Single-site and monolayer surface hydration energy of anatase and rutile nanoparticles using density functional theory

Models of 1, 2, and 3 nm diameter anatase and rutile particles, with either a surface monolayer of water or a single water molecule at different surface sites, were subjected to energy minimizations using density functional theory (DFT). The optimized structures show that H₂O molecules bind covalently to both anatase and rutile particles via undercoordinated Ti atoms at the surface of the particle, with a significant degree of hydrogen bonding to other surface waters and bridging oxygens. Ti-OH₂ bonds are more highly ordered on anatase surfaces but are stronger on rutile surfaces. Energies of the fully optimized structures with and without water show that hydration of rutile surfaces is more strongly exothermic than is the hydration of anatase surfaces, and this effect becomes more pronounced as particle size decreases. Individual surface sites exhibit a wide range of hydration energies, explaining the strong experimental dependence of hydration energy on water coverage. The less exothermic surface hydration energy of anatase offsets its lower vacuum surface energy and makes rutile nanoparticles thermodynamically competitive with anatase in low-temperature aqueous solutions.