Fabrication of Robust Hydrogen Evolution Reaction Electrocatalyst Using Ag2Se by Vacuum Evaporation

Much research has been done on reliable and low-cost electrocatalysts for hydrogen generation by water splitting. In this study, we synthesized thin films of silver selenide (Ag2Se) using a simple thermal evaporation route and demonstrated their electrocatalytic hydrogen evolution reaction (HER) activity. The Ag2Se catalysts show improved electrochemical surface area and good HER electrocatalytic behavior (367 mV overpotential @ 10 mA·cm−2, exchange current density: ~1.02 × 10−3 mA·cm−2, and Tafel slope: 53 mV·dec−1) in an acidic medium). The reliability was checked in 0.5 M sulfuric acid over 20 h. Our first-principles calculations show the optimal energy of hydrogen adsorption, which is consistent with experimental results. The works could be further extended for finding a new catalyst by associating the selenide, sulfide or telluride-based materials without complex catalyst synthesis procedures.


Introduction
Due to the environmental problems caused by the huge amount of fossil fuels, it is imperative to advance renewable and environmentally-friendly energy sources. Hydrogen generation from water splitting has been considered as a replacement for traditional fossil fuels. Electrochemical water splitting from various catalysts is an efficient way to generate hydrogen [1]. For decades, Pt and noble metals have been used as catalyst materials for hydrogen generation [2][3][4][5][6][7]. From an economic point of view, high prices of those noble metals are a fundamental issue. Therefore, exploring robust, cost-effective, earth-rich, and non-noble metal catalysts to replace Pt is very crucial for hydrogen generation from electrochemical water splitting. Until now, many researchers have made great efforts to search for efficient electrocatalysts, using earth-abundant materials such as metals (Mo, Co, W, Ni, Fe, etc.) along with selenides, sulfides, carbides, phosphides and hybrids [6][7][8]. However, most of the catalysts presented much poor catalytic performance compared to Pt. Besides, many catalysts are synthesized using a complicated preparation procedure to enhance the catalytic properties which, in turn, increases costs.
Since silver (Ag) has the highest electrical conductivity and is rather richer than Pt [9,10], Ag could be considered as a potential candidate. However, the biggest issue of pristine Ag is poor hydrogen evolution reaction (HER) activity, especially in acidic environments. Even though Ag exhibits poor electrocatalytic activity, recently the Ag and S combination (Ag 2 S) exhibited promising, good HER properties [11][12][13][14]. The synergetic chemical coupling behavior between Ag and S elements are assumed to provide the enrichment in the HER activity. Ag 2 S possesses a bandgap of 1.1-2.1 eV with the excellent electrical conductivity, and also theoretically proves its ability for HER reaction [12][13][14]. A few reports for Ag 2 S or Ag 2 S/Ag have been presented [13,15]. Ren et al. [15] observed good HER performance of a porous Ag 2 S/CuS electrocatalyst (200 mV overpotential @ 10 mA·cm −2 , Tafel slope: 75 mV·dec −1 ). Basu et al. [13] designed Ag 2 S/Ag catalyst (199 mV overpotential, Tafel slope: 102 mV·dec −1 ). Being inspired by excellent electrical properties of Ag 2 S, we combined Ag and Se to implement Ag 2 Se as catalysts. To the best of our knowledge, this is the first report of Ag 2 Se as the electrocatalyst for HER. To overcome previously mentioned obstacles, in this paper, we prepared thin films of Ag 2 Se using a single-step process (vacuum evaporation). Ag 2 Se films with various thicknesses were deposited onto Au/Si in vacuum at room temperature. Ag 2 Se catalysts show good HER properties (367 mV overpotential @ 10 mA·cm −2 , exchange current density:~1.02 × 10 −3 mA·cm −2 , and Tafel slope: 53 mV·dec −1 ).

Material Synthesis
Highly pure Ag 2 Se powder (99.99%) was procured from Sigma Aldrich (St. Louis, MO, USA). Initially, Au (10-20 nm)/Si substrates were cleaned by deionized (DI) water, methanol, acetone, and isopropyl alcohol. The substrates were loaded on the sample holder and the evaporation chamber was vacuumed by diffusion pump. Ag 2 Se thin films (100-250 nm) were coated at a rate of 1.0 Å·s −1 . The rate of deposition and film thickness were monitored by a quartz crystal thickness monitor during the film deposition.

Electrochemical Measurements
Electrochemical properties were analyzed using Biologic SP-300 potentiostat. To estimate the HER electrochemical performance for different electro-catalysts, LSV polarization analyses were employed in three-electrode electrochemical cells, in which the silver electrode (Ag/AgCl) used as the reference electrode, the graphite rod as the counterpart electrode, and the Ag 2 Se as the working electrode in 0.5 M H 2 SO 4 solution. The scan rate was at 10 mV/s. The CV measurements were conducted between +0.1 and −0.4 V vs. Ag/AgCl. LSV curves were recorded using Ag/AgCl, then potentials were changed into the reversible hydrogen electrode (RHE) scale with the support of subsequent calculation: E (sat. Ag/AgCl) + E • (sat. Ag/AgCl) + 0.059 pH. Besides, the working electrode was performed for 20 h to obtain stable polarization curves. EIS studies were performed in the frequencies from 1 Hz to 1 MHz (10 mV). All the polarization measurements were carried out with iR correction using ohmic loss.

Characterizations
Physical morphology and structure of catalysts were studied with scanning electron microscopy (SEM, HITACHI S-4700, Tokyo, Japan). X-ray diffraction (Rigaku, Tokyo, Japan) with Cu-Kα radiation was used to check the film crystallinity. The Raman spectral results were collected using an Ar laser (512 nm) (Renishaw inVia RE04, Gloucestershire, United Kingdom) with a scan speed of 30 s and 1 µm spot size. X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe (25 W Al Kα), Kanagawa, Japan) was used for the chemical compositions. Figure 1 illustrates the schematic diagram of thermally evaporated Ag 2 Se films with various thicknesses (100, 150, and 200 nm), from the bulk polycrystalline Ag 2 Se source. Raman scattering analysis was performed for Ag 2 Se thin films, and their spectra are shown in Figure 2a. The Raman spectra show a rather broad peak @ 232 cm −1 , which is the characteristics of Ag 2 Se [16,17]. Further, structural characterizations were employed by XRD. From the XRD spectra (Figure 2b Figure 1 illustrates the schematic diagram of thermally evaporated Ag2Se films with various thicknesses (100, 150, and 200 nm), from the bulk polycrystalline Ag2Se source. Raman scattering analysis was performed for Ag2Se thin films, and their spectra are shown in Figure 2a. The Raman spectra show a rather broad peak @ 232 cm −1 , which is the characteristics of Ag2Se [16,17]. Further, structural characterizations were employed by XRD. From the XRD spectra (Figure 2b   SEM examinations were carried out to reveal the surface structure and homogeneity of Ag2Se thin films. As shown in Figure 3, the evaporated Ag2Se films have uniform morphologies over the surfaces of the film. The smooth surface is found in a 100 nm-thick Ag2Se film (labeled as Ag2Se-100) due to low film thickness, as shown in Figure 3a,b. For 150 nm-thick-Ag2Se (labeled as Ag2Se-150) in Figure 3c,d, the film surface is covered with an irregular spatial type of grains. The different sizes of grain clusters are exhibited by agglomerations which tend to form larger size grains due to increased  Figure 1 illustrates the schematic diagram of thermally evaporated Ag2Se films with various thicknesses (100, 150, and 200 nm), from the bulk polycrystalline Ag2Se source. Raman scattering analysis was performed for Ag2Se thin films, and their spectra are shown in Figure 2a. The Raman spectra show a rather broad peak @ 232 cm −1 , which is the characteristics of Ag2Se [16,17]. Further, structural characterizations were employed by XRD. From the XRD spectra (Figure 2b   SEM examinations were carried out to reveal the surface structure and homogeneity of Ag2Se thin films. As shown in Figure 3, the evaporated Ag2Se films have uniform morphologies over the surfaces of the film. The smooth surface is found in a 100 nm-thick Ag2Se film (labeled as Ag2Se-100) due to low film thickness, as shown in Figure 3a,b. For 150 nm-thick-Ag2Se (labeled as Ag2Se-150) in Figure 3c,d, the film surface is covered with an irregular spatial type of grains. The different sizes of grain clusters are exhibited by agglomerations which tend to form larger size grains due to increased SEM examinations were carried out to reveal the surface structure and homogeneity of Ag 2 Se thin films. As shown in Figure 3, the evaporated Ag 2 Se films have uniform morphologies over the surfaces of the film. The smooth surface is found in a 100 nm-thick Ag 2 Se film (labeled as Ag 2 Se-100) due to low film thickness, as shown in Figure 3a,b. For 150 nm-thick-Ag 2 Se (labeled as Ag 2 Se-150) in Figure 3c,d, the film surface is covered with an irregular spatial type of grains. The different sizes of grain clusters are exhibited by agglomerations which tend to form larger size grains due to increased film thickness. For the 200 nm-thick Ag 2 Se film (labeled as Ag 2 Se-200) in Figure 3e-f, the surface of the film consists of different sizes of grain domains with different shapes, possessing pinholes, and hillocks which may affect the conductivity of the film. The existence of Ag and Se stoichiometric ratio, and atomic homogeneity (Ag 2 Se-200) were proved by energy-dispersive X-ray spectroscopy (EDS) ( Figure S1, and Figure S2 of Supplementary Materials).  The chemical compositions of Ag2Se were further investigated by XPS analysis. XPS spectra are shown for Ag2Se-200 ( Figure 4). For Ag 3d orbitals, as shown in Figure 4a, binding energies at 368 and 374 eV are ascribed to the spin-orbit doublet Ag 3d5/2 and Ag 3d3/2, respectively [18]. The energy peaks observed at 53.88 and 54.78 eV are assigned to the Se 2p1/2 and Se 2p3/2 orbital divalent selenide (Se 2− ), respectively, as shown in Figure 4b. The calculated stoichiometric ratio is 2:1 and observed XPS results are coordinated with the literature for Ag2Se crystal [18]. The survey spectrum is given to verify all the elements in the Ag2Se film ( Figure S3). The chemical compositions of Ag 2 Se were further investigated by XPS analysis. XPS spectra are shown for Ag 2 Se-200 ( Figure 4). For Ag 3d orbitals, as shown in Figure 4a, binding energies at 368 and 374 eV are ascribed to the spin-orbit doublet Ag 3d 5/2 and Ag 3d 3/2 , respectively [18]. The energy peaks observed at 53.88 and 54.78 eV are assigned to the Se 2p 1/2 and Se 2p 3/2 orbital divalent selenide (Se 2− ), respectively, as shown in Figure 4b. The calculated stoichiometric ratio is 2:1 and observed XPS results are coordinated with the literature for Ag 2 Se crystal [18]. The survey spectrum is given to verify all the elements in the Ag 2 Se film ( Figure S3). The catalytic activity of the Ag2Se and Pt electrode for the HER was examined using a threeelectrode system in 0.5 M H2SO4 electrolyte solution (linear sweep voltammetry (LSV), 10 mV/s scan rate). As expected, the commercial Pt wire exhibited very efficient HER activity (54 mV overpotential @ 10 mA·cm −2 ). Ag2Se-200 showed 367 mV overpotential @ 10 mA·cm −2 , which is 15 and 23 mV lower potential than that of Ag2Se-100 and Ag2Se-150 (Table 1). HER electrocatalytic activities revealed that Ag2Se thickness plays a vital role because of the variation of active edges and morphological properties with deposition time. For Ag2Se-250 sample, the lower catalytic activity may be due to the surface continuity of the film ( Figure S4). Thick films could disturb electrolyte penetration and diminish the role of inherent Ag2Se active edges thereby decrease the electrocatalytic activity. A pure 200 nm-thick Ag film (labeled as Ag) exhibited 588 mV at 10 mA·cm −2 , as shown in Figure 5a. In earlier studies, HER properties were demonstrated for the various layer materials including, bulk and their exfoliated layers of TMD materials such as high-quality monolayer MoS2(1-x)Se2x (η10 mA·cm −2 = 273-300 mV) [19], S-doped MoSe2-x nanotubes and MoSe2 nanocaterpillars (η2 mA·cm −2 = 95-318 mV) [20], tungsten selenide thin films deposited on tungsten foils (η10 mA·cm −2 = 350 mV) [21], electrodeposition of amorphous cobalt-cobalt oxide/cobalt selenide (CoOx-CoSe) composite film on a 3-dimensional porous Ni foam (NF) (η10 mA·cm −2 = 300-380 mV) [22], and electrodeposited cobalt selenide nanostructures (η10 mA·cm −2 = 472-481 mV) [23].  The catalytic activity of the Ag 2 Se and Pt electrode for the HER was examined using a three-electrode system in 0.5 M H 2 SO 4 electrolyte solution (linear sweep voltammetry (LSV), 10 mV/s scan rate). As expected, the commercial Pt wire exhibited very efficient HER activity (54 mV overpotential @ 10 mA·cm −2 ). Ag 2 Se-200 showed 367 mV overpotential @ 10 mA·cm −2 , which is 15 and 23 mV lower potential than that of Ag 2 Se-100 and Ag 2 Se-150 (Table 1). HER electrocatalytic activities revealed that Ag 2 Se thickness plays a vital role because of the variation of active edges and morphological properties with deposition time. For Ag 2 Se-250 sample, the lower catalytic activity may be due to the surface continuity of the film ( Figure S4). Thick films could disturb electrolyte penetration and diminish the role of inherent Ag 2 Se active edges thereby decrease the electrocatalytic activity. The catalytic activity of the Ag2Se and Pt electrode for the HER was examined using a threeelectrode system in 0.5 M H2SO4 electrolyte solution (linear sweep voltammetry (LSV), 10 mV/s scan rate). As expected, the commercial Pt wire exhibited very efficient HER activity (54 mV overpotential @ 10 mA·cm −2 ). Ag2Se-200 showed 367 mV overpotential @ 10 mA·cm −2 , which is 15 and 23 mV lower potential than that of Ag2Se-100 and Ag2Se-150 (Table 1). HER electrocatalytic activities revealed that Ag2Se thickness plays a vital role because of the variation of active edges and morphological properties with deposition time. For Ag2Se-250 sample, the lower catalytic activity may be due to the surface continuity of the film ( Figure S4). Thick films could disturb electrolyte penetration and diminish the role of inherent Ag2Se active edges thereby decrease the electrocatalytic activity. A pure 200 nm-thick Ag film (labeled as Ag) exhibited 588 mV at 10 mA·cm −2 , as shown in Figure 5a. In earlier studies, HER properties were demonstrated for the various layer materials including, bulk and their exfoliated layers of TMD materials such as high-quality monolayer MoS2(1-x)Se2x (η10 mA·cm −2 = 273-300 mV) [19], S-doped MoSe2-x nanotubes and MoSe2 nanocaterpillars (η2 mA·cm −2 = 95-318 mV) [20], tungsten selenide thin films deposited on tungsten foils (η10 mA·cm −2 = 350 mV) [21], electrodeposition of amorphous cobalt-cobalt oxide/cobalt selenide (CoOx-CoSe) composite film on a 3-dimensional porous Ni foam (NF) (η10 mA·cm −2 = 300-380 mV) [22], and electrodeposited cobalt selenide nanostructures (η10 mA·cm −2 = 472-481 mV) [23].   The extracted Tafel  HER kinetics can be processed via a Volmer-reaction (discharge step, Equation (1)) tailed by either an Heyrovsky reaction (ion-atom step, Equation (2)), or combination process (Tafel-reaction, Equation (3)) [33,34].

Results
The Tafel slope of 51 mV·dec −1 for Ag 2 Se-150 signifies that HER occurs through the Volmer-Heyrovsky kinetics, where a rapid protons discharge (Equation (1)) is tailed by a Heyrovsky reaction (Equation (2)) or combination reaction step (Equation (3)) [35,36]. Exchange current density (J 0 ), another HER characteristic assessment of electrocatalysts, was obtained by extrapolation of Tafel plot towards cathodic current ( Table 1). The reported values of J 0 for ternary selenide-or sulfide-based catalysts for HER are presented in Supplementary Materials. Cyclic voltammetry (CV) was measured for the double layer capacitances (C dl ) and the assessment of electrochemically effective surface area (ECSA), assuming that those quantities are directly proportionate [11,37]. The C dl of Ag 2 Se samples are measured by CV at non-faradaic potential regions (Supplementary Figure S5), and is extracted at a given potential (0.24 V vs. RHE) from the differences between cathodic and anodic current density values (∆j = j a − j c ) against the various scan rates (20-100 mV·s −1 ). Ag 2 Se-150 displays 55.9 µF·cm −2 of C dl value and it is greater than that of the Ag 2 Se-100 (41.1 µF·cm −2 ) and Ag 2 Se-200 (50.5 µF·cm −2 ), indicating the propagation of more active edges in the catalyst, resulting in the enhanced catalytic activity. From these capacitance values, the obtained ECSA values are 2.34 cm 2 (Ag 2 Se-200), 3.19 cm 2 (Ag 2 Se-150), 2.88 cm 2 (Ag 2 Se-100) [38]. The results indicate that the Ag 2 Se-150 has the largest ECSA among the three samples.
For an efficient HER catalyst, cycling stability is also important. To probe the stability of Ag 2 Se in an acidic medium, an incessant HER cycling test at a static overpotential was conducted. The robustness of Ag 2 Se-200 was further proved by employing continuous hydrogen evolution in 0.5 M H 2 SO 4 for 20 h, and then, the polarization curve was recorded between −0.6 and 0.2 V vs. RHE. The polarization curves of the three electrodes even after the 20 h successive HER operation, were changed little from their initial curves, as shown in Figure 6a-c. For the chronoamperometric (j-t) profile, a static overpotential of 367 mV (Ag 2 Se-200) was used for 20 h, as shown in Figure 6d. The current density exhibits a robust stability performance in a continuous HER operation (over 20 h). Electrochemical impedance spectroscopy (EIS) was carried out to examine the junction reactions and electrocatalyst kinetics of HER process over the frequency range between 1 Hz and 1 MHz. Nyquist plots (Figure 6e) reveal the charge-transfer resistance (R ct ) values at 16.6 Ω (Ag 2 Se-100), 22.1 Ω (Ag 2 Se-150), and 30.2 Ω (Ag 2 Se-200) for Ag 2 Se catalyst. The small series resistance (4.1-4.5 Ω) for all catalysts confirms the significance of the preparation of high conductive Au substrate, which allows comfort and operative electrical integration that diminishes ohmic losses. Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 12 polarization curves of the three electrodes even after the 20 h successive HER operation, were changed little from their initial curves, as shown in Figure 6a-c. For the chronoamperometric (j-t) profile, a static overpotential of 367 mV (Ag2Se-200) was used for 20 h, as shown in Figure 6d. The current density exhibits a robust stability performance in a continuous HER operation (over 20 h). Electrochemical impedance spectroscopy (EIS) was carried out to examine the junction reactions and electrocatalyst kinetics of HER process over the frequency range between 1 Hz and 1 MHz. Nyquist plots (Figure 6e) reveal the charge-transfer resistance (Rct) values at 16.6 Ω (Ag2Se-100), 22.1 Ω (Ag2Se-150), and 30.2 Ω (Ag2Se-200) for Ag2Se catalyst. The small series resistance (4.1-4.5 Ω) for all catalysts confirms the significance of the preparation of high conductive Au substrate, which allows comfort and operative electrical integration that diminishes ohmic losses. XPS spectra taken after the 20 h continuous HER operation revealed the slight alteration in the Se binding energy spectrum intensity due to the Se active facets contribution to produce and sustain HER properties in Ag2Se-200 film ( Figure S6), supporting the long-time HER robustness of Ag2Se. An SEM image taken after the 20 h stability test is shown in Figure S7. XPS spectra taken after the 20 h continuous HER operation revealed the slight alteration in the Se binding energy spectrum intensity due to the Se active facets contribution to produce and sustain HER properties in Ag 2 Se-200 film ( Figure S6), supporting the long-time HER robustness of Ag 2 Se. An SEM image taken after the 20 h stability test is shown in Figure S7.
To validate the capability of Ag 2 Se [39,40] acts as a catalyst for decomposing water molecules, we performed ab initio total-energy calculations based on the density functional theory (DFT) [41] within the generalized gradient approximation [42] for the exchange-correlation functional. The wave functions were extended using a plane-wave basis set [43,44], and the cutoff energy was 600 eV. Various model structures were considered: A water molecule and a hydroxyl (-OH) group were adsorbed on a 2 nm-thick slab of Ag 2 Se. We used the nudged elastic band (NEB) method to calculate the activation energy barrier. Six replicas were chosen, including the initial and final configurations. For a reaction path of decomposition, as shown in Figure 7, we considered several steps (stage A, stage Bi (i = 1, 2, 3), and stage C To validate the capability of Ag2Se [39,40] acts as a catalyst for decomposing water molecules, we performed ab initio total-energy calculations based on the density functional theory (DFT) [41] within the generalized gradient approximation [42] for the exchange-correlation functional. The wave functions were extended using a plane-wave basis set [43,44], and the cutoff energy was 600 eV. Various model structures were considered: A water molecule and a hydroxyl (-OH) group were adsorbed on a 2 nm-thick slab of Ag2Se. We used the nudged elastic band (NEB) method to calculate the activation energy barrier. Six replicas were chosen, including the initial and final configurations. For a reaction path of decomposition, as shown in Figure 7, we considered several steps (stage A, stage Bi (i = 1, 2, 3), and stage C). At stage A, an H2O molecule is located 3.5 Å from the Ag2Se surface. Stages B1, B2, and B3 are intermediate configurations, and stage C is the final configuration in which a hydroxyl (-OH) group is adsorbed onto the Ag2Se surface. At stage B1, the H2O molecule is bonded to a Selenium atom in Ag2Se. At stage B2, the HO-H bond of a water molecule (H2O) becomes dissociated. At stage B3, the separated H atom hops to an adjacent Se atom and is bonded. We calculated the total energy of each system and found that the reaction energy (   The activation energy barrier is calculated to be 0.77 eV. The activation energy is dependent on the path with initial and final configurations, but we can expect that the actual energy barrier could be less than 1 eV because we did experiments at room temperature. We note that dissociation of HO-H bond of a water molecule (H 2 O) requires 5.03 ± 0.2 eV [45] in the absence of the catalyst. The geometrical information about the absorption process is presented in the Supplementary Figure S8. For the binding of the hydroxyl group, the bond length is slightly longer than the well-known covalent bond lengths [46] of Se and O atoms because the O atom also has weak interaction with a neighbor Ag atom. The covalent radius of Se is 1.2 Å, and that of O is 0.66 Å.

Conclusions
Ag 2 Se thin films with various thicknesses (100-200 nm) were deposited by a simple thermal evaporation route. The electrochemical surface area values assessment revealed the mechanism of HER electrocatalytic properties in Ag 2 Se thin films. The Ag 2 Se catalysts showed good HER electrocatalytic performances in an acidic medium (367 mV overpotential at 10 mA·cm −2 , 53 mV·dec −1 Tafel slope, and~1.02 × 10 −3 mA·cm −2 exchange current density). Using first-principles calculations, we investigated the water splitting mechanism for the Ag 2 Se catalysts. The activation energy barrier was estimated to be 0.77 eV. From the observed results, we believe that Ag 2 Se-based catalyst will pave a novel pathway to improve the favorable use as the heterogeneous catalysis for catalytic electrochemical reactions and hydrogen-driven solar energy.