Enhancing the Photocatalytic Activity of BiVO4 for Oxygen Evolution by

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Enhancing the Photocatalytic Activity of BiVO4 for Oxygen Evolution by Ce Doping: Ce3+ Ions as Hole Traps Zaiyong Jiang,† Yuanyuan Liu,*,† Tao Jing,‡ Baibiao Huang,*,† Xiaoyang Zhang,† Xiaoyan Qin,† Ying Dai,*,‡ and Myung-Hwan Whangbo§ †

State Key Laboratory of Crystal Materials and ‡School of Physics, Shandong University, Jinan 250100, P. R. China § Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: To enhance the photocatalytic activity of monoclinic BiVO4 for O2 evolution from water, Ce-doped BiVO4 was prepared using the one-pot facile solvothermal method and characterized via XRD, Raman, XPS, and electrochemical impedance spectroscopy (EIS). The XPS spectra confirm that Ce component is Ce3+ ions instead of Ce4+ ions. From the structural characterization and the calculations of formation energies it has been stated that the doping of Ce3+ ions takes place at Bi3+ sites without changing the host structure. The as-prepared Cedoped BiVO4 samples display significantly enhanced photocatalytic O2 evolution activities from water compared to pristine BiVO4. Density of states calculations indicate that Ce3+ ions act as hole traps, thereby delaying the recombination of photogenerated electrons and holes. The results demonstrate that the substitution of the remaining monoclinic crystal structure may offer an attractive alternative approach for the doping of BiVO4 to enhance the evolution activity of photocatalytic O2. ization,27 which is very advantageous for O2 evolution. The distortion of [VO4] tetrahedron chains plays a negative effect on the O2 evolution by BiVO4. Therefore, it is important to explore a new doped BiVO4 system, which not only keeps the monoclinic crystal structure but also slows the recombination of photogenerated electron−hole pairs. These considerations led us to choose Ce3+ (f1) ions as dopants to improve the photocatalytic activity of BiVO4. The ionic radius of Ce3+ is similar to that of Bi3+ (s2) ion but is quite different from that of V5+ ion (102, 103, and 54 pm for Ce3+, Bi3+, and V5+, respectively). Consequently, Ce3+ doping is most likely lead to occupy the Bi3+ site so that the distortion of [VO4] tetrahedron chains is largely suppressed, which is favor of keeping the monoclinic structure. In our work, we prepare Ce3+ (f1)-doped BiVO4 photocatalysts using one-pot facile solvothermal method and show that Ce3+ dopants in Ce-BiVO4 are uniformly dispersed with almost no distortion of the crystal lattice. We characterize CeBiVO4 samples with various amounts of Ce3+. In addition, we carried out density functional theory (DFT) calculations on Ce-BiVO4 to characterize the defect states the Ce3+ dopants produce. Our work shows that Ce-BiVO4 has a substantially higher photocatalytic activity than the pristine BiVO4 does and that the Ce3+ dopants form states above the VBM. Therefore, the enhanced photocatalytic activity of Ce-BiVO4 indicates that Ce3+ ions act as hole traps, thereby slowing the recombination of photogenerated electrons and holes.

1. INTRODUCTION Since the first report of the photocatalytic water splitting by TiO2 in 1972,1 semiconductor photocatalysts have been extensively studied in meeting renewable energy demands and solving environmental pollution problems,2,3 leading to a variety of semiconductor photocatalysts. For example, TiO2,4 BiVO4,5 ZnS,6 Bi2WO6,7 SrTiO3,8 and BiOX (Cl, Br, I)9,10 are widely applied to water/air purification or water splitting under solar light irradiation. Among them, monoclinic bismuth vanadate (hereafter BiVO4) is promising for solar energy conversion because it is a nontoxic, low-cost, photostable, and eco-friendly photocatalyst with a relatively narrow bandgap of about 2.4 eV.11 However, it has been reported that BiVO4 usually exhibits poor photocatalytic property because of low mobility of photogenerated charge carriers and high recombination rates of photogenerated electron−hole pairs.12−14 These two major disadvantages significantly limit the practical applications of BiVO4. Therefore, it is of great interest to find how the photocatalytic activity of BiVO4 can be enhanced. Various methods have been proposed to enhance the photocatalytic activity of BiVO4, including doping,15 semiconductor recombination,16 depositing the cocatalysts,17,18 crystal-facet control, and morphology control.19 The doping method is an efficient strategy for improving the photocatalytic performance, as has been reported in recent studies on Cdoped BiVO4,20 Mo-doped BiVO4,21 W-doped BiVO4,22 Pdoped BiVO4,23 B-doped BiVO4,24 and V-doped BiVO4.25 All these doped BiVO4 systems exhibit outstanding activities under visible light irradiation. The substitution at the V-sites is very common in BiVO4, which tends to distort the [VO4] tetrahedral chains triggering the phase transition from the monoclinic to the tetragonal structure.26 The monoclinic BiVO4 exhibits weak hole local© XXXX American Chemical Society

Received: November 5, 2015 Revised: January 6, 2016

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DOI: 10.1021/acs.jpcc.5b10856 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2.4. Electrochemical Impedance Spectroscopy (EIS). EIS measurements were performed with a CHI 660C electrochemical workstation. A 300 W Xe arc lamp was utilized as the light source. Samples (Ce-BiVO4 (C) and pristine BiVO4) were spin-coated on a 1.5 × 1.5 cm2 ITO glass electrode. The ITO glass was used as working electrodes. In addition, a Pt was employed as the counter electrode, and a saturated calomel electrode was used as the reference electrode with 0.2 M Na2SO4 solution as the electrolyte. The pH value of the Na2SO4 solution is 5.83. The photocurrents of the samples (Ce-BiVO4 (C) and pristine BiVO4) were also measured via the same methods. 2.5. Computational Method. A 2 × 2 × 1 supercell with 96 atoms was used to simulate the doping effects, corresponding to the atomic concentration of 1.04%. The kspace integrations were carried out utilizing the Monkhorst− Pack grid with 2 × 2 × 2 k-points in the Brillouin zone.29 The convergence threshold for self-consistent iteration is set at 10−5 eV. The lattice parameters were fixed to the experiment values, and all atomic structures were fully relaxed until the residual forces on all atoms were smaller than 0.02 eV/Å. The electronic structure of Ce-doped BiVO4 was examined by performing DFT calculations as implemented in the Vienna ab initio simulation package (VASP)30 with the PBE exchangecorrelation functional31 and the plane wave cutoff energy of 400 eV. The effect of strong electron correlation at the Ce site was treated by performing DFT+U calculations with effective onsite repulsion on Ce, U(Ce) = 3−8 eV.

2. EXPERIMENTAL SECTION 2.1. Hydrothermal Synthesis of Ce-BiVO4. Bi(NO3)3· 5H2O (0.32 g, 0.66 mmol) and polyvinylpyrrolidone (PVP, K30) were dissolved in 50 mL of ethylene glycol and continually stirred for 30 min; meanwhile, an appropriate amount of Ce(NO3)3·6H2O was added into the above mixed solution to form A-solution, and stoichiometric NH4VO3 was added into 30 mL of deionized water and stirred for 40 min to form Bsolution. Then the B-solution was dropped into A-solution under magnetic stirring and continued to stir the suspension for about 30 min. The mixed suspension was transferred into a 100 mL autoclave, which was heated and maintained at 453 K for 10 h and cooled down to room temperature. The product was washed three times with deionized water and three times with ethyl alcohol to remove any ionic residual and then dried in an oven at 333 K for 6 h.28 Ce-BiVO4 samples with different Ce3+ concentrations were synthesized by adding 0.0066 mmol (Ce/ Bi = 0.01), 0.033 mmol (Ce/Bi = 0.05), 0.066 mmol (Ce/Bi = 0.1), and 0.132 mmol (Ce/Bi = 0.2) of Ce(NO3)3·6H2O, and the corresponding products were marked as Ce-BiVO4(A), CeBiVO4(B), Ce-BiVO4(C), and Ce-BiVO4(D). The ratio of Ce/ Bi is the molar ratio. Likewise, pristine BiVO4 was also prepared keeping the same procedure described above without adding Ce(NO3)3·6H2O. 2.2. Characterization. Powder X-ray diffraction (XRD) analyses were carried out with a Bruker AXS D8 advance powder diffractometer with Cu Kα X-ray radiation. The morphologies of the samples were investigated on a Hitachi S-4800 microscope with an accelerating voltage of 7.0 kV. Raman spectra were measured on a Horiba LabRAM HR system. The chemical element mapping was performed by an energy dispersive X-ray spectrometer equipped in the SEM machine. UV−vis diffuse reflectance spectra were carried out with the wavelength range 800−200 nm on a Shimadzu UV 2550 recording spectrophotometer equipped with an integrating sphere. The XPS was analyzed using a Thermo Fisher Scientific (ESCALAB 250) X-ray photoelectron spectrometer, and C 1s (284.6 eV) was used to calibrate the peak positions of the elements. 2.3. Photocatalytic Water Oxidation Activity Measurement. The photocatalytic water oxidation activity of the prepared Ce-BiVO4 powder samples was carried out in 100 mL of aqueous AgNO3 solution (0.015 M) under UV−vis from a 300 W Xe lamp (PLS-SXE300, Beijing Trusttech Co., Ltd.). 100 mg of the photocatalyst was poured into the AgNO3 aqueous solution kept at 20−25 °C in a quartz reactor, subsequently sealed the reactor, and vacuum-dried for 30 min to remove the residual oxygen. The quantities of O2 production were analyzed by gas chromatography with a thermal conductivity detector. For comparison, the photocatalytic water oxidation activity of pristine BiVO4 was also investigated under the same conditions. In addition, the photocatalytic oxygen evolution reaction of Ce-BiVO4 (C) and pristine BiVO4 photocataltysts were also performed under visible light irradiation using the same method as described above. The visible light source is the same as the UV−vis light irradiation except the use of a 420 nm cutoff filter, which prevents the UV light (wavelength shorter than 420 nm) from reaching the solution. The irradiance of the lamp just above the solution has been measured, and the values with and without the cutoff filter are 138.85 and 285.60 mW/cm2, respectively.

3. RESULTS AND DISCUSSION The XRD patterns of pristine BiVO4 and all Ce-BiVO4 samples are shown in Figure 1. As can be seen, all diffraction peaks are

Figure 1. XRD patterns of pristine BiVO4 and Ce-BiVO4.

perfectly indexed to the monoclinic BiVO4 (JCPDS, No. 751866). There are no diffraction peaks of impurities such as CeO2, Ce2O3, or tetragonal BiVO4. Furthermore, Ce-doping does not change the peak position or induce a phase transition from the monoclinic to the tetragonal (scheelite) structure. In other words, Ce-doping does not change the crystal structure of BiVO4. The same oxidation state and similar ionic radii of Bi3+ and Ce3+ play essential roles in stabilizing the monoclinic structure as mentioned above. The strains of the pristine BiVO4 and Ce-BiVO4 (C) were further analyzed using the program Jade 6, and the results are shown in Figures S1 and S2. A slight increase of strain from 0.364% (BiVO4) to 0.64% (Ce-BiVO4 (C)) is observed, which suggest the effective insertion of Ce in the BiVO4 structure. B

DOI: 10.1021/acs.jpcc.5b10856 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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V, and Ce are homogeneously distributed, demonstrating a uniform distribution of Ce in Ce-BiVO4. To identify the valence state of Ce, XPS spectra of Ce-BiVO4 (C) were carried out (Figure 4). For comparation, XPS spectra

To investigate the doping sites of Ce in Ce-BiVO4, Raman spectra of pristine BiVO4 and Ce-BiVO4 samples were carried out, and the results are shown in Figure 2a. All samples exhibit

Figure 2. (a) Raman spectra for pristine BiVO4 and Ce-BiVO4. (b) UV−vis absorption spectra of pristine BiVO4 and Ce-BiVO4. Inset of (b): plots of (ahν)2 vs hv reveal the Eg of BiVO4 and Ce-BiVO4.

three typical vibrational peaks at 815, 133, and 200 cm−1, which are assigned to the symmetric stretching, translation, and rotation modes of VO43− units, respectively. In terms of both peak position and shape, no difference can be discerned. This indicates that the VO4 tetrahedra do not change after Ce doping, most probably because Ce3+ ions occupy the Bi3+ sites rather than V5+ sites of BiVO4. The smaller formation energy of Ce in the Bi site (−3.85 eV) than that in the V site (1.85 eV) further confirms the Raman conclusion theoretically. Figure 2b illustrate the UV−vis absorption spectra of BiVO4 and CeBiVO4 samples, and their band gaps can be estimated based on the formula ahν = A(hν − Eg)n/2,32 where a and A represent the absorption coefficient and a constant, respectively. BiVO4 is a direct band gap semiconductor.33,34 Therefore, the value of n is 1. As shown in the inset of Figure 2b, the absorption edges of Ce-BiVO4 are similar to that of BiVO4, and Ce doping does not obviously change the band gap of BiVO4. In addition, no photoluminescence can be observed. The morphologies of BiVO4 and Ce-BiVO4 samples were examined by scanning electron microscopy (SEM). As shown in Figure 3a−d and Figure S3, the morphologies are almost

Figure 4. High-resolution XPS spectra of Ce-BiVO4 (C) and pristine BiVO4: (a) Bi 4f, (b) O 1s, (c) V 2p, and (d) Ce 3d.

of pristine BiVO4 were also performed. The binding energies of Bi 4f5/2 and Bi 4f7/2, located at 164.2 and 158.9 eV, respectively, are observed for both BiVO4 and Ce-BiVO4 (C) (Figure 4a). Similarly, the binding energies of the O 2p show a negligible difference between BiVO4 and Ce-BiVO4 (C). The two peaks at 529.5 and 531.3 eV are ascribed to the lattice oxygen of BiVO4 crystal and the OH groups formed on the surface of both samples, respectively (Figure 4b). In contrast, the V 2p states at 516.3 and 523.9 eV for BiVO4 are slightly shifted to 516.5 and 524.3 eV in Ce-BiVO4 (C) (Figure 4c). This shift may be caused by the different electronegativities of Bi3+ and Ce3+. The above results support the conclusion that Ce3+ ions substitute for Bi3+ ions in BiVO4. The XPS peak of Ce3+ ions can be fitted into two sets of two Gaussian peaks at (881.7, 885.6 eV) and (900.1, 904.2 eV) (Figure 4d). These can be assigned to the Ce3+ 3d5/2 and 3d3/2, respectively.35 No peaks corresponding to the Ce4+ 4f states are observed. In addition, the Ce content of the Ce-BiVO4 (C) was determined to be 1.12% via the XPS spectra. The photocatalytic water oxidation activities over pristine BiVO4 and Ce-BiVO4 samples with different Ce3+ concentration were investigated in 100 mL of aqueous AgNO3 solution under UV−vis light irradiation. Here, AgNO3 is used as a sacrificial reagent. As shown in Figure 5a, it is observed that the O2 evolution activities for all Ce-BiVO4 samples are significantly improved compared to pristine BiVO4. Among them, the Ce-BiVO4(C) sample exhibits the highest activity of O2 evolution. After 4 h irradiation, the amount of O2 evolution is significantly higher than that from pristine BiVO4, about 4 times. Meanwhile, the photocatalytic oxygen evolution rates of Ce-BiVO4 (C) and pristine BiVO4 were also compared under visible light irradiation (Figure 5b). After 4 h irradiation, CeBiVO4 (C) also displays a higher photocatalytic activity than does BiVO4 (about 4 times as well). These results show that the substitution of Ce3+ ions for Bi3+ enhances the photocatalytic activity of BiVO4. Electrochemical impedance spec-

Figure 3. (a, b) SEM images of pristine BiVO4. (c, d) Ce-BiVO4(C). (e) Chemical element mapping data of Ce-BiVO4 (C).

identical, indicating the morphology of BiVO4 is retained after Ce doping. The EDS spectra and TEM images are shown in Figures S4−S7 and Figure S8. In addition, to confirm the distribution of Ce element over Ce-BiVO4, chemical element mapping analyses were carried out (Figure 3e and Figures S9− S11). In selected regions of the samples, the elements of Bi, O, C

DOI: 10.1021/acs.jpcc.5b10856 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 6. (a) PDOS plots of Ce-BiVO4 calculated using U(Ce) = 6 eV. (b) Schematic view of the essential electronic structure of Ce-BiVO4. The filled and empty boxes represent the VB and CB, respectively. The up-spin f1 level of a Ce3+ dopant ion lies above, but close to, the VBM. (c) The role of a dopant Ce3+ ion as a hole trap in Ce-BiVO4.

essential feature of the electronic structure of Ce-BiVO4 can be represented as in Figure 6b. Then, the enhanced photocatalytic activity of Ce-BiVO4 is explained by considering that the upspin f1 level of Ce3+ lying slightly above the VBM can trap the photogenerated holes (Figure 6c). This will prevent a quick recombination of photogenerated electrons and holes. The hole on Ce4+ can capture an electron from the VB leading to Ce3+, making a hole on the VBM. Therefore, Ce doping enhance the photocatalytic water oxidation via prolonging the lifetime of photogenerated holes, rather than Ce4+ directly oxidize water. Moreover, that Ce dopants in Ce-BiVO4 remain the monoclinic structure, which also plays an important role in the improved photocatalytic performance. An excessive amount of Ce3+ ions increases the amount of doping-induced defects that act as electron−hole recombination sites. This explains why the O2 evolution rate of Ce-BiVO4 (D) is lower than that of Ce-BiVO4 (C).

Figure 5. (a) Photocatalytic O2 evolution for BiVO4 and Ce-BiVO4 under UV−vis light irradiation and (b) photocatalytic O2 evolution for BiVO4 and Ce-BiVO4 (C) under visible light irradiation. (c) EIS of Ce-BiVO4 (C)/ITO and BiVO4/ITO under UV−vis light illumination. The electrolyte solution was 0.2 M Na2SO4 aqueous solution.

troscopy (EIS) measurements of Ce-BiVO4 (C) and pristine BiVO4 were further carried out to investigate the charge transfer resistance and the separation efficiency. As shown in Figure 5c, the arc radius of Ce-BiVO4 (C) under UV−vis light is smaller than that of BiVO4, indicating that Ce-BiVO4 (C) has a lower resistance than does BiVO4 and has an accelerated interfacial charge-transfer process.36 The photocurrent measurements are presented in Figure S12. To understand the main reason for the photocatalytic activity, the BET surface areas of pristine BiVO4 and Ce-BiVO4 (C) were determined to be 11.19 and 14.59 m2 g−1, respectively. This results imply that the surface area is not the primary cause for affecting the photocatalytic activities of the samples. To clarify the mechanism of the enhanced photocatalytic activity of Ce-BiVO4, its electronic structure was examined. The PDOS plot obtained for Ce-BiVO4 by GGA+U calculations with U(Ce) = 6 eV is presented in Figure 6a. The corresponding plots obtained for other U(Ce) values (3−8 eV) are presented in Figure S13. In all cases of U(Ce), the VB and CB of Ce-BiVO4 are practically the same as those of BiVO4. With U(Ce) = 3 eV, the up-spin f1 level of Ce3+ lies slightly below the CBM. With increasing the value of U(Ce), the up-spin f1 level is gradually lowered in energy such that it lies lightly above the VBM at U(Ce) = 6 eV but lies below the VBM for U(Ce) ≥ 7 eV. Since Ce is slightly less electronegative than Bi (1.12 vs 2.02), the up-spin f1 level should lie above the VBM of BiVO4. In addition, the 4f orbital is more contracted than 3d orbital; the on-site repulsion should be greater for Ce than that for a 3d element such as Fe for which U(Fe) = 4−5 eV. Consequently, for Ce-BiVO4, our GGA+U results obtained with U(Ce) ≥ 6 eV would be more appropriate. Therefore, the

4. CONCLUSIONS In conclusion, the substitution of Ce3+ for Bi3+ in BiVO4 was achieved and confirmed via XRD, Raman, XPS, electrochemical impedance spectroscopy (EIS), and the density of states calculation. Because of the unique properties of Ce3+ (the same valence state and similar ion radius with Bi3+), Ce3+ doping remains the crystal structure of BiVO4, and no defect state is observed. Compared to pristine BiVO4, the photocatalytic water oxidation activity of Ce-BiVO4 is enhanced. Density of states calculation for Ce-BiVO4 indicates that the enhancement of photocatalytic performance for water oxidation most probably because the dopant Ce3+ ions act as traps for photogenerated holes. Engineering the substitution of Ce3+ for Bi3+ on the BiVO4 may open a new avenue for the modification of BiVO4 photocatalyst, which is expected to show more potential applications in water splitting of BiVO4 photocatalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10856. Additional data giving more details about the SEM images, TEM patterns, EDS, photocurrent, chemical element mapping data, and PDOS plots of Ce-BiVO4 (PDF) D

DOI: 10.1021/acs.jpcc.5b10856 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*Tel +86-531-8836-6324; e-mail [email protected] (Y.L.). *Tel +86-531-8836-6324; e-mail [email protected] (B.H.). *Tel +86-531-8836-6324; e-mail [email protected] (Y.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (the 973 Program, No. 2013CB632401), the National Natural Science Foundation of China (No. 21573135, 21333006, 21007031, 11374190, and 51321091), Taishan Scholar Foundation of Shandong Province, China, and the Shandong Province Natural Science Foundation (ZR2014JL008).

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DOI: 10.1021/acs.jpcc.5b10856 J. Phys. Chem. C XXXX, XXX, XXX−XXX