J. Phys. Chem. C 2007, 111, 2953-2958
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Controlled Synthesis of β-AgI Nanoplatelets from Selective Nucleation of Twinned Ag Seeds in a Tandem Reaction Choon Hwee Bernard Ng and Wai Yip Fan* Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed: NoVember 7, 2006; In Final Form: December 27, 2006
Recent research has established the influence of seed nature in determining the final shapes of metallic (Ag, Au) nanocrystals. In this work, we utilize and extend this knowledge to produce anisotropic β-AgI nanoplatelets in high yield: Ag seeds (∼20 nm) with twin defects were first selectively nucleated and then reacted with Iin a one-pot tandem reaction. I- performs multiple roles of oxidative dissolution regulator and reactant and surface stabilizing agent, thereby reducing the complexity of the synthesis. Examination of the nanocrystals at different stages of the reaction via electron microscopy confirmed that the AgI nanoplates grew from twinned Ag seeds and the initial Ag seed formation step was found to be vital to the appearance of the anisotropic platelike AgI nanocrystals. In general, the reaction of preformed metallic seeds with other reactive species may provide a convenient channel for the shape-controlled production of semiconductor nanostructures, with the final shapes being determined by the structure (twinning) of the seed. Aging of the reaction mixture in the presence of light resulted in the dissolution of AgI nanoplates to form Ag dendritic nanostructures. We explained its growth based on a diffusion-limited aggregation model.
1. Introduction Architectural control of nanocrystals with well-defined shapes is desirable since it is known that their physical and chemical properties are strongly influenced by their size and shape.1,2 The ability to control the shape of nanocrystals is not only of technological interest, but access to well-defined nanostructures is also essential for unraveling novel intrinsic properties effected by shape. As illustrated by several groups, a convenient channel for the shape-controlled preparation of semiconductor nanocrystals is by reacting metallic templates with reactive species to yield a variety of nanostructures that adopts the shapes of the templates.3,4 However, such a synthesis would inevitably necessitate the preliminary synthesis of metallic templates with well-defined shapes, which can be tedious. We are therefore interested in exploring other simpler alternatives to achieve shape-controlled growth of semiconductor nanocrystals. Recently, several illuminating pieces of work have highlighted the pivotal role(s) of the seed nature in determining the shape of metallic nanocrystals:5 In particular, Xia et al. reported the exquisite control of Ag nanocrystal shapes by employing halides or Fe(II)/Fe(III) cations to regulate the oxidative etching process during nucleation, producing Ag seeds with single crystalline, single-twinned, and multiply twinned structures which grow into nanocubes,6 right bipyramids,7 and nanowires.8 In this work, we utilize and extend this knowledge in a pioneering attempt to produce anisotropic β-AgI nanoplatelets from spherical Ag seeds (∼20 nm). In the synthesis, shape control is believed to be achieved by the selective nucleation of twinned, spherical seeds, which directs the subsequent growth and formation of AgI triangular nanoplates, instead of the traditional approach of employing preformed metallic templates with the desired shapes. Transmission electron microscopy (TEM) monitoring of the growth process reveals the initial formation of twinned * To whom correspondence should be addressed. E-mail: chmfanwy@ nus.edu.sg. Fax : 6567791691.
Ag seeds, followed by a tandem reaction with I- species in solution. As shown in tandem organic reactions, the ability to carry out a series of reactions without a need for the addition of new reactants or a change in reaction conditions can offer numerous advantages including reduced labor, resources, and time.9 Silver iodide (AgI) is a direct-gap semiconductor that exhibits a rich phase diagram:10 Under ambient conditions, it may exist in two distinct phases,11 (1) wurtzite β-AgI with a hexagonal close-packed (hcp) lattice and (2) zinc-blende γ-AgI with a facecentered cubic (fcc) lattice. At 419 K, β-AgI undergoes a firstorder phase transition into the superionic R-phase, in which iodine ions form a body-centered cubic (bcc) lattice, with highly mobile Ag+ ions randomly distributed through the equivalent interstices. AgI has drawn much attention as a model compound for the study of solid-state ionic transport following the discovery of anomalously large Ag+ conductivity in the R-phase12 and the possibility for applications in solid-state battery and electrochemical sensing systems.13-16 Several synthetic routes to nanocrystalline AgI have been developed: AgI nanoparticles have been synthesized via wet chemical methods by employing stabilizing agents like gelatin,17 organic polymers,18-21 and polyelectrolytes.22 Laser-based methods23 and alumina membrane templating24 have also been successful in production of AgI particles and AgI nanowire arrays. 2. Experimental Section Synthesis of AgI Nanoplatelets. The reagent-grade chemicals were obtained from Sigma Aldrich and used without further purification. Sodium borohydride (20 mL, NaBH4, 6 × 10-3 M) was added dropwise to a 20 mL aqueous solution of silver nitrate (AgNO3, 10-3 M) and potassium iodide (KI, 10-3 M) under ambient conditions. The mixture was stirred vigorously for 30 min and then allowed to age in the dark for 24 h. The particles were then purified by centrifugation at
10.1021/jp0673260 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/31/2007
2954 J. Phys. Chem. C, Vol. 111, No. 7, 2007
Ng and Fan
Figure 1. (A) UV-visible absorption spectra of reaction mixture at various reaction times: (I) t ) 1 h; (II) t ) 8 h; (III) t ) 20 h. Inset: Photo of AgI colloid which exhibits a pink color. (B) Twinned Ag seeds (as indicated by arrows), (C) mixture of AgI nanocrystals and Ag seeds, (D) AgI nanoplatelets.
4000 rpm for 20 min and redispersed in water to give a pink solution. For the formation of dendrites, the solution was allowed to age in the presence of light for up to 3 days. Instrumentation. The progress of the oxidation was followed by a UV-visible absorption spectrometer (Shimadzu UV-2550). Aliquots of the reaction mixture were retrieved at various oxidation times for TEM and energy dispersive X-ray (EDX) analyses. The TEM images (JEOL-2010 or JEOL-3010) were taken of direct sampling of the solution on carbon-coated copper grids, and the EDX spectra were taken during TEM (JEOL3010) imaging process. XRD patterns were recorded from a powder AgI sample supported on a glass slide using SIEMENS D5005 diffractometer (Cu KR λ ) 0.15418 nm) at a scanning rate of 0.01° s-1 for 2θ in the range from 20 to 70°. 3. Results and Discussion The essence of our synthesis lies in the reduction of an AgNO3-KI mixture, leading to the production of ∼20 nm Ag nanoparticles, which serves as the substrate (seeds) for the reaction with I- to form AgI nanoplatelets. Spectroscopic and TEM Monitoring of Growth Process. UV spectroscopy and TEM monitoring of the reaction progress revealed the formation of Ag seeds and AgI nanoplates in two
separate but tandem reactions. Addition of NaBH4 led to the appearance of two absorption bands centered at 388 and 225 nm, which corresponds to the surface plasmon resonance (SPR) band of Ag nanoparticles and the absorption of free Iin solution (Figure 1A). After aging for in the dark for t ) 8 h, a weak exciton absorption peak emerges at 427 nm, commensurate with the formation of AgI nanoparticles. The Ag SPR band was extinguished after t ) 20 h, and the final spectrum reveals four bands at 429, 335, 279, and 240 nm which can be assigned to W3, W2, and W1 excitons of β-AgI.25 The reaction was accompanied by a color change from brown (Ag seeds) to pink (AgI nanoplatelets). Parts B-D of Figure 1 show the images of the sample taken from the reaction mixture after t ) 1, 8, and 20 h, respectively. Because of the strong reducing ability of NaBH4, ∼20 nm spherical Ag nanoparticles were formed after t ) 1 h and TEM images show that the seeds are characterized by multiple twin planes (as indicated by arrows in Figure 1B. During aging, AgI nanoparticles formed from the initial ambient oxidation of Ag0 atoms followed by the reaction of Ag+ with I-, which grew via atomic addition and/or aggregation. It may be initially puzzling how Ag0 may be oxidized in a reductive environment, but that may be rationalized by the depletion of excess NaBH4 via its well-known reaction
Controlled Synthesis of β-AgI Nanoplatelets
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2955
Figure 2. (A) TEM image of β-AgI nanoplatelets, (B) SAED pattern of the nanoplatelets which consists of rings that could be indexed to β-AgI, (C) size distribution (edge-length) histograms of β-AgI nanoplatelets, (D) EDX, and (E) XRD of β-AgI nanoplatelets. The Cu signals of the EDX plots belong to the copper grid used for nanoparticle deposition. The XRD peaks are indexed to β-AgI.
with water, producing H2 and metaborate ion.26 Hence the reducing environment only persists in the initial stages. At t ) 20 h, no Ag seeds can be observed and the reaction mixture consisted of only AgI nanoplatelets (Figure 1D). Structural Analysis of the AgI Nanoplatelets. TEM analysis of the products revealed that a large proportion (∼80%) of the nanocrystals exhibits a triangular platelet morphology, with the remaining crystals appearing as polygonal plates and nanorods/ wires (Figure 2A). It is important to point out that AgI decomposes rapidly under the electron beam and thus the samples were viewed under low magnifications and electron intensities. All images were acquired of the nanocrystals in less than 5 seconds of exposure to the electron beam to minimize the extent of decomposition. As shown in Figure 2B, selected area electron diffraction (SAED) of the nanoplatelets gave rings that could be indexed to Miller planes of wurtzite β-AgI. Statistical analysis of the sizes of the nanocrystal (Figure 2C), obtained from counting ∼100 nanoparticles from different parts of the grids, indicate that the triangular nanoplates have an average edge length of 125 nm. The identity of the triangular nanoplates was confirmed to be hexagonal β-AgI by EDX and XRD. EDX analysis gave peaks due to Ag and I only, and elemental ratios of Ag:I of unity is in good agreement well with the stoichometric ratio (Figure 2D). The Cu signals observed in the spectrum belong to the Cu grids used for nanoparticle
Figure 3. (A) TEM image of a single triangular nanoplate, (B) SAED aligned perpendicular to the nanoplate surface revealing a (001) pattern of hexagonal AgI.
deposition. Powder XRD patterns of the as-prepared samples (Figure 2E) revealed the formation of a single hexagonal phase β-AgI with a wurtzite structure (JCPDF #00-003-0940). Diffraction peaks arising from other AgI phases or metallic Ag could not be observed in the XRD pattern. In addition, the high intensity of the (002) diffraction peak suggests that the nanoplates are mainly dominated by (002) facets, and the (002) planes tend to be preferentially oriented parallel to the surface of the supporting substrate. TEM image of a single triangular nanoplate (Figure 3A) reveals its equilateral symmetry and that the edges of the nanoplates were slightly truncated. The electron diffraction pattern obtained by aligning the electron beam perpendicular
2956 J. Phys. Chem. C, Vol. 111, No. 7, 2007
Figure 4. TEM images of: (A) Ag dendrites, (B) a single Ag dendrite, (C) SAED pattern of the dendrites which gave a (110) pattern of fcc Ag, (D) HRTEM of the dendrite revealing a (110) lattice orientation. Similar images were observed in other sections of the dendrites, (E) EDX of Ag dendrites. The Cu signals of the EDX plots belong to the copper grid used for nanoparticle deposition.
to the planar surface shows a 6-fold rotational symmetry of the diffraction spots (Figure 3B), which indicates that the surface is single crystalline with a (002) lattice plane as the basal plane. Unfortunately, direct evidence by HRTEM was not possible due to the susceptibility of AgI toward decomposition under the
Ng and Fan electron beam. Two sets of spots can be identified based on their d spacing: spacings of 3.91 and 2.24 Å are due to the (100) and (110) Bragg reflections of β-AgI, respectively. Structural Analysis of the Ag Dendrites. TEM analysis of AgI colloids after an aging period of ∼3 days revealed the formation of dendritic nanostructures around the nanoplates (parts A and B of Figure 4). The relative stability of the dendrites toward the electron beam hints that it could be composed of Ag instead of AgI. As shown in Figure 4C, SAED encompassing the fully grown dendrites yielded a regular pattern of diffraction spots that suggests that the dendrites are characterized by (110) facets parallel to the substrate. HRTEM imaging confirmed that the dendrites were oriented with their (110) planes preferentially exposed (Figure 4D). Similar lattice patterns were observed throughout the dendrite with different sections being displaced by 1-2° to one another. This accounts for the observation that the diffraction spots were slightly smeared. EDX analysis by focusing the electron beam on the dendrites (without sampling the residual AgI nanocrystals) gave peaks due to Ag only, which confirmed that the dendrites are composed of Ag. However, we were unable to detect Ag in the UV spectrum and XRD patterns of the aged samples. This could be due to the relatively low Ag concentration in the samples and the inability of these methods for local sampling as in TEM and EDX. Effect of Seed Nature and Role of Iodide. The growth mechanism of anisotropic nanostructures has been primarily explained in terms of the selective adsorption of surfactants or capping agents that regulate the growth of the crystal in a particular direction.27-29 However, the nucleation and kinetics of the growth mechanism have drawn much attention, in addition to thermodynamics or physical restrictions imposed by the surface stabilizing agent. Lofton and Sigmund first proposed a growth mechanism which suggests that the formation of twin planes promotes the creation of favorable re-entrant sites for further growth, leading to anisotropic growth.30 Recent research has established that, for the solution-phase synthesis of nanocrystals, the structure of the seeds determines the morphology of the final product, which highlights that the kinetics of growth and the structure of the nuclei are critical in determining the habit of the final product. Control of the seed structure was first achieved by Xia et al. In the polyol synthesis, the addition of Cl-, Br-, or Fe3+ leads to the formation of single-crystalline, single-twinned, and multiple-twinned Ag seeds which grows to produce nanocubes, right bipyramids, or nanowires of silver, respectively.5-8 This phenomenon was explained by the influence of the additives on the extent of oxidative dissolution of the Ag nuclei. They found that nanoparticles with twin defects
Figure 5. TEM images of (A) a single-twinned seed and (B) a triple-twinned seed that were commonly found for the Ag nuclei produced.
Controlled Synthesis of β-AgI Nanoplatelets
Figure 6. TEM images of: (A) AgI nanorods/wires found in the reaction mixture and (B) AgI cluster prepared from the direct mixing of AgNO3 and KI. Inset: ED of the nanocluster revealing polycrystalline rings, which can be indexed to β-AgI.
are preferentially etched in the presence of Cl-, Br-, and air, leading to high yields of single-crystal and single-twinned seeds. Without the addition of Cl- or Br-, 100-300-nm multiply twinned particles were formed. In this work, we utilize the findings of these researches to design a synthetic pathway to anisotropic AgI nanostructures using twin Ag seeds as substrates to initiate anisotropic growth. The elegance of our synthesis lies in the use of I- as an oxidative etching regulator for the selective nucleation of twinned seeds and subsequently as a reactant to effect a tandem reaction to form AgI nanoplatelets. Furthermore, the ability of I- to serve as a stabilizing agent overcomes the need for the introduction of an external capping agent, which greatly reduces the complexity of the reaction. Twins (stacking faults) are commonly formed for Ag and Au due to their low stacking fault energy and appear as a result of atoms attaching erroneously to a growing crystal such that two crystals appear to be growing out of each other during crystal growth.31 The presence of twin planes is evident in HRTEM imaging of the Ag seeds (parts A and B of Figure 5), which adopts single- and triple-twinned structures, formed from contact lamellar or cyclic twinning during the nucleation process. The appearance of many singleand triple-twinned seeds may be due to the fact that I- is less corrosive than Cl- and Br-. Hence, I- enables sufficient etching to eliminate the seeds with multiple twin defects but not so much as to form single-crystalline seeds. The I--induced formation of twinned Ag seeds was followed by a tandem reaction with the free I- in solution. We believe that the twinning observed in the Ag seeds could have directed the formation of AgI nanoplatelets in a manner similar to previously reported formation mechanisms of metallic nanoplates.32,33 The stacking fault of the twin planes may lead to the appearance of concave and convex facets; atoms added to the convex facets have limited stability due to the presence of only three nearest atomic neighbors and likely to redissolve into the reaction media. Because of extra stability arising from an increase in the number of nearest atomic neighbors, atoms would
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2957 preferentially attach to the re-entrant grooves present in concave facets, leading to accelerated growth along these sites. The hexagonal platelets observed in the system might be explained as nuclei for triangular platelets whose growth was inhibited due to the exhausted amount of monomers while the nanorods/ wires (Figure 6A) could have grown from the small amounts of five-twinned seeds that survive the I--catalyzed oxidative etching. The ability of I- to act as a stabilizing agent has been reported for the synthesis of Cu colloids34 and the formation of various Cu2O nanostructures.35 It is believed that the surface adsorbed I- and the counter ions (Na+ or K+) creates an electrical double layer, which is responsible for generating a repulsive interaction between the Cu nanoparticles, affording electrostatic stabilization against aggregation. To consolidate the fundamental influence of the Ag seed structure in directing the formation of AgI nanoplatelets, we observed the morphology of the AgI nanocrystals by simply mixing AgNO3 and KI. As shown in Figure 6B, the dropwise addition of KI to a AgNO3 solution resulted in the formation AgI clusters with no definite morphology. This confirms that the preformation of twinned Ag seeds was vital to the appearance of the AgI nanoplatelets. The success of this attempt to dictate the shape of a semiconductor nanocrystal via control of the structure of preformed metallic seeds could stimulate research in this aspect. In a general sense, the reaction of preformed metallic seeds with other reactive species (e.g., halides, chalcogenides) might provide a convenient channel for the shape-controlled production of semiconductor nanostructures, with the final shapes being determined by the structure (twinning) of the seed. To investigate the effect of surfactant concentration on the AgI nanocrystal shapes, control experiments were performed in which the ratio (R) of [KI]:[AgNO3] was varied, while carefully keeping other reaction parameters the same. When R < 0.1, Ag nanoclusters with no definite morphology were observed (Figure 7A), which could have formed due to insufficient I- for the reaction with Ag seeds to form AgI nanocrystals. When R was varied from R ) 1 (Figure 7B) to R ) 10 (Figure 7C), the product morphology was observed to be invariant, suggesting that a change in iodide concentration was inadequate to effect a change in particle morphology. However, it is noted from the observation of nanocrystal sizes for R ) 1.0, 5.0, and 10 that an increase in I- concentration correlates with the formation of larger nanoplates. This is not unexpected considering that I- is likely to be the limiting reagent (due to its multiple roles) and an increase in its concentration would lead to enhanced growth. Formation Mechanism of Dendrites. The AgI nanoplatelets were found to be stable for more than 8 weeks when kept in the absence of light, with no observable change in its morphol-
Figure 7. TEM images of: (A) Ag nanoclusters obtained when R ([KI]/[AgNO3]) ) 0.1. Inset: ED of the nanocluster revealing polycrystalline rings, which can be indexed to fcc Ag, (B) AgI nanoplatelets (∼125 nm) obtained when R ) 1.0, and (C) AgI nanoplatelets (∼400 nm) obtained when R ) 10.
2958 J. Phys. Chem. C, Vol. 111, No. 7, 2007 ogy and chemical composition. However, on aging in the presence of light, Ag dendrites were observed to form. We have previously found that the decomposition of a colloidal solution of Ag2Se nanowires resulted in the appearance of dendrites and explained its formation based on a diffusion-limited aggregation (DLA) model.36 We believe that a similar mechanism is responsible for the appearance of dendrites in this work. DLA was originally introduced by Tom Witten and Len Sander as a model for irreversible colloidal aggregation and was found to be applicable to a variety of physical phenomenon including bacterial colonies, viscous fingering, and electrochemical deposition.37-39 In the DLA model, particles are released one by one from sites arbitrarily far from a central cluster and sticking irreversibly at first contact with the growing cluster. During aging in the presence of light, AgI will decompose giving rise to Ag nanoparticles over a period of 3 days. A slow decomposition process would mean that the concentration of Ag nanoparticles in solution at any one time is low. Hence, aggregation may be perceived to occur one particle at a time. The initial particles diffuse randomly in solution and adhere irreversibly on contact with seed particles believed to be the numerous nucleation sites on surface of the partially decomposed nanoplates. Subsequent diffusing nanoparticles may stick to any part of the cluster where it first strikes but are more likely to encounter the tips of the cluster than to penetrate deep into its inner regions. This causes the dendrites to be formed, which grow outward from initial location of seed particles. It is initially puzzling that a regular electron diffraction pattern should be obtained from the dendrites since it is intuitive that the dendrites, which form primarily due to the inability of particles to rearrange, should be polycrystalline. In fact, the dendrites reported in our previous work were determined to be polycrystalline.36 We believe that the explanation for this phenomenon lies in the nature of the surface adsorbing agent used. In the previous case, PVP was employed as the adsorbing agent which coordinates strongly via chemisorption by the presence of numerous coordinating groups (CdO) on the polymer. Hence, the rearrangement of Ag nanoparticles in the dendrites was not possible, giving rise to its polycrystalline nature. On the other hand, the surface adsorbing I- used in this work functions via electrostatic interactions and interparticle attractions are therefore not expected to be strong. This permits the diffusive rearrangement process similar to annealing, which results in the dendrites having uniform single-crystalline facets. The observation that the computer simulations for DLA model with sticking coefficients of 1.0 and 0.5 generates structures that resemble closely to the Ag(PVP) dendrites and Ag(I-) dendrites, respectively, further support our proposal.40 The results of both work suggests that the decomposition of photosensitive semiconductor nanocrystals in solution could present a pathway to the synthesis of dendritic metal nanostructures. 4. Conclusion We have shown that reaction of preformed multiple-twinned Ag seeds with I- could lead to the formation of anisotropic AgI nanoplatelets. The selective nucleation of twinned Ag seeds (∼20 nm) and the formation of AgI nanoplate occur via two separate but tandem reactions. In addition, the ability of I- to perform multiple roles of oxidative regulator and reactant and surface stabilizing agent effects the simplification of the entire synthesis. In the synthesis, the initial Ag seed formation step was found to be crucial to the formation of the AgI nanoplates. This work may pioneer the development of methods for the
Ng and Fan shape-controlled syntheses of semiconductor nanostructures based on the reaction of preformed metallic seeds with other reactive species. Photodecomposition of the AgI nanoplates led to the formation of Ag dendritic nanostructures with uniform single-crystalline exposed facets. The DLA model was used to describe and analyze its formation. Acknowledgment. The project was supported by a National University of Singapore research grant under Grant No. 143-000-298-112. We thanked B. Liu, G. L. Loy, and P. L. Chong for their help in TEM microscopy. References and Notes (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Markovich, G.; Collier, C. P.; Henrichs, S. E.; Remacle, F.; Levine, R. D.; Heath, J. R. Acc. Chem. Res. 1999, 32, 415. (3) Gates, B.; Myers. B.; Wu, Y.; Sun, Y.; Cattle, B.; Yang, P.; Xia, Y. AdV. Funct. Mater. 2002, 12, 679. (4) Cao, H.; Qian, X.; Wang, C.; Ma, X.; Yin, J.; Zhu, Z. J. Am. Chem. Soc. 2005, 127, 16024. (5) Wiley, B. J.; Im, S. H.; Li, Z.; McLellan, J.; Siekkinen, A.; Xia, Y. J. Phys. Chem. B 2006, 110, 15666. (6) Wiley, B. J.; Herricks, T.; Sun, Y.; Xia, Y. Nano Lett. 2004, 4, 1733. (7) Wiley, B. J.; Xiong, Y.; Li, Z.; Yin, Y.; Xia, Y. Nano Lett. 2006, 6, 765. (8) Wiley, B. J.; Sun, Y.; Xia, Y. Langmuir 2005, 21, 8077. (9) Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G. Angew. Chem., Int. Ed. 2006, 45, 7134. (10) Keen, D. A.; Hull, S.; Hayes, W.; Gardner, N. J. Phys. ReV. Lett. 1996, 77, 4914. (11) Nagai, M.; Nishino, T. Solid State Ionics 1999, 117, 317. (12) Cava, R. J.; Rietman, E. A. Phys. ReV. B 1986, 30, 6896. (13) Ida, T.; Saeki, H.; Hamada, H.; Kimura, K. Surf. ReV. Lett. 1996, 3, 41. (14) Ida, T.; Kimura, K. Solid State Ionics 1998, 107, 313. (15) Guo, Y.; Lee, J.; Maier, J. AdV. Mater. 2005, 17, 2815. (16) Guo, Y.; Hu, Y.; Lee, J.; Maier, J. Electrochem. Commun. 2006, 8, 1179. (17) Berry, C. R. Phys. ReV. 1967, 161, 848. (18) Chen, S.; Ida, T.; Kimura, K. J. Phys. Chem. B 1998, 102, 6169. (19) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, 6435. (20) Tanaka, T.; Saijo, H.; Matsubara, T. J. Photogr. Sci. 1979, 27, 60. (21) Saijo, H.; Iwasaki, M.; Tanaka, T.; Matsubara, T. Photogr. Sci. Eng. 1982, 26, 92. (22) Guo, Y.; Lee, J.; Maier, J. Solid State Ionics 2006, 177, 2467. (23) Tan, H.; Fan, W. Y. Chem. Phys. Lett. 2005, 406, 289. (24) Wang, Y.; Mo, J.; Cai, W.; Yao, L.; Zhang, L. J. Mater. Res. 2001, 16, 990. (25) Cardona, M. Phys. ReV. 1967, 161, 848. (26) Schlesinger, H. I.; Brown, H. C.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra, H. R.; Hyde, E. K. J. Am. Chem. Soc. 1953, 75, 215. (27) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (28) Pastoriza-Santos, I.; Liz-Marzan, L. M. Nano Lett. 2002, 2, 903. (29) Gou, L.; Murphy, C. J. Nano Lett. 2003, 3, 231. (30) Lofton, C.; Sigmund, W. AdV. Funct. Mater. 2005, 15, 1197. (31) Dahmen, U.; Hetherington, C.; Radmilovic, V.; Johnson, E.; Xiao, S.; Luo, C. Microsc. Microanal. 2002, 8, 247. (32) Elechiguerra, J. L.; Reyes-Gasgab, J.; Yacaman, M. J. J. Mater. Chem. 2006, 16, 3906. (33) Salzemann, C.; Urban, J.; Lisiechi, I.; Pileni, M. P. AdV. Funct. Mater. 2005, 15, 1277. (34) Kapoor, S.; Joshi, R.; Mukherjee, T. Chem. Phys. Lett. 2002, 354, 443. (35) Ng, C. H.; Fan, W. Y. J. Phys. Chem. B 2006, 110, 20801. (36) Ng, C. H.; Tan, H.; Fan, W. Y. Langmuir 2006, 22, 9712. (37) Witten, T. A.; Sander, L. M. Phys. ReV. Lett. 1981, 47, 1400. (38) Ben-Jacob, E.; Shmueli, H.; Shochet, O.; Tenenbaum, A. Physica 1992, 187, 378. (39) Daccord, G.; Nittmann, J.; Stanley, H. E. Phys. ReV. Lett. 1986, 56, 336. (40) Meakin, P. Phys. ReV. A 1983, 27, 1495.
