⇦Chinese Physics Letters, 2019, Vol. 36, No. 7, Article code 077202 Pressure-Induced Ionic-Electronic Transition in BiVO$_{4}$ * Shu-Peng Lyu (吕舒鹏), Jia Wang (王佳), Guo-Zhao Zhang (张国召), Yu-Fei Wang (王宇飞), Min Wang (王敏), Cai-Long Liu (刘才龙), Chun-Xiao Gao (高春晓), Yong-Hao Han (韩永昊)** Affiliations State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012 Received 27 February 2019, online 20 June 2019 *Supported by the National Natural Science Foundation of China under Grant Nos 11774126, 11774174 and 11404133.
**Corresponding author. Email: hanyh@jlu.edu.cn Citation Text: Lv S P, Wang J, Zhang G Z, Wang Y F and Wang M et al 2019 Chin. Phys. Lett. 36 077202    Abstract Electrical transport properties of bismuth vanadate (BiVO$_{4}$) are studied under high pressures with electrochemical impedance spectroscopy. A pressure-induced ionic-electronic transition is found in BiVO$_{4}$. Below 3.0 GPa, BiVO$_{4}$ has ionic conduction behavior. The ionic resistance decreases under high pressures due to the increasing migration rate of O$^{2-}$ ions. Above 3.0 GPa the channels for ion migration are closed. Transport mechanism changes from the ionic to the electronic behavior. First-principles calculations show that bandgap width narrows under high pressures, causing the continuous decrease of electrical resistance of BiVO$_{4}$. DOI:10.1088/0256-307X/36/7/077202 PACS:72.90.+y, 61.50.Ks, 66.10.Ed © 2019 Chinese Physics Society Article Text Bismuth vanadate (BiVO$_{4}$) is a kind of scheelite ABO$_{4}$ material and is widely used in photoelectrodes because of its narrow band gap, wide absorption wave length range and good chemical stability.[1–6] Electrical transport properties are fundamental factors that determine the photoelectric functions of BiVO$_{4}$. For example, the photoelectric conversion efficiency of BiVO$_{4}$ strongly depends on its carrier concentration.[7–10] It can be increased by 40% in nanocrystalline BiVO$_{4}$ films with grain size in 300–400 nm and even by 10 times in W and Mo-doped BiVO$_{4}$ bulk materials.[11,12] High pressure is an effective way to regulate the properties of materials by inducing complex changes in crystalline, energy-band and electronic structures, and then endows the materials with novel electrical transport properties.[13–17] Under high pressure, CaMoO$_{4}$ changes from tetragonal phase to monoclinic phase with electrical resistance decreasing by three orders of magnitude at 8.0 GPa and with the carrier concentration increasing.[18] With the generation of oxygen vacancies in SrCrO$_{4}$ under high pressure, the carrier concentration in conduction band increases, causing the decrease of electrical resistivity correspondingly.[19] After the phase transition of CuWO$_{4}$ to monoclinic structure at 7.7 GPa the activation energy of carriers decreases, leading to the decrease of electrical resistance.[20] In a word, high pressure can improve the conductivity and carrier concentration of scheelite-type ABO$_{4}$ materials, and therefore we expect that it also has a positive effect on the transport properties of BiVO$_{4}$. However, the electrical transport properties of BiVO$_{4}$ under high pressure have not been studied yet. Motivated by the above issues, in this Letter we mainly study the electrical properties of BiVO$_{4}$ under high pressure with electrochemical impedance spectroscopy measurement and use first-principles calculation to explain the corresponding changes from the viewpoint of energy band structures. The sample in our experiment was made from BiVO$_{4}$ polycrystalline powder in purity of 99.98%, which was purchased from Alfa Aesar. X-ray diffraction (XRD) shows that BiVO$_{4}$ has a monoclinic scheelite structure with space group of $I2/a$. High pressure was generated by a diamond anvil cell (DAC) with the top facet having a diameter of 300 µm. A piece of T301 stainless steel 250 µm thick was pre-indented to 40 µm thick by the DAC, and then an insulating sample chamber of 170 µm diameter was prepared within the center of indentation. Platinum foil electrodes were placed tightly on both sides of the sample, forming a pair of parallel plate electrodes. The sample thickness was measured with an electronic micrometer.[21] Pressure was calibrated by the shift of ruby fluorescence R1 peak. To avoid introducing additional and unnecessary resistance, no pressure transmitting medium was added into the sample chamber. The electrochemical impedance spectra were measured with a Solartron 1260 impedance spectrometer and a 1296 impedance analyzer. The applied voltage amplitude and frequency range were 3 V and $1\times 10^{-3}$–$1\times 10^{6}$ Hz, respectively. First-principles calculations were performed on BiVO$_{4}$ with the CASTEP module. Structure optimization was conducted based on the density functional theory and plane wave pseudopotential method, and exchange correlation function was described with the GGA-PBZ model. The impedance spectra of BiVO$_{4}$ in Nyquist representation under different pressures are shown in Figs. 1(a)–1(f). At ambient pressure, the polycrystalline BiVO$_{4}$ exhibits a typical ionic transport spectrum, which is composed of two semi-circular arcs in the high-frequency region and a tilted straight tail in the low-frequency region. The semi-circular arcs represent the O$^{2-}$ ion's vibration within the lattice and around the grain boundaries of BiVO$_{4}$ along the high-frequency electric field respectively, and the straight tail represents the long-range diffusion of O$^{2-}$ ions along the low-frequency electric field. From ambient pressure to 1.4 GPa, the ionic conduction is the main transport mechanism in BiVO$_{4}$. However, the tilted straight tail in the low-frequency region, which results from the ionic conduction, disappears as pressure is higher than 3 GPa, indicating that the diffusion of O$^{2-}$ ions in BiVO$_{4}$ is prohibited. The transport mechanism has transformed from the ionic to the electronic behavior. Because the lattice constant of monoclinic BiVO$_{4}$ decreases continuously under high pressure,[22] the pressure-induced electronic-ionic transformation of conduction mechanism can be attributed to the closure of ion transport channels.
cpl-36-7-077202-fig1.png
Fig. 1. Nyquist impedance spectra for BiVO$_{4}$ at different pressures.
In typical ion transport spectra as shown in Fig. 1(a), the point which connects the semicircular arc and the straight tail can be used to analyze the transport behaviors of O$^{2-}$ ions in BiVO$_{4}$. We define the frequency of the connecting point as $f_{\rm w}$.[23] It denotes the frequency at which the long-range diffusion of O$^{2-}$ ions begins. The higher the $f_{\rm w}$ is, the faster the O$^{2-}$ ions migrate. As shown in Fig. 1(a), $f_{\rm w}$ increases with the pressure, indicating that the migration rate of O$^{2-}$ ions increases correspondingly. Generally, the increase of carrier migration rate is usually accompanied by the increase of carrier concentration.[24–26] Thus, it can be expected that the concentration of migrating O$^{2-}$ ions increases under high pressure. To quantify the transport behavior of BiVO$_{4}$, we choose corresponding equivalent circuits to fit the impedance spectra for different transport mechanisms. Figures 2(a) and 2(b) are representative fitting results with equivalent circuits for ionic conduction and electronic conduction, respectively.
cpl-36-7-077202-fig2.png
Fig. 2. Equivalent circuit diagrams and selected fitting results for ionic conduction at 0.3 GPa and electronic conduction at 3.0 GPa for BiVO$_{4}$.
For ionic conduction as shown in Fig. 2(a), the impedance of BiVO$_{4}$ can be expressed as $$\begin{align} Z=\frac{1}{\frac{1}{R_{\rm i_{1}}+Z_{\rm W}}+\frac{1}{Z_{Q_{1}}}}+\frac{1}{\frac{1}{R_{\rm i_{2}}}+\frac{1}{Z_{Q_{2}}}},~~ \tag {1} \end{align} $$ where $R_{\rm i_{1}}$ is the transfer resistance of O$^{2-}$ ions through grains, $R_{\rm i_{2}}$ is the transfer resistance of O$^{2-}$ ions through grain boundaries, $Z_{\rm W}$ is the impedance of Warburg element (W) to describe the long-range diffusion of O$^{2-}$ ions in BiVO$_{4}$, and $Z_{\rm Q_1}$ and $Z_{\rm Q_2}$ are the impedances of the constant phase elements (CPE1 and CPE2) to describe the charge/discharge processes in grains and grain boundaries, respectively. The formulas for $Z_{\rm W}$ and $Z_{\rm Q}$ are $$\begin{align} Z_{\rm W}=\,&\sigma_{_{\rm W}}\omega^{-1/2}(1-j)\coth\Big[\delta \Big(\frac{j\omega }{D}\Big)^{1/2}\Big],~~ \tag {2} \end{align} $$ $$\begin{align} Z_{\rm Q}=\,&\sigma_{_{\rm Q}}(j\omega)^{-n},~~ \tag {3} \end{align} $$ where $\sigma_{_{\rm W}}$ and $\sigma_{_{\rm Q}}$ are the Warburg coefficient and the CPE coefficient, respectively, $\omega$ is the angular frequency ($\omega =2\pi f$), $D$ is the diffusion coefficient of O$^{2-}$ ions, and $\delta$ is the average diffusion length of O$^{2-}$ ions. From the above formula, it can be seen that the imaginary part of impedance $Z$ will approach infinity if the frequency $f$ is reduced to 0 Hz. For electronic conduction in Fig. 2(b), the impedance can be expressed as $$\begin{align} Z=\frac{1}{\frac{1}{R_{\rm e_{1}}}+\frac{1}{Z_{\rm Q_{1}}}}+\frac{1}{\frac{1}{R_{\rm e_{2}}} +\frac{1}{Z_{\rm Q_{2}}}},~~ \tag {4} \end{align} $$ where $R_{\rm e_{1}}$ is the transfer resistance of electrons through grains, $R_{\rm e_{2}}$ is the transfer resistance of electrons through grain boundaries, and $Z_{\rm Q_1}$ and $Z_{\rm Q_2}$ are the impedances of constant phase elements respectively. It can be seen that the impedance imaginary part will approach zero when the frequency ($f$) is reduced to 0 Hz. With the equivalent circuits, the experimental impedance spectra can be well fitted and the pressure dependence of electrical transport parameters can be deduced as shown in Figs. 3(a) and 3(b). It can be seen from Fig. 3(a) that both grain and grain boundary resistances decrease as pressure increases, causing the decrease of total resistance. Below 1.4 GPa, the transport mechanism in BiVO$_{4}$ is ionic. Although the channels for O$^{2-}$ ion migration become much narrower and are closed completely at 3.0 GPa, the ionic resistance still decreases with increasing the pressure. One most reasonable reason is that ion migration scattering by crystal lattice is restrained under high pressure, leading to the increase of O$^{2-}$ ion migration rate as revealed by Fig. 1(a), where the starting frequency of O$^{2-}$ ion diffusion ($f_{\rm w}$) is increased with the pressure. Figure 3(b) shows the pressure-dependent relaxation frequency which can characterize the speed of BiVO$_{4}$ recovering to its equilibrium state after being disturbed electrically. At 3.0 GPa a jump with one order of magnitude in relaxation frequency can be found, indicating that the recovering speed for BiVO$_{4}$ is accelerated after the pressure-induced ionic-electronic transition, because electrons have a much higher migration rate than ions. It can be considered as more evidence to support the occurrence of pressure-induced ionic-electronic transition in BiVO$_{4}$.
cpl-36-7-077202-fig3.png
Fig. 3. (a) Pressure dependence of grain, grain boundary and bulk resistances of BiVO$_{4}$. (b) Pressure dependence of relaxation frequency of BiVO$_{4}$.
cpl-36-7-077202-fig4.png
Fig. 4. Energy band structures of BiVO$_{4}$ at (a) 1 GPa and (b) 5 GPa, respectively. (c) Pressure dependence BiVO$_{4}$ bandgap width.
The energy band structure and bandgap widths of BiVO$_{4}$ under high pressure are calculated with the first-principles calculation method. Figures 4(a) and 4(b) show the band structures of monoclinic BiVO$_{4}$ at 1 GPa and 5 GPa, respectively. We find that the narrowing of the BiVO$_{4}$ bandgap under high pressure is caused by the shift of conduction band minimum to the Fermi level. In Fig. 4(c) the bandgap width decreases linearly with increasing pressure. The narrowing bandgap is beneficial for electrons jumping from valence band to conduction band, which leads to the continuous decrease of BiVO$_{4}$ resistance under high pressure as shown in Fig. 3(a). In summary, the electrical transport properties of BiVO$_{4}$ have been investigated at high pressure with electrochemical impedance spectroscopy. A pressure-induced ionic-electronic transition is found to occur at 3.0 GPa. Below 3.0 GPa, the electrical transport mechanism is ionic. The ionic resistance decreases under high pressure, which is caused by the increasing migration rate of O$^{2-}$ ions. Above 3.0 GPa the channels for ion migration is closed. The transport mechanism changes from the ionic to the electronic. First principles calculations show that the bandgap width narrows under high pressures, leading to the continuous decrease of electrical resistance of BiVO$_{4}$. The pressure-induced ionic-electronic transition in BiVO$_{4}$ is obvious and the transition pressure is moderate and easy to detect, indicating that BiVO$_{4}$ can be applied as a new type of pressure calibrator or stress controlled ionic-electronic switch. References Band Edge Electronic Structure of BiVO 4 : Elucidating the Role of the Bi s and V d OrbitalsDoping properties of monoclinic BiVO 4 studied by first-principles density-functional theoryPhotocatalytic O2 evolution under visible light irradiation on BiVO4 in aqueous AgNO3 solutionPhotoelectrochemical water splitting using visible-light-responsive BiVO4 fine particles prepared in an aqueous acetic acid solutionEfficient photoelectrochemical hydrogen production from bismuth vanadate-decorated tungsten trioxide helix nanostructuresNature and Light Dependence of Bulk Recombination in Co-Pi-Catalyzed BiVO 4 PhotoanodesEfficient BiVO 4 Thin Film Photoanodes Modified with Cobalt Phosphate Catalyst and W-dopingHighly Improved Quantum Efficiencies for Thin Film BiVO 4 PhotoanodesThe Origin of Slow Carrier Transport in BiVO 4 Thin Film Photoanodes: A Time-Resolved Microwave Conductivity StudyFacile fabrication of BiVO 4 nanofilms with controlled pore size and their photoelectrochemical performancesFactors in the Metal Doping of BiVO 4 for Improved Photoelectrocatalytic Activity as Studied by Scanning Electrochemical Microscopy and First-Principles Density-Functional CalculationNew high-pressure phases of lithiumPressure-driven dome-shaped superconductivity and electronic structural evolution in tungsten ditelluridePressure-induced semiconducting to metallic transition in multilayered molybdenum disulphideStructural Investigation of Solid Methane at High PressureStructures and novel superconductivity of hydrogen-rich compounds under high pressuresHigh-pressure dielectric behavior of BaMoO 4 : a combined experimental and theoretical studyHigh Pressure Raman, Optical Absorption, and Resistivity Study of SrCrO 4Effect of crystallization water on the structural and electrical properties of CuWO 4 under high pressureNew diamond anvil cell system for in situ resistance measurement under extreme conditionsStructural stability, band structure and optical properties of different BiVO4 phases under pressureIonic transport properties in AgCl under high pressuresModeling of carrier mobility against carrier concentration in arsenic-, phosphorus-, and boron-doped siliconCharge-carrier concentration dependence of the hopping mobility in organic materials with Gaussian disorderCharge carrier mobility in doped semiconducting polymers
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