Self-Assembled Gold Nanoparticle–Mixed Metal Oxide

Article pubs.acs.org/Langmuir

Self-Assembled Gold Nanoparticle−Mixed Metal Oxide Nanocomposites for Self-Sensitized Dye Degradation under Visible Light Irradiation Seungho Cho,†,§ Ji-Wook Jang,‡,§ Sekyu Hwang,† Jae Sung Lee,‡ and Sungjee Kim*,† †

Department of Chemistry, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, Korea 790-784 ‡ Department of Chemical Engineering, POSTECH, San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, Korea 790-784 S Supporting Information *

ABSTRACT: Gold nanoparticle (Au NP)−mixed metal oxide (MMO) nanocomposite photocatalysts for efficient selfsensitized dye degradations under visible light were prepared by an electrostatically driven self-assembly. Dihydrolipoic acid (DHLA)-capped Au NPs (building block I) were synthesized through a room temperature reaction. Their hydrodynamic size was determined as being around 4.9 nm by dynamic light scattering measurements. MMO nanoplates with lateral dimensions of 100−250 nm (building block II) were prepared by a calcination of zinc aluminum layered double hydroxides at 750 °C for 2 h in air. In a pH 7.0 aqueous solution, the DHLAcapped Au NPs had a negative zeta potential (−22 ± 3 mV); on the other hand, the MMO nanoplates had a positive zeta potential (15 ± 2 mV). Electrostatic self-assembly was achieved by stirring an aqueous solution (pH 7.0) containing DHLAcapped Au NPs and MMO nanoplates at room temperature for 1 h. The self-assembled and sequentially calcined nanocomposites exhibited the superior self-sensitized dye degradation efficiency under visible light to that of ZnO, TiO2 (P25), or pure MMO nanoplates. The enhanced degradation efficiency could be attributed to strong coupling interactions of ZnO and ZnAl2O4 phases of the MMO and the role of Au as an electron sink and mediator for formations of reactive oxidation species and as a light concentrator. photocatalysts has been widely researched.15−18 Essentially, decomposition of pollutants by using solar energy, an inexhaustible resource, is an environmentally promising approach. Visible light accounts for 44−47% of the solar energy spectrum, whereas UV light accounts for only 3−5%. Efficient use of visible light, therefore, is a prerequisite for the efficient use of solar energy for water purification process. Metal oxide nanostructures are promising materials as building blocks of hybrid nanostructures for photocatalysis, photoelectrochemical processes, and photovoltaics using solar energy. They are highly stable, their small size is comparable to their carrier scattering length, they display strong absorption coefficients due to an increased oscillator strength, and their synthetic routes are well-established.19 Mixed metal oxides (MMOs) synthesized by calcination of layered double hydroxides (LDHs), a class of ionic lamellar solids,20 are composed of two metal oxide phases homogeneously dispersed. Adequate combinations of two metal oxide phases can render MMOs unique and useful optical and electronic properties for

1. INTRODUCTION Self-assembly is the autonomous organization of components into patterns or structures without human intervention.1 It is a delightful concept, essentially one in which the complex work of engineering is performed by nature’s own intent.2 Nanostructures as building blocks can be self-assembled using natural forces such as electrostatics, van der Waals interactions, and entropy.3−7 In particular, self-assembled hybrid nanostructures that comprise two or more different components are at the forefront of research on nanomaterials due to their spontaneity and the possibility of combination and integration of material properties together. These can promise a wide range of applications including photocatalysts, which may not be easily attainable by single-component nanostructures.8−11 Organic dyes are one of the largest groups of pollutants discharged into wastewater from textile and other industrial processes.12 In particular, azo dyes are considered a threat to the surrounding ecosystem due to their nonbiodegradability, toxicity, and potential carcinogenicity.13,14 Environmental concerns and the need to meet stringent international standards for wastewater safety have encouraged the development of efficient processes for purifying aqueous effluents.13 Photoinduced degradations or detoxifications of the pollutants over © 2012 American Chemical Society

Received: October 30, 2012 Revised: November 27, 2012 Published: November 28, 2012 17530

dx.doi.org/10.1021/la304303h | Langmuir 2012, 28, 17530−17536

Langmuir

Article

water by sonication for 10 min. 3 μM gold nanoparticle solution (1 mL, pH 7.0) was added to the MMO powder-dispersed solution and stirred for 1 h at room temperature. The powders were gathered with a centrifuge (4000 rpm, 3 min). The powders were washed several times with DI water and then dried in an oven at 60 °C for 12 h. The dried powders were placed in the furnace. The furnace temperature was increased to 500 °C (the ramping rate: 10 °C/min) and maintained at 500 °C for 2 h in air. Then the powders were cooled naturally. Characterization. The morphology, crystallinity, crystalline nature, chemical composition, and optical properties of the synthesized materials were determined using field-emission scanning electron microscopy (FESEM, JEOL JMS-7401F, operated at 10 keV), high-resolution scanning transmission electron microscopy (Cscorrected HR-STEM, JEOL JEM-2200FS with an energy-dispersive X-ray spectrometer operating at 200 kV, National Center of Nanomaterials Technology (NCNT)), X-ray diffraction (XRD, Mac Science, M18XHF by scanning the 2θ range between 5° and 80° using Cu Kα (λ = 0.154 06 nm) radiation), and UV−vis diffuse reflectance spectroscopy (Shimadzu, UV2501PC). Brunauer−Emmett−Teller (BET) nitrogen adsorption−desorption was measured using a Micromeritics analyzer (ASAP 2020 V3.01 H analyzer). The particle hydrodynamic size and zeta potentials were measured using a Malvern zetasizer Z and S. Photocatalytic Activity Measurements. Orange II (4-(2hydroxy-1-naphthylazo)benzenesulfonic acid, Aldrich) was used as a model azo dye. 50 mg of powders was transferred to 100 mL of a 50 μM Orange II aqueous solution. The photocatalytic reactions in the solution were carried out at room temperature in a closed system using a mercury lamp (1 W cm−2, Model 66905, Newport Co.) with a cutoff filter (λ ≥ 420 nm) placed in an inner irradiation-type 100 mL Pyrex reaction cell. Prior to visible light exposure, the suspension was aged in the dark to equilibrate the adsorption and desorption of dye molecules. With stirring, the suspensions were placed under visible light. The quantity of Orange II in solution was determined by measuring the UV−vis absorption intensity at 486 nm, the main absorption peak of the dye.

efficient photocatalysis. Meanwhile, the photocatalytic activities were found to be significantly enhanced when noble metal nanoparticles such as Au, Ag, and Pt were deposited on metal oxides.21−23 In particular, Au−metal oxide hybrid nanostructures have become an active frontier because of their remarkable optical, electrical, and catalytic properties.24−30 However, conventional methods, such as electrochemical deposition,31 hydrothermal,32 and precipitation−deposition,33 for the synthesis of Au−metal oxide hybrid nanostructures, generally lead to the formation of gold with uncontrollable size, morphology, and distribution on the metal oxide. In contrast, the use of presynthesized Au nanoparticles (NPs) capped with ligands for the preparation of Au−semiconductor hybrid nanostructures offers a number of advantages.34 For example, Au NP size can be independently controlled; particularly, a very narrow size distribution can be obtained via well-established synthetic methods. As a result, various Au nanostructures with controlled morphologies can be taken advantage of as the building blocks. In this paper, we report a method for synthesizing selfassembled photocatalysts which can be used for efficient selfsensitized dye degradations based on the excitation of dyes under visible light irradiation. Two kinds of building blocks were synthesized separately. Dihydrolipoic acid (DHLA)capped Au NPs (building block I) were synthesized through a room temperature reaction. MMO nanoplates (building block II) were prepared by a calcination of zinc aluminum LDHs. In a pH 7.0 aqueous solution, the DHLA-capped Au NPs had a negative zeta potential; on the other hand, the MMO nanoplates had a positive zeta potential. Electrostatic selfassembly was achieved by simple stirring an aqueous solution containing building blocks (pH 7.0). The self-assembled nanocomposites exhibited enhanced self-sensitized dye degradation efficiency. The degradation mechanism and the reason for their superior efficiency will be discussed.

3. RESULTS AND DISCUSSION Figure 1A shows a TEM image of the synthesized DHLAcapped Au NPs. They had pseudospherical shapes, and their average diameter is 3.6 nm. Figure 1B is a histogram of hydrodynamic diameters measured by dynamic light scattering. The nanoparticles had an average hydrodynamic diameter of 4.7 nm. Figure 1C is an SEM image of MMO powders synthesized by calcinations of zinc aluminum LDH nanoplates as a single-source precursor (Figure S1 in the Supporting Information) at 750 °C for 2 h in air. The lateral dimensions of the nanoplates were 100−250 nm, similar to those of zinc aluminum LDHs as the precursor. In general, calcination of LDH nanostructures has been reported to be an alternative to the traditional chemical and physical methods for the fabrication of a wide variety of MMO nanomaterials composed of two metal oxide phases, homogeneously dispersed.20,35,36 A TEM analysis revealed their hexagonal shapes at large (Figure 1D) and ZnAl2O4 crystal grains were homogeneously dispersed inside a network of ZnO crystal grains (see Figure S2). We preformed zeta potential measurements separately on the DHLA-capped Au NPs and MMO nanoplates to investigate whether charges are present on the particles in the aqueous solution (pH 7.0). The results (Figure 1E) clearly demonstrated the negatively charged state of the DHLA-capped Au NPs (−22 ± 3 mV). The MMO nanoplates exhibited a positive zeta potential (15 ± 2 mV). Exploiting the opposite surface charges possessed by the Au NPs and MMO nanoplates, the Au NP−MMO nanocomposites could be self-assembled via electrostatic interactions.

2. EXPERIMENTAL DETAILS Preparation of Dihydrolipoic Acid (DHLA)-Capped Au NP Solution. All chemicals used in this study were analytical grade and were used without further purification. 20 μmol of hydrogen tetrachloroaurate hydrate (HAuCl4, 99.999%, Aldrich) was dissolved in 10 mL of deionized (DI) water with stirring at room temperature, and the pH was adjusted to 7.2 using 2 M NaOH aqueous solution. 1 μmol of (±)-α-lipoic acid (C8H14O2S2, 99%, Sigma-Aldrich) and 40 μmol of sodium borohydride (NaBH4, 99%, Sigma-Aldrich) were added to the solution, and the solution was stirred at room temperature for 3 h. For preparing an additional aqueous DHLA solution, 4 μmol of (±)-α-lipoic acid and 8 μmol of sodium borohydride were dissolved in 0.8 mL of DI water at room temperature for 30 min. The aqueous DHLA solution was added to the reaction solution. The reaction solution was stirred at room temperature for 3 h. After the reaction, the solution was dialyzed three times using Amicon ultra 30 kDa Mw cutoff centrifugal filters for purification. Preparation of MMO Nanoplates. An aqueous solution containing 0.01 M zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99%, Samchun), 0.0033 M aluminum chloride (AlCl3, 99.99%, Aldrich), and 0.35 M ammonia (NH3, Samchun) was prepared and maintained with stirring at room temperature for 24 h and then filtered through a polycarbonate membrane filter (ISOPORE). The filtered powders were washed several times with DI water then dried in an oven at 60 °C for 12 h. The dried powders (see the Supporting Information, Figure S1) were placed in an annealing furnace and maintained at 750 °C for 2 h in air. Preparation of Au NP−MMO Nanocomposites by SelfAssembly. 0.3 g of MMO powders was dispersed in 25 mL of DI 17531

dx.doi.org/10.1021/la304303h | Langmuir 2012, 28, 17530−17536

Langmuir

Article

indexed. All LDH peaks (Figure S1B) disappeared on the XRD pattern, which implies the complete conversion of LDHs into MMOs. No peaks corresponding to any other phases or impurities were detected on the XRD and TEM analyses, indicating that the powders were pure MMO crystals. The selfassembly process involves the solution phase mixing of the building blocks. DHLA-capped Au NPs- and MMO nanoplatescontaining aqueous solution were stirred for 1 h. The resulting products were calcined at 500 °C in air for 2 h in order to remove the DHLAs on Au nanocrystals for better contacts between Au NPs and MMO matrices. The adsorbed organic molecules on building blocks are detrimental to electronic and photonic applications because they result in indirect contact between the different phases and increase the barrier for electron transport.37 The XRD pattern of the self-assembled and sequentially calcined nanocomposites (Figure 2B) shows the Au (111) peak (JCPDS No. 04-0784) in addition to ZnO and ZnAl2O4 peaks. Figure 3A shows a TEM image of a portion of a nanocomposite. Nanoparticles were well-distributed on MMO

Figure 1. Characterizations of dihydrolipoic acid (DHLA)-capped Au nanoparticles (NPs) and zinc aluminum mixed metal oxides (MMOs) as building blocks: (A) TEM image of DHLA-capped Au NPs. (B) Histogram of hydrodynamic diameters of DHLA-capped Au NPs measured by dynamic light scattering measurements. (C) SEM image and (D) TEM image of MMO nanoplates. (E) Zeta potential measurements performed separately on the aqueous DHLA-capped Au NP and MMO solutions (pH 7.0).

Figure 2A shows the XRD pattern of the synthesized MMO nanoplates. The wurtzite ZnO phase (JCPDS No. 35-1451) and the cubic ZnAl2O4 phase (JCPDS No. 05-0669) can be

Figure 3. (A, C, D) TEM images and (B) high-angle annular dark-field (HAADF) image of Au NP−MMO nanocomposites. (E, F) EDX patterns of the areas marked by the circle and the triangle in (A), respectively.

plates. High-angle annular dark-field (HAADF) imaging in scanning transmission electron microscope (STEM) mode was utilized (Figure 3B). The method is highly sensitive to the atomic number (Z) of the materials, scaling proportionally to ∼Z2.38,39 Thus, HAADF imaging is also referred to as Z contrast imaging. Zn, Al, O, and Au have Z = 30, 13, 8, and 79, respectively. Bright nanoparticles shown in Figure 3B imply that they contained high atomic number elements. A higher magnification image (Figure 3C) reveals that spherical

Figure 2. XRD patterns of the powders: (A) MMOs; (B) Au NP− MMO nanocomposites. 17532

dx.doi.org/10.1021/la304303h | Langmuir 2012, 28, 17530−17536

Langmuir

Article

UV light, the first mechanism is ruled out, and the second mechanism is dominant in the case of the dye degradations over the wide band gap semiconductors under visible light irradiation. To investigate visible light photodegradation activities, Orange II was chosen as a model compound. Orange II is a nonbiodegradable azo dye and is widely used in the textile industry.14a,46 The concentration of Orange II in the solutions under exposure to visible light (λ ≥ 420 nm) was monitored (Figure 5). The Orange II concentration was almost constant

nanoparticles were attached on the crystalline matrix without organic layers. Figure 3D shows that the matrix was highly crystalline with a lattice spacing of about 0.28 nm, which corresponds to the distance between the {100} planes in a wurtzite ZnO crystal lattice. A lattice spacing of 0.24 nm on a single crystalline nanoparticle indicated the distance between (111) planes of a cubic Au phase. The composition of the synthesized materials was investigated by energy dispersive Xray spectroscopy (EDX). The EDX pattern (Figure 3E) of the area without a nanoparticle marked by the circle in Figure 3A shows only Zn, Al, and O signals; the Cu signals are attributed to the copper mesh used for TEM imaging. In contrast, the area with nanoparticles marked by the triangle in Figure 3A has the Au signal in addition to Zn, Al, and O signals (Figure 3F). No evidence of other impurities was found. The synthesized nanostructures were therefore found to be Au NP−MMO nanocomposites. The diffused reflectance spectra and a photograph of the pure MMO nanoplate powders (i) and the Au NP−MMO nanocomposite powders (ii) are presented in Figure 4. The

Figure 5. Normalized concentrations of the 100 mL Orange II solution without a catalyst, with 50.0 mg of ZnOs, with 50.0 mg of P25, with 50.0 mg of Au NP−ZnO nanocomposites, with 50.0 mg of MMO nanoplates, and with 50.0 mg of Au NP−MMO nanocomposites as a function of the visible light irradiation time.

during visible light irradiation for the solution without any catalyst, which confirmed the photostability of the dye. In each experiment with a catalyst, the suspension was aged in the dark to equilibrate the adsorption and desorption of dye molecules before exposure to visible light. A comparison was made with pure ZnO and TiO2 (P25, Degussa) nanoparticles and AuNP− ZnO nanocomposites. The pure ZnOs were synthesized from the reaction of an aqueous solution containing 0.03 M zinc acetate dihydrate and 0.1 M sodium peroxide at room temperature for 5 h. The AuNP−ZnO nanocomposites were synthesized by the same procedures for synthesis of Au NP− MMO structures except building block II, the as-prepared ZnOs instead of MMO plates. The specific surface areas of powder samples were measured using the micromeritics analyzer and the BET equation. The specific areas of ZnO, TiO2, Au NP−ZnO structures, MMO, and Au NP−MMO structures were 24.77, 47.22, 25.73, 22.89, and 23.65 m2/g. The Orange II concentrations decreased to 0.739C0 and 0.832C0 after visible light irradiation for 2 h in the presence of the ZnO and P25, respectively. The visible light photocatalytic degradation of the dye was better for the MMO nanoplates than for the ZnO and P25. The concentration of Orange II dye molecules decreased for the MMO nanoplates to 0.364Ci after 2 h visible light irradiation. With Au NP−ZnO nanocomposites, the concentration of Orange II dye was reduced to 0.683Ci under visible light irradiation for 2 h. In the case of the Au NP−MMO nanocomposites used as photocatalysts, Orange II dyes almost disappeared (∼10−6Ci) after 2 h visible light irradiation. The durability of photocatalytic activity was also studied by reuse of Au NP−MMO nanocomposites in fresh Orange II solution under visible light irradiation. Figure S3 shows the photocatalytic results for five cycles using the Au

Figure 4. Diffused reflectance spectra and digital camera image of MMO nanoplate powders and Au NP−MMO nanocomposite powders.

powders of MMO nanoplates and Au NP−MMO nanocomposites were white and purple in color, respectively. The pure MMO nanoplates exhibited a strong UV adsorption band characteristic of the wide band gap ZnO and ZnAl2O4. The small tail extended to 450 nm on the absorption spectrum may be attributed to defects of MMOs. In the case of Au NP− MMO nanocomposites, the surface plasma resonance peak of Au was also observed in the visible light region. The degradation of dyes over semiconductors as photocatalysts can be conducted following two different possible mechanisms.40−42 The first is based on the excitation of the semiconductor under light irradiation to form various reactive oxidation species (ROS), such as hydroxyl radical, hydroperoxyl radical, hydrogen peroxide, and superoxide.43 Dyes were oxidized by these ROS.44,45 The other mechanism is that the dye rather than a semiconductor is excited under light irradiation, followed by electron transfer from the excited dye (dye*) to the conduction band of a semiconductor.46 Then, the electron is trapped by surface adsorbed O2 to generate various ROSs. The dye•+ subsequently self-degrades or is degraded by ROSs.40,44 Dye degradations on a semiconductor under light irradiation may occur based on one or both of these two mechanisms. Because wide band gap semiconductors, widely used as photocatalysts, such as TiO2 and ZnO, can absorb only 17533

dx.doi.org/10.1021/la304303h | Langmuir 2012, 28, 17530−17536

Langmuir

Article

photocatalytic activity of the Au NP−ZnO nanocomposites was comparable to that of the Au NP−MMO nanocomposites. We cannot rule out the possibility of a role of Au NPs as light concentrators. Au nanostructures as plasmonic materials have shown promise in manipulating and concentrating light in photocatalytic and photovoltaic systems, which can lead to enhanced photon absorption in the surrounding materials.58−60 The Au NPs can also act as light concentrators which lead to enhanced photon absorption in dyes near the Au NPs. Enhanced photon absorption of dyes can expedite selfsensitized dye degradations.

NP−MMO nanocomposites (120 min irradiation for each cycle). There are no significant reductions in the photocatalytic activities even after five cycles. Hence, these results imply that the Au NP−MMO nanocomposites are exhibiting good stability and recyclability. The degradation efficiency greatly depends on the electron transfer between the dye* and the catalysts as well as the electron recombination between dye•+ and catalyst−.40,42,47 The rate of the electron injection is determined by the redox potentials of the adsorbed dye* and the catalysts.40,42,48,49 Figure 6 illustrates the energy levels of the nanocomposite

4. CONCLUSIONS In this paper, we have shown that Au NP−MMO nanocomposites could be prepared by using the electrostatic selfassembly of building blocks. The self-assembly process involves the simple solution phase mixing of the two building blocks (Au NPs with the negative zeta potential and MMOs with the positive zeta potential). The Au NP−MMO nanocomposites exhibited significantly enhanced self-sensitized dye degradation efficiency under visible light compared to pure ZnO, P25, or pure MMO nanostructures. Such a high efficiency could be attributed to good contact and appropriate energy band alignment of ZnO and ZnAl2O4 phases of the MMOs, which enabled the strong coupling interaction. The Au NPs acted as electron sinks, which led to spatial separations of dye•+ and electrons, and also as efficient electron mediators which facilitate the formation of various ROS. The Au NPs can also act as light concentrators which lead to enhanced photon absorption in dyes. These results warrant efforts to extend this strategy to synthesize other metal−MMO hybrid nanostructures, allowing a myriad of combinations of different metal nanostructures and divalent and trivalent cations from various LDH nanostructures, which can open a window for a wide range of applications by the uniquely tailored properties.

Figure 6. Schematic illustration of the energy levels of the nanocomposite components and Orange II dye and electron transfers.

components and an Orange II dye and electron transfers between them. In the case of the ZnO, P25, or pure MMO, the dye rather than the ZnO, P25, or the MMO is excited by visible light as discussed above, followed by the electron transfer from the dye* to the conduction band of the semiconductors because the dye* have a higher energy level than that of the conduction band edge of the semiconductors. Such transferred electrons on semiconductor generate ROS, which degrade dyes. In the case of the MMO which consisted of ZnO and ZnAl2O4 phases, the conduction band edge of ZnO is located between the conduction band edge and the valence band edge of ZnAl2O4 (type II energy band alignment). This type II energy band alignment of ZnO and ZnAl2O4 induced formation of a diffusion potential. This potential acted as an electromotive force to favor electron injection.50,51 If dyes were adsorbed on the ZnAl2O4 phase, conduction band electrons transferred from dye* could be transferred to the conduction band of ZnO by electron injection, which facilitated spatial separation of the electron and dye•+ before recombination. In particular, MMO nanostructures derived from LDHs have an advantage of good contact between two phases (e.g., metal oxide and spinel phases). The good photocatalytic performance of the MMO may be attributed to stronger coupling interactions at the interface in the network of ZnO with homogeneously dispersed ZnAl2O4 phases, which was derived from structural features of LDHs.36 In the case of the Au NP−MMO nanocomposites, since the energy level of the conduction band edge of ZnO or ZnAl2O4 is higher than the new Fermi energy level of the semiconductor−Au heterostructures after Fermi-level equilibration, the photoelectrons could transfer from the semiconductors to Au driven by the energy difference.33,52−54 Thus, Au NPs act as an electron sink, which leads to spatial separations of dye•+ and electrons, thus retarding the recombination process. Moreover, Au can act as an efficient electron mediator,42,55−57 and the surface adsorbed O2 can easily trap the electrons from the Au NPs and form various ROS. Therefore, these roles of Au NPs were closely related to the interaction with MMOs. If the enhancement of the photocatalytic activity resulted solely from the Au NPs, the

■

ASSOCIATED CONTENT

S Supporting Information *

SEM image and XRD pattern of the zinc aluminum double hydroxide nanostructures used as precursors for the synthesis of mixed metal oxide nanoplates. (Figure S1); HR-TEM image of a portion of a mixed metal oxide structure (Figure S2); the decomposition ratios of Orange II dye after cyclic photocatalytic reaction under visible light irradiation by reuse of Au NP−MMO nanocomposites (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by KOSEF grant funded by MOST (20120006280), the Priority Research Center Program through NRF (2011-0031405 and 20110027727), (20120005973), and the Basic Science Research Programs (20110027236). 17534

dx.doi.org/10.1021/la304303h | Langmuir 2012, 28, 17530−17536

Langmuir

■

Article

(d) Zhao, F.; Li, X.; Zheng, J.-G.; Yang, X.; Zhao, F.; Wong, K. S.; Wang, J.; Lin, W.; Wu, M.; Su, Q. ZnO Pine-Nanotree Arrays Grown from Facile Metal Chemical Corrosion and Oxidation. Chem. Mater. 2008, 20, 1197. (e) Cho, S.; Jang, J.-W.; Jung, A.; Lee, S.-H.; Lee, J.; Lee, J. S.; Lee, K.-H. Formation of Amorphous Zinc Citrate Spheres and Their Conversion to Crystalline ZnO Nanostructures. Langmuir 2011, 27, 371. (20) Cavani, F.; Trifirò, F.; Vaccari, A. Hydrotalcite-Type Anionic Clays: Preparation, Properties and Applications. Catal. Today 1991, 11, 173. (21) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Formation of Asymmetric One-Sided Metal-Tipped Semiconductor Nanocrystal Dots and Rods. Nat. Mater. 2005, 4, 855. (22) Zeng, H.; Cai, W.; Liu, P.; Xu, X.; Zhou, H.; Klingshirn, C.; Kalt, H. ZnO-Based Hollow Nanoparticles by Selective Etching: Elimination and Reconstruction of Metal-Semiconductor Interface, Improvement of Blue Emission and Photocatalysis. ACS Nano 2008, 2, 1661. (23) Gu, C.; Chen, C.; Huang, H.; Wong, T.; Wang, N.; Zhang, T. Y. Growth and Photocatalytic Activity of Dendrite-like ZnO@Ag Heterostructure Nanocrystals. Cryst. Growth Des. 2009, 9, 3278. (24) Zhang, J. T.; Tang, Y.; Lee, K.; Ouyang, M. Science 2010, 327, 1634. (25) Menagen, G.; Macdonald, J. E.; Shemesh, Y.; Popov, I.; Banin, U. Au Growth on Semiconductor Nanorods: Photoinduced versus Thermal Growth Mechanisms. J. Am. Chem. Soc. 2009, 131, 17406. (26) Lee, J. S.; Shevchenko, E. V.; Talapin, D. V. Au−PbS Core− Shell Nanocrystals: Plasmonic Absorption Enhancement and Electrical Doping via Intra-Particle Charge Transfer. J. Am. Chem. Soc. 2008, 130, 9673. (27) Herring, N. P.; AbouZeid, K.; Mohamed, M. B.; Pinsk, J.; ElShall, M. S. Formation Mechanisms of Gold−Zinc Oxide Hexagonal Nanopyramids by Heterogeneous Nucleation using Microwave Synthesis. Langmuir 2011, 27, 15146. (28) Kuo, C. H.; Hua, T.; Huang, M. H. Au Nanocrystal-Directed Growth of Au−Cu2O Core−Shell Heterostructures with Precise Morphological Control. J. Am. Chem. Soc. 2009, 131, 17871. (29) Zeng, J.; Huang, J. L.; Liu, C.; Wu, C. H.; Lin, Y.; Wang, X. P.; Zhang, S. Y.; Hou, J. G.; Xia, Y. N. Gold-Based Hybrid Nanocrystals Through Heterogeneous Nucleation and Growth. Adv. Mater. 2010, 22, 1936. (30) Li, P.; Wei, Z.; Wu, T.; Peng, Q.; Li, Y. Au−ZnO Hybrid Nanopyramids and Their Photocatalytic Properties. J. Am. Chem. Soc. 2011, 133, 5660. (31) Ahmad, M.; Yingying, S.; Nisar, A.; Sun, H.; Shen, W.; Wei, M.; Zhu, J. Synthesis of Hierarchical Flower-Like ZnO Nanostructures and Their Functionalization by Au Nanoparticles for Improved Photocatalytic and High Performance Li-Ion Battery Anodes. J. Mater. Chem. 2011, 21, 7723. (32) Wang, Q.; Geng, B. Y.; Wang, S. Z. ZnO/Au Hybrid Nanoarchitectures: Wet-Chemical Synthesis and Structurally Enhanced Photocatalytic Performance. Environ. Sci. Technol. 2009, 43, 8968. (33) Souza, K. R.; Lima, A. F. F.; Sousa, F. F.; Appel, L. G. Preparing Au/ZnO by Precipitation−Deposition Technique. Appl. Catal., A 2008, 340, 133. (34) Xiao, F.; Wang, F.; Fu, X.; Zheng, Y. A Green and Facile SelfAssembly Preparation of Gold Nanoparticles/ZnO Nanocomposite for Photocatalytic and Photoelectrochemical Applications. J. Mater. Chem. 2012, 22, 2868. (35) Evans, D. G.; Slade, R. C. T. Structural Aspects of Layered Double Hydroxides. Struct. Bonding (Berlin) 2006, 119, 1. (36) Zhao, X.; Wang, L.; Xu, X.; Lei, X.; Xu, S.; Zhang, F. Fabrication and Photocatalytic Properties of Novel ZnO/ZnAl2O4 Nanocomposite with ZnAl2O4 Dispersed Inside ZnO Network. AIChE J. 2012, 58, 573. (37) Chen, P.; Gu, L.; Cao, X. From Single ZnO Multipods to Heterostructured ZnO/ZnS, ZnO/ZnSe, ZnO/Bi2S3 and ZnO/Cu2S Multipods: Controlled Synthesis and Tunable Optical and Photoelectrochemical Properties. CrystEngComm 2010, 12, 3950.

REFERENCES

(1) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295, 2418. (2) Chen, Z.; O’Brien, S. Structure Direction of II−VI Semiconductor Quantum Dot Binary Nanoparticle Superlattices by Tuning Radius Ratio. ACS Nano 2008, 2, 1219. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices. Science 1995, 270, 1335. (4) Rogach, A. L.; Talapin, D. V.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. Organization of Matter on Different Size Scales: Monodisperse Nanocrystals and Their Superstructures. Adv. Funct. Mater. 2002, 12, 653. (5) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale Forces and Their Uses in Self-Assembly. Small 2009, 5, 1600. (6) Talapin, D. V. LEGO Materials. ACS Nano 2008, 2, 1097. (7) Sun, H.; Wei, H.; Zhang, H.; Ning, Y.; Tang, Y.; Zhai, F.; Yang, B. Self-Assembly of CdTe Nanoparticles into Dendrite Structure: A Microsensor to Hg2. Langmuir 2011, 27, 1136. (8) Lee, J.-H.; Jun, Y.-W.; Yeon, S.-L.; Shin, J.-S.; Cheon, J. DualMode Nanoparticle Probes for High-Performance Magnetic Resonance and Fluorescence Imaging of Neuroblastoma. Angew. Chem., Int. Ed. 2006, 45, 8160. (9) Pellegrino, T.; Kudera, S.; Liedl, T.; Javier, A. M.; Manna, L.; Parak, W. J. On the Development of Colloidal Nanoparticles towards Multifunctional Structures and their Possible Use for Biological Applications. Small 2005, 1, 48. (10) Huh, Y. M.; Jun, Y. M.; Song, H. T.; Kim, S.; Choi, J. S.; Lee, J. H.; Kim, K. S.; Shin, J. S.; Suh, J. S.; Cheon, J. In Vivo Magnetic Resonance Detection of Cancer by Using Multifunctional Magnetic Nanocrystals. J. Am. Chem. Soc. 2005, 127, 12387. (11) Zhang, W.-Q.; Lu, Y.; Zhang, T.-K.; Xu, W.; Zhang, M.; Yu, S.H. Controlled Synthesis and Biocompatibility of Water-Soluble ZnO Nanorods/Au Nanocomposites with Tunable UV and Visible Emission Intensity. J. Phys. Chem. C 2008, 112, 19872. (12) Lam, S.; Sin, J.; Abdullah, A. Z.; Mohamed, A. R. Degradation of Wastewaters Containing Organic Dyes Photocatalysed by Zinc Oxide: A Review. Desalin. Water Treat. 2012, 41, 131. (13) Tada, H.; Kubo, M.; Inubushic, Y.; Itob, S. NN Bond Cleavage of Azobenzene through Pt/TiO2 Photocatalytic Reduction. Chem. Commun. 2000, 977. (14) (a) Jung, S.; Yong, K. Fabrication of CuO-ZnO Nanowires on a Flexible Mesh for Highly Efficient Photocatalytic Applications. Chem. Commun. 2011, 47, 2643. (b) Niu, P.; Hao, J. Fabrication of Titanium Dioxide and Tungstophosphate Nanocomposite Films and Their Photocatalytic Degradation for Methyl Orange. Langmuir 2011, 27, 13590. (15) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269. (16) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium Dioxide Photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1. (17) Cho, S.; Jang, J.-W.; Lee, J. S.; Lee, K.-H. Exposed Crystal Face Controlled Synthesis of 3D ZnO Superstructures. Langmuir 2010, 26, 14255. (18) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. Highly Ordered TiO2 Nanotube Arrays with Controllable Length for Photoelectrocatalytic Degradation of Phenol. J. Phys. Chem. C 2008, 112, 253. (19) (a) Cho, S.; Jang, J.-W.; Lim, S.-H.; Kang, H. J.; Rhee, S.-W.; Lee, J. S.; Lee, K.-H. Solution-Based Fabrication of ZnO/ZnSe Heterostructure Nanowire Arrays for Solar Energy Conversion. J. Mater. Chem. 2011, 21, 17816. (b) Zhao, F.; Zheng, J.-G.; Yang, X.; Li, X.; Wang, J.; Zhao, F.; Wong, K. S.; Liang, C.; Wu, M. Complex ZnO Nanotree Arrays with Tunable Top, Stem and Branch Structures. Nanoscale 2010, 2, 1674. (c) Niu, K. Y.; Yang, J.; Kulinich, S. A.; Sun, J.; Du, X. W. Hollow Nanoparticles of Metal Oxides and Sulfides: Fast Preparation via Laser Ablation in Liquid. Langmuir 2010, 26, 16652. 17535

dx.doi.org/10.1021/la304303h | Langmuir 2012, 28, 17530−17536

Langmuir

Article

(38) Sanchez, S. I.; Small, M. W.; Sivaramakrishnan, S.; Wen, J.-G.; Zuo, J.-M.; Nuzzo, R. G. Visualizing Materials Chemistry at Atomic Resolution. Anal. Chem. 2010, 82, 2599. (39) Abel, K. A.; FitzGerald, P. A.; Wang, T.-Y.; Regier, T. Z.; Raudsepp, M.; Ringer, S. P.; Warr, G. G.; van Veggel, F. C. J. M. Probing the Structure of Colloidal Core/Shell Quantum Dots Formed by Cation Exchange. J. Phys. Chem. C 2012, 116, 3968. (40) Kamat, P. V. Photochemistry on Nonreactive and Reactive (Semiconductor) Surfaces. Chem. Rev. 1993, 93, 267. (41) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69. (42) Xiong, Z.; Zhang, L. L.; Ma, J.; Zhao, X. S. Photocatalytic Degradation of Dyes over Graphene−Gold Nanocomposites under Visible Light Irradiation. Chem. Commun. 2010, 46, 6099. (43) Gaya, U. I.; Abdullah, A. H. Heterogeneous Photocatalytic Degradation of Organic Contaminants over Titanium Dioxide: A Review of Fundamentals, Progress and Problems. J. Photochem. Photobiol. C 2008, 9, 1. (44) Wu, T. X.; Liu, G. M.; Zhao, J. C.; Hidaka, H.; Serpone, N. Photoassisted Degradation of Dye Pollutants. V. Self-Photosensitized Oxidative Transformation of Rhodamine B under Visible Light Irradiation in Aqueous TiO2 Dispersions. J. Phys. Chem. B 1998, 102, 5845. (45) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515. (46) Li, J.; Wang, J.; Huang, L.; Lu, G. Photoelectrocatalytic Degradation of Methyl Orange over Mesoporous Film Electrodes. Photochem. Photobiol. Sci. 2010, 9, 39. (47) Zhao, D.; Chen, C.; Wang, Y.; Ma, W.; Zhao, J.; Rajh, T.; Zang, L. Enhanced Photocatalytic Degradation of Dye Pollutants under Visible Irradiation on Al(III)-Modified TiO2: Structure, Interaction, and Interfacial Electron Transfer. Environ. Sci. Technol. 2008, 42, 308. (48) Chatterjee, D.; Dasgupta, S. Visible Light Induced Photocatalytic Degradation of Organic Pollutants. J. Photochem. Photobiol. C 2005, 6, 186. (49) Hagfeldt, A.; Gratzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95, 49. (50) Kim, S.; Fisher, B.; Eisler, H.-J.; Bawendi, M. Type-II Qunatum Dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) Heterostructures. J. Am. Chem. Soc. 2003, 125, 11466. (51) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices; John Wiley & Sons: Edison, NY, 2007. (52) Zheng, Y. H.; Zheng, L. R.; Zhan, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K. M. Ag/ZnO Heterostructure Nanocrystals: Synthesis, Characterization, and Photocatalysis. Inorg. Chem. 2007, 46, 6980. (53) Wood, A.; Giersig, M.; Mulvaney, P. Fermi Level Equilibration in Quantum Dot−Metal Nanojunctions. J. Phys. Chem. B 2001, 105, 8810. (54) Subramanian, V.; Wolf, E.; Kamat, P. V. Green Emission to Probe Photoinduced Charging Events in ZnO−Au Nanoparticles. Charge Distribution and Fermi-Level Equilibration. J. Phys. Chem. B 2003, 107, 7479. (55) Subramanian, V.; Wolf, E.; Kamat, P. V. Semiconductor−Metal Composite Nanostructures. To What Extent Do Metal Nanoparticles Improve the Photocatalytic Activity of TiO2 Films? J. Phys. Chem. B 2001, 105, 11439. (56) 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. (57) Ismail, A. A.; Bahnemann, D. W.; Bannat, I.; Wark, M. Gold Nanoparticles on Mesoporous Interparticle Networks of Titanium Dioxide Nanocrystals for Enhanced Photonic Efficiencies. J. Phys. Chem. C 2009, 113, 7429. (58) Gao, H.; Liu, C.; Jeong, H. E.; Yang, P. Plasmon-Enhanced Photocatalytic Activity of Iron Oxide on Gold Nanopillars. ACS Nano 2012, 6, 234.

(59) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103, 8410. (60) Yang, J.; You, J.; Chen, C.-C.; Hsu, W.-C.; Tan, H.-R.; Zhang, X. W.; Hong, Z.; Yang, Y. Plasmonic Polymer Tandem Solar Cell. ACS Nano 2011, 5, 6210.

17536

dx.doi.org/10.1021/la304303h | Langmuir 2012, 28, 17530−17536