On the Crystallography of Silver Nanoparticles with Different Shapes

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On the crystallography of silver nanoparticles with different shape Jens Helmlinger, Oleg Prymak, Kateryna Loza, Martin Gocyla, Marc Heggen, and Matthias Epple Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00178 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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Crystal Growth & Design

On the crystallography of silver nanoparticles with different shape

Jens Helmlinger,1 Oleg Prymak,1 Kateryna Loza,1 Martin Gocyla,2 Marc Heggen,2 Matthias Epple1,*

1

Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (CeNIDE), University of

Duisburg-Essen, Universitaetsstr. 5-7, 45117 Essen, Germany. E-mail: [email protected]; Fax: +49 201 1832621; Tel: +49 201 1832402 2

Ernst Ruska-Center and Peter Grünberg Institute, Forschungszentrum Jülich GmbH, 52425

Jülich, Germany

KEYWORDS Silver nanoparticles, powder diffraction, electron microscopy, pole figures

ABSTRACT The crystallographic properties of silver nanoparticles with different morphologies (two different kinds of spheres, cubes, platelets, and rods) were derived from X-ray powder diffraction and electron microscopy. The size of the metallic particle core was determined by scanning electron microscopy, and the colloidal stability and the hydrodynamic particle diameter were analyzed by dynamic light scattering. The preferred crystallographic orientation (texture) as obtained by Xray powder diffraction, including pole figure analysis, and high resolution transmission electron

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microscopy showed the crystallographic nature of the spheres (almost no texture), the cubes (terminated by {100} faces), the platelets (terminated by {111} faces), and the rods (grown from pentagonal twins along [110] and terminated by {100} faces). The crystallite size was determined by Rietveld refinement of X-ray powder diffraction data and agreed well with the transmission electron microscopic data.

Introduction Silver nanoparticles are well known for their interesting optical, biological, and electronic properties. There are many possible applications, ranging from catalysis1,2 over photonics,3-5 medical research,6-11 imaging and sensing by surface-enhanced Raman spectroscopy (SERS)12-14 to energy storage and conversion.15 Many synthetic methods exist for the synthesis of silver nanoparticles, using either top-down methods such as lithography,16,17 ultrasonication18-20 and laser ablation21-26 or bottom-up approaches like the reduction of silver ions in organic solution in the presence of shape-defining polymers,27-31 also induced by microwave irradiation.32-35 The particle properties do not only depend on their size, but also on their morphology, therefore a shape-controlled synthesis of silver nanoparticles is of special interest. Xia et al. and others have described the structural evolution of silver nanoseeds to nanoparticles with defined shapes30 like cubes,36-43 platelets,44-46 rods,47,48 rings,49 and bipyramids.50 It must be emphasized that the reproducible synthesis of silver nanoparticles with a uniform shape is synthetically challenging and often not reproducible. In particular, the formation of silver nanoparticles with defined shape highly depends on a number of reaction parameters that cannot be easily controlled, e.g. the surface of the glass vessel, the shape and the size of stirring bar, or variations between different batches of parent compounds from the same manufacturer.42,51-53


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Concerning the analysis of metallic nanoparticles, it is necessary to apply more than one method to obtain reliable and comparable results.54,55 Colloid-chemical methods such as dynamic light scattering (DLS) and analytical disc centrifugation give information about the size and agglomeration state in dispersion. However, they are not sensitive to the morphology of shapedefined nanoparticles, therefore microscopic methods like transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) are used to analyze the shape of nanoparticles. Another powerful method is X-ray powder diffraction (XRPD). It is frequently used to confirm the crystal structure and the crystallographic phase of crystalline nanoparticles. Furthermore, unit cell parameters, crystallite size, and microstrain can be determined.56-60 This is especially interesting in the case of nanoparticles because their crystallographic properties often deviate from the bulk material.61-63 If the diffraction angle Θ is fixed and the diffracted intensity is collected during variation of the tilt angle χ of the sample and the rotation angle ϕ around the sample, pole figures (stereographic projections) can be obtained.64 Pole figures give information on the texture of a material, which is especially interesting for individual nanoparticles. This technique is often used in materials science to analyze thin films,65 but was seldom applied to crystalline nanoparticles with anisotropic morphology. For instance, Paris et al. have determined the crystallographic orientation of biomineral nanoparticles in human vertebral bone by pole figure analysis.66 The method was also used to characterize nanoparticles of ZnFe2O4

67

and

CuInS2,68 and for the analysis of oriented cobalt and nickel nanocrystals in ZnO.69 Concerning face-centered cubic (fcc) metals, there are several reports on pole figure measurements with nickel nanocubes70 and silver coatings.71-73


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Here, we report on a thorough crystallographic analysis of silver nanoparticles with different morphology,74 correlated to electron microscopic data, presenting data on two kinds of spheres, cubes, platelets, and rods. It will be shown that X-ray powder diffraction is ideally suited to describe not only the size, but also the shape and the crystallographic orientation of metallic nanoparticles.

Results and discussion Silver nanoparticles with defined morphologies (spheres, platelets, cubes and rods) were synthesized by variations of the standard polyol process and colloidally stabilized with poly(Nvinylpyrrolidon) (PVP) (see also Ref.74) All particles were carefully purified by ultracentrifugation (especially removal of counter ions and excess PVP) and redispersed in ultrapure water before characterization. Dynamic light scattering (DLS) was used to determine the particle size distribution in aqueous dispersion (Figure 1). In DLS, the particles are considered as spherical, which gives reliable data for spheres, cubes, and platelets. However, rods are highly anisotropic, therefore they do not give reliable DLS results due to their large size up to the µm range. Taking into account the fact that dispersed nanoparticles are covered by a hydration shell, the DLS results are in good agreement with the results from scanning electron microscopy (SEM). All analytical results are given in Table 1. In the following, we discuss all particles types separately.


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Figure 1. Particle size distribution of nanospheres from glucose reduction (Spheres GL), nanospheres from polyol microwave reduction (Spheres MW), nanoplatelets, and nanocubes, derived from DLS (particle size distribution by intensity). The polydispersity index (PDI) was below 0.3 in all cases, indicating a good monodispersity of the particles.


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d (DLS)

d (SEM)

Crystallite size

Lattice parameter

/ nm

/ nm

(XRPD) / nm

a (XRPD) / Å

108

55 (15)

31

4.088 (1)

128

150 (30)

30

4.087 (1)

Cubes

211

160 (20)

203

4.088 (1)

Platelets

48

30 (10)

15

4.088 (1)

117

4.087 (1)

Particle shape

Spheres from glucose reduction Spheres from polyol microwave reduction

100 (50) (diameter) Rods

up to 20 µm (length)

Table 1. Comparison of particle size and lattice parameters obtained by different methods for all investigated kinds of silver nanoparticles. X-ray powder diffraction data were refined by the Rietveld method, assuming an fcc lattice of silver. The standard deviations are given in parentheses.

Spheres Figure 2 shows SEM and high-resolution transmission electron microscopy (HRTEM) images of silver nanospheres. Particle populations from polyol microwave reduction and from glucose reduction are shown.


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Figure 2. Electron microscopic images of silver nanospheres, obtained by polyol microwave reduction (A, B), and by glucose reduction (C, D). The HRTEM images (B, D) show magnifications of selected particles with an orientation perpendicular to the domain boundaries.

Nanospheres from glucose reduction are smaller than nanospheres obtained by polyol microwave reduction. The particles are not perfectly spherical, but often slightly elongated in at least one direction. The regular fivefold symmetry of a twinned particle from glucose reduction (Figure


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2D) indicates a regular crystal growth process, starting from an already twinned nucleus.75 A similar symmetry was reported also for multiple twinned crystals, consisting of C60 and C70,76 and also for gold nanoparticles.77

Figure 3. X-ray powder diffractogram of silver nanospheres from the polyol microwave reduction (Bragg-Brentano geometry). There is no preferred orientation.

Figure 3 shows the X-ray powder diffractogram of nanospheres from the polyol microwave reduction, including Rietveld refinement. Phase analysis was performed, and the fcc lattice parameter a was determined (Table 1). The peaks were assigned to pure fcc silver without preferred orientation. A small peak at 31.2 °2Θ indicates the presence of a small amount of silver chloride (ICDD PDF4 #031-1238) as a synthetic impurity. Note that this peak is narrow, indicating large AgCl microcrystals that are present only to a very small extent (low peak intensity). All peaks of silver are broadened due to the nanocrystalline size. Rietveld refinement


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gave a crystallite size of 30 nm (isotropic refinement). This value is significantly smaller than the size of the individual crystals determined by SEM (Table 1), but in good agreement with the domain size measured by HRTEM (Figure 2). This underscores that the crystals are twinned.

(111)

(200)

(220)

Figure 4. Pole figures of the (111), (200), and (220) crystallographic planes of silver nanospheres obtained by the polyol microwave reduction.

The pole figures were measured to determine the crystallographic projections at the most prominent diffraction peaks (111), (200), and (220) (Figure 4). Again, no preferred orientation was found, i.e. the nanospheres were randomly oriented on the sample holder. Contrary to the nanospheres from polyol microwave reduction, the nanospheres from the glucose reduction showed a distinct preferred orientation of the {111} facets. This indicates that they are not isotropically oriented in a crystallographic sense, probably because they are capped by distinct crystallographic faces. No AgCl is present, and the sample consists of pure fcc silver (Figure 5). The crystallite size from Rietveld refinement was 31 nm (isotropic refinement), in good agreement with the TEM data and identical to the nanospheres from glucose synthesis. The


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pole figures did not show a clear texture, only a few spikes suggesting an inhomogeneous deposition of the particles on the sample holder (Figure 6).

Figure 5. X-ray powder diffractogram of silver nanospheres from the glucose reduction (BraggBrentano geometry). There is a preferred orientation (increased relative peak intensity) of the (111) and (222) planes.


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(111)

(200)

(220)

Figure 6. Pole figures of the (111), (200) and (220) crystallographic planes of silver nanospheres from the glucose reduction.

Cubes The edge lengths of the nanocubes are between 140-180 nm (Figure 7). Minor amounts of spherical and prismatic particles occurred as by-product (less than 10%; Figure 7A). Notably, the edges of the cubes are not sharp but rounded as clearly seen in the TEM. This appears to be a general phenomenon for nanoparticles (see the discussion in Ref.

77

). The nanocubes were too

thick to permit electron beam diffraction or lattice plane imaging by TEM. The nanocubes lay on a {100} face on the surface of the sample holder and showed a tendency to form regular twodimensional assemblies as in earlier reports.78


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Figure 7. SEM (A) and TEM image (B) of silver nanocubes.


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6.0 (200)

5.5

Intensity / log10(counts)

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


5.0 4.5 4.0 (111) 3.5 3.0 (220)

(311) (222)

2.5 2.0 20

30

40

50

60

70

80

90

Diffraction angle / °2θ Figure 8. X-ray powder diffractogram of silver nanocubes, showing an extreme texture of the (200) peak due to the cubes lying on a flat {100} face. Note the very high intensity of the (200) peak, also in terms of counts on the y-axis (in logarithmic representation).


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Figure 9. X-ray powder diffractogram of silver nanocubes measured in grazing incidence geometry (GIXRD) with an incident beam angle of 1.0°.

A very strong texture in the [200] direction was observed in Bragg-Brentano geometry because the nanocubes are terminated by {100} planes and are mostly orientated horizontally on the sample holder. For a better representation of the weak peaks besides the dominant (200) peaks, a grazing incidence diffraction experiment (GIXRD) was carried out at an incident angle of 1° (Figure 9). In this case, the cubes that lie flat on the sample holder are out of the diffraction condition. We see only cubes that lie on their edges in a more or less pronounced way. Phase analysis of the GIXRD data showed that the particles consist of pure fcc silver (Table 1). It was now possible to calculate the crystallite size from the peak profiles by Rietveld refinement from GIXRD data. In this case, the crystallite size of the nanocubes (203 nm) was in good agreement


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with their particle size (edge length 160±20 nm; Table 1), demonstrating that the cubic particles consist of single crystals with only one domain.

(111)

(200)

(220)

Figure 10. Pole figures of the strongest peaks in the X-ray powder diffractogram of silver nanocubes.

The horizontal arrangement of the nanocubes with their {100} faces parallel to the sample holder surface defines the theoretical angle between the different crystallographic planes in fcc silver with respect to the sample holder. As expected, the pole figures (Figure 10) showed a strong texture for (200) planes at a tilt angle χ=0°, whereas the (111) and (220) planes were presented by specific cylindrical stereographic patterns at tilt angles of 55° and 45°, respectively. This agrees well with the theoretical values for the angle Φ between two crystallographic planes as calculated for a cubic system (Equation 1; Table 2):

cos

∙

(1)

with h, k, l the Miller indices.


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Angle between two

Φ calculated

χ experimental

(200) and (111)

54.8°

55°

(200) and (200)

0°

0°

(200) and (220)

45°

45°

crystallographic planes

Table 2. Comparison between theoretically calculated and experimentally measured angles between the different crystallographic planes in silver nanocubes from pole figure analysis.

The circular diffraction intensity for (200) and (111) pole figures showed that the nanocubes are distributed in a random orientation on the sample holder, but with one face flat on the sample holder. The small central peak at a tilt angle of 0° in the (111) and (220) orientations can be explained by the small amount of cubes that are not parallel with their {100} faces to the sample holder surface.

Platelets The edge length of nanoplatelets is about 20-40 nm as determined by electron microscopy (Figure 11). By SEM and DLS, it is impossible to confirm their flat shape, i.e. to distinguish them from spherical particles. However, this is possible by a combination of TEM and X-ray powder diffraction as we will demonstrate in the following. Seen from above in the SEM, the platelets appear either round or triangular. HRTEM showed that the platelets are single crystals and not twinned. Notably, the edges were again round as in the case of the cubes.


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Figure 11. SEM (A), HRTEM images (B, C) and FFT pattern of the HRTEM image (D) of silver nanoplatelets.

Figure 11D shows a fast Fourier transformation (FFT) of the HRTEM image (Figure 11B) where the spots have a six-fold symmetry consistent with a diffraction pattern for fcc crystals in the [111] beam direction.79 This was observed for all nanoplatelets, indicating that they lie flat with


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the (111) plane parallel to the sample holder. This was confirmed by X-ray powder diffraction (Figure 12).

Figure 12. X-ray powder diffractogram in Bragg-Brentano geometry of silver nanoplatelets, showing a strong preferred orientation of the (111) and (222) peaks and the complete absence of all other diffraction peaks of silver. The sample contained a minor impurity of microcrystalline AgCl as synthesis byproduct.


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(111)

(200)

(220)

Figure 13. Pole figures of the (111), (200) and (220) crystallographic planes of silver nanoplatelets.

In agreement with the HRTEM data, the platelets showed a complete texture for the (111) and (222) reflections (Figures 12 and 13), confirming that the particles are terminated by {111} faces parallel to the surface of the sample holder. The other crystallographic planes did not show any texture effects in the pole figures, like the data obtained for nanospheres. The crystallize size of the silver nanoplatelets was determined by Rietveld refinement for the (111) peaks with 15 nm, corresponding to the thickness of the platelets. The crystallite size in the lateral dimension could not be determined by X-ray powder diffraction due to the absence of all other peaks. However, the lateral dimension by electron microscopy and by DLS was in the range of 30-50 nm, i.e. the thickness is about 1/3 of the edge length of a nanoplatelet, a fact that obviously leads to a very high degree of preferential orientation.


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Rods Figure 14 shows SEM and HRTEM images of silver nanorods.

Figure 14. SEM (A, B) and HRTEM images (C, D) of silver nanorods.

By electron microscopy, silver nanorods had a diameter between 50 and 150 nm and a length of up to 20 µm. They were typically arranged parallel to the surface of the sample holder. Their cross section was pentagonal and the tip was not flat but rounded. Moiré fringes were observed


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in the HRTEM images (Figure 14D) which can be ascribed to double diffraction from crystalline domains with different orientation. This can occur either for domains with parallel lattice planes with different d-spacing or from double-diffraction at domains with a specific azimuthal rotation between the lattice planes with different d-spacing.80,81 The Moiré fringes indicate that the rods consist of several crystallites and are not single crystals.

Figure 15. X-ray powder diffractogram in Bragg-Brentano geometry of silver nanorods, showing a preferred orientation (increased relative intensity of the (111) and (200) peaks) for face-centered cubic (fcc) silver. An asymmetric peak profile was employed to account for the body-centered tetragonal (bct) Ag phase (see text).

An X-ray powder diffractogram of nanorods is shown in Figure 15. All peaks are assigned to fcc silver. The profile of the (200) peak shows an asymmetric shape that indicates the presence of a


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body-centered tetragonal silver phase (bct) as described earlier by Sun et al.82 The lattice parameters of this tetragonal nanocrystalline silver phase (crystallite size 25 nm derived from peak broadening) obtained by Rietveld refinement were abct=2.879(1) Å and cbct=4.136(1) Å (space group I4/mmm). This leads to an anisotropic distortion of the main fcc silver phase in aand c-directions (cbct ≠ afcc, √2abct ≠ afcc). The calculated volume of the unit cell of bct silver was 34.283 (9) Å3, i.e. a small increase of the volume of fcc silver (∆V=+0.45 %) was detected. Like the platelets, the rods were oriented in parallel to the sample holder, and a similar crystallographic interpretation was applied. The isotropic crystallite size was 117 nm.

(111)

(200)

(220)

Figure 16. Pole figures of the (111), (200) and (220) crystallographic planes of nanorods.

The pole figures for silver nanorods show a complicated, specific stereographic pattern (Figure 16). In addition to the sharp peak at χ=0°, a broad, wavelike intensity at tilt angle ranges between 55° and 70° was observed for the (111) and (200) pole figures. The (220) pole figure did not show a preferred orientation. The following conclusions can be drawn with respect to the crystallographic nature of the nanorods. The two intensive sharp peaks at the tilt angle χ=0° for (111) and (200) pole figures confirm that the (111) and (200) planes are both oriented parallel to the surface of the sample


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holder. This is confirmed by the increased relative intensity of (111) and (200) peaks (Figure 15). This construction is only possible in the configuration shown in Figure 17.

Figure 17. Schematic model for the crystallographic orientation of nanorods, based on the previous work of Xia et al.48 and Xie et al.83 The rod is growing in the [110] direction, consisting of five pentagonally twinned domains, and terminated by (001) planes. One pentagonal rod (A). Schematic cross section of one of the five pentagonally oriented crystallites (B).

As it can be seen from Figure 17 and confirmed by the SEM images, each rod consists of five pentagonally twinned crystals with an internal angle of 70.5°. A combination of these five units leads to a total angle of 352.5° and results in a gap of 7.5° (360° minus 5·70.5°) as it was already described in the literature.48,83 Following this model, the side facets of the rods are parallel to the crystallographic (100) planes of the depicted five tetrahedra. Only one of the five facets is parallel to the sample holder (lying on the sample holder with the flat surface) and produces the sharp peak at χ=0° in the (200) pole figure. In the (111) pole figure, the sharp peak at the tilt angle χ=0° is caused by the inner (111) planes of the tetrahedra that are radially distributed from


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the center to the outside of the rods. As described above, the rods are lying parallel to the sample holder due to their anisotropic shape. Following this, one part of the rods produces a reflection for (200) and another part for (111), both parallel to the sample holder. The appearance of the wavelike texture at a tilt angle range of 55-70° is caused by the other corresponding planes that are in reflection when the rod is tilted along the long axis. The calculated tilt angle for (100) is 70.5° and for (111) 54.75°. The width of the reflection intensity can be explained by the described twinning region. The pentagonal cross section can also be seen in the spherical particles (Figure 2). Figure 18 shows this crystallographic construction, based on the magnified pentagonal face of the spherical crystal in Figure 2D.


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Figure 18. HRTEM image of a pentagonally twinned nanosphere (Figure 2D), showing one pentagonally oriented crystallite. The yellow lines indicate the (110) lattice plane distance of 2.7 Å.

Summary Silver nanoparticles with different shape (two kinds of spheres, cubes, platelets and rods) were synthesized, purified and subsequently analyzed by DLS, SEM, HRTEM and different powder diffraction methods. In general, all particles can be considered as very homogeneous in size and shape. Bragg-Brentano geometry was used to confirm the phase purity and to determine the lattice parameters and crystallite size. Pole figures showed characteristic patterns for nanoparticles with different shapes and was very well suited to characterize such structures. The


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nanospheres showed only a small texture, but consisted of twinned particles. The nanocubes were terminated by (100) faces and are single crystals. The nanoplatelets lay flat on the sample holder with their dominant {111} faces. The nanorods consisted of five pentagonally oriented crystallites, elongated in the [110] direction and terminated by {100} faces and contained a body-centered tetragonal silver phase (bct)

Experimental section Polyol microwave synthesis of spherical silver nanoparticles Spherical silver nanoparticles were synthesized by a microwave-assisted polyol process.84 Briefly, silver nitrate was reduced by diethylene glycol in the presence of PVP at 160 °C (200 W) in the microwave. After 20 min, the reaction was stopped and the particles were collected by ultracentrifugation (66,000 g, 30 min) and washed with acetone and ultrapure water several times.

Glucose reduction synthesis of spherical silver nanoparticles Spherical silver nanoparticles with smaller diameters than in the microwave synthesis were obtained by reduction of silver nitrate with glucose in aqueous solution.75,85 After cooling to room temperature, the product was collected by ultracentrifugation (29,400 g; 30 min) and washed with ultrapure water several times.

Synthesis of silver nanoplatelets Silver nanoplatelets were synthesized by microwave-assisted reduction of silver nitrate with ethylene glycol monoethyl ether in the presence of PVP. In this case, the crystal growth was


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kinetically controlled by the weaker reducing agent (2-ethoxyethanol), leading to nanoplatelets.86 It was necessary to increase the microwave input power manually from 25 W to 100 W at about 100 °C to achieve a reaction temperature of 140 °C after about 10 min. For purification, the particles were collected by centrifugation (6,000 g, 30 min) and subsequently washed with acetone, ethanol and ultrapure water.

Synthesis of silver nanocubes The synthesis of silver nanocubes was extensively described in the literature,36-41,43,78,87-89 but often difficult to reproduce.42,51 We have used a protocol developed by Xia et al.37 and modified it slightly to optimize the particle quality. 6 mL ethylene glycol (≥99.8%, Sigma-Aldrich) were placed in a 50 mL round bottom flask, sealed with a glass stopper and heated to 140 °C under constant stirring (1300 rpm). After 60 min, 30 µL of 0.1 M HCl in water were added. With a two-channel syringe pump (KDS-200, KD Scientific), freshly prepared solutions of silver nitrate (3 mL, 16.0 mg mL-1, Carl Roth) and PVP (3 mL, 16.2 mg mL-1, MW ~55,000 g mol-1, SigmaAldrich) in ethylene glycol were simultaneously added at a rate of 45 mL h-1 each. The flask was then sealed with a glass stopper. Heating and stirring were continued. After 20 h, the reaction was stopped and particles were collected by ultracentrifugation (29,400 g, 30 min). To remove silver nanowires that typically occur as by-product, filtration was performed with 0.45 µm cellulose syringe filters. The particles were isolated by ultracentrifugation (29,400 g, 30 min) and washed several times with acetone and ultrapure water.

Synthesis of silver nanorods


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Silver nanorods were obtained by a modified polyol process. First, silver chloride nanocubes were generated in situ by the reaction of silver nitrate with sodium chloride as described by Schuette et al.47 For purification, the nanorods were collected by centrifugation (666 g, 30 min), washed with aqueous ammonia (2 mL 30% aqueous NH3 given to 5 mL particle dispersion) to remove excess silver chloride, centrifuged again and rinsed with ultrapure water several times.

Purification and storage All nanoparticles were separated from the reaction solution by centrifugation as described above. For final purification, centrifugation and redispersion in ultrapure water were repeated under the same conditions. To avoid dissolution74,90-93 or chemical aging, all particles were stored in ultrapure water under argon and kept in the dark.

Dynamic light scattering and electron microscopy The size distribution in aqueous suspension was measured by DLS (Malvern Zetasizer Nano ZS, 633 nm laser) at a concentration of 50 mg mL-1, whereas the size and shape of the metallic particle core was determined by scanning electron microscopy (ESEM Quanta 400 FEG, FEI). 10 µL of each suspension were placed on a silicon wafer, dried under ambient condition and analyzed at accelerating voltages between 15 and 30 kV. To obtain a cross section of silver nanorods (Figure 14B), an aqueous solution of the particles was dripped on a silicon wafer, cooled in liquid nitrogen and then mechanically fractioned. High-resolution imaging was performed with an aberration-corrected FEI Titan transmission electron microscope operated at 300 kV.


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Powder diffraction For crystallographic investigations, the synthesized nanoparticles were studied with a Panalytical Empyrean instrument with Cu Kα radiation (λ=1.54 Å; 40 kV and 40 mA). Similar to the preparation of SEM samples, the nanoparticles were placed on a single crystal silicon surface and dried at ambient temperature in air. All nanoparticles were analyzed in Bragg-Brentano geometry to record the powder diffractograms in the 2Θ range from 5 to 90° with a step size of 0.05°. Pole figures were obtained with the program X’Pert Texture 1.2 from PANalytical. Furthermore, the silver nanocubes were investigated by grazing incidence diffraction (GIXRD) with a beam angle of 1° with respect to the sample holder surface due to the strong texture of the (100) peaks and the absence of other reflection peaks. For the calculation of the average crystallite size and the lattice parameters, Rietveld refinement was performed with the program TOPAS 4.2 (Bruker). For each Rietveld refinement, the instrumental correction as determined with a LaB6 standard powder sample from NIST (National Institute of Standards and Technology) as standard reference material (SRM 660b). Note that the instrumental peak broadening was different for the two diffractometers and determined separately (including the GIXRD setup). Silver (#4-0783) patterns from the ICDD database were used as reference material.

ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (DFG) for funding in framework of the projects Ep 22/44-1, He 7192/2-1, and He 7192/1-1.


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For Table of Contents Use Only

Title: On the crystallography of silver nanoparticles with different shape

Authors: Jens Helmlinger, Oleg Prymak, Kateryna Loza, Martin Gocyla, Marc Heggen, Matthias Epple

TOC graphic

Synopsis X-ray powder diffraction as a crystallographic tool is well suited to study silver nanoparticles with different shape. Especially a detailed texture analysis and the analysis peak broadening can shed light on their morphology and on their crystal growth process.


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On the Crystallography of Silver Nanoparticles with Different Shapes

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