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Fabrication of Two-Dimensional ZnO Nanostructures from Nanoparticles Ming Yang, Guangsheng Pang,* Jixue Li, Linfeng Jiang, Daxin Liang, and Shouhua Feng* State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: July 3, 2007; In Final Form: September 12, 2007
Two-dimensional ZnO nanostructures have been prepared with nanoparticles as building blocks either by a solvothermal method or a liquid transport process at room temperature. ZnO nanoparticles can agglomerate into particulate nanosheets with area size in the micrometer range by directly heating in n-hexane. A stack of ultrathin ZnO particulate nanosheets can be obtained via two-dimensional aggregation of nanoparticles when the vapor-liquid-solid interface acts as a template at room temperature. The products are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, nitrogen adsorption isotherm, and photoluminescence spectroscopy. The formation mechanism of ZnO nanostructures is discussed.
1. Introduction Control of particle size, shape, and crystalline structure has been among the key issues in today’s nanochemistry.1,2 The ability of tailoring the dimensional regime of nanoparticles represents a landmark achievement in materials science, since at the nanoscale both size and shape dictate the peculiar chemical-physical (such as optical, electrical and catalytic) properties of materials.3,4 Nanoparticles can be used as building blocks to obtain well-defined assemblies and superstructures, which can lead to novel and unique properties that are not found in the individual components.5,6 Self-assembly driven by various interactions is an effective strategy for forming versatile “soft” nanoparticle-assembly motifs.7 Understanding factors governing the creation of nanoparticle assemblies would allow the design of desired nanostructures for optical, microelectronic, chemical, and biological applications.8 Solution-based methods have become widely used for the synthesis of various crystalline nanoparticles with narrow size distributions and well-defined morphologies.9,10 In general, when synthesizing nanoparticles from solution, nucleation is very fast, and subsequent growth occurs by two primary mechanisms: coarsening (also known as Ostwald ripening) and growth involving aggregation.11 For the aggregation process, primary particles may aggregate in an oriented fashion to produce a larger single crystal, or they may aggregate randomly and reorient, recrystallize, or undergo phase transformations to produce larger single crystals.12,13 The aggregation process has attracted increased interest in recent years as a new means for the fabrication and self-organization of nanocrystalline materials. Recent work has shown the universality of the aggregation process in the formation and growth of nanoparticles.11-17 As a versatile semiconductor, ZnO is especially attractive in recent years.18,19 Various ZnO nanostructures have been prepared on the basis of the solution method.18-26 In our previous work, we have reported the preparation of ZnO nanowires based on a concentration-controlled orientation attachment process.17 Here, we show that solvents play an important role in determining * To whom correspondence should be addressed. E-mail: panggs@ jlu.edu.cn (G.P.);
[email protected] (S.F.).
the growth habit of ZnO nanoparticles. Heat treatment of ZnO nanoparticles in n-hexane results in the formation of particulate nanosheets along with the formation of larger aggregates. Roomtemperature two-dimensional aggregation of ZnO nanoparticles can be achieved via a liquid transport process when the vaporliquid-solid interface acts as a template. A stack of ultrathin ZnO particulate nanosheets is obtained. 2. Experimental Section 2.1. Preparation Method. 2.1.1. Preparation of ZnO Nanoparticles. ZnO nanoparticles are prepared by a conventional solgel method.27,28 In a typical procedure, zinc acetate dehydrate (2.195 g) is dissolved in ethanol (100 mL) and heated at reflux for 2 h. The solution is then cooled to room temperature, and lithium hydroxide hydrate (0.588 g) is added. The suspension is then placed in an ultrasonic bath in order to solubilize the weakly soluble powder. After 30 min, the suspension is filtered through a glass fiber (0.1 µm) frit to remove the insoluble solids and then n-hexane (300 mL) is added into the as-obtained ZnO colloids to precipitate the ZnO nanoparticles. The as-prepared nanoparticles are dried at 25 °C in air. The average size of asprepared nanoparticles is ca. 4.0 nm. 2.1.2. Preparation of ZnO Particulate Nanosheets by SolVothermal Method. A 0.02 g amount of ZnO nanoparticles is transferred into a 40 mL Teflon-lined stainless steel autoclave containing 25 mL of n-hexane and heated at 160 °C for 12 h. The products are collected from the bottom of autoclaves and dried at 60 °C in air for 2 h. 2.1.3. Preparation of a Stack of ZnO Nanosheets at Room Temperature. A 0.02 g amount of ZnO nanoparticle powder is added to the bottom of a 100 mL beaker. A 20 mL aliquot of n-hexane is carefully added; then, 250 µL of n-butylamine is added dropwise. Evaporation is processed at 25 °C in open air. The formation of a stack of ZnO nanosheets can be found on the wall of the beaker after the evaporation is finished. The products are carefully removed from the beaker and are dried at 60 °C in air for 2 h. 2.2. Characterization Method. Powder X-ray diffraction (XRD) analysis is performed with a Rigaku D/MAX 2500/PC diffractometer with a Cu KR radiation. Scanning electron
10.1021/jp075189g CCC: $37.00 © 2007 American Chemical Society Published on Web 10/26/2007
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Figure 1. (A) XRD pattern of ZnO nanoparticles (inset: TEM image) and (B) XRD pattern of the products obtained by heating ZnO nanoparticles in n-hexane at 160 °C for 12 h.
Figure 2. SEM images with different magnification of sheetlike structures obtained by heating ZnO nanoparticles in n-hexane at 160 °C for 12 h.
Figure 3. (A, B) TEM and (C) HRTEM images of sheetlike structures obtained by heating ZnO nanoparticles in n-hexane at 160 °C for 12 h.
microscopy (SEM) is performed with a JSM-6700F electron microscope. High-resolution transmission electron microscopy (HRTEM) is performed with a JEM-3010 electron microscope. For the measurement of radial HRTEM images of ZnO nanorods, samples are first enwrapped in epoxide resin and then sliced by a Leica slicing machine. The fragments obtained by this procedure are used for further HRTEM observation. The optical image is obtained using an Olympus BX-51 microscope. Nitrogen adsorption isotherms are obtained on a Micromeritics ASAP 2020. Samples are degassed at 120 °C under vacuum until a final pressure of 1 × 10-3 Torr is reached. Pore size distributions are calculated using the Barett-Joyner-Halenda (BJH) method. The infrared (IR) spectrum is recorded on a Nicolet Impact 410 FTIR spectrometer. Photoluminescence (PL) spectra mesurements are performed at room temperature by irradiation with a 290 nm laser. 3. Results and Discussion 3.1. Solvothermal Method. 3.1.1. Morphological and Structural Characterization. Figure 1A shows the XRD pattern of ZnO nanoparticles. The average size of ZnO nanoparticles is ca. 4.0 nm by using the Debey-Scherrer formula according to
the XRD line broadening. The inset of Figure 1A shows that the nanoparticles are well-dispersed. Figure 1B shows the XRD pattern of products obtained by heating ZnO nanoparticles in n-hexane at 160 °C for 12 h. All diffraction peaks can be wellindexed to hexagonal phase zinc oxide (JCPDC card no. 361451). No impurities such as Zn(OH)2 can be detected. During the heat treatment, most nanoparticles grow into larger irregular aggregates. However, some sheetlike structures can also be found in the products. SEM images of such sheetlike structures with different magnification are shown in Figure 2. We can see the thickness of the nanosheets is uniform and the area size of such nanosheets is in the micrometer range. TEM investigation of this sample is further carried out. An individual nanosheet is shown in Figure 3A. The nanosheet is so thin that the underlay is clearly seen although it is covered. An intact image of two overlapping nanosheets is given in Figure 3B. Some nanoparticle aggregates can also be detected. The area size of the nanosheets is ca. 1.5 µm × 3 µm. HRTEM result is shown in Figure 3C, which demonstrates that the nanosheets are not single crystalline but are formed by the aggregation of ZnO nanoparticles. The formation of such sheetlike structures is concomitant with the formation of irregular aggregates. Figure 4A shows a TEM
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Figure 4. (A) TEM image of sheetlike structures with large amounts of irregular aggregates and (B, C) HRTEM images of an irregular aggregate.
Figure 5. SEM images of products obtained by heating ZnO nanoparticles in n-hexane at 160 °C for (A) 2 and (B) 4 h, respectively.
Figure 6. SEM images of products obtained by heating ZnO nanoparticles in n-hexne at (A, B) 100 and (C) 200 °C, respectively. (D) XRD pattern of products obtained by heating ZnO nanoparticles in n-hexane at [A] 100, [B] 160, and [C] 200 °C, respectively.
image of sheetlike structures with a large amount of irregular aggregates. Parts B and C of Figure 4 show the HRTEM image of an irregular aggregate. At least four attached ZnO nanoparticles can be detected, which is direct proof for the occurrence of the aggregation process. The reaction time has some influence on the formation of sheetlike structures. Parts A and B of Figure 5 show SEM images of products obtained by heat treatment of ZnO nanoparticles at 160 °C for 2 and 4 h, respectively. At 2 h, there is no obvious formation of sheetlike structures. When reaction time is increased to 4 h, we found some sheetlike structures. However, the sheetlike structures are not isolated,
and they interweave to form a porous structure. Temperature is another important parameter. Parts A-C of Figure 6 show SEM images of products obtained by heat treatment of ZnO nanoparticles for 12 h at 100 and 200 °C, respectively. We could see that when the temperature is 100 °C, sheetlike structures can also be obtained. However, the thickness is smaller than that obtained at 160 °C, and the area size of the sheetlike structures is very small. It should be noted that the sheetlike structures are enclosed in the big nanoparticle aggregates, and there exist holes near the sheetlike structures. When the temperature is increased to 200 °C, no sheetlike structures are
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Figure 7. TEM images of products obtained by heating ZnO nanoparticles in n-hexane with n-butylamine as additive at (A) 100 and (B) 180 °C for 2 h. The insets are the corresponding HRTEM images.
Figure 8. (A, B) TEM images of products obtained by heating ZnO nanoparticles in distilled water at 160 °C for 12 h. (C, D) axial HRTEM image and radial HRTEM image of nanorods, respectively.
observed. Figure 6D shows XRD patterns of products obtained at different temperature. It is found that all products are pure hexagonal phase zinc oxide. According to the difference in the full width of the peak at half-maximum height in XRD patterns, we can know that when the temperature is 100 °C, smaller crystallites form compared with that when the temperature is 160 and 200 °C. When n-butylamine is added into n-hexane during the heat treatment, no sheetlike structures can be found in the products. Instead, monodisperse ZnO nanoparticles with different sizes can be obtained by changing the heat temperature. Parts A and B of Figure 7 show TEM images of ZnO nanoparticles with average sizes ca. 10 and 20 nm, respectively. The insets show corresponding HRTEM images. The general morphology of nanoparticles with average sizes ca. 20 nm is hexagonal. If n-hexane is replaced by distilled water, onedimensional nanostructures with good uniformity can be obtained. Parts A and B of Figure 8 show TEM images of ZnO nanorods with average diameter ca. 20 nm obtained by hydrothermal treatment of ZnO nanoparticles in distilled water. The axial and radial HRTEM images of nanorods are given in Figure
8C,D, respectively. The preferred growth direction of nanorods is [0001], and the cross-section of nanorods is typically hexagonal. 3.1.2. Discussion of the Formation Process of ZnO Nanostructures. For the formation of nanorods, it can be explained on the basis of the structure characteristic of ZnO. A normal dipole moment and spontaneous polarization along the c-axis favor the formation of rodlike structures.16 For the formation of sheetlike structures in n-hexane, the condition is different. It should be noted that such nanosheets are obtained without the use of structure-directing reagents such as surfactants. Banfield et al. provided strong evidence that some natural minerals and synthetic nanocrystals could grow through a mechanism of oriented attachment of nanocrystals.12,13 In our condition, due to the nonpolar character of n-hexane, the dispersion of ZnO nanoparticles is difficult. However, the high-heat temperature may favor this process. More importantly, the adsorbent materials on the surface of nanoparticles such as adsorbent water play an important role. Figure 9A shows the IR spectrum of ZnO nanoparticles dried at 25 °C in air. The strong and wide adsorption peak around 3500 cm-1 indicates that there exists a
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Figure 9. (A) IR spectrum of ZnO nanoparticles dried at 25 °C in air and (B) TEM image of products obtained by heating ZnO nanoparticles in n-hexane with the addition of ca. 200 µL of distilled water.
large amount of adsorbent water on the surface of ZnO nanoparticles. The strong and narrow peaks at 1579 and 1425 cm-1 can be ascribed to the stretching vibration of CdO and C-O in acetates, respectively. In our solvothermal conditions, ZnO nanoparticles surrounded by adsorbent water in a nonpolar solvent are forced to aggregate in a two-dimensional manner during the heat treatment to form sheetlike structures. This process is similar with the formation process of porous Mg(OH)2 nanoplates, during which primary particles form first and then aggregate into nanoplates.29 The formation of porous structures (Figure 5B) or holes (Figure 6A,B) near the sheetlike structures in the big aggregates may be due to the absence of nanoparticles at that position after the formation of sheetlike structures. With increasing reaction time, sheetlike structures can grow into bigger ones and finally become isolated (Figure 2). If the temperature is too high (200 °C), the adsorbent water may depart from the surface of nanoparticles and sheetlike structures do not form (Figure 6C). The amount of adsorbent water is an important factor in the formation of sheetlike structures. If the nanoparticles are dried at 60 °C in vacuum overnight to remove most of the adsorbent water, the following heat treatment in n-hexane cannot result in the formation of sheetlike structures. If additional water exists in n-hexane besides adsorbent water, one-dimensional growth happens and results in the formation of nanorods instead. In fact, if we add only one drop of water (approximately 200 µL) into n-hexane before heating, ZnO nanorods with poor uniformity form instead of sheetlike structures (Figure 9B). The addition of n-butylamine in the solvothermal condition will improve the solubility of ZnO nanoparticles in n-hexane. In such case, the coarsening process is dominant, which involves the growth of larger crystals at the expense of smaller crystals. Higher temperature will favor the fast growth of crystals. Consequently, monodisperse ZnO nanoparticles with different sizes can be prepared. 3.2. Liquid Transport Process at Room Temperature. 3.2.1. Description of Liquid Transport Process. If no heat treatment is performed, the addition of n-butylamine can only disperse ZnO nanoparticles into n-hexane by ligand-exchange reaction, which has also been used to improve the dispersion of ZnO nanorods into ethanol solution.30 Spin coating of the ZnO nanorods suspension can result in the self-alignment of the nanorods along the substrate.30 Stimulated by this work, we believe that our hexane-butylamine-ZnO nanoparticles system can also be used to assemble nanoparticles with the assistance of the substrate. In fact, when this system is evaporated, we observe the formation of ZnO whiskers consisting of a stack of particulate nanosheets with the vapor-liquid-solid interface as a template. Such a process can be described as the aggregation of ZnO nanoparticles via a liquid transport process (Scheme 1). ZnO nanoparticles settle down to the bottom of a beaker containing n-hexane. The n-butylamine serves as a transport
SCHEME 1: Simplified Illustration of the Liquid Transport Process To Prepare a Stack of ZnO Particulate Nanosheetsa
a (A) Without n-butylamine, assembly cannot happen, and (B) the formation of a stack of ZnO particulate nanosheets is shown with the presence of n-butylamine.
agent and improves the dispersion of ZnO nanoparticles in n-hexane by capping the nanoparticles via a ligand-exchange reaction. Moving to the liquid-gas interface, ZnO nanoparticles deposit on the beaker wall due to the evaporation of n-hexane and n-butylamine. ZnO particulate nanosheets form during this evaporation process. The concentration gradient of ZnO nanoparticles is kept throughout the formation of nanosheets, which makes more nanoparticles transfer through the bottom to the liquid-gas interface. This process results in the formation of a stack of ZnO nanosheets. The following experimental results favor the above suggested mechanism. We find that high concentration of n-butylamine will result in the formation of dense whiskers, whereas low concentration will result in the formation of sparse longer whiskers. Furthermore, the extent of the blank zone can be reduced by increasing the amount of n-butylamine leaving a smaller quantity of powders at the bottom. The evaporation speed is another important factor. Beakers with a bigger orifice favor the formation of whiskers. In the same conditions, beakers with small enough orifice give
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Figure 10. (A) Optical image of ZnO whiskers, (B) SEM image of a ZnO whisker showing regular stripes, (C) SEM image of a ZnO whisker showing distinct formation of sheetlike structures, and (D) corresponding XRD pattern of ZnO whiskers.
Figure 11. (A, B) TEM images of a stack of ZnO particulate nanosheets and (C) HRTEM image of ZnO particulate nanosheets.
no whiskers. If the evaporation proceeds at 50 °C, ZnO whiskers cannot form. In a closed system, there are no ZnO whiskers found. 3.2.2. Morphological and Structural Characterization. Figure 10A shows an optical image of ZnO samples after being removed from the beaker wall, in which ZnO whiskers can be observed. Figure 10B shows an SEM image of an individual whisker. The existence of stripes in the whisker may be due to the regular stacking of nanosheets. The more distinct observation of sheetlike structures can be detected in Figure 10C. The corresponding XRD pattern in Figure 10D confirms that the sample is composed of ZnO nanoparticles with an average size of ca. 4.0 nm estimated from the XRD line broadening by using the Debey-Scherrer formula. The average particle size is the same as that of the initial ZnO nanoparticles, which indicates that there is no detectable crystal growth during the evaporation process at room temperature. Wrinkled nanosheets are obtained after spallation of ZnO whiskers in ethanol under ultrasonic radiation. Parts A and B of Figure 11 show typical TEM images of the nanosheets. Many streaklike or ripple-type contrasts are observed over the thin nanosheet. This is an electron diffraction phenomenon and is most frequently observed in thin TEM samples due to deformation and bending.31 An HRTEM image
indicates that the nanosheet is composed of ZnO nanoparticles, as shown in Figure 11C. There is no obvious preferred orientation of connecting nanoparticles in two-dimensional manner, and the nanoparticles are in close contact with each other. The arrow in Figure 11C indicates the wrinkled position where higher contrast is observed. The TEM result suggests that the sample is formed by a stack of ZnO particulate nanosheets, and the nanosheets are stable enough to be removed from the sample under sonication. During the formation of nanosheets, the vapor-liquid-solid interface acts as a template to restrict the two-dimensional aggregation process. The stabilization effect of n-butylamine also plays an important role. Without n-butylamine, the process cannot happen. Evaporation of transparent ZnO colloids in ethanol, which are prepared by a sol-gel method,16 can only result in the formation of irregular aggregates. 3.2.3. Adsorption and Optical Properties. Figure 12 shows N2 adsorption/desorption isotherms of ZnO whiskers which are treated at 120 °C for 2 h under vacuum before testing. The isotherms can be identified as type IV, which is characteristic of mesoporous materials. According to the classification of de Boer, such an isotherm is identified as type B in various shaped hysteresis loops.32 The explanation of this hysteresis loop is
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J. Phys. Chem. C, Vol. 111, No. 46, 2007 17219 SCHEME 2: Simplified Illustration of the Generation of Ultrabroad PL Emission
Figure 12. N2 adsorption/desorption isotherms of a stack of ZnO particulate nanosheets. Inset: BJH pore size distribution.
Figure 13. PL spectra of ZnO nanoparticles (a) before and (b) after assembly irradiated with a 290 nm laser.
based on the model of slit-shaped pores which are usually formed by platelike particles.33 For slit-shaped pores, the process of adsorption and desorption is different. The adsorption process is related to multilayered sorption, whereas the desorption process is based on the capillary evaporation. When we use adsorption isotherm to calculate BJH pore size distribution, the pore volume is minus. In fact, for slit-shaped pores, only desorption isotherm can be used to calculate BJH pore size distribution. Correspondingly, the BJH pore size distribution shows a narrow pore size distribution at 4.0 nm and a neglectable broad distribution from 5.0 to 10.0 nm (the inset of Figure 12). The generation of mesoporosity with narrow size distribution is presumably due to the interspace between ZnO nanosheets. However, during the desorption process, there may be a “pore blocking” effect,34,35 which probably results in the narrow size distribution of slit-shaped pores. The optical property is sensitive to the variation of local structures. The strong interaction between nanoparticles in the nanosheets is confirmed by the change of ZnO PL spectra. Figure 13 shows the PL spectra of ZnO nanoparticles before and after evaporation. The PL spectrum of ZnO nanoparticles consists of a weak UV peak and a broad visible emission band. The weak UV peak centered at ca. 385 nm is due to the exciton recombination.36 The visible emission band centered at ca. 550 nm ranging from 450 to 700 nm is generally associated with intrinsic and surface defects.37,38 However, the PL spectrum of the stack of ZnO nanosheets changes into a broad emission band
centered at ca. 448 nm ranging from 350 to 700 nm (full width at half-maximum ca. 180 nm). The emission covers the whole visible spectral region with calculated CIE coordinates of 0.2506 and 0.2755, which fall within the white region (the 1931 CIE diagram). There is inevitably adsorbed n-butylamine, which has influence on the luminescence of ZnO nanoparticles.39 It has been demonstrated that a second emission at 440 nm is observed when an amine functional group is present. A possible hole trapping effect by the amine groups on the surface of ZnO nanoparticles is suggested.39 In the case of our sample, nbutylamine plays a similar role for the presence of a new emission centered at ca. 448 nm. However, the continuity of an ultrabroad emission band should be related to the generation of radiative deep levels due to the interaction between ZnO nanoparticles within the nanosheets. Distortion of the semiconductor’s lattice structure will result in the appearance of a broad PL band.40 Figure 11C shows that ZnO nanoparticles are in close contact with each other, and the near-surface lattice is expected to be strongly distorted. Such structural defects destroy the symmetry of the lattice and perturb the energy band structure. In this situation, a local energy potential appears in the lattice and traps the carriers. Consequently, a broad PL spectrum covering the whole visible spectral region can be observed. This local perturbation effect could be described by a series of energy levels which lie within the band gap. A simplified illustration is shown in Scheme 2. 4. Conclusion In summary, we show that fabrication of two-dimensional ZnO nanostructures can be achieved by controlling the growth habit of nanoparticles. The crystal growth mechanism involving an aggregation process is used to explain the formation of nanostructures. ZnO nanoparticles can agglomerate into particulate nanosheets in n-hexane with area size in the micrometer range. Evaporation of the hexane-butylamine-ZnO nanoparticle system results in the formation of a stack of ultrathin ZnO particulate nanosheets at room temperature. Acknowledgment. This research was supported by the National Natural Science Foundation of China (Grant Nos. 20671039 and 20121103) and the National High Technology Research and Development Program of China (863 Program). References and Notes (1) Schmidt, G. Nanoparticles: from Theory to Applications; Wiley: Weinheim, Germany, 2004.
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