Growth Kinetics of Individual Au Spiky Nanoparticles Using Liquid-Cell

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Growth kinetics of individual Au spiky nanoparticles using liquid-cell transmission electron microscopy Wan-Gil Jung, Jeung Hun Park, Yong-Ryun Jo, and Bong-Joong Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03718 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Journal of the American Chemical Society

Growth kinetics of individual Au spiky nanoparticles using liquid-cell transmission electron microscopy

Wan-Gil Jung1, Jeung Hun Park2,3, Yong-Ryun Jo1, Bong-Joong Kim1*

1

School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju, Korea

2

Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544, USA 3

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

ABSTRACT: A precise control over size and morphology of the Au spiky nanoparticle (SNP) is essential to obtain narrow and tunable surface plasmon resonance (SPR). However, these challenges require a fundamental understanding of the particle growth mechanism and kinetics, as well as its morphological transition, which can only be achieved by real time observation at nanometer resolution. Here, we report in situ liquid cell transmission electron microscopy studies of single and multiple Au SNP growth at various conditions of such parameters as size and dose rate of electron beam, and HAuCl4 solution concentration. The particle evolves via a mixture of reaction and Au formationlimited growth, followed by Au formation-limited growth ‒ a transition from facetted to roughened surfaces, confirmed by the analysis with different beam sizes and the UV-vis spectra that feature a unique trend of short and long wavelength

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plasmon band shift. Quantitative analyses combined with a theoretical model determine the transition time (tc) of the two regimes and estimate the surface concentration (ci) of the particle with time. Interestingly, the tc can be manipulated by the particle density which alters surface roughening rate, and the density is modulated by tuning the aforementioned parameters based on DLVO theory. These results suggest a new method for synthesizing a Au SNP whose size, morphology, SPR and density can be sensibly manipulated without adding reducing or capping agents.

INTRODUCTION Au spiky nanoparticles (SNPs), so called nanostars (NSs) or nanourchins (NUs) have attracted intense attention due to their unique plasmonic resonance, which covers a wide range of spectra from visible to mid-infrared regions1-7. In particular, the size and shape-dependent physicochemical properties of Au SNPs, which are attributed to anisotropic distribution of the electromagnetic field near the surface of the particle,8-11 enable various interdisciplinary research via such functionalities as surface enhanced Raman scattering (SERS) detection12-16, surface plasmon resonance (SPR) sensing17, bionanosensing18-21, target drug delivery22, and catalysis23. Typically, to efficiently functionalize the Au SNP via tuning its diameter and surface roughness, the reactivity of Au ions needs to be manipulated by controlling the amount of reducing agents (ascorbic acid, dimethylformamide, HEPES, hydrazine, etc.) with the assistance of capping agents (CTAB, CTAC, PVP, citrate, etc.) which promote the anisotropic growth of branches. Using the reducing agents alone is for one-pot methods24 whereas adding both of the agents into the solution is for seed-mediated methods25,26. Nevertheless, a judicious


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control over the diameter and surface roughness of the Au SNP can only be achieved by understanding the growth mechanism and kinetics, and morphology evolution, all of which are highly sensitive to the specific growth conditions in different growth methods. In situ liquid-cell transmission electron microscopy (LCTEM) offers such insights at unique combinations of spatial and temporal resolutions by acquiring quantitative information on the growth of the particle which is precipitated from a solution of the metal ions via radiolysis as evidenced by previous reports27-33. However, the research using LCTEM is restricted to the following examples: First, most of the prior works were conducted using scanning transmission electron microscopy (STEM)27,28 under which consistent particle growth is interrupted, and the estimation of the hydrate electron concentration and solution concentration gradient are complicated due to the rastering electron beam on the sample with a fine spot size in consideration of radiolysis33. These conditions make it difficult to determine growth mechanisms and kinetics. Next, all the quantitative works using LCTEM are limited to multiple particle growth27,28,29-32, which prevents the understanding of intrinsic growth properties, and complicates the effects of the growth parameters including e-beam dose rates and solution concentrations on the properties. Finally, the quantitative analysis using TEM mode (continuous beam pose) in LCTEM with consideration of radiolysis was reported. However, this previous work in Ref. 33 only focused on facetted particle growth defined by bubbles which are centers for heterogeneous nucleation. Here, we use in situ LCTEM (in bright field mode and with no bubbles in solution) to provide direct insights into the growth mechanism and kinetics of single and multiple Au SNPs under different electron beam cylinder sizes, dose rates and solution concentrations. For the first time, we establish the condition for homogeneously



growing a single Au particle in a beam cylinder, which is essential to understand intrinsic growth properties, and observe the particle grows via two consecutive mechanisms of mixed (reaction and Au formation-limited growth together) and Au formation-limited growth. Our quantitative analyses combined with a kinetic model determine the transition time (tc) and calculate the surface concentration (ci) of the single particle as the particle grows, clarifying both properties are insensitive to dose rates and solution concentrations. Next, enlarging the cylinder size leads to multiple particle growth with increasing density. Surprisingly, the Au SNPs stably grow without interactions among them, thus undergoing the same growth modes as single SNPs, and the tc decreases with increasing the cylinder size due to faster surface roughening. The growth mechanism is confirmed by UV-vis absorption spectra ‒ initially the short and long wavelength plasmon band peaks are red-shifted up to tc, and then the shift quickly diminishes, and that cover a wide range of SPR regions (530 ‒ 1120 nm). Furthermore, we find that particle density is proportional to the dose rate but it remains nearly constant for a broad range of solution concentrations. Accordingly, the tc decreases with increasing dose rates whereas it remains approximately constant with solution concentration. Additionally, the Au formation rate, estimated by the growth rate at the Au formation-limited regime, can be limited by the control of dose rates and solution concentrations.

RESULTS AND DISCUSSION A series of TEM bright field images of single Au particle growth with time is shown in plan view in Figures 1a-e together with the schematics (Figures 1f-j) corresponding to the images, depicting the morphologies of the particle during growth.


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This growth experiment was performed within an e-beam irradiated cylinder (the diameter and thickness are 1.5 and 1.25 μm, respectively, in BF mode with a parallel and continuous beam) fully filled (no bubbles) with 10 mM HAuCl4 solution and at the e-beam dose rate of 127 e-/nm2·s. We observed that the particle nucleated homogeneously in the middle of the cylinder volume (see the rotating particle in Movie S1), and the morphology of the particle transformed during growth. The initial (~ 1 sec) structure of the particle is icosahedral quasicrystalline (Figures 1a,f) reflected by the three- (Figures 1k,l) and five-fold (Figures 1m,n) symmetries34, as presented in the HRTEM images and their corresponding FFT patterns of the particles. The three-fold symmetry is rotated by 37 degrees along the five-fold axis. Subsequently, the faceted particle starts to form spikes on its surface, and the size and number of the spikes increase as the growth progresses. A typical TEM BF image of the particle that begins to develop spikes is exhibited in Figure 1o with the related SAED pattern, indicating that the particle is polycrystalline due to the spikes. Such a state, in which both facets and spikes coexist, persists for some time, about 10 seconds (Figures 1b,c,g,h), followed by the condition in which the spikes fully cover the surface of the particle with extended lengths (Figures 1d,e,i,j). The SAED pattern obtained from a representative particle grown after 30 seconds shows an increased number of diffracted spots of the spikes (Figures 1 q,r) compared to the particle grown for a few seconds such as the one in Figure 1o. It is identified that the spikes of the particle in Figure 1q are more blunt than those in Figure 1e. Such reshaping of the spikes could be induced by the surface diffusion of Au atoms from the tips to the concave surface due to the chemical potential differences of the surface curvatures.26,35 Convexity calculations (i.e., convex hull perimeter divided by the actual



perimeter) also confirm the reshaping of the spikes as the convexity (0.84) of the Au SNP in Figure 1q is higher than that (0.63) in Figure 1e. (Figure S1) Figure 1s shows a SEM image of the fully grown particle for 40 seconds. The top and bottom of the particle become flat due to the limited height of the cylinder, allowing us to estimate the height of the cylinder ~ 1.25 μm. This evaluation should be accurate because the Au SNP growth is led by radiolysis in the solution, thus it could exclusively occur within the liquid volume as shown in Ref. 33 where the facetted Au plate laterally grows within ~ 100 nm thick liquid layer. Additionally, the spike of the Au SNP looks like a cone with no facets, which could also be associated with the reshaping of the facetted spikes as mentioned above. Figures 2a,b show quantitative measurements of r3 and r (the particle radius which is defined in the figure caption) versus time of the particle in Figures 1a-e, respectively. The particle growth took place within approximately 2 seconds after ebeam exposure onto the solution, and the time at which the growth started was estimated by extrapolating the data36. The particle initially grew linearly (Figure 2b) in size (~ r) but within approximately 11 seconds, reached a crossover (or transition) point (tc) after which the volume (~r3) of the particle linearly increased (Figure 2a). The tc at which the growth mechanism changes was determined by the fitted plots of the two regimes, as shown in Figure 2b. To interpret the growth regimes, we enlarged the beam size to form larger irradiated volumes of solution; 1.5, 2 and 2.5 times larger in volume than the one used for the single particle growth in Figure 1, as shown in Figures 3a-c, Figures 3d-f and Figures 3g-i, respectively. We used the identical beam dose rate and concentration of solution for the particle growth and observed multiple particle growth. As increasing the


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beam size, the number of particles increases from 4 to 6 and then 11 (including the particles cut by the edge of the beam circle). The plots of rtot3 versus t for the sum of all the particles in the irradiated cylinders in different volumes (Figure 3p) demonstrate that the total volume linearly increases after slow growth where the linear dimension to the total size, rtot , of the particles linearly increases with time (Figure S2) − the identical trend as for single particle growth. Importantly, the slope of the plot is linearly proportional to the irradiated volume (inset of Figure 3p) at a one-to-one ratio, indicating that the fast growth after tc is controlled by the formation rate of Au atoms33 within the beam cylinder where theoretically no surface saturation of Au exists. (If this regime is controlled by diffusion limited growth, the slope should remain constant regardless of beam cylinder sizes.) As well known in the absence of capping agent, the growth of the particle is controlled by one or more of the following processes31,33,37,38: the reaction rate at the particle surface, the attachment of Au atoms to the particle surface, the diffusion of Au atoms to the particle surface, or the formation rate of Au atoms. Here, from the structural and quantitative analyses shown above, we explain that the particle growth is limited by initially the mixture of surface reaction and Au formation rate, which is referred to here as the "mixed regime". This regime is where the portion of the Au formation limited growth progressively dominates that of the reaction limited growth as the number and size of the spikes on the growing particle increase, prior to the crossover point after which the Au formation rate solely governs the growth. The spiky surface being developed during the mixed regime leads to larger surface area to accommodate increased amount of Au, and the sharp points of the spikes collect more charges than concave or flat area, which could be related to the instability of facets and the initial



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growth of spikes. To elucidate these kinetics, we derive a model that predicts the solution concentration at the particle surface, ci

as a function of time, t. Assuming that the

neutral Au atoms are created as radiolytically produced eh s reduce Au3+ ions which diffuse in from the unirradiated solution, we find

  Rc   M t   M2 (1) ci   t Vm D 4 D M2 t 2 4 3 F  APF ,  M and  F are dr/dt (the slope of the mixed where  = VAu atom  Avogadro number regime) and dr3/dt (the slope of the Au formation limited regime) for slow and fast growth, respectively, APF is the atomic packing factor of Au crystal, VAu atom is the volume of an Au atom, Rc is the radius of the irradiated cylinder, D is the diffusivity of Au, and Vm is the molar volume of Au (see Supplementary Information SI 1 for the details of deriving the model). The fitted values of  M and  F directly let us evaluate ci as a function of time, as shown in Figure 2c. ci is extremely high when the particle is small, but it quickly drops as the particle grows and reaches nearly zero (0.11 ± 0.01 mM) at ~ 11 ± 0.2 seconds (see the orange line) corresponding to the point where the mixed growth regime transforms to the Au formation limited growth regime, which is consistent with our interpretation of growth mechanisms by experimental observation. Subsequent to the transition point, the composition remains nearly constant (deviating by only ~ 5% at the maximum size of the particle) due to the negligible size dependence for the large size of the particle based on the following relation37,38:  2 Vm   2 Vm   ci  ce  c exp  c exp   13 13    rRT   F t RT 

(2)

where ce is the equilibrium concentration, γ is the average surface energy of a Au


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particle (approximately 1.4 J/m2)39, R is the gas constant and T is temperature. We now discuss the dependence of dose rate and solution concentration on the particle growth in the same size irradiated cylinder (Rc = 0.75 μm) as shown in Figure 1. For the ranges of dose rates from 92 to 156 e-/nm2·s and solution concentrations from 5 to 20 mM, single particles nucleated and their growth mechanisms were identical to the case in Figure 1, as shown in Figures 2d,e. (see Figure S3 for r vs. t plots to identify the mixed regime.) The plots show that the growth rate of the Au formation controlled regime increases up to 2.5-fold with dose rates and solution concentrations (see the insets of Figures 2d,e). The ci of all the particles for two varied parameters are also calculated with time in Figure 2f. Intriguingly, it is observed that all the values of ci are collapsed, indicating the tc remains nearly the same regardless of the large variation in Au formation rates. This is because the roughening at higher dose rates and solution concentrations is faster (reflected by the larger volumes, r3, at tc), and therefore, in our experimental conditions, the growth rates in the two growth regimes strike a balance between the two terms in equation (1). From the aforementioned information, it could be reasonably assumed that a typical branching process plays a major role in growing the Au SNP: Spikes enclosed by high-index facets grow on {111} surfaces of icosahedral crystals26,35,40. We suggest that the driving force for the formation of the spikes would be the supersaturation at the surface of the Au particle caused by the flux of Au atoms to lower the surface energy of the particle. Compared to the single particle growth, multiple particle growth exhibits unique features, as observed in the TEM BF images in Figure 3. The plots of transition time, tc, versus total volume of all particles, rtot3 , in Figure 3p show that tc decreases with increasing irradiated cylinder volume. Such a decline in tc should be related to increased



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particle density and their growth kinetics. Interestingly, each particle grows via the same mechanism, mixed and Au formation controlled regimes as single particle growth throughout the entire growth process, reflected by the linearity in r versus t and r3 versus t plots in Figures 3j-o. The degree of roughening at particles differs due to the various fluxes of Au atoms to the particles, which are simply calculated by J ~

F

 M 2t 2

(see details in the Supporting Information, and the relevant plots of the particles grown in the three cylinders in Figure S4). This phenomenon could be affected by such factors as the location and nucleation time of the particle. Surprisingly, these trends represent no significant particle interaction during the growth, which will be discussed below. Notably, the values of tc of all the particles in a cylinder are nearly identical, thus corresponding to the tc for their total volume, as indicated by the vertical red lines in Figures 3j-o. The different volumes/sizes of the particles at tc indicate various degrees of roughening accomplished at the particles. However, the growth of all particles simultaneously enters the Au formation limited regime at tc, implying that the progress of roughening essentially stops at this point because the roughened surfaces of the particles can sufficiently absorb all the Au atoms created at each time in the beam cylinder. Therefore, we can explain that the decreasing tc with increasing the irradiated beam size is attributed to the faster surface roughening of the particles. Such faster roughening is visually identified by the rtot3 versus t plots of various sizes of cylinders, as shown in Figure 3p (and the inset of Figure 3q), which demonstrates that the growth rate of the total volume (~ rtot3 ) in the cylinder of the larger beam size is faster (the dotted line indicates tc of the particle growth in each cylinder.). Specifically, the dependence of the growth rate (drtot3/dt3) at the mixed regime on tc in


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the case of single particle growth is estimated using equation (1) when ci  0 , as shown in Figure 3q (see equation (7) in Supporting Information), indicating that the tc decreases as the growth rate increases with slower rates for larger cylinders. From this, to have the same tc of ~ 11 seconds, (the same tc for the single particle growth mentioned above) we need higher growth rates (thus faster surface roughening) in larger cylinders, as indicated by the dotted lines. These higher growth rates expected at single particle growth are compared to those experimentally obtained from the cylinders of different sizes where multiple particle growth takes place, as shown in the inset of Figure 3q, indicating the distance between the two cases becomes larger with increasing the cylinder size. This analysis confirms that the faster surface roughening at the larger cylinder gives rise to the observed trend of the reduced tc. Additionally, from the modified form of equation (1) acquired at ci  0 , Rc-eff, the effective irradiated cylinder size of each particle is evaluated as shown by blue circles in Figures 3a,d,g (see equation (8) in Supporting Information). We further measured the SPR properties of the Au particles grown at the same condition as in Figures 3g-i using UV-vis spectroscopy. The spectra in Figure 3r show a series of short and long wavelength plasmon bands in the visible and mid-infrared regions (530 ‒ 1120 nm) with increasing particle size, which are associated with the inner cores and spikes41. Intriguingly, the short plamson peaks red-shift rapidly up to the tc (marked by the red line), and then the shift becomes slower, corresponding to the growth trend of the core size (Figure 3s). Similarly, the long plasmon peaks increase quickly up to tc and subsequently the shift becomes negligible, indicating that initially the nucleation and lengthening of the spikes are rapid but after tc, the aspect ratio of the spike remains nearly constant42 (Figure 3s). Thus, the surface roughening is active only



up to tc, which agrees with the growth mechanisms illustrated above. Next, we focus on the multiple particle growth at varied dose rates and solution concentrations when the irradiated cylinder size is kept constant (Rc = 1.06 μm). Figure 4a shows that the number of particles increases with increasing the dose rate (1.0 ‒ 30 mM) while Figure 4b shows that the number of particles decreases with the solution concentration (57 ‒ 360 e-/nm2·s). The representative TEM BF images of the particle growth at the conditions which the vertical red lines in Figures 4a,b designate are shown in Figure 4e. These trends can be explained by the DLVO theory43 where the sum of the Van der Walls (attractive) and the EDL (electric double layer, repulsive) forces determine the total interaction potential (TIP) ‒ the activation energy for particle aggregation and the thickness of EDL. The TIP values relative to the interparticle distances are qualitatively plotted at typical cases of high eh  /low HAuCl4, mid

eh  /mid HAuCl4, and low eh  /high HAuCl4 concentrations as exhibited in Fig. 4c. These plots illustrate that with increasing eh  and decreasing HAuCl4 concentrations, both the TIP and EDL enlarge due to the enhanced surface charge and minimal shielding effect of the solvated positively charged ions such as Au3+, which explains the observed trends in the particle density. The trends in the particle density at varied eh  and HAuCl4 concentrations are depicted using the average values of the data in Figures 4a,b and shown in Figure 4d to guide the tendency. These trends are only effective for a beam cylinder whose radius is 1.06 μm. Reducing the radius should push down the plots, thereby lowering the particle density. Therefore, it is reasonable that the single particle growth discussed above can occur in a cylinder whose radius is 0.75 μm; the dose rate and solution concentration are marked by the dotted lines in Figure 4d. Additionally, DLVO theory predicts that increasing the particle size generally


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increases the TIP, allowing the particle to be more stable43. This is especially true as the surface roughens as in our case of spiky particle growth (surface potential should be more constant)44 although the stability of the particle is a complex function of such parameters as surface roughness, eh  and HAuCl4 concentrations, and inter-distance between particles. Hence, the interplay among the parameters with increasing particle size can account for the independent growth of particles in multiple particle growth, as discussed in Figure 3. For instance, at small particle sizes, the TIP is small but the interparticle distance is large whereas at large sizes, the opposite occurs, thereby providing the condition for the colloidal stability of the particle throughout the entire growth process. The dependence of beam dose rates and solution concentrations on the morphological transition and growth kinetics of the particles is more complicated than the case at various irradiated cylinder sizes. We extracted the quantitative data for growth kinetics of the particles discussed in Figure 4. Typical cases of dependence on both parameters are presented in Figures 5a,b (the data for the entire ranges of the two parameters are shown in Figures S5,6). It is observed that the growth rates of the total volume of the particles drastically increase with beam dose rate (Figure 5a), whereas they increase much less sensitively with solution concentration (Figure 5b). Concretely, the growth rate of the mixed growth regime rapidly increases with beam dose rate, leading to extensive reduction of tc. However, in the oppose sense, it slightly increases with solution concentration, resulting in minimal deviation of tc, as indicated by the dotted lines. These trends are also demonstrated in the plots of dose rate versus tc in Figure 5c, where entire sets of the data with the variation of the two parameters are included. These results demonstrate that tc is inversely correlated with particle density,



which is consistent with the results for various cylinder sizes mentioned above, suggesting that growth regimes and kinetics, and particle morphology and density can be precisely controlled by such parameters as beam size, dose rate, and solution concentration. To understand the reduction kinetics under the variation in dose rates and concentrations, the growth rates of the total volumes in the Au formation limited regime are compared as shown in Figures 5d,e. From the data in Figure 5d, it can be seen that the growth rates are saturated at low concentrations and high dose rates (effectively, the exponent β( eh ) from the power law fitting is 0.48 at 2.5 mM), where Au3+ concentration becomes dominated by eh concentration, which limits Au formation rates. In that respect, the dose rate for the saturation is delayed with increasing solution concentration (the β( eh ) increases from 0.55 to 0.68), and ultimately, this tendency vanishes at the highest concentration of our experiments, 20 mM (lowest pH), where a near linear dependence is obtained (β( eh ) ~ 0.95), which is in agreement with the Ref. 33. A similar situation also occurs with respect to solution concentration, as shown in Figure 5e. At low dose rates, the growth rates reach saturations at high concentrations (β(Au3+) ~ 0.367 at 52 e-/nm2·s), in which the eh concentration becomes deficient to the Au3+ concentration, thereby restricting Au formation. Increasing dose rate straightens the dependence (β(Au3+) ~ 0.66 at 360 e-/nm2·s). It is intriguing that all the power law fits to the data in each of Figures 5d,e nearly converge to a point at which the growth rate (drtot3/dt) is zero, called a threshold dose rate (or a threshold solution concentration) below which the particle growth is


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prohibited possibly due to the presence of a nucleation barrier33,43. For the dose rate dependence, the threshold dose rate is 37 ± 1 e-/nm2·s, which is approximately two orders smaller than the reported value obtained from the Au crystal growth via the same solution as our work in an irradiated cylinder whose thickness was in the range of 50 100 nm33. This range of the solution thickness is considered within the mean free path ( ~ 260 nm)45 of the incoming electrons in water. We presume that the significantly low threshold dose rate from our experiments might be mainly associated with the multiple scattering of the electrons with water molecules46, which generates increased numbers of hydrated electrons, eh , due to the thick solution layer (1.25 µm). Based on the model in Ref. 46, the dose rate increases with increasing liquid thickness, and the liquid layer which is located at 1.25 μm away from the top surface of the e-beam cylinder provides a dose rate which is 5.7 times larger, compared to zeroth order estimate. We note that the secondary electrons ejected from the Si3N4 membranes increase the dose rate in liquid merely ~ 5 % within 10 nm apart from the membrane surface, and thus their contribution to the particle growth is minimal. For the solution concentration dependence, the threshold concentration is 0.9 ± 0.05 mM.

CONCLUSIONS

We believe that the results shown here are of fundamental interest in advancing the current understanding of Au NP grown via radiolysis within a liquid cell using in situ TEM, owing to the wide ranges of experimental conditions using the parameters of cylinder size, dose rate, and solution concentration, as well as a thorough quantitative analyses with theoretical modeling of the single and multiple Au NP growth, all of which have never been carried out. Moreover, we provide the comprehensive



information for controlling the key properties of the Au SNP including growth regimes and kinetics, and particle morphology and density, which will be useful for tuning the broad range of plasmon resonance absorption by modulating the growth parameters without adding reducing agents or capping ligands. The methods illustrated here also open avenues to optimize the functionalities of applications in other research fields, including renewable energy47, catalysis23, optics12-16, and biomedicine22.

EXPERIMENTAL SECTION Solution preparation: Hydrogen tetrachloroaurate (III) (HAuCl4·3H2O) solution and

deionized (DI) water (Milli-Q) were purchased from Alfar Asear and Fisher Chemical, respectively. A stock solution of HAuCl4 was diluted with DI water to form solutions of 1, 2.5, 5, 10, 20 and 30 mM. Because of their light-sensitive nature, the solutions were stored in opaque glass bottles and covered with aluminum foil.

Preparation for a LCTEM holder for in situ experiments: To observe the process of

spiky Au particle growth induced by electron beam radiolysis, we used a liquid cell transmission electron microscope (LCTEM) holder purchased from Hummingbird Scientific. A LCTEM holder includes an inlet and an outlet liquid path lines both of which are built for the continuous flow of the solution into the micro-size aquarium. The aquarium is fabricated by an assembly of two (top and bottom) chips of silicon nitride (Si3N4) membranes whose dimensions are 50 × 200 μm, and the liquid thickness can be manipulated by a choice of spacers that are inserted between the two chips. Prior to assembling the chips, we cleaned the two chips with ethanol and deionized water, and then dried them in a desiccator. The Si3N4 membrane was turned hydrophilic by plasma


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cleaning for 30 seconds. Thereafter, the chips were assembled in the holder, and then they were inserted into the vacuum station (1.6 × 10-6 Torr) for several hours while the deionized water flowed at 10 μl/min to confirm the vacuum safety within the TEM (Tecnai 300 KeV) and to remove the bubbles in the solution. Finally, the holder was loaded into our TEM to perform experiments.

Experimental procedure and other details: In this work, the electron beam is used to

grow Au particles from a solution of Au ion via radiolysis, in which the electron beam forms hydrated electron ( eh ), among other species. To adjust the dose rate of the transmitted electron beam, which is an important factor for Au particle formation, we changed spot sizes and gun lenses. The dose rate of the transmitted electron beam was measured using a viewing screen. Electron beam sizes and dose rates used for our experiments were in the ranges of 1.84 – 2.37 μm and 57 – 360 e-/nm2·s, respectively. Because the particle growth takes place only within electron beam irradiated areas, we carried out numerous experiments by moving the beam to unirradiated areas. For each growth experiment, we refreshed the aquarium by flowing the solution at 10 μl/min for 30 min but liquid circulation was not applied during the growth experiment. The pH of HAuCl4 maintains below 2. The time for each experiment is less than 1 min. The conditions mentioned above also helped maintain the no bubble condition in the cell. We performed our experiments near the edge of the membranes that had constant liquid thickness induced by the difference in pressure between the inside and the outside of the cell. Movies were recorded at the speed of approximately 4 frames per second with 1024 × 1024 pixels in bright-field (BF) TEM imaging. TEM beam current is ≤ 1nA at 300 keV which would result in a maximum temperature rise of lower than 4 °C (ref. 43).



Beam-induced temperature rise is insignificant in our typical TEM imaging conditions.

UV-vis spectroscopy measurement: For UV-vis spectroscopy measurements, we

formed a high density of spiky Au particles for each growth time at the condition in which the beam radius was 2.37 μm, the dose rate was 127 e-/nm2·s and the solution concentration was 10 mM and moved the beam on the membrane to generate sufficient signals for detection. Then, the solution was dried, and the chips were separated to be loaded to the instrument. UV-visible absorption spectra were measured using a Beckman Coulter 530 spectrophotometer.

ASSOCIATED CONTENT Supporting Information

SI 1. Models for calculating the surface concentration of the Au particle and other properties. References for Supplementary Information. Supplementary Figures 1 -5.

AUTHOR INFORMATION Corresponding Authors *

Bong-Joong Kim

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS


Page 18 of 33

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B.-J. Kim acknowledges financial support from the “GIST-Caltech Research Collaboration” grant funded by the GIST in 2019, and NRF-2017R1A2B4003615. J. H. Park acknowledges funding support from the Princeton Catalysis Initiative.

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Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Main figures

(a)

(f)

1.04

(b)

2.59

(g)

(c)

4.14

(h)

(d) 12.69

(e)

(i)

(j)

(k)

(l)

(m)

(n)

(o)

(p)

(q)

(r)

33.94

(s)

Figure 1. In situ observation of a single Au particle growth, and Ex situ TEM and SEM analyses of particle morphology.

(a-j) A series of TEM BF images of single Au particle growth as time progresses with



the schematics corresponding to each image. The number at the top of each image is the time (in seconds) passed since the growth started. The insets in Figures 1(a-c) are expanded views of the Au SNP at early stage of growth. The scale bar is 1 µm. (k, l) A HRTEM image and FFT pattern of an iscosahedral Au crystal grown at the early stage, showing a three-fold symmetry. The scale bar is 5 nm. (m, n) A HRTEM image and FFT pattern of an iscosahedral Au crystal grown at the early stage, showing a five-fold symmetry. The scale bar is 10 nm. (o, p) A BF image and SAED pattern of a Au particle on which small spikes have just developed. The scale bar is 100 nm. (q, r) A BF image and SAED pattern of a Au particle on which spikes have grown extensively. The scale bar is 500 nm. (s) A SEM image of a fully grown Au spiky particle with its spikes whose surfaces are smooth. The three insets in Figure 1(s) are the SEM images taken from the three different view-directions as marked by the dotted arrows, clarifying the thickness of the particle which corresponds to the height of the beam cylinder (particularly the image at the bottom left was taken at 30 degrees of beam tilt from the side-view and clearly demonstrates the thickness of the liquid) and the rounded shape of the spike's surface. The scale bar is 1 μm.


Page 26 of 33

Page 27 of 33

20 (d) 2

10 5

3

r (10 nm )

3 6

100 120 140 160 2 Dose rate (e/nm )

9 6 3 0 0

5 10 15 20 Mole conc (mM)

(e)

25

5 mM 10 mM 15 mM 20 mM

0 100

(f)

60

-

20

0

0 0

40

2

156 ee-/nm /nm2 s 156 2 127 ee-/nm /nm2 s 127 2 /nm2 s 106 ee-/nm 106 2 92 ee-/nm /nm2 s 92 mM 55 mM 10 mM mM 10 15 mM mM 15 20 mM mM 20

40

20 20 30 t (sec)

4

5

Ci (mM)

40

10

6

12

80

60

0

8

10

(c)

80

10

7 3

Au formation Limited growth

12

3

15

dr /dt (10 nm /sec)

20

2

r (nm x 10 )

0 7 6 (b) 5 Mixed growth 4 3 2 1 0 100

0 25

3

3

5

-

156 e /nm s 2 127 e /nm s 2 106 e /nm s 2 92 e /nm s

3

7

10

15

6

3

15

dr /dt (10 nm /sec)

(a) r (10 nm )

20

3

3

7

r (nm x 10 )

25

Ci (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20 30 t (sec)

40

50

Figure 2. Kinetic analyses of single Au particle growth with the estimated composition at the surface of the particle.

(a-c) r3, r and ci vs. t plots of the particle shown in Figures 1(a-e), respectively. (d-f) r3, r and ci versus t plots of the single particles at various e-beam dose rates and solution compositions, respectively. Approximating the particle image by an ellipse, the radius r is calculated as the geometric mean of the semi-major and semi-minor axes; that is, the radius of a circle with an equivalent area.



(b)

(a)

(c)

3

1 4

2

9.3

2.6

(e)

(d)

13.7

(f)

4 5 6

1 2 2.6

(h) 10 9

11.1 40

(l)

2.12 μm 3 6

1 2 3 4 5 6

20 10

0

(k)

300

200 100 0

3

6 9 t (sec)

12

15

2.37 μm

0

(m)

300

200 100

(o)

200 100

0

0

(n)

1 2 3 4 5 6 7 8 9 10 11

30

3

3 6 3

10

6 7.5

r (nm)

300

1 2 3 4

r (nm)

3

0 400

20

r (10 nm )

40 20

30

4

2 7 3

r (10 nm )

40

1.84 μm

6

3

r (10 nm )

(j)

(i)

1 8

3.9

11.2 5

11

60

3 6.7

(g)

r (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

0

3

6 9 t (sec)

12

15


0

0

3

6 t (sec)

9

12

3 6

12

3

15

3

4

5

63

6

(r)

0.4 0.2 4

6 8 10 12 14 2 Wavelength (10 nm)

3 3

1

3

0 Wavelength (10 nm)

25 nm 50 nm 70 nm 98 nm 112 nm 132 nm 150 nm 180 nm 210 nm 250 nm

0.6

0.0

9

30

2

10 20 t (sec)

Single particle Multiple particles

0.1 1.4 1.6 1.8 2.0 2.2 2.4 Cylinder size (μm)

1.5 μm 1.84 μm 2.12 μm 2.37 μm

6

6 Irradiated volume (10 nm )

0

0.8

12

9 2

10

5

18

0

1.0

tc (sec)

100 50

(q)

3

15

1.5 μm 1.84 μm 2.12 μm 2.37 μm

dr /dt (10 nm /sec)

3

6

3

rtot (10 nm )

150 (p)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


drtot /dt (10 nm /s )

Page 29 of 33

5 10 15 20 3 3 4 3 3 dr /dt (mixed) (10 nm /s )

12 11 (s) 10 Au 9 Mixed formation 8 growth Limited growth 7 6 5 0 1 2 3 2 r (10 nm)

Figure 3. Kinetic analyses of multiple Au particle growth in various sized e-beam cylinders at a constant dose rate (127 e-/nm2s) and solution concentration (10 mM), and UV-vis spectroscopy study for growth kinetics.

(a-i) Three series of BF images of multiple Au particle growth in e-beam irradiated cylinders whose diameters are 1.84, 2.12 and 2.37 μm, respectively. The number at the bottom (in black) of each image indicates the time (in seconds) passed since the growth started. Numbers (in blue) are assigned to Au SNPs (see Figures 3(b,e,h)) to correspond to their plots in Figures 3(j-o). The blue circle indicates the calculated effective beam cylinder of each particle. The scale bars are 500 nm. (j, k) r3 and r vs. t plots of the particles in Figures 3(a-c), respectively. (l, m) r3 and r vs. t plots of the particles in Figures 3(d-f), respectively. (n, o) r3 and r vs. t plots of the particles in Figures 3(g-i),



respectively. (p) rtot3 vs. t plots for the sum of all the particles in the e-beam cylinders with different sizes. The inset shows the dependence of the cylinder volume on the growth rate (drtot3/dt3) for the Au formation limited regime. (q) Calculated plots of dr3/dt3 at the mixed regime vs. tc of single particle growth in various sized cylinders. The inset compares the estimated dr3/dt3 values of single particles at the mixed regime and at a constant tc of 11 seconds with the measured drtot3/dt3 values of multiple particles at the mixed regime and at tcs of the various sized cylinders. (r) UV-vis spectra of the particles grown in LCTEM under the identical condition to Figures 3(g-i). Each plot is acquired from the particles at different growth time. The average sizes of the particles at different growth times are labeled in the legend. (s) Plots of short and long wavelength plasmon band peaks vs. linear dimension r of total volume of the multiple particles based on Figure 3(r).


Page 30 of 33

1.0 mM 2.5 mM 5 mM 10 mM 20 mM 30 mM

(a)

# of Au particles

100

10

0

(b)

100

0

h-

(c)

0

Distance

(e)

# of Au particles

0 High e conc hMid e & HAuCl4 conc High HAuCl4 conc

_

2

57 e /nm s _ 2 127 e /nm s _ 2 194 e /nm s _ 2 265 e /nm s _ 2 360 e /nm s

10

100 200 300 400 2 Dose rate (e /nm )

TIP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


# of Au particles

Page 31 of 33

5 10 15 20 25 30 Sol. conc. (mM) Sol. Conc. (mM) 5 10 15 20 25 30

100 (d)

HAuCl4 conc he conc

10 Smaller beam size

0

100 200 300 400 Dose rate (e-/nm2s)

194 e-/nm2s 1.0 mM

360 e-/nm2s

57 e-/nm2s

2.5 mM 20 mM

Figure 4. Density analyses of the particles grown at various dose rates and solution concentrations in a constant sized irradiated cylinder (Rc = 1.06 μm).

(a) Plots of number of particles vs. dose rates with various solution concentrations. (b) Plots of number of particles vs. solution concentration with various dose rates. (c)



Qualitative plots of TIP vs. inter-particle distance at different eh  /low HAuCl4, ratios. (d) Qualitative plots of number of particles vs. dose rate with dotted lines indicating the growth condition for single particle growth. (e) TEM BF images of the particles grown at the conditions designated by the lines in Figures 4(a,b).

2.5 mM 5 mM 10 mM 20 mM

10

20 30 t (sec)

60 (d) 40

3 6

3

2.5 mM 5 mM 10 mM 20 mM

20

3

0

0

100 200 300 400 2 Dose rate (e/nm )

40 7 6 5 4 3 2 1 0

tc (sec)

10

0

50

(c)

(b)

127 e-/nm2s

drtot /dt (10 nm /sec)

6 3

40

300 250 200 150 100 50 0

3

6

2

3

_


rtot (nm x 10 )

300 10 mM 250 (a) 200 150 100 50 0 0 10 20 30 t (sec) drtot /dt (10 nm /sec)

3

3

6

rtot (nm x 10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

1 0

50

_

2

2.5 mM 5 mM 10 mM 20 mM

100 200 300 _ 2 Dose rate (e /nm )

400

(e)


0

4

8 12 16 20 Sol. conc. (mM)

Figure 5. Kinetic analyses of the particle growth at various dose rates and solution concentrations in a constant sized irradiated cylinder (Rc = 1.06 μm).

(a) rtot3 vs. t plots at various dose rates and at 10 mM. (b) rtot3 vs. t plots at various solution concentrations and at 127 e-/nm2·s. (c) tc at various dose rates and solution concentrations. (d, e) Plots of drtot3/dt3 at Au formation limited regime vs. dose rate, and solution concentration, respectively.


Page 33 of 33

1.04

2.59

12.69

4.14

33.94

1 μm

Absorbance

18

3

15

6

12

3

3

50

dr /dt (10 nm /sec)

6

100

1.0

1.5 μm 1.84 μm 2.12 μm 2.37 μm

150

3

rtot (10 nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

9 6 2

3

4

5

6

63

0

10 20 t (sec)

0.6 0.4 0.2 0.0

Irradiated volume (10 nm )

30

25 nm 50 nm 70 nm 98 nm 112 nm 132 nm 150 nm 180 nm 210 nm 250 nm

0.8

4

6 8 10 12 14 2 Wavelength (10 nm)

Table of Contents Graphic


Growth Kinetics of Individual Au Spiky Nanoparticles Using Liquid-Cell

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