Titania Trilayered Nanorods and Their

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Preparation of Gold/Silver/Titania Trilayered Nanorods and Their Photocatalytic Activities Yoshimasa Horiguchi,† Takashi Kanda,† Kanjiro Torigoe,*,† Hideki Sakai,†,‡ and Masahiko Abe†,‡ †

Department of Pure and Applied Chemistry and ‡Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510 Japan S Supporting Information *

ABSTRACT: Gold/silver/titania trilayered nanorods have been prepared by the successive deposition of silver and titania layers on gold nanorod cores, and their photocatalytic activities were investigated under visible-light illumination (λ > 420 nm). The photocatalytic activity of the trilayered nanorods in the oxidation of 2-propanol depends on both the Au/Ag composition and the thickness of the TiO2 shell. It increases with increasing Ag content up to [Au]/[Ag] = 1:5 (molar ratio) and then decreases with further increasing Ag content. The photocatalytic activity also increases with increasing TiO2 shell thickness up to 10 nm and then decreases with further increases in the shell thickness. These effects were explained by electrontransfer and energy-transfer mechanisms.

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INTRODUCTION Titanium dioxide or titania is an excellent photocatalyst with high chemical stability.1,2 However, only UV light is utilizable as a light source for photocatalytic reactions because its bandgap energy is 3.2 eV for anatase (3.0 eV for rutile). To expand the wavelength range available for photocatalysis, a variety of strategies have been applied that involve a combination with photosensitizing dyes,1,2 semiconductor quantum dots,1,2 and doping light elements1,3−9 or metal ions10 into the TiO2 lattice. Meanwhile, noble metal nanoparticles exhibit great light absorptivity as a result of resonance between the incident electromagnetic wave and the collective motion of conduction (free) electrons at metal surfaces, called localized surface plasmon resonance (LSPR).12,13 The peak wavelength of the LSPR band depends not only on the metal species but also on the size12,13 and the shape of each nanoparticle.14 For instance, spherical gold nanoparticles of 5−20 nm in diameter exhibit an LSPR peak at about 520 nm, and gold nanorods with a 10−15 nm shorter axis length exhibit two LSPR peaks, one at about 510 nm (transverse mode) and the other at a longer wavelength depending on their aspect ratio (longitudinal mode).15,16 The longitudinal LSPR band is more intense than the transverse one. Therefore, gold nanorods should be more appropriate than spherical ones with regard to effective light absorption. Meanwhile, spherical silver nanoparticles exhibit an SPR band at about 390 nm.12,13 Although the SPR band of silver nanoparticles is located at a shorter wavelength than that of gold nanoparticles, the former is more intense: ε(Ag) ≈ 13 000 M−1 cm−1 (at 390 nm) and ε(Au) ≈ 3300 M−1 cm−1 (at 520 nm) based on the atomic concentration. However, the LSPR wavelength of gold/silver composite nanosphere systems © 2014 American Chemical Society

Figure 1. TEM images of (a) Au nanorods and (b−d) Au/Ag core− shell nanorods. [Au]/[Ag] loadings (molar ratios): (b) 1:2.5, (c) 1:5, and (d) 1:20.

is tunable by varying their composition.17 These properties are advantageous for utilizing gold/silver composite systems as an Received: November 12, 2013 Revised: January 6, 2014 Published: January 8, 2014 922

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preparation of the trilayered nanorods involves the formation of gold nanorods by a seed-mediated growth method,15 followed by covering with a silver shell by the reduction of AgCl adsorbed on the gold nanorods’ core36 and finally the formation of a titania shell on the silver layer using a preferential deposition of titania on the cationic surfactant layers on the silver.32 The photocatalytic properties were investigated by selecting the oxidation of 2-propanol as a model reaction, and the effects of the metal composition and titania shell thickness were studied. The photooxidation of 2-propanol was selected here because the reaction pathway is well-defined (2-propanol → acetone → carbon dioxide). We found that an optimal thickness exists both for the silver and titania shells on the gold nanorod core.

Table 1. Mean Dimensions of Au Nanorods and Au/Ag Core−Shell Nanoparticles

a b c d

[Au]/[Ag] (molar ratio)

longer length (nm)

shorter length (nm)

aspect ratio

1/0 1/2.5 1/5 1/20

44.2 47.3 47.8 51.1

10.4 14.2 17.0 24.7

4.30 3.33 2.81 2.07

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EXPERIMENTAL SECTION

Materials. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O), silver nitrate, ethanol, and 2-propanol were purchased from Wako Pure Chemical Industries. Cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), and L-ascorbic acid were purchased from Tokyo Chemical Industries. Titanium(IV) tetraisopropoxide (TTIP) was purchased from Sigma-Aldrich. All of the chemicals were reagent grade and used without further purification. Water was deionized to a resistivity of 18.2 MΩ cm using a Barnstead NanoPure system. Methods for Sample Preparation. Gold Nanorods. Gold nanorods were prepared by the seed-mediated growth method developed by Nikoobakht and El-Sayed.15 In a typical preparation, first seed solution was prepared by mixing 5 mL of 200 mM aqueous CTAB with 2.5 mL of 1 mM HAuCl4 and 2.5 mL of water, followed by the swift addition of 10 mM NaBH4 (0.6 mL) with vigorous stirring. Subsequently, growth solution was prepared by adding 6.25 mL of 4 mM AgNO3, 125 mL of 1 mM HAuCl4, 1.375 mL of 100 mM L-ascorbic acid, and 2.5 mL of water to 125 mL of 200 mM CTAB in this order under vigorous stirring. Once the orange color had vanished, 3 mL of the seed solution was added. The sample was gently stirred for about 30 min at 27 °C to ensure that the temperature was above the Krafft point of CTAB (∼25 °C). Silver Coating on Gold Nanorods. The gold nanorods were coated with silver via the procedure by Niidome et al.36 Prior to the silver coating, gold nanorods were centrifuged once (8000 rpm, 60 min) to remove excess CTAB and other impurities. Then, to substitute the CTAB bilayer on the metal surface with CTAC, the precipitate was

Figure 2. UV−vis extinction spectra of Au nanorods and Au/Ag core− shell nanorods. The Au loading concentration is 0.1 mM.

effective light absorber for white light. Moreover, Fermi levels of Au (−5.1 eV) and Ag (−4.7 eV) are close to the lowest energy of the conduction band of titania (−4.3 eV).18 Consequently, when gold, silver, or their composite nanoparticles and titania are in contact with each other, a metal-to-semiconductor electron transfer should readily take place, whereby LSPR-assisted photocatalytic reactions under visible light become possible.18−28 Although the preparation and photocatalytic properties of gold and silver nanosphere/titania systems have been reported,29−34 little is known regarding nanorod-based metal/titania composite nanoparticles,35 in particular, Au/Ag/TiO2 ternary nanorod systems. We report herein a facile preparation of Au/Ag/TiO2 trilayered nanorods and their photocatalytic properties. The

Figure 3. TEM images of Au/Ag/TiO2 trilayered nanorods at different magnifications. (a, d) [Au]/[Ag] = 1:2.5, (b, e) 1:5, and (c, f) 1:20. 923

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Table 2. Photocatalytic Activity and Peak Area of Extinction Spectra sample

k (min−1)

peak area (arb. units)a

TiO2 shell thickness (nm)

[Au]/[Ag] = 1: 20 [Au]/[Ag] = 1: 5 [Au]/[Ag] = 1: 2.5 Au

0.051 0.444 0.189 0.053

290 279 181 57

5 6 8 10

a

Area for 420 nm ≤ λ ≤ 900 nm.

Figure 4. UV−vis extinction spectra of Au/TiO2 bilayered nanorods and Au/Ag/TiO2 trilayered nanorods. The Au loading concentration is 0.1 mM. dispersed in 30 mL of 80 mM aqueous CTAC solution, followed by two centrifugations at 8000 rpm for 60 min, and the final precipitates were redispersed in 30 mL of 80 mM CTAC. Separately, AgCl nanoparticles were prepared by reacting 10 mL of 80 mM CTAC with 17 mg of AgNO3 under vigorous stirring. Subsequently, 5 mL of the CTAC-capped Au nanorod dispersion and 0.5 mL of 100 mM L-ascorbic acid were added to 10 mL of 80 mM CTAC with vigorous stirring, followed by the addition of 0.25 mL of 10 mM AgCl with vigorous stirring. Finally, to promote the reduction of Ag+ with L-ascorbic acid, 0.5 M NaOH was added to adjust the pH to 5.4 with vigorous stirring. The solution was left stirring for 3 h at ambient temperature. Titania Coating on Silver/Gold Nanorods. First, 60.4 μL of TTIP was dissolved in 20 mL of ethanol to make a 10 mM TTIP/ethanol solution. Then 20 mL of a Ag/Au nanorods dispersion was added with vigorous stirring, and the stirring was continued for 60 min at ambient temperature. Characterization of Nanorods. For dispersed nanorod samples, UV−visible extinction spectra were recorded on an Agilent 8453A diode array spectrophotometer using a quartz vessel with a 1 cm path length. Transmission electron micrographs (TEM) were taken with a Hitachi H-7650 operating at 120 kV. Energy-dispersive X-ray analysis (EDX) was performed with a Horiba EDX system attached to the TEM. High-resolution TEM (HRTEM) observation was performed with a Hitachi H-9500 at 200 kV. Photocatalytic Reaction. The photocatalytic properties of Au/Ag/ TiO2 nanorods were investigated by the oxidation of 2-propanol under visible-light illumination. The photocatalytic reaction was performed with a 300 mL custom-built cylindrical glass vessel with a quartz ceiling. As a photocatalyst, 50 mg of Au/Ag/TiO2 nanorods was dispersed on a Petri dish from an ethanol dispersion. The photocatalyst was dried in advance overnight in vacuo at ambient temperature, and then it was placed in the reaction vessel. Prior to the introduction of the substrate, the photocatalyst was irradiated with UV light (10 mW cm−2) from above for 2 h to activate the particle surface. Subsequently, the atmosphere was replaced with air of 50% relative humidity, and then 0.4 μL of 2-propanol was injected into the vessel with a glass syringe and left to stand in the dark for 2 h at 25 °C to attain adsorption equilibrium. Finally, the sample was irradiated by visible light (λ > 420 nm) with a 200 W Xe lamp equipped with a UV

Figure 5. (a) Photocatalytic degradation of 2-propanol and (b) production of acetone by Au/Ag/TiO2 trilayered nanorods under visible light irradiation (λ > 420 nm). The logarithmic plot of a is presented in c. Note that the sample was left standing in the dark for the first 2 h to reach adsorption equilibrium. cut filter (Kenko U-420). The irradiance at the sample position was 500 mW cm−2, and the reaction was performed at 25 °C. At a fixed time interval, 20 μL of the gas phase was collected with a glass syringe and analyzed with a Shimadzu GC-2014 gas chromatograph equipped with an FID detector using a 5-m-long silica column and He carrier gas. Calibration for 2-propanol and acetone in the gas phase was performed in the concentration range of 0−500 ppm in advance, and the sample concentration was determined by the peak area.

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RESULTS AND DISCUSSION Figure 1a displays a TEM image of Au nanorods employed as the core. The average lengths are 44.2 and 10.4 nm for the longer axis and the shorter axis, respectively, hence the mean aspect ratio is 4.3. By coating with a Ag shell of increasing concentration, the aspect ratio of nanorods decreases, as shown in Figure 1b−d and Table 1. The reason for the decreasing 924

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Figure 6. TEM images of Au/Ag/TiO2 trilayered nanorods prepared at different TTIP concentrations: (a) 0.5, (b) 1.0, (c) 5.0, (d) 10, and (e) 50 mM.

reduced there to Ag(0) by ascorbic acid. This leads to the preferential growth of the Ag shell in the direction normal to the longer axis of the core. Subsequently, the Au/Ag core−shell nanorods were further coated with the TiO2 shell to make the photocatalyst. Figure 3 presents TEM images of Au/Ag/TiO2 trilayered nanorods prepared at different loading Ag+ concentrations. A trilayered structure is clearly observed in high-resolution images for each nanorod. EDX analysis verified the TiO2 shell for the outermost layer showing a lighter contrast compared with the Ag layer (Figure S1 in Supporting Information). The mean thickness of the titania layer is 5−8 nm, as listed in Table 2. UV−visible extinction spectra of the Au/Ag/TiO2 trilayered nanoparticles are presented in Figure 4. Compared to Au/Ag samples shown in Figure 2, one finds that all SPR bands are red shifted when coated with titania. This red shift can be attributed to the increase in the dielectric constant of the medium (εm) because titania’s εm value (4.8 for anatase) is larger than that of water (1.77).40 Their photocatalytic properties were then studied by the oxidation of 2-propanol in the gas phase under visible-light illumination (λ > 420 nm). First the effect of Au/Ag composition was investigated. As shown by the results in Figure 5 and Table 2, the photocatalytic activity is increased by combining titania with a gold core and is further enhanced by introducing a silver shell between the gold and titania. The degree of enhancement is in the following order: [Au]/[Ag] = 1:5 ≳ 1:2.5 > 1:20 > Au alone. This order can be correlated with the extinction cross section for 420 nm ≤ λ ≤ 900 nm, as shown in Table 1, except for the [Au]/[Ag] = 1:20 sample. This enhancement in the photocatalytic activity can be attributed to electron transfer from the metal to semiconductor. Because the Fermi energy of Ag is less negative (−4.7 eV) than that of Au (−5.1 eV) and closer to the conduction band of TiO2 (−4.3 eV),17 electron transfer from silver to titania proceeds more readily than that from gold to titania. As an extreme case, in the sample composed of [Au]/[Ag] = 1:20, dissolution of the Ag shell has taken place. This is indirect proof of the electron

aspect ratio is apparent from the TEM images, which indicate the preferential growth of the Ag shell in the direction normal to the longer axis of the core. Also important to note is that the particle shape evolves from rod to cuboid with the thickening of the Ag shell.37,38 The change in the metal species on the surface and the transformation significantly affect the optical extinction spectra. As shown in Figure 2, the Au nanorod core displays two LSPR bands, a transverse band at 510 nm and a longitudinal band at 735 nm. With increasing thickness of the Ag shell, the longitudinal LSPR band undergoes a blue shift. Another important remark is the disappearance of the transverse LSPR band of Au nanorods at 510 nm and the appearance of two peaks at shorter wavelengths (e.g., 342 and 394 nm for the sample comprising [Au]/[Ag] = 1:2.5). Apart from these two bands, one faint shoulder is observed at about 420 nm for the sample at [Au]/[Ag] = 1:2.5, which develops into a prominent peak with increasing Ag content with a slight red shift. At [Au]/ [Ag] = 1:20, four distinct LSPR bands are observed at 347, 400, 450, and 524 nm. This spectral evolution has two different origins. The blue shift in the longitudinal LSPR band with increasing Ag content is mainly due to the evolution of metal species on the surface from Au to Ag,17 and the appearance of the other three bands is mostly due to the morphological evolution.37 The four SPR bands observed here are very similar to those of Au/Ag core−shell nanocubes studied by Tang et al.,37 who assigned them to the edge-associated plane multipole (EAPM) mode, the edge-associated corner multipole (EACM) mode, the corner quadrupole (CQ) mode, and the corner dipole (CD) mode from shorter- to longer-wavelength peaks, respectively. The anisotropic growth of the Ag shell is related to the CTAB layers on the surface of the Au nanorod core. Initially, the longer axis of Au nanorods is coated with a CTAB bilayer,39 and the surface close to the tip appears not to be completely covered with CTAB molecules. Because Ag+ has a high affinity for Br− being the counterion of CTA+, the Ag+ ions preferentially adsorb along the longer axis of the core and is 925

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the TiO2 shell as a result of the increasing probability of trapping electrons before their arrival at the shell surface. This result also eliminates the hypothesis that the oxidation of 2-propanol would proceed at the metal surface for the same reason. Consequently, a mechanism other than the electron transfer should be operative, and the photooxidation of 2-propanol has most probably taken place on the titania surface. Some recent papers18−22 account for the LSPR-assisted photocatalysis by the following three mechanisms: (1) electron transfer from metal to semiconductor, in which excited surface electrons of the metal are transferred to the conduction band of the semiconductor; (2) energy transfer from metal to semiconductor, in which the energy of the electric field enhanced at the surface of metal nanoparticles is conveyed to the semiconductor, leading to the promotion of electron/hole pair production, and (3) enhanced light scattering by metal nanoparticles, leading to effective light absorption by the semiconductor. Larger metal nanoparticles exhibit a greater scattering intensity. The effect of the titania shell thickness could be explained by the energy-transfer mechanism as follows. With increasing shell thickness, a larger volume of the titania shell can receive the energy of incident light with enhanced power by LPR of the metal. However, the power of the light’s electric field decreases exponentially with the distance from the metal surface. Consequently, the available power for the production of electron/hole pairs takes a maximum at a certain shell thickness of the semiconductor, which corresponds to about 10 nm in this system. Meanwhile, on a titania surface, oxidative radicals (•OH) are generated by the interaction of electrons and holes with water or oxygen in the atmosphere by the following reactions:1

transfer from Ag to titania ending up with the oxidation of Ag to Ag+. Note that the dissolution of Ag is not observed for the other samples containing less Ag, which is probably due to the mixing of electronic energy states for the gold core. However, electron transfer is not the only mechanism operating to enhance the photocatlytic activity, as discussed below. In the following section, the Au/Ag/TiO2 trilayered nanorods with different TiO2 shell thicknesses were prepared and their photocatalytic activities were studied. Figure 6 shows their TEM images, indicating that the TiO2 shell can be varied from 5 to ∼50 nm with increasing TTIP concentration. The photocatalytic activity, shown in Figure 7 and Table 2, increases

TiO2 + hν → h+ + e−

(1)

H 2O + h+ →• OHads + H+aq

(2)

O2 + e− → O2−ads

(3)

O2−ads + H+aq → HO2•ads

(4)

Here, both •OH and HO2• radicals are strongly oxidative and capable of oxidizing 2-propanol. An important remark is related to the distance from the metal surface available for electron transfer or energy transfer. Although we do not have exact values for it, it would be reasonable to assume that electron transfer is effective only at a short distance (within several nanometers) whereas energy transfer will reach farther. Because the effect of the Au/Ag composition was investigated by using nanorods with thin titania shells, the metal-to-semiconductor electron transfer would prevail over energy transfer. Because the effect on the titania shell was studied with a thicker shell, the metal-to-semiconductor energy-transfer mechanism could be dominant. This issue will be addressed in more detail in a forthcoming paper. Another remark to be noted is that titania used in this study is amorphous. At least in the XRD profile (not shown), the Au/Ag/TiO2 trilayered nanorods show no distinct peaks corresponding to crystallite titania (anatase, rutile, and brookite). However, by combining with Au/Ag nanorods, they show photocatalytic activity. Moreover, these nanorods are stable, and no reshaping was observed during visible-light irradiation. This could be attributed to the core−shell structure.

Figure 7. (a) Photocatalytic degradation of 2-propanol and (b) production of acetone with Au/Ag/TiO2 trilayered nanorods under visible-light illumination (λ > 420 nm). (c) Logarithmic plot for the consumption of 2-propanol.

with increasing thickness of the titania shell up to 10 nm and then decreases with further increases in the thickness to ∼50 nm. Apparently these results are not consistent with the electron-transfer mechanism because in that case the photocatalytic activity should decrease with increasing thickness of 926

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their transformation at the nanoscale. J. Phys. Chem. B 2004, 108, 1230−1240. (7) Shi, J. W.; Zong, X.; Wu, X.; Cui, H. J.; Xu, B.; Wang, L.; Fu, M. L. Carbon-doped titania hollow spheres with tunable hierarchical macroporous channels and enhanced visible light-induced photocatalytic activity. ChemCatChem 2012, 4, 488−491. (8) Stewart, S. J.; Fernández-García, M.; Belver, C.; Mun, B. S.; Requejo, F. G. Influence of N-doping on the structure and electronic properties of titania nanoparticle photocatalysis. J. Phys. Chem. B 2006, 110, 16482−16486. (9) Wu, X. W.; Wu, D. J.; Liu, X. J. Optical investigation on sulfurdoping effects in titanium dioxide nanoparticles. Appl. Phys. A: Mater. Sci. Process. 2009, 97, 243−248. (10) Goswani, P.; Ganguli, J. N. A novel synthetic approach for the preparation of sulfated titania with enhanced photocatalytic activity. RSC Adv. 2013, 3, 8878−8888. (11) Vijayan, B. K.; Dimitrijevic, N. M.; Wu, J.; Gray, K. A. The effects of Pt doping on the structure and visible light photoactivity of titania nanotubes. J. Phys. Chem. C 2010, 114, 21262−21269. (12) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (13) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (14) Tréguer-Delapierre, M.; Majimel, J.; Mornet, S.; Duguet, E.; Ravaine, S. Synthesis of non-spherical gold nanoparticles. Gold Bull. 2008, 41/2, 195−207. (15) Nikoobakht, B.; El-Sayed, M. A. Preparation and growth mechanism of gold nanorods (NRs) using seed mediated growth method. Chem. Mater. 2003, 15, 1957−1962. (16) Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L. M.; Mulvaney, P. Gold nanorods: synthesis, characterization and applications. Coord. Chem. Rev. 2005, 249, 1870−1901. (17) Tréguer, M.; de Cointet, C.; Remita, H.; Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J.; de Keyser, R. Dose-rate effect on radiolytic synthesis of gold-silver bimetallic clusters in solution. J. Phys. Chem. B 1998, 102, 4310−4321. (18) Zhang, X.; Chen, Y. L.; Liu, R. S.; Tsai, D. P. Plasmonic photocatalysis. Rep. Prog. Phys. 2013, 76, 046401. (19) Cushing, S. K.; Li, J.; Meng, F.; Senty, T. R.; Suri, S.; Zhi, M.; Li, M.; Bristow, A. D.; Wu, N. Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor. J. Am. Chem. Soc. 2012, 134, 15033−15041. (20) Primo, A.; Corma, A.; Garcia, H. Titania supported gold nanoparticles as photocatalyst. Phys. Chem. Chem. Phys. 2011, 13, 886−910. (21) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy. Nat. Mater. 2011, 10, 911−921. (22) Zhu, H.; Chen, X.; Zheng, Z.; Ke, X.; Jaatinen, E.; Zhao, J.; Guo, C.; Xie, T.; Wang, D. Mechanism of supported gold nanoparticles as photocatalysis under ultraviolet and visible light irradiation. Chem. Commun. 2009, 7524−7526. (23) Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M. H. Facile in situ synthesis of visible-light plasmonic photosynthesis M@TiO2 (M = Au, Pt. Ag) and evaluation of their photocatalytic oxidation of benzene to phenol. J. Mater. Chem. 2011, 21, 9079−9087. (24) Zielinska-Jurek, A.; Kowalska, E.; Sobczak, J. W.; Lisowski, W.; Ohtani, B.; Zaleska, A. Preparation and characterization of monometallic (Au) and bimetallic (Ag/Au) modified-titania photocatalysts activated by visible light. Appl. Phys. B 2011, 101, 504−514. (25) Kowalska, E.; Mahaney, O. O. P.; Abe, R.; Ohtani, B. Visiblelight-induced photocatalysis through surface plasmon excitation of gold on titania surfaces. Phys. Chem. Chem. Phys. 2010, 12, 2344−2355. (26) Subramanian, V.; Wolf, E. E.; Kamat, P. V. Catalysis with TiO2/ gold nanocomposites. Effect of metal particle size on the Fermi level equilibration. J. Am. Chem. Soc. 2004, 126, 4943−4950. (27) Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M. Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles. J. Am. Chem. Soc. 2007, 129, 14852−14853.

CONCLUSIONS In this study, Au/Ag/TiO2 trilayered nanorods were prepared and their photocatalytic properties were investigated. Optical extinction spectra were recorded for Au/Ag core−shell nanorods and Au/Ag/TiO2 trilayered nanorods with different Au/Ag compositions and TiO2 shell thicknesses. Regarding the photocatalytic activity of these nanorods in the oxidation of 2-propanol, the following results were obtained: (1) The photocatalytic activity for the oxidation of 2-propanol increases with increasing Ag content up to a [Au]/[Ag] = 1:5 molar ratio. At higher Ag content ([Au]/[Ag] = 1:20), the activity decreases because of the dissolution of the Ag shell. (2) The photocatalytic activity increases with increasing thickness of the titania shell up to 10 nm. Further increases in the thickness of the titania shell to 50 nm lead to a significant decrease in the activity. The first result correlates with the extinction cross section of the LSPR band, supporting the metal-to-semiconductor electron-transfer mechanism. Meanwhile, the second result cannot be explained by the electron-transfer mechanism but is successfully accounted for by the energy-transfer mechanism. It is possible that the electron-transfer mechanism is predominant for the composite nanorods with thin titania shells of only a few nanometers’ thickness, whereas energy transfer prevails for thicker semiconductor shells.

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ASSOCIATED CONTENT

S Supporting Information *

EDX profile for the Au/Ag/TiO2 trilayered nanorods. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Prof. Iwao Hashimoto (TUS) and Prof. Makoto Tadokoro (TUS) for allowing us to use their highresolution TEM facility (Hitachi H-9500).

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REFERENCES

(1) Anpo, M., Kamat, P. V., Eds. Environmentally Benign Photocatalysis: Applications of Titanium Oxide-Based Materials; Springer: New York, 2010. (2) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem. Rev. 1995, 95, 735−758. (3) Wang, Z.; Yang, C.; Lin, T.; Yin, H.; Chen, P.; Wan, D.; Xu, F.; Huang, F.; Lin, J.; Xie, X.; Jiang, M. H-doped black titania with very high solar absorption and excellent photocatalysis enhanced by localized surface plasmon resonance. Adv. Funct. Mater. 2013, 23, 5444−5450. (4) Štengl, V.; Houšková, V.; Bakardjieva, S.; Murafa, N. Photocatalytic activity of boron-morified titania under UV and visible-light illumination. ACS Appl. Mater. Interfaces 2010, 2, 575−580. (5) Wang, X.; Blackford, M.; Prince, K.; Caruso, R. A. Preparation of boron-doped porous titania networks containing gold nanoparticles with enhanced visible-light photocatalytic activity. ACS Appl. Mater. Interfaces 2012, 4, 476−482. (6) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y.; Chen, X. Highly efficient formation of visible light tunable TiO2−xNx photocatalysis and 927

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dx.doi.org/10.1021/la404370s | Langmuir 2014, 30, 922−928