Optimization of BiVO4 photoelectrodes made by electrodeposition for sun-driven water oxidation

In this work, the synthesis of cheap BiVO4 photoanodes for the photoelectrochemical water splitting reaction was optimized via the scalable thin film electrodeposition method. Factors affecting the photoelectrochemical activity, such as the electrodeposition time, the ratio of the Bi-KI to benzoquinone-EtOH in the deposition bath, and the calcination temperature, have been investigated by using the Central Composite Design of Experiments. Pristine monoclinic scheelite BiVO4 photoanodes having a photocurrent density of 0.45 ± 0.05mA/cm2 at 1.23 V vs RHE have been obtained. It was shown that a high photocurrent density is generally dictated by the following physico-chemical properties: a higher crystallite size, optimal thickness and a porous morphology, which give rise to a low charge transfer resistance, low onset potential and a high donor density. Moreover, to the best of our knowledge, this is the first report on the depth profile XPS analysis performed in BiVO4 photoanodes made by electrodeposition technique, from which it was concluded that the surface V species exist as V4+ while the bulk V species are V5+. The V4+ induces a higher amount of surface oxygen vacancies, which was found to be beneficial for the photoactivity

Catchment geology preconditions spatio-temporal heterogeneity of ecosystem functioning in forested headwater streams

Catchment geology can affect water chemistry and groundwater influence, eventually affecting macroinvertebrate communities, but its effects on stream functions such as leaf decomposition have been scarcely investigated. To understand the effects of geology on leaf decomposition, we conducted leaf litter experiments in streams with volcanic and non-volcanic substrata using fine and coarse mesh bags. Volcanic spring-fed streams showed lower temperature in summer and higher temperature in winter (with temperature difference being more pronounced later in incubation) than non-volcanic streams. Macroinvertebrate communities captured inside coarse litter bags differed in the two stream types in both seasons, mainly because of shredder communities. Shredder abundance and biomass were higher in volcanic streams in both seasons. Geology-dependent temperature influenced microbe-mediated decomposition in both seasons, with total phosphorus as an additional driver in winter. Summer temperature was associated with an overall positive effect on the abundance of shredders, which affected invertebrate-mediated decomposition, but this was not evident in winter. Shredder activity in volcanic streams compensated for temperature-dependent microbial activity resulting in an overall balance in leaf decomposition. Spring-fed systems are valuable ecosystems, particularly for cold-adapted species. Thus, understanding these understudied ecosystems will significantly aid in their appropriate conservation

Correction: Core-substituted naphthalenediimides anchored on BiVO4 for visible light-driven water splitting

Correction for 'Core-substituted naphthalenediimides anchored on BiVO4 for visible light-driven water splitting' by Simelys Hernández et al., Green Chem., 2017, 19, 2448–2462

Single-Source Bismuth (Transition Metal) Polyoxovanadate Precursors for the Scalable Synthesis of Doped BiVO4 Photoanodes.

Single-source precursors are used to produce nanostructured BiVO4 photoanodes for water oxidation in a straightforward and scalable drop-casting synthetic process. Polyoxometallate precursors, which contain both Bi and V, are produced in a one-step reaction from commercially available starting materials. Simple annealing of the molecular precursor produces nanocrystalline BiVO4 films. The precursor can be designed to incorporate a third metal (Co, Ni, Cu, or Zn), enabling the direct formation of doped BiVO4 films. In particular, the Co- and Zn-doped photoanodes show promise for photoelectrochemical water oxidation, with photocurrent densities >1 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE). Using this simple synthetic process, a 300 cm2 Co-BiVO4 photoanode is produced, which generates a photocurrent of up to 67 mA at 1.23 V vs RHE and demonstrates the scalability of this approach.We thank the following for financial support: China Scholarship Council (H.L.), the Cambridge Trusts (Vice Chancellor’s Award) and the Winton Programme for the Physics of Sustainability (V.A.), A*STAR Graduate Scholarship (Overseas) (N.L.), Imperial College Research Fellowship (A.R.), Christian Doppler Research Association and the OMV Group (E.R), Herchel Smith Research Fund (S.D.P

Hierarchical bismuth vanadate/reduced graphene oxide composite photocatalyst for hydrogen evolution and bisphenol A degradation

Bismuth vanadate (BiVO4) is a widely studied photocatalyst for the depollution of contaminated wastewater, production of hydrogen by water splitting, and organic synthesis. The photophysical properties of BiVO4 are sensitive to morphology and quantum confinement effects, and can exhibit enhanced photocatalytic performance in nanocomposites with graphene. Synthesis of hierarchical BiVO4 plates decorated by nanoparticles (h-BiVO4) in contact with reduced graphene oxide (RGO) is reported via a facile one-pot solution phase approach using ethanolamine and a polyethylene glycol stabilizer. The resulting h-BiVO4/RGO photocatalyst exhibited superior photoactivity for bisphenol A (BPA) degradation and hydrogen evolution under visible light irradiation compared to single component h-BiVO4 or a μm-sized block-like BiVO4 morphology. Rates of BPA photocatalytic degradation and apparent quantum efficiency (AQE) decreased in the order h-BiVO4/RGO (4.5 × 10−2 mmol.g−1.min−1; 15.1% AQE) > h-BiVO4 (3.5 × 10−2 mmol.g−1.min−1; 11.7% AQE) > BiVO4 (1 × 10−2 mmol.g−1.min−1; 3.4% AQE), representing a 4.5 fold enhancement for h-BiVO4/RGO versus BiVO4. Liquid phase photodegradation products included benzene-1,4-diol, cyclohexa-2,5-diene-1,4-dione and (2Z)-but-2-enedioic acid. The rate of photocatalytic hydrogen production under visible light was 11.5 µmol.g−1.h−1 for h-BiVO4/RGO, ~383.3 times greater than for BiVO4 (0.03µmol.g−1.h−1). The superior photocatalytic performance of h-BiVO4/RGO is largely attributed to its higher surface area, aided by enhanced visible light absorption and charge separation across the semiconductor-RGO interface, which together confer a higher density and lifetime of photoexcited charge carriers

Latitude dictates plant diversity effects on instream decomposition

Running waters contribute substantially to global carbon fluxes through decomposition of terrestrial plant litter by aquatic microorganisms and detritivores. Diversity of this litter may influence instream decomposition globally in ways that are not yet understood. We investigated latitudinal differences in decomposition of litter mixtures of low and high functional diversity in 40 streams on 6 continents and spanning 113 degrees of latitude. Despite important variability in our dataset, we found latitudinal differences in the effect of litter functional diversity on decomposition, which we explained as evolutionary adaptations of litter-consuming detritivores to resource availability. Specifically, a balanced diet effect appears to operate at lower latitudes versus a resource concentration effect at higher latitudes. The latitudinal pattern indicates that loss of plant functional diversity will have different consequences on carbon fluxes across the globe, with greater repercussions likely at low latitudes

Photocatalyseurs actifs dans le visible pour l'oxydation de l'eau : vers les bioraffineries solaires

Photoelectrochemical (PEC) water splitting is a direct way of producing a solar fuel like hydrogen from water. The bottleneck of this process is in the photoanode, which is responsible for the water oxidation side of the reaction1,2. In this work, the use of BiVO4 as a photoanode was extensively studied in order to improve its photoactivity. The optimization of BiVO4 photoanode synthesis via thin film electrodeposition on FTO was performed. The factors affecting the photoelectrochemical activity such as the electrodeposition time, ratio of the Bi-KI to benzoquinone-EtOH in the deposition bath, and the calcination temperature, have been investigated by using the Central Composite Design of Experiments.Surface states on the BiVO4 surface give rise to defect levels, which can mediate electron-hole recombination via the Shockley-Read-Hall mechanism5. In order to protect the BiVO4 surface and minimize the inefficiencies due to electron-hole recombination and passivate the surface states, ultrathin overlayers of Al2O3 and TiO2 were deposited to the BiVO4 thin film electrodes in an ALD-like manner. A photocurrent density of 0.54 mA/cm2 at 1.23 V vs RHE was obtained for the 2 cycles Al2O3-modified BiVO4, which was a 54% improvement from the bare BiVO4 that demonstrated a photocurrent density of 0.35 mA/cm2 at 1.23 V vs RHE. A 15% increase in stability of the Al2O3- modified BiVO4 electrode was also observed over 7.5 hours of continuous irradiation. Moreover, through surface capacitance measurements, it was shown that the Al2O3 overlayer was indeed passivating the surface states of the BiVO4 electrodes. The nature of the BiVO4 surface was studied by investigating the reactivity of powder BiVO4 with a chemical titrant. The existence of surface hydroxyl groups on BiVO4 was confirmed and quantified (max 1.5 OH/nm2) via chemical titration. The reaction of the BiVO4 powder with one pulse of AlMe3 and 1 pulse of H2O showed that there were 1.2 molecules of CH4 evolved per Bi-OH. In this work, we were able to highlight which factors are important in the synthesis of BiVO4, and how they affect the resulting photoactivity. We have also achieved the passivation of the BiVO4 surface states using Al2O3, which is not well-explored in literature. Moreover, we were able to probe and discuss the nature of the BiVO4 surface. This is a very fundamental knowledge and the first report of such, to the best of our knowledge. A good understanding of this important semiconductor surface and its interactions will aid in the design of a more efficient BiVO4 photoanodeLa séparation photoélectrochimique de l'eau (PEC) est un moyen direct de produire un combustible solaire tel que l'hydrogène à partir de l'eau. Le goulot d'étranglement de ce processus se situe dans la photoanode, qui est responsable du côté oxydation de la réaction1,2. Dans ce travail, l'utilisation de BiVO4 en tant que photoanode a été largement étudiée afin d'améliorer sa photoactivité. L’optimisation de la synthèse de photoanodes BiVO4 par électrodéposition en couche mince sur du FTO a été réalisée. Les facteurs influant sur l'activité photoélectrochimique, tels que le temps d'électrodéposition, le rapport Bi-KI/benzoquinone-EtOH dans le bain de dépôt et la température de calcination, ont été étudiés à l'aide de la conception composite centrale d'expériences. Les états de surface sur la surface de BiVO4 donnent lieu à des niveaux de défaut pouvant induire une recombinaison électron-trou via le mécanisme de Shockley-Read-Hall5. Afin de minimiser les inefficacités dues à la recombinaison électron-trou et passiver les états de surface, des couches de recouvrement ultra-fines d'Al2O3 et de TiO2 ont été déposées sur les électrodes en film mince BiVO4 d'une manière analogue à l'ALD. Cela a également été réalisé afin de protéger la surface de BiVO4 de la photocorrosion et d’augmenter sa stabilité. Une densité de photocourant de 0,54 mA/cm2 à 1,23 V vs RHE a été obtenue pour les 2 cycles de BiVO4 modifié par Al2O3, comme le montre la Figure 2, soit une amélioration de 54% par rapport à la BiVO4 nue qui démontrait une densité de photocourant de 0,35 mA/cm2. à 1,23 V vs RHE. Une augmentation de 15% de la stabilité de l'électrode de BiVO4 modifiée par Al2O3 a également été observée au cours de 7,5 heures d'irradiation continue. De plus, grâce aux mesures de capacité de surface présentées à la Figure 3, il a été montré que la surcouche de Al2O3 passivait effectivement à passiver les états de surface des électrodes de BiVO4. La nature de la surface de BiVO4 a été étudiée en étudiant la réactivité de la poudre de BiVO4 avec un titrant chimique. L’existence de groupes hydroxyle de surface sur BiVO4 a été confirmée et quantifiée (max. 1,5 OH / nm2) par titrage chimique. La réaction de la poudre de BiVO4 avec une impulsion de AlMe3 et une impulsion de H2O a montré qu'il existait 1,2 molécules de CH4 dégagées par Bi-OH. Dans ce travail, nous avons pu mettre en évidence les facteurs importants dans la synthèse de BiVO4 et leur incidence sur la photoactivité résultante. Nous avons également réussi à passiver les états de surface de BiVO4 en utilisant Al2O3, ce qui n’est pas bien exploré dans la littérature. De plus, nous avons pu sonder et discuter de la nature de la surface de BiVO4. Ceci est une connaissance très fondamentale et le premier rapport à ce sujet, à notre connaissance. Une bonne compréhension de cette surface semi-conductrice importante et de ses interactions facilitera la conception d'un photoanode BiVO4 plus efficac

Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges

Photoelectrochemical (PEC) water splitting, which is a type of artificial photosynthesis, is a sustainable way of converting solar energy into chemical energy. The water oxidation half-reaction has always represented the bottleneck of this process because of the thermodynamic and kinetic challenges that are involved. Several materials have been explored and studied to address the issues pertaining to solar water oxidation. Significant advances have recently been made in the use of stable and relatively cheap metal oxides, i.e., semiconducting photocatalysts. The use of BiVO4 for this purpose can be considered advantageous because this catalyst is able to absorb a substantial portion of the solar spectrum and has favourable conduction and valence band edge positions. However, BiVO4 is also associated with poor electron mobility and slow water oxidation kinetics and these are the problems that are currently being investigated in the ongoing research in this field. This review focuses on the most recent advances in the best-performing BiVO4-based photoanodes to date. It summarizes the critical parameters that contribute to the performance of these photoanodes, and highlights so far unresolved critical features related to the scale-up of a BiVO4-based PEC water-splitting device