Abstract: The promise of solar water splitting as a sustainable route to renewable energy storage has motivated many device designs. One design that mimics natural photosynthesis is termed Z-Scheme. It consists of two artificial photosystems that each absorbs in different regions of the solar spectrum to drive H2 evolution via water reduction and O2 evolution via water oxidation. When these reactions occur in one compartment an explosive, and therefore dangerous, oxyhydrogen gas mixture results. A safer alternative is to separately compartmentalize each reaction, which then requires the use of an aqueous redox shuttle, such as Fe(III/II), to mediate charge between compartments. While safer, this strategy requires exquisite control over redox selectivity. Fe(III) reduction and Fe(II) oxidation are each more thermodynamically and kinetically favored over their desired counterparts of H2 evolution and O2 evolution, respectively. Overcoming this challenge requires new and innovative means to achieve exceptional redox reaction selectivity on the nanoscale. This is underscored by the poor photoactivity of state-of-the-art H2 evolving nanoparticles, such as SrTiO3 doped with transition metals and with metallic cocatalysts on their surface whose solar-to-hydrogen energy conversion efficiency is only of the order of 1%. Improving reaction selectivity is one of the ways to improve solar-to-hydrogen energy conversion efficiency in Z-Scheme solar water splitting systems. In this talk I will describe results obtained for Rh-doped SrTiO3 photocatalyst particles coated with a conformal layer of ultrathin oxide coatings (i.e. TiOx or SiOx). These nanoparticles were used to unequivocally demonstrate enhanced reaction selectivity for visible-light-driven H2 evolution in the presence of significant amounts of facile-to-reduce aqueous Fe(III) oxidation products. Comparison of results using coatings consisting of amorphous TiOx versus SiOx, variable cocatalyst loading, and practical electrolyte conditions indicate the role of the ultrathin oxide coatings to selectively inhibit permeation of Fe(III), thus affording a higher selectivity for H2 evolution. Although our data suggest that permeation of Fe(II) through the coatings is also inhibited to some degree, thus attenuating desired rates of Fe(II) oxidation by photogenerated oxidizing equivalents, we believe that our approach and results validate a new platform for designing enhanced reaction selectivity into photocatalytic systems. Furthermore, I will also present recent results that begin to shed light on the cause of the commonly observed, yet currently unexplained, delay in H2 evolution activity that is widely observed during photodeposition of Pt cocatalysts on these nanoparticles. My ability to glean new information into these technoeconomically important designs was supported by use of a mass spectrometry instrument for rapid detection of H2 and a home-built time-resolved microwave conductivity apparatus to measure photogenerated transient mobile charge carriers in wetted semiconductor nanoparticles.