The use of thin Ta3N5 films in tandem Si-Ta3N5 photoelectrochemical (PEC) devices motivates understanding of the surface Ta3N5 properties, as they may have a strong effect on the device performance. The bulk and surface properties can change as a function of nitridation temperature; thus its effect is studied, ranging from 700 to 1000 °C, on the PEC performance, morphology, and composition of thin (10 nm) Ta3N5 films deposited on planar and nanostructured Si substrates. Scanning electron microscopy (SEM), scanning Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) are employed to gain fundamental understanding in the differences of the Ta3N5 films. By controlling Ta3N5 morphology and composition with nitridation temperature, it is determined that Ta3N5 with high crystallinity and surface N/Ta ratio, synthesized at 800 °C, yields the highest PEC performance with the earliest photocurrent onset and highest photocurrent. Samples nitrided at 700 °C have lower crystallinity and that likely leads to lower performance. For samples nitrided at temperatures above 800 °C, the N/Ta ratio decreases forming chemically reduced tantalum nitride phases, as well as N-deficient and correspondingly O-rich morphological domains that can adversely affect the PEC performance as hole-blocking layers or O trap-mediated recombination centers at the surface of Ta3N5.