Influence of Cetyltrimethylammonium Bromide on Gold Nanocrystal

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Influence of CTAB on Gold Nanocrystal Formation Studied by In Situ Liquid Cell Scanning Transmission Electron Microscopy Silvia A. Canepa, Brian T. Sneed, Hongyu Sun, Raymond R. Unocic, and Kristian Molhave J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06383 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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The Journal of Physical Chemistry

Influence of CTAB on Gold Nanocrystal Formation Studied by In Situ Liquid Cell Scanning Transmission Electron Microscopy a

b

a

b,*

Silvia A. Canepa, Brian T. Sneed, Hongyu Sun, Raymond R. Unocic,

Kristian Mølhave

a,*

a

Department of Micro- and Nanotechnology, Technical University of Denmark, 2800 Kongens, Lyngby, Denmark

b

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA

ABSTRACT: The synthesis of monodisperse size- and shape-controlled Au nanocrystals is often achieved with cetyltrimethylammonium bromide (CTAB) surfactant; however, its role in the growth of such tailored nanostructures is not well understood. To elucidate the formation mechanism(s) and evolution of the morphology of Au nanocrystals in the early growth stage, we present an in situ liquid-cell scanning transmission electron microscopy (STEM) investigation using electron beam induced radiolytic species as the reductant. The resulting particle shape at a low beam dose rate is shown to be strongly influenced by the surfactant; the Au nanocrystal growth rate is suppressed by increasing the CTAB concentration. At a low CTAB concentration, the nanoparticles (NPs) follow a reaction-limited growth mechanism, while at high a CTAB concentration the NPs follow a diffusion-limited mechanism, as described by the Lifshitz-Slyozov-Wagner (LSW) model. Moreover, we investigate the temporal evolution of specific NP geometries. The amount of Au reduced by the electron beam outside the irradiated area is quantified to better interpret the nanocrystal growth kinetics, as well as to further develop an understanding of electron beam interactions with nanomaterials towards improving the interpretation of in situ measurements.

growth is necessary to advance the production of tailored nanomaterials with optimized structure and properties.

INTRODUCTION Noble metal nanoparticles (NPs) have applications in catalysis,1,2 sensing,3 and spectroscopy,4 due to their physico-chemical properties.5 These properties are highly dependent on structural parameters such as NP size, shape, and chemical composition. There is a need to improve our understanding of the nucleation and growth mechanisms occurring at the nanoscale to optimize properties. Many synthesis methods have been developed, including wet chemical and solvothermal strategies, electrochemical deposition in templates, electrochemical synthesis in solution, photochemical methods, and thermal evaporation.6–9 Among these strategies, colloidal methods offer the advantage of a high degree of control over NP size, shape, and composition. In the colloidal synthesis of shape-controlled NPs, a reducing agent is employed together with a surfactant to induce and regulate the nucleation of seed crystals and the growth rates of the evolving facets.10,11 However, it is challenging to decouple the preferential adsorption of surfactant molecules on specific crystal facets from the effects of the reducing agent and other shape-directing additives, such as halide salts.12,13 Thus, exploring the direct effect of capping surfactant molecules and reductants on NP formation and

Gold (Au) has been investigated in this regard due to local surface plasmon resonance (LSPR), which has opened new potential applications.14 Because of LSPR and other shape-dependent properties, as in catalysis, much effort has been placed on the synthesis of NPs with different morphologies such as spheres, cubes, rods, octahedra, and decahedra, for example.15–17 The synthesis of welldefined Au NPs is achieved by means of seed-mediated growth, with a large number of methods employing cetyltrimethylammonium bromide (CTAB) as the surfactant.18 CTAB is a “face-blocking” agent in the formation of Au nanocrystals.19 Moreover, the use of CTAB can affect the crystallinity of the seeds and the overall reaction kinetics by changing the reduction rate of the Au ions.20 The reduction potential of Au+ is also modified by the presence of bromide in the growth solution, as Aubromide complexes exhibit a lower value rate. Ex situ characterization methods, as well as a wide range of in situ spectral techniques, have been employed to study the dynamic evolution of Au NPs.21,22 The in situ liquid cell scanning transmission electron microscopy (S/TEM) technique23 allows for the direct observation of dynamic processes in a liquid layer, such as the nuclea-

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tion and growth of NPs from liquid phase precursor solutions,24 towards elucidating growth mechanisms, crystallization processes, and self-assembly.25–30 Many unseen and unpredicted mechanisms of colloidal nanocrystal growth and morphological evolution have been reported.31 For example, by imaging through thin droplets (~30 nm) of an aqueous solution inside a liquid cell, Loh et al.29 reported each step in the growth of gold and Ag NPs in water, from the initial supersaturated homogeneous solution, spinodal decomposition, solidification, and crystallization, resulting in Ag NPs ranging between 10-38 Å in size. These studies demonstrate that in situ liquid STEM can be used to investigate the fundamental kinetic, thermodynamic, and self-assembly rules that govern NP synthesis. However, several experimental factors related directly to the STEM imaging conditions can affect the growth kinetics of the NPs. For example, the effect of imaging mode (TEM vs. STEM), the electron beam accelerating voltage, and the electron dose on the final NP morphology have been investigated.24,32 Therefore, selecting the optimal experimental conditions that minimize deleterious effects is necessary to fully realize quantitative analysis in liquid STEM experiments. Strategies to understand, quantify, diminish, and/or mitigate the damage resulting from the beam-induced reactions within the liquid, both within and outside the field of view, are being addressed.33,34 In this work, HAuCl4 and CTAB are used as a model system to investigate the effect of CTAB surfactant on Au nanocrystal growth and morphological evolution during electron beam-induced reduction. We use time dependent measurements, derived from real-time observations, to show the influence of surfactant on the Au NP growth rate and shape evolution by using different CTAB concentrations at the same precursor concentration. The effect of using different electron beam dose rates is also investigated. Due to STEM image resolution limitations, NP growth was analyzed and quantified only when small “seed” NPs were observed with in the electron beam irradiated area. Nanocrystal shape evolution was analyzed by combining quantitative image analysis methods with a 2D geometric shape factor. The difference between spherical and non-spherical nanoparticles is well described by the 2D shape factor and can be used to predict the final NP shape (in this work, a twinned bipyramidal or decahedral structure) while monitored during the NP growth process. Reactions occurring outside the field of view influence the processes occurring within the field of view, and are also quantified to interpret the observed processes.

■ MATERIALS AND METHODS

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Precursor Solution Preparation. Stock solutions of chloroauric acid (Gold(III) chloride trihydrate, HAuCl4, Sigma-Aldrich Co.) were made by dissolving the Au precursor in deionized (DI) water at concentrations of 0.5 mM and 1 mM. A 100 mM CTAB solution was prepared by dissolving 0.364 g of cetrimonium bromide surfactant (cetyltrimethylammonium bromide, CTAB, SigmaAldrich Co.) in 10 mL of DI water. The high concentration CTAB solution was then diluted to 50 mM and 7.5 mM by the addition of DI water and 1 mM HAuCl4 solution to obtain the final concentration of 0.5 mM HAuCl4, which is in the range 0.2-10 mM of typical synthesis concentrations in publications. In situ Liquid STEM Experiments. The microscopy experiments were performed using a commercial in situ liquid cell system (Protochips Poseidon 500) in an FEI Titan STEM operated at 300 kV. A thin liquid layer of the precursor solution was encapsulated between two silicon microchips, each with a 50 nm thick, 50 µm × 200 μm electron-transparent silicon nitride (SiNx) membrane, which serves as the imaging window. In order to maintain a stable liquid layer thickness, one of the microchips had a 500 nm thick SU-8 spacer layer to maintain the 500 nm distance when the two microchip devices are stacked together; however, due to some membrane bulging, imaging likely occurs through a 0.5-1µm thick liquid layer (see Figure S1). High Angle annular dark field (HAADF) STEM imaging was performed at different electron dose rates by varying the magnification, with a pixel dwell time (4 µs pixel-1), a pixel scan area (1024 x 1024 pixels), and an additional ~25% dose deposited on the edge of the image due to flyback in the STEM raster generator. The electron beam current (60 pA) was measured prior to the in situ experiments and was maintained during NP growth. Videos of the nanocrystal growth were captured using the screen recorder software Camtasia; post-processing and quantitative image analyses were performed using ImageJ software.

Particle Analysis and Shape factor. The effects of CTAB on the nucleation and growth kinetics of Au NPs were investigated through quantitative image processing. During the liquid STEM experiments, the data acquired are 2D projections recorded using a HAADF STEM detector. To assess the 2D shape of the NPs such that 3D structures can be ascertained, a 2D shape factor, f, is defined as: ݂=

‫´ܥ‬ ‫ܥ‬

where C´ is the experimentally observed shape circumference (perimeter) of a particle with area A, and C=2√πA is the circumference of a spherical plate with the same area

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as the imaged NP. By definition, f=1 for a circular particle and f>1 for non-circular geometries. Examples of the shape factors calculated for regular shapes are presented in the SI Figure S2 and Table S1.

■ RESULTS AND DISCUSSION The formation of Au NPs was accomplished by electron beam-induced reduction. In the absence of the electron beam, Au growth is not observed, but under electron beam irradiation, a variety of complex chemical reactions occur that induce the breakdown of water molecules and the formation of aqueous electrons (eaq−) and other radiolytic species, thereby shifting the balance of reducing and oxidizing radical species.24,35 Au3+ reduces to Au0 (depending on the particular imaging conditions used),33 resulting in Au NP nucleation and growth, with etching observed at higher dose rates.36 Au nanocrystals were visible in the STEM when growing NPs attached on the SiNx window; Au clusters and particles moving freely within the liquid are not visible in STEM, and thus, later growth phases than those discussed by Loh et al.29 are considered here. The smallest detectable particle size measured via intensity thresholding/segmentation was 3.5 nm at the lowest magnification and electron dose rate (EDR), which is ~3X the pixel size of 1.2 nm. Figure 1 shows a series of HAADFSTEM images after 300 s of e-beam-induced reduction of the HAuCl4 solutions with controlled concentrations of CTAB and EDR. By varying the EDR (65 e-nm-2s-1 to 1040 enm-2s-1), the growth rate and morphology of Au NPs changes significantly. At the higher EDR and CTAB concentration, as in Figure 1c, NPs appeared, fluctuated in size, and ultimately disappeared, while some larger ones remained. This behavior is likely due to Ostwald ripening with oxidative dissolution of smaller, less stable particles.36 At the left edge of the images, where the STEM scan generator creates a higher localized dose, the thicker deposit obscures the view at higher EDR (Supporting Movie S1).

Figure 1. HAADF-STEM images acquired after 300 s of ebeam irradiation from an aqueous solution of 0.5 mM HAuCl4 with (a) no CTAB, (b) 7.5 mM CTAB, and (c) 50 mM CTAB. Beam current was 60 pA. From left to right, the EDR increases from 65 e-nm-2s-1 (1st column, scale bar 500 nm), 260 e-nm-2s-1 (2nd column, scale bar 300 nm), and 1040 e-nm-2s-1 (3rd column, scale bar 100 nm). Based on these observations, the relatively low EDR condition of 65 e-nm-2s-1 was deemed as a the most representative environment for comparison with ex situ Au NP synthesis, and match or are below what is presented as low dose conditions in other liquid cell TEM studies.37,38Observations of Au NPs formed with the high CTAB concentration are consistent with conditions that match those for ex situ synthesis of seed particles, which result in small, uniform Au NPs. The low CTAB concentration matches those of shaped particle synthesized with lower CTAB in a single step or by injection of seed particles.39,40 Growth of dendritic structures was observed in the absence of CTAB and at high EDR, in agreement with in situ liquid studies of Au nanocrystal growth without CTAB.41,42 While several studies discussed the distribution of radiolytic products and quantified details of the outflux of reducing species from the field of view (FOV),36,42 to the best of our knowledge, direct measurement of the actual products, such as eaq−, has not yet been attempted in situ. There have been few results of the influx of the metal ions from unirradiated solution in the FOV and outward flux of radiolysis species from the FOV.43 The complex and dynamic diffusive mixing of species creates a boundary layer outside the FOV, where metal ions can precipitate before reaching the FOV and precipitate NPs. Analyzing the non-irradiated area is essential to investigate possible limitations for the diffusion of ions into the FOV, result-

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ing in depletion effects that could influence the reaction mechanisms within the FOV. Radiolytic reduction occurs up to ~1 µm from the beam source,33 but here we observed considerable NP deposition occurring several micrometers from the FOV, as shown in Figure S3. We consider the important possibility of depletion in the imaged region by quantifying the amount of Au deposited inside and outside the irradiated area (see Table S2). The Au NPs have well-defined shapes, and the number of moles of deposited Au was determined from Au NP volumes assuming hemispherical geometry based on 2D projected areas from the HAADF-STEM images. The number of moles of Au deposited during the course of acquiring a single image series is very small compared to the total quantity of Au initially available in the liquid within the cell and experiments were limited to acquiring five image series during a single in situ liquid STEM session (SI section 2). The bulk Au concentration can be considered constant between all the experiments where NP growth is being observed, as confirmed by reproducibility tests. Given that some FOV deposits contain up to ~1000X more Au than available in the FOV region, Au3+ must be diffusing into the FOV. Since primary deposits were created over t ~200 s, and in this time period Au3+ can diffuse L= sqrt(Dt) ~300 µm with D ~ 1000 (µm)2/s,43 we do not expect any diffusion limitations for the Au3+ reagent supply. Often, comparable amounts of Au were deposited inside and outside of the FOV even at low EDR, which likely results from a combination of the diffusion of radicals escaping from the imaging area and scattered electrons reaching the same region. The outflux of reducing species from the FOV may, however, reduce the diffusive Au3+ influx by causing a ring-like deposit outside the FOV, as shown in Figure S3. This is most visible for high CTAB concentrations, where very little Au deposited inside the FOV, as shown in Figure 1c. The ratio between moles of Au3+ deposited outside and inside the FOV increases with increasing EDR. The ring-like deposits indicate that the strong reducing environment generates concentration gradients, not only of the radiolytic species but also of the incoming ionic species that become depleted by depositing before reaching the FOV at high EDR. This is similar to ring deposits observed for e-beam-induced deposition based on surface diffusing species,44 when the FOV is so intensely irradiated that the incoming species never reach the FOV. One could refer to this effect as a ‘liquid cell proximity effect’ as observed in e-beam lithography when nearby regions become strongly affected by their vicinity to the irradiated FOV. At high EDR, deposits are formed ahead of the rastering STEM e-beam during acquisition of the first frame, which indicates that the low EDR of 65 e-

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nm-2s-1 provides the most reliable imaging condition based on limited deposits forming outside the FOV and indicates [Au3+] remains fairly constant during each experiment, as in ex situ work. The time-dependent evolution of Au NPs at the lowest EDR was studied, as shown in Figure 2, which reflected conditions for ex situ NP synthesis. For pure solutions of the HAuCl4 precursor (no CTAB), Au NPs exhibit a blurred dendritic morphology (Supporting Movie S2), while faceted Au NPs formed under the same imaging conditions when CTAB was present in the solution. It is evident that the number and growth of Au NPs for the 50 mM CTAB addition is less than that for the 7.5 mM CTAB. These results demonstrate the important role of CTAB on Au NP growth kinetics and morphological evolution. For ex situ NP growth with e.g. injection of sodium borohydride as reducing agent, NP formation occurs within seconds, which is comparable to the observations here.22

Figure 2. Formation and size evolution of Au NPs from 0.5 mM HAuCl4 with (a) no CTAB, (b) 7.5 mM CTAB, and (c) 50 mM CTAB. The 500 nm scale bar is for all images. In order to trace the size evolution of the Au NPs, a series of time-lapse HAADF-STEM images were acquired, as shown in Figure 2 (Supporting Movie S3 and S4), which capture the full NP nucleation and growth process. Due to the resolution limitations for imaging within a liquid layer, the typical nucleation from the initial supersaturated homogeneous solution to the final crystalline solid seed29 (10-38 Å) could not be observed or captured. The growth could be captured and quantified only when small Au “seed” NPs were observed in the FOV. In this context, we define nucleation differently from the classical term; here nucleation refers to the first observable instance of a NP growing, which can be detected via intensity thresholding against the background. Seed is defined as the smallest nucleated NP appearing in the FOV. Thus, three stages of growth were observed: during the first ~70 s

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(initial nucleation stage) with CTAB, an increasing number of NPs were observed for all conditions, with slightly faster nucleation observed in the solution with no CTAB. The nucleation and growth of Au NPs were dynamic with the addition of 7.5 mM CTAB to the solution; while most of the NPs increased in size over time for the entirety of the irradiation period, ~ 3% of the NPs disappeared from the FOV. After 300 s irradiation, the FOV was covered by faceted Au NPs with an average size of ~50 nm, and they remained separate with a few NPs merging after 600 s. This is in contrast to the no-CTAB case where dendritic particles merge into a filamentous-dendritic structure. At 300 s for the 7.5 mM CTAB solution, the average distance between particles is 11 nm, which is larger than the CTAB double layer distance of 2.5 nm.45 The NPs counting rates for the Au NPs are shown in Figure 3a, and the corresponding nucleation rate is shown in Figure S4, which were quantified by intensity thresholding/segmentation using ImageJ during the entire irradiation time for each of the CTAB concentrations. For the case of no-CTAB, counting was not performed beyond the initial detection due to filamentous particles growing into contact with one another. The nucleation of Au NPs exhibits a significant increase within ∼63 s and then stabilizes to a slow rise. At the high CTAB concentration, nucleation rates were reduced to 8 NPs/s (50 mM) from 16 NPs/s (7.5 mM), noting that 20 NPs/s formed with no CTAB. After the initial frames were acquired within 63 s, it is clear that the number of new NPs forming in the FOV was reduced by the CTAB (Figure 3a). For the 50 mM CTAB solution, very few new growth events were observed after the initial particles formed within 63 s, indicating this system is far below the nucleation threshold. To study the growth kinetics and identify the mechanisms for growth of CTAB-capped Au NPs, we tracked the projected area changes of individual Au NPs over time (Figure 3b) by averaging the diameter for all particles formed during short- and long-term experiments, which also serves to verify reproducibility. The growth processes can be divided into three distinct regimes: initial NP nucleation and growth from 0-63 s (Figure 3a), continued NP growth from 63-240 s (Figure 3b), and slow, sustained NP growth beyond 240 s (Figure 3b). The growth rate (in area/sec) during the second stage of growth is 10 times slower for 50 mM than 7.5 mM CTAB. Apart from the influential proximity effect, the likely explanation for this behavior is that the Au seed surfaces are capped by the CTAB, which limits further crystal growth and results in the formation of Au NPs with smaller sizes; an effect also noted in the ex situ synthesis of seed nanoparticles.46 A large proportion of Au ions will form bromide complexes,

which have a lower reduction potential47,48 and are likely lower the reduction rate.

Figure 3. (a) Number of NPs detected per frame at a given time for 7.5 mM (black) and 50 mM (red) CTAB. Detection only counts particles staying in view, i.e., does not include particles appearing or disappearing shortly in the FOV. (b) In situ quantification of Au NP growth. Evolution of the average area of all NPs present in the FOV for different CTAB concentrations, 7.5 mM (black) and 50 mM (red). Two data sets analyzed for 7.5 mM CTAB: short-term experiment (300 s, black triangle) and longterm experiment (600 sec, black square). EDR of 65 e-nm2 -1 s used. T=0 is the time when the beam was turned on. Further insights regarding the morphological evolution of faceted Au NPs were extracted by quantifying the deviation from spherical particle geometry via the quantitative shape factor parameter, f. The f values for the NP populations were used to determine the nanocrystal shape distribution at the end of the irradiation times (Figure 4). For 50 mM CTAB, 80% of the NPs had a shape factor between 1-1.05, suggesting a highly spherical shape, while for 7.5 mM CTAB, only 14 % of NPs had a value of f between 1-1.05, whereas 72 % had a value of f between 1.11.25. The 50 mM CTAB resulted in a predominantly spherical shape (f ~ 1) while 7.5 mM CTAB resulted in NPs with

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mostly rhombic and/or “pointed” shapes (f > 1). At low CTAB concentration, the polydispersity of the imaged particles appears to be somewhat large compared to optimized liquid synthesis procedures, likely a cause of the strong substrate interaction with the silicon nitride membrane for the immovable particles.

Figure 4. Histogram of shape factors, f, for the all Au NPs at a dose of 65 e- nm-2s-1for the two CTAB concentrations: 7.5 mM CTAB (black) and 50 mM CTAB (red) from images shown in Fig. 2b and 2c at t=300 s. The scale bars are 500 nm.

Figure 5. Au NP shape evolution over time. Shape factor f vs. time for three NPs at 50 mM CTAB (red) and six (black) at 7.5 mM CTAB at a dose rate of 65 e-nm-2s-1. As shown in Figure 5, when the CTAB concentration is low, individual NPs evolved from a roughly circular/spherical shape (f ~ 1.02) to final rhombus (f ~ 1.24) and pentagon (f ~ 1.07) shapes over the 300 s period. During the initial nucleation stage (~56 s from Figure 3a) and early growth stage up to 100 s, a clear shape for most of the NPs was not clearly discernable. The Au NPs started as spherical After 120 s, it is possible to observe the NPs transforming into well-defined shapes (see SI Figure S5 and S6). The effect of CTAB addition on the growth rate for the Au NPs was studied as a function of morphology and time.

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The formed Au NPs could be categorized into three well defined shapes: sphere, rhombus, and pentagon for the 7.5 mM CTAB and spheres for 50 mM CTAB (Figure S7 and S8). The growth kinetics for each category were fitted by according to power law behavior for the equivalent radius, approximated as R ~ (A / π )½ with a time, t, dependence R∼ Kt β , where K is a reaction rate constant and β is the growth exponent.49 A t1/3 scaling is predicted for a purely diffusion limited process49 in the Lifshitz−Slyozov−Wagner (LSW) model.35 In these experiments, the β value was obtained by a linear fitting of the logarithmic relationship between R and t. Power law fits for the growth rates for an average of three NP growth trajectories in each category show that for a concentration of 50 mM CTAB, the mean radius R grows as t0.23 ± 0.03, and for a concentration of 7.5 mM CTAB, R grows as t0.54 ±0.02 for pentagon shapes, t0.55 ±0.03 for spheres, and t0.70 ±0.01 for rhombus NPs (Figure 6). Interestingly, even though we cannot resolve the rhombic shape in the early stage, the growth rate indicates these particles are a different class in terms of growth rate. At longer times the final slow growth sets in with lower overall growth and exponent around 0.15.

Figure 6. Mechanism of growth of selected Au NPs in the presence of 7.5 mM CTAB (black) and 50 mM CTAB (red). Logarithmic relationship exists between equivalent particle radius, R, of average size NPs and time (see SI section 2). These growth exponent values are different from those previously reported for in situ liquid TEM studies, where β values were 0.33 for Pt NPs,27 0.32 for zinc oxide NPs,50 0.495 for spinel ferrite NPs,51 0.4 for Pd deposited on Au,25 and 1.0 for linear growth of Au.52 In our case, it is clear from Figure 5 that surfactant CTAB concentration played an important role in modifying the nanocrystal growth exponent to lower values of 0.23 for a high CTAB concen-

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tration and higher values 0.54-0.77 at low CTAB concentration. The high CTAB LSW power fit shows that R scales as = 0.23, which is smaller than the diffusion limited t1/3 scaling, and this magnitude is often considered a reasonable match to β = 1/3. However, to fully understand and quantify the diffusion-limited growth model, Woehl et al.53 demonstrated that the LSW model prediction of β= 1/3 is valid both for Ostwald ripening and Smoluchowski aggregation.54 These two different processes can be distinguished by analyzing the normalized particle size distributions (PSDs), where the shape of the experimental PSD should be symmetric about R/=1 for Smoluchowski aggregation and asymmetric and skewed towards larger NP sizes for Ostwald ripening. The PSDs for Au NPs measured at different times in FOV and all CTAB concentrations exhibit symmetric profiles around the average radius (Figure S9-12), supporting a NP aggregation mechanism, which is also supported by the fact that even the smallest particles grow with time at low EDR. Ostwald ripening may still be present but at a much lower rate; in fact, the 7.5 mM CTAB PSD indicates a skew in the Ostwald distribution (see SI Figure S12) between 300 to 600 s, and it is possible that longer term observations would indicate a low Ostwald ripening rate that is only measurable once the initial fast NP aggregation is reduced. One possible explanation for the observed difference between the theoretical β=1/3 and experimental β=0.23 growth exponent is a consequence of the CTAB growth solution. Growth at the lower CTAB concentration mainly follows a R~t½ power law, consistent with the characteristics of reaction-limited growth predicted by the LSW model.32 In this case, nanocrystal growth is limited by the surface area available for the reduction reaction. The surface addition of Au atoms to the nanocrystal seeds may be limited by the presence of CTAB in the solution. At high CTAB concentrations, the proximity effect was pronounced and the FOV was likely Au-depleted as far more Au deposited outside the FOV, which likely even became increasingly depleted with time. This could explain diffusion-limited growth with a very low exponent of less than 1/3. When the 2D shape factor stabilizes over the course of the experiment, it can be used to analyze the final 3D shape. The determination of a high number of pentagonal and rhombic 2D projections agrees well with a decahedral crystal model, where five tetrahedra join faces about a 5fold symmetry axis (sharing one common edge) to create a penta-twinned bipyramidal crystal structure.15 This structure, when viewed from the direction parallel to the

5-fold axis will take a pentagonal shape, whereas when viewed 90° from this orientation, appears rhombic due to the two poles of the symmetry axis and the lengthier vertices along the equator of the crystal.55 While post mortem was not conducted on to confirm this structure, the shape has been well documented, and the projections from the low magnification images (Figure 2b) fit well with those from the literature.15 We further speculate the spherical particles may also be twinned bipyramidal decahedra that have been truncated or etched on the energy vertices during the growth. Because the proposed formation mechanism for the penta-twinned decahedra is based on the formation of crystal twins as a kinetically controlled product, usually coupled to an oxidant added during nucleation and growth of seeds,5 we suspect that these multiply twinned crystals dominate because of a relatively fast growth rate in the presence of the mixed reductive and oxidative radicals produced by the electron beam35,36 Furthermore, our work is preliminary evidence decahedral growth mechanism from twinned seed. Future work is aimed at tuning the effects of the beam to generate and study the formation of alternate shapes through introduction of various mild reducing agents and halide salts.

■ CONCLUSIONS We have investigated the effect of CTAB on the formation of Au NPs by in situ liquid STEM. The nucleation and crystal growth processes for Au NPs are directly observed. The addition of CTAB and its concentration play an important role in the growth kinetics by controlling the size and shape of the final Au NPs from HAuCl4. At 7.5 mM CTAB the nanocrystals grow by a reaction limited mechanism, supported by LSW growth rate exponent β ~ 1/2 and the faceted morphology of the nanocrystal. Based on the shape factor analysis the evolution of the faceted NPs, the formation of penta-twinned bipyramidal decahedra. The NP shape changes over time towards a fixed shape by the end of observations at 300 sec. When the CTAB concentration is increased to 50 mM, the growth rate is drastically reduced and the follow a diffusion-limited process β=1/3 and form round particles which is likely a result of precursors being limited in supply deposited outside the by an outflux of reducing species. The experimental PSDs at different times suggest Smoluchowski aggregation is the dominant growth mechanism while Ostwald ripening be present at a low rate and only become significant long times. Importantly our results show that it is essential to document what is outside the FOV since considerable deposition can take place the influx of precursor molecules to the imaged region aspect in situ studies to date. Our findings illustrate the ability of in situ TEM com-

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bined with quantitative analysis to obtain important information the controllable synthesis of metal NPs via wet chemical surfactant-assisted methods.

ASSOCIATED CONTENT

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Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org with the following content: Shape factor Liquid volumes in the experiment Amount of Au deposited Inside and Outside the Irradiated FOV Area Nanoparticle formation LSW model and PSD size distribution In situ follow-up of the growth of selected NPs: rhombus and pentagon Supporting movie captions.

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AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]

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Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT The in situ liquid STEM experiments were conducted as part of a user proposal at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences, a U.S. Department of Energy Office of Science User Facility. This work is supported by Funding from the Danish Research Council for Technology and Production Case No. 12-126194.

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TOC Graphic


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Influence of Cetyltrimethylammonium Bromide on Gold Nanocrystal

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