Anatase $\mathrm{Ti}{\mathrm{O}}_2$ nanoparticles with average particle size ranging between 12 and 23 nm were synthesized by metalorganic chemical vapor deposition. The structure and particle size were determined by x-ray diffraction and transmission electron microscopy. The specific surface areas were measured by Brunauer-Emmett-Teller and ranged from 65 to $125{\mathrm{m}}^2∕\mathrm{g}$ . The size effects on the stability of $\mathrm{Ti}{\mathrm{O}}_2$ in the air were studied by x-ray diffraction and transmission electron diffraction for isochronally annealed samples in the temperature range of 700–800 °C. Only anatase to rutile phase transformation occurred. With the decrease of initial particle size the onset transition temperature was decreased. An increased lattice compression of anatase with the raising of temperature was observed by the x-ray peak shifts. Larger distortions existed in samples with smaller particle size. The calculated activation energy for phase transformation decreased from 299 to 180 kJ∕mol with the decrease of initial anatase particle size from 23 to 12 nm. The decreased thermal stability in finer nanoparticles was primarily due to the reduced activation energy as the size related surface enthalpy and stress energy increased.

The metallorganic chemical vapor deposition method was successfully used to synthesize pure TiO ₂ and Nd ³⁺ -, Pd ²⁺ -, Pt ⁴⁺ -, and Fe ³⁺ -doped TiO ₂ nanoparticles. Polycrystalline TiO ₂ structure was verified with x-ray diffraction, which showed typical characteristic anatase reflections without any separate dopant-related peaks. Transmission electron microscopy observations confirmed the existence of homogeneously distributed 22 ± 3 nm TiO ₂ nanoparticles. The particle size remained the same for the doped samples. The doping level of transition metals was kept at ≈1 atomic percent, which was determined by x-ray photoelectron spectra and energy dispersive x-ray spectroscopy. The effects of different types of dopants on the photocatalytic activity were revealed by the degradation of 2-chlorophenols with an UV light source. The photocatalytic efficiency was remarkably enhanced by the introduction of Pd ²⁺ and Nd ³⁺ . Nd ³⁺ -doped TiO ₂ showed the largest enhancement. However, Pt ⁴⁺ changed the 2-chlorophenol degradation rate only slightly, and Fe ³⁺ was detrimental to this process. These effects were related to the position of the dopants in the nanoparticles and the difference in their ionic radii with respect to that of Ti ⁴⁺ .

Large band gap semiconductors are typically doped in order to enhance their photocatalytic, photovoltaic, and other chemical and optoelectronic properties. The identification of dopant position and its local environment are essential to explore the effect of doping. X ray techniques, including extended x ray absorption fine structure, x ray photoelectron spectroscopy, and x ray diffraction, were performed to analyze the Nd $(0\text{to}1.5\mathrm{at.}\%)$ dopant location and the structural changes associated with the doping in anatase $\mathrm{Ti}{\mathrm{O}}_2$ nanoparticles, which were synthesized by metalorganic chemical vapor deposition. Nd ions were determined to have a trivalent chemical state and substitute for ${\mathrm{Ti}}^{4+}$ in the $\mathrm{Ti}{\mathrm{O}}_2$ structure. The substitutional ${\mathrm{Nd}}^{3+}$ ions cause anatase lattice expansion along $c$ direction with a maximum value of $0.15\mathrm{\AA }$ at 1.5 % Nd doping level and the local structure of the dopants changes towards rutile like configuration. The lengths of the nearest neighbor $\mathrm{Nd}\text{−}\mathrm{O}$ and $\mathrm{Nd}\text{−}\mathrm{Ti}$ bonds increase by $0.5–0.8\mathrm{\AA }$ compared to their counterparts in the pure $\mathrm{Ti}{\mathrm{O}}_2$ host structure. The substitutional nature of ${\mathrm{Nd}}^{3+}$ dopants explains why they are efficient not only for charge carrier separation but also for visible light absorption in $\mathrm{Ti}{\mathrm{O}}_2$.