Aqueous TiO2 Nanoparticles React by Proton-Coupled Electron Transfer

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 (zeta) 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 zeta 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 O-2 (air) and 4-MeO-TEMPO (4-methoxy-2,2,6,6-tetramethylpiperine-1-oxy radical). The difference in the number of protons released with KI3 vs O-2 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 center dot 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 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 zeta 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.

Automated summary

This study investigates the relationship between the redox reactions of aqueous colloidal TiO2 nanoparticles and pH. The results show that the reduction potentials of the nanoparticles are linearly correlated to the pH values, with a coefficient of -60 +/- 5 mV/pH. Additionally, it is found that both capped and uncapped nanoparticles exhibit similar reduction potentials at low pH. Furthermore, proton-coupled electron transfer is suggested to be the preferred mode of redox reactions.