Photoelectrochemical (PEC) water splitting devices are envisioned to play a key role in the societal transition towards sustainable energy sources. In this context, multinary metal oxide semiconductors are the most promising candidates for the role of photoanodes to drive the water oxidation reaction. Nevertheless, the ideal material has yet to be found. Furthermore, when aiming at maximizing PEC performance, the ability to tailor-make material platforms with tunable morphological characteristics in an unrestricted compositional range is critical for achieving optimal design specifications, as well as for understanding the sensitivities of performance parameters to different compositional and structural parameters. This thesis focuses on the mechanistic studies and development of synthesis routes for ternary metal oxides as photoanodes for the water splitting reaction. A nanocrystal (NC) seeded-growth approach is introduced and demonstrated to attain a material tunability not always accessible via other synthesis techniques. In the first example, size-controlled NCs were reacted with a molecular precursor to synthesize ternary metal oxides. Specifically, surface oxidized Cu NC seeds with diameter changing from 6 nm to 70 nm were combined with vanadium acetylacetonate to form Cu2V2O7 . The seed size was found to regulate the grain size of the obtained ternary oxide, which was indeed obtained with tunable grain dimensions ranging from 29 nm to 63 nm. In-situ X-Ray diffraction studies explained this result as they evidenced the occurance of a solid state reaction between the NCs and the reacted molecular precursor upon annealing. This achieved tunability allowed us to correlate the PEC performance to the grain dimensions in a size regime close to the charge carrier diffusion length of Cu2V2O7 (20 - 40 nm). Moving forward, the nanocrystal seeded growth approach was further extended to Cu2V2O7/WO3 heterostructured nanocomposites. Here, Cu and WO3 NCs were combined in different ratio and simultaneously reacted with vanadium acetylacetonate. The result was a material platform with tunable composition and mesostructure which helped us to identify the optimal conditions to achieve better charge extraction at the interfacial region, which led to improved PEC performance. Finally, the molecular precursor was substituted with a second NC reactant and solid state chemistry between NCs was explored in the general framework of copper based ternary metal oxides. Scaling down the size of the reactants compared to traditional solid state chemistry among powders reduces the reaction time and temperature. More importantly, the ternary metal oxide products were NCs with tunable size and shape, a control not straightforward via other approaches. Binary superlattices of Cu and Fe3O4 NCs were used as a platform to monitor the reaction mechanism by the combination of various electron microscopy techniques. This study showed that having only one of the two NC precursor dissolving and diffusing toward the other is crucial to obtain a final nanocrystalline product with homogeneous size and shape. The latter are regulated by the NC precursor which is the most stable at the reaction temperature. Considering the variety of controlled NCs available, our findings open up a new avenue for the synthesis of functional and tunable polyelemental nanomaterials.