Amorphous molybdenum sulfide (MoSx ) is an attractive Pt-free catalyst for the hydrogen evolution reaction (HER) in both neutral and acidic pH electrolytes. Among the available approaches for the preparation of MoSx , the electrochemical oxidation and reduction of a tetrathiomolybdate salt ([MoS4 ] 2-) represents the most convenient. Herein we describe new insights onto the electrochemical oxidation of [MoS4 ]2- to grow MoSx thin films by employing an advanced technique called electrochemical quartz crystal microbalance (EQCM). These new findings enrich the current understanding of the structure, growth mechanism, redox property and catalytic operation of the MoSx material.
Physical Sciences | Physics Doi: 10.31276/VJSTE.61(4).09-13 New insights into the formation of amorphous molybdenum sulfide from a tetrathiomolybdate precursor Anh D Nguyen1, 2*, Phuong T Pham2, An T Dam2, Phong D Tran2* Graduate University of Science and Technology, Vietnam Academy of Science and Technology University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology Received 30 August 2019; accepted November 2019 Abstract: Amorphous molybdenum sulfide (MoSx) is an attractive Pt-free catalyst for the hydrogen evolution reaction (HER) in both neutral and acidic pH electrolytes Among the available approaches for the preparation of MoSx, the electrochemical oxidation and reduction of a tetrathiomolybdate salt ([MoS4]2-) represents the most convenient Herein we describe new insights onto the electrochemical oxidation of [MoS4]2- to grow MoSx thin films by employing an advanced technique called electrochemical quartz crystal microbalance (EQCM) These new findings enrich the current understanding of the structure, growth mechanism, redox property and catalytic operation of the MoSx material One sentence summary: a new mechanism for growth of amorphous molybdenum sulfide thin films via the electrochemical polymerization of [MoS4]2- is discussed Keywords: EQCM, MoSx formation, [MoS4]2-, [Mo3S11] building block Classification number: 2.1 Introduction Amorphous molybdenum sulfide, usually denoted as MoSx, represents one of the most promising catalysts under investigation as the replacement for the Pt catalyst in the hydrogen evolution reaction (HER) of water It shows excellent catalytic activity in both acidic and neutral pH electrolytes [1] Its preparation can be achieved by different approaches like reactive magnetron sputtering [2], acidification of a [MoS4]2- solution, electrochemical oxidation or electrochemical reduction of a deposition solution composed of [MoS4]2- [3, 4], [Mo2S12]2- [5], or [Mo3S13]2- [5, 6] In our previous work, we have demonstrated that MoSx is a coordination polymer made of discrete [Mo3S13]2- building block clusters [4] In the ideal circumstance where no structural defects like Mo- site are present within the polymeric structure of MoSx, it could be described as a (Mo3S11)n polymer Treating MoSx thin films or nanoparticles in an alkaline solution causes depolymerization that generates [Mo3S13]2- clusters that could be easily isolated, e.g by adding Et4N+ cation [4] Inversely, we recently demonstrated that electrochemical oxidation of the [Mo3S13]2- cluster via a two-electron process generated the MoSx material [6] A key elemental step was proposed to be the electrochemical elimination of the terminal disulfide ligand within the [Mo3S13]2- as the source of Mo- defects that subsequently served as anchoring sites for other [Mo3S13]2- clusters, thus driving polymerization This means that during the growth of MoSx materials, the [Mo3S7] core skeleton is conserved [MoS4]2- → MoS3 + 1/8 S8 + 2e- (eq 1) We then aim to revisit the polymerization of [MoS4]2-, namely a mononuclear species, into MoSx which is made of [Mo3S13]2- building blocks We note that the first discussion of the [MoS4]2- to MoSx polymerization mechanism was reported by Hu and co-workers [3, 7] In that work, a rather simple reaction (eq 1) was proposed based on the establishment of a relationship between the mass loss of the [MoS4]2- precursor and the net charge consumed during the oxidation process Furthermore, the amorphous molybdenum sulfide was described as MoS3 which was not appropriate, as recent analysis has revealed the S: Mo atomic ratio within MoSx should be closer to 4.0, e.g ca 3.7, rather than to 3.0 [4, 8] We hypothesize the actual mechanism could be much more complicated than what has been described In any case, the fundamental question of how the [MoS4]2- mononuclear species can assemble into a * Corresponding authors: Email: nguyen-duc.anh@usth.edu.vn; tran-dinh.phong@usth.edu.vn DECEMBER 2019 • Vol.61 Number Vietnam Journal of Science, Technology and Engineering Physical Sciences | Physics [Mo3S7] cluster within the MoSx structure remains unclear Herein, employing an Electrochemical Quartz Crystal Microbalance (EQCM) analysis we show that the electrochemical oxidation of [MoS4]2- precursor into MoSx thin film occurs via a 10-electron process Spectroscopic analyses clearly confirm the creation of the [Mo3S13]2building block within MoSx A mechanism describing how [MoS4]2- fragments assemble into the (Mo3S11)n polymer, namely the MoSx, is discussed Materials and methods Materials Ammonium tetrathiomolybdate ((NH4)2[MoS4]) 99% and fluorine-doped tin oxide (FTO) coated glass were purchased from Sigma-Aldrich while Sulfuric acid (H2SO4), K2HPO4, KH2PO4, K3[Fe(CN)6] and K4[Fe(CN)6] were purchased from Xilong (99% purity) These chemicals were used as received without any further purification Electrochemical deposition and analyses Electrochemical deposition and measurements were performed using a Biologic SP-50 potentiostat in a conventional three-electrode system The working electrode was made of an FTO substrate for deposition of the MoSx film which was then used for morphology and chemical composition characterization For electrode mass change analysis, a AT-cut MHz Au/Ti quartz QCM (S 1.31 cm2) working electrode was used A Pt plate was used as the counter electrode whereas the reference electrode was an Ag/AgCl (1 M KCl) electrode All the potentials were reported on a normal hydrogen electrode (NHE) The electrolyte solution consisted of mM (NH4)2[MoS4] in a pH phosphate buffer solution Prior to use, the solution was filtered to remove any precipitates and then degassed by an N2 flux for 30 Cyclic voltammograms were recorded on the Au QCM electrode immersed in the electrolyte solution The potential was polarized from V towards 1.4 V then backwards to -1 V vs NHE with a potential scan rate of mV/s The deposition of MoSx was conducted using the chronoamperometric technique wherein the Au QCM electrodes were held at 0.33, 0.36 or 0.41 V vs NHE The total amount of charge passed through the working electrode for the MoSx deposition was set at 10 mC/cm2 on the electrode when the recorded mass was stable We define a stable mass as changes less than 10-2 μg/cm2 or lower The deposition time was set for 500 s per cycle The mass change was recorded in several repeated cycles Spectroscopic and microscopic characterization The surface morphology of the MoSx thin film was characterized by using a field emission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan) Raman spectra were collected using a LabRAM HR Evolution Raman Microscope (Horiba) with the 532 nm green laser excitation XPS analysis was conducted on a ULVAC PHI 500 (Versa Probe II) equipped with a monochromatic Al Kα (1486.6 eV) X-ray source Results and discussion Electrochemical property of a [MoS4]2- solution We first re-investigated the electrochemical property of the [MoS4]2- from the perspective of identification of suitable conditions for the electrochemical polymerization effect that generates MoSx Fig shows the first three consecutive cyclic voltammograms recorded on a clean FTO electrode immersed in a 1.0 mM [MoS4]2- solution in pH phosphate buffer The potential polarization direction was set from the open circuit voltage towards the anodic potential with a potential scan rate of 50 mV/s In the first nm green laser excitation XPS analysis was conducted on a ULVAC PHI 500 (Versa cycle, two Probe oxidation events are observed potentials of II) equipped with a monochromatic Al Kαat (1486.6 eV) X-ray source and discussion V and 0.95Results V, while a reduction event is observed at -0.8 V 2property a [MoS 4] solution vs NHE (Fig.Electrochemical 1, blue trace) Inofthe second and subsequent We first re-investigated the electrochemical property of the [MoS4]2- from the scans, the perspective V oxidation and the -0.8 V reduction events polymerization of identification of suitable conditions for the electrochemical effect that generates MoSthe shows the first three consecutive cyclic x Fig are unchanged However, 0.95 V oxidation event is novoltammograms recorded on a clean FTO electrode immersed in a 1.0 mM [MoS 4]2- solution in pH longer observable, and a potential new oxidation event at PHI 0.41 Vthe open circuit phosphate buffer polarization direction was set from nm green laser The excitation XPS analysis was conducted on a ULVAC 500 (Versa II) equipped with a monochromatic Al Kα a (1486.6 eV) X-ray source voltageProbe towards the anodic potential with potential scan rate of 50 mV/s In the first emerges (Fig 1, red and green traces) Thus, to investigate Results and discussion cycle, two oxidation events are observed at potentials of V and 0.95 V, while a Electrochemical property a [MoS reduction event is observed atof-0.8 V ] vs NHE (Fig.]21,,blue In the second and wetrace) chose the electrochemical polymerization ofsolution [MoS We first re-investigated the electrochemical property 4of the [MoS ] from the subsequent scans, the V of oxidation and the -0.8 V reduction events are unchanged perspective of identification suitable conditions for the electrochemical polymerization three potentials for chronoamperometry (CA) deposition, However, the 0.95 V oxidation event is no longer observable, and a new effect that generates MoS Fig shows the first three consecutive cyclic voltammograms oxidation event recorded on a clean electrode in a 1.0 Thus, mM [MoS ] solution in pH at 0.41 V emerges (Fig.FTO 1, V red and immersed green traces) to investigate the7electrochemical namely 0.33, 0.36 and 0.41 vs NHE, which corresponds phosphate buffer The potential direction was set from the open circuit polymerization of [MoS4]2-,potential wepolarization chose three potentials for chronoamperometry (CA) voltage towards the anodic with a potential scan rate of 50 mV/s In the first to the foot-wave potential, half-wave potential theV,peak deposition, namely 0.33,events 0.36areand 0.41 V vs NHE, cycle, two oxidation observed at potentials of which and V andcorresponds 0.95 while to a the foot-wave reduction event is observed at -0.8 (Fig potential 1, blue trace) the second potential, half-wave potential andV vs theNHE peak ofInthe latter and oxidation event, potential ofrespectively the subsequent latter oxidation event, respectively scans, the V oxidation and the -0.8 V reduction events are unchanged 2- x 2- 2- However, the 0.95 V oxidation event is no longer observable, and a new oxidation event at 0.41 V emerges (Fig 1, red and green traces) Thus, to investigate the electrochemical polymerization of [MoS4]2-, we chose three potentials for chronoamperometry (CA) deposition, namely 0.33, 0.36 and 0.41 V vs NHE, which corresponds to the foot-wave potential, half-wave potential and the peak potential of the latter oxidation event, respectively Bulk electrolysis The number of electrons used per cluster during the oxidative deposition was determined using bulk electrolysis (chronoamperometric technique as aforementioned) During the electrolysis, the evolution of electrode mass was recorded Because of the high sensitivity of QCM, a potential (e.g 0.33 V) was only applied to deposit amorphous MoSx 10 Vietnam Journal of Science, Technology and Engineering nd Fig Subsequent cyclic voltammograms (1st scan:st blue trace; 2nd scan: red trace Fig Subsequent cyclic voltammograms(1 (1st scan: blue trace; scan: red trace Fig Subsequent voltammograms scan: blue scan: green trace) recorded on an FTO electrode immersed in atrace; 1.0 mM and 3cyclic scan: green an The FTO electrode and 3rd(NH ) [MoS ] solution in pHrecorded phosphateon buffer potential scan rateimmersed was mV/s in a 1.0 mM rdtrace) 2nd scan: red trace and scan: green trace) recorded on towards4the anodic scanin direction ] solution pH phosphate buffer The potential scanan rate was mV/s (NH4)2[MoS FTO electrode immersed indirection a 1.0 mM (NH4)2[MoS4] solution towards the anodic scan characterization of deposited MoS films in pH phosphateInbuffer potential scan rate was mV/s the same 1.0The mM [MoS ] solution in pH phosphate buffer, a5clean FTO electrode was held at of 0.33, 0.36 or 0.41MoS V vs.xNHE for hours to deposit brown-coloured characterization deposited films towards the anodic 2MoS scan thin films.direction DECEMBER 2019 • Vol.61 Number rd 4 2- x In thex same 1.0 mM [MoS4] solution in pH phosphate buffer, a clean FTO electrode was held at 0.33, 0.36 or 0.41 V vs NHE for hours to deposit brown-coloured MoSx thin films Physical Sciences | Physics Characterization of deposited MoSx films In the same 1.0 mM [MoS4]2- solution in pH phosphate buffer, a clean FTO electrode was held at 0.33, 0.36 or 0.41 V vs NHE for hours to deposit brown-coloured MoSx thin films MoO3, within the deposit is also likely as a doublet having Mo3d5/2 of 232.44 eV is observed The (S-S)br/sh disulfide ligand and the apical sulfide are characterized by a doublet having S2p3/2 of 163.18 eV While the doublet at S2p3/2 of 162.07 eV is assigned to the terminal (S-S)t disulfide ligand (Fig 4) The binding energies of Mo and S from XPS results are in consensus with the reported literatures [4, 6] and energies of Mo and S from XPS results are in consensus with the reported literatures [4, thetheobtained materials amorphous molybdenum 6] proving and proving obtained materials is is amorphous molybdenum sulfide, labeled as sulfide, labeled as MoSx MoS x Figure shows the SEM images of these films It can be seen that the obtained MoSx films consist of clumps arranged close together in different shapes and sizes The variation in clump shape and size are likely due to the different in Figure shows the SEM images of these films It can be seen that the obtained the rate of MoSx at different There arranged close applied together inpotential different shapes and sizes The MoSgrown x films consist of clumps variation in clump shape size are likely due to the different in the grown is no difference of and morphologies observed between filmsrate of MoSx Figure shows images of these films can be seen that the obtained at different applied potential Therethe is noSEM difference of morphologies observedItbetween prepared at various anodic potentials films prepared various anodic potentials consist of clumps arranged close together in different shapes and sizes The MoSx atfilms variation in clump shape and size are likely due to the different in the grown rate of MoSx at different applied potential There is no difference of morphologies observed between 2.00 μmat various anodic potentials films prepared (A) (B) 0.36 V Fig.Fig (A) Mo-Moand and (B) Slevels levels of a MoS (A) (B)core S- core of ax film MoSgrown film at grown at vs NHE x Fig SEM images of MoSx grown at 0.33 (A), 0.36 (B) and 0.41V vs NHESimilar (C) features were recorded for the films grown at 0.33 V and 0.41the V films Fig SEM images of MoS grown at 0.33 (A), 0.36 (B), and 0.36 V vs NHE Similar features were recorded for x 0.41Raman V vs NHE grown thinat 0.33 V and 0.41 V spectra(C) clearly show all the characteristic features of a MoSx polymeric 2film with [Mo3S13]2- building block clusters, as previously reported (Fig 3)These [4] results confirm that at the potentials applied, [MoS4] is 2-electrochemically -1 -1 which grows the MoSx thin film consisting of [Mo3S13] oxidized, building block Vibrations at 284-382 cm clearly are assigned to the Mo-S the one at 450 cm is Raman spectra show all bond, the whereas characteristic These results confirm that at the potentials applied, clusters In following section, we employ EQCM analysis to identify the number of attributed to the Mo3-Sapical vibration mode Vibrations of bridged or shared disulfide (Sfeatures of a MoSx polymeric thin film with [Mo3S13]2- electrons involved in each elemental electrochemical oxidation event 2S)br/sh and terminal disulfide (S-S)t are observed at 555 and 525 cm-1, respectively [MoS ] is electrochemically oxidized, which grows the Fig.block SEM images MoSx grown at 0.33 (A), 0.41V vs quartz NHEcrystal (C) microbalance analysis building clusters, as of previously reported (Fig 3)0.36 [4] (B) and Electrochemical 2MoS thin film consisting [Mo3S13in ]2-a 1.0 building -1 A clean Au QCM electrode was firstofequilibrated mM [MoSblock x 4] solution in a Vibrations at 284-382 cm are assigned to the Mo-S bond, pHclusters phosphate 30 minutes atthin the circuitEQCM voltage Subsequently, following section, weopen employ analysis to an anodic show alltothe of In a buffer MoS -1 xforpolymeric whereas theRaman one atspectra 450 cm2-clearly is attributed thecharacteristic nMo3-Sapical features potential of 0.33 V vs NHE iselectrons applied forinvolved a period of 500 s elemental We observed a linear film with [Mo S ] building block clusters, as previously reported (Fig 3) [4] 13 identify the number of in each vibration mode Vibrations of -1bridged or shared disulfide increment of the electrode mass (Fig 5A) -1 that indicated growth, by electrodeposition, of Vibrations at 284-382 cm are assigned to the Mo-S bond,electrochemical whereas the oneoxidation at 450 cm is event on the Au QCM electrode surface When the applied potential was removed, no (S-S)br/sh and terminal disulfide (S-S)t are observed at 555 material attributed to the Mo -S vibration mode Vibrations of bridged or shared disulfide (S3 apical -1 mass increment was recorded This clearly confirmed that the MoSx deposition is solely and 525 cm , respectively -1 Electrochemical quartz crystal microbalance S)br/sh and terminal disulfide (S-S)t are observed at 555 and 525 cm , respectively driven by the applied oxidation potential From the mass increment analysis we are able to deduce the amount of MoSx deposited in moles under the assumption that the MoSx is a perfect clean Au QCM was first equilibrated in a time, the (Mo3S11A )n polymer without any electrode structural defects or impurities At the same 2amount of charge involved in the process was recorded We then plot the 1.0 mM [MoS ] solution in a pH phosphate buffer for 30number of electrons (in mol) against the amount of Mo S clusters deposited (Fig 5B) 11 minutes at the open circuit voltage Subsequently, an anodicA slope of 9.65 is found The same value was determined when we repeated the same depositionpotential of 0.33 V vs NHE is applied for a period of 500 s relaxation at: 0.33process on the same electrode for several cycles (Figs 5C, D) Fig Raman spectra recorded on the MoSx films grown on FTO electrode (A) 2.00 μm (B) (C) We observed a linear increment of the electrode mass (Fig 5A) that indicated growth, by electrodeposition, of material In XPS analysis, MoIV species is characterized by a doublet having Mo3d5/2 of on thetoAu QCM electrode surface When the applied potential 229.38 eV The doublet at higher binding energy, Mo3d5/2 of 230.30 eV, is attributed VI Moremoved, MoV species, e.g due to the presence of MoV=O defects The presence of some was no mass increment was recorded This clearly species, like excess [MoS4]2- or MoO3, within the deposit is also likely as a doublet confirmed that the MoSx deposition is solely driven by the having Mo3d5/2 of 232.44 eV is observed The (S-S)br/sh disulfide ligand and the apical sulfide are characterized by a doublet having S2p3/2 of 163.18 eV While the doublet at oxidation potential From the mass increment we applied S2p3/2 of 162.07 eV is assigned to the terminal (S-S)t disulfide ligand (Fig 4) The binding are able to deduce the amount of MoSx deposited in moles under the assumption that the MoSx is a perfect (Mo3S11)n Fig Raman spectra recorded on the MoSx films grown on FTO electrode at: 0.33 V (red trace), 0.36 V (blue trace), and 0.41 V on FTO electrode at: 0.33defects or impurities At the Fig Raman spectra recorded on the MoSx films grown polymer without any structural vs NHE (green trace).0.36 V (blue trace) and 0.41 V vs NHE (green trace) V (red trace), same time, the amount of charge involved in the process recorded We then plot the number of electrons (in mol) In XPSInanalysis, MoIV species is characterized by a bywas XPS analysis, MoIV species is characterized a doublet having Mo3d5/2 of against the amount of Mo3S11toclusters deposited (Fig 5B) doublet having Mo3d of 229.38 eV The doublet at higher 229.38 eV The5/2doublet at higher binding energy, Mo3d5/2 of 230.30 eV, is attributed V V V A slope of 9.65 isoffound TheVIsame value was determined binding Mo3d of 230.30 is attributed Modefects species, e.g.5/2 due to the eV, presence of Mo to=O The presence some Mo Moenergy, 2V whenis we deposition-relaxation process species, liketoexcess [MoS4] oforMo MoO within the alsorepeated likely asthea same doublet species, e.g due the presence =O3, defects Thedeposit VI 232.44 eV is observed The (S-S) 2having Mo3d of disulfide ligand and the apical presence of some 5/2 Mo species, like excess [MoS4] orbr/sh on the same electrode for several cycles (Figs 5C, D) sulfide are characterized by a doublet having S2p3/2 of 163.18 eV While the doublet at S2p3/2 of 162.07 eV is assigned to the terminal (S-S)t disulfide ligand (Fig 4) The binding V (red trace), 0.36 V (blue trace) and 0.41 V vs NHE (green trace) DECEMBER 2019 • Vol.61 Number Vietnam Journal of Science, Technology and Engineering 11 Physical Sciences | Physics At a higher applied potential, namely 0.36 V and 0.41 V vs NHE, we observed the same phenomenon and a similar value of ca 10 electrons over a (Mo3S11) cluster was determined (Figs 6A, B) When the deposited film got thicker, e.g after longer deposition time, the number of electrons involved was higher to generate the same (Mo3S11) cluster (Figs 6C-F) This observation can be explained by the fact that MoSx is not a good conductor Thus, a thick layer of MoSx may inhibit the electron transfer process A similar phenomenon was also observed for the case of MoSx films grown via an electrochemical oxidation of [Mo3S13]2clusters [6] Thus, based on the available data, we conclude that the MoSx thin film is grown from the [MoS4]2- solution via a 10-electron oxidation process during the early stages of deposition when the MoSx film is not too thick In other words, each Mo3S11 unit comprising the MoSx film is Fig 5 EQCM EQCM analysis analysis conducted conducted on mM [MoS via4]a2- 10-electron oxidation This result is different Fig onan anAu Au QCM QCM electrode electrodeheld heldin a 1created 2solution at 0.33 NHE (A) of NHE the mass the electrodecompared as functionwith of that reported by Hu, et al [3, 7] where a in a mM [MoSV4]vs solution at evolution 0.33 V vs (A)ofevolution deposition (B,electrode C, D) plots electrode mass increment the amount of of the masstime; of the as the function of deposition time; against (B, 2-electron oxidation process was proposed (eq 1) [3, 7] electrons involved for different deposition periods C, D) plots the electrode mass increment against the amount of The 10-electron oxidation process is given below: electrons involved for different deposition periods At a higher applied potential, namely 0.36 V and 0.41 V vs NHE, we observed the (eq 2) 3[MoS4]2- → Mo3S11 + 1/8S8 + 10 esame phenomenon and a similar value of ca 10 electrons over a (Mo3S11) cluster was determined (Figs 6A, B) When the deposited film got thicker, e.g after longer mechanism for the growth of MoS film Proposed x deposition time, the number of electrons involved was higher to generate the same We MoS propose a mechanism for the growth of the (Mo3S11)n (Mo3S11) cluster (Figs 6C-F) This observation can be explained by the fact that is x not a good conductor Thus, a thick layer of MoSx may inhibit the electron transfer polymer, namely the MoSx film, via the electrochemical process A similar phenomenon was also observed for the case of MoSx films grown via oxidation of [MoS4]2- Our primary focus is on the an electrochemical oxidation of [Mo3S13]2- clusters [6] mechanism behind the construction of a [Mo3S11] skeleton by assembling three [MoS4]2- fragment species through a 10-electron oxidation process Firstly, a [MoS4]2- molecule adsorbs to the electrode surface and loses electron to create a Au-S covalent bond (Fig 7A) Two other [MoS4]2molecules approach (Fig 7B) and remove electrons, which create three (S-S)2- ligands as well as a Mo3-Sapical mode (Fig 7C) Actually, each (S-S)2- is generated from two S2- ligands via a two-electron reaction: 2S2- → (S-S)2- + 2e- (eq 3) MoVI + 2e- → MoIV (eq 4) Here, the [Mo3S7] skeleton grafted onto the electrode surface via a sulfide covalent bond is readily (Fig 7D) In this [Mo3S7] species, two Mo atoms are bound to two terminal S2- ligands These S2- ligands could also be oxidized via a two-electron reaction, creating the terminal (S-S)2- ligand This oxidation can also occur in parallel with the reduction of MoVI into MoIV (eq 4) Alternatively, a S2- ligand is oxidized by a two-electron process producing elemental sulfur and leaving a coordination vacancy on the Mo atom (Fig 7E) Indeed, the presence of an elemental Fig of electrode mass against the amount electronsofinvolved (A) MoSx film Fig.6.6.Plot Plot of electrode mass against theofamount electrons sulfur impurity in the MoSx film has been discussed grown at 0.36 V MoS and (B)film 0.41grown V vs NHE for a deposition at 0.36 V and (B)period 0.41ofV500-1,000 vs NHEs (C, D, E, involved (A) x elsewhere [9, 10] This vacancy then acts as an anchoring F) at 0.41 Vof vs 500-1,000 NHE for longer x film grown period forMoS a deposition s deposition (C, D, E,periods F) MoSx film grown at 0.41 V vs NHE for longer deposition periods position to host a new [MoS4]2- molecule At this step, a Thus, based on the available data, we conclude that the MoSx thin film is grown from the [MoS4]2- solution via a 10-electron oxidation process during the early stages of deposition when the MoSx film is not too thick In other words, each Mo3S11 unit comprising the MoSx film is created via a 10-electron oxidation This result is different Vietnam JournalbyofHu, Science, compared with that reported et al [3, 7] where a 2-electron oxidation process was DECEMBER 2019 • Vol.61 Number 12 (eq.Technology proposed 1) [3, 7] The process is given below: and10-electron Engineeringoxidation Mo atom (Fig 7E) Indeed, the presence of an elemental sulfur impurity in the MoSx film has been discussed elsewhere [9, 10] This vacancy then acts as an anchoring position to host a new [MoS4]2- molecule At this step, a new (S-S)2- ligand is generated after removing two more electrons (Fig 7F) The newly [MoS4]- species grafted on the [Mo3S7] cluster now continues its reaction in the same manner as the [MoS4]- grafted on the Au electrode described above (Fig 7G) Through such a reaction sequence, the (Mo3S11)n polymer is grown on the Au electrode surface via a 10-electron oxidation process as determined experimentally in this work Physical Sciences | Physics REFERENCES [1] D.E López, Z Lou, N.V Rees (2019), “Benchmarking the activity, stability, and inherent electrochemistry of amorphous molybdenum sulfide for hydrogen production”, Adv Energy Mater., 9, 1802614, Doi: 10.1002/ aenm.201802614 [2] F Xi, P Bogdanoff, K Harbauer, P Plate, C Höhn, J Rappich, B Wang, X Han, R van de Krol, S Fiechter (2019), “Structural transformation identification of sputtered amorphous MoSx as an efficient hydrogen-evolving catalyst during electrochemical activation”, ACS Catal., 93, 2368, Doi: 10.1021/acscatal.8b04884 ]2-]2Fig.7.7.Proposed Proposedmechanism mechanismofofelectro-oxidation electro-oxidation synthesis synthesis of of a-MoS a-MoSxxfrom Fig from[MoS [MoS 4 precursor precursor Conclusions [3] D Merki, S Fierro, H Vrubel, X Hu (2011), “Amorphous molybdenum sulfide films as catalysts for electrochemical hydrogen production in water”, Chemical Science, 2(7), pp.1262-1267 22- more new ligand is generated oxidation after removing two [4] Phong D Tran, T.V Tran, M Orio, S To(S-S) conclude, the electrochemical of a [MoS 4] solution in neutral pH can electrons (Fig 7F) The newlysulfide [MoS(MoS ] species grafted on Torelli, Q.D Truong, K Nayuki, Y Sasaki, S.Y Chiam, R Yi, I grow an amorphous molybdenum ) thin film, which is constructed of x the [Mo3S7] cluster now continues its reaction in the same Honma, J Barber, V Artero (2016), “Coordination polymer structure manner as the [MoS4]- grafted on the Au electrode described and revisited hydrogen evolution catalytic mechanism for amorphous above (Fig 7G) Through such a reaction sequence, the molybdenum sulfide”, Nature Materials, 15, pp.640-646 (Mo3S11)n polymer is grown on the Au electrode surface via a [5] O Mabayoje, Y Liu, M Wang, A Shoola, A.M Ebrahim, A.I 10-electron oxidation process as determined experimentally Frenkel, C.B Mullins (2019), “Electrodeposition of MoSx hydrogen in this work evolution catalysts from sulfur-rich precursors”, ACS Appl Mater Interfaces, 11(36), pp.32879-32886, Doi: 10.1021/acsami.9b07277 Conclusions To conclude, the electrochemical oxidation of a [MoS4]2solution in neutral pH can grow an amorphous molybdenum sulfide (MoSx) thin film, which is constructed of [Mo3S13]2building blocks Employing an electrochemical quartz crystal microbalance (EQCM) analysis, we revealed that the [MoS4]2- molecule goes through a 10-electron oxidation process to create the (Mo3S11) structure unit and subsequently the (Mo3S11)n polymer A mechanism has been proposed to describe the elemental steps of such a 10-electron oxidation process This work enriches the current knowledge of the formation, structure, and attractive redox property of the MoSx ACKNOWLEDGEMENTS This work is supported by Graduated University of Science and Technology (GUST - VAST) via project GUST STS.DT 2017 - HH11 We acknowledge Dr Nguyen Thu Loan and Prof Ung Thi Dieu Thuy (Institute of Materials Science - VAST) for experimental support The authors declare that there is no conflict of interest regarding the publication of this article [6] Tuan M Duong, Anh D Nguyen, Ly T Le, Loan T Nguyen, Phong D Tran (2019), “Insights into the electrochemical polymerization of [Mo3S13]2- generating amorphous molybdenum sulfide”, Chem Eur J., 25(60), pp.13676-13682 [7] H Vrubel, X Hu (2013), “Growth and activation of an amorphous molybdenum sulfide hydrogen evolving catalyst”, ACS Catalysis, 3(9), pp.2002-2011 [8] J.D Benck, Z Chen, L.Y Kuritzky, A.J Forman, T.F Jaramillo (2012), “Amorphous molybdenum sulfide catalysts for electrochemical hydrogen production: insights into the origin of their catalytic activity”, ACS Catal., 2(9), pp.1916-1923 [9] J Tan W Yang, Y Oh, H Lee, J Park, J Moon (2018), “Controlled electrodeposition of photoelectrochemically active amorphous MoSx Cocatalyst on Sb2Se3 photocathode”, ACS Appl Mater Interfaces, 10(13), pp.10898-10908, Doi: 10.1021/ acsami.8b00305 [10] X.J Chua, M Pumera (2018), “Molybdenum sulfide electrocatalysis is dramatically influenced by solvents used for its dispersions”, ACS Omega, 3(10), pp.14371-14379 DECEMBER 2019 • Vol.61 Number Vietnam Journal of Science, Technology and Engineering 13 ... potential polarization direction was set from the open circuit voltage towards the anodic potential with a potential scan rate of 50 mV/s In the first nm green laser excitation XPS analysis was conducted... by the applied oxidation potential From the mass increment analysis we are able to deduce the amount of MoSx deposited in moles under the assumption that the MoSx is a perfect clean Au QCM was... 6] and energies of Mo and S from XPS results are in consensus with the reported literatures [4, thetheobtained materials amorphous molybdenum 6] proving and proving obtained materials is is amorphous