Effect of Temperature and Humidity on Coarsening Behavior of Au

Jun 19, 2012 - Coarsening behavior of the Au nanoparticles produced by thermal evaporation of Au onto a liquid crystalline lipid (1 ...
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Effect of Temperature and Humidity on Coarsening Behavior of Au Nanoparticles Embedded in Liquid Crystalline Lipid Membrane Seung Jae Lee, Hyeun Hwan An, Won Bae Han, Hee-Soo Kim, and Chong S. Yoon* Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea S Supporting Information *

ABSTRACT: Coarsening behavior of the Au nanoparticles produced by thermal evaporation of Au onto a liquid crystalline lipid (1,2-dioleoyl-3trimethylammonium-propane, DOTAP) membrane was investigated by subjecting the nanoparticle-embedded DOTAP membrane to two different annealing conditions (at 100 °C under no humidity and at 20 °C and 80% relative humidity). Although the coarsening rate was relatively slow because of the low temperature (from 5.6 nm in the asdeposited state to ∼7 nm after 30 h), it was identified that at 100 °C without humidity the Au nanoparticles resulted in shape refinement whereas the high humidity at 20 °C induced self-organization of the nanoparticles into a monolayer. It was also found that annealing in both cases tended to segregate the lipid molecules from the nanoparticle array and forced the nanoparticles into a tighter area. In the case of the high-humidity sample, the lipid segregation eventually led to extensive coalescence of the Au nanoparticles.

1. INTRODUCTION In recent years, the synthesis of noble metal nanoparticles has reached a stage where both particle size and distribution can be controlled with great precision. Today, metal nanoparticles with size distributions below 10% are routinely produced by state-ofthe-art techniques.1−3 In many cases, noble metal nanoparticles are obtained through solution chemistry, and the shapes of these nanoparticles are mostly spherical and typically encapsulated by surfactants which often dictate the particle geometry. Because of the exceedingly large surface area of the nanoparticles, however, the particle size and shape can change rapidly and relax into their equilibrium shapes when the nanoparticles are treated by heat or even when irradiated by light.4−8 In spite of the wealth of work on synthesis of nanoparticles, there exists a relatively little amount of work on thermally induced changes in the particle morphology through postannealing because postannealing often quickly leads to irreversible aggregation of the nanoparticles9 before detailed observation of the individual particles can be made. Previously, a solid-supported lipid membrane in a liquid crystalline state was used as a template to synthesize nanosized noble metal particles (Ag and Au) embedded in the membrane in a near-monolayer configuration. The mechanism for the formation of the Au nanoparticle monolayer by thermal evaporation of a noble metal layer onto a liquid crystalline lipid membrane has been previously discussed in detail.10,11 In short, noble metal atoms such Au or Ag were able to penetrate into the soft lipid membrane during the deposition process and nucleated within the membrane. Subsequent growth and coalescence of the nuclei were limited by spontaneous encapsulation of the lipid molecules so that a monolayer of discrete nanoparticles was formed inside the membrane. © 2012 American Chemical Society

Because these nanoparticles are embedded in a liquid crystalline medium in which lipid molecules have high mobility, a relatively low-temperature heat treatment can induce changes in the particle morphology, and the kinetics is slow enough to allow observation of the coarsening of the nanoparticle in detail. By analyzing the morphological changes of the nanoparticles during postannealing, the coarsening mechanism for the metal nanoparticles embedded in a semiliquid medium can be studied. In addition, the coarsening and shape relaxation of the nanoparticles in a lipid membrane during postannealing can potentially provide an alternative means to control the size and shape of the synthesized nanoparticles. The nanoparticle coarsening is of interest since tailoring the size and shape of the nanoparticles can alter both optical12,13 and electronic properties14−17 of the synthesized nanoparticles as well as their catalytic efficiency.15−17 In this paper, Au nanoparticles embedded in a lipid membrane were heat-treated under two different conditions: at 100 °C under no humidity and at 20 °C with high relative humidity. The ensuing changes in the particle size and shape were traced using transmission electron microscopy (TEM). Based on the observed morphological changes, the coarsening mechanism for the nanoparticles was elucidated.

2. EXPERIMENTAL METHODS Au nanoparticles embedded in a liquid crystalline matrix were fabricated by thermal evaporation of the metallic Au onto a solidsupported 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) Received: March 16, 2012 Revised: June 18, 2012 Published: June 19, 2012 10980

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membrane that was prepared by spin-coating the lipid solution onto a Si substrate following the method devised by Mennicke and Salditt.18 The lipid solution was made by dissolving the lipid in chloroform (10 mg/mL). 100 μL of the lipid solution was used to form the multilayer by spin-coating the lipid solution at 5000 rpm onto a 2 cm × 2 cm Si substrate. After spin-coating, the solvent was allowed to evaporate in a freeze-dryer for 12 h. A 5 nm thick Au layer was evaporated onto the lipid multilayer using a thermal evaporator. The film thickness was monitored using a quartz balance. The nanoparticle-embedded DOTAP membrane was heat-treated at 100 °C in a tube furnace with continuous flow of dry Ar gas. For the humidity effect, the DOTAP membrane was hydrated in a humidity-controlled furnace. Transmission electron microscopy (TEM, JEOL2010) was mainly used to characterize the particle morphology. For preparing the TEM specimen, a TEM Cu grid covered by amorphous carbon film was placed on the substrate. The lipid spin-coating and Au deposition were carried out directly onto the TEM Cu grid. TEM observation was made without further treatment of the specimen.

3. RESULTS AND DISCUSSION Figure 1a shows a TEM image of the dense layer of discrete Au nanoparticles embedded in the DOTAP membrane produced by thermally evaporating a 5 nm thick layer of metallic Au onto a DOTAP membrane. The Au nanoparticles were distributed uniformly throughout the membrane in a near-monolayer configuration inside the membrane (cross-sectional TEM image of the embedded Au nanoparticles is shown in Figure S1 of the Supporting Information). The membrane integrity after the Au evaporation was also confirmed by X-ray reflectivity (see Figure S2). The shape of the Au nanoparticles embedded in the DOTAP membrane was nearly spherical as can be seen from a magnified TEM image of the Au nanoparticles in Figure 1b; however, some of the nanoparticles appeared to have been joined to form elongated particles. In addition, judging from the contrast difference, there were some nanoparticles stacked on top of each other as indicated by the arrows in Figure 1b. The particle size distribution curve estimated from a series of TEM images is shown in Figure 1c. The average particle size was 5.6 ± 1.5 nm, and the size distribution was far from being a Gaussian distribution. There was an extended tail toward the lower end with a subsidiary peak centered around 1.8 nm (small particles well below the average size interspersed among larger particles can be clearly seen in Figure 1b). In addition, a shoulder observed at 6.5 nm was attributed to the population of the elongated particles discussed earlier. In order to isolate the effect of temperature on the coarsening behavior of the Au nanoparticles embedded in the DOTAP membrane, the nanoparticle-embedded membrane was annealed at 100 °C under dry Ar gas flow for duration of up to 30 h. A typical TEM image of the Au nanoparticles annealed at 100 °C for 5 h in Figure 2a indicates that all of the nanoparticles have become spherical without the elongated particles that were typically observed in the as-synthesized state. The disappearance of the elongated particles were likely manifestation of Raleigh shape instability by which elongated shapes with relatively high surface-to-volume ratio were broken apart into a string of spherical shapes to reduce the overall surface energy.19 The particle size distribution curves before and after annealing at 100 °C for 5 h are compared in Figure 1cthe shoulder peak in the as-deposited distribution curve assigned to the elongated particles has disappeared after annealing at 100 °C for 5 h. In fact, the average particle size decreased slightly to 5.1 ± 1.2 nm after annealing at 100 °C for 5 h. The subsidiary peak at 1.8 nm has shifted to the right in the

Figure 1. (a) TEM image of as-deposited Au nanoparticle embedded in the DOTAP membrane, (b) magnified image of (a), and (c) particle size distribution curve of as-deposited and after heat treatment for 5 h at 100 °C with no humidity. Arrows in (b) indicate the vertically misaligned Au nanoparticles.

annealed distribution curve and nearly merged into the main peak. It is likely that the small particles with high surface-tovolume ratio coarsened through either by Ostwald ripening or by coalescence during annealing.20 No drastic changes in the average particle size or in their shape were observed up to 10 h at 100 °C. A typical TEM image of annealed Au nanoparticles after 30 h is shown in Figure 2b (TEM images for intermediate annealing hours are included in Figures S3−S12). The Au nanoparticles after 30 h of annealing at 100 °C remained spherical in shape and retained the vertical misalignment as a number of Au nanoparticles appeared to be stacked on top of each other, similar to the as-deposited state. The average particle size of the Au nanoparticles and the particle number density are plotted as a function of the annealing period in Figure 2c. The average particle size increased incrementally 10981

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Figure 2. TEM images of the Au nanoparticles deposited on the DOTAP membrane: after heat treatment for (a) 5 h and (b) 20 h at 100 °C with no humidity, (c) average particle size and particle number density as a function of annealing period at 100 °C with no humidity, (d) normalized particle size distribution curves annealed at dry at 100 °C for different periods, and (e) schematic drawing of morphology change of the Au nanoparticles during heat treatment at 100 °C with no humidity.

after 5 h and plateaued at 7.2 nm after 25 h. The particle size distribution curves normalized by their respective average particle size, ⟨D⟩, after annealing for 10−30 h are shown in Figure 2d in which the distribution curves coincided onto a single distribution curve. The coincidence of the normalized distribution suggests that the coarsening mechanism of the nanoparticles at 100 °C under dry condition be self-similar throughout the annealing period.21 It was shown that the coarsening mechanism of Au nanoparticles capped with a selfassembled monolayer of organic molecules tended to coarsen in two stages. The organic molecules initially prevented aggregation of the nanoparticles as designed to do so. Once the encapsulating molecules are freed from the particle surface, the particle growth proceeded exclusively through coalescence.21,22 In our case, because the Au nanoparticles were completely embedded in the DOTAP membrane, the coarsening mechanism likely remained unchanged during annealing. Both coalescence and Ostwald ripening can act as an active coarsening mechanism simultaneously, and the relative contributions of each mechanism can be estimated

from the moments of the particle size distribution curves as defined below: μ1 =

(∑i R i 3/N )1/3 N ∑i (1 / R i)

,

μ3 =

⟨R ⟩ (∑i R i 3/N )1/3

(1)

where R is the particle radius, ⟨R⟩ is the arithmetic mean radius, and N is the number of particles sampled. For growth of particles exclusively through coalescence, μ1 should greater than 1.25 and μ3 less than 0.905.23 For the distribution curves in Figure 2c, the calculated μ1 and μ3 moments were 1.1 and 0.97, which suggests that both mechanisms were responsible for the Au nanoparticle growth. The evidence for coalescence was indirectly provided from the increasing fraction of regions where the Au nanoparticles were missing as shown in Figure 2d. It is conjectured that the DOTAP molecules in vicinity of the nanoparticles diffused away from the Au nanoparticles and tended to form a multilayer free of Au nanoparticles. The clustering of the lipid molecules created increasingly large fraction of regions free of the Au nanoparticles as the annealing period was extended. The clustering of the DOTAP molecules 10982

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Figure 3. TEM images of the Au nanoparticles deposited on the DOTAP membrane: (a) after hydration for 5 h at 20 °C and 80% RH, (b) magnified image of (a), (c) after hydration for 20 h at 20 °C and 80% RH, (d) magnified image of (c), (e) pair correlation functions of the Au nanoparticles annealed at dry 100 and at 20 °C, 80% RH for 15 h, (f) average particle size and particle number density as a function of annealing period at 20 °C and 80% RH, and (g) normalized size distribution curves for the Au nanoparticles hydrated at 20 °C and 80% RH for different periods. Circled regions I and II in (b) indicate the elongated nanoparticles and the nanoparticles stacked on top of each other, respectively.

led to crowding of the Au nanoparticles into an increasingly smaller area as evidenced by the progressive increase of the particle number density with longer annealing periods as shown in Figure 2c. It can be summarized that annealing of the Au nanoparticles embedded in a DOTAP membrane under low humidity conditions resulted in shape refinement through which irregular-shaped particles were converted into spherical shapes. The shape refinement which occurred at the initial stage suggests that the interfacial energy between DOTAP molecules

and the nanoparticles surface appears to be high and dominates the coarsening behavior of the nanoparticles. The spherical shape was maintained throughout 30 h. A similar particle refinement was also observed for the Ag nanoparticles embedded in a 1,2-dioleoyl-sn-glycero-3-phosphocholine membrane.10 The particle growth rate (⟨D⟩ ∼ t0.18) was much slower than that predicted by the mean-field theory,24 possibly due to the relatively slow diffusion of the Au atoms at 100 °C. The vertical misalignment of the nanoparticles shown in Figure 2c 10983

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Figure 4. (a) TEM image of the Au nanoparticles after heat treatment for 30 h at 20 °C and 80% RH, (b) magnified image of (a), and (c) schematic image of morphology change the Au nanoparticle during hydration at 20 °C and 80% RH. Arrows in (b) highlight the coalesced particle.

should also contribute to the retardation of the particle growth. Even though the lipid segregation during annealing brought the Au nanoparticles closer to each other, the three-dimensional distribution of the nanoparticles arising from the vertical misalignment would not decrease the particle-to-particle distance as effectively as the case in which the Au nanoparticles were distributed in a monolayer. Figure 2e schematically illustrates the coarsening behavior of the Au nanoparticles at 100 °C under a low humidity condition, which goes through immediate shape refinement, lipid segregation, and limited particle growth due to the isolation of the nanoparticles in the vertical direction. In order to determine the effect of humidity on the coarsening behavior of the Au nanoparticles embedded in the DOTAP membrane, the Au nanoparticles were kept at 20 °C and 80% relative humidity (RH). As can be seen from the TEM images from the samples annealed for 5 h in Figure 3a,b, the Au nanoparticles unlike the heat-treated nanoparticles did not undergo extensive shape refinement. The circled region I in Figure 3b indicates the elongated nanoparticles observed in the as-deposited state retained their shapes after annealing in a high-humidity condition. Nanoparticles aggregated into a cluster were also found in the circled region II. The magnified image in Figure 3b clearly shows some of the elongated particles whose length extended up to 10 nm, much larger than the adjacent spherical nanoparticles. After 20 h of annealing at 20 °C and 80% RH, the Au nanoparticle layer in Figure 3c contains much less fraction of elongated or aggregated particles compared to Figure 3a. The magnified image in Figure 3d verifies the persistent presence of the elongated nanoparticles. Also noticed from the TEM micrograph is that in some regions the Au nanoparticles were organized in a hexagonal array and there were no evidence for extensive overlaying of the nanoparticles. Pair correlation functions calculated from the

TEM images of the Au nanoparticles treated at dry 100 and 20 °C with 80% RH for 15 h are compared in Figure 3e which evinces clearly defined first- and second-nearest neighbors for the 20 °C with 80% RH sample. Meanwhile, the Au nanoparticles treated at dry 100 °C were arranged more or less in a random manner. The ordering of the nanoparticles suggests that high humidity conditions tend to favor the vertical alignment of the Au nanoparticles so that the nanoparticles were forced into a tight monolayer arrangement. The particle growth rate was again extremely slow with (⟨D⟩ ∼ t0.087) as can be seen from the plot of the average particle size vs annealing time in Figure 3f. Similar to the case annealed at 100 °C under dry conditions, migration of the DOTAP molecules led to segregation of the Au nanoparticles which created increasingly large areas in the membrane devoid of the Au nanoparticles. At the same time, the particle number density continuously decreased as shown in Figure 3f. Meanwhile, the normalized particle size distribution curves presented in Figure 3g show that the distribution curve became progressively symmetric with annealing while the population of the smaller particles on the left side of the main peak persisted up to 20 h. Also visible is the shoulder peak on right side, corresponding to the elongated particles observed in the TEM micrographs. Calculated moments μ1 and μ3 were 1.2 and 0.94 (at 20 h), which were well below and above the critical criteria for exclusive coalescence so that the shape of the distribution curves are still too narrow to suggest substantial amount of coalescence. At 30 h of annealing at 20 °C and 80% RH, cracks developed through the nanoparticle layer became accentuated as shown in Figure 4a. The nanoparticle layer was also punctuated by intermittent voids. A magnified image in Figure 4b clearly shows that the two or three nanoparticles were impinged and coalesced into a single particle as indicated by the arrows. It is 10984

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also noted that even with a large fraction of voids and cracks in the nanoparticle layer at 30 h, the TEM image in Figure 4a suggests that the monolayer configuration was maintained. The schematic drawing in Figure 4c illustrates the changes in the Au nanoparticles morphology and the organization of the nanoparticles in the vertical direction during annealing at 20 °C and 80% RH. Unlike the high-temperature and dry case, the Au nanoparticles underwent little change in shape, retaining the elongated nanoparticles initially synthesized while the annealing enhanced the vertical alignment which resulted in particle impingement and subsequent particle coalescence. The above results demonstrate that temperature and humidity had different effects on the coarsening behavior of the Au nanoparticles embedded in the DOTAP membrane. Water molecules that were absorbed into the lipid membrane led to swelling of the membrane25 and increased the lateral mobility. The increased lipid mobility under high humidity conditions more effectively accelerated the lipid segregation and forced the Au nanoparticles into a tighter space whose proximity led to the eventual massive coalescence of the nanoparticles. Figure 5 shows AFM images of the DOTAP membrane treated at dry 100 and 20 °C with 80% RH for 15 h. The DOTAP membrane treated at dry 100 °C in Figure 5a was nearly flat with intermittent protrusions on the membrane surface, whereas the membrane surface kept at 20 °C with 80% RH shown in Figure 5b had a strikingly different surface with multiple flat terraces or steps (4 nm in height) which were likely produced by uneven swelling of the membrane. Such terraces suggest vertical rearrangement of the lipid molecules which would expedite the migration of the Au nanoparticle in the vertical direction to allow the Au nanoparticles to organize into a monolayer. It has shown by computer simulation that water molecules trapped in the hydrophobic region can form stable channels in a lipid bilayer26 which can provide a conduit for rapid migration of the Au nanoparticles. On the other hand, heat-treating the DOTAP membrane at dry 100 °C actually flattened the membrane surface as the lipid mobility was confined in the lateral direction. The limited vertical migration of the lipid molecules kept the nanoparticles at different heights in the lipid multilayer, inherited from the as-synthesized state. The vertical misalignment prevented or delayed the coalescence of the Au nanoparticles under dry conditions. As for the shape change incurred when annealed at dry 100 °C, it is surmised that the interfacial energy between the lipid molecules and the particles surface was high enough to induce the Raleigh shape instability, whereas under high humidity conditions the water molecules penetrated into the interface and lowered the interfacial energy so that the interfacial energy was no longer critical. In fact, when the Au nanoparticles were heat treated at high humidity and temperature conditions, the Au nanoparticles developed facets and grew into polygonal shapes. It appears that at high water content the anisotropy of the intrinsic surface energy of the Au nanoparticle dictated the particle growth, rather than the interfacial energy between the lipid molecules and the particles surface. Lastly, to better demonstrate the effect of the moisture on the coarsening of the Au nanoparticles on a solid substrate, DOTAP-stabilized Au nanoparticles were synthesized by dissolving the nanoparticle-embedded DOTAP membrane in isooctane and by removing the excess lipid molecules through centrifugation. 3 μL of the nanoparticle solution was dropped on a carbon grid to induce self-assembly of DOTAP-stabilized Au nanoparticles. The self-assembled Au nanoparticles were

Figure 5. AFM images of the DOTAP membrane: (a) after heat treatment for 15 h at 100 °C with no humidity; (b) after hydration for 15 h at 20 °C and 80% relative humidity.

subjected to annealing at 20 °C and 80% RH up to 20 h. A series of TEM images of as-prepared Au nanoparticles and after annealing at 20 °C and 80% RH for different periods are shown in Figure 6. The TEM images show that the Au nanoparticles that were self-assembled on a substrate decreased in size mostly because the elongated nanoparticles became progressively spherical during hydration. The average particle size, which was 8.5 ± 2.1 nm prior to annealing, decreased to 7.5 ± 1.4 nm after 20 h of hydration at 80% RH. It appears that without the surrounding lipid matrix the particle migration was not possible and remained fixed in the superlattice. Since the limited particle migration excluded the coalescence of the nanoparticles, the nanoparticles solely underwent the shape refinement. The 10985

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lipid mobility and degree of freedom to migrate in the membrane.

4. CONCLUSION It was demonstrated that temperature and humidity had different effects on the coarsening behavior of the Au nanoparticles embedded in a liquid crystalline lipid membrane (DOTAP). Although relatively low temperatures used in the experiment (20 and 100 °C) substantially slowed the coarsening rate, it was identified that high temperature without humidity resulted in shape refinement whereas humidity alone induced self-organization of the nanoparticles into a monolayer. It was also found that annealing in both cases tended to segregate the lipid molecules, creating a large fraction of areas devoid of the Au nanoparticles as the increasing lipid segregation forced the nanoparticles into a tighter area. The lipid segregation eventually led to the coalescence of the nanoparticles, especially in the case of the high humidity sample where the monolayer configuration accelerated the particle-toparticle impingement.

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ASSOCIATED CONTENT

S Supporting Information *

Cross-sectional TEM image and X-ray reflectivity of the assynthesized Au nanoparticles and a series of TEM images of the annealed Au nanoparticles for intermediate annealing hours at dry 100 °C and at 20 °C, 80% RH. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone +82-2-2220-0384; Fax +82-2-2220-1838; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2012000815).

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REFERENCES

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Figure 6. (a) TEM image of the DOTAP-stabilized Au nanoparticles that were self-assembled on a carbon grid, (b) after annealing at 20 °C and 80% RH for 10 h, and (c) after annealing at 20 °C and 80% RH for 20 h.

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