Photocatalytic hydrogen generation using two-phase anatase/brookite titanium dioxide nanostructures
Date of Issue2015
School of Materials Science and Engineering
Solar-driven water splitting with the use of photocatalyst is an attractive method to produce clean and sustainable hydrogen to meet the growing demand of energy. Highly crystalline brookite and two-phase anatase/brookite TiO2 nanostructures were synthesized via a simple hydrothermal method with titanium sulfide as the precursor in sodium hydroxide. The control of phase composition has been demonstrated via varying the sodium hydroxide concentration and by studying the TiO2 phases formed with time-evolved hydrothermal reaction. Results have shown that anatase and brookite are formed from the transformation of sodium titanate. From the Mott-Schottky analysis, brookite phase is found to have a more cathodic conduction band potential than anatase phase which leads to higher hydrogen production. In addition, with two-phase anatase/brookite TiO2, the hydrogen production is further enhanced due to effective electron-hole separation as a result of charge transfer from brookite to anatase TiO2. In comparison with the highly active commercial benchmark P25, two-phase anatase/brookite TiO2 is 220 % more active when measured by the hydrogen yield per unit area of the photocatalyst surface. To further improve the charge separation, rGO was coupled with the two-phase anatase/brookite TiO2. Graphene with its superior electrical conductivity can act as electron collector to further separate the photogenerated charge carriers in TiO2. With rGO-anatase/brookite TiO2 mixture, the hydrogen production is 2.3 times the amount of hydrogen produced with rGO-P25 mixture. This is due to the more cathodic conduction band potential of brookite than anatase and rutile TiO2 which is energetically more favourable to reduce water to produce hydrogen. To increase the solar to hydrogen conversion efficiency by enabling visible light harvesting, graphite-like carbon nitiride (g-C3N4) which is visible light active was synthesized. Herein, a novel approach to tune the photophysical properties of g-C3N4 for efficient photocatalytic hydrogen production was developed by introducing hydrogen gas in the polycondensation reaction of g-C3N4 to create N vacancy and H substitution at N vacancy. With H doping in g-C3N4, the band gap is narrowed to 2.0 eV and produces open pore structures with threefold increase of surface area. The electronic structure calculations show that the band gap of H2 treated g-C3N4 is narrowed by pushing the valence band to negative values as well as conduction band to positive values. Detailed chemical analysis using X-ray photoelectron spectroscopy and elemental analysis results confirm the stoichiometry corresponding to N vacancy and H substitution at N vacancy. With increased optical absorption in the visible range, higher surface area and open pore structure, the hydrogen production under visible light illumination is fivefold enhanced with g-C3N4 synthesized in pure H2 [g-C3N4 (H2)] as compared to g-C3N4 synthesized from the conventional method [g-C3N4 (air)]. Two-phase anatase/brookite TiO2 was then coupled with g-C3N4 (H2) to create a heterojunction which gives the highest hydrogen evolution as compared to g-C3N4 (H2) with single phase anatase, brookite, two-phase anatase/rutile and three-phase anatase/brookite/rutile. With 20 wt% of two-phase anatase/brookite TiO2 and 80 wt% of g-C3N4 (H2), hydrogen production under visible light is around 363 times (36.91 µmole h-1 g-1) that of g-C3N4 (air).