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Langmuir 1998, 14, 1528-1531
Structural Characterization of Colloidal Ag2Se Nanocrystals V. Buschmann* and G. Van Tendeloo EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Ph. Monnoyer and J. B. Nagy Faculte´ s Universitaires Notre-Dame de la Paix, Laboratoire de RMN, rue de Bruxelles 61, B-5000 Namur, Belgium Received December 2, 1997. In Final Form: February 2, 1998 Ag2Se nanocrystals of less than 15 nm have been investigated by means of high-resolution electron microscopy and nano-energy-dispersive X-ray spectroscopy. Two groups of clusters are distinguished, depending upon their preparation technique. Those obtained by double jet precipitation show a limited stability both in time and under the electron beam. Their structure is orthorhombic, although a cubic phase is encountered if an excess Ag is present. Those prepared by microemulsion mixing have an orthorhombic structure as well but show a greater stability.
Introduction Its low activation energy for diffusion and conduction makes Ag2Se a typical superionic conductor, similar to the high-temperature phases of AgI and Ag2S. Ag2Se nanocrystals may be used as chemical sensitizers in the photographic industry, equivalent to the traditional Ag2S.1 The high-temperature phase, above 406 K, of Ag2Se films was determined by Ralphs et al.2 as body-centered cubic (bcc) with a lattice parameter of 0.498 nm. As to the low-temperature phase, the influence on the structure of both stoichiometry and preparation conditions has resulted in a huge amount of structural data. Nevertheless, most authors agree that in the case of a correct stoichiometry the structure of the investigated films is orthorhombic with a ) 0.433 nm, b ) 0.706 nm, and c ) 0.776 nm, belonging to the space group P212121 (19).3 When the compound is nonstoichiometric, as a result of processing conditions, a different structure may be encountered. Okabe et al.4 used the flash evaporation technique for preparing Ag2Se thin films and sandwiched them between two carbon layers in order to maintain their initial composition throughout the processing and investigation. Using this procedure, they found for a correct stoichiometry the orthorhombic phase, as described above. Films containing an excess of silver were face-centered cubic (FCC) with a lattice parameter of 1.09 nm. On the other hand, in the presence of an excess of selenium, the structure was monoclinic with a ) 0.705 nm, b ) 0.817 nm, c ) 0.434 nm, and R ) 101.0°. Also a metastable phase, only found for grains measuring less than 50 nm, was characterized exhibiting the tetragonal structure.4 The influence of the thickness on the film structure has been studied by Gu¨nter et al.5 and Sa´fra´n et al.6 This was * Corresponding author, present address: TU Darmstadt, Fachgebiet Materialwissenschaften-Fachbereich Strukturforschung, Petersenstrasse 23, D-64287 Darmstadt, Germany. (1) Buschmann, V.; Schryvers D.; Van Landuyt J.; Van Roost C.; De Keyzer R. J. Imaging Sci. Technol. 1996, 40, 189. (2) Ralphs, P. Z. Phys. Chem. 1936, 31B, 157. (3) Wiegers, G. A. Am. Mineral. 1971, 56, 1882. (4) Okabe, T.; Ura, K. J. Appl. Crystallogr. 1994, 27, 140. (5) Gu¨nter, J. R.; Keusch, P. Ultramicroscopy 1993, 49, 293. (6) Sa´fra´n, G.; Keusch, P.; Gu¨nter, J. R.; Barna, P. B. Thin Solid Films 1992, 215, 147.
done by selenizing silver films on 〈100〉-cleaved NaCl substrates, meanwhile gradually increasing the selenium and silver deposition. For the thinnest films a cubic together with a tetragonal phase was formed. A metastable monoclinic, pseudotetragonal phase was encountered when investigating films with a thickness of about 20-30 nm. Thicker Ag2Se films transformed into the stable orthorhombic phase. Many other structures are also described in the literature, e.g., fcc with lattice parameter 0.565 nm,7 a triclinic - pseudo-orthorhombic phase by De Ridder et al.8 Molecular dynamics simulation at constant pressure and temperature revealed an orthorhombic phase with lattice parameters a ) 0.429 nm, b ) 0.682 nm, and c ) 0.825 nm and space group Pmnb.9,10 In the present study individual Ag2Se nanocrystals with diameter less than 15 nm are investigated. The clusters are prepared in solution, in regard to which two groups may be distinguished, depending on the preparation method. One group is made by double jet precipitation, the other by microemulsion mixing. In the first group a protective colloid, in the latter a surfactant, helps to maintain the correct stoichiometry during the examination of the clusters. The close relationship between stoichiometry and structure favors a high-resolution electron microscopy (HREM) study in combination with nano-energy-dispersive X-ray spectroscopy (nano-EDX) analyses. This indeed enables us to obtain maximal information about individual nanocrystals. Experimental Section As mentioned above, the Ag2Se nanoclusters are divided into two groups depending on their origin: double jet precipitation and microemulsion mixing. Double jet precipitation is a well-established technique in the photographic industry for the manufacture of silver halides.1 For the preparation of the Ag2Se nanocrystals 1,1-dimethyl-2selenurea and AgNO3 are precipitated in a reaction vessel in the (7) De Ridder, R.; Amelinckx, S. Phys. Status Solidi 1973, 18, 99. (8) De Ridder, R.; De Sitter, J.; Amelinckx, S. Phys. Status. Solidi A 1974, 23, 615. (9) Shimojo, F.; Okazaki, H. J. Phys. Soc. Jpn. 1991, 60, 3745. (10) Shimojo, F.; Okazaki, H. J. Phys. Soc. Jpn. 1993, 62, 179.
S0743-7463(97)01321-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/04/1998
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Figure 1. Size distribution for the different samples studied: (a) double jet precipitation, (b-d) microemulsion mixing, AgNO3 concentration 0.063, 0.125, and 0.250 M. presence of 0.1% gelatine, which acts as a protective colloid. The precipitation takes place under controlled conditions of pAg ()log[AgNO3]) and temperature. The second technique is the mixing of a microemulsion containing AgNO3 as precursor salt and a selenium salt solution, Aerosol OT (AOT or sodium bis(2-ethylhexyl)sulfosuccinate), acting as surfactant, and water.11 An aqueous solution of AgNO3 is added to the AOT solution in n-heptane. The thus formed microemulsion contains the precursor salt in its water droplet. A solution of bis(trimethyl)selenium in heptane is added to the initial microemulsion and stirred until a stable state is reached. Different concentrations of AgNO3 (0.25, 0.125, and 0.063 M) are used in order to influence the final size of the nanocrystals. The amount of bis(trimethyl)selenium in heptane is of course each time adjusted for a stoichiometric matching with respect to the AgNO3 concentration. For both techniques processing and preparation are carried out at room temperature. The specimens for the transmission electron microscopy (TEM) investigations are obtained by
Langmuir, Vol. 14, No. 7, 1998 1529 depositing a drop of the respective solutions upon a carbon support. The reference standard for the EDX analysis is a Ag2Se crystalline thin film made by successive evaporation of Ag and Se onto a NaCl substrate. To check for the representatives of the film, X-ray diffraction (XRD), energy dispersive X-ray (EDX), and selected area diffraction (SAD) are used. As the spectra from the clusters originate from a limited number of counts, compared to those resulting from bulk material, optimal calibration of the EDX system is cared for. The effect of such low-count spectra on the results of the quantitative analysis was examined beforehand, for that matter.12 A Philips CM20 microscope, operating at 200 kV, with a point resolution of 0.24 nm and a JEOL 4000 EX microscope, operating at 400 kV, with point resolution of 0.17 nm, are used for the structural investigation of the clusters. Image simulations are calculated using the MacTempas program based on the multislice method. A LINK EDX-system with ultrathin window is attached to the Philips CM20 microscope. Making the electron beam as narrow as a few nanometers in the nanoprobe mode, individual clusters may be analyzed.
Results Ag2Se Nanocrystals Made by Double Jet Precipitation. HREM overview images taken at low magnification show a non-Gaussian cluster size distribution profile. A 63% portion of the entire population lies within the interval 3-7 nm (see Figure 1). EDX analysis of clusters, using a beam diameter of 7 nm in the nanoprobe mode, with different sizes reveals a slight silver excess of about 2 atom % for the majority of them. In Figure 2, a HREM image of an orthorhombic Ag2Se cluster is presented viewed along the [021] direction. Lattice fringes of 0.43 and 0.32 nm, corresponding to the (100) and (01 h 2) planes viz., can be seensthe calculated FFT is shown as inset.
Figure 2. (a) Orthorhombic Ag2Se nanocrystal, made with double jet precipitation, viewed along the [021] direction. The corresponding calculated FFT as inset. (b) HREM simulations at different defoci and thickness. (c) Schematic representation of the structure along the [021] zone axis.
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Figure 3. Cubic Ag2Se nanocrystal, made with double jet precipitation, 60 days after preparation. Notice the metallic Ag speck on the cluster.
The structural data, used for the simulation giving an optimal fit between experimental and calculated image, are a ) 0.433 nm, b ) 0.706 nm, and c ) 0.776 nm, with the space group P212121. The resulting simulated images, calculated for different defoci and thickness, are presented in Figure 2b, while a schematic representation of the structure along this zone axis is shown in Figure 2c. At a defocus of -70 nm and a thickness of 5 nm, the white dots correspond to Ag atoms, while the weaker dots represent the Se positions. The limited stability of these clusters both in time and under the influence of the electron beam is demonstrated in Figure 3 representing a cluster 60 days after its preparation. A metallic Ag0-speck has formed on the Ag2Se cluster. Remarkably, the Ag2Se cluster is no longer orthorhombic, but cubic instead. The orientational relationship between both Ag0-speck and cluster is [001]Ag/ /[010] Ag2Se and [010]Ag//[010]Ag2Se. During the observation, the metallic Ag speck dissolved while the Ag2Se cluster remained cubic. Ag2Se Nanocrystals Made by Double Jet Precipitation. As for the previous samples, these clusters also show a deviation from the Gaussian profile (Figure 1). Although the majority of the clusters have a diameter between 6 and 8 nm, what ever the AgNO3 concentration, the mean size of the nanocrystals increases with augmenting precursor concentration. These nanocrystals also exhibit a greater stability in time as well as under the electron beam. Nano-EDX analysis exhibits the correct stoichiometry for most of the clusters analyzed, but occasionally a Se excess is detected. The Debye-Scherrer rings, present in the selected area diffraction (SAD) patterns taken from the sample, may consistently be indexed as those of the orthorhombic phase; moreover, the presence of metallic silver is not present. These clusters show very few defects and a typical example of a Ag2Se cluster made by microemulsion mixing is presented in Figure 4 (again the calculated FFT is shown as inset). This particle is viewed along the [001] zone axis of the orthorhombic structure. (11) Monnoyer, Ph.; Nagy, J. B.; Buschmann, V.; Fonseca, A.; Jeunieau, L.; Piedigrosso, P.; Van Tendeloo, G. In Nanoparticles in Solids and Solutions; Fendler, J. H., De´ka´ny, I., Eds.; Kluwer Academic Publishers: The Netherlands, 1996; p 505. (12) Buschmann, V.; Monnoyer, Ph. To be published.
Figure 4. Orthorhombic Ag2Se nanocrystal, made by microemulsion mixing, [001] zone axis, corresponding calculated FFT as inset.
Discussion Both preparation techniques, double jet precipitation and microemulsion mixing, are suitable for the production of nanosized Ag2Se crystals. The second technique, however, produces clusters with a much narrower distribution profile and better stability in time and under the electron beam. This may be the result of the AOT surfactant used, which also prevents the clusters from coalescing. It is well-known that gelatin melts already at a temperature of 40 °C. As much higher temperatures are induced in the specimen by constant observation under electron beam irradiation, choosing a more suitable surfactant may improve the stability of the clusters. With the aid of nano-EDX, a more accurate profile of the true stoichiometry of individual clusters can be obtained. The results show that both techniques provide a stoichiometric correct Ag2Se population. Whenever a deviation is present, though, a slight excess of silver tends to be more readily encountered in the case of double jet precipitation, whereas a slight selenium excess may be expected in the case of the microemulsion technique. For both preparation methods, HREM images prove the structure of the nanocrystals to be orthorhombic, although a cubic modification is observed in the presence of a silver excess. However, some nanoclusters could not be uniquely catalogued as belonging to one of the known Ag2Se phases. The limited size of these crystals prevents us from obtaining more structural information: tilting the specimen in order to examine the crystal along different zone-axes is impossible. No phase changes or vibrations under the influence of the electron beam could be detected, a phenomenon often associated with nanoclusters. However, effects such as coalescing were witnessed in both samples. Another difference with the more frequently studied metallic and bimetallic nanocrystals is the absence of multiple twinned particles (MTPs). Summary We report on the structural investigation of colloidal Ag2Se clusters by means of high-resolution electron
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microscopy. Two groups of nanoclusters may be distinguished: the first one is made by double jet precipitation and the second one by microemulsion mixing. With these techniques individual nanocrystals are obtained sized less than 15 nm and having a correct stoichiometry for the majority of the clusters. Moreover they exhibit the expected orthorhombic phase with a high crystalline quality.
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Acknowledgment. V. Buschmann acknowledges J. O. Malm, Department of Inorganic Chemistry, University of Lund, Sweden, for the use of the JEOL 4000 EX microscope. R. De Keyzer of Agfa N.V. (Belgium) is acknowledged for providing the Ag2Se clusters made by double jet precipitation. LA9713210
