The inorganic titanium dioxide TiO2 and metal-free graphitic carbon nitride (g-C3N4) are the two most important photocatalysts owing to their low cost, non-toxicity, high thermal and chemical stability, as well as high reactivity. However, these two materials also suffer from certain drawbacks such as a relatively high rate of electron-hole pair recombination, a slightly large bandgap that requires UV-light for activation (TiO2), and a relatively small surface area (g-C3N4).This thesis focuses to overcome above drawbacks by combining TiO2 with low bandgap metal chalcogenide semiconducting nanoparticles as a strategy to create an efficient charge separation at the interface of the heterojunctions and to extend the absorption to visible region. This thesis also attempts to improve the photocatalytic efficiency of g-C3N4 by synthesizing it with high surface area and, finely, by combining it with TiO2. After describing briefly the motivation behind the work and the existing literature on the subject matter in the first chapter, the Chapter 2 describes precursor-mediated synthesis and photocatalytic activity of binary coinage metal chalcogenide nanoparticles and their composites with TiO2 under mild conditions. We successfully isolate and characterize kinetically and/or thermally unstable molecular species during the reactions, thus providing an insight into the molecule-to-nanoparticle mechanisms. The metal chalcogenide-titania nanocomposites show superior activity for the photodegradation of formic acid under ultraviolet light, as compared to titania (P25), which is a well-established benchmark for photocatalysis under UV light. Chapter 3 describes the synthesis of ternary coinage metal chalcogenides and their composites with titania. A direct room temperature reaction of tBu2Se with copper and silver trifluoroacetates gives copper-silver-selenide nanoparticles that are formed via a highly reactive intermediate molecular species [Ag2Cu(TFA)4(tBu2Se)4]. We also employed the pre-formed copper chalcogenide NPs as precursors and reacted it with di-tertiary butyl chalcogenide to obtain ternary metal chalcogenide nanoparticles and their composites with TiO2 in pure phase and with high yield. Photocatalytic studies for the degradation of formic acid under ultraviolet radiations show that the ternary CuAgSe-TiO2 nanocomposites are even better photocatalysts than the binary chalcogenide-TiO2 nanocomposites described in Chapter 2. The second part of this thesis described in Chapter 4 diverges from the earlier chapters to concentrate on the coupling of titania with graphitic carbon nitride. In the first step, we develop a simple single-step calcination approach to synthesize graphitic nanoparticles with a high surface area (200 m2/g). We test the prepared photocatalyst for the degradation of formic acid and phenol under visible light, and analyze the effect of factors such as surface area, irradiance and concentration of carbon nitride on the photocatalytic performance. Results show that the performance increases linearly with surface area and irradiance, whereas it first increases and gradually reaches plateau as concentration of the photocatalyst is increased. In the second step, we couple carbon nitride with titania using mechanical mixing. The prepared nanocomposites are then evaluated for the photodegradation of formic acid under both ultraviolet and visible lights. Finally, the Chapter 5 describes the experimental details of the synthesis and characterization of the molecular precursors, metal chalcogenide nanoparticles, g-C3N4 nanosheets as well as their composites with TiO2. It also describes in detail the experimental setup for the photocatalytic studies.