The possibility of controlling the photocatalytic activity of TiO2 nanoparticles by tailoring their crystalline structure and morphology is a current topic of great interest. In this study, a broad variety of well-faceted particles with different phase compositions, sizes, and shapes have been obtained from concentrated TiOCl2 solutions by systematically changing temperature, pH, and duration of the hydrothermal treatment. The guide to select the suitable experimental conditions was provided by thermodynamic modeling based on available thermochemical data. By combining the results of TEM, HRTEM, XRD, density, and specific surface area measurements, a complete structural and morphological characterization of the particles was performed. Correlation between the photocatalytic activity in the UV photodegradation of phenol solutions and the particle size was established. Prismatic rutile particles with length/width ratio around 5 and breadth of 60-100 nm showed the highest activity. The surface chemistry of the particles was also investigated. Treatments that decrease the surface acidity, such as washing the powders with ammonia solution and/or calcining at 400 degrees C, have detrimental effect on photocatalytic activity. The overall results suggest correlation between particle morphology and photocatalytic activity and indicate that both electron-hole recombination and adsorption at the surface can be rate-controlling processes. The systematic approach presented in this study demonstrates that a substantial improvement of the photocatalytic activity of TiO2 can be achieved by a careful design of the particle morphology and the control of the surface chemistry.