Effect of Ag2S Nanocrystals/Reduced Graphene Oxide Interface on Hydrogen Evolution Reaction

The development of efficient electrocatalyst to produce molecular hydrogen from water is receiving considerable attention, in an effort to decrease our reliance on fossil fuels. The prevention of the aggregation of active sites during material synthesis, in order to increase charge transport properties of electrocatalysts, is needed. We have designed, synthesized, and studied a Ag2S/reduced graphene oxide (rGO) electrochemical catalyst (for hydrogen evolution) from water. The Ag2S nanocrystals were synthesized by the solvothermal method in which the rGO was added. The addition of the rGO resulted in the formation of smaller Ag2S nanocrystals, which consequently increased the electrical conductivity of the composite catalyst. The composite catalyst showed a higher electrochemical catalytic activity than the one with an absence of rGO. At a current density of 10 mA/cm2, a low overpotential of 120 mV was obtained. A Tafel slope of 49.1 mV/dec suggests a Volmer–Herovsky mechanism for the composite catalyst. These results may provide a novel strategy for developing hydrogen evolution reaction (HER) electrocatalysts, via the combining of a nano-semiconductor catalyst with a 2D material.


Introduction
Hydrogen is an energy carrier with the highest combustion heat known. When produced from a renewable source, it has no harmful environmental effect. Among the strategies for producing hydrogen, electrocatalytic water splitting is an efficient, green (if electricity originates from renewable sources), and potentially low-cost method.
Among various types of semiconductors, Ag 2 S has recently attracted interest due to its negligible toxicity and broad absorption spectrum that extends into the near infrared. Bulk Ag 2 S has a band gap of 1.06 ± 0.2 eV [1], which can be further adjusted to 1.44 eV through size-control or surface modification. Ag 2 S has been studied because of its possible applications as a photodetector [2][3][4], as a photo-catalyst [5,6], in near-infrared fluorescence imaging in vivo [7][8][9], in surface-enhanced Raman scattering [10,11], and as a nanocrystal-sensitized solar cell [12][13][14][15][16]. In some work, nanomaterials,

Result and Discussion
The crystals of free Ag 2 S nanocrystal are shown in Figure 1a; they have a mean particle size of about 15 nm. Figure 1b shows a TEM image of the GO, and the Ag 2 S particles on top of rGO are shown in Figure 1c. Smaller aggregates of Ag 2 S are present when the Ag 2 S is deposited onto the rGO (mean particle size of about 7 nm). The results show that the presence of the rGO sheets prevented further growth of the Ag 2 S nanocrystals during the synthesis. A high-resolution TEM (HRTEM) image of the Ag 2 S nanocrystals on the rGO sheets is shown in Figure 1d. The lattice spacing of 0.16 and 0.19 nm corresponded to the spacing of the (141) and (212) faces of the Ag 2 S, respectively.
The optical, diffraction, and vibrational characteristics of the Ag 2 S/rGO composite are presented in Figure 2. The UV-Vis-IR absorbance spectra showed an onset at 880 nm (Figure 2a), from which the band gap was extracted to be ca. 1.4 eV. The first excitonic absorbance peak was located at 800 nm. The fluorescence spectra of the Ag 2 S/rGO composite, Ag 2 S/GO composite, Ag 2 S, and GO are shown in Figure 2b. One can see a fluorescence centered at 825 nm in all three of the samples containing the Ag 2 S nanocrystals (which corresponds to 1.5 eV). After the addition of the GO and rGO, the peak Catalysts 2020, 10, 948 3 of 12 position did not shift within the experimental errors (±3 nm). A direct comparison between the PL and UV-vis absorbance may not be possible, particularly because of the presence of excitonic absorbance and quantum size effects. Figure S3 shows the changes in photoluminescence at the different ratios of Ag 2 S to rGO; the decrease in the PL with the decreasing Ag 2 S wt. % is due to the dilution effect, yet the trend is not linear. This might be due to experimental uncertainty but may also be evidence of some charge transfer between the Ag 2 S and rGO. The XRD patterns of the Ag 2 S/rGO composite are given in Figure 2c; they matched well with its orthorhombic structure [7]. The black curve refers to the XRD reference card of the Ag 2 S (PDF#03-0844). The optical, diffraction, and vibrational characteristics of the Ag2S/rGO composite are presented in Figure 2. The UV-Vis-IR absorbance spectra showed an onset at 880 nm (Figure 2a), from which the band gap was extracted to be ca. 1.4 eV. The first excitonic absorbance peak was located at 800 nm. The fluorescence spectra of the Ag2S/rGO composite, Ag2S/GO composite, Ag2S, and GO are shown in Figure 2b. One can see a fluorescence centered at 825 nm in all three of the samples containing the Ag2S nanocrystals (which corresponds to 1.5 eV). After the addition of the GO and rGO, the peak position did not shift within the experimental errors (±3 nm). A direct comparison between the PL and UV-vis absorbance may not be possible, particularly because of the presence of excitonic absorbance and quantum size effects. Figure S3 shows the changes in photoluminescence at the different ratios of Ag2S to rGO; the decrease in the PL with the decreasing Ag2S wt. % is due to the dilution effect, yet the trend is not linear. This might be due to experimental uncertainty but may also be evidence of some charge transfer between the Ag2S and rGO. The XRD patterns of the Ag2S/rGO composite are given in Figure 2c; they matched well with its orthorhombic structure [7]. The black curve refers to the XRD reference card of the Ag2S (PDF#03-0844). The dynamic measurements were performed using a time resolved photoluminescence (TRPL) system. Figure 2d shows TRPL spectra of the free Ag 2 S and Ag 2 S/rGO; the lifetime data are shown in Table S1. The fluorescence lifetime of the Ag 2 S was found to be about 7.7 ns. It decreased by about half upon the deposition on rGO from 7.7 to 4.3 ns, which might be related to the increasing electron-hole pair separation due to the rapid charge transfer in Ag 2 S/rGO. This might result in a more efficient H 2 elevating reaction on the surface of Ag 2 S nanoparticle nanocrystals, and the interface between the Ag 2 S nanocrystals and rGO.
The electrocatalytic behavior is shown in Figure 3. Experiments were carried out using a standard three-electrode system, in a 0.5 M H 2 SO 4 aqueous solution. An overpotential of the Ag 2 S/rGO composite for HER (of about 120 mV) was obtained from the polarization curve, shown in Figure 3a. In contrast, the Ag 2 S/GO composite, Ag 2 S, and rGO (used as the electrocatalysts) showed overpotentials of 131, 207, and 240 mV, respectively. The Tafel curves, extracted from the linear region of the polarization curves, are shown in Figure 3b. From the Tafel equation (η = b log j + a, in which j is the current density and b is the Tafel slope), the slopes of the Ag 2 S/rGO composite, Ag 2 S/GO composite, Ag 2 S, and rGO were found to be 49.1, 62.3, 99.2, and 120.6 mV/dec, respectively. The performance of catalysts mentioned above is listed in Table 1. The oscillation of the Tafel curve of the Ag 2 S can be attributed to the moderate stability of the Ag 2 S as an electrocatalyst. The polarization curve and the Tafel curve confirmed the improvement performance of the Ag 2 S/rGO composite catalyst. The dynamic measurements were performed using a time resolved photoluminescence (TRPL) system. Figure 2d shows TRPL spectra of the free Ag2S and Ag2S/rGO; the lifetime data are shown in Table S1. The fluorescence lifetime of the Ag2S was found to be about 7.7 ns. It decreased by about half upon the deposition on rGO from 7.7 to 4.3 ns, which might be related to the increasing electronhole pair separation due to the rapid charge transfer in Ag2S/rGO. This might result in a more efficient H2 elevating reaction on the surface of Ag2S nanoparticle nanocrystals, and the interface between the Ag2S nanocrystals and rGO.
The electrocatalytic behavior is shown in Figure 3. Experiments were carried out using a standard three-electrode system, in a 0.5 M H2SO4 aqueous solution. An overpotential of the Ag2S/rGO composite for HER (of about 120 mV) was obtained from the polarization curve, shown in Figure 3a. In contrast, the Ag2S/GO composite, Ag2S, and rGO (used as the electrocatalysts) showed overpotentials of 131, 207, and 240 mV, respectively. The Tafel curves, extracted from the linear region of the polarization curves, are shown in Figure 3b. From the Tafel equation (η = b log j + a, in which j is the current density and b is the Tafel slope), the slopes of the Ag2S/rGO composite, Ag2S/GO composite, Ag2S, and rGO were found to be 49.1, 62.3, 99.2, and 120.6 mV/dec, respectively. The performance of catalysts mentioned above is listed in Table 1. The oscillation of the Tafel curve of the Ag2S can be attributed to the moderate stability of the Ag2S as an electrocatalyst. The polarization curve and the Tafel curve confirmed the improvement performance of the Ag2S/rGO composite catalyst.   To further compare the samples, in particular, those of the Ag2S/rGO and Ag2S/GO, we  To further compare the samples, in particular, those of the Ag 2 S/rGO and Ag 2 S/GO, we measured the CV curves continuously for 1000 cycles. The polarization characteristics were tested before and after the 1000 cycles CV test, respectively, showing very small changes (Figure 3d). In addition, the typical CV scans (the 1st and 1000th) are shown in Figure S4a. The stability test of the Ag 2 S/GO composite is shown in Figure S4b; it decreased much faster than that of the Ag 2 S/rGO composite. The Ag 2 S and rGO, alone, were unstable; they often fell off from the electrode and could not complete 1000 cycles during the CV test.
To further extrapolate the stability of the Ag 2 S/rGO catalyst, XPS analysis before and after electro-catalytic reaction was carried out to investigate the possible compositional change of the Ag 2 S/rGO (Figure 4 and Figure S5). The binding energy was calibrated with respect to the C1s (284.5 eV). Before the electrocatalytic reaction, the Ag 3d 3/2 and Ag 3d 5/2 were at 373.7 and 367.7 eV, respectively, which corresponded to the Ag + cations in the Ag 2 S [43][44][45]. After the electrocatalytic reaction, the Ag 3d 3/2 and Ag 3d 5/2 moved to 374.1 and 368.1 eV, respectively, which indicated the presence of Ag on the surface of the Ag 2 S/rGO after the reaction [20]. As shown in Figure 4b, the peaks at 161.1 and 162.2 eV correspond to the S 2p of the Ag 2 S, and the peaks at 162.0 and 163.3 eV correspond to the S 2p of the C-S in the MPA ligand. After the electrocatalytic reaction, the peak intensity of the S-C decreased, which might have been caused by the oxidation of the ligand [5,46,47]. The details of the fitting peaks of S before and after the HER reaction are listed in Tables 2 and 3, respectively. The atomic ratio of the Ag/S changed slightly from 1/1.94 to 1/1.86, which further proves the stability of the Ag 2 S/rGO after the electrocatalytic reaction. We then further explored the reasons for the higher activity of the Ag 2 S/rGO composite catalysts for HER. This higher performance may be attributed to three points: (i) the Ag 2 S nanocrystals grown on the surface of the rGO were better dispersed than the free-grown Ag 2 S nanocrystals; (ii) the sizes of the Ag 2 S nanocrystals on the rGO were slightly smaller, which may have had a different intrinsic electrocatalytic activity; and (iii) rGO can have an enhanced conductivity compared to Ag 2 S alone, which would lead to less electrochemical impedance than that in the other samples (Figure 3c). correspond to the S 2p of the C-S in the MPA ligand. After the electrocatalytic reaction, the peak intensity of the S-C decreased, which might have been caused by the oxidation of the ligand [5,46,47]. The details of the fitting peaks of S before and after the HER reaction are listed in Tables 2 and 3, respectively. The atomic ratio of the Ag/S changed slightly from 1/1.94 to 1/1.86, which further proves the stability of the Ag2S/rGO after the electrocatalytic reaction.   In the presence of the rGO, the free electrons generated on the Ag 2 S nanocrystals may be transferred to the rGO [48]. The proton interaction and further hydrogen evolution reaction are enhanced because the rGO provides the needed large surface area. At the same time, it does not block the active sites on the Ag 2 S nanocrystals' surfaces, meaning it can continue generating the electrons needed for the hydrogen evolution reaction. In the absence of an rGO substrate, all the reactions would be carried out on the surface of the Ag 2 S nanocrystals. It is, thus, plausible that a synergistic effect between the Ag 2 S nanocrystals and rGO is present, leading to an improvement in the efficiency of molecular hydrogen production.
Three processes may occur in the electrocatalytic HER, in the acidic electrolytic condition. Firstly, the hydrated protons are combined with electrons (e − ) and then form an adsorbed hydrogen (H ad )-where ad stands for adsorbed (the Volmer reaction) [18,49]: where R is the ideal gas constant, T is the absolute temperature, α is the symmetry coefficient (approximately equal to 0.5), and F is the Faraday constant. Following the first step, two possible reactions for H 2 production may occur: an electrochemical desorption reaction (Heyrovsky reaction) or a recombination reaction (Tafel reaction).
In the Heyrovsky reaction, the H ad combines with a second hydrated protons and a second electron from the catalyst to form a hydrogen molecule: In the Tafel reaction, two H ad directly combine to form a hydrogen molecule: In the case of the Ag 2 S/rGO-catalyzed electrochemical HER reaction, two processes may occur, as shown in Figure 5. The Volmer process may occur on both the Ag 2 S and rGO. The adsorbed hydrogen was formed on the surface of the catalysts waiting for the next reaction process. The Tafel slope was found to be equal, 49 and 120 mV/dec for the Ag 2 S/rGO and rGO, respectively. Thus, the reaction on the surface of the Ag 2 S/rGO, here, can be determined following the Volmer-Herovsky mechanism [48,49]. The decrease in the Tafel slope of the Ag 2 S/rGO composite (49 mV/dec), relative to that of the free Ag 2 S nanocrystals (99 mV/dec) and Ag 2 S/GO composite (63 mV/dec), can be attributed to the improvement in two steps, in the presence of rGO. The Ag 2 S nanocrystals grown on the rGO were more uniform and of smaller size than the free Ag 2 S nanocrystals, which provided more active sites for the adsorption of the protons and, therefore, the generation of molecular hydrogen. The better charge transfer ability of the rGO facilitated the electrical coupling between the Ag 2 S nanocrystals and the underlying rGO sheets.

Chemicals and Materials
The AgNO3 powder and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co., Ltd. Shanghai,China The 3-mercaptopropionic acid was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. The Na2S was purchased from Beijing Chemical Works, Beijing, China. The GO and electrochemical adhesive Nafion were purchased from Sigma Aldrich Co., Ltd., St. Louis, MO, the United States.

Synthesis of Ag2S Nanocrystals
Fifty-one milligrams of AgNO3 was dissolved in 90 mL of ethylene glycol (EG) under nitrogen at 60 °C, as shown in Scheme 1a. Then, 0.06 mL (7 mmol) of 3-mercaptopropionic acid (MPA) was added. The mixture was stirred vigorously for 10 min and then heated to 145 °C. The solution color

Chemicals and Materials
The AgNO 3 powder and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China The 3-mercaptopropionic acid was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. The Na 2 S was purchased from Beijing Chemical Works, Beijing, China. The GO and electrochemical adhesive Nafion were purchased from Sigma Aldrich Co., Ltd., St. Louis, MO, the United States.

Synthesis of Ag 2 S Nanocrystals
Fifty-one milligrams of AgNO 3 was dissolved in 90 mL of ethylene glycol (EG) under nitrogen at 60 • C, as shown in Scheme 1a. Then, 0.06 mL (7 mmol) of 3-mercaptopropionic acid (MPA) was added. The mixture was stirred vigorously for 10 min and then heated to 145 • C. The solution color changed from a white to yellow clarification. After the reaction was finished, the solution was cooled to room temperature. The product was separated by centrifugation, then dispersed in the deionized water. The free Ag 2 S nanocrystals were obtained.

Synthesis of Ag2S/rGO Composite
The method of synthesizing Ag2S/rGO is presented in Scheme 1b. Fifty-one milligrams of AgNO3 was dissolved in 90 mL of ethylene glycol (EG) under nitrogen at 60 °C. Then, 0.06 mL (0.7 mmol) of 3-mercaptopropionic acid (MPA) was added. The mixture was stirred vigorously for 10 min, then heated to 145 °C. The reaction solution changed from white to yellow. Two milliliters (0.1 M) of the Na2S aqueous solution and 2.325 mL (4 mg/mL) of the GO were quickly injected into the mixture. The Na2S was added together with the GO as a reducing agent. After the reaction was finished, the solution was cooled to room temperature. The product was separated through centrifugation, then dispersed in the deionized water.
In the thus-synthesized Ag2S/rGO composite, Ag2S nanocrystals were uniformly distributed in the rGO according to the EDS spectra of the Ag2S/rGO composite, shown in Figure S2. The size of the GO is on the micrometer scale, as shown in Scheme 1a.

Characterizations
The microstructure of the composite was characterized with a TEM (H-800 electron microscope with a charged coupled device camera with a 200 kV acceleration voltage). The high-resolution TEM (HRTEM) image was recorded with a JEM-2100F electron microscope at 200 kV. The energy dispersive analysis (EDX) of the composite catalyst was performed through the Phenom Element Identification application, attached to the Phenom Pro-X, at room temperature. The X-ray photoelectron spectra (XPS) were recorded with the Thermo Scientific Escalab 250Xi, Thermo Fisher Scientific (China) Co., Ltd., Shanghai, China. The crystal structure of the composite was obtained from X-ray diffraction (XRD) spectra recorded using a BRUKER D8 FOCUS, Bruker Daltonics Inc. that was operated in air at room temperature. The photoluminescence and UV-Vis spectra were recorded using a fluorescence spectrometer (a Cary Eclipse Fluorescence Spectrophotometer from Agilent Technologies Inc., Santa Clara, CA, USA) and a Cary 100 Conc UV-Vis spectrophotometer, respectively, in air at room temperature. A self-made dynamic fluorescence system was used for the

Synthesis of Ag 2 S/rGO Composite
The method of synthesizing Ag 2 S/rGO is presented in Scheme 1b. Fifty-one milligrams of AgNO 3 was dissolved in 90 mL of ethylene glycol (EG) under nitrogen at 60 • C. Then, 0.06 mL (0.7 mmol) of 3-mercaptopropionic acid (MPA) was added. The mixture was stirred vigorously for 10 min, then heated to 145 • C. The reaction solution changed from white to yellow. Two milliliters (0.1 M) of the Na 2 S aqueous solution and 2.325 mL (4 mg/mL) of the GO were quickly injected into the mixture. The Na 2 S was added together with the GO as a reducing agent. After the reaction was finished, the solution was cooled to room temperature. The product was separated through centrifugation, then dispersed in the deionized water.
In the thus-synthesized Ag 2 S/rGO composite, Ag 2 S nanocrystals were uniformly distributed in the rGO according to the EDS spectra of the Ag 2 S/rGO composite, shown in Figure S2. The size of the GO is on the micrometer scale, as shown in Scheme 1a.

Characterizations
The microstructure of the composite was characterized with a TEM (H-800 electron microscope with a charged coupled device camera with a 200 kV acceleration voltage). The high-resolution TEM (HRTEM) image was recorded with a JEM-2100F electron microscope at 200 kV. The energy dispersive analysis (EDX) of the composite catalyst was performed through the Phenom Element Identification application, attached to the Phenom Pro-X, at room temperature. The X-ray photoelectron spectra (XPS) were recorded with the Thermo Scientific Escalab 250Xi, Thermo Fisher Scientific (China) Co., Ltd., Shanghai, China. The crystal structure of the composite was obtained from X-ray diffraction (XRD) spectra recorded using a BRUKER D8 FOCUS, Bruker Daltonics Inc. that was operated in air at room temperature. The photoluminescence and UV-Vis spectra were recorded using a fluorescence spectrometer (a Cary Eclipse Fluorescence Spectrophotometer from Agilent Technologies Inc., Santa Clara, CA, USA) and a Cary 100 Conc UV-Vis spectrophotometer, respectively, in air at room temperature. A self-made dynamic fluorescence system was used for the fluorescence lifetime.
The tests were carried out on a solution dropped onto a glass slide. The excitation light was a 532 nm picosecond laser, with a repetition rate of 2 MHz. Each sample was tested at five different locations; the results are listed in Table S2.

Electrochemical HER Performance Test
The electrocatalytic HER reactions were carried out on a standard three-electrode biologic electrochemical workstation (VMP3), which was assembled using a Rotating Ring Disk Electrode (RRDE) apparatus at room temperature. For the Ag 2 S and Ag 2 S/rGO catalysts, 10 mg of powder was dispersed in 1 mL of a deionized water/ethanol mixture (= 1:1). Then, 50 µL of 5 wt% of Nafion 117 was added and followed by ultra-sonication. Five microliters of the as-prepared material was then pipetted onto a glassy carbon electrode (GCE) with an effective area of 0.07065 cm 2 and dried. Thus, the catalyst loaded onto the GCE was about 0.708 mg/cm 2 . For the GO and rGO catalysts, 50 µL of 5 wt% of Nafion 117 was added into 1 mL of the 4 mg/mL aqueous solution and followed by ultra-sonication. Then, 12.5 µL (divided into two: 6 µL and 6.5 µL) of the as-prepared material was pipetted onto the GCE, then dried. Thus, the catalyst loaded onto the GCE was about 0.708 mg/cm 2 . The electrolyte was a 0.5 M aqueous solution of sulfuric acid. Using a saturated calomel electrode as a reference electrode (and a platinum wire as a counter electrode), the cyclic voltammetries (CVs) were tested from −2 to 2 V, with a scanning speed of 100 mV/s. The polarization curves were acquired within a range of −0.7 V to 0 V, and at a scanning speed of 20 mV/s. The Nyquist plots of the composite were tested from 100 kHz to 0.01 Hz. The stability tests were carried out by comparing the polarization curves before and after 1000 cycles of the voltammograms cycle.

Conclusions
Introducing the rGO micro-sheets by the solvothermal method improves the electrocatalytic (HER) activity of the Ag 2 S nanocrystals. A Tafel slope of 49.1 mV/dec and an overpotential of 120 mV were obtained for the Ag 2 S/rGO composite electrocatalysts. Compared to the free Ag 2 S nanocrystals and the Ag 2 S/GO, the Ag 2 S/rGO composite showed better dispersion and a uniform size distribution. The rGO also enhanced the electrocatalyst conductivity (PEIS Nyquist plots), as well as the charge transfer efficiency (the TR luminescence). The hydrogen production mechanism of the composite catalyst was found to follow the Volmer-Herovsky mechanism. This work may provide a new strategy for developing active HER electrocatalysts, via the combining of a semiconductor with 2D materials.