Synergistic-Effect-Controlled CoTe2/Carbon Nanotube HybridMaterial for Efficient Water OxidationTzu-Hsiang Lu,† Chih-Jung Chen,† Ying-Rui Lu,§,⊥ Chung-Li Dong,*,∥ and Ru-Shi Liu*,†,‡

†Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan‡Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University ofTechnology, Taipei 10608, Taiwan§National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan∥Department of Physics, Tamkang University, Tamsui 25137, Taiwan⊥Program for Science and Technology of Accelerator Light Source, National Chiao Tung University, Hsinchu 30010, Taiwan

*S Supporting Information

ABSTRACT: In anode, electrocatalytic water splitting involves oxygenevolution reaction (OER), which is a complex and sluggish reaction, andthus the efficiency to produce hydrogen is seriously limited by OER. Wereport that CoTe2 exhibits optimized OER activity for the first time.Multiwalled carbon nanotube (MWCNT) is utilized to support CoTe2 ingenerating a synergistic effect to enhance OER activity and improvestability by tuning different loading amounts of CoTe2 on CNT. In 1.0 MKOH, bare CoTe2 needed overpotential of 323 mV to produce 10 mA/cm2 with Tafel slope of 85.1 mV/dec, but CoTe2/carbon nanotube(CNT) with optimized loading amount of CoTe2 required only 291 mVto produce10 mA/cm2 with Tafel slope of 44.2 mV/dec. X-ray absorptionnear edge structure (XANES) was applied to prove that an electrontransfer from eg band of CoTe2 to CNT caused a synergistic effect. Thiselectron transfer modulated the bond strength of oxygen-related intermediate species on the surface of catalyst and optimizedOER performance. In situ XANES was used to compare CoTe2/CNT and pristine CoTe2 during OER. It proved the transitionstate of CoOOH more easily existed by adding CNT in hybrid material during OER to enhance the efficiency of OER. Moreover,bare CoTe2 is unstable under OER, but the CoTe2/CNT hybrid materials exhibited improved and exceptional durability by time-dependent potentiostatic electrochemical measurement for 24 h and continuous cyclic voltammetry for 1000 times. Our resultsuggests that this new OER electrocatalyst for OER can be applied in various water-splitting devices and can promote hydrogeneconomy.

■ INTRODUCTION

The energy crisis and environmental issues have been takenmore seriously at present. Most energy demands come fromfossil fuel, increasing the risk of air pollution and globalwarming because of the product after combustion.1 Hydrogenwith large mass storage and long storage time is considered aclean and promising energy carrier.2,3 Electrocatalytic watersplitting is a simple method to produce hydrogen withoutinvolving fossil fuel and producing greenhouse gases. Electro-catalytic water splitting involves two half reactions. One ishydrogen evolution reaction (HER) in cathode, 2H+ + 2e− →H2, and the other is oxygen evolution reaction (OER) in anode,2H2O → O2 + 4H+ + 4e−. However, OER is a complex andsluggish reaction with four-electron transfer that contains O−Hbond breaking and O−O bond formation.4 OER causes highpotential of electrocatalytic water splitting, and thus theefficiency to produce hydrogen is seriously limited by OER.A few studies in the past have demonstrated some noble-metalcatalysts such as RuO2 and IrO2, which can overcome slow

OER kinetics and can exhibit high activity.5,6 However, the lowstability, low abundance, and high cost of these catalysts limittheir commercial utilization. In recent years, Co as a 3dtransition metal has been developed to most popular non-noblemetal catalysts including metal oxides, hydro(oxy)oxides,phosphate, and perovskite to synthesize robust catalyst; thereason is that Co is Earth abundant and environmentalfriendly.7−10 Cobalt dichalcogenides as bifunctional catalystshave been proven as HER and OER active catalyst in muchresearch.11 According to a previous research by Shao-Horn etal., the ideal eg occupancy of 3d transition-metal cation in OERcatalyst should be close to unity.12 Xie et al. followed theprinciple of Shao-Horn’s principle in developing cubic andorthorhombic CoSe2 with t2g

6eg1 electronic configuration,

showing low overpotential and low Tafel slope.13 Meanwhile,

Received: October 3, 2016Revised: November 9, 2016Published: November 10, 2016

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Yu et al. reported cubic CoSe2 hybridized nitrogen-dopedgraphene, showing high activity and improved stability.14 Theyproposed that a synergistic effect caused by the electron transferfrom graphene to CoSe2 at the interface was found. This effectchanged the bond strength between CoSe2 and oxygen speciesimproving the OER kinetics. In general, the stability becomeshigher because of the conductivity of carbon materials. In ourprevious work, we have reported that CoTe2 is HER active anda robust catalyst.15 In the presented study, CoTe2 with t2g

6eg1

electronic configuration exhibits optimized OER activity.Multiwalled carbon nanotube (MWCNT) was utilized tosupport CoTe2 in generating synergistic effect to enhance OERactivity and improve stability.

■ EXPERIMENTAL SECTIONChemicals and Materials. MWCNT, sodium borohydride

(NaBH4), and ruthenium dioxide (RuO2) were purchased fromAldrich. Sodium nitrate (NaNO3), hydrogen peroxide (H2O2),hydrochloride acid (HCl), sulfuric acid (H2SO4), urea(CH4N2O), and potassium hydroxide (KOH) were purchasedfrom Sigma−Aldrich. Potassium permanganate (KMnO4) andcobalt nitrate hexahydrate (Co(NO3)2·6H2O) were purchasedfrom Acros. Ammonium fluoride (NH4F) was purchased fromMerck. Te powder was purchased from Alfa Aesar.Synthesis of Oxidized MWCNTs. MWCNTs were

oxidized by modified Hummers’ method.16,17 About 1 g ofMWCNT was calcined for 1 h at 500 °C in a furnace followingthe removal of metal residues by 70 mL of HCl (10%). Theproduct was filtered, washed with alcohol and deionized water,and dried in an oven at 50 °C. Then, 23 mL of H2SO4 (98%)was added to the MWCNT solution in a 250 mL round-bottomed flask, and the mixture was stirred at roomtemperature overnight. The solution was transferred to an oilbath, the temperature was raised to 40 °C, and 350 mg ofNaNO3 was added. After slowly adding 1 g of KMnO4 andmaintaining the temperature below the reaction temperature of45 °C, the solution was stirred for 30 min at 40 °C, and 3 mLof deionized water was added. Then, 3 mL of deionized waterwas added again after 5 min until 40 mL of deionized water wasobtained. After 15 min, oil bath was removed and 140 mL ofdeionized water and 10 mL of H2O2 (30%) were added.MWCNTs were collected, and HCl (5%), followed bydeionized water, was used to wash the sample. Finally, thesolution was oven-dried at 50 °C.Synthesis of Co(OH)F/CNT. About 20 mg of oxidized

MWCNTs, 0.1455 g of Co(NO3)2·6H2O (0.5 mmol), 0.0556 gof NH4F (1.5 mmol), 0.1502 g of CH4N2O (2.5 mmol), and 40mL of deionized water were added to a beaker, and the mixturewas sonicated for 5 min. The solution was transferred to a 125mL Teflon bottle, which was placed in a stainless-steelautoclave, and hydrothermal method was carried out at 120°C for 12 h.18 After removing the alcohol and washing it withdeionized water, the compound was oven-dried at 50 °C. Ifonly Co(OH)F was synthesized; then, the above-mentionedsteps were conducted using 0.2910 g of Co(NO3)2·6H2O (1.0mmol), 0.1112 g of NH4F (3.0 mmol), 0.3004 g of urea (5.0mmol), and 40 mL of deionized water. To synthesize differentproportions of Co(OH)F/CNT, Co(NO3)2·6H2O, NH4F, urea(mole ratio = 1:3:5), and 20 mg of oxidized MWCNTs wereusedSynthesis of CoTe2/CNT. About 20 mg of Co(OH)F/

CNTs and 37.62 mg of Te powder with 40 mL of deionizedwater were added to a beaker, and the mixture was sonicated

for 30 min. The solution was transferred to a 125 mL Teflonbottle, which was placed in a stainless-steel autoclave, and 33.77g of NaBH4 was added before performing hydrothermalmethod at 180 °C for 15 h.19,20 If we used 20 mg ofCo(OH)F/CNT synthesized by 0.75 or 1.00 mmol Co(NO3)2·6H2O, Te powder and NaBH4 were 41.52 mg of Te and 37.28mg of NaBH4 or 44.17 mg of Te and 39.65 mg of NaBH4,respectively. The solution was washed with deionized water andalcohol and dried in an oven at 50 °C. If only CoTe2 wassynthesized, then the steps described above was used by 20 mgof Co(OH)F (2.1 mmol), 53.47 mg of Te (4.2 mmol), and48.00 mg of NaBH4 (1.26 mmol).

Decoration of Catalyst on Glassy Carbon Electrode.To measure the electrochemical properties of the catalyst usingan electrochemical analyzer, the slurry must be dropped toglassy carbon electrodes. First, 20 mg of the catalyst powder,1.5 mL of ethanol, and 0.5 mL of Nafion (0.5%) in a centrifugetube were obtained. Thus the solution was sonicated for 1 h todisperse the slurry. The slurry of 10 μL was dropped on glassycarbon electrode with a diameter of 5.61 mm by utilizing apipet. The slurry covered the surface of the glassy carbonelectrode at room temperature. Finally, the catalyst powderadhered to the surface of the glassy carbon electrode.

Characterization of Materials. X-ray diffraction (XRD;Bruker D2 PHASER) using Cu Kα as the source radiation wascarried out to determine the crystallinity and the crystalstructure. The morphology of the samples was investigatedusing scanning electron microscopy (SEM; JEOL JSM-6700F)and transmission electron microscopy (TEM; JEOL, Japan).The atomic ratio of our samples was determined by energy-dispersive spectrometry (EDS) with SEM. The X-rayabsorption near edge structure (XANES) of Co K- and L-edge was conducted at the beamline of 17C1 and 20A1 fromthe National Synchrotron Radiation Research Center(NSRRC) in Hsinchu City, Taiwan.

Electrochemical Measurement. All electrochemical anal-yses were carried out in a three-electrode system by a RRDE asa working electrode with an electrochemical instrument (CHI760D). The slurry of CoTe2/CNT was dropped on the RRDE.The analysis was conducted in 1.0 M KOH(aq) at roomtemperature. The three-electrode system contained a workingelectrode of RRDE, counter electrode of Pt-foil, and referenceelectrode of Hg/HgO. In 1.0 M KOH(aq), the potentialchanged to be at reversible hydrogen electrode (RHE) wascalculated by the following equation: ERHE = EHg/HgO + 0.9316V. The voltage we mentioned was at RHE. Linear sweepvoltammetry (LSV) was carried out at 1.0 to 1.7 V at a scanningrate of 10 mV/s. The linear portion at the low overpotential inthe Tafel plot was fitted to the Tafel equation, and the Tafelslope was shown. All data in LSV were corrected by iR loss,which is mainly from the electrolyte between the working andreference electrode based on Rs on electrochemical impedancespectroscopy (EIS) at 1.45 V. To compare the active surfaceareas, cyclic voltammetry (CV) was applied from 0.90 to 0.96 Vwith scanning rates of 2, 4, 6, 8, and 10 mV/s. ΔJ was calculatedat 0.93 V and plotted against the scan rate. The slope of the linewas equal to twice the capacitance of the double layer (Cdl).The long-term stability was measured by time-dependentpotentiostatic electrochemical measurement at 1.52 V andcontinuous CV at a scanning rate of 10 mV/s for 1000 cyclesbetween 1.10 and 1.70 V.

In Situ XANES of Co K-Edge. In situ Co K-edge X-rayabsorption spectroscopy (XAS) spectra were carried out with

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the beamline BL17C of the NSRRC.21 Scheme 1 presents aschematic of the substrate−film−sample holder assembly in

contact with the 1 M KOH electrolyte. The sample holder wasmade of PVC with outer dimensions of 6 cm × 5.5 cm × 0.8cm and was used to collect the total fluorescence yield (TFY).The samples were deposited onto the Au-coated Si3N4membrane window and were used as the working electrodein a three-electrode setup. In addition, the saturated Ag/AgClelectrode and Pt electrode were the reference and counterelectrodes. The Si3N4 membrane window was transparent forX-rays; then, the membrane was attached to a PVC supportingframe by an Araldite adhesive that creates a tight seal.In Situ XANES of Co L-Edge. In situ soft X-ray absorption

experiments were performed at the BL20A1 of the NSRRC.22

The chemical cell of the substrate−film−cell assembly incontact with the 1 M KOH electrolyte was used to collect theTFY with base pressure of 5 × 10−8 Torr. The cell wasseparated from UHV by the Au-coated Si3N4 membranewindow. The membrane area was chosen to be 1.0 mm × 1.0mm, and the thickness was 100 nm. The thin film of ourmaterial with a 5 nm thick adhesive Au layer was depositedonto the cell side of the membrane and was used as the workingelectrode in a three-electrode setup. A Pt wire used as thecounter electrode and a Pt wire used as the reference electrodewere inserted in the cell through small holes on the sides.

■ RESULTS AND DISCUSSIONFabrication and Characterization. In the current work,

commercial MWCNT was functionalized through a modifiedHummers’ method.16,17 Oxidized CNT had stronger inter-action with metal ions in the subsequent hydrothermal reactionfor synthesizing Co(OH)F/CNT, which was the precursor forpreparing CoTe2/CNT electrocatalyst.18 The powder wasapplied to substitute the anions of Co(OH)F/CNT forproducing CoTe2/CNT material by hydrothermal method,19,20

and excessive sodium borohydride also reduced oxidized CNTto CNT. Various loading amounts of CoTe2 were prepared bytuning different concentration ratios between cobalt nitrate andoxidized CNT. For convenience, we abbreviated CNT withdifferent CoTe2 loading amounts as “CoTe2/CNT-X,” in whichX represents the millimoles of cobalt nitrate. The TEM imagesof CoTe2/CNT with different loading amounts of CoTe2 andCoTe2 reveal the morphology (Figure 1a−d). The morphologyof bare CoTe2 is identified as nanowire (NW), and thediameter of CoTe2 NW is ∼70 nm, as shown in Figure 1d.Oxidized CNT, which contains more functional groups ofoxygen on CNT, can interact with Co ion in the synthesis of

Co(OH)F/CNT to cause stronger chemical adsorption.Accordingly, the CoTe2 materials became separated nano-particles. The diameters of CoTe2/CNT hybrid materials arearound 40−50 nm. The XRD pattern reveals the diffractionpeaks of all CoTe2/CNT and CoTe2 NW (Figure 2). The

strong peaks of CoTe2 NW were observed at 21.8, 28.3, 31.7,32.9, 33.6, 43.5, 46.4, and 58.2°. These peaks are characteristicpeaks of orthorhombic marcasite CoTe2 (JCPDS-89-2091).The diffraction peaks of all CoTe2/CNT at 21.8 and 28.3° aredifficult to be observed because a broad peak that originatedfrom CNT in the orange region exists. In our preparation, CNTis oxidized by modified Hummers’ method. Then, the oxidizedCNT was reduced to CNT. After these processes, the phase ofCNT becoming more amorphous caused broader peak than theoriginal peak of CNT only by calcination and washing with acidsolution in the Supporting Information (Figure S1). Moreover,as loading amount of CoTe2 decreases, the diffraction peak ofCNT becomes more obvious. The lowest amount of cobaltnitrate we used to prepare CoTe2/CNT is 0.50 mmol. If <0.50mmol cobalt nitrate is used, then the peak of Te side productwill be observed in XRD. The compositions of CoTe2/CNTwith different loading amounts of CoTe2 and CoTe2 NW weredetermined by energy-dispersive X-ray (EDX) in theSupporting Information (Figure S2a−d). EDX reveals thesignal of Co and Te in all samples and the additional signal of Cin CoTe2/CNT with different loading amounts of CoTe2. Thepeak of high intensity at 1.8 eV is from Si wafer, which was used

Scheme 1. Scheme of the Substrate−Film−Sample HolderAssembled in Contact with the 1 M KOH Electrolyte

Figure 1. TEM images of (a) CoTe2/CNT-0.50, (b) CoTe2/CNT-0.75, (c) CoTe2/CNT-1.00, and (d) CoTe2 NW.

Figure 2. XRD patterns of CoTe2/CNT-0.50, CoTe2/CNT-0.75,CoTe2/CNT-1.00, and CoTe2 NW.

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as substrate. As the loading amount of CoTe2 rises, theintensity of C compared with the intensity of Co becomessignificantly lower. The atomic ratio of C was also lower. Theresult demonstrates that the loading amount of CoTe2 is higherwhen higher concentration of cobalt nitrate with same amountof oxidized CNT is used to synthesize Co(OH)F/CNT. Thevalence states of Co ion in CoTe2/CNT with different loadingamounts of CoTe2 and CoTe2 NW were investigated bymeasuring the Co K-edge XANES (Figure 3). Co foil and CoO

with chemical state of 0 and 2+ charge were used as standardcompound. The oxidation state of Co ion in CoTe2 was 2+because of the Te2

2− dimer. The Co K-edge jump of our foursamples is between the absorption edge of Co foil and CoO. Achemical negative shift attributed to the lower electronegativityof Te compared with that of O. The phenomenon can also beobserved in the Co K-edge jump of CoS2.

23 The hybridmaterials containing CNT show edge jump at more positiveenergy as compared with pristine CoTe2 NWs. Therefore, acharge transfer at the interface between CoTe2 and CNTcaused a more significant positive absorption energy shiftobserved on Co of CoTe2/CNT. This effect will be discussedlater.Electrochemical Measurement. All electrochemical

measurements of OER were conducted in 1.0 M KOH(aq).The IR-corrected polarization curve and Tafel plot for CoTe2/CNT with different loading amounts of CoTe2 and CoTe2 NWwere obtained by using LSV (Figure 4a,b). In this study, RuO2was prepared to compare its electrochemical performance withthat of samples. RuO2 exhibited low overpotential of only 239mV to produce 10 mA/cm2, but its high Tafel slope of 65.3mV/dec caused the current density to rise slowly at highervoltage. The overpotential of bare CoTe2 NW was 323 mV togenerate 10 mA/cm2. After integrating CoTe2 on CNT, theoverpotential of CoTe2/CNT-0.50 for producing 10 mA/cm

2 isimproved to 291 mV, but the overpotential of CoTe2/CNT-0.75 and CoTe2/CNT-1.00 is 301 and 313. This result revealsthat as the loading amount of CoTe2 increases the overpotentialfor producing 10 mA/cm2 becomes lower. CNT reveals noobvious OER activity before 1.6 V, and thus CoTe2 was a maincatalyst in our hybrid materials. CNT should serve as asynergist to enhance the activity of CoTe2. In the Tafel plot

(Figure 4b), the slope of these samples becomes lower with thedecrease in loading amount of CoTe2, indicating fast OERkinetics. In the two electrochemical measurements fordetermining the OER activity, the synergistic effect at theinterface between CoTe2 and CNT induced the differences ofactivity in our samples, not the high conductance of CNT. Thereason is that the difference of activity was not only foundbetween hybrid materials and CoTe2 NW but also observedamong hybrid materials. The synergistic effect should be relatedto the phenomenon of charge transfer observed in Co K-edgeXANES. Notably, different strengths of synergistic effect causedifferent OER performance. The EIS can be used as anelectrical model to investigate the OER efficiency in theSupporting Information (Figure S3). The Rs represents theseries resistance, which is from the resistance of component insystem and electrolyte between working and referenceelectrode. Rs was defined as the onset point of the semicirclein EIS spectra. Given that all electrochemical measurements areconducted in 1.0 M KOH(aq), the onset points should beclose. Rct represents the charge-transfer resistance, whichdepends on the capability of passing electron at the interfacebetween catalyst and electrolyte. Rct leads to different semicirclediameters in EIS spectra. CoTe2/CNT-0.50 shows the smallestsemicircular diameter, indicating that this sample is the mostefficient electrocatalyst because of the best capability oftransferring electron between electrode and electrolyte.Notably, the semicircular diameter became larger with theincrease in loading amount of CoTe2 on CNT. Highconductance of CNT as a physical property is not the reasonbehind the tendency in EIS because this property does notaffect the capability of passing electron at the interface betweencatalyst and electrolyte. This property should only affect Rs, andthus the change of Rct in EIS can prove the presence of thesynergistic effect caused by the charge transfer shown in Co K-edge XANES. Moreover, the strongest effect is observed onCoTe2/CNT-0.50 because of its lowest semicircular diameter.The active-site surface area can be estimated by measuring CVto calculate double-layer capacitance (Cdl) at the interfacebetween solid and liquid phases.24 The Cdl values of CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00, and CoTe2NW are 25.61, 24.52, 24.40, and 15.99 mF/cm2 in theSupporting Information (Figures S4 and S5). Moreover, we canfind that the maximum of capacitance contribution of CNT isonly 4.21 mF/cm2, which is still much lower than thecapacitance of CoTe2/CNT in the Supporting Information(Figure S6). Because the amount of CNT in CoTe2/CNT isless than the sample of CNT, the capacitance contribution ofCNT in CoTe2/CNT should be <4.21 mF/cm2. Therefore, the

Figure 3. XANES spectra of Co K-edge for CoTe2/CNT-0.50,CoTe2/CNT-0.75, CoTe2/CNT-1.00, CoTe2 NW, and the oxidestandards containing Co.

Figure 4. (a) Polarization curves show the performance of CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00, and CoTe2 NWcompared with RuO2 and CNT. (b) Tafel plots show the Tafel slopesthat exhibit OER kinetics for CoTe2/CNT-0.50, CoTe2/CNT-0.75,CoTe2/CNT-1.00, and CoTe2 NW compared with RuO2.

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capacitance of CoTe2/CNT is contributed from CoTe2 mainly,and the Cdl of CoTe2 integrating on CNT is higher than that ofpristine CoTe2 NW. This result shows that CoTe2 nano-particles of hybrid material generate more active sites on thesurface, as compared with bare CoTe2 NW. This phenomenonis the other reason why the OER activity of hybrid materialscontaining CNT is higher than that of CoTe2 NW. However,the difference between the Cdl values of CoTe2/CNT withdifferent loading amounts of CoTe2 was slight. Therefore, thevarying OER active surface area of these catalysts was not thedominant factor that affected the activity discrepancy.X-ray Absorption Near Edge Structure. The electronic

configuration of Co ion in CoTe2 is t2g6eg

1, and the Fermi levelis at eg band, and thus an unoccupied state exists in eg band in3d orbital. XANES of Co L-edge absorption was the excitationfrom 2p to 3d orbital. Therefore, Co L-edge spectra was appliedto investigate the charge transfer that causes the synergisticeffect of CoTe2 hybridized with CNT to exhibit different OERperformances (Figure 5a). Two peaks in the Co L-edge

spectrum of CoTe2 corresponded to L2 and L3 near 795 and779 eV. The intensity of L3 absorption was enlarged, and theintensity of L3 absorption is higher with the decreasing loadingamount of CoTe2 on CNT (Figure 5b). If the intensity ishigher, it indicates that the possibility of the excitation from 2porbital to 3d unoccupied states is higher. CoTe2/CNT-0.50shows the highest intensity of L3 accompanying strongestsynergistic effect because of the lowest loading amount ofCoTe2 on the surface of CNT and the lowest Rct. Therefore,stronger synergistic effect with the decrease in loading of CoTe2was observed as the intensity of L3 became higher. This resultreveals that the charge-transfer direction of electrons is fromCoTe2 to CNT. This parameter is the main cause of thedifferent OER performances of CoTe2/CNT compositecatalysts and bare CoTe2 NW. As the effect of charge transferbecomes stronger, there more vacancies at the eg band in the 3dorbital of CoTe2 are observed. Given that the orbitals in the egband of transition-metal ions are the active sites for interactingwith anion adsorbate to generate σ-bonding, the bond strengthof oxygen-related intermediate species is modulated by electrondonation from CoTe2 to CNT.12 This phenomenon causes thebonding between the Co ion in CoTe2 and oxygen-relatedintermediate species to become stronger when catalystsundergo OER. With stronger bonding to strengthen thesurface−oxygen interaction, the kinetics and activity of OERwill be optimized. Meanwhile, the result of CoTe2 in Co L-edgeXANES can correspond to the result in Co K-edge XANES.The charge-transfer direction of electrons is from CoTe2 toCNT. Thus the chemical state of Co ion in hybrid materialsshould be more positive than that of bare CoTe2 NW.

In Situ X-ray Absorption Near Edge Structure. Torealize the function of charge transfer by CNT in our hybridmaterial during OER, the oxidation states of CoTe2 NW andCoTe2/CNT-0.75 were monitored by in situ XANES in 1.0 MKOH (Figure 6a,b and Figure S7a,b of the Supporting

Information).21,22 The in situ XANES of Co K-edge and L-edge spectra was conducted with applied bias from 1.00 to 1.80V and 1.00 to 1.60 V, respectively. The XANES of Co K-edgefor CoTe2/CNT-0.75 in Figure 5a exhibited an obviouslypositive change of energy of white line from 7727.4 to 7730.3eV with applied bias from 1.00 to 1.80 V, so the valence of Coin CoTe2/CNT-0.75 was changed during OER. The XANES ofCo L-edge for CoTe2/CNT-0.75 in Figure 5b also showed anincrease in energy of L3 absorption from 778.8 to 780.7 eV withapplied bias from 1.00 to 1.60 V. The absorption of Co2+ andCo3+ is near 778.8 and 780.7 eV, so the oxidation state of Co inCoTe2/CNT-0.75 was changed from Co2+ to Co3+ duringOER, which was accompanied by a formation of transition stateof Co(III)OOH.25 There was not a distinct change of energy inthe XANES of Co K-edge and L-edge for CoTe2 NW in theSupporting Information (Figure S7a,b), so the charge-transfereffect by CNT in our hybrid material can be an advantageousfactor of generating transition state of CoOOH to enhance theefficiency of water oxidation.

Stability. Apart from high OER activity, the long-termstability is also an important parameter to consider indeveloping electrocatalysts for electrocatalytic water splitting.A time-dependent potentiostatic electrochemical measurementto detect durability at 1.52 V is executed within 24 h, in whichCoTe2/CNT-0.50 exhibits ∼10 mA/cm2 (Figure 7). The result

Figure 5. (a) XANES spectra of Co L-edge for CoTe2/CNT-0.50,CoTe2/CNT-0.75, CoTe2/CNT-1.00, and CoTe2 NW. (b) EnlargedXANES spectra of Co L3 absorption.

Figure 6. (a) In situ XANES spectra of Co K-edge for CoTe2/CNT-0.75. (b) In situ XANES spectra of Co L-edge for CoTe2/CNT-0.75.

Figure 7. Time dependence of anodic current density of CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00, and CoTe2 NW.

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depicts that the current density of hybrid materials containingCNT does not obviously decay but the current density ofCoTe2 NW without CNT decays around 45% The stability ofCoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00, andCoTe2 NW was also examined by continuous CV between 1.10and 1.70 V at a scanning rate of 10 mV in the SupportingInformation (Figure S8). After 1000 cycles, CoTe2/CNT-0.50,CoTe2/CNT-0.75, and CoTe2/CNT-1.00 only need 21, 28,and 31 mV more to drive 10 mA/cm2 in polarization curves butCoTe2 NW needs 69 mV more. This finding results from theassistance of CNT material, which improves electronconductivity of CoTe2 catalysts. When conductance of CoTe2was enhanced through integrating on CNT, charge accumulat-ing on catalysts was reduced to further increase its OERstability.

■ CONCLUSIONS

In this work, we have successfully synthesized CoTe2/CNThybrid materials functionalized as OER catalysts by using thehydrothermal method. The optimum loading amount of CoTe2on CNT was achieved by CoTe2/CNT-0.50. This electro-catalyst exhibited outstanding OER activity with a smalloverpotential at 0.291 V, generating current density of 10mV/cm2 and a small Tafel slope of 44.2 mV/dec. AlthoughCoTe2 is unstable under OER, the CoTe2/CNT hybridmaterials exhibited improved and exceptional durability bytime-dependent potentiostatic electrochemical measurementfor 24 h and continuous CV characterization for 1000 times.The synergistic effect between CoTe2 and CNT is thedominant factor that boosts the OER performance. Co L-edge XANES was applied to prove that a electron transfer fromeg band of CoTe2 to CNT exists. This electron transfermodulated the bond strength of oxygen-related intermediatespecies on the surface of catalyst and optimize OERperformance. In situ XANES was used to compare CoTe2/CNT and pristine CoTe2 during OER. It proved the transitionstate of CoOOH was easier to exist by adding CNT in hybridmaterial during OER to enhance efficiency of OER. The designof new electrocatalyst for OER can be applied in various water-splitting devices and promote hydrogen economy.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.6b10000.

XRD pattern of CNT only by calcination and washingwith acid solution, EDS of our materials showing thepresence of Co, Te, and C as element, EIS of ourmaterials, CV of our materials, the plots showing thedouble-layer capacitance (Cdl), in situ XANES spectra ofCo K-edge and L-edge for CoTe2 NW, and polarizationcurves for our materials before and after 1000 cycles bycontinuous CV between 1.10 and 1.70 V. (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*C.-L.D.: E-mail: cldong@mail.tku.edu.tw.*R.-S.L.: E-mail: rsliu@ntu.edu.tw.

ORCIDRu-Shi Liu: 0000-0002-1291-9052

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Ministry of Science andTechnology of Taiwan (Contract No. MOST 104-2113-M-002-012-MY3), Academia Sinica (Contract No. AS-103-TPA06),and National Taiwan University (104R7563-3). We appreciateMs. Chia-Ying Chien, who helped us to perform TEM at theInstrumentation Center in Nation Taiwan University.

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