Hydrogen-Induced Reduction Improves the Photoelectrocatalytic Performance of Titania.

One of the main challenges to expand the use of titanium dioxide (titania) as a photocatalyst is related to its large band gap energy and the lack of an atomic scale description of the reduction mechanisms that may tailor the photocatalytic properties. We show that rutile TiO2 single crystals annealed in the presence of atomic hydrogen experience a strong reduction and structural rearrangement, yielding a material that exhibits enhanced light absorption, which extends from the ultraviolet to the near-infrared (NIR) spectral range, and improved photoelectrocatalytic performance. We demonstrate that both magnitudes behave oppositely: heavy/mild plasma reduction treatments lead to large/negligible spectral absorption changes and poor/enhanced (×10) photoelectrocatalytic performance, as judged from the higher photocurrent. To correlate the photoelectrochemical performance with the atomic and chemical structures of the hydrogen-reduced materials, we have modeled the process with in situ scanning tunneling microscopy measurements, which allow us to determine the initial stages of oxygen desorption and the desorption/diffusion of Ti atoms from the surface. This multiscale study opens a door toward improved materials for diverse applications such as more efficient rutile TiO2-based photoelectrocatalysts, green photothermal absorbers for solar energy applications, or NIR-sensing materials.

Advanced Nanostructured Conjugated Microporous Polymer Application in a Tandem Photoelectrochemical Cell for Hydrogen Evolution Reaction.

Solar energy conversion through photoelectrochemical cells by organic semiconductors is a hot topic that continues to grow due to the promising optoelectronic properties of this class of materials. In this sense, conjugated polymers have raised the interest of researchers due to their interesting light-harvesting properties. Besides, their extended π-conjugation provides them with an excellent charge conduction along the whole structure. In particular, conjugated porous polymers (CPPs) exhibit an inherent porosity and three-dimensional structure, offering greater surface area, and higher photochemical and mechanical stability than their linear relatives (conjugated polymers, CPs). However, CPP synthesis generally provides large particle powders unsuitable for thin film preparation, limiting its application in optoelectronic devices. Here, a synthetic strategy is presented to prepare nanostructures of a CPP suitable to be used as photoelectrode in a photoelectrochemical (PEC) cell. In this way, electronic and photoelectrochemical properties are measured and, attending to the optoelectronic properties, two hybrid photoelectrodes (photoanode and photocathode) are designed and built to assemble a tandem PEC cell. The final device exhibits photocurrents of 0.5 mA cm-2 at a 0.7 V in the two electrode configuration and the hydrogen evolution reaction is observed and quantified by gas chromatography, achieving 581 µmol of H2 in a one-hour reaction.

Mechanistic View of the Main Current Issues in Photocatalytic CO2 Reduction.

After 40 years of research on photocatalytic CO2 reduction, there are still many unknowns about its mechanistic aspects even for the most common TiO2-based photocatalytic systems. These uncertainties include the pathways inducing visible-light activity in wide-band gap semiconductors, the charge transfer between semiconductors and plasmonic metal nanoparticles, the unambiguous determination of the origin of C-bearing products, the very first step in the activation of the CO2 molecule, the factors determining the selectivity, the reasons for photocatalyst deactivation, the closure of the catalytic cycle by the hole-scavenging reagent, and the detailed reaction pathways and the most suitable techniques for their determination. This Perspective discusses these controversial issues based on the most relevant investigations reported so far. For that purpose, we have tried to view the complex CO2 reduction in a holistic manner, considering today's state-of-the-art approaches, strategies, and techniques for the study of one of the hottest topics in energy research.