a 100 mL two-neck round bottom flask. The mixture was degassed for 30 min at 120 ℃ and flushed with argon. Meanwhile, Se powder (1 mmol, 78.96 mg) was mixed with 1 mL of 1-DDT and 3 mL of OLA in a 25 mL two-neck round bottom glass flask. The Se solution was stirred under vacuum at room temperature for 30 min and then purged with argon. Next, the temperature of vanadium precursor was raised to 140 ℃, followed by the swift injection of the Se solution. The reaction was further heated to 250 ℃ and maintained at this temperature for two hours to form the VSe 2 nanosheets. Upon cooling, a mixture of OLA (9 mL) and I-DDT (1 mL) was added to remove the Se residues and the precipitated VSe 2 nanosheets were collected by centrifugation. The nanosheets were subsequently washed twice with the same CHCl 3 and C 2 H 5 OH mixture (V:V, 1:3). The final precipitates were collected and dried overnight in a vacuum oven. Ligand exchange with S 2− . A ligand exchange process was conducted to replace the OLA coordinated to Cu 3 VSe 4 nanosheet surface with S 2− . In a typical experiment, Cu 3 VSe 4 nanosheets were suspended in chloroform (8 mg mL −1 ) and 10 mL of the suspension was transferred to a 50 mL tube containing 10 mL of Na 2 S solution in formamide (0.2 M). Next, the mixture was vigorously shaken for 1 min and further allowed to rest until ligand exchange completion, when Cu 3 VSe 4 NSs were fully transferred from the chloroform phase (lower) to the formamide phase (upper). Afterward, the clear chloroform phase was removed, and 5 mL distilled water and 20 mL of ethanol was added to the aqueous phase to precipitate the Cu 3 VSe 4 NSs. The precipitate was purified by washing twice with a mixture of ethanol and distilled water (V:V, 4:1) followed by washing with a mixture of toluene and ethanol (V:V, 1:3). The resulting product was collected and dried in a vacuum oven overnight.

Preparation of Cu
Cu 3 VSe 4 inks and thin film fabrication. The Cu 3 VSe 4 NSs inks were prepared via dispersing 10 mg of ligand exchanged-Cu 3 VSe 4 NSs into 1 mL of ethanol and ultrasonicating the dispersion for 10 min with a probe sonicator. The substrate used for ink deposition, 1″ × 1″ FTO-coated glass, were freshly cleaned using an ultrasonic bath using, in sequence: distilled water, methanol, and acetone, for 10 min each. All Cu 3 VSe 4 thin films used in this work were fabricated by bar-coating in ambient conditions. An amount of 20 μL of Cu 3 VSe 4 ink was dispensed and coated on the FTO-coated glass. The film was dried for 1 min in air at 100 ℃, using a hot plate. The coating and drying process was repeated twice to complete the glass/FTO/ Cu 3 VSe 4 thin film.
Photoelectrochemical measurements. To evaluate the photoelectrochemical performance, a threeelectrode photoelectrochemical cell has been set up, consisting of an Ag/AgCl reference electrode, a platinum counter electrode, and the Cu 3 VSe 4 NSs-FTO thin film as the photoelectrode. An aqueous solution of KCl (0.6 M) was used as the electrolyte. The pH of the KCl electrolyte was adjusted to either 4 or 10 using HCl or 2 M NaOH aqueous solution, respectively. The current density-voltage (J-V) dependence was investigated in the − 0.5 to 0 V range, with a cycle of 10-s light-off and 10-s light-on. The sweep rate was 2 mV s −1 and the LED light power was 2000 lumens. The photocurrent of the Cu 3 VSe 4 NSs-FTO thin film using the same 10 s light-on/10 s light-off cycle recorded with a Pine Research Potentiostat.
Characterization. The crystal structure and purity of prepared VSe 2 and Cu 3 VSe 4 nanosheets were determined by X-ray powder diffraction (XRD) using a Siemens Diffractometer D5000 (Cu Kα radiation, λ = 1.5405 Å). To confirm product purity by Raman spectroscopy, Raman spectra were obtained with a Renishaw Raman microscope equipped with a 633 nm laser. The morphology and size of the synthesized VSe 2 and Cu 3 VSe 4 were determined by transmission electron microscopy (TEM) imaging, using a Philips CM200, and scanning electron microscopy (SEM) imaging, with a JEOL 6330F. Elemental distribution of the Cu 3 VSe 4 nanosheets was determined with the energy dispersive spectroscopy (EDS) feature of the JEOL 6330F SEM. The oxidation states of the Cu, V, and Se elements of the synthesized Cu 3 VSe 4 nanosheets were confirmed via X-ray photoelectron spectroscopy (XPS) in a VG Escalab 220i-XL equipped with an Al Kα source. Photoluminescence Scientific Reports | (2020) 10:21679 | https://doi.org/10.1038/s41598-020-78649-9 www.nature.com/scientificreports/ (PL) measurements of Cu 3 VSe 4 nanosheets were carried out with a Perkin Elmer LS-55 Luminescence Spectrometer. The absorption spectrum of Cu 3 VSe 4 nanosheets was collected using an Agilent Cary 5000 UV-Vis-NIR spectrophotometer. The thermal stability of the Cu 3 VSe 4 nanosheets was determined by thermogravimetric analysis (TGA) using a TA Instrument SDT-Q600 Simultaneous TGA/DSC. The WaveNow Potentiostat (PINE research) was used to determine the photoelectrochemical behavior of the Cu 3 VSe 4 NSs-FTO thin film.

Data availability
All data generated or analyzed during this study are included in this article (and its "Supporting Information" file).