1. 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, 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
1.11S
2 phase is one of nonstoichiometric 1T-V
1 +
XS
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
1.11S
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
1.11S
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
1.11S
2 were synthesized by a simple hydrothermal synthesis and subsequent calcination (
Figure 1). The electrochemical catalytic properties of the resultant V
1.11S
2 materials were systematically investigated.
2. Results and Discussion
Figure 2 shows the XRD patterns of the obtained V
1.11S
2 materials. All the diffraction peaks can be assigned to the V
1.11S
2 (33–1445) phase without discernible impurities. It was found that both V
1.11S
2-1 and V
1.11S
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
1.11S
2 at high-temperature conditions [
28].
Figure 3 shows the FE-SEM images of V
1.11S
2 materials.
Figure 3a,b display that flower-like V
1.11S
2 is stacked by a large number of V
1.11S
2 nanoplates in different directions. It is worth noting that the average radius of a single V
1.11S
2 nanoflower is about 10 μm. Moreover, the formation of flowerlike V
1.11S
2 probably involves two steps [
29]. First, in a weak alkaline environment, the −SH functional group produced by C
2H
5NS 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
1.11S
2 nanosheets, which are stacked together to form flower-like structures. Irregular flake V
1.11S
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
1.11S
2-1. As depicted in
Figure 4a, flower-like V
1.11S
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
1.11S
2 (
Figure 4b). The corresponding selected area electron diffraction (SAED) pattern also confirmed the crystal structure of the V
1.11S
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 V
1.11S
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
1.11S
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
1.11S
2 materials with different morphologies have been successfully prepared.
The electrocatalytic HER activities of V
1.11S
2 materials were assessed by linear sweep voltammetry (LSV) using a three-electrode system under 0.5 M H
2SO
4 acidic aqueous condition. From
Figure 6a, V
1.11S
2-1 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 VS
2 nanodiscs (420 mV) [
32], CFP supported V
1.11S
2 (259.7 mV) [
33], non−templated VS
2 (378 mV) [
34], and Co-N-doped single-crystal V
3S
4 nanoparticles (268 mV) [
35]. As illustrated in
Figure 6b, the calculated Tafel slopes of V
1.11S
2-1, V
1.11S
2-2, V
1.11S
2-3, 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, V
1.11S
2-1 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 V
1.11S
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
1.11S
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
1.11S
2 material has superior electrochemical activity and stability for HER.
According to previous studies, the catalytic active H−adsorption site of the V
1.11S
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
1.11S
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
1.11S
2-1 has higher electric double-layer capacitance (3.4 mF cm
−2) than V
1.11S
2-2 (0.45 mF cm
−2) and V
1.11S
2-2 (1.9 mF cm
−2). Moreover, the fitting value R−Squares is listed in
Table S1, suggesting V
1.11S
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
1.11S
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
1.11S
2 nanomaterials with different morphologies, V
1.11S
2-1 (49.54 Ω) < V
1.11S
2-3 (60.8 Ω) < V
1.11S
2-2 (114.3 Ω), indicating that V
1.11S
2-1 has better conductivity. Overall, we can conclude that the enhanced catalytic HER activity of flower-like V
1.11S
2 compared to the other two structures can be accountable for both the abundant catalytically active sites and preferable low charge transfer resistance.