Sulfuric acid (H₂SO₄), chloroauric acid (HAuCl₄·4H₂O), sodium citrate, L-cysteine, sodium molybdate (Na₂MoO₄), and other reagents were purchased from Alfa Aesar. Deionized water was used for preparing all solutions.

The morphologies of catalysts were characterized using field emission scanning electron microscopy (SEM) with an accelerating voltage of 15 kV (Hitachi S-4800, Japan). Transmission electron microscopy (TEM) images were achieved by a JEM-1400 operating at 120 kV. The x-ray photoelectron spectroscopy (XPS) measurement was carried on PHI Quantera (PHI, Japan), and the binding energy is calibrated with C 1s = 284.8 eV.

All electrochemical measurements were performed on a glass carbon electrode (GCE)by using a three-electrode CHI660D electrochemical workstation (Chenhua Instrument, Shanghai, China) in 0.5-M H₂SO₄ solution, platinum wire was used as a counter electrode, and a Ag/AgCl electrode served as a reference electrode. Typically, 6-μL AuNPs, MoS₂, and Au–MoS₂ composite inks were drop-casted onto the conventionally pretreated 3-mm-diameter GCEas working electrodes. In this study, all the potentials measured were verified by the reversible hydrogen electrode (RHE) according to the equation: E_RHE = E_Ag/AgCl + 0.197 + 0.059 pH. Linear sweep voltammetry (LSV) were swept from −0.8 V to 0.4 V versus RHE was swept with a scan rate of 0.05 V/s. The current density was calculated based on the geometric area of the GCE to be 7.07 × 10⁻² cm².

In a typical synthesis of the MoS₂, 0.2-g Na₂MoO₄ and 0.4-g L-cysteine were dissolved in 40-mL deionized (DI) water. The mixture was sonicated for 15 min. The homogeneous solution was then transferred into a 100-mL Teflon-lined stainless steel autoclave reactor, and hydrothermally treated in an oven at 180 °C for 12 h. The autoclave was then removed from the oven and naturally cooled to room temperature. The black product formed in the solution was then collected by centrifugation, washed with DI water and ethanol repeatedly many times, and then dried by lyophilization. Finally, the dried black product was dispersed in DI water with a concentration of 8 mg/mL.

Au nanoparticles were synthesized according to our previously reported method.^[38] Briefly, 500-μL HAuCl₄ (25.4 mM) was added into 40-mL boiling distilled water under vigorous stirring, and 900-μL trisodium citrate (0.1 M) was dropwise added into the boiling solution under constant stirring for 20 min, the color changed to wine red when the reaction was completed. Finally, the suspension was cooled down to room temperature under continuous stirring and stored at 4 °C.

Au nanoparticles (NPs) were in situ synthesised on the MoS₂ nanoflower. Briefly, 0.75-mL MoS₂ aqueous solution was added into 20-mL DI water followed by the addition of HAuCl₄ (25.4 mM). HAuCl₄ solution volumes were set to be 10, 25, 50, 100, and 150 μL. The mixture was sonicated for 15 min. Then, the solution was heated to boiling under vigorous stirring, and excess trisodium citrate (0.1 M) was dropwise added into the mixture under constant stirring for another 20 min. After the suspension was cooled down to room temperature, the products were centrifugally washed with DI water four times. Finally, the black products were dispersed in 20-mL DI water. The composites were denoted as Au–MoS₂-10, Au–MoS₂-25, Au–MoS₂-50, Au–MoS₂-100, and Au–MoS₂-150, respectively.

Figure 1 shows TEM images of MoS₂, Au–MoS₂-10, Au–MoS₂-25, Au–MoS₂-50, Au–MoS₂-100, Au–MoS₂-150, SAED pattern, and high-resolution TEM image of Au–MoS₂-100 composite. Few small dark spots were found to adhere to the surface as a consequence of Au loading and not observed outside the supporter. Distinct lattice fringes of Au and MoS₂ can be observed in the high-resolution TEM images with the fringe spaces of 0.18 nm and 0.64 nm, confirming that the (111) plane of AuNPs and the (002) plane of MoS2 exist respectively, which indicates the formation of Au–MoS₂ composites.^[6,20] The AuNPs prepared by sodium citrate reduction have good monodispersity and an average diameter of 10 nm. The morphologies and crystal structures of MoS₂ and Au–MoS₂ composites were also studied by SEM (Fig. 2). MoS₂ exhibits the sphere-like aggregate morphology and the spheres are composed of thin nanosheets. The thin nanosheet will increase the specific surface area of MoS₂ and introduce more active sites, which is significantly important for the electrocatalytic activity of MoS₂. With the different volumes (10 μL–150 μL) of HAuCl₄ added into MoS₂ solution, the amount of AuNPs grown on the surface of the MoS₂ nanosheet increases. The right amount of AuNPs may improve the electronic property of MoS₂, but excess AuNPs can influence the active site on the MoS₂ nanosheets then reduce the catalytic performance of the system.

Fig. 1.

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	Fig. 1. TEM images of (a) pure MoS2, (b) Au–MoS2-10, (c) Au–MoS2-25, (d) Au–MoS2-50, (e) Au–MoS2-100, and (f) Au–MoS2-150. (g) SAED patterns of Au–MoS2-100. (h) High-resolution TEM of AuNPs/MosS2.

Fig. 2.

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	Fig. 2. (color online) SEM images of (a) pure MoS2, (b) Au–MoS2-10, (c) Au–MoS2-25, (d) Au–MoS2-50, (e) Au–MoS2-100, and (f) Au–MoS2-150.

The elemental mapping images based on Fig. 3(a) are displayed in Figs. 3(b)–3(d), demonstrating that Mo, S, and Au are uniformly distributed all over the nanosheets. Moreover, energy dispersive x-ray spectrum (Fig. 3(e)) shows that the as-synthesized Au–MoS₂ composite sample is mainly composed of Mo, S, and Au elements, indicating the successful synthesis of Au–MoS₂ composite.

Fig. 3.

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	Fig. 3. (color online) EDS elemental mapping images of (a) Au–MoS2-100 composite, (b) Mo, (c) S, (d) Au, and (e) energy dispersive x-ray spectrum of Au–MoS2-100 composite.

To explore the composition and chemical bonding configuration, XPS spectra of MoS₂ are recorded before and after the introduction of Au. The XPS survey spectra of samples are shown in Fig. 4, which confirm the presence of Mo, S, and Au elements, no impurity phase is found. Two characteristic peaks at 227.1 eV and 230.4 eV are typical values for Mo 3d_5/2 and Mo 3d_3/2 orbits (Fig. 4(b)) respectively, suggesting the dominance of the +4 oxidation state of the Mo element in MoS₂. The peaks at around 160.1 eV and 161.1 eV (Fig. 4(c)) are attributed to the coexistence of S 2p_3/2 and 2p_1/2 respectively, suggesting the −2 reduction state for the S element. The characteristic peak at 224.4 eV is related to S 2s. Detailed compositional analysis results reveal that the atomic ratio of Mo:S is about 1:2.11.^[39] The peaks at 227.3 eV, 230.4 eV (Fig. 4(d)) and 160.2 eV, 161.3 eV (Fig. 4(e)) are all close to the binding energies of pure MoS₂ (Figs. 4(b) and 4(c)). This indicates that a similar chemical environment for the two elements in pure MoS₂ and Au–MoS₂ composite and the small binding energy shift may be caused by the electron transfer between AuNPs and MoS₂ sheets. The Au 4f peaks at 82.6 eV and 86.4 eV (Fig. 4(f)) can be detected, which indicates the successful synthesis of Au–MoS₂ composite.

Fig. 4.

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	Fig. 4. (color online) (a) XPS survey spectra of pure MoS2 and Au–MoS2-100 composite. High-resolution XPS survey spectra of (b) Mo 3d and (c) S 2p. High-resolution XPS survey spectra of (d) Mo 3d, (e) S 2p, and (f) Au 4f.

In order to evaluate the electrocatalytic HER activities of the Au–MoS₂ composites, glassy carbon electrodes modified with pure MoS₂, Au–MoS₂-10, Au–MoS₂-25, Au–MoS₂-50, Au–MoS₂-100, and Au–MoS₂-150 compositions are prepared for polarization curves in 0.5-M H₂SO₄ electrolyte. As shown in Fig. 5(a), the pure MoS₂ has a high onset overpotential of 300 mV, while all the Au–MoS₂ composites have a lower onset overpotential between 120 mV and 270 mV. The enhanced HER activity can be mainly attributed to the introduction of AuNPs, which improves electron transfer between electrode and MoS₂. The onset overpotential decreases with the increase of AuNPs, and Au–MoS₂-100 has the lowest onset overpotential of 120 mV, but when the concentration of AuNPs increases to Au–MoS₂-150, the overpotential reaches to 180 mV. This is most probably because too many AuNPs have grown on the surface of MoS₂, thereby reducing the active sites on the edge nanosheets. That is, Au–MoS₂-100 has the best HER catalytic activity, which is better than Au–MoS₂-10 (270 mV), Au–MoS₂-25 (240 mV), Au–MoS₂-50 (230 mV), and Au–MoS₂-150 (180 mV).

Fig. 5.

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	Fig. 5. (color online) (a) LSV curves for pure MoS2, Au–MoS2-10, Au–MoS2-25, Au–MoS2-50, Au–MoS2-100, and Au–MoS2-150. Corresponding (b) Tafel plots and (c) Nyquist plots. (d) LSV curves of the Au–MoS2-100 at initial and 200 cycles for durability test.

The Tafel slope is an intrinsic nature of the electrocatalyst material. The Tafel slope derived from the polarization curves is shown in Fig. 5(b). The Au–MoS₂-10, Au–MoS₂-25, Au–MoS₂-50, Au–MoS₂-100, and Au–MoS₂-150 compositions are found to show Tafel slopes of 289 mV·dec⁻¹, 287 mV·dec⁻¹, 239 mV·dec⁻¹, 163 mV·dec⁻¹, and 284 mV·dec⁻¹, respectively. The smaller Tafel slope signifies a faster increase of HER rate with increasing overpotential. Therefore, the Au–MoS₂-100 has the smallest Tafel slope, supporting its best electrocatalystic activity for HER. Furthermore, electrochemical impedance spectroscopy (EIS) is used to study the interfacial reactions of the obtained samples under HER process. The EIS measurements are carried out in 0.5-M H₂SO₄ at an overpotential of 0.25 V. As shown in Fig. 5(c), the reaction resistance of Au–MoS₂ composite increases in the sequence of Au–MoS₂-150<Au–MoS₂-100<Au–MoS₂-50<Au–MoS₂-25<Au–MoS₂-10, which is similar to the above HER results. The reaction resistances are 95 Ω for Au–MoS₂-150 and 177 Ω for Au–MoS₂-100, respectively, which are much smaller than those of Au–MoS₂-10 (360 Ω), Au–MoS₂-25 (270 Ω), and Au–MoS₂-50 (260 Ω), indicating that the AuNPs grown on MoS₂ can improve the conductivity and ensure high-quality catalytic activity in the HER process. Finally, the stability of the Au–MoS₂-100 catalytic response is also evaluated by conducting the cyclic voltammetry (CV). As can be seen from Fig. 5(d), after 200-CV cycles, the catalyst affords similar curves to the initial cycle with slight anodic current loss at a certain overpotential. These results indicate that the Au–MoS₂-100 has good stability in a long-term electrochemical process and has a promising potential for practical applications.