Synthesis, characterisation and potential employment of Pt–modified TiO2 photocatalysts towards laser induced H2 production
The photocatalytic production of H2 from water as well as from a 1:1 methanol:water solution employing pre–treated TiO2 and various Pt–TiO2 photocatalysts was studied by using an Nd:YAG laser as irradiation source. The photocatalysts (0.5–, 1–, 1.5– and 2 wt% Pt–TiO2) were prepared by utilizing a photocatalytic reduction method after which characterisation by various analytical techniques, i.e. XRD, TEM, ICP, SEM, and EDX, were conducted. XRD clearly indicated that platinum was not present in the crystal structure of TiO2, but was rather loaded onto the surface of TiO2. TEM analysis confirmed the presence of Pt on the surface with a particle/cluster size between 11 nm and 22 nm. SEM showed that repeatable results in respect of surface appearance were obtained. ICP and EDX indicated that the loading method was successful with only a slight deviation between the actual amount loaded and the calculated amount loaded. The impact of the loaded Pt on the band gaps of the different photocatalysts was investigated by diffuse reflectance spectroscopy (DRS) and calculated by employing the Kubelka–Munk method. The band gap values shifted sequentially from 3.236eV to 3.100 eV as the loading increased, moving closer to the absorbance region for visible light. The amount of hydrogen produced from the individual photocatalysts dispersed in both pure water and aqueous methanol solutions, was measured manually with a gas chromatograph. As soon as irradiation was initiated, a distinct colour change from shades of grey to dark blue–grey was observed for all the photocatalysts. XRD confirmed that it was due to the anatase phase transforming to produce more rutile phase. No H2 was detected for the various photocatalysts suspended in water, i.e. in the absence of methanol. The amount of hydrogen produced from the various Pt photocatalysts suspended in the aqueous methanol solution was found to be the highest for the 0.5wt%– and 1.5wt% Pt–TiO2 photocatalysts and the lowest for the 2wt% Pt–TiO2. This could be due to loading Pt above the optimum amount to such an extent, preventing sufficient light from reaching the TiO2 surface. Pt particles can also touch and overlap which will decrease Pt contact with TiO2 thus decreasing effective charge transfer.