Abstract:
Increase in the world population has led to notable increase in the energy consumption rate, resulting in a significant dependence on carbonaceous fuels. Limited availability along with environmental hazards related to carbonaceous fuel highlight the need for the clean and renewable sources of energy.[1] Since it is known that hydrogen is a clean source of energy (H2O as a byproduct) with energy density value as high as 142 MJ/kg, motivates researchers to find methods to produce hydrogen on a large scale using economically viable routes.[2] Almost 85 to 90 % of H2 is presently being produced by natural gas reforming which requires high temperature (up to 900 ºC) and pressure (1.5-3 MPa) and results in the evolution of like hazardous byproducts CO and CO2.[3] It is therefore, highly desirable to produce hydrogen by employing a renewable source of energy with minimum environmental hazard. In doing so, natural photosynthesis involving sunlight inspires us to use abundant solar energy. The well-known process of photosynthesis in plants involves a redox reaction wherein CO2 is converted into carbohydrate and H2O is oxidize into O2 by the photoexcited electrons and holes respectively. The process of photosynthesis involves single or two photosystems (Z-scheme). There have been strategies to convert solar energy into chemical energy mimicking natural photosynthesis wherein CO2 and H2O are used as reactants.[4] These processes involve the use of photocatalysts with action similar to the chloroplast in plants which absorbs sunlight and generates excited electron and hole pairs which then take part in the reduction and oxidation of H2O respectively. These materials can be directly used as a suspension in the pool of water (Photochemical water splitting) or can be coated on a conductive substrate in the form of a film and used in the assistance of minimum voltage (photoelectrochemical H2O splitting (Figure 1).[5] It should be noted that splitting of H2O into H2 and O2 is a thermodynamically uphill reaction (ΔG = 237 kJ/mol) and requires careful experimental strategies.[6] Since the discovery of hydrogen production from n-TiO2 using the photoelectrochemical (PEC) method by Honda and Fujisima's, there have been enormous efforts to develop technologies for semiconductor-based hydrogen evolution by water splitting.
