Macroporous Semiconductors Tantalum Oxide, (Oxy)nitride and Nitride for Photocatalytic Hydrogen Evolution

Due to the serious climatic consequences of CO2 pollution and increasing global energy demand, a clean and sustainable energy source is required. Perhaps the ideal clean fuel is hydrogen, which would be sustainable if it could be sourced efficiently from water. Photocatalysis using metal-semiconductor composites is potentially a feasible way to make use of solar energy to drive the water splitting reaction to product hydrogen and oxygen. A significant number of studies have been reported in recent decades on the development of new photocatalytic materials, ion doping, co-catalyst addition and modification of the morphology to enhance the light harvesting, and increase the number of active sites in order to improve the photocatalytic activity.
In this project, three-dimensional ordered macroporous (3DOM) Ta2O5, TaON and Ta3N5 have been prepared and loaded with Pt co-catalyst (0.5 wt% and 3 wt %). Subsequently the photocatalytic activities with respect to hydrogen production using methanol as a sacrificial reagent were measured and compared with bulk analogues. A colloidal crystal templating method using polystyrene (PS) monodispersed spheres with diameter 500±20 nm was used for the synthesis of the macroporous materials. Characterizing data of the macroporous materials was obtained by powder X-ray diffraction (PXRD), scanning electron microscopy(SEM), transmission electron microscopy (TEM), BET surface area measurement and UV-Vis reflectance and absorbance spectroscopy. Pore sizes of macroporous Ta2O5, TaON and Ta3N5 are 370±10, 380±10 and 400±10 nm, respectively. The
wall thicknesses are 70±5, 60±5 and 60±10 nm, respectively. Spectroscopy showed that the macroporous Ta2O5, TaON and Ta3N5 structures are photonic and stop bands are observed at 721, 683 and 748 nm, respectively. Surface areas were
measured to be 11.53, 12.12, 22.98 m2g-1 for macroporous Ta2O5, TaON and Ta3N5 respectively whereas bulk materials were between 1.35, 3.22 and 7.91 m2g-1,respectively. The microstructure of the macroporous materials was determined by PXRD and electron microscopy which showed increasing crystallite fragmentation as the level of nitridation increases. Calculated crystallite size as determined by PXRD are 60, 36, 35 nm for bulk Ta2O5, TaON and Ta3N5 and 40, 33, 18 nm for macroporous Ta2O5, TaON and Ta3N5, respectively. Electron microscopy of 0.5 wt% Pt loaded Ta2O5 showed evidence for deposition of Pt on the surface of the bulk Ta2O5 and on the inner walls of the macroporous Ta2O5, respectively, but some aggregation occurred. Comparing the photocatalytic activities for hydrogen production showed that Pt addition enhances the activities
for both bulk materials and macroporous Ta2O5, however, less activity was observed for macroporous TaON and Ta3N5. Normalizing for the increase in surface area of macroporous materials Ta2O5 and Ta3N5 show ca 50% less activity whereas TaON shows a 40% increase in activity. It should be noted errors have not been estimated and the surface chemistry of the materials is at present unknown. However, the activities strongly indicate that photocatalysis is occurring throughout the porous material.
In addition, because of difficulties comparing photocatalytic reactions in the literature a series of calibration experiments was performed using P25 and methanol. Activity as a function of photocatalyst mass and concentration were performed. The results indicate that the concentration of P25 does not have significant effect for the photocatalytic activities and the optimum amount of
photocatalyst in our reaction system is ca 80 mg (in the system of 100 ml H2O + 10 ml MeOH solution).