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Carbon-Nanotube-Guiding Oriented Growth of Gold Shrubs on TiO2 Nanotube Arrays Lixia Yang,†,‡,§ Shenglian Luo,*,†,‡,§,| Fang Su,| Yan Xiao,†,‡,| Yufang Chen,| and Qingyun Cai| College of EnVironmental Science and Engineering, Key Laboratory of EnVironmental Biology and Pollution Control, Ministry of Education, and State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan UniVersity, Changsha 410082, People’s Republic of China, and School of EnVironment and Chemical Engineering, Nanchang Hangkong UniVersity, Nanchang 330063, People’s Republic of China ReceiVed: December 20, 2009; ReVised Manuscript ReceiVed: March 26, 2010
Shape-controlled synthesis of gold nanoparticles is common, while the crystal growth of dendritic gold architectures is rare, especially the perpendicular growth of shrub-like gold crystals on substrate. Herein, a simple potential-static electrodeposition method was developed to fabricate shrub-like gold nanostructures on TiO2 nanotube (NT) arrays assisted by acidified carbon nanotubes (CNTs). Au crystals were electrostacked along the electric field lines that are perpendicular to the TiO2 substrate, constructing beautiful shrubs on TiO2 NT arrays. In this process, the acidified CNTs with rich electrons attract Au3+ through electric attraction and work as primers guiding the Au growth along the direction of the electric field. The Au shrubs are single crystalline with intertwined symmetrical nanostructures. The as-prepared Au shrub/TiO2 NTs exhibited a more sensitive response in sensing highly toxic As3+ than Au films due to the higher surface area and unique three-dimensional structures. 1. Introduction Controllable growth of crystals according to predesigned size, shape, and structures is fascinating and attracts high interests as their applications in devices with designed functions in areas of electronics, sensing, optoelectronics, catalysis, and ceramics.1,2 The synthesis of gold nanocrystals with special morphology is such a subject of wide research due to their unique optical, electric, and catalytic properties.3–5 Au nanostructures with variable shapes such as nanoparticles,6,7 nanoplates,8,9 nanowires,10 nanorods/chains,11,12 nanopeanuts,13 nanoflowers,14 and dendritic structures15,16 were synthesized mainly by chemical reduction of Au3+ in the presence of surfactants, capping agents, ionic liquid, or heavy metal ions which tailor the morphology of Au crystals. For example, Au nanochains were formed in the presence of glutamic acid and histidine12 and a singlecrystalline dendritic Au nanostructure was synthesized under the assistance of the ionic liquid [BMIM][PF6].16 The additives or capping agents generally have functional groups such as -SH, -OH, or -NH2. It was generally accepted that the functional groups coordinate with the central metal ions through their lone pair electrons, selectively adsorbing on some crystal planes whose growth is therefore inhibited, resulting in the final product with special morphology. In another situation, the introduction of Pb2+ into gold plating baths induced a marked cathodic depolarization effect, resulting in the formation of dendritic Au nanostructure through an in suit galvanic replacement of lead with AuCl4- during the course of gold electrodeposition.15 Although great progress has been achieved on the synthesis of materials with special nanostructures, it is still difficult to fabricate nanomaterials according to a predesigned structure. In addition, the present methods are generally time†
College of Environmental Science and Engineering, Hunan University. Key Laboratory of Environmental Biology and Pollution Control, Hunan University. § Nanchang Hangkong University. | State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University. ‡
consuming and complicated, and need expensive and toxic reagents which are harmful to the environment. Herein, a facile, structure-controllable, and green electrodeposition method is demonstrated for the fabrication of hyperbranched and shrub-like gold nanostructures on anodized TiO2 NT substrate. The gold shrubs at a height of 11 µm were grown within 40 s. The highly orientated TiO2 NT arrays with uniform morphology provided a homogeneous environment for the crystal growth in the electric field. The growth direction of the shrub was guided by acidified CNTs which aligned along the electric field lines. The CNTs can be collected by centrifugation after electrodeposition. 2. Experimental Procedures The titania NTs serving as substrates with a 90 nm pore diameter and 320 nm in length17 were fabricated by anodization of a Ti sheet at 15 V in an electrolyte containing 0.1 M NaF and 0.5 M NaHSO4 at room temperature for 3 h in a two-electrode configuration with a platinum cathode. Singlewalled CNTs with 5 nm in diameter and 10 µm in length purchased from Shenzhen Nanotechnologies Co. Ltd. were acidified with the aid of ultrasonication in the conventional HNO3-H2SO4 mixed acid (1:3 v/v) at 60 °C for 5 h to increase the water solubility. The acidified CNTs were collected by centrifugation and washed with pure water until pH 7 for application. The electrodeposition of Au crystals was performed in a standard three-electrode configuration with a titania/Ti working electrode, a platinum wire auxiliary electrode, and a saturated calomel electrode (SCE) reference electrode at constant potential in a plating solution containing 5 mM HAuCl4 and 2 mg/L CNTs or 0.5 M KCl. The morphology of the samples was characterized using a field emission scanning electron microscope (FESEM, Hitachi, model S-4800) and a transmission electron microscope (TEM, Tecnai F20). Energy dispersive X-ray (EDX) spectrometers fitted to SEM were used for elemental analysis. The crystal structure of the Au/TiO2 sample was characterized with use of
10.1021/jp912007g 2010 American Chemical Society Published on Web 04/14/2010
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Figure 1. (a, b) SEM images of Au particles electrodeposited at -1 V with prolonged duration time of (a) 4 s and (b) 40 s in 5 mM HAuCl4 (conductivity: 4.01 mS cm-1). (c, d) SEM images of Au clusters electrodeposited at -1 V with prolonged duration time of (c) 40 s and (d) 120 s in 5 mM HAuCl4 and 0.5 M KCl (conductivity: 37.2 mS cm-1).
an X-ray diffractometer (XRD, M21X, MAC Science Ltd., Japan) with Cu Ka radiation (λ ) 1.54178 Å). 3. Results and Discussion 3.1. Electrodeposition of Gold Crystals in the Absence of CNTs. Figure 1a and b shows the representative scanning electron microscope (SEM) images of the Au crystals electrodeposited at -1 V with a duration of (a) 4 s and (b) 40 s in 5 mM HAuCl4 with an electrical conductivity of 4.01 mS cm-1. In Figure 1a, Au nanoparticles with about 10 nm diameter are dispersed with a good distribution on the top surface, inner space of the TiO2 NTs. Nucleation preferentially happened at the boundaries between TiO2 tubes (marked by red arrows) due to the higher current densities there.18,19 Countless boundaries provided countless nucleation sites for the Au crystals. It is more important that the highly uniform TiO2 NT arrays supplied a homogeneous electrodepositing environment for the nucleation and growth of the crystals. No uniform Au nanoparticle was formed on Ti sheet in the absence of the TiO2 NT arrays (data was not shown). For a given deposition potential, the particle size deposited increased with the prolonging deposition time. In Figure 1b, some particles were connected and fused together when the deposition time was 40 s. The solution containing 0.5 M KCl and 5 mM HAuCl4 (electrical conductivity, 37.2 mS cm-1) was employed to investigate the influence of the improved mass transport rate on the Au crystal morphology. Figure 1c shows the SEM images of Au clusters deposited at -1 V for 40 s. No Au particles in nanoscale but irregular shaped Au crystals were formed under this condition. Few 2-fold symmetrical Au crystals marked by a red rectangle are observed in Figure 1c. The SEM image in low magnification is provided in Figure S1 in the Supporting Information. Much more Au particles with large size were formed on the TiO2 NTs when prolonging the electrodeposition
duration to 120 s, shown as panel d of Figure 1. Moreover, 1 µm high Au shrubs with dendrite structure were grown on the substrate. These observations suggest that improved conductivity of the plating solution accelerates the nucleation rate and growing rate of the Au crystals, facilitating the formation of large sized Au particles and the Au dendrites. 3.2. Electrodeposition of Gold Shrub under the Assistance of CNTs. Significantly different results were obtained when employing the plating solution containing 2 mg/L CNTs and 5 mM HAuCl4. The final conductivity of the mixed solution is 36.7 mS cm-1 which is almost the same as that of 0.5 M KCl and 5 mM HAuCl4. In panels a and b of Figure 2, Au crystals were deposited at -1 V for 40 s, which exhibit hierarchical dendrites with 2-fold symmetrical leaves 50-200 nm in length (marked by red rectangles) originated from the backbone. Panels c and d with low magnification of Figure 2 show that the orientable growth characteristic of the Au crystals deposited at -1 V for 80 s, especially panel c depicting a 4 µm high leaflike Au crystal which is perpendicular to the TiO2 NT substrate. The Au dendrites grown in 80 s with more than 11 µm in height in panel d look like thick shrubs, which is 10 times taller than those obtained in 0.5 M KCl and 5 mM HAuCl4 even with a prolonged reaction time of 120 s (see panel d of Figure 1). It looks like the Au shrub is too high and heavy for the trunk to bear, resulting in the final lean. Those observations indicate that the Au dendrites grown assisted by the CNTs can reach a greater height along the electric field lines. In addition to the height, the Au shrubs formed under the assistance of CNTs are with more beautiful symmetrical structures, which may result from the higher mass transfer rate on CNTs. Although similar structures with intertwined Au crystals electrodeposited under the introduction of Pb2+ have been shown previously, the Au
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Figure 2. SEM images of Au crystals electrodeposited at -1 V in the solution containing 5 mM HAuCl4 and 2 mg/L CNTs (conductivity: 36.7 mS cm-1). (a, b) SEM images of Au shrubs deposited at -1 V for 40 s in high magnification showing the 2-fold symmetry nanostructures. (c, d) Lateral view of the typical Au shrubs deposited at -1 V for 80 s, depicting the perpendicular growth characteristic on TiO2 NTs.
Figure 3. XRD (a) and EDS (b) spectra of the dendritic Au crystal modified TiO2 sample.
dendrites were separated from each other, looked like feathers lying on the substrate,15 which are different from our investigation results. Figure 3 shows the XRD and EDX spectra of the as-prepared Au shrubs/TiO2 NTs. Besides the characteristic peaks assigned to Ti and TiO2, the XRD pattern in Figure 3a exhibits sharp reflections or characteristic peaks (marked by black triangles) which are indexed to the cubic Au (JCPDS 02-1095), indicating that Au crystals are single crystalline. The EDS spectrum depicted in Figure 3b shows the Au signals as well as those of Ti and O. The TEM image shown in Figure 4a depicts a typical Au shrub, further confirming the growth characteristic of the Au shrubs. The Au shrubs obtained in 0.5 M KCl and 5 mM HAuCl4 cannot reach such a height. The selected electron diffraction (ED) depicted in Figure 4b illustrates that the Au branch is a single crystal, which is consistent with the XRD spectrum in Figure 3a.
3.3. Detection of As(III) Using the Au Shrub/TiO2 NT Array as a Sensor. Arsenic is highly toxic. The arsenic guideline level in drinking water prescribed by the World Health Organization is 10 µg/L. Anodic stripping techniques (ASV) provide accurate measurements for the determination of As3+ with rapid analysis.20–22 Gold is believed to be the most sensitive electrode material for detecting As3+, since stable Au-As intermetallic compounds can be formed during the predeposition step.21,22 Consequently, the Au shrub modified TiO2 NT arrays shown in Figure 2a and b would be a promising electrode material for As3+ detection. The analysis of As(III) on Au shrub and Au film (the sample depicted in Figure S1, Supporting Information) modified TiO2 NT arrays by ASV was performed with 8 min of predeposition at -0.6 V. As is depicted in Figure 5, the response current density to 5 µg/L As3+ on the Au shrubs is 25.7 µA/cm2, which is much higher than the value of 10.6 µA/cm2 on the Au films. As compared with the anodic peak potential (Epa) of As0 on the Au film/TiO2 electrode,
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Figure 4. TEM image and the selected electron diffraction (ED) patterns corresponding to the dendritic Au crystals.
Figure 6. Electrophoresis of the acidified CNTs in polyacrylamide gel at an applied voltage of 50 V.
Figure 5. ASV responses of As3+ in 1 M HNO3 on Au shrub and Au film modified TiO2 NT electrodes. The arrows indicate the scanning direction.
TABLE 1: Comparison of Conductivity (σ) and Final Au Products for the Various Plating Solutions σ (mS cm-1)
final product
5 mM HAuCl4 5 mM HAuCl4 and 0.5 M KCl
4.01 37.2
5 mM HAuCl4 and 2 mg/L CNTs
36.7
Au films irregular shaped Au crystals dominate with few 2-fold symmetrical structure hierarchical, shrub-like Au dendrites
plating solutions
Epa ) 0.112 V, the Epa negative shift by a value of 48 mV was observed on Au shrubs/TiO2 electrodes, on which the Epa value is 0.064 V. Furthermore, the Au films showed no response to 1 µg/L As3+, while the Au shrubs exhibited 6.4 µA/cm2 response current. The enhanced electrocatalytic activity of the Au shrubs can be attributed to the larger surface area and the unique 3D construction which facilitates the electron transfer. 3.4. Investigation on the Formation Mechanism of Gold Shrub Guided by CNTs. To shed light on the formation of the Au shrubs, Table 1 summarizes the compositions and conductivity (σ) for the various plating solutions as well as the corresponding final products. It can be seen that improving the conductivity of the plating solution can facilitate the rapid growth of Au crystals in seconds based on the nucleation and growth theory of nanocrystals.2 Increased conductivity is important to the crystallization of Au but not the key factor for the perpendicular growth of the Au shrubs on TiO2 NTs substrates; only the CNTs play the determinative role. We proposed that the acidified CNTs exhibited two functions, which
include (i) bridges for electrotransfer with improved mass transport rate and (ii) guides for orientable growth of the Au shrubs along the electric field lines, just as the DNA primer works in the polymerase chain reaction (PCR). As already known, electrophoresis is generally applied in separating proteins according to their size. We utilized electrophoresis to investigate the transfer action of the acidified CNTs in the electric field to verify our deduction. Because the CNTs have been enriched with -COO- groups through the acidification procedure, sodium dodecyl sulfate (SDS), a detergent applied to make the proteins negatively charged, was not employed during the polyacrylamide gel preparation. Figure 6a shows the view of the CNTs before beginning their journey through the gel. After turning on the current and setting the voltage to 50 V, light black lanes of the CNTs toward the positive pole of the electric field can be observed 15 min later, depicted in Figure 6b, suggesting that the CNTs can travel along the electric field lines driven by electric force. A similar phenomenon was observed by Hongjie Dai’s group in the electric-field-aligned growth of single-walled CNTs catalyzed by Mo on SiO2/Si substrate.23 The control experiments revealed that the CNTs grew in random orientations in the absence of SCHEME 1: Proposed Growth Mechanism of Gold Shrubs Guided by the Carbon Nanotubes in the Electric Field
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Figure 7. SEM images of Pt crystals electrodeposited at -1 V in plating solution containing (a) 2 mM H2PtCl6 for 10 s, (b) 2 mM H2PtCl6 and 0.5 M NaCl for 40 s, and (c-f) 2 mM H2PtCl6 and 2 mg/L CNTs for 40 s.
electric fields, while a high degree of alignment was observed for CNTs growing under applied electric fields. The CNTs showed clear alignment of the direction of the electric field, perpendicular to the electrode, which would affect the electrochemical behaviors of the samples in the electric field.23 Scheme 1 depicts the CNT-guided formation process of dendritic Au clusters on TiO2 NTs. As is shown in the scheme, the concentration gradient of [AuCl4]- depicts that the [AuCl4]concentration close to the TiO2 NT surface is lower than that in the solution which is far from the electrode. The electroionization equation of [AuCl4]- in the plating solution can be expressed as
[AuCl4]- h Au3+ + 4Cl-
(1)
The electrodeposition of the Au crystals on the TiO2 NTs can be depicted as follows:
Au3+ + 3e f Au0
(2)
The depletion of Au3+ due to the electroreduction results in the right movement of the ionization balance based on eq 1 and the diffusion of much more [AuCl4]- from the solution with
high concentration. In the Au3+ electroreduction process, the negatively charged CNTs with rich COO- groups can attract Au3+ through electric attraction. Since the CNTs can travel along the electric field lines driven by electric force, the Au crystal grows along the electric field lines. With the Au crystal growth, the negatively charged CNTs were pushed away from the Au shrub cathode by static repulsion. In this process, the CNTs work as primers guiding the Au crystal growth with no CNTs left in the Au crystal, which was confirmed by the EDX analysis. 3.5. Verifying Investigation: Electrodeposition of Pt Shrubs Guided by CNTs. Pt crystals were electrodeposited under the condition similar to that of Au crystals to testify the function of CNTs. As shown in Figure 7a, Pt nanopariticles with 10 nm diameter were obtained at -1 V with a duration time of 10 s in 2 mM H2PtCl6 solution. In Figure 7b, the diameter of Pt nanoparticles increased to 50 nm with some small crystalline sheets (100 nm in length, 20 nm in thickness) embedded in when performing the electrodeposition in the plating solution of 0.5 M NaCl and 2 mM H2PtCl6 at -1 V for 40 s. Beautiful flowerlike, shrub-like Pt crystals were electrodeposited on the TiO2 NTs using the plating solution containing 2 mM H2PtCl4 and 2 mg/L CNTs, shown as panels c-f of Figure 7. In panel c, Pt “leaves” of 200 nm in length originated from the same Pt crystal core, constructing a beautiful Pt nanoflower. Figure 7c and d shows that the surface of the Pt leaf is coarse and covered with
CNT-Guiding Oriented Growth of Gold Shrubs islands, differing from the Au dendrites, which may be attributed to the inherent crystal properties of Pt. Continuous bifurcating, growing, and stacking of Pt crystalline “leaves” resulted in the final formation of Pt shrubs (see Figure 7e and f). All of the Pt shrubs are perpendicular to the TiO2 NTs under the assistance of the CNTs, confirming our view on the actions of CNTs in electrodepositon. EDS spectra of the Pt shrubs and SEM images of the shrubs in low magnification were provided as Supporting Information. The green dyed Pt shrubs look like beautiful forests. Conclusions Single-crystalline, hierarchical, shrub-like gold clusters were fabricated by a simple electrodeposition method under the assistance of acidified carbon nanotubes which served as primers for the perpendicular growth of Au dendrites on the TiO2 nanotube arrays. The highly orientated TiO2 nanotubes are ideal supports, providing a homogeneous environment and countless nucleation sites for the crystal growth. The negatively charged carbon nanotubes attracted Au3+ due to electric attraction and guided the Au shrubs through growing along the electric field lines. The gold shrub/TiO2 NT composite is with much higher surface area as compared with those irregular gold crystals, which makes it a promising material in areas as diverse as electrics, sensing, electro-catalysis, and photoelectric catalysis due to the unique three-dimensional structures. The successful application of carbon nanotubes as additives in crystal growth is less costly, green, facile, and controllable, which could open new avenues to synthesize hierarchical nanostructures based on a carbon-nanotube-directed crystallization mechanism. Acknowledgment. This work was supported by the National Science Foundation for Distinguished Young Scholars (No. 50725825), the National Basic Research Program of China (No. 2009CB421601), the Natural Science Foundation of China (No. 50878079 and No. 50830301), the Key Special Science and Technology Project for Energy Saving and Emission Reduction of Hunan Province (No.2008SK1002) and the Natural Science Foundation of Hunan Province (No. 08JJ3113).
J. Phys. Chem. C, Vol. 114, No. 17, 2010 7699 Supporting Information Available: Figures showing a SEM image of Au crystals, EDS spectra of Pt shrubs, and SEM images in low magnification of Pt shrubs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yin, Y. D.; Alivisatos, A. P. Nature 2005, 437, 664. (2) Tang, J.; Alivisatos, A. P. Nano Lett. 2006, 6, 2701. (3) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (4) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (5) Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marguez, M.; Xia, Y. Chem. Soc. ReV. 2006, 35, 1084. (6) Quinn, B. M.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2005, 127, 6146. (7) Brown, K. R.; Andrew Lyon, L.; Fox, A. P.; Reiss, B. D.; Natan, M. J. Chem. Mater. 2000, 12, 314. (8) Huang, Y. Z.; Wang, W. Z.; Liang, H. Y.; Xu, H. X. Cryst. Growth Des. 2009, 9, 858. (9) Huang, W L.; Chen, C. H.; Huang, M. H. J. Phys. Chem. C 2007, 111, 2533. (10) Wang, C.; Hu, Y. J.; Lieber, C. M.; Sun, S. H. J. Am. Chem. Soc. 2008, 130, 8902. (11) Shibu Joseph, S. T.; Ipe, B. I.; Pramod, P.; Thomas George, K. J. Phys. Chem. B 2006, 110, 150. (12) Polavarapu, L.; Xu, Q. H. Nanotechnology 2008, 19, 075601. (13) Xie, W.; Su, L.; Donfack, P.; Shen, A. G.; Zhou, X. D.; Sackmann, M.; Materny, A.; Hu, J. M. Chem. Commun. 2009, 5263. (14) Zhao, L. L.; Ji, X. H.; Sun, X. J.; Li, J.; Yang, W. S.; Peng, X. G. J. Phys. Chem. C 2009, 113, 16645. (15) O’Mullane, A. P.; Ippolito, S. J.; Sabri, Y. M.; Bansal, V.; Bhargava, S. K. Langmuir 2009, 25, 3845. (16) Qin, Y.; Song, Y.; Sun, N. J.; Zhao, N. N.; Li, M. X.; Qi, L. M. Chem. Mater. 2008, 20, 3965. (17) Yang, L. X.; Cai, Q. Y.; Yan, Yu. Inorg. Chem. 2006, 45, 9616. (18) Yang, L. X.; Yang, W. Y.; Cai, Q. Y. J. Phys. Chem. C 2007, 111, 16613. (19) Grimes, C. A.; Mor, G. K. TiO2 NT Arrays Synthesis, Properties, and Applications; Springer: Norwell, MA, 2009. (20) Dai, X.; Nekrassova, O.; Hyde, M. E.; Compton, R. G. Anal. Chem. 2004, 76, 5924. (21) Yamada, D.; Ivandini, T. A.; Komatsu, M.; Fujishima, A.; Einaga, Y. Electroanal. Chem. 2008, 615, 145. (22) Dai, X.; Compton, R. G. Electroanalysis 2005, 17, 1325. (23) Ural, A.; Li, Y. M.; Dai, H. J. Appl. Phys. Lett. 2002, 81, 3464.
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