Structural Defects and Their Role in the Growth of Ag Triangular

J. Phys. Chem. C 2007, 111, 6989-6993

6989

Structural Defects and Their Role in the Growth of Ag Triangular Nanoplates Tu´ lio C. R. Rocha†,‡ and Daniela Zanchet*,† Laborato´ rio Nacional de Luz Sı´ncrotron, C.P. 6192, 13083-970 Campinas (SP), Brazil, and IFGW-UniVersidade Estadual de Campinas, C.P. 6165, 13083-970 Campinas (SP), Brazil ReceiVed: January 12, 2007; In Final Form: March 2, 2007

The influence of structural defects in the formation of silver nanoplates synthesized by the photochemical seed-mediated method is addressed. The results show that anisotropic growth starts at the very beginning of the reaction by epitaxial anisotropic deposition of Ag atoms on defective seeds. Insights into the role of capping molecules and the minor role played by surface plasmon excitation regarding the origin of the anisotropy in this method are provided.

Introduction Control of the shape of metallic nanoparticles has been a subject of intensive research in the past few years because it provides an alternative means, in addition to size, for tailoring the properties of a metal. Shape effects are particularly important in the case of the optical properties of metal nanostructures. It has been demonstrated theoretically1 and experimentally2 that the control over both the size and shape of Au and Ag particles allows fine-tuning of the surface plasmon resonance (SPR) over the visible-near-infrared spectral range. The ubiquitous characteristics of surface plasmons to confine incident light as near fields in the surface of the metal and the high sensitivity of the resonance to small changes in the local environment have been exploited in several applications.3 These applications have been a primary driving force for the development and improvement of synthetic routes that produce particles with controlled sizes and shapes in high yields. Nevertheless, despite the efforts that have been made and the significant advances already achieved, several issues related to the mechanisms of size and shape control still remain unsolved. It is worth recalling that the external shape of a crystal is intimately related to its internal structure. This correspondence is valid for different crystals, from minerals to macromolecules and also supercrystals, formed by the crystallization of colloidal particles.4 Some recent works have more carefully addressed the influence of crystalline structure on the growth mechanism of anisotropic nanoparticles. As an example, the formation of Au nanorods synthesized by seed-mediated methods has been related to the unidirectional growth of multiply twinned decahedral particles.5 Willey et al.6 demonstrated the influence of the structure in Ag nanowire growth by a selective etching of the defective seeds that inhibited the formation of anisotropic particles. In the case of planar morphologies, Germain et al.7 suggested that structural defects observed in Ag truncated nanoplates could be related to formation of these particles. More recently, Lofton et al.8 applied the growth model of silver halide * To whom correspondence should be addressed. Address: LNLS Brazilian Synchrotron Light Laboratory, C.P. 6192, Campinas, SP, 13083970 Brazil, Tel.: +55 19 3512-1010. Fax: +55 19 3512-1004. E-mail: [email protected]. † Laborato ´ rio Nacional de Luz Sı´ncrotron. ‡ IFGW-Universidade Estadual de Campinas.

micrometer-sized particles to explain the influence of defects on the growth of nanoplates. Yacaman et al.9 also reviewed the subject and presented selected examples of different systems in which the shape is closely related to the internal structure of the crystals. However, additional experimental data on a broad set of systems are still required to confirm this correlation. In this work, we address the growth mechanism of Ag triangular nanoplates produced by photochemical growth by detailed structural characterization at the early stages of reaction. In this method, Ag seeds are irradiated by visible light in the presence of excess Ag+ and citrate ions.10-12 In a previous work,12 we pointed out that the excitation of the particles’ SPR determines the nanoplate size but does not appear to be the main factor determining the particle anisotropy at the initial stages. We show here that structural defects, namely, twins and stacking faults, play a major role in the growth mechanism determining the flat triangular morphology of the particles obtained by this method. It is worth remarking that photochemical synthesis provides ideal conditions for studying anisotropic growth because the reaction rate can be varied by the intensity and wavelength of the incident light and the reaction can be stopped at any moment by simply interrupting the irradiation. This allows a careful examination of intermediate steps even at the early stages of the synthesis. Experimental Section The silver triangular nanoplates were photochemically synthesized using a two-step seed-mediated approach. The synthesis methodology was adapted from the literature10 and is described in our previous work.12 Briefly, previously formed citratestabilized small spherical nanoparticles, known as seeds, are irradiated by visible light in the presence of an excess of silver nitrate and sodium citrate and anisotropically grow to form the nanoplates. The wavelength of the incident light controls the final size of the nanoplates, and in this work, we used incident wavelengths of either 500 or 600 nm (full width at halfmaximum of 80 nm). Transmission electron microscopy (TEM) characterization was performed with a JEOL JEM 3010 instrument. The samples were prepared by complete evaporation of 7-10 µL of nanoparticle solution over carbon-coated copper grids. Then, a

10.1021/jp0702696 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/20/2007

6990 J. Phys. Chem. C, Vol. 111, No. 19, 2007

Figure 1. Structural characterization of the final triangular nanoplates and initial seeds. HRTEM images of (a) a final nanoplate (inset shows the low-magnification image) and (b) an initial seed. X-ray diffractograms of (c) the nanoplates and (d) the initial seeds. See text for details.

droplet of pure water was deposited on the grids for 30 s and was carefully removed by capillarity using filter paper. This procedure was adopted to remove the excess of reactants deposited on the carbon and to minimize the agglomeration induced by evaporation. X-ray diffraction (XRD) patterns were measured using 1.7711 Å (7 keV) synchrotron radiation in a conventional reflection geometry. The samples were prepared by complete evaporation of 500-800 µL of the previously concentrated nanoparticle solution on a miscut Si substrate under vacuum. Ultraviolet-visible (UV-vis) extinction spectra were measured with an HP/Agilent model 8453 series spectrophotometer using quartz cuvettes with a 10 mm optical path. Results and Discussions Structural characterization by high-resolution TEM (HRTEM) of the final nanoplates confirmed that they all presented the same crystallographic structure, corresponding to an fcc lattice with the 〈111〉 direction perpendicular to the flat faces (Figure 1a and inset), that is independent of the incident wavelength used in the reaction. The lattice fringes observed in most of the HRTEM images of 〈111〉-oriented nanoplates showed a periodicity of 2.49 Å, as previously observed by other authors for nanoplates and nanodisks of Au and Ag produced by different methods.13 This periodicity also appears in the selected area diffraction (SAD) image, as faint spots close to the direct beam (see the Supporting Information, S1). The unusual periodicity revealed by HRTEM and the corresponding reflection in the SAD pattern have been proposed in the literature to be 3 × 〈422〉 lattice fringes associated with 1/3{422} forbidden reflections. The observation of these kinematically forbidden reflections has been attributed to different factors, such as the presence of atomically flat surfaces, fractionary unit cells, surface reconstructions, twin planes, and stacking faults parallel to the flat facets associated with dynamical effects of electron diffraction.13 Interestingly, we found that, in fact, many of the initial seeds already presented the same 2.49 Å lattice fringes (Figure 1b). Seed particles containing parallel planar defects in the 〈110〉 direction were also found (see S2 in the Supporting Information for additional images).

Rocha and Zanchet To better understand the crystallographic structure of both the formed nanoplates and the initial spherical seeds, XRD patterns were obtained. The intensity as a function of the modulus of the scattering vector, s ) 2 sin θ/λ ) 1/d (where d is the interplanar distance, θ is one-half of the scattering angle, and λ is the incident wavelength), is presented in Figure 1c,d. A peak at s ) 0.40 Å-1, which corresponds to the 2.49 Å periodicity in real space was observed for both samples, as evidenced by the circles in Figure 1c,d. The better definition of this peak in the case of the nanoplates can be associated with their larger size, flat surface, and also preferential deposition for lying flat on the substrate. These results show that the unusual periodicities measured by electron microscopy are not related to dynamical effects, which are not significant in the case of X-ray diffraction in this type of system. It can be demonstrated that the presence of twins and stacking faults in an fcc lattice (parallel planar defects in the 〈111〉 direction) generates hcp lamellas that leads to the appearance of extra reflections (see the structural scheme and simulated diffractograms in the Supporting Information, S3 and S4). In addition to the 0.40 Å-1 reflection, an additional weak shoulder at s ) 0.45 Å-1 can be seen in the XRD pattern of the initial seeds (marked with an arrow in Figure 1d), which corresponds to a periodicity of 2.22 Å. Both periodicities, 2.22 and 2.49 Å, are in agreement with an hcp lamellar region with a ) b ) r and c ) x6r (where a, b, and c are the hexagonal cell parameters and r is the first-neighbor separation in fcc Ag). The 2.49 Å lattice fringes in the HRTEM images or, equivalently, the 0.40 and 0.45 Å-1 peaks in XRD diffractograms can be used as fingerprints for the presence of defects in the particles. Bearing this in mind, the initial stages of the photochemical reaction were carefully studied by HRTEM. Two aliquots were taken at short times (few minutes) of the reaction using incident wavelengths of 500 and 600 nm (Ag500 and Ag600). We showed in a previous work12 that the photochemical growth can be characterized by two regimes: during the first, the SPR is not significantly excited, and the Ag+ consumption rate is slow (stage I), whereas during the second, the SPR is excited by the incident wavelength, and the Ag+ consumption rate increases by 1 order of magnitude (stage II). In this work, we analyzed the aliquots taken at early stages of reaction using these two different wavelengths and found similar results. It is important to remark that photochemical growth is wavelengthdependent in terms of the final size of the particles.10-12 In the beginning of the growth process, all of the observed particles appeared to be nearly spherical at first, resembling the initial seeds. However, a more detailed and careful analysis of both their structure and lateral faceting revealed that some of the particles were indeed small triangular nanoplates, at the beginning of their formation. Figure 2a shows two examples of well-aligned nanoplates observed in sample Ag500. The 〈111〉-oriented nanoplate on the left of Figure 2a presents a slight triangular faceting and 2.49-Å lattice fringes, due to the presence of defects. In the 〈110〉-oriented particle on the right of Figure 2a, the formation of small 〈111〉 facets can be observed. The core-shell contrast in both particle might be related to an anisotropic shell grown over the spherical seed. Figure 2b,c corresponds to the Ag600 sample and shows particles at the same orientations as presented in Figure 2a. In this case, welldefined nanoplates can be clearly observed, with the triangular projection for a 〈111〉-oriented particle (Figure 2b) and wellformed flat facets in the 〈110〉-oriented particle (Figure 2c). Moreover, an enlargement of the border of the nanoplate in

Growth of Ag Triangular Nanoplates

Figure 2. HRTEM images of nanoplates in the initial stages of the reaction and schematic representation of their position relative to the electron beam: (a) Ag500 and (b,c) Ag600. The scale bar is the same for all images. The inset in c shows an enlargement of the square marked region, in which is possible to visualize many planar defects.

Figure 2c (inset) confirms the presence of several defects parallel to the flat facets of the nanoplates. These HRTEM observations clearly show that, in the photochemical growth process, nanoplates are formed in the initial stages of the synthesis by anisotropic deposition of Ag atoms on the defective initial spherical seeds. It is important to mention that, in the case of the photoinduced conversion of spheres to nanoplates, where no excess Ag+ ions is added to the initial seeds, the mechanism of nanoplate formation is still subject to debate.14 The presence of planar defects both in the seeds and in the formed nanoplates suggests their influence in the anisotropic deposition of the atoms on the spherical seeds. Recently, the growth mechanism of micrometer-sized silver halide triangular particles has been used to support the importance of defects in the anisotropic growth of metal nanoparticles.8,9 However, much care needs to be taken in the simple extrapolation of this model to the case of metallic nanometer-sized particles, because of their much larger surface area-to-volume ratios, which make surface effects more pronounced. This is particularly important for the case of colloidal nanoparticles, in which molecules are attached to the surface of the particles to prevent agglomeration and to stabilize them in solution. As already shown by several groups,16 the surface chemistry of the seeds has a great influence in shape determination. In addition, in silver halide, the chemical bonds are ionic. Finally, the presence of single or double twinning events in this model is enough to lead to the formation triangular or hexagonal nanoplates, respectively. However, as shown in the inset of Figure 2c, the structure of Ag nanoplates can be very complex, with many twins and stacking faults. The

J. Phys. Chem. C, Vol. 111, No. 19, 2007 6991 additional reflections in the XRD patterns corroborate the presence of highly defective particles in these samples. To better understand the role of the crystalline structure and surface chemistry of the seeds, we performed the same photochemical synthesis using citrate-capped Au seeds instead of Ag. The photochemical reaction occurred, as evidenced by the changes in the UV-vis spectra, but the main final product consisted of several large faceted but isotropic particles and large decahedral particles, with the nanoplates being a rare minor product (Supporting Information, S5). These striking differences in the final product of the photochemical reaction can be expected if the crystalline structure plays a role, as explained below. The Au seeds were synthesized exactly in the same way as the Ag seeds and, hence, were capped with citrate molecules. The size distribution of the Au seeds was similar to that of the Ag seeds, but more importantly, the crystalline structure of the Au seeds was very different, with decahedra and icosahedra being the most abundant defective structures. It has been reported in the literature that Ag and Au, despite their similarities in the bulk, show different structural features on the nanometer scale.17,18 We observed that, whereas the Ag seeds predominantly presented parallel defects, the Au seeds presented defective particles with a 5-fold axis (multiply twinned particles, MTPs). Figure 3a shows an HRTEM image of some MTPs easily found in the Au seed sample. The same trend was previously observed for the case of alkanethiol-capped Au and Ag nanoparticles synthesized in organic solvents.18 HRTEM images at initial stages of the reaction showed that the growing particles were indeed core-shell Au@Ag, confirming that the particles grow by deposition of photochemically reduced Ag atoms on the initial Au seeds. This is shown in Figure 3b, in which a decahedral particle and a nearly triangular particle can be identified. The darker Au core and the Ag shell can be clearly distinguished in both particles. This image also shows that the Ag shell grows epitaxially over the Au core, following the inner structure. This strongly supports the importance of the structure of the initial seeds in the shape of the growing particles. The nearly triangular particle presents 2.4-Å lattice fringes with approximately hexagonal symmetry, suggesting the initial growth of a Ag nanoplate from a Au seed with planar defects, as in the case of Ag seeds. It is worth recalling that, during photochemical growth, the metallic seeds (Au or Ag) act as catalysts and electron sinks to promote the surface reduction of the Ag ions, so the difference in the photoactivity of the Au and Ag would not be relevant in this case. From another point of view, this experiment also reveals other important issues about the role of sodium citrate. Willey et al.19 proposed that, in the photoinduced conversion method, the presence of citrate is required for the formation of nanostructures bounded by 〈111〉 facets. This is in agreement with the presence of several large Au@Ag decahedra and icosahedra under our synthesis conditions, given that, in these cases, the surface is ideally formed by 〈111〉 facets. Another important point is that no rod-like particles were observed in our synthesis. Several groups have shown that Au nanorods with pentagonal profiles (twinned rods) can be grown by the slow reduction of Au atoms over defective 5-fold seeds in the presence of polymers6,16 or others surfactants.5 These rods are laterally bounded by 〈100〉 facets with caps formed by 〈111〉 facets. It has been proposed16 that a preferential stabilization of 〈100〉 facets by the capping molecules induces anisotropic growth. In our case, the isotropic growth of the multiply twinned particles instead of the formation of nanorods may provide indirect evidence of a preferential

6992 J. Phys. Chem. C, Vol. 111, No. 19, 2007

Rocha and Zanchet

Figure 4. (a) X-ray diffractograms of the O2 seed sample (open circles) and N2 seed sample (solid squares). (b) UV-vis spectra of the final solution, after 120 min of irradiation. Note that a higher yield of nanoplates is achieved for the sample with more defective particles.

Figure 3. HRTEM images of (a) the initial spherical Au seeds, showing a high population of multiply twinned particles, and (b) the Au@Ag core-shell nanoparticles formed after irradiation.

stabilization of 〈111〉 facets by sodium citrate. It is important to remark, however, that this likely selective-binding effect of sodium citrate alone is not enough to explain the anisotropic growth of the Ag nanoplates. Another interesting point is that, concerning the SPR, the reaction of Ag seeds using a wavelength of 500 nm would be equivalent to the reaction of Au seeds with a wavelength of 600 nm because the SPRs of Ag and Au nanoparticles are at about 400 and 500 nm, respectively. Nevertheless, the final products are very different, strongly supporting the conclusion that the SPR defines the final size of the nanoplates but does not determine the anisotropic growth at the beginning of the reaction.12 A complementary experiment to evaluate the role of crystallographic structure in anisotropic growth would be the production of defective seeds in a controllable way. Recently, Willey and co-workers6 nicely demonstrated a methodology to produce samples of single-crystal Ag particles at high yields based on the preferential oxidative etching of defective particles in the Polivynilpirrolidone (PVP) method. We performed similar experiments in which Ag seeds were synthesized under air bubbling (called O2 seed sample here) and compared the results to the original method (called N2 seed sample). It is important to point out that no fine control of the dilute oxygen in solution was attempted, so the results are only comparative. XRD measurements of these samples (Figure 4a,b) indicated that,

although effective control of the defective particles in the citrate method was not completely achieved, a qualitative change in the population of defective particles could be observed. Surprisingly, in our case, the presence of larger amounts of oxygen induced an increase in the number of particles with structural defects in contrast to the results reported in ref 6. This can be verified by the more prominent shoulders at 0.4 and 0.45 Å-1 in the XRD profile of the O2 seed sample. These samples were used as seeds for photochemical growth, and the UV-vis spectra of the final product suggest that the number of triangular nanoplates relative to spherical particles was larger when using the O2 seed sample, as evidenced by the absorbance ratios of the SPR peaks at 430 nm (spheres) and 600 nm (nanoplates) for the two samples (Figure 4b). This is in agreement with the relevance of the defects in the anisotropic growth of the nanoplates. The influence of oxygen was restricted only to the determination of the structural distribution of the seeds during their formation. The presence of oxygen during the photochemical growth itself was demonstrated to play a minor role in the growth process, as evidenced by a control experiment in which the same O2 seed sample were used and the amount of dilute oxygen in the growth solution (seeds + water + Ag+ + citrate) was varied; no significant differences in the final product were observed. Finally, it is worth mentioning that these results may provide relevant information for other systems/methods, such as the formation of Ag nanoplates in the photoinduced conversion approach,14 where no excess Ag+ ions is added to the seed solution. Nevertheless, in our case, the presence of large amounts of Ag+ ions at the beginning of the reaction dominates the kinetics of nanoplate growth, making other factors such as the

Growth of Ag Triangular Nanoplates amount of dissolved oxygen or the photoactivity of the metal seeds less critical. Conclusions In summary, the present work shows strong experimental evidence that stacking faults and twin planes present in the initial seeds used for photochemical seed-mediated growth play a major role in the anisotropic growth of triangular nanoplates. We observed that the crystalline structures of the initial seeds and the final nanoplates can be very complex, with many defects parallel to the 〈111〉 direction, making XRD a suitable choice for obtaining comparative statistical data about defects among samples. HRTEM observations at the early stage of the reaction revealed that nanoplates are formed at shorter times by epitaxial anisotropic deposition of photoreduced Ag atoms on the defective seeds. By using different seeds (Au and Ag), we clearly demonstrated that the distribution of crystalline structures (Au or Ag) had a striking effect on the final product. Equally important, by using Au seeds, we observed indirect evidence of the role of the capping agent and confirmed that excitation of the SPR appears to play a minor role in the origin of anisotropic growth in this system. These results suggest that the correct combination of suitable capping agents and nanoparticle structure can be a critical factor in the controlled growth of anisotropic metal nanoparticles, which means that more than one factor appears to determine the origin of the anisotropy and the weights of the various factors may depend on the synthetic route. Further controlled experiments and theoretical models will be important in developing a full understanding of the common factors behind the anisotropic growth in metal nanoparticles and other systems. Acknowledgment. We thank Prof. Daniel Ugarte and Prof. Herbert Winnischofer for the instructive discussions and suggestions. LME/LNLS is acknowledged for the use of the TEM instrument (JEM-3010). LNLS is acknowledged for use of the XRD2 and XPD beamlines. This research was supported by FAPESP procs. 01/07715-8 and 02/12720-3. Supporting Information Available: Single nanoplate SAD. Additional HRTEM images of initial seed sample showing defective particles. Schematic geometry of planar defects in an

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