Hydrogen produced from water using photocatalysts driven by sunlight is a sustainable way to overcome the intermittency issues of solar power and provide a green alternative to fossil fuels. TiO₂ has been used as a photocatalyst since the 1970s due to its low cost, earth abundance, and stability. There has been a wide range of research activities in order to enhance the use of TiO₂ as a photocatalyst using dopants, modifying the surface, or depositing noble metals. However, the issues such as wide bandgap, high electron-hole recombination time, and a large overpotential for the hydrogen evolution reaction (HER) persist as a challenge. Here, we review state-of-the-art experimental and theoretical research on TiO₂ based photocatalysts and identify challenges that have to be focused on to drive the field further. We conclude with a discussion of four challenges for TiO₂ photocatalysts—non-standardized presentation of results, bandgap in the ultraviolet (UV) region, lack of collaboration between experimental and theoretical work, and lack of large/small scale production facilities. We also highlight the importance of combining computational modeling with experimental work to make further advances in this exciting field.

Titanium dioxide (TiO₂) is a commonly used wide bandgap semiconductor material for energy and environmental applications. Although it is a promising candidate for photovoltaic and photocatalytic applications, its overall performance is still limited due to low mobility of porous TiO₂ and its limited spectral response. This limitation can be overcome by several ways, one of which is doping that could be used to improve the light harvesting properties of TiO₂ by tuning its bandgap. TiO₂ doped with elements, such as alkali-earth metals, transition metals, rare-earth elements, and nonmetals, were found to improve its performance in the photovoltaic and photocatalytic applications. Among the doped TiO₂ nanomaterials, transition metal doped TiO₂ nanomaterials perform efficiently by suppressing the relaxation and recombination of charge carriers and improving the absorption of light in the visible region. This work reports the possibility of enhancing the performance of TiO₂ towards Dye Sensitised Solar Cells (DSSCs) and photocatalytic degradation of methylene blue (MB) by employing Zn doping on TiO₂ nanomaterials. Zn doping was carried out by varying the mole percentage of Zn on TiO₂ by a facile solvothermal method and the synthesized nanomaterials were characterised. The XRD (X-Ray Diffraction) studies confirmed the presence of anatase phase of TiO₂ in the synthesized nanomaterials, unaffected by Zn doping. The UV-Visible spectrum of Zn-doped TiO₂ showed a red shift which could be attributed to the reduced bandgap resulted by Zn doping. Significant enhancement in Power Conversion Efficiency (PCE) was observed with 1.0 mol% Zn-doped TiO₂ based DSSC, which was 35% greater than that of the control device. In addition, it showed complete degradation of MB within 3 h of light illumination and rate constant of $1.5466\times {10}^{-4}{s}^{-1}$ resembling zeroth order reaction. These improvements are attributed to the reduced bandgap energy and the reduced charge recombination by Zn doping on TiO₂.

This study focuses on employing cuprous iodide (CuI) as a hole-transporting material (HTM) in fabricating highly efficient perovskite solar cells (PSCs). The PSCs were made in air with either CuI or 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) as HTMs. A simple and novel pressing method was employed for incorporating CuI powder layer between perovskite layer and Pt top-contact to fabricate devices with CuI, while spiro-OMeTAD was spin-coated between perovskite layer and thermally evaporated Au top-contact to fabricate devices with spiro-OMeTAD. Under illuminations of 100 mW/cm² with an air mass (AM) 1.5 filter in air, the average short-circuit current density (J_SC) of the CuI devices was over 24 mA/cm², which is marginally higher than that of spiro-OMeTAD devices. Higher J_SC of the CuI devices can be attributed to high hole-mobility of CuI that minimizes the electron-hole recombination. However, the average power conversion efficiency (PCE) of the CuI devices were lower than that of spiro-OMeTAD devices due to slightly lower open-circuit voltage (V_OC) and fill factor (FF). This is probably due to surface roughness of CuI powder. However, optimized devices with solvent-free powder pressed CuI as HTM show a promising efficiency of over 8.0 % under illuminations of 1 sun (100 mW/cm²) with an air mass 1.5 filter in air, which is the highest among the reported efficiency values for PSCs fabricated in an open environment with CuI as HTM.

Earth–abundant transition metal chalcogenide materials are of great research interest for energy production and environmental remediation, as they exhibit better photocatalytic activity due to their suitable electronic and optical properties. This study focuses on the photocatalytic activity of flower-like SnS₂ nanoparticles (composed of nanosheet subunits) embedded in TiO₂ synthesized by a facile hydrothermal method. The materials were characterized using different techniques, and their photocatalytic activity was assessed for hydrogen evolution reaction and the degradation of methylene blue. Among the catalysts studied, 10 wt. % of SnS₂ loaded TiO₂ nanocomposite shows an optimum hydrogen evolution rate of 195.55 µmolg⁻¹, whereas 15 wt. % loading of SnS₂ on TiO₂ exhibits better performance against the degradation of methylene blue (MB) with the rate constant of 4.415 × 10⁻⁴ s⁻¹ under solar simulated irradiation. The improved performance of these materials can be attributed to the effective photo-induced charge transfer and reduced recombination, which make these nanocomposite materials promising candidates for the development of high-performance next-generation photocatalyst materials. Further, scavenging experiments were carried out to confirm the reactive oxygen species (ROS) involved in the photocatalytic degradation. It can be observed that there was a 78% reduction in the rate of degradation when IPA was used as the scavenger, whereas around 95% reduction was attained while N₂ was used as the scavenger. Notably, very low degradation (<5%) was attained when the dye alone was directly under solar irradiation. These results further validate that the •OH radical and the superoxide radicals can be acknowledged for the degradation mechanism of MB, and the enhancement of degradation efficiency may be due to the combined effect of in situ dye sensitization during the catalysis and the impregnation of low bandgap materials on TiO₂.

Transition metal chalcogenides have intensively focused on photocatalytic hydrogen production for a decade due to their stronger edge and the quantum confinement effect. This work mainly focuses on synthesis and hydrogen production efficiencies of cobalt disulfide (CoS₂)-embedded TiO₂ nanocomposites. Materials are synthesized by using a hydrothermal approach and the hydrogen production efficiencies of pristine CoS₂, TiO₂ nanoparticles and CoS₂/TiO₂ nanocomposites are compared under UV irradiation. A higher amount of hydrogen production (2.55 mmol g⁻¹) is obtained with 10 wt.% CoS₂/TiO₂ nanocomposite than pristineTiO₂ nanoparticles, whereas no hydrogen production was observed with pristine CoS₂ nanoparticles. This result unveils that the metal dichalcogenide–CoS₂ acts as an effective co-catalyst and nanocrystalline TiO₂ serves as an active site by effectively separating the photogenerated electron–hole pair. This study lays down a new approach for developing transition metal dichalcogenide materials with significant bandgaps that can effectively harness solar energy for hydrogen production.

A set of titania–silica mixed oxide materials were prepared by a cosolvent-induced gelation method using ethanol and toluene as solvent and cosolvent, respectively. These materials were extensively characterized by utilizing several characterization techniques and assessed for phenol degradation under UV illumination. The degradation of phenol follows first-order kinetics, and fragmented products formed during the phenol degradation were qualitatively identified by using high performance liquid Chromatographic (HPLC) and atomic pressure chemical ionization mass spectroscopic (APCI-MS) techniques. The complete mineralization of phenol was further evidenced by the measurement of the total organic contents that remained in the solution after irradiation. The pore diameter of the materials was found to be the key factor for phenol degradation, whereas surface area and pore volume play a role among the mixed oxide materials. In addition, in the mixed oxide system there was an inverse correlation obtained with the particle size of the materials and the degradation efficiency. The smaller particle size of titania in the mixed oxide material was found to be a requirement for an effective degradation of phenol.