Silver nanocube and nanobar growth via anisotropic monomer

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Silver nanocube and nanobar growth via anisotropic monomer addition and particle attachment processes Dongdong Xiao, Zhigang Wu, Miao Song, Jaehun Chun, Gregory K. Schenter, and Dongsheng Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02870 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Silver nanocube and nanobar growth via anisotropic monomer addition and particle attachment processes Dongdong Xiao1#, Zhigang Wu2#, Miao Song1, Jaehun Chun1, Gregory K Schenter1, Dongsheng Li1* 1

Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory,

Richland, WA 99352, USA 2

School of Science, North University of China, Taiyuan 030051, China

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ABSTRACT Understanding the growth mechanism of noble metal nanocrystals during solution synthesis is of significant importance for shape and property control. However, much remains unknown about the growth pathways of metal nanoparticles due to lack of direct observation. Using an in-situ transmission electron microscopy technique, we directly observed Ag nanocube and nanobar growth in aqueous solution through both classical monomer-by-monomer addition and nonclassical particle attachment processes. During the particle attachment process, Ag nanocubes and nanobars were formed via both oriented and non-oriented attachment. Our calculations, along with dynamics of the observed attachment, showed that van der Waals force overcomes hydrodynamic and friction forces and drives the particles toward each other at separations of 10100 nm in our experiments. During classical growth, an anisotropic growth was also revealed, and the resulting unsymmetrical shape constituted an intermediate state for Ag nanocube growth. We hypothesized that the temporary symmetry breaking resulted from different growth rates on (001) surfaces due to a local surface concentration variation caused by the imbalance between the consumption of Ag+ near the surface and the diffusion of Ag+ from bulk to surface.


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Introduction The morphology of colloidal nanocrystals critically determines their physical and chemical properties. The synthesis of nanocrystals with control of size and shape has become an important focus of both fundamental study and applications1-3. Understanding the growth mechanism is therefore a critical prerequisite to mastering the shape-controlled synthesis of nanocrystals. Generally, the crystal growth can be classified into two categories, namely classical and non-classical growth. In classical formation, crystal growth proceeds through monomer addition (MA) onto the surface of the growing particles 4. The growth kinetics is well described by the Lifshitz-Slyozov-Wagner (LSW) theory, which predicts two well-defined limits for nanocrystal growth: diffusion-limited and reaction-limited growth 5. Non-classical crystal growth is achieved by particle attachment (PA) and shows multiple pathways resulting from the interplay of thermodynamics and reaction dynamics 6. Among the various pathways, oriented attachment (OA), which proceeds by a crystallographic co-alignment of two particles on a specific crystal face, is a special one because it helps increase our understanding the formation of nanocrystals with complex shapes

7, 8

. However, it should be noted that most OA phenomena

have been reported on oxides and sulfides rather than metal systems 9, due to lack of definitive evidence showing the apparent assembly of co-aligned metal nanocrystals. Furthermore, our current understanding of the growth mechanisms of nanocrystals is mainly inferred from postmortem observation of particles; the intermediate states or growth dynamics remain elusive

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.

Therefore, comprehensive understanding of the nucleation and growth mechanisms calls for appropriate in situ techniques to illuminate the reaction kinetics involved in nanocrystal synthesis. In-situ X-ray scattering and /or X-ray absorption spectroscopy have been extensively adopted to probe the nucleation and growth processes of nanocrystals in real time11, 12, which


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enhanced our mechanistic understanding of crystal growth. However, these methods are incapable of directly following the morphology evolution, attachment and /or growth trajectories of nanoparticles at high spatial resolution. The advent of liquid cell transmission electron microscopy (TEM) fills the gap and allows us to directly observe the dynamic process of materials in a liquid environment at high spatial resolution, providing an unprecedented opportunity to advance our understanding of growth mechanisms in solution13-16. In a liquid cell, the interaction between the electron beam and the aqueous solution can create hydrated electrons (݁௛ି ) and free radicals (including H•, OH•). These species either reduce the metal precursor or change the pH and/or ionic strength of the solution, thereby enabling real-time observation of metal nanocrystal growth and particle attachment (PA) 17. Here we investigated the growth process of Ag nanocube and nanobar, both of which are of great importance in surface plasmon resonance scattering

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18

, catalysis

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, and surface-enhanced Raman

, in an aqueous solution through in-situ observation on a TEM. By tracking the

dynamic process, we disclosed different growth behaviors of Ag nanocubes, including both classical and non-classical growth pathways. During classical growth, an interesting shape transformation from nanocube to nanobar was revealed; it can be considered an intermediate state for further Ag nanocube growth. During non-classical growth, both non-OA and OA processes were observed, wherein the OA process leads to the formation of nanobar. Experimental Methods Materials: AgNO3 (purity > 99.0%) was purchased from Sigma-Aldrich Co. PVP (K30, MW = 40 k in terms of monomeric units) was supplied by EM Science. Trisodium citrate dihydrate (Na3C6H5O7•2H2O, >99.8%) was obtained from J.T. Baker Chemical Co. Homemade deionized water (DI water) was used. All reagents were used without further purification. An aqueous


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mixture of DI water (21.00 mL), PVP (0.15 M, 10 µL), and Na3C6H5O7•2H2O (0.01 M, 18.875 mL) was prepared. Then AgNO3 (0.1 M, 125 µL) was added to the solution with magnetic stirring. TEM experiments: We used a static liquid cell holder (Hummingbird Scientific, USA) for insitu observation. Two electron transparent SiNx (~50 nm) membranes of liquid cell were separated by a 100 nm spacer. Prior to loading the solution, the membranes were oxygen plasma cleaned for 1 min to remove organic contamination and render the surfaces hydrophilic. Up to a few µl of the precursor solution was loaded into a liquid TEM cell. The in-situ observation of nanocrystal growth was performed on a Tecnai F20 (200 keV, FEI, USA) at a dose rate from 190 to 220 e/Å2·s. The images were recorded using an FEI Eagle charge-coupled device (CCD) camera. Continuous movies were captured using a freeware screen grabber (VirtualDub) with a time resolution of about 2 s, which is limited by the CCD camera. Friction force measurement: The friction force between an Ag-coated tip and the silicon nitride membrane was evaluated by an atomic force microscope (AFM) (Cypher, Asylum Research) in the precursor solution as used in in-situ TEM. A silver-coated silicon nitride AFM tip (MLCT, B cantilever, Bruker) with a typical length of 210 µm was used. The real normal spring constant of the AFM tip was calibrated to be 14.85 pN/nm. Results and Discussion Using a liquid cell that operates inside a TEM, we followed the dynamic growth process of Ag nanocubes. It was found that they can grow either by classic monomer-by-monomer addition or by PA including OA and non-OA processes. In the classical growth pathway, an Ag nanocube can exhibit both isotropic (equal lengths, Figure 1A) and anisotropic (unequal lengths, Figure 2A and 2B) growth behaviors. The isotropic growth (movie S1) of an Ag nanocrystal 5 ACS Paragon Plus Environment

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induced by an electron beam in an aqueous solution after rapid nucleation was clearly demonstrated by both morphology evolution (Figure 1A) and size change (Figure 1B). The square morphology in Figure 1A has been frequently reported in the shape-controlled synthesis of Ag nanocrystals and corresponds to the two dimensional projection of an Ag nanocube with (100) facets exposed on the surface in the TEM images3,

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. Our ex-situ TEM results also

showed the resulting Ag nanocubes with (100) facets (Figure S1). It is worth noting that one of the surfaces of nanocube sometimes contacts the membrane and may grow slower compared to other faces due to limited diffusion of Ag+ and the nanoparticle may not be a perfect nanocube unless otherwise stated in the following discussion.

Figure 1. (A) In-situ TEM imaging of isotropic growth behavior of a Ag nanocube in aqueous solution; (B) The increase in edge length of the Ag nanocube with time, wherein edge L1 and edge L2 were equal during in-situ observation; the inset shows the relationship between growth rate and reaction time. (C) The cube of the edge length of the Ag nanocube shown as a function


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of time; the linear dependence L3 on time indicates diffusion-limited growth. The slope is the rate constant K that is proportional to the diffusion coefficient D. Quantitative analysis of the Ag nanocube size as a function of time showed that the lengths of edge L1 and edge L2 remain equal during in-situ observation (Figure 1B). During the initial 100 s, edge L1 and edge L2 showed no obvious growth, which is probably related with surface concentration of precursor. The edge length increased gradually with time after 100 s and the growth rate exhibited a volcano-type behavior as a whole (inset in Figure 1B), which was caused by the increasing particle size 24. Based on the LSW theory for nanocrystal growth 5, we considered the growth kinetics of the nanocube after the first 100 s because no prior growth was observed. We found that the cube of the edge length with time follows a linear relationship (Figure 1C and Figure S2). The slope corresponded to the rate constant (see SI for details); the averaged rate constant was ~333 nm3/s, leading to a calculated Ag+ ion diffusion coefficient of 2.9 ×10-10 ~ 2.9×10-11 m2/s (see the detailed calculation in SI), which was smaller than the reported value (1.7×10-9 m2/s) in water

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. The lower diffusion coefficient in our experiment

would be due to the existence of citrate and PVP, which formed complexes with Ag+ and led to the slower diffusion. Therefore, we can conclude that the linear time dependence of the cube of the particle size indeed suggested a diffusion-limited growth process for the Ag nanocube based on the diffusion limited growth model in the LSW theory. Silver precursor can be reduced rapidly under electron dose rate of ~200 e/ Å2⋅s in our experiment, the nanocrystal growth was limited by diffusion of the precursor from solution to near the surface of the growing nanoparticle

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. It is noted that the morphology of nanocube changed during growth process at

higher electron dose per unit area (~3800 e/ Å2⋅s) (see Figure S3)


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According to the fcc lattice symmetry, the exposed (100) facets are equivalent and should grow isotropically. However, we observed multiple anisotropic growth processes and large fluctuation of growth rate rather than a constant rate (Figure 2). This asymmetry in the growth process (breaking the symmetry of the cubic shape) has also been observed in the shapecontrolled synthesis of Pd and Ag nanocrystals

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. The particle growth (Figure 2A and movie

S2) showed repeating transitions between nanocubes and nanobars. At the initial stage (from 0 s to 73 s), a nanobar was formed from the nanocube due to different growth rates of edges L1 and L2, probably due to the difference in local solution chemistry under the electron beam

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. With

time, the anisotropic nanobar developed back into nearly nanocube shape at 88 s with sudden, simultaneous length decrease in L1 and increase in L2 (highlighted by the first dotted line in Figure 2C top). It is worth noting that the particle in Figure 2A only showed small displacement and rotation during the whole growth process (see Movie S2 for the details) due to the interaction (friction force) between particle and membrane, which is similar to the reported “stick” motion mode of Au nanocube 30. In addition, we also compared the contrast profile of Ag nanoparticles to confirm that the formation of nanobar is not caused by rotation or imaging artifact (See discussion in SI and Figure S4). From the viewpoint of energetics, the nanocube is thermodynamically favored due to its smaller specific surface area than that of a nanobar under the same volume. The reduction of overall surface energy drove the shape change of the nanobar to nanocube. Therefore, the nanobar with symmetry-broken shape constituted an intermediate state for the Ag nanocube; this was also confirmed in another shape transformation event (Figure S5). In the subsequent growth stage (from 88 s to 320 s), the nanoparticle manifested the conversion between nanocube and nanobar accompanying an increase of edge length (Figure 2C top), and the growth rate of the


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nanoparticle showed large fluctuation over the entire growth period (Figure 2C bottom). We propose that the interplay of the transport of precursor in the solution and its consumption on the surface of nanoparticle may be the root cause of the complex shape transformation of the nanocube (Figure S6). Before the growth (t = 0 s), the bulk concentration (CB) of reactant was assumed to be the same as that of reactant on the flat surface of the nanocube. As the growth proceeded, the rapid consumption of reactant resulted in a decrease of local surface concentration (CS), leading to the slowed growth. With time, the accumulated concentration difference (∆C = CB − CS) and large concentration gradient finally drove the diffusive flux of reactant from bulk solution to the particle surface. This, in turn, increased the local concentration on the flat surface, and accordingly enhanced the growth rate.

Figure 2. Shape transformation of Ag nanocube during the growth process. (A) Nanocube growth showing different intermediate shapes; (B) Formation of a nanobar by anisotropic growth of the Ag nanocube, showing a length reduction in edge L2 and increase in edge L1; (C) The


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length (top) and growth rate (bottom) of edges L1 and L2 of the nanocube in (A) as a function of time, the dotted lines show Ag nanoparticle growth rate and shape change; (D) Edge length as a function of time for nanocube in (B), the green rectangle highlights the time domain of energetically un-favored shape change. Consequently, the variation in local near-surface concentration around the particle was not monotonous. Such recurring decrease and increase of local concentration with time on the flat surface of the nanocube led to the fluctuating growth rate over an extended time. This may also explain no apparent growth from 0 s to 100 s as shown in Figure 1. The growth rate variation on each surface and its fluctuation (Figure 2C) gave rise to a non-uniform growth and shape change. Also, because the macromolecular PVP in the precursor solution can adsorb on the particle surface, the interplay of PVP adsorption time and the exposure time of the terraces on the particle to the surrounding solution can be possibly related to the fluctuation in growth rate that we observed 31. Unexpectedly, we also observed an energetically unfavorable (increase of surface energy) shape change of a nanocrystal (Figure 2B and movie S3) in a narrow time domain (highlighted in Figure 2D). It transformed from nanocube to nanobar rapidly by the simultaneous length increase of edge L2 and shrinkage of edge L1. In order to understand this unusual phenomenon, we made a closer inspection of the surface of the nanoparticle and observed that the surface curvature changed after the shape change (Figure S7). Compared to the contour of the particle before transformation, the left surface (indicated by pink arrows in Figure S7) became flat at 47 s and the surface curvature decreased after the shape transformation, leading to a reduced chemical potential of the surface according to the dependence of chemical potential on the surface curvature of the nanoparticle 32. Similarly, from 47 s to 80.7 s, the right convex particle surface


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also became flat (indicated by red arrows in Figure S7). We propose that the reduction of chemical potential during shape change may drive the atom diffusion away from the surface with higher chemical potential, leading to the shrinking of edge L1 (from 35 nm to 32 nm in Figure 2B). The capping agent PVP in the solution can preferably absorb onto the flat (001) facets due to stronger binding to Ag (001) and hence leads to lower total surface energy

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, although the

surface area increased during shape change. It is interesting to note that the edge L2 continued to increase regardless of the shrinkage of edge L1 (Figure 2D). This indicated that edge L1 and L2 grew independently as also reflected in Figure 2C. The observed isotropic growth in Figure 1 can be considered as a special case, wherein the growth rate for both edges were almost the same.

Figure 3. Different modes of Ag nanocrystal attachment. (A) Corner-to-corner attachment (Type I); (B) Oriented attachment of two particles of different size (Type II); (C) Oriented attachment of Ag nanocubes of similar size leading to the formation of a nanobar (Type III). The curved arrows in the figures indicate the direction of rotation. (D) The change in length of edge L1 and


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L2 of particle 2 in (C) with time before and after OA. The green dashed lines in (D) indicate the occurrence of attachment. Besides the classical crystal growth by atom addition onto an existing particle observed for an Ag nanocube, non-classical growth by PA also occurred during our in-situ experiment, and different types of attachment (non-OA and OA) were observed (Figure 3). During attachment type I (Figure 3A and movie S4), particle 1 rotated with respect to particle 2 and consequently the relative angle between two edges of the particles (highlighted by yellow lines, unless otherwise stated in the following) decreased with time. An approximately corner-to-corner (non-OA) contact at about 20° produced a dimer formation and the imperfect match resulted in the formation of a high energy interface that had high mobility, and finally a large nanocube was formed rapidly within 2 seconds. Attachment type II (Figure 3B and movie S5) showed side-toside contact, in which the (100) surfaces of two nanocubes were crystallographically co-aligned, indicating an OA process. However, because of different sizes of the nanocubes, a step-like structure was formed during PA, giving rise to a concave region with negative curvature and therefore lower chemical potential than a flat surface 32. The difference in chemical potential led to smoothing of the particles by extensive diffusion and finally transformation into a large single particle. Particle 3, which seemed in contact with the newly formed one (Figure 3B 20.7 s), didn’t attach with it. Instead, particle 3 moved away from it (Figure S8). The possible reasons are: (1) Particles 1, 2 and 3 are at different height, which can be seen by the white shadow caused by delocalization effect in Figure 3B (from 4.3 s to 20.7 s), so they were not actually in contact with each; (2) The attraction from other particles (not in the TEM image) and/or Brownian motion caused particle 3 moved to other directions instead of toward the newly formed one. We also observed nanobar formation (attachment type III) by OA of two nanocubes of similar size


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(Figure 3C and movie S6), and the length of the nanobar was the sum of the edge lengths L1 of the two particles (Figure 3D). Due to the similar length of the two particles, no negative curvature was formed after attachment, resulting in nanobar formation. The resulting nanobar continued to grow in a classical manner but maintained the nanobar shape. The observed “pits” in the Ag nanocubes (Figure 3B and 3C) are the nanopore probably due to spatially confined growth in the liquid cell (Figure S9). Analysis of particle motion showed most of the particles undergo “jump-to-contact” with accelerated translational and rotational velocities (Figure 4A-D) while some showed smaller velocity increase compared to the velocities during the approaching process. Particles involved in attachment mostly moved towards each other with a relatively constant translational speed of approximately 0.2 to 4 nm/s, while in a few cases, particles moved away from each other slightly (Figure S10). Within the last 2 seconds before attachment, the translational speed increased by a factor of 1.4 to 60. The particles rotated in random directions in 2D (Figure 4B and S10) at a speed within ±4 degrees/s. This random rotation in 2D implied Brownian motion of the nanoparticles (i.e., rotary Brownian motion), and occasional negative translational velocities indicated that the force due to Brownian motion was comparable to other forces (e.g., van der Waals and friction forces). Larger particles (~60 nm) moved and rotated relatively slower compared with smaller particles (~30 nm) (Figure 4C and D). Upon contact, the rotational speed increased by a factor of 1–45. The “jump-to-contact” separation ranged from 11 nm to 82 nm, indicating that a long-range force drove the approach and attachment of particles. The large jump distance and large range of jump speeds were quite different from the metal oxide system reported previously, where particle jump-to-contact within 1 nm 15 indicated a stronger attractive force at larger separation compared to metal oxide systems 34. This is also supported by the fact


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that Hamaker constant for metals is typically one order of magnitude higher than that for metal oxides.34 To estimate the strength of this long-range attraction, we determined the translational and angular acceleration by measuring the frame-to-frame velocity and rotation rate (Figure 4) and calculated the forces (~10-26 N) and torques (10-31 to 10-34 N·m) acting on the approaching particles.

Figure 4. (A) and (B) Translational (v) and rotational (ω) velocity of the approaching particle during attachment as a function of time; the dash-dotted lines in the figures were drawn to guide the eye, and the last data point indicates the jump-to-contact. The last data points are the attachment velocities. (C) and (D) The relationship between translational (v) and rotational (ω) velocity of the approaching particle and particle size during attachment. Note that the bars in (C) and (D) indicate the variation range of velocities and size of the approaching particles. Due to the time resolution of ~2 s, the contact angle of attachment was not observed in some cases, so some data points of attachment rotational velocity are not shown.


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The interaction forces involved during movement and attachment of nanoparticles in solution include van der Waals, electrostatic, solvation, and hydrodynamic forces 35. In addition, from the in-situ liquid cell TEM observation, particles are more or less bound to the silicon nitride membranes, showing slow 2D movement30,

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; therefore, the friction force between

particle and SiN membrane was involved. The existence of solvent molecules also exerts a force on the particles themselves—Brownian force. The averaged zeta potential of Ag nanoparticles under reaction solution conditions is close to zero (-0.84 mV, Table S1), indicating the electrostatic interaction was very small and can be neglected. Therefore, the main forces involved in the system were van der Waals force, Brownian force, hydrodynamic force, and friction (see details about force calculation in supporting information). We calculated the van der Waals force per unit area during attachment based on the jump-to-contact distance, and the order of magnitude of the force varied from 5.2 N/m2 to 2.2 × 104 N/m2, depending mainly on the separation between two particles. The Brownian force per unit area can be estimated to be 19.1 N/m2 ~ 256 N/m2, which was larger than the van der Waals force at a large separation and may lead to a separation increase occasionally and to random rotation as two particles approach each other. The resisting hydrodynamic force experienced by the approaching cubic particle was calculated to be 7.2 × 10-3 N/m2 ~1.0 × 10-2 N/m2, which was much smaller than van der Waals force. We used AFM to estimate the friction force using an Ag-coated tip and SiN substrate in the precursor solution; the maximum magnitude of the friction force is in the order of 10-3 N/m2, which is in the same order of hydrodynamic force. Based on the force calculation (Table S2 and S3), the attractive van der Waals force was larger than the friction and hydrodynamic forces. During particle attachment process, van der Waals force overcame friction force from SiNx


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membrane and hydrodynamic forces, approached each other, and finally made contact and attached with each other. Conclusion With in-situ observation of Ag nanocrystal growth in a liquid cell TEM, we provided direct visualization of Ag nanocubes in aqueous solution, and demonstrated that both classical MA and non-classical PA contributed to the growth of Ag nanocubes and the formation of nanobar. In the classical scenario, isotropic and anisotropic growth behaviors of Ag nanocubes were observed. The growth was limited by diffusion of the precursor, and the growth rate varied due to the fluctuation of surface concentration caused by the interplay of diffusion of the precursor from the bulk to the particle surface and consumption of the precursor on the particle surface during growth. Non-uniform concentration on the surfaces caused the anisotropic growth of Ag cubes. In the non-classical scenario, Ag nanocubes and nanobars formed via both OA and non-OA processes. The jump-to-contact at large separations suggested that a long-range force drove the particle approach and attachment. Our analysis of the dynamics of Ag particles showed that the force due to Brownian motion was comparable with other forces. However, due to a long-range attraction force, Ag particles approached each other and made jump-to-contact, forming single cubic crystals. The balance between the interaction forces indicated that the attractive van der Waals force was a driving force for particles approaching each other and attachment. Our in-situ observation of dynamic crystal growth improves our understanding of growth mechanisms and provides insights into the synthesis of nanocrystals with tunable size and shape for functional applications.


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ASSOCIATED CONTENT Supporting Information TEM images and videos; evaluating diffusion coefficient of Ag+; force calculation, friction force measurement using AFM and discussion. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions #D. Xiao and Z. Wu contributed equally. Acknowledgement The research was supported by the U.S. Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences (BES) Early Career Research program under Award #67037. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for the DOE by Battelle under Contract No. DE-AC05-76RL01830. Theoretical analysis for different forces was partially supported by the DOE BES Synthesis Science and Processing Program. Development of the numerical scheme to calculate hydrodynamic forces was supported by Interfacial Dynamics in Radioactive Environments and Materials (IDREAM), an Energy Frontier Research Center funded by DOE BES. Initial work on the theory of hydrodynamic forces was supported by the MS3 (Materials Synthesis and Simulation Across Scales) Initiative, a Laboratory Directed Research and Development Program at PNNL. Notes The authors declare no competing financial interests.


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Silver nanocube and nanobar growth via anisotropic monomer

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