Room Temperature Electrodeposition of Molybdenum Sulfide for Catalytic and Photoluminescence

thickness and demonstrates high photoluminescence activity with a decrease in the

evolution reaction.photoluminescence

while their basal planes remain

catalytically inert.7There have been many

to improve the catalytic activity

lms by synthesizing materials in

orientations to increase the number

planes.8à11

current synthesis methods for MoS2

high temperature,high pressure,

oxygen,and other extreme conditions.

temperature to 100°C,the redox peaks associated with molybdenum ions disappear.Reduction at potentials negative of à2V leads to the direct deposition of MoS 2.26The electrodeposited ?lms at various temperatures were further characterized by Raman spectroscopy (Figure 1B)to access crystallinity.The ?lms electrodeposited at room temperature showed no distinct peaks and suggest that the ?lms are amorphous on the GC electrode.With an increase in temperature,the Raman spectrum changes signi ?cantly.At 100°C distinct Raman peaks start to appear at 385and 404cm à1,corresponding to the E 12g and A 1g vibrational modes of crystalline MoS 2,respectively.28E 12g indicates planar vibration and A 1g is associated with the vibration of sul ?des in the out-of-plane direction.The Raman shift in the peaks compared to bulk MoS 2samples may be due the existence of only a few layer thickness ?lm of MoS 2.This phenomenon has been reported for exfoliated layers MoS 2.4,29,30

MoS 2formation was further examined and quanti-Figure 1.(A)Potentiodynamic deposition of MoS 2?lm over polished GC electrodes at di ?erent temperatures (a,room temperature (rt);b,50°C;and c,100°C)with 100μL of 1,4-butanedithiol and 100μL of molybdenum glycolate in 1mL of PP 13TFSI ionic liquid.Cycled between 0to à2.7V vs Pt (QRE)at scan rate of 100mV/s.(B)Raman spectrum of electrodeposited MoS 2at di ?erent temperatures.a,rt;b,50;c,100°C;and d,commercial bulk MoS 2for comparison.(C)Mo 3d XPS analysis of electrodeposited MoS x ?lms at 100°C.(D)S 2p XPS analysis of MoS 2.

of MoS2.To understand the mechanism of MoS2for-mation,depositions were performed as a function of temperature from rt to100°C.Figure2shows the SEM analysis of MoS x depositions at various temperatures. Room-temperature deposition leads to the forma-tion of nanoparticles of amorphous MoS x,which supported by Raman(Figure1Ba)analysis showing no distinct peaks.EDS analysis con?rms these particles constitute Mo and S(Figure S1,Supporting Information). With an increase in temperature,the nanoparticles of MoS x agglomerate and fuse together to form big clus-

ters of MoS x(Figure2B).Furthermore,with an increase in temperature at75and100°C(Figure2C,D),these fused particles form thin and smooth porous?lms of MoS x.The presence of a uniform layer suggests that layered deposition via sulfur complexation is analogous to the growth of larger particles via Oswald ripening, wherein smaller crystalline particles coalesce and form larger particles in a solvent mediated medium.26,33 These porous smooth?lms were characterized as MoS through Raman,XPS,and SEM-EDS analysis.

Di?erent concentrations of S precursor were added to Mo precursor in1:1,1:2,and1:3ratios,and deposi-tions were performed at100°C potentiodynamically from0toà2.7V vs Pt(QRE).Raman analysis of electrodeposited samples con?rms that the increase in concentration produces extra sulfur peak around 310cmà1,and this peak grows with an increase in concentration(Figure S2,Supporting Information). Only1:1ratio of S and Mo precursors forms stoichio-metric MoS2without any impurities.To understand the MoS2formation at100°C,chronoamperometric depositions were performed at various reduction potentials(à1.5,à2,andà2.7V vs Pt).Deposition at à2.7V shows the irregularity in the iàt curve con?rm-ing the growth of MoS2(Figure S3,Supporting Information).This irregular current response may be due to the complex nature of the process involving heterogeneous nucleation of MoS2from the ionic liquid mediated complex and also in?uenced by a resistive contact with the GC substrate.26,34However, use of metal surfaces for the deposition can show di?erent behavior.Metals exhibit partial Fermi level pinning with MoS2and results in a strong interaction.16 The growth of MoS2takes place after the potential à2V vs Pt con?rmed by the Raman spectrum(Figure S4, Supporting Information).The peaks at380and405cmà1 grow with an increase in potential andà2.7V vs Pt sharp peaks were obtained,con?rming the formation of crys-talline MoS2.Figure3shows the MoS2?lm deposited on GC atà2.7V vs Pt(QRE)at100°C.The SEM images show that the?lm is quite uniform with a distinct?ower-like morphology(Figure3A),and the enlarged image shows that the surface contains small spherical?ower-like particles(Figure3B,C).The agglomeration of MoS x par-ticles continued into a layered?ower-like morphology of up to2μm,which are con?rmed to be MoS x by EDS analysis(Figure3DàF).This agrees well with the forma-tion and growth mechanism of metal chalcogenides with the complexation of ionic liquid precursors.Pre-sumably,the loss of moisture in the Mo precursor by heating at100°C helps the reaction of S precursors.This is further supported by CVs at100°C,which show the loss of oxidation peaks as compared to the rt and50°C depositions that show predominant peaks atà1.25and 0.65V.The?ower-like morphology promotes more edge sites compared to a uniform?lm.These edge sites are

Figure2.SEM images of potentiodynamically electrode-

posited MoS2at various temperatures.(A)rt,(B)50,(C)75,

and(D)100°C.

Figure3.(A)SEM-EDS analysis of chronoamperometrically

atà2.7V vs Pt electrodeposited MoS2over glassy carbon

electrodes showing the formation of?ower morphology.

(B,C)Enlarged and closer view of MoS2?ower morphology.

(D)Elemental analysis of MoS2showing the presence of

Mo and S in the secondary electron(SE)image.(E)Elemental

analysis of showing the presence of Mo.(F)Elemental

analysis showing the presence of S.

essential for the hydrogen evolution reaction as it mimics electronically and structurally biologically formed genase https://www.360docs.net/doc/303040753.html,ually this MoS2?ower morphology been found to form at high temperature(1000 during sulfurization of MoO2.35Our synthesis method shown that these active sites can be obtained

electrodeposition process.This kind of?ower-like morphology has also been reported by chemical deposition growth of metal chalcogenides through boundary layer di?usion mediated process such as GeS.

evaluate the catalytic activity of electrodeposited

over GC,the HER was examined in0.5M H

Linear sweep voltammetry(LSV)was used to determine kinetic activity of the deposited?lms.Figure shows the results of LSVs compared to those conducted bare GC to?nd the reduction overpotential hydrogen evolution.From the polarization measure-ments,maximum current density of22mA/cm2 obtained atà0.6V vs RHE.These MoS2?lms display onset potential for hydrogen evolution with catalytic current densities of0.18and0.34mA/cm 100and200mV,respectively(Figure4B).This potential for HER is similar to the MoS2reported in literature.9,22,37The polarization curves remain stable after25scans con?rming the electrodeposited

are stable as HER catalysts(Figure S5,Supporting Information).A Tafel analysis was conducted on polarization curves to determine the catalytic activity electrochemical reduction process.The overpoten-

(V)observed during an experiment is given

atb ln j where a(V)is the Tafel constant,

decà1)is the Tafel slope and j(mA cmà2) current density.The Tafel slope is an inherent property catalyst determined by the rate limiting

HER reaction(Figure S6,Supporting Information).

linear portions of the Tafel curve were?t to the equation at the point of hydrogen evolution.The response was then determined to be106mV/decade corresponding to the slope of the linear response The low overpotential region shows a di

slope,which is possibly due to the potential dependent surface coverage of hydrogen and change in the activation barrier.Presumably, morphology increases the number of catalytically

edge sites for enhanced hydrogen evolution.

evolution by MoS2with the?ower-like morphology appears to proceed via a di?erent mechanism observed at single crystal or nanoparticulate

This Tafel slope is di?erent from those of crystals(55à60mV per decade)or MoS2nanoparticles

Figure4.(A)Hydrogen evolution reaction(HER)in

at the scan rate of2mV/s at GC(a)and at

electrodeposited over GC(b).(B)Onset potential of GC

MoS2over GC electrode(b).

5.(A)Fluorescence scanning confocal optical

image of electrodeposited MoS2at100°C

electrodes.(B)Accumulated spectra collected in the

uorescence imaging.(C)Bulk?uorescence spectrum

collected over the large area exposed sample in

mode for electrodeposited MoS2for600s

(b)atà2.7V vs Pt(QRE).