Study On Doping Of Calcium Titanate Photocatalyst And Its Electronic Structure

Solar water Spliiting into H₂ using new photocatalysts is one of low-cost and convenient methods for hydrogen obtaining, and obtaining the photocatalysts with high efficiency is the key technology in this method. Very recently, it is found that doping is a feasible and effective means to improve the activity of photocatalysts under visble light, which beomes an active aspect in photocatalytic research field.In this paper, CaTiO₃ nanopowders were prepared by a microwave-assisted sol-gel self-propagating method which is a combination of the combustion and the chemical gelation processes. Doping of foreign elements into UV-active CaTiO₃ with wide band gaps in order to introduce a donor or an acceptor level in the forbidden band has been done to design the visible-light-driven photocatalyst. A study on the synthesis, doping improvements, structure characteristics, photocatalytic performance and electronic structure were carried out systemically and in detail through test measures such as TG-DTA (Thermogravimetry analysis-Differential Thermal Anylysis), XRD (X-ray radial diffraction), SEM (Scanning electron microscope), BET, Photcatalytic activity and DFT (Density functional theory) method, etc.In the present work, the process of microwave-assisted sol-gel self-propagating method was studied to optimize the synthesis process of perovskite CaTiO₃, in order to prepare the photocatalyst with good crystallinity, pattern and photocatalytic activity. The effect of calcination temperature and reaction time on the structure, suface pattern and photocatalytic water splitting into H2 has been studied. Experiment results indicated that the calcination temperature and reaction time played an important role in the photocatalytic activity. The optimized condition for microwave-assisted sol-gel self-propagating method was: The raw materials with the appropriate ratio of Ca/Ti were calcinated at 850℃for 7h.A study was carried out about first series transition metal (V, Cr, Fe, Co, Ni, Cu and Zn) doping on the B site of CaTiO₃. XRD results showed that V, Cr, Fe, Co, Ni, Cu and Zn have been doped into B site of CaTiO₃. From the UV-vis diffuse reflect spectra, we found that optical ability of CaTiO₃ is improved by doping all transition metal, and the doped sample can obsorb visible light except the Zn doped CaTiO₃. The SEM images showed that the grain sizes become bigger by V, Fe doping, which decreased the BET surface area, leading to the decrease of photocatalytic activity. On the contrary, grain size becomes smaller by Co, Ni, and Cu doping, which increases the BET surface area, leading to increase of photocatalytic activity.The electronic structures of doped CaTiO₃ with B site were calculated by using CASTEP software based on density functional theory. The calculated results showed V, Cr, and Fe doping introduced some doping levles, which can change the transition model of electron, and increase the optical activity of samples, however, the position of conduction band of CaTiO₃ shifts to lower energy, which means the reduction of samples, was decreased, resulting in the increase of photocatalytic activity. Co, Ni, Cu doping imporved the photocatalytic activity becasue they have not changed the position of conduction band, and introduced some doping levles in the band gap, which can provide a passway for electoron transition, and improved the seperated rate of photogenerated electron-hole. Zn 3d and O 2p orbitals mixed together and composed a new valence band, which reduces band gap and improved the photocatalytic activity.The effect of Mn doping on the photocatalytic activity was appraised according to the electronic structure calculations. The calculated results showed Mn doping decreased the reduction ability and changed the optical ability of photocatalyst.Doping Ag, La, and Ag/La into A site of CaTiO₃ were carried out and XRD results showed Ag, La, and Ag/La has been doped into A site. The optical absorption abilities of Ag, La, Ag/La doped samples were tested and the results showed Ag and Ag/La doping can imporve the optical absorption ability of CaTiO₃, on the contrary, La doping decreased the optical ability of CaTiO₃. Ag and Ag/La doping can imporve the photocatalytic activity of CaTiO₃ because the band gap is narrowed by their doping. La doping decreased the activity of photocatalyst.Doping N and N/La on O site of CaTiO₃ were carried out. Experiment results indicated that the calcination temperature and ratio of nitrogen source and titanium source are the key effect infactors. N doped CaTiO₃ showed higher activity when the calcination temperature is 400℃and the ratio of nitrogen source/titanium source is 4:1. N and N/La doped CaTiO₃ showed higher photocatalytic activity, because N and N/La doping increased the band gap of CaTiO₃, and improved the seperated rate of electron-hole.Photocatalytic activities Ag/La and N/La codoped CaTiO₃ are higher than that of N or Ag single doping. Because the charge balance is maintained by N/La or Ag/La codoping, and the band gap is decreased without introducing any impurity energy levels which can become recombination center of electron and hole in case of high doping amount.