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Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution Dong Myung Jang, In Hye Kwak, El Lim Kwon, Chan Su Jung, Hyung Soon Im, Kidong Park, and Jeunghee Park J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Jan 2015 Downloaded from http://pubs.acs.org on January 6, 2015
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The Journal of Physical Chemistry
Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution Dong Myung Jang, In Hye Kwak, El Lim Kwon, Chan Su Jung, Hyung Soon Im, Kidong Park, and Jeunghee Park* Department of Chemistry, Korea University, Jochiwon 339-700, Korea 3) KEYWORDS. Electrocatalysts, nanocrystals, oxides, metal doping, water oxidation, oxygen evolution ABSTRACT: Catalysts for the oxygen reduction and evolution reactions are central to key renewable-energy technologies including fuel cells and water splitting. Despite tremendous effort, the development of oxygen electrode catalysts with high activity at low cost remains a great challenge. In this study, we report a generalized sol-gel method for the synthesis of various oxide nanocrystals (TiO2, ZnO, Nb2O5, In2O3, SnO2, and Ta2O5) with appropriate transition metal dopants for an efficient electrocatalytic oxygen evolution reaction (OER). Although TiO2 and ZnO nanocrystals alone have little activity, all the Mn-, Fe-, Co-, and Ni-doped nanocrystals exhibit greatly enhanced OER activity. A remarkable finding is that Co dopant produces higher OER activity than the other doped metals. X-ray photoelectron and X-ray absorption spectroscopies revealed the highly oxidized metal ions that are responsible for the enhanced catalytic reactivity. The excellent OER activity of the Co-doped nanocrystals was explained by a synergistic effect in which the oxide matrix effectively guards the most active Co dopants at higher oxidation states by withdrawing the electrons from the metal dopants. The metal-doped NCs exhibit the enhanced catalytic activity under visible light irradiation, suggesting the potential as efficient solar-driven OER photoelectrocatalysts.
1. INTRODUCTION The electrochemical or solar-driven photoelectrochemical splitting of water offers an attractive means of generating hydrogen fuel, which is free from environmental issues related to the combustion of fossil fuels. However, large-scale water splitting is greatly hampered by the kinetically sluggish four-electron oxidation reaction, i.e., oxygen evolution reaction (OER, 4OH-→2H2O + 4e- + O2 in alkaline media).1,2 Catalyst development is critical in addressing this challenge in order to efficiently couple multiple proton and electron transfers for the evolution of O2 under low overpotentials. To date, the most efficient electrochemical OER catalysts are known to be ruthenium (RuO2) and iridium oxides (IrO2), despite their limited availability and high cost.3–7 As a consequence, the search for robust and efficient alternative nanosize catalysts based on earth abundant 3d metals (e.g., Co, Mn) has been vigorously pursued, but substantial progress is still needed.8–35 Recently, the Yang group showed that transition-metaldoping (2%) of TiO2 nanowires decreased the OER overpotential in the electrolysis of water.25 Pfrommer et al. demonstrated the excellent OER activity of Co (30%)-substituted ZnO nanoparticles.32 TiO2 and ZnO are rather ideal for practical applications due to their low cost and good environmental compatibility. In the present work, a simple route was used to synthesize various metal oxide nanocrystals (NCs) with effective metal doping. The metal oxides were TiO2, ZnO, Nb2O5, In2O3, SnO2, and Ta2O5, and the first-row transition metals, Mn, Fe, Co, and Ni, were chosen as dopants. The metal-doped NCs exhibited surprisingly high electrocatalytic OER activities, but were especially enhanced by Co doping. In order to comprehend the enhanced efficiency of the OER catalysts during operation, the oxidation state of the doped metals was thoroughly investigated using UVvisible absorption, X-ray photoelectron (XPS), and X-ray absorption (XAS) spectroscopies. Furthermore we compared systematically catalytic ability for a series of metal-doped TiO2 and ZnO
NCs, which could provide valuable insight on the mechanism of the electrocatalytic OER. Doped oxide NCs have been used as photocatalysts in the conversion of solar energy to chemical energy (e.g., the production of H2 and O2 by water splitting) as well as the degradation of toxic water pollutants.36–38 The doping by transition metals has improved visible-range absorption, which could be useful for the photocatalytic reactions. Nonetheless, because of mechanistic complexity, the photocatalytic process of transition-metal doped oxide NCs is still not sufficiently understood. Herein, we demonstrated that the OER activity is increased under the visible light irradiation. This is a first observation of the photocatalytically active OER electrocatalysts. 2. RESULTS AND DISCUSSION We developed a convenient sol-gel route to synthesize large quantities of high-purity TiO2, ZnO, Nb2O5, In2O3, SnO2, and Ta2O5 NCs, by adopting the method developed for N-doped Ta2O5.39 For metal-doped oxide NCs, metal chloride salts were chosen as dopant precursors. This synthetic route enabled homogeneous doping, as described below. TEM images and energy-dispersive Xray fluorescence (EDX) spectra of the uniformly sized TiO2, ZnO, Nb2O5, In2O3, SnO2, and Ta2O5 NCs, with average diameters of 7, 10, 17, 7, 23, and 10 nm, respectively (Supporting Information, Figure S1). Their X-ray diffraction (XRD) patterns confirmed high-purity anatase TiO2, wurtzite ZnO, hexagonal Nb2O5, cubic In2O3, tetragonal SnO2, and orthorhombic Ta2O5 (Supporting Information, Figure S2). Mn-, Fe-, Co-, and Ni-doped TiO2 (and ZnO) NCs were synthesized using the corresponding precursors. The metal ( 1.0
> 1.0
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Fe
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a
Onset overpotential, b Overpotential at j=0.5 mA/cm2, c Tafel equation: η = b log(j/j0), η is the overpotential (measured), defined as Eapplied (vs. RHE) − 1.229 V, b is the Tafel slope (mV/decade), j is the current density (measured), and j0 is the exchange current density.
We prepared 1–10 at%-doped NCs and evaluated their OER activities as a function of dopant concentrations. All the NCs exhibit maximum efficiency at the 2−5 at% doping level. The catalysis kinetics for the OER was examined using Tafel plots (Figure 4c). Tafel slope (= b) and exchange current density (= -log j0), obtained from the linearity portion of LSV curve (as fitted by the dotted lines), corresponding to activation-controlled current density regions, are summarized in Table 1. All Tafel parameters have
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to stabilize the intermediate (metal–OH or –OOH) species and to assist the deprotonation pathway. During the OER, the metal ions could be oxidized to the higher oxidation states. The electron transfer can also keep continuously the population of highly oxidized metal ions during the catalysis reaction. This model is supported by the better performance exhibited by Co-ZnO (lower b value and higher current) than Co-TiO2, since Zn is more electronegative than Ti. The electron transfer from the Co or Mn ions to the Au nanoparticles was previously suggested for the enhanced OER activity of Au-Co3O4, and Au-MnOx.16,30,54 The Ti and Zn XPS 2p peaks of undoped and doped NCs show that the Ti and Zn 2p electron binding energies are reduced after metal doping (see SI, Figure S5). Such red shifts probably support the electron withdrawal ability of the Ti and Zn ions. The electron donation from the metal dopant will make the oxide more electron-rich, which should facilitate the activation of H2O molecules through Lewis acid−base or hydrogen bonding interactions and lead to the improved OER activity of the catalyst. The synergy effect would be maximized when the catalytic ability of the metal ions becomes larger, which is the case for the Co doping. A deeper examination of this experimentally observed synergistic enhancement of OER activity is ongoing. 3
(a)
14 12 10
Co-TiO2
Light On
Co-ZnO
Dark Light On Dark
8 6 4 2 0 1.3
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10 % fitting errors. A lower Tafel slope value indicates more favorable kinetics; Co-TiO2 and Co-ZnO have the lowest slopes, 67 and 63 mV/decade, respectively. These results agree with the lower overpotential values, which definitely suggest the better catalytic activity of the Co dopants than the others. The Tafel slopes are near the 59 mV/decade value that corresponds to characteristic of an O2 evolution mechanism involving a reversible one-electron transfer.12 The chronoamperometric response results demonstrate the high stability of the Co-ZnO NCs, showing a slight anodic current attenuation of 5% within 4 h (Supporting Information, Figure S7). For comparison, the LSV curves and Tafel plots were obtained for RuO2 and IrO2 under the same experimental condition (Supporting Information, Figure S8). Although these materials exhibit the lower overpotentials than the metal-doped NCs, the Co-doped NCs are comparable with them in terms of Tafel slope. In accordance with the generalized OER pathway of metal oxides in alkaline solution (4OH− → O2 + 2H2O + 4e−), metal ions can facilitate the adsorption of OH− ions, promote electron transfer between the catalyst surface and reaction intermediates, and assure the facile recombination of two adsorbed oxygen atoms for O2 evolution. It has been emphasized that the highly oxidized Mn, Co, and Ni ions (+3, +4, or +5 oxidation states) are particularly important in enabling the OER.10,16,17,20,24,29,30,33 The presence of these cations is believed to facilitate the formation of metal–OH or hydroperoxo species (metal–OOH) from adsorbed O and promote the deprotonation to produce O2. Our XPS and XAS data provide conclusive evidence for the oxidation states of the doped metals. The Mn-ZnO and Fe-ZnO materials contain more highly oxidized Mn and Fe ions (Mn3+, Mn4+, and Fe3+) than the TiO2 counterparts. In the case of Co and Ni doping, the populations of Co3+ and Ni3+ are nearly the same for both oxides. As shown by the overpotentials and Tafel plots, the OER activities of the Mn-, Fe-, and Ni-doped NCs are not significantly different for either of the oxide NCs, and all are lower than those of Co-doped NCs. This indicates that the populations of the highly oxidized Mn and Fe ions are less effective than the catalytic ability of the metal species in the oxide matrix. On the other hand, the lower OER activity of the Ni-doped NCs is ascribed to the small quantity of Ni3+ ions. Question remains for how Co doping enhances the OER activity more significantly than the other dopants. One hypothesis is that Co2+/Co3+, having the highest reduction potential among the dopants; 1.81 V (versus the normal hydrogen electrode) for Co2+/Co3+, 1.51 eV for Mn2+/Mn3+, and 0.77 eV for Fe2+/Fe3+, enables the most efficient catalysis along the thermodynamic pathway for O2 production at reduced overpotentials. Recently, Bajdich et al. reported a theoretical study, showing the highest activity of Co-OOH intermediates due to the Co3+ ions.53 Trasatti reported an OER volcano plot that correlated η with the metaloxygen bond strength on the surface of the oxides.4 It showed a lower η for Co3O4 compared to Fe2O3, since the metal-oxygen bond strength is too strong for Fe2O3. The mechanism by which the doped metal ions catalyze the OER would be expected to be similar to those proposed for the metal oxide. Metal doping at as low as 5% enabled the oxide NCs to exhibit excellent electrocatalytic activity toward water oxidation. This suggests that TiO2 and ZnO matrix provides structural support and/or cooperativity, although they play no role as an active catalytic site. The enhancement effect of the redox-inactive Zn ions was recently reported for an analogous catalyst, Zn-Co layered double hydroxides as well as 30% Co:ZnO nanoparticles.28,32 The Ti and Zn ions can withdraw the electrons from the metal dopants
Current density (mA/cm2)
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Co-ZnO 2
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Figure 5. (a) LSV of the OER for Co-doped TiO2 and ZnO NCs in the dark and under visible light irradiation. (b) Photocurrent– time curves for Co-doped NCs. The photocatalytic OER activity of NCs was investigated under visible light irradiation (>420 nm, 800 mW/cm2 Xe lamp); all measure process was under illumination. Catalysis films, prepared by depositing on the FTO substrates, exhibited negligible difference in the LSV curve and Tafel plots for all the NCs (under dark condition). The experimental conditions were the same as the previous OER. The catalytic current increased obviously by the light (Figure 5a). For Co-TiO2 and Co-ZnO, the current 8 and 13 mA/cm2 (at 1.95 V versus RHE) increased to 10 and 15 mA/cm2 with illumination, respectively, illustrating that visible-light irradiation could effectively promote the OER. The Tafel slope value remains close to 60 mV/decade. The enhancement of metal-doped NCs follows the same sequence of electrocatalytic OER: the Mn, Fe, and Ni-doped NCs showed a current increase of 0.2~0.5 mA/cm2 (at 1.95 V versus RHE). There is no increase upon the UV irradiation (without UV cut-off filter), except undoped TiO2 and ZnO NCs, where the enhancement increases by about 4 times (0.04 mA/cm2 versus 0.15 mA/cm2). The unique property of metal-doped NCs makes them promising to be used for solar-driven OER in photoelectrochemical cells. The chronoamperometric enhancement were also monitored at 1.95 V (versus RHE) as the light source turned on and off with a time interval of 1 min (Figure 5b). Remarkable photo-response was observed from on/off light cycles; current level increased quickly under irradiation. The current difference induced by irradiation (photocurrent) is about 2 mA/cm2. After extended cycles, the photocurrent can be still changed distinctly with repeatedly
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The Journal of Physical Chemistry
turning the light on/off; demonstrating that the NCs has excellent reversibility and stability of OER activity. The photocatalysis principle is suggested as follows: as soon as the Co-TiO2 or CoZnO is photoexcited (presumably electron of Co dopants in the valence band jumped into conduction band), the OH- will capture the holes and promote OER. 3. CONCLUSIONS We developed a generalized sol-gel method for the large-scale synthesis of TiO2, ZnO, Nb2O5, In2O3, SnO2, and Ta2O5 NCs, which can be conveniently doped with various metals. A series of transition metal-doped (5 at% Mn, Fe, Co, and Ni) TiO2 and ZnO NCs were synthesized, in order to find robust, cheap, and efficient electrocatalysts for the OER. The electronic structures of the metal dopants were thoroughly investigated using UV-visible absorption spectroscopy, XPS, and XAS (with XMCD). The analysis provided robust evidence for the oxidized states of the dopants. We compared the electrocatalytic performance of two metaldoped oxide series for the OER. While undoped TiO2 and ZnO alone had little activity, all the metal-doped materials exhibited higher OER activity within the same sequence. The metal ions would facilitate the formation of the intermediate species (e.g., metal–OH, –OOH) and enhance the OER activity. The oxide matrix would guard the catalytically active metal ion sites at higher oxidation states, by withdrawing the electrons from the metal dopants. Greater enhancement upon Co doping was observed for both oxide NCs. The high catalytic OER activity of the Co-doped NCs arises from a remarkable synergism between the high reduction potential Co3+ ions and the oxide matrix. Under visible-light irradiation, the catalytic OER activity is enhanced, suggesting that these electrocatalysts have the potential as efficient OER photoelectrocatalysts. 4. MATERIALS AND METHODS TiCl4, ZnCl2, (C4H4N)NbO9 (ammonium niobate oxalate), InCl3, SnCl4, and TaCl5 hydrate were purchased from Aldrich. 2.8 mmol was dissolved in ethanol (100 mL). For 1–10 mol% metal doping, one of the metal chloride hydrates (MnCl2, FeCl2, CoCl2, or NiCl2, 0.028–0.28 mmol) was added to the starting ethanol. Then, an ammonia solution (5%) was added dropwise under magnetic stirring for a total volume of 300 mL. A colloidal solution was obtained after continuous stirring of the reaction mixture. The resulting gel was washed several times with water and ethanol, and then dried in air. Calcination of the obtained precipitate was performed in an O2 (100 sccm)/Ar (100 sccm) flow at 500–800°C for 2 h, resulting in the formation of the oxide NCs. The structures and compositions of the products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), fieldemission transmission electron microscopy (TEM, FEI TECNAI G2 200 kV), high voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), and energy-dispersive X-ray fluorescence spectroscopy (EDX). High resolution XRD patterns were obtained using the 9B beam lines of the Pohang Light Source (PLS) with monochromatic radiation. XPS measurements were performed using the 8A1 beam line of the PLS, as well as a laboratory-based spectrometer (Thermo Scientific Theta Probe) using a photon energy of 1486.6 eV (Al Kα). The XAS and XMCD measurements were carried out at the PLS elliptically polarized undulator beamline, 2A. A 0.6 T electromagnet was used to switch the magnetization direction. UV-visible absorption spectra of the samples were recorded using a spectrometer (Cary 5000, Agilent Tech.). Electrochemical experiments were carried out at room temperature in a three-electrode cell connected to an electrochemical analyzer (CompactStat, Ivium Technologies). The working electrodes were prepared by drop-casting the samples (100 µg dispersed in
Nafion using isopropyl alcohol) over a glassy carbon electrode (area = 0.1963 cm2, Pine Instruments Model No. AFE5T050GC) or by spin-coating the samples (100 µg in isopropyl alcohol) on a fluorine-doped tin oxide (FTO) substrate (area = 0.25 cm2). A Ag/AgCl electrode (Aldrich, saturated KCl) and a Pt wire were used as reference and counter electrodes, respectively. RuO2 (99.9%, Aldrich) and IrO2 (99.9%, Aldrich) were used as standard materials. The potentials reported in our work were referenced to the reversible hydrogen electrode (RHE) through standard RHE calibration, and in 0.1 M KOH, E(RHE) = E(Ag/AgCl) + 0.9673 V. The reference was also calibrated in the same electrolyte by measuring hydrogen oxidation/evolution currents on a Pt wire and defining the potential of zero current as the reversible hydrogen electrode (RHE). All the potentials in this study were referenced to the RHE potential scale and correspond to the applied potentials. The overpotential (η) is defined as Eapplied (vs. RHE) − 1.229 V. Before the electrochemical measurement, the electrolyte (0.1 M KOH, 99.99% metal purity, pH = 13) was purged by O2 (ultrahigh grade purity) for at least 0.5 h to ensure the saturation of the electrolyte. The photoelectrochemical properties were investigated using a 450 W xenon lamp (Oriel). An UV-cutoff filter (>420 nm) was used for visible light source.
Supporting Information Figures S1-S8: TEM, EDX, XRD, XPS, XMCD, and OER data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author parkjh@korea.ac.kr
ACKNOWLEDGMENT This study was supported by NRF (20110020090; 2014R1A6A1030732) funded by the Ministry of Education, and the Industrial Strategic Technology Development Program (10043929) funded by MOTIE (Korea). The HVEM (Dejeon) and XPS (Pusan) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH.
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Table of Contents
TiO2 -
4OH
ZnO 0.8
Mn
-
O2 + 2H2O + 4e
Fe
Co
Ni
0.4
ZnO
0.6
TiO2
Overpotential (V)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.2 0.0
We report a generalized sol-gel synthesis of metal-doped TiO2 and ZnO nanocrystals, and the remarkable finding that the Co doping produces the higher OER activity than the Mn-, Fe-, and Ni doping.
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