Nanoscale Ag2S Hollow Spheres and Ag2S Nanodiscs Assembled to

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Nanoscale Ag2S Hollow Spheres and Ag2S Nanodiscs Assembled to Three-Dimensional Nanoparticle Superlattices Peter Leidinger,† Radian Popescu,‡ Dagmar Gerthsen,‡ and Claus Feldmann*,† †

Institut für Anorganische Chemie, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, Karlsruhe, D-76131, Germany Laboratorium für Elektronenmikroskopie, Karlsruhe Institute of Technology (KIT), Engesserstraße 7, D-76131 Karlsruhe, Germany

‡

S Supporting Information *

ABSTRACT: Nanoparticle superlattices are built up with Ag2S hollow spheres (outer diameter, 37 nm; wall thickness, 10 nm; inner cavity size, 17 nm) and Ag2S nanodiscs (diameter, 20 nm; thickness, 7 nm) as building blocks. Both types of Ag2S superstructures are formed via microemulsion-based synthesis, followed by a phase-separation reaction. The nanoparticle superlattices exhibit dimensions of 5−20 μm. Herein, the Ag2S hollow spheres are arranged like a closest packing of hard spheres, whereas the Ag2S nanodiscs are stacked in parallel rows. Both as-prepared building blocks crystallize with the α-Ag2S/acanthite type of structure. The here described nanoparticle superlattices with nanoscale Ag2S hollow spheres are some of the first examples employing nanoscale hollow spheres (diameter 10 nm). With these conditions, Ag2S hollow spheres are formed with water remaining as an inner cavity (Figure 1). In sum, the formation of the different Ag2S building blocks is well in accordance with the underlying concepts describing the stability of micellar systems (hydrophilic−lipophilic balance, mixed-film theory, and solubilization theory),15−17 which supports the here observed nanostructures. Formation of Nanoparticle Superstructures of Ag2S Hollow Spheres and Ag2S Nanodiscs. For obtaining the nanoscale Ag2S hollow spheres as building blocks, a water-tosurfactant ratio of ω = 22 turned out as optimal and was applied to synthesize all hollow spheres discussed in the following (Figure 2, Table 1). In contrast, the Ag2S nanodiscs were best obtained at ω = 5 (Table 1). For both building blocks, the base reaction of ammonia and the hydrolysis of thiourea (TU) in the polar phase are decisive for controlling the course of the reaction,13 which can be followed even by the naked eye due to the color change from yellow via orange-brown to deep black (cf. ESI: Figure S1). On the basis of the microemulsion

Figure 4. Electron microscopy (TEM) of single Ag2S hollow spheres: (A,C) HRTEM images; (B) Fourier transform of the hollow sphere shown in (A) and calculated indexed diffraction pattern of monoclinic Ag2S with [123] zone axis (blue circles); (D,E) BF-STEM images; (F) HAADF-STEM image.

The self-assembly of the individual building blocks and the formation of nanoparticle superlattices for both the Ag2S hollow spheres and the Ag2S nanodiscs were maintained by a phase transfer-induced separation of remaining salts and polar molecules (e.g., NH4+, NO3−, TU, NH3, and H2O) subsequent to the microemulsion-based synthesis (Figure 3). After intense stirring for 24 h, phase separation occurred with the black Ag2S nanoparticles being located in the upper nonpolar toluene phase. Remaining salts and polar molecules, on the other hand, were dissolved and separated by the polar bottom-DEG phase (cf. ESI: Figure S1). In accordance with the accepted mechanisms and concepts for self-assembly,2,3 this measure resulted in a controlled colloidal destabilization. After separation of the toluene phase, the formation of nanoparticle 4176

dx.doi.org/10.1021/cm401668g | Chem. Mater. 2013, 25, 4173−4180

Chemistry of Materials

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Figure 5. Electron microscopy (SEM) of the Ag2S hollow sphere superstructures: (A) overview; (B−D) top and side view of hollow sphere superstructrues; (E,F) high-magnification image indicating the dense packing of hollow spheres.

nature of the Ag2S building blocks is also demonstrated by calculated diffractograms (Figure 4b) that were obtained by Fourier transformation of the hollow sphere image shown in Figure 4a. The calculated diffraction pattern of bulk monoclinic α-Ag2S exactly agrees with the experimentally observed arrangement of reflections (Figure 4b).17 In addition to selected particles, the composition and crystallinity are evidenced for powder samples with a statistically relevant number of Ag2S hollow spheres based on selected-area electron diffraction (SAED), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDXS). Here, the observed Bragg peaks can be readily identified as monoclinic α-Ag2S/acanthite (Figure 8, cf. ESI: Figure S3).18 EDXS indicates an Ag:S ratio of 1.96:1 (calculated 2:1, cf. ESI: Figure S3). In addition, a certain amount of bromine, carbon, and oxygen is observed and can be ascribed to CTABremaining as a surface capping on the hollow spheresas well as to the carbon (Lacey) film used for sample deposition. Nanoparticle superlattices of nanoscale Ag2S hollow spheres show regularly assembled building blocks similar to a dense packing of hard spheres with overall dimensions of 10−30 μm (Figure 5). Herein, the individual Ag2S hollow spheres are nevertheless clearly visible. The as-prepared Ag2S hollow spheres turned out to be highly sensitive to the electron beam, especially if the current density was increased at high magnification to typical values up to 100 A cm−2. Perforation and collapse of the hollow sphere structure are observed on a time scale of some seconds (cf. ESI: Figure S2). Such decomposition of fragile nanostructures has been frequently reported in the literature and is attributed to local charging and

superlattices occurred on a time scale of 2−6 h for the Ag2S hollow spheres and a period of 4−6 weeks for the Ag2S nanodiscs. During self-assembly the concentration of the nanoparticles remaining in suspension decreases continuously as indicated by the brightening of the liquid phase. The significantly longer period needed for the self-assembly of the nanodiscs can be rationalized based on the fact thatin contrast to the hollow spheres as spherical building blocksa defined orientation of the nanodiscs is a prerequisite (Figure 3). This preferred orientation of the nanodiscs is validated by HRTEM (Figure 7) as well as by XRD (Figure 8). The hollow spheres, in contrast, do not show a preferred orientation (Figures 4 and 8). Characterization of Nanoscale Ag2S Hollow Spheres and Nanoparticle Superlattices Thereof. Electron microscopy is naturally the most relevant analytical tool for characterizing the Ag2S hollow spheres and the hollow sphere superlattices. Since inelastic electron scattering of the micrometer-sized superstructures is too strong for showing the inner cavity of the hollow spheres with transmission electron microscopy, first of all, individual Ag2S hollow spheres were studied. To this end, HRTEM (Figure 4a,c), BF-STEM (Figure 4d,e), and HAADF-STEM (Figure 4f) images clearly evidence the hollow sphere structure. The nanoparticles exhibit an average outer diameter of 37(4) nm, a wall thickness of 10(2) nm, and an inner cavity diameter of 17(3) nm. Lattice fringes extend through the whole hollow sphere and indicate the single-crystalline structure of the sphere wall (Figure 4a). The measured fringe distance of 2.8(1) Å is compatible with α-Ag2S/acanthite (d(1̅12) with 2.84 Å).18 The single-crystalline 4177



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Figure 7. High-resolution electron microscopy of the Ag2S nanodiscs: (A,B) top view of a single nanodisc with lattice fringes (HRTEM); (C,D) stack of nanodiscs (SEM); (E,F) stack of nanodiscs (HRTEM).

Figure 6. Electron microscopy (SEM) of the Ag2S nanodisc superstructures: (A) overview; (B) side view of a nanodisc superstructure; (C−E) top views of hollow tube-shaped nanodisc superstructures with helix-like winding; (F) high-magnification image indicating the stacked packing of the helix-like winded nanodiscs (turning of the helix-like winding indicated by arrows).

heating due to the high-energy electron bombardment.8g,19 With regard to the electron energy, the hollow spheres are even more sensitive to high-resolution TEM analysis (≥80 keV) due to additional knock-on damage than to SEM analysis (≤30 keV) where knock-on damage is negligible. During the collapse of the hollow-sphere structure under high-energy electron bombardment, the sphere wall is partly broken up so that the inner cavity becomes visible as a hole. The damaged hollow spheres located on the outer surface of the nanoparticle superlattice with its opened inner cavity look like a perforated structure (cf. ESI: Figure S2). The collapse and occurrence of a characteristic hole in the center of each hollow sphere, on the other hand, again evidence the presence of the inner cavity. In contrast, solid Ag2S nanoparticles of similar size do not show such collapse and formation of a perforated structure. 3.3. Characterization of Nanoscale Ag2S Nanodiscs and Nanoparticle Superlattices Thereof. The as-prepared Ag2S nanodiscs form huge quantities of tubelike superstructures,

Figure 8. XRD diffraction pattern of the Ag2S hollow sphere (top) and Ag2S nanodisc (bottom) superstructures (reference: α-Ag2S/acanthite).17

5−20 μm in length and about 500 nm in diameter (aspect ratio of 10−60, Figure 6a). Figure 6b displays a side view of a single tubelike superstructure. A top view of Ag2S nanodisc arrays shows a tubelike structure with a uniform outer diameter of 400−500 nm and a lens-shaped inner channel with dimensions of about 200 × 50 nm (Figures 6c−e). Figure 6f clearly indicates the substructure of the nanodisc superstructure array. Accordingly, the tube-like superstructure is composed of individual nanodiscs that are deposited on top of each other like a stack of drops. Notably, the parallel rows of stacked nanodiscs are slightly twisted along the longitudinal axis of the superstructure, resulting in a helix-like winding. As expected, this helix-like winding is observed with a right turn for some 4178



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like winding. In general, such nanoparticle superlattices of hollow spheres or nanodiscs as building units are rare. As the microemulsion approach for hollow spheres and nanodiscs as well as the self-assembly of nanoparticle superlattices are successful in the case of Ag2S, this strategy can be a useful option for obtaining further nanoparticle superlattices and compositions as well. Besides transferring the here applied microemulsion approach to other types of nanoparticles and nanoparticle superlattices, the Ag2S hollow sphere and nanodisc superlattices can be interesting in view of their physical properties. To this concern, the intrinsic electron and ion conductivity of Ag2S as well as its semiconducting properties can be relevant in view of solar cells or sensors. Moreover, the mechanical properties of the nanoparticle superlattices can be interesting. However, a reliable characterization of the electrical and the mechanical properties of the nanoparticle superlattices is not easy, on one hand, due to the small dimensions that need special equipment, and on the other hand, due to the huge number of grain boundaries in the nanocrystal.

superstructures and with a left turn for others (Figures 6b−f). One can assume that such helix-like winding increases the structural stability of the nanodisc stacks. The formation of an inner channel is a direct structural consequence of the observed helix-like winding. A densely packed rod of nanodiscs, in comparison, would exhibit significant stress and strain. HRTEM images illustrate the substructure of a single Ag2S nanodisc in detail (Figure 7). Accordingly, the nanodiscs are characterized by an average diameter of 20(3) nm and a thickness of 7(2) nm. Again, parallel rows of nanodiscs stacked on top of each other are clearly visible (Figure 7c−f). Similar to the above-discussed Ag2S hollow spheres, lattice fringes across the whole nanodisc confirm the single crystallinity of the nanodiscs (Figure 7a). The measured fringe distance of 2.4(2) Å is again compatible with the presence of α-Ag2S/acanthite (d(121) with 2.44 Å).18 Furthermore, the composition of the nanodiscs is validated by EDXS analysis, indicating a ratio Ag:S of 2.17:1 that is well in accordance with the expectation (2:1). Certain amounts of bromine, carbon, and oxygen can be again ascribed to CTAB remaining as a surface capping and to carbon (Lacey) film used for sample deposition (cf. ESI: Figure S5). SAED patterns recorded from single nanodisc superlattices indicate a remarkably high intensity of the (1̅12) diffraction peak (cf. ESI: Figure S5). The higher peak intensity of (1̅12) is also evident when comparing the nanodiscs with the hollow sphere superlattices (cf. ESI: Figure S3). XRD patterns also confirm the difference between the nanoparticle superlattices of Ag2S hollow spheres and Ag2S nanodiscs (Figure 8). Again, the (11̅ 2) Bragg peak occurs with much higher intensity. Since the single-crystalline nanodiscs are well-assembled along the longitudinal axis of the tubelike superstructure such textural effect is not a surprise and can be reliably explained with (1̅12) in parallel to the predominant layer of the nanodisc (Figure 7b). Taking this as a model, lattice planes perpendicular to (11̅ 2) should exhibit a reduced intensity. Indeed, this holds for several Bragg peaks (including (111), (112), (121), (031)) showing a significantly lower intensity as compared to the hollow sphere superlattices or the bulk reference (Figure 8). The orientation of the nanodiscs with (11̅ 2) parallel to the predominant disc layer is also in agreement with (121) as the as-observed lattice distance on the HRTEM images (Figure 7a), since (121) is almost rectangular to (11̅ 2). On the basis of the (11̅ 2) diffraction peak, the volume-averaged diameter of the nanodiscs can be finally calculated to Dv = 16 nm via the Scherer equation. This value is in sufficient agreement with the real diameter of the nanodiscs (20(2) nm) and confirms their orientation once more. In sum, the lattice orientation of the nanodiscs is independently derived from XRD and HRTEM. Both data match exactly to each other and reliably indicate the identical lattice orientation of each individual nanodisc as well as the parallel orientation of all nanodiscs in the macroscopic hollow tube superstructure.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information regarding analytical tools and materials characterization, the course of the reaction, damage/collapse of Ag2S hollow spheres and nanodiscs under the influence of highenergy electrons, and the chemical composition of the Ag2S hollow spheres and nanodiscs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Center for Functional Nanostructures (CFN) of the Deutsche Forschungsgemeinschaft (DFG) at the Karlsruhe Institute of Technology (KIT) for funding.

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REFERENCES

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CONCLUSIONS Nanoparticle superlattices consisting of nanoscale Ag2S hollow spheres and Ag2S nanodiscs are selectively obtained via a microemulsion approach. Whereas the individual building units exhibit dimensions on the nanoscale (Ag2S hollow spheres: outer diameter 37 nm, wall thickness 10 nm, inner cavity size 17 nm; Ag2S nanodiscs: diameter 20 nm, thickness 7 nm), both types of nanoparticle superlattices show micrometer-size dimensions. While the Ag2S hollow spheres are arranged like a close packing of hard spheres, the Ag2S nanodiscs are stacked to parallel rows, forming a tubelike superstructure with a helix4179



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Nanoscale Ag2S Hollow Spheres and Ag2S Nanodiscs Assembled to

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