| Environmental pollution caused by volatile organic compounds(VOCs)is becoming more and more serious all over the world.Formaldehyde(HCHO),as a widespread toxic VOC in our daily life,is harmful to people’s health and endangers our living.Formaldehyde can be released by the use of interior building materials.Thus,it is urgent to remove formaldehyde from the indoor atmosphere.Some traditional methods have their limitation,while photocatalytic oxidation decomposition of formaldehyde has drawn much attention because of its environment-friendly and energy-saving.As a good photocatalyst,TiO2 has received much attention due to its strong oxidizing power and low cost.Doping of TiO2 with non-metal ions can enhance the photocatalytic activity of TiO2 surface,which has great potential for the development of the removal of formaldehyde.In this paper,the electronic structure and optical properties of anatase S-TiO2(001)and S-TiO2(101)surfaces,and the adsorption interactions between HCHO molecule and S-TiO2(001)and S-TiO2(101)surfaces without and with water and oxygen were studied by using the density functional theory(DFT)method,the main results are as follows:(1)The doping S atom can reduce the band gap of TiO2(001)and(101)surfaces and expand their optical response range.The S 3p impurity state is introduced near Fermi level,and the whole DOS curve shifts towards the low energy level,resulting in the red shift of absorption edge.As a result,the absorption range of visible light is increased effectively and the optical absorption intensities of TiO2(001)and TiO2(101)surfaces are enhanced in the visible region.(2)When HCHO molecule is adsorbed on TiO2(001)surface,there are two situations:one is that HCHO is not dissociated,the other is that HCHO is dissociated.When HCHO is not dissociated,the adsorption energy is-277.28 kJ·mol-1.The O atom of HCHO is bonded to Ti atom of the surface,while C atom is bonded to O2c atom,and a CH2O2 structure is formed;when HCHO is dissociated,the adsorption energy is-32.34 kJ·mol-1,and two C-H bonds of HCHO are broken.Two H atoms are bonded to two O atoms of the surface,respectively,and then two O-H bonds can be oxidized to H2O.Both C and O atoms of HCHO are bonded with one Ti atom at the same time,and C-O separated from the surface and was oxidized to CO2.When HCHO molecule is adsorbed on TiO2(101)surface,the adsorption energy is-360.96 kJ·mol-1,Oand C atoms of HCHO are bonded to Ti and O2c atoms of the surface,respectively.As a result,a CH2O2 structure is formed.(3)VWhen HCHO molecule is adsorbed on S-TiO2(001)surface,the adsorption energy is-312.14 kJ·mol-1,one H atom is bonded to S atom with the fracture of one C-H bond of HCHO.At the same time,C atom is also bonded to S atom,while O atom of HCHO is bonded to Ti atom.In the end,a CH2OS structure is formed.When HCHO molecule is adsorbed on S-TiO2(101)surface,the adsorption energy is-389.90 kJ·mol-1,C atom and O atom of HCHO are bonded to S atom and Ti atom of the surface,respectively,and a CH2OS structure is formed.The adsorption energy of the system is increased by the doping S.Besides,S atom can directly interact with formaldehyde molecule,and promote the degradation of formaldehyde.(4)When HCHO+H2O+O2 are coadsorbed on S-TiO2(001)surface,the adsorption energy is-709.62 kJ·mol-1.Two C-H bonds of HCHO are broken,H1 atom is bonded to S atom,while H2 atom combines with O atom of O2,the O=O bond of O2 is elongated,and the species HO2 · is formed.Besides,One H atom of H2O is bonded to O1 atom of the surface,and forms the species OH ·.HCHO molecule can be directly oxidized into CO2 and H2O due to their strong oxidizing ability.When HCHO+H2O+O2 are coadsorbed on S-TiO2(101)surface,the adsorption energy is-788.12 kJ·mol-1,two C-H bonds of HCHO are broken,H1 atom is bonded to O atom of the surface,while H2 atom combines with O atom of O2,and the species HO2 · is formed.Besides,One H atom in H2O is separated,and forms the species OH·.Water and oxygen molecules can provide the necessary conditions for the formation of the strong oxidizing free radicals and accelerate the degradation process of formaldehyde molecules. |
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