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Aqueous TiO2 Nanoparticles React by Proton-Coupled Electron Transfer.

Jennifer L Peper1, Noreen E Gentry1, Benjamin Boudy1

  • 1Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States.

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Redox potentials of titanium dioxide nanoparticles (TiO2 NPs) are pH-dependent and involve proton-coupled electron transfer (PCET), not surface charge. This PCET mechanism is crucial for understanding TiO2 NP reactivity in aqueous systems.

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Area of Science:

  • Materials Science
  • Electrochemistry
  • Nanotechnology

Background:

  • Aqueous colloidal titanium dioxide nanoparticles (TiO2 NPs) exhibit redox activity.
  • Understanding the factors influencing TiO2 NP redox potential is critical for their application.
  • Previous explanations for pH dependence often relied on surface charge, which may be insufficient.

Purpose of the Study:

  • To investigate the redox reactions of citrate-capped (c-TiO2) and uncapped (uc-TiO2) 4 nm TiO2 NPs in aqueous solution.
  • To determine the relationship between TiO2 NP reduction potential and pH.
  • To elucidate the mechanism governing the pH dependence of TiO2 NP redox behavior.

Main Methods:

  • Photoreduction of TiO2 NPs to create stable blue colloidal NPs.
  • Equilibration of reduced NPs with soluble redox reagents (e.g., methylviologen) to measure colloid reduction potential.
  • Oxidation of reduced NPs using electron-transfer oxidants (e.g., KI3, O2, 4-MeO-TEMPO) to observe pH changes and proton release.

Main Results:

  • TiO2 NP reduction potentials varied linearly with pH (slope of -60 ± 5 mV/pH) from pH 2-9.
  • Reduction potentials did not correlate with zeta potentials or surface charge, and were similar for capped and uncapped NPs.
  • Oxidation released protons, indicating proton-coupled electron transfer (PCET) as the dominant mechanism over pH 2-13.

Conclusions:

  • The pH dependence of TiO2 NP redox potential is governed by PCET, not surface charging.
  • The measured redox potentials correspond to TiO2-H bond dissociation free energy (BDFE), determined to be 49 ± 2 kcal mol-1.
  • PCET is likely the norm for redox-active oxide/water interfaces.