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Ag and CdS nanoparticles co-sensitized TiO2 nanotubes for enhancing visible photoelectrochemical performance
Electrochimica Acta 83 (2012) 140–145
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Ag and CdS nanoparticles co-sensitized TiO2 nanotubes for enhancing visible photoelectrochemical performance Qingyao Wang, Xiuchun Yang ∗ , Dan Liu, Lina Chi, Junwei Hou School of Materials Science and Engineering, Tongji University, Shanghai 201804, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 May 2012 Received in revised form 25 July 2012 Accepted 25 July 2012 Available online 9 August 2012 Keywords: TiO2 nanotubes Successive ionic layer adsorption and reaction Ag and CdS nanoparticles Photocurrent Photocatalysis
a b s t r a c t The Ag and CdS nanoparticles co-sensitization of TiO2 nanotubes (CdS–Ag/TiO2 NTs) were prepared by successive ionic layer adsorption and reaction (SILAR) technique. The phase composition, morphology and optical property were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV–vis diffusion reflection spectroscopy (DRS). The comodification of Ag and CdS nanoparticles expanded the photoresponse range of TiO2 NTs from ultraviolet region to 668.7 nm, and the CdS–Ag/TiO2 NTs prepared by SILAR deposition of 5 cycles exhibited higher visible photocurrent and stability against photocorrosion. The detailed electrons transfer mechanism of CdS–Ag/TiO2 NTs was proposed, and photocatalytic activity toward degradation of methyl orange (MO) under visible-light irradiation was also investigated. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, titanium dioxide nanotubes have attracted great interest due to their unique electronic and optical properties, which can be applied in photocatalyst, biosensor, photo-chromic devices and other applications [1–5]. Many methods, such as anodization [6–8], template technique [9], hydrothermal process [10], and soft chemical process [11], have been used to prepare TiO2 NTs. Among these fabrication techniques, the electrochemical anodization process, since its discovery, has been considered as the most prominent one in producing high-aspect-ratio nanotubes with controllable tube size and morphology [12,13]. However, the intrinsic band gap of TiO2 (3.2 eV) and high recombination rate are the two major factors limiting the further improvement of photoelectrochemical properties. Various attempts, such as transition metal cation doping [14], nonmetal doping [15,16] and surface modification with semiconductor [17] have been tried to overcome these obstacles. Coupling TiO2 NTs with narrow band gap semiconductors was found to be the most effective method to improve the visible light activity [18,19]. CdS (2.5 eV), a well-known narrow band gap semiconductor with excellent visible absorption and rapid electron injection rate, has attracted considerable attention [20–24]. Various methods such as
∗ Corresponding author. E-mail address: [email protected] (X. Yang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.07.092
layer-by-layer deposition [25], chemical bath deposition [26] and electrochemical atomic layer deposition [27] were used to modify TiO2 NTs with CdS nanoparticles. Nevertheless, the SILAR method is considered the simplest and most controllable way of synthesizing CdS/TiO2 NTs. Recently, Chen et al. [27] synthesized CdS nanoparticles sensitized TiO2 NTs by SILAR technique, which caused the absorption to red-shift from 388 to 494 nm. However, poor conductivity of CdS/TiO2 NTs results in a low photo-generated electron transfer rate, facilitating the recombination of the electrons–holes pairs. A typical strategy of solving this obstacle is to deposit noble metal on the surface of TiO2 NTs to achieve co-sensitization with CdS nanoparticles. Owning to its high carriers transfer rate and plasmonic effect, Ag nanoparticle deposited on CdS/TiO2 NTs surface acts as a sink for photo-generated electrons, promoting charge carrier separation [28,1,29]. On the basis of above considerations, such an advantageous combination of the deposition of CdS and Ag nanoparticles on TiO2 NTs could simultaneously realize enhancement of visiblelight response and effective separation of electron–hole pairs. Last year, Xie et al. [30] successfully fabricated CdS–Ag/TiO2 NTs by electrochemical methods. Nevertheless, electrochemical methods could only provide CdS nanoparticles coverage on the top surface of TiO2 NTs, and little work has been reported on the electron transport mechanism of CdS–Ag/TiO2 NTs in detail. Therefore, the SILAR fabrication of CdS–Ag/TiO2 NTs with effective coverage of CdS nanoparticles is still challenging and demanding. In this paper, TiO2 nanotubes co-sensitized by Ag and CdS nanoparticles were successfully prepared using the SILAR method.
Q. Wang et al. / Electrochimica Acta 83 (2012) 140–145
The effects of Ag and CdS nanoparticles on the morphology and photoelectrochemical property of the CdS–Ag/TiO2 NTs were systematically investigated.
141
2. Experimental
the dye on catalyst surface was established by mechanical stirring in the dark for 30 min. After visible-light irradiation for 5 h, the remaining dye concentration was determined with a UV1700 UV–vis spectrophotometer by detecting the maximum absorption wavelength for MO at 464 nm.
2.1. Experimental materials
3. Results and discussion
NH4 F, AgNO3 , CdCl2 , Na2 S, NaBH4 , mercaptoacetic acid and ethylene glycol were purchased from Shanghai Chemical Reagent Co. Ltd. All reagents are of analytical grade and used without further purification.
The XRD patterns of TiO2 NTs and CdS–Ag/TiO2 NTs are shown in Fig. 1. All diffraction peaks in Fig. 1a besides Ti can be indexed to the anatase TiO2 , which is in good agreement with the standard spectrum (JCPDS, card no: 211272). The diffraction peaks of hexagonal CdS (JCPDS, card no: 41-1049) could be observed after CdS nanoparticles sensitization, and the peaks intensity increases with increasing SILAR cycles. However, Ag nanoparticles deposited on TiO2 NTs were very small and highly dispersed, so no characteristic peaks attributed to Ag nanoparticles were detected. From the SEM image of TiO2 NTs (Fig. 2a), the vertically oriented TiO2 NTs are with a diameter of approximately 137 nm and a wall thickness of about 30 nm. A cross-section view of the TiO2 NTs as shown in Fig. 2b indicates that all walls are entirely smooth. No obvious changes of the morphologies were observed in the top (Fig. 2d) and cross-section view (Fig. 2e) of Ag/TiO2 NTs. Nevertheless, the EDS result (Fig. 2f) verified the presence of Ag nanoparticles. Fig. 2g and h shows SEM images of Ag and CdS nanoparticles filled TiO2 NTs after 5 cycles, and the right views show the corresponding enlarged images. It was apparent that Ag and CdS nanoparticles with a uniform diameter were distributed on surface and inner walls of the TiO2 NTs, and the deposition process did not damage the ordered tubular structure. The EDS spectra (Fig. 2c and i) indicate that the bare TiO2 NTs are composed of Ti and O elements, and the CdS(5)–Ag/TiO2 NTs are composed of elements Ti, O, 0.6 at.% Ag and 7.18 at.% S and 6.33 at.% Cd. The atomic ratio of S versus Cd is little larger than stoichiometry, and the excess S element is considered from the linker mercaptoacetic acid. The detailed microscopic structure of the CdS(5)–Ag/TiO2 NTs was further investigated by TEM. Fig. 3a and b shows the lowmagnification TEM images of the CdS(5)–Ag/TiO2 NTs. It could be clearly seen that many nanoparticles with diameters of approximately 10 nm were deposited into the TiO2 NTs. Fig. 3c is a HRTEM image taken from the white square area in Fig. 3b. In the nanoparticle observed, the crystallinity of CdS nanoparticle was seen clearly with the interplane distance of 0.316 nm, which corresponds to the
2.2. Preparation of Ag nanoparticles sensitized TiO2 nanotubes TiO2 NTs were prepared according to a two-step anodization as reported previously [31]. Ag nanoparticles sensitized TiO2 nanotubes were fabricated by using SILAR technique, similar to our previous method [32]. The TiO2 NTs were immersed in 0.3 M mercaptoacetic acid solution for 30 min. Afterwards, the TiO2 NTs containing mercaptoacetic acid were dried at 60 ◦ C for 10 h. TiO2 NTs containing mercaptoacetic acid were immersed in 0.2 M AgNO3 solution for 5 min. After rinsed with deionized water, the nanotubes were immersed in 0.15 M NaBH4 solution for another 5 min. 2.3. Synthesis of Ag and CdS nanoparticles co-sensitized TiO2 nanotubes The Ag/TiO2 NTs were sequentially sensitized with CdS nanoparticles by using SILAR technique. First, Ag/TiO2 NTs were dipped into 0.1 M CdCl2 solution for 5 min, rinsed with deionized water, then dipped for another 5 min into 0.1 M Na2 S solution and again rinsed with deionized water. The two-step dipping procedure was termed as one SILAR cycle. This SILAR processes for 3, 5 and 7 cycles were denoted as CdS(3)–Ag/TiO2 NTs, CdS(5)–Ag/TiO2 NTs and CdS(7)–Ag/TiO2 NTs, respectively. For comparison, bare TiO2 NTs modification by CdS nanoparticles for 5 cycles was carried out and denoted as CdS(5)/TiO2 NTs. 2.4. Characterization The phase compositions of samples were determined by a Rigaku D/Max 2400 X-ray diffractometer (XRD) equipped with graphite monochromatized Cu K␣ radiation. The morphology of the prepared sample was observed using scanning electron microscopy (SEM, Quanta 200 FEG) and transmission electron microscopy (TEM, Tecnai-F30). UV–vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV-2550 UV–vis spectrophotometer with an integrating sphere attachment. Photoelectrochemical performances of the as-prepared samples were measured with LK98II electrochemical system (Lanlike, China) using a three-electrode system composed of the samples as a working electrode, a Pt foil as a counter electrode, a saturated calomel electrode (SCE) as a reference electrode in 0.1 M Na2 SO4 solution. The working electrode was illuminated with a solar simulator equipped with a 500 W Xe lamp with a visible-light filter (<420 nm). The photocurrent dynamics of the working electrode were recorded according to the responses to sudden switching on and off at 0.25 V bias. 2.5. Photocatalytic activity test The photocatalytic activity of the prepared samples was evaluated by photocatalytic decomposition of 10−5 M methyl orange (MO) solution under a 500 W Xe lamp with a visible-light filter (<420 nm). Before photodegradation, adsorption equilibrium for
Fig. 1. XRD patterns of TiO2 NTs (a), CdS(3)–Ag/TiO2 NTs (b), CdS(5)–Ag/TiO2 NTs (c) and CdS(7)–Ag/TiO2 NTs (d).
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Fig. 2. SEM images of TiO2 NTs (left), Ag/TiO2 NTs (middle) and CdS(5)–Ag/TiO2 NTs (right): (a, d, and g) top-view, (b, e, and h) cross-sectional view and (c, f, and i) EDS spectra.
(1 0 1) plane in the CdS nanoparticles. Close observation by HRTEM (Fig. 3d) demonstrated that the CdS nanoparticles were markedly adsorbed on the surface of Ag nanoparticles with a diameter of about 5 nm. The interplanar spacings of 0.236, 0.358, 0.337 and 0.352 nm of crystal lattices are fairly close to those of the (1 1 1) plane of Ag, (1 0 0) and (0 0 2) planes of CdS and (1 0 1) plane of anatase, respectively. Fig. 4 shows a series of SEM images of Ag and CdS nanoparticles co-sensitized TiO2 NTs prepared by SILAR for 3, 5 and 7 cycles, respectively. As shown in Fig. 4a, few CdS nanoparticles were deposited on the nanotubes. With the deposition up to 5 cycles, CdS nanoparticles were almost uniformly distributed on the surface of each nanotube. However, when deposition was increased to 7 cycles, the entire surface of the nanotubes was covered with aggregated CdS particles, but the tubes remained well open. The UV–vis absorption spectra of TiO2 NTs, CdS/TiO2 NTs and CdS–Ag/TiO2 NTs are shown in Fig. 5. The spectrum of the bare TiO2 NTs (Fig. 5a) shows that the absorption edge is observed at about 380 nm, corresponding to the band-gap energy of 3.2 eV. After the deposition of Ag and CdS nanoparticles, it is also found that the absorption edge is shifted significantly toward the visible region, and the absorption edge red-shifts slightly with increasing CdS nanoparticles deposition cycles. For CdS(7)–Ag/TiO2 NTs, the absorption edge is extended to 668.7 nm, which is larger than that of CdS(5)–Ag/TiO2 NTs (616.7 nm) and CdS(3)–Ag/TiO2
NTs (553.3 nm). It is observed that Ag and CdS nanoparticles cosensitized TiO2 NTs extend noticeably the absorption in visible light area than that of only CdS sensitization, and such improvement is mainly attributed to the overlap with the surface plasmon resonance absorption of Ag nanoparticles. Fig. 6a shows the transient photocurrent responses of the TiO2 NTs before and after modification with Ag and CdS nanoparticles. Upon visible light illumination, the TiO2 NTs do not show any photocurrent density, and photocurrent increases with the increasing of SILAR cycles due to more CdS nanoparticles on the CdS–Ag/TiO2 NTs. The photocurrent of only CdS modified TiO2 NTs for 5 cycles is low because of fast charge recombination. By employing Ag nanoparticles, it could be possible to improve the efficiency of charge separation through charge rectification. However, since CdS(7)–Ag/TiO2 NTs are subject to long-time light illumination, the photocurrent decays rapidly and is even lower than that of CdS(5)–Ag/TiO2 NTs after the third turn-on cycle. It implies that the excess CdS nanoparticles covered on the surface of TiO2 NTs are liable to suffer photocorrosion, causing low conversion from photo-generated electron to photocurrent. Moreover, the chalcogenide electrodes are usually subjected to photocorrosion phenomenon that limits long-term stability. Electron transfer occurs, leaving behind a hole in the CdS lattice (reaction (1)): CdS(h+ + e− ) + Ag . . . . . . . . . . . . CdS(h+ ) + Ag(e− )
(1)
Q. Wang et al. / Electrochimica Acta 83 (2012) 140–145
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Fig. 3. TEM (a and b) and HRTEM (c and d) images of CdS(5)–Ag/TiO2 NTs.
The excess holes remaining in the valence band of the CdS nanoparticles induce anodic corrosion (reaction (2)): +
2+ −
CdS(h ) . . . . . . . . . . . . (Cd
S )m (CdS)n−m . . . . . . . . . . . . S0m/2 (CdS)n−m/2
(2)
The photocurrent attenuation rate of CdS(5)–Ag/TiO2 NTs is much slower than that of CdS(7)–Ag/TiO2 NTs, and the reason maybe related to the effective utilization of photo-generated electrons. As a result of good contact between CdS and Ag nanoparticles in CdS(5)–Ag/TiO2 NTs, and it greatly improves the charge collection efficiency. For CdS(7)–Ag/TiO2 NTs, excess CdS nanoparticles
contain a large number of interfacial centers and grain boundaries with a high interfacial resistance, which trap the photo-generated electrons and delay the transportation of these electrons into the electrode side. That is to say, equivalent photocurrent means losing more CdS nanoparticles in CdS(7)–Ag/TiO2 NTs. Therefore, the CdS(5)–Ag/TiO2 NTs are the best candidate as a visible light photoelectrode. When CdS–Ag/TiO2 NTs are excited to generate electrons and holes under visible light, the band bending rectifies the flow of photo-generated charge carriers to produce photocurrent, and the
Fig. 4. SEM images of CdS(3)–Ag/TiO2 NTs (a), CdS(5)–Ag/TiO2 NTs (b) and CdS(7)–Ag/TiO2 NTs (c).
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Fig. 5. UV–vis spectra of TiO2 NTs (a), CdS(5)/TiO2 NTs (b), CdS(3)–Ag/TiO2 NTs (c), CdS(5)–Ag/TiO2 NTs (d) and CdS(7)–Ag/TiO2 NTs (e).
detailed mechanism is schematized in Fig. 6b. The charge transfer mechanism is found to interpret the high photocurrent in the hybrid photoelectrode, where CdS, TiO2 semiconductors and the plasmonic Ag nanoparticles contact with each other, allowing an ideal stepwise band structure for the rapid transport of excited electrons across the hybrid photoelectrode. The driving force for the electron transfer between CdS, Ag nanoparticles and TiO2 NTs is dictated by the energy difference between the conduction band energies. Because the energy level of the conduction band (CB) of CdS is higher than the Ag Fermi energy level, the electrons will continuingly transfer from the CdS to the Ag nanoparticles until the equilibrium between the Fermi level and CB of CdS. Since the electron accumulation prompts the Fermi level of the Ag nanoparticles to more negative potential, the resultant Fermi level of Ag nanoparticles up-shifts [33]. Once the new Fermi level higher than the CB of TiO2 NTs, the stored electrons will rapidly inject into TiO2 , and transfer along vertically oriented nanotube walls to the conducting Ti-metal support to generate a photocurrent. The photocatalytic activity of CdS–Ag/TiO2 NTs was evaluated by measuring the decomposition of MO under visible light irradiation for 5 h. The comparable experiment with bare TiO2 NTs was also investigated, and the less than 10% of degradation rate could be neglected. Fig. 7a shows that sample CdS(5)–Ag/TiO2 NTs has the highest photocatalytic activity among these samples, and the decomposition rate achieved 68.8%, whereas 59.7%, 44.4% and 38.7% of MO were decomposed by CdS(7)–Ag/TiO2 NTs,
Fig. 7. (a) Efficiency of adsorption and degradation and (b) degradation rate constants of MO on different samples: CdS(5)/TiO2 NTs (a), CdS(3)–Ag/TiO2 NTs (b), CdS(5)–Ag/TiO2 NTs (c) and CdS(7)–Ag/TiO2 NTs (d).
CdS(3)–Ag/TiO2 NTs and CdS(5)/TiO2 NTs under the same illumination time, respectively. Fig. 7a also shows that the adsorption amount of the MO for all samples is very small in comparison with the photocatalytic decomposition percentage. Assuming a pseudo-first-order reaction, the rate constant for MO decay is plotted against the SILAR cycles of CdS on the TiO2 NTs, and presented in Fig. 7b. The CdS(5)–Ag/TiO2 NTs show a maximum value with a rate of 0.131 h−1 , which is approximately two times greater than that of CdS(5)/TiO2 . Previous report has proved that composite
Fig. 6. (a) The transient photocurrent response of TiO2 NTs (a), CdS(5)/TiO2 NTs (b), CdS(3)–Ag/TiO2 NTs (c), CdS(5)–Ag/TiO2 NTs (d) and CdS(7)–Ag/TiO2 NTs (e), and (b) schematic diagram of the charge separation of CdS–Ag/TiO2 NTs.
Q. Wang et al. / Electrochimica Acta 83 (2012) 140–145
plasmonic-metal/semiconductor photocatalysts achieved significantly higher rates in various photocatalytic reactions compared with bare semiconductor counterparts [34]. In our system, Schottky barriers formed at CdS–Ag junction, which served as efficient electron traps to avoid electron–hole recombination, and the separated electrons and holes were then free to initiate reactions with the MO adsorbed on the photocatalyst surfaces. 4. Conclusions In summary, highly aligned TiO2 NTs were co-sensitized with Ag and CdS nanoparticles by SILAR technique for 3, 5 and 7 cycles, respectively. The optical and photoelectrochemical properties of the CdS–Ag/TiO2 NTs were examined. The results indicated that CdS(5)–Ag/TiO2 NTs showed higher visible light photocurrents and photocatalytic activity. The enhanced photoelectrochemical properties can be attributed to the extended absorption in the visible light region and effective separation of photo-generated carriers by the co-sensitization of Ag and CdS nanoparticles. Acknowledgments This work was financially supported by Key Item for Basic Research of Shanghai (No. 05JC14058), the National Natural Science Foundation of China (No. 50672069) and the Nanotechnology Special Foundation of Shanghai (No. 11nm0500700). References [1] X.L. He, Y.Y. Cai, H.M. Zhang, C.H. Liang, Journal of Materials Chemistry 21 (2011) 475. [2] P. Xiao, B.B. Garcia, Q. Guo, D.W. Liu, G.Z. Cao, Electrochemistry Communications 9 (2007) 2441. [3] L.X. Yang, Y. Xiao, S.H. Liu, Y. Li, Q.Y. Cai, S.L. Luo, Applied Catalysis B: Environmental 94 (2010) 142. [4] Y.Y. Song, F.S. Stein, S. Berger, P. Schmuki, Small 6 (2010) 1180. [5] Z.H. Zhang, M.F. Hossain, T. Takahashi, International Journal of Hydrogen Energy 35 (2010) 8528. [6] D.A. Wang, Y. Liu, B. Yu, F. Zhuo, W.M. Liu, Chemistry of Materials 21 (2009) 1198.
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Ag and CdS nanoparticles co-sensitized TiO2 nanotubes for enhancing visible photoelectrochemical performance Qingyao Wang, Xiuchun Yang ∗ , Dan Liu, Lina Chi, Junwei Hou School of Materials Science and Engineering, Tongji University, Shanghai 201804, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 May 2012 Received in revised form 25 July 2012 Accepted 25 July 2012 Available online 9 August 2012 Keywords: TiO2 nanotubes Successive ionic layer adsorption and reaction Ag and CdS nanoparticles Photocurrent Photocatalysis
a b s t r a c t The Ag and CdS nanoparticles co-sensitization of TiO2 nanotubes (CdS–Ag/TiO2 NTs) were prepared by successive ionic layer adsorption and reaction (SILAR) technique. The phase composition, morphology and optical property were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV–vis diffusion reflection spectroscopy (DRS). The comodification of Ag and CdS nanoparticles expanded the photoresponse range of TiO2 NTs from ultraviolet region to 668.7 nm, and the CdS–Ag/TiO2 NTs prepared by SILAR deposition of 5 cycles exhibited higher visible photocurrent and stability against photocorrosion. The detailed electrons transfer mechanism of CdS–Ag/TiO2 NTs was proposed, and photocatalytic activity toward degradation of methyl orange (MO) under visible-light irradiation was also investigated. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, titanium dioxide nanotubes have attracted great interest due to their unique electronic and optical properties, which can be applied in photocatalyst, biosensor, photo-chromic devices and other applications [1–5]. Many methods, such as anodization [6–8], template technique [9], hydrothermal process [10], and soft chemical process [11], have been used to prepare TiO2 NTs. Among these fabrication techniques, the electrochemical anodization process, since its discovery, has been considered as the most prominent one in producing high-aspect-ratio nanotubes with controllable tube size and morphology [12,13]. However, the intrinsic band gap of TiO2 (3.2 eV) and high recombination rate are the two major factors limiting the further improvement of photoelectrochemical properties. Various attempts, such as transition metal cation doping [14], nonmetal doping [15,16] and surface modification with semiconductor [17] have been tried to overcome these obstacles. Coupling TiO2 NTs with narrow band gap semiconductors was found to be the most effective method to improve the visible light activity [18,19]. CdS (2.5 eV), a well-known narrow band gap semiconductor with excellent visible absorption and rapid electron injection rate, has attracted considerable attention [20–24]. Various methods such as
∗ Corresponding author. E-mail address: [email protected] (X. Yang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.07.092
layer-by-layer deposition [25], chemical bath deposition [26] and electrochemical atomic layer deposition [27] were used to modify TiO2 NTs with CdS nanoparticles. Nevertheless, the SILAR method is considered the simplest and most controllable way of synthesizing CdS/TiO2 NTs. Recently, Chen et al. [27] synthesized CdS nanoparticles sensitized TiO2 NTs by SILAR technique, which caused the absorption to red-shift from 388 to 494 nm. However, poor conductivity of CdS/TiO2 NTs results in a low photo-generated electron transfer rate, facilitating the recombination of the electrons–holes pairs. A typical strategy of solving this obstacle is to deposit noble metal on the surface of TiO2 NTs to achieve co-sensitization with CdS nanoparticles. Owning to its high carriers transfer rate and plasmonic effect, Ag nanoparticle deposited on CdS/TiO2 NTs surface acts as a sink for photo-generated electrons, promoting charge carrier separation [28,1,29]. On the basis of above considerations, such an advantageous combination of the deposition of CdS and Ag nanoparticles on TiO2 NTs could simultaneously realize enhancement of visiblelight response and effective separation of electron–hole pairs. Last year, Xie et al. [30] successfully fabricated CdS–Ag/TiO2 NTs by electrochemical methods. Nevertheless, electrochemical methods could only provide CdS nanoparticles coverage on the top surface of TiO2 NTs, and little work has been reported on the electron transport mechanism of CdS–Ag/TiO2 NTs in detail. Therefore, the SILAR fabrication of CdS–Ag/TiO2 NTs with effective coverage of CdS nanoparticles is still challenging and demanding. In this paper, TiO2 nanotubes co-sensitized by Ag and CdS nanoparticles were successfully prepared using the SILAR method.
Q. Wang et al. / Electrochimica Acta 83 (2012) 140–145
The effects of Ag and CdS nanoparticles on the morphology and photoelectrochemical property of the CdS–Ag/TiO2 NTs were systematically investigated.
141
2. Experimental
the dye on catalyst surface was established by mechanical stirring in the dark for 30 min. After visible-light irradiation for 5 h, the remaining dye concentration was determined with a UV1700 UV–vis spectrophotometer by detecting the maximum absorption wavelength for MO at 464 nm.
2.1. Experimental materials
3. Results and discussion
NH4 F, AgNO3 , CdCl2 , Na2 S, NaBH4 , mercaptoacetic acid and ethylene glycol were purchased from Shanghai Chemical Reagent Co. Ltd. All reagents are of analytical grade and used without further purification.
The XRD patterns of TiO2 NTs and CdS–Ag/TiO2 NTs are shown in Fig. 1. All diffraction peaks in Fig. 1a besides Ti can be indexed to the anatase TiO2 , which is in good agreement with the standard spectrum (JCPDS, card no: 211272). The diffraction peaks of hexagonal CdS (JCPDS, card no: 41-1049) could be observed after CdS nanoparticles sensitization, and the peaks intensity increases with increasing SILAR cycles. However, Ag nanoparticles deposited on TiO2 NTs were very small and highly dispersed, so no characteristic peaks attributed to Ag nanoparticles were detected. From the SEM image of TiO2 NTs (Fig. 2a), the vertically oriented TiO2 NTs are with a diameter of approximately 137 nm and a wall thickness of about 30 nm. A cross-section view of the TiO2 NTs as shown in Fig. 2b indicates that all walls are entirely smooth. No obvious changes of the morphologies were observed in the top (Fig. 2d) and cross-section view (Fig. 2e) of Ag/TiO2 NTs. Nevertheless, the EDS result (Fig. 2f) verified the presence of Ag nanoparticles. Fig. 2g and h shows SEM images of Ag and CdS nanoparticles filled TiO2 NTs after 5 cycles, and the right views show the corresponding enlarged images. It was apparent that Ag and CdS nanoparticles with a uniform diameter were distributed on surface and inner walls of the TiO2 NTs, and the deposition process did not damage the ordered tubular structure. The EDS spectra (Fig. 2c and i) indicate that the bare TiO2 NTs are composed of Ti and O elements, and the CdS(5)–Ag/TiO2 NTs are composed of elements Ti, O, 0.6 at.% Ag and 7.18 at.% S and 6.33 at.% Cd. The atomic ratio of S versus Cd is little larger than stoichiometry, and the excess S element is considered from the linker mercaptoacetic acid. The detailed microscopic structure of the CdS(5)–Ag/TiO2 NTs was further investigated by TEM. Fig. 3a and b shows the lowmagnification TEM images of the CdS(5)–Ag/TiO2 NTs. It could be clearly seen that many nanoparticles with diameters of approximately 10 nm were deposited into the TiO2 NTs. Fig. 3c is a HRTEM image taken from the white square area in Fig. 3b. In the nanoparticle observed, the crystallinity of CdS nanoparticle was seen clearly with the interplane distance of 0.316 nm, which corresponds to the
2.2. Preparation of Ag nanoparticles sensitized TiO2 nanotubes TiO2 NTs were prepared according to a two-step anodization as reported previously [31]. Ag nanoparticles sensitized TiO2 nanotubes were fabricated by using SILAR technique, similar to our previous method [32]. The TiO2 NTs were immersed in 0.3 M mercaptoacetic acid solution for 30 min. Afterwards, the TiO2 NTs containing mercaptoacetic acid were dried at 60 ◦ C for 10 h. TiO2 NTs containing mercaptoacetic acid were immersed in 0.2 M AgNO3 solution for 5 min. After rinsed with deionized water, the nanotubes were immersed in 0.15 M NaBH4 solution for another 5 min. 2.3. Synthesis of Ag and CdS nanoparticles co-sensitized TiO2 nanotubes The Ag/TiO2 NTs were sequentially sensitized with CdS nanoparticles by using SILAR technique. First, Ag/TiO2 NTs were dipped into 0.1 M CdCl2 solution for 5 min, rinsed with deionized water, then dipped for another 5 min into 0.1 M Na2 S solution and again rinsed with deionized water. The two-step dipping procedure was termed as one SILAR cycle. This SILAR processes for 3, 5 and 7 cycles were denoted as CdS(3)–Ag/TiO2 NTs, CdS(5)–Ag/TiO2 NTs and CdS(7)–Ag/TiO2 NTs, respectively. For comparison, bare TiO2 NTs modification by CdS nanoparticles for 5 cycles was carried out and denoted as CdS(5)/TiO2 NTs. 2.4. Characterization The phase compositions of samples were determined by a Rigaku D/Max 2400 X-ray diffractometer (XRD) equipped with graphite monochromatized Cu K␣ radiation. The morphology of the prepared sample was observed using scanning electron microscopy (SEM, Quanta 200 FEG) and transmission electron microscopy (TEM, Tecnai-F30). UV–vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV-2550 UV–vis spectrophotometer with an integrating sphere attachment. Photoelectrochemical performances of the as-prepared samples were measured with LK98II electrochemical system (Lanlike, China) using a three-electrode system composed of the samples as a working electrode, a Pt foil as a counter electrode, a saturated calomel electrode (SCE) as a reference electrode in 0.1 M Na2 SO4 solution. The working electrode was illuminated with a solar simulator equipped with a 500 W Xe lamp with a visible-light filter (<420 nm). The photocurrent dynamics of the working electrode were recorded according to the responses to sudden switching on and off at 0.25 V bias. 2.5. Photocatalytic activity test The photocatalytic activity of the prepared samples was evaluated by photocatalytic decomposition of 10−5 M methyl orange (MO) solution under a 500 W Xe lamp with a visible-light filter (<420 nm). Before photodegradation, adsorption equilibrium for
Fig. 1. XRD patterns of TiO2 NTs (a), CdS(3)–Ag/TiO2 NTs (b), CdS(5)–Ag/TiO2 NTs (c) and CdS(7)–Ag/TiO2 NTs (d).
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Fig. 2. SEM images of TiO2 NTs (left), Ag/TiO2 NTs (middle) and CdS(5)–Ag/TiO2 NTs (right): (a, d, and g) top-view, (b, e, and h) cross-sectional view and (c, f, and i) EDS spectra.
(1 0 1) plane in the CdS nanoparticles. Close observation by HRTEM (Fig. 3d) demonstrated that the CdS nanoparticles were markedly adsorbed on the surface of Ag nanoparticles with a diameter of about 5 nm. The interplanar spacings of 0.236, 0.358, 0.337 and 0.352 nm of crystal lattices are fairly close to those of the (1 1 1) plane of Ag, (1 0 0) and (0 0 2) planes of CdS and (1 0 1) plane of anatase, respectively. Fig. 4 shows a series of SEM images of Ag and CdS nanoparticles co-sensitized TiO2 NTs prepared by SILAR for 3, 5 and 7 cycles, respectively. As shown in Fig. 4a, few CdS nanoparticles were deposited on the nanotubes. With the deposition up to 5 cycles, CdS nanoparticles were almost uniformly distributed on the surface of each nanotube. However, when deposition was increased to 7 cycles, the entire surface of the nanotubes was covered with aggregated CdS particles, but the tubes remained well open. The UV–vis absorption spectra of TiO2 NTs, CdS/TiO2 NTs and CdS–Ag/TiO2 NTs are shown in Fig. 5. The spectrum of the bare TiO2 NTs (Fig. 5a) shows that the absorption edge is observed at about 380 nm, corresponding to the band-gap energy of 3.2 eV. After the deposition of Ag and CdS nanoparticles, it is also found that the absorption edge is shifted significantly toward the visible region, and the absorption edge red-shifts slightly with increasing CdS nanoparticles deposition cycles. For CdS(7)–Ag/TiO2 NTs, the absorption edge is extended to 668.7 nm, which is larger than that of CdS(5)–Ag/TiO2 NTs (616.7 nm) and CdS(3)–Ag/TiO2
NTs (553.3 nm). It is observed that Ag and CdS nanoparticles cosensitized TiO2 NTs extend noticeably the absorption in visible light area than that of only CdS sensitization, and such improvement is mainly attributed to the overlap with the surface plasmon resonance absorption of Ag nanoparticles. Fig. 6a shows the transient photocurrent responses of the TiO2 NTs before and after modification with Ag and CdS nanoparticles. Upon visible light illumination, the TiO2 NTs do not show any photocurrent density, and photocurrent increases with the increasing of SILAR cycles due to more CdS nanoparticles on the CdS–Ag/TiO2 NTs. The photocurrent of only CdS modified TiO2 NTs for 5 cycles is low because of fast charge recombination. By employing Ag nanoparticles, it could be possible to improve the efficiency of charge separation through charge rectification. However, since CdS(7)–Ag/TiO2 NTs are subject to long-time light illumination, the photocurrent decays rapidly and is even lower than that of CdS(5)–Ag/TiO2 NTs after the third turn-on cycle. It implies that the excess CdS nanoparticles covered on the surface of TiO2 NTs are liable to suffer photocorrosion, causing low conversion from photo-generated electron to photocurrent. Moreover, the chalcogenide electrodes are usually subjected to photocorrosion phenomenon that limits long-term stability. Electron transfer occurs, leaving behind a hole in the CdS lattice (reaction (1)): CdS(h+ + e− ) + Ag . . . . . . . . . . . . CdS(h+ ) + Ag(e− )
(1)
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Fig. 3. TEM (a and b) and HRTEM (c and d) images of CdS(5)–Ag/TiO2 NTs.
The excess holes remaining in the valence band of the CdS nanoparticles induce anodic corrosion (reaction (2)): +
2+ −
CdS(h ) . . . . . . . . . . . . (Cd
S )m (CdS)n−m . . . . . . . . . . . . S0m/2 (CdS)n−m/2
(2)
The photocurrent attenuation rate of CdS(5)–Ag/TiO2 NTs is much slower than that of CdS(7)–Ag/TiO2 NTs, and the reason maybe related to the effective utilization of photo-generated electrons. As a result of good contact between CdS and Ag nanoparticles in CdS(5)–Ag/TiO2 NTs, and it greatly improves the charge collection efficiency. For CdS(7)–Ag/TiO2 NTs, excess CdS nanoparticles
contain a large number of interfacial centers and grain boundaries with a high interfacial resistance, which trap the photo-generated electrons and delay the transportation of these electrons into the electrode side. That is to say, equivalent photocurrent means losing more CdS nanoparticles in CdS(7)–Ag/TiO2 NTs. Therefore, the CdS(5)–Ag/TiO2 NTs are the best candidate as a visible light photoelectrode. When CdS–Ag/TiO2 NTs are excited to generate electrons and holes under visible light, the band bending rectifies the flow of photo-generated charge carriers to produce photocurrent, and the
Fig. 4. SEM images of CdS(3)–Ag/TiO2 NTs (a), CdS(5)–Ag/TiO2 NTs (b) and CdS(7)–Ag/TiO2 NTs (c).
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Fig. 5. UV–vis spectra of TiO2 NTs (a), CdS(5)/TiO2 NTs (b), CdS(3)–Ag/TiO2 NTs (c), CdS(5)–Ag/TiO2 NTs (d) and CdS(7)–Ag/TiO2 NTs (e).
detailed mechanism is schematized in Fig. 6b. The charge transfer mechanism is found to interpret the high photocurrent in the hybrid photoelectrode, where CdS, TiO2 semiconductors and the plasmonic Ag nanoparticles contact with each other, allowing an ideal stepwise band structure for the rapid transport of excited electrons across the hybrid photoelectrode. The driving force for the electron transfer between CdS, Ag nanoparticles and TiO2 NTs is dictated by the energy difference between the conduction band energies. Because the energy level of the conduction band (CB) of CdS is higher than the Ag Fermi energy level, the electrons will continuingly transfer from the CdS to the Ag nanoparticles until the equilibrium between the Fermi level and CB of CdS. Since the electron accumulation prompts the Fermi level of the Ag nanoparticles to more negative potential, the resultant Fermi level of Ag nanoparticles up-shifts [33]. Once the new Fermi level higher than the CB of TiO2 NTs, the stored electrons will rapidly inject into TiO2 , and transfer along vertically oriented nanotube walls to the conducting Ti-metal support to generate a photocurrent. The photocatalytic activity of CdS–Ag/TiO2 NTs was evaluated by measuring the decomposition of MO under visible light irradiation for 5 h. The comparable experiment with bare TiO2 NTs was also investigated, and the less than 10% of degradation rate could be neglected. Fig. 7a shows that sample CdS(5)–Ag/TiO2 NTs has the highest photocatalytic activity among these samples, and the decomposition rate achieved 68.8%, whereas 59.7%, 44.4% and 38.7% of MO were decomposed by CdS(7)–Ag/TiO2 NTs,
Fig. 7. (a) Efficiency of adsorption and degradation and (b) degradation rate constants of MO on different samples: CdS(5)/TiO2 NTs (a), CdS(3)–Ag/TiO2 NTs (b), CdS(5)–Ag/TiO2 NTs (c) and CdS(7)–Ag/TiO2 NTs (d).
CdS(3)–Ag/TiO2 NTs and CdS(5)/TiO2 NTs under the same illumination time, respectively. Fig. 7a also shows that the adsorption amount of the MO for all samples is very small in comparison with the photocatalytic decomposition percentage. Assuming a pseudo-first-order reaction, the rate constant for MO decay is plotted against the SILAR cycles of CdS on the TiO2 NTs, and presented in Fig. 7b. The CdS(5)–Ag/TiO2 NTs show a maximum value with a rate of 0.131 h−1 , which is approximately two times greater than that of CdS(5)/TiO2 . Previous report has proved that composite
Fig. 6. (a) The transient photocurrent response of TiO2 NTs (a), CdS(5)/TiO2 NTs (b), CdS(3)–Ag/TiO2 NTs (c), CdS(5)–Ag/TiO2 NTs (d) and CdS(7)–Ag/TiO2 NTs (e), and (b) schematic diagram of the charge separation of CdS–Ag/TiO2 NTs.
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plasmonic-metal/semiconductor photocatalysts achieved significantly higher rates in various photocatalytic reactions compared with bare semiconductor counterparts [34]. In our system, Schottky barriers formed at CdS–Ag junction, which served as efficient electron traps to avoid electron–hole recombination, and the separated electrons and holes were then free to initiate reactions with the MO adsorbed on the photocatalyst surfaces. 4. Conclusions In summary, highly aligned TiO2 NTs were co-sensitized with Ag and CdS nanoparticles by SILAR technique for 3, 5 and 7 cycles, respectively. The optical and photoelectrochemical properties of the CdS–Ag/TiO2 NTs were examined. The results indicated that CdS(5)–Ag/TiO2 NTs showed higher visible light photocurrents and photocatalytic activity. The enhanced photoelectrochemical properties can be attributed to the extended absorption in the visible light region and effective separation of photo-generated carriers by the co-sensitization of Ag and CdS nanoparticles. Acknowledgments This work was financially supported by Key Item for Basic Research of Shanghai (No. 05JC14058), the National Natural Science Foundation of China (No. 50672069) and the Nanotechnology Special Foundation of Shanghai (No. 11nm0500700). References [1] X.L. He, Y.Y. Cai, H.M. Zhang, C.H. Liang, Journal of Materials Chemistry 21 (2011) 475. [2] P. Xiao, B.B. Garcia, Q. Guo, D.W. Liu, G.Z. Cao, Electrochemistry Communications 9 (2007) 2441. [3] L.X. Yang, Y. Xiao, S.H. Liu, Y. Li, Q.Y. Cai, S.L. Luo, Applied Catalysis B: Environmental 94 (2010) 142. [4] Y.Y. Song, F.S. Stein, S. Berger, P. Schmuki, Small 6 (2010) 1180. [5] Z.H. Zhang, M.F. Hossain, T. Takahashi, International Journal of Hydrogen Energy 35 (2010) 8528. [6] D.A. Wang, Y. Liu, B. Yu, F. Zhuo, W.M. Liu, Chemistry of Materials 21 (2009) 1198.
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