One-Pot Synthesis of Hollow PbSe Single-Crystalline Nanoboxes via

Feb 2, 2010 - One-pot self-assembly of flower-like Cu2S structures with near-infrared photoluminescent properties. Na Li , Xiaoling Zhang , Shutang Ch...
0 downloads 0 Views 3MB Size
Download PDF
DOI: 10.1021/cg901280a

One-Pot Synthesis of Hollow PbSe Single-Crystalline Nanoboxes via Gas Bubble Assisted Ostwald Ripening

2010, Vol. 10 1257–1262

Shutang Chen,† Xiaoling Zhang,*,† Xiaomiao Hou,† Qi Zhou,† and Weihong Tan*,‡ †

Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China, and ‡Center for Research at Bio/nano Interface, Department of Chemistry and Shands Cancer Center, University of Florida Genetics Institute and McKnight Brain Institute, University of Florida, Gainesville, Florida 32611-7200 Received October 15, 2009; Revised Manuscript Received November 14, 2009

ABSTRACT: We report a simple one-pot route to fabricate PbSe hollow single-crystalline nanoboxes with the presence of trioctylphosphine as the structure-directing agent and stabilizer. Various controlling parameters were examined, such as trioctylphosphine amounts, reaction temperature, reaction time, and lead(II) precursors. On the basis of the experimental results, the gas bubbles assisted Ostwald ripening process was proposed to explain the formation of hollow PbSe nanoboxes. The morphology and composition of the products were characterized by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy.

Introduction Nanostructured materials have attracted tremendous attention in recent years due to their unique electronic and optical properties as well as their potential applications in nanoscale devices.1 PbSe is an important semiconductor material with a narrow band gap energy (0.28 eV) and large bulk exciton Bohr radius (46 nm), which results in a strong confinement of the electron-hole pair and large optical nonlinearity.2 Moreover, the recent discoveries of highly efficient multiexciton generation in PbSe nanocrystals have drawn intense interest from applied and fundamental research fields.3 Thus, a number of synthetic strategies of PbSe nanocrystals based on well-known or newly discovered phenomena in the formations of nanocrystals with different morphologies have been developed. For example, PbSe pyramid nanocrystals were prepared by a onestep synthetic method.4 Crystalline PbSe nanowires and nanorings have been produced through oriented attachment of nanocrystals building blocks.5 PbSe nanotubes were synthesized by biomolecule-assisted self-assembly of nanocrystals at room temperature.6 Cubic-shaped PbSe nanocrystals have been obtained using a dynamic injection technique.7 Hierarchical superstructures of PbSe have been fabricated by a microwave-assistant method.8 Ligand-tailored PbSe nanocrystals were prepared through cation-exchange-mediated nucleation method.9 Besides those, hollow nanostructures have been considered as an emerging class for nanoelectronics, catalysis, thermoelectricity, and drug-delivery applications due to their high surface areas and low density.10 Currently, one of main approaches for hollow nanostructures is a template-free process, which includes oriented attachment,11 Kirkendall-effect-induced growth,12 and direct solid evacuation with Ostwald ripening.13 Very recently, hollow PbSe nanospheres were prepared in situ by a cationexchange approach14 and Kirkendall-effect-induced growth.15 However, all these synthetic schemes require using an amorphous Se sphere as the core material and the hollow PdSe

nanostructures are not single-crystalline. Herein we report a synthetic strategy that allows the one-pot synthesis of hollow PbSe single-crystalline nanoboxes. Although hollow PbTe single-crystalline hollow nanoboxes have been prepared by a solvothermal technique,16 to the best of our knowledge, this is the first report on hollow PbSe single-crystalline nanoboxes prepared in one-pot. The reaction revealed that lead nitrate as the metal precursors is critical for the formation of hollow nanocrystals in cubic shape as trioctylphosphine (TOP) was used as the morphology controller. Materials and Methods Materials. TOP (90%) and selenium powder were purchased from Alfa Aesar Chemicals and used without further purification. Lead nitrate, lead(II) acetate trihydrate, and solvents used were purchased from Beijing Chemical Reagents Co. Synthesis of Hollow PbSe Nanoboxes. In brief, lead nitrate (0.15 mmol), Se powder (0.75 mmol), and TOP (4 mL) were added into a 25 mL three-neck flask. Under nitrogen flow, the resulting solution was heated up to 200 °C at a rate of 5 °C/min. After the temperature reached 200 °C, serial aliquots were taken for mechanism studies. The samples were separated by centrifugation and decantation prior to any further measurements. Characterization of Hollow PbSe Nanoboxes. The phases of as-prepared products were characterized using powder X-ray diffraction (XRD, Rigaku D/MAX 2400) with Cu KR radiation (λ = 1.5405 A˚) from 20° to 80° at a scanning rate of 8°/min. The morphologies of samples were characterized by field-emission scanning electron microscopy (FESEM, XL-SFEG, FEI Corp, with an accelerating voltage of 5 kV), electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM, FET TECNAI F30, with an accelerating voltage of 200 kV), respectively. The optical diffuse reflectance spectra were performed on a UV-vis-NIR scanning spectrophotometer (Cary-5000, Varian) using an integrating sphere accessory.

Results and Discussion

*Corresponding authors. Phone: 86-010-88875298. Fax: 86-010-88875298. E-mail: [email protected] (X.Z.), [email protected] (W.T.).

Figure 1 shows the XRD pattern of sample synthesized at 200 °C for 6 h in a 8 mL TOP solution. All the diffraction peaks can be well indexed to the cubic structure of PbSe (JCPDS 78-1903), and no any impurities were detected in the XRD pattern, indicating the formation of pure products.

r 2010 American Chemical Society

Published on Web 02/02/2010

pubs.acs.org/crystal

1258

Crystal Growth & Design, Vol. 10, No. 3, 2010

Figure 1. XRD pattern of as-synthesized PbSe hollow nanoboxes prepared at 200 °C for 6 h in 8 mL TOP solution.

Figure 2. (a) Low magnification and (b) high magnification SEM images of PbSe hollow nanoboxes; (c) low magnification TEM image of PbSe hollow nanoboxes; and (d) enlarged TEM image of an individual PbSe hollow nanobox prepared at 200 °C for 6 h in 8 mL TOP solution; the inset shows the related selected-area electron diffraction (SAED) pattern, demonstrating a singlecrystalline nature.

The sharp and strong diffraction peaks also confirm the good crystallization of the products. The morphology of as-synthesized products was investigated by SEM and transmission electron microscopy (TEM) as shown in Figure 2. Figure 2a gives the representative SEM image of obtained products at 200 °C for 6 h in 8 mL TOP solution, which suggests that a large quantity of nanocrystals has a regular cubic shape. The magnified image shown in Figure 2b exhibits detailed morphology of the obtained products, indicating the formation of uniform cubes, and some slits of cube’s surface could also be observed in the image, confirming that the cubes have a hollow interior. The hollow nanostructures of the as-synthesized PbSe nanoboxes were further studied by the TEM image as shown in Figure 2c. All these nanocrystals show a uniform cubic shape with edge lengths of about 90-120 nm, and a strong contrast difference in all of the cubes with a bright center surrounded by a much darker edge confirms their hollow architecture. The select-area

Chen et al.

electron diffraction (SAED) pattern in Figure 2d indicates that the PbSe nanoboxes are single crystalline and can be indexed based on a face-centered-cubic cell with a lattice parameter, R = 6.105 A˚, consistent with the XRD results. Figure 2d gives a typical TEM image of an individual hollow nanobox, and the average edge length and shell thickness could be determined as 106 and 28 nm, respectively. The experimental parameters have a great effect on the formation of the hollow PbSe nanocrystals. Figure 3 shows the TEM and SEM images of samples obtained at different TOP additions while keeping all the other experimental parameters the same. When 2 mL of TOP was used, only irregular solid PbSe nanocrystals were obtained (Figure 3a,b,g). After increasing the amount of TOP to 4 mL, hollow PbSe nanoboxes were obtained (Figure 3c,d,h). When the TOP amount is 8 mL, a hollow nanosphere emerges among the dominating hollow microstructures (Figure 3e,f,i). These experimental results indicate that the surfactant TOP plays multiple roles in the formation of hollow structures. It acts not only as the reaction regulators and weak reducing reagent to adjust the nucleation and growth of PbSe nanocrystals, but also as the structure-directing agent and stabilizer to control morphology of PbSe naocrystals. As shown in experimental results, a decrease in the amount of TOP led to an obvious shape evolution of the PbSe nanocrystals from regular hollow boxes (Figure 3c-f,h,i) to irregular solid nanocrystals (Figure 3a,b,g). Possible reasons for this change are given as follows: The precursor concentration is increased gradually with a decrease of TOP amounts. Under a high concentration of precursors, many nuclei form at the beginning of the reaction. Although TOP has high viscosity, small quantities of TOP solution cannot prevent the particles from aggregation. Moreover, a decrease in TOP amounts results in ineffective adsorption of the TOP on the nanocrystals’ surface, which favors the formation of irregular solid PbSe nanocrystals. To shed light on the formation mechanism of these novel PbSe hollow nanoboxes, temperature-dependent experiments were carried out in 4 mL of TOP solution for 3 h, and the products were analyzed by TEM as shown in Figure 4. At a relatively lower temperature (such as 150 °C), only solid PbSe nanospheres were obtained (Figure 4a). After increasing the temperature to 180 °C, hollow nanospheres emerged and were predominant in the final products (Figure 4b). At the higher temperature (such as 200 and 220 °C), the hollow PbSe nanoboxes with a narrow size distribution and smooth surface are favored (Figures 3c and 4c). With the reaction temperature increasing, morphologies and sizes of the PbSe nanocrystals changed from regular solid nanospheres with an average diameter of 325 nm (Figure 4a) to irregular hollow nanoboxes with an average diameter of 119 nm (Figure 4b) and uniform hollow nanoboxes with an average edge length of 61 nm (Figure 4c). These experimental results indicate that the shape of the PbSe nanostructures is sensitive to the reaction temperature. Taking account of the formation process of hollow interiors, it is recognized that the Ostwald ripening should be the underlying mechanism for the formation of hollow PbSe nanoboxes. If the reaction proceeds at a higher temperature, the crystal growth rate will increase with respect to the diffusion rate of the cores.17 In other words, at a higher reaction temperature, the Ostwald-ripening process proceeds at a rather higher rate and core-hollowing takes place much faster. As a result, hollow PbSe nanoboxes are formed at a relatively higher reaction temperature.

Article

Crystal Growth & Design, Vol. 10, No. 3, 2010

1259

Figure 3. TEM and SEM images of the as-synthesized PbSe products at 200 °C for 3 h with different TOP amounts: 2 mL (a, b, and g), 4 mL (c, d, and h), 8 mL (e, f, and i).

Figure 4. TEM images of PbSe nanocrystals prepared in 4 mL of TOP solution for 3 h at different reaction temperatures: (a) 150 °C, (b) 180 °C, (c) 220 °C, and the close views of an individual for each hollow PbSe nanobox sample are in the insets.

To study void evolution of the hollow PbSe single-crystalline nanoboxes during the controlled Ostwald-ripening process, we took aliquots of reaction mixtures at different times during the reaction, quickly precipitated the nanoparticles, and redispersed them into toluene for TEM study. Figure 5 shows detailed time-dependent evolutions of morphology and crystallinity. The loose solid nanosphere with an average diameter of 102 nm obtained at 150 °C for 1 min contains nanosized colloidal particles (Figure 5a). Moreover, the inset shows the related SAED pattern, demonstrating the polycrystalline nature because of the nucleation and aggregation of nanosized colloidal particles. When the reaction temperature was increased to 180 °C, as shown in Figure 5b, a high

yield of metastable solid nanospheres with diameters of about 140 nm grew from the nanosized colloidal spheres, and it was also found that small surface cubes formed shells on the solid cores. After 1 min at 200 °C of reaction, the diameters of the small nanocubes coated solid nanospheres increased to about 190 nm (Figure 5c). The formation of samples with different sizes may be attributed to the fact that the crystal growth process proceeded at a rather higher rate as the reaction temperatures increased. After 30 min of reaction, the diameters of compact metastable nanospheres decreased to about 131 nm due to gas bubble assisted Ostwald-ripening, and it was also found that the surface became smoother and the void was formed in the center of the nanospheres

1260

Crystal Growth & Design, Vol. 10, No. 3, 2010

Chen et al.

Figure 5. TEM image of PbSe nanocrystals prepared in 8 mL of TOP solution at (a) 150 °C for 1 min, (b) 180 °C for 1 min, (c) 200 °C for 1 min, (d) 200 °C for 30 min, (e) 200 °C for 90 min, (f) the corresponding XRD patterns; the SAED patterns in the inset show their single-crystalline nature.

(Figure 5d). Interestingly, after heating at 200 °C for 90 min, the void size increased to about 31 nm and the morphology transformed from a solid metastable nanosphere to hollow nanoboxes (Figure 5e). When the reaction time was prolonged to 6 h, the final void size and the average edge length of hollow nanoboxes could be determined as 54 and 106 nm (Figure 2c,d), respectively. The SAED patterns (the insets of Figure 5b-e) show their single-crystalline nature. XRD patterns of the samples obtained at different stages (Figure 5f) display the gradual enhancement of the crystallinity of PbSe products with increasing temperature and time. By using the Scherrer-Debye formula for an estimation of PbSe nanocrystals, we found that the sizes of the crystallite were in agreement with what has been observed in the TEM study. As far as the growth mechanism is concerned, the effects of lead(II) nitrate must be taken into account. Under the condition of our experiments, Pb(NO3)2 and Se powders were used as precursors and TOP as solvent and stabilizer, and no surfactants were used. The possible reaction processes for the formation of PbSe hollow nanoboxes can be proposed by eqs 1-3. TOP þ Se f TOPSe ð1Þ PbðNO3 Þ2 f PbO þ NO2 þ O2

ð2Þ

PbO þ TOPSe f TOPO þ PbSe

ð3Þ

At beginning of the process, transparent solution was formed because Se could be chelated with an excess amount of TOP according to reaction 1. It is known that lead nitrate is a mild oxidizer and may decompose above 200 °C. In our experiments, it was found surprisingly that gas bubbles were produced at 120 °C in 4 mL of TOP solution. This indicated that lead nitrate decomposed and gave off gas at 120 °C. Because there was no special reducing agent added in our reaction solution, it could be deduced that TOP had an obvious reducing action for the induced decomposition of lead nitrate

Figure 6. TEM images and XRD pattern of PbSe nanocrystals obtained using lead acetate as a lead precursor at 200 °C for different reaction times, (a) 1 min, (b) 3 h, and (c) corresponding to the XRD pattern.

at the relative lower reaction temperature. As illustrated by reaction 2, when a certain concentration of gas was generated, simultaneously, a large amount of PbSe nuclei were produced in solution according to reaction 3 (see Figure S1, Supporting Information). These nanocrystals have a tendency to aggregate,

Article

Crystal Growth & Design, Vol. 10, No. 3, 2010

1261

Figure 7. Diffuse reflectance spectra of (a) hollow PbSe nanospheres with an average diameter about 190 nm and (b) hollow PbSe nanoboxes with average edge lengths about 106 nm; the insets are the corresponding curves plotted as normalized (F(R)*hv)2 versus hv (eV).

resulting from their high surface energy. At the same time, a large amount of gas bubbles generated in the reaction may also provide the nucleation and agglomeration centers for the nanocrystals, which is favorable for the formation of hollow nanoboxes by Ostwald ripening. While lead(II) acetate trihydrate served as a precursor with other experimental parameters unchanged, no gaseous bubbles were generated during whole reaction process and the final products were irregular solid PbSe nanocrystals as shown in Figure 6. This result accords with the preparation of solid PbSe pyramidal nanocrystals using lead(II) acetate as a precursor.4 Thus, as elucidated above, the production of gas bubbles is helpful for the formation mechanism of hollow PbSe nanoboxes. Figure 7 shows the optical diffuse reflection spectra of both the hollow PbSe nanospheres and nanoboxes in order to resolve the excitonic or interband (valence-conduction band) transitions, which allows us to calculate their band gaps. The spectral envelopes are clearly the summations of a number of subspectra. The estimates of the optical band gap (Eg) can be obtained using the following eq 4 for a semiconductor: RðvÞ ¼ Aðh0 v=2 -EgÞm=2

ð4Þ

where A is a constant, R is the absorption coefficient, and m is equal to 1 for a direct allowed transition. Since R is proportional to F(R), the Kubelka-Munk function, the energy intercept of a plot of (F(R)*hv)2 versus hv yields Eg for a direct allowed transition when the linear region is extrapolated to zero ordinate.18 From the spectra (insets to panels A and B in Figure 7), the band gaps of the PbSe nanospheres and nanoboxes are calculated to be 0.79 and 0.83 eV, respectively. Both are larger than those of the reported values for bulk materials (0.29 eV for PbSe).19 The increase in the band gaps of the as-prepared materials are indicative of size quantization effects that lead to a series of discrete states in the conduction and valence bands.20 On the basis of the above results, we propose that the gas bubbles assist in the Ostwald ripening process to form hollow PbSe nanoboxes, similar to the preparation of hollow Fe3O4 nanospheres.21 The whole process is illustrated in Figure 8. At an earlier stage, a large amount of gas bubbles generated in the reaction may provide nucleation centers for the freshly formed crystalline nanoparticles. Simultaneously, the minimization of nanoparticles’ interfacial energy causes their self-aggregation followed by the formation of solid metastable spheres. These kinds of uniform metastable spheres can be obtained in large quantities by limiting the reaction time to 3 h (150 °C) or 0.5 h (180 °C). At this stage, the crystallites are located in the inner

Figure 8. Schematic illustration of the possible formation process of hollow PbSe nanoboxes.

cores, compared to those in the exteriors, and they have a smaller crystallite size and higher surface energies because the initial high supersaturation results in a faster nucleation rate and the production of more crystallites with smaller sizes.22 Thus, during the evacuation of the solid spheres, the large crystallites located in the outer surface serve as crystal seeds for the subsequent recrystallization process and durative crystal growth. As a result, the outer crystallites become larger at the expense of the smaller crystallites, which results in a vacant space and thus the formation of hollow nanospheres. Finally, after prolonging the reaction further to 6 h, the cores within the hollow spheres are consumed completely and hollow nanoboxes with a cubic shape are formed. Considering the inner space occupied by the gas bubbles, this solid evacuation process is faster than the general ripening process. Conclusions In summary, we demonstrated that the single-crystalline PbSe hollow nanoboxes were created by a simple one-pot strategy. Using this method, nucleation and growth of hollow nanocrystals can be carried out in TOP solution without the presence of nucleation initiators. The experimental results indicate that the surfactant TOP plays important multiple roles in the formation of hollow structures. Moreover, the selection of lead nitrate as the metal precursors is critical to the formation of hollow nanocrystals. Thus, through controlling the reaction parameter, the interior-cavity size of the PbSe hollow nanoboxes can be adjusted. The formation of the hollow nanoboxes probably is through a gas-bubble assisted Ostwald ripening process. We envisage that this simple method is general and can be applied to the synthesis of hollow structures of many other semiconductors. Acknowledgment. This work was supported by the National Nature Science Foundation of China (No. 20975012) and the 111 Project (B07012). Supporting Information Available: The 31PNMR spectrum of reaction solution obtained at 150 °C for reaction 1 min (Figure S1). This material is available free of charge via the Internet at http:// pubs.acs.org.

1262

Crystal Growth & Design, Vol. 10, No. 3, 2010

Chen et al.

References (1) (a) Pietryga, J. M.; Schaller, R. D.; Werder, D.; Stewart, M. H.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2004, 126, 11752. (b) Peng, X. G.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (2) (a) Efros, Al. L.; Efros, A. L. Sov. Phys. Semicond. 1982, 16, 772. (b) Chen, M.; Xie, Y.; Lu, J. C.; Zhu, Y. J.; Qian, Y. T. J. Mater. Chem. 2001, 11, 518. (c) Leitsmann, R.; Bechstedt, F. ACS Nano 2009, DOI: 10.1021/nn900987j. (3) (a) Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2004, 92, 186601/1. (b) Luque, A.; Marti, A.; Nozik, A. J. MRS Bull. 2007, 32, 236. (c) Trinh, M. T.; Houtepen, A. J.; Schins, J. M.; Hanrath, T.; Piris, J.; Knulst, W.; Goossens, A. P. L. M.; Siebbeles, L. D. A. Nano. Lett. 2008, 8, 1713. (4) Khanna, P. K.; Jun, K. W.; Gokarna, A.; Baeg, J. O.; Seok, S. Mater. Chem. Phys. 2006, 96, 154. (5) Cho, K.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (6) Tong, H.; Zhu, Y. J.; Yang, L. X.; Li, L.; Zhang, L. Angew. Chem., Int. Ed. 2006, 45, 7739. (7) Lu, W. G.; Fang, J. Y.; Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2005, 109, 19219. (8) Cao, H. L.; Gong, Q.; Qian, X. F.; Wang, H. L.; Zai, J. T.; Zhu, Z. K. Cryst. Growth Des. 2007, 7, 425. (9) Kovalenko, M. V.; Talapin, D. V.; Loi, M. A.; Cordella, F.; Hesser, G.; Bodnarchuk, M. I.; Heiss, W. Angew. Chem., Int. Ed. 2008, 47, 3029. (10) (a) Venkatasubramaniam, R.; Siivola, E.; Colpitts, T.; O’Ouinn, B. Nature 2001, 413, 597. (b) Schuth, F. Annu. Rev. Mater. Res. 2005, 35, 209. (c) Harman, T. C.; Taylor, P. J.; Walsh, M. P.; Laforge, B. E. Science 2002, 297, 2229. (d) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (e) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (f) Velev, O. D.; Kaler, E. W. Adv. Mater. 2000, 12, 531. (g) Ovsyannikov, S. V.;

(11) (12)

(13) (14) (15) (16) (17) (18) (19) (20)

(21) (22)

Shchennikov, V. V.; Ponosov, Y. S.; Gudina, S. V.; Guk, V. G.; Skipetrov, E. P. J. Phys. D 2004, 37, 1151. (h) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (i) Im, S. H.; Jeong, U.; Xia, Y. Nat. Mater. 2005, 4, 671. (a) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (b) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707. (a) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (b) Peng, S.; Sun, S. H. Angew. Chem., Int. Ed. 2007, 46, 4155. (c) Kim, S. J.; Ah, C. S.; Jang, D. J. Adv. Mater. 2007, 19, 1064. (d) Fan, H. J.; Gosele, U.; Zacharias, M. Small 2007, 3, 1660. (a) Zeng, H. C. J. Mater. Chem. 2006, 16, 649. (b) Zeng, H. C. Curr. Nanosci. 2007, 3, 177. (c) Li, J.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 15839. (a) Camargo, P. H. C.; Lee, Y. H.; Jeong, U.; Zou, Z. Q.; Xia, Y. N. Langmuir 2007, 23, 2985. (b) Zhu, W.; Wang, W. Z.; Shi, J. L. J. Phys. Chem. B 2006, 110, 9785. Zhang, G. Q.; Wang, W.; Yu, Q. X.; Li, X. G. Chem. Mater. 2009, 21, 969. Wang, W. Z.; Poudel, B.; Wang, D. Z.; Ren, Z. F. Adv. Mater. 2005, 17, 2110. Shi, L.; Xu, Y. M.; Li, Q. Cryst. Growth Des. 2009, 9, 2214. Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650. Streltov, E. A.; Osipovich, N. P.; Ivashkevich, L. S.; Lyakhov, A. S.; Sviridov, V. V. Electrochim. Acta 1998, 43, 869. (a) Pejova, B.; Najdoski, M.; Grozdanov, I.; Dey, S. K. Mater. Lett. 2000, 43, 269. (b) Gorer, S.; Hodes, G. J. Phys. Chem. 1994, 98, 5338. (c) Gorer, S.; Yaron, A. A.; Hodes, G. J. Phys. Chem. 1995, 99, 16442. (d) Mikhailovskij, I. M.; Sadanov, E. V.; Mazilova, T. I.; Ksenofontov, V. A.; Velicodnaja, O. A. Phys. Rev. B 2009, 80, 165404. Hu, P.; Yu, L. J.; Zou, A. H.; Guo, C. Y.; Yuan, F. L. J. Phys. Chem. C 2009, 113, 900. Liu, B.; Zeng, H. C. Small 2005, 1, 566.