Diffusional Mediation of Surface Electron Transfer on TiO<sub>2</sub>

Trammell, Scott A.; Meyer, Thomas J.

doi:10.1021/jp9825258

Cited by 120 publications

(207 citation statements)

References 10 publications

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“…S4). A related phenomenon was reported earlier for ½OsðbpyÞð4;4 0 -ðPO 3 H 2 Þ 2 bpyÞ 2þ surfacebound to nanoTiO 2 and attributed to OsðII → IIIÞ oxidation by cross-surface, diffusional electron transfer (28). In this process, site-to-site electron transfer hopping and counter ion transport occur between surface couples providing a cross-surface electron transfer channel to the underlying electrode.…”

supporting

confidence: 73%

Proton-coupled electron transfer at modified electrodes by multiple pathways

Chen

Vannucci

Concepcion

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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In single site water or hydrocarbon oxidation catalysis with polypyridyl Ru complexes such as ½Ru II ðMebimpyÞðbpyÞðH 2 OÞ 2þ [where bpy is 2,2′-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog ½Ru II ðMebimpyÞ ð4,4 0 -bis-methlylenephosphonato-2,2 0 -bipyridineÞðOH 2 Þ 2þ 2-PO 3 H 2 , accessing the reactive states, Ru V ¼O 3þ ∕Ru IV ¼O 2þ , at the electrode interface is typically rate limiting. The higher oxidation states are accessible by proton-coupled electron transfer oxidation of aqua precursors, but access at inert electrodes is kinetically inhibited. The inhibition arises from stepwise mechanisms which impose high energy barriers for 1e − intermediates. Oxidation of the Ru III -OH 2þ or Ru III -OH 2 3þ forms of 2-PO 3 H 2 to Ru IV ¼O 2þ on planar fluoride-doped SnO 2 electrode and in nanostructured films of Sn(IV)-doped In 2 O 3 and TiO 2 has been investigated with a focus on identifying microscopic phenomena. The results provide direct evidence for important roles for the nature of the electrode, temperature, surface coverage, added buffer base, pH, solvent, and solvent H 2 O∕D 2 O isotope effects. In the nonaqueous solvent, propylene carbonate, there is evidence for a role for surface-bound phosphonate groups as proton acceptors.concerted electron-proton transfer | metal oxide electrode | surface modification | electrocatalysis | spectroelectrochemistry E lectron transfer reactions involving pH-dependent protoncoupled electron transfer (PCET) couples with a change in proton content between oxidation states, such as quinone/hydroquinone (Q∕H 2 Q) or M¼O∕M-OH∕M-OH 2 oxo/hydroxo/aqua transition metal couples, are often slow at inert electrodes (1-5). For these couples, the change in protonation state and the requirement to add or lose protons adds to the normal kinetic barrier to electron transfer. The PCET effect arises from the fact that oxidation or reduction at the electrode occurs by electron transfer without a change in proton content. This effect restricts interfacial mechanisms to electron transfer followed by proton transfer (ET-PT) or proton transfer followed by electron transfer (PT-ET). Both involve high-energy intermediates in nonequilibrium protonation states.As an example, oxidation of H 2 Q, H 2 Q − e − → H 2 Q þ• , occurs at E o0 ¼ 1.10 V vs. normal hydrogen electrode (NHE) independent of pH (E o0 is the formal potential) (2). The thermodynamic potential for H 2 Q oxidation at pH 7,. An inert electrode at pH ¼ 7 creates an overpotential of 0.47 V for the initial electron transfer in the ET-PT sequence. Following electron transfer, thermodynamic equilibrium is reached by proton loss from H 2 Q þ• to the surrounding medium at the prevailing pH with ΔE o0 ¼ 1.10 V − 0.059fpH − ðpK a ðH 2 Q þ• Þg with pK a ðH 2 Q þ• Þ ¼ −0.95. Oxidation can also occur by PT-ET with initial loss of a proton from H 2 Q to give HQ − followed by oxidation to HQ• at E o0 ðHQ • ∕HQ − Þ ¼ 0.46 V. For this mechanism, an inhibition to rate arises from the pH depende...

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supporting

confidence: 73%

Proton-coupled electron transfer at modified electrodes by multiple pathways

Chen

Vannucci

Concepcion

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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“…Impedance spectroscopy has proved to be a very useful technique for the characterization of properties of dye solar cells 1,[19][20][21][22] and electrochromic windows. 23 Recently, a new approach has been taken [24][25][26][27][28] toward electric and electrooptic devices formed by molecular functionalized mesoscopic oxides, such as photovoltaic cells, sensors, and Figure 1. Scheme of the two-channel transmission line model for a porous electrode in electrolyte solution.…”

Section: Introductionmentioning

confidence: 99%

Three-Channel Transmission Line Impedance Model for Mesoscopic Oxide Electrodes Functionalized with a Conductive Coating

et al. 2006

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A three-channel transmission line (TL) impedance model is proposed to address the charge transport behavior of molecular functionalized mesoscopic oxide electrodes at different bias conditions. A full general solution of the three-channel TL for the system is provided in this paper. Selected experimental results of impedance spectroscopy of mesoscopic Al 2 O 3 and TiO 2 networks, covered with a monolayer of Ru complex cis-RuLL′-(NCS) 2 (L ) 2,2′-bipyridyl-4,4′-dicarboxylic acid, L′ ) 4,4′-dinonyl-2,2′-bipyridyl) (Z907), are briefly discussed. It shows that the model constitutes a useful tool for characterizing nanoporous electrodes functionalized with organic conducting layers in the surface. The model makes it possible to determine the separate conductivity of substrate oxide and molecular layer, and interfacial charge transfer, in the functionalized nanostructured electrodes.

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“…Previous studies have shown that Ru II and other sensitizers anchored to mesoporous nanocrystalline TiO 2 thin films can be reversibly oxidized in standard electrochemical cells provided that the sensitizer surface coverage exceeds a percolation threshold. [74][75][76] Cyclic voltammetry and spectroelectrochemistry are thus powerful in situ tools for determining formal reduction potentials and absorption spectra of relevant redox states.…”

Section: Charge Separationmentioning

confidence: 99%

Solar Photochemistry with Transition Metal Compounds Anchored to Semiconductor Surfaces

Meyer

2010

Physical Inorganic Chemistry

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There exists a critical need for sustainable power on a terawatt (TW ¼ 10 12 W) scale. 1,2 As the world's need for energy is related to the number of people on Earth, a staggering 45% population growth over the last quarter-century equates to roughly 2 billion people and a 6 TW ($63%) increase in needed power. 3 In addition, the urbanism of third-world and industrialized nations and cities has led to an increase in the demand for fuel that has driven gas and oil prices to record highs. 3 Regardless of price, the continued use of fossil fuels cannot be a long-term solution as they come from a limited stock and the deleterious environmental consequences of their combustion have become self-evident. 4,5 The increased average global temperature and rates of glacial melting measured over the past few decades are telling signs. [6][7][8][9] Concern should be elicited as ice-core data correlate temperature with greenhouse gas concentrations over the past half-a-million years. The present 380 ppm atmospheric CO 2 levels exceed any values attained over the same time period. 5,6 Further, other than natural photosynthesis, there exist no obvious means by which we can decrease today's level. Thus, population, energy demand, and fuel prices do not convey the severity of the need for sustainable energy.It is our belief that molecular assemblies, particularly molecules arranged at semiconductor interfaces, will one day efficiently harvest and convert energy from the Sun, thereby providing sustainable, carbon-neutral power for future generations. The Sun is in fact the only source that on its own can provide the TWs of power needed. It has been shown that the amount of solar energy reaching the Earth in one day could power the planet for an entire year. 7,8 Remaining is the important challenge of harvesting, converting, and storing this energy in a cost-effective way. Coordination Physical Inorganic Chemistry: Reactions, Processes, and Applications Edited by Andreja Bakac

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Diffusional Mediation of Surface Electron Transfer on TiO₂

Cited by 120 publications

References 10 publications

Proton-coupled electron transfer at modified electrodes by multiple pathways

Proton-coupled electron transfer at modified electrodes by multiple pathways

Three-Channel Transmission Line Impedance Model for Mesoscopic Oxide Electrodes Functionalized with a Conductive Coating

Solar Photochemistry with Transition Metal Compounds Anchored to Semiconductor Surfaces

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Diffusional Mediation of Surface Electron Transfer on TiO2

Cited by 120 publications

References 10 publications

Proton-coupled electron transfer at modified electrodes by multiple pathways

Proton-coupled electron transfer at modified electrodes by multiple pathways

Three-Channel Transmission Line Impedance Model for Mesoscopic Oxide Electrodes Functionalized with a Conductive Coating

Solar Photochemistry with Transition Metal Compounds Anchored to Semiconductor Surfaces

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Diffusional Mediation of Surface Electron Transfer on TiO₂