Hematite (α-Fe₂O₃) is a promising photoanode in solar water splitting devices with a set of intrinsic limitations that lessen its maximum performance; among the methods used for improving its photoactivity, titanium doping has witnessed an intensive research during recent years. However, the origin of the Ti-induced enhancement remains elusive to date with the lack of a systematic mechanistic study. In this contribution, we prepared mesoporous hematite (host)–titania (guest) composite films by mixing the respective preformed nanoparticles obtained by a non-aqueous sol–gel route in a wide range of loading levels (0–20 mol%) up to the solid state solubility limits of both components. Voltammetric and impedance measurements were performed observing an optimum 10% doping, with a 15-fold photocurrent increase (up to 1.3 mA cm⁻² at 1.23 V_RHE) and a 100-fold decrease in the charge transfer resistance. The roles of surface states and charge donor (dopant) densities were also assessed, assuming a charge transfer mechanism through hole trapping at surface states and its isoenergetic transfer to water; an optimum 10–15% doping range was obtained similarly to photocurrent, where the maximum overlapping between surface and water states is prevalent. Finally, HR-TEM and EELS measurements were employed to detect the presence of pseudobrookite and titania phases (20% doping), evincing that hematite–pseudobrookite heterojunctions have a beneficial cascade of charge transfers but titania-pseudobrookite heterojunctions depict a deleterious “hole mirror” mechanism that prevents water photooxidation. Tailoring the combined effect of donor states (conductivity), phase coexistence (solubility), heterojunctions (energetics) and surface states (kinetics) in the composite paves the way for understanding the mechanism of other dopant-induced changes and could be extended to further photoactive materials.

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (µLP) to achieve a NO2-parts-per-billion conductimetric gas sensor operating at room temperature. In this µLP configuration, an InGaN-based LED (emitting at 455 nm) is integrated at a few hundred nanometers distance from the sensor material, leading to sensor photoacti-vation with well controlled, uniform and high irradiance condi-tions, and very low electrical power needs. The response curves to different NO2 concentrations as a function of the irradiance displayed a bell‐like shape. Responses of 20% to 25 ppb of NO2 were already observed at irradiances of 5 mWatts·cm-2 (applying an electrical power as low as 30 µW). In the optimum illumination conditions (around 60 mWatts · cm-2, or 200 µW of electric power), responses of 94% to 25 ppb were achieved, corresponding to a lower detection limit of 1 ppb of NO2. High-er irradiance values worsened the sensor response in all the parts-per-billion range of NO2 concentrations. The responses to other gases such as NH3, CO and CH4 were much smaller, showing a certain selectivity towards NO2. The effects of hu-midity on the sensor response are also discussed.

TiO₂ nanorod arrays grown on conductive substrates were converted using chemical strategies into CdS@TiO₂ and CdS@anatase@rutile TiO₂ heterostructures to fabricate visible-light harvesting assemblies. Compared to pure TiO₂ nanorods, CdS@TiO₂ heterostructures evidently extended the absorption edge and exhibited enhanced photoelectrochemical (PEC) response in the visible region. Further enhancement of PEC performance was achieved by introducing an intermediate anatase TiO₂ layer in the CdS@rutile TiO₂ heterostructures. An excitonic cascade of band alignment (CdS, anatase-TiO₂ and rutile-TiO₂) was constituted by arranging different semiconductors in order to align the edges of their conducting band, which improved charge separation and suppressed the recombination processes by facilitating the transfer of forward electrons and limiting the reverse processes due to spatial separation of the electron and hole in different material regions.