ARTICLE pubs.acs.org/JPCC
One-Step Fabrication of CdS Nanoparticle-Sensitized TiO2 Nanotube Arrays via Electrodeposition Zhibin Shao, Wei Zhu,* Zhi Li, Qianhui Yang, and Guanzhong Wang* Hefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China
bS Supporting Information ABSTRACT: CdS nanoparticle-sensitized TiO2 (CdS-TiO2) nanotube arrays are synthesized with a facile one-step electrodeposition technique. In these composited nanostructures, CdS nanoparticles uniformly distribute in the TiO2 nanotubes and partially embed in the shell of TiO2 nanotubes. These structures effectively prevent CdS nanoparticles assembling or clogging the nanotubes and improve the contact area between CdS nanoparticles and the TiO2 shells. Furthermore, the size and distribution density of CdS nanoparticles can be tuned easily by controlling the concentration of electrolyte. Coupling TiO2 nanotubes with CdS nanoparticles extends the optical absorption from ultraviolet into the visible-light region up to 580 nm. An 11fold enhancement in photoelectrochemical (PEC) activity is observed for CdS-TiO2 nanotube arrays compared with plain TiO2 nanotube arrays. This unique method is also suitable for the synthesis of other narrow band gap semiconductor-sensitized TiO2 nanotubes.
’ INTRODUCTION In recent years, TiO2 nanotubes have received extra attention because of their superior performances in fields such as photocatalysis,14 water photoelectrolysis,58 gas sensors,9,10 environmental purification,1113 and dye-sensitized solar cells.1419 However, the wide band gap (3.2 eV) restricts the photoresponse of TiO2 to only ultraviolet region with the wavelength below ∼390 nm of the solar spectrum and depresses vastly utilization ratio of solar power. For extention of the optical absorption into the visible-light region, various strategies have been designed, including doping TiO2 with a metal or nonmetal2022 and coupling it with organic dye or narrow band gap semiconductors such as CdS,2334 CdSe,35,36 and CdTe.37,38 Among those strategies, CdS-TiO2 nanotube arrays have attracted more attention because CdS nanoparticles are important narrow band gap semiconductor quantum dots and the photoresponse can be modulated in the visible-light spectrum by controlling their size. In general, two steps is necessary to synthesis of CdS-TiO2 nanotube arrays: (1) obtaining TiO2 nanotube arrays by Ti anodization and (2) decorating TiO2 nanotube arrays with CdS nanoparticles by means of electrochemical deposition,2325 spray pyrolysis deposition,26 successive ionic layer adsorption and reaction (SILAR),27,28 sequential chemical bath deposition (S-CBD),2932 a chemical bath method with bifunctional linker molecules,33 or close space sublimation (CSS).34 These methods have some disadvantages, for example, the anodization consumes a long time, the nanotube length of CdS loading is short, CdS nanoparticles distribute nonuniformly in the nanotubes, and they aggregate or even plug the nanotubes. r 2011 American Chemical Society
In our previous report, the fabrication of Ni/TiO2 core/shell nanorod arrays and Ni nanoparticle chains embedded in TiO2 nanotubes in anodic aluminum oxide (AAO) membranes by a one-step electrodeposition method was demonstrated.39,40 This method is also expected to fabricate narrow band gap semiconductors-sensitized TiO2 nanotube arrays. Here we report an investigation of CdS (CdSe, CdTe)-sensitized TiO2 nanotube arrays synthesized by a fast, one-step electrodeposition method and the performance of the composited nanotube arrays in PEC cells. Via codeposition with TiO2, CdS nanoparticles are partially embedded in the shell of TiO2 nanotubes. This is different from other reports that CdS nanoparticles are attached to the surface of TiO2 nanotubes. Furthermore, the CdS nanoparticles disperse uniformly inside the TiO2 nanotubes, and the nanoparticle size and distribution density can be controlled expediently. Fabricating by this method, the CdS-TiO2 nanotubes can be as long as more than 30 μm. These CdS-TiO2 composited nanotube arrays exhibit much better PEC performance than plain TiO2 nanotube arrays.
’ MATERIALS AND METHODS Materials. Titanium fluoride (TiF4, Alfa Aesar, 98%), cadmium chloride (CdCl2, SCRC, 99%), sodium thiosulfate (Na2S2O3, SCRC, 99%), sodium hydroxide (NaOH, SCRC, 96%), and sodium sulfide (Na2S, Sigma-Aldrich, 98%) are used as received. Received: August 14, 2011 Revised: December 12, 2011 Published: December 20, 2011 2438
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Scheme 1. Schematic Illustration of the Synthesis Process of Fabricating CdS-TiO2 Nanotube Arrays
Synthesis of CdS-TiO2 Nanotube Arrays. The electrolyte was prepared by dissolving TiF4, CdCl2, and Na2S2O3 in 100 mL of stirring deionized water. The stirring time is 30 min. The concentration of TiF4, CdCl2, and Na2S2O3 is 0.04, 0.1, and 0.05 M, respectively. The pH of the solution was maintained at ca. 1.8. As shown in Scheme 1, AAO membranes (Whatman Anodisc 47) were used as the templates. The diameter and length of the channels are about 250 nm and 60 μm, respectively, according to our SEM observation. The electrodeposition was performed with a standard three-electrode system (CH instruments, model 620B) with a platinum coil counter electrode and a saturated Ag/AgCl reference electrode. The electrodeposition potential was 0.6 VAg/AgCl, the temperature was 60 °C, and the time was 20 min. Before electrodeposition, a thin layer of Au (ca. 40 nm) was sputtered on the back of the AAO membranes as the working electrode. After electrodeposition, the AAO membrane was removed with 1 M NaOH at 40 °C for 30 min. The as-obtained highly ordered CdS-TiO2 nanotube arrays were annealed under an argon or air atmosphere at 500 °C for 3 h. Characterization. CdS-TiO2 nanotube arrays are characterized by various analytical and spectroscopic techniques. Field-emission scanning electron microscope (FESEM; JEOL JSM-6700F) and high-resolution transmission electron microscope (HRTEM; JEOL model 2010) were used to characterize the morphology and structure of the CdS-TiO2 nanotube arrays. Energy dispersive spectroscopy (EDS) and selected area electron diffraction (SAED) were used to determine the chemical composition and crystal structure of the CdS-TiO2 nanotube arrays. UVvisible spectroscopy (Shimadzu Solidspec-3700DUV) was used to characterize the optical absorption property of the CdS-TiO2 nanotube arrays. PEC Performance Measurements. The PEC measurement was carried out using a conventional three-electrode system. A 0.1 M Na2S aqueous solution was used as the electrolyte, and the working electrode had an area of 0.4 cm2 immerged into the solution. A 350 W Xe lamp (Shanghai LanSheng Electronic, model XQ350W) was used as the light source. The light intensity on the photoanode calibrated by a thermopile detector (Beijing Jinghe Luneng Electronic Technology, model YFJ-10) is 75 mW/cm2. The PEC response of the sample was recorded using an electrochemical analyzer from 1.2 to 0.2 VAg/AgCl with a scanning rate of 5 mV/s. The photocurrent dynamics of the working electrode were recorded according to the responses to sudden switching on and off at 0.3 VAg/AgCl bias.
’ RESULTS AND DISCUSSION Figure 1 shows SEM images of the CdS-TiO2 nanotube arrays prepared by electrodeposition at 0.6 VAg/AgCl. The backscattered
Figure 1. CdS-TiO2 nanotube arrays electrodeposited in electrolyte containing 0.04 M TiF4, 0.1 M CdCl2, and 0.05 M Na2S2O3 at 0.6 V for 20 min. (a) Typical side view BSE image of the sample after removing the AAO membranes; the inset is a top view SEM image of the arrays. (b) Cross-sectional SEM image of the film.
electron (BSE) images, shown in Figure 1a,b with enhanced atomic contrast, reveals that the CdS-TiO2 nanotubes are wellaligned parallel to each other. The TiO2 nanotubes are ca. 30 μm in length, in which CdS nanoparticles are dispersed uniformly all over their inner surface. The inset of Figure 1a, a cross-sectional image, shows that the outer diameter of TiO2 nanotubes is ca. 250 nm and CdS particles are slightly smaller than the inner diameter of TiO2 nanotubes. To avoid blocking the nanotubes, CdS particles are expected to be smaller and more discrete. The size and density of CdS nanoparticles could be tuned by modulating the concentration of Cd precursor (CdCl2) and S precursor (Na2S2O3) in the electrolyte, as demonstrated in Figure S1 in the Supporting Information. TEM is used to characterize further the structure of CdS-TiO2 nanotubes fabricated via the one-step electrodeposition. A representative TEM image shown in Figure 2a indicates that CdS nanoparticles are dispersed on the inner surface of TiO2 nanotube, in accordance with the SEM observations. Interestingly, CdS nanoparticles are found to be partially embedded in the shell of TiO2 nanotubes. The sample annealed in air (Figure S2, Supporting Information) shows more clearly this structure. By means of annealing in air, CdS nanoparticles sublimate, decompose, and are oxidized, leaving holes in the TiO2 nanotubes. Such a structure can help improve the contact area between CdS nanoparticles and the TiO2 shell, exciting the photoelectrons in the sensitizer and smoothly injecting them into the conduction band of TiO2 nanotubes.41 The fringes with lattice spacing of 0.359 nm are observed in the HRTEM image (inset in the bottom right corner of Figure 2a) of nanoparticles, which correspond to the (100) lattice plane of CdS (JCPDS file no. 75-1545). SAED patterns support a mixed 2439
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TiðOHÞ4x Fx þ xOH f TiðOHÞ4 þ xF f TiO2
Figure 2. (a) TEM image and (b) EDS spectrum of a CdS-TiO2 nanotube. The top inset is the enlarged image of the marked part, and the bottom inset is the HRTEM image of the CdS nanoparticle.
polycrystalline/amorphous microstructure with d spacings (Figure S3, Supporting Information), suggesting the presence of hexagonal CdS. The sample composition was identified by EDS analysis (Figure 2b) performed inside TEM. As shown by the EDS spectrum, there are six major elements: Ti, O, Cd, S, C, and Cu. Obviously, Ti and O peaks result from the TiO2 NTs, whereas Cu and C peaks are derived from the Cu grid and the supporting amorphous carbon film. Moreover, the quantitative analysis reveals that the atomic ratio of Cd to S is approximately 1:1, which is consistent with the atomic ratio in CdS. According to SEM, TEM, and EDS analysis, we postulate the growth mechanism of CdS-TiO2 nanotube arrays in AAO membranes by an electrochemical deposition technique. The chemical reactions in the deposition process are described as follows 2H2 O f 2Hþ þ 2OH
ð1Þ
2Hþ þ 2e f H2 v
ð2Þ
S2 O3 2 þ 2e f S2 þ SO3 2 S2 þ Cd2þ f CdS TiF4 þ ð4 xÞOH f TiðOHÞ4x Fx þ ð4 xÞF
ð3Þ
ð4Þ
We have discussed the growth mechanism of Ni/TiO2 core/ shell nanorod arrays in AAO membranes in our previous publication.39 Similarly, the formation process of the CdSTiO2 nanotube arrays is illustrated in Scheme 1. In the initial stage of the reaction, when a negative potential was applied, hydroxyl ion would be produced and diffuse rapidly along the inner surface of the channels because of the electric field driving. With the hydrolysate Ti(OH)4xFx diffusing into the channels, the reaction process 4 would occur, and TiO2 deposited on the inner surface of the channels to form TiO2 nanotubes. At the same time, S2O32 ions in the electrolyte would react with the electrons, and S2 ions produced in the process 3 would diffuse and react with Cd2+, leading to the nucleation and growth of CdS nanoparticles. Compared with the sequential deposition of TiO2 that formed the nanotubes, CdS formed discrete nanoparticles due to the low concentration of Cd and S precursor. With electrochemical deposition continuing, the thickness of TiO2 nanotube shell and the diameter of CdS nanoparticles were largened, causing CdS nanoparticles to be partially embedded in the shell of TiO2 nanotubes. At last, AAO membrane was removed with 1 M NaOH solution, and the CdS-TiO2 nanotube arrays were obtained. Figure 3 shows the absorption spectra recorded from the pure TiO2 nanotube arrays (a) and the CdS-TiO2 nanotube arrays (b). These spectra show that the pure TiO2 nanotube arrays absorb mainly UV light with a wavelength below 350 nm, whereas the band edge of the CdS-TiO2 nanotube arrays is extended to visible region at the wavelength of ∼580 nm. It is suggested that the TiO2 nanotubes are effectively sensitized by CdS nanoparticles. A comparison between the PEC performances of as-synthesized the plain TiO2 nanotube arrays and CdS-TiO2 nanotube arrays was made using a three-electrode PEC cell. Figure 4 shows the currentvoltage (IV) curves measured from the plain TiO2 nanotube array photoelectrode and the as-synthesized CdS-TiO2. The open circuit photovoltage (Voc) is 1.06 VAg/AgCl for the plain TiO2 nanotube array electrode, but the Voc shifts to 1.14 VAg/AgCl for the as-prepared CdS-TiO2 nanotube array electrode. The short-circuit photocurrent of the CdS-TiO2 nanotube array electrode (0.89 mA/cm2) is 11 times higher than that of the plain TiO2 nanotube array electrode (0.08 mA/cm2). The photoresponse of the CdS-TiO2 nanotube arrays was carried out by potentiostatic (current vs time, It) measurements under intermittent illumination. Figure 5 shows the It curve obtained from the PEC cell under a 0.3 VAg/AgCl bias, which also demonstrates that the photocurrent of the CdS-TiO2 nanotube array electrode is much higher than that of the plain TiO2 nanotubes electrode. Moreover, the CdS-TiO2 nanotube array electrode is prompt in generating photocurrent under intermittent illumination, indicating that charge-transport properties are rapid.25 Not only CdS, but also other narrow band gap semiconductors could be used to sensitize TiO2 nanotubes with this one-step electrodeposition method. We also have fabricated CdSe and CdTe nanoparticle-sensitized TiO2 nanotube arrays. (See the Supporting Information for experimental details.) The TEM image (Figure 6a) and the SEM image (Figure S4a, Supporting Information) show that the CdSe nanoparticle-sensitized TiO2 nanotubes are successfully synthesized. The lattice fringes of 0.37 nm observed in the HRTEM image (Figure 6c) of the 2440
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Figure 3. UVvisible absorption spectra of (a) TiO2 nanotubes and (b) CdS-TiO2 nanotubes.
Figure 6. (a,b) TEM images, (c,d) HRTEM images, and (e,f) EDS spectra of the CdSe and CdTe nanoparticle-sensitized TiO2 nanotubes prepared by the one-step electrodeposition, respectively. Figure 4. (a) Current obtained for CdS-TiO2 nanotubes without illumination, (b) TiO2 nanotube arrays under illumination, and (c) CdS-TiO2 nanotubes under illumination. A xenon lamp (75 mW/cm2) is used for illumination.
Figure 5. Photocurrent dynamics of (a) the TiO2 nanotube arrays and (b) CdS-TiO2 nanotube arrays in response to onoff irradiation.
nanoparticles can be indexed to the (100) plane of hexagonal CdSe, which is consistent with the SAED patterns (Figure S5, Supporting Information). The EDS analysis shown in Figure 6e demonstrates the composition of nanoparticles is CdSe. In a similar way, as shown in Figure 6b,d,f and Figures S4b and S6 in the Supporting Information, the CdTe nanoparticle-sensitized TiO2 nanotubes can also be synthesized. These results suggest that the one-step electrodeposition process is a general method to fabricate various semiconductor nanoparticle-sensitized TiO2
nanotubes. The PEC performance of CdSe (and CdTe) nanoparticle-sensitized TiO2 nanotube arrays (Figure S7, Supporting Information) is worse than that of CdS-TiO2 nanotube arrays; it could be caused by low deposited density of sensitizers.
’ CONCLUSIONS In summary, CdS nanoparticle-sensitized TiO2 nanotube arrays are fabricated by a one-step electrodeposition method. Compared with other CdS-TiO2 nanotube arrays reported, in our composited nanotube arrays, CdS nanoparticles uniformly distribute and partially embed in the shell of TiO2 nanotubes instead of adsorbing on the surface of TiO2 nanotubes. Moreover, the size and distribution density of CdS nanoparticles could be controlled by modulating the concentration of CdCl2 and Na2S2O3 in electrolyte. As-prepared CdS-TiO2 nanotube arrays could harvest solar light in UV as well as visible light region up to 580 nm. The photocurrent of CdS-TiO2 nanotube array electrode in PEC cells is 11 times higher than that of pure TiO2 nanotubes. Besides CdS, we also fabricated other semiconductor nanoparticle-sensitized TiO2 nanotube arrays. The one-step electrodeposition is a general strategy for synthesizing TiO2 composited nanostructure. ’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details of synthesis of CdSe and CdTe-TiO2 nanotube arrays, SEM images, TEM images, SAED patterns, and EDS spectra of CdS-TiO2 nanotubes prepared under different conditions and TEM images, SAED patterns, and PEC performance of CdSe and CdTe-TiO2 2441
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (W.Z.);
[email protected] (G.W.).
’ ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (grant nos. 50772110, 50721091) and the National Basic Research Program of China (2007CB925202, 2009CB939901, 2011CB921400). ’ REFERENCES (1) Zheng, Q.; Zhou, B.; Bai, J.; Li, L.; Jin, Z.; Zhang, J.; Li, J.; Liu, Y.; Cai, W.; Zhu, X. Adv. Mater. 2008, 20, 1044–1049. (2) Zhang, J.; Zhou, B.; Zheng, Q.; Li, J.; Bai, J.; Liu, Y.; Cai, W. Water Res. 2009, 43, 1986–1992. (3) Tan, L. K.; Kumar, M. K.; An, W. W.; Gao, H. ACS Appl. Mater. Interfaces 2010, 2, 498–503. (4) Ryu, J.; Lee, S. H.; Nam, D. H.; Park, C. B. Adv. Mater. 2011, 23, 1883–1888. (5) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2005, 6, 24–28. (6) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2004, 5, 191–195. (7) Allam, N. K.; Shankar, K.; Grimes, C. A. J. Mater. Chem. 2008, 18, 2341–2348. (8) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707–065717. (9) Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A.; Ong, K. G. Nanotechnology 2006, 17, 398–402. (10) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. Adv. Mater. 2003, 15, 624–627. (11) Zhao, W.; Sun, Y.; Castellano, F. N. J. Am. Chem. Soc. 2008, 130, 12566–12567. (12) Hayden, S. C.; Allam, N. K.; El-Sayed, M. A. J. Am. Chem. Soc. 2010, 132, 14406–14408. (13) Baram, N.; Starosvetsky, D.; Starosvetsky, J.; Epshtein, M.; Armon, R.; Ein-Eli, Y. Electrochim. Acta 2009, 54, 3381–3386. (14) Zhu, K.; Vinzant, T. B.; Neale, N. R.; Frank, A. J. Nano Lett. 2007, 7, 3739–3746. (15) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2006, 7, 69–74. (16) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. D: Appl. Phys. 2006, 39, 2498–2503. (17) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Hardin, B.; Grimes, C. A. Nanotechnology 2006, 17, 1446–1448. (18) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 6, 215–218. (19) Macak, J. M.; Tsuchiya, H.; Ghicov, A.; Schmuki, P. Electrochem. Commun. 2005, 7, 1133–1137. (20) Khan, M. A.; Yang, O. B. Catal. Today 2009, 146, 177–182. (21) Liu, Z.; Pesic, B.; Raja, K. S.; Rangaraju, R. R.; Misra, M. Int. J. Hydrogen Energy 2009, 34, 3250–3257. (22) Ghicov, A.; Macak, J. M.; Tsuchiya, H.; Kunze, J.; Haeublein, V.; Frey, L.; Schmuki, P. Nano Lett. 2006, 6, 1080–1082. (23) Wang, C. L.; Sun, L.; Yun, H.; Li, J.; Lai, Y. K.; Lin, C. J. Nanotechnology 2009, 20, 295601–295606. (24) Chen, S.; Paulose, M.; Ruan, C.; Mor, G. K.; Varghese, O. K.; Kouzoudis, D.; Grimes, C. A. J. Photochem. Photobiol., A 2006, 177, 177–184. (25) Banerjee, S.; Mohapatra, S. K.; Das, P. P.; Misra, M. Chem. Mater. 2008, 20, 6784–6791. (26) Shin, K.; Il Seok, S.; Im, S. H.; Park, J. H. Chem. Commun. 2010, 46, 2385–2387.
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