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Small Gold Nanoparticles as Crystallization ‘catalysts’: Effect of Seed Size and Concentration on Au-ZnO Hetero Nanoparticles Joseph F. S. Fernando, Matthew P. Shortell, Kristy C Vernon, Esa A Jaatinen, and Eric R. Waclawik Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00619 • Publication Date (Web): 10 Aug 2015 Downloaded from http://pubs.acs.org on August 12, 2015

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Crystal Growth & Design

Small Gold Nanoparticles as Crystallization ‘catalysts’: Effect of Seed Size and Concentration on Au-ZnO Hetero Nanoparticles Joseph F. S. Fernando, Matthew P. Shortell, Kristy C. Vernon, Esa A. Jaatinen and Eric R. Waclawik* School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, Queensland, 4000, Australia

ABSTRACT

This work reports the effect of seed nanoparticle size and concentration effects on heterogeneous crystal nucleation and growth in colloidal suspensions. We examined these effects in the Au nanoparticle-seeded growth of Au-ZnO hetero-nanocrystals under synthesis conditions that generate hexagonal, cone-shaped ZnO nanocrystals. It was observed that small (~ 4 nm) Au seed nanoparticles form one-to-one Au-ZnO hetero dimers and that Au nanoparticle seeds of this size can also act as crystallization ‘catalysts’ that readily promote the nucleation and growth of ZnO nanocrystals. Larger seed nanoparticles (~9 nm, ~ 11 nm) provided multiple, stable ZnOnucleation sites, generating multi-crystalline hetero trimers, tetramers and oligomers.

*Corresponding author: [email protected]

ACS Paragon Plus Environment

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Small Gold Nanoparticles as Crystallization ‘catalysts’: Effect of Seed Size and Concentration on Au-ZnO Hetero Nanoparticles Joseph F. S. Fernando, Matthew P. Shortell, Kristy C. Vernon, Esa A. Jaatinen and Eric R. Waclawik* School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, Queensland, 4000, Australia

ABSTRACT

This work reports the effect of seed nanoparticle size and concentration effects on heterogeneous crystal nucleation and growth in colloidal suspensions. We examined these effects in the Au nanoparticle-seeded growth of Au-ZnO hetero-nanocrystals under synthesis conditions that generate hexagonal, cone-shaped ZnO nanocrystals. It was observed that small (~ 4 nm) Au seed nanoparticles form one-to-one Au-ZnO hetero dimers and that Au nanoparticle seeds of this size can also act as crystallization ‘catalysts’ that readily promote the nucleation and growth of ZnO nanocrystals. Larger seed nanoparticles (~9 nm, ~ 11 nm) provided multiple, stable ZnOnucleation sites, generating multi-crystalline hetero trimers, tetramers and oligomers.


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1. Introduction Colloid hybrids in the form of binary heterostuctures are an interesting class of material. This is because new collective properties often emerge in colloid hybrids that differ significantly from a simple linear combination of the individual component’s properties.1,2 Potentially useful phenomena can be manifested in the optical, electrical and catalytic properties of these complex colloid systems and so be harnessed in a diverse range of applications.2,3 Application examples include photocatalysis,4-6 nonlinear optics,7 solar energy conversion,8,9 and biomedical imaging.13

Control over the connections between binary nanohybrids and therefore the properties of these

composite colloid materials for technology applications is a major challenge, because it requires the development of robust and reproducible methodologies.10 These new developments will of necessity, require control over the separate processes of nucleation and growth which must be carefully optimized to achieve a desired nanocrystal size and shape.11 In the hybrid nanocrystal field, a recent focus of attention has been controlled chemical synthesis, with the intent to produce shape-controlled binary heterostructures.4 A number of studies have proven that this can be done using faceted nanoparticles as seeds.10,12 In this area, considering their promise in a range of optoelectronic applications, colloid hybrids formed from plasmonic metal colloids and semiconductors is a particular focus,12 where Au-ZnO hetero-nanocrystals have proven to be a useful test-bed for different synthetic approaches (and applications).4,13-15 Photochemical reactions can also provide an alternative pathway for the production of these nanocrystal hybrids, where metal nanoparticles can be selectively reduced on the surface of semiconductor nanocrystals.15,16 The defining of a crystal growth environment to produce novel heterogeneous nanostructures with facet-selective growth remains a significant challenge. Heterogeneous particle formation will be preferred if crystal nucleation and growth is anisotropic and centered at


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a particular lattice plane of the seed crystal. Uniform, isotropic growth over all facets of the seed leads to a core-shell structure instead and this is most likely when the heterojunction is formed between materials of a similar type and where there is a good match of two materials’ lattice constants.17-20

If homogeneous nucleation can be suppressed by maintaining the precursor

concentration below the critical supersaturation value throughout the synthesis process, then heterogeneous nucleation on a nanocrystal seed should be favored.21 When there is a large lattice mismatch between the two materials, growth of one component can also occur at specific positions or sites on the second component. Recent experimental research on metalsemiconductor (frequently II-VI type) colloid heterostructures has revealed that when there is a high lattice mismatch at the interface or heterojunction, an energetically unfavorable situation, stability can be gained if the mismatch is compensated by coincident lattice matching at periodic intervals.22,23 Such “commensurate fitting” or “coincidence site epitaxy” is one way these types of systems can gain, or maintain stability.17 Other mechanisms that could play a role in conferring stability to a particular hybrid structure are bonding affinity, surface de-wetting processes and electron transfer at the interface between the two components of the system.21,22,24,25 With these considerations in mind, in the present work we decided to investigate how Au nanoparticle seed crystal size and concentration influence the crystal growth and thus design of Au-ZnO hereostructures. Au nanocrystal seeds and ZnO nanocrystals were prepared using intentionally similar precursors and synthesis conditions. The face centered cubic (fcc) Au seeds were enclosed by low index (111) surfaces, thus presenting a specific atomic arrangement and lattice spacing upon which the seeded growth could occur.12 Under the limitations of ligandmediated growth of the ZnO nanocrystals on the Au seeds under mild conditions, the hybrid


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synthesis was performed in the presence of labile, (L-type) amine ligands which are distinguished by their stoichiometric, dative binding with the colloid crystals’ surfaces in an adsorption/desorption dynamic equilibrium.26 The wurtzite crystal structure that ZnO colloids usually form under these typical solution synthesis conditions can generate a wide range of crystal morphologies.27 This is because the anisotropic crystal structure of a wurtzite-phase II-VI semiconductor drives anisotropic growth along certain facets and this growth can be modified using ligands and/or temperature for a given solvent/precursor combination. Anisotropy is intrinsic to the wurtzite crystal structure, so the physical and chemical properties of the crystal’s facets in this case are deeply direction-dependent.12 ZnO in various morphologies have previously been shown to preferentially grow along the c-axis with the hexagonal, basal (0001) facet attached to (111) facets of noble metal fcc crystals.4,12,22,27,28 A solvent, ligand and temperature combination conducive to the formation of hexagonal, cone-shaped ZnO nanocrystals was selected. Depending on the reaction conditions, preferred ZnO nanocrystal product shapes can be generated, but how might this anisotropic growth be affected by the heterogeneous seeding process? Although the seeding of a supersaturated solution to nucleate crystals is a well understood process, at least qualitatively using classical nucleation theory in the case of ‘homogeneous’ crystal seeds, the parameters that define heterogeneous colloid-seeded crystallization are not so well known. Numerical simulations predict that to be an effective crystallization promotor, a seed must exceed a minimum size and that above this critical size, the seed particles can effectively act as crystallization ‘catalysts’: forming new heterogeneous crystals which detach from the seed’s surface after they exceed a particular size. The detachment process being driven by strain in the heterogeneous structure as the new crystal grows. This


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means that a low concentration of such small Au seeds might crystallize a large number of ZnO crystals in the system studied here. Large seed particles on the other hand, are more stable surfaces and are predicted to remain covered in the crystallites they spawn.29 The present work is consistent with these predictions based on hard-sphere models, confirming that a careful selection of a crystallization promoter, or (heterogeneous) crystal seed’s size requires consideration because of its importance in determining the size and structure/composition of a heterogeneous, seeded metal-semiconductor hybrid. 2. Experimental 2.1 Materials Zinc acetate dihydrate (Zn(OAc)2.2H2O, 98%), Gold(III) chloride trihydrate (HAuCl4.3H2O, 99.99%), Oleylamine (cis-1-amino-9-octadecene, 70 %), 1-Dodecanol (CH3(CH2)11OH, 98%), tert-butylamine borane complex (TBAB, 97%) were purchased from Sigma-Aldrich. All chemicals were used without further purification. 2.2 Sample synthesis Synthesis of Au Nanoparticles. Monodisperse Au nanoparticles were prepared by reduction of HAuCl4 as reported by Peng et al.30 with some modifications. Typically to synthesize 4 nm Au nanoparticles a precursor solution containing 50 mg HAuCl4.3H2O, 5 mL oleylamine and 5 mL dodecanol was prepared at room temperature. A reducing solution containing 0.25 mmol TBAB, 0.5 mL oleylamine and 0.5 mL dodecanol was added quickly to the precursor solution under N2 environment. After 1 hour stirring the nanoparticles were precipitated by addition of ethanol followed by centrifugation. To prepare larger Au nanoparticles the reducing solution was omitted. Instead the reaction temperature was raised to 80 ˚C and stirred for 1 hour and 3 hours


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under N2 environment to obtain ~9 nm and ~11 nm Au nanoparticles respectively. The final products were redispersed in hexane. Synthesis of Au-ZnO Heterostructures. Heterogeneous nucleation and growth of ZnO on oleylamine capped Au seed particles was used for the preparation of Au-ZnO heterostructures with varying size of ZnO domains.4 In a typical synthesis a precursor solution containing 0.5 mmol Zn(OAc)2.2H2O, 6 mL dodecanol and 3 mL oleylamine was prepared. Ten milligrams of oleylamine capped Au seeds in 5 mL of hexane was added to the precursor solution under constant stirring at room temperature. Next, the reaction mixture was degassed at 100 ˚C for 15 minutes and slowly heated up to 175 ˚C. The reaction mixture was kept at reflux for 4 hours at this temperature before cooling down to room temperature. The as-obtained products were repeatedly washed with ethanol and dried in air. To evaluate the effect of seed concentration the same procedure was repeated with 0, 2.5 and 6 mg of ~4 nm Au seeds in 5 mL of hexane. To evaluate the effect of seed size the same procedure was repeated with 10 mg of ~9 nm and ~11 nm Au seeds in 5 mL of hexane. Particle characterization. The structure, morphology and crystalline nature of particles were characterized by (high resolution) transmission electron microscopy (HRTEM), X-ray diffraction spectrometry (XRD), selected area electron diffraction (SAED) and ultraviolet-visible spectroscopy (UV-Vis). HRTEM imaging was performed using a JEOL JEM-2100 instrument with an acceleration voltage of 200 kV, low-resolution TEM images and SAED patterns were recorded using JEOL JEM-1400 instrument with an acceleration voltage of 100 kV. Samples for TEM and HRTEM were prepared by drop casting a dilute solution of nanoparticles dispersed in the appropriate solvent (hexane, ethanol) onto a carbon-coated copper grid (ProSciTech; Australia). XRD patterns were recorded using PANanalytical XPert Pro Multi Purpose


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Diffractometer using Co-Kα radiation (λ = 0.179095 nm). UV-Vis spectra were obtained using a Varian Cary 50 UV-Vis spectrometer. 3. Results and discussion Temperature- and Time-Dependent Studies of Au-ZnO hexagonal nanocrystal hybrid Growth To determine the range of synthesis conditions under which Au-seeded, Au-ZnO nanopyramid crystal growth might be complete and stable, temperature- and time-dependent studies were performed using 4 nm diameter Au crystal seeds. In the absence of these heterogeneous crystal seeds, hexagonal ZnO pyramids normally form at temperatures of between 160 - 180 °C.31 In the presence of 4 nm seed crystals, we observed that attached, seeded, ZnO nanocrystals were nucleated at reaction temperatures well below 175 °C (see Figure S1). Examination of the TEM images of products produced at lower temperatures of 130 °C, 140 °C and 150 °C revealed that Au-ZnO hexagonal nanocrystal hybrids can form at these temperatures, but it was noted that the ZnO-component morphologies were progressively less complete the lower the temperature was maintained. At 150 °C and above, Au-ZnO nanohybrid products were indistinguishable from those produced at 175 °C. A kinetic study was also performed using 4 nm Au seed crystals, where other reaction conditions were identical to that described in Section 2.2. At reaction times shorter than 4 hours, obvious differences in the morphology of the seeded ZnO-component of the hetero-nanocrystals were observed, where the ZnO nanopyramids possessed sharper, more defined corners. Between 4 -7 hours, the ZnO nanopyramid tips became progressively more rounded and blunt with increased reaction time. Above 8 hours, we observed that the Au seed crystals recrystallized and grew much larger than the original 4 nm diameter, presumably through an Ostwald ripening-type of process (see Figure S2).


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Typical structures of isolated Au-ZnO hexagonal nanocrystal hybrids; We have demonstrated that Au nanoparticle seeds can initiate growth of ZnO hexagonal nanopyramids. These hexagonal nanopyramids form because ZnO is a polar crystal consisting of tetrahedrally coordinated O2- and Zn2+ ions that stack in alternating layers in the c-axis direction. Since the Zn2+ terminated (0001) surface is more reactive than the O2- terminated ( 000 1 ) surface, the crystal grows faster in this direction and eventually forms a pyramid tip.22 The Au seed nanoparticles frequently remain attached to the center of the hexagonal base of ZnO pyramid nanocrystals, as can be readily observed from inspection of the TEM images of Figure 1a-c. The Au nanoparticles were synthesized in a polycrystalline form, consisting of multitwinned crystals with the (111) facets exposed. At these exposed facets, the ZnO nanopyramids were attached. As can be seen in Figure S3, the lattice spacing of the (111) facets of these crystals were measured at 0.235 nm. The Au facets have hexagonal symmetry, as does the (hexagonal) ZnO basal plane, so ZnO nanocrystals prefer to directly grow from Au (111) facets along the ZnO c-axis direction.


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Figure 1. (a) TEM image of the as prepared Au-ZnO hybrid nanopyramid product. (b) Magnified TEM image of the side-view of Au-ZnO hybrid single particle. (c) Magnified TEM image of the basal view of Au-ZnO hybrid single particle; the inset shows the HRTEM image of the marked area of the ZnO component of the particle. (d) SAED pattern of Au-ZnO hybrid particle along the [0001] axis. The SAED pattern (Figure 1d) confirmed that grown nanopyramids were single-crystalline in nature. The pattern shows that the 0001 zone axis of the hexagonal cone was surrounded by 1010 planes. The HRTEM image of the marked basal surface of the nanopyramids (inset of Figure 1c) reveals these structures were highly crystalline with a lattice spacing of 0.28 nm, corresponding to the distance between 1010 planes in the ZnO crystal lattice. These observations identified the nano-pyramid structures formed with ( 000 1 ) basal surface exposed. Using HRTEM and other methods, it has been shown previously that ZnO hexagonal nanopyramids consist of six oxygen terminated { 10 1 1} facets as side surfaces and one oxygen


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terminated ( 000 1 ) facet, where this ( 000 1 ) basal plane is perpendicular to the [0001] growth direction of the crystal, the c axis of the wurtzite hexagonal structure.32 The 0.28 nm lattice spacing between 1010 planes therefore presents a ~ 16 % lattice mismatch with the (111) facet of the Au nanoparticle it is attached to. A large lattice mismatch between the Au and ZnO facets is thought to drive formation of a hetero nanostructure such as we see in Figure 1a-c, rather than formation of a core-shell nanoparticle.33 If an epitaxial heterojunction is formed, a large lattice mismatch is likely to generate strain at the interface between the two materials during crystal growth. A theoretical study might prove this is the case here. One way by which such strain could be accommodated, leading to the stable heterostructures observed in Figure 1, is if a commensurate fit of lattices stabilizes the structure. In this Au-ZnO system, a coincident lattice match between 6d(111) fcc Au = 5d( 000 1 ) wz ZnO at the interface is possible. This type of heteroepitaxy has been observed in other similar systems.17,23 The effect of gold nanoparticle size Figures 2a-c clearly shows that Au nanoparticle size has a significant impact on the morphology and crystallinity of the final Au-ZnO hybrid product. The measured size distributions of the Au nanoparticles in the seed solutions were 4 nm ± 1 nm, 9 nm ± 1 nm and 11 nm ± 1 nm respectively, as confirmed by TEM analysis of the products in Figure S4. We observed that a small ~4 nm diameter Au nanoparticle seed solution nucleated complete Au-ZnO hexagonal nanopyramid heterocrystals with diameters of 35 nm ± 4 nm across the hexagonal base and 21 nm ± 4 nm high along the [0001] ZnO growth direction, with the Au nanoparticle seeds located at the center of the ( 000 1 ) ZnO base. Larger Au nanoparticle seeds generated what appeared to be incomplete Au-ZnO hybrids. The TEM images of these incomplete pyramids for the case of ~9 nm Au nanoparticle seeded hybrids were analyzed, where it is


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readily apparent that the seeding of multiple ZnO nanocrystals off the same Au nanoparticle frequently occurred. The appearance of incomplete crystallization in Figure 2b is therefore actually a manifestation of rapid and effective ZnO crystal nucleation. We interpret this as an indication that on a given large Au nanoparticle seed, the multiple ZnO crystallites seeded at different Au facets which are in fact stable and thus compete for limited space around the central Au nanoparticle seed. This would be expected to generate smaller ZnO nanocrystals, with a high proportion of the product containing multiple ZnO nanocrystals per Au nanoparticle seed. This heterogeneous crystal seeding mode appears to occur quite readily. Given the correct synthesis conditions, in a variety of closely related systems, this behavior has often been described in recent literature by the rather nugatory term “nanoflower” synthesis.34 This type of multiplenucleation and crystallization phenomenon has been observed in Au-ZnO hybrid synthesis studies, an example of which is the study by Chen et al.13 Although this study employed one-pot synthesis rather than a seeded approach used here, with minor variations in the materials and reaction conditions, when Chen et al. synthesized their hybrids under conditions where 6-9 nm Au nanoparticles were prevalent, a composite structure identical to those observed in Figure 2b was obtained. In hybrid crystallization then, obtaining a desired structure is critically dependent on the seed size and polydispersity. It appears that within a narrow Au nanoparticle seed size range, complete, one-to-one Au-ZnO hexagonal nanopyramids form. Hybrid samples seeded from ~11 nm Au nanoparticle seeds produced tiny ZnO nanocrystallites of 9 nm ± 1 nm diameter (Figure 2c). It is interesting to note that the hybrid sample seeded from ~11 nm Au seeds, hardly have any free Au nanoparticles. Monte Carlo simulations have proven that large seed particles are less likely to detach from the crystallites from whence they spawn.29


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Figure 2. TEM images of Au-ZnO hybrids formed by different sizes of Au. (a) ~4 nm. (b) ~9 nm. (c) ~11 nm. Figure 3a shows the HRTEM image of a hybrid system (inset of Figure 3a) formed by an ~8 nm Au seed. This hybrid system consists of three ZnO crystallites. The HRTEM image reveals that these structures were highly crystalline with lattice spacings of 0.158 nm and 0.247 nm at the basal surface corresponding to the distance between 1120 planes and 1011 planes respectively. The 1120 planes are parallel to the [0001] direction, which indicates that each crystallite has grown along the c-axis of the [0001] direction. The SAED pattern along the [0001] direction (Figure S5a) confirmed that an individual ZnO crystallite is single crystalline in nature. Similar to the Au-ZnO heterostructures formed by a ~4 nm Au seed, each ZnO crystallite in the hybrid system formed by an ~8 nm Au seed consists of a single 0001 basal surface and six 1011 side surfaces. Figure 3c shows the HRTEM image of a hybrid system formed by a ~12 nm Au seed which is composed of 5 or more tiny ZnO fragments. HRTEM image (Figure 3c) reveals that individual fragments are crystalline in nature. Lattice spacings of 0.247 nm and 0.258 nm can be clearly identified in these structures, corresponding to 1011 and 0002 planes respectively. The 0002 planes are perpendicular to [0001] direction, which indicates that each fragment has grown along the c-axis of the [0001] direction. The SAED pattern (Figure


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S5b) reveals that the hybrid system is multi-crystalline, suggesting that there are multiple nucleation sites of ZnO at the surface of Au.

Figure 3. (a) HRTEM of a Au-ZnO hybrid system (inset) seeded by an ~8 nm Au nanoparticle. (b) Atomic model of the hybrid system formed by an 8 nm Au seed. (c) HRTEM of a Au-ZnO hybrid system formed by a ~12 nm Au seed. (e) Atomic model of the hybrid system formed by a 12 nm Au seed. Figure 4a-c gives the powder XRD patterns of Au-ZnO hybrid samples and these were consistent with that of wurtzite crystal structure of ZnO (P63 mc, JCPDS 36-1451) and fcc crystal structure of Au (Fm-3m, JCPDS 4-784). It is evident that ZnO diffraction peaks have broadened with increasing size of Au seed particle, indicating that ZnO crystallites have become smaller. Domain size calculations of ZnO crystallites were performed using TOPAS 4.2 software (detailed in supporting information). The calculated domain sizes of ZnO crystallites were 38.4(7) nm, 17.2(2) nm and 8.7(2) nm for ~4 nm, ~9 nm and ~ 11 nm Au seeds respectively, which is in good agreement with the data obtained from TEM.


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Figure 4. Powder XRD patterns of Au-ZnO hybrids seeded from different sizes of Au. (a) ~11 nm, (b) ~9 nm (c) ~4 nm The effect of gold seed concentration; We studied the effect that the concentration of Au nanocrystal seeds can have on Au-ZnO heterostructure growth, focusing on smallest 4 nm diameter Au seed particles which were observed to generate complete hexagonal ZnO pyramids at the base. Generally a low concentration of Au nanoparticles was observed to produce a low ratio of Au-ZnO hybrids compared to ZnO nanopyramids in the final product mixture compared to when higher concentrations of Au seeds were used. With increased Au nanoparticle concentration, the proportion of Au-ZnO hybrids in the final product clearly increased. Qualitatively this trend can


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be verified by comparing the TEM images of the hybrid products in Figure 5b-d, which were generated from different Au seed concentrations in the reactant solution. At the highest Au nanoparticle concentration studied here, large numbers of free Au nanoparticles can be discerned to be present alongside Au-ZnO hybrids in Figure 5d. Although it is difficult to precisely quantify the number concentration of Au nanoparticles present in these samples, we found that 6 mg of Au nanoparticles yielded a high ratio of Au-ZnO hexagonal nanopyramids when reacted with the ZnO precursor solution under the conditions specified in Section 2.2. It is interesting to note that at any (low to high) Au seed concentration, the ZnO nanopyramids have the same morphology and particle size (Figure 5b-d). However under the same reaction conditions when no Au seeds were employed, the ZnO nanopyramids appear to be incomplete and significantly larger in particle size (Figure 5a). It is evident that Au seed particles greatly increased the nucleation rate of ZnO crystals. The classical theory of nucleation outlines that in order for a crystal to grow, crystallites of the stable phase need to exceed a critical size and crystallites that are smaller than this nucleus dissolve again.35 It is also reported that crystallization can proceed spontaneously if we add a seed nanoparticle that is larger than the critical nucleus. This explains why independently grown ZnO nanopyramids are significantly larger and incomplete compared to seed mediated ZnO nanopyramids.29 From Figure 5b it is clear that using low concentrations of Au seeds, the number concentration of ZnO-only pyramids are far greater than Au-ZnO hybrid pyramids. This observation mirrors the theoretical phenomenon described as ‘crystallization catalysis’ proposed by Cacciuto et al.29 They reported that seed particles need to exceed a critical size to be functional crystallization promoters and that above this critical size, the seed particle can act as a catalytic surface to form a new crystallite. Subsequently the same seed particle may detach and seed a new crystal.


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Figure 5. TEM images of Au-ZnO hybrids formed at different concentrations of Au seeds (~ 4 nm). (a) 0 mg. (b) 2.5 mg. (c) 6 mg. (d) 10 mg. We highlight that unattached Au seeds can be readily separated from the ZnO nanocrystal and Au-ZnO hybrid component in these mixtures, using high-speed centrifuge separation. UV-Vis spectra: The optical extinction spectra (Figure 6a-b) of the three Au nanoparticle samples and their corresponding heterostructures were taken in hexane and ethanol respectively. For the pure Au samples, the plasmon peak at ~ 505 nm red shifts with increasing size as expected. The introduction of ZnO into the sample has two effects on the extinction spectra. Firstly, both the location and shape of the plasmon peak changed. Secondly, attachment to ZnO introduced strong extinction in the UV, producing a typical ZnO ‘absorption edge’. There are various reasons for the change in the extinction of the heterostructures that can’t readily be explained as a simple linear combination of ZnO and Au extinction.


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Figure 6. (a) UV-Vis spectra of Au seed particles of different sizes. (b) UV-Vis spectra of AuZnO heterostructures formed by Au seeds of different sizes. The heteroepitaxial growth of ZnO domains on Au seeds changes the local dielectric function of their surrounding medium, and therefore the extinction spectrum of the samples.3 In a simple core-shell, Au-ZnO arrangement, we would expect the plasmon peak to red shift compared to the pure Au nanoparticle seed.36 However, the strong chemical bonds at the Au-ZnO interface could also modify the dielectric function of the Au nanoparticles themselves further complicating the interpretation of the extinction spectra.37


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The UV peak for the hybrids containing the larger ZnO nanocrystals (4 and 9 nm Au seeds) was near 370 nm. In comparison, the UV peak for the hybrids containing the smallest ZnO nanocrystals (11 nm Au seed) was blue shifted compared to this, toward 350 nm. Although these small ZnO nanocrystals (~8 nm) were close enough to the bulk ZnO exciton Bohr radius (~ 1 nm) to display quantum confinement effects, it is unlikely quantum confinement is the dominant cause for the blue shift. A more likely reason is decreased scattering of the smaller nanoparticles (Figure S6). This effect is similar to the red shift observed in Au nanoparticles of increasing size. Conclusions In summary we report a method to synthesize Au-ZnO heterostructures by a seed mediated growth technique with accurate control over morphology and domain size of ZnO crystallites. The present work confirms that a careful selection of the size of the crystallization promotor can be of crucial importance in determining the final product’s form, at least for the case of heterogeneous nucleation.29 Au seed particles facilitate the nucleation and growth of ZnO crystallites. The large lattice mismatch between 1010 planes of ZnO and (111) plane of Au drives the formation of hetero dimers, trimers or oligomers. We have shown that Au seed particles can act as crystallization ‘catalysts’ where seeds detach from newly formed crystallites and become free to seed a new crystal. Seed particle size plays a crucial role in determining the structure and morphology of newly forming crystallites. Small seed particles (~ 4 nm) have only one nucleation site and form one-to-one Au-ZnO hexagonal nanopyramids. Large seed particles have multiple nucleation sites and form three or more ZnO crystallites around a single Au nanoparticle. Further, the size of ZnO crystallites around a seed particle depends on the size of the seed particle; larger seeds spawn smaller ZnO crystallites. The growth of ZnO crystallite on


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Au seed particle, greatly affects the frequency of plasmon resonance which may be used to tune the optical properties of the multi-component system. ASSOCIATED CONTENT Supporting information. TEM images of Au-ZnO hetero nanoparticles synthesized at different growth temperatures, TEM images of as-prepared Au-ZnO hetero nanoparticles collected at different reaction stages, estimation of domain size using TOPAS version 4.2 software, HRTEM image of a Au nanoparticle, TEM images of as prepared Au nanoparticles of different sizes, SAED patterns of Au-ZnO hetero nanostructures and theoretical absorption and scattering cross-section of spherical ZnO nanoparticles. This information is available free of charge via the internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written with contributions from all the authors. All the authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS J. F. S. Fernando gratefully acknowledges the Queensland University of Technology (QUT) for providing a PhD scholarship. The authors also wish to thank Dr. H. Spratt for assistance with domain size calculations and other staff at the Central Analytical Research Facility (CARF) at QUT for their support. This material is based on research sponsored by the Air Force Research


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“For Table of Contents Use Only”

Small Gold Nanoparticles as Crystallization ‘catalysts’: Effect of Seed Size and Concentration on Au-ZnO Hetero Nanoparticles Joseph F. S. Fernando, Matthew P. Shortell, Kristy C. Vernon, Esa A. Jaatinen and Eric R. Waclawik*

Synopsis Au seed particles facilitate the nucleation and growth of ZnO crystallites. Small seed particles (~4 nm) can act as crystallization ‘catalysts’. The seed particle size is of crucial importance in determining the final product’s form.


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Small Gold Nanoparticles as Crystallization ... - ACS Publications

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