Titanium dioxide nanoparticles (TiO2 NPs) are photocatalytical active materials of great interest for energy management and environmental applications. Among others, they enable enhanced conversion of solar energy into hydrogen as a strategic future energy carrier or the oxidative degradation of contaminants in water, thus addressing critical issues of water remediation. Combined with the large surface area of TiO2 nanoparticles, their relevance relates to their processing ability to form sustainable waterborne inks towards the fabrication of films serving as photoelectrodes in photoelectrochemical or photocatalytic processes. In this chapter, we systematically characterize the physicochemical properties of TiO2 NPs from various sources. We show how these directly influence the optical properties in respective inks as well as the morphological characteristics when processed into films. Analyzing the photoelectrochemical behavior of respective photoelectrodes under water-splitting conditions, we disclose the close relationship between the physicochemical properties of TiO2 NPs regarding their ability to form aggregates in the liquid phase and the photoelectrochemical performance of respective photoelectrodes. These findings highlight the need for an appropriate choice of TiO2 NPs for a specific photoelectrochemical or photocatalytic application.