BiVO4 Single Crystal Research Articles

Surface recombination velocities for the (100) and (001) crystal faces of bismuth vanadate single crystals

Bismuth vanadate (BiVO4) is widely used as a photocatalyst for water splitting, and its carrier lifetime is the most essential parameter for photocatalysts. In this study, we characterized the carrier lifetime in BiVO4 single crystals on the (100) and (001) crystal faces using the microwave photoconductivity decay (μ-PCD) method. For the (001) face, the observed μ-PCD curves with excitation by a 266 nm laser had no injected photon density dependence. In contrast, those excited using a 355 nm laser had an injected photon density dependence. The decay at a high injected photon density was faster than that at a low injected photon density. For the (100) face, the decay curves at both excitations of 266 and 355 nm were not significantly different, and they depended on the injected photon density. These results indicate that the carrier lifetime is dominated by surface recombination only under the 266 nm excitation condition for the (001) face, whereas under other conditions, this is dominated by the Schokley–Read–Hall recombination. The temperature independence of the μ-PCD curves indicates that the recombination center is sufficiently deep in the bandgap. We estimated the surface recombination velocities and bulk lifetimes of the samples by fitting the experimental results to the calculations. We believe that the estimated surface recombination velocity and bulk lifetime will aid in the design of BiVO4 photocatalysts.

Mass-transfer control for selective deposition of well-dispersed AuPd cocatalysts to boost photocatalytic H2O2 production of BiVO4

Owing to excellent activity and selectivity for two-electron O2 reduction to H2O2, well-dispersed AuPd nanoalloy cocatalyst is high desired for photocatalytic H2O2 production. In practice, however, it is hardly realized due to technical difficulty. Utilizing the directed transfer of photocarriers in BiVO4 single crystal with crystal-facet exposure, in this work, a mass-transfer control approach is developed for selective photodeposition of well-dispersed AuPd cocatalysts to boost photocatalytic H2O2 production. The as-obtained BiVO4 (010)-AuPd photocatalysts exhibit improved photocatalytic activity for H2O2 production compared with BiVO4 (010)-Au (1.15 mmol L−1) and BiVO4 (010)-Pd (1.37 mmol L−1). Therein, the optimum BiVO4 (010)-AuPd photocatalyst (the mass ratio of Au to Pd for 19:1) presents the highest H2O2 production (2.29 mmol L−1 in 2 h under 420 nm light irradiation) and the corresponding AQE value is 11.38 %. In the case, the synergistic effect of electronic effects and geometric effects for the well-dispersed AuPd nanoalloy cocatalysts contribute to excellent activity and selectivity for two-electron O2 reduction to H2O2 and thus boost the photocatalytic H2O2 production of BiVO4. The present work also provides an insight into the modification of well-dispersed nanoalloy cocatalysts for photocatalytic H2O2 production.

Selective modification of ultra-thin g-C3N4 nanosheets on the (110) facet of Au/BiVO4 for boosting photocatalytic H2O2 production

Au/BiVO4 is a promising photocatalyst for H2O2 production owing to its appropriate energy-band structure and excellent two-electron O2 reduction ability. However, easy H2O2 decomposition by photogenerated holes limits its high-efficient H2O2 production. To alleviate the issue, in the study, BiVO4 single crystal with selective co-modification of ultra-thin g-C3N4 and Au was prepared for the first time. Specifically, ultra-thin g-C3N4 nanosheets are mainly modified on the (110) facet of BiVO4 to facilitate effective transfer of photogenerated holes and depress H2O2 decomposition, while Au is modified on its (010) facet to effectively catalyze the two-electron O2 reduction. Consequently, the photocatalyst presents excellent H2O2-production performance with 1.35 mmol L−1 in 2 h which is 2.65-fold compared with that for Au/BiVO4, and shows the best performance in the literatures related with BiVO4 based photocatalysts for H2O2 production. The present strategy provides a new insight into the design of highly efficient photocatalysts for H2O2 production.

Precise location and regulation of active sites for highly efficient photocatalytic synthesis of ammonia by facet-dependent BiVO4 single crystals

Based on the characterizations of photo-deposition experiment, environmental transmission electron microscopy (ETEM) and scanning fluorescence X-ray microscopy (SFXM) for the in-situ photocatalytic nitrogen reduction reaction (pNRR) on different facets as well as theoretical calculation, we confirmed that the cycle of oxygen vacancy-V4+/V5+ (OV-V4+/V5+) on (040) facet is the exact active site for the photocatalytic NRR (pNRR) by bismuth vanadate (BiVO4) single crystal. V4+ plays the role of chemisorption of N2, V5+ is electron transfer bridge, and the photogenerated electrons trapped in OV provide the driving force for ammonia synthesis. (110) facet has only V5+, which can not produce the start-up step of chemisorption of N2, so it is the inert facet of pNRR. Thus, NRR activity increases linearly with the increase of (040)/(110) facet ratio by adjusting the active sites on (040) facet, and the highest activity can reach 103.4 μmol g−1 h−1 without sacrifice reagent. Based on the discussion of real active sites and their transformation in photocatalytic NRR by facet-dependent BiVO4, we not only analyze the essential reason why exposing specific facets can greatly improve the photocatalytic activity, but also put forward a new viewpoint on the activity control of photocatalytic ammonia synthesis.

Role of the ferroelastic strain in the optical absorption of BiVO4

Bismuth vanadate (BiVO4) has recently been under focus for its potential use in photocatalysis thanks to its well-suited absorption edge in the visible light range. Here, we characterize the optical absorption of a BiVO4 single crystal as a function of temperature and polarization direction by reflectance and transmittance spectroscopy. The optical bandgap is found to be very sensitive to the temperature, and to the tetragonal-to-monoclinic ferroelastic transition at 523 K. The anisotropy, as measured by the difference in the absorption edge for the light polarized parallel and perpendicular to the principal axis, is reduced from 0.2 eV in the high-temperature tetragonal phase to 0.1 eV at ambient temperature. We show that this evolution is dominantly controlled by the ferroelastic shear strain. These findings provide a route for further optimization of bismuth vanadate-based light absorbers in photocatalytic devices.

Chemical, Structural, and Electronic Characterization of the (010) Surface of Single Crystalline Bismuth Vanadate

We have structurally, chemically and electronically characterized the most stable (010) surface of a Mo-doped BiVO4 single crystal. Low energy electron diffraction (LEED) reveals that the surface is not significantly reconstructed from a bulk termination of the crystal. Synchrotron based X-ray spectroscopies indicate no surface enhancement of any of the crystal constituents and that the Mo dopant occupies tetrahedral sites by substituting for V at the surface. Using resonant photoemission to study the valence band structure as the V L3 edge is scanned we observe an intraband gap state associated with reduced vanadium formed by the Mo doping. This state is likely associated with small polaron formation. This feature is enhanced at a photon energy that is not resonant with any of the main features in the absorption spectrum of the pristine BiVO4. This indicates that the additional electron from Mo doping likely induces further distortion of the VO4 tetrahedral units and generates a new conduction band state...

Twin Structure in BiVO4 Photoanodes Boosting Water Oxidation Performance through Enhanced Charge Separation and Transport

AbstractThe homojunction induced by twin structure in semiconductors is found to be highly effective in decreasing the bulk recombination. Theoretical and experimental results have demonstrated the “back to back” potentials and directional diffusion that originate from the homojunction could facilitate the charge separation and transport of BiVO4 (BVO) single crystals. As expected, the resultant homojunction can lead to significantly enhanced photocatalytic activity. To eliminate the significant reduction of performance caused by solution mediated interface recombination on exposed fluorine‐doped tin oxide (FTO), a strategy for improving substrate coverage by a new “n‐step” method is applied. The BVO photoanode with preferable twin structure and high substrate coverage ratio reaches a photocurrent density of ≈3.1 mA cm−2 at 1.23 V versus reversible hydrogen electrode (RHE) and exhibits over 1.35% half‐cell solar‐to‐hydrogen conversion efficiency at 0.6 V versus RHE. This finding offers a new and effective way of fabricating twin structures to reduce bulk recombination of BVO photoanodes, which can also be applied to other semiconductors.

Ion beam modification of single crystalline BiVO4

A single crystalline BiVO4 sample was investigated. Angular resolved Rutherford backscattering spectrometry (arRBS) was performed as a function of two orthogonal angles perpendicular to the surface. The crystal planes appearing in the angular charts are compared with the crystal structure of monoclinic BiVO4. By this comparison the crystal axis being almost normal to the surface was identified to be 〈001〉. These measurements support the control of orientation and quality of the grown BiVO4 crystal. Additionally it is found that during prolonged analysis the He ions produce a considerable amount of damage. As the nuclear energy loss of the He ions is negligibly low within the corresponding depth region, the damage is mainly caused by the electronic energy loss of the ions. For studying radiation resistance and damage formation, the BiVO4 single crystal was implanted with 200keV Ar ions. The damage production in the Bi sublattice was analysed by RBS applying 1.8MeV He ions in channelling configuration. The damage profiles determined from the channelling RBS spectra can be well represented by the electronic energy loss of the implanted Ar ions. From this it is concluded that, in agreement with the finding mentioned above, this energy mainly triggers damage formation in ion irradiated BiVO4. The energy for producing one displaced Bi atom as seen by RBS decreases with increasing damage concentration and varies between 33 and 3.4eV.

A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals.

Doped BiVO4 is a promising photoelectrochemical water splitting anode, whose activity is hampered by poor charge transport. Here we use a set of X-ray spectroscopic methods to probe the origin and nature of localized electron states in W:BiVO4. Furthermore, using the polarized nature of the X-rays, we probe variations in the electronic structure along the crystal axes. In this manner, we reveal aspects of the electronic structure related to electron localization and observations consistent with conductivity anisotropy between the ab-plane and c-axis. We verify that tungsten substitutes as W6+ for V5+ in BiVO4. This is shown to result in the presence of inter-band gap states related to electrons at V4+ sites of e symmetry. The energetic position of the states in the band gap suggest that they are highly localized and may act as recombination centres. Polarization dependent X-ray absorption spectra reveal anisotropy in the electronic structure between the ab-plane and c-axis. Results show the superior hybridization between V 3d and O 2p states, higher V wavefunction overlap and broader conduction bands in the ab-plane than in the c-axis. These insights into the electronic structure are discussed in the context of existing experimental and theoretical reports regarding charge transport in BiVO4.

Electronic Transport Characterization of BiVO4 Using AC Field Hall Technique

ABSTRACTBismuth vanadate (BiVO4) is a photoelectrode for the oxidation of water. It is of fundamental importance to understand the electrical and photoelectrochemical properties of this material. In metal oxides, the electronic transport is described by the small polaron model, first described by Mott. In this model, the resistivity varies with temperature as $\rho \,\left( T \right)\, \propto \,Te^{({{E_a } \mathord{\left/ {\vphantom {{E_a } {(k_B T))}}} \right. \kern-\nulldelimiterspace} {(k_B T))}}} $, where Ea is the hopping activation energy, kB is the Boltzmann constant and T is the absolute temperature. Resistivity measurements confirm that small polaron hopping dominates in temperature ranges from 250 K to 300 K. In addition measurements from 175K to 250K show the variable range hopping dominates the transport. To this end, the electronic transport properties of BiVO4 single crystal were characterized using resistivity measurements and Hall effect measurements over temperatures ranging from 175 K to 300 K.

Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide

Bismuth vanadate (BiVO4) is a promising photoelectrode material for the oxidation of water, but fundamental studies of this material are lacking. To address this, we report electrical and photoelectrochemical (PEC) properties of BiVO4 single crystals (undoped, 0.6% Mo, and 0.3% W:BiVO4) grown using the floating zone technique. We demonstrate that a small polaron hopping conduction mechanism dominates from 250 to 400 K, undergoing a transition to a variable-range hopping mechanism at lower temperatures. An anisotropy ratio of ~3 was observed along the c axis, attributed to the layered structure of BiVO4. Measurements of the ac field Hall effect yielded an electron mobility of ~0.2 cm(2) V(-1) s(-1) for Mo and W:BiVO4 at 300 K. By application of the Gärtner model, a hole diffusion length of ~100 nm was estimated. As a result of low carrier mobility, attempts to measure the dc Hall effect were unsuccessful. Analyses of the Raman spectra showed that Mo and W substituted for V and acted as donor impurities. Mott-Schottky analysis of electrodes with the (001) face exposed yielded a flat band potential of 0.03-0.08 V versus the reversible H2 electrode, while incident photon conversion efficiency tests showed that the dark coloration of the doped single crystals did not result in additional photocurrent. Comparison of these intrinsic properties to those of other metal oxides for PEC applications gives valuable insight into this material as a photoanode.

EPR study of Gd3+ in a ferroelastic BiVO4 single crystal

Electron paramagnetic resonance of the Gd3+ ion in a ferroelastic BiVO4 single crystal with a single domain, grown by the Czochralski method, has been investigated at room temperature using a Q-band spectrometer. The rotation patterns of the resonance fields measured in the crystallographic planes are analyzed using a monoclinic spin Hamiltonian. The principal Z-axis of the second-order zero-field splitting tensorD is found to be along the crystallographicb-axis. Spin Hamiltonian parameters together with the principal axes ofg andD tensors in the monoclinic plane show that the local site symmetry of Gd3+ ion in BiVO4 crystal is monoclinic and that the Gd3+ ion substitutes for Bi3+ ion.

Monoclinic Spin Hamiltonian Analysis of EPR Spectra of Mn2+ in BiVO4 Single Crystals

AbstractX‐band EPR spectra of Mn2+ in ferroelastic BiVO4 single crystals at room temperature taken in three orthogonal planes are analyzed using a twestage least‐squares fitting procedure. The absence of site splitting from the spectra shows unequivocally that Mn2+ lies on a site with two‐fold rotation symmetry, whose two fold axis is parallel to the monoclinic crystal axis. The low symmetry effects arising from the monoclinic site symmetry are evidenced by the observed coincidence of the extrema for transitions between different Zeeman levels along the monoclinic axis, and by the 180° rotational symmetry in the monoclinic plane and a small noncoincidence of the turning points in this plane. In the first stage, the spectra are fitted using the monoclinic form of the electronic Zeeman and fine structure terms (of second and fourth degree) appropriate for the crystallographically determined orientation of the monoclinic axis. In the second stage, fitting of the nuclear hyperfine tensor (A) and the nuclear quadrupole tensor (P) is carried out with fixed values of the electronic Zeeman tensor (g) and fine structure terms, i.e. the tensor (D) and the fourth‐degree terms, as obtained in the first stage. The principal axes of g, D, A, and P are found to be non‐coincident in the monoclinic plane, as would be generally expected for a monoclinic site.

Phase transition of ferroelastic BiVO4 crystals under stress at various temperatures

AbstractThe stress‐strain hysteresis curves of BiVO4 single crystals have been studied by thermal mechanical analysis in the ferroelastic and paraelastic phases. The phase transition temperatures are obtained as afunction of stress at various temperatures. The phase transition temperature is found to decrease as the stress is increased. The experimental resultsindicate that the phase transition temperature of the ferroelastic BiVO4 shifts to the lower temperature side under the influence of externalstress.

Magnetic Resonances of 51V, 209Bi, Mn2+, and Fe3+in BiVO4Single Crystals

Abstract Nuclear magnetic resonance (NMR) of 51V and 209Bi, and electron paramagnetic resonance (EPR) of Mn2+ and Fe3+ impurities in ferroelastic BiVO4 single crystals are reviewed. The nuclear quadrupole parameters and principal axes of the two host nuclei, 51V and 209Bi, and the second-or-der zero field splitting tensor of the isoelectronic 3 d5 ions, Mn2+ and Fe3+ , in BiVO4 are determined for the first time. Temperature dependence studies on 51V NMR and Mn2+ EPR confirmed that this crystal undergoes a second order phase transition. The substitutional sites of Mn2+ and Fe3+ in BiVO4 are Bi3+ and V5+, respectively.

Crystal growth and conductivity investigations on BiVO4 single crystals

Some oxides with the scheelite structure are well known mixed ionic and electronic conductors. This structure family, with the general composition ABO4, can be described as a layer structure derivative of the fluorite structure. The relations between the structural and the electrical transport properties of these materials have been investigated. The model system used in this work was BiVO4, which shows both electronic and ionic conduction.

The domain structure of ferroelastic BiVO4 studied by magnetic resonances

The domain structure of the ferroelastic BiVO4 single crystal has been investigated using the nuclear magnetic resonance (NMR) of 51V and by the electron paramagnetic resonance (EPR) of the Mn2+ ions contained in the crystal as an impurity. The 14 resonance lines of 51V (I=7/2) in a BiVO4 crystal with the twin domain were measured in the crystallographic a-b (or c-b) and c-a (or a-c) planes with an FT NMR spectrometer. Two sets of Mn2+ EPR signals were also obtained in the c-a plane. These two sets of NMR and EPR signals originate from the twin-domain structure. From these two sets of experimental data from 51V NMR and Mn2+ EPR, it is confirmed that the BiVO4 single crystal has the prominent (W-plane) domain wall reported previously. The investigated domain structure is found to be stable with time in contrast with a previous report by Baran et al. The previous model of the twinning mechanism derived from NMR and X-ray diffraction data has been improved by employing the EPR results. The observed W plane of the domain wall in BiVO4 can be explained by the ferroelastic species 4/mmm F 2/m instead of 4/m F 2/m.

A theoretical investigation of the zero-field splitting parameters for an Mn2+ centre in a BiVO4 single crystal

The zero-field splitting (ZFS) parameters D, E and F for an Mn2+ centre in a BiVO4 single crystal are calculated using the point-charge electrostatic model and the superposition model. The calculated ZFS parameters at the Bi and V sites are compared with the experimental values for Mn2+. The calculated second-order axial and rhombic ZFS parameters at the Bi site turn out to be similar to those from experiment. The superposition model and the point-charge model give similar results. This supports the notion that the Mn2+ impurity substitutes for the Bi3+ ion in BiVO4.

Nuclear magnetic resonance of 209Bi in a BiVO4 single crystal

Nuclear magnetic resonance spectra of 209Bi(I=9/2) in the ferroelastic BiVO4 single crystal has been investigated by employing a wide-line Varian spectrometer. Only three lines due to a large quadrupole interaction were measured at a fixed frequency of 6 MHz in the principal planes at room temperature. From the experimental data the quadrupole coupling constant E2qQ/h=79.2+or-0.1 MHz and asymmetry parameter eta =0.70+or-0.01 are determined; however, these values turn out to be quite different from the previous report obtained with a ceramic sample at liquid-nitrogen temperature. The principal axes of the electric field gradient tensor are determined for the first time, the x, y and z axes being the crystallographic c+25 degrees , a+25 degrees , and b axes, respectively. Thes directions are discussed in terms of the Bi-O bonds in the Bi-O polyhedra.

Ferroelastic phase transition of bivo4studied by51v nmr

Abstract Nuclear magnetic resonance (NMR) of 51V in the ferroelastic BiVO4 single crystals has been investigated at a fixed frequency of 6 MHz up to 533 K just above its Curie temperature, Tc = 528 K. The seven resonance lines of 51V(I = 7/2) due to the quadrupole interaction were measured in the three mutually perpendicular crystal planes. While the quadrupole coupling constant (e2qQ/h) is found to be staying nearly the same, whereas the asymmetry parameter (η) is decreased with the increasing temperature. Above Tc, e2qQ/h = 5.031 MHz, and η = 0 were obtained at T = 533 K and the principal Z axis of the electric field gradient tensor is along the crystallographic b axis, same as in the ferroelastic phase. These results are consistent with the phase transition from the monoclinic ferroelastic phase below Tc to the tetragonal paraelastic phase above Tc .