Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic

Subscriber access provided by ERCIYES UNIV

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 8

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

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

110

330

6.6

3.0

Mn

0.49

0.46

0.67

0.60

140

70

9.7

6.7

Metal Onset η (V)a

Fe

0.52

0.49

0.61

0.63

65

97

9.1

5.5

Co

0.38

0.39

0.43

0.46

67

63

6.3

6.3

Ni

0.47

0.43

0.55

0.62

65

74

8.3

5.8

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

3

ACS Paragon Plus Environment


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

(b)

Co-TiO2

2

16

Photocurrent (mA/cm )

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)

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

Page 4 of 8

Co-ZnO 2

1

0 1.4

1.5

1.6

1.7

E (V) vs. RHE

1.8

1.9

0

2

4

6

8

10

Time (min)

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

4


Page 5 of 8

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


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 [email protected]

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.

REFERENCES (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S.

W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (2) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 64746502. (3) Gnanamuthu, D. S.; Petrocelli, J. V. A Generalized Expression for the Tafel Slope and the Kinetics of Oxygen Reduction on Noble Metals and Alloys. J. Electrochem. Soc. 1967, 114, 10361041. (4) Trasatti, S. Electrocatalysis by Oxides-Attempt at a Unifying Approach. J. Electroanal. Chem. 1980, 111, 125-131. (5) Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. Electrolysis of Water on Oxide Surfaces. J. Electroanal. Chem. 2007, 607, 83-89. (6) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399-404.

5


The Journal of Physical Chemistry (7) Sanchez Casalongue, H. G.; Ng, M. L.; Kaya, S.; Friebel, D.;

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

Ogasawara, H.; Nilsson, A. In Situ Observation of Surface Species on Iridium Oxide Nanoparticles during the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2014, 53, 7169-7172. (8) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolution Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072–1075. (9) Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342-345. (10) Li, Y.; Hasin, P.; Wu, Y. NixCo3-xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution. Adv. Mater. 2010, 22, 1926– 1929. (11) Gorlin, Y.; Jaramillo. T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612-13614. (12) Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Mechanistic Studies of the Oxygen Evolution Reaction by Cobalt-Phosphate at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501-16509. (13) Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J. Rapid Room-Temperature Synthesis of Nanocrystalline Spinels as Oxygen Reduction and Evolution Electrocatalysts. Nat. Chem. 2011, 3, 79-84. (14) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780–786. (15) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383–1385. (16) Yeo, B. S.; Bell, A. T. Enhanced Activity of Gold-Supported Cobalt Oxide for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2011, 133, 5587–5593. (17) Gerken, J. B.; McAlpin, J. G.; Chen, J. Y. C.; Rigsby, M. L.; Casey, W. H.; Britt, R. D.; Stahl, S. S. Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0-14: The Thermodynamic Basis for Catalyst Structure, Stability, and Activity. J. Am. Chem. Soc. 2011, 133, 14431–14442. (18) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550–557. (19) Cobo, S.; Heidkamp, J.; Jacques, P. A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; et al. A Janus Cobalt-Based Catalytic Material for Electro-Splitting of Water. Nat. Mater. 2012, 11, 802-807. (20) Gao, M. R.; Xu, Y. F.; Jiang, J.; Zheng, Y. R.; Yu, S. H. Water Oxidation Electrocatalyzed by an Efficient Mn3O4/CoSe2 Nanocomposite, J. Am. Chem. Soc. 2012, 134, 2930-2933. (21) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution. J. Am. Chem. Soc. 2012, 134, 17253-17261. (22) Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K. J.; Trudel, S.; Berlinguette, C. P. Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis. Science 2013, 340, 60– 63. (23) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An Advanced Ni−Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452-8455. (24) Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S.

Page 6 of 8

M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. In Situ X‑ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525−8534. (25) Liu, B.; Chen, H. M.; Liu, C.; Andrews, S. C.; Hahn, C.; Yang, P. Large-Scale Synthesis of Transition-Metal-Doped TiO2 Nanowires with Controllable Overpotential. J. Am. Chem. Soc. 2013, 135, 9995-9998. (26) Chen, S.; Duan, J.; Jaroniec, M.; Qiao, S. Z. ThreeDimensional N-Doped Graphene Hydrogel/NiCo Double Hydroxide Electrocatalysts for Highly Efficient Oxygen Evolution. Angew. Chem., Int. Ed. 2013, 52, 13567-13570. (27) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. (28) Zou, X.; Goswami, A.; Asefa, T. Efficient Noble Metal-Free (Electro) Catalysis of Water and Alcohol Oxidations by Zinc−Cobalt Layered Double Hydroxide. J. Am. Chem. Soc. 2013, 135, 17242-17245. (29) Han, X.; Cheng, F.; Zhang, T.; Yang, J.; Hu, Y.; Chen, J. Hydrogenated Uniform Pt Clusters Supported on Porous CaMnO3 as a Bifunctional Electrocatalyst for Enhanced Oxygen Reduction and Evolution. Adv. Mater. 2014, 26, 2047-2051. (30) Zhuang, Z.; Sheng, W.; Yan, Y. Synthesis of Monodispere Au@Co3O4 Core-Shell Nanocrystals and Their Enhanced Catalytic Activity for Oxygen Evolution Reaction. Adv. Mater. 2014, 26, 3950-3955. (31) Jung, J. I.; Jeong, H. Y.; Lee, J. S.; Kim, M. G.; Cho, J. A Bifunctional Perovskite Catalyst for Oxygen Reduction and Evolution Angew. Chem. Int. Ed. 2014, 53, 4582-4586. (32) Pfrommer, J.; Lublow, M.; Azarpira, A.; Göbel, C.; Lücke, M.; Steigert, A.; Pogrzeba, M.; Menezes, P. W.; Fischer, A.; SchedelNiedrig, T.; et al. A Molecular Approach to Self-Supported Cobalt-Substituted ZnO Materials as Remarkably Stable Electrocatalysts for Water Oxidation. Angew. Chem. Int. Ed. 2014, 53, 5183-5187. (33) Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.; Grützke, S.; Weide, P.; Muhler, M.; Schuhmann, W. MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen-Doped Carbon Matrix for High-Performance Bifunctional Oxygen Electrodes. Angew. Chem., Int. Ed. 2014, 53, 8508-8512. (34) Zhao, A.; Masa, J.; Xia, W.; Maljusch, A.; Willinger, M. G.; Clavel, G.; Xie, K.; Schlögl, R.; Schuhmann, W.; Muhler, M. Spinel Mn-Co Oxide in N‑Doped Carbon Nanotubes as a Bifunctional Electrocatalyst Synthesized by Oxidative Cutting. J. Am. Chem. Soc. 2014, 136, 7551-7554. (35) Meng, Y.; Song, W.; Huang, H.; Ren, Z.; Chen, S. Y.; Suib, S. L. Structure-Property Relationship of Bifunctional MnO2 Nanostructures: Highly Efficient, Ultra-Stable Electrochemical Water Oxidation and Oxygen Reduction Reaction Catalysts Identified in Alkaline Media. J. Am. Chem. Soc. 2014, 136, 11452-11464. (36) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemannt, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96. (37) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Direct Splitting of Water under Visible Light Irradiation with an Oxide Semiconductor Photocatalyst. Nature 2001, 414, 625-627. (38) Choi, J.; Park, H.; Hoffmann, M. R. Effects of Single MetalIon Doping on the Visible-Light Photoreactivity of TiO2. J. Phys. Chem. C 2010, 114, 783-792. (39) Morikawa, T.; Saeki, S.; Suzuki, T.; Kajino, T.; Motohiro, T. Dual Functional Modification by N Doping of Ta2O5: p-Type

6


Page 7 of 8

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


Conduction in Visible-Light-Activated N-Doped Ta2O5. Appl. Phys. Lett. 2010, 96, 142111-142113. (40) Mizushima, K.; Tanaka, M.; Asai, A.; Iida, S.; Goodenough, J. B. Impurity Levels of Iron-Group Ions in TiO2 (II). J. Phys. Chem. Solids 1979, 40, 1129-1140. (41) Malati, M. A.; Wong, W. K. Doping TiO2 for Solar Energy Applications. Surf. Tech. 1984, 22, 305-322. (42) Koidl, P. Optical Absorption of Co2+ in ZnO. Phys. Rev. B 1977, 15, 2493-2499. (43) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.; Chastain, J.; King, R. C., Jr., Eds. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, MN, 1995. (44) Descostes, M.; Mercier, F.; Thromat, N.; Beaucaire, C.; Gautier-Soyer, M. Use of XPS in the Determination of Chemical Environment and Oxidation State of Iron and Sulfur Samples: Constitution of a Data Basis in Binding Energies for Fe and S Reference Compounds and Applications to the Evidence of Surface Species of an Oxidized Pyrite in a Carbonate Medium. Appl. Surf. Sci. 2000, 165, 288–302. (45) Mekki, A.; Holland, D.; Ziq, K.; McConville, C. F. XPS and Magnetization Studies of Cobalt Sodium Silicate Glasses. J. NonCryst. Sol. 1997, 220, 267-279. (46) Moses, A. W.; Garcia Flores, H. G.; Kim, J. G.; Langell, M. A. Surface Properties of LiCoO2, LiNiO2 and LiNi1-xCoxO2. Appl. Surf. Sci. 2007, 253, 4782-4791. (47) Kumar, S.; Gautam, S.; Kim, G. W.; Ahmed, F.; Anwar, M. S.; Chae, K. H.; Choi, H. K.; Chung, H.; Koo, B. H. Structural, Magnetic and Electronic Structure Studies of Mn Doped TiO2 Thin Films. Appl. Surf. Sci. 2011, 257, 10557-10561. (48) Thakur, P.; Chae, K. H.; Kim, J, -Y.; Subramanian, M.; Jayavel, R.; Asokan, K. X-Ray Absorption and Magnetic Circular Dichroism Characterizations of Mn-Doped ZnO. Appl. Phys. Lett. 2007, 91, 162503-16505. (49) Kataoka, T.; Yamazaki,Y.; Singh, V. R.; Sakamoto, Y.; Ishigami, K. ; Verma, V. K.; Fujimori, A.; Chang, F.-H.; Lin, H.J.; Huang, D. J.; et al. X-Ray Absorption Spectroscopy and X-Ray Magnetic Circular Dichroism Studies of Transition-MetalCodoped ZnO Nano-Particles. Surf. Sci. Nanotech. 2012, 10, 594598. (50) Zafeiratos S.; Dintzer, T.; Teschner, D.; Blume, R.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R. Methanol Oxidation over Model Cobalt Catalysts: Influence of the Cobalt Oxidation State on the Reactivity. J. Catal. 2010, 269, 309-317. (51) Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S. Y.; Koinuma, H. Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide. Science

2001, 291, 854-856. (52) Gu, W.; Wang, H.; Wang, K. Nickel L-edge and K-edge X-ray

Absorption Spectroscopy of Non-Innocent Ni[S2C2(CF3)2]2n Series (n = −2, −1, 0): Direct Probe of Nickel Fractional Oxidation State Changes. Dalton Trans. 2014, 43, 6406-6413. (53) Bajdich, M.; García-Mota, M.; Vojvodic, A.; Nørskov, J. K.; Bell, A. T. Theoretical Investigation of the Activity of Cobalt Oxides for the Electrochemical Oxidation of Water. J. Am. Chem. Soc. 2013, 135, 13521-13530. (54) Kuo, C. –H.; Li, W.; Pahalagedara, L.; El-Sawy, A. M.; Kriz, D.; Genz, N.; Guild, C.; Ressler, T.; Suib, S. L.; He, J. Understanding the Role of Gold Nanoparticles in Enhancing the Catalytic Activity of Manganese Oxides in Water Oxidation Reactions. Angew. Chem. Int. Ed. DOI: 10.1002/anie.201407783.

7



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

Page 8 of 8

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.

8


Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic

Recommend Documents