Abstract
The adsorption properties of water molecules on TiO2 nanotubes (TiO2NT) and the interaction mechanisms between water molecules are studied by first principles calculations. The adsorption preferences of water molecules in molecular or dissociated states on clean and H-terminated TiO2NT are evaluated. Adsorption of OH clusters on (0, 6) and (9, 0) TiO2 nanotubes are first studied. The smallest adsorption energies are −1.163 eV and −1.383 eV, respectively, by examining five different adsorption sites on each type of tube. Eight and six adsorption sites were considered for OH adsorbtion on the H terminated (0, 6) and (9, 0) nanotubes. Water molecules are reformed with the smallest adsorption energy of −4.796 eV on the former and of −5.013 eV on the latter nanotube, respectively. For the adsorption of a single water molecule on TiO2NT, the molecular state shows the strongest adsorption preference with an adsorption energy of −0.660 eV. The adsorption of multiple (two and three) water molecules on TiO2NT is also studied. The calculated results show that the interactions between water molecules greatly affect their adsorption properties. Competition occurs between the molecular and dissociated states. The electronic structures are calculated to clarify the interaction mechanisms between water molecules and TiO2NT. The bonding interactions between H from water and oxygen from TiO2NT may be the reason for the dissociation of water on TiO2NT.
Highlights
The demands of energy and the control of environment pollution are factors that inspire us to develop clean and renewable energy sources
The adsorption energies of the water molecule and OH cluster on TiO2 nanotubes (TiO2 NT) are evaluated via the following definition: Eads = Ent+a − Ent − Ea where Ent+a and Ent denote the total energies of the adsorbed water/hydroxyl and clean TiO2 NTs, respectively
The Ea is the total energy of the adsorbate evaluated using a 1.0 nm × 1.0 nm ×1.0 nm cell
Summary
The demands of energy and the control of environment pollution are factors that inspire us to develop clean and renewable energy sources. Hydrogen is one of the most promising energy carriers due to its high energy density and zero pollution. The production of hydrogen with low cost still hinders its application as an energy carrier. Splitting of water on photocatalysts with solar energy is a desirable way to produce hydrogen without pollution and with low-cost. Honda and Fujishima proved photocatalytic hydrogen production from water and demonstrated the decomposition concept using a photo-electrochemical cell [1]. Many photocatalysts have been subsequently developed with high quantum efficiency for water splitting under UV and visible light illumination [2,3,4], such as
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