Several application fields can benefit from solar-hydrogen technologies via specific short-term and long-term pathways.

The recent surge in investigating electrocatalysts for the H2 evolution reaction is based on finding a cheap alternative to Pt. However platinum's excellent catalytic activity means very little catalyst needs to be used. The present study combines model experiments with numerical modeling to determine exactly how little catalyst is needed. Specifically we investigate ultra-low Pt loadings for use in photoelectrochemical H2 evolution using TiO2-Ti-pn + Si photocathodes. At a current density of 10 mA/cm 2 , we photocathodically evolve H2 at +465, +450, +350 and +270 mV vs, RHE at Pt loadings of 1000, 200, 50, and 10 ng/cm 2 corresponding to HER overpotentials of η1000ng = 32 mV, η200ng = 46 mV, η50ng = 142 mV, and η10ng = 231 mV. To put this in perspective, if 30% of the world's current annual Pt production was used for H2 evolution catalysis, using a loading of 100 ng/cm 2 and a current of 10 mA/cm 2 would produce 1 TWaverage of H2. The photoelectrochemical data matched the modeling calculations implying that we were near the fundamental maximum in performance for our system. Furthermore modeling indicated that the overpotentials were dominated by mass transfer effects, rather than catalysis unless catalyst loadings were less than 1,000 ng/cm 2 .

Solar-light driven CO2 reduction into value-added chemicals and fuels emerges as a significant approach for CO2 conversion. However, inefficient electron-hole separation and the complex multi-electrons transfer processes hamper the efficiency of CO2 photoreduction. Herein, we prepare ferroelectric Bi3TiNbO9 nanosheets and employ corona poling to strengthen their ferroelectric polarization to facilitate the bulk charge separation within Bi3TiNbO9 nanosheets. Furthermore, surface oxygen vacancies are introduced to extend the photo-absorption of the synthesized materials and also to promote the adsorption and activation of CO2 molecules on the catalysts’ surface. More importantly, the oxygen vacancies exert a pinning effect on ferroelectric domains that enables Bi3TiNbO9 nanosheets to maintain superb ferroelectric polarization, tackling above-mentioned key challenges in photocatalytic CO2 reduction. This work highlights the importance of ferroelectric properties and controlled surface defect engineering, and emphasizes the key roles of tuning bulk and surface properties in enhancing the CO2 photoreduction performance.