Investigations on photocatalytic H2 generation using modified TiO2 and g-C3N4 semiconductors
Generation of hydrogen from renewable sources like water and solar energy is considered to be an attractive and viable solution for replacing fossil fuels. Photocatalytic water splitting has attracted much attention as it involves conversion of solar energy into useful chemical energy. The major challenge involved in the process is the development of stable and visible light active photocatalyst with required solar to fuel energy conversion efficiency (SFE). In the present work, studies were undertaken to improvise the optical and photocatalytic properties of several photocatalysts; conventionally known UV active TiO2 and novel organic semiconductor, graphitic carbon nitride (g-C3N4). Various strategies such as cationic doping by Cu in TiO2, composite formation with NiO and CuO inducing pn heterojunctions, carbon and TiO2 heterojunction to improve electronic conductivity, surface modification of g-C3N4 by dispersing carbon nanodots (CND) and noble metal (Pt, Pd, Cu, Ag and Au) were adopted to limit the e-/h+ recombination reaction and to enhance the photoresponse under visible light illumination. All samples were thoroughly characterized by relevant techniques and their potential for H2 generation was evaluated under sunlight and UV-visible light in presence of sacrificial reagent. Density functional theory calculations were performed and life time of e-/h+ from PL decay curves was measured to support the activity trend. Parameters such as illumination area, catalyst concentration, form of catalyst (powder/films) and different sacrificial reagents were optimized for maximum H2 yield. Performance of the screened photocatalysts was also tested in up-scaled photoreactors (volume = 0.5, 1 and 2 L). H2 yield @ 16 ml/h/g with apparent quantum efficiency (AQE) of 7.5 % and SFE of 3.9 % over Cu0.02Ti0.98O2-δ (without cocatalyst) was observed under sunlight suggesting that 0.96 m2 illumination area will yield H2 @ 1 L/h photocatalytically. Among modified g-C3N4 photocatalysts, maximum H2 yield of 398 μmol/h over 80 mg of Pt/CND/ g-C3N4 (0.48 wt %) under sunlight with AQE of 4.0 % and SFE of 2.0 % was achieved as compared to almost negligible yield over pristine carbon nitride. The present study is targeted to provide valuable inputs for actual large scale solar photocatalytic H2 production.