Morphology-Controlled Synthesis of V1.11S2 for Electrocatalytic Hydrogen Evolution Reaction in Acid Media

High-performance low-cost catalysts are in high demand for the hydrogen evolution reaction (HER). In the present study, we reported that V1.11S2 materials with flower-like, flake-like, and porous morphologies were successfully synthesized by hydrothermal synthesis and subsequent calcination. The effects of morphology on hydrogen evolution performance were studied. Results show that flower-like V1.11S2 exhibits the best electrocatalytic activity for HER, achieving both high activity and preferable stability in 0.5 M H2SO4 solution. The main reason can be ascribed to the abundance of catalytically active sites and low charge transfer resistance.


Introduction
Nowadays, the development of green and sustainable energy has become an important research topic due to the depletion of fossil fuels, as well as increasing serious environmental issues [1,2]. Therefore, it is urgent to develop renewable and clean alternatives. Electrochemical water splitting is considered to be a clean and sustainable way to produce hydrogen fuel [3]. Moreover, the abundance of protons in acid electrolytes facilitates the acceleration of the hydrogen evolution reaction [4]. However, the acidic electrolyte can cause severe chemical corrosion of electrolyzers, which limits the use of non-platinum group metals or their compounds as catalysts [5]. In particular, the high cost and insufficient reserves of precious metals have greatly restricted their large-scale commercial applications [6,7]. Therefore, a lot of research has been focused on exploring low-cost electrocatalysts [8][9][10][11][12].
Among the various hydrogen evolution reaction (HER) catalysts, transition metal chalcogenides (TMDs) have made tremendous progress due to their high catalytic activity toward the HER, as well as their low-cost [13,14]. MoS 2 is one of the most excellent electrocatalytic materials among transition metal sulfides, and the catalytic activity and mechanism of MoS 2 for HER have been widely understood [15][16][17][18]. MoS 2 with a two-dimensional (2D) layered structure is known to contain both active edge sites and chemically inert basal plane. Lots of work has been conducted to improve the activity by increasing the edge sites of MoS 2 and/or exploiting the inert basal plane to create additional active sites [19,20]. Hexagonal 1T-phase VS 2 (1T-VS 2 ) as a group TMDs is a promising HER electrocatalyst. The structure of 1T-VS 2 is similar to that of MoS 2 , which is assembled by stacked S-V-S monolayers via weak van der Waals interaction, which also has excellent structural stability. For the first time, Pan demonstrated by density functional theory calculation that the catalytic performance of single-layer VS 2 is equivalent to that of Pt at low hydrogen coverage [21]. Zhang and his colleagues further explained the role of intrinsic point defects in HER activity of monolayer VS 2 catalyst [22]. After that, Liang et al. developed a facile hydrothermal calcination method to synthesize self-supported VS 2 on carbon paper, Molecules 2022, 27, 8019 2 of 10 which shows excellent HER properties [23]. Qu and his colleagues also prepared VS 2 with flower-like morphology, obtaining superior HER performance in acid solution [24]. V 1.11 S 2 phase is one of nonstoichiometric 1T-V 1 + X S 2 (0 < X< 0.17) with V atoms in the interstitial site between adjacent layers (X is the concentration of V atoms) [25,26]. Both theoretical and experimental results indicate the excellent HER activity of self-intercalated V 1.11 S 2 , which shows a much faster proton/electron adsorption and hydrogen release process than the VS 2 [26]. Despite these advances, there were few reports focused on the morphology-controlled synthesis of V 1.11 S 2 , as well as their effect on HER performance. It is well known that electrocatalytic activities are highly reliant on the catalyst morphology, which is given more edge sites and lowly coordinated surface atoms that often determine the catalytic performance [27].
Herein, different morphologies of V 1.11 S 2 were synthesized by a simple hydrothermal synthesis and subsequent calcination ( Figure 1). The electrochemical catalytic properties of the resultant V 1.11 S 2 materials were systematically investigated.
calculation that the catalytic performance of single-layer VS2 is equivalent to that of Pt at low hydrogen coverage [21]. Zhang and his colleagues further explained the role of intrinsic point defects in HER activity of monolayer VS2 catalyst [22]. After that, Liang et al. developed a facile hydrothermal calcination method to synthesize self-supported VS2 on carbon paper, which shows excellent HER properties [23]. Qu and his colleagues also prepared VS2 with flower-like morphology, obtaining superior HER performance in acid solution [24].
V1.11S2 phase is one of nonstoichiometric 1T-V1 + XS2 (0 < X< 0.17) with V atoms in the interstitial site between adjacent layers (X is the concentration of V atoms) [25,26]. Both theoretical and experimental results indicate the excellent HER activity of self-intercalated V1.11S2, which shows a much faster proton/electron adsorption and hydrogen release process than the VS2 [26]. Despite these advances, there were few reports focused on the morphology-controlled synthesis of V1.11S2, as well as their effect on HER performance. It is well known that electrocatalytic activities are highly reliant on the catalyst morphology, which is given more edge sites and lowly coordinated surface atoms that often determine the catalytic performance [27].
Herein, different morphologies of V1.11S2 were synthesized by a simple hydrothermal synthesis and subsequent calcination ( Figure 1). The electrochemical catalytic properties of the resultant V1.11S2 materials were systematically investigated.  Figure 2 shows the XRD patterns of the obtained V1.11S2 materials. All the diffraction peaks can be assigned to the V1.11S2 (33-1445) phase without discernible impurities. It was found that both V1.11S2-1 and V1.11S2-2 have a well-crystalline phenomenon. It can be seen from Figures 2 and S1 that the XRD diffraction peaks before and after calcination are quite different, which is mainly due to the transformation of VS4 and VS2 to V1.11S2 at high-temperature conditions [28].  Figure 2 shows the XRD patterns of the obtained V 1.11 S 2 materials. All the diffraction peaks can be assigned to the V 1.11 S 2 (33-1445) phase without discernible impurities. It was found that both V 1.11 S 2 -1 and V 1.11 S 2 -2 have a well-crystalline phenomenon. It can be seen from Figure 2 and Figure S1 that the XRD diffraction peaks before and after calcination are quite different, which is mainly due to the transformation of VS 4 and VS 2 to V 1.11 S 2 at high-temperature conditions [28].  Figure 3 shows the FE-SEM images of V1.11S2 materials. Figure 3a,b display that flower-like V1.11S2 is stacked by a large number of V1.11S2 nanoplates in different directions. It is worth noting that the average radius of a single V1.11S2 nanoflower is about 10 μm.   Figure 3a,b display that flower-like V 1.11 S 2 is stacked by a large number of V 1.11 S 2 nanoplates in different directions. It is worth noting that the average radius of a single V 1.11 S 2 nanoflower is about 10 µm. Moreover, the formation of flowerlike V 1.11 S 2 probably involves two steps [29]. First, in a weak alkaline environment, the −SH functional group produced by C 2 H 5 NS reacts with the precursor of V to form V-S intermediate complexes followed by decomposing to shape VSx (x = 2, 4) nuclei for further growth. Then, VSx nanoplates are transformed at high temperatures into V 1.11 S 2 nanosheets, which are stacked together to form flower-like structures. Irregular flake V 1.11 S 2 prepared by process B is shown in Figure 3c and Figure  S2a. It can be seen from Figure S3b that the precursor obtained in process B is closely stacked by nanosheets, which are dispersed and smaller after calcination. When the solvent change to ethanol, a porous structure can be observed (Figure 3d). Compared with the powder before calcination shown in Figure S3c, the morphology changes dramatically, which is mainly due to the slight solubility of NaVO 3 in ethanol solution and the calcination procedure [30]. These results suggest the morphology can be easily controlled by altering the hydrothermal solvent and the source of vanadium. Transmission electron microscope (TEM) measurements were performed to analyze the physical structure of the V 1.11 S 2 -1. As depicted in Figure 4a, flower-like V 1.11 S 2 -1 can be exfoliated into a nanosheet structure under long−time ice bath ultrasound. The high-magnification TEM image shown in Figure 4b exhibits the periodic lattice fringe pattern, and the inter-planar spacing was measured to be 0.163 nm, which agrees with that of the (110) facet of V 1.11 S 2 ( Figure 4b).

Results and Discussion
The corresponding selected area electron diffraction (SAED) pattern also confirmed the crystal structure of the V 1.11 S 2 phase (inset in Figure 4b). Moreover, the EDS pattern in Figure S4 also reveals that V and S can be detected.  To detect the surface chemical state and element composition, X-ray photoelectron spectroscopy (XPS) analysis was performed on V1.11S2 with different morphologies. Investigation of the XPS spectrum clearly shows the presence of V and S ( Figure S5). The V 2p spectra can be fitted with two sets of doublet peaks (Figure 5a), and the spectrum of V1.11S2(V 2p) shows two additional broad peaks at a lower binding energy of 513.4, 516.3, 520.9, and 523.8 eV, which can be respectively assigned to V 2+ 2p3/2, V 4+ 2p3/2, V 2+ 2p1/2, and V 4+ 2p1/2 [26]. The peak fitting analysis of S 2p (Figure 5b) confirms the presence of S 2− with two peaks located at 160.8 and 162 eV that can be assigned to S2p3/2 and S2p1/2 [26,31]. The To detect the surface chemical state and element composition, X-ray photoelectron spectroscopy (XPS) analysis was performed on V 1.11 S 2 with different morphologies. Investigation of the XPS spectrum clearly shows the presence of V and S ( Figure S5). The V 2p spectra can be fitted with two sets of doublet peaks (Figure 5a), and the spectrum of V 1.11 S 2 (V 2p) shows two additional broad peaks at a lower binding energy of 513.4, 516.3, 520.9, and 523.8 eV, which can be respectively assigned to V 2+ 2p 3/2 , V 4+ 2p 3/2 , V 2+ 2p 1/2 , and V 4+ 2p 1/2 [26]. The peak fitting analysis of S 2p (Figure 5b) confirms the presence of S 2− with two peaks located at 160.8 and 162 eV that can be assigned to S2p 3/2 and S2p 1/2 [26,31]. The combined above-mentioned data indicate that the V 1.11 S 2 materials with different morphologies have been successfully prepared.
The electrocatalytic HER activities of V1.11S2 materials were assessed by linear sweep voltammetry (LSV) using a three-electrode system under 0.5 M H2SO4 acidic aqueous condition. From Figure 6a, V1.11S2-1 exhibits the best catalytic performance, achieving a current density of 10 mA cm −2 with an overpotential of 252 mV, which is superior to the previously reported vanadium sulfide acidic HER electrocatalysts, such as VS2 nanodiscs (420 mV) [32], CFP supported V1.11S2 (259.7 mV) [33], non−templated VS2 (378 mV) [34], and Co-N-doped single-crystal V3S4 nanoparticles (268 mV) [35]. As illustrated in Figure 6b, the calculated Tafel slopes of V1.11S2-1, V1.11S2-2, V1.11S2-3, and 5 wt.% Pt/C are 71.7 mV dec −1 , 264.1 mV dec −1 , 102.5 mV dec −1 , and 30.6 mV dec −1 , respectively. It is worth noting that the Tafel slope of 5 wt.% Pt/C is as low as 30.6 mV dec −1 , which is consistent with previous studies [36,37]. Therefore, V1.11S2-1 shows lower overpotential and Tafel slope, indicating its high HER activities. The stability of the catalyst plays an important role in practical application. The stability test of V1.11S2-1 was also carried out by chronopotentiometry test. From Figure S6, the potential remains stable at the current density of 10 mA cm −2 . For comparison, the potential of 5 wt.% Pt/C drops dramatically with the extension of test time, which is consistent with previous studies [38,39]. After the chronopotentiometry test, the LSV curves of V1.11S2-1 show a negligible recession phenomenon ( Figure S7), suggesting that the catalyst maintains a highly stable catalytic performance. In brief, the above electrochemical The stability of the catalyst plays an important role in practical application. The stability test of V 1.11 S 2 -1 was also carried out by chronopotentiometry test. From Figure  S6, the potential remains stable at the current density of 10 mA cm −2 . For comparison, the potential of 5 wt.% Pt/C drops dramatically with the extension of test time, which is consistent with previous studies [38,39]. After the chronopotentiometry test, the LSV curves of V 1.11 S 2 -1 show a negligible recession phenomenon ( Figure S7), suggesting that the catalyst maintains a highly stable catalytic performance. In brief, the above electrochemical test results confirm the flower-like V 1.11 S 2 material has superior electrochemical activity and stability for HER.
According to previous studies, the catalytic active H−adsorption site of the V 1.11 S 2 catalyst is S in the outermost layer [24,40]. Generally, the electrocatalytic activity is highly dependent on the catalyst morphology with more active sites. In order to further clarify the origination of excellent HER performance for V 1.11 S 2 materials, both the electrochemical surface area (ECSA) of the samples were tested. The corresponding current in the applied potential window of 0.06-0.16 V vs. the reversible hydrogen electrode (RHE) should be originated from the charging of the double-layer, and the calculated capacitance (C dl ) should be proportional to the ECSA [41]. As shown in Figure 7 and the corresponding cyclic voltammograms in Figure S8, V 1.11 S 2 -1 has higher electric double-layer capacitance (3.4 mF cm −2 ) than V 1.11 S 2 -2 (0.45 mF cm −2 ) and V 1.11 S 2 -2 (1.9 mF cm −2 ). Moreover, the fitting value R−Squares is listed in Table S1, suggesting V 1.11 S 2 -1 has a larger surface area with more exposed active sites. This may be one of the reasons for its high HER performance.
EIS measurements were performed to examine the kinetic differences between V 1.11 S 2 in different morphologies during the electrochemical process [42]. As shown in the illustration in Figure 8, the semicircle in the Nyquist plots was fitted by using the Randles equivalent circuit, in which Rs represents the equivalent series resistance, Rct 1 represents the charge transfer resistance of the electrode, and CPE represents the constant phase element [43,44]. It is worth noting that the charge transfer resistance (Rct 1 ) is related to the electrocatalytic kinetics, and a lower value corresponds to a faster reaction rate, which can be quantified from the diameter of the semicircle in the low-frequency zone [45]. Table 1 demonstrates the changing trend of the Rct 1 value for V 1.11 S 2 nanomaterials with different morphologies, V 1.11 S 2 -1 (49.54 Ω) < V 1.11 S 2 -3 (60.8 Ω) < V 1.11 S 2 -2 (114.3 Ω), indicating that V 1.11 S 2 -1 has better conductivity. Overall, we can conclude that the enhanced catalytic HER activity of flower-like V 1.11 S 2 compared to the other two structures can be accountable for both the abundant catalytically active sites and preferable low charge transfer resistance.
alyst is S in the outermost layer [24,40]. Generally, the electrocatalytic activity is highly dependent on the catalyst morphology with more active sites. In order to further clarify the origination of excellent HER performance for V1.11S2 materials, both the electrochemical surface area (ECSA) of the samples were tested. The corresponding current in the applied potential window of 0.06-0.16 V vs. the reversible hydrogen electrode (RHE) should be originated from the charging of the double-layer, and the calculated capacitance (Cdl) should be proportional to the ECSA [41]. As shown in Figure 7 and the corresponding cyclic voltammograms in Figure S8, V1.11S2-1 has higher electric double-layer capacitance (3.4 mF cm −2 ) than V1.11S2−2 (0.45 mF cm −2 ) and V1.11S2-2 (1.9 mF cm −2 ). Moreover, the fitting value R−Squares is listed in Table S1, suggesting V1.11S2-1 has a larger surface area with more exposed active sites. This may be one of the reasons for its high HER performance. EIS measurements were performed to examine the kinetic differences between V1.11S2 in different morphologies during the electrochemical process [42]. As shown in the illustration in Figure 8, the semicircle in the Nyquist plots was fitted by using the Randles equivalent circuit, in which Rs represents the equivalent series resistance, Rct1 represents the charge transfer resistance of the electrode, and CPE represents the constant phase element [43,44]. It is worth noting that the charge transfer resistance (Rct1) is related to the electrocatalytic kinetics, and a lower value corresponds to a faster reaction rate, which can be quantified from the diameter of the semicircle in the low-frequency zone [45]. Table 1 demonstrates the changing trend of the Rct1 value for V1.11S2 nanomaterials with different morphologies, V1.11S2-1 (49.54 Ω) < V1.11S2-3 (60.8 Ω) < V1.11S2-2 (114.3 Ω), indicating that V1.11S2-1 has better conductivity. Overall, we can conclude that the enhanced catalytic HER activity of flower-like V1.11S2 compared to the other two structures can be accountable for both the abundant catalytically active sites and preferable low charge transfer resistance.

Synthesis of V1.11S2 Materials
The schematic for the synthesis of V1.11S2 with different morphology is shown in Figure 1. In process A, 1 mmol NH4VO3 and 10 mmol CH3CSNH2 were first dissolved in a solution containing 38 mL deionized water and 2 mL NH3·H2O, which was stirred for 1 h  O), N-methyl pyrrolidone (C 5 H 9 NO). The above chemicals and reagents were purchased from Chengdu Kelong Co., Ltd., Chengdu, China. All reagents were used directly without further purification. The commercial 5 wt.% Pt/C catalyst was purchased from Macleans. Carbon paper (TGP-H-060) was purchased from yilongsheng Energy Technology Co., Ltd., Suzhou, China.

Synthesis of V 1.11 S 2 Materials
The schematic for the synthesis of V 1.11 S 2 with different morphology is shown in Figure 1. In process A, 1 mmol NH 4 VO 3 and 10 mmol CH 3 CSNH 2 were first dissolved in a solution containing 38 mL deionized water and 2 mL NH 3 ·H 2 O, which was stirred for 1 h to form a uniform solution. Afterward, the prepared solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and maintained at 160 • C for 24 h. After natural cooling to room temperature, the black precipitates were collected by centrifugation, washed several times with deionized water and absolute ethanol, and dried under a vacuum for 6 h. In process B, 1 mmol NaVO 3 was used as a vanadium source, and 15 mmol CH 3 CSNH 2 as a sulfur source, dissolved in 25 mL deionized water; other reaction conditions were the same. In process C, 4 mmol NaVO 3 , and 24 mmol CH 3 CSNH 2 were dissolved in 25 mL C 2 H 6 O, and the reaction temperature was raised to 180 • C. All final precipitates were calcined at 400 • C for 2 h to obtain V 1.11 S 2 . These V 1.11 S 2 materials obtained by different preparation procedures were labeled as V 1.11 S 2 -1, V 1.11 S 2 -2, and V 1.11 S 2 -3, respectively.

Materials Characterization
The phase constitutes of the obtained samples were characterized by an X-ray diffractometer (XRD, DX-2700B) with Cu Kα radiation. The microstructure of V 1.11 S 2 with different morphology was examined by a field emission scanning electron microscope (FESEM, FEI Insect F50). The TEM images of V 1.11 S 2 −1 were obtained by high-resolution transmission electron microscopy (TEM, FEI Talos F200S Super). The surface valence states and elemental compositions were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Fischer, ESCALAB Xi + ).

Electrochemical Measurements
All electrochemical data were measured by an electrochemical workstation (CHI660E, CH Instrument, Shanghai, China) with a typical three-electrode electrochemical cell in the acidic electrolyte (0.5 M H 2 SO 4 ). A graphite rod was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. The working electrode was prepared as follows: 3 mg catalysts and 50 µL Nafion solution (5 wt.%) were dispersed in 500 µL mixed solvent of deionized water−isopropanol (volume ratio of 3:1), then sonication to form a homogeneous ink solution. Ink with a volume of 15 µL was loaded onto the carbon paper (0.25 cm −2 ) electrode and dried at ambient temperature. A loading density of about 0.343 mg cm −2 was obtained. Linear sweep voltammograms (LSV) were measured from 0.10 to −0.90 V (vs. RHE) at a scan rate of 5 mV s −1 . Electrochemical impedance spectroscopy (EIS) was conducted under −238 mV (vs. RHE) over a frequency range from 100 kHz to 0.01 Hz with a 5mV amplitude potential. The cyclic voltammograms (CV) measurements at various scan rates from 20, 40, 60, 80, and 100 mV s −1 were performed in the potential range of 0.06~0.16 V (vs. RHE) for the electrochemical double−layer capacitance (C dl ) estimation. It should be noted that no iR compensation was applied to our testing data. According to E corrected = E measured − i × R s , all potentials are corrected by iR, where i is the test current and Rs is the equivalent series resistance, which is determined by the Nyquist plots fitting.

Conclusions
In conclusion, V 1.11 S 2 was successfully synthesized by hydrothermal synthesis and subsequent calcination. The morphology can be easily controlled by altering the hydrothermal solvent and the source of vanadium. The electrocatalysis results show that the flower−like V 1.11 S 2 has the best catalytic activity, which can be ascribed to abundant catalytically active sites and preferable low charge transfer resistance. This research provides us with a strong basis for the morphology dependent of V 1.11 S 2 materials towards HER. Further works will be performed to enhance the intrinsic activity of V 1.11 S 2 and/or supported on conductive substrates such as carbon cloth and metal foam.
Author Contributions: Methodology, X.L. and W.Y.; Software, X.A., J.Z. and L.X.; Formal analysis, Q.K.; Data curation, Q.C. and S.T. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to restrictions eg privacy or ethical.