Optical and Photocatalytic Properties of Heavily F--Doped SnO2 Nanocrystals by a Novel Single-Source Precursor Approach

Heavily F-doped SnO2 nanocrystals were successfully prepared by a novel synthetic approach involving low-temperature oxidation of a Sn2+-containing fluoride complex KSnF3 as the single-source precursor with H2O2. The F-doped SnO2 powder was characterized by powder X-ray diffraction, TG-MS, BET surface area, diffuse reflectance spectroscopy, XPS, PL, FTIR spectroscopy, Raman spectroscopy, EPR spectroscopy, SEM, and TEM. Broadening of the diffracted peaks, signifying the low crystallite size of the products, was quite evident in the powder X-ray diffraction pattern of SnO2 obtained from KSnF3. It was indexed in a tetragonal unit cell with lattice constants a = 4.7106 (1) angstrom and c = 3.1970 (1) angstrom. Agglomeration of particles, with an average diameter of 5-7 nm, was observed in the TEM images whose spotwise EDX analysis indicated the presence of fluoride ions. In the core level high-resolution F Is spectrum, the peak observed at 685.08 eV was fitted by the Gaussian profile yielding the fluoride ion concentration to be 21.23% in the SnO2 lattice. Such a high fluoride ion concentration is reported for the first time in powders. SnO2:F nanocrystals showed greater thermal stability up to 300 degrees C when heated in a thermobalance under flowing helium, after which generation of small quantities of HF was observed in the TG coupled mass spectrometry analysis. The band gap value, estimated from the Kubelka-Munk function, showed a large shift from 3.52 to 3.87 eV on fluoride ion doping, as observed in the diffuse reflectance spectrum. Such a large shift was corroborated to the overdoped situation due to the Moss-Burstein effect with an increase in the carrier concentration. In the photoluminescence (PL) spectrum, SnO2:F nanocrystals exhibited a broad green emission arising from the singly ionized oxygen vacancies created due to higher dopant concentration. The evidence for singly ionized vacancies was arrived from the presence of a signal with a g value of 1.98 in the ESR spectrum of SnO2:F at room temperature. The disordered nature of the rutile lattice and the enormous oxygen vacancies created due to fluoride ion doping were evident from the broad bands observed at 455, 588, and 874 cm(-1) in the room-temperature Raman spectrum of SnO2:F. As the consequence of the oxygen vacancies, F-doped SnO2 was examined for the function as a photocatalyst in the degradation of aqueous RhB dye solution under UV irradiation. A very high photocatalytic efficiency was observed for the F-doped SnO2 nanocrystals as compared to pure SnO2. The BET surface area of pure SnO2 was quite high (207.81 m(2)/g) as compared to the F-doped SnO2 nanocrystals (45.16 m(2)/g). Pore size analysis showed a mean pore diameter of 1.97 and 13.97 nm for the pure and doped samples. The increased photocatalytic efficiency was related to the very high concentration of oxygen vacancies in SnO2 induced by F doping.