Characterization of the Mobility and Reactivity of Water Molecules on TiO₂ Nanoparticles by ¹H Solid-State Nuclear Magnetic Resonance

Abstract

Understanding interfacial water behavior is essential to improving our understanding of the surface chemistry and interfacial properties of nanomaterials. Here using 1H solid-state nuclear magnetic resonance (1H SSNMR), we successfully monitored ligand exchange reaction between oleylamine (OLA) and adsorbed water on titanium dioxide nanoparticles (TiO2 NPs). Three different types of interfacial waters with different reactivities were distinguished. The mobility of the adsorbed water molecules was characterized by dipolar filtered 1H SSNMR. Our experimental results demonstrate that the adsorbed water can be categorized into three different layers: (i) rigid water species with restricted mobility closest to the surface of TiO2 NPs, (ii) less mobile water species weakly confined on TiO2 NPs, and (iii) water molecules with high mobility. Water in the third layer could be replaced by OLA, while water in the first and second layers remained intact. The finding that the interfacial water with the highest mobility has the strongest reactivity has guiding significance for tailoring the hydrophilic and hydrophobic properties of TiO2 NPs.

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

“…As mentioned previously, a simple and green way to fabricate TiO 2 NPs with uniform particle size and high surface hydroxyl groups is absolutely required for industrial production. On the basis of the results of this study and the literature [20,43,49,50,51,52,53,54,55,56], a pictorial representation of the structure of water layers adsorbed on the surface of TiO 2 NPs is proposed in Scheme 1.…”

Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics

“…similar to the three-domain models (with irreversibly bound waters, partially mobile waters, and bulk waters) that have been used to interpret solid-state nuclear magnetic resonance (NMR) data for hydrated TiO 2 nanoparticles. 10,63 The total hydration layer thickness (defined as the difference in positions of the outermost L i -peak and the top oxygen layer in the TiO 2 slab) is 5 to 7.5 Å for the different surfaces (Fig. 5), with anatase (100) being a notable exception, as discussed below.…”

“…7,8 Understanding water adsorption on TiO 2 is the key not only to efficiently split water molecules (as made use of in hydrogen gas production 9 ) but also to optimize adsorption on, and functionalization of, TiO 2 by larger molecules as mediated via strongly bound surface waters. [10][11][12][13][14][15] On the other hand, concerns have been raised with respect to environmental safety and health when TiO 2 is used in medical implants 16 and as engineered nanoparticles in consumer products. 17 TiO 2 nanoparticles are small enough to penetrate the blood-brain barrier, 18 can aggregate in organisms and the environment, and yield a nanotoxic response by increasing the levels of intracellular reactive oxygen species, leading to apoptosis.…”

“…[24][25][26][27][28]62 Although the exact structure of the fully hydrated TiO 2 -water interface is still not fully resolved, 32 the surface interactions are strong enough to induce three ordered solvent layers (∼10 Å boundary layer). 10,[63][64][65] Here, we use ab initio molecular dynamics (AIMD) simulations to determine the structure of the hydration layer for six different TiO 2 surfaces at full water coverage (Fig. 1).…”

Reactive wetting properties of TiO2 nanoparticles predicted by ab initio molecular dynamics simulations

“…22). 32,33 For example, Zhang et al 34 found low Ti coordination numbers (5.1 vs. 6 for bulk) for small-sized (2 nm) amorphous nanoparticles using synchrotron wideangle X-ray scattering (WAXS) and molecular modeling. Much less is known about the irregular surfaces of wet nanoparticles, where the higher frequency of surface defects may lead to higher reactivity.…”