Adsorption Of CO Molecules On Functionalized TiO₂Nanotubes A DFT Study

The nanostructured titanium dioxide (TiO2) materials have attracted extensivelyexploration due to their high photocatalytic activity, strong oxidising power, goodchemical inertness and large surface area. A wide range of applications have beenproposed in TiO2materials, such as photocatalysis, water and air purifier, electrodesfor lithium batteries, and gas-sensor. Compared with the other two phases—rutile andbrookite, TiO2in the anatase is more dominant in solar energy conversion andgas-sensing application because of its high photocatalytic activity and chemicalstability. Due to its high surface-to-volume ratio, TiO2nanotubes (TiO2NTs) haveattracted more and more attention in recent years. We proposed two modelstructures((0,3) and (6,0)) TiO2NTs by rolling the anatase (101) sheet along both the[101] and [010] directions.In this work, we investigate the (0,3)TiO2NTs((6,0)TiO2NTs) decorated(substituted) with Co(Cu) by DFT method, and thenexplorethe adsorption of CO molecule onto Co(Cu)/TiO2NTs. Through analyzing thestructure, adsorption energy and the electronic properties, we further compare theroles of Co and Cu atoms for the CO molecule adsorption. The main contents are asfollows:1. We use density functional theory (DFT) to investigate the binding of Co(Cu)atom on the two different types of nanotubes, i.e.(0,3)TiO2nanotube and(6,0)TiO2nanotube. It is found that the binding ability of Co atoms on TiO2NTs isstronger than that of Cu atoms.When the Co(Cu) atom adsorbedon(0,3)TiO2NTand (6,0)TiO2NT, it carries some positive charge. Our study indicates that thesignificant interaction between2cO-2p orbital and Co(Cu)-3d orbital takes placein the valence band. In addition,when the Co(Cu) atoms are adsorbed on TiO2NTs,there are certain impurity states inside the band gap near the Fermi level, whichmakes the band gap decreases. Notably, the Fermi level of Co/(0,3)TiO2NT andCo/(6,0)TiO2NTshifts upward to the original conduction band region.2. The Co(Cu)atom lost electrons and were positively charged after doping on the (0,3)TiO2NT and (6,0)TiO2NT. Co(Cu) doped on the materialsshows thatthepreseneeof impuritystates of Co(Cu)-3d on the upper edge of valence band wasdecreasingthe band gap of TiO2nanotubes. By comparison, more impurity statesare introduced via the doped Co atom, with the bandgap of nanotube significantlynorrowed. Inaddition, both types of the doped metal atoms are responsible for thedecline of the Fermi level, which even make the Fermi level at the valence band.3. There are two different adsorption modes for Co(Cu)-decoratedTiO2NTs: one isthe adsorption of CO molecule through C atom bonding with Co atom and theother adsorptionis through O atom connecting with Co atom. However the COmolecule can merely adsorb on Co(Cu)-dopedTiO2NTs through C atom bondingwith Co(Cu) atom. The CO molecule and Co atom interact mainly through thehybridization between the C-2p orbital and Co-3d orbital below Fermi level, andthose electron hybridizations cause charge redistributions.Therefore, theinteraction between CO and Co follows the Blyholder model. The CO5σ orbitalis broken, and charges are transferred from CO-5σ orbital to Co-3dorbital.Simultaneously, the Co-3d orbital reverse this process by back-donating chargesinto CO-2orbital, resulting in the formation of Co-CO complex and theelongation of the C-O bond.