Deciphering the Anomalous Acidic Tendency of Terminal Water at Rutile(110)–Water Interfaces

Abstract

Understanding the mechanism of the oxygen evolution reaction (OER) is essential to improve the efficiency of photocatalysis for TiO2. Previous studies have highlighted the importance of terminal hydroxide radical (TiOH•) in the OER. Ab initio molecular dynamics simulations (AIMD) with hybrid functional have revealed that this radical readily loses its proton, creating the key intermediate, oxygen radical anion (Ti5cO•–). Herein, we combine machine-learning-accelerated molecular dynamics with density functional theory calculations to demonstrate that the Ti5cO•– can alternatively be generated through the trapping of a hole in a terminal oxygen anion (Ti5cO2–) at rutile(110)–water interfaces. Further examination reveals that the Ti5cO2– results from the deprotonation of Ti5cOH– and remains stable at the charge-neutral interfaces for a transient time period of ca. 100 ps. The AIMD-based free energy perturbation method predicts that the acidity constant of Ti5cOH– is even smaller than that of Ti5cOH2, thereby rationalizing the stability of Ti5cO2–. Structural analyses show that this anomalous acidic tendency of terminal water originates from the decrease of Ti–O bond length and the transition of Titanium’s coordination from octahedral to pyramidal in Ti5cO2–. Our findings provide valuable insights into the surface acid–base chemistry and a potential explanation for the pH-dependent behavior of photogenerated holes for TiO2.

Accelerating Computation of Acidity Constants and Redox Potentials for Aqueous Organic Redox Flow Batteries by Machine Learning Potential-Based Molecular Dynamics

“…This problem can be solved by binding the dummy atom with the conjugated base to maintain the approximate geometry of the acid with a harmonic restraint potential V r , in which the force constants are set to 0.05 au For η = 1.00, spontaneous proton transfer from the water molecule to the conjugate base would occur during AIMD simulation. To keep the hydrogen atoms in water molecules from being transferred to the conjugate base, a Buckingham-type repulsive potential V normalr normale normalp = normalA normale − d / B is applied, and the parameters A and B are 1254.7 eV and 0.1724 Å. , Then, the mapping potential is expanded as E η = ( 1 − η ) E I + η E F + V r + V normalr normale normalp Note that the applied restraint and repulsive potentials show negligible effect on the computed thermodynamic integrals, which has been well established in previous studies …”

“…is applied, and the parameters A and B are 1254.7 eV and 0.1724 Å. 58,59 Then, the mapping potential is expanded as…”

Molecular Mechanisms of Oxygen Evolution Reactions for Artificial Photosynthesis

“…Having consensus on the assumption that the first oxidation takes place at bridged oxygen Ti-O-Ti, the reaction mechanism of OER was discussed [2]. Recently, Zhuang and Cheng [121] reported that at the rutile (110) surface, pKa of Ti 5C OH 2 is larger than that of Ti 5C OH-, and then the coordination structure Ti 5C O 2− exists stably in water and easily becomes Ti-O•. These results in the calculation seem to contradict the fact that the isoelectric point of rutile TiO 2 is around pH 6.…”

Molecular Mechanisms of Oxygen Evolution Reactions for Artificial Photosynthesis

“…It has had a consensus that first oxidation takes place at bridged oxygen Ti-O-Ti, and the reaction mechanism of OER was discussed [2]. Recently, Zhuang and Cheng [121] reported that at the rutile (110) surface, pKa of Ti5COH2 is larger than that of Ti5COH-, and then the coordination structure, Ti5CO 2-exist stably in water and easily become Ti-O•. This results in calculation seems to contradict to the fact that isoelectric point of rutile TiO2 is around pH 6.…”