Molybdenum disulfide (MoS₂), a well-known two-dimensional (2D)-layered transition metal dichalcogenide, has been extensively studied for various applications, including optoelectronic devices, energy storage, and catalysis [1–9]. In particular, MoS₂ exhibits a high activity for photocatalyzed and electrocatalyzed hydrogen evolution reactions (HER) [4–8,10,11]. However, the MoS₂-assisted catalytic CO₂ reduction process has been rarely experimentally explored [12–27]. According to previous studies, CO is obtained as the major product of the photocatalytic reduction of CO₂ over MoS₂ (prepared by chemical vapor deposition) [12]. Hybrid catalysts, such as Bi₂S₃/MoS₂ (p–n junction), produce CO as the major product along with minor amounts of CH₄ and CH₃OH [13]. Similarly, a MoS₂/TiO2 hybrid structure produces CH₄ and CH₃OH [14]. The selectivity of the CO₂ reduction products over MoS₂ depends on the nature of the loaded metals. Sun et al. [15] reported that CH₄ and CO are the major products of CO₂ reduction via photocatalytic reactions over MoS₂ and that the selectivities for CH₄ and CO significantly increase to 80 and 98 %, respectively, upon Au and Ag loading.

MoS₂ exhibits a low activity for electrochemical CO₂ reduction (EC CO₂R) because of its poor electron mobility [25]. Density functional theory calculations show that surface modification through metal doping can suppress the HER and enhance the CO₂ reduction activity and selectivity [10,16–18]. For example, theoretically, the catalytic activity of a single-atom Ni₁/MoS₂ catalyst for electrocatalytic CO₂ reduction to produce methanol is higher than that of other metals [26]. Moreover, ionic liquids are used in EC CO₂R experiments [19–23]. Asadi et al. [20] demonstrated that MoS₂ exhibits a high Faradaic efficiency (FE) for CO production in an ionic liquid owing to the presence of Mo-terminated edges with a high density of d electrons and low work function (3.9 eV) [20]. N, P-co-doped MoS₂ nanosheet arrays also exhibit a high FE (91.5 %) for CO production [21]. Lv et al. [22] showed that the CO/H₂ syngas can be produced with a tunable ratio using fluorosilane-decorated MoS₂ via EC CO₂R in a CO₂- saturated ionic liquid. According to Asadi et al. [24], MoS₂ produces CO with a high CO₂ conversion efficiency in an aqueous electrolyte, such as a choline chloride/KOH electrolyte. MoS₂-based hybrid materials have also been studied for EC CO₂R. For example, a Cu₂O–MoS₂ composite produces methanol and ethanol with FE values of 12.3 and 7.9 %, respectively [25]. Further, MoS₂–polyethylenimine-modified graphene produces CO as a CO₂ reduction product in an NaHCO3 electrolyte [27].

As mentioned earlier, the application of MoS₂ in EC CO₂R has been rarely studied, and its use as an electrode support remains hitherto unexplored, which motivated us to explore the potential of MoS₂ as a support material for EC CO₂. Additionally, we investigated the possible synergistic effects of using Au, Ag, and Cu as overlayer metals to improve CO₂ reduction activity. Furthermore, the formation of an interface between the metal and MoS₂ could provide new paths for CO₂ reduction. Au and Ag were selected as preferred materials because of their high FE values for CO production during EC CO₂R, as demonstrated in previous studies [28–32]. By contrast, Cu is a promising material for the production of C₂₊ (oxygenated) hydrocarbons [33–37]. These three metals served as excellent model overlayer metals for the EC CO₂R experiments. In this study, MoS₂ was directly grown on a Mo metal support and used as an electrode without applying further electrode preparation methods (such as mixing with a Nafion solution or pasting the sample on a carbon electrode), in stark contrast to the methods reported in previous studies [20–25]. The results of the EC CO₂R tests, including the information on the reduction products, provide valuable insights into the development of MoS₂ electrocatalysts.

To prepare the MoS₂/Mo sheets, 30 mm × 50 mm × 0.1 mm Mo sheets were first cleaned by sonication in deionized water for 10 min and then subjected to ozone cleaning. The cleaned Mo sheets were dipped in a thiourea solution (0.84 g in 70 mL of deionized water) and tightly secured in a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was then heated to 180 °C and maintained at this temperature for 24 h. After the reaction, the autoclave was allowed to cool naturally, and the sheet was removed, gently cleaned using deionized water, and dried under an infrared lamp. This resulted in the growth of ultrathin MoS₂ layers on the Mo metal sheet (MoS₂/Mo electrode). To prepare the Au/MoS₂/Mo, Cu/MoS₂/M, and Ag/MoS₂/Mo electrodes, Au, Cu, and Ag metals were deposited on the MoS₂/Mo sheets, respectively, using an SPT-20 ion sputter coater (COXEM Co., Korea) for 600 s at 3 mA. For reference characterization, a MoS₂ nanopowder sample was prepared by completely dissolving 1 mmol of (NH₄)₆Mo₇O₂₄·4 H₂O (≥ 99.0 %, Sigma-Aldrich) and 10 mmol of thiourea (≥ 99.0 %, Sigma-Aldrich) in 30 mL of deionized water mixed with 5 mL of 1-methyl-2-pyrrolidinone (≥ 99.0 %, Sigma-Aldrich). The resulting solution was then placed in a Teflon-lined stainless-steel autoclave at 180 °C for 24 h. After the reaction, the resulting powder sample was completely cleaned using deionized water and ethanol, repeatedly centrifuged, and dried in an oven at 80 °C for 24 h for further characterization. The procedure for preparing the MoS₂/Mo electrodes is illustrated in Fig. 1.

Figure 1. Schematic of the MoS₂/Mo electrode fabrication process and threeelectrode electrochemical reaction system.

The surface morphology of the MoS₂/Mo sheet was examined by scanning electron microscopy (SEM; S-4800, Hitachi, Ltd.) and compared with that of the MoS₂ powder. High-resolution transmission electron microscopy (HRTEM; FEI Tecnai G2 F30 S-TWIN TEM) was performed at 300 kV. X-ray diffraction (XRD) patterns were obtained using a MiniFlex II X-ray diffractometer (Rigaku Corp., CNU Chemistry Core Facility center) with Cu Kα (40 kV and 30 mA) radiation to examine the crystal structures of the samples. Raman spectra were obtained using an ultraviolet (UV)–visible–near infrared Raman spectrometer (Horiba Jobin Yvon LabRAM HR-800, 514 nm laser, 100× objective, and 1800 grating). X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo Scientific K-alpha+ spectrometer (a hemispherical energy analyzer and monochromated Al Kα X-rays) to investigate the chemical composition and electronic structure of the samples.

For the EC CO₂R reaction, a three-electrode system, comprising a Pt counter electrode, Ag/AgCl/3.0 M KCl reference electrode, and MoS₂ working electrode, was used (Fig. 1). The dimensions of the working electrode were 5 mm × 30 mm × 0.1 mm. A potentiostat/galvanostat (WizECM-1200 Premium, WizMAC, Inc.) was used to control the potential, and 0.1 M KHCO₃ (50 mL) in a 100 mL glass cell was used as the electrolyte. Prior to the EC CO₂R, CO₂ gas (99.999 %) was fully bubbled through the electrolyte. Further, amperometry experiments were conducted for 1 h at a fixed negative potential. Next, the photoelectrochemical (PEC) CO₂R process was carried out under a 395-nm UV light source (6.54 mW·cm⁻²), and the corresponding experimental conditions were the same as those in the dark.

The gas and liquid products were analyzed after the EC CO₂R and PEC CO₂R reactions using gas chromatography (GC) and nuclear magnetic resonance (NMR) spectroscopy, respectively. For the GC analysis, the gas phase (0.5 mL) was injected into a YL 6500 gas chromatograph (Young In Chromass Co., Ltd.) equipped with a Ni catalyst methanizer assembly, thermal conductivity detector, and flame ionization detector. The gases were separated using 40/60 Carboxen-1000 and HP-Plot Q-PT columns. The liquid phase (0.5 mL) was analyzed using a 600 MHz FT-NMR spectrometer (AVANCE III, Bruker Corp.).

Figure 2(a) shows the linear sweep voltammetry (LSV) curves obtained from a MoS₂ sheet immersed in N₂ and CO₂-saturated 0.1 M KHCO₃ electrolytes. Evidently, the current density (CD) is slightly higher under the CO₂-saturated condition possibly because of the CO₂ reduction current. To investigate these observations in more detail, EC CO₂R amperometry tests were performed at applied potentials of −2.0, −2.2, and −2.4 V (vs. Ag/AgCl); the corresponding CD values were determined to be in the range of 10–15 mA cm⁻². Further, the corresponding GC profiles of the gaseous products are shown in Figs. 2(b) and 2(c). The NMR spectra shown in Fig. 2(d) indicate that formate is a liquid product, and Fig. 2(e) reveals that H₂, with a concentration range of 40,000–100,000 ppm, is the dominant product. Other minor products include CO, C₂H₄, C₂H₆, and C₃H₈. Figure 2(f) shows that the FE of H₂ is between 23 and 44 % and increases with the increasing applied potential. Further, the concentrations of the products CO, C₂H₄, C₂H₆, and C₃H₈ are in the ranges of 3.3–4.7, 1.7–2.7, 0.28–1.2, and 0.4–0.6 ppm, respectively [Fig. 2(g)]. In contrast, CH₄ is produced in larger amounts, ranging from 17.5 to 23.8 ppm, which indicates that CH₄ is the main CO₂ reduction product. The FE values of CH₄ and formate are the same (only 0.1 %) [Fig. 2(h)]. These results suggest that although the EC CO₂R activity of MoS₂ is extremely low, the minor products are crucial for understanding the CO₂ adsorption and hydrogenation characteristics of the fabricated electrode system.

Figure 2. (a) LSV and amperometry i–t curves of the MoS₂/Mo electrode, (b) and (c) GC profiles, (d) NMR spectra, (e) H₂ production amount, (f) FE values of the EC products, (g) CO and hydrocarbon concentrations, and (h) FE values of CO, the hydrocarbons, and formate.

Subsequently, the PEC CO₂R tests were performed to evaluate the feasibility of using the MoS₂ sheets for CO₂ reduction [7,8,38]. Figure 3(a) shows the results of the PEC CO₂R amperometry test performed over the MoS₂ sheet at −2.0 V (vs. Ag/AgCl) in a 0.1 M KHCO₃ electrolyte; additionally, the sheet was irradiated by light of wavelength 395 nm for 1 h during the PEC test. The resulting GC profiles are shown in Figs. 3(b) and 3(c), and peaks corresponding to formate production appear in the NMR spectra in Fig. 3(d). Figures 3(e) and 3(f) indicate that the H₂ signal intensity decreases from 34,873 ppm (FE = 23.2 %) to 23,191 ppm (FE = 14.7 %) upon photoirradiation, whereas the intensities of the CO and C₂H₄ signals drastically increase as shown in Fig. 3(g). Additionally, upon photoirradiation, the production of CH₄ increases, whereas that of C₂H₆ and C₃H₈ slightly decreases. These results suggest that a decrease in the H₂ production leads to a corresponding decrease in the saturated alkane production owing to the reduction in the number of hydrogen atoms on the electrode surface. Further, the CO concentration increases from 3.3 to 92.8 ppm, indicating a 28-fold production enhancement, consistent with previously reported results [39–41]. Similarly, the C₂H₄ and CH₄ concentrations increase from 0.9 to 8.6 ppm and 19.2 to 27.5 ppm, indicating 9.6 and 1.4 times production enhancements, respectively. Figure 3(h) indicates that the FE of formate slightly decreases from 0.09 to 0.08 % upon photoirradiation. These results collectively validate the feasibility of using MoS₂ in PEC CO₂R reactions. Further- Figure 3. (a) Amperometry i–t curves of the MoS₂/Mo electrode under dark conditions and a 395-nm light source, (b) and (c) GC profiles, (d) NMR spectra, (e) H₂ concentration, (f) FE values of the EC products, (g) CO and hydrocarbon concentrations, and (h) FE values of CO, the hydrocarbons, and formate. more, the CO and C₂H₄ productions appear to be correlated, whereas the H₂ production is correlated with that of the saturated alkanes.

Figure 3. (a) Amperometry i–t curves of the MoS₂/Mo electrode under dark conditions and a 395-nm light source, (b) and (c) GC profiles, (d) NMR spectra, (e) H₂ concentration, (f) FE values of the EC products, (g) CO and hydrocarbon concentrations, and (h) FE values of CO, the hydrocarbons, and formate.

The surface of MoS₂ was modified by sputter-deposition of Au, Cu, and Ag for 600 s, and their EC CO₂R performances were examined at an applied potential of −2.0 V (vs. Ag/AgCl) in a 0.1 M KHCO₃ electrolyte. The gaseous and liquid products can be identified from the corresponding GC and NMR profiles shown in Figs. 4(a)–(c). The deposition of Au, Cu, and Ag decreases the amount (or FE) of H₂ produced from 23.2 to 14.2, 15.6, and 13.5 %, respectively [Figs. 4(d) and 4(e)]. The CH₄ concentration also decreases from 19 to 12–13 ppm upon the metal deposition [Fig. 4(f)]. By contrast, the produced concentration and FE of CO are enhanced by the deposited metals; the highest increase in CO concentration (12×) is observed upon Au deposition (from 3.3 to 39.7 ppm) [Fig. 4(f)]. Interestingly, the production of C₂H₄ and C₂H₆ on the Au/MoS₂ electrode shows different behaviors. The GC profiles in the inset in Fig. 4(a) indicate that upon Au deposition, the concentration of C₂H₄ substantially increases from 0.9 to 3.5 ppm, while that of C₂H₆ decreases from 1.9 to 0.6 ppm. This concentration trend suggests different production pathways for these two compounds. Conversely, the deposition of Cu and Ag results in a slight decrease in the concentrations of the products C₂H₄ and C₂H₆. The amount of C₃H₈, which is extremely low, decreases upon the metal deposition. However, the hydrocarbons C₃H₆, C₄H₁₀, and C₄H₈ are detected after the Au deposition [Figs. 4(b) and 4(g)].

Figure 4. (a) and (b) GC profiles of different electrodes obtained at −2.0 V in 0.1 M KHCO₃, (c) NMR spectra, (d) H₂ concentration, (e) FE values of the EC products, (f) CO and hydrocarbon concentrations, (g) C₂₋₄ hydrocarbon concentrations, and (h) FE values of CO, the hydrocarbons, methanol, and formate.

The NMR spectra shown in Fig. 4(c) reveal that compared to bare MoS₂ (FE = 0.09 %), the Au- and Cu-deposited MoS₂ sheet exhibits an enhanced formate production, and the corresponding FE values increase by 1.3 and 2.1 times, respectively [Fig. 4(h)]. In addition, methanol is detected on Au/MoS₂, which is confirmed by the amperometry test results obtained at various applied potentials. Further, EC CO₂R amperometry tests were conducted over the Au/MoS₂ sheet in a 0.1 M KHCO₃ electrolyte at applied potentials of −2.0, −2.2, and −2.4 V (vs. Ag/AgCl). The corresponding CD values were in the range of 8–14 mA cm⁻². The gas and liquid products can be identified from the corresponding GC and NMR profiles shown in Figs. 5(a)–(c). Figures 5(d) and 5(e) show that the concentration of H₂ decreases more at −2.2 V than at −2.0 V, whereas it substantially increases at −2.4 V. In contrast, Fig. 5(f) indicates that concentrations of CO and CH₄ significantly increase by 8.1 and 2.1, respectively, compared to those (39.7 and 12.3 ppm) at −2.0 V, and are maximized at −2.2 V; the corresponding FE values are 0.29 and 0.09 %, respectively. At an applied potential of −2.4 V, 133.0 and 21.6 ppm of CO and CH₄ are produced, respectively, indicating that the concentrations of CO and CH₄ produced at −2.4 V are higher than those produced at −2.2 V. As shown in Fig. 5(g), the concentration of C₂H₄ decreases with the increasing potential, whereas those of C₂H₆ and C₃H₈increase with the increasing potential, implying that the amount of C₂H₄produced is inversely proportional to that of C₂H₆ and C₃H₈.

Figure 5. (a) and (b) GC profiles of the Au(600s)/MoS₂/Mo electrode at different applied potentials, (c) NMR spectra, (d) H₂ concentration, (e) FE values of the EC products, (f) CO and hydrocarbon concentrations, (g) C₂₋₄ hydrocarbon concentrations, and (h) FE values of CO, the hydrocarbons, methanol, and formate.

The resulting NMR spectra [Fig. 5(c)] and determined FE values of the liquid products [Fig. 5(h)] indicate that the maximum amount of formate is produced at −2.2 V with an FE of 0.49 %. The intensity of the NMR peak of methanol increases with the increasing potential, and the maximum FE of methanol (0.49 %) is achieved at −2.2 V. The FE values of CO, CH₄, methanol, and formate maximize at −2.2 V, while that of H₂ minimizes and that of C₂H₄ is the highest at −2.0 V. Conversely, the FE values of C₂H₆, C₃H₈, and C4H10 maximize at the highest potential of −2.4 V.

The selected MoS₂/Mo and Au/MoS₂/Mo electrodes were further characterized by XRD, Raman spectroscopy, SEM, and HRTEM. The XRD patterns of the MoS₂/Mo sheet [Fig. 6(a)] show peaks at 2θ = 40.5 ° (vw), 58.6 ° (vs), and 73.6 ° (vs), which can be assigned to the (011), (002), and (112) crystal planes of the cubic-phase metallic Mo, respectively. These XRD signals can be mainly attributed to the Mo support. Further, no XRD signal of MoS₂ is observed because of the extremely small thickness of the MoS₂ layer on the Mo support. Thus, to examine the XRD peaks of the ultrathin MoS₂ nanosheets, a bulk powder sample was synthesized. The corresponding XRD patterns [Fig. 6(a)] show two major broad peaks around 2θ = 32 and 57 °, which can be attributed to the (100) and (110) planes of MoS₂, respectively [7,8,25]. The Raman spectrum of MoS₂/Mo [Fig. 6(b)] shows two peaks at 378 and 405 cm⁻¹, which can be ascribed to the E_2g and A_1g modes of MoS₂, respectively [20,25,38].

Figure 6. (a) XRD profiles of the MoS₂ powder and MoS₂/Mo electrode, (b) Raman spectrum of the MoS₂/Mo electrode; inset shows the electrode, (c) and (d) SEM images of the MoS₂/Mo electrode, (e) SEM image of the MoS₂ powder, (f) SEM image of the electrochemically treated MoS₂/Mo electrode, (g) and (h) SEM images of the Au(600s)/MoS₂/Mo electrode before and after the EC treatment, respectively, and (i) and (j) TEM and HRTEM images of the MoS₂ powder, respectively.

The SEM image of the bare MoS₂/Mo electrode [Figs. 6(c) and 6(d)] exhibits a uniform morphology of the ultrathin 2D nanosheets [7,8], indicating the successful and uniform growth of MoS₂ nanosheets on the Mo support. Additionally, the SEM image [Fig. 6(e)] of the MoS₂ powder sample shows a microstructure similar to that of the MoS₂/Mo electrode and thus confirms the successful growth of ultrathin MoS₂ nanosheets on the Mo support. The morphology of the MoS₂/Mo electrode remains stable after the EC CO₂R reaction at −2.0 V, indicating the stability of the electrode during EC CO₂R [Fig. 6(f)]. The SEM image of Au/MoS₂ [Fig. 6(g)] shows small clusters (sputter-deposited Au) that cover the nanosheets, indicating a well-formed interface between Au and MoS₂. The morphology remains stable after the EC CO₂R at −2.0 V [Fig. 6(h)]. The MoS₂ nanosheet powder sample was further characterized by TEM [Figs. 6(i) and 6(j)], and the corresponding results reveal an ultrathin morphology with a clear lattice spacing [Fig. 6(j)]. The interlayer spacing of 0.65 nm corresponds to the (002) plane of MoS₂ [42]. The XRD, Raman, SEM, and TEM/HRTEM characterization results demonstrate the successful growth of ultrathin MoS₂ nanosheets on the Mo support as well as the formation of a uniform interface between Au and MoS₂.

Figure 7 presents the results of the XPS analysis, showing the oxidation states of the MoS₂/Mo electrode and MoS₂ powder sample. In Fig. 7(a), two major Mo 3d peaks appear at the binding energies (BEs) of 231.6 (Mo 3d_3/2) and 228.6 eV (Mo 3d_5/2), which indicate the Mo(IV) oxidation state of MoS₂ [7,8]. The same Mo 3d peaks are observed in the XPS profile of the MoS₂ powder sample, suggesting that the oxidation states of the MoS₂ film and powder are the same. Additionally, the peak at approximately 226 eV originates from S 2s [38,42]. In Fig. 7(b), two S 2p peaks are observed at 162.7 (S 2p_1/2) and 161.5 eV (S 2p_3/2), with a spin–orbit (S-O) splitting of 1.2 eV; these peaks can be attributed to S in MoS₂ [7,8,38,42]. However, for the Au(600s)/MoS₂/Mo electrode, the Mo 3d and S 2p peak intensities drastically decrease because of the Au overlayer on MoS₂. The BEs are the same as those of the MoS₂/Mo electrode. The Au 4f_7/2 and 4f_5/2 peaks, observed at 84.0 and 87.7 eV with an S-O splitting of 3.7 eV, can be attributed to metallic Au [43]. The Au 4f peak position remains unchanged after the EC treatment. Furthermore, after the EC treatment, the corresponding Mo 3d profile shows two additional Mo 3d_3/2 and Mo 3d_5/2 pairs at 235.5 and 232.3 eV, respectively, which can be attributed to the Mo(VI) oxide species [42,44], and at 232.9 and 229.8 eV, respectively, which can be ascribed to MoS₂ with a different phase [7,8]. This drastic change in the oxidation state may be related to the CO₂ reduction activity. Similarly, one additional S 2p_1/2 and S 2p_3/2 XPS pair is observed at 164.2 and 163.1 eV, respectively, which originate from a different phase of Mo 3d. The valence-band (VB) XPS profile of the MoS₂/Mo electrode [Fig. 7(d)] indicates that the density of states appears near the Fermi level, mainly owing to Mo 4d. The VB profile shows a slight change after the EC treatment because of the contribution of O 2p. The spectrum of the Au(600s)/MoS₂/Mo electrode shows highly intense Au 5d peaks at approximately 4 and 6 eV, indicating that the characteristics of the Au overlayer become dominant. The VB profile of the Au(600s)/MoS₂/Mo electrode does not show any substantial change after the EC treatment, indicating that the electrode remains stable during the EC operation.

Figure 7. XPS profiles: (a) Mo 3d, (b) S 2p, (c) Au 4f , and (d) VB region.

Figure 8 illustrates the reaction pathways of the products obtained in this study. H₂, the dominant product, is generated through the adsorbed H (H_ad) process, which involves the initial formation of H_ad via H⁺ + e⁻ → H_ad, followed by either H_ad + H⁺ + e⁻ → H₂ or H_ad + H_ad → H₂ reactions [45]. Abundant surface H is expected during this process. Further, CO₂ is adsorbed as either OCHO* or HOOC* [39–41]; OCHO* is converted to formate (HCOO⁻), which is observed over MoS₂, while HOOC* converts to surface O≡C* via the reaction HOOC* + H⁺ + e⁻ → O≡C* + H₂O. When O≡C* strongly adheres to the MoS₂ surface, CO desorption is suppressed [18], which results in a low CO production as shown in Fig. 2. Further, O≡C* is expected to be converted to CHO via the reaction O≡C + H⁺ + e⁻ → *CHO [39–41]; additional processes include *CHO + 4H⁺ + 4e⁻ → *CH₃ + H₂O and *CHO + 2H⁺ + 2e⁻ → *OCH₃. The surface *CHx species may associate to generate C2−4 hydrocarbon compounds via C–C coupling [39–41], and CH₄ is produced via *CH₃ + H⁺ + e⁻ → CH₄. The surface *OCH₃ is desorbed as CH₃OH, which is mainly observed over the Au/MoS₂/Mo electrode [46].

Figure 8. Proposed reaction pathways.

As previously elucidated, the MoS₂ support demonstrated a weak synergistic effect on the EC CO₂R process, resulting in an FE of < 2 % for the carbon products. In contrast, for the EC CO₂R process over Au/TiC, the corresponding CO FE was higher than 40 % [47]. Our results for the EC CO₂R process and products obtained using the Au/MoS₂ electrode provide valuable insights that can be implemented to realize highly efficient MoS₂-based electrodes.

In summary, we successfully synthesized ultrathin MoS₂ nanosheets on a Mo support using a hydrothermal method and investigated the feasibility of applying them in EC CO₂R reactions. Under dark conditions, CH4 was the predominant CO₂R product over MoS₂, whereas CO production dominated under photoirradiation. Furthermore, UV light significantly enhanced CO desorption, resulting in a 28-fold increase in CO production and a 10-fold increase in C₂H₄ production. The deposition of Au also increased the production of CO. The alkanes (C_nH_2n+2, n = 2, 3) to alkenes (C_nH_2n, n = 2, 3) ratio was affected by the applied potential and overlayer metal. The production pathways for C₂H₄ and C₂H₆ were found to be different. The XPS data showed that the poor CO₂ reduction performance of MoS₂ was due to a significant change in the oxidation state during the EC CO₂R reaction and the density of states near the Fermi level. Methanol production was also observed when Au was deposited on a MoS₂ electrode. Overall, our results provide valuable insights for the development of MoS₂-based materials for EC CO₂R.

This research was supported by a research grant awarded by Chungnam National University (Project No. 2020-0690-01).

The authors declare no conflicts of interest.