DFT Study on Atomic Layer Deposition of Cerium Dioxide on Hydroxylated Titanium Dioxide As Efficient Photocatalyst
Authors: Liu Ji , Saedy Saeed , Verma Rakshita , Ommen J.Ruud van , Rhatigan Stephen , Nolan Michael 
Published: May 2020

Design of photocatalyst materials for hydrogen production is of great interest due to a lack of cost-effective candidates. Titanium dioxide (TiO2) is a well-known photocatalyst with good stability, reasonable photoactivity, and low cost. To improve the efficiency and selectivity, we apply a surface modification approaches using nanostructured Cerium dioxide (CeO2) deposited onto TiO2 by atomic layer deposition (ALD). The deposition of CeO2 onto TiO2 uses the precursor Tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)cerium (IV) (Ce(TMHD)4) and co-reactant ozone.

The atomic layer deposition (ALD) of CeO2 on TiO2 was carried out by experiments with an atmospheric pressure fluidized bed ALD reactor. The TiO2 substrate was pre-treated using a combination of ozone and humidified nitrogen flow pulses to investigate the effect of the increase of surface functional groups on CeO2 deposition. The physicochemical properties of the obtained CeO2/TiO2 photocatalysts were characterized using different methods and their photocatalytic activity was evaluated during photocatalytic hydrogen evolution reaction.

We have also performed density functional theory (DFT) calculations to investigate the reaction mechanism of metal precursors Ce(TMHD)4 and tris(methylcyclopentadienyl)cerium (III) (Ce(MeCp)3) on hydroxylated rutile and anatase TiO2 surfaces. The hydroxyl saturation coverage on different TiO2 facets was first addressed. These facets include anatase a-TiO2(001) and a-TiO2(101) and rutile r-TiO2(100) and r-TiO2(110). Hydroxyl groups OH and H are initially binding to surface Ti and O atoms, respectively. On a-TiO2(001) hydroxyl recombination to form water results in a final coverage of 0.81ML, due to the shortest neighboring HO-H distance on all surfaces. The Gibbs energy is then calculated to elucidate the effect of pressure and temperature on the hydroxylated termination of TiO2 surfaces. The final hydroxyl coverage is as follows: 0.81ML on a-TiO2(001) surface, 1ML on a-TiO2(101) and r-TiO2(100) surfaces, and 0.75ML on r-TiO2(110) surface.

Two precursors (Ce(TMHD)4 and Ce(MeCp)3) are adsorbed on hydroxylated TiO2 surfaces. After precursor adsorption and relaxation, the surface hydroxyl coverage changes with water formation. After Ce(THMD)4 adsorbed on r-TiO2(100) surface, the hydroxyl coverage is decreased from 1ML to 0.25ML with 12 H2O molecules formation. After Ce(MeCp)3 adsorbed on a-TiO2(101) surface, although the hydroxyl coverage is not changed, the hydroxyl orientation is modified, breaking the symmetry. For Ce(TMHD)4, direct ligand dissociation is applied to study the mechanism of eliminating the ligands. For Ce(MeCp)3, the ligand can be eliminated by hydrogen transfer. The barrier for hydrogen transfer is calculated using climbing image nudged elastic band (CI-NEB) method. The results show that for Ce(MeCp)3, the reaction is endothermic on a-TiO2(101) surface, but it is overall exothermic on the other three surfaces. The facets play an important role in the eliminating the ligand. We present DFT results on the activity of ceria-modified rutile TiO2 towards water dissociation and CO2 activation showing that this heterostructured catalyst can promote both reactions. This work will be important to reveal the mechanism and feasibility of atomic layer deposition of ceria on different TiO2 surface facets.