Curved TiO₂ Nanoparticles in Water: Short (Chemical) and Long (Physical) Range Interfacial Effects

Abstract

In most technological applications, nanoparticles are immersed in a liquid environment. Understanding nanoparticles/liquid interfacial effects is extremely relevant. This work provides a clear and detailed picture of the type of chemistry and physics taking place at the prototypical TiO2 nanoparticles/water interface, which is crucial in photocatalysis and photoelectrochemistry. We present a multistep and multiscale investigation based on hybrid density functional theory (DFT), density functional tight-binding, and quantum mechanics/molecular mechanics calculations. We consider increasing water partial pressure conditions from ultra-high vacuum up to the bulk water environment. We first investigate single water molecule adsorption modes on various types of undercoordinated sites present on a realistic curved nanoparticle (2–3 nm) and then, by decorating all the adsorption sites, we study a full water monolayer to identify the degree of water dissociation, the Brønsted–Lowry basicity/acidity of the nanoparticle in water, the interface effect on crystallinity, surface energy, and electronic properties, such as the band gap and work function. Furthermore, we increase the water coverage by adding water multilayers up to a thickness of 1 nm and perform molecular dynamics simulations, which evidence layer structuring and molecular orientation around the curved nanoparticle. Finally, we clarify whether these effects arise as a consequence of the tension at the water drop surface around the nanosphere by simulating a bulk water up to a distance of 3 nm from the oxide surface. We prove that the nanoparticle/water interfacial effects go rather long range since the dipole orientation of water molecules is observed up to a distance of 5 Å, whereas water structuring extends at least up to a distance of 8 Å from the surface.

Impact of surface curvature, grafting density and solvent type on the PEGylation of titanium dioxide nanoparticles

“…The long-range effect of the TiO 2 nanoparticles surface on water has been already shown in other classical molecular dynamics study [54,56,84]. In particular, it has been reported in a recent publication by some of us [57], that the effect is propagated even to longer distances if a correct quantum chemical explicit description of the solvent, with a DFT-based method, is employed. Our simulation clearly shows that water tends to gather around the NP surface, increasing its density up to a distance of, at least, 6.0 Å.…”

Interfacing CRYSTAL/AMBER to Optimize QM/MM Lennard–Jones Parameters for Water and to Study Solvation of TiO2 Nanoparticles

“…As mentioned in Section 1 , it is important to explore the effects of including increasing amounts of water around NPs, since the nanoparticle/water interfacial effects are quite long range. Here, we have studied three systems: (A) the bare NP (TiO 2 ) 223 ·10H 2 O ( Figure 4 a); (B) the NP with a first water monolayer (ML) adsorbed on the surface containing 134 molecules, ∼20% of which are dissociated, as discussed and detailed in Section 4.2.5 and in [ 28 ] ( Figure 4 b); and (C), comprised of system B, with a molecular mechanic (MM) region composed by 824 surrounding waters added around it ( Figure 4 c). The figure shows the structures after geometry optimizations.…”

“…The figure shows the structures after geometry optimizations. Systems A and B were originally prepared for a previous study by some of us [ 28 ]. The geometry optimization for system C was carried out twice, once with the water in the MM region described with the CF LJ parameters, and once with the TIP3P LJ parameters, to assess any possible effects on the NP.…”

“…The exact exchange term partially corrects for the electronic self-interaction error that is present in standard GGA functionals, such as PBE, and causes a large underestimation of the band gap, as is evident from Figure 6 . We wish to comment on the fact that the DFTB+/AMBER approach used in previous work [ 28 ] is clearly faster than the current approach that does not employ the Tight-Binding approximation. The previous approach allows introducing a much larger number of water molecules to describe the bulk of water around the nanoparticle.…”

“…The procedure is described in detail in ref. [ 28 ]. The most stable water monolayer around the NP is constituted by 134 water molecules and has an extent of dissociation = 0.21 (see Figure 4 b).…”