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Facile method for fabricating silver-doped TiO2 nanotube arrays with enhanced photoelectrochemical property
Materials Letters 122 (2014) 248–251
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile method for fabricating silver-doped TiO2 nanotube arrays with enhanced photoelectrochemical property Yang Wang a,b, Zhen Li a,b,n, Yunfeng Tian a,b, Wen Zhao a,b, Xueqin Liu a,b, Jianbo Yang a,b a b
Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 December 2013 Accepted 14 February 2014 Available online 22 February 2014
A facile sensitization route was adopted to construct well-dispersed Ag nanoparticles on TiO2 nanotube arrays (TNAs) which were prepared by the anodization method. The Ag content loaded on the arrays was controlled by changing the concentration of AgNO3 solution. Ag/TiO2 nanotube arrays (Ag/TNAs) were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) pattern, UV–vis spectroscopy and photoluminescence (PL) spectroscopy. The effect of Ag content on the photoelectrochemical property of TNAs was studied. It showed significant enhancement in photoelectrochemical property compared to pure TNAs. & 2014 Elsevier B.V. All rights reserved.
Keywords: TiO2 nanotube arrays Composite materials Nanoparticles Photoelectrochemical property
1. Introduction Since Zwilling et al. firstly showed that Ti could be converted to highly ordered nanotube arrays using self-assembly during anodic oxidation [1], there have been lots of efforts to study the morphology of the TNAs and discover their potential applications [2–4]. It has been well recognized that TNAs have important application in solar cells, environmental purification, photolysis water, gas sensor and bio-application due to its unique highly ordered array structure, good mechanical and chemical stability, excellent corrosion resistance and high specific surface area [5–7]. Nevertheless, the practical application of TNAs was limited by several defects. On the one hand, the wide band gap (3.2 eV) of TiO2 can only be excited by UV light, which only accounts for 5% of the solar spectrum. On the other hand, the rapid recombination of photoinduced electrons and holes (h þ /e ) greatly lowered the quantum efficiency [8]. In order to effectively extend the photoresponse of TiO2 into visible light region and separate the photoinduced charge carriers, a great deal of attempts has been performed. An efficient way to improve their performance is to complex with noble metal [9]. The specific interactions between the noble metal nanoparticles and TiO2 materials have been recognized to have Schottky barriers, which significantly prohibits the recombination of photogenerated electrons and holes [10]. Among modified noble metals, Ag is one of the most suitable for
n
Corresponding author. Tel.: þ 86 27 678 83737; fax: þ 86 27 678 83732. E-mail address: [email protected] (Z. Li).
http://dx.doi.org/10.1016/j.matlet.2014.02.053 0167-577X & 2014 Elsevier B.V. All rights reserved.
industrial applications due to its low cost, nontoxicity and easy preparation [11,12]. In this work, Ag/TNAs were prepared by the wet-chemical method. This method not only obtained highly dispersed Ag nanoparticles on TNAs, but produced high quality material with advanced properties. Combined with TNAs, the reason for the excellent photocurrent performance of the Ag/TNAs was also discussed.
2. Experimental Preparation of TNAs: TNAs were prepared by anodization of Ti foils in a conventional two electrode configuration at a constant potential of 50 V at room temperature for 2 h in ethylene glycol electrolyte containing 0.3% NH4F electrolyte, with iron sheet as the counter electrode. After anodization, the samples were rinsed by distilled water, and then air-dried. The as-prepared TNAs were directly annealed at 500 1C for 2 h in air atmosphere. Ag deposition: Firstly, the prepared TNAs were carefully immersed into 50 mL of ethanol solution containing 0.02 g of SnCl2 with vigorous stirring for about 30 min at room temperature to obtain the activated TNAs. Secondly, the activated TNAs were immersed into an AgNO3 solution with a certain concentration. After that, the obtained Ag/TNAs were taken out from the solution and washed with deionized water for 5 times. Finally, the product was dried under nitrogen atmosphere at room temperature. Characterization: The morphologies of the samples were observed by FESEM (JEOL-6300F). The crystalline structure of the samples was identified by XRD (Philips, Panalytical X'pert, Cu Kα
Y. Wang et al. / Materials Letters 122 (2014) 248–251
radiation). UV–vis Spectroscopy (UV–vis, Perkin-Elmer Lambda 35) was employed to investigate the optical properties of the TNAs and Ag/TNAs samples. The photoluminescence (PL) was carried using an F-4500 fluorescence spectrophotometer at room temperature. The photoelectrochemical current response was measured by an electrochemical workstation (CHI660C) in 0.10 M Na2SO4 under visible light illumination. The current–voltage characteristics were performed using a Keithley 2400 source meter under simulated AM 1.5 G illumination (100 mWcm 2) provided by a solar light simulator (Oriel, Model: 91192). Electrochemical impedance spectra (EIS) of the thin film made from these as-made materials were measured via an EIS spectrometer (EC-Lab SP-150, BioLogic Science Instruments) in a three-electrode cell [13].
3. Results and discussion Fig. 1a and b shows the typical top-view and cross-sectional SEM images of self-assembly TNAs. It can be seen that crystalline TNAs are highly ordered with an average inside diameter of 100 nm and an average tube thickness of 20 nm. The average tube length of the TNAs is 1.3 mm. The crystalline TNAs with highly ordered structure and vertical orientation were used as
249
substrate for Ag nanoparticles deposition. Fig. 1c–e shows SEM images of Ag nanoparticles deposition on TNAs in different AgNO3 solution. High dispersed Ag nanoparticles with the diameter of 10–15 nm on the sample surface are obtained. The density of Ag particles increases gradually with the concentrations of AgNO3. Fig. 1f is the EDX spectra of the entire area of the 1d. Obviously, it can be seen that the EDX spectrum of Ag/TNAs exists Ag beside Ti and O, and the atomic percentage of Ag is about 3.19%. The structure of the samples is analyzed with XRD. Fig. 2 shows XRD patterns of TNAs and Ag/TNAs. It is clear that all samples show anatase structure. Notably, when the concentration of AgNO3 is below 0.010 M, the intense peak corresponding to the metallic Ag (111) plane at 2θ¼38.11 could be covered up by peaks attributed to anatase at 2θ¼37.81, and other diffraction peaks could not be seen due to low Ag content (Fig. 2b). However, the intense peak could be observed as the concentration of AgNO3 increased. Especially, when the concentration of AgNO3 increases to 0.020 M, two additional peaks at 44.31 and 64.51 attributed to Ag (200) and (220) planes are observed (Fig. 2d), which proves that TNAs surface is loaded with Ag nanoparticles. Fig. 3a displays the optical absorption of TNAs and Ag loaded TNAs with different Ag contents. The absorption edge at wavelength lower than 387 nm (ca. 3.2 eV) can be assigned to the intrinsic band gap
Fig. 1. SEM images of TNAs (a, b), Ag/TNAs (c, d, e) obtained in the presence of AgNO3 (0.005 M, 0.010 M, 0.020 M) and EDX spectra of Ag/TNAs (f) obtained under the electrodeposited charge density of 900 mC cm 2.
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Y. Wang et al. / Materials Letters 122 (2014) 248–251
absorption of pure anatase TiO2. After depositing Ag nanoparticles, the absorption band edges of Ag/TNAs are red-shift. It is also found that with the increasing of the concentrations of AgNO3, the peak at around 400–650 nm becomes broader. This may be attributed to the surface plasma resonance (SPR) effect of spatially confined electrons in metallic Ag nanoparticles [14,15]. However, when the concentration of Ag increases to 0.020 M, the visible region absorption effect reduced. Compared to the pure TNAs, the Ag/TNAs exhibit strong and broad absorption in the visible region, which make them a promising material for solar cells. Fig. 3b shows photoluminescence (PL) spectra for the TNAs and the Ag/TNAs synthesized at different Ag contents. It can be clearly seen that the relative PL intensity of the TNAs is the highest, which means that electrons and holes of TNAs have a higher tendency to recombine. And the PL intensity of Ag/TNAs is smaller than that of TNAs. When the concentration of AgNO3 is below 0.010 M, the PL intensity of the nanostructures becomes lower with the increasing of Ag contents. Photoelectrochemical property was also investigated by measuring the transient photocurrent in 0.10 M Na2SO4 electrolyte without any sacrificial reagents. Fig. 4a shows the photocurrent under visible light irradiation. In the dark, there is no current flow. However, the responsive photocurrent density drastically increased under visible light irradiation. And the responsive
photocurrent density of TNAs is much less than that of Ag/TNAs under the same condition. Moreover, when the concentration of AgNO3 is 0.010 M, the photocurrent density of Ag/TNAs electrode is almost 1.7 times higher than that of pure TNAs. The samples obtained at different conditions were then exploited as photoanodes to assemble dye-sensitized solar cells (DSSCs). Fig. 4b shows the current–voltage (J–V) characteristics of the resulting N719 dye-sensitized solar cells. Compared to TNAs, Ag/TNAs have a higher short circuit current density (Jsc¼4.81 mA/ cm2 for Ag/TNAs vs. Jsc ¼1.78 mA/cm2 for TNAs), The increase of Jsc is largely due to a large surface area of Ag/TNAs, which absorb much more dye molecules and allows more injection of photogenerated electrons. Additionally, the open-circuit photovoltages (Voc) of Ag/TNAs increased apparently. It can be ascribed to the blocking of interfacial recombination by the Ag nanoparticles [16] and the fast electron transport of one-dimensional TiO2 nanotubes. As shown in Fig. 4c, the typical EIS were presented as Nyquist plots. It is observed that, with the introduction of Ag, the semicircle in the plot became shorter, which indicated a decrease in the solid state interface layer resistance and the charge transfer resistance on the surface [17]. Overall, both the electron accepting and transporting properties of Ag in the composite could contribute to the suppression of charge recombination [18], and thereby a rapid transport of charge carriers and an effective charge separation in the photoelectrochemical property would be achieved. There are two main factors for the decreased photoelectrochemical property of TNAs compared to Ag/TNAs. One is that the photogenerated charge carriers in the Ag/TNAs might be separated more efficiently than in pure TNAs due to the Schottky barriers formed at the interface between Ag nanoparticles and TNAs [19]. The other one is that that the photogenerated electrons are transferred from Ag nanoparticles to TNAs, which obviously reduces the recombination of photogenerated electrons and holes, and improves photocurrent response of TNAs electrode [20,21]. Additionally, the reason for the decrease of the photocurrent with increase of Ag loading is that the excessive surface Ag particles may act as recombination centers at high silver deposition.
4. Conclusions
Fig. 2. XRD spectra of TNAs (a) and Ag/TNAs obtained in the presence of AgNO3: 0.005 M (b), 0.010 M (c), 0.020 M (d).
Highly ordered TNAs loaded with Ag nanoparticles have been synthesized by the wet-chemical method. It is evidenced that appropriate amount of Ag nanoparticles loaded on TNAs are able to significantly improve the photoelectrochemical property. Compared
Fig. 3. (a) UV–vis absorption spectra of TNAs and Ag/TNAs obtained in the presence of different concentrations of AgNO3. (b) PL emission spectra of TNAs and the as-prepared samples.
Y. Wang et al. / Materials Letters 122 (2014) 248–251
251
Fig. 4. (a) Short-circuit photocurrent density vs. time for the TNAs and Ag/TNAs obtained in the presence of different concentrations of AgNO3 in 0. 10 M Na2SO4 solution under visible light irradiation. (b) Photocurrent–photovoltage (J–V) curves of the resulting DSSCs. (c) Nyquist plots of the EIS data of TNAs and Ag/TNAs (0.010 M AgNO3).
with pure TNAs, the Ag/TNAs obtained in 0.010 M AgNO3 have 1.7 times higher photocurrent. It is indicated that the route is a very facile and promising approach to synthesize noble nanoparticles loaded nanotube array materials. References [1] [2] [3] [4] [5] [6] [7] [8]
Zwilling V, Aucouturier M, Darque-Ceretti E. Electrochim Acta 1999;45:921–9. Pan DY, Li JH, Wang L, Xi C, Xue Q, Wu MH, Li Z. Mater Lett 2013;100:82–5. Liu SW, Yu JG, Jaroniec M. J Am Chem Soc 2010;132:11914–6. Chen KS, Feng XR, Hu R, Li YB, Xie K, Li Y, Gu HS. J Alloys Compd 2013;554:72–9. Bian HD, Shu X, Zhang JF, Yuan B, Wang Y, Liu LJ, Xu GQ, Chen Z, Wu YC. Chem Asian J 2013;8:2746–54. Macak JM, Zlamal M, Krysa J, Schmuki P. Small 2007;3:300–4. Paulose M, Mor GK, Varghese OK, Shankar K, Grimes CAJ. Photochem Photobiol A: Chem 2006;178:8–15. Xing MY, Zhang JL, Chen F. Appl Catal B: Environ 2009;89:563–9.
[9] Chen KS, Feng XR, Hu R, Li YB, Xie K, Li Y, Gu HS. J Alloys Compd 2013;554:72–9. [10] Xie KP, Sun L, Wang C, Lai YK, Wang MY, Chen HB, Lin CJ. Electrochim Acta 2010;55:7211–8. [11] Liu X, Liu ZQ, Hao SX, Chu W. Mater Lett 2012;80:66–8. [12] Chang WT, Hsueh YC, Huang SH, Liu KI, Kei CC, Perng TP. J Mater Chem A 2013;1:1987–91. [13] Pan X, Zhao Y, Liu S, Korzeniewski CL, Wang S, Fan ZY. Appl Mater Interfaces 2012;4:3944–50. [14] Wang SL, Qian HH, Hu Y, Dai W, Zhong YJ, Chen JF, Hu X. Dalton Trans 2013;42:1122–8. [15] Atwater HA, Polman A. Nat Mater 2010;9:205–13. [16] Lewis NS. Science 2007;315:798–801. [17] He BL, Dong B, Li HL. Electrochem Commun 2007;9:425–30. [18] Hou Y, Li XY, Zhao QD, Quan X, Chen GH. J Mater Chem 2011;21:18067–76. [19] Shan ZC, Wu JJ, Xu FF, Huang FQ, Ding HM. J Phys Chem C 2008;112:15423–8. [20] Zou XX, Silva R, Huang XX, Al-Sharab JFT, Asefa T. Chem Commun 2013;49:382–4. [21] Su CY, Liu L, Zhang MY, Zhang Y, Shao CL. CrystEngComm 2012;14:3989–99.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile method for fabricating silver-doped TiO2 nanotube arrays with enhanced photoelectrochemical property Yang Wang a,b, Zhen Li a,b,n, Yunfeng Tian a,b, Wen Zhao a,b, Xueqin Liu a,b, Jianbo Yang a,b a b
Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 December 2013 Accepted 14 February 2014 Available online 22 February 2014
A facile sensitization route was adopted to construct well-dispersed Ag nanoparticles on TiO2 nanotube arrays (TNAs) which were prepared by the anodization method. The Ag content loaded on the arrays was controlled by changing the concentration of AgNO3 solution. Ag/TiO2 nanotube arrays (Ag/TNAs) were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) pattern, UV–vis spectroscopy and photoluminescence (PL) spectroscopy. The effect of Ag content on the photoelectrochemical property of TNAs was studied. It showed significant enhancement in photoelectrochemical property compared to pure TNAs. & 2014 Elsevier B.V. All rights reserved.
Keywords: TiO2 nanotube arrays Composite materials Nanoparticles Photoelectrochemical property
1. Introduction Since Zwilling et al. firstly showed that Ti could be converted to highly ordered nanotube arrays using self-assembly during anodic oxidation [1], there have been lots of efforts to study the morphology of the TNAs and discover their potential applications [2–4]. It has been well recognized that TNAs have important application in solar cells, environmental purification, photolysis water, gas sensor and bio-application due to its unique highly ordered array structure, good mechanical and chemical stability, excellent corrosion resistance and high specific surface area [5–7]. Nevertheless, the practical application of TNAs was limited by several defects. On the one hand, the wide band gap (3.2 eV) of TiO2 can only be excited by UV light, which only accounts for 5% of the solar spectrum. On the other hand, the rapid recombination of photoinduced electrons and holes (h þ /e ) greatly lowered the quantum efficiency [8]. In order to effectively extend the photoresponse of TiO2 into visible light region and separate the photoinduced charge carriers, a great deal of attempts has been performed. An efficient way to improve their performance is to complex with noble metal [9]. The specific interactions between the noble metal nanoparticles and TiO2 materials have been recognized to have Schottky barriers, which significantly prohibits the recombination of photogenerated electrons and holes [10]. Among modified noble metals, Ag is one of the most suitable for
n
Corresponding author. Tel.: þ 86 27 678 83737; fax: þ 86 27 678 83732. E-mail address: [email protected] (Z. Li).
http://dx.doi.org/10.1016/j.matlet.2014.02.053 0167-577X & 2014 Elsevier B.V. All rights reserved.
industrial applications due to its low cost, nontoxicity and easy preparation [11,12]. In this work, Ag/TNAs were prepared by the wet-chemical method. This method not only obtained highly dispersed Ag nanoparticles on TNAs, but produced high quality material with advanced properties. Combined with TNAs, the reason for the excellent photocurrent performance of the Ag/TNAs was also discussed.
2. Experimental Preparation of TNAs: TNAs were prepared by anodization of Ti foils in a conventional two electrode configuration at a constant potential of 50 V at room temperature for 2 h in ethylene glycol electrolyte containing 0.3% NH4F electrolyte, with iron sheet as the counter electrode. After anodization, the samples were rinsed by distilled water, and then air-dried. The as-prepared TNAs were directly annealed at 500 1C for 2 h in air atmosphere. Ag deposition: Firstly, the prepared TNAs were carefully immersed into 50 mL of ethanol solution containing 0.02 g of SnCl2 with vigorous stirring for about 30 min at room temperature to obtain the activated TNAs. Secondly, the activated TNAs were immersed into an AgNO3 solution with a certain concentration. After that, the obtained Ag/TNAs were taken out from the solution and washed with deionized water for 5 times. Finally, the product was dried under nitrogen atmosphere at room temperature. Characterization: The morphologies of the samples were observed by FESEM (JEOL-6300F). The crystalline structure of the samples was identified by XRD (Philips, Panalytical X'pert, Cu Kα
Y. Wang et al. / Materials Letters 122 (2014) 248–251
radiation). UV–vis Spectroscopy (UV–vis, Perkin-Elmer Lambda 35) was employed to investigate the optical properties of the TNAs and Ag/TNAs samples. The photoluminescence (PL) was carried using an F-4500 fluorescence spectrophotometer at room temperature. The photoelectrochemical current response was measured by an electrochemical workstation (CHI660C) in 0.10 M Na2SO4 under visible light illumination. The current–voltage characteristics were performed using a Keithley 2400 source meter under simulated AM 1.5 G illumination (100 mWcm 2) provided by a solar light simulator (Oriel, Model: 91192). Electrochemical impedance spectra (EIS) of the thin film made from these as-made materials were measured via an EIS spectrometer (EC-Lab SP-150, BioLogic Science Instruments) in a three-electrode cell [13].
3. Results and discussion Fig. 1a and b shows the typical top-view and cross-sectional SEM images of self-assembly TNAs. It can be seen that crystalline TNAs are highly ordered with an average inside diameter of 100 nm and an average tube thickness of 20 nm. The average tube length of the TNAs is 1.3 mm. The crystalline TNAs with highly ordered structure and vertical orientation were used as
249
substrate for Ag nanoparticles deposition. Fig. 1c–e shows SEM images of Ag nanoparticles deposition on TNAs in different AgNO3 solution. High dispersed Ag nanoparticles with the diameter of 10–15 nm on the sample surface are obtained. The density of Ag particles increases gradually with the concentrations of AgNO3. Fig. 1f is the EDX spectra of the entire area of the 1d. Obviously, it can be seen that the EDX spectrum of Ag/TNAs exists Ag beside Ti and O, and the atomic percentage of Ag is about 3.19%. The structure of the samples is analyzed with XRD. Fig. 2 shows XRD patterns of TNAs and Ag/TNAs. It is clear that all samples show anatase structure. Notably, when the concentration of AgNO3 is below 0.010 M, the intense peak corresponding to the metallic Ag (111) plane at 2θ¼38.11 could be covered up by peaks attributed to anatase at 2θ¼37.81, and other diffraction peaks could not be seen due to low Ag content (Fig. 2b). However, the intense peak could be observed as the concentration of AgNO3 increased. Especially, when the concentration of AgNO3 increases to 0.020 M, two additional peaks at 44.31 and 64.51 attributed to Ag (200) and (220) planes are observed (Fig. 2d), which proves that TNAs surface is loaded with Ag nanoparticles. Fig. 3a displays the optical absorption of TNAs and Ag loaded TNAs with different Ag contents. The absorption edge at wavelength lower than 387 nm (ca. 3.2 eV) can be assigned to the intrinsic band gap
Fig. 1. SEM images of TNAs (a, b), Ag/TNAs (c, d, e) obtained in the presence of AgNO3 (0.005 M, 0.010 M, 0.020 M) and EDX spectra of Ag/TNAs (f) obtained under the electrodeposited charge density of 900 mC cm 2.
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Y. Wang et al. / Materials Letters 122 (2014) 248–251
absorption of pure anatase TiO2. After depositing Ag nanoparticles, the absorption band edges of Ag/TNAs are red-shift. It is also found that with the increasing of the concentrations of AgNO3, the peak at around 400–650 nm becomes broader. This may be attributed to the surface plasma resonance (SPR) effect of spatially confined electrons in metallic Ag nanoparticles [14,15]. However, when the concentration of Ag increases to 0.020 M, the visible region absorption effect reduced. Compared to the pure TNAs, the Ag/TNAs exhibit strong and broad absorption in the visible region, which make them a promising material for solar cells. Fig. 3b shows photoluminescence (PL) spectra for the TNAs and the Ag/TNAs synthesized at different Ag contents. It can be clearly seen that the relative PL intensity of the TNAs is the highest, which means that electrons and holes of TNAs have a higher tendency to recombine. And the PL intensity of Ag/TNAs is smaller than that of TNAs. When the concentration of AgNO3 is below 0.010 M, the PL intensity of the nanostructures becomes lower with the increasing of Ag contents. Photoelectrochemical property was also investigated by measuring the transient photocurrent in 0.10 M Na2SO4 electrolyte without any sacrificial reagents. Fig. 4a shows the photocurrent under visible light irradiation. In the dark, there is no current flow. However, the responsive photocurrent density drastically increased under visible light irradiation. And the responsive
photocurrent density of TNAs is much less than that of Ag/TNAs under the same condition. Moreover, when the concentration of AgNO3 is 0.010 M, the photocurrent density of Ag/TNAs electrode is almost 1.7 times higher than that of pure TNAs. The samples obtained at different conditions were then exploited as photoanodes to assemble dye-sensitized solar cells (DSSCs). Fig. 4b shows the current–voltage (J–V) characteristics of the resulting N719 dye-sensitized solar cells. Compared to TNAs, Ag/TNAs have a higher short circuit current density (Jsc¼4.81 mA/ cm2 for Ag/TNAs vs. Jsc ¼1.78 mA/cm2 for TNAs), The increase of Jsc is largely due to a large surface area of Ag/TNAs, which absorb much more dye molecules and allows more injection of photogenerated electrons. Additionally, the open-circuit photovoltages (Voc) of Ag/TNAs increased apparently. It can be ascribed to the blocking of interfacial recombination by the Ag nanoparticles [16] and the fast electron transport of one-dimensional TiO2 nanotubes. As shown in Fig. 4c, the typical EIS were presented as Nyquist plots. It is observed that, with the introduction of Ag, the semicircle in the plot became shorter, which indicated a decrease in the solid state interface layer resistance and the charge transfer resistance on the surface [17]. Overall, both the electron accepting and transporting properties of Ag in the composite could contribute to the suppression of charge recombination [18], and thereby a rapid transport of charge carriers and an effective charge separation in the photoelectrochemical property would be achieved. There are two main factors for the decreased photoelectrochemical property of TNAs compared to Ag/TNAs. One is that the photogenerated charge carriers in the Ag/TNAs might be separated more efficiently than in pure TNAs due to the Schottky barriers formed at the interface between Ag nanoparticles and TNAs [19]. The other one is that that the photogenerated electrons are transferred from Ag nanoparticles to TNAs, which obviously reduces the recombination of photogenerated electrons and holes, and improves photocurrent response of TNAs electrode [20,21]. Additionally, the reason for the decrease of the photocurrent with increase of Ag loading is that the excessive surface Ag particles may act as recombination centers at high silver deposition.
4. Conclusions
Fig. 2. XRD spectra of TNAs (a) and Ag/TNAs obtained in the presence of AgNO3: 0.005 M (b), 0.010 M (c), 0.020 M (d).
Highly ordered TNAs loaded with Ag nanoparticles have been synthesized by the wet-chemical method. It is evidenced that appropriate amount of Ag nanoparticles loaded on TNAs are able to significantly improve the photoelectrochemical property. Compared
Fig. 3. (a) UV–vis absorption spectra of TNAs and Ag/TNAs obtained in the presence of different concentrations of AgNO3. (b) PL emission spectra of TNAs and the as-prepared samples.
Y. Wang et al. / Materials Letters 122 (2014) 248–251
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Fig. 4. (a) Short-circuit photocurrent density vs. time for the TNAs and Ag/TNAs obtained in the presence of different concentrations of AgNO3 in 0. 10 M Na2SO4 solution under visible light irradiation. (b) Photocurrent–photovoltage (J–V) curves of the resulting DSSCs. (c) Nyquist plots of the EIS data of TNAs and Ag/TNAs (0.010 M AgNO3).
with pure TNAs, the Ag/TNAs obtained in 0.010 M AgNO3 have 1.7 times higher photocurrent. It is indicated that the route is a very facile and promising approach to synthesize noble nanoparticles loaded nanotube array materials. References [1] [2] [3] [4] [5] [6] [7] [8]
Zwilling V, Aucouturier M, Darque-Ceretti E. Electrochim Acta 1999;45:921–9. Pan DY, Li JH, Wang L, Xi C, Xue Q, Wu MH, Li Z. Mater Lett 2013;100:82–5. Liu SW, Yu JG, Jaroniec M. J Am Chem Soc 2010;132:11914–6. Chen KS, Feng XR, Hu R, Li YB, Xie K, Li Y, Gu HS. J Alloys Compd 2013;554:72–9. Bian HD, Shu X, Zhang JF, Yuan B, Wang Y, Liu LJ, Xu GQ, Chen Z, Wu YC. Chem Asian J 2013;8:2746–54. Macak JM, Zlamal M, Krysa J, Schmuki P. Small 2007;3:300–4. Paulose M, Mor GK, Varghese OK, Shankar K, Grimes CAJ. Photochem Photobiol A: Chem 2006;178:8–15. Xing MY, Zhang JL, Chen F. Appl Catal B: Environ 2009;89:563–9.
[9] Chen KS, Feng XR, Hu R, Li YB, Xie K, Li Y, Gu HS. J Alloys Compd 2013;554:72–9. [10] Xie KP, Sun L, Wang C, Lai YK, Wang MY, Chen HB, Lin CJ. Electrochim Acta 2010;55:7211–8. [11] Liu X, Liu ZQ, Hao SX, Chu W. Mater Lett 2012;80:66–8. [12] Chang WT, Hsueh YC, Huang SH, Liu KI, Kei CC, Perng TP. J Mater Chem A 2013;1:1987–91. [13] Pan X, Zhao Y, Liu S, Korzeniewski CL, Wang S, Fan ZY. Appl Mater Interfaces 2012;4:3944–50. [14] Wang SL, Qian HH, Hu Y, Dai W, Zhong YJ, Chen JF, Hu X. Dalton Trans 2013;42:1122–8. [15] Atwater HA, Polman A. Nat Mater 2010;9:205–13. [16] Lewis NS. Science 2007;315:798–801. [17] He BL, Dong B, Li HL. Electrochem Commun 2007;9:425–30. [18] Hou Y, Li XY, Zhao QD, Quan X, Chen GH. J Mater Chem 2011;21:18067–76. [19] Shan ZC, Wu JJ, Xu FF, Huang FQ, Ding HM. J Phys Chem C 2008;112:15423–8. [20] Zou XX, Silva R, Huang XX, Al-Sharab JFT, Asefa T. Chem Commun 2013;49:382–4. [21] Su CY, Liu L, Zhang MY, Zhang Y, Shao CL. CrystEngComm 2012;14:3989–99.