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Abstract
In the present work, Zn-doped TiO2 thin films were deposited on soda-lime-silica glass substrates by spin coating. Titanium isopropoxide and zinc chloride were used as sources of Ti and Zn, respectively. The levels of doping were 0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0 mol% (metals basis). Solutions were dropped on substrates spun at 2000 rpm. Afterwards, the films were annealed in air at 500 °C for 5 h, using a heating rate of 5 °C per min, followed by natural cooling.
Laser Raman microspectroscopy and glancing angle X-ray diffraction (GAXRD) indicated that all of the films consisted of polycrystalline anatase only, without other contaminant phases, such as ZnO. Single-beam focused ion beam milling (FIB) data showed that the thickness, which was consistently ~350 nm, was not affected by Zn. Field emission scanning electron microscopy (FESEM) showed that the films were microstructurally homogenous, pore-free and crack-free. Increasing the Zn doping level did not enhance or suppress grain growth and all of the films showed a consistent grain size of ~40 nm. UV-VIS spectrophotometry transmission data in the visible region indicated that all of the films were of high transparency, with >80% transmission in visible region (400 800 nm). Further, the interference fringes shown in the transmission spectra suggested that the surfaces of the films were smooth and flat, thereby confirming the FESEM results.
The optical indirect band gap of the films, which was calculated using the transmission data, showed three phenomena:
Undoped TiO2 gave a band gap of ~3.40 eV,
TiO2 doped at what is assumed to be the solubility limit of Zn (0 0.8 mol% Zn) in TiO2 gave a band gap of ~3.10 eV, and
TiO2 doped beyond the solubility limit (1.0 2.0 mol%) gave a band gap of ~3.20 eV.
This can be explained by the potential effects of precipitation and/or segregation, both of which would serve to deposit a monolayer or extremely thin surface coating on the TiO2.
Testing of the photoactivity by the decomposition of methylene blue supported the preceding interpretation in that:
The band gaps showed three values: (a) undoped, (b) below the solubility limit of Zn in TiO2 of 0.8 mol% Zn, and (c) above the solubility limit. The constancy of the band gaps were consistent with the presence of homogeneous solutions (where any graduated differences were beyond the sensitivity of the method) and surface precipitation and/or segregation.
The methylene blue testing showed two trends: (a) below the solubility limit of Zn in TiO2, the photoactivity increased owing to the generation of oxygen vacancies (here, the differences could be detected) and (b) above the solubility limit, the degree of coverage and/or thickness of the ZnO surface coating increased with increasing Zn level, consequently inhibiting the photoactivity.