Aqueous TiO₂ Nanoparticles React by Proton-Coupled Electron Transfer

Journal Article · Thu Dec 30 00:00:00 EST 2021 · Inorganic Chemistry

DOI:https://doi.org/10.1021/acs.inorgchem.1c03125· OSTI ID:1872429

Peper, Jennifer L. ^[1]; Gentry, Noreen E. ^[1]; Boudy, Benjamin ^[1]; Mayer, James M.  ^[1]

---

1. Yale University, New Haven, CT (United States)

Redox reactions of aqueous colloidal TiO2 4 nm nanoparticles (NPs) have been examined, including both citrate-capped and uncapped NPs (c-TiO2 and uc-TiO2). Photoreduction gave stable blue colloidal c-TiO2R NPs with 10-60 electrons per particle. Equilibration of these reduced NPs with soluble redox reagents such as methylviologen (MV2+) provided measurements of the colloid reduction potential as a function of pH. The potentials of c-TiO2 from pH 2-9 varied linearly with pH, with a slope of –60 ± 5 mV/pH. Estimates of the potential at pH 12 were consistent with extrapolating that line to high pH. The reduction potentials did not correlate with the zeta potentials ( or the surface charge of the NPs across this pH range. Similar reduction potentials were observed for c- and uc-TiO2 at low pH even though they have quite different  potentials. These results show that the common surface-charging explanation of the pH dependence is not tenable in these systems. Oxidation of reduced c-TiO2R with the electron-transfer oxidant potassium triiodide (KI3) occurred with a significant drop in pH, showing that protons were released when the electrons were removed from the NPs. Smaller pH drops were observed for the proton-coupled electron transfer (PCET) reagents O2 (air) and 4-MeO-TEMPO (4-methoxy-2,2,6,6-tetramethyl-piperine-1-oxy radical). The difference in the number of protons released with KI3 vs. O2 and 4-MeO-TEMPO was roughly one proton per electron removed. Thus, the thermodynamically preferred reactivity of these colloidal TiO2 NPs is PCET over the pH 2-13 range studied. The measured redox potentials refer to the chemical process TiO2 + H+ + e–  TiO2•e–,H+, and therefore they do not correspond with an electronic energy such as a conduction band edge or flat band potential. The 1e–/1H+ stoichiometry means that the reduction TiO2 reduction potentials correspond to a TiO2–H bond dissociation free energy (BDFE), determined to be 49 ± 2 kcal mol-1. The PCET description is consistent with the pH dependence of E(TiO2/TiO2•e–,H+), the release of protons upon oxidation, the lack of correlation with  potentials, the similarity of capped and uncapped NPs, and the small change in the potential and BDFE from the first to the last electron/proton pair (H-atom) removed. This behavior is suggested to be the norm for redox-active oxide/water interfaces.

• Research Organization: Yale Univ., New Haven, CT (United States); Energy Frontier Research Centers (EFRC) (United States). Center for Light Energy Activated Redox Processes (LEAP)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); US-Israel Binational Science Foundation
• Grant/Contract Number: SC0021298; SC0001059; 2016123
• OSTI ID: 1872429
• Alternate ID(s): OSTI ID: 1840672; OSTI ID: 1992180
• Journal Information: Inorganic Chemistry, Vol. 61, Issue 2; ISSN 0020-1669
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Redox reactions
Oxides
Proton coupled electron transfer
Colloid
pH