Formamidinium Halide Perovskite and Carbon Nitride Thin Films Enhance Photoreactivity under Visible Light Excitation.

Photochemical and photocatalytic activity of adsorbates on surfaces is strongly dependent on the nature of a given substrate and its resonant absorption of the (visible) light excitation. An observation is reported here of the visible light photochemical response of formamidinium lead bromide (FAPbBr3) halide perovskite and carbon nitride (CN) thin-film materials (deposited on a SiO2/Si(100) substrate), both of which are known for their photovoltaic and photocatalytic properties. The goal of this study was to investigate the role of the substrate in the photochemical reactivity of an identical probe molecule, ethyl chloride (EC), when excited by pulsed 532 nm laser under ultrahigh vacuum (UHV) conditions. Postirradiation temperature-programmed desorption (TPD) measurements have indicated that the C-Cl bond dissociates following the visible light excitation to form surface-bound fragments that react upon surface heating to form primarily ethane and butane. Temperature-dependent photoluminescence (PL) spectra of the FAPbBr3 films were recorded and decay lifetimes were measured, revealing a correlation between length of PL decay and the photoreactivity yield. We conclude that the FAPbBr3 material with its absorption spectrum in resonance with visible light excitation (532 nm) and longer PL lifetime leads to three times faster (larger cross-section) photoproduct formation compared with that on the CN substrate. These results contrast the behavior under ambient conditions where the CN materials are photochemically superior due, primarily, to their stability within humid environments.

XPS studies on dispersed and immobilised carbon nitrides used for dye degradation.

Liquid phase adsorption is a common technique in waste water purification. However, this process has some downsides. The removal of environmentally harmful contaminants from organic liquids by adsorption produces secondary waste which has to be treated afterwards. The treatment can be e.g. high temperatures or a landfill. Photocatalysts such as CN6 can remove the dye under light irradiation but most times they have to be separated afterwards. Immobilisation of these photocatalysts can be one way to address this problem. The resulting photocatalyst layers were analysed in operando by near-ambient pressure XPS. This enabled us to detect the active species, i.e. oxygen radicals, at the surface, responsible for the dye degradation.