Investigation of photocatalytic activity of boron and nitrogen doped TiO2 thin films

Heterogeneous photocatalysis has been intensively studied in recent decades for a wide variety of application such as hydrogen production from water splitting, water and air treatment and self-cleaning surfaces. Titanium dioxide (TiO2) is undoubtedly the best semiconductor for photocatalytic applications among all photocatalytic materials due to its relatively high photocatalytic efficiency under UV irradiation, abundance, low cost, photo-corrosion resistance and biocompatibility. Although TiO2 photocatalyst is widely used in powder form for many applications, TiO2 thin films have attracted a great deal of attention owing to their flexibility in treating wastes. For instance, they require no separation and they can be reused after photocatalytic process. Nevertheless, their major problems are dramatic loss of surface area when prepared in film form and the deteriorative effect of the substrate. TiO2 is not a strong optical absorber, it allows them to be coated without compromising the optical properties of the transparent materials like glass items. Considering the architectural trends for larger windows and the difficulties to clean windows in high-rise buildings, high efficiency self-cleaning glasses will become more important. Being a wide-band gap semiconductor, titanium dioxide can utilize only a limited (ultraviolet) portion (for anatase <387 nm, 2-5 %) of the solar spectrum. Lack of the photocatalytic activity of TiO2 under visible light irradiation limits its indoor applications. Shifting the absorption edge of TiO2 from the UV band toward the visible spectrum (red shift) can expand the usage area of photocatalyst and increase the photocatalytic efficiency under solar irradiation as well. Various strategies have been adopted for improving the photocatalytic efficiency of TiO2. They can be summarized as either morphological modifications, such as increasing surface area and porosity, or as chemical modifications, by incorporation of additional components in the TiO2 structure. Although visible light active TiO2 photocatalysts require chemical modifications, their overall efficiencies have been significantly enhanced by controlling the semiconductor morphology. Doping of pure TiO2 is the most widely used method to increase the visible light activity, while non-metal doping of TiO2 has shown great success in achieving visible light active photocatalysis. Several methods have been reported for the preparation of TiO2 films, such as sol-gel, chemical vapour deposition, magnetron sputtering, spray pyrolysis, direct deposition and layer-by-layer coating. In particular, sol-gel is an attractive method because of the capability to coat materials with various shapes and ease of control over the composition of the films with relatively simple and inexpensive equipment. In this dissertation work, B, N doped and B/N co-doped TiO2 photocatalysts were synthesized in powder and thin film form by sol-gel method. In a typical preparation process of solution, titanium isopropoxide precursor (99,999 % purity, Sigma Aldrich) was dissolved in ethanol and hydrolysed in the presence of hydrochloric acid. Acetylacetone was added to TiO2 solution as a stabilizer to prevent the uncontrolled polycondensation reaction. For doped TiO2 solution, appropriate amounts of boric acid and urea were dissolved separately in ethanol and rapidly added to the main solution. After aging process, solutions were coated on soda-lime glass (Corning® 2947) substrates and SiO2 precoated soda-lime glass substrates by dip coating method and dried at 100 °C for 20 minutes in an oven. SiO2 solution was synthesized by a similar method with TiO2 solution, where tetraethyl orthosilicate (TEOS, Sigma Aldrich) precursor was used. SiO2 coating as interlayer is prepared for preventing alkali ion (Na+) diffusion into sol-gel prepared film during heat treatment and annealed at 500 °C for 1 hour before the TiO2 coating procedure. Sodium ions suppress crystallization of TiO2 and degrade the photocatalytic activity. The solutions used in the coating process were kept in the oven at 100 °C for 24 hours to obtain powder samples. Thin films and powder samples were annealed at 500 °C for 1 hour with heating rate of 10 °C/min in air atmosphere to achieve anatase TiO2. Doped and undoped TiO2 gels were obtained from TiO2 sols and thermal characterizations were carried out by using differential thermal analysis technique (DTA). According to the DTA results, crystallization temperature of anatase phase, which has the highest photocatalytic activity, is determined. Crystal structure of powders was analysed by X-ray diffraction (XRD) and optical spectra of thin films were collected by UV-Vis Spectrophotometer. Surface characterizations of thin films were conducted with SEM. Effect of B, N dopant and B/N co-dopant in the TiO2 structure to the photocatalytic activity of TiO2 is explored with methylene blue degradation under UV and visible light. For this purpose, coated slides (1 cm x 3 cm) and powder samples (2.5 g/l) were placed in petri dishes containing 10 ml 20 μmol/l aqueous methylene blue solution and kept in dark for 30 min (for adsorption equilibrium), and then illuminated with UVC light (15 W, λ = 254 nm, OSRAM) or visible light (15 W) for different hours. Representative samples were taken every hour from the solutions and dye concentration was measured using UV-Vis Spectrophotometer. Visible light activity of TiO2 was very low compared with UV activity, so the samples were illuminated with visible light much longer. Photocatalytic activity kinetics were investigated and photocatalytic activity rate constants were calculated. In order to calculate the rate constants a first-order kinetic rate was assumed. According to XRD analysis powder samples have anatase crystal structure. N and B doped amounts used in TiO2 synthesis did not cause any phase transformation or formation a new phase related with additives. When annealing temperature increased the peak intensities were also increased as expected. N, B doping and B-N co-doping shifted absorption edge of TiO2 films to the longer wavelengths but they decreased light absorption of TiO2. Optical transmittance of TiO2 films with or without SiO2 interlayer were decreased with increased number of layers as expected. Degradation of methylene blue under UV light without any photocatalyst was observed during photocatalytic experiments. This result confirms the literature knowledge, which specifies photolysis of methylene blue by UV irradiation. It was detected with photocatalytic test results that SiO2 interlayer enhanced the photocatalytic activity of TiO2 thin films under UV irradiation. TiO2 in the anatase form is the most efficient of photocatalysts for many applications. However, during the heat treatment step, the diffusion of sodium oxide from the soda lime glass into the TiO2 layer deteriorates the photocatalytic activity of TiO2. Preventing sodium migration through introducing a SiO2 barrier layer between the glass substrate and TiO2 film proved to be useful. Undoped TiO2 powder has the highest photocatalytic activity between all powder photocatalysts both UV and visible illumination. Similarly, undoped TiO2 thin film sample, which contain SiO2 interlayer showed best photocatalytic activity performance under UV irradiation. Unfortunately, no visible light activity was detected on doped and undoped thin films. Non-metal doping reduced photocatalytic activity of thin films and powder photocatalysts.