Indirect Bandgap and Optical Properties of Monoclinic Bismuth

Jan 15, 2015 - ABSTRACT: Monoclinic scheelite bismuth vanadate (m-BiVO4) is a promising semiconductor photoanode for photoelectrochemical...
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Indirect Bandgap and Optical Properties of Monoclinic Bismuth Vanadate Jason K. Cooper,†,‡ Sheraz Gul,§ Francesca M. Toma,†,∥ Le Chen,†,‡ Yi-Sheng Liu,⊥ Jinghua Guo,⊥ Joel W. Ager,†,‡ Junko Yano,†,§ and Ian D. Sharp*,†,§ †

Joint Center for Artificial Photosynthesis, ‡Materials Sciences Division, §Physical Biosciences Division, ∥Chemical Sciences Division, Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

⊥

S Supporting Information *

ABSTRACT: 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. density of states. While the first reported density functional calculations identified BiVO4 as a direct bandgap semiconductor, this assignment was based on a limited exploration of the band structure that analyzed only three k-points within the Brillouin zone.10 Subsequent calculations of the extended band structure, which considered an expanded set of k-points, revealed the material to be an indirect bandgap semiconductor with the minimum transition lying within the L−M k-path.8 A recent study by Ma and Wang9 demonstrated that accurate calculations of the band structure, as well as the atomic structure of local distortions around Bi that are not accurately predicted by DFT methods, are complicated by the energetic location of the Bi 6s orbitals. It is important to note that the calculations presented in all of these reports are in broad agreement; the apparent confusion regarding the nature of the bandgap stems from the locations of the band extrema in reciprocal space that were not initially examined. Furthermore, comprehensive theoretical treatment of the material revealed that a direct transition is present at a slightly higher energy,

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onoclinic scheelite phase bismuth vanadate (m-BiVO4) has been the focus of intensive research as a promising visible light absorbing semiconductor photoanode for application in solar water splitting devices.1 With a bandgap of ∼2.4− 2.5 eV, this material can achieve water oxidation with photoanodic current onset potentials as low as 0.1−0.3 V vs. RHE and has a theoretical maximum photocurrent density of ∼7 mA/cm2.2−4 Half-cell saturation photocurrent densities as high as 4−4.5 mA/cm2 in the presence of sacrificial hole acceptor,5 as well as nearly 5% efficiency in a tandem overall water splitting device,6 have been achieved. While advanced impurity doping and mesostructuring strategies have enabled significant progress, further improvement toward achieving the theoretical efficiency limit can be aided by understanding of the fundamental optoelectronic properties of BiVO4. Despite the considerable attention that BiVO4 has attracted, there remains controversy over the nature of its fundamental bandgap, with various computational and experimental studies showing it to be either indirect7−9 or direct.10,11 Knowledge of this property is critical for understanding material characteristics such as the absorption coefficient and excited state lifetime. First-principles electronic structure calculations of mBiVO4 have yielded generally consistent predictions of the © 2015 American Chemical Society

Received: December 6, 2014 Revised: January 14, 2015 Published: January 15, 2015 2969

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The Journal of Physical Chemistry C which makes experimental assignment of the bandgap challenging and, at times, ambiguous. Here, we utilize a combination of traditional methods, such as UV−vis absorption spectroscopy and spectroscopic ellipsometry, together with an advanced synchrotron X-ray method, resonant inelastic X-ray scattering spectroscopy (RIXS), to provide experimental verification that the fundamental bandgap of BiVO4 is indirect, with a value of 2.5 eV. Furthermore, we find that a direct transition is present near 2.7 eV, just 200 meV above the minimum indirect transition. These properties have a profound positive impact on the photoelectrochemical behavior of this material compared to many other metal oxides. Indeed, the indirect nature of the material yields relatively long photocarrier relaxation times, as verified by transient absorption spectroscopy, while the presence of a direct transition just above the indirect bandgap ensures strong optical absorption near the fundamental edge. The most commonly utilized method for characterization of semiconductor bandgaps is UV−vis optical absorption spectroscopy. Under the assumption of parabolic band dispersion, the energy dependence of optical transition strengths can be expressed by the Tauc equation, which is given by (αhν)n = A(hν − Eg )

Figure 1. Optical absorption data from BiVO4 thin films on fused silica. (a) Transmission (purple) and reflection (blue) data were collected using an integrating sphere as a function of wavelength. These data were used to calculate the absorption coefficient as a function of wavelength. (b) Tauc plots for different powers of n corresponding to indirect (n = 1/2, left axis) and direct (n = 2, right axis), along with linear fits of the absorption edges.

(1)

where α is the measured optical absorption coefficient, hν is the photon energy, Eg is the transition energy, and A is a proportionality constant. Values of n equal to 1/2 and 2 are indicative of indirect and direct transitions, respectively. Therefore, the linearity of plots of (αhν)1/2 and (αhν)2 vs hν allows determination of the nature of transitions and extrapolation of the linear region to the x-axis intercept provides a measure of the transition energy. Despite the apparent simplicity of this technique, practical complications can lead to uncertainty in the analysis. As described above, previous experimental optical absorption studies have yielded inconclusive results, with a variety of reports that the bandgap is either direct or indirect. As a starting point for investigating the origin of this discrepancy, we recorded the optical absorption spectrum of a high quality m-BiVO4 thin film spin-coated onto a fused silica substrate (see SI for synthesis details). Both transmission and reflection measurements were performed using an integrating sphere over the range from 250−800 nm, as shown in Figure 1a. The uniform planar thin films of BiVO4 were characterized by a high reflectivity, which was collected at near normal incidence (17.5° with respect to normal incidence) and peaked at a value of 38% near the band edge. The absorption coefficient (α) was calculated from these data using the film thickness, d, which was determined to be 51 nm by Rutherford backscattering spectrometry (RBS), by α = −ln[(%T⊥ + %R17.5 °)/100]/d

negligible diffuse reflectance/scattering background and was principally dominated by specular reflectance, indicating a high degree of uniformity, low roughness, and high surface quality. These characteristics were confirmed by atomic force microscopy (AFM), which showed the surface to have an rms roughness of 3.5 nm (Figure S1). We note that for the Tauc analysis, the fit was performed without subtracting the background response. Considering that the small contribution from diffuse specular reflectance was accounted for in the measurement, the remaining absorption in the subgap region is attributed to a broad Urbach tail, rather than a scattering response as is often observed. This result was also confirmed in the spectroscopic ellipsometry measurements, described below. Photoluminescence (PL) spectroscopy was performed as a function of temperature from 9 to 298 K using a solid state 405 nm cw laser in an attempt to observe PL related to band-toband carrier recombination, which would be expected for a direct bandgap semiconductor. No such band edge emission was observed at any temperature (data not shown). For an indirect semiconductor, relaxation is typically dominated by nonradiative processes since momentum conservation requires phonon interaction during band-to-band recombination events. Therefore, this finding further supports the conclusion that BiVO4 has an indirect fundamental bandgap. While optical absorption spectroscopy can be an effective method for bandgap determination, broadening from subgap states associated with lattice disorder, as well as scattering, may yield ambiguous results. Resonant inelastic X-ray scattering, however, offers the unique advantage of specifically probing vertical transitions in E−k space, allowing for a more direct measure of the band dispersion. RIXS is a photon-in/photonout bulk sensitive measurement in which incident synchrotron soft X-ray radiation of energy hν is used to resonantly promote a core level electron to the CB, rather than to vacuum as in conventional nonresonant X-ray emission spectroscopy (XES).

(2)

Tauc plots reveal that the fundamental transition exhibits a (αhν)1/2 dependence on energy, suggesting that monoclinic scheelite BiVO4 is an indirect bandgap semiconductor (Figure 1b). However, a plot of (αhν)2 provides evidence that a direct transition is also present at 2.7 eV, just 200 meV above the fundamental absorption edge. Consequently, the material is strongly absorbing at and above its fundamental indirect bandgap, which may explain the discrepancy between prior experimental and theoretical determinations of the nature of the bandgap. Additionally, the material studied here had a 2970

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The Journal of Physical Chemistry C

ray absorption occurs according to the E−k dispersion of the CB and X-ray emission occurs from states at identical k-points in the VB. Therefore, direct bandgap semiconductors are characterized by emission at the highest energy for excitation to the CBM and a redshift of the X-ray emission with increasing excitation energy. In contrast, for the case of an indirect bandgap semiconductor, resonant X-ray absorption to the CBM yields X-ray emission from energetically lower-lying VB states away from the VBM. As the excitation energy is increased, absorption occurs to energetic states higher above the CBM, but emission occurs from energetic states closer to the VBM (Figure 2a). As a consequence, indirect bandgap semiconductors are characterized by a blueshift of the X-ray emission with increasing excitation energy in the vicinity of the absorption threshold, as seen in diamond,20 Be chalcogenides,21 and Fe2O3.22 In the present work, RIXS was applied to a high quality thin film of m-BiVO4 deposited on a FTO/glass substrate by chemical vapor deposition (CVD), which provided a thicker sample than by spin coating. As previously demonstrated, O 2p states contribute to both the VB and CB of BiVO4 through hybridization with Bi 6s and V 3d orbitals, respectively.23 Therefore, determination of the nature of the fundamental bandgap of BiVO4 is accomplished by resonant excitation at energies across the O K-edge (O 1s) to unoccupied conduction band states (Figure 2b). The corresponding O K-edge emission spectrum arises from relaxation of occupied VB states to fill O 1s core holes. Figure 2c shows the emission spectra from BiVO4 as a function of X-ray excitation energy. Excitation with 529.1 eV marked the onset of emission arising from the VB → O 1s transitions. As the excitation energy was increased, a clear blueshift of the emission edge was observed. This result is consistent with optical absorption measurements and conclusively indicates that the bandgap of m-BiVO4 is indirect. In addition to determination of the nature of the fundamental bandgap, the value of the bandgap was assessed by comparing the nonresonant X-ray emission spectrum to the X-ray absorption spectrum of the O K-edge (Figure S2(a)). The zero-crossings of the first derivatives of these spectra indicate a bandgap energy of 2.48 eV (Figure S2(b)), which is in good agreement with the value reported by Payne et al. of 2.38 eV also obtained by X-ray spectroscopies,24 as well as by optical absorption spectroscopy reported herein. This indicates that core-hole effects are minimal in this system. Finally, we note that analysis of the O K-edge requires care to ensure that the FTO-coated glass substrate does not contribute to the measured spectra. Considering the attenuation length at the O K-edge is approximately 50 nm,25 all measurements were performed on a continuous, CVD grown 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 (Figure S3). Variable angle spectroscopic ellipsometry (VASE) of BiVO4 allows for further characterization of the nature of optical transitions and for determination of the optical constants (n and κ), which are important for predictive modeling of solar water splitting devices. Recently, VASE was applied to BiVO4 grown by molecular beam epitaxy (MBE). In that work, the authors utilized a four critical point parabolic band (CPPB) oscillator model that was consistent with a direct bandgap of 2.5 eV.11 However, the possibility of an indirect transition was not considered in development of the model, which was

Radiative annihilation of the core hole with a VB electron (direct RIXS mechanism) results in X-ray emission with energy hν′.12 As a result, the final state is comprised of an electron in the CB and a hole in the VB. Since the momentum transfer by the soft X-ray photon is negligible compared to the electron momentum and because momentum conservation between the initial and final states is required, transitions that are vertical in k-space contribute preferentially toward RIXS spectrum within a few eV above the absorption threshold.12,13 Consequently, RIXS provides a unique and powerful method for probing band dispersion and electronic structure, as demonstrated for graphite,14 YBa2Cu3Ox,15 and ZnO,16 as well as for discerning direct versus indirect bandgap semiconductors without further theoretical input, as shown in the schematic illustration in Figure 2a. For the case of a direct bandgap semiconductor, resonant excitation from a core level to the conduction band minimum (CBM) leads to X-ray emission from the valence band maximum (VBM). As reported for CdO17 and GaN,18,19 the emitted X-rays possess the highest energy upon resonant excitation to the CBM. As the excitation energy is increased, X-

Figure 2. (a) Schematic representation illustrating the excitation energy dependence of the X-ray emission energy in resonant inelastic X-ray scattering (RIXS), which is governed by momentum conservation. For the case of an indirect bandgap material, a blueshift of emission occurs with increasing excitation energy in the vicinity of the absorption threshold. In contrast, a redshift occurs for the case of a direct bandgap semiconductor. (b) O K-edge X-ray absorption spectrum of BiVO4 with vertical lines indicating the excitation energies used to collect the RIXS spectra. (c) O K-edge emission spectra from BiVO4 as a function of X-ray excitation energy. The shift of the emission edge to higher energies with increasing excitation energy close to the absorption onset proves conclusively that the bandgap of BiVO4 is indirect. 2971

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The Journal of Physical Chemistry C reasonable in light of existing literature that assumed the material to have a direct bandgap. In the present work, VASE is performed on the spin-coated thin films of BiVO4 on fused silica substrates and a model is developed to determine optical constants and investigate the nature of the fundamental transition. The change in polarization of incident light after interaction with the material, measured as the intensity ratio of p and s polarized light (denoted Ψ), as well as the phase shift (denoted Δ), is recorded as a function of incident angle between 45° and 75° with 5° increments and as a function of wavelength between 190−1690 nm (6.526−0.734 eV), as shown in Figure S4. A fit of the Ψ and Δ data was performed using a three oscillator model to account for the indirect, direct, and UV regions of the absorption spectrum. The indirect gap was modeled with a Cody−Lorentz (CL) oscillator, which includes a component to describe the Urbach tail associated with disorder-induced subgap absorption. Additional broadening in the subgap region is also expected for an indirect bandgap semiconductor due to phonon emission and absorption that are required for momentum conservation.26 The direct gap was modeled with PSemi-M0, which is effective for modeling the E0 critical point in direct semiconductors such as CdO27 and ZnO.28 The remaining UV absorption was fit with a PSemi-Tri oscillator. To improve the description of the absorbance from the model, the transmission spectrum was included in the VASE fit. This enabled the model to accurately describe the dip in the transmission spectrum near 390 nm. The fit result of the VASE Ψ and Δ data is shown in Figure S4(a) and the εi contributions from individual oscillators are plotted in Figure S4(b). The oscillator energies from this fit were 2.52, 2.70, and 3.65 eV. Using these three oscillators, we achieve an excellent fit to experimental data at all incident angles and wavelengths. Therefore, we conclude that VASE data are well described by an ellipsometric model of an indirect bandgap semiconductor that also possesses an energetically low lying direct transition (∼200 meV above the CBM). This assignment is consistent with the UV−vis absorption and RIXS data described above, as well as previous DFT calculations. The real and imaginary components of the dielectric function, εr and εi, respectively, as well as the refractive index and extinction coefficient, n and κ, are shown in Figure 3a,b. The absorption coefficient, α, was determined from the ellipsometry data according to the standard definition of α = 4πκ/λ and is plotted as a function of wavelength in Figure 3c. As described above, absorption below the bandgap is observed as a consequence of disorder in polycrystalline thin films. The band edge absorption onset occurs near 500 nm and plateaus between 430 and 390 nm, with a large value of absorption coefficient of approximately 1 × 105 cm−1. At shorter wavelengths, the absorption coefficient again rises. The results presented here are in good agreement with computational work by Zhao et al.8 and Ding et al.,7 which show the absorption onset at ∼490 nm and an absorption coefficient at 440 nm (2.82 eV) of 4 × 104 cm−1. We note that the absorption data derived from the UV−vis spectrometer, the spectroscopic ellipsometer in transmission mode, and the ellipsometry model are all in good agreement, as shown in Figure S4(c,d). The thickness determined from the ellipsometry model was 53.1 nm, which is in excellent agreement (3% variation) with that measured by RBS (51.4 nm). Additionally, the surface roughness was determined by ellipsometry to be 6.43 nm, compared to 3.5 nm by AFM. These results demonstrate that

Figure 3. Variable angle spectroscopic ellipsometry of spin-coated BiVO4 deposited on quartz substrate. (a) Photon energy dependence of the complex dielectric constant where ε̃ = εr + iεi, εr = n2 − κ2, and εi = 2nκ. (b) Wavelength dependence of the index of refraction (n), red, and the extinction coefficient (κ), turquoise, where ñ = n + iκ. (c) Wavelength dependence of the absorption coefficient determined from spectroscopic ellipsometry. In all plots, vertical dashed lines indicate the wavelengths corresponding to the indirect and direct transitions described in the text.

fitting of the ellipsometry data is achieved with physically justifiable parameters. Ensuring that the optical absorption depth is well matched to the carrier diffusion lengths is critical for efficient carrier collection efficiencies from semiconductor photoelectrodes. Optical characterization of the material indicates that above bandgap photons are absorbed within approximately 100 nm of the surface (α ∼ 1 × 105 cm−1). While this means that light is strongly absorbed, it is still important to maximize carrier diffusion lengths to achieve efficient charge extraction. The minority carrier diffusion length (L) is defined by L = (Dτ)1/2, where D is the diffusion constant, which is proportional to the carrier mobility, and τ is the minority carrier lifetime, which is highly sensitive to material quality and recombination channels that may be present. Because of the indirect bandgap of BiVO4, band edge PL is not observed and direct measurement of the minority carrier lifetime via time-resolved PL is not possible. Therefore, we have utilized transient absorption pump−probe spectroscopy to determine characteristic relaxation times. Measurements were performed using excitation pulses at 350 nm with durations of 100 fs and a repetition rate of 1 kHz. A white light continuum pulse (360−700 nm) was used to probe the ground and excited state absorption as a function of time following excitation. The time dependence of the recovery of the probe signal allows for determination of the dynamics associated with relaxation to the ground state, though does not yet allow unambiguous assignment of the minority carrier lifetime. Detailed analysis of excited state absorption spectra and ultrafast recombination channels in our BiVO4 thin films is 2972

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The Journal of Physical Chemistry C beyond the scope of the present work. However, we find that the characteristic relaxation time for the near band-edge absorption response, defined as the 1/e decay time, is approximately 20 ns, as shown in Figure 4.

terized by small absorption coefficients and require long minority carrier diffusion lengths for charge extraction. In the case of BiVO4, the presence of a direct transition close to the indirect fundamental edge ensures strong absorption and eases, but does not eliminate, the requirement for long diffusion lengths. Despite the low mobilities expected for BiVO4, the indirect nature of the bandgap appears to suppress band-toband recombination and increase carrier lifetimes. In conclusion, we have utilized a range of spectroscopic measurements to determine that the fundamental bandgap of monoclinic scheelite BiVO4 is indirect. This finding is derived from RIXS measurements, which allow unambiguous assignment of the indirect transition based on momentum conservation requirements. Complementary characterization by UV−vis absorption, PL, and spectroscopic ellipsometry supports this conclusion and allows determination of the spectral dependence of the optical properties of this semiconductor light absorber. Determination of these parameters is important for enabling predictive optoelectronic modeling of solar energy conversion devices and architectures. Furthermore, these measurements reveal an additional direct transition at ∼200 meV above the fundamental indirect bandgap, which is responsible for the large absorption coefficient (105 cm−1) of BiVO4. Transient absorption spectroscopy reveals relatively long photocarrier lifetimes and the presence of multiple recombination pathways. These nonradiative recombination dynamics are consistent with the indirect nature of the bandgap and suggest that improved material quality is important for approaching theoretical efficiency limits. The electronic structure of this material, which simultaneously allows strong optical absorption and long phototcarrier lifetimes, is desirable for efficient charge carrier extraction for photoelectrochemical energy conversion

Figure 4. Transient absorption of BiVO4 on quartz showing the recovery of the bleach signal (425 nm) after excitation with a 375 nm pulse (1 mJ cm−2).

Fitting was performed with a double exponential function plus a stretched exponential, to account for a broad population distribution of trap states, according to 2

dA(t ) =

∑ Ai i=1

exp( −x /τi) + A3 exp(− (x /τ3)β )

(3)

where Ai and τi are the amplitude and lifetimes of the ith component and β is the stretching parameter, which here was found to be 0.5. This function provides an adequate fit of recombination dynamics in the time domain from 0.5 ns to 10 μs, as shown in Figure 4. The lifetimes found by this method were 6, 55, and 600 ns. The 55 ns relaxation time is consistent with a previous report of photoconductivity decay in spraypyrolyzed BiVO4 thin films29 and is significantly longer than for many other visible light absorbing metal oxides used for photoelectrochemical energy conversion, in which lifetimes are in the tens to hundreds of ps regime. This result is consistent with the assignment of BiVO4 as an indirect bandgap material. Unlike direct bandgap semiconductors in which rapid band-toband recombination often dominates photocarrier relaxation, indirect semiconductors are characterized by much longer band-to-band recombination times and photocarrier relaxation typically occurs via trap-assisted recombination at defect sites. Multiple parallel recombination channels can contribute to the relaxation dynamics, as found in the present case. While much longer lifetimes may be achieved for indirect materials, the values are also extremely sensitive to material quality and doping. Compared to high quality, indirect bandgap, single crystal semiconductors (e.g., Si or Ge), the relaxation times presented here are very fast. Additional research on this material system, with a focus on passivating or eliminating recombination centers, holds promise for achieving improved carrier extraction efficiency. Recently, nanostructured BiVO4 thin films have been produced to decouple the optical absorption depth and minority carrier length, thereby significantly increasing photoelectrochemical performance in undoped materials in which these characteristic length scales are not well matched.2 Understanding the fundamental material and defect properties that define carrier transport is important for establishing rational approaches to improving efficiency. As described above, indirect bandgap semiconductors are typically charac-

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, atomic force micrograph, X-ray absorption and emission spectra, model fitting of spectroscopic ellipsometry data, and X-ray diffraction data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Jeremy Van Derslice for insightful discussions about ellipsometry. This material is based on work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DESC0004993. XAS, XES, and RIXS experiments were performed at the Advanced Light Source (BL 8.0.1), Berkeley, which is operated under DOE (DE-AC02-05CH11231).

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REFERENCES

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DOI: 10.1021/jp512169w J. Phys. Chem. C 2015, 119, 2969−2974