Monoclinic scheelite bismuth vanadate (m-BiVO4) is a promising semiconductor photoanode for photoelectrochemical (PEC) water splitting. Despite considerable recent progress in achieving improved ...photocurrents and photovoltages, there remain open questions about the basic optoelectronic properties of this material. Indeed, there is disagreement about the nature of its fundamental bandgap, with theoretical predictions and some experimental observations pointing to an indirect bandgap and other experimental studies to a direct bandgap. Knowledge of this property is critical for understanding light absorption and photocarrier properties, as well as for establishing rational approaches to improved efficiency. Here, experimental spectroscopic techniques are used to resolve this issue and provide a fundamental portrait of the optical properties of the material. Resonant inelastic X-ray scattering proves conclusively that m-BiVO4 is an indirect bandgap semiconductor. These measurements are supported by UV–vis absorption spectroscopy and spectroscopic ellipsometry, which confirm this finding and also indicate the presence of a direct transition located at 200 meV above the indirect one. The spectral dependence of the optical constants is determined by development of a photophysical model for the ellipsometric data. Photogenerated carrier dynamics are probed by transient absorption spectroscopy, which reveals a relatively long lifetime compared to other commonly utilized metal oxide photoanodes and is attributed to the indirect nature of the fundamental gap. The combination of strong visible light absorption and relatively long excited state lifetime provides the basis for the high performance that has been achieved from BiVO4 photoanodes for water splitting.
Herein, we report a colloidal wet-chemical approach enabling control on dopant concentration and location in a nanocrystal host lattice. Growth-doping and nucleation-doping, driven by primary and ...tertiary amines, respectively, were identified as predominant doping mechanisms responsible for the introduction of nitrogen impurities in interstitial and substitutional sites in highly branched rutile TiO2 nanostructures. High-resolution X-ray photoelectron spectroscopy was used to distinguish the two nitrogen occupational lattice sites and, in combination with UV–vis absorption spectroscopy, to investigate the impact of the nitrogen impurities on the optoelectronic properties. The implementation of the nitrogen-doped titania nanostructures in photoelectrodes for water oxidation suggests that these atomically defined building blocks can function as a platform to investigate the impact of the nitrogen occupational sites on the photocatalytic properties. By deliberately choosing precursors and reaction conditions, instead of relying on the most common high temperature annealing of preformed metal oxide in ammonia, we emphasize the importance of understanding the chemistry behind doping to achieve an unprecedented level of control on effective dopant introduction and, therefore, property tunability.
Development of an efficient yet durable photoelectrode is of paramount importance for deployment of solar-fuel production. Here, we report the photoelectrochemically self-improving behaviour of a ...silicon/gallium nitride photocathode active for hydrogen production with a Faradaic efficiency approaching ~100%. By using a correlative approach based on different spectroscopic and microscopic techniques, as well as density functional theory calculations, we provide a mechanistic understanding of the chemical transformation that is the origin of the self-improving behaviour. A thin layer of gallium oxynitride forms on the side walls of the gallium nitride grains, via a partial oxygen substitution at nitrogen sites, and displays a higher density of catalytic sites for the hydrogen-evolving reaction. This work demonstrates that the chemical transformation of gallium nitride into gallium oxynitride leads to sustained operation and enhanced catalytic activity, thus showing promise for oxynitride layers as protective catalytic coatings for hydrogen evolution.
Formation of planar heterojunction perovskite solar cells exhibiting both high efficiency and stability under continuous operation remains a challenge. Here, we show this can be achieved by using a ...defective TiO2 thin film as the electron transport layer. TiO2 layers with native defects are deposited by electron beam evaporation in an oxygen-deficient environment. Deep-level hole traps are introduced in the TiO2 layers and contribute to a high photoconductive gain and reduced photocatalytic activity. The high photoconductivity of the TiO2 electron transport layer leads to improved efficiency for the fabricated planar devices. A maximum power conversion efficiency of 19.0% and an average PCE of 17.5% are achieved. In addition, the reduced photocatalytic activity of the TiO2 layer leads to enhanced long-term stability for the planar devices. Under continuous operation near the maximum power point, an efficiency of over 15.4% is demonstrated for 100 h.
The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Here we map the correlation between morphological and functional ...heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity-dependent current-voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. These findings support the fabrication of highly efficient bismuth vanadate nanostructures and provide insights into how local functionality affects the macroscopic performance.
Electronic Structure of Monoclinic BiVO4 Cooper, Jason K; Gul, Sheraz; Toma, Francesca M ...
Chemistry of materials,
09/2014, Letnik:
26, Številka:
18
Journal Article
Recenzirano
A comprehensive approach to understanding the electronic structure of monoclinic scheelite bismuth vanadate (ms-BiVO4), including both valence band (VB) and conduction band (CB) orbital character, is ...presented. Density functional theory (DFT) calculations are directly compared to experimental data obtained via X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS), and X-ray photoelectron spectroscopy to provide a complete portrait of the total and partial density of states (DOS) near the bandgap. DFT calculations are presented to confirm the VB maximum and CB minimum are comprised primarily of O 2p and V 3d orbitals, respectively. Predicted triplet d-manifold splitting of V 3d CB states, arising from lone pair-induced lattice distortions, is quantified by V L- and O K-edge XAS. Furthermore, the partial contributions to the total DOS within both the CB and VB, determined by RIXS, are found to be in excellent agreement with DFT calculations. Energy levels are placed relative to the vacuum level by photoemission spectroscopy, which provides a measure of the work function and electron affinity of the investigated BiVO4 thin film. The implications of the fundamental electronic structure of ms-BiVO4 on its photocatalytic behavior, as well as considerations for improvements by substitutional incorporation of additional elements, are discussed.
Increasing anthropogenic carbon dioxide emissions have prompted the search for photoelectrochemical (PEC) methods of converting CO2 to useful commodity products, including fuels. Ideally, such PEC ...approaches will be sustained using only sunlight, water, and CO2 as energetic and reactant inputs. However, low peak conversion efficiencies (<5%) have made commercialization of fully‐integrated PEC devices prohibitive. Here, a 4 cm2 monolithic PEC device exceeding 10% solar‐to‐fuel efficiency with principal fuel products of carbon monoxide and hydrogen is reported. The corresponding solar‐to‐CO and solar‐to‐H2 efficiencies are 7% and 3.5%, respectively. Screening of a range of operating conditions reveals a tunable product mixture of H2 and CO using a gold electrocatalyst. Accordingly, it is shown that device optimization yields a H2‐to‐CO ratio of 1:2 commonly present in synthesis gas (syngas). Notably, the modularity and facile fabrication of this device permit the incorporation of a broad array of materials for various applications. For example, the electrocatalyst may easily be swapped to target a different set of products.
Photoelectrochemical conversion of carbon dioxide into fuels and chemicals is an appealing method to combat rising CO2 concentrations in the atmosphere. A modular, fully‐integrated device architecture is presented, following design principles developed for dark electrolysis. The result is a compact device, allowing for efficient and stable conversion of water and CO2 into carbon monoxide and hydrogen (synthesis gas).
Transient absorption (TA) spectroscopy is uniquely suited for understanding kinetic processes initiated by light over vast ranges of time. In combination with white light probes, the recorded ...differential absorption spectra can contain spectroscopic signatures characteristic of specific charge carrier population densities. However, disentangling the often-complex and convoluted spectra is made challenging without robust analysis methods relating the underlying physical mechanisms to the spectral components. In this work, we address the origin of the transient spectra of a model system of emerging solar energy harvesting materials using a monoclinic BiVO4 thin film. Using ground-state optical properties of the semiconductor, we find the main derivative-like spectral response to be related to shifting and broadening of oscillators, rather than specific carrier-related transitions. However, by using the Drude optical model of free carriers, we also identify the transient response related to free-hole density. Importantly, sample heating from the optical pumping, which begins at ∼10 ps and plateaus by ∼200 ps, dominates the overall spectral response at longer times. On the basis of a physical model of the spectral response, a kinetic model is developed that describes the pump power dependence of the free-hole density, as well as the temporal evolution of the spectral changes associated with shifting and broadening of oscillators. First-principles density functional theory calculations are used to rationalize experimental measurements. This comprehensive approach to analyzing and modeling the TA spectra offers a generalizable basis for understanding the complex pump–probe data, reveals thermal heating artifacts that are frequently erroneously assigned to long-lived photocarriers, and offers a path to eliminating ambiguity in analysis of photocarrier dynamics in solid-state systems.
Surface ligands of semiconductor quantum dots (QDs) critically influence their properties and functionalities. It is of strong interest to understand the structural characteristics of surface ligands ...and how they interact with the QDs. Three quantum dot (QD) systems (CdSe, ZnSe, and ZnS) with primary aliphatic amine capping ligands were characterized primarily by FT-IR spectroscopy as well as NMR, UV–vis, and fluorescence spectroscopy, and by transmission electron microscopy (TEM). Representative primary amines ranging from 8 to 16 carbons were examined in the vapor phase, KBr pellet, and neat and were compared to the QD samples. The strongest hydrogen-bonding effects of the adsorbed ligands were observed in CdSe QDs with the weakest observed in ZnS QDs. There was an observed splitting of the N–H scissoring mode from 1610 cm–1 in the neat sample to 1544 and 1635 cm–1 when bound to CdSe QDs, which had the largest splitting of this type. The splitting is attributed to amine ligands bound to either Cd or Se surface sites, respectively. The effect of exposure of the QDs dispersed in nonpolar medium to methanol as a crashing agent was also examined. In the CdSe system, the Cd-bound scissoring mode disappeared, possibly due to methanol replacing surface cadmium sites. The opposite was observed for ZnSe QDs, in which the Se-bound scissoring mode disappeared. It was concluded that surface coverage and ligand bonding partners could be characterized by FT-IR and that selective removal of surface ligands could be achieved through introduction of competitive binding interactions at the surface.
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