First Principles Study Of Anatase-phase TiO₂Nanotubes

TiO₂nanomaterials are widely used in the fields of heterogeneous photocatalyticreaction, gas sensors, etc. because of their unique physical and chemical properties,and have great potential to build dye-sensitized solar cells with high conversionefficiency. In this paper, we calculate and study the structure and energy of a varietyof single-walled titanium dioxide nanotubes built by rolling titania monolayer and theCu atoms and CO coadsorption on nanotubes by the first-principles methods. Themain contents are as follows:DFT calculations are performed to obtain stable structures of the （0, m）,（n,0） and （n,m） nanotubes. Three types of TiO₂nanotubes:（0, m）,（n,0） and （n, m）, aresemiconductors with band gaps of2.95eV-3.4eV. We find all nanotubes have similarwall-structure, but there are significant differences in the charges distribution andenergy band structure due to the different chiralities. The three types of TiO₂nanotubes also have something in common: with the increase of their diameters, theband gaps increases, and the strain energy can be reduced. Thus, TiO₂nanotubesstructure of large diameter will be more stable. But the wavelength of their absorbinglight is reduced, which makes it difficult to absorb sunlight. Therefore, the smalldiameter of the nanotubes have more applications on the absorption of sunlight, forexample,（3,2）,（4,2） and （6,3）. Their band gap is close to the bulk material, potentialfor effectively absorbing visible light, and to be applied to heterogeneousphotocatalytic reaction and the preparation of the solar cell. In addition, the walls oftitanium oxide nanotubes contain many unsaturated atoms:2-fold-coordinated oxygenand titanium atoms, provides a large number of active sites for the catalytic reaction,and has a good adsorption capacity of the reactants or small molecules, such as carbonmonoxide, hydrogen, methanol, etc.We then study adsorption of Cu atom on the （0,3） and （6,0） tubes. The Cu atoms, astransition metal, are adsorbed or doped on titanium dioxide nanotubes in the forms ofatoms and ions. In our study, after Cu adsorbed on the two nanotubes, Cu atoms will lose electrons with positive charges. And the band gaps of the systems are reduced.The band gap of the （0,3）tubes changes more apparent and energy of adsorption canbe higher, mainly owing to large overlap populations of that the Cu atoms and theoxygen atoms, implying stronger bond energy.In order to further study of the sensing properties of the Cu-nanotubes systems, theCO are adsorbed on the surface of the Cu-nanotubes systems. The density functionaltheory calculations are performed on the charge configuration and transfer of theCO-Cu-nanotubes systems. We find that the CO cannot be adsorbed on the surface of（0,3） and （6,0） tubes, but binding on Cu atom. Adsorption energy of CO with a Cu-Cbond can be higher, the structure of system is more stable. The band gaps increase forthe four configurations after the CO adsorption.In all adsorption systems, the bandgap of the configuration （0,3）-Cu-CO （1） increase most obviously.