Due to the environmental problems, development of alternative energies has become an important issue. One of the most effective solutions for using solar energy is the use of semiconductor-based systems. Among the plentiful semiconductor materials, titanium dioxide (TiO2) is well known as a material with a high photocatalytic activity, good stability, low cost, and easy availability. However, due to its large band gap (about 3.2 eV) and high rate of electron-hole recombination, improvement the photocatalytic activity of the traditional TiO2, always has been considered as one of the promising ways. In this thesis, several methods have been used to improve photocatalytic and photo-electrocatalytic activity of titanium TiO2. In the first part, for the first time, a string shape of Ag(4,4' dicyanamidobiphenyl) complex (Ag(DCBP)) was used as a modifier to develop a new photoanode structure with TiO2 nanoparticles. Its application summarily led to achieving powerful light scattering, a high specific surface area, subsequent increases in the amount of dye adsorption, long electron lifetime, facile electron traort and finally less charge recombination properties. In the string-shaped Ag(DCBP) complex, the efficiency of electron-traort was increased due to the presence of strong intermolecular ?–? interaction. Under optimal conditions, the efficiency of the solar cell made based on Ag(DCBP) (1.5%)/TiO2 photoanode was 5.17%, which was higher than that for pure TiO2 (3.06%) by 69.0% increasing. In the second part, carbon nitride nanotubes doped with platinum nanoparticles were prepared (Pt/C3N4 NTs) to decorate and activate the Ag/TiO2 by a simple, “green” and economic hydrothermal synthesis for the first time. On the fabricated photoanode, due to the presence of Pt/C3N4 NTs, more dye adsorbed and following that charge separations and electron lifetimes in the semiconductor were improved. It should be noted that the increase in the efficiency can also be attributed to the surface plasmonic effects, caused by the presence of silver nanoparticles. The power conversion efficiency was 6.98% which indicated a 161% increase in the conversion efficiency compared to those of TiO2 DSSC. In the third part, a simple hydrothermal method for the green synthesis of water-soluble CDs has been reported using Rosemary leaves as a precursor. On the designed TiO2 photoanode, the electron hopping at the boundaries improved in the presence of CDs caused by the interconnection of the TiO2 nanoparticles. The effective separation of charge carriers and increases the lifetime of the electrons are owing to the well-transferred electron to the conduction band of TiO2 and accumulated holes in the valence band of CDs. In the presence of CDs (5%)/TiO2 as a photoanode, the power conversion efficiency was 7.32%, which indicated a 125% increase in the conversion efficiency compared to those of TiO2 DSSC. In the last part, a comprehensive study was conducted on the performance of three-dimensional TiO2 nanostructures including, I. Photovoltaic performance of DSSC, II. photocatalysis measurements, III. photoelectrochemical performance in the bifunctionalized TiO2 film, and IV. photoelectrochemical performance in the bifunctionalized TiO2 film modified with MIP. The results of this study showed that the designed photoanodes, due to the creation of favorable pathways and reduction of lattice imperfection and grain boundaries exhibit a good photon to the current conversion efficiency (4.9%) in DSSC. Eventually, we achieved the highest percentage of destruction in the shortest possible time by modifying the degradation zone with the MIP in the photoelectrochemical system. The application of a bifunctionalized NF-3.0 TiO2 films consisting of dye-sensitized solar cell and degradation zone, at best (NF-3.0 TiO2), due to the excellent performance of the DSSC in the electron injection to the degradation zone and effective transfer of electrons to the surface of the layer, followed by the production of more active radicals, degradation percentage of 99.0% with the degradation rate of 2.9×10-2 min-1 in 160 min were achieved. Finally, by modifying the degradation zone with MIP degradation percentage of 98.1% with the degradation rate of 4.9×10-2 min-1 in 80 min were achieved.