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Blacking FTO by strongly cathodic polarization with enhanced photocurrent
Applied Surface Science 347 (2015) 321–324
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Blacking FTO by strongly cathodic polarization with enhanced photocurrent Yun Xie, Xiaoqing Lu, Wei Huang ∗ , Zelin Li ∗ Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China
a r t i c l e
i n f o
Article history: Received 2 February 2015 Received in revised form 2 April 2015 Accepted 10 April 2015 Available online 20 April 2015 Keywords: Strongly cathodic polarization Black FTO Sn nanoparticle Photocurrent
a b s t r a c t Transparent fluorine-doped tin oxide (TFTO) coating on quartz glass is widely used as substrate in photoelectrochemistry for solar energy transformation, sensing and so on. We observed that the TFTO could become blackish by strongly cathodic polarization. Characterization of the black FTO (BFTO) by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy revealed that part of SnO2 on the TFTO was reduced into metal Sn nanoparticles during the cathodic polarization. The BFTO greatly increased solar absorption and enhanced photocurrent responses in comparison with TFTO. It might be necessary to take caution in photoelectrochemical measurements while the FTO is strongly cathodically polarized. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Recently, black semiconductor nanomaterials from their white precursors have attracted increasing attention due to their excellent performance in photocatalysis and photoelectrochemistry. For example, hydrogenated black TiO2 nanomaterials increased solar absorption, photocatalytic activity, and photoelectrochemical efficiency in water splitting [1–3]. Other black semiconductors such as BiOCl [4,5] and WO3 [6] have also been prepared from their white precursors. We previously obtained black Cd/CdS films from yellow CdS with enhanced photoelectrochemical performance [7]. Transparent fluorine-doped tin oxide (TFTO) coating on quartz glass has been widely used as substrate in photoelectrochemistry for solar energy transformation [3,7], sensing [8] and so on. Although we know that SnO2 on the FTO could be reduced into metal Sn at a considerably negative potential, no detailed reports on it can be found in literature, especially on how the produced metal Sn influences the photoelectrochemistry of FTO. We observed that TFTO could become black FTO (BFTO) by strongly cathodic polarization due to formation of Sn nanoparticles, which greatly increased solar absorption and photocurrent responses. In this paper, a detailed experimental investigation has been carried out to reveal these phenomena for the first time.
All reagents were in analytical grade and were used without further purification. A platinum wire, a saturated mercurous sulfate electrode (SMSE), and a piece of TFTO (NSG group, 16 /square, 1 cm × 1.5 cm) or BFTO was used as the counter, reference and working electrode, respectively. Typically, the BFTO was fabricated by strongly cathodic reduction of TFTO at −2.1 V for 200 s in a solution of 1 M Na2 SO3 using a CHI 660C electrochemical workstation (Shanghai, Chenhua, China). We denote this BFTO as BFTO-200, where the supplementary number represents the cathodic reduction times (s). Photoelectrochemical measurements were carried out on the electrochemical station in a self-made spectroelectrochemical cell with the prepared BFTO as working electrode. A 500 W Xe lamp (CHF-XM-500W, Changtuo Beijing) served as the simulative solar light, which illuminated at regular intervals through the electrolyte with an illumination power of 100 mW cm−2 . Images of surface morphology for TFTO and BFTO were taken with scanning electron microscopy (SEM) performed on a JEOL JSM-6360 microscope. Crystalline phases of TFTO and BFTO were characterized by X-ray diffraction (XRD) on a Bruker D8 Discover X-ray diffractometer using Cu K␣ radiation ( = 0.1542 nm). Surface compositions of TFTO and BFTO were analyzed by X-ray photoelectron spectroscopy (XPS) on an ESCALab250 spectroscope. UV–Visible (UV–Vis) absorption spectra of the BFTO relative to TFTO were measured by a UV–Vis spectrophotometer (UV-2450, Shimadzu), using a piece of TFTO as the reference.
∗ Corresponding author. Tel./fax: +86 731 88872531. E-mail addresses: [email protected] (W. Huang), [email protected] (Z. Li). http://dx.doi.org/10.1016/j.apsusc.2015.04.078 0169-4332/© 2015 Elsevier B.V. All rights reserved.
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Y. Xie et al. / Applied Surface Science 347 (2015) 321–324
3. Results and discussion 3.1. Comparison of electrochemistry and photoelectrochemistry between TFTO and BFTO The electrochemical behavior of TFTO and BFTO in 1 M Na2 SO3 between −2.1 V and 0.4 V is compared in Fig. 1a. At −2.1 V, cathodic current occurred, due to reduction of water and SnO2 into hydrogen gas and Sn nanoparticles, respectively. While the potential went positively, two anodic peaks appeared successively around −1.35 V (Ia ) and −1.2 V (IIa ), which were related to formation of SnO and SnO2 , respectively [9]. Notably, the peak current at the BFTO-200 was much larger than that at the TFTO because more Sn nanoparticles were produced on the BFTO at −2.1 V for 200 s. While the initial potential was positively shifted to about −1.8 V, no apparent anodic peaks were observed on the TFTO because SnO2 was not reduced into Sn at a potential positive to −1.8 V in this solution. In Fig. 1b, greatly enhanced photocurrent responses were observed on the BFTO (curves 1-4) prepared by strongly cathodic polarization at −2.1 V for different times (100–400 s) in comparison with TFTO (curve 8) for the first potential scan. However, the photocurrent substantially decreased during the second potential scan (curve 5) for the BFTO-300, which can be ascribed to the surface oxidation of Sn nanoparticles into SnO2 . The photocurrent at the BFTO-300 then gradually decreased for continuous potential scans (curves 6 and 7), which was only slightly larger than that at the TFTO after ten times of successive potential scans because Sn nanoparticles were deeply oxidized into SnO2 . At the same time, the background current decreased simultaneously with the photocurrent decay for successive potential scans (curves 2, 5–7). Besides, the background current for the first potential scan also became smaller when the cathodic reduction time got shorter from 400 to 100 s in preparing the BFTO. Both of these facts evidenced the gradual electrooxidation of Sn nanoparticles on the BFTO during the potential scan.
3.2. Morphologies, crystalline phases, surface compositions and solar absorption of TFTO and BFTO SEM, XRD, XPS, and UV–Vis absorption spectroscopy were employed to characterize the BFTO to confirm the presence of Sn nanoparticles from cathodic reduction and the enhanced solar absorption.
By comparing the SEM images between TFTO and BFTO in Fig. 2a and b, it can be clearly seen that the surface of BFTO was covered with Sn nanoparticles sizing 10–20 nm after cathodic reduction of TFTO at −2.1 V for 200 s. The BFTO with Sn nanoparticles appeared blackish in color (the inset of Fig. 2b) due to the quantum size effect of Sn nanopartices. The UV–Vis absorption spectrum of BFTO relative to TFTO in Fig. 2c displayed that the presence of black Sn nanoparticles remarkably increased solar absorption, noting that a piece of TFTO was used here as the reference to directly demonstrate the difference between the BFTO and TFTO in UVVis absorption spectrum. The enhanced solar absorption can be ascribed to the surface plasmon effect of Sn nanoparticle in BFTO [10], which is beneficial to improve the photoelectrochemical performance. The comparison of XRD patterns for the TFTO and BFTO is shown in Fig. 3. Both of TFTO and BFTO comprised cassiterite SnO2 (JPCDS No. 46-1088) and tetragonal rutile structured SnO2 (JPCDS 88-0287) with strong and sharp diffraction peaks from the crystalline planes of (1 1 0), (2 0 0), (2 1 1) and (3 1 0). Moreover, weak and broadened diffraction peaks of (2 0 0), (1 0 1), (2 2 0) and (2 2 1) crystalline planes from metal Sn (JPCDS 89-4898) could be discerned for the BFTO. A more sensitive surface analysis technique, XPS, was further performed to compare the surface composition between TFTO and BFTO (Fig. 4). Besides the peaks at 487.2/495.7 eV for the binding energies of Sn 3d3/2 /Sn 3d5/2 of SnO2 observed on the both FTO, the deconvoluted peaks centered at 485/493.5 for the BFTO could be assigned to the binding energies of metal Sn [11,12].
3.3. The main reasons for enhanced photocurrent of BFTO We observed that the conductance of the BFTO decreased somewhat in comparison with that of TFTO (Fig. S1 in Supplementary data), which could also be attributed to the quantum size effect of Sn nanoparticles [13]. Therefore, the enhanced photocurrent in the presence of Sn nanoparticles for the BFTO could be mainly ascribed to improvements in solar absorption (Fig. 2c), charge separation and transportation of light-excited holes and electrons [14]. The work function of Sn is 4.42 eV [15], smaller than that of SnO2 (4.75 eV) [16]. Therefore, the electrons would transfer from Sn to SnO2 to achieve the Fermi level balance of both. This means that the presence of metallic Sn lifts the surface energy levels of SnO2 , which would promote the transportation of photo-excited electrons from
Fig. 1. (a) Polarization curves and (b) photocurrent responses at TFTO and BFTO-200 electrodes in 1 M Na2 SO3 solution. The numbered curves in (b) were from (1–7) BFTO and (8) TFTO. The BFTO was prepared by strongly cathodic reduction of TFTO at −2.1 V for different times: (1) 400 s, (2) 300 s, (3) 200 s, and (4) 100 s. The curves of (1–4, 8) were from the first potential scan, and those of (5–7) were from the second, third and tenth potential scans of the BFTO-300 numbered (2).
Y. Xie et al. / Applied Surface Science 347 (2015) 321–324
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Fig. 2. SEM images of (a) TFTO and (b) BFTO-200, and (c) UV–Vis absorption spectrum of BFTO-200 with TFTO as the reference. The inset of Fig. 2b was a photograph for the BFTO-200 (the bottom half of TFTO).
would depress the re-combination of light-excited holes and electrons. Similar phenomena on photocurrent enhancement have also been observed previously for composites of Cd/CdS [7] and Bi/Bi2 O3 [17]. 4. Conclusions We have confirmed that SnO2 in FTO can be reduced into Sn nanoparticles under strongly cathodic polarization, and the transparent FTO becomes blackish. The black FTO greatly increases solar absorption and enhances photocurrent responses in comparison with the transparent FTO. However, the black FTO is not so stable in photoelectrochemical performance because of gradual electrooxidation of Sn nanoparticles. Take care while the FTO is strongly cathodically polarized, which might influence the photoelectrochemical measurements.
Fig. 3. XRD patterns for the TFTO and BFTO-200.
Acknowledgements We are grateful for the financial supports of this research from National Natural Science Foundation of China (Grant Nos. 21173075 and 21003045), Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Construct Program of the Key Discipline in Hunan Province.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.04. 078 References
Fig. 4. Sn 3d XPS spectra for the TFTO and BFTO-200.
the BFTO photoanode to the Pt cathode. The fact that the photocurrent appeared at more negative potential on the BFTO (−0.78 V) than on the TFTO (−0.62 V) in Fig. 1b supports the rising of the surface energy levels of SnO2 in the presence of metal Sn. Meanwhile, an internal electric field forms between Sn and SnO2 , which
[1] X.B. Chen, L. Liu, P.Y. Yu, S.S. Mao, Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals, Science 331 (2011) 746–750. [2] Z. Wang, C.Y. Yang, T.Q. Lin, H. Yin, P. Chen, D.Y. Wan, F.F. Xu, F.Q. Huang, J.H. Lin, X.M. Xie, M.H. Jiang, H-doped black titania with very high solar absorption and excellent photocatalysis enhanced by localized surface plasmon resonance, Adv. Funct. Mater. 23 (2013) 5444–5450. [3] G.M. Wang, H.Y. Wang, Y.C. Ling, Y.C. Tang, X.Y. Yang, R.C. Fitzmorris, C.C. Wang, J.Z. Zhang, Y. Li, Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting, Nano Lett. 11 (2011) 3026–3033. [4] Y.T. Li, C.M. Li, X.G. Sun, Z.F. Zhang, Z. Peng, J.M. Zhang, J.J. Zhao, Preparation of black BiOCl with visible light photocatalytic activity by Fe reduction, Mater. Lett. 116 (2014) 98–100.
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[5] L.Q. Ye, K.J. Deng, F. Xu, L.H. Tian, T.Y. Peng, L. Zan, Increasing visible-light absorption for photocatalysis with black BiOCl, Phys. Chem. Chem. Phys. 14 (2012) 82–85. [6] G. Liu, J.F. Han, X. Zhou, L. Huang, F.X. Zhang, X.L. Wang, C.M. Ding, X.J. Zheng, H.X. Han, C. Li, Enhancement of visible-light-driven O2 evolution from water oxidation on WO3 treated with hydrogen, J. Catal. 307 (2013) 148–152. [7] X.Q. Lu, X.L. Yin, F. Pu, Y. Xia, W. Huang, Z.L. Li, Blacking of nano-CdS thin film from gas/liquid interface for enhanced photoelectrochemical performances, Appl. Surf. Sci. 313 (2014) 26–30. [8] S.S. Zhang, S.Q. Zhang, B.Y. Peng, H.J. Wang, H. Yua, H.H. Wang, F. Peng, High performance hydrogenated TiO2 nanorod arrays as a photoelectrochemical sensor for organic compounds under visible light, Electrochem. Commun. 40 (2014) 24–27. [9] Y.C. Yang, J. Liu, C. Li, L. Fu, W. Huang, Z.L. Li, Fabrication of pompon-like and flower-like SnO microspheres comprised of layered nanoflakes by anodic electrocrystallization, Electrochim. Acta 72 (2012) 94–100. [10] D. Gaspar, A.C. Pimentel, M.J. Mendes, T. Mateus, B.P. Falcão, J.P. Leitão, J. Soares, A. Araújo, A. Vicente, S.A. Filonovich, H. Águas, R. Martins, I. Ferreira, Ag and Sn nanoparticles to enhance the near-infrared absorbance of a-Si:H thin films, Plasmonics 9 (2014) 1015–1023.
[11] V.M. Jimenez, J.P. Espinós, A.R. González-Elipe, Effect of texture and annealing treatments in SnO2 and Pd/SnO2 gas sensor materials, Sens. Actuat. B Chem. 61 (1999) 23–32. [12] C.D. Wagner, W.M. Riggs, L.E. David, J.F. Mouider, G.E. Muilenberg (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979, p. 118. [13] L.W. Tu, G.K. Wong, J. Ketterson, Observation of quantum size effect in the resistivity of thin, gray tin epilayers, Appl. Phys. Lett. 55 (1989) 1327–1329. [14] A.K. Sinha, M. Pradhan, S. Sarkar, T. Pal, Large-scale solid-state synthesis of Sn-SnO2 nanoparticles from layered SnO by sunlight: a material for dye degradation in water by photocatalytic reaction, Environ. Sci. Technol. 47 (2013) 2339–2345. [15] H.B. Michaelson, The work function of the elements and its periodicity, J. Appl. Phys. 48 (1977) 4729–4933. [16] M.N. Islam, M.O. Hakim, Electron affinity and work function of polycrystalline SnO2 thin film, J. Mater. Sci. Lett. 5 (1986) 63–65. [17] C.Z. Li, J.Y. Zhang, K.J. Liu, A new method of enhancing photoelectrochemical characteristics of Bi/Bi2 O3 electrode for hydrogen generation via water splitting, Int. J. Electrochem. Sci. 7 (2012) 5028–5034.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Blacking FTO by strongly cathodic polarization with enhanced photocurrent Yun Xie, Xiaoqing Lu, Wei Huang ∗ , Zelin Li ∗ Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China
a r t i c l e
i n f o
Article history: Received 2 February 2015 Received in revised form 2 April 2015 Accepted 10 April 2015 Available online 20 April 2015 Keywords: Strongly cathodic polarization Black FTO Sn nanoparticle Photocurrent
a b s t r a c t Transparent fluorine-doped tin oxide (TFTO) coating on quartz glass is widely used as substrate in photoelectrochemistry for solar energy transformation, sensing and so on. We observed that the TFTO could become blackish by strongly cathodic polarization. Characterization of the black FTO (BFTO) by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy revealed that part of SnO2 on the TFTO was reduced into metal Sn nanoparticles during the cathodic polarization. The BFTO greatly increased solar absorption and enhanced photocurrent responses in comparison with TFTO. It might be necessary to take caution in photoelectrochemical measurements while the FTO is strongly cathodically polarized. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Recently, black semiconductor nanomaterials from their white precursors have attracted increasing attention due to their excellent performance in photocatalysis and photoelectrochemistry. For example, hydrogenated black TiO2 nanomaterials increased solar absorption, photocatalytic activity, and photoelectrochemical efficiency in water splitting [1–3]. Other black semiconductors such as BiOCl [4,5] and WO3 [6] have also been prepared from their white precursors. We previously obtained black Cd/CdS films from yellow CdS with enhanced photoelectrochemical performance [7]. Transparent fluorine-doped tin oxide (TFTO) coating on quartz glass has been widely used as substrate in photoelectrochemistry for solar energy transformation [3,7], sensing [8] and so on. Although we know that SnO2 on the FTO could be reduced into metal Sn at a considerably negative potential, no detailed reports on it can be found in literature, especially on how the produced metal Sn influences the photoelectrochemistry of FTO. We observed that TFTO could become black FTO (BFTO) by strongly cathodic polarization due to formation of Sn nanoparticles, which greatly increased solar absorption and photocurrent responses. In this paper, a detailed experimental investigation has been carried out to reveal these phenomena for the first time.
All reagents were in analytical grade and were used without further purification. A platinum wire, a saturated mercurous sulfate electrode (SMSE), and a piece of TFTO (NSG group, 16 /square, 1 cm × 1.5 cm) or BFTO was used as the counter, reference and working electrode, respectively. Typically, the BFTO was fabricated by strongly cathodic reduction of TFTO at −2.1 V for 200 s in a solution of 1 M Na2 SO3 using a CHI 660C electrochemical workstation (Shanghai, Chenhua, China). We denote this BFTO as BFTO-200, where the supplementary number represents the cathodic reduction times (s). Photoelectrochemical measurements were carried out on the electrochemical station in a self-made spectroelectrochemical cell with the prepared BFTO as working electrode. A 500 W Xe lamp (CHF-XM-500W, Changtuo Beijing) served as the simulative solar light, which illuminated at regular intervals through the electrolyte with an illumination power of 100 mW cm−2 . Images of surface morphology for TFTO and BFTO were taken with scanning electron microscopy (SEM) performed on a JEOL JSM-6360 microscope. Crystalline phases of TFTO and BFTO were characterized by X-ray diffraction (XRD) on a Bruker D8 Discover X-ray diffractometer using Cu K␣ radiation ( = 0.1542 nm). Surface compositions of TFTO and BFTO were analyzed by X-ray photoelectron spectroscopy (XPS) on an ESCALab250 spectroscope. UV–Visible (UV–Vis) absorption spectra of the BFTO relative to TFTO were measured by a UV–Vis spectrophotometer (UV-2450, Shimadzu), using a piece of TFTO as the reference.
∗ Corresponding author. Tel./fax: +86 731 88872531. E-mail addresses: [email protected] (W. Huang), [email protected] (Z. Li). http://dx.doi.org/10.1016/j.apsusc.2015.04.078 0169-4332/© 2015 Elsevier B.V. All rights reserved.
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Y. Xie et al. / Applied Surface Science 347 (2015) 321–324
3. Results and discussion 3.1. Comparison of electrochemistry and photoelectrochemistry between TFTO and BFTO The electrochemical behavior of TFTO and BFTO in 1 M Na2 SO3 between −2.1 V and 0.4 V is compared in Fig. 1a. At −2.1 V, cathodic current occurred, due to reduction of water and SnO2 into hydrogen gas and Sn nanoparticles, respectively. While the potential went positively, two anodic peaks appeared successively around −1.35 V (Ia ) and −1.2 V (IIa ), which were related to formation of SnO and SnO2 , respectively [9]. Notably, the peak current at the BFTO-200 was much larger than that at the TFTO because more Sn nanoparticles were produced on the BFTO at −2.1 V for 200 s. While the initial potential was positively shifted to about −1.8 V, no apparent anodic peaks were observed on the TFTO because SnO2 was not reduced into Sn at a potential positive to −1.8 V in this solution. In Fig. 1b, greatly enhanced photocurrent responses were observed on the BFTO (curves 1-4) prepared by strongly cathodic polarization at −2.1 V for different times (100–400 s) in comparison with TFTO (curve 8) for the first potential scan. However, the photocurrent substantially decreased during the second potential scan (curve 5) for the BFTO-300, which can be ascribed to the surface oxidation of Sn nanoparticles into SnO2 . The photocurrent at the BFTO-300 then gradually decreased for continuous potential scans (curves 6 and 7), which was only slightly larger than that at the TFTO after ten times of successive potential scans because Sn nanoparticles were deeply oxidized into SnO2 . At the same time, the background current decreased simultaneously with the photocurrent decay for successive potential scans (curves 2, 5–7). Besides, the background current for the first potential scan also became smaller when the cathodic reduction time got shorter from 400 to 100 s in preparing the BFTO. Both of these facts evidenced the gradual electrooxidation of Sn nanoparticles on the BFTO during the potential scan.
3.2. Morphologies, crystalline phases, surface compositions and solar absorption of TFTO and BFTO SEM, XRD, XPS, and UV–Vis absorption spectroscopy were employed to characterize the BFTO to confirm the presence of Sn nanoparticles from cathodic reduction and the enhanced solar absorption.
By comparing the SEM images between TFTO and BFTO in Fig. 2a and b, it can be clearly seen that the surface of BFTO was covered with Sn nanoparticles sizing 10–20 nm after cathodic reduction of TFTO at −2.1 V for 200 s. The BFTO with Sn nanoparticles appeared blackish in color (the inset of Fig. 2b) due to the quantum size effect of Sn nanopartices. The UV–Vis absorption spectrum of BFTO relative to TFTO in Fig. 2c displayed that the presence of black Sn nanoparticles remarkably increased solar absorption, noting that a piece of TFTO was used here as the reference to directly demonstrate the difference between the BFTO and TFTO in UVVis absorption spectrum. The enhanced solar absorption can be ascribed to the surface plasmon effect of Sn nanoparticle in BFTO [10], which is beneficial to improve the photoelectrochemical performance. The comparison of XRD patterns for the TFTO and BFTO is shown in Fig. 3. Both of TFTO and BFTO comprised cassiterite SnO2 (JPCDS No. 46-1088) and tetragonal rutile structured SnO2 (JPCDS 88-0287) with strong and sharp diffraction peaks from the crystalline planes of (1 1 0), (2 0 0), (2 1 1) and (3 1 0). Moreover, weak and broadened diffraction peaks of (2 0 0), (1 0 1), (2 2 0) and (2 2 1) crystalline planes from metal Sn (JPCDS 89-4898) could be discerned for the BFTO. A more sensitive surface analysis technique, XPS, was further performed to compare the surface composition between TFTO and BFTO (Fig. 4). Besides the peaks at 487.2/495.7 eV for the binding energies of Sn 3d3/2 /Sn 3d5/2 of SnO2 observed on the both FTO, the deconvoluted peaks centered at 485/493.5 for the BFTO could be assigned to the binding energies of metal Sn [11,12].
3.3. The main reasons for enhanced photocurrent of BFTO We observed that the conductance of the BFTO decreased somewhat in comparison with that of TFTO (Fig. S1 in Supplementary data), which could also be attributed to the quantum size effect of Sn nanoparticles [13]. Therefore, the enhanced photocurrent in the presence of Sn nanoparticles for the BFTO could be mainly ascribed to improvements in solar absorption (Fig. 2c), charge separation and transportation of light-excited holes and electrons [14]. The work function of Sn is 4.42 eV [15], smaller than that of SnO2 (4.75 eV) [16]. Therefore, the electrons would transfer from Sn to SnO2 to achieve the Fermi level balance of both. This means that the presence of metallic Sn lifts the surface energy levels of SnO2 , which would promote the transportation of photo-excited electrons from
Fig. 1. (a) Polarization curves and (b) photocurrent responses at TFTO and BFTO-200 electrodes in 1 M Na2 SO3 solution. The numbered curves in (b) were from (1–7) BFTO and (8) TFTO. The BFTO was prepared by strongly cathodic reduction of TFTO at −2.1 V for different times: (1) 400 s, (2) 300 s, (3) 200 s, and (4) 100 s. The curves of (1–4, 8) were from the first potential scan, and those of (5–7) were from the second, third and tenth potential scans of the BFTO-300 numbered (2).
Y. Xie et al. / Applied Surface Science 347 (2015) 321–324
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Fig. 2. SEM images of (a) TFTO and (b) BFTO-200, and (c) UV–Vis absorption spectrum of BFTO-200 with TFTO as the reference. The inset of Fig. 2b was a photograph for the BFTO-200 (the bottom half of TFTO).
would depress the re-combination of light-excited holes and electrons. Similar phenomena on photocurrent enhancement have also been observed previously for composites of Cd/CdS [7] and Bi/Bi2 O3 [17]. 4. Conclusions We have confirmed that SnO2 in FTO can be reduced into Sn nanoparticles under strongly cathodic polarization, and the transparent FTO becomes blackish. The black FTO greatly increases solar absorption and enhances photocurrent responses in comparison with the transparent FTO. However, the black FTO is not so stable in photoelectrochemical performance because of gradual electrooxidation of Sn nanoparticles. Take care while the FTO is strongly cathodically polarized, which might influence the photoelectrochemical measurements.
Fig. 3. XRD patterns for the TFTO and BFTO-200.
Acknowledgements We are grateful for the financial supports of this research from National Natural Science Foundation of China (Grant Nos. 21173075 and 21003045), Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Construct Program of the Key Discipline in Hunan Province.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.04. 078 References
Fig. 4. Sn 3d XPS spectra for the TFTO and BFTO-200.
the BFTO photoanode to the Pt cathode. The fact that the photocurrent appeared at more negative potential on the BFTO (−0.78 V) than on the TFTO (−0.62 V) in Fig. 1b supports the rising of the surface energy levels of SnO2 in the presence of metal Sn. Meanwhile, an internal electric field forms between Sn and SnO2 , which
[1] X.B. Chen, L. Liu, P.Y. Yu, S.S. Mao, Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals, Science 331 (2011) 746–750. [2] Z. Wang, C.Y. Yang, T.Q. Lin, H. Yin, P. Chen, D.Y. Wan, F.F. Xu, F.Q. Huang, J.H. Lin, X.M. Xie, M.H. Jiang, H-doped black titania with very high solar absorption and excellent photocatalysis enhanced by localized surface plasmon resonance, Adv. Funct. Mater. 23 (2013) 5444–5450. [3] G.M. Wang, H.Y. Wang, Y.C. Ling, Y.C. Tang, X.Y. Yang, R.C. Fitzmorris, C.C. Wang, J.Z. Zhang, Y. Li, Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting, Nano Lett. 11 (2011) 3026–3033. [4] Y.T. Li, C.M. Li, X.G. Sun, Z.F. Zhang, Z. Peng, J.M. Zhang, J.J. Zhao, Preparation of black BiOCl with visible light photocatalytic activity by Fe reduction, Mater. Lett. 116 (2014) 98–100.
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