Article pubs.acs.org/JPCC
Sonochemical Deposition of Au Nanoparticles on Different FacetsDominated Anatase TiO2 Single Crystals and Resulting Photocatalytic Performance Kun Cheng, Wenbin Sun, Hai-Ying Jiang, Jingjing Liu, and Jun Lin* Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China S Supporting Information *
ABSTRACT: The dependence of Au adhesion and photocatalytic performance on the support’s exposed facets in the Au/TiO2 system is an important consideration in the design of Au/TiO2 heterogeneous catalysts. In this article, we prepared anatase TiO2 single crystals with different percentages of exposed {101} and {001} facets by a hydrothermal process. The uniform deposition of crystalline metallic Au nanoparticles with the mean diameter of 10 to 11 nm onto both exposed {101} and {001} facets of the prepared anatase TiO2 single crystals has been achieved with the aid of power ultrasound. The Au-deposited TiO2 single crystals with the dominant {001} facet have been demonstrated to have much higher plasmon-induced photocatalytic activity than those with the dominant {101} facet for the degradation of 2,4-dichlorophenol. It was revealed that the dominant facet of anatase TiO2 crystal plays a critical role in the plasmon-induced photocatalytic performance of Au/TiO2 system. The {001} facet enhances the efficiency of the interfacial charge-transfer process by offering a stronger adherence for Au nanoparticles than the {101} facet, which promotes the generation of the reactive oxygen species O2•− by the plasmon-induced electron transfer from Au nanoparticles to anatase TiO2 substrate. This work would provide useful guidance for the design of Au/TiO2 heterogeneous photocatalysts.
1. INTRODUCTION Noble-metal−oxide composite systems, especially involving TiO2, have been a subject of intensive research because of their important applications as photocatalysts, chemical sensors, and heterogeneous catalysts.1−4 These systems are very amazing and interesting. For example, neither metallic Au particles nor TiO2 semiconductor oxide can behave as an efficient photocatalyst upon visible-light irradiation, but a proper combination of both generates excellent visible-light photocatalytic performances, even significantly enhancing UV photocatalysis of TiO2.4 This indicates, for this composite system, that a key to achieve the excellent catalytic performances is the interfacial interaction between the metal particles and the TiO2 semiconductor.3,4 In the case of the Au/TiO2 photocatalytic system, the TiO2 surface as support not only helps us to load Au particles but also transfers the electrons from its conduction band (CB) to Au or accepts the electrons from Au,5 depending on whether the excitation occurs on TiO2 or on the surface plasmon band of Au. Apparently, the interaction is definitely affected by the surface atomic structures of TiO2 because surface atomic arrangement and coordination intrinsically determine the adsorption energy and states of the loaded Au, and the electron transfer between TiO2 support and the loaded Au, as a result, influences the final catalytic properties. Surface atomic arrangement and coordination vary with exposed crystal facets in different orientations. The thorough understanding of the dependence of noble-metal adhesion and photocatalysis on the support’s exposed crystal facets in noble-metal−TiO2 systems is © 2013 American Chemical Society
very important in the design of noble metal−TiO2 composite catalysts. With the development of works controlling crystal facets,6 intensive studies have been conducted on the dependence of reactivity, in particular, photocatalytic activity, on the exposed crystal facets of metal oxides such as TiO2.7−10 Meanwhile, the interaction dependence of noble metal-oxide composites on oxide crystal facets has also been brought to some attention. Typically, on the basis of the different adsorption behaviors of molecules and ions on crystal facets, the atomic facet-selective deposition of Au nanoparticles on Cu2O or ZnO oxides was achieved by preferential preadsorption of additives.11 Another representative study demonstrated that Pt particles from the photoreduction of Pt+ can be deposited only on the {110} facets of rutile TiO2 while the photo-oxidation of Pd+ to PdO occurs mostly on {011} facets.12 A similar trend was also found in anatase facets. These results clearly indicated the deposition dependence of these noble metals on the exposed crystal facets of oxide supports. More recently, it was reported that selectively depositing Pt onto {101} facets of anatase TiO2 crystals with different percentages of exposed {001} and {101} facets can effectively enhance the photocatalytic activity of TiO2 in both photoreduction and photooxidation processes.13 To date, however, similar experimental studies seldom occur on Received: April 9, 2013 Revised: June 26, 2013 Published: June 26, 2013 14600
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
The Journal of Physical Chemistry C
Article
X-ray radiation under 40 kV and 30 mA. The morphology and microstructure of TiO2(101) and TiO2(001) samples before and after Au loading were characterized by field-emission scanning electron microscopy (FESEM) (JEOL JSM-7401F) and high-resolution transmission electron microscopy and selected area electron diffraction (HRTEM/SAED) (JEOL JEM-2010), respectively. The optical absorbance spectra of all samples were recorded with a UV−vis spectrophotometer (Hitachi U-3900) using BaSO4 as reflectance standard. The surface chemical states of the elements in all samples were analyzed by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi) using 300 W Al Kα radiation. All binding energies were referenced to the C1s peak (284.6 eV) of the surface adventitious carbon. To study the generation of the reactive oxygen radical O2•− in the visible-light irradiated suspensions of Au/TiO2(101) and Au/TiO2(001), we employed the spin trap method using diamagnetic DMPO to produce a stable paramagnetic spin-adduct with O2•− radicals in methanol solution. The spin-adduct was detected using an electron spin resonance (ESR) spectrometer (JEOL, JES-FA200) equipped with a 100 W short arc mercury lamp as the irradiation light source (λ > 420 nm). The settings for the ESR spectrometer were center field = 336.93 mT, microwave frequency = 9438 MHz, and power = 3.0 mW. 2.4. Photocatalytic Activity Test. The degradation of 2,4dichlorophenol (2,4-DCP) was carried out to evaluate photocatalytic activities of the prepared catalysts upon visiblelight irradiation. Typically, 10 mg catalyst was suspended in 10 mL of aqueous solution of 2,4-DCP (∼10−4 M). The light source was a 300 W Xe-arc lamp (CHF-XM150, Beijing Trusttech) equipped with a wavelength cutoff filter for λ > 420 nm and positioned about 8 cm above the aqueous suspension. Prior to irradiation, the suspension of the catalyst in the 2,4DCP aqueous solution was continuously stirred in the dark for more than 1 h to ensure the establishment of an adsorption− desorption equilibrium between the catalyst surface and 2,4DCP. The degradation of 2,4-DCP was followed chromatographically by a high-performance liquid chromatograph (Agilent, 1260 Infinity LC) with a C18 column (5 μm, 4.6 × 250 mm). The eluent consisted of 65% acetonitrile, 35% water, and 0.1% phosphoric acid.
Au/TiO2, except for several theoretical reports predicting the active nucleation sites for Au clusters on the different facets of anatase and rutile TiO2.14,15 The Au/TiO2 composite as an important photocatalytic system can efficiently degrade organic pollutants upon light irradiation with operation mechanisms different from the Pt/TiO2 system.16 To well design Au/TiO2 composite system and further optimize its photocatalytic properties, it is highly desirable and necessary to systematically investigate the influences of the support’s exposed crystal facets on Au loading, electron transfer between TiO2 support and loaded Au, and final photocatalytic properties. In the present work, anatase TiO2 single crystals with different percentages of exposed {101} and {001} facets were synthesized through a hydrothermal method. Crystalline metallic Au nanoparticles with the mean diameter of 10 to 11 nm were uniformly deposited on both exposed {101} and {001} facets of the prepared anatase TiO2 single crystals by a facile sonochemical approach. The Au-loaded anatase TiO2 single crystals with the dominant {001} facets have been shown to have much higher photocatalytic activities for the degradation of 2,4-dichorophenol than those with the dominant {101} facets. Furthermore, the role of the TiO2 support’s exposed facets in the Au deposition and the electron transfer between TiO2 and loaded Au during photocatalysis was investigated according to various physicochemical characterization results. The process of the ultrasound-driven synthesis employed for depositing Au on TiO2 is also briefly discussed.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Anatase TiO2 Single Crystals with Dominant Exposed {101} or {001} Facets. Anatase TiO2 single crystals with the dominant exposed {101} or {001} facets, denoted as TiO2(101) and TiO2(001), respectively, were synthesized according to a reported hydrothermal method using TiOSO4 as Ti source and HF as a morphology controlling agent.6,7 In detail, 64 and 70 mg of titanium oxysulfate (TiOSO4·xH2O Sigma-Aldrich, 159.9 g·mol−1) powder were dissolved in the 40 mL aqueous solutions of HF with the concentration of 110 and 175 mM, respectively, to produce the TiOSO4 aqueous solution precursor for TiO2(101) and TiO2(001) samples. Then, the obtained aqueous solution was transferred to a Teflon-lined autoclave and treated at 180 °C for 12h. After reaction, the products were collected by centrifugation, washed with deionized water several times, and finally dried at 80 °C for 10h. 2.2. Sonochemical Deposition of Au Nanoparticles. Before Au loading, the as-prepared TiO2(101) and TiO2(001) samples were calcined at 600 °C in air for 2 h for the removal of the surface fluorine.7 In a typical sonochemical deposition, 10 mg of the clean sample was dispersed into 20 mL of aqueous solution of methanol (2.5 M) and HAuCl4 (0.01 mM) in a beaker. Then, the sonication of the dispersion with the ultrasonic irradiation was carried out by the direct immersion of the beaker in an ultrasonic cleaning bath (40 kHz, 150 W) at room temperature for 20 min. After the ultrasonic irradiation, the product was collected by centrifugation, thoroughly washed with deionized water, and finally dried at 100 °C for 8h. The Au-deposited TiO2(101) and TiO2(001) samples were denoted as Au/TiO2(101) and Au/TiO2(001), respectively. 2.3. Characterization. The crystal phases of the TiO2(101) and TiO2(001) samples before and after the removal of the surface fluorine were identified at room temperature with an Xray diffractometer (SHIMADZU, XRD-7000) using Cu Kα as
3. RESULTS AND DISCUSSION 3.1. Phase Structure and Morphology. XRD was used to identify the phase structures of the synthesized samples. Figure 1 shows the XRD patterns of the synthesized TiO2(101) and TiO2(001), in which all diffraction peaks of two samples match well with the crystal structure of the anatase TiO2 phase (JCPDS no. 21-1272, space group I41/amd). In addition, the intensities of the (004) diffraction peaks for the two samples are significantly enhanced as compared with the corresponding one of the reference (JCPDS No. 21-1272), suggesting a preferential orientation growth along the [001] direction for the two samples. Figure 2a,b shows the representative FESEM images of the synthesized TiO2(101) and TiO2(001), respectively. From Figure 2a,b, both TiO 2 (101) and TiO2(001) anatase exhibit the well-faceted crystals with a truncated octahedral bipyramidal structure. However, TiO2(001) crystals have a short main axis of the truncated bipyramid, resulting in a smaller aspect ratio than for the {101} counterpart. On the basis of the related reports and the symmetries of anatase TiO2,7,17 the two flat sides of two samples shown in Figure 2a,b; that is, the square surfaces are 14601
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
The Journal of Physical Chemistry C
Article
HAuCl4 and methanol. Power ultrasound induces chemical changes as a result of cavitation phenomena involving the formation, growth, and implosive collapse of bubbles in liquid.19 Meanwhile, the power ultrasound has been proven to reduce the metal precursor in situ through the generation of H• radicals (eq 1).20 During sonochemical deposition, very high-speed microjets and shock waves with a scale of several hundred meters/second are produced when the bubbles in liquid are created and collapse. The high-speed jets can effectively push the metal particles toward the surface of a substrate so as to form substrate-supported metal. By now, the sonochemical phenomena have been exploited for anchoring metallic nanocrystals (Ag, Au, Pd, and Pt) onto the surfaces of various substrates such as silica spheres, titania, alumina, and polystyrene spheres.20−23 Figure 3a,b shows the FESEM images Figure 1. X-ray diffraction patterns of TiO2(101) and TiO2(001) samples.
{001} facets, while the eight isosceles trapezoidal surfaces are {101} facets of anatase TiO2. The representative HRTEM image and SAED pattern shown in Figure 2c further confirm the single-crystal characteristics of the truncated octahedral bipyramidal structure. The HRTEM image viewed from the [001] crystallographic direction clearly demonstrates the atomic planes of (200) and (020) crystal facets with a lattice spacing of 0.19 nm and an interfacial angle of 90°. The SAED pattern (insert, Figure 2c) can be indexed as diffraction spots of the [001] zone, supporting that the square surfaces are {001} facets of the single-crystalline anatase TiO2. These results reveal that the synthesized samples are composed of large-sized and well-faceted anatase TiO2 single crystals. According to the morphologies and sizes of anatase TiO2 single crystals in the two samples, it can be found that both samples have similar surface area. Moreover, the percentages of {101} and {001} facets in the anatase single crystal could be estimated to be ca. 70 and 30% for TiO2(101) and ca. 40 and 60% for TiO2(001), respectively. Thus, anatase TiO2 single crystals with dominant exposed {101} or {001} facets have been readily prepared to load Au nanoparticles on them. Considering the effects of the surface fluorine termination on various facets nature7,18 and the fluorine-free surface in the practical application of anatase TiO2, we removed the surface fluorine atoms from the two samples by calcination at 600 °C in air for 2 h before Au loading.7 The removal of the surface fluorine atoms on sample crystals at 600 °C in air did not change the crystal form and crystallinity of TiO2(101) and TiO2(001) samples. (See Figure S1 in the Supporting Information.) After removal of the surface fluorine, Au nanoparticles were deposited on the clean surface of the two samples with the aid of power ultrasound in the presence of
Figure 3. FESEM images of (a) Au/TiO2(101) and (b) Au/ TiO2(001) and their individual crystals (right).
of TiO2(101) and TiO2(001), respectively, after Au deposition by sonication. The individual crystals (right) of two samples clearly show that Au nanoparticles are uniformly dispersed and deposited on both exposed {101} and {001} facets in the sonochemical process. No selective and obvious preferential deposition on {101} or {001} facets are observed, indicating that the Au clusters hit {101} and {001} facets nonselectively and then load on both of them under the effect of high-speed jets during the ultrasonic irradiation. According to the mechanism of the sonochemical reduction,20 H• free-radical species are produced from water molecules by the absorption of ultrasound (eq 1). The formed H• radical can act as a reducing agent to react with AuCl4− to form Au° (eq 2).
Figure 2. FESEM images of anatase (a) TiO2(101) and (b) TiO2(001) samples. (c) A representative HRTEM image and SAED pattern (insert). 14602
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
The Journal of Physical Chemistry C
Article
H 2O → H• + •OH
(1)
AuCl4 − + 3H• → Au o + 3H+ + 4Cl−
(2)
Noticeably, in our case, the sonochemical deposition of Au on two samples was carried out within 20 min, which is much faster than a traditional sonochemical process (60 min). This might be attributed to the fact that the methanol in our sonication solution can consume •OH, allowing more H• to reduce AuCl4− to Au° based on eqs 1 and 2. Figure 4a,b shows the typical TEM images (left) and the deposited Au particle size distribution (right) for Au/
Figure 5. Well-crystalline Au nanoparticle deposited on a (001) facet of TiO2 a nanocrystal.
Figure 4. TEM images (left) and Au particle size distribution (right) of (a) Au/TiO2(101) and (b) Au/TiO2(001).
TiO2(101) and Au/TiO2(001), respectively. From Figure 4, it is obvious that the dark-gray small Au particles are uniformly loaded on the surfaces of both light TiO2 substrates. The Au particles loaded on both samples are of uniform and spherical shape, and the particle size of the loaded Au is mainly in the range of 10−11 nm. As observed in a representative HRTEM image (Figure 5), Au nanoparticles with well-developed crystallinity are deposited closely on the {001} of TiO2 substrate. The HRTEM analysis of the Au nanoparticles also indicates that the lattice fringes of 0.20 and 0.23 nm match the crystallographic planes of metallic cubic-phase Au (200) and (111), respectively. The interfacial angle between Au (200) and (111) is ∼54.7°, which is identical to the theoretical value for the angle between the cubic-phase bulk-like Au (200) and (111) planes.24,25 The characterization results of crystal phase and morphology clearly reveal that crystalline metallic Au nanoparticles with a mean diameter of 10 to 11 nm are uniformly deposited on both exposed {101} and {001} facets of anatase TiO2 single crystals with dominant {101} or {001} facets with the aid of power ultrasound. These conditions allow us to further study the dependence of electron transfer and photocatalytic performance in the Au/TiO2 system on the dominant facet. 3.2. XPS and UV−vis Diffuse Reflectance Spectra. To further investigate the chemical states of the elements in our samples, we characterized anatase TiO2(101) and TiO2(001) single crystals before and after Au loading by XPS analysis. Figure 6 displays the XPS Au4f spectra of Au/TiO2(101) and Au/TiO2(001), respectively. Au4f spectra of both samples are
Figure 6. Au4f XPS spectra from Au/TiO2(101) and Au/TiO2(001).
composed of two peaks at the binding energies of 83.2 and 86.9 eV, assigned to Au4f7/2 and Au4f5/2, respectively, suggesting that the Au species in the two samples are present in the metallic state.26,27 No oxidized gold species were detected. This result is in agreement with the HRTEM observation reported above. On the basis of literature data,28,29 the binding energies of Au4f7/2 and Au4f5/2 for metallic Au are centered at 83.8 and 87.5 eV, respectively. As reported,3,30 the Fermi level of TiO2 is higher than that of Au nanoparticles. The electron transfer from TiO2 substrate to Au nanoparticles occurs for Fermi level equilibration when the TiO2 substrate and Au nanoparticles are in contact, which results in the observed slight shift of Au4f peaks in the samples toward lower binding energy. Figure 7a,b demonstrates the chemical states of Ti in TiO2(101) and TiO2(001) before and after Au loading, respectively. Two peaks for all Ti2p spectra are found at about 458 and 464 eV, assigned to Ti2p3/2 and Ti2p1/2, respectively, suggesting the presence of Ti(IV) species.28,31 A careful comparison of Ti2p spectra in Figure 7a,b shows that there are no measurable changes in the peak positions for Ti2p in TiO2(101) before and after Au loading, while the doublet from the TiO2(001) sample shifts visibly toward higher binding energy after Au loading. This indicates that the interfacial interaction between Au and 14603
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
The Journal of Physical Chemistry C
Article
Figure 8. UV−vis diffuse reflectance spectra of TiO2(101) and TiO2(001) before and after Au loading.
peak shoulder centered at ca. 537 nm, which is due to the absorption of the Au surface plasmon. This absorption is caused by the collective oscillation of free CB electrons of the gold particles in response to optical excitation.36,37 Moreover, two spectra that both Au/TiO2(101) and Au/TiO2(001) exhibit in visible-light region appear to be very similar in shape and intensity in addition to the absorption band. It was reported that the band position and intensity of the plasmon absorption strongly depend on the particle size and amount of the deposited gold, respectively.38−40 Thus, it can be concluded that the particle sizes and amount of the Au nanoparticles deposited onto both TiO2(101) and TiO2(001) crystals are similar, as partially evidenced by the TEM images shown above (Figure 4). 3.3. Photocatalytic Performance and Mechanism. The degradation of 2,4-DCP under visible-light irradiation (λ≥420 nm) was performed to evaluate the photocatalytic performances of TiO2(101) and TiO2(001) before and after Au loading. For a comparison, the TiO2(101) and TiO2(001) samples were also treated by ultrasonic irradiation in the absence of methanol and HAuCl4 before the photocatalytic activity test. The experimental results are shown in Figure 9. Upon visible-light irradiation, both TiO2(101) and TiO2(001) exhibit very low photocatalytic activities for the degradation of 2,4-DCP. Au loading onto both samples significantly enhances catalytic activity, which is attributed to the localized surface
Figure 7. Ti2p XPS spectra from (a) TiO2(101) and (b) TiO2(001) before and after Au loading.
TiO2(001) is stronger than that between Au and TiO2(101) due to different surface atomic arrangement and coordination of {101} and {001} facets of anatase TiO2. As compared with fully coordinated atoms on the {101} surface facets of anatase TiO2, all surface atoms on the (001) facet with five-foldcoordinated Ti (Ti5c) and two-fold-coordinated O (O2c) are unsaturated, resulting in a much higher adsorption energy over {001} than {101} facets.32−34 Theoretical studies indicated that the adsorption of gold particles on the {001} facets dominated by unsaturated atoms is much stronger than that on {101} facets featuring fully coordinated atoms.14 Hence, the electron transfer between Au nanoparticles and {001}-dominated TiO2 substrate is enhanced, causing the observed shift in Ti2p binding energy. This strong interfacial interaction could be beneficial for the enhancement of photocatalytic performance over Au/TiO2(001). Figure 8 shows the UV−vis diffuse reflectance spectra of the TiO2(101) and TiO2(001) crystals before and after Au loading. It can be seen that both TiO2(101) and TiO2(001) crystals before Au loading exhibit comparable absorbance, but the absorption edge of TiO2(101) shows a blue shift of 5 to 6 nm as compared with that of TiO2(001). Previous reports indicated that the band gap of {101} facets is larger than that of {001} facets because of different atomic structure.7,35 Therefore, the observed blue shift is attributed to the high percentage of {101} facets in TiO2(101). After Au loading, both samples present a
Figure 9. Degradation of 2,4-DCP over various samples upon visiblelight irradiation. 14604
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
The Journal of Physical Chemistry C
Article
plasmon resonance of Au. Interestingly, Au/TiO2(001) shows much higher photocatalytic activity than Au/TiO2(101). Approximately 60% 2,4-DCP degradation is reached over Au/ TiO2(001), whereas only 420 nm). Moreover, the production of DMPO-O2•− is apparently more pronounced in Au/TiO2(001) than Au/TiO2(101) suspensions. No such signals are detected in the dark. This nicely parallels the results of the photocatalytic activity test
Figure 10. ESR signal of DMPO-O2•− in the Au/TiO2(101) and Au/ TiO2(001) methanol suspensions before and after visible-light irradiation.
reported above and strongly supports the fact that more O2•− radicals are generated by the plasmon-induced electron transfer from Au nanoparticles to the CB of {001} facets in Au/ TiO2(001). Taking into account the high adsorption energy over {001} facets, we also cannot rule out the fact that the better adsorption of 2,4-DCP pollutant over {001} than {101} facets contributes to the higher photocatalytic activity observed over Au/TiO2(001). In summary, in this work, we revealed the effects of TiO2exposed facets on Au adhesion and resulting plasmon-induced photocatalytic performance in a Au/TiO2 system characterized by different percentages of {101} and {001} facets. Anatase TiO2 single crystals with the dominant exposed {101} or {001} facets have been prepared by a hydrothermal process. Crystalline metallic Au nanoparticles with a mean diameter of 10 to 11 nm were uniformly deposited on both exposed {101} and {001} facets of prepared TiO2 single crystals with the aid of power ultrasound. The Au-deposited anatase TiO2 single crystals with the dominant {001} facets have been shown to have much higher photocatalytic activity than those with the dominant {101} facets under visible-light irradiation. The dominant {001} facets of anatase TiO2 play a critical role in the high photocatalytic activity by offering a stronger adherence for Au nanoparticles than {101} facets, which promotes the generation of reactive oxygen species O2•− by the plasmoninduced electron transfer from Au nanoparticles to the CB of the anatase TiO2 substrate. This work, we believe, will be useful for the design of Au/TiO2 heterogeneous catalysts.
■
ASSOCIATED CONTENT
S Supporting Information *
X-ray diffraction patterns of TiO2(101) and TiO2(001) after the removal of surface fluorine by calcination in air for 2 h. This material is available free of charge via the Internet at http:// pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +8610-62514133. Fax: +8610-62516444. E-mail: jlin@ chem.ruc.edu.cn. Notes
The authors declare no competing financial interests. 14605
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
The Journal of Physical Chemistry C
■
Article
(18) Zhao, Y.; Ma, W.; Li, Y.; Ji, H.; Chen, C.; Zhu, H.; Zhao, J. The Surface-Structure Sensitivity of Dioxygen Activation in the AnatasePhotocatalyzed Oxidation Reaction. Angew. Chem., Int. Ed. 2012, 51, 3188−3192. (19) Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: Germany, 1988. (20) Pol, V. G.; Gedanken, A.; Calderon-Moreno, J. Deposition of Gold Nanoparticles on Silica Spheres: A Sonochemical Approach. Chem. Mater. 2003, 15, 1111−1118. (21) Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Gedanken, A. Sonochemical Deposition of Silver Nanoparticles on Silica Spheres. Langmuir 2001, 18, 3352−3357. (22) Pol, V. G.; Grisaru, H.; Gedanken, A. Coating Noble Metal Nanocrystals (Ag, Au, Pd, and Pt) on Polystyrene Spheres via Ultrasound Irradiation. Langmuir 2005, 21, 3635−3640. (23) Zhong, Z.; Mastai, Y.; Koltypin, Y.; Zhao, Y.; Gedanken, A. Sonochemical Coating of Nanosized Nickel on Alumina Submicrospheres and the Interaction between the Nickel and Nickel Oxide with the Substrate. Chem. Mater. 1999, 11, 2350−2359. (24) Jiang, H.-Y.; Cheng, K.; Lin, J. Crystalline Metallic Au Nanoparticle-loaded α-Bi2O3 Microrods for Improved Photocatalysis. J. Phys. Chem. Chem. Phys. 2012, 14, 12114−12121. (25) Li, H.; Bian, Z.; Zhu, J.; Huo, Y.; Li, H.; Lu, Y. Mesoporous Au/ TiO2 Nanocomposites with Enhanced Photocatalytic Activity. J. Am. Chem. Soc. 2007, 129, 4538−4539. (26) Kielbassa, S.; Kinne, M.; Behm, R. J. Thermal Stability of Au Nanoparticles in O2 and Air on Fully Oxidized TiO2(110) Substrates at Elevated Pressures. An AFM/XPS Study of Au/TiO2 Model System. J. Phys. Chem. B 2004, 108, 19184−19190. (27) Mottet, C.; Treglia, G.; Legrand, B. Electronic Structure of Pd Clusters in the Tight-binding Approximation: Influence of Spdhybridization. Surf. Sci. 1996, 352, 675−679. (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.; Chastain, J. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp.: Eden Prairie, MN, 1992; p191. (29) Liu, Y. C.; Juang, J. Electrochemical Methods for the Preparation of Gold-Coated TiO2 Nanoparticles with Variable Coverages. Langmuir 2004, 20, 6951−6955. (30) Wu, Y.; Liu, H.; Zhang, J.; Chen, F. Enhanced Photocatalytic Activity of Nitrogen-Doped Titania by Deposited with Gold. J. Phys. Chem. C 2009, 113, 14689−14695. (31) Lignier, P.; Comotti, M.; Schüth, F.; Rousset, J.-L.; Cap, V. Effect of the Titania Morphology on the Au/TiO2-catalyzed Aerobic Epoxidation of Stilbene. Catal. Today 2009, 141, 355−360. (32) Selloni, A. Crystal Growth: Anatase Shows Its Reactive Side. Nat. Mater. 2008, 7, 613−615. (33) Gong, X.-Q.; Selloni, A. Reactivity of Anatase TiO2 Nanoparticles: The Role of the Minority (001) Surface. J. Phys. Chem. B 2005, 109, 19560−19562. (34) Vittadini, A.; Selloni, Rotzinger, F. P.; Grätzel, M. Structure and Energetics of Water Adsorbed at TiO2 Anatase (101) and (001) Surfaces. Phys. Rev. Lett. 1998, 81, 2954−2957. (35) Liu, G.; Sun, C. H.; Yang, H. G.; Smith, S. C.; Wang, L. Z.; Lu, G. Q.; Cheng, H.-M. Nanosized Anatase TiO2 Single Crystals for Enhanced Photocatalytic Activity. Chem. Commun. 2010, 46, 755−757. (36) Li, X. Z.; Li, F. B. Study of Au/Au3+-TiO2 Photocatalysts toward Visible Photooxidation for Water and Wastewater Treatment. Environ. Sci. Technol. 2001, 35, 2381−2387. (37) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Superamolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. (38) Zanella, R.; Giorgio, S.; Shin, C. H.; Henry, C. R.; Louis, C. Characterization and Reactivity in CO Oxidation of Gold Nanoparticles Supported on TiO2 Prepared by Deposition-precipitation with NaOH and Urea. J. Catal. 2004, 222, 357−367. (39) Dozzi, M. V.; Prati, L.; Canton, P.; Selli, E. Effects of Gold Nanoparticles Deposition on the Photocatalytic Activity of Titanium
ACKNOWLEDGMENTS The research work was supported by the National Natural Science Foundation of China (21273281) and National Basic Research Program of China (973 Program, No. 2013CB632405).
■
REFERENCES
(1) Tauster, S. Strong Metal-Support Interactions. J. Acc. Chem. Rec. 1987, 20, 389−394. (2) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. Low-Temperature Oxidation of CO over Gold Supported on TiO2, α-Fe2O3, and Co3O. J. Catal. 1993, 144, 175−192. (3) Subramannian, 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. (4) Silva, C. G.; Juarez, R.; Marino, T.; Molinari, R.; Garicia, H. Influence of Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water. J. Am. Chem. Soc. 2011, 133, 595−602. (5) Du, L.; Furube, A.; Yamamoto, K.; Hara, K.; Katoh, R.; Tachiya, M. Plasmon-Induced Charge Separation and Recombination Dynamics in Gold−TiO2 Nanoparticle Systems: Dependence on TiO2 Particle Size. J. Phys. Chem. C 2009, 113, 6454−6462. (6) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H.-M.; Lu, G. Q. Anatase TiO2 Single Crystals with A Large Percentage of Reactive Facets. Nature 2008, 453, 638−642. (7) Pan, J.; Liu, G.; Lu, G. Q.; Cheng, H.-M. On the True Photoreactivity Order of {001}, {010}, and {101} Facets of Anatase TiO2 Crystals. Angew. Chem., Int. Ed. 2011, 50, 2133−2137. (8) Liu, G.; Yu, J. C.; Lu, G. Q.; Cheng, H.-M. Crystal Facet Engineering of Semiconductor Photocatalysts: Motivations, Advances and Unique Properties. Chem. Commun. 2011, 47, 6763−6783. (9) Liu, S. W.; Yu, J. G.; Jaroniec, M. Tunable Photocatalytic Selectivity of Hollow TiO2 Microspheres Composed of Anatase Polyhedra with Exposed {001} Facets. J. Am. Chem. Soc. 2010, 132, 11914−11916. (10) Liu, G.; Yang, H. G.; Wang, X.; Cheng, L.; Lu, H.; Wang, L.; Lu, G. Q.; Cheng, H.-M. Enhanced Photoactivity of Oxygen-Deficient Anatase TiO2 Sheets with Dominant {001} Facets. J. Phys. Chem. C 2009, 113, 21784−21788. (11) Read, C. G.; Steinmiller, E. M. P.; Choi, K.-S. Atomic PlaneSelective Deposition of Gold Nanoparticles on Metal Oxide Crystals Exploiting Preferential Adsorption of Additives. J. Am. Chem. Soc. 2009, 131, 12040−12041. (12) Ohno, T.; Sarukawa, K.; Matsumura, M. Crystal Faces of Rutile and Anatase TiO2 Particles and Their Roles in Photocatalytic Reactions. New J. Chem. 2002, 26, 1167−1170. (13) Liu, C.; Han, X.; Xie, S.; Kuang, Q.; Wang, X.; Jin, M.; Xie, Z.; Zheng, L. Enhancing the Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet-Induced Spontaneous Separation of Photogenerated Electrons and Holes. Chem. Asian J. 2013, 8, 282− 289. (14) Sun, C.; Smith, S. C. Strong Interaction between Gold and Anatase TiO2(001) Predicted by First Principle Studies. J. Phys. Chem. C 2012, 116, 3524−3531. (15) Gong, X. Q.; Selloni, A.; Dulub, O.; Jacobson, P.; Diebold, U. Small Au and Pt Clusters at the Anatase TiO2(101) Surface: Behavior at Terraces, Steps, and Surface Oxygen Vacancies. J. Am. Chem. Soc. 2008, 130, 370−381. (16) Wood, A.; Giersig, M.; Mulvaney, P. Fermi Level Equilibration in Quantum Dot−Metal Nanojunctions. J. Phys. Chem. B 2001, 105, 8810−8815. (17) Zhang, D.; Li, G.; Wang, H.; Chan, K. M.; Yu, J. C. Biocompatible Anatase Single-Crystal Photocatalysts with Tunable Percentage of Reactive Facets. Cryst. Growth Des. 2010, 10, 1130− 1137. 14606
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
The Journal of Physical Chemistry C
Article
Dioxide under Visible Light. Phys. Chem. Chem. Phys. 2009, 11, 7171− 7180. (40) Yogi, C.; Kojima, K.; Hashishin, T.; Wada, N.; Inada, Y.; Gaspera, E. D.; Bersani, M.; Martucci, A.; Liu, L.; Sham, T.-K. Size Effect of Au Nanoparticles on TiO2 Crystalline Phase of Nanocomposite Thin Films and Their Photocatalytic Properties. J. Phys. Chem. C 2011, 115, 6554−6560. (41) Kochuveedu, S. T.; Kim, D.-P.; Kim, D. H. Surface-PlasmonInduced Visible Light Photocatalytic Activity of TiO2 Nanospheres Decorated by Au Nanoparticles with Controlled Configuration. J. Phys. Chem. C 2012, 116, 2500−2506. (42) Maitani, M. M.; Tanaka, K.; Mochizuki, D.; Wada, Y. Enhancement of Photoexcited Charge Transfer by {001} FacetDominating TiO2 Nanoparticles. J. Phys. Chem. Lett. 2011, 2, 2655− 2659.
14607
dx.doi.org/10.1021/jp403489r | J. Phys. Chem. C 2013, 117, 14600−14607
