Titanium and Zinc Oxide Nanoparticles Are Proton-Coupled Electron Transfer Agents

Pluses and Minuses Because of their high surface area and tunable electronic structure, nanoparticles are of interest for next-generation photocatalysis and light-harvesting applications. Many of these applications involve electron transfer events at the particle surface after light absorption. Schrauben et al. (p. 1298) now show, through a series of radical trap kinetic studies, that proton transfer can concurrently accompany electron transfer at the solvent interfaces of two common nanoparticle formulations (oxides of titanium and of zinc). The results may help in the optimization of particle structure and energetics. Protons can play more of a role than previously appreciated in charge-transfer events at nanoparticle interfaces. Oxidation/reduction reactions at metal oxide surfaces are important to emerging solar energy conversion processes, photocatalysis, and geochemical transformations. Here we show that the usual description of these reactions as electron transfers is incomplete. Reduced TiO2 and ZnO nanoparticles in solution can transfer an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e– and H+ are coupled in this interfacial reaction. These proton-coupled electron transfer (PCET) reactions are rapid and quantitative. The identification of metal oxide surfaces as PCET reagents has implications for the understanding and development of chemical energy technologies, which will rely on e–/H+ coupling.

[1]  J. Moser,et al.  Dynamics of interfacial charge-transfer reactions in semiconductor dispersions. Reduction of cobaltoceniumdicarboxylate in colloidal titania , 1985 .

[2]  B. Robinson,et al.  Room-temperature electron spin dynamics in free-standing ZnO quantum dots. , 2007, Physical review letters.

[3]  A. Henglein,et al.  Flash photolysis observation of the absorption spectra of trapped positive holes and electrons in colloidal titanium dioxide , 1984 .

[4]  D. A. Schwartz,et al.  Magnetic quantum dots: synthesis, spectroscopy, and magnetism of Co2+ - and Ni2+-doped ZnO nanocrystals. , 2003, Journal of the American Chemical Society.

[5]  J. Mayer,et al.  Understanding hydrogen atom transfer: from bond strengths to Marcus theory. , 2011, Accounts of chemical research.

[6]  P. Cozzoli,et al.  Low-temperature synthesis of soluble and processable organic-capped anatase TiO2 nanorods. , 2003, Journal of the American Chemical Society.

[7]  A. Bard.  Inner-sphere heterogeneous electrode reactions. Electrocatalysis and photocatalysis: the challenge. , 2010, Journal of the American Chemical Society.

[8]  Xiaobo Chen,et al.  Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals , 2011, Science.

[9]  J. Ashby.  References and Notes , 1999 .

[10]  F. Marken,et al.  Electrochemical reactivity of TiO2 nanoparticles adsorbed onto boron-doped diamond surfaces , 2004 .

[11]  J. Mayer,et al.  The first crystal structure of a monomeric phenoxyl radical: 2,4,6-tri-tert-butylphenoxyl radical. , 2008, Chemical communications.

[12]  K. Honda,et al.  Measurement of the extinction coefficient of the methyl viologen cation radical and the efficiency of its formation by semiconductor photocatalysis , 1982 .

[13]  J. Gladysz,et al.  Synthesis, crystal structure, and reactions of the 17-valence-electron rhenium methyl complex [(η5-C5Me5)Re(NO)(P(4-C6H4CH3)3)(CH3)]+ B(3,5-C6H3(CF3)2)4−: experimental and computational bonding comparisons with 18-electron methyl and methylidene complexes , 2000 .

[14]  D. Gamelin,et al.  Electron transfer between colloidal ZnO nanocrystals. , 2011, Journal of the American Chemical Society.

[15]  K. Ingold,et al.  Photoinduced electron transfer from dialkyl nitroxides to halogenated solvents , 1990 .

[16]  Cheol-Eui Lee,et al.  Evidence of hydrogen-mediated ferromagnetic coupling in Mn-doped ZnO , 2011 .

[17]  L. Kavan,et al.  Pseudocapacitive Lithium Storage in TiO2(B) , 2005 .

[18]  C. Sanchez,et al.  Characterization of titanium dioxide nanoparticles dispersed in organic ligand solutions by using a diffusion-ordered spectroscopy-based strategy. , 2007, Chemistry.

[19]  S. Hammes-Schiffer,et al.  Introduction: Proton-coupled electron transfer. , 2010, Chemical reviews.

[20]  Optical transitions in artificial few-electron atoms strongly confined inside ZnO nanocrystals. , 2003, Physical review letters.

[21]  D. A. Schwartz,et al.  Synthesis of colloidal Mn2+:ZnO quantum dots and high-TC ferromagnetic nanocrystalline thin films. , 2004, Journal of the American Chemical Society.

[22]  C. Costentin.  Electrochemical approach to the mechanistic study of proton-coupled electron transfer. , 2008, Chemical reviews.

[23]  S. Raychaudhuri,et al.  The network of receptors characterize B cell receptor micro- and macroclustering in a Monte Carlo model. , 2010, The journal of physical chemistry. B.

[24]  A. Bard,et al.  Dynamic potential–pH diagrams application to electrocatalysts for water oxidation , 2012 .

[25]  Xiaobo Chen,et al.  Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. , 2007, Chemical reviews.

[26]  M. Mahanthappa,et al.  Synthesis, structure, and ethylene/α-olefin polymerization behavior of (cyclopentadienyl)(nitroxide)titanium complexes , 2004 .

[27]  M. Mahanthappa,et al.  Synthesis and molecular structure of titanium complexes containing a reduced TEMPO radical. , 2002, Chemical communications.

[28]  J. Hupp,et al.  Energetics of the Nanocrystalline Titanium Dioxide/Aqueous Solution Interface: Approximate Conduction Band Edge Variations between H0 = −10 and H- = +26 , 1999 .

[29]  I. Shkrob,et al.  Light Induced Fragmentation of Polyfunctional Carboxylated Compounds on Hydrated Metal Oxide Particles: From Simple Organic Acids to Peptides , 2009 .

[30]  Donald Fitzmaurice,et al.  Spectroscopic determination of flatband potentials for polycrystalline TiO2 electrodes in mixed solvent systems , 1994 .

[31]  D. Bahnemann,et al.  Kinetic and mechanistic investigations of multielectron transfer reactions induced by stored electrons in TiO2 nanoparticles: a stopped flow study. , 2011, The journal of physical chemistry. A.

[32]  M. Pourbaix.  Atlas of Electrochemical Equilibria in Aqueous Solutions , 1974 .

[33]  P. Kamat.  Dominance of Metal Oxides in the Era of Nanotechnology , 2011 .

[34]  Donald Fitzmaurice,et al.  Spectroscopic determination of flatband potentials for polycrystalline titania electrodes in nonaqueous solvents , 1993 .

[35]  S. Morrison.  Electrochemistry at Semiconductor and Oxidized Metal Electrodes , 1980 .

[36]  P. Bruce,et al.  Lithium intercalation into mesoporous anatase with an ordered 3D pore structure. , 2010, Angewandte Chemie.

[37]  H. Petek,et al.  Ultrafast interfacial proton-coupled electron transfer. , 2010, Chemical reviews.

[38]  Joseph T. Hupp,et al.  Energetics of Semiconductor Electrode/Solution Interfaces: EQCM Evidence for Charge-Compensating Cation Adsorption and Intercalation during Accumulation Layer Formation in the Titanium Dioxide/Acetonitrile System , 1995 .

[39]  Kaixue Wang,et al.  Light-induced formation of porous TiO2 with superior electron-storing capacity. , 2010, Chemical communications.

[40]  A. Fujishima,et al.  TiO2 photocatalysis and related surface phenomena , 2008 .

[41]  Eric A. Meulenkamp,et al.  Synthesis and Growth of ZnO Nanoparticles , 1998 .

[42]  Shane Ardo,et al.  Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces. , 2009, Chemical Society reviews.

[43]  Pengyuan Yang,et al.  Determination of metal impurities in titanium dioxide using slurry sample introduction by axial viewing inductively coupled plasma optical emission spectrometry , 2004 .

[44]  J. Mayer,et al.  Thermochemistry of proton-coupled electron transfer reagents and its implications. , 2010, Chemical reviews.

[45]  J. Durrant,et al.  Kinetic and energetic paradigms for dye-sensitized solar cells: moving from the ideal to the real. , 2009, Accounts of chemical research.