Effect of Water Adsorption on Carrier Trapping Dynamics at the Surface of Anatase TiO₂ Nanoparticles

Abstract

Charge carrier trapping plays a vital role in heterogeneous photocatalytic water splitting because it strongly affects the dynamics of photogenerated charges and hence the photoconversion efficiency. Although hole trapping by water at water/photocatalyst interface is the first step of oxygen evolution in water splitting, little has been known on how water adsorbate itself is involved in hole trapping dynamics. To clarify this point, we have performed infrared transient and steady-state absorption spectroscopy of anatase TiO2 nanoparticles as a function of the number of water adsorbate layers. Here, we demonstrate that water molecules reversibly adsorbed in the first layer on TiO2 nanoparticles are capable to trap photogenerated holes, while water in the second layer hydrogen bonding to the first-layer water makes hole trapping less effective.

Markedly Enhanced Surface Hydroxyl Groups of TiO2 Nanoparticles with Superior Water-Dispersibility for Photocatalysis

“…The relative amounts of surface OH groups were normalized to Degussa P25 as follows: Degussa:ST-01:AHP-ST-01 = 1:1.9:3.2, listed in Table 1. However, it should be noted that the water adsorbates observed under the vacuum were very strongly bound through oxygen of water to a less coordinated Ti cation and a hydrogen bond to a surface oxygen atom [8]. In the FTIR study of ST-01 at 0.01 Pa, 80% of the intensity of the OH stretching band at room temperature was contributed by molecularly adsorbed water, while only 20% was by surface OH.…”

“…The surface hydroxyl group density of TiO 2 is considered to play a significant role in the photocatalytic processes [3,4,5]. Surface hydroxylated centers have been proposed to act as trapping sites for both electrons and holes migrated to the surface [6,7,8]. Several investigations have demonstrated that the additional hydroxyl groups on the TiO 2 surface can improve the adsorption capacity, the formation of mesoporous structure, and the catalytic efficiency [9,10,11].…”

Role of Adsorbed Water on Charge Carrier Dynamics in Photoexcited TiO2

“…8 On the other hand, a recent study of photogenerated electron dynamics at the picosecond to nanosecond time scale shows that adsorbed water suppresses charge carrier recombination in anatase TiO 2 . 27,28 In order to evaluate whether adsorbed water has a positive or an adverse effect on the charge carrier recombination at the millisecond to minute time scale, we studied photogenerated electron absorption dynamics in the material which was dehydrated under dynamic vacuum ( p < 10 –3 mbar) at 623 K (sample iii). The room-temperature dark DRIFT spectrum of this sample is shown in Figure 1 (red curve).…”

Influence of Surface‐Related Phenomena on Mechanism, Selectivity, and Conversion of TiO2‐Induced Photocatalytic Reactions

“…This mechanism is in accord with other relevant reports. [24] Water in the second layer weakens the interaction between first-layer water and TiO 2 ,t hus reducing the hole-trappingc apability. It has been found that hydrogen bondingw ith adsorbed water ontoa natase TiO 2 (1 01)s tabilizes surfacetrapped holes, lowers the barrier for hole migration (which enhances carrier mobility), and suppresses chargesrecombination on the timescale of minutes.…”

“…[24,72] Blockingthese sites by surfacemodification statistically reduces subsequent interfacial electron transfer,s ot hat the most probablef ate of the charges is recombination. [24,72] Blockingthese sites by surfacemodification statistically reduces subsequent interfacial electron transfer,s ot hat the most probablef ate of the charges is recombination.…”