pubs.acs.org/Langmuir © 2009 American Chemical Society
Synthesis of Core-Shell Au@TiO2 Nanoparticles with Truncated Wedge-Shaped Morphology and Their Photocatalytic Properties Xiao-Feng Wu,† Hai-Yan Song,† Jeong-Mo Yoon,† Yeon-Tae Yu,*,† and Yun-Fa Chen‡ † Division of Advanced Materials Engineering and Research Centre for Industrial Technology, College of Engineering, Chonbuk National University, Chonju 561-756, South Korea, and State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100080, China
Received January 6, 2009. Revised Manuscript Received February 25, 2009 Core-shell Au@TiO2 nanoparticles with truncated wedge-shaped TiO2 morphology have been synthesized successfully by a simple and flexible hydrothermal route. Morphological evolution of TiO2 shells was investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction technique. It has been revealed that the truncated wedge-shaped TiO2 shells experience an epitaxially segmented orientation growth. Also, the (101) crystal planes of TiO2 crystals grow preferentially on the surface of gold nanocrystals stabilizing the heterointerfaces, then faster [001] growth results in the “budding” process occurs, producing growth sites on the initial deposition TiO2 layers, where the TiO2 crystals grow up into truncated wedge-shaped morphologies. It is also found that morphological evolution of TiO2 shells is dependent on the produced F- ion concentration from hydrolyzed TiF4 precursors. The produced F- ions not only facilitate the formation of well-defined wedge-like TiO2 shells, but also contribute to the externally exposed truncated crystal {004} facets. As the representative photocatalyst, the catalytic activities of the resultant core-shell Au@TiO2 nanoparticles were investigated by photoinitiated oxidation degradation of gaseous acetaldehyde. It has been indicated that the nanostructured core-shell Au@TiO2 photocatalyst represents high photocatalytic activity when exposed to UV or visible light irradiation. The high phototocatalytic performance is also largely attributed to the preferentially grown TiO2 shell structures and metal (Au)-TiO2 heterointerfaces.
Introduction Semiconductor-based heterostructures with desirable chemical compositions and/or morphologies have always attracted intensive research interest due to their tailorable physical and chemical properties and potential technical applications in biomedicine, photocatalysis, energy conversion, and storage and nanodevices.1-12 Driven by these technical applications, many efforts have been devoted to designing and modulating the compositions (semiconductor-semiconductor, metal-semiconductor and metal-metal), structures (anisotropic and core-shell), and dimensions in recent years.13-15 By now, noble metal-semiconductor is one of the most popular heterostructures because of its wide application in *Corresponding author. E-mail:
[email protected]. (1) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47. (2) Wang, D. Y.; Rogach, A. L.; Caruso, F. Nano Lett. 2002, 2, 857. (3) Kim, H. G.; Borse, P. H.; Choi, W.; Lee, J. S. Angew. Chem., Int. Ed. 2005, 44, 4585. (4) Park, W. I.; Yi, G. C.; Kim, M. Y.; Pennycook, S. J. Adv.Mater. 2003, 15, 526. (5) Wu, J. J.; Tseng, C. H. Appl. Catal. B-Environ. 2006, 66, 51. (6) Iliev, V.; Tomova, D.; Todorovska, R.; Oliver, D.; Petrov, L.; Todorovsky, D.; Uzunova-Bujnova, M. Appl. Catal. A-Gen. 2006, 313, 115. (7) Zheng, J. Y.; Yu, H.; Li, X. J.; Zhang, S. Q. Appl. Surf. Sci. 2008, 254, 1630. (8) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943. (9) Yang, Z. X.; Wu, R. Q. Phys. Rev. B 2000, 15, 14066. (10) Zheng, Y. H.; Zheng, L. R.; Zhan, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K. M. Inorg. Chem. 2007, 46, 6980. (11) Zheng, Y. H.; Chen, C. Q.; Zhan, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K.; Zhu, J. F. J. Phys. Chem. C 2008, 112, 10773. (12) Lam, S. W.; Chiang, K.; Lim, T. M.; Amal, R.; Low, G. K. C. Appl. Catal. B-Environ. 2007, 72, 363. (13) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, P. Nature (London) 2004, 430, 190. (14) Kwon, K. W.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269. (15) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. Rev. 2006, 35, 1195.
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photocatalysis,5-8,11,12 where the metal in contact with the semiconductor greatly enhances the overall photocatalytic redox process, because the metal acts as a reservoir of photoelectrons, improving the interfacial charge-transfer process and retarding the recombination of photoexcited electronholes of semiconductors.8,12,16-21 In most cases, the metal nanocrystals were anchored on the surfaces of the semiconductors as isolated “islands” to produce heterointerfaces due to catalytic activity dependent on the size of the metal nanocrystals.5,6,9,12,16,21 Despite effective catalytic activity, this structural drawback results in exposing metal and oxide surfaces to reactants, products, and surrounding medium. Corrosion or dissolution of the noble metal particles during a photocatalytic reaction becomes problematic in practical applications.21,22 It is necessary to design new strategies to either improve the chemical stability or attain higher photoconversion efficiency.21 Accordingly, heterostructures with metal core and semiconductor shell become increasingly among the strongest candidates as photocatalysts due to their controllable chemical and colloidal stability within the shell and charge transfer between metal cores and semiconductors.21 TiO2 semiconductor has been always regarded as one of the most promising photocatalysts in practical applications, especially water cleaning and removal of volatile organic compounds (16) Lee, M. S.; Hong, S. S.; Mohseni, M. J. Mol. Catal. A-Chem. 2005, 242, 135. (17) Hirakawa, T.; Kamat, P. V. Langmuir 2004, 20, 5645. (18) Hirakawa, T.; Kamat, P. V. J. Am. Chem. Soc. 127, 2005, 3928. (19) Oldfield, G.; Ung, T.; Mulvaney, P. Adv. Mater.12, 2000, 1519. (20) Berger, T.; Sterrer, M.; Diwald, O.; Kno1zinger, E. J. Phys. Chem. B 2005, 109, 6061. (21) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (22) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 11439.
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(VOCs) in air, due to its high photocatalytic activity, chemical stability, low cost, and nontoxicity.23-28 Recently, many preparation routes have been developed to synthesize metal@TiO2 core-shell nanoparticles.32-37 Until now, this research was largely focused on synthesis and optical applications.32-38 Besides, some advances proved that the high Fermi energy level of metal cores can separate efficiently photoexcited electron-hole pairs of TiO2 semiconductor, thus heightening the TiO2 redox capability.8,31,39 However, these core-shell metal@TiO2 photocatalysts always possess dull and unstructured shells due to the inherent limitations of core-shell nanoparticles,32-37,39 which results in low catalytic activity from the exposure of lowreactivity crystal planes within TiO2 shells and small specific surface area.29,30,39 Accordingly, structurizing shell texture has increasingly become an interesting and meaningful subject because of the availability in coupling morphologyl with structural properties of bulk TiO2 crystals.15,34 To our knowledge, only a few successful examples have been reported, in which the Au@TiO2 nanoreactors with movable Au cores and nanochannels in the TiO2 shell and flower-like Au@TiO2 nanoparticles with shell built by packed TiO2 nanoparticles were presented.34 Efforts to utilize nanostructured metal@TiO2 nanoparticles as photocatalysts are still limited. The slow advance is chiefly attributed to the difficulty in tailoring the shell structures and morphologies due to the uncontrollable hydrolysis process of TiO2 precursors and physical constraints imposed by the structure.37,40 In this present work, the interesting core-shell Au@TiO2 nanoparticles with truncated wedge-shaped morphology have been successfully synthesized by a relatively simple and flexible hydrothermal route using TiF4 as the precursor. Different from the above-mentioned reported results,34 the truncated wedgeshaped TiO2 shells experience an epitaxially segmented orientational growth. The developed approach is also extended to synthesize other metal (Ag, Pt)@TiO2 core-shell nanoparticles with nanostructured shells. As the representative photocatalyst, the photocatalytic activities of Au@TiO2 nanoparticles with truncated wedge-shaped shells have been evaluated by UV or visible light-induced catalytic oxidation decomposition of gaseous acetaldehyde, and its relationship to the structure and photocatalytic property is also discussed. (23) Schmidt, C. M.; Buchbinder, A. M.; Weitz, E.; Geiger, F. M. J. Phys. Chem. A 2007, 111, 13023. (24) Liu, Z. Y.; Zhang, X. T.; Nishimoto, S.; Murakami, T.; Fujishima, A. Environ. Sci. Technol. 2008, 42, 8547. (25) Auvinen, J.; Wirtanen, L. Atmos. Environ. 2008, 42, 4101. (26) Yang, X. X.; Cao, C. D.; Hohn, K.; Erickson, L.; Maghirang, R.; Hamal, D.; Klabunde, K. J. Catal. 2007, 252, 296. (27) Kim, H.; Choi, W. Appl. Catal. B-Environ. 2007, 69, 127. (28) Ohno, T.; Murakami, N.; Tsubota, T.; Nishimura, H. Appl. Catal. A-Gen. 2008, 349, 70. (29) Fan, X. X.; Chen, X. Y.; Zhu, S. P.; Li, Z. S.; Yu, T.; Ye, J. H.; Zou, Z. G. J. Mol. Catal. A-Chem. 2008, 284, 155. (30) Zhao, Y.; Zhang, X. T.; Zhai, J.; Jiang, L.; Liu, Z. Y.; Nishimoto, S.; Murakami, T.; Fujishima, A.; Zhu, D. B. Microporous Mesoporous Mater. 2008, 116, 710. (31) Hirakawa, T.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 3928. (32) Tom, R. T.; Nair, A. S.; Singh, N.; Aslam, M.; Nagendra, C. L.; Philip, R.; Vijayamohanan, K.; Pradeep, T. Langmuir 2003, 19, 3439. (33) Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 2000, 16, 2731. (34) Li, J.; Zeng, H. C. Angew.Chem. Int. Ed. 2005, 44, 4342. (35) Sakai, H.; Kanda, T.; Shibata, H.; Ohkubo, T.; Abe, M. J. Am. Chem. Soc. 2006, 128, 4944. (36) Zhang, L.; Xia, D.; Shen, Q. J. Nanoparticle Res. 2006, 8, 23. (37) Mayya, K. S.; Gittins, D. I.; Caruso, F. Chem. Mater. 2001, 13, 3833. (38) Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2003, 107, 7312. (39) Wang, W. J.; Zhang, J. L.; Chen, F.; He, D. N.; Anpo, M. J. Colloid Interface Sci. 2008, 323, 182. (40) Giersig, M.; Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Adv. Mater. 1997, 9, 570.
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Experimental Section Material Synthesis. A typical procedure for synthesizing the Au@TiO2 core-shell nanoparticles is carried out as in the literature, but modified:34,46 0.6-4.5 mL of 0.01 M HAuCl4 solution (Sigma-Aldrich, USA) was mixed with 4.5 mL of 0.01 M sodium citrate solution (Yakuri, Japan).The mixture was stirred vigorously for 2 min. Then, 0.6-5 mL of 0.01 M ascorbic acid (Showa, Japan) was added dropwise. The mixture appeared turned orchid and then rapidly to reddish brown. After stirring continuously for 5 min, 4.5-6.0 mL of 0.04 M TiF4 solution (not acidified before use, Aldrich, USA) was added. The mixture was subsequently diluted to 90 mL with deionized water and transferred to Teflon-lined stainless steel autoclaves. The hydrothermal reaction was conducted at 180 °C for 48 h. After that, the products were cooled to room temperature and separated by centrifuge at 7000 rpm for 10 min, then washed with distilled water. The cycles of separation and washing were repeated 5 times to remove the remaining ions. The final samples were dried in an 80 °C oven and collected for further use. The hollow TiO2 nanoparticles were produced by aerial oxidation of as-prepared Au@TiO2 core-shell nanoparticles in KSCN solution according to the reported literature.40,44 0.07 g of the above Au@TiO2 nanoparticles were first dispersed into 20 mL deionized water. 5 mL of 0.01 M KSCN solution was added and the pH value was modified to 10.5 using 0.01 M NaOH solution. The reaction mixture was stirred in air for 3 h. The resultant white suspension was separated by centrifuge at 9500 rpm for 15 min and then washed with distilled water. The cycles of separation and washing were repeated 5 times. The final white product was dried in an 80 °C oven and then collected. Material Characterization. The transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy (HRTEM) images, and electronic diffraction (ED) patterns were taken on a JEM-2010 JEOL instrument with a tungsten filament, using an accelerating voltage of 200 kV. The corresponding fast Fourier transfer (FFT) patterns were obtained using the DigitalMicrograph (DM) software to treat the related HRTEM images. The crystallographic structures of the solid samples were determined with D/Max 2005 Rigaku X-ray diffractometer equipped with graphite monochromatized high-intensity Cu KR radiation (λ = 1.541 87 A˚). The UV-visible spectra were conducted on a UV-visible spectroscope (UV-2550, Shimadzu). Photocatalysis Experiments. As additional referenced photocatalyst, the hollow TiO2 nanoparticles were obtained by complete oxidation removal of 0.050 g core-shell Au@TiO2 nanoparticles, as described in the Material Synthesis section. The as-obtained white product was carefully separated by centrifuge and meticulously collected. The white slurry was dried at 80 °C overnight. The weight of final white powder was 0.043 g. The same amount of commercial P-25 TiO2 nanoparticles (Degussa, Germany) was used as another referenced photocatalyst. 0.050 g of core-shell Au@TiO2 and two referenced photocatlysts (0.043 g, respectively) were separately dispersed on three quartz glass substrates (220 cm2), then transferred to a 120 °C oven for 12 h. After that, these three substrates for coating photocatalysts were exposed to UV light for 10 h to remove any possible organic residues. The photocatalytic experiments were conducted in a closedcirculation reactor at ambient conditions. A quartz reactor (volume, 18 cm3) with a window was connected to a circulation pump using plastic pipes. The substrate loaded with photocatalyst was placed on the reactor and then put on the quartz window and fixed using four mechanical fasteners. A low-pressure Hg lamp (Sankyo Denki Japan, 352 nm, 6 W) or halide lamp (Venture Lighting Korea, MH 175 WUB, 175 W) was used as UV or visible light source, respectively. The acetaldehyde concentration in all experiments was modified by diluting the DOI: 10.1021/la900035a
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standard acetaldehyde gas (500 ppm in N2) with dry air at a ratio of 1:4 through two mass flow controllers. The total flow rate was 250 mL/min. The mixed gas was first charged into a humidifier (RH 60% at 25 °C), then flowed into reactor for 20 min to stabilize acetaldehyde concentration. After that, the inlet of mixed gas was shut, and the circulation pump was started to circulate the gas for 3 h in the dark. The gas exiting the reactor was analyzed by a gas chromatograph (Shimadzu GC-2010) equipped with two flame ionization detectors and one with an additional Methanizer (Shimazu Model MTN-1).
Results and Discussion Morphologies and Structures. The morphology of the final products shown in Figure 1a,b represents core-shell nanoparticles with radial wedge-shaped antennae when observed using transmission electron microscopy (TEM). Compared with the Au nanoparticles (Figure 1S, Supporting Information), the final product possesses a radial wedge-shaped morphology after hydrothermal reaction using TiF4 as precursor, indicative of the formation of heterogeneous TiO2 layers on Au nanoparitcles. The dimension of the Au cores is about 37.5 nm and that of the wedge-like TiO2 antennae about 110 nm in length, and the size of Au cores is adjustable by changing the concentration of HAuCl4 and ascorbic acid (Figure 2S, Supporting Information). The electron diffraction pattern in the inset of Figure 1a shows the discrete diffraction spots, indicative of high-quality crystallinity of the individual core-shell nanoparticles. XRD patterns in Figure 2 reveal further that the TiO2 shells have formed on the surface of face-centered-cubic (fcc) metallic Au nanocrytals (JCPDS No. 01-1174) after 48 h under hydrothermal conditions and have anatase-type crystalline phase (JCPDS No. 21-1272) without amorphous or additional phases. Interestingly, it can be seen that the preferential growth of individual TiO2 antenna occurs for single Au@TiO2 core-shell nanoparticles as marked by the arrow in Figure 1b. In order to obtain more detailed growth information of shell formation of the core-shell Au@TiO2 nanoparticles, HRTEM was applied and the results are shown in Figure 1c,d. Figure 1c shows the local crystalline lattices at the heterointerface of the Au and TiO2 shell as indicated by the white circle in Figure 1b. It is revealed that well-defined interface and continuity between metallic Au nanoparticles and TiO2 shells is indicative of strong interaction between exposed Au atoms and -O-Ti-O- bonds in TiO2 crystals.10 The spacing between adjacent lattice fringes is 0.241 nm for the Au core (Figure 1S, Supporting Information) and 0.363 nm for TiO2 antennae (shell), which is close to the d spacing of the (111) plane of fcc metallic Au (d = 0.235 nm; JCPDS No. 01-1174) and anatase-type TiO2 (d = 0.352nm; JCPDS No. 21-1272), revealing that the epitaxial formation of (101) crystal planes of TiO2 near the Au-TiO2 interface due to matching with exposed Au (111) planes keeps interfacial energy low.13-15,43The fast Fourier transform (FFT) pattern in the upper right inset of Figure 1c shows a diffuse diffraction circle attributed to (101) planes41 and anisotropic diffraction of (004) planes,41-43 respectively. Interestingly, compared with the root region, the external region of TiO2 antenna marked by the white rectangle in Figure 1b presents a significant oriented growth as disclosed in Figure 1d. The interplanar spacing of adjacent lattice fringes is 0.243 nm, attributed to d spacing of (41) Masuda, Y.; Kato, K. Cryst. Growth Des. 2008, 8, 3213. (42) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (43) Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Curri, M. L.; Innocenti, C.; Sangregorio, C.; Achterhold, K.; Parak, F. G.; Agostiano, A.; Cozzoli, P. D. J. Am. Chem. Soc. 2006, 128, 16953.
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the (004) plane (d = 0.238nm; JCPDS No. 21-1272) of anatasetype TiO2. It is indicated that TiO2 crystals grow preferentially along the [001] direction in the external region of the TiO2 antenna, which is also affirmed by the FFT pattern in this region as shown in the upper right inset of Figure 1d. The FFT pattern of this part shows anisotropic (004) diffraction and an unclear but closed (101) diffraction circle, verifying oriented growth along the [001] direction and random growth of (101) planes in this region. In order to estimate the relative growth rates of the above-mentioned crystal planes of wedge-shaped TiO2 shells, it is also necessary to remove Au cores and simultaneously preserve the growth information of TiO2 shells due to overlapping of anatase TiO2 (004) diffraction peak with that of Au (111) crystal planes as shown in Figure 2. The representative TEM observation (inset in Figure 3S, Supporting Information) shows that wedge-shaped morphology of the remaining product is retained, and a hollow interior can be clearly detected after etching Au@TiO2 core-shell nanoparticles shown in Figure 1 using KSCN solution,40,44 indicative of the formation of hollow particles. XRD pattern reveals (Figure 3S, Supporting Information) that no characteristic diffraction peaks are identified at 2θ = 38.27°, 44.60o, 64.68o, 77.55o, 82.35o, reflecting complete removal of the Au cores, and only TiO2 characteristic diffraction remained. In compared with the JCPDS data (No. 21-1272), the (004) diffraction intensity is 0.37 times the (101) diffraction intensity, which is generally 0.2 times that for spherical TiO2 particles, suggesting that the TiO2 crystal elongates anisotropically along the c-axis,41 which is consistent with the above HRTEM observation in Figure 1. Faster [001] growth results in (004) crystal planes surrounded by low-index {101} facets,45,46 whereby the TiO2 shells retain the appearance of wedge-shaped morphology (shown in Figure 1). From these results, it can be revealed that a segmented process within radial wedge-shaped shells occurs during TiO2 shell formation, by which, adjacent to the local region of Au nanocrystals, the (101) crystal planes of TiO2 crystals first grow epitaxially to stabilize the Au-TiO2 heterojunction, then transfer gradually to preferential orientational growth along the [001] direction into the wedge-shaped morphology. It can be reasonably anticipated that an intermediate state or “growth sites” for wedge-shaped TiO2 shells may exist during this preferentially segmented orientational growth process. Remarkably, it is also found that wedge-like TiO2 antennae have blunt tips at their ends, and some become flatter due to the exposure of (004) crystal planes as shown in Figure 1b, attributed to a truncation process due to energetically more stable F-terminated (004) planes than F-terminated (101) planes.46,47The external exposure of the {004} facets of TiO2 crystal, as radial antennae for individual core-shell Au@TiO2 nanoparticles, can provide potentially highly reactive {004} facets and large specific surface area, therefore possessing high activity and reactivity when exposed to reagents or medium.29,30,46,48 To acquire more concrete growth information for the abovementioned epitaxially segmented orientational growth within the TiO2 antennae of Au@TiO2 core-shell nanoparticles, it is necessary to verify the existence of a so-called intermediate state or “growth sites”. According to LaMer’s theory,49 it is well-known (44) Lee, J.; Park, J. C.; Bang, J. U.; Song, H. Chem. Mater. 2008, 20, 5839. (45) Tao, A. R.; Habas, S.; Yang, P. D. Small 2008, 4, 310. (46) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature (London) 2008, 453, 638. (47) Barnard, A. S.; Curitiss, L. A. Nano Lett. 2005, 5, 1261. (48) Jun, Y. W. J. Am. Chem. Soc. 2003, 125, 15981. (49) LaMer, V. K.; Dinegar, R. J. J. Am. Chem. Soc. 1950, 72, 4847.
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Figure 1. TEM images of the as-prepared core-shell Au@TiO2 nanoparticles (a) and individual particle image (b).The inset in part b is the electronic diffraction (ED) pattern of the individual particle. Local HRTEM images of individual TiO2 antenna: root region (c) and external edge of individual antenna (d); the insets in parts c and d are their corresponding FFT patterns.
Figure 2. X-ray diffraction (XRD) patterns of Au nanoparticles (a) and core-shell Au@TiO2 nanoparticles (b).
that sufficient supersaturation of hydrolyzed species is necessary to feed nucleus for further growth. Therefore, by changing the added amount of TiF4 precursor, the TiO2 shell growth can be controlled to investigate the formation process of TiO2 shells, and the corresponding TEM and HRTEM results are listed in Figure 3. It can be seen that, when the added amount of TiF4 solution (0.04 M) is 1.5 mL, the TiO2 shells appear as a uniform spherical shape with 47 nm in thickness, and the corresponding HRTEM observation shows d spacing of 0.362 nm within the whole TiO2 shell, near that of the (101) planes of anatase-type TiO2 with a value of 0.352 nm (JCPDS No. 21-1272). The corresponding local FFT pattern in the inset in Figure 3d shows Langmuir 2009, 25(11), 6438–6447
an unclear and open diffraction cycle attributed to (101) diffraction. It can be indicated that poor crystallinity of TiO2 shell and preferential growth of (101) planes are dominant. It can be certain that, if sufficient supersaturation is supplied when enough TiF4 added, reasonable TiO2 shell growth can be expected. Figure 3b,e shows the TEM and local HRTEM results of samples obtained when the added amount of TiF4 solution (0.04 M) increases to 3 mL. In Figure 3b, some premature wedge-like and spin-like “unfledged antennae” are identified clearly at outside edges of TiO2 shells, and “budding” of antennae appears, indicative of evolution of “growth sites” during structural transition of nonstructured TiO2 shells into wedge-shaped morphology. Further, the related HRTEM observation in Figure 3e reveals the local crystal lattice of the TiO2 shell near the Au cores with d spacing of 0.364 nm, attributed to (101) crystal planes of anatase-type TiO2 crystals. However, HRTEM observation of these “unfledged antennae” was unsuccessful, which may be due to poor crystallinity. The corresponding FFT pattern in the inset of Figure 3e represents a similar unclosed discrete (101) diffraction circle, indicative of preferential growth of (101) crystal planes. When more TiF4 solution is introduced up to 4.6 mL, those “unfledged antennae” grow into “adult antennae”. This can be seen by TEM and HRTEM observations listed in Figure 3c,f. In Figure 3c, it is apparent that the well-defined wedge-shaped antennae have formed radially. HRTEM observation in Figure 3f shows clear TiO2 crystal lattices in the local region near the Au cores and wedge-like antennae as indicated by a white circle and white rectangle in the upper-right inset of Figure 3c, respectively. Similarly, at the local region near the Au cores, the TiO2 crystal lattices have d spacing of 0.364 nm attributed to the (101) plane of anatase-type TiO2 crystals as shown in Figure 3f part 1 (left), DOI: 10.1021/la900035a
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Figure 3. TEM images of as-prepared Au@TiO2 core-shell nanoparticles obtained with different added amounts of TiF4 solution (0.04 M): (a) 1.5 mL; (b) 3 mL; and (c) 4.6 mL. The local HRTEM mages of TiO2 shells of samples (a), (b), and (c) are listed in (d), (e), and (f), respectively. The average Au core diameter is 71.2 nm.
and the related FFT pattern clearly shows (101) diffraction and (004) diffraction, indicative of better crystallinity at this region. The wedge-like part of TiO2 antenna in Figure 3f part 2 (right) represents TiO2 crystal lattices with d spacing of 0.241 nm attributed to (004) crystal planes of anatase-type TiO2 crystals, and the corresponding FFT pattern shows clear anisotropic (004) diffraction and unclear (101) diffraction, indicative of preferentially oriented growth along the [001] direction. When the added amount of TiF4 solution increases further to 6 mL, the more perfect wedged-shaped morphology and better crystallinity is present as in Figure 1. From all these facts, it can be ascertained that the “budding” process occurs on the substrate of spherically deposited TiO2 layers built by preferential (101) crystal planes during the formation of radial TiO2 shells for individual core-shell Au@TiO2 nanoparticles, simultaneously producing “ growth sites” where the wedge-like TiO2 antennae grow further up preferentially along the [001] direction under adequate supersaturation. 6442
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Time-dependent morphological evolution at fixed added amount of TiF4 (6 mL, 0.04 M) is also collected to investigate this segmented orientation growth of radial wedge-like TiO2 shells of Au@TiO2 core-shell nanoparticles, and the corresponding results obtained from an earlier reaction period (3-24 h) are shown in Figure 4. It can be seen that, after a relatively short time (3 h) of hydrothermal reaction, hydrolysates of TiF4 are deposited on the surface of Au nanoparticles without welldefined shapes in Figure 4a, and some core-shell particles appear to have a somewhat wedged-shaped morphology. It can be suggested that initial TiO2 deposition has been complete in this stage and wedge-shaped antennae begin to grow, i.e., the “budding” process occurs. When the time is protracted to 6 h, the morphology of TiO2 appears to be a clear wedge-shaped profile (Figure 4b). With the longer reaction time, the TiO2 shells grow gradually into more perfect wedge-like antennae as shown in Figure 4c (12 h) and d (24 h), and Figure 1 (48 h), and the radial antennae also become longer in length from 70 to 110 nm Langmuir 2009, 25(11), 6438–6447
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Figure 4. TEM images of Au@TiO2 core-shell nanoparticles obtained by different hydrothermal reaction times of 3 h (a); 6 h (b); 12 h (c); 24 h (d). The average diameter of the Au cores is 71.2 nm.
when the reaction time is extended from 6 to 48 h. From those results, it can be seen that the initial TiO2 deposition layer forms first, followed by preferential growth at the external rim into wedge-like shapes. Noticeably, compared with the sharp tips of TiO2 antennae in Figure 4b (6 h), it is also found that truncated planes at the end of TiO2 antennae begin to appear from 12 h in Figure 4c, and more antennae with flat tops are clearly detected when the reaction time is protracted to 24 h. The morphological evolution of TiO2 shells of core-shell particles can be attributed to its surface chemistry, in which different adsorbate atoms impose significant effects on the relative stability of different crystal planes of TiO2 crystals.46,47 Theoretical calculation and experimental results have affirmed that fluorine-terminated {001} facets of anatase-type TiO2 are energetically more stable than {101} facets.46 In our reaction system, the TiF4 precursor is hydrolyzed into TiO2, producing HF.42,46 The produced HF is believed to have a significant role in forming truncated wedge-like antennae of core-shell nanoparticles, because absorbed F- ions reduce surface energy to promote isotropic growth along [010] and [100] axes.46,48 In fact, when produced HF is partly in situ cleaned using H3BO3 as F- scavenger,50 the well-defined wedge-shaped morphology remains with sharp tips, and a few truncated tops are also identified (Figure 4Sa, Supporting Information). However, for the case of completely scavenged F- ions (Figure 4Sb, Supporting Information) the as-obtained core-shell particles represent poor wedge-like morphology and severe aggregation due to an excessively rapid hydrolysis rate and significant decrease of ionic intensity, indicating that F- irons have significant roles in formation of wedge-like TiO2 antennae. It can be revealed that morphological evolution of the TiO2 antennae is dependent on (50) Wamser, C. A. J. Am. Chem. Soc. 1951, 73, 409.
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produced F- concentration in the reaction system. From these results in Figure 4S and Figure 1 (no F- cleaned), it can be anticipated that, when produced HF is too low, absorbed F- ions are not able to stabilize (004) crystal planes due to excessively high surface energy;46 the lower-energy (101) crystal planes initially grow on the surface of Au nanocrystals to stabilize the heterointerface. With more HF produced during hydrolysis of TiF4, more F- ions are adsorbed and gradually accumulate to reduce (004) surface energy until it is below that of (101) surface planes (referenced F-terminated surface energy (γ):46 γF-101 ≈ -0.32 J/m2, γF-001 = -0.66 J/m2. clean surface energy (γ):51 γ101 = 0.44 J/m2, γ001 = 0.90 J/m2, respectively), whereby truncation occurs due to more stable F-terminated (004) crystal planes than F-terminated (101) planes.46 From the above discussion, the possible formation process of the core-shell Au@TiO2 nanoparticles with truncated wedgeshaped morphology is proposed as shown in Figure 5. At the earlier stage of TiF4 hydrolysis, the produced TiO2 species are first deposited on the surfaces of the Au nanoparticles. An insufficient concentration of F- produced at this stage is adsorbed on different crystal planes to reduce their surface energy, but still not enough to reverse the surface energy of (004) crystal planes below that of the (101) crystal planes, so the low-energy (101) planes grow preferentially on the heterogeneous surface of Au nanocrystals to stabilize the Au-TiO2 heterointerface. Faster growth along the [001] direction results in budding TiO2 antennae as TiF4 hydrolysis continues, whereby “unfledged antennae” are formed and provide growth sites for further growth into wedge-like morphology. When the adsorbed F- ions accumulate enough to reduce the (004) surface energy below that of the (101) planes, the more stable F-terminated (51) Ohko, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1998, 102, 2699.
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Figure 5. Schematic diagram of the proposed formation process of core-shell Au@TiO2 nanoparticles with truncated wedge-shaped morphology.
(004) crystal planes form during the development of TiO2 antennae; also, truncation occurs, producing the truncated wedge-shaped morphology of TiO2 shells of the obtained Au@TiO2 core-shell nanoparticles. Additionally, it can be found that this segmented orientation growth is more dependent on the crystal structure of metal cores and the amount of F- ions produced from hydrolysis of TiF4 precursor. Accordingly, for fcc metallic nanocrystals, it can be reasonably expected that the preparation procedure developed can be extended to synthesize other metal@TiO2 core-shell nanoparticles with wedge-shaped morphology. Figure 6 shows some successful examples using Ag and Pt nanocrystals as metallic cores (preparation procedure and the corresponding EDX patterns available in S5, Supporting Information). By introducing some metal salts (such as Fe3+, Cr3+) in hydrothermal reaction systems, Fe3+-doping Au@TiO2 and Cr3+-doping Au@TiO2 nanoparticles with wedge-shaped morphology also have been obtained successfully (Figure 6S1-4, Supporting Information), indicative of a relatively generic and flexible preparation procedure to synthesize metal@TiO2 nanoparticles with identical morphologies. These tentative efforts make it possible to further investigate the structure-property (photocatalytic) relationship including the heterointerface of metal-TiO2 semiconductor, crystal structures (oriented planes, defect types), nanostructured TiO2 shells, and so on. The related results will be generalized for possible publication in the future. Photocatalytic Degradation of Acetaldehyde. The photocatalytic activity of the representative Au@TiO2 nanoparticles with wedge-shaped morphology was investigated by decomposition of gas-phase acetaldehyde. Photocatalytic oxidation of acetaldehyde using TiO2-based catalysts was considered as a radical chain reaction mechanism.30,51,52 R-Carbon atom tracing labeled with C13 revealed that gaseous acetaldehyde was oxidized preferentially into acetic acid, which immediately transferred to CO2, and formaldehyde followed by fast oxidation into CO2 through a formic acid intermediate.52 The overall process of acetaldehyde photodecomposition over TiO2-based catalysts can be depicted by the following equation:29 TiO2
2CH3 CHO þ 5O2 f 4CO2 þ 4H2 O hν
Besides the kinetic factors of gas-phase reaction, it is more important to design and modify chemical compositions, morphologies, and structures to improve their physical and chemical properties of TiO2-based photocatalysts for degradation of organic compounds (such as acetaldehyde).21,26-30 (52) Muggli, D. S.; McCue, J. T.; Falconer, J. L. J .Catal. 1998, 173, 470.
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Herein, in order to determine the relationship between the structures and the catalytic properties of Au@TiO2 nanoparticles, the photocatalytic reaction conditions are kept constant, and the core-shell Au@TiO2 nanoparticles with 37.5 nm Au cores (Figure 1) were selected as targeted photocatalyst and commercial Degussa (P-25) TiO2 nanoparticles (∼33 nm) as a reference. Figure 7 shows the concentration changes of acetaldehyde and concurrent generated CO2 as a function of irradiation time using an ultraviolet or visible light resource, respectively. It can be seen in Figure 7a that acetaldehyde concentration using core-shell Au@TiO2 as the photocatalyst takes a sharp nosedive with 65% margin in a short time (15 min) under ultraviolet irradiation, then a fast fall with only 13 ppm of acetaldehyde remaining within 30 min, followed by a slow decline until only 2% of the acetaldehyde is left when the time is prolonged to 210 min. The concurrent generated CO2 concentration has a steep jump and then quickly reaches a plateau near 205 ppm CO2. It can also be seen that the amount of evolved CO2 slightly exceeds the 2-fold amount of decomposed acetaldehyde (initial concentration, 96.7 ppm) attributed to the preabsorbed acetaldehyde molecules on photocatalyst when blowing mixed gas into the reactor.27 As the referenced P-25 photocatalyst, acetaldehyde decomposition behaves as a nearly linear decay with prolonged irradiation time, and 20 ppm of acetaldehyde are still not decomposed after 210 min. The corresponding evolved CO2 increases slowly. It is very evident that core-shell Au@TiO2 nanoparticles represent a higher decomposition rate of gaseous acetaldehyde, revealing excellent photocatalytic activity when exposed to ultraviolet light. Besides, the visible light (λ > 400 nm) induced photocatalytic properties have also been investigated, and the corresponding results are shown in Figure 7b. It can be observed that the referenced P-25 TiO2 shows poor catalytic activity with only about 6% acetaldehyde degraded when irradiation time is protracted to 210 min. However, using the prepared core-shell Au@TiO2 nanoparticles as photocalalyst for visible light initiated degradation of gaseous acetaldehyde, it can be found that acetaldehyde degradation first experiences a short induction period, followed by degradation acceleration. The occurrence of an induction period may be attributed to the photoinduced (heat-induced) desorption of pro-adsorbed acetaldehyde on the surface of catalysts.27 The related evolved CO2 accordingly experienced a slow increase during the induction period. Thereafter, the CO2 concentration has a significant rise and gradually reaches a plateau. After 210 min exposure under visible light irradiation, the residual acetaldehyde is about 15 ppm, which is far lower than that using P-25 as catalyst. Therefore, it can be revealed that the synthesized core-shell Au@TiO2 catalyst presents better catalytic performance for the decomposition of Langmuir 2009, 25(11), 6438–6447
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Figure 6. TEM images of core-shell Ag@TiO2 nanoparticles (a) and Pt@TiO2 nanoparticles (b).
Figure 7. Time-dependent ultraviolet (a) and visible light (b) photocatalytic removal of acetaldehyde and the concurrently evolved CO2 using the commercial Degussa (P-25) TiO2 (dashed line) and core-shell Au@TiO2 nanoparticles (solid line) as photocatalysts, respectively.
gaseous acetaldehyde when illuminated by visible light. From the above catalytic performance of the targeted sample, it can be thought that, in contrast to the TiO2 nanoparticles (P-25), the Au-TiO2 heterointerfaces and externally exposed truncated wedge-shaped TiO2 shells with highly reactive crystal {004} facets exert a joint influence to enhance the photocatalytic activity of the nanostructured core-shell Au@TiO2 nanoparticles. In order to approximate the value of the respective contributions of those above-mentioned structural factors to improve photocatalytic properties, the ultraviolet-induced decomposition behavior of acetaldehyde is also measured using hollow TiO2 with wedge-shaped morphology as an additional reference, obtained by completely removing the Au cores of as-prepared core-shell Au@TiO2 with KSCN solution (shown as inset in Langmuir 2009, 25(11), 6438–6447
Figure 3S, Supporting Information). The comparative data are shown in Figure 8. It can be found that the core-shell Au@TiO2 photocatalyst represents the highest degradation rate of gas-phase acetaldehyde and the hollow TiO2 particles also have photocatalytic activity higher than the commercial P-25 TiO2 nanoparticles. In addition, for core-shell and derivative hollow TiO2 catalysts, their concurrently evolved CO2 concentrations are higher than the theoretical value of 193.4 ppm derived from completely degraded acetaldehyde in gas-phase flow. The increments of CO2 can be attributed to the preabsorption of acetaldehyde on the surface of photocatalysts due to their bigger surface areas than P-25 nanoparticles. Compared with core-shell Au@TiO2 catalytst, its derivative hollow TiO2 catalyst has a bigger CO2 increment, due to its bigger specific surface area than core-shell nanoparticles with the same structured TiO2 shells.29,30 Accordingly, the surface area contributions of different photocatalytic activities can be approximately estimated using the relative evolved CO2 increments (Au@TiO2 19.8 ppm; hollow TiO2 70.1 ppm; P-25 1.6 ppm), which are derived from the deduction of 2-fold decomposed acetaldehyde. Thus, for core-shell Au@TiO2 photocatalyst, it can be seen that the higher photocatalytic activity may contribute to the roles played by Au-TiO2 heterointerfaces, because of the smaller contribution of surface area to photocatalytic activity than that of its derivative hollow TiO2. However, these rough estimates only give qualitative approximate insights into the effects of structures on photocatalytic activity, because of not taking into account other important factors, including reactivity, crystal structure, and kinetics of adsorption and desorption in gas-solid interface,27,29,52 and so on. Accordingly, it is difficult to determine the contribution of exposed higher-reactivity crystal (004) facets of truncated TiO2 antennae to photocatalytic activity for core-shell Au@TiO2 photocatalyst. It can be certain that the Au-TiO2 heterointerface interaction and the externally exposed highly reactive {004} facets act synergistically to enhance the photocatalytic activity. The “antennae” built by truncated wedgelike TiO2 crystals actually provide rougher interfaces and bigger surface area where the kinetics of the gas reaction are improved.29,30 For the Au-TiO2 heterointerfaces within the produced Au@TiO2 nanoparticles, it is well-known that the high Fermi level (Au, +0.45 V vs NHE21) induces the photoexcited electron transfer from the conduction band (-0.66 V vs NHE53) of the TiO2 semiconductor to metal cores through the contact (53) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2004, 126, 5851.
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Figure 8. Comparative diagram of photocatalytic activity using core-shell Au@TiO2 nanoparticles, derivative hollow TiO2 nanoparticles, and Degussa (P-25) as photocatalysts, respectively. The hollow TiO2 nanoparticles were obtained by completely etching the Au cores with KSCN solution. The dot-dash line labeled with 193.4 ppm is a theoretically generated CO2 value when the initial 96.7 ppm acetaldehyde is completely decomposed in gas-phase flow. The reaction time is 3.5 h.
interface until equilibrium is reached,17-20,54 whereby it retards efficiently the recombination of excited charged electron-hole pairs,7,17-20,55 producing long-lived charged holes with strong oxidizing power to initiate the redox reaction.56 The electrontransfer kinetics is dependent on the nature of heterointerfaces, electronic structures of semiconductor, and the potentials of redox pairs,54,62,63 and so on. Additionally, it is also known that the crystal size and anisotropic dimensions exert important effects on the band-gap shift of semiconductor.53,57-61 The equation governing the band-gap shift (ΔEg) in the anisotropic semiconductors has the form below:57,58 h2 2π2 ΔEg ¼ 2μxy Lxy 2
! þ
h2 π2 8μz Lz 2
where h is Plank’s constant, μxy, μz are the reduced effective mass of electron-hole pairs, respectively, and Lxy, Lz are the corresponding crystallite dimensions. On the basis of those above insights, the electric structure changes of the targeted samples can accordingly be characterized by UV-vis spectroscopy. Figure 9 shows UV-vis spectra of the produced core-shell Au@TiO2 photocatalyst and two referenced samples. It can be found that, unlike the featureless and broad onset of TiO2 nanoparticles (Figure 9b), the absorption of core-shell Au@TiO2 nanoparticles and their derivative hollow TiO2 presents sharp absorption peaks at 329.7 nm accompanying a red-shift, which is comparable to those observations of colloidal TiO2 nanosheets,59 indicative of the asymmetric crystalline structures of TiO2 shells. This result is consistent with the TEM results in Figure 1. The absorption red-shift may be attributed to bigger sizes of truncated wedge-shaped TiO2 (54) Wood, A.; Giersig, M.; Mulvaney, P. J. Phys. Chem. B. 2001, 105, 8810. (55) Tang, H.; Berger, H.; Schmid, P. E.; Levy, F. Solid State Commun. 1993, 87, 847. (56) Ribbens, S.; Meynen, V.; Tendeloo, G. V.; Ke, X.; Mertens, M.; Maes, B. U. W.; Cool, P.; Vansant, E. F. Microporous Mesoporous Mater. 2008, 114, 401. (57) Standroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. Rev. B 1986, 33, 5953. (58) Standroff, C. J.; Kelty, S. P.; Hwang, D. M. J. Chem. Phys. 1986, 85, 5337. (59) Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997, 101, 10159. (60) Ohno, T; Tagawa, S.; Itoh, H.; Suzuki, H.; Matsuda, T. Mater. Chem. Phys. 2009, 113, 119. (61) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196. (62) Curran, J. S.; Lamouche, D. J. Chem. Phys. 1983, 87, 5405. (63) Gerischer, H. J. Phys. Chem. 1984, 88, 6093.
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Figure 9. UV-vis adsorption spectra of gold (a), TiO2 (b) nanoparticles, derivative hollow TiO2 particles (c), and core-shell Au@ TiO2 nanoparticles (d). Hollow TiO2 particles obtained by removing the Au cores of sample (d, Figure 1) with KSCN solution in air.
antennae of core-shell Au@TiO2 nanoparticles due to the orientation growth along the c-axis direction. Compared with their derivatives (Figure 9c), core-shell Au@TiO2 nanoparticles have a weak absorption peak at 529.4 nm, attributed to the surface plasmon resonance (SPR) response of Au cores, and a significant red-shift against pure Au nanocrystals (shown as Figure 9 a), reflecting Au nanocrystals surrounded by high-refractive-index TiO2 shells.32-38 The weak SPR response is attributed to the thick TiO2 shells for individual core-shell Au@ TiO2 nanoparticles.64 3 Remarkably, for core-shell Au@TiO2 nanoparticles, the TiO2 characteristic absorption peak becomes broader than that of the two references. The absorption edges of core-shell Au@TiO2 and two references can be determined using the linear fitting method,60 and the corresponding results are 402.8 nm (P-25), 467.8 nm (hollow TiO2), and 484.4 nm (core-shell Au@TiO2), and the corresponding band gap edge is 3.08 eV (P-25), 2.65 eV (hollow TiO2), and 2.56 eV (core-shell Au@TiO2), respectively. From these results, it can be approximately deduced that the truncated wedge-shaped structure of TiO2 shells results in about 0.43 eV decrease of band-gap energy of anatase-type TiO2 due to the anisotropic crystal structure,53,57-61 and the Au-TiO2 heterointerface has approximately 0.09 eV contribution in narrowing the band gap of TiO2, which may be attributed to the modification of electronic states due to a heterojunction-induced changetransfer interaction.54,65 From these estimates, it can be suggested, for the as-synthesized core-shell Au@TiO2 photocatalyst, the externally exposed highly reactive crystal facets resulted from the segmented orientation growth in shells, and metal-TiO2 heterointerfaces synergistically narrow the band gap of the anatasetype TiO2 semiconductor. Additionally, the high Fermi energy level of Au cores induces the transfer of excited photoelectrons within the conduction band of TiO2 shells into Au cores, thus reducing the recombination of electrons in the conduction band and holes in the valence band of TiO2.8,12,16-21 The changes of the electronic band structure and photoexcited charge-transfer process are schematically depicted in Figure 7S (Supporting Information) Accordingly, the long-lived electric holes on the surface of the wedge-shaped TiO2 antennae of core-shell Au@TiO2 nanoparticles initiate acetaldehyde
(64) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (65) Okumura, M.; Kinoshita, M.; Yabushita, H.; Kitagawa, Y.; Kawakami, T.; Yamaguchi, K. Int. J. Quantum Chem. 2008, 108, 2888.
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decomposition with high catalytic activity when exposed to ultraviolet or visible light.
Conclusions In summary, core-shell Au@TiO2 nanoparticles with truncated wedge-shaped morphology have been synthesized successfully. TEM, HRTEM, and XRD results confirm that the wedgelike TiO2 shells experience an epitaxially segmented orientation growth. Also, the TiO2 crystals from hydrolysis of TiF4 precursors preferentially grow (101) crystal planes on the surface of gold nanocrystals to maintain low interface energy stabilizing the heterointerfaces. Faster [001] growth results in the budding process, and the growth sites form on the initial TiO2 layers, where the TiO2 crystals grow into truncated wedge-shaped morphologies. The morphological evolution of TiO2 shells is dependent on the produced F- ion concentration. The produced F- ions not only facilitate the formation of well-defined wedgelike TiO2 shells, but also contribute to the truncated crystal {004} facets. The preparation procedure developed is also extended successfully to synthesize other nanostructured core-shell metal (Ag or Pt) @TiO2 nanoparticles and metal (Fe3+ or Cr3+) iondoping Au@TiO2 nanoparticles, indicative of generic and flexible features. As the representative photocatalyst, the catalytic activities of as-synthesized core-shell Au@TiO2 nanoparticles are investigated by photoinitiated oxidation degradation of gaseous acetaldehyde. It is revealed that, under ultraviolet irradiation (λ = 352 nm), degradation behavior of acetaldehyde has rapid decay over a short time (15 min), and 98% acetaldehyde has been decomposed after 210 min. Similarly, when exposed to visible light (λ > 400 nm), the degradation of acetaldehyde has a
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photoinduction period followed by a acceleration of decomposition. About 15 ppm of acetaldehyde remains when irradiation time reaches 210 min. Comparative experiments suggest that the high photocatalytic activities of as-synthesized Au@ TiO2 nanoparticles are derived from the Au-TiO2 heterointerfaces and externally exposed truncated wedge-shaped morphology within TiO2 shells, both of which act synergistically to narrow the band gap of anatase-type TiO2. Additionally, the high Fermi level of gold nanocrystals induces the photoelectron transfer from the conduction band of TiO2, reducing the recombination of excited electron-hole pairs. Therefore, the long-lived charged holes on the surface of truncated wedge-like TiO2 antennae initiate the oxidative degradation of gaseous acetaldehyde with high catalytic activity when exposed to ultraviolet or visible light. Acknowledgment. This work was supported by the grant of Postdoctoral Program, Chonbuk National University. Supporting Information Available: TEM and HTEM images of Au nanoparticles; TEM images of core-shell Au@TiO2 nanoparticles with Au cores; XRD patterns of derivative hollow TiO2 nanoparticles; TEM images of core-shell Au@TiO2 nanoparticles obtained using H3BO3 as scavengers; preparation procedures, TEM images and energy-dispersive X-ray (EDX) patterns of core-shell Ag@TiO2 or Pt@TiO2 nanoparticles; TEM images of Fe3+, or Cr3+-doping core-shell Au@TiO2 nanoparticles; schematic charge-transfer process of core-shell Au@TiO2 photocatalyst. This material is available free of charge via the Internet at http://pubs.acs.org.
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