Tuning thermally treated graphitic carbon nitride for H₂ evolution and CO₂ photoreduction: The effects of material properties and mid-gap states

Graphitic carbon nitride (g-C3N4) is regarded as an attractive photocatalyst for solar fuel production, i.e., H2 evolution and CO2 photoreduction. Yet, its structural, chemical and optoelectronic properties are very much dependent on the synthesis method and are likely to contribute differently whether H2 evolution or CO2 reduction is considered. Little is known about this aspect making it difficult to tailor g-C3N4 structure and chemistry for a specific photoreaction. Herein, we create g-C3N4 of varying chemical, structural and optical features by applying specific thermal treatments and investigating the effects of the materials properties on solar fuel production. The samples were characterized across scales using spectroscopic, analytical and imaging tools, with particular attention given to the analyses of trap states. In the case of H2 evolution, the reaction is controlled by light absorption and charge separation enabled by the presence of trap states created by N vacancies. In the case of CO2 photoreduction, reactant adsorption appears as a dominating factor. The analyses also suggest that the thermal treatment leads to the formation of trap states located close to the valence band of g-C3N4

The effect of nanoparticulate PdO co-catalysts on the faradaic and light conversion efficiency of WO3 photoanodes for water oxidation

WO3 photoanodes offer rare stability in acidic media, but are limited by their selectivity for oxygen evolution over parasitic side reactions, when employed in photoelectrochemical (PEC) water splitting. Herein, this is remedied via the modification of nanostructured WO3 photoanodes with surface decorated PdO as an oxygen evolution co-catalyst (OEC). The photoanodes and co-catalyst particles are grown using an up-scalable aerosol assisted chemical vapour deposition (AA-CVD) route, and their physical properties characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and UV-vis absorption spectroscopy. Subsequent PEC and transient photocurrent (TPC) measurements showed that the use of a PdO co-catalyst dramatically increases the faradaic efficiency (FE) of water oxidation from 52% to 92%, whilst simultaneously enhancing the photocurrent generation and charge extraction rate. The Pd oxidation state was found to be critical in achieving these notable improvements to the photoanode performance, which is primarily attributed to the higher selectivity towards oxygen evolution when PdO is used as an OEC and the formation of a favourable junction between WO3 and PdO, that drives band bending and charge separation

Rate law analysis of water oxidation and hole scavenging on a BiVO4 photoanode

Spectroelectrochemical studies employing pulsed LED irradiation are used to investigate the kinetics of water oxidation on undoped dense bismuth vanadate (BiVO4) photoanodes under conditions of photoelectrochemical water oxidation and compare to those obtained for oxidation of a simple redox couple. These measurements are employed to determine the quasi-steady-state densities of surface-accumulated holes, ps, and correlate these with photocurrent density as a function of light intensity, allowing a rate law analysis of the water oxidation mechanism. The reaction order in surface hole density is found to be first order for ps 1 nm–2. The effective turnover frequency of each surface hole is estimated to be 14 s–1 at AM 1.5 conditions. Using a single-electron redox couple, potassium ferrocyanide, as the hole scavenger, only the first-order reaction is observed, with a higher rate constant than that for water oxidation. These results are discussed in terms of catalysis by BiVO4 and implications for material design strategies for efficient water oxidation

Optimizing the Activity of Nanoneedle Structured WO3 Photoanodes for Solar Water Splitting: Direct Synthesis via Chemical Vapor Deposition

Solar water splitting is a promising solution for the renewable production of hydrogen as an energy vector. To date, complex or patterned photoelectrodes have shown the highest water splitting efficiencies, but lack scalable routes for commercial scale-up. In this article, we report a direct and scalable chemical vapor deposition (CVD) route at atmospheric pressure, for a single step fabrication of complex nanoneedle structured WO3 photoanodes. Using a systematic approach, the nanostructure was engineered to find the conditions that result in optimal water splitting. The nanostructured materials adopted a monoclinic γ-WO3 structure and were highly oriented in the (002) plane, with the nanoneedle structures stacking perpendicular to the FTO substrate. The WO3 photoanode that showed the highest water splitting activity was composed of a ∼300 nm seed layer of flat WO3 with a ∼5 μm thick top layer of WO3 nanoneedles. At 1.23 VRHE, this material showed incident photon-to-current efficiencies in the range ∼35–45% in the UV region (250–375 nm) and an overall solar predicted photocurrent of 1.24 mA·cm–2 (∼25% of the theoretical maximum for WO3). When coupled in tandem with a photovoltaic device containing a methylammonium lead iodide perovskite, a solar-to-hydrogen efficiency of ca. 1% for a complete unassisted water splitting device is predicted

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis

It has been more than 40 years since Fujishima and Honda demonstrated water splitting using TiO2, yet there is still no clear mechanism by which surface holes on TiO2 oxidize water. In this paper, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent and transient absorption spectroscopy, we measure both the kinetics of electron extraction (t50% ≈ 200 μs, 1.5VRHE) and the kinetics of hole oxidation of water (t50% ≈ 100 ms, 1.5VRHE) as a function of applied potential, demonstrating the water oxidation by TiO2 holes is the kinetic bottleneck in this water-splitting system. Photoinduced absorption spectroscopy measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2 holes under conditions of photoelectrochemical water oxidation. Under these conditions, we find that the surface density of these holes increases nonlinearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is third order with respect to surface hole density, with a rate constant kWO = 22 ± 2 nm4·s–1. Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions, the rate law was second order with respect to surface hole density, indicative of a change in reaction mechanism. Although a change in reaction order was observed, the rate of reaction did not change significantly over the wide pH range examined (with TOFs per surface hole in the region of 20–25 s–1 at ∼1 sun irradiance). This showed that the rate-limiting step does not involve OH– nucleophilic attack and demonstrated the versatility of TiO2 as an active water oxidation photocatalyst over a wide range of pH

Manufacturing and Supply Chain Flexibility: Building an Integrative Conceptual Model Through Systematic Literature Review and Bibliometric Analysis

The purpose of this study is twofold: first, to establish the current themes on the topic of manufacturing and supply chain flexibility (MSCF), assess their level of maturity in relation to each other, identify the emerging ones and reflect on how they can inform each other, and second, to develop a conceptual model of MSCF that links different themes connect and highlight future research opportunities. The study builds on a sample of 222 articles published from 1996 to 2018 in international, peer-reviewed journals. The analysis of the sample involves two complementary approaches: the co-word technique to identify the thematic clusters as well as their relative standing and a critical reflection on the papers to explain the intellectual content of these thematic clusters. The results of the co-word analysis show that MSCF is a dynamic topic with a rich and complex structure that comprises five thematic clusters. The value chain, capability and volatility clusters showed research topics that were taking a central role in the discussion on MSCF but were not mature yet. The SC purchasing practices and SC planning clusters involved work that was more focused and could be considered more mature. These clusters were then integrated in a framework that built on the competence–capability perspective and identified the major structural and infrastructural elements of MSCF as well as its antecedents and consequences. This paper proposes an integrative framework helping managers keep track the various decisions they need to make to increase flexibility from the viewpoint of the entire value chain

Impact of synthesis route on the water oxidation kinetics of hematite photoanodes

Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. Whilst these photoanodes exhibit very different current / voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential towards less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intraband-gap states, indicating these intraband-gap states do not contribute directly to water oxidation

Porous boron nitride for combined CO2 capture and photoreduction

Porous and amorphous materials are typically not employed for photocatalytic purposes, like CO2 photoreduction, as their high number of defects can lead to low charge mobility and favour bulk electron–hole recombination. Yet, with a disordered nature can come porosity, which in turn promotes catalyst/reactant interactions and fast charge transfer to reactants. Here, we demonstrate that moving from h-BN, a well-known crystalline insulator, to amorphous BN, we create a semiconductor, which is able to photoreduce CO2 in the gas/solid phase, under both UV-vis and pure visible light and ambient conditions, without the need for cocatalysts. The material selectively produces CO and maintains its photocatalytic stability over several catalytic cycles. The performance of this un-optimized material is on par with that of TiO2, the benchmark in the field. For the first time, we map out experimentally the band edges of porous BN on the absolute energy scale vs. vacuum to provide fundamental insight into the reaction mechanism. Owing to the chemical and structural tunability of porous BN, these findings highlight the potential of porous BN-based structures for photocatalysis particularly solar fuel production

Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface

Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a pectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to directly compare the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving the homolytic reductive elimination of hydrogen

The effect of residual palladium catalyst contamination on the photocatalytic hydrogen evolution activity of conjugated polymers

The effect of residual Pd on hydrogen evolution activity in conjugated polymer photocatalytic systems is systematically investigated using colloidal poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles as a model system. Residual Pd, originating from the synthesis of F8BT via Pd catalyzed polycondensation polymerization, is observed in the form of homogeneously distributed Pd nanoparticles within the polymer. Residual Pd is essential for any hydrogen evolution to be observed from this polymer, and very low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the hydrogen evolution reaction (HER) rate. The HER rate increases linearly with increasing Pd concentration from <1 ppm to approximately 100 ppm, at which point the rate begins to saturate. Transient absorption spectroscopy experiments support these conclusions, and suggest that residual Pd mediates electron transfer from the F8BT nanoparticles to protons in the aqueous medium