Ni-Doped BiVO4 with V4+ Species and Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting
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Bismuth vanadate is a promising photoanode material for photoelectrochemical (PEC) water splitting, but its activity and stability need to be further improved. In this work, we synthesized Ni-doped BiVO4 abundant with V4+ species and oxygen defects through an in situ electrodeposition method. The effective doping can decrease the particle size of BiVO4 and lead to the formation of V4+ species/oxygen defects. Accordingly, the doped and defective BiVO4 showed high optical absorption and rapid charge transfer, and further showed much higher PEC activity than pure BiVO4. Specifically, 5-Ni-BiVO4 exhibits the highest activity in PEC water splitting, with a photocurrent of 2.39 mA/cm2 at 1.23 V versus RHE (the reversible hydrogen electrode), which is 2.5 times higher than pure BiVO4 (0.94 mA/cm2), and much higher incident photon-to-current efficiency (IPCE) value of 45% (while only 25% for BiVO4 at ca. 400 nm). This work provides an in situ method for the development of a high-performance photoanode.
KeywordsBiVO4 Doping Photoelectrochemistry V4+ species Oxygen vacancies
As the energy crisis and environmental pollution have become serious issues in the past decades , sustainable energy needs to be explored, and the utilization of solar energy has drawn much attention recently [2, 3, 4]. Specifically, photoelectrochemical (PEC) water splitting has been regarded as a promising pathway to convert solar energy to hydrogen fuel .
Since Fujishima and Honda  first applied TiO2 as a photoanode material for PEC water splitting, much research has been carried out to explore the active photoanodes, such as WO3, Fe2O3, ZnO, BiVO4 [7, 8, 9, 10, 11, 12]. Among them, bismuth vanadate (m-BiVO4) is an ideal material, attributed to its suitable band gap of 2.4 eV [13, 14, 15, 16] and the ideal PEC current density of 7.6 mA/cm2 under solar light .
However, the rapid recombination of the photo-induced electron–hole pairs in BiVO4 restricts its PEC properties. Recently, several methods have been applied to improve its PEC performance, such as morphology control [15, 16], heterojunction [17, 18, 19, 20], surface modification [12, 21], and element doping [22, 23, 24, 25, 26, 27]. Among them, doping by metal ions is a promising approach to tune the electronic and band structures, which may also cause surface defects and further improve the charge separation [26, 27, 28, 29, 30, 31]. Recently, Ni ions were applied to narrow the band gap of BiVO4 [32, 33, 34, 35], and the atom radii difference between Ni2+ (0.78 Å) and Bi3+ (1.03 Å) could lead to abundant surface defects, which are beneficial to optical absorption and charge transfer .
In this work, we used an in situ electrodeposition method to fabricate a Ni-doped BiVO4 photoanode for PEC water splitting. The doping of Ni ions into the BiVO4 lattice decreases the particle size and causes abundant V4+ species and oxygen defects, which shorten the migration length of charge carriers and enhance optical absorption. Therefore, 5-Ni-BiVO4 exhibits very high PEC performance, with a photocurrent of 2.39 mA/cm2 at 1.23 V versus RHE (the reversible hydrogen electrode) and an incident photon-to-current conversion efficiency (IPCE) value of 45% (at 400 nm).
Bi(NO3)3·H2O, Ni(NO3)2·6H2O, VO(acac)2, KI, p-benzoquinone, KOH, and absolute ethanol were all obtained from Aladdin Chemicals Co., Ltd. (China). Dimethyl sulfoxide (DMSO) and HNO3 were purchased from Tianjin Jiangtian Chemical Institute. Milli-Q ultrapure water (> 18 mΩ/cm) was used in all experiments. All reagents were of analytical grade and used without further purification.
Synthesis of Doped BiVO4
0.4 mol/L KI solution (pH = 1.7), Bi(NO3)3·H2O (0.97 g), and Ni(NO3)2·6H2O (0%, 3%, 5%, 10% of Bi source) were dissolved in ultrapure water, which was then mixed with 20 mL of absolute ethanol containing 0.497 g p-benzoquinone (as an electrolyte). Then, a piece of fluorine-doped tin oxide (FTO), a saturated Ag/AgCl electrode, and a platinum counter were used as a working electrode, reference electrode, and counter electrode in a three-electrode cell for electrodeposition, respectively. The electrodeposition of BiOI electrode was then operated potentiostatically at − 0.1 V versus Ag/AgCl for 3 min.
Then, 0.14 mL of DMSO solution containing 0.106 g VO(acac)2 was dripped on the BiOI electrode, which was dried in a vacuum at 40 °C for 8 h. Afterward, the electrodes were calcined at 450 °C for 2 h with an increasing rate of 2 °C/min to obtain BiVO4. Then, 1.0 mol/L KOH was used to rinse the surface V2O5. The Ni-doped BiVO4 with varied proportions was named as 3-Ni-BiVO4, 5-Ni-BiVO4, and 10-Ni-BiVO4.
X-ray diffraction (XRD) characterization was applied to analyze the crystalline phase of samples with a diffractometer by Germany Brooke AXSCO. A field emission scanning electron microscope (FE-SEM, Hitachi S-4800) was used to investigate the morphology. High-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F-20) and the attached energy dispersive spectrometer (EDS) were applied to obtain the microstructures and the elemental distribution. The X-ray photoelectron spectrum (XPS) was used to characterize the surface composition of samples, which was conducted with a PHI-1600 X-ray photoelectron spectroscope equipped with Al Kα radiation. Raman spectra were carried out with a Raman spectrometer (DXR Microscope), and the excitation source was a green semiconductor laser (532 nm). Steady-state photoluminescence (PL) spectra were measured by a Horiba Jobin–Yvon Fluorolog 3-21 with the excitation light at 420 nm. The UV–Vis diffuse reflectance spectra (UV–Vis DRS) were recorded on a Shimadzu U-2600 spectrometer equipped with a 60-mm-diameter integrating sphere of BaSO4. The concentration of Ni was detected by inductively coupled plasma-mass spectrometry (ICP-MS) (ICP-MS XSERIES 2, Thermo Fisher Scientific).
A typical three-electrode cell and a CHI 660E electrochemical workstation were applied to evaluate the PEC performance of the BiVO4-based photoanodes. The simulated solar illumination source was a 300 W Xe arc lamp (100 mW/cm2, PLS-SXE300UV, Beijing Trusttech. Co. Ltd). The electrolyte for the photocurrent measurements was a 0.2 mol/L Na2SO4 aqueous solution. Linear sweep voltammetry (LSV) was monitored while sweeping the potential to the positive direction with a scan rate of 10 mV/s. The potential versus Ag/AgCl reference electrode was converted to the reversible hydrogen electrode (RHE) according to the Nernst equation: E (vs. RHE) = E (vs. Ag/AgCl) + 0.0591 pH + 0.197. Electrochemical impedance spectroscopy (EIS) measurement was taken with a sinusoidal ac perturbation of 10 mV applied over the frequency range of 0.1–105 Hz.
Results and Discussion
Crystal Structure and Morphology
In the Raman spectra (Fig. 1c), the characteristic vibration peaks of all samples refer to the crystal structure of monoclinic scheelite BiVO4, and the Ni doping gradually decreased the crystallization of BiVO4, which may have caused surface defects. The peak at 215.9 cm−1 was attributed to the external vibration of BiVO4. The peaks at 330.6 cm−1 and 373.1 cm−1 were caused by V–O asymmetrical vibration and V–O bending vibration in the VO43− unit, while the peaks at 717 and 834.0 cm−1 could be attributed to two different modes of stretching vibration of V–O [39, 40, 41, 42]. Ni-BiVO4 samples showed similar Raman peaks with BiVO4, but the band position at 834.0 cm−1 right-shifted gradually with increasing Ni doping concentration (Fig. 1d), indicating the gradual deformation of VO43– . This should be caused by the much smaller atomic radius of doped Ni2+ (0.78 Å) rather than Bi3+ (1.03 Å), leading to the crystal deformation and surface defects.
It is worth noting that the concentrations of V4+ species and oxygen vacancies increased with the doping concentration, but stayed constant when the doping concentration was ≥ 5%. From Fig. 3d, the binding energies of the dominant Ni 2p XPS peaks were found to locate at 855.7, 861.3, 873.6 and 879.8 eV, and contributed to the Ni 2p3/2 main peak, Ni 2p1/2 main peak, Ni 2p3/2 satellite peak, and Ni 2p1/2 satellite peak, respectively. The peak intensity strengthened with the amount of Ni doping, confirming the effective Ni doping in the BiVO4 lattice. The decreasing concentration of free electrons in Ni-BiVO4 has been reported to stabilize the oxygen defects .
As shown in Fig. 4a, the band gap of BiVO4 was ca. 2.43 eV, while those of Ni-doped BiVO4 were 2.42 eV for 3-Ni-BiVO4, 2.39 eV for 5-Ni-BiVO4, and 2.38 eV for 10-Ni-BiVO4. The decrease in band gap should be attributed to the introduced band levels of Ni-doping-mediated V4+ species and oxygen defects.
Photoluminescence (PL) spectra were performed to investigate the charge carrier recombination. As shown in Fig. 4b, the main emission peaks of all samples were at ca. 530 nm, for the recombination of extrinsic radiative transitions . Notably, Ni-doped BiVO4 samples exhibited a lower emission peak intensity than BiVO4, suggesting that the recombination of charge pairs was inhibited and more holes transferred to the surface to trigger the water oxidation reaction . However, too many dopants (10-Ni-BiVO4) lead to an increase in emission peak intensity, attributed to the fact that part of the foreign Ni species acts at the recombination sites and suppresses the PEC performance .
In electrochemical impedance spectra (EIS), the arc radius of Nyquist plots can be used to evaluate the charge transfer resistance (Rct) at the semiconductor/electrolyte interface, and the smaller arc radius implies lower Rct. The Rct was calculated based on the fitting model, as seen in the inset of Fig. 5d, and the data are shown in Table S1. The Rct was in the order of Rct(5-Ni-BiVO4) < Rct(3-Ni-BiVO4) < Rct(10-Ni-BiVO4) < Rct(BiVO4). Obviously, the conductivity of BiVO4 was significantly decreased by Ni doping and surface defects, and 5-Ni-BiVO4 showed the lowest resistivity, consistent with the PEC performances.
Therefore, the optimal Ni doping (5-Ni-BiVO4) can decrease the BiVO4 particle size and form abundant surface V4+ and oxygen defects, which then introduces new band levels in the band gap and accelerates the charge transfer , benefiting optical absorption, charge separation efficiency, and finally the PEC performance.
We synthesized Ni-doped BiVO4 through an in situ electrodeposition method. The doping can decrease the particle size of BiVO4, and lead to the formation of V4+ species and oxygen defects. Accordingly, the doped BiVO4 shows high optical absorption and rapid charge transfer. Therefore, Ni-doped BiVO4 shows a much higher PEC activity than the pure form. Specifically, 5-Ni-BiVO4 exhibits the best PEC performance, with the photocurrent as high as 2.39 mA/cm2 at 1.23 V versus RHE, a high IPCE (400 nm) value of 45%, and good PEC stability.
The authors are grateful for the support from the National Natural Science Foundation of China (Nos. 51661145026, 21506156, 21676193) and the Tianjin Municipal Natural Science Foundation (No. 16JCQNJC05200).
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