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Synthesis, Optical Properties, and Ethanol-Sensing Properties of Bicone-like ZnO Microcrystals via a Simple Solution Method Qingjiang Yu,†,‡ Cuiling Yu,§ Wuyou Fu,† Mingxia Yuan,† Jin Guo,† Minghui Li,† Shikai Liu,† Guangtian Zou,† and Haibin Yang†,* State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, P. R. China, State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Jilin Institute of Metrology, Changchun 130022, P. R. China ReceiVed: March 8, 2009; ReVised Manuscript ReceiVed: May 7, 2009
Large-scale uniform bicone-like ZnO microcrystals were successfully synthesized via a simple solution method at lower temperature (80 °C). The as-prepared bicones, composed of two cones, possess a single-crystal hexagonal structure and grow along the [0001] direction. The influence of the reactant concentration on the size and shape of the ZnO samples has been studied in detail, and the results revealed that reactant concentration plays a crucial role in determining the final morphologies of the samples. The growth process of the biconelike ZnO microcrystals was viewed by field emission scanning electron microscopy (FESEM) characterization, and a possible formation mechanism was proposed. Optical properties of the bicone-like ZnO microcrystals were also investigated by photoluminescence (PL) spectroscopy. The room-temperature PL spectrum of the bicone-like ZnO microcrystals shows a strong UV emission peak. The UV emission is further identified to originate from the radiative free exciton recombination by temperature-dependent PL. In addition, the biconelike ZnO microcrystals exhibit excellent sensing properties toward ethanol gas at an operating temperature of 300 °C. The response and recovery times are 2 and 3 s, respectively. These results indicate that the biconelike ZnO microcrystals have potential applications in fabricating optoelectronic devices and gas sensors. Introduction Semiconductor nanomaterials have attracted much attention due to their novel properties and potential applications as building blocks for electronic and optoelectronic nanodevices.1-3 Among these materials, the n-type semiconductor material ZnO, with a wide direct band gap of 3.37 eV and large exciton binding energy (60 meV) that guarantees stablity of excitons at room temperature, is considered to be a promising candidate for applications in UV light-emitting diodes and room-temperature UV-lasing.4,5 Because of its piezoelectric properties, ZnO is being explored for fabricating surface acoustic wave devices for delay lines, filters, resonators in wireless communication, and signal processing.6 Moreover, ZnO is also one of the promising materials for application in photovoltaic devices,7 transparent conductors,8 thin-film transistors,9 varistors,10 catalysts,11 solar cells,12 gas sensors,13,14 and hydrogen-storage devices.15 Control over size, crystallization, and morphology of semiconductor materials has become a hot topic in recent material research fields. This is because electrical, optical, magnetic, and thermoelectric properties of these materials, which is crucial for their potential applications, can be adjusted by varying their size, crystallization, and morphology.16-18 Because of its remarkable physical properties and versatile applications, ZnO has been prepared recently into various nano- or microstructured crystal morphologies, which include wires,5 rods,19 needles,20 columns,21 tubes,22 springs,23 urchins,24 cones,25 disks,26 towers,27 * To whom correspondence should be addressed. E-mail: yanghb@ jlu.edu.cn. † Jilin University. ‡ Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. § Jilin Institute of Metrology.
belts,28 nails,29 helices,30 branches,31 dendrites,32 tetrapods,33 dumbbells,34 hourglasses,35 pyramids,36 and ellipsoids.37-39 In the aspect of material synthesis, different methods such as metal-organic chemical vapor deposition (MOCVD),20 vaporphase transport (VPT),26 thermal evaporation,29 radio frequency magnetron sputtering,36 pulsed laser deposition (PLD),40 and molecular beam epitaxy (MBE)41 have been introduced to prepare ZnO nano- or microstructured materials with various morphologies. For these methods, high-temperature, vacuum technique catalysts or the use of noxious gas compounds may be required, which will increase the cost and limit the choice of substrates. Large-scale use will require the development of simple, low-cost approaches to the synthesis of inorganic functional nanomaterials. The solution-based chemical methods may be the most simple and effective way to prepare largescale and well-crystallized ZnO nano- or microstructured materials at a relatively low temperature. Recently, Oliveira and co-workers have synthesized ellipsoidal ZnO particles, also called bicone-like ZnO particles, by a double-jet precipitation method.42 However, the biconelike particles synthesized are irregular and nonuniform. Xie et al. have prepared bicone-like ZnO particles in an aqueous solution mediated by sonication.39 Nevertheless, the two subunits of the bicone-like particles are asymmetrical because one subunit is larger than the other. In addition, as far as we know, lowtemperature PL and gas-sensing properties of the bicone-like particles have rarely been investigated, thus limiting them to be used for few applications. Herein, we present a simple wet chemical approach to the fabrication of bicone-like ZnO microcrystals at lower temperature (80 °C). The crystallinity, morphology, and structure of bicone-like ZnO microcrystals are examined, the effects of reactant concentration and reaction time on the size and shape of the ZnO products are analyzed, and
10.1021/jp9020849 CCC: $40.75 2009 American Chemical Society Published on Web 06/09/2009
Properties of Bicone-like ZnO Microcrystals
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Figure 1. XRD pattern of the bicone-like ZnO microcrystals.
the formation mechanism of bicone-like ZnO microcrystals is discussed from the angle of nucleation and morphology. Furthermore, room-temperature and low-temperature PL and gas-sensing properties of the bicone-like ZnO microcrystals were also investigated. Experimental Section All chemicals (analytical grade reagents) were purchased from Beijing Chemicals Co., Ltd. and used as received without further purification. Deionized water with a resistivity of 18.0 MΩ cm was used in all experiments. In a typical synthesis process, 100 mL aqueous solution of zinc nitrate hexahydrate [Zn(NO3)2 · 6H2O] and 100 mL monoethanolamine (MEA) aqueous solution of equal concentration (0.02 M) were mixed together and kept under mild magnetic stirring for 5 min. Then the solution was transferred into a 500 mL flask and heated at 80 °C for 2 h with refluxing. Subsequently, the resulting white products were centrifuged, washed with deionized water and ethanol, and dried at 60 °C in air for further characterizations. X-ray power diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). Field emission scanning electron microscopic (FESEM) images were performed on a JEOL JEM-6700F microscope operating at 5 KV. Energy dispersive X-ray (EDX) spectra were measured with a FESEM attachment. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM2000EX microscope with an accelerating voltage of 200 KV. The PL emission spectra were recorded with HR-800 LabRam Infinity spectrophotometer excited by a continuous He-Cd laser with a wavelength of 325 nm. The sensor-measuring device belongs to the sintered block type, the fabrication process of which is similar to the result reported elsewhere.43 Gas-sensing properties of the bicone-like ZnO microcrystal-based sensor were measured by using a RQ1 intelligent test system. Gas sensitivity Sg was determined by the relative resistance, Sg ) Ra/Rg, where Ra and Rg are the resistance of the gas sensor in air and in test gas, respectively. Results and Discussion Structure and Morphology. The typical XRD pattern of the bicone-like ZnO microcrystals is shown in Figure 1. It is shown that all of the diffraction peaks can be indexed as pure hexagonal ZnO with lattice constants of a ) 3.249 Å and c ) 5.206 Å, which are consistent with the values in the standard card (JCPDS 36-1451). The relative sharp peaks in the XRD pattern confirm that the bicone-like ZnO microcrystals are well-crystallized.
Figure 2. Morphological and structural characterizations of the biconelike ZnO microcrystals: (a) low-resolution FESEM image, (b) highresolution FESEM image, (c) TEM image and SAED pattern (inset) of a bicone-like ZnO microcrystal, and (d) EDX spectrum of the biconelike ZnO microcrystals.
Figure 2 shows the morphological and structural characterizations of the bicone-like ZnO microcrystals. The low-resolution image (Figure 2a) shows that the obtained ZnO product is a typical bicone structure composed of two cones. The as-prepared bicones have an average length of about 2 µm and a mean diameter of about 800 nm (aspect ratio is about 2.5). The highresolution image in Figure 2b clearly reveals that the obtained product exhibits the well-defined bicone-like morphology. It is obvious that there is a joint boundary perpendicular to the length of the bicones that divides the bicones into two symmetrical cones. Moreover, particle-assembled surfaces of the bicones are observed. The nanoparticles serving as building blocks are in the quasi-spherical shape and about 15 nm in diameter. It is noteworthy to stress that these bicone-like structures are sufficiently stable and could not be destroyed even after longtime ultrasonication. The structure of the bicone-like ZnO microcrystals was further investigated by TEM. As shown in Figure 2c, the bicone-like morphology can still be observed, which agrees with the FESEM results. The inset SAED pattern indicates that the bicones possess a single-crystal hexagonal structure and grow along the [0001] direction. The EDX spectrum of the bicones is shown in Figure 2d, where except for the peaks corresponding to Zn and O, no trace amount of other impurities can be seen in the detection limit of the EDX. The contents of Zn and O are determined to be 50.68 and 49.32 atom %, respectively, which is close to the stoichiometry. To investigate effects of the reactant concentration, we first simply changed the concentration of Zn(NO3)2 and MEA while keeping their ratio constant. When the reactant concentration was 0.002 M, the obtained ZnO products exhibited the typical bipyramid structure with an aspect ratio of about 3.6 and an average diameter of 450 nm. The well-resolved hexagonal edges and corners can be observed in Figure 3a. Furthermore, the joint boundary can also be observed at the middle part of the bipyramids, which implys that the bipyramid structure consists of two pyramids, namely, twinned pyramids. When the concentration was increased to 0.01 M, the obtained ZnO products displayed the twinned cone structure (Figure 3b), which is similar to the morphology of the ZnO products prepared at the reactant concentration of 0.02 M. As the concentration was
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Figure 4. FESEM and TEM images of the ZnO products prepared at different reaction times: (a) 10 min, (b) 30 min, (c) 1 h, and (d) 8 h. Reactant concentration is 0.02 M.
Figure 3. FESEM images of the synthesized ZnO products with different reactant concentrations: (a) 0.002 M Zn(NO3)2 and 0.002 M MEA, (b) 0.01 M Zn(NO3)2 and 0.01 M MEA, (c) 0.05 M Zn(NO3)2 and 0.05 M MEA, (d) 0.1 M Zn(NO3)2 and 0.1 M MEA, (e) 0.02 M Zn(NO3)2 and 0.05 M MEA, and (f) 0.02 M Zn(NO3)2 and 0.1 M MEA. The reaction time is 2 h.
added to 0.05 M, the morphology of the obtained ZnO products was changed to the elongated rugby ball-like ellipsoid (Figure 3c), and the aspect ratio decreased to 1.8, having a length of about 1.7 µm and a diameter of about 940 nm. By further increasing the concentration to 0.1 M, interestingly, the typical sphere structure, comprised of two hemispheres with an aspect ratio of about 1.0 and an average diameter of about 1.1 µm, was achieved, as shown in Figure 3d. It is obvious that the aspect ratio of the obtained ZnO products gradually decreased with an increase in the reactant concentration when the concentration of Zn(NO3)2 and MEA is uniform. That is to say, the ZnO preferential growth difference between (0001) and other directions would diminish with an increase in reactant concentration. A possible reason will be described in detail below. In addition, we fixed the concentration of Zn(NO3)2 (0.02 M) while changing the concentration of MEA. When the concentration of MEA was 0.05 M, the ZnO product still displayed a typical bicone structure (Figure 3e). Further increasing the concentration of MEA to 0.1 M, the obtained ZnO product displayed the elongated bipyramid structure with an aspect ratio of about 4.3. This indicates that ZnO crystals will quickly grow along the [0001] direction when the ratio of the concentration of MEA and Zn(NO3)2 achieved a certain value. Growth Process. To understand the growing mechanism, the ZnO products were prepared at different reaction times. Figure 4 shows the evolution of the ZnO structures as a function of reaction time. FESEM observation of the products, which aged at 80 °C for only 10 min, revealed that smaller twinned cones, with an average length of 240 nm and a mean diameter of 150 nm, were obtained. Although these twinned cones were asymmetrical, there were two well-developed ZnO (0001j) planes at
the joint boundary, which provide some clues for the understanding of the growth mechanism. Moreover, it was found that some floccules were randomly scattered in these twinned cones, as illustrated in Figure 4a. As seen from the TEM image in the inset of Figure 4a, the floccules are comprised of nanoparticles with an average diameter of 4 nm. As the reaction time was prolonged to 30 min, the nanoparticles disappeared, while the size of the twinned cones obviously increased (Figure 4b). This suggests that the nanoparticles are only an intermediate and will gradually form twinned cones with increasing reaction time. When the reaction proceeded to 1 h, the twinned cones grew larger and longer and showed better crystal perfection. On further prolonging the reaction to 2 h, the smaller subunit of the twinning had evolved to be equivalent with the larger one, and typical bicone-like ZnO microcrystals formed as shown in Figure 2a,b. To determine the appropriate reaction time, we investigated the effect of longer reaction times, up to 8 h, as shown in Figure 4d. Results show that reaction time exceeding 2 h will not bring about obvious structural and morphological modifications. Therefore, a reaction time of 2 h is sufficient to obtain the well-defined bicone-like ZnO microcrystals. Growth Mechanism. The overall reaction for the growth of ZnO crystals may be expressed by the following equations.
Zn2+ + NH2 · (CH2)2 · OH T Zn2+ amino complex
(1) NH2 · (CH2)2 · OH + H2O T [NH3 · (CH2)2 · OH)]+ + OH(2) Zn2+ + 4OH- f [Zn(OH)4]2- f ZnO + 2OH- + H2O (3) Zn2+ amino complex + H2O f ZnO + [NH3 · (CH2)2 · OH)]+
(4)
When MEA and zinc nitrate aqueous solutions were mixed at room temperature, the chemical equilibrium in eq 1 shifts to the right, and hence, a large number of Zn2+ amino complexes
Properties of Bicone-like ZnO Microcrystals were produced in the solution. In the hydrothermal condition, the equilibrium in eq 1 shifts to the left and the equilibrium in eq 2 shifts to the right, respectively, corresponding to thermolysis of Zn2+ amino complexes and the hydrolysis of MEA. MEA serves as a pH buffer to release OH-, which will facilitate the chemical equilibrium in eq 3 to shift to the right. Additionally, a part of the Zn2+ amino complexes will be directly hydrolyzed, and ZnO can be formed as shown in eq 4. To understand the observed behaviors of ZnO, it is necessary to investigate its growth mechanism. It is well-known that the hexagonal ZnO crystal has polar and nonpolar faces. The typical crystal habit exhibits a basal polar oxygen plane (0001j), a top tetrahedron corner-exposed polar zinc plane (0001), and lowindex faces (parallel to the c-axis) consisting of a nonpolar {101j0} face. Polar faces with surface dipoles are thermodynamically less stable than nonpolar faces and often undergo rearrangement to reduce their surface energy.44 At the early stage of the reaction system, [Zn(OH)4]2- ions, the growth units in the solution near the surface of the ZnO nanoparticles, are likely adsorbed onto the positive polar face of the (0001) surface, resulting in faster growth along the [0001 direction, and thus solid ZnO cones were obtained. Moreover, the Zn2+ amino complexes, with positive charges, would adsorb onto the negative polar face of the (0001j) surface by the coulomb interaction. Thus, by the hydrolysis of Zn2+ amino complexes, smaller ZnO crystals would grow on the larger end of the cones. The cones and smaller crystals gradually grew larger and longer with an increase in reaction time and finally formed the typical bicone-like ZnO microcrystals. Moreover, in the solution, NO3ions may also be adsorbed onto the positive polar face of the (0001) surface. When the reactant concentration is lower, the concentration of NO3- ions is also lower and the adsorption of [Zn(OH)4]2- ions on the (0001) surface are dominant, thus resulting in a faster growth rate along the [0001] direction than along other directions. Therefore, the ends of the products become cuspate as shown in Figure 3a. Contrarily, when the reactant concentration is higher, many ZnO nuclei appear simultaneously at the beginning of the reaction. With the reaction proceeding, the concentration of [Zn(OH)4]2- ions rapidly reduces. While the concentration of NO3- ions is still higher, NO3- ions will compete with [Zn(OH)4]2- ions to absorb on the (0001) surface, which prevents the contact of [Zn(OH)4]2on the (0001) surface and thus limits the crystal growth along the [0001] direction, resulting in a decrease of the ZnO preferential growth difference between (0001) and other directions as shown in Figure 3c,d. Photoluminescence. PL from ZnO consists of two emission bands at room temperature: a near-band-edge (UV) emission and a broad, deep-level (visible) emission. The visible emission is usually considered to be related to various intrinsic defects produced during ZnO preparation and posttreatment.45,46 Normally, these defects are located at the surface of the ZnO structure.45-47 Figure 5 shows the PL spectrum of the biconelike ZnO microcrystals, which is excited by 325 nm UV light from a He-Cd laser at room temperature. There appears to be a strong UV emission at ∼386 nm and a weak broad green emission around 525 nm. The UV emission is the band-edge emission resulting from the recombination of free excitons. Though the origin of the green emission is controversial, generally, it is attributed to the singly ionized oxygen vacancy and the emission results from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy.48
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Figure 5. PL spectrum of the bicone-like ZnO microcrystals at room temperature.
To better understand the PL property of the bicone-like ZnO microcrystals, the PL spectrum measured at 79 K is shown in Figure 6a. The dominant peak at 3.356 eV is known as the donor bound exciton (D0X) emission.49 The emission observed at 3.368 eV on the higher-energy shoulder of the D0X peak is assigned to the free exciton (FX) emission. The difference between 3.356 and 3.368 eV is close to the activation energy of D0X.50 The lower-energy side of the D0X peak arises at two shoulders located at 3.295 and 3.223 eV, which are attributed to the 1-LO and 2-LO photon replica of FX, respectively. Figure 6b shows the temperature-dependent PL spectra of the bicone-like ZnO microcrystals measured from 79 to 299 K. With an increase in the measurement temperature, the intensities of all emissions are decreased and the intensity of the D0X emission decreases more rapidly than that of the FX emission. The decrease in intensity is due to the result of the thermal ionization of the bound excitons. In contrast, the FX emission gets relatively stronger and dominates the emission spectra when the temperature is above 239 K. The FX phonon replicas have different temperature dependence than excitons as they show a slower intensity decrease with an increase in temperature. Gradually, they get weaker and broadened at higher temperatures and finally merge into the low-energy tail of the FX peak. In addition, the exciton emission of FX and D0X show an obvious red shift with increasing measurement temperature. The temperature dependence of the peak energy of FX and D0X is shown in Figure 6c. The temperature dependence of the exciton emission energy is related to the temperature dependence of the band energy, which is expressed in terms of the following semiempirical formula51
Ex(T) ) Ex(0) - RT2 /(T + β)
(5)
where Ex(0) is the peak energy at absolute zero temperature and R and β are the fitting parameters. Ex(0) is 3.375 and 3.362 eV for FX and D0X emissions, respectively. In Figure 6c, the lines represent the calculated temperature dependences for each emission mode, and it is shown that the calculated lines fit well with the experimental values. Ethanol Sensing Properties. Figure 7a shows the response and recovery of the bicone-like ZnO microcrystals upon exposure to ethanol with various concentrations at the operating temperature of 300 °C. The heating voltage is 5 V and the environment humidity is about 20% RH (relative humidity at
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Figure 6. (a) PL spectrum of the bicone-like ZnO microcrystals at 79K. (b) Temperature-dependent PL spectra of the bicone-like ZnO microcrystals. (c) Temperature dependence of excitonic emission energies of the bicone-like ZnO microcrystals.
Figure 7. (a) Response and recovery characteristics of the bicone-like ZnO microcrystals exposed to ethanol with various concentrations at 300 °C. (b) Relationship between the sensitivity and ethanol concentration.
20 °C). In the measurements, four periods were examined at each fixed concentration. The response values are about 3.3, 7.4, 22.5, 40.4, 70.2, 141.3, 196.5, 220.3, and 231.1 to 1, 10, 50, 100, 200, 500, 1000, 1500, and 2000 ppm ethanol, respectively. The sensing properties are better than that of other ZnO nanomaterials reported previously.13,52-54 The response and recovery time are around 2 and 3 s, respectively. In fact, the sensitivity of oxide semiconductors can usually be empirically depicted as Sg ) APβg, where Pg is the target gas partial pressure, which is in direct proportion to its concentration, and the sensitivity is characterized by the prefactor A and exponent β.55 β may have some rational fraction value (usually 1 or 1/2), depending on the charge of the surface species and stoichiometry of the elementary reactions on the surface. As shown in Figure 6b, at lower ethanol concentrations, the sensitivity increases nearly linearly. With an ethanol concentration in the range of 1-200 ppm, β is found to be 1. Above 200 ppm, the sensitivity slowly increases with ethanol concentration. The sensitivity saturates with an ethanol concentration up to 2000 ppm.
Therefore, the sensor is very suitable for low-concentration ethanol gas detection. Most metal oxide semiconductor gas sensors work on the basis of the change of the conductance of the sensing materials arising from reactions between the active surface complexes and gas molecules.56,57 Hence, the sensing mechanism of ZnO to ethanol gas may be described as follows. When the bicone-like ZnO microcrystals are surrounded by air, oxygen molecules will adsorb onto the ZnO surface and extract electrons from the ZnO conduction band to generate chemisorbed oxygen species (O2-, O2-, and O-). Thus, the ZnO microcrystals will become highly resistive. When ethanol gas is introduced at moderate temperature, the ZnO microcrystals are exposed to the reductive gas. By reacting with the oxygen species on the ZnO surface, the reductive gas will reduce the concentration of oxygen species on the ZnO surface, resulting in an increase in the concentration of electrons in the conduction band. This eventually increases the conductivity of the ZnO microcrystals.
Properties of Bicone-like ZnO Microcrystals Conclusion Large-scale uniform bicone-like ZnO microcrystals were successfully synthesized via a facile solution method at lower temperature (80 °C). The as-prepared bicones, composed of two cones, possess a single-crystal hexagonal structure and grow along the [0001] direction. The influence of the reactant concentration on the size and shape of the ZnO products is studied, and the results reveals that when the concentration of the Zn(NO3)2 and MEA is uniform, the ZnO preferential growth difference between (0001) and other directions will diminish with an increase in reaction concentration. The growth process and growth mechanism of the bicone-like ZnO microcrystals are discussed from the angle of nucleation and morphology. Room-temperature PL indicates that the bicone-like ZnO microcrystals have a very strong UV emission at ∼386 nm, while low-temperature PL shows that primary emission is from bound and free exciton recombination. Gas sensors fabricated from these bicone-like ZnO microcrystals are very sensitive to ethanol gas, and the response and recovery processes are very rapid. The bicone-like ZnO microcrystals have promise in their potential applications to optoelectronic devices and gas sensors. References and Notes (1) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Liber, C. M. Nature 2001, 409, 66. (2) Golderger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H.-J.; Yang, P. Nature 2003, 422, 599. (3) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (4) Bao, J.; Zimmler, M. A.; Capasso, F.; Wang, X.; Ren, Z. F. Nano. Lett. 2006, 6, 1719. (5) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (6) Gorla, C. R.; Emanetoglu, N. W.; Liang, S.; Mayo, W. E.; Lu, Y.; Wraback, M.; Shen, H. J. Appl. Phys. 1999, 85, 2595. (7) Zhao, Q.; Xie, T.; Peng, L.; Lin, Y.; Wang, P.; Peng, L.; Wang, D. J. Chem. Phys. C 2007, 111, 17136. (8) Xu, H. Y.; Liu, Y. C.; Mu, R.; Shao, C. L.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. Appl. Phys. Lett. 2005, 86, 123107. (9) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Nunes, G. Appl. Phys. Lett. 2003, 82, 117. (10) Mahan, G. D. J. Appl. Phys. 1983, 54, 7. (11) Yoshimoto, M.; Takagi, S.; Umemura, Y.; Hada, M.; Nakatsuji, H. J. Catal. 1998, 173, 53. (12) Katoh, R.; Furube, A.; Hara, K.; Murata, S.; Sugihara, H.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2002, 106, 12957. (13) Feng, P.; Wan, Q.; Wang, T. H. Appl. Phys. Lett. 2005, 87, 213111. (14) Wang, J. X.; Sun, X. W.; Yang, Y.; Huang, H.; Lee, Y. C.; Tan, O. K.; Vayssieres, L. Nanotechnology 2006, 17, 4995. (15) Wan, Q.; Lin, C. L.; Yu, X. B.; Wang, T. H. Appl. Phys. Lett. 2004, 84, 124. (16) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayer, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (17) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3, 199. (18) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L.-W.; Alivisatos, A. P. Nature 2004, 430, 190. (19) Guo, L.; Ji, Y. L.; Xu, H. J. Am. Chem. Soc. 2002, 124, 14864. (20) Park, W. I.; Yi, G. C.; Kim, M.; Pennycook, S. J. AdV. Mater. 2002, 14, 1841. (21) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954.
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