Although large research efforts have been devoted to photoelectrochemical (PEC) water splitting in the past several decades, the lack of efficient, stable and Earth-abundant photoelectrodes remains a bottleneck for practical application. Here, we report a photocathode with a coaxial nanowire structure implementing a Cu2O/Ga2O3-buried p–n junction that achieves efficient light harvesting across the whole visible region to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%. With a photocurrent onset over +1 V against the reversible hydrogen electrode and a photocurrent density of ~10 mA cm−2 at 0 V versus the reversible hydrogen electrode, our electrode constitutes the best oxide photocathode for catalytic generation of hydrogen from sunlight known today. Conformal coating via atomic-layer deposition of a TiO2 protection layer enables stable operation exceeding 100 h. Using NiMo as the hydrogen evolution catalyst, an all Earth-abundant Cu2O photocathode was achieved with stable operation in a weak alkaline electrolyte. To show the practical impact of this photocathode, we constructed an all-oxide unassisted solar water splitting tandem device using state-of-the-art BiVO4 as the photoanode, achieving ~3% solar-to-hydrogen conversion efficiency. The generation of hydrogen fuel from water and visible light requires photoelectrodes that are inexpensive, stable and highly active. Now, Luo, Gratzel and co-workers report Cu2O photocathodes that reach these goals. Incorporation into an unassisted solar water splitting device gives ~3% solar-to-hydrogen conversion efficiency.

/pdf/boosting-the-performance-of-cu2o-photocathodes-for-2vak1qfc20.pdf

Boosting the performance of Cu2O photocathodes for unassisted solar water splitting devices

A family of titania derived nanocomposites synthesized via sol-gel and hydrothermal routes exhibit excellent performance for the photocatalytic degradation of two important exemplar water pollutants, oxytetracycline and Congo Red. Low loadings of Co3O4 nanoparticles dispersed over the surfaces of anatase TiO2 confer visible light photoactivity for the aqueous phase decomposition of organics through the resulting heterojunction and reduced band gap. Subsequent modification of these Co3O4/TiO2 composites by trace amounts of graphene oxide nanosheets in the presence of a diamine linker further promotes both oxytetracycline and Congo Red photodegradation under simulated solar and visible irradiation, through a combination of enhanced photoresponse and consequent radical generation. Radical quenching and fluorescence experiments implicate holes and hydroxyl radicals as the respective primary and secondary active species responsible for oxidative photodegradation of pollutants.

Cobalt promoted TiO2/GO for the photocatalytic degradation of oxytetracycline and Congo Red

Kesterite Cu2ZnSnS4 (CZTS) thin-film solar cells have drawn worldwide attention because of outstanding performance and earth-abundant constituents. However, problems such as coexistence of complex secondary phases, the band tailing issue, short minority lifetime, bulk defects, and undesirable band alignment at p–n interfaces need to be addressed for further efficiency improvement. In this regard, Cd alloying shows promise for dealing with some of these problems. In this work, a beyond 11% efficient Cd-alloyed CZTS solar cell is achieved, and the effects of Cd-alloying and mechanism underpinning the performance improvement have been investigated. The introduction of Cd can significantly reduce the band tailing issue, which is confirmed by the reduction in the difference between the photoluminescence peak and optical band gap (Eg) as well as decreased Urbach energy. The microstructure, minority lifetime, and electrical properties of CZTS absorber are greatly improved by Cd alloying. Further XPS analyses sho...

Beyond 11% Efficient Sulfide Kesterite Cu2ZnxCd1–xSnS4 Solar Cell: Effects of Cadmium Alloying

Biogenic synthesis of copper oxide nanoparticles using plant extract and its prodigious potential for photocatalytic degradation of dyes

ZnO for application in photocatalysis: From thin films to nanostructures

One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near-edge region can be fitted to a simple expression in which the intercept gives the band-gap and the fitting exponent identifies the electronic transition as direct or indirect (see Tauc et al., Phys. Status Solidi 15, 627 (1966); these are often called “Tauc” plots). While the technique is powerful and simple, the accuracy of the fitted band-gap result is seldom stated or known. We tackle this question by refitting a large number of Tauc plots from the literature and look for trends. Nominally pure zinc oxide (ZnO) was chosen as a material with limited intrinsic deviation from stoichiometry and which has been widely studied. Our examination of the band gap values and their distribution leads to a discussion of some experimental factors that can bias the data and lead to either smaller or larger apparent values than would be expected. Finally, an easily evaluated figure-of-merit is defined that may help guide more accurate Tauc fitting. For samples with relatively sharper Tauc plot shapes, the population yields Eg(ZnO) as 3.276 ± 0.033 eV, in good agreement with data for single crystalline material.

Benjamin E. Davis

Papers

Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system

Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system (Phys. Status Solidi B 8/2015)