Decoration of Ag nanoparticles on CoMoO4 rods for efficient electrochemical reduction of CO2

Hydrothermal and photoreduction/deposition methods were used to fabricate Ag nanoparticles (NPs) decorated CoMoO4 rods. Improvement of charge transfer and transportation of ions by making heterostructure was proved by cyclic voltammetry and electrochemical impedance spectroscopy measurements. Linear sweep voltammetry results revealed a fivefold enhancement of current density by fabricating heterostructure. The lowest Tafel slope (112 mV/dec) for heterostructure compared with CoMoO4 (273 mV/dec) suggested the improvement of electrocatalytic performance. The electrochemical CO2 reduction reaction was performed on an H-type cell. The CoMoO4 electrocatalyst possessed the Faraday efficiencies (FEs) of CO and CH4 up to 56.80% and 19.80%, respectively at  − 1.3 V versus RHE. In addition, Ag NPs decorated CoMoO4 electrocatalyst showed FEs for CO, CH4, and C2H6 were 35.30%, 11.40%, and 44.20%, respectively, at the same potential. It is found that CO2 reduction products shifted from CO/CH4 to C2H6 when the Ag NPs deposited on the CoMoO4 electrocatalyst. In addition, it demonstrated excellent electrocatalytic stability after a prolonged 25 h amperometric test at  − 1.3 V versus RHE. It can be attributed to a synergistic effect between the Ag NPs and CoMoO4 rods. This study highlights the cooperation between Ag NPs on CoMoO4 components and provides new insight into the design of heterostructure as an efficient, stable catalyst towards electrocatalytic reduction of CO2 to CO, CH4, and C2H6 products.

multiple oxidation states of cobalt involved at the intermediate state for CO 2 RR 26 .Furthermore, rods like structure or nanorods can contribute a higher contact area along with great electron pathways than other morphologies 21 .Although lots of paper has been published for Ag NPs towards CO 2 RR, Ag NPs decorated CoMoO 4 rod heterostructure has rarely been reported for CO 2 reduction to CO, C1 and multi-carbon compounds.
In this study, Ag/CoMoO 4 heterostructure was synthesized by hydrothermal and photoreduction/deposition methods.The hydrothermal method revealed several advantages as compared to others, such as low cost, mass efficiency, high product purity, mild preparation conditions, and simple equipment 20,24,[27][28][29][30][31] .In addition, photoreduction process is simple and inexpensive and can be operated at room temperature 32 .The formation of heterostructure is well characterized by X-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), elemental mapping, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and Inductively coupled plasma optical emission spectroscopy (ICP-OES).The electrochemical measurements (cyclic voltammetry, electrochemical impedance spectroscopy, linear sweep voltammetry, and chronoamperometry) of catalysts were performed in an H-type cell for electrochemical CO 2 RR.Tafel plots were analyzed.The gaseous products were detected by gas chromatography (GC).The Faradic efficiencies (FEs) of CO 2 RR was calculated, and possible mechanisms were proposed.

Experimental section Materials
All chemicals consist of analytical grade.These were used without any further purification.Copper foil (CF) with 0.1 mm thickness was purchased from Merck, Germany.Molybdic acid (H 2 MoO 4 ) and cobalt nitrate hexahydrate [Co(NO 3 ) 2 .6H 2 O], potassium bicarbonate (KHCO 3 ), and aqueous ammonia (aq.NH 3 ) were used for the synthesis of samples and obtained from the Sigma-Aldrich.Silver nitrate (AgNO 3 ) was purchased from Fisher chemical, Belgium.

Synthesis of CoMoO 4 and Ag/CoMoO 4
CoMoO 4 was synthesized from hydrothermal process.In this synthesis technique, 2 × 10 -2 mol of H 2 MoO 4 was placed in 40 mL of water.In addition, 2 × 10 -2 mol of Co(NO 3 ) 2 •6H 2 O was dissolved in 40 mL of water.These were magnetically stirred until clear solution was obtained.Then, the prepared solutions were mixed dropwise by using pipette under magnetic stirring and precipitation was occurred.The pH of the solution was adjusted at pH 7 by using aqueous ammonia (NH 3 ).It was magnetically stirred for 4 h.After that, the suspension solution was transferred into a 100 mL Teflon-lined stainless autoclave and kept at 200 °C for 4 h.After completion of hydrothermal treatment, the solution was centrifuged and washed with water and ethanol multiple times.Subsequently, it was dried in a vacuum oven at 70 °C for 8 h.The powder was obtained and calcined at 400 °C for 5 h.At last, the powder sample (CoMoO 4 ) was grounded with the help of mortar and piston.
Ag/CoMoO 4 was fabricated by hydrothermal followed by photoreduction/deposition techniques.According to this technique, hydrothermal synthesized 1 g of CoMoO 4 powder was taken and placed in 100 mL beaker.80 mL ethyl alcohol (C 2 H 5 OH) was put in a beaker and magnetically stirred for 2 h. 5 wt% of Ag (source: AgNO 3 ) was placed in a beaker and magnetically stirred for 4 h under UV light irradiation.After this step, it was centrifuged and washed with water/C 2 H 5 OH several times.The obtained sample was dried in a vacuum oven for 70 °C for 4 h.Finally, it was grounded.The schematic illustration of material synthesis was presented in Fig. S1.

Material characterization
The crystal phase was determined using a powder X-ray diffractometer (Rigaku, Miniflex 600) with Cu Kα radiation (2θ: 20 to 80°, continuous rate: 1°/minute, and step: 0.02).The morphologies of samples were investigated by field emission scanning electron microscopy (FESEM, JEOL, JSM-IT800).Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area diffraction patterns (SAED), EDS elemental mapping images were obtained by JEOL 1230.The X-ray photoelectron spectroscopy (XPS) analysis of the samples was performed using Thermo Scientific ESCALAB™ XI (Al Kα and 200 eV).The Raman spectra of samples were measured on Horiba Raman confocal microscope.Fouriertransform infrared spectroscopy (FTIR) of samples were measured on IRTracer-100 (Shimadzu).Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to find the leaching of Ag NPs after performance of electrochemical CO 2 RR by samples.It is also used to find out the metal ions in the samples.The detail explanation was provided in supporting information (Figs.S2, S3, and S4).Zeta potential of powder samples was measured by Zetasizer Nano ZS (Malvern Instruments, Malvern, UK).The powder was dispersed in 70% ethanol (15 mL) and placed in an ultrasonic bath for 1 h.The zeta potential was measured after diluting the samples with distilled water.

Electrochemical characterizations
All the electrochemical measurements were carried out on a CH Instruments with a typical three-electrode system in 0.5 M KHCO 3 electrolyte solution at room temperature, a platinum electrode (counter electrode), Ag/ AgCl electrode (reference electrode), and working electrodes (CoMoO 4 and Ag/CoMoO 4 ).For the synthesis of working electrodes, 0.5 mL of C 2 H 5 OH, 50 μL nafion, and 4 mg of powder sample were dispersed via ultrasonic processing.As a substrate, CF (2 cm × 2 cm) was washed with water and ethanol for 60 min under ultrasonication and dried at 70 °C for 4 h in a vacuum oven.The well-dispersed ink was placed in CF via controllable drop casting.The available working area in the electrode was 1 cm 2 .Then, it was dried in an oven at 70 °C for 4 h.
Cyclic voltammetry (CV) with scan rate 20 to 150 mV/s of samples was measured.In addition, the electrical conductivity of the samples was performed through the electrochemical impedance spectroscopy (EIS)

Characterization of synthetic materials
As shown in Fig. 1, XRD patterns of samples were well matched with monoclinic structure of pure α-CoMoO 4 with space group C2/m (JCPDS No. 21-0868) 25 .After deposition of Ag NPs on CoMoO 4 , new crystal plane (111) was appeared that suggests the existence of cubic Ag NPs with JCPDS No. 4-0783 34 .In addition, the intensities of XRD peaks were slightly reduced in Ag/CoMoO 4 sample.Besides, it should be noted that the peak position of CoMoO 4 was not shifted, which suggested no substitutional doping.No impurities diffraction peaks were found in samples.The existence of Ag and CoMoO 4 in Ag/CoMoO 4 suggest the successful fabrication of heterostructures.
The morphologies of synthesized materials were investigated by FESEM and TEM.The samples presented rodlike morphology with dimensions of 1-3 μm in length and 0.3-1 μm in width (Fig. 2a and b).The existence of Ag NPs on the CoMoO 4 rods was also observed in Fig. 2b.In addition, the loading of Ag NPs on CoMoO 4 rods did not change the morphology of materials.Figure 2c and d S1).According to the results of XRD, FESEM, TEM, HRTEM, SAED, and TEM-EDS elemental mapping images, it was concluded that the heterostructure was successfully formed between CoMoO 4 and Ag NPs.
The existence of elements and oxidation states in CoMoO 4 and Ag/CoMoO 4 were investigated using XPS technique (Fig. 3).The Co 2p spectra of samples could be deconvoluted into 2p 3/2 , (CoMoO 4 : 781.12 eV and     3c and g) 38 .As shown in Fig. 3h, the presence of metallic Ag NPs in Ag/CoMoO 4 was proved by 3d 5/2 and 3d 3/2 peaks at 366.83 eV and 372.80 eV, respectively 39 .Furthermore, the survey spectra suggested the confirmation of Co, Mo, O, and Ag in samples (Fig. 3c and g).Also, the results of ICP-OES indicated the presence of metallic ions (Co, Mo, and Ag) in samples (Supporting information).To find the surface charge in samples, the zeta potential of CoMoO 4 and Ag/CoMoO 4 was evaluated.The zeta potential of CoMoO 4 and Ag/CoMoO 4 was − 12.30 mV and − 15.33 mV, respectively.This results suggests that the surface of both samples are negatively charged.Raman spectra of CoMoO 4 and Ag/CoMoO 4 was shown in Fig. S6.The vibrational modes were found at 926, 869, 808.70, and 355.61 cm −1 .The Raman mode located at 926.51 cm −1 was associated with symmetric stretching mode of doubly coordinated bridging oxygen in Mo-O 40 .The band at 869.20 cm −1 was related to the symmetric stretching of Co-O-Mo bond.In addition, the band observed at 808.08 cm −1 can be attributed to the asymmetric stretching mode of oxygen in O-Mo-O 41 .The symmetry bending modes of O-Mo-O was observed at 355.61 cm −142 .The decoration of Ag NPs on CoMoO 4 did not alter the Raman bands that suggest the fabrication of heterojunction between Ag NPs and CoMoO 4 .Furthermore, FTIR studies were performed of CoMoO 4 and Ag/CoMoO 4 over the range 500-4000 cm −1 (Fig. S7).The band in lower frequency region (CoMoO 4 : 692.90 cm −1 and Ag/CoMoO 4 : 632.96 cm −1 ) was associated with Co-Mo-O stretching vibrations 43 .The peaks appeared in CoMoO 4 (779.75,832.78, and 926.54 cm −1 ) and Ag/CoMoO 4 (779.76,846.01, and 933.40 cm −1 ) were assigned to Mo-O stretching bands 44 .These bands provided the evidence of CoMoO 4 in samples.

Electrochemical CO 2 reduction
As shown in Fig. 4a and b, CV curves of CoMoO 4 and Ag/CoMoO 4 nanorods were recorded in a potential window of − 0.6 to 0.6 V at different sweeping rates (20 mV/s, 40 mV/s, 60 mV/s, 80 mV/s, 100 mV/s, and 150 mV/s).The observed redox peaks may be attributed to reversibly changing their oxidation states of Co 2+ and Co 3+45 .These redox peaks were obtained from redox mechanism that reveals the Faradic capacitive behavior of the CoMoO 4 and Ag/CoMoO 4 electrodes.In addition, the enhancement of conductivity by molybdenum (Mo) can improve the electrochemical performances of electrodes 28 .Also, an increase in sweep rate provided the shifting of the oxidation and reduction peaks of electrodes towards right and left, respectively due to higher internal diffusion resistance.The CV curve area and current increased with increase in scan rate because of www.nature.com/scientificreports/fast reaction kinetics 27,46 .The shape of CV peaks did not change at high scan rate that suggests the good rate performance of catalyst.
As depicted in Fig. 4c, compared with CoMoO 4 electrode, the increased loop of CV curves was observed for Ag/CoMoO 4 electrodes.In addition, presence of Ag NPs in CoMoO 4 enhanced the reduction ability.These factor indicate the improvement of charge transfer and transportation of ions by making heterostructure between Ag NPs and CoMoO 4 .So, the fabrication of heterojunction between Ag NPs and CoMoO 4 rods enhanced the electrocatalytic performance which is beneficial for CO 2 reduction.To observe the effect of Cu-foil in fabricated electrodes, CV curve of Cu-foil was carried out (Fig. S8).It demonstrates the negligible current as compared to CoMoO 4 and Ag/CoMoO 4 CV curves.Also, insignificant contribution of Cu-foil was noted.Also, EIS was measured to observe the interfacial charge transfer on catalysts (Fig. 4d) 47 .The obtained data was fitted, and equivalent circuit was made (Fig. S9).It was composed of solution resistance (R1), charge transfer resistance (R2), electric double layer capacitance (C2), Warburg impedance, and constant phase element (Q).According to the Nyquist plots, Ag/CoMoO 4 (41.57Ω) demonstrated lower charge transfer resistance in comparison to CoMoO 4 (309.50Ω) suggesting its rapid electron transfer between the interface of electrolyte and electrocatalyst that may allow efficient electron, Ag NPs and CoMoO 4 interactions (Table S2).Therefore, decoration of Ag NPs on CoMoO 4 rods could promote the electron transportation between the electrocatalyst and CO 2 molecules that provides the electrochemical reduction capability of heterostructure.
The accelerated CO 2 RR conversion kinetics upon the heterostructure was further conformed by Tafel plots (Fig. 5a).The Tafel slope for CoMoO 4 and Ag/CoMoO 4 were estimated to be 273 mV/dec and 112 mV/dec, respectively.The lowest Tafel slope for Ag/CoMoO 4 suggests the enhancement of electrocatalytic activity by fabricating heterostructure between Ag NPs and CoMoO 4 because of rapid electron transfer from the electrode to electrocatalyst.This result also indicates that the transfer of first electron to adsorbed CO 2 molecules.It facilitates the production *CO 2 that can improve a second electron-transfer for *COOH generation 48 .To compare the electrochemical performance of Ag NPs with other non-precious metal particles, Tafel slope was evaluated (Fig. S10).Ag NPs showed lower Tafel slope than Cu indicating great electrochemical performance of Ag NPs that is accordance to published report 49 .Furthermore, the CO 2 RR performance of the as-synthesized electrocatalysts was investigated by LSV graphs (Fig. 5c and d).Ag/CoMoO 4 revealed higher current density than CoMoO 4 at − 0.6 V. Ag NPs decorated CoMoO 4 rods showed approximately fivefold enhancement of current density in comparison with CoMoO 4 rods (Fig. 5b).The current density of samples along with CO 2 -saturated 0.1 M KHCO 3 electrolyte suggest demonstrated higher current density suggesting better reactivity in CO 2 RR (Fig. 5c).
Steady-state current responses in a CO 2 -saturated electrolyte for 400 s at − 1.3 V versus RHE of samples were presented in Fig. S11.The obtained current densities for CoMoO 4 and Ag/CoMoO 4 were − 3.35 mA and − 4.62 mA, respectively.After that, the gas-phase products were detected by using a GC (Table S3).The FEs for various gas formation of CoMoO 4 and Ag/CoMoO 4 were calculated (Supporting Information) 33 .According to Fig. 5d, CoMoO 4 presented FEs for CO and CH 4 were 56.80% and 19.80%, respectively.In this case, CO and CH 4 act as a major and minor gaseous products during CO 2 RR, respectively.In addition, Ag/CoMoO 4 revealed FEs for CO and CH 4 , and C 2 H 6 were 35.30%, 11.40%, and 44.20%, respectively.It was noted that loading of Ag  In addition, electrocatalytic stability of cu-foil was evaluated after a 400 s amperometric test at − 1.3 V versus RHE (Fig. S12).Low current density along with unstable nature were observed.It suggests the Cu-foil did not contribute the significantly for steady-state current responses.Stability curves of Ag/CoMoO 4 at − 1.3 V versus RHE was shown in Fig. S13.Notably, the electrocatalyst exhibited outstanding stability even upto 25 h.Also, the current density was not changed during stability.ICP-OES analysis of electrolyte solution revealed no leaching of Ag ions after 25 h stability test.Table 1 revealed the comparison of FEs of various Ag-based electrocatalysts with Ag/CoMoO 4 [16][17][18][19]34,48,50,51 . This  26 . At first, Co 2+ is reduced to Co + under the applied potential.When CO 2 is adsorbed on the (002) catalyst surface, oxidation of Co + to Co 2+ is occurred. Du to this reason, electron is transferred to the adsorbed CO 2 and stabilization of CO 2 radical is happened.Also, the presence of oxygen vacancy in catalyst can improve the stabilization of CO 2 radical 15 .After that, CO 2 radical has ability to capture the proton (H + ) and electron (e − ) that may dissociate from HCO 3 − ion to produce COOH* intermediate because of small potential obstacles.Then, this intermediate reacted with H + /e − continuously to generate CO molecule.The possible reasons for formation of CO as a major product is related with existence of π-back donation between the center of Co metal and CO 2 ligand that can enhance the C-O cleavage 52 .Furthermore, *CO may transform into *CHO via hydrogenation 53 53,55 .The double bond between the C and O is broken and proton can attack the O site to form -OH during reaction with H + /e − .In addition, -OH functional group is eliminated with the reaction with proton to produce H 2 O. Due to this reaction, double bonds are created between C to C and C to O.After transfer of two H + /e − , 2H + react with carbon to form HC=CH along with attachment of O on the surface of catalyst.Then, H + may react with double bond containing C to form single bond between carbon along with attachment of O with surface and -CH 2 .At last, surface attached O reacts with H + to form H 2 O and C 2 H 6 is produced.The possible reason for obtaining the higher FEs of C 2 H 6 than CO and CH 4 for Ag/CoMoO 4 may associate with higher chance of protonation (*CO → *COH) than desorption of *CO on interface 56 .Also, decrease in FEs for CO and CH 4 of Ag/CoMoO 4 than COMoO 4 could be related with covering the Ag NPs on CoMoO 4 rods.The synergistic effect between Ag NPs and CoMoO 4 rods can be attributed to generate the CO, CH 4 and C 2 H 6 .

Conclusion
Ag/CoMoO 4 electrocatalyst was prepared through hydrothermal and photoreduction/deposition methods.The existence of heterostructure between Ag NPs and CoMoO 4 rods was shown by structural and physicochemical characterization techniques.The excellent electrochemical behaviors of catalysts were proved by CV, EIS, LSV, chronoamperometry, and Tafel plots.The electrochemical CO 2 RR of CoMoO 4 favored for CO (FEs: 56.80) and CH 4 (FEs: 19.80) at − 1.3 V versus RHE in a H-type cell containing 0.5 M KHCO 3 .However, the heterostructure revealed selectivity for reducing mainly CO 2 to C 2 H 6 (FEs: 56.80) along with lower FEs for CO and CH 4 at same condition.The selectivity for reducing CO 2 to CO, CH 4 , and C 2 H 6 by electrocatalyst was attributed to adequate active sites, oxygen vacancies, and excellent conductivities.In addition, the synergistic effect of Ag-CoMoO 4 active sites provided the C-C coupling for reduction of CO 2 to C 2 H 6 electrocatalytically.The electrocatalyst showed excellent stability upto 25 h without reduction of current density that can be applied for practical application towards electrocatalytic CO 2 reduction.The possible mechanisms/pathways were proposed for CO 2 RR.Finally, the outcomes of this work present a new approach for improving electrochemical performances/reduction of CO 2 to CO and hydrocarbon by using the Ag/CoMoO 4 heterostructure catalyst.
presented the TEM image of Ag/CoMoO 4 .This image revealed the decoration of Ag NPs on the surface of CoMoO 4 rods.It also indicates the uniform distribution of Ag NPs on rods.The interplanar spacing of 0.33 nm and 0.23 nm calculated from Fig. 2e were corresponds to the (002) and (111) crystal planes of CoMoO 4 and Ag, respectively which are also strongest peak in the XRD spectrum.All the interplanar spacing calculated from HRTEM image are well consistent with crystallographic plane of CoMoO 4 and Ag.SAED patterns suggested the poly-crystalline nature of Ag/CoMoO 4 (Fig. 2f).As shown in Figs.2g-k and S5, the TEM-EDS mapping/spectrum of Ag/CoMoO 4 indicated the existence as well as homogenous distribution of Co, Mo, O, and Ag (Table
Vol.:(0123456789) Scientific Reports | (2024) 14:1406 | https://doi.org/10.1038/s41598-024-51680-wwww.nature.com/scientificreports/ Table suggests the fabrication of various morphologies of Ag and Ag-based heterostructure by several techniques for electrochemical reduction of CO 2 to CO and C 2 H 4 under different applied potential.Although several gaseous products were found on Ag-based electrocatalysts, electrocatalytic reduction of CO 2 into C 2 H 6 by using Ag/CoMoO 4 heterostructure has not yet been reported in the literature.Based on the above results, the possible reaction mechanisms/pathways were purposed.In CoMoO 4 , Co consists of loosely bonded d-electrons that provides the multiple oxidation state.Moreover, transition of Co (II) to Co (I) is considered as an intermediate state for CO 2 reduction . The stabilization of *CHO intermediate play significant role for mitigating the overpotential for CH 4 production.This *CHO intermediate may convert into *CH 2 O and *CH 3 O during transfer of H + /e − during CO 2 RR.Finally, *CH 3 O intermediate transforms into CH 4 53,54 .The decoration of Ag NPs (111) on the surface of CoMoO 4 rod may reduce the energy barrier for conversion of CO 2 to CO, CH 4 and C 2 H 6 products that can change the reactions pathways.C-C coupling mechanism plays a vital role to achieve the high selectivity of C 2 H 6 species.The possible reason for generation of C 2 H 6 species may associate with existence of active sites in catalyst.According to this mechanism, *CO dimerization process is occurred at the catalyst surface

Table 1 .
The comparison of the electrochemical CO 2 RR results of this work with other Ag-based electrocatalysts.