In-situ liquid cell transmission electron microscopy investigation on oriented attachment of gold nanoparticles

Inside a liquid solution, oriented attachment (OA) is now recognized to be as important a pathway to crystal growth as other, more conventional growth mechanisms. However, the driving force that controls the occurrence of OA is still poorly understood. Here, using in-situ liquid cell transmission electron microscopy, we demonstrate the ligand-controlled OA of citrate-stabilized gold nanoparticles at atomic resolution. Our data reveal that particle pairs rotate randomly at a separation distance greater than twice the layer thickness of adsorbed ligands. In contrast, when the particles get closer, their ligands overlap and guide the rotation into a directional mode until they share a common {111} orientation, when a sudden contact occurs accompanied by the simultaneous expulsion of the ligands on this surface. First-principle calculations confirm that the lower ligand binding energy on {111} surfaces is the intrinsic reason for the preferential attachment at this facet, rather than on other low-index facets.


Supplementary Figures
Supplementary Figure 1. Preparation of a carbon film based liquid cell. a Schematic illumination for the fabrication of liquid cell: A droplet of as-prepared solution was sandwiched by two TEM copper grids with formvar stabilized carbon support films face-to-face. The sample was then left under atmosphere until the extra liquid volatized. b An enlarged scheme shows the liquid encapsulated by carbon films. c TEM image of a sealed liquid pocked with the size of 80 nm. Figure 2. Free rolling of "L" shape nanostructure. To ensure the occurrence of OA is in solution rather than in vacuum, we have monitored the movement behaviors of different large particles. This figure is the TEM sequences to show the motion of a "L" shape nanostructure in the liquid pocket. It can be seen that the particle undergo free translation and rotation in solution. It does not lay on the carbon films, but keeps rolling freely (not only along the viewing axis but also off axis) because it is suspended in solution. NaCl. This image is recorded at location of a liquid pocket, which indicates that the observed solid-state NaCl is precipitation from supersaturated solution and adsorbed onto surfaces of carbon films. b EDX result confirms that the components of this material are only Na and Cl elements. Figure 5. Nanoparticle morphology for tracking of their motions and orientations, as well as measurement of their surface area. a The equal proportional atomic model of a truncated gold nanoparticle. Theoretical calculations have predicted that gold nanoparticles in the 1 − 3 nm size range, prefer to keep a truncated octahedral or cubeoctahedral appearance rather than other symmetries such as Ino-decahedral and icosabedral [20][21][22] . b 2D projection of the model viewed along [011] zone axis. c TEM sequences show the motion trajectories of a particle. The hexagonal morphology of this particle along [011] zone axis confirms its truncated octahedral 3D constructions. Thus, rotation of the particle can be accurately monitored by a combination of its morphology and crystal orientation. As an example here, the particle clockwise rotate about 6 degree during 5.5 seconds (Arrows indicate the rotation direction and white dashed lines are horizontal references). Moreover, we find that the surfaces of particles smaller than 3 nm are all composed of {111} and {100} surfaces, but no any obvious {110} surfaces. It is reasonable because in such a small size, surface energy may contribute more than volume energy, so the unstable {110} surface with highest energy is not a favorable surface. d Snapshots captured from videos show that most of the small particles have a highly symmetrical truncated octahedron morphology, which is composed of 8 equivalent {111} surfaces and 6 equivalent {100} surfaces as illustrated in (a). Then we estimated their surface area for each particle and the ratio between {111} and {100} is shown at the top of images, ranging from about 1 : 1.2 to 1 : 0.9. Figure 6. A single crystal structure formed through oriented attachment. Image sequences show the OA process of gold nanoparticles at {111} surfaces. The contact at their aligned {111} surfaces with parallel {100} facets lead to a final single crystal structure. Figure 7. A particle pair with on occurrence of OA. The particle pair is close enough for more than 230 seconds, during which the right particle keeps rotating but still fails to get a pre-alignment of their {111} facets. Finally, it coalesces with another incoming particle (noted by the blue arrow) instead of the left one to form a perfect single crystal structure. The lower rows are corresponding filtered images. Figure 8. OA process and the changes of their relative angle between {111} facets of a particle pair. In this case, the angle changes randomly when D > 1.2 nm. Then as the particle pair gets closer (D > 1.2 nm), the directional rotation begins to make the angle gradually decrease to approximate 0º. Figure 9. OA process and the changes of their relative angle between {111} facets of a particle pair. In this case, the angle does not change much as D > 1.3 nm. But when the particle pair approaches to a smaller distance (D < 1.3 nm), it quickly decreases to nearly 0º. Figure 10. Distribution of separation distance. Statistical distribution of the separation distance for the observed OA events by 21 particle pairs and the fitting result using Boltzmann distribution as given in Supplementary Equation (5). Figure 11. Oriented attachment at {111} surfaces. TEM snapshots from videos show the contact moment of different particle pairs, illustrating that all OA events occur at their {111} surfaces. The lower sequences are corresponding FFT images of the neck location of each particle pair. The d-spacing is confirmed to be 0.24 nm for Au {111} facets.

Supplementary Notes Note 1. Dissolution of large particles and generation of small ones
During in-situ observation, it is found that if we apply a relative high dose of electron beam Our solution just provides such an environment to facilitate the dissolution. First of all, the TEM irradiation offers sufficient electrons for this reaction. What's more, the oxidation agents such as H2O2, OH and O that are produced by the interaction between incident electrons and water molecules also promote the oxidization of Au particles 6 . Therefore under electron beam irradiation, gold atoms on the surfaces of large particles are captured by Cl − to become AuCl4 − ions and diffuse around in the solution. Consequently, these ions are reduced again by the excessive citrates to generate small particles.

Note 2. Interaction potential between gold particle pairs.
The interaction potential between two particles contains four parts:  summarized here: (1) extremely small permanent dipole moments for individual particle; (2) nearly no redistribution of dipole moments as the particles approach. We can deduce that dipole evolution rarely plays a role in OA growth. Therefore, electric dipole-dipole interaction can be ignored in our gold particle systems, in accordance with the conclusion from other literature 10  ( ) where ρ is number density, z is the valence of ions, e is electron, 0 ε and ε are vacuum and relative permittivity respectively, B k is Boltzmann constant, and T is temperature. Although the concentrations of some ions are difficult to estimate due to the evaporation of solvent in liquid cell fabrication process, the observed solid phase precipitation of NaCl crystals ( Supplementary Fig. 4) suggests the solution is saturated of NaCl. Accordingly, the Debye screen length in Supplementary Equation (3)  This indicates that both dipole-dipole and charge-charge interactions are screened at the separation distance larger than 0.12 nm.
As a result, the final interaction between the gold nanoparticles in our system is the steric-hydration and van der Waals potentials, described as follow 7,11 : where 0 W depends on the hydration of the surfaces, D is the distance between particle surfaces, λ is the decay length, A is the Hamaker constant and R is the radius of the interacting gold particles. Apparently the combined potential between two particles is only related to their separation distance, which indicates that the work done during relative motion of the particle pair is independent with the taken path. In other words, this combined potential satisfies the definition of a typical conservative force field. Accordingly, the distribution of the separation distance should be described by Boltzmann statistics 12 : By adopting Supplementary Equation (4) and (5), the relationship between the separation distance and the corresponding distribution can be expressed as: Supplementary Fig. 10 shows the combined separation distance distribution of 21 particle pairs during their approaching process (Figure 3a) and the corresponding fitting curve using Supplementary Equation (6). Here 1.41 nm = R is the mean radius of these particle pairs,  Figure 3b.

Computational Models and Methods
Au{111}, and Au{100} surfaces were modeled as p (4×6), p (4×4) and p (4×5) periodic slab model includes four, four and three atomic layers respectively. Atoms in the top two layers were relaxed, and all other atoms were fully frozen. The neighboring layers were separated in the direction perpendicular to the surface by a vacuum distance of 15 Å. DFT calculations were performed using projector augmented wave (PAW) potentials and the PBE functional implemented in the Vienna ab initio simulation package (VASP) [13][14][15][16] . Relaxations were carried out using conjugate-gradient algorithm, and stopped if all forces were smaller than 0.03 eV Å -1 . The kinetic-energy cutoff of plane wave was set to 400 eV. Brillouin-zone was integrated using Monkhorst-Pack-generated sets of k points. A 2×2×1 k-points grid was used for the surface calculations.

Surface energy
The surface energy ( surface E ) was calculated as follows: where slab n E is the total energy of the slab, bulk E is the total energy of the bulk per unit cell, n is the number of bulk unit cells contained in the slab, and A is the surface area of each side for the slab. The calculated results are summarized in Supplementary Table 1.

Ligand configuration
For citrate configurations, infrared and X-ray photoelectron spectroscopy analysis have demonstrated that in aqueous solution citrates will bond with two hydrogen atoms to form dihydrogen citrate anions (H2Citrate¯), which are adsorbed on gold surfaces by the central carboxylate group ( 2 η -COO¯ ) 17 . In this consideration, we propose three possible initial configurations depending on the position of oxygen atoms, described as follow: In our previous work 18 , it is found that polycrystalline nano-chains can be built by imperfect attachment of nanoparticles when the dipolar interaction compels the particles to arrange end-by-end. In that work, the drift velocity increases greatly at very close distance because the attractive dipolar interaction (U ~ 1/r 2 ) increases dramatically as the particles get closer.
This increase of dipolar interaction also leads to an apparent acceleration process when the distance decreases to 3 nm for particle-particle pair and 6 nm for particle-chain pair ( Figure   4A, Science 2012, 336, 1011). However, the situation is different in this work. The attractive force does not change much as the particles get closer, and then even becomes repulsive force when the distance is smaller than about 1 nm (see the dashed magenta curves in Figure 3b, this manuscript). This is why the drift velocity slightly decreases as they closer ( Supplementary Fig. 14). In addition, the abrupt increase of velocity before the final contact ( Supplementary Fig. 14) well indicates that the expulsion of surface ligands leads to the vanishing of repulsive force and then quick contact. Therefore the jump to contact in this work should be distinguished from the increase of drift velocity in our previous work, similar behavior but different mechanism.
In another previous work 19 , the pathways of OA in solution has been in-situ observed.
Although the authors have imaged the successive pre-alignment, jump to contact and interface elimination process, they have not performed quantitatively statistical analysis about the detailed evolvement laws of particle's movement and rotation before contact. In comparison, here in our work, the small size of gold particles allows us to study the movement and rotation behavior more easily, and a large number of statistical data also allow us to investigate the nature of driving force. More details about OA have been discovered in our work.
Most importantly, numerous literature about nano-synthesis have noticed the crucial role of surface ligands in OA, whereas there is still no direct evidence to elucidate this mechanism.
Our results have, for the first time, revealed how the surface ligands control OA and provided a new mechanism of the binding energy related crystal facet selection. These new findings were enabled by the liquid cell TEM with significant improved spatial resolution as compared to the previous reports.