The Effect Of N-Doping And Nano-PdO On The Optical Properties Of TiO₂ Thin Films

Titanium dioxide has been extensively applied in the field of photocatalyst,due to its efficient photoactivity, high chemical stability, safty to human beings,nontoxicity and low cost. Nanosized TiO₂ is a new promising candidate inpotocatalyzer to degrading enviromental contamination. In this paper, nanosizedTiO₂ and that with N and PdO doped thin films were prepared. The component,microstructure, and optical absorption properties of these thin-films wereanalysed by XPS (X-ray photoelectron spectroscopy), TEM (TransmissionElectron Microscope) and UV-visble spectrum. Further more, the electronenergy bands structure and the optical absorption were simulated by the firstprinciple calculation, and the visble light absorption mechanism was discussedfor heavy nitrogen-doped TiO₂.The TiO₂ thin-films, about 45 nm thick, were prepared by ion-beam-assisteddepositon technology, including TiO₂ thin film, slightly nitrogen-doped, heavynitrogen-doped, and both nitrogen and PdO-doped ones.The composition and structure of thin films were charaterized by XPS, HRTEM(high resolution transmission electron microscope), STEM(scanningtransmission electron microscope) and so on. SAED (selected area electrondiffraction) and HRTEM analysis show that the structure of thin-film withoutnitrogen is anatase TiO₂ with the grain size of about 20～30 nm; contrastly, thegrain size of the nitrogen-doped ones is smaller, approximately 10～20 nm. Whenthe quantity of doped nitrogen was below 1.7 at%, the thin film consisted ofsingle TiO₂ phase, under our experimental condition with more nitrogen doped,the TiN phase appreared in the thin film. By the HAADF (high angle annulardark field) observation and the EDS analysis showed that the PdO-doped thinfilm consisted of anatase TiO₂ with grain size of 10～20 nm in which dispersedPdO with grain size of 2～3 nm.The test results of Uv-visble spectrophotometer showed that nitrogendoping extended the optical absorption range of TiO₂ thin films fromultraviolet(＜400 nm) to visible light(～550 nm). The thin film doped withnitrogen and PdO present extrimely fine optical absorption properties, it almostabsorbs the full rang visible light.We calculated the TiO₂ models of N free and N doping using the firstprinciples method by the visible software MS (Material Studio), discussed theeffects of nitrogen doping concentration on the optical absorption properties ofTiO₂, and researched the mechanism. Based on the calculate results of energyband struture and density of states, we concluded that the band gap between thevalence bands and conduct bands was narrowed, resulting in improvement in the visible light rang absorption of TiO₂, and there was a critical value of thequantity of nitrogen doping. Under the critical value, the narrowed energy gapwas contributed by the isolated N2p states; above the critical value, the energygap narrowing was not only beacase of the isolated N2p stated, but theheightening of O energy band. The simulation results on the optical absorptionproperties indicated that the nitrogen doping could improve the visible lightabsorption of TiO₂, the doped nitrogen concentration greatly affect the opticalobsorption properties, especially the heavy nitrogen doping obviously improvedthe optical properties in the visible light range of the TiO₂ thin film.