Clean water and the increased use of renewable energy are considered to be two of the main goals in the effort to achieve a sustainable living environment. The fulfillment of these goals may include the use of solar-driven photocatalytic processes that are found to be quite effective in water purification, as well as hydrogen generation. H2 production by water splitting and photocatalytic degradation of organic pollutants in water both rely on the formation of electron/hole (e−/h+) pairs at a semiconducting material upon its excitation by light with sufficient photon energy. Most of the photocatalytic studies involve the use of TiO2 and well-suited model compounds, either as sacrificial agents or pollutants. However, the wider application of this technology requires the harvesting of a broader spectrum of solar irradiation and the suppression of the recombination of photogenerated charge carriers. These limitations can be overcome by the use of different strategies, among which the focus is put on the creation of heterojunctions with another narrow bandgap semiconductor, which can provide high response in the visible light region. In this review paper, we report the most recent advances in the application of TiO2 based heterojunction (semiconductor-semiconductor) composites for photocatalytic water treatment and water splitting. This review article is subdivided into two major parts, namely Photocatalytic water treatment and Photocatalytic water splitting, to give a thorough examination of all achieved progress. The first part provides an overview on photocatalytic degradation mechanism principles, followed by the most recent applications for photocatalytic degradation and mineralization of contaminants of emerging concern (CEC), such as pharmaceuticals and pesticides with a critical insight into removal mechanism, while the second part focuses on fabrication of TiO2-based heterojunctions with carbon-based materials, transition metal oxides, transition metal chalcogenides, and multiple composites that were made of three or more semiconductor materials for photocatalytic water splitting.

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In this study, sandwich‐type composites made of commercial TiO2‐P25 and α‐Fe2O3 are obtained by spin coating thin films with different layer configurations, namely: i) TiO2 layer over α‐Fe2O3 (TiO2@α‐Fe2O3), ii) α‐Fe2O3 layer over TiO2 (α‐Fe2O3@TiO2), and iii) physically mixed 50% (w/w) of TiO2/Fe2O3. Photocatalytic activity under simulated solar irradiation of the aforementioned composites and their pure components is investigated for the degradation of amoxicillin (AMX) in the presence and absence of persulfate (PS). In both cases, TiO2@α‐Fe2O3 sandwich‐type achieve the highest degradation rates of AMX and a marked effect of PS addition on the AMX degradation rate is noted. The influence of pH and PS concentration on AMX degradation rate is established by means of experimental design and response surface modeling. The AMX degradation pathway is studied by means of reactive oxygen species scavenging and identification of intermediates by liquid chromatography with tandem mass spectrometry. Their evolution is directly correlated with an increased toxicity assessed by Daphnia magna and Vibrio fischeri assays. Furthermore, biodegradability changes are correlated with the mineralization profile of AMX solution. The influence of water matrix constituents (Cl−, CO32−, NO3−, PO43− and Suwannee river natural organic matter) on AMX degradation is established as well.