Carbon Nanotube Hybrid Material

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Synergistic-Effect-Controlled CoTe/Carbon Nanotube Hybrid Material for Efficient Water Oxidation Tzu-Hsiang Lu, Chih-Jung Chen, Ying-Rui Lu, Chung-Li Dong, and Ru-Shi Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10000 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Synergistic-Effect-Controlled CoTe2/Carbon Nanotube Hybrid Material for Efficient Water Oxidation Tzu-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

Chemistry, National Taiwan University, Taipei 10617, Taiwan ‡

Department

of

Mechanical

Engineering

and

Graduate

Institute

of

Manufacturing

Technology, National Taipei University of Technology, 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

ACS Paragon Plus Environment

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ABSTRACT

In anode, electrocatalytic water splitting involves oxygen evolution (OER) which is a complex and sluggish reaction, and thus, the efficiency to produce hydrogen is seriously limited by OER. In this communication, we are reporting that CoTe2 exhibits optimized OER activity for the first time. Multi-walled carbon nanotube (MWCNT) is utilized to support CoTe2 in generating synergistic effect to enhance OER activity and improve stability by tuning different loading amount of CoTe2 on CNT. In 1.0 M KOH, bare CoTe2 needed overpotential of 323 mV to produce 10 mA/cm2 with Tafel slope of 85.1 mV/dec but of CoTe2/carbon nanotube (CNT) with optimized loading amount of CoTe2 required only 291 mV to producing 10 mA/cm2 with Tafel slope of 44.2 mV/dec. X-ray absorption near edge structure (XANES) was applied to prove that an electron transfer from eg band of CoTe2 to CNT caused a synergistic effect. This electron transfer modulated the bond strength of oxygen-related intermediate species on the surface of catalyst and optimized OER performance. In-situ XANES was used to compare CoTe2/CNT and pristine CoTe2 during oxygen evolution reaction (OER). It proved the transition state of CoOOH was easier to exist by adding CNT in hybrid material during OER to enhance 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 result suggests that this new OER electrocatalyst for OER can be applied in various water-splitting devices and can promote hydrogen economy.


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INTRODUCTION The energy crisis and environmental issues have been taken more seriously at present. Most energy demands come from fossil fuel, increasing the risk of air pollution and global warming because of the product after combustion.1 Hydrogen with large mass storage and long storage time is considered a clean and promising energy carrier.2,3 Electrocatalytic water splitting is a simple method to produce hydrogen without involving fossil fuel and producing greenhouse gases. Electrocatalytic water splitting involves two half reaction. One is hydrogen 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 and sluggish reaction with fourelectron transfer that contains O-H bond breaking and O-O bond formation.4 OER causes high potential of electrocatalytic water splitting, and thus, the efficiency to produce hydrogen is seriously limited by OER. A few studies in the past have demonstrated some noble metal catalysts such as RuO2 and IrO2, which can overcome slow OER kinetics and can exhibit high activity. 5,6 However, the low stability, low abundance, and high cost of these catalysts limit their commercial utilization. In recent years, Co as a 3d transition metal has been developed to most popular non-noble metal catalysts including metal oxides, hydro(oxy)oxides, phosphate, and perovskite to synthesize robust catalyst; the reason is that Co is Earth abundant and environmental friendly.7-10 Cobalt dichalcogenides as bifunctional catalysts have been proven as HER and OER active catalyst in much research.11 According to a previous research by ShaoHorn et al., the ideal eg occupancy of 3d transition metal cation in OER catalyst should be close to unity.12 Xie et al. followed the principle of Shao-Horn’s principle in developing cubic and orthorhombic CoSe2 with t2g6eg1 electronic configuration, showing low overpotential and low


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Tafel slope.13 Meanwhile, Yu et al. reported cubic CoSe2 hybridized nitrogen-doped graphene, showing high activity and improved stability.14 They proposed that a synergistic effect caused by the electron transfer from graphene to CoSe2 at the interface was found. This effect changed the bond strength between CoSe2 and oxygen species improving the OER kinetics. In general, the stability becomes higher because of the conductivity of carbon materials. In our previous work, we have reported that CoTe2 is HER active and a robust catalyst.15 In the presented study, CoTe2 with t2g6eg1 electronic configuration exhibits optimized OER activity. Multi-walled carbon nanotube (MWCNT) was utilized to support CoTe2 in generating synergistic effect to enhance OER activity and improve stability. EXPERIMENTAL Chemicals and Materials Multi-walled carbon nanotube, sodium borohydride (NaBH4), and ruthenium dioxide (RuO2) were purchased from Aldrich. Sodium nitrate (NaNO3), hydrogen peroxide (H2O2), hydrochloride acid (HCl), sulfuric acid (H2SO4), urea (CH4N2O), and potassium hydroxie (KOH) were purchased from Sigma–Aldrich. Potassium permanganate (KMnO4) and cobalt nitrate hexahydrate (Co(NO3)2･6H2O) were purchased from Acros. Ammonium fluoride (NH4F) was purchased from Merck. Te powder was purchased from Alfa Asesar. Synthesis of oxidized MWCNTs MWCNTs were oxidized by modified Hummer’s method.16,

17

About 1 g of MWCNT was

calcined for 1 h at 500 ℃ in a furnace, following removal of metal residues by 70 mL of HCl (10%). The product was filtered, washed with alcohol and deionized water, and dried in an oven


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at 50 °C. Then 23 mL of H2SO4 (98%) was added to the MWCNT solution in a 250 mL bottom flask, and the mixture was stirred at room temperature overnight. The solution was transferred to an oil bath, the temperature was raised to 40 °C, and 350 mg of NaNO3 was added. After slowly adding 1 g of KMnO4 and maintaining the temperature below the reaction temperature of 45 °C, the solution was stirred for 30 minutes at 40 °C, and 3 mL of deionized water was added. Then 3 mL of deionized water was added again after 5 min until 40 mL deionized water was obtained. After 15 min, oil bath was removed, 140 mL of deionized water and 10 mL of H2O2 (30%) were added. MWCNTs was collected, and HCl (5%) followed by deionized water were used to wash the sample. Finally, the solution was oven dried at 50 ℃.

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 g of NH4F (1.5 mmol), 0.1502 g of CH4N2O (2.5 mmol), and 40 mL of deionized water were added to a beaker, and the mixture was sonicated for 5 min. The solution was transferred to a 125 mL Teflon bottle, which was placed in a stainless-steel autoclave, and hydrothermal method was carried out at 120 °C for 12 h.18 After removing the alcohol and washing it with deionized water, the compound was oven dried at 50 °C. If only Co(OH)F was synthesized, the above-mentioned steps were conducted using 0.2910 g of Co(NO3)2 · 6H2O (1.0 mmol), 0.1112 g of NH4F (3.0 mmol), 0.3004 g of urea (5.0 mmol), and 40 mL of deionized water. To synthesize different proportions of Co(OH)F/CNT, Co(NO3)2·6H2O, NH4F, urea (mole ratio = 1: 3: 5), and 20 mg of oxidized MWCNTs were used


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Synthesis of CoTe2/CNT About 20 mg of Co(OH)F/CNTs and 37.62 mg of Te powder with 40 mL of deionized water were added to a beaker, and the mixture was sonicated for 30 min. The solution was transferred to a 125 mL Teflon bottle, which was placed in a stainless-steel autoclave, and 33.77 g of NaBH4 was added before performing hydrothermal method at 180 °C for 15 h. 19, 20 If we use 20 mg of Co(OH)F/CNT synthesized by 0.75 mmol or 1.00 mmol Co(NO3)2 · 6H2O, Te powder, and NaBH4 were 41.52 mg of Te and 37.28 mg of NaBH4, or 44.17 mg of Te and 39.65 mg of NaBH4, respectively. The solution was washed with deionized water and alcohol, and dried in an oven at 50 °C. If only CoTe2 was synthesized, the steps described above using 20 mg of Co(OH)F (2.1 mmol), 53.47 mg of Te (4.2 mmol), and 48.00 mg of NaBH4 (1.26 mmol). Decoration of Catalyst on Glassy Carbon Electrode To measure the electrochemical properties of the catalyst using an electrochemical analyzer, the slurry must be dropped to glassy carbon electrodes. First, 20 mg of the catalyst powder, 1.5 mL of ethanol, and 0.5 mL of Nafion (0.5%) in a centrifuge tube were obtained. Thus, the solution was sonicated for 1 h to disperse the slurry. The slurry of 10 µL was dropped on glassy carbon electrode with a diameter of 5.61 mm by utilizing a pipet. The slurry will cover the surface of the glassy carbon electrode at room temperature. Finally, the catalyst powder will adhere to the surface of the glassy carbon electrode. Characterization of Materials


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X-ray Diffraction (XRD; Bruker D2 PHASER) using Cu Kα as the source radiation was carried out to determine the crystallinity and the crystal structure. The morphology of the samples was investigated using 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-ray absorption near edge structure (XANES) of Co K- and L-edge was conducted at the beamline of 17C1 and 20A1 from the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu City, Taiwan. Electrochemical Measurement: All electrochemical analyses were carried out in a three electrode system by a RRDE as a working electrode with an electrochemical instrument (CHI 760D). The slurry of CoTe2/CNT was dropped on the RRDE. The analysis was conducted in 1.0 M KOH(aq) at room temperature. The three-electrode system contains a working electrode of RRDE, counter electrode of Pt-foil, and reference electrode of Hg/HgO. In 1.0 M KOH(aq), the potential changed to be at reversible hydrogen electrode (RHE) was calculated by following equation: ERHE = EHg/HgO + 0.9316 V. The voltage we mentioned will be at RHE. Linear sweep voltammetry (LSV) was carried out at 1.0 V to 1.7 V at a scanning rate of 10 mV/s. The linear portion at the low overpotential in the Tafel plot was fitted to the Tafel equation, and the Tafel slope was shown. All the data in LSV were corrected by iR loss, which is mainly from the electrolyte between the working and reference electrode based on Rs on electrochemical impedance spectroscopy at 1.45 V. To compare the active surface areas, cyclic voltammetry (CV) was applied 0.90 V to 0.96 V with scanning rates of 2, 4, 6, 8, and 10 mV/s. ∆J was calculated at 0.93 V and plotted against the scan rate. The slope of the line is equal to twice the capacitance of the double layer (Cdl). The long-term stability was measured by time-dependent potentiostatic electrochemical measurement


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at 1.52 V and continuous CV at a scanning rate of 10 mV/s for 1000 cycles between 1.10 and 1.70 V. In-situ XANES of Co K-edge In-situ Co K-edge XAS spectra were carried out with the beamline BL17C of the NSRRC.21 Scheme 1 presents a schematic of the substrate-film-sample holder assembly in contact with the 1M KOH electrolyte. The sample holder is made of PVC with outer dimensions of 6 cm × 5.5 cm × 0.8 cm and was used to collect the total fluorescence yield (TFY). The samples were deposited onto the Au-coated Si3N4 membrane window and was used as working electrode in a three-electrode setup. In addition, the saturated Ag/AgCl electrode and Pt electrode were the reference and counter electrodes. The Si3N4 membrane window is transparent for X-rays and then the membrane is attached to a PVC supporting frame 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 in contact with the 1 M KOH electrolyte was used to collect the total fluorescence yield (TFY) with base pressure of 5 × 10−8 Torr. The cell was separated from UHV by the Au-coated Si3N4 membrane window. The membrane area was chosen to be 1.0 mm × 1.0 mm and the thickness was 100 nm. The thin film of our material with a 5 nm-thick adhesive Au layer was deposited onto the cell side of the membrane and was used as working electrode in a three-electrode setup. A Pt wire as the counter electrode and a Pt wire was used as reference electrode were inserted in the cell through small holes on the sides.


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RESULTS AND DISCUSSION Fabrication and characterization In the current work, commercial MWCNT was functionalized through a modified hummer method.16,17 Oxidized CNT had stronger interaction with metal ions in the subsequent hydrothermal reaction for synthesizing Co(OH)F/CNT which was the precursor for preparing CoTe2/CNT electrocatalyst.18 Te powder was applied to substitute the anions of Co(OH)F/CNT for producing CoTe2/CNT material by hydrothermal method.19,20 And excessive sodium borohydride also reduced oxidized CNT to CNT. Various loading amounts of CoTe2 were prepared by tuning different concentration ratios between cobalt nitrate and oxidized CNT. For convenience, we abbreviated CNT with different CoTe2 loading amounts as “CoTe2/CNT-X,” in which X represents the millimoles of cobalt nitrate. The transmission electron microscopy (TEM) images of CoTe2/CNT with different loading amounts of CoTe2 and CoTe2 reveal the morphology (Figure 1a,b,c,d). The morphology of bare CoTe2 is identified as nanowire (NW), and the diameter of CoTe2 NW is approximately 70 nm, as shown in Figure 1d. Oxidized CNT which contain much functional group of oxygen on CNT, and it can interact with Co ion in the synthesis of Co(OH)F/CNT to caused stronger chemical adsorption. Accordingly, the CoTe2 materials became separated-nanoparticles. The diameters of CoTe2/CNT hybrid materials are around 40-50 nm. The X-ray diffraction (XRD) pattern reveals the diffraction peaks 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 characteristic peaks of orthorhombic marcasite CoTe2 (JCPDS-89-2091). The diffraction peaks of all CoTe2/CNT at 21.8° and 28.3° are difficult to be observed because a broad peak that originated from CNT in the orange region exists. In our preparation, CNT is oxidized by modified hummer method.


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Then, the oxidized CNT was reduced to CNT. After these processes, the phase of CNT becoming more amorphous caused broader peak than the original peak of CNT only by calcination and washing with acid solution in the Supporting Information (Figure S1). Moreover, as loading amount of CoTe2 decreases, the diffraction peak of CNT becomes more obvious. The lowest amount of cobalt nitrate we used to prepare CoTe2/CNT is 0.50 mmol. If less than than 0.50 mmol cobalt nitrate is used, then the peak of Te side product will be observed in XRD. The compositions of CoTe2/CNT with different loading amount of CoTe2 and CoTe2 NW were determined by energy-dispersive X- ray (EDX) in the Supporting Information (Figure S2a,b,c,d). EDX reveals the signal of Co and Te in all samples and the additional signal of C in CoTe2/CNT with different loading amounts of CoTe2. The peak of high intensity at 1.8 eV is from Si wafer, which was used as substrate. As the loading amount of CoTe2 rises, the intensity of C compared with the intensity of Co becomes significantly lower. The atomic ratio of C was also lower. The result demonstrates that the loading amount of CoTe2 is higher when higher concentration of cobalt nitrate with same amount of oxidized CNT is used to synthesize Co(OH)F/CNT. The valence states of Co ion in CoTe2/CNT with different loading amount of CoTe2 and CoTe2 NW was investigated by measuring the Co K-edge X-ray absorption near edge structure (XANES; Figure 3). Co foil and CoO with chemical state of 0 and 2+ charge were used as standard compound. The oxidation state of Co ion in CoTe2 was 2+ because of Te22- dimer. The Co Kedge jump of our four samples are between the absorption edge of Co foil and CoO. A chemical negative shift attributed to the lower electronegativity of Te compared with that of O. The phenomenon can also be observed in the Co K-edge jump of CoS2.23 The hybrid materials containing CNT show edge jump at more positive energy as compared with pristine CoTe2 NWs. Therefore, a charge transfer at the interface between CoTe2 and CNT caused more significant


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positive absorption energy shift observed on Co of CoTe2/CNT. This effect will be discussed later. 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 NW were obtained by using linear sweep voltammetry (LSV, Figure 4a,b). In this study, RuO2 was prepared to compare its electrochemical performance with that of samples. RuO2 exhibited low overpotential of only 239 mV to produce 10 mA/cm2, but its high Tafel slope of 65.3 mV/dec caused the current density to rise slowly at higher voltage. The overpotential of bare CoTe2 NW was 323 mV to generate 10 mA/cm2. After integrating CoTe2 on CNT, the overpotential of CoTe2/CNT-0.50 for producing 10 mA/cm2 is improved to 291 mV, but the overpotential of CoTe2/CNT-0.75 and CoTe2/CNT-1.00 are 301 and 313. This result reveals that, as the loading amount of CoTe2 increases, the overpotential for producing 10 mA/cm2 became lower. CNT reveals no obvious OER activity before 1.6 V, and thus, CoTe2 was a main catalyst in our hybrid materials. CNT should serve as a synergist to enhance the activity of CoTe2. In the Tafel plot (Figure 4b), the the slope of these samples becomes lower with the decrease in loading amount of CoTe2, indicating fast OER kinetics. In the two electrochemical measurements for determining the OER activity, the synergistic effect at the interface between CoTe2 and CNT to induced the differences of activity in our samples not the high conductance of CNT. The reason is that the difference of activity was not only found between hybrid materials and CoTe2 NW but also observed among hybrid materials. The synergistic effect should be related to the phenomenon of charge transfer observed in Co K-edge XANES. Notably, different strengths of synergistic effect cause different OER performance. The electrochemical impedance


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spectroscopy (EIS) can be used as an electrical model to investigate the OER efficiency in the Supporting Information (Figure S3). The Rs represents the series resistance, which is from the resistance of component in system and electrolyte between working and reference electrode. Rs was defined as the onset point of the semicircle in EIS spectra. Given that all electrochemical measurements are conducted in 1.0 M KOH(aq), the onset points should be close. Rct represents the charge transfer resistance, which depends on the capability of passing electron at the interface between catalyst and electrolyte. Rct leads to different semicircle diameters in EIS spectra. CoTe2/CNT-0.50 shows the smallest semicircular diameter, indicating that this sample is the most efficient electrocatalyst because of the best capability of transferring electron between electrode and electrolyte. Notably, the semicircular diameter became larger with the increase in loading amount of CoTe2 on CNT. High conductance of CNT as a physical property is not the reason to behind the tendency in EIS because this property does not affect the capability of passing electron at the interface between catalyst and electrolyte. This property should only affect Rs, and thus, the change of Rct in EIS can prove the presence of the synergistic effect caused by the charge transfer shown in Co K-edge XANES. Moreover, the strongest effect is observed on CoTe2/CNT-0.50 because of its lowest semicircular diameter. The active site surface area can be estimated by measuring cyclic voltammetry (CV) to calculate double layer capacitance (Cdl) at the interface between solid and liquid phase.24 The Cdl value of CoTe2/CNT0.75, CoTe2/CNT-1.00, and CoTe2 NW are 25.61, 24.52, 24.40 and 15.99 mF/cm2 in the supporting information (Figure S4, S5). Moreover, we can find the maximum of capacitance contribution of CNT is only 4.21 mF/cm2 which is still much lower than the capacitance of CoTe2/CNT in the supporting information (Figure S6). Because the amount of CNT in CoTe2/CNT is less than the sample of CNT, the capacitance contribution of CNT in CoTe2/CNT


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should be less than 4.21 mF/cm2. Therefore, the capacitance of CoTe2/CNT is contributed from CoTe2 mainly, and the Cdl of CoTe2 integrating on CNT is higher than that of pristine CoTe2 NW. This result shows that CoTe2 nanoparticles of hybrid material generate more active sites on the surface as compared with bare CoTe2 NW. This phenomenon is the other reason why the OER activity of hybrid materials containing CNT is higher than that of CoTe2 NW. However, the difference between the Cdl values of CoTe2/CNT with different loading amounts of CoTe2 was slight. Therefore, the varying OER active surface area of these catalysts was not the dominant factor that affected the activity discrepancy. X-ray Absorption Near Edge Structure (XANES) The electronic configuration of Co ion in CoTe2 is t2g6eg1 and the fermi level is at eg band, and thus, an unoccupied state exists in eg band in 3d orbital. XANES of Co L-edge absorption was the excitation from 2p to 3d orbital. Therefore, Co L-edge spectra was applied to investigate the charge transfer that causes the synergistic effect of CoTe2 hybridized with CNT to exhibit different OER performances (Figure 5a). Two peaks in the Co L-edge spectrum of CoTe2 corresponded to L2 and L3 near 795 and 779 eV. The intensity of L3 absorption was enlarged, and the intensity of L3 absorption is higher with the decreasing loading amount of CoTe2 on CNT (Figure 5b). If the intensity is higher, it indicates that the possibility of the excitation from 2p orbital to 3d unoccupied states is higher. CoTe2/CNT-0.50 shows the highest intensity of L3 accompanying strongest synergistic effect because of the lowest loading amount of CoTe2 on the surface of CNT and the lowest Rct. Therefore, stronger synergistic effect with the decrease in loading of CoTe2 was observed as the intensity of L3 becomes higher. This result reveals that the charge transfer direction of electrons is from CoTe2 to CNT. This parameter is the main cause of the different OER performances of CoTe2/CNT composite catalysts and bare CoTe2 NW. As the


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effect of charge transfer became stronger, there are more vacancies at eg band in 3d orbital of CoTe2 are observed. Given that the orbitals in eg band of transition metal ions are the active sites for interacting with anion adsorbate to generate σ-bonding, the bond strength of oxygen-related intermediate species are modulated by electron donation from CoTe2 to CNT.12 This phenomenon causes the bonding between Co ion in CoTe2 and oxygen-related intermediate species to become stronger when catalysts undergo OER. With stronger bonding to strengthen surface-oxygen interaction, the kinetics and activity of OER will be optimized. Meanwhile, the result of CoTe2 in Co L-edge XANES can be corresponded to the result in Co K-edge XANES. The charge transfer direction of electrons is from CoTe2 to CNT. Thus, the chemical state of Co ion in hybrid materials should be more positive than that of bare CoTe2 NW. In-situ X-ray Absorption Near Edge Structure (XANES) To realize the function of charge transfer by CNT in our hybrid material during OER, the oxidation states of CoTe2 NW and CoTe2/CNT-0.75 were monitored by in-situ XANES in 1.0 M KOH (Figure 6a,b and S7a,b of the Supporting Information) 21,22. The in-situ XANES of Co Kedge and L-edge spectra was conducted with applied bias from 1.00 V to 1.80 V and 1.00 V to 1.60 V, respectively. The XANES of Co K-edge for CoTe2/CNT-0.75 in Figure 5a exhibited an obviously positive change of energy of white line from 7727.4 eV to 7730.3 eV with applied bias from 1.00 V to 1.80 V so the valence of Co in CoTe2/CNT-0.75 was changed during OER. The XANES of Co L-edge for CoTe2/CNT-0.75 in Figure5b also showed an increase in energy of L3 absorption from 778.8 eV to 780.7 eV with applied bias from 1.00 V to 1.60 V. The absorption of Co2+ and Co3+ are near 778.8 eV and 780.7 eV so the oxidation state of Co in CoTe2/CNT0.75 was changed from Co2+ to Co3+ during OER which was accompanied by a formation of transition state of Co(III)OOH.25 There was not distinct change of energy in the XANES of Co


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K-edge and L-edge for CoTe2 NW in the Supporting Information (Figure S7a,b) so the charge transfer effect by CNT in our hybrid material can be an advantageous factor of generating transition state of CoOOH to enhance the efficiency of water oxidation. Stability Apart from high OER activity, the long-term stability is also important parameter to consider in developing electrocatalysts for electrocatalytic water splitting. A time-dependent potentiostatic electrochemical measurement to detect durability at 1.52 V is executed within 24 h, in which CoTe2/CNT-0.50 exhibits approximately 10 mA/cm2 (Figure 7). The result depicts that the current density of hybrid materials containing CNT does not decay obviously but the current density of CoTe2 NW without CNT decays around 45% The stability of CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00, and CoTe2 NW was also examined by continuous CV between 1.10 and 1.70 V at a scanning rate of 10 mV in the Supporting Information (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 but CoTe2 NW needs 69 mV more. This finding results from the assistance of CNT material which improves electron conductivity of CoTe2 catalysts. When conductance of CoTe2 was enhanced through integrating on CNT, charge accumulating on catalysts was reduced to further increase its OER stability. CONCLUSIONS In this work, we have successfully synthesized CoTe2/CNT hybrid materials functionalized as OER catalysts by using the hydrothermal method. The optimum loading amount of CoTe2 on CNT was achieved by CoTe2/CNT-0.50. This electrocatalyst exhibited outstanding OER activity with a small overpotential at 0.291 V, generating current density of 10 mV/cm2 and a small Tafel


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slope of 44.2 mV/dec. Although CoTe2 is unstable under OER, the CoTe2/CNT hybrid materials exhibited improved and exceptional durability by time-dependent potentiostatic electrochemical measurement for 24 h and continuous CV characterization for 1000 times. The synergistic effect between CoTe2 and CNT is the dominant factor that boosts the OER performance. Co L-edge XANES was applied to prove that a electron transfer from eg band of CoTe2 to CNT exists. This electron transfer modulated the bond strength of oxygen-related intermediate species on the surface of catalyst and optimize OER performance. And In-situ XANES was used to compare CoTe2/CNT and pristine CoTe2 during oxygen evolution reaction (OER). It proved the transition state of CoOOH was easier to exist by adding CNT in hybrid material during OER to enhance efficiency of OER. The design of new electrocatalyst for OER can be applied in various watersplitting devices and promote hydrogen economy. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology 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 appreciate Ms. Chia-Ying Chien who help us to perform TEM at the Instrumentation Center in Nation Taiwan University. SUPPORTING INFORMATION Information includes XRD pattern of CNT only by by calcination and washing with acid solution, EDS of our materials showing the presence of Co, Te, and C as element, EIS of our materials, CV of our materials, the plots show the double layer capacitance (Cdl), In-situ XANES spectra of Co K-edge and L-edge for CoTe2 NW, and polarization curves for our materials before


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and after 1000 cycles by continuous CV between 1.10 and 1.70 V. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors Chung-Li Dong, Department of Physics, Tamkang University, Tamsui 25137, Taiwan. E-mail: [email protected] Ru-Shi Liu, Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. E-mail: [email protected] Notes The authors declare no competing financial interest. REFERENCES (1) Rausch, B.; Symes, M. D.; Chisholm, G.; Cronin, L. Decoupled Catalytic Hydrogen Evolution from a Molecular Metal Oxide Redox Mediator in Water Splitting. Science 2014, 345, 1326-1330. (2) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974. (3) Gray, H. B. Powering the Planet with Solar Fuel. Nat. Chem. 2009, 1, 7-7. (4) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem 2010, 2, 724-761.


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TOC Graphic


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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 1 82x50mm (150 x 150 DPI)



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Figure 2. XRD patterns of CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00, and CoTe2 NW Figure 2 82x56mm (150 x 150 DPI)


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Figure 3. XANES spectra of Co K-edge for CoTe2/CNT-0.75, CoTe2/CNT-1.00, CoTe2 NW and the oxide standards containing Co. Figure 3 82x62mm (150 x 150 DPI)



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Figure 4. (a) polarization curves show the performance of CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT1.00 and CoTe2 NW compared with RuO2 and CNT; (b) Tafel plots show the Tafel slopes that exhibit OER kinetics for CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00 and CoTe2 NW compared with RuO2. Figure 4 82x31mm (150 x 150 DPI)


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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) Enlarged XANES spectra of Co L3 absorption. Figure 5 82x30mm (150 x 150 DPI)



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Figure 6. (a) In-situ XANES spectra of Co K-edge for CoTe2/CNT-0.75; (b) In-situ XANES spectra of Co Ledge for CoTe2/CNT-0.75. Figure 6 82x33mm (150 x 150 DPI)


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Figure 7. Time dependence of anodic current density of CoTe2/CNT-0.50, CoTe2/CNT-0.75, CoTe2/CNT-1.00 and CoTe2 NW. Figure 7 82x61mm (150 x 150 DPI)


Carbon Nanotube Hybrid Material

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