Photoelectrocatalytic hydrogen evolution reaction stimulated by the surface plasmon resonance effect of copper and silver surrounded with MoS2

Developing plasmonic metal-based photocatalysts can improve the photoelectrocatalytic hydrogen evolution reaction (HER) and overcome the limitations of semiconductor-based photocatalysts. A MoS2@Ag–Cu foam composite electrode was fabricated on a copper foam and employed for photoelectrocatalytic HER. The optical behavior and photoelectrocatalytic HER test results indicate that the surface plasmon resonance effect of copper and silver is the primary source of light absorption. Additionally, molybdenum sulfide was employed as a hot-electron trap to capture the energetic electrons generated from copper and silver, thereby promoting the hydrogen evolution reaction. Its binder-free electrode exhibits the better HER performance and excellent stability. Low-cost plasmonic metals, copper and silver, were used as the source of photocatalysis, providing a novel perspective for enhancing photoelectrocatalytic HER performance.


Introduction
Photoelectrocatalytic HER has been identied as one of the best ways to address the energy crisis and environmental pollution. 1 The rst-generation semiconductor-based photoelectrocatalyst materials were proposed by scientists Fujishima and Honda in the 1970s. 2 In the following decades, they were continuously utilized for enhancing photoelectrocatalytic reactions through modication 3-6 and decoration. [7][8][9][10][11] However, low electric conductivity and narrow window for light absorption 12,13 became fatal aws that limited the HER performance. In recent years, plasmonic metal-based materials, such as precious metal Au, [14][15][16][17] have been considered a novel and reliable means of boosting the photoelectrocatalytic HER. 18,19 Research has shown that plasmonic metals can effectively improve photoelectrocatalytic performance due to their three advantages: the antenna effect, adjustable resonance energy, and ultra-sensitive sensing. [20][21][22] Thus, plasmonic metals are expected to become the next generation of photocatalysts. Besides, metals with high conductivity promote electron transfer and rapid electrocatalytic reactions. 23 However, the cost of photoelectrodes has increased due to the use of precious metals in the preparation process. Therefore, it is imperative to utilize low-cost plasmonic metals such as copper and silver for efficient light harvesting and hot electron generation. The photogenerated carriers resulting from plasmon resonance effects are more prone to recombination compared to those in semiconductor-based materials. 24,25 The most signicant subject now is how to capture and utilize the hot electrons generated for stimulating HER.
Molybdenum sulde exhibits a free energy of adsorbed H comparable to that of Pt, [26][27][28] which is considered the best HER catalyst and can replace noble catalysts. Furthermore, amorphous molybdenum sulde has been demonstrated to exhibit superior catalytic performance due to the presence of catalytically active sites located at lattice defects. [29][30][31] The twodimensional layered molybdenum sulde, with a band gap of 1.8 eV, can generate photo-induced carriers under visible light illumination. 32,33 The appropriate band structure of molybdenum sulde can trap the hot electrons 34 originated from surface plasmon resonance effect, which synergistically participate in photoelectrocatalytic HER.
In this paper, a hierarchical porous copper foam (Cu foam) was used as a conductive substrate. Ag nano-layers and amorphous molybdenum sulde were successively grown in situ on the Cu foam by means of wet chemical and electrochemical deposition methods (MoS 2 @Ag-Cu foam). The prepared electrodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectroscopy, and photoluminescence spectroscopy (PL). The results of the photoelectrocatalytic HER test indicate that the MoS 2 @Ag-Cu foam electrode exhibits the strongest photoelectrocatalytic performance, with a photo response current density (DJ) 2.88 times higher than that of Cu foam and 1.83 times higher than that of Ag-Cu foam. Additionally, MoS 2 @Ag-Cu foam electrode demonstrated superb electrochemical stability. It offers a new perspective for the eld of photoelectric HER.

Preparation of the composite electrode
As shown in Scheme 1, MoS 2 @Ag-Cu foam electrode was synthesized on the copper foam with 3D hierarchical porous structure through the following two-step procedure.
Step 1: Ag nano-layers grown in situ on the Cu foam by wetting chemical deposition method (Ag-Cu foam).
Firstly, Cu foam was cut into rectangle with the area of 2 × 4 cm, and it was washed by acetone, ethanol, hydrochloric acid, and deionized water respectively. Aer that, 0.75 g AgNO 3 was dissolved in the mixture of 12 mL NH 3 $H 2 O, 8 mL CH 2 O and 100 mL deionized waster. Next, the cleaned Cu foam was immersed into the solution with stirring for 15 minutes. At last, the grown in situ Ag nano-layer on Cu foam was washed by deionized water and dried in the N 2 stream. The as-obtained electrode was named Ag-Cu foam.
Step 2: molybdenum sulde grown in situ on the Ag-Cu foam by electrochemical deposition method (MoS 2 @Ag-Cu foam).
Molybdenum sulde coated on Ag-Cu foam via a facile electrochemical deposition method described by previous work. 35 Specically, 0.104 g (NH 4 ) 2 MoS 4 , 1.5 g KCl and 0.096 g Na 2 S$9H 2 O were dissolved in the 200 mL deionized waster. Above solution was used as electrolyte, and carbon rod and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The Ag-Cu foam electrode was directly used as working electrode. The electrodeposition mood was set to chronopotentiometry with 0.075 mA for 10 minutes. Aer that, the as-obtained electrode was washed by deionized water and dried in the N 2 stream, and it was denoted as MoS 2 @Ag-Cu foam.

Characterization
Scanning electron microscopy (SEM, JEOL JSM-7800F) was employed to investigate the morphology and microstructure of the as-prepared electrodes. A small piece of electrodes can be directly employed as the sample for characterization of SEM and energy dispersive spectrum. X-ray diffraction (XRD) patterns were taken on an X-ray diffractometer (2500VB2+PC, Japan) with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) analysis was detected on an Thermo ESCALAB 250. The optical behaviours were detected by ultraviolet-visible spectrophotometer (Shimadzu UV3600, Japan). Photoluminescence measurements (PL) were examined on a spectrophotometer (Hitachi F-7000) at an excitation light of 410 nm. The electrodes prepared were directly utilized as the detected samples.

Photoelectrochemical measurements
Photoelectrocatalytic HER measurements were conducted using a standard three-electrode system in 0.5 M H 2 SO 4 aqueous solution on the electrochemical workstation (CHI, 760E). The as-prepared electrodes were cut into pieces with a surface area of 0.25 cm 2 and directly employed as the working electrode. A saturated calomel electrode (SCE) and a carbon rod were employed as the reference and counter electrode. The electrocatalytic activity was evaluated by recording linear sweep voltammetry (LSV) with a scan rate of 5 mV s −1 . The potentials in the LSV curves were IR-corrected based on the ohmic resistance of the solution. The electrochemical impedance spectroscopy (EIS) measurements were performed at −0.2 V vs. RHE over potential in frequency from 10 −1 to 10 4 Hz. The cyclic voltammetric method was taken to evaluate the electrochemical double layer capacitance (C dl ) in the non-Faraday region from 0 to 0.1 V vs. RHE with the scan rates are 150, 120, 100, 80, and 60 mV s −1 , respectively. The visible light source was simulated by a 300 W Xe arc lamp (CEL-HXF300) assembled with an AM 1.5 lter.

Structure characterization
The morphologies of Ag-Cu foam and MoS 2 @Ag-Cu foam were examined via SEM analysis, as illustrated in Fig. 1. As observed, the Cu foam frame was coated with a layer of Ag nanoparticles Scheme 1 Schematic illustration of the synthesis process of MoS 2 @Ag-Cu foam. ( Fig. 1a and b), while still maintaining its initial hierarchical porous structure (Fig. S1a †) aer the wetting chemical deposition process. As depicted in Fig. 1c, the skeletons of the Cu foam are uniformly coated with composite catalysts. A higher-magnication SEM image (Fig. 1d) revealed that the diameter of nanoparticles located on Cu foam increased following molybdenum sulde coating. Furthermore, the corresponding elemental mapping images shown that Mo, S, and Ag elements were uniformly spatially distributed on the Cu foam (Fig. 1f 1-4 ), which suggested that the MoS 2 @Ag-Cu foam composite electrode can be synthesized through the two-step procedure. For comparison, MoS 2 @Cu foam were prepared by similar method, as shown in the Fig. S1b and c. † The composition and structure of the synthesized electrodes were also characterized using X-ray diffraction technique, as illustrated in Fig. 2a 36 It should be noted that the diffraction peak of MoS 2 @Ag-Cu foam electrode was signicantly attenuated compared to that of Ag-Cu foam electrode, which can be attributed to the amorphous molybdenum sulde coating covering the diffraction signal of copper and silver.
The chemical states of elements in MoS 2 @Ag-Cu foam was analyzed in detail by XPS, as depicted in Fig. 2b-f. The XPS survey spectra revealed the presence of major elements including Cu, Ag, S, and Mo (Fig. 2b). In Fig. 2c, two prominent peaks observed at 952.5 eV and 932.8 eV corresponded to Cu 2p 1/2 and Cu 2p 3/2 orbitals of Cu, respectively. The peaks at 954.4 eV and 934.4 eV were ascribed to Cu 2+ 2p 1/2 and Cu 2+ 2p 3/2 of CuO, respectively, which resulted from its oxidation in the air. 37 The peaks observed at 374.4 eV and 368.3 eV can be attributed to the Ag 3d 3/2 and Ag 3d 5/2 orbitals of silver, respectively, as depicted in Fig. 2d. Meanwhile, the peaks at 163.0 eV and 161.8 eV are assigned to the S 2− 2p 1/2 and S 2− 2 p 3/2 orbitals of sulde, respectively (Fig. 2e). In Fig. 2f, the Mo 4+ 3d 3/2 and Mo 4+ 3d 5/2 peaks were detected at 231.6 eV and 228.8 eV, respectively. The peaks located at 235.0 eV and 232.1 eV were attributed to Mo 6+ 3d 3/2 and Mo 6+ 3d 5/2 , respectively, because of the reoxidation process of MoS 2 . Additionally, two peaks appearing at 226.6 eV and 225.9 eV can be assigned to S 2 2− 2s and S 2− 2s orbitals of amorphous MoS 2 . 36 The SEM, XRD and XPS analyses provided comprehensive evidence that the Cu foam substrate was effectively coated with a bilayer of silver nanoparticles and molybdenum sulde.

Optical property
The rate of photoinduced carrier recombination is a crucial factor that impacts the photoelectrocatalytic performance, which can be evaluated through photoluminescence (PL) emission spectra analysis. Fig. 3a displayed the normalized PL emission spectra of the samples obtained, such as pure Cu foam, Ag-Cu foam, MoS 2 @Cu foam, and MoS 2 @Ag-Cu foam, through uorescence spectroscopy with an excitation wavelength of 410 nm. Evidently, Cu foam and Ag-Cu foam, without MoS 2 layer, exhibited the most prominent diffraction signal at 600 nm, indicating that it can generate a signicant number of photogenerated carriers originated from the plasmon resonance effect of copper and undergoes rapid recombination. 8,15 Signicantly weaker emission signals were observed from MoS 2 @Ag-Cu foam and MoS 2 @Cu foam, both with a layer of MoS 2 , compared to those from Cu foam and Ag-Cu foam. This suggested that the presence of MoS 2 can effectively trap hot carriers and hinder their recombination. The UV-vis absorption  spectra of the obtained samples was analyzed, as illustrated in Fig. 3b. The absorption peak at 580 nm observed in the pristine Cu foam electrode was attributed to the surface plasmon resonance (SPR) absorption of copper. 38 In comparison with Cu foam, the additional absorption in Ag-Cu foam at wavelength shorter than 350 nm was ascribed to the SPR absorption of silver. 39 More importantly, the light absorption for MoS 2 @Ag-Cu foam was signicantly extended to visible absorption region due to the narrow band gap of MoS 2 . 40,41 To demonstrate the effective photoelectrocatalytic properties facilitated by the SPR effect and molybdenum sulde enhancement, we conducted a comparative analysis of the photo response current density for the obtained samples. As illustrated in Fig. 4a, the photocurrent chronoamperometry was performed with chopped light irradiation at potential of −200 mV. The photoresponsivity of Cu foam was evident, while that of Ag-Cu foam electrode exhibited even higher enhancement, and their photo response current densities were 0.091 and 0.143 mA cm −2 , respectively. Aer applying a molybdenum sulde trap, the photo response current density of MoS 2 @Ag-Cu foam electrode increased twofold compared to that of Ag-Cu foam, as illustrated in Fig. 4b. On the contrary, MoS 2 /FTO electrode, in the absence of plasmonic metal, exhibited a negligible photo response current density under identical conditions (Fig. S2 †).

The stability of the electrode
The superior stability of MoS 2 @Ag-Cu foam composite electrode was conrmed via electrochemical and physical techniques. As shown in Fig. 5a, the composite electrode exhibited exceptional potential stability over 28 hours at 10 mA cm −2 . Fig. 5b showed that the LSV curve aer cyclic testing exhibited a negligible decline compared to the pristine curve. The SEM image (Fig. 5c), XRD pattern (Fig. 5d), and EDS mapping images (Fig. S3 †) of MoS 2 @Ag-Cu foam aer long-term cyclic testing indicated structural and morphological stability.

Mechanism analysis
The electrochemical measurements were analyzed to elucidate the mechanism underlying the enhancement of photoelectrocatalytic hydrogen evolution. The electrochemical activity measurements of MoS 2 @Ag-Cu foam for HER in 0.5 M H 2 SO 4 solution were carried out using a typical three-electrode cell setup in the dark and under illumination, as depicted in Fig. 6a. For comparison, the HER activities of bare Cu foam, Ag-Cu foam, and MoS 2 @Cu foam were investigated. The LSV curves of the composite electrodes exhibited improved HER performance under illuminated conditions compared to those observed in the absence of light. And the composite electrodes coated with molybdenum sulde indicated enhanced   electrocatalytic activity towards HER. The MoS 2 @Ag-Cu foam electrode exhibited an overpotential of 218 mV at current density of 10 mA cm −2 , surpassing those of bare Cu foam, Ag-Cu foam, and MoS 2 @Cu foam, whether in the dark or under illumination. The specic overpotential values at 10 mA cm −2 and 50 mA cm −2 under illumination were presented in Table S1. † The Tafel slope, obtained from the LSV curves of prepared electrodes, serves as an indicator of the intrinsic catalytic activity. As is shown in Fig. 6b, the Tafel slope of MoS 2 @Ag-Cu foam (54.2 mV dec −1 ) was comparable to that of MoS 2 @Cu foam (68.5 mV dec −1 ), yet signicantly lower than that of Ag-Cu foam (115 mV dec −1 ) and pure Cu foam (153 mV dec −1 ). This suggested that the outer layer of molybdenum sulde can serve as an electron mediator between plasmonic meatal and H + in aqueous solution, stimulating rapid kinetics for HER.
The enhanced photoelectrocatalytic performance of MoS 2 @Ag-Cu foam was also attributed to its higher electrochemically active surface area (ECSA). The estimation of the ECSA at the solid-liquid interface can be achieved through the measurement of electrochemical double-layer capacitance (EDLC) via CV test. 42 The EDLC values were determined by calculating the slope of the linear regression analysis of the capacitive current plotted against scan rate (Fig. S4 †). As shown in Fig. 6c, the EDLC value of MoS 2 @Ag-Cu foam (53.2 mF cm −2 ) was ∼1.4 times than that of MoS 2 @Cu foam (38.5 mF cm −2 ), ∼4.8 times than that of Ag-Cu foam (11.2 mF cm −2 ) and ∼30 times than that of Cu foam (1.8 mF cm −2 ). This indicated that the MoS 2 @Ag-Cu foam composite electrode possessed the highest ECSA. The values of Tafel slop and ECSA were exhibited in Table S1. † The Nyquist plot was generated through EIS analysis to evaluate the surface kinetics of the electrodes prepared, as illustrated in Fig. 6d. The diameter of the tted semicircle represented the charge transfer resistance (R ct ), which denotes the resistance encountered during electron transport. The Nyquist plots of the electrodes with layer of molybdenum sulde (MoS 2 @Ag-Cu foam and MoS 2 @Cu foam) exhibited a narrower semicircle diameter than that of the other electrodes. The as prepared electrodes displayed the smaller R ct values under illumination, especially for electrode with silver nano-layer.
As illustrated in Scheme 2a, upon exposure to resonant optical radiation, the plasmonic metal exhibited oscillation of its free electrons within the conductive band. Then the hot electron-hole pairs were generated via non-radiation attenuated Landau damping within a time frame of 1 to 100 fs. 44 The plasmon-induced electric eld promotes the transition of electrons from occupied states to the higher energy SPR states. 45,46 Unfortunately, the photoelectrocatalytic performance of pure plasmonic metals is severely limited by their high recombination probability for photogenerated carriers (shown in Fig. 3a) and sluggish kinetics for HER (shown in Fig. 6). For MoS 2 @Ag-Cu hybrids catalyst, on the one hand, silver and copper metals produced a huge number of hot electrons through the SPR effect. On the other hand, the band structure of molybdenum sulde has been engineered to enable efficient capture and utilization of photoelectrons generated from the SPR effect for stimulating HER reaction, as depicted in Scheme 2b.

Conclusions
In conclusion, the self-supported MoS 2 @Ag-Cu foam electrode was successfully fabricated as a photocathode through a twostep procedure. The enhanced performance of the MoS 2 @Ag-Cu foam photocathode originated from surface plasmon resonance (SPR) effect of silver and copper, which served as primary optical absorbers. Molybdenum sulde acted as a hot electron trap, enhancing the separation efficiency of photogenerated carriers and expediting rapid kinetics, to promote hydrogen evolution reaction. Besides, the superior stability of composite electrode is ascribed to the in situ growth method employed for its preparation. The development of low-cost plasmonic metalbased materials, the next generation photocatalysts, is key to breaking through the bottleneck of photoelectrocatalytic performance improvement.

Conflicts of interest
There are no conicts to declare.