Enhanced Visible Light Absorption TiO2 Based Catalysts for Photocatalytic H2 Generation

Abstract

Photocatalytic H2 generation from water, by utilizing semiconductors, is attractive process as the energy needed to break two chemical bonds of water and to form a chemical bond between hydrogen atoms is provided by the sun, a free and abundant energy source. The formation of solar H2 does not only decrease human dependency on conventional fossil fuels but also alleviate atmospheric pollution. For the practical application of photocatalysts for H2 generation, there is a need to identify an ideal catalyst that possesses efficient light-harvesting, charge separation and migration properties, and optimized reactive sites which can lead to higher photocatalytic activity and stability.

With the ambition to design a cost-effective and highly stable photocatalyst with improved photocatalytic activity towards H2 generation by water splitting, a two-step facile synthesis approach is reported for core-shell nanostructure of TiO2 and CdS, i.e. CdS@TiO2 nanocomposite. The most optimized sample exhibited an enhanced photocatalytic activity by generating 954 µmol g-1 h-1 of hydrogen which are ~1.4 and ~1.7 times higher than pure CdS nanoparticles and pure TiO2, respectively. Motivated to avail of the benefits of placing the conductor layer in the heterojunction of two semiconductors and to further enhance the photocatalytic activity of CdS@TiO2, we have introduced a conductive layer of graphene (G) between the CdS core and TiO2 shell (CdS@G@TiO2). The most optimized sample, i.e. CdS@50G@TiO2 generated 1510 µmole g-1 h-1 of H2 with AQE of 5.78% from water.

The combined effect of doping and heterojunction formation between C-doped CdS and graphene in the form of a core-shell structure has been simultaneously manipulated to achieve elevated H2 generation. The effect of doping and the use of graphene as co-catalyst for CdS is studied for photocatalytic H2 generation. The most active sample consists of CdS and graphene (CdS-0.15G) exhibits promising photocatalytic activity, producing 3.12 mmol g-1 h-1 of H2 under simulated solar light giving AQY of 11.7%.

Finally, to accomplish the enhanced photocatalytic activity for H2 generation from water, we engineer novel heterojunction nanocomposite between reduced TiO2 and reduced CdS by NaBH4 treatment at relatively low temperature in an inert atmosphere. The reduction of nanocomposite of TiO2 and CdS sample with NaBH4 leads to the generation of oxygen vacancies (Vo)/Ti3+ and Cd3+ states in TiO2 and CdS respectively. These defect states alter the band structure of both the semiconductors by narrow downing the bandgaps which in-term increases the light absorption. By utilizing the above advantages of the heterojunction formation between two reduced semiconductors, an optimized reduced 2P25-4CdS sample generates 1395 µmol g-1 h-1 of photocatalytic H2 with AQE of 5 % without the use of any noble metal or co-catalysts along with promising stability.