Title: Pathways Following Electron Injection: Medium Effects and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst Assembly on TiO₂

Abstract

Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO₃H₂)₂bpy)₂Ru^II(bpy-bimpy)Ru^II(tpy)(OH₂)]⁴⁺, -[Ru_a^II–Ru_b^II–OH₂]⁴⁺, on nanocrystalline TiO₂ electrodes, starting from either -[Ru_a^II–Ru_b^II–OH₂]⁴⁺ or -[Ru_a^II–Ru_b^III–OH₂]⁵⁺, have been investigated. Transient absorption measurements for TiO₂–[Ru_a^II–Ru_b^II–OH₂]⁴⁺ in 0.1 M HPF₆ or neat trifluoroethanol reveal that electron injection occurs with high efficiency but that hole transfer to the catalyst, which occurs on the electrochemical time scale, is inhibited by local environmental effects. Back electron transfer occurs to the oxidized chromophore on the microsecond time scale. Photoexcitation of the once-oxidized assembly, TiO₂–[Ru_a^II–Ru_b^III–OH₂]⁵⁺, in a variety of media, generates -[Ru_a^III–Ru_b^III–OH₂]⁶⁺. The injected electron randomly migrates through the surface oxide structure reducing an unreacted -[Ru_a^II–Ru_b^III–OH₂]⁵⁺ assembly to -[Ru_a^II–Ru_b^II–OH₂]⁴⁺. In a parallel reaction, -[Ru_a^III–Ru_b^III–OH₂]⁶⁺ formed by electron injection undergoes proton loss giving -[Ru_a^II–Ru_b^IV$$=$$O]⁴⁺ with possible conversion to -[Ru_a^II–Ru_b^II–OH₂]⁴⁺ by an electrolyte-mediated reaction. In the following slow step, re-equilibration on the surface occurs either by reaction with added Fe^III/II or by cross-surface electron transfer between spatially separated -[Ru_a^II–Ru_b^IV$$=$$O]⁴⁺ and -[Ru_a^II–Ru_b^II–OH₂]⁴⁺ assemblies to give -[Ru_a^II–Ru_b^III–OH₂]⁵⁺ with a half-time of t_1/2 ~ 68 μs. Finally, these results and analyses show that the transient surface behavior of the assembly and cross-surface reactions play important roles in producing and storing redox equivalents on the surface that are used for water oxidation.

Authors:

Brennaman, M. Kyle  ^[1];

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• Search DOE PAGES for ORCID "0000-0003-2364-3044"

Gish, Melissa K. ^[1]; Alibabaei, Leila ^[1]; Norris, Michael R. ^[2]; Binstead, Robert A. ^[1]; Nayak, Animesh ^[1]; Lapides, Alexander M. ^[1]; Song, Wenjing  ^[3];

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Brown, Robert J. ^[1]; Concepcion, Javier J.  ^[4];

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• Search DOE PAGES for ORCID "0000-0002-9296-0943"

Templeton, Joseph L. ^[1]; Papanikolas, John M.  ^[1];

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• Search DOE PAGES for ORCID "0000-0002-0902-6990"

Meyer, Thomas J.  ^[1]

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• Search DOE PAGES for ORCID "0000-0002-7006-2608"

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+ Show Author Affiliations

1. Univ. of North Carolina, Chapel Hill, NC (United States). Dept. of Chemistry
2. Univ. of North Carolina, Chapel Hill, NC (United States). Dept. of Chemistry; Univ. of Richmond, VA (United States). Dept. of Chemistry
3. Univ. of North Carolina, Chapel Hill, NC (United States). Dept. of Chemistry; Chinese Academy of Sciences (CAS), Beijing (China). Inst. of Chemistry, Beijing National Lab. for Molecular Sciences, Lab. of Photochemistry
4. Univ. of North Carolina, Chapel Hill, NC (United States). Dept. of Chemistry; Brookhaven National Lab. (BNL), Upton, NY (United States). Dept. of Chemistry

Publication Date:

Fri Jun 08 00:00:00 EDT 2018

Research Org.:

Energy Frontier Research Centers (EFRC) (United States). Center for Solar Fuels (UNC EFRC)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1470650

Grant/Contract Number:  

SC0001011

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Physical Chemistry. C

Additional Journal Information:

Journal Volume: 122; Journal Issue: 24; Related Information: UNC partners with University of North Carolina (lead); Duke University; University of Florida; Georgia Institute of Technology; University; North Carolina Central University; Research Triangle Institute; Journal ID: ISSN 1932-7447

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; catalysis (homogeneous); catalysis (heterogeneous); solar (photovoltaic); solar (fuels); photosynthesis (natural and artificial); hydrogen and fuel cells; electrodes - solar; charge transport; materials and chemistry by design; synthesis (novel materials); synthesis (self-assembly)