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Controlled Synthesis of Ag/TiO2 Core-Shell Nanowires with Smooth and Bristled Surfaces via a One-Step Solution Route Jimin Du, Jianling Zhang, Zhimin Liu, Buxing Han,* Tao Jiang, and Ying Huang Center for Molecular Science, CAS Key Laboratory of Colloid, Interfacial and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed August 26, 2005 Ag/TiO2 core-shell nanowires were synthesized via a one-step solution method without using a template. Interestingly, the shell morphologies can be controlled to be smooth or bristled by altering the reaction temperature. Moreover, the TiO2 shell thickness and bristle length can be tuned by changing the AgNO3 concentration. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selectedarea electron diffraction (SAED), energy-dispersive X-ray analysis (EDS), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the resultant Ag/TiO2 core-shell nanowires. Moreover, the absorption peaks of our samples are significantly red-shifted compared with those of the uncoated pure silver nanowires, indicating that interaction between the core and shell occurred. On the basis of the experimental results, we proposed a template-induced Oswald ripening mechanism to explain the formation of the Ag/TiO2 core-shell nanowires.
Introduction Recently, core-shell nanostructures have been a very attractive topic because of their potential applications in different fields, such as microelectronics, optoelectronics, catalysis, and optical devices.1 Among such nanocomposite structures, the Ag/TiO2 core-shell motif has attracted attention not only because TiO2 is a promising material for various applications including photoelectrochemical activity, solar energy conversion, and photocatalysis2 but also because silver nanomaterials display some unique activities in chemical and biological sensing, which are based on surface-enhanced Raman scattering, localized surface plasmon resonance, and metal-enhanced fluorescence.3 Moreover, Ag/TiO2 core-shell nanostructures may lead to enhanced optical and catalytic properties because of the fabrication of a silver core and TiO2 sheath as well as new chemical activities owing to the electron transfer between photoexcited TiO2 and silver.4 There has been some research on the synthesis and functionality of Ag/TiO2 core-shell nanostructures. For instance, Pradeep and co-workers prepared Ag/TiO2 core-shell nanoparticles in organic solvents.5 More recently, Kamat et al. prepared Ag/TiO2 coreshell colloids via the titanium-(triethanolaminato) isopropoxide reduction of AgNO3 in DMF solution and investigated the charge separation and catalytic activity of Ag/TiO2 core-shell clusters under UV irradiation.6 To the best of our knowledge, the synthesis of Ag/TiO2 coreshell nanowires has not been reported. Here, we report a onestep solution method to prepare Ag/TiO2 core-shell nanowires. The morphology of TiO2-coated silver nanowires can be changed * Corresponding author. E-mail:
[email protected]. (1) Cui, Y.; Lieber, C. Science 2001, 291, 851. (2) (a) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960. (b) Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 2000, 16, 2731. (c) Rodriguez-Gattorno, G.; Diaz, D.; Rendon, L.; Hernandez-Segura, G. O. J. Phys. Chem. B 2002, 106, 2482. (3) (a) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (b) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 1999, 103, 6770. (4) (a) Zhan, J.; Bando, Y.; Hu, J.; Li, Y.; Golberg, D. Chem. Mater. 2004, 16, 5158. (b) Cozzoli, P. D.; Comparelli, R.; Fanizza, E.; Curri, M. L.; Agostiano, A. Laub, D. J. Am. Chem. Soc. 2004, 126, 3868. (5) Tom, R.; Nair, A.; Singh, N.; Aslam, M.; Nagendra, L.; Philip, R.; Vijayamohanan, K.; Pradeep, T. Langmuir 2003, 19, 3439. (6) Hirakawa, T.; Kamat P. V. J. Am. Chem. Soc. 2005, 127, 3928.
Figure 1. XRD patterns of Ag/TiO2 core-shell nanowires with (a) smooth and (b) bristled surfaces.
from a smooth surface to bristled by tuning the reaction temperature, and the TiO2 shell thickness and bristle length can be tuned by changing the AgNO3 concentration. The possible chemical reactions can be formulated as follows: heating
HOCH2CH2OH y\z CH3CHO + H2O
(1)
CH3CHO + AgNO3 + H2O f Ag + HNO3 + CH3COOH (2) Ti[O(CH2)3CH3]4 + 2H2O f TiO2 + 4CH3(CH2)3OH (3) Reaction 1 shows that ethylene glycol (EG) can be reversibly converted into an aldehyde through heat treatment.7 Then, silver can be formed via the silver mirror reaction (eq 2). Meanwhile, tetrabutyl titanate (TBT) can react with water to form TiO2 (eq 3). Experimental Section All chemicals (analytical grade) were purchased from Beijing Chemical Reagents Co. and used without further purification. In a typical synthesis of Ag/TiO2 core-shell nanostructures, tetrabutyl (7) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736.
10.1021/la052337q CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006
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Figure 2. XPS (a) survey, (b) Ag 3d, (c) Ti 2p, (d) O 1s, and (e) O 1s deconvolution spectra of Ag/TiO2 core-shell nanowires with (A) smooth and (B) bristled surfaces, respectively. titanate (TBT) (0.5 mmol, 170 mg) was added dropwise to 6 mL of ethylene glycol (EG) containing the desired amount of AgNO3 (0.25, 0.5, 1, or 1.5 mmol). The above solutions were loaded into a 20 mL stainless steel cell and maintained at 240 °C or 270 °C for 14 h. Then the reaction solution was left to cool to room temperature. The precipitate depositing on the bottom of the cell was collected and washed with ethanol four times and then dried at 80 °C for 12 h in a vacuum oven. X-ray diffraction (XRD) patterns were collected on a Rigaku D/max-rA X-ray diffractometer equipped with graphite-monochromatized high-intensity Cu KR (λ ) 1.54 Å) radiation. Scanning electron microscopy (SEM) images were taken on a JEOL JSM-
6700FSEM. Transmission electron microscopy (TEM) images and selected-area electronic diffraction (SAED) patterns were taken on a Hitachi H-800 instrument with a tungsten filament using an acceleration voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images and energy-dispersive X-ray analysis (EDS) were recorded on a JEOL-2010 TEM at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was carried out on an Escalab M K11 X-ray photoelectron spectrometer with Mg KR X-rays as the excitation source. UV-vis absorption spectra of the samples dispersed in ethanol were recorded on a TU-1201 spectrophotometer.
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Figure 3. (a) SEM and (b) TEM images of core-shell nanowires with smooth surfaces synthesized with AgNO3 (0.5 mmol, 85 mg) and TBT (0.5 mmol, 170 mg) in 6 mL of EG at 240 °C for 14 h. The inset in b gives the SAED of the silver core. (c) EDS spectrum of the nanowires in b. HRTEM image (d) of the TiO2 shell and (e) the silver core of the nanowires.
Figure 4. TEM images of the core-shell nanowires synthesized with (a) 0.25 mmol of AgNO3 and 0.5 mmol of TBT and (b) 1.5 mmol of AgNO3 and 0.5 mmol of TBT in 6 mL of EG at 240 °C for 14 h.
Results and Discussions X-ray diffraction (XRD) patterns of the Ag/TiO2 core-shell nanowires with (a) smooth and (b) bristled surfaces are shown in Figure 1. The peaks were assigned to the diffraction of (111), (200), (220), and (311) planes of face-centered cubic (fcc) silver (JCPDS card no. 4-783, a ) 4.08 Å).8 However, there are no TiO2 diffraction peaks in the patterns, verifying that the TiO2 shell is mainly amorphous. (8) Xiao, J.; Xie, Y.; Tang, R.; Chen, M.; Tian. X. AdV. Mater. 2001, 13, 1887.
To confirm the composition of the products obtained with AgNO3 (0.5 mmol, 85 mg) and TBT (0.5 mmol, 170 mg) in 6 mL of EG at 240 and 270 °C for 14 h, quantitative XPS analysis was performed on the core-shell nanowires with smooth and bristled surfaces, respectively. The survey XPS spectrum (Figure 2a) indicates that the core-shell nanowires are mainly composed of Ag, Ti, and O. A weak C emission peak can be observed, which may result from the ex situ preparation process or the transfer process of the sample into the UHV chamber.9 The peaks observed at 368.4 and 374.4 eV (Figure 2b) can be ascribed to Ag 3d3/2 and Ag 3d5/2 of the metallic silver.10 Both of the products exhibit two Ti 2p1/2 (464.1 eV) and Ti 2p3/2 (458.4 eV) peaks as shown in Figure 2c, which are assigned to the Ti4+ oxidation state according to reported XPS data.11 The peak separation between the 2p1/2 and 2p3/2 lines is 5.7 eV, which is also consistent with the +4 oxidation state.12 The XPS spectra of O 1s of the core-shell nanowires with smooth and bristled surfaces are similar, with an asymmetric characterization (Figure 2) that can be fitted with the nonlinear leastsquares fitting program using Gaussian-Lorentzian peak shapes. (9) Liu, G.; Jaegermann, W.; He, J.; Sundstro1m, V.; Sun, L. J. Phys. Chem. B 2002, 106, 5814. (10) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perking-Elmer Corp., Physical Electronics Division: Eden Prairie, MN, 1979. (11) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (12) Zhang, Q. L.; Du, L. C.; Weng, Y. X.; Wang, L.; Chen, H. Y.; Li, J. Q. J. Phys. Chem. B 2004, 108, 15077.
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Figure 5. (a) SEM and (b) TEM images of the bristled nanowires synthesized with AgNO3 (0.5 mmol, 85 mg) and TBT (0.5 mmol, 170 mg) in 6 mL of EG at 270 °C for 14 h. The inset in b gives the SAED of the silver core. HRTEM images of (c) the silver core and (d) the TiO2 shell of the nanowires.
Figure 6. (a) SEM and (b) TEM images of the core-shell nanowires synthesized from 1 mmol of AgNO3 and 0.5 mmol of TBT in 6 mL of EG at 270 °C for 14 h.
After deconvolution, three O 1s peaks appear and are attributed to surface bridging oxygen (530.0 eV),13 surface hydroxyl oxygen (531.5 ( 0.5 eV) in TiO2,14 and adsorbed O2 (533 ( 1 eV),15
(Figure 2e). On the basis of the data of Figure 2a-e, it can be concluded that the core-shell nanowires are composted of TiO2 and silver.
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Our experimental results showed that the morphology of the synthesized products also depends on the reaction temperature. Parallel experiments were performed with AgNO3 (0.5 mmol, 85 mg) and TBT (0.5 mmol, 170 mg) in 6 mL of EG at 270 °C for 14 h, and the SEM image of the as-prepared sample is given in Figure 5a. The surface of the products is rough in comparison with the Ag/TiO2 core-shell nanowires prepared at 240 °C (Figure 3a). The TEM image (Figure 5b) shows that the products exhibit the bristled shape and the length and diameter of the bristles are about 100 nm and several nanometers, respectively.
It is interesting that the numerous TiO2 bristles are almost perpendicular to the surface of the silver nanowires. SAED shows that nanowires with a bristled TiO2 layer have a crystalline silver core. The HRTEM image (Figure 5c) of the silver core demonstrates that the marked interplanar d spacing of 0.24 nm correspond to that of the (111) lattice planes of the fcc silver. The HRTEM image of the TiO2 shell (Figure 5d) indicates that the spacing of adjacent fringes of the bristles is 0.32 nm, corresponding to the spacing of the (110) planes of the rutile TiO2.16 Discontinuous fringes indicate that TiO2 bristles are mainly in the amorphous phase. It is worth noting that the TiO2 bristles are generally thinner and shorter when 1 mmol of AgNO3 (170 mg) was employed under similar conditions. The SEM image (Figure 6a) suggests that the products have a relatively rough surface. The TEM image (Figure 6b) reveals that the silver core is coated with TiO2 bristles with a length of around 60 nm. It is well known that the absorption peak shifts toward the red as the silver cluster surface is covered with an oxide layer, resulting from the dielectric constant of the surrounding matrix.17 We characterized the optical response of the Ag/TiO2 core-shell nanowires with smooth and bristled surfaces by UVvis spectroscopy, and the spectra are given in Figure 7. In comparison with the absorption spectrum of the pure TiO2 (inset in Figure 7), the Ag/TiO2 core-shell nanowires with smooth and bristled surfaces present absorption centered at about 486 nm in the visible region, as can be seen from Figure 7a and b. The wide absorption is attributed to the transverse plasmon response of the core silver nanowires. Obviously, wide bands of our samples are significantly red-shifted compared with those of uncoated pure silver nanowires of similar dimensions, verifying that the core-shell motif was formed.18 It is worth noting that Ag/TiO2 core-shell nanowires with absorption in the visible region may present higher activity in photocells and photocatalysis.19 We also carried out a series of comparative experiments to understand the formation mechanism of Ag/TiO2 core-shell nanowires. The results showed that the silver microparticles were obtained from AgNO3 (0.5 mmol, 85 mg) in 6 mL of EG at 240 °C in the absence of TBT (Figure 8a), verifying that TBT and/or TiO2 played an important role in forming the nanowires. Meanwhile, TiO2 nanoaggregates with irregular shapes were formed without AgNO3 under similar conditions (Figure 8b). On the basis of our experimental results, we proposed a template-induced Oswald ripening mechanism to explain the formation of the Ag/TiO2 core-shell nanowires, which is demonstrated in Figure 9. EG is converted to an aldehyde and water through reaction 1. AgNO3 can be reduced by an aldehyde to form silver (a in Figure 9), as shown by reaction 2. At the same time, TBT can be adsorbed on the surface of the produced silver particles because of O‚‚‚Ag interaction (b in Figure 9). TiO2 is formed on the surface of the silver particles through the in situ hydrolyzation of TBT with the produced water in reaction 1 (c in Figure 9). TiO2 is coated onto the silver nanoparticles and acts as a template, which results in anisotropic growth to form 1D silver nanowires via a process known as Ostwald ripening (d in Figure 9). This is confirmed by the TEM images in which the
(13) Reddy, B. M.; Sreekanth, P. M.; Reddy, E. P.; Yamada, Y.; Xu, Q.; Sakurai, H.; Kobayashi, T. J. Phys. Chem. B 2002, 106, 5695. (14) Xin, B.; Jing, L.; Ren, Z. F.; Wang, B.; Fu, H. J. Phys. Chem. B 2005, 109, 2805. (15) Sanjine´s, R.; Tang, H.; Berger, H.; Gozzo, F.; Margaritondo, G.; Le´vy, F. J. Appl. Phys. 1994, 75, 2945.
(16) Hosono, E.; Fujihara, S.; Kakiuchi, K.; Imai, H. J. Am. Chem. Soc. 2004, 126, 7790. (17) Mayya, K.; Gittins, D.; Caruso, F. Chem. Mater. 2001, 13, 3833. (18) Xiong, Y.; Xie, Y.; Wu, C.; Yang, J.; Li, Z.; Xu, F. AdV. Mater. 2003, 15, 405. (19) Jang, S. R.; Vittal, R.; Kim, K. J. Langmuir 2004, 20, 9807.
Figure 7. UV-visible absorption spectra of Ag/TiO2 core-shell nanowires with (a) smooth and (b) bristled surfaces. (Inset) Absorption spectrum of pure TiO2.
The morphologies of the as-prepared sample synthesized with AgNO3 (0.5 mmol, 85 mg) and TBT (0.5 mmol, 170 mg) in 6 mL of EG solution at 240 °C for 14 h were characterized by scanning electron microscopy (SEM). The SEM image (Figure 3a) reveals that the obtained products with smooth surfaces feature the core-shell nanostructures because of the contrast difference between the center and the edge of the nanowire images. The low-magnification TEM image (Figure 3b) also shows that the silver nanowires are encapsulated by a layer of TiO2. Typically, the thickness of the TiO2 shell is about 35 nm, and the diameter of the silver core is around 40 nm. The corresponding selected-area electron diffraction (SAED) pattern is illustrated in the inset of Figure 3b, which shows a set of diffraction spots, indicating that the silver core is the crystal phase. Furthermore, the composition of the sample was determined with energy-dispersive X-ray spectroscopy (EDS). The EDS spectrum (Figure 3c) exhibits O, Ag, and Ti peaks, showing that the core/shell nanowires are composed of Ag and TiO2. HRTEM images (Figure 3d) illustrate that the TiO2 shells exhibit an amorphous phase whereas the silver core (Figure 3e) is single crystal, corresponding to the fcc silver (d111 ) 0.24 nm). The Ag/TiO2 core-shell nanowires were also prepared with AgNO3 (0.25 mmol, 42.5 mg or 1.5 mmol, 212.5 mg) and TBT (0.5 mmol, 170 mg) in 6 mL of EG solution at 240 °C for 14 h. The TEM images demonstrate that, as expected, with increasing amounts of AgNO3 used the diameter of the silver core increases and the thickness of the TiO2 layer decreases as shown in Figure 4a and b.
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Figure 8. (a) SEM image of silver particles synthesized with 0.5 mmol of AgNO3 (85 mg) in the absence of TBT. (b) SEM image of TiO2 nanoaggregates synthesized from 0.5 mmol of TBT (170 mg) in the absence of AgNO3 in 6 mL of EG at 240 °C for 14 h.
temperature (270 °C), the (110) crystal plane of TiO2 is relatively active, which is favorable to forming TiO2 nanobristles on the surface of the silver particles (e in Figure 9). As the process continues, the TiO2 nanobristles also control the growth orientation of the silver particles to form 1D bristled nanowires (f in Figure 9).
Conclusions In summary, we demonstrate a simple thermal solution route to synthesize Ag/TiO2 core-shell nanowires without using a template. The shell morphologies can be controlled to be smooth or bristled by altering the reaction temperature. Moreover, the TiO2 shell thickness and bristle length can be tuned by changing the AgNO3 concentration. On the basis of the experimental results, the template-induced Oswald ripening mechanism to explain the formation of Ag/TiO2 core-shell nanowires is discussed. We believe that this one-step route for the synthesis of core-shell nanowires can also be used to synthesize other core-shell nanowires. Furthermore, Ag/TiO2 core-shell nanowires with absorption in the visible region may present higher activity in photocells and photocatalysis. Figure 9. Proposed mechanism to form Ag/TiO2 core-shell nanowires with smooth and bristled surfaces.
ends of silver nanowires are largely uncovered and remain to be attractive toward new silver atoms (not shown). At the higher
Acknowledgment. This work was supported by the National Natural Science Foundation of China (50472096, 20133030). LA052337Q
