Study On Surface Modification Of TiO₂ Photocatalytic Materials By Plasma-enhanced Atomic Layer Deposition Technique

With the industry rapid development, human society is facing the increasingly serious problems of environmental pollution. How to utilize the inexhaustible sunlight and to develop the environmentally friendly photocatalysts become popular topics. TiO2 has been widely studied due to its low price, safety and non-toxic, good stability, and high photocatalytic activity. However, as TiO2 is a wide band gap (3.2 eV) semiconductor material, it can only absorb ultraviolet light. The absorption efficiency of the vast majority of the visible light in the sunlight is relatively low, so the visible light photocatalytic activity is very low. In order to make full use of visible light, the doping and modification of traditional photocatalytic semiconductor materials have drawn great concerns.Atomic layer deposition (ALD) is a novel thin film deposition technique based on sequential self-limited and complementary surface chemisorption reactions using precursor vapor with large area uniformity, excellent three-dimensional conformality, and precise control of film-thickness. Recently, ALD has shown increasing prospects and wide applications in various fields such as semiconductor, new energy and photocatalysis, especially in the surface modification of nanomaterials.Here we focused on new-type semiconductor photocatalytic materials and investigated the surface doping and modification of the commercial TiO2 nanopowders (P25) by plasma-enhanced ALD (PEALD) technique. Several samples such as PEALD-derived TiN, iron oxide, and cobalt oxide-coated P25 powders have been prepared. And their structures, morphologies, compositions, and bandgaps have been characaterized by means of a series of analytical methods. The photocatalytic acitivity of degradation of organic dyes and related mechanism have been explored.The main achievements are summarized as follows:1. TiN-coated P25 powders with various cycles have been fabricated by PEALD. A series of analyses show that the surface deposition of 20,50 and 100 cycles of TiN does not change the crystal structure, grain size, and band gap of P25 nano powders, but forms an ultrathin disordered defect layer of N-doped TiO2 on the powder surfaces. Due to the limitation of PEALD processing, the surface N-doping has been performed on about five ten thousandths P25 powders, however, because the photocatalytic reaction main relies on the powder surface effect, the surface N-doped P25 samples exhibit excellent photocatalytic degradation on methyl orange under the visible light with good photocatalytic stability compared to pure P25 powders. 100-cycle TiN-caoted P25 powders show optimal photocatalytic acitivity with the degradation of 97% in 2 hours, much higher than that of pure P25 powder of 18%. The improvement of photocatalytic activity is attributed to the successful small amount of doping of N ions into the TiO2 lattice of P25 powder surfaces, which causes the lattice distortion and charge defects, forms the oxygen vacancies, and further shows better photocatalytic activity.2. The surface trace Fe-doped TiO2 powders have been successfully prepared using PEALD method. The surface deposition of 700-cycle iron oxide produces an ultrathin trace-Fe doped TiO2 defect layer on the surfaces of P25 powder instead of changing the anatase structure, grain size, and band gap of P25 nanopowders. Although the Fe dopant amount is only 668 ppm due to the limitation of PEALD processing, the photodegradation of methyl orange under the visible light is significantly improved with 72.6% in 2 hours, much faster than that of the pure P25 powders with 42.8% in 4 hours. It is ascribed to the trace Fe3+ ions doped into TiO2 lattice of the P25 powder surfaces with Fe3+ substitution to Ti4+. In order to keep the electrical neutrality, oxygen vacancies are formed in the TiO2 lattice, leading to better photocatalytic activity.3. The surface cobalt oxide nanoparticles-modified TiO2 powders have been successfully prepared by PEALD. The trace Co ion does not occupy the Ti4+ position of TiO2 lattice without effect on the crystal structure, grain size, and band gap of P25 nanopowder. The photocatalytic tests show that under visible light, surface cobalt oxide-modified P25 powders have the same visible light degradation of methyl orange as pure P25 ones. However in ultraviolet light, surface cobalt oxide-modified P25 powders display enhanced photocatalytic efficiency for almost 100% degradation of methylene blue in 90 mins, compared with P25 for 80%. A possible photocatalytic mechanism is proposed.