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Liu H.,City University of Hong Kong | Liu H.,Wuhan University | Liu H.,PLA Military Economics Academy | Liew K.M.,City University of Hong Kong | Pan C.,Wuhan University
RSC Advances | Year: 2014

We performed a systematic investigation of adsorption of small gas molecules (O2, CO, NO, NO2 and SO2) on pristine and fluorine doped (F-doped) anatase TiO2 (001) surface using density functional theory (DFT). Three kinds of F-dopants, which were achieved by substituting a surface O2C atom (FI), or a surface O3C (FII), or an O3C atom below a surface Ti5C with an F atom (FIII), were studied to investigate the effects on the surface properties as well as the adsorption of molecules. The influence of F-dopants on the adsorption energy, charge transfer and magnetic moment of the most stable adsorption configurations of these molecules on the surfaces are thoroughly discussed. Three types of F-dopants were found to significantly promote the adsorption of O2, NO and NO2. However, the promotive effect of FI and FII dopants was not found upon the adsorption of CO and SO2. Only the FIII dopant was found to have a promotive effect on the two molecules. The mechanisms of interactions between molecules and surfaces are examined by analyzing their electronic structure and charge transfer. The results show that Ti3+ induced by F-dopants plays an important role in enhancing the interaction between gas molecules and TiO2 surfaces. This journal is © the Partner Organisations 2014.

Liu H.,City University of Hong Kong | Liu H.,Wuhan University | Liu H.,PLA Military Economics Academy | Liew K.M.,City University of Hong Kong | Pan C.,Wuhan University
Physical Chemistry Chemical Physics | Year: 2013

The adsorption of formaldehyde (HCHO) on both clean and hydroxylated TiO2-B(100) surfaces with terminal and bridging hydroxyl groups is systematically investigated by using first principles density functional theory calculations. The discussion is mainly focused on the two different chemical adsorption configurations of HCHO in periodicity (2 × 2), in which the C atom of HCHO is bonded with two coordinated O atoms on a step (Structure I) or on a terrace (Structure II). The study indicates that bridging hydroxyl groups on most of the adsorption sites near to HCHO will weaken the adsorption of HCHO, while terminal hydroxyl groups on most of adsorption sites will facilitate it. The investigation of the effects of hydroxyl groups and H2O molecule on HCHO in different periodicities shows that the terminal hydroxyl groups or H2O molecules have significantly facilitated the adsorption of H 2O at larger periodicities, while bridging hydroxyl groups do not have this trend. The analysis of the adsorption mechanisms of HCHO molecules on both clean and hydroxylated surfaces indicate that the terminal hydroxyl groups can extract electrons from the surface and facilitate adsorption of HCHO due to the adsorption energy being higher than that on the clean surface, while the bridging hydroxyl groups donate electrons to the surface and weaken the adsorption. In all chemical adsorption configurations, HCHO acts as an electron acceptor. Interestingly, though the adsorptions are weaker, HCHO in Structure II gains more electrons on both the clean and hydroxylated surfaces than in Structure I. This unique mechanism provides a novel angle to understand the interaction of HCHO with the hydroxylated TiO2 surface. © the Owner Societies 2013.

Liu H.,Wuhan University | Liu H.,City University of Hong Kong | Liu H.,PLA Military Economics Academy | Wang X.,City University of Hong Kong | And 2 more authors.
Journal of Physical Chemistry C | Year: 2012

This study investigated adsorption and reactions of formaldehyde (HCHO) on TiO 2 rutile (110) and anatase (001) surfaces by first-principles calculation. The structure, vibrational frequencies, and electronic properties of the interaction system are studied to investigate the adsorption mechanisms of HCHO on TiO 2 surfaces. It is found that HCHO can chemically adsorb on all surfaces to form into a dioxymethylene structure with O of HCHO bonding to a coordinatively unsaturated surface Ti atom (Ti 4C or Ti 5C) and C bonding to a surface O 2C. The anatase (001) surface is found to be more active in HCHO adsorption with lower adsorption energy and larger charge transfer. In addition, the (1 × 4) reconstructed anatase (001) surfaces are found to have higher adsorption ability and more stable surface properties than that on (1 × 1) unreconstructed ones. These findings indicate that the (001) surface holds the potential for the improvement of sensitivity to reductive HCHO gas, in which the (1 × 4) reconstructed surface may play an important role for further improving gas-sensing properties of TiO 2-based sensors while keeping the stability of them. © 2012 American Chemical Society.

Liu H.,Wuhan University | Liu H.,PLA Military Economics Academy | Zhao M.,Wuhan University | Lei Y.,Wuhan University | And 3 more authors.
Computational Materials Science | Year: 2012

The adsorptions of formaldehyde molecule on the stoichiometric anatase TiO2 (1 0 1) surface have been studied by first principles calculations. Four types of adsorption have been investigated at 0.25 ML coverage. Two of them are chemical adsorptions and the other two are physical adsorptions. For the chemical adsorptions, C, O atoms in the formaldehyde molecule form two bonds with the O2c/O3c and Ti 5c on the anatase (1 0 1) surface. The CO bond in the formaldehyde molecule is elongated and a dioxymethylene structure forms in the two chemical adsorptions. The OTi5c interaction can be found in the two physical adsorptions and it is the only contacting point at the interface. No serious internal distortion in the formaldehyde molecule can be found in the physical adsorptions. The LDOS and the difference of the charge density are calculated to investigate the interface bonds of the adsorption. As the adsorption coverage increase, the molecules on the surface repel each other and weaken the adsorptions. For example, the chemical adsorption may become physical adsorption at high coverage. © 2011 Elsevier B.V. All rights reserved.

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