TiO2 nanorods based self-supported electrode of 1T/2H MoS2 nanosheets decorated by Ag nano-particles for efficient hydrogen evolution reaction

Chngzheng Lin,Yunpeng Liu,Yxing Sun,Zhenyu Wng,Ho Xu,Mingto Li,Jingto Feng,?,Bo Hou,Wei Yn,c,?

a Department of Environmental Science & Engineering,Xi’an Jiaotong University,Xi’an 710049,China

b School of Physics and Astronomy,Cardiff University,The Parade,Cardiff,CF24 3AA,Wales,United Kingdom

c State Key Laboratory of Multiphase Flow in Power Engineering,Xi’an Jiaotong University,Xi’an 710049,China

d International Research Center for Renewable Energy (IRCRE),State Key Laboratory of Multiphase Flow in Power Engineering (MFPE),Xi’an Jiaotong University,Xi’an 710049,China

Keywords:Molybdenum disulfide Silver nanoparticles Hydrogen evolution reaction Density functional theory Hydrogen spillover

ABSTRACT Molybdenum disulfide (MoS2) has shown significant promise as an economic hydrogen evolution reaction (HER) catalyst for hydrogen generation,but its catalytic performance is still lower than noble metalbased catalysists.Herein,a silver nanoparticles (Ag NPs)-decorated 1T/2H phase layered MoS2 electrocatalyst grown on titanium dioxide nanorod arrays (Ag NPs/1T(2H) MoS2/TNRs) was prepared through acid-tunable ammonium ion intercalation.Taking advantage of MoS2 layered structure and crystal phase controllability,as-prepared Ag NPs/1T(2H) MoS2/TNRs exhibited ultrahigh HER activity.As-proposed strategy combines facile hydrogen desorption (Ag NPs) with efficient hydrogen adsorption (1T/2H MoS2) effectively circumventes the kinetic limitation of hydrogen desorption by 1T/2H MoS2.The as-prepared Ag NPs/1T(2H) MoS2/TNRs electrocatalyst exhibited excellent HER activity in 0.5 mol/L H2SO4 with low overpotential (118 mV vs.reversible hydrogen electrode (RHE)) and small Tafel slope (38.61 mV/dec).The overpotential exhibts no obvious attenuation after 10 h of constant current flow.First-principles calculation demonstrates that as-prepared 1T/2H MoS2 exhibit a large capacity to store protons.These protons can be subsequently transferred to Ag NPs,which significantly increases the hydrogen coverage on the surface of Ag NPs in HER process and thus change the rate-determining step of HER on Ag NPs from water dissociation to hydrogen recombination.This study provides a unique strategy to improve the catalytic activity and stability for MoS2-based electrocatalyst.

Hydrogen is an extremely clean and renewable energy source,which is an ideal substitute of fossil fuels for environmental protection.Among the clean energy conversion methods,hydrogen evolution reaction (HER) is one of the most promising methods for commercial application,which has attracted extensive attentions[1,2].However,the main obstacle to hydrogen production from water electrolysis is a slow HER and large kinetic hindrance [3].Platinum (Pt) has been widely studied as an excellent catalyst with extremely high electrical conductivity and excellent hydrogen adsorption and desorption for HER [4–7].Unfortunately,the broad application of Pt-based catalysts are the significantly limited by their high price and limited natural Pt proven reserves [8].Thus,the development of Pt-free electrocatalysts with comparable performance,better stability and cost-effectiveness preparation process is imminent.

Researchers have recently investigated many low-cost and highperformance catalysts,mainly including transition metal dichalcogenides (TMDCs) [9–11],metal carbides [12],metal nitrides[13] and metal phosphides [14].Among these candidates,molybdenum disulfide (MoS2) has attracted a lot of attention due to its two-dimensional layered structure and abundant catalytic active sites.Due to differences in the structure of layers (1,2 and 3) and crystals (hexagonal,trigonal and rhombohedral) in a single unit cell,MoS2has three natural or synthetic polymorphisms,namely 1-trigonal (1T),2-hexagonal (2H),and 3-rhombohedral (3R).Unlike 2H-MoS2phase,the 1T-MoS2phase exhibits metallic properties,so it has a high conductivity facilitating its HER performance [15].Metastable 1T-MoS2can only be obtained under harsh synthetic strategies such as alkali metal intercalation-exfoliation[16,17],doping [18],mechanical strain [19],and electron beam irradiation [20].However,the yields of the above methods are low,which severely limits the application of 1T-MoS2.It is a great challenge to obtain high-purity 1T-MoS2by a facile method.To address this issue,an acid regulation strategy is employed to efficiently induce phase transition from 2H-MoS2to 1T-MoS2for enhancing its HER performance [8,21].To improve the conductivity of MoS2,the most widely used approache is coating conductive carbon on MoS2or loading MoS2on conductive carriers to reduce the charge transfer resistance in the electrochemical process [22,23].Although the high 1T phase MoS2catalyst synthesized by the above method improved the HER activity,its stability and surface charge transfer and internal resistance still have great challenges [24,25].Therefore,it is necessary to develop a new type of MoS2-based catalyst with a facile growth approach but,high HER performance and stability.

Herein,a novel flower rod-like catalyst stacked by nanosheet MoS2was synthesized by hydrothermal grow MoS2on the surface of TiO2nanorods (TNRs) (Fig.S1a in Supporting information).The organic acid plays a major role in modulating the conversion efficiency of MoS2from 2H phase to the 1T phase which promotes electron transfer.Meanwhile,the internal resistance of charge transfer can be reduced by electrodepositing of Ag NPs.The resulting electrocatalyst exhibited excellent HER activity in 0.5 mol/L H2SO4with low overpotential (118 mVvs.RHE) and small Tafel slope (38.61 mV/dec).Furthermore,as-prepared Ag NPs/MoS2/TNRs shows robust cycle stability and there is negligible overpotential attenuation after 10 h of constant current flow.

Fig.S1b and Eq.S1 (Supporting information) show the simple hydrothermal synthesis steps of MoS2in H2O as solvent (HMoS2) nanoparticles and MoS2nanosheets on TNRs.In this experiment,thiourea as both sulfur source and reductant was employed to promote the formation of molybdenum blue (MB) from Mo-O-Mo bond condensation of protonated Mo-O-Mo under the action of propionic acid.As shown in Fig.S2 (Supporting information),MB species have the typical absorption band around 600–1100 nm which is attributed to the intervalence charge transfer(IVCT) [26].The maximum absorbance is reached at the propionic acid volume fraction of 58.3 vol%.Fig.S3 (Supporting information)is the Fourier transform infrared spectrometer (FT-IR) spectrum of MB powder,Mo-O bonds with different coordination oxygens have different characteristic absorption bands in the range of 1000–500 cm-1[27].The band at 1414 cm-1corresponds to the bending vibration of the N–H in ammonium ions (NH4+),indicating the presence of NH4+bound to MB through strong electrostatic interaction[28,29].The result of X-ray photoelectron spectroscopy (XPS) spectrum of the Mo 3d in Fig.S4 (Supporting information) confirms the presence of reduced Mo(V) species in the MB powder.As shown in Fig.S5 (Supporting information),graph element mapping analysis reveals a uniform distribution of Mo,S,C and O elements,further validating the formation of polyoxometalates (POMs) [29].These data demonstrate that MB was successfully obtained by adding thiourea to sodium molybdate in the mixture of propionic acid and water.

As shown in Fig.1a,uniformly dense TNRs with an average diameter of 125 ± 5 nm are grown vertically on SnO2conductive glass doped with fluorine (FTO) surface.As shown in Fig.1b,the MoS2/TNRs show a lamellae MoS2intercalated on the TNRs with a diameter of about 400 nm.As shown in Fig.1c,the electrodeposition of Ag NPs is uniformly loaded on MoS2/TNRs.Meanwhile,Mo,S,Ti,O and Ag species are detectable and distributed uniformly over the entire sample (Figs.S6 and S7 in Supporting information).Comparing with MoS2/TNRs,the MoS2/FTO sample showed nanoflower morphology with a diameter of 1.2 μm,but the coverage of MoS2was also lower (Figs.S8a and b in Supporting information).MoS2on the H-MoS2/TNRs presents a rod-like stack on the surface of the TNRs,and the layered of MoS2has a larger electrochemically active area than the rodlike (Figs.S8c and d in Supporting information).As shown in Fig.1d,the prepared Ag NPs/MoS2/TNRs electrodes are composed of TNR with a diameter of about 125 nm and MoS2(Ag NPs) with a thickness of 50 nm.The HRTEM (Figs.1e,f and h) results further confirmed that the as-prepared TiO2nanorods possess the (110) plane for rutile TiO2[30].The lattice fringe spacing of 0.24 nm (Figs.1e,g and i) in the shell corresponds to the (111) plane of Ag NPs [31].As shown in Fig.S9 (Supporting information),MoS2was successfully loaded on TNRs,and Ag element is uniformly distributed on MoS2.SEM and TEM analysis confirm that the AgNO3precursor was successfully reduced to Ag NPs by electrodeposition.The close contact between Ag NPs and MoS2enables Ag NPs to efficiently transport electrons from Ag NPs to layered MoS2,which is crucial for the high HER performance of Ag NPs/MoS2/TNRs electrodes.

Fig.1.Top view SEM images of (a) TNRs,(b) MoS2/TNRs and (c) Ag NPs/MoS2/TNRs.(d,e) high-resolution TEM (HRTEM) images of Ag NPs/MoS2/TNRs.The enlarged area denoted in (e) corresponding to the HRTEM images of (f) TiO2 and (g) Ag,respectively.(h,i) Profile plots of the calibration for measuring the spacings of TiO2 and Ag.

As shown in Fig.2a,the peaks at 3133 and 1400 cm-1are due to the stretching and bending vibrations of the N–H bond,revealing the presence of intercalated NH4+in the MoS2/TNRs [32].The XPS spectra of N 1s (Fig.S10 in Supporting information) indicate the presence of intercalated NH4+.The Intercalation of NH4+as electron donors lead to the formation and stabilization of 1T-phase MoS2[33].As shown in Fig.2b,a broad molybdenum sulfide peak is observed only at 13.8° when an aqueous solution of propionic acid was used as the solvent for the hydrothermal preparation of MoS2[34].The Ag NPs peaks in the X-ray diffraction (XRD) pattern of Ag NPs/MoS2/TNRs are detected at 2θ=38.22° and 44.35°consistent with (111) and (200) plane (JCPDS card No.04–0783)[35].As shown in Fig.2c,at H-MoS2/TNRs,the characteristic Raman shifts at 408 and 452 cm-1expected for the E2g1and A1gmodes of 2H-MoS2are clearly observed [15,36].At MoS2/TNRs,the vibration of bridging/shared disulfide (ν(S-S)br/sh) and terminal disulfide (ν(S-S)t) are found at 555 and 525 cm-1,respectively.Molybdenum sulfide bonds [37,38] are found atν(Mo-S) of 382–284 cm-1whereas theν(Mo3-μ3S) vibration is detected at 450 cm-1.Raman vibration signatures of Ag NPs/MoS2/TNRs indicate that the disulfide ligands are not displaced after the electrodeposition of silver.As shown in Fig.S11 (Supporting information),H-MoS2/TNRs,MoS2/TNRs and Ag NPs/MoS2/TNRs contain Ag(Ag NPs/MoS2/TNRs),S,Mo,C and O peaks without any impurity.As can be seen from the curve in Fig.S12 (Supporting information),the high-resolution Mo 3d spectrum of the MoS2/TNRs sample contains three spin-splitting doublets (Mo 3d5/2and Mo 3d3/2),where Mo 3d5/2peaks at ≈228.8 eV,≈229.5 eV,and ≈233.8 eV.The feature at 228.8 eV and 229.5 eV is assignable to Mo4+,which is compatible with the binding energy of the 1T and 2H phase of MoS2[15,31].Mo6+originates from the MoOyor MoSxOyregions in the electrodes (Figs.S11 and S13 in Supporting information)[39,40].The high-resolution S 2p spectra in Fig.S14 (Supporting information) further demonstrate the generation of 1T/2H MoS2.However,these peaks in the MoS2/TNRs and Ag NPs/MoS2/TNRs samples are red-shifted.This result proves the existence of electronic interaction between Ag NPs and MoS2.Furthermore,for the Ag 3d of Ag NPs/MoS2/TNRs (Fig.2d),two peaks located at 368.3 eV and 374.3 eV prove the existence of metallic Ag,because the difference between the two peaks is 6.0 eV [41].In Figs.2e and f,H-MoS2/TNRs prepared with water as the only solvent have 2H-MoS2but no 1T phase MoS2.Therefore,XPS results along with electron microscopy,FT-IR,XRD,and Raman demonstrate the successful formation of the acid-controlled ammonium ion intercalated Ag NPs/MoS2/TNRs hybrid structure with high 1T phase MoS2and more active sites.

Fig.2.(a) FT-IR spectra of H-MoS2 and MoS2 samples.(b) XRD patterns and (c) Raman spectra of Ag NPs/MoS2/TNRs and each component.(d) The high-resolution XPS spectra of Ag 3d from Ag NPs/MoS2/TNRs.The high-resolution XPS spectra of (e) Mo 3d and (f) S 2p from MoS2/TNRs and H-MoS2/TNRs.

The experimental results of Figs.S15 and S16 (Supporting information) showed that the overpotential was lowest at a propionic acid volume fraction of 58.3 vol% (Fig.S17 in Supporting information) and 3 mmol/L sodium molybdate and 15 mmol/L thiourea(48 mL solution).Fig.3a shows that FTO,TNRs and Ag NPs/TNRs hardly exhibit the performance of electrocatalytic hydrogen evolution.Compared with H-MoS2/TNRs,MoS2/TNRs have higher electrocatalytic hydrogen evolution performance,which may be due to the high catalytic activity and high electrochemical active area of the 1T phase [15,18,39,42].After silver electrodeposited,its electrocatalytic hydrogen evolution performance will be further improved,which may be due to electrocatalytic performance and high electrical conductivity of Ag [43].The electrochemical doublelayer capacitance (Cdl) value of Ag NPs/MoS2/TNRs is determined to be 28.34 mF/cm2,which is 1.2,2.6 and 69.1 times higher than that of MoS2/TNRs (24.01 mF/cm2),H-MoS2/TNRs (11.04 mF/cm2) and MoS2/FTO (0.14 mF/cm2),respectively (Fig.3b and Fig.S18 in Supporting information).The maximumCdlvalue of Ag NPs/MoS2/TNRs indicates the highest electrochemically active region with exposed active sites,which greatly enhances the HER performance [39].The Nyquist curve (Fig.3c) and equivalent circuit fitting (Fig.S19 and Table S1 in Supporting information) results show that MoS2/FTO and H-MoS2/TNRs have greater charge transfer resistance (Rct=4.83×1011Ωand 6.28×104Ω) compared with MoS2/TNRs.These results demonstrate that the crystal phase tuning and Ag NPs deposition can greatly facilitate charge transfer,thereby enhancing the reaction efficiency and promoting efficient electrical integration to reduce parasitic ohmic losses [44,45].To get into the HER mechanism of these samples,we calculate the Tafel curves based on their linear sweep voltammetry (LSV) (Fig.3d).The Tafel slope of Ag NPs/MoS2/TNRs is only 38.61 mV/dec,which is smaller than that of MoS2/TNRs (40.36 mV/dec),HMoS2/TNRs (91.62 mV/dec) and MoS2/FTO (74.64 mV/dec),indicating that it is more consistent with the Heyrovsky-Tafel mechanism(Eq.S2 in Supporting information).Smaller Tafel slopes show faster HER reaction kinetics,resulting in efficient H2generation [42].The stability of Ag NPs/MoS2/TNRs,MoS2/TNRs and MoS2/FTO are analysed by performing chronoamperometry test (Fig.3e) at constant potentials (?10) of 120 mV,210 mV and 280 mVvs.RHE,respectively.The presence of TNRs (MoS2/TNRs) and Ag NPs deposited on MoS2surface significantly enhance the stability.As shown in Fig.3e and Fig.S20 (Supporting information),the polarization curves of Ag NPs/MoS2/TNRs after 10 h constant voltage test almost overlap,the overpotential at 10 mA/cm2changes from the initial 118 mVvs.RHE to 123 mVvs.RHE,and the overpotential at 50 mA/cm2changes from the initial 163 mVvs.RHE to 169 mVvs.RHE.The above results indicate that TNRs provide good loading sites for MoS2,which has great advantages over FTO and the enhanced stability of 1T-MoS2is related to its substrate and surface electrodeposited Ag NPs.As summerised in Fig.3f,the HER performance of the as-prepared Ag NPs/MoS2/TNRs is also better than previous reported Mo-based materials.

Fig.3.(a) Polarization curves of electroplating silver in 0.5 mol/L H2SO4 solution with a scan rate of 5 mV/s.Capacitive currents with (b) various sweeping velocities,(c)Nyquist plot and (d) tafel plots of the electrodes.(e) Constant voltage response of MoS2/FTO,MoS2/TNRs and Ag NPs/MoS2/TNRs.(f) Comparison Tafel slope and ?10 with other HER electrocatalysts reported recently.Values were plotted from references (Table S2 in Supporting information).

Hydrogen spillover,the migration of activated hydrogen atoms generated by the dissociation of di-hydrogen adsorbed on a metal surface onto a reducible metal oxide support,is a common phenomenon in heterogeneous catalysis [3].To gain theoretical insights into whether hydrogen spillover can take place from MoS2to Ag NPs,density functional theory (DFT) calculation was carried out to determine the hydrogen transfer energy barriers.As shown in Fig.4a,the adsorption of hydrogen is extremely weak on the surface of Ag (111),while the adsorption onto MoS2(002) is significantly enhanced,indicating that MoS2(002) is prone to hydrogen adsorption.As shown in Fig.4b and Fig.S21 (Supporting information),the Gibbs free energy (ΔGH?) of adsorbed hydrogen in MoS2surface with the Ag absence (site 1′) tends to be negative.The thermodynamic energy barrier of adsorbed hydrogen desorption to free hydrogen is 0.35 eV,indicating that hydrogen is diffi-cult to desorbed from site 2′ to site 4′.At high hydrogen coverage,theΔGH?is 0.54 eV,and the thermodynamic energy barrier with the adsorbed hydrogen on the MoS2surface near Ag is 0.2 eV,indicating that the hydrogen transfer process from MoS2(site 3′) to MoS2near Ag (site 1) is greatly promoted.Additionally,hydrogen adsorption is stronger on Ag NPs (site 4) which is combined to MoS2surface,which is more negative at site 4 than at site 1′′.Thus,adsorbed hydrogen can be spontaneously transferred to Ag from the MoS2adsorption site covered with high density hydrogen(from site 3′ to site 5).To unravel the facilitated hydrogen transfer process on Ag NPs/MoS2,the charge density difference was calculated to explore the charge distribution at the interface.As shown in Fig.4c,electron accumulation is observed below the surface layer of Ag.High density electrons are favorable to trap hydrogen atoms by interacting with unsaturated electrons in the H 1s orbital.As a result,hydrogen spillover from MoS2to Ag is thermodynamically and kinetically facilitated.To investigate charge transfer between Ag and MoS2,the work functions (?) of Ag and MoS2were calculated.The work function of MoS2is determined to be 4.25 eV,smaller than that of Ag (4.33 eV),revealing electron transfer from MoS2to Ag (Fig.4d and Fig.S22 in Supporting information).Combining with the above analyses,a reasonable explanation for hydrogen spillover from MoS2to Ag is given as follows: the difference in work function between Ag and MoS2leads to electron accumulation at the subsurface of Ag,which enhances the hydrogen adsorption on Ag surface and weakens the hydrogen adsorption on the MoS2surface,driving the desorption of hydrogen.As shown in Fig.S23 (Supporting information),it is difficult for adsorbed hydrogen on MoS2to evolve molecular hydrogen (Pathway 1).As a result,MoS2serves as an adsorbed hydrogen reservoir,which form hydrogen through Pathways 2–5.

Fig.4.(a) Calculated free energy diagram for HER on MoS2 and Ag.(b) Free energies of HER on MoS2 and Ag were calculated for different hydrogen coverage and adsorption sites.(c) Electron density difference plot across the Ag-MoS2 interface.Electron accumulation and depletion are indicated in blue and purple,respectively.(d) Work function calculations for various Ag and MoS2.

In conclusion,we proposed a layered 1T/2H phase Ag NPs/MoS2/TNRs as a high-performance and high-stability electrode for hydrogen evolution in acidic water electrolysis.The composite electrodes have excellent hydrogen evolution performance and low charge transfer resistance.The resulting composite electrodes exhibit good HER activity in 0.5 mol/L H2SO4solution with a low overpotential (118 mVvs.RHE) and a small Tafel slope(38.61 mV/dec).More importantly,after electrodeposition of Ag NPs,not only the performance of electrocatalytic hydrogen evolution is increased,but also its stability is significantly increased.These results suggest that Ag NPs,lamellar MoS2,and TNRs composites have a good synergy effect,which enables each component to play a unique role in efficient-performance of HER applications.DFT simulation and comprehensive characterisations suggest that the high HER catalytic activity of Ag NPs/MoS2/TNRs in acid possibly results from an unusual hydrogen spillover effect between multiple catalytic sites,whereby MoS2site captures proton,then proton diffuses from MoS2site to Ag site,and eventually forming H2and releases from MoS2-Ag boundary and Ag site.Our proof-of-concept study of unique molybdenum disulfide supported noble metal structure is expected to be a general strategy to improve the catalytic activity and stability of TMDCs.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No.52270078) and the Royal Society IECNSFC211201-International Exchanges 2021 Cost Share (NSFC).The authors thank Zijun Ren at the Instrument Analysis Center of Xi’an Jiaotong University for their assistance with SEM analysis.

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2023.108265.