Titanium improves water oxidation yields over hematite photoanodes, tailoring its surface state density (kinetics) and hematite-pseudobrookite heterojunctions (energetics).

Water splitting with hematite is negatively affected by poor intrinsic charge transport properties. However, they can be modified by forming heterojunctions to improve charge separation. For this purpose, charge dynamics of TiO2:α-Fe2O3 nanocomposite photoanodes are studied using transient absorption spectroscopy to monitor the evolution of photogenerated charge carriers as a function of applied bias voltage. The bias affects the charge carrier dynamics, leading to trapped electrons in the submillisecond time scale and an accumulation of holes with a lifetime of 0.4 ± 0.1 s. By contrast, slower electron trapping and only few long-lived holes are observed in a bare hematite photoanode. The decay of the long-lived holes is 1 order of magnitude faster for the composite photoanodes than previously published for doped hematite, indicative of higher catalytic efficiency. These results illustrate the advantages of using composite materials to overcome poor charge carrier dynamics, leading to a 30-fold enhancement in photocurrent.

Nanocomposites represent an interesting class of materials because their applications are of multidisciplinary importance. Herein, interactions at the interface of heterostructures, leading to superior performance and sometimes to synergistic interactions, are highlighted. Although the types of junctions seem to be similar in different fields, the requirements for their function is device‐dependent and thus no generally valid recipe can be utilized. Nonetheless, the overlap between research fields unveils the unique opportunity to combine the different knowledge and expertise as an inspiration to designing new composites for photoelectrochemical water splitting, photocatalysis, chemoresistive sensors, batteries, and composite multiferroics. A non‐comprehensive overview is provided with focus on selected examples to highlight the concepts behind these nanojunctions.

Doping of TiO2 enables tailoring of the phase composition, microstructure and topography of titania–hematite photoanodes during annealing, resulting in remarkable photocurrents.

We present a sol–gel processed hematite–titania-based photoanode, which exhibits a photocurrent of up to 2.5 mA/cm 2 at 1.23 V RHE under simulated AM 1.5 G illumination (100 mW/cm 2 ) thanks to the addition of an amorphous cocatalyst with the nominal composition Fe 20 Cr 40 Ni 40 O x . To unveil the role of the cocatalyst interconnected to the photoanode, we performed impedance measurements. According to the one order of magnitude higher value for the capacitance associated with surface states ( C SS ) compared to the bare photoanode, the function of the catalyst−photoanode interface resembles that of a p−n-like junction. In addition, the charge transfer resistance associated with charge transfer processes from surface states ( R ct,ss ) was unchanged at potentials between 0.8 and 1.1 V RHE after adding the cocatalyst, indicating that the catalyst has a negligible effect on the hole transport to the electrolyte. The understanding of the role of oxygen evolution catalysts (OECs) in conjunction with the photoanodes is particularly important for water splitting because most OECs are studied separately at considerably higher potentials compared to the potentials at which photoanode materials are operated.