Scavenging of photogenerated holes in TiO2-based catalysts uniquely controls pollutant degradation and hydrogen formation under UVA or visible irradiation

Abstract

The use of heterogeneous photocatalysts to degrade hard-to-remove pharmaceuticals from polluted water offers a promising approach for efficient water treatment. These contaminants, commonly found in rivers and lakes, can be traced back to wastewater effluents containing harmful chemical species, including endocrine disruptors. Our research provides valuable insights into the functioning of known TiO₂-based photocatalysts as well as novel materials designed to expand the catalyst's absorption range into the visible region. This includes TiO₂, Pd@TiO₂, Cu@TiO₂, and Au@TiO₂. Among the emerging photocatalysts, black TiO₂ (b-TiO₂) was selected as the starting point, and subsequently decorated with metal nanoparticles to produce Pd@b-TiO₂, Cu@b-TiO₂, and Au@b-TiO₂. Mechanistic findings reveal that hole trapping consistently emerges as the yield-determining step, with electron scavenging following closely behind. Consequently, oxygen or proton trapping of electrons has no significant impact on the overall efficiency of removal of pollutants. Additionally, we present a methodology for screening the capability of newly designed materials to photodegrade pollutants by measuring the output of H₂. This eliminates the need for series of experimental trials specific to each target pollutant; thereby, streamlining conventional processes upheld in this field. Each of these materials was first tested for their hydrogen generating ability under UV and visible light using methanol and formic acid as sacrificial electron donors, as well as estradiol, ibuprofen, and acetaminophen. Following this, ibuprofen was selected for extended studies where it would be photooxidized and/or release H₂ gas as a by-product that is readily detectable using gas chromatography.