Electronic Structure of Monoclinic BiVO4 - American Chemical Society

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Electronic Structure of Monoclinic BiVO4 Jason K. Cooper, Sheraz Gul, Francesca M. Toma, Le Chen, PerAnders Glans, Jinghua Guo, Joel W. Ager, Junko Yano, and Ian D. Sharp Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5025074 • Publication Date (Web): 18 Aug 2014 Downloaded from http://pubs.acs.org on August 20, 2014

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Chemistry of Materials

Electronic Structure of Monoclinic BiVO4 Jason K. Cooper1,2, Sheraz Gul,3 Francesca M. Toma,1,4 Le Chen,1,2 Per-Anders Glans,5 Jinghua Guo,5 Joel W. Ager,1,2 Junko Yano,1,3 Ian D. Sharp1,3* 1

Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

2

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

3

Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

4

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

5

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

* Corresponding author. ABSTRACT 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 (XES), resonant inelastic X-ray spectroscopy (RIXS), and X-ray photoelectron spectroscopy (XPS) 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 to be 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 LIII- 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 thin film BiVO4. 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.

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I.

INTRODUCTION Intensive research is currently devoted to developing n-type semiconductors as photoanodes for

efficient conversion of solar energy to chemical fuel via photoelectrochemical water splitting. The material requirements of the photoanode are stringent: the semiconductor must be stable under aqueous operating conditions, exhibit favorable band alignment with the water oxidation potential, and possess a band gap that is small enough to absorb visible photons. Some of the most actively studied systems are based on metal oxides such as titania (TiO2), hematite (Fe2O3), and bismuth vanadate (BiVO4). Of these materials, monoclinic BiVO4 offers considerable promise. It possesses a moderate bandgap (Eg) of 2.4-2.5 eV,1,2 which corresponds to a theoretical maximum saturation photocurrent density of approximately 7 mA cm-2, is composed of Earth-abundant elements, can be deposited using scalable and inexpensive methods,3-6 and has been reported to enable water oxidation with onset potentials as small as 0.1-0.3 V vs. RHE.7-10 State of the art BiVO4 thin films often incorporate Mo or W dopants to improve conductivity and to promote internal charge separation, and can achieve photocurrent densities between 3-4 mA cm-2 at 1 sun illumination.9,11,12 However, the obtained performance remains far below the theoretical limit. Improvements of the charge carrier separation, transport, and extraction, as well as aqueous stability, benefit from fundamental knowledge of the electronic structure of this material. Monoclinic scheelite bismuth vanadate (ms-BiVO4) is characterized by a layered structure containing cations with formal oxidation states of Bi3+ (6s2) and V5+ (3𝑑0 ) in coordination with O2- (2p6). In ms-BiVO4, distortion of the VO4 tetrahedra and BiO8 dodecahedra leads to two and four distinct oxygen neighbors in each subunit, respectively. These distortions, which have been attributed to effects of hybridization of Bi 6s/O 2p lone pair states at the top of the valence band (VB), were observed by both neutron and X-ray diffraction (XRD) and result in a C2 space group.13,14 Electronic structure calculations by density functional theory (DFT) have shown that the VB is composed primarily of O 2p states, with Bi 6p states contributing to the bottom and V 3d to the middle of the VB.15 Previous models of the local structural distortions around Bi in 𝛼-Bi2O3, as well as other analogous systems, was explained by a 6s-6p ACS Paragon Plus Environment

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hybrid orbital near the Fermi energy pointing into the void of the crystal. Conversely, experimental and computational observations of Bi 6s states have found them to be far below the Fermi energy. In addition, the 6p states have been found to be located at the bottom of the VB, rather than the top, in both 𝛼-Bi2O3

and BiVO4.16 The model proposed by Payne et al. describes the electronic asymmetry around Bi arising predominantly from interaction between the Bi 6p, in the middle of the VB, and antibonding Bi 6s and O 2p at the VB maximum (VBM), which creates a Bi “lone pair.”16 This interaction reduces antibonding destabilization, resulting in the room temperature stable monoclinic phase. Importantly, the Bi 6s/O 2p VBM has been implicated in the relatively high hole diffusion lengths in BiVO4 (100 – 200 nm) compared to other metal oxides (e.g. 10-20 nm for rutile-TiO2).15,17,18 In addition, these lattice distortions are expected to affect the conduction band (CB) electronic structure. While Zhao et al. have proposed a molecular orbital model,19 in which the t2 and e orbitals in a Td ligand field are further split into e, 𝜋 ∗, and 𝜎 ∗ orbitals as a consequence of the C2 symmetry, the V d-orbital degeneracy in the CB remains experimentally unresolved.

In this work, a comprehensive approach to understanding VB and CB orbital character using a combination of X-ray spectroscopies, in conjunction with DFT calculations, is presented. X-ray absorption spectroscopy (XAS) at the V L- and O K-edges is used to resolve the triplet splitting of the V 3d states within the CB, which is a consequence of V-O bond length anisotropy and yields splitting energies of 1.0 and 1.9 eV. Resonant inelastic X-ray scattering (RIXS) measurements are performed to confirm the d-manifold splitting of the CB. In addition, combined RIXS and X-ray photoelectron spectroscopy (XPS) measurements provide a detailed portrait of the VB density of states (DOS), including the Bi 6p partial DOS contribution. Experimental probes of the electronic structure are compared to the partial and total DOS, calculated using DFT, to provide a clear picture of the VBM and CB minimum (CBM). Finally, photoemission spectroscopy is used to determine work function and electron affinity of the investigated BiVO4 thin films. Together, these results provide a basis for



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elucidating and improving the photocatalytic activity of this material, as well as for validating electronic structure calculations with detailed spectroscopic measurements. II.

EXPERIMENTAL Thin films of BiVO4 were grown on substrates composed of fluorine-doped tin oxide (FTO) on

glass (Sigma Aldrich) via chemical vapor deposition using a previously reported method.4 In brief, growth was carried out in a two zone furnace under flowing air, with Bi metal and V2O5 precursors held at 900 °C and the substrate held at 460 °C. Continuous films with thicknesses in the range of 150 – 200 nm, as determined by Rutherford backscattering spectrometry (RBS), and grain sizes of 200 nm to 1 µm, as determined by scanning electron microscopy (SEM), were obtained. X-ray diffraction of CVD films was performed to confirm deposition of pure-phase monoclinic BiVO4 (Fig. S1). Although BiVO4 can be deposited using a variety of methods, CVD offers material with large grain sizes and excellent crystallinity. Therefore, this material was selected for the present study. We note that X-ray absorption measurements on material deposited using other methods (e.g. reactive co-sputtering, spray pyrolysis, electrodeposition, and spin coating) showed no significant differences from those presented here. Density functional theory calculations of monoclinic BiVO4 were performed using the Quantum Espresso package.20 All calculations were carried out with a kinetic energy cutoff of 816 and 8,163 Ry for wavefunctions and charge density, respectively, with an 8×8×8 k-point grid, tetrahedral occupations, ultra-soft pseudopotentials, and the Perdew-Burke-Ernzerhof (PBE) functional. Calculations were performed on the structurally optimized monoclinic BiVO4 unit cell (a = 7.330 Å, b = 11.811 Å, c = 5.149 Å; α = β = 90°, γ = 134.227°) prior to both scf and nscf calculations; the unit cell parameters are provided in the Supporting Information (Fig. S2). Calculation of the density of states was performed with projwfc.x and integrated local density of states with pp.x codes. Visualization and analysis of the isosurfaces was carried out using Vesta.21


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V L-edge and O K-edge X-ray absorption spectroscopy (XAS) measurements were performed at beamlines 8.0.1 and 6.3.1 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL). Beamline 6.3.1 uses a variable line spacing plane-grating monochromator on a bending magnet source,22 whereas the undulator beamline 8.0.1 is equipped with a spherical grating monochromator.23 Absorption data were collected in total electron yield (TEY), total fluorescence yield (TFY), and partial fluorescence yield (PFY) by monitoring the photoemission current, using a channeltron electron multiplier and silicon drift detector (SII Nano Technology Inc., USA), respectively. The resolution for XAS spectra was better than 0.15 eV at the O K-edge. The spectra were normalized to the incident photon flux monitored by measuring the photocurrent from a clean gold mesh. Non-resonant X-ray emission spectroscopy (XES) and resonant inelastic X-ray spectroscopy (RIXS) data were collected at beamline 8.0.1 using a Rowland-circle grating spectrometer equipped with a two dimensional multichannel plate detector. The instrumental resolution for X-ray emission was 0.4 eV and the emission energy was calibrated using the positions of the elastic features in the emission spectra. XAS and XES data fitting was performed with Igor Pro using a sigmoidal line shape to account for the background and Voigt line shapes for the peaks. X-ray photoelectron spectroscopy (XPS) was performed using a monochromatized Al Kα source (hν = 1486.6 eV), operated at 225 W, on a Kratos Axis Ultra DLD system at a takeoff angle of 0º relative to the surface normal, and pass energy for narrow scan core level and valence band spectra of 20 eV. Spectral fitting was done using Casa XPS analysis software. Spectral positions were corrected using adventitious carbon by shifting the C 1s core level position to 284.8 eV and curves were fit with quasiVoigt lines following Shirley background subtraction. For work function measurements, a bias of -9 V was applied to the sample and UPS lens mode with pass energy of 5 eV was utilized to ensure extraction and collection of low-energy electrons. A linear fit of the secondary electron cutoff was extrapolated to the background intercept to determine the cutoff energy.



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III.

RESULTS AND DISCUSSION

Density Functional Theory To provide a basis for assignment and interpretation of experimental X-ray spectra, the orbital energetics, density of states (DOS), and partial density of states (pDOS) were modeled via DFT. Shown in Fig. 1(a) are the total DOS and pDOS for O 2p, V 3d, V 4s, V 4p, Bi 6s, and Bi 6p, which are consistent with previous reports.19,24 The Eg was found to be 2.05 eV, which is an underestimate of the real material, as expected from this level of DFT calculation. The VB is primarily of O 2p character where O, having a distorted trigonal planar geometry, possesses unhybridized 2pπ and hybridized sp2 orbitals. At the VBM, non-bonding states from O 2pπ mostly contribute, together with a small contribution from Bi 6s orbital mixing. The middle of the VB contains V 3d hybridized with O sp2, while the lowest energy edge of the VB has increased contribution from Bi 6p mixed with O sp2 (Fig. 1a).


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Figure 1. (a) Density of states (DOS) and partial DOS of monoclinic scheelite BiVO4 projected onto atomic wavefunctions. (b) Conduction band density of states, including V 3d and Bi 6p component contributions, which are significantly affected by the distorted polyhedra of monoclinic scheelite BiVO4, as described in the text. Computational results are presented with 30 meV broadening.

The CB is primarily composed of antibonding V 3d states, with significant contribution from antibonding O sp2* due to hybridization. The upper region of the CB also contains strong Bi 6p character. To elucidate the specific d-orbital contributions to the CB, the pDOS for all V d-orbitals are presented in Fig. 1(b). In addition to V 𝑑𝑧2 and V 𝑑𝑥 2 −𝑦2 states, the lower region of the CB includes contribution from 𝑑𝑧𝑥 states, which are mixed with the 𝑑𝑧2 orbital near 3 eV. This finding is consistent with the known

distortion of VO4 tetrahedron within ms-BiVO4 (see above), which yields C2 symmetry and reduces the d-

orbital degeneracy. The predicted charge density on the oxygen atoms is also affected by the lattice distortions within ms- BiVO4, with a calculated partial charge on half of the oxygen atoms in the lattice of -0.4824 and of 0.4847 on the other half (Fig. S3 red and blue, respectively). The integrated local DOS (ILDOS) within various energetic windows of the VB and CBM are presented in Fig. S4. In particular, the ILDOS at the CBM (2.04 to 2.14 eV), as well as a cross section of the ILDOS through the (101) plane, highlight the primary contribution from V 𝑑𝑧2 and 𝑑𝑥 2 −𝑦2 orbitals (Figs. S4(d) and S5). Poor wavefunction overlap between V neighbors is likely to cause a detrimental localization of CB electrons. Indeed, previous

photoelectrochemical measurements have found larger saturation current densities for backside, compared to frontside, illumination of the material. 5,25-27 This observation has been attributed to poor majority carrier transport in n-type ms-BiVO4; the majority electron mobility, rather than the minority hole mobility, limits photocarrier transport and extraction.5,17,26,27 Localization of photogenerated electrons, together with the polar nature of ms-BiVO4, suggests that self-trapping and small electron polaron formation may occur within this material. Indeed, a small polaron hopping conduction mechanism has been proposed to describe majority carrier transport within n-type BiVO4 single crystals.17 In contrast,



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recent theoretical calculations on minority carrier transport ms-BiVO4 suggest weak hole localization.28 The relative orbital delocalization at VBM, compared to CBM, as shown in Fig. S4, is consistent with this picture of majority electron-limited charge transport in this material. Finally, distortion of BiO8 dodecahedra also leads to lifting of Bi 6p orbital degeneracy in the upper region of the CB, which results in degenerate px and pz orbitals, with the py orbital located at higher energies (Fig. 1b). Conduction band electronic structure In order to understand the fundamental electronic structure of the unoccupied states within the CB of ms-BiVO4, we present a comparison between XAS and the DFT-determined DOS. As a starting point for this analysis, the CB was studied by XAS at the V LIII- and O K-edges. XAS offers the unique advantage of being element specific; the electronic structure of constituent atoms in the BiVO4 lattice may be probed directly. Considering orbital selection rules (Δℓ± 1), the V L-edge offers a probe of 2p →

3d transitions, while the O K-edge resolves 1s → 2p excitations. For the case of BiVO4, the CB DOS may be studied from both V L-edge and O K-edge spectroscopy due to hybridization of O 2p with V 3d, as

well as O 2p with Bi 6p. The XAS spectra collected in total electron yield (TEY) and in partial fluorescence yield (PFY) were very similar in both line shape and spectral features so only the TEY is reported herein.


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Figure 2. X-ray absorption spectra measured in total electron yield (TEY) at the V LIII (a) and O K (b) absorption edges. Data points are shown as open grey circles. Deconvolution of the spectral features was accomplished using Voigt functions and the individual fit components are shown as colored traces. The resulting envelopes describe the data well and are shown as the solid black lines. Vertical dashed lines indicate the center positions of main features. The top right shows bond lengths and symmetry of VO4 tetrahedra and BiO8 dodecahedra. (c) Density functional theory calculated total density of conduction band states, as well as the partial density of states of V 3d and Bi 6p orbitals, generated using 400 meV broadening to aid comparison with experimental data.

The V LIII absorption edge is shown in Fig. 2(a). Three peaks are clearly resolved as a consequence of a dipole allowed transition from V 2p3/2 to the unoccupied V 3d orbitals, which dominate the CB DOS. Fitting with Voigt functions, to account for both core-hole and instrument broadening, revealed three major peaks at 515.89, 516.86, and 518.12 eV, which originate from the ligand field splitting of V 3d states. The fitting result suggests that the d-orbital splitting energies of the CB are 0.97 and 2.22 eV. An additional feature was observed at 3.18 eV above the edge, the origin of which will be described below. Additional features in the LIII spectra could not be observed because of the LII edge, whose onset lies approximately 6.6 eV above the LIII edge. These results provide direct evidence for a



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triplet d-manifold in BiVO4, and are consistent with the distorted local tetrahedral environment of V. However, we note that quantification of d-orbital splitting from the metal absorption edge is less accurate than from the ligand edge, in which core hole effects are reduced and the excited electron more directly probes the d-orbital splitting.29 Analysis of the O K-edge XAS requires care to ensure that the FTO-coated glass substrate does not contribute to the measured spectrum. Considering the attenuation length at the O K-edge is approximately 50 nm,30 all measurements were performed on a continuous BiVO4 film with a thickness of >150 nm. Comparison of spectra from the BiVO4 film with bare FTO-coated glass confirms that the substrate signal is entirely attenuated and does not interfere with the measurement (Fig. S6). The O Kedge absorption spectrum from the BiVO4 film is shown in Fig. 2(b). The spectrum was deconvoluted into five components, at 529.42, 530.41, 531.35, 532.50, and 534.61 eV. Knowledge of the locations of V 3d and Bi 6p states from DFT predictions (Fig. 2(c)) provides significant insight into the XAS assignments; the first three peaks in the O K-edge spectrum are attributed to triplet splitting of the dorbitals, with separation of 1.00 eV and 1.91 eV. The presence of two additional spectral components at energies above the O K-edge is consistent with DFT predictions of Bi 6p orbital contributions to the upper CB. Therefore, peaks located at 532.50 and 534.61 eV in the experimental spectrum are assigned to Bi 6p (CBM + 3.07 eV and +5.23 eV). As described above, anisotropy of the Bi 6p states is expected as a result of significant distortion in the BiO8 dodecahedra, which like V, possess local C2 symmetry. The ILDOS from 4.96-5.50 eV and 6.61-7.03 eV clearly demonstrate the p orbital degeneracy breaking as well (Fig. S7). The Bi-O bond distances have been reported to be 2 × 2.628, 2 × 2.516, 2 × 2.372, and 2 × 2.354 Å by neutron diffraction13 and to be 2 × 2.640, 2 × 2.502, 2 × 2.379, and 2 × 2.343 Å by powder x-ray diffraction.14 All V-O and Bi-O bond distances, as well as the crystal structure of monoclinic BiVO4, are summarized in Fig. S8. We note that the feature at CBM + 3.08 eV in the O K-edge spectrum is also observed in the V LIII edge spectrum at CBM + 3.18 eV as a consequence of hybridization.


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In addition to XAS, resonant inelastic X-ray scattering (RIXS)31 measurements were performed to directly probe the V 3d orbital splitting energies through observation of d-d excitations in the CB of BiVO4. In this photon-in/photon-out scattering technique, incident soft X-ray synchrotron radiation of energy ℎ𝜈 is used to resonantly promote a core level electron to the CB. Radiative annihilation of the

core hole with a VB electron via a direct RIXS mechanism results in X-ray emission with energy ℎ𝜈′. The

final state is comprised of an electron in the CB and a hole in the VB. This mechanism of X-ray scattering by exciton generation provides a means of resonantly probing filled states of the VB. In addition, inelastically scattered radiation from an indirect RIXS process, in which energy transfer between the corehole and CB electrons occurs during the energetic excited intermediate state, possesses information pertaining to fundamental transitions in the material, such as charge transfer excitations, d-d transitions, phonon energies, and magnon excitations (Fig. S9). In the absence of defect states, BiVO4 is a 𝑑0 system possessing an unoccupied CB and indirect RIXS transitions that probe the d-manifold should not

contribute to the spectrum. However, the as-grown material is characterized by n-type conductivity and partial occupation of the CB, presumably as a consequence of native defects.32 Indeed, high resolution V 2p X-ray photoelectron spectroscopy (XPS) of our films reveals the expected major component from lattice V5+ at 516.74 eV, as well as a low binding energy shoulder at 515.45 eV that can be assigned to V4+ (Fig. S10). While XPS is surface sensitive and the V4+ contribution to the XPS cannot be used for quantification of the bulk character, its presence does suggest that V4+ ([Ar]3s23p63d1) may contribute to RIXS. Therefore, the presence of electrons in the CB is expected to enable the indirect RIXS mechanism, thereby providing an additional probe of CB d-manifold.



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Figure 3. (a) X-ray emission spectrum of BiVO4 at the O K-edge obtained under resonant excitation of V LIII with energy of 515.90 eV, as indicated by the vertical arrow in the X-ray absorption spectrum shown in the inset of (a). The red box in (a) shows the region of interest, presented in (b), for inelastic scattering associated with d-d transitions in the material. Experimental data are presented as open circles, along with Voigt component fits (solid green and red lines) and the corresponding envelope function (solid black line). The peak at approximately 516 eV is due to elastic scattering of the incident X-rays and the energetic splittings of the inelastically scattered peaks are indicated. The instrument resolution for this measurement was 0.36 eV, determined by fitting of the elastic peak.

The RIXS spectrum from BiVO4, collected by resonant excitation of the V LIII-edge at 515.9 eV, shows a prominent feature at 510 eV due to emission from VB states with O sp2/V 3d hybrid character (Fig. 3a). At higher energy, the peak at 515.9 eV is due to elastically scattered radiation; the resolution of the measurement, determined from the FWHM of this feature, was 0.36 eV. In addition, lower amplitude inelastic scattering is observed in the 514-515 eV region, which appears as a flat topped, multi-component contribution to the emission spectrum (Fig. 3). Fitting of the spectrum in this region reveals two inelastic scattering features at 514.20 and 514.74 eV, as shown in Fig. 3b, corresponding to an energy loss of 1.16 eV and 1.71 eV, respectively. The energy loss measured by RIXS correlates well with the triplet d-orbital


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splittings measured in the O K-edge XAS spectrum (1.00 and 1.91 eV) and lie within the d-d excitation energy range typically reported for similar vanadium oxide compounds.33 Given the agreement between the energy loss features in RIXS with spectral components in XAS, predictions of theory, and the XPS observation of V4+ on thin film materials, we assign the observed inelastic peaks within the RIXS spectrum to d-orbital splitting in the CB of ms-BiVO4. Valence Band Electronic Structure RIXS was also collected as a function of excitation energy spanning the O K-edge absorption in order to provide detailed information regarding the element specific pDOS within the VB (Fig. 4(a,b)). Two representative spectra obtained at excitation energies of 529.5 eV and 534.8 eV, along with fit components, are shown in Fig. 4c. Tentative assignments of the three spectral contributions to the emission spectra can be made based on DFT calculations and previous reports:1,19,24 (i) the high energy peak near 526 eV originates from O 2pπ occupied states at and below the VB edge, (ii) the peak at ~524.2eV corresponds to hybridized O sp2/V 3d orbitals within the VB, and (iii) the low energy peak appearing at 522.5 eV stems from hybridized O sp2/Bi 6p orbitals deep within the VB. For reference purposes, the DFT-calculated VB DOS and pDOS, with 400 meV broadening for comparison to experimental VB spectra, are shown in Fig. 4e. The excitation-energy dependence of RIXS provides additional confirmation of these assignments, as described below.



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Figure 4. (a) O K-edge X-ray absorption spectrum of BiVO4 with the excitation energies used for RIXS measurements shown as vertical dashed lines. (b) Resonantly excited X-ray emission spectra as a function of excitation energy. Regions (i), (ii), and (iii) represent the upper, middle, and lower regions of the VB, as described in the text. The arrow indicates resonant emission assigned to occupied Bi 6p states. (c) Characteristic RIXS spectra, together with three component fits used for analysis, that were obtained by excitation with 529.5 and 534.8 eV X-rays. (d) Relative peak areas of each fit component of the RIXS spectra as a function of excitation energy. Based on results presented here, features (i), (ii), and (iii) are assigned to O 2pπ, V 3d hybridized with O sp2, and Bi 6p hybridized with O sp2 states, respectively. (e) DFT-calculated DOS and of the VB generated with 400 meV broadening to aid comparison to experimental data. The energy scale is relative to VBM. (f) Valence band X-ray photoelectron spectrum, along with component fits (solid color lines) and envelope function (solid black line). The energy scale is relative to the Fermi energy of the measured film.

Figure 4(d) shows the change of relative integrated spectral weight of each emission component as a function of excitation energy. The spectral weight of the high energy feature (i) initially increases and then plateaus as the excitation energy moves through the O K-edge. While this result is consistent


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with the model proposed by Zhao et al.,19 emission specifically related to O 2p/Bi 6s mixing at the VBM could not be resolved by this technique due to spectral broadening. A more pronounced excitation energy dependence was observed for features (ii) and (iii). Progressing from 529.5 to 532.5 eV, the emission intensity from the middle of the VB (ii) declined. This observation is consistent with the DFT-derived assignment of this emission to hybridized O sp2/V 3d states within the VB. Further, this emission is most intense upon resonant excitation to unoccupied O sp2/V 3d antibonding states, which have the highest pDOS near the CB edge. As the excitation moves off resonance at higher energies, emission from occupied states in the VB that possess significant V 3d character is reduced. Thus, RIXS measurements are in excellent agreement with DFT predictions and confirm assignments of both occupied and unoccupied hybridized O sp2/V 3d orbitals within the VB and CB, respectively. The emission intensity from feature (iii), which is highlighted by the arrow in Fig. 4b, exhibits a maximum for excitation in the range of 532.5 to 534.8 eV (Fig. 4d). This can be understood by considering that the X-ray absorption in this range is predominantly to unoccupied Bi 6p/O sp2 antibonding states of the CB. Therefore resonant excitation to these states corresponds to increased emission intensity from Bi-related states within the VB. From DFT calculations, occupied Bi 6p states are expected near the bottom of the VB, in excellent agreement with the RIXS results presented here. At higher excitation energy, the emission intensity of this feature declines, indicating that the X-ray absorption peak at 538.3 eV contains less Bi character, as predicted from the DFT-calculated pDOS. Indeed, increased emission intensity from (ii) upon resonant excitation at 538.3 eV is consistent with predictions that this feature is dominated by unoccupied V 4s and 4p orbital contributions. Valence band XPS was used to further study the VB total DOS of BiVO4. We note that high energy X-ray excitation, rather than lower energy ultraviolet light excitation, was selected in order to �⃑ conservation without introducing additional structure to the eliminate final state modulation and ensure 𝑘 measured VB spectrum.34 As shown in Fig. 4(f), the spectrum is composed of broad multicomponent photoemission between 9 and 2 eV, with an additional isolated feature centered at 12.0 eV.



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Deconvolution of these features yielded peaks at 2.65, 3.67, 5.63, 7.45, and 11.94 eV. The observed features are in good agreement with measured RIXS spectra and the calculated DOS, with the exception of the Bi 6s core orbitals, which are calculated to be 2.6 eV lower in energy than experiment. Comparison of the VB photoemission spectrum to both RIXS data and DFT calculation of the VB pDOS allows these features to be assigned to unhybridized O 2pπ mixed with Bi 6s near 2.65 eV, O 2pπ near 3.67 eV, hybridized O sp2/V 3d near 5.63 eV, hybridized O sp2/Bi 6p near 7.45 eV, and Bi 6s at 11.94 eV. The energies reported here are with respect to the Fermi level, as is typical in XPS. Bandgap and band edge positions The magnitude of the bandgap of ms-BiVO4 was assessed by comparing the non-resonant XES spectrum to the XAS spectrum at the O K-edge (Fig. 5a). The valence band emission onset and conduction band absorption onset were determined from the local minimum and maximum of the corresponding first derivative spectra, respectively (Fig. 5b). The result yields a bandgap of 2.48 eV. This finding is in excellent agreement with a previous report by Payne et al., who reported the bandgap of solid state synthesized bulk ms-BiVO4 powder to be 2.48 eV by diffuse reflectance and to be 2.38 eV by X-ray spectroscopies.1


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Figure 5. (a) Non-resonant O K-edge X-ray emission spectrum, black, and O K-edge x-ray absorption spectrum (XAS), blue. (b) First derivative of the spectra in (a). The bandgap of monoclinic scheelite BiVO4 is estimated to be 2.48 eV from analysis of the valence band emission and conduction band absorption onsets, as indicated by the vertical dashed lines. The result is in good agreement with similar analysis by Payne et al. on monoclinic scheelite BiVO4 bulk powder.1

From XPS, the onset of photoemission indicates that the energetic difference between the VB edge and the Fermi energy, at the surface, is 2.0 eV, which is consistent with the n-type character of the material. It is important to note that this value is sample specific; it depends on the carrier concentration within the deposited material. Furthermore, photoemission spectroscopy is surface sensitive and the measured Fermi energy with respect to the VB edge can vary due to surface band bending. In the present work, XPS measurements were performed with and without external illumination of the semiconductor with above bandgap light and only minor (

Electronic Structure of Monoclinic BiVO4 - American Chemical Society

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