Effects of Surface Electrochemical Pretreatment on the

Article pubs.acs.org/JPCC

Effects of Surface Electrochemical Pretreatment on the Photoelectrochemical Performance of Mo-Doped BiVO4 Wenjun Luo,† Zhaosheng Li,*,†,‡ Tao Yu,† and Zhigang Zou*,† †

Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing National Laboratory of Microstructures, Department of Physics, and ‡College of Engineering and Applied Sciences, Nanjing University, 22 Hankou Road, Nanjing 210093, People' Republic of China ABSTRACT: The surface pretreatment by electrochemical cyclic voltammetry (CV) in the dark was found to remarkably enhance the photocurrent of Mo-doped BiVO4 from the front side illumination. The variation of the samples before and after the surface pretreatment was investigated by scanning electron microscopy, X-ray photoelectron spectroscopy, and Mott− Schottky methods. The results showed that the photocurrent enhancement came from both the removal of the surface recombination center, including Mo6+ ions, and reoxidation of the reduced species. The part of the reduced ions can be reoxidized in air. However, the photocurrent enhancement from the Mo6+ dissolution can be kept at high potential under illumination. A possible mechanism was also proposed to explain the reason for the photocurrent enhancement.

1. INTRODUCTION Solar-driven electrolysis of water has been a divertive issue, because it is considered as a clean and renewable method of hydrogen production by utilizing solar energy.1 In terms of solar hydrogen production through a photoelectrochemical cell, photoelectrodes are required to be highly stable and cheap and to absorp visible light. Some simple oxides with narrower band gaps, WO3 and Fe2O3, have been studied intensively for decades as photoelectrode materials due to their visible light response and fairly good stability.2−6 Very recently, monoclinic BiVO4 (band gap ∼ 2.4 eV), as a photoanode, has received intensive and growing attention, owing to its high theoretical conversion efficiency of 9.1%.7−16 The surface modification of cocatalysts, such as Co-Pi, Co3O4, and RhO2, is one desirable approach to ameliorate the photoelectrochemical performance of photoelectrodes, including BiVO4. Doping with ions, such as Mo6+ and W6+, has also been carried out to increase the carrier concentration and enhance the photoelectrochemical performance of BiVO4.15,16 There are two kinds of dopant levels, including deep level and shallow level. A deep level usually acts as a recombination center, whereas a shallow level does not. The Mo6+ dopant level is a shallow level; the electrons at the Mo6+ dopant level can be excited at room temperature, which is the main reason for the lower resistance and improved performance in Mo6+-doped BiVO4. The results have been verified by precious calculation and experimental results.15−18 Therefore, a shallow dopant level is usually better than a deep level for a photoelectrode. However, we further found that a surface segregation acted as a recombination center and limited the performance of Modoped BiVO4 photoelectrode. Up to date, there are few reports on the surface recombination on a doped photoelectrode with a shallow level. © 2012 American Chemical Society

Note that segregation, referring to the enrichment of a material constituent, often emerges at a free surface or an internal interface of a material during the preparation. The segregation of the photoelectrode will influence its photoelectrochemical properties. For example, we found that In-rich segregation existed on the surface of the In0.20Ga0.80N electrode, which acted as surface recombination centers of photoinduced electrons and holes. The electrochemical surface treatment was utilized to carry out an ∼150% increase in photocurrent of In0.20Ga0.80N at 400 nm, because the segregation on the surface was removed by surface electrochemical treatment.18 In this study, we focused on a simple pretreatment process to remove the surface recombination center and investigated the details of the surface treatment effect on the Mo-doped BiVO4, in order to apprehend the mechanism of the pretreatment for improving the photoelectrochemical performance. It is also expected that one can develop a universal way to increase the photoelectrochemical properties of the photoelectrode.

2. EXPERIMENTAL SECTION 2.1. Preparation of Samples. The Mo-doped BiVO4 photoelectrode was synthesized by a metal−organic decomposition method.7,16 Bi(NO3)·5H2O in glacial acetic acid (0.2 M), vanadyl acetylacetonate (0.03 mol/L), and molybdenyl acetylacetonate (0.01 M) in acetylacetone were mixed with a 1:1 mole ratio of Bi/(V + Mo). The optimum concentration of Mo doping was 3%. The solution was obtained after 30 min of ultrasonication at room temperature. The solution was coated Received: October 24, 2011 Revised: December 22, 2011 Published: January 24, 2012 5076

dx.doi.org/10.1021/jp210207q | J. Phys. Chem. C 2012, 116, 5076−5081

The Journal of Physical Chemistry C

Article

Figure 1. Cyclic voltammetry curves with different threshold reduction potentials of (a) pure BiVO4 and (b) Mo-doped BiVO4 in the dark. Electrolyte: 0.5 M Na2SO4. pH = 6.5. Threshold reduction potentials: black line, −0.3 V; green line, −0.6 V; blue line, −0.9 V.

Figure 2. Photocurrent variation trend with threshold reduction potential (−0.3, −0.6, and −0.9 V) on the Mo-doped BiVO4 samples under a Xe lamp (λ > 420 nm): (a) from the front side and (b) from the backside. Electrolyte: 0.5 M Na2SO4. pH = 6.5.

respectively. The electrolyte was 0.5 M Na2SO4 aqueous solution (pH = 6.5). Before photoelectrochemical properties of the samples were measured, the pretreatment was carried out as follows. The sample was put into the electrolyte and scanned by cyclic voltammetry for 30 cycles in the dark. The cyclic voltammetry scans were performed at the scan speed of 30 mV/s and with various different threshold reduction potentials (−0.3, −0.6, and −0.9 V). For photoelectrochemical measurement, a xenon lamp was used as a light source and a 420 nm cutoff filter was used to obtain visible light. The samples were illuminated from the back side (FTO substrate/semiconductor interface) and the front side (electrolyte/semiconductor interface), respectively. To investigate the stability of the surface states in vacuum, the Modoped BiVO4 photoelectrode after the pretreatment was immediately put into a cell that was evacuated by a mechanical vacuum pump, and the pressure was about 1 Pa.

on FTO (SnO2:F on glass) substrates by a spin-coater (500 rpm, 10 s) and dried at 150 °C for 10 min, then calcined at 470 °C for 30 min for each layer. The thickness of the films was controlled by the layers of spin-coating and four layers were deposited in a typical sample. An undoped BiVO4 thin film was prepared by the same process as the reference. 2.2. Characterization of Samples. The chemical states and ion ratios of the surface were investigated by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The X-ray source is Al Kα (1486.6 eV). The depth analysis was done by Ar+ etching. The binding energy was calibrated by C 1s (284.8 eV). The Mo-doped BiVO4 was dissolved into 0.5 M HCl solution, and the Mo concentration in the bulk was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima-5300DV, PE, USA).To avoid the surface states reoxidized in air, the Mo-doped BiVO4 was put in N2 as a shielding gas and was removed quickly into the XPS sample chamber to measure after the electrochemical CV pretreatment. The Mott−Schottky curves were measured by using an electrochemical analyzer (Princeton Applied Research, 2273). The electrolyte was 0.5 M Na2SO4 aqueous solution (pH = 6.5). The ac amplitude was 10 mV, and the frequency was 500 Hz. 2.3. Photoelectrochemical Measurement. The photoelectrochemical properties were investigated in a conventional three-electrode cell by using an electrochemical analyzer (CHI633C, Shanghai Chenhua). The prepared films were used as the working electrode, while a Pt wire and an Ag/AgCl electrode were used as the counter and the reference electrodes,

3. RESULTS AND DISCUSSION 3.1. Effect of the Pretreatment on the Photocurrent. Figure 1 shows the cyclic voltammetry curves with different threshold reduction potentials of pure and doped BiVO4 electrodes in Na2SO4 aqueous solution in the dark. The threshold reduction potential is defined as the negative potential maximum during the cyclic voltammetry scan pretreatment. High reduction current is observed at −0.9 V in both pure and Mo-doped BiVO4. The reduction current of Mo-doped BiVO4 is a little higher than that of pure BiVO4. The reduction current at −0.9 V may come from the photoelectrode 5077



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Figure 3. Photocurrent variation trend with threshold reduction potential (−0.3, −0.6, and −0.9 V) on the pure BiVO4 samples under a Xe lamp (λ > 420 nm): (a) from the front side and (b) from the backside. Electrolyte: 0.5 M Na2SO4. pH = 6.5.

or electrolyte reduction. When 1 M KNO3 or 1 M NaCl aqueous solution was used as the electrolyte, the similar oxidation and reduction currents were also observed, which suggested that the reduction or oxidation reactions came from the electrodes themselves instead of the electrolytes. Moreover, unlike pure BiVO4, a much lower oxidation peak of −0.06 V occurred in Mo-doped BiVO4 (see Figure 1b), which may be related to Mo6+ ions. Figure 2 indicates the photocurrent variation trend with threshold reduction potential on the Mo-doped BiVO4 under visible light (λ > 420 nm) irradiation. The photocurrent depends sensitively on the threshold reduction potential. For Mo-doped BiVO4, the photocurrent increases when the threshold reduction potential becomes more negative from the front side (see Figure 2a), whereas the photocurrent shows no obvious change from the back side (see Figure 2b). The photocurrent at 0.6 V vs Ag/AgCl after the pretreatment of a −0.9 V CV scan becomes about 2 times higher than that before the pretreatment. The photocurrent after the electrochemical pretreatment with a more negative potential than −0.9 V, for example, −1 V vs Ag/AgCl, was also measured in order to find its saturated CV potential. However, the photocurrent did not increase further when the CV potential of the electrochemical pretreatment is more negative than −0.9 V, indicating that the saturated CV potential is −0.9 V. However, in the case of pure BiVO 4 samples, the photocurrent whether from the front side or from the back side did not change obviously before and after the electrochemical pretreatment with various threshold reduction potentials (see Figure 3a,b). The results suggest that the photocurrent enhancement may be associated with Mo6+. From the above discussion, the photocurrent of only doped samples from the front side can be improved significantly after enough negative reduction potential pretreatment. Therefore, we will focus on what happens on the Mo-doped BiVO4 when they are scanned by reduction scan pretreatment. 3.2. Effect of Pretreatment on the Electrodes. The morphologies of Mo-doped BiVO4 were investigated by SEM, and the results are shown in Figure 4. The morphology of the photoelectrode does not change obviously, indicating that the photocurrent enhancement has little relation to the morphology. Moreover, to investigate the effect of the electrochemical pretreatment on the photoelectrochemical properties, we tested the Mott−Schottky plots of Mo-doped BiVO4 after different threshold reduction potential pretreatments in Na2SO4 aqueous solution, the results of which are shown in Figure 5. The Mott− Schottky plots are linear when the capacity of the surface states

Figure 4. SEM images of Mo-doped BiVO4 before (a) and after (b) the pretreatment in Na2SO4 solution; the threshold potential was −0.9 V.

Figure 5. Mott−Schottky plots of Mo-doped BiVO4 samples after different threshold reduction potential pretreatments. Electrolyte: 0.5 M Na2SO4. pH = 6.5.

and the Helmholz layer are negligible and only the capacity of the space-charge layer is included, which means that the Mott− Schottky plots are relative mainly to the bulk not to the surface of the samples.19,20 The space-charge capacitance C varies with the potential drop V over the depletion layer according to the Mott−Schottky equation 1 C

2

=

2(V − Vfb − kT /e0) e0 εr ε0Nd

(1)

where C is the capacitance of the space-charge layer; V and Vfb are the electrode potential and the flat potential of the semiconductor electrode, respectively; k is the Boltzmann constant; T is the temperature; e0 is the electron charge; εr is the dielectric constant; ε0 is the permittivity of vacuum; and Nd is the carrier concentration. The variation of the Mott− 5078



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Figure 6. XPS of different metal ions on the surface of Mo-doped BiVO4 before and after CV scans in N2 and 48 h in air: (a) Bi3+, (b) V5+, (c) Mo6+. The pretreatment was carried out in Na2SO4 solution, and the threshold potential was −0.9 V.

Schottky plots is negligible after different threshold reduction potential pretreatments (−0.6 and −0.9 V CV scan), which means that the carrier concentration in the bulk does not change after the pretreatment. Figure 6 indicates the XPS of Bi, V, and Mo on the surface of Mo-doped BiVO4 before and after the electrochemical pretreatment. In Figure 6a, the binding energies of 164.6 and 159.3 eV are assigned to Bi3+ before the reduction scan, according to the previous data.8,21,22 The binding energy of Bi 4f shifts to lower values, 164.2 and 158.9 eV, respectively, after the samples were pretreated and preserved in the N2. The results suggest that the Bi3+ was reduced to Bi+(3−x) with lower valence states.23,24 When the sample after the electrochemical pretreatment was put in air for 48 h, the valence states of Bi+(3−x) can be restored to the initial values. Moreover, the peak intensity of the Bi3+ ion does not change obviously. In Figure 6b, the binding energy of 516.9 eV is assigned to V5+, and the results are in good agreement with the previous data.8 Similar to Bi3+, the V5+ was also reduced after the pretreatment and the valence states can be also restored after the sample was put in air for 48 h.25 However, the peak intensity of V5+ deceases a little after the pretreatment. The binding energy of Mo6+ is indicated in Figure 6c. The peaks at 235.6 and 232.4 eV are assigned to Mo6+.26 The Mo6+ was reduced after the pretreatment and could be restored after the sample was put in air for 48 h.27 The peak intensity of Mo6+ deceases very much after the pretreatment. Table 1 shows the V/Bi and Mo/Bi ratios by XPS on the surface and in the 40 nm depth profile of Mo-doped BiVO4 before and after the pretreatment scan in the dark. Before the pretreatment, the V/Bi ratio is 0.8, which is much less than 1.1 in the bulk. A Bi-rich layer exists on the surface of the BiVO4 electrode. Similar phenomena were also observed in Sayama’s report.7 The Mo/Bi ratio on the surface before the pretreatment is 6%, higher than 4% in the 40 nm depth, which suggests the Mo6+ ions are segregated on the surface. The ICP results show that the Mo concentration in the bulk was 3.5%, close to the value (4%) by XPS in the 40 nm depth profile. The V/Bi

Table 1. V/Bi and Mo/Bi Ratios by XPS on the Surface and in the 40 nm Depth Profile of Mo-Doped BiVO4 before and after the Pretreatment (CV Scan) in the Dark V/Bi ratio before after

Mo/Bi ratio

surface

40 nm depth

surface

40 nm depth

0.8 0.6

1.1 1.1

0.06 0.02

0.04 0.04

and Mo/Bi ratios on the surface decrease after the pretreatment, whereas V/Bi and Mo/Bi do not change in the 40 nm depth. Especially for Mo6+, the ratio of Mo/Bi on the surface decreases from 6% to 2% after the pretreatment. The results suggest that part of V5+ and Mo6+ ions on the surface of the Mo-doped BiVO4 were dissolved into the electrolyte after the pretreatment. We also investigated the pure BiVO4 electrode after the same pretreatment process by XPS and found that the V5+ ions were also dissolved, but the photocurrent did not increase. Therefore, the photocurrent enhancement comes from the dissolution of Mo6+ on the surface. 3.3. Stability of the Photocurrent after the Pretreatment. Since the photocurrent of the Mo-doped BiVO4 from the front side can be enhanced after the pretreatment, the stability of the photocurrent after the pretreatment should be investigated carefully for practical application. Figure 7 shows the stability of the photocurrent in air or in vacuum after the pretreatment. The photocurrent of the Mo-doped BiVO4 after the pretreatment from the front side decreased from 2.6 mA/ cm2 at 0.6 V vs Ag/AgCl to about 2.1 mA/cm2 after the sample was put in air for 12 h, while the photocurrent only decreased to 2.5 mA/cm2 after the sample was put vacuum for the same time. As mentioned above, a part of the photocurrent comes from the reoxidation of the reduced species on the surface. Therefore, the photocurrent from the front side deceased until the reduced species was reoxidized completely. The photocurrent also decreases slowly in vacuum than in air because a trace of O2 exists in vacuum. The photocurrent of the 5079



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3.4. A Possible Mechanism of Photocurrent Enhancement after the Pretreatment. From the above discussion, a possible sketch of the transfer of photogenerated electrons in Mo-doped BiVO 4 is drawn in Figure 9. Some MoO x segregation occurs on the surface of the Mo-doped BiVO4 electrode during the preparation of the photoelectrode. MoOx

Figure 7. Stability of the photocurrent of Mo-doped BiVO4 under a full-arc Xe lamp from the front side before and after pretreatment in air and vacuum for 12 h. The pretreatment was carried out in Na2SO4 solution, and the threshold potential was −0.9 V.

Figure 9. Possible sketch of the transfer of photogenerated electrons in Mo-doped BiVO4 electrodes illuminated from the front side and the back side after the pretreatment.

Mo-doped BiVO4 from the front side increases to 3 times after the pretreatment, while the photocurrent only deceases to 80% in air after 12 h and still much higher than the photocurrent before the pretreatment. The results suggest that the pretreatment makes part of the surface states change irreversibly, that is, the dissolution of Mo6+. In a previous study, we have reported the photocurrent stability of RhO2 loaded Mo-doped BiVO4 from the back side in seawater splitting. It is more meaningful to investigate the photocurrent stability of Mo-doped BiVO4 under continuous illumination from the front side before and after the pretreatment. Figure 8 shows i−t curves of Mo-doped BiVO4 illuminated from the front side in Na2SO4 under visible light

acts as a recombination center and has a negative effect on the photocurrent. After the pretreatment, MoOx on the surface was dissolved into the electrolyte. From the front side, most photogenerated carriers are near the surface and some of them will recombine at the MoOx, thus reducing the photocurrent. After the pretreatment, the photogenerated carriers can be separated and the electrons can transfer to the substrates and form the photocurrent; therefore, the photocurrent from the front side is enhanced after the pretreatment. However, from the back side, most photogenerated carriers are close to the substrates and there is no MoOx (see Table 1). Therefore, photocurrent enhancement is obvious from the front side, whereas there is no photocurrent improvement from the backside.

4. CONCLUSION The photocurrent of Mo-doped BiVO4 under visible light from the front side can be improved significantly after the electrochemical surface pretreatment. The pretreatment makes some metal ions on the surface become reduced and others dissolved into the electrolyte. The photocurrent enhancement mainly comes from the dissolution of Mo6+ ions, which can be kept at high potential and under illumination.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.L.), [email protected] (Z.Z.).

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 50902068, 11174129, 51102132, and 21073090), the National Basic Research Program of China (Grant No. 2011CB933303), the Jiangsu Provincial Science and Technology Research Program (Grant No. BK2011056), the Fundamental Research Funds for the Central Universities, and the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University.

Figure 8. i−t curves of Mo-doped BiVO4 illuminated from the front side in Na2SO4 (a) before the pretreatment and (b) after the pretreatment. Electrode potential: 0.6 V vs Ag/AgCl; Xe lamp (λ > 420 nm). The pretreatment was carried out in Na2SO4 solution, and the threshold potential was −0.9 V.

illumination (λ > 420 nm). In Na2SO4 solution, the photocurrent before the pretreatment decreases sharply within the initial seconds and then a stable photocurrent is obtained. After the sample is pretreated, the stable photocurrent is much higher than that before the pretreatment (see Figure 8, curve b).

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REFERENCES

(1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Grätzel, M. Nature 2001, 414, 338.

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Article

(3) Alexander, B. D.; Kulesza, P. J.; Rutkowska, I.; Solarska, R.; Augustynski, J. J. Mater. Chem. 2008, 18, 2298. (4) Cesar, I.; Kay, A.; Gonzalez Martinez, J. A.; Grätzel, M. J. Am. Chem. Soc. 2006, 128, 4582. (5) Kay, A.; Cesar, I.; Grätzel, M. J. Am. Chem. Soc. 2006, 128, 15714. (6) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. J. Am. Chem. Soc. 2001, 123, 10639. (7) Sayama, K.; Nomura, A.; Arai, T.; Sugita, T.; Abe, R.; Yanagida, M.; Oi, T.; Iwasaki, Y.; Abe, Y.; Sugihara, H. J. Phys. Chem. B 2006, 110, 11352. (8) Long, M.; Cai, W.; Kisch, H. J. Phys. Chem. C 2008, 112, 548. (9) Luo, H.; Mueller, A. H.; McCleskey, T. M.; Burrell, A. K.; Bauer, E.; Jia, Q. X. J. Phys. Chem. C 2008, 112, 6099. (10) Sayama, K.; Wang, N.; Miseki, Y.; Kusama, H.; OnozawaKomatsuzaki, N.; Sugihara, H. Chem. Lett. 2010, 39, 17. (11) Ye, H.; Lee, J.; Jang, J.; Bard, A. J. J. Phys. Chem. C 2010, 114, 13322. (12) Iwase, A.; Kudo, A. J. Mater. Chem. 2010, 20, 7536. (13) Li, M.; Zhao, L.; Guo, L. Int. J. Hydrogen Energy 2010, 35, 7127. (14) Hong, S. J.; Lee, S.; Jang, J. S.; Lee, J. S. Energy Environ. Sci. 2011, 4, 1781. (15) Zou, Z.; Luo, W.; Li, Z. Chinese Patent CN101775615A, Jan. 20, 2010. (16) Luo, W.; Yang, Z.; Li, Z.; Zhang, J.; Liu, J.; Zhao, Z; Wang, Z.; Yan, S.; Yu, T.; Zou, Z. Energy Environ. Sci. 2011, 4, 4046−4051. (17) Zhao, Z.; Luo, W.; Li, Z.; Zou, Z. Phys. Lett. A 2010, 374, 4919. (18) Yin, W. J.; Wei, S. H.; Al-Jassim, M. M.; Turner, J.; Yan, Y. Phys. Rev. B 2011, 83, 155102. (19) Li, M.; Luo, W.; Liu, B.; Zhao, X.; Li, Z.; Chen, D.; Yu, T.; Xie, Z.; Zhang, R.; Zou, Z. Appl. Phys. Lett. 2011, 99, 112108. (20) Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1975, 97, 7427. (21) McGregor, K. G.; Calvin, M.; Otvos, J. W. J. Appl. Phys. 1979, 50, 369. (22) Dunkle, S. S.; Suslick, K. S. J. Phys. Chem. C 2009, 113, 10341. (23) Shi, R.; Lin, J.; Wang, Y.; Xu, J.; Zhu, Y. J. Phys. Chem. C 2010, 114, 6472. (24) Chu, M.; Ganne, M.; Caldes, M. T.; Brohan, L. J. Appl. Phys. 2002, 91, 3178. (25) Hu, Z.; Gu, H.; Hu, Y.; Zou, Y.; Zhou, D. Mater. Chem. Phys. 2009, 113, 42. (26) Gao, W.; Wang, C. M.; Wang, H. Q.; Henrich, V. E.; Altman, E. I. Surf. Sci. 2004, 559, 201. (27) Yao, W.; Iwai, H.; Ye, J. Dalton Trans. 2008, 11, 1426. (28) Greiner, M. T.; Helander, M. G.; Wang, Z. B.; Tang, W. M.; Qiu, J.; Lu, Z. H. Appl. Phys. Lett. 2010, 96, 213302.

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Effects of Surface Electrochemical Pretreatment on the

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