Sonochemical preparation of SbSI gel

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Ultrasonics Sonochemistry 15 (2008) 709–716 www.elsevier.com/locate/ultsonch

Sonochemical preparation of SbSI gel M. Nowak b

a,*

, P. Szperlich a, Ł. Bober a, J. Szala b, G. Moskal b, D. Stro´z_

c

a Solid State Physics Section, Silesian University of Technology, ul. Krasin´skiego 8, 40-019 Katowice, Poland Department of Materials Science, Silesian University of Technology, ul. Krasin´skiego 8, 40-019 Katowice, Poland c Institute of Material Science, University of Silesia, ul. Bankowa 12, 40-007 Katowice, Poland

Received 2 June 2007; received in revised form 24 August 2007; accepted 12 September 2007 Available online 20 September 2007

Abstract A novel sonochemical method for direct preparation of nanocrystalline antimony sulfoiodide (SbSI) has been established. The SbSI gel was synthesized using elemental Sb, S and I in the presence of ethanol under ultrasonic irradiation (35 kHz, 2 W/cm2) at 50 C for 2 h. The products were characterized by using techniques such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and optical diffuse reflection spectroscopy (DRS). The SEM and HRTEM investigations exhibit that the as-prepared samples are made up of large quantity nanowires with diameters of about 10–50 nm and lengths reaching up to several micrometers and singlecrystalline in nature.  2007 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 82.70.Gg; 71.20.Nr Keywords: Sonochemistry; Antimony sulfoiodide; Nanocrystals; Ferroelectric materials

1. Introduction The first description of the synthesis of antimony sulfoiodide (SbSI) was given by Henry and Garot [1] in 1824. The next papers on this material were published by Schneider [2], Ouvrard [3,4] and Franc¸ois [5]. In 1950, the crystal structure of SbSI was established by Do¨nges [6]. In 1958, Mooser and Person predicted the semiconducting properties of AVBVICVII compounds [7]. However, the intensive investigation of SbSI started after discovery in 1960 by Nitsche and Merz [8] of its photoconductivity and in 1962 by Fatuzzo et al. [9] of its ferroelectric properties. The SbSI being a semiconductor ferroelectric has an unusually large number of interesting properties. Among them there are the pyroelectric, pyro-optic, piezoelectric, electromechanical, electrooptic and other nonlinear optical

*

Corresponding author. Tel.: +48 32 603 41 67; fax: +48 32 603 43 70. E-mail address: [email protected] (M. Nowak).

1350-4177/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2007.09.003

effects. They are influenced by light leading to photoferroelectricity and to photorefractive effect. The main properties of SbSI have been reviewed in a few monographs [10–15]. However, the properties of these materials are still investigated [16–19]. Due to these properties it is an attractive and suitable material for thermal imaging [12,13,20–23], light modulator [12,13,24], ferroelectric field effect transistor (FeFET) [25,26], gas sensors [27], piezoelectric elements used in certain types of electromechanical transducers [13,28–30], temperature auto stabilized nonlinear dielectric elements (TANDEL) [31,32], time-controlling devices [12,13,33] and other applications. The SbSI is taken into consideration as a valuable material for photonic crystals [34,35]. Being a promising material with potential applications, SbSI was synthesized in a variety of ways. Since SbSI melts without decomposing [13] at comparatively low temperature (673 K), one of the most convenient methods of preparing its specimens is to fuse stoichiometric amounts of antimony iodide and antimony sulphide [8,36,37];

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antimony, sulfur and iodine [8,37,38]; antimony iodide, antimony and sulfur [37] or antimony sulphide, iodine and antimony [13]. All the above procedures require high temperature (450–600 C) and long reaction time (1–3 days) [39]. The composition of the SbSI compound lies outside the glass-forming region of the non-stoichiometric glass compositions containing antimony, sulfur and iodine and sometimes in addition some other elements [40–44]. The glass-forming tendency of SbSI is so small that even when very small quantities of the stoichiometric melt are quickly cooled e.g. by plunging quartz capsules into liquid nitrogen, the resulting material is always crystalline [45]. SbSI crystals can be obtained (see e.g. [13]) using the melt or flux growth (slowly cooling and unidirectional solidification), the vapour growth (sublimation–condensation and chemical transport reactions) and the hydro-thermal method. Polycrystalline samples obtained from the melt consist of many needles. Monocrystalline samples having mirror like surfaces were obtained from the vapour phase in a closed tube. Unfortunately, the crystals of SbSI are remarkably needle-like; accordingly, it is difficult to form a large single crystal. Bigger samples of hot-pressed SbSI bulk ceramic were prepared but their physical properties were not satisfying [45]. On the other hand, SbSI in a thin-film form have been made by e-beam evaporation, flash evaporation, thermal evaporation and laser evaporation [21,46,47]. Unfortunately, these films fail to fully exhibit the physical properties of SbSI. Currently, nanostructure materials have received great attention because of their potential application in mesoscopic research, the development of nanodevices and in heterogeneous catalysis. The review paper [48] provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructures – wires, rods, belts, and tubes – whose lateral dimensions fall anywhere in the range of 1–100 nm. The authors of [48] highlight a range of unique properties (e.g., thermal, mechanical, electronic, optoelectronic, optical, nonlinear optical and field emission) associated with different types of 1D nanostructures. Compared with bulk materials, low-dimensional nanoscale materials, with their large surface areas and possible quantum-confinement effects, exhibit distinct electronic, optical, chemical and thermal properties. In many cases, 1D nanostructures are superior to their counterparts with larger dimensions [49]. Thus, the attempts have been made to produce SbSI nanocrystals. In 1999, Xu et al. reported the synthesis of SbSI quantum dots in the Na2O–B2O3–SiO2 ORMOSILs (organic modified silicates) matrix by the sol–gel technique (bulk or thin film) [49]. Nanocrystals of SbSI were produced also in organically modified titanium dioxide (TiO2) glass [50– 52]. In 2001, Wang et al. reported [53] synthesis of SbSI nanorods by hydro-thermal method for the first time. It should be mentioned that the SbSI nanocrystals were obtained by ball milling [54–56], too. To the best of our knowledge, preparation of SbSI by sonochemical method has never been reported before. In

sonochemistry powerful ultrasound (i.e. sound with frequencies above 20 kHz) is used to stimulate chemical reactions and physical changes in liquids. It is successfully applied to produce nano-structured metals, alloys, oxides, carbides and sulfides, or nanometer colloids [57–60]. Ultrasound irradiation can be used at room temperature and ambient pressure to promote heterogeneous reactions that normally occur only under extreme conditions of hundreds of atmospheres and degrees. An acoustic pressure wave consists of alternate compressions and rarefactions in the transmitting medium along the wave propagation direction. When a large negative pressure is applied to a liquid, intermolecular van der Waals forces are not strong enough to maintain cohesion and small cavities or gas-filled microbubbles are formed. The rapid nucleation, growth and implosive collapse of these micrometer-scale bubbles constitutes the phenomenon of cavitation. According to the thermal ‘‘hot spot’’ theory, extreme local temperatures and pressures are produced inside the cavitating bubbles and at their interfaces when they collapse. The effective temperature of the resulting transient, local ‘‘hot spots’’ was estimated to be in the range of 5200 ± 650 K [57]. Assuming such value, the pressure during collapse, as inferred from the van der Waals equation, would be approximately 1700 atm. These exceptional local conditions can be used to generate high surface area nano-structured materials. In fact, using the sonication nanorods of the following compounds had been prepared: Bi2S3 [61], Sb2S3 [62–64]. The aim of this paper was to synthesize the SbSI nanomaterial using ultrasound radiation. 2. Experiment The compound was prepared essentially quantitatively from the constituents (the elements: antimony, sulphur and iodine), weighed in the stoichiometric ratio without any excess of iodine or sulphur. Ethanol served as the solvent for this reaction. All the reagents used in our experiments were of analytical purity and were used without further purification. Antimony (99.95%) was purchased from Sigma–Aldrich. Sublimated sulfur (pure p.a.), iodine (pure p.a.), and absolute ethanol (pure p.a.) was purchased from POCH S.A. (Gliwice, Poland). In a typical procedure, the elemental mixture with stoichiometric ratio of e.g. 1.84280 g Sb, 0.48535 g S and 1.92082 g I, was immersed at room temperature and ambient pressure in 8 ml absolute ethanol, which was contained in a 54 ml Pyrex glass cylinder of 20 mm inside diameter. The vessel was closed during the experiment to prevent volatilization of the precipitant in long time tests. The bottom of the vessel was planar or semispherical and 1 mm in thickness. The cylinder was partly submerged in water in a commercial ultrasonic cleaning bath (InterSonic IS-UZP-2, frequency 35 kHz, with 80 W electrical power). In this cleaner, transducers are fixed to the underside of a tank, filled with water continuously pumped through thermostated reservoir to main-

M. Nowak et al. / Ultrasonics Sonochemistry 15 (2008) 709–716

tain constant temperature. The sonolysis experiments were carried out at various temperatures in the range of 20– 75 C. The object to be sonicated, a reaction cylinder in particular, was placed in the area of maximal irradiation, visualized by the strong rippling at the water surface in the cleaner bath. The bottom of the cylinder containing the reactants slurry was positioned 3 mm above the base of the bath. The level of the ethanol slurry inside the tube was maintained to be the same as that of the water in the sonication bath, in order to obtain reproducible sonochemical yields. The time of sonification depended on the temperature of the water in the ultrasonic bath (Fig. 1). During the sonification a sol was formed. It was observed that the color of the slurry changed gradually from red (before sonification) into olive, green, yellow and then into red-orange after 45 min of sonication, indicating the growth process of SbSI nanorods, similarly as in the growth process of Bi2S3 nanorods [61]. The sonochemical reaction was continued so as to complete the gelation of SbSI. When the sonification process was finished, a red-orange gel was obtained (Fig. 2). It is noteworthy that the SbSI sol converted into gel after relatively short sonication (Fig. 1). It happens only under the ultrasound irradiation. To make a comparison, we have also carried out the reaction with vigorous stirring

Fig. 1. Duration of the sonochemical synthesis of SbSI ethanogel as a function of temperature of the bath.

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at room temperature instead of ultrasound irradiation. As a result, no SbSI gel could be obtained even after 8 h of reaction. Characterization of the SbSI gel was accomplished using various different techniques, such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and optical diffuse reflection spectroscopy (DRS). All the investigated samples were dried in air at 40 C. The masses of the samples after drying were practically equal to the masses of the elements Sb, S and I used for the sonochemical synthesis. The ethanol was evaporated from the SbSI gel during the drying, so a so-called xerogel was practically obtained. During this process the syneresis was observed. The volume of the dried samples decreased about 40%. XRD measurements were carried out to determine the crystalline phase of the as-prepared powders of SbSI gel. The powder XRD measurements were performed on a JEOL JDX-7S X-ray diffractometer with graphite monochromatized Cu Ka radiation (k = 0.154056 nm). The acceleration voltage was 40 kV with a 20 mA current flux. A scan rate of 0.05 s1 was used to record the patterns in the 2h range of 10–80. Scanning electron micrograph and EDAX patterns were taken on a Hitachi S-4200 scanning electron microscope with Noran Instruments EDS Voyager 3500 spectrometer. The sizes and structure of the SbSI nanowires were further characterized with HRTEM on a JEOL-JEM 3010 microscope, working at an 300 kV accelerating voltage. The point-to-point resolution was 0.17 nm, and lattice resolution was 0.14 nm. The samples used for HRTEM observations were prepared by dispersing a small quantity of the SbSI gel in ethanol followed by ultrasonic vibration for 120 min. One or two drops of the nanoparticle solution were deposited on a carbon-coated copper grid and dried in 30 min at room temperature in air and then in 120 min at 100 C in vacuum. The crystallinity and crystallography of the product was proven by SAED conducted using a JEOL-JEM 3010 microscope. The DRS measurements were carried out on a spectrophotometer SP-2000 (Ocean Optics Inc.) equipped with an integrating sphere ISP-REF (Ocean Optics Inc.). Spectra were recorded at room temperature, from 1000 to 350 nm. The standard WS-1 (Ocean Optics Inc.) was used as a reference. The diffuse reflectance values were converted to the Kubelka–Munk function [65] (known to be proportional to the absorption coefficient) shown by 2

F K–M ðRd Þ ¼

Fig. 2. SbSI ethanogel obtained by the sonochemical method.

ð1  Rd Þ a 2Rd

ð1Þ

where Rd describes the coefficient of diffuse reflectance and a is the absorption coefficient of light in the investigated material.

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3. Results The powder XRD pattern of the SbSI prepared by the sonochemical method is shown in Fig. 3. It shows welldefined, sharp peaks. So, it indicates that the product can be obtained with high purity and well crystallized. All the diffraction peaks can be indexed to be a pure orthorhombic phase for SbSI with the cell constants a = 0.858 nm, b = 1.017 nm and c = 0.414 nm. The identification was done using the PCSIWIN computer program and the data from JCPDS-International Centre for Diffraction Data 2000. In each XRD pattern, the reflections can be indexed to those of the corresponding pure phases, and all the lattice parameters are very close to the reported data [66]. The intensities and positions of the peaks are in good agreement with literature values for SbSI [66]. These peaks correspond to the 0 2 0, 1 2 0, 2 0 0, 2 1 0, 2 2 0, 1 3 0, 1 2 1, 2 1 1, 3 1 0, 2 3 0, 0 3 1, 0 4 0, 1 3 1, 3 1 1, 2 4 0, 1 4 1, 0 0 2, 1 5 0, 2 4 1, 4 1 1, 2 5 0, 1 5 1, 3 4 1, 5 1 0, 4 3 1, 0 4 2 and 2 6 0 reflections. No peaks of any other phases are detected, indicating the high purity of the product. Usually, the XRD data can be applied to estimate the crystallite average size from the halfwidth of the reflection profiles. However, for needle-shaped crystallites such method can hardly be applicable since the notion ‘‘average size’’ cannot be clearly specified. The analysis of XRD investigation on SbSI nanocrystalite size will be presented in a future publication. The particle sizes and morphology of the SbSI gel were analyzed with SEM and HRTEM. The SEM micrograph of sonochemically prepared SbSI is shown in Fig. 4. The sonochemical SbSI exists as a porous gel composed of nanowhiskers, filaments or nanorods with average lateral dimensions of 10–50 nm and average lengths up to several micrometers. The EDAX analysis performed on this SbSI gel was also done, and characteristic peaks for antimony, sulfur and iodide were observed (Fig. 5) and confirmed an elemental atomic ratio of 0.36:0.30:0.34 for Sb, S and I averaged over

Fig. 3. The powder XRD pattern of orthorhombic phase of dried SbSI ethanogel.

Fig. 4. The typical SEM micrograph of sonochemically prepared SbSI ethanogel.

Fig. 5. The EDAX spectrum of sonochemically prepared SbSI ethanogel.

the SbSI gel. So, it indicates within the experimental error a stoichiometric SbSI. The HRTEM image (Fig. 6) of sonochemically prepared SbSI reveals that the product consists of needle-shaped nanorods. The lateral dimension of the nanorods (see the representative HRTEM image in Fig. 6) is in the range from 10 to 50 nm, and the lengths up to several micrometers. The HRTEM image of a single SbSI nanorod (Fig. 6) exhibits good crystalline and clear (1 1 0) lattice fringes parallel to the rod axis. It indicates that the growth velocity in [0 0 1] direction is larger than in the [1 1 0] direction. The interplanar spacing is about 0.6510(40) nm, which corresponds to the interplanar spacing 0.64989 nm of (1 1 0) planes of the orthorhombic structure of conventional SbSI [66]. In addition, we have observed that the sonochemically produced SbSI nanorods have about 6 nm thick fuzzy shell (Fig. 6). This is probably due to the amorphous species absorbed on the surface of the crystalline nanorods. Similar but thinner fuzzy shell on the surface of sonochemically produced Sb2S3 nanorod has also been reported in [63]. The SAED pattern (Fig. 7) recorded on an individual SbSI nanorod indicates that the sonochemically produced

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Fig. 8. Comparison of the diffuse reflectance spectra of the dried SbSI gel and the powdered single crystals of SbSI. Fig. 6. Typical HRTEM image of an individual nanorod from sonochemically prepared SbSI ethanogel. The fringe spacings of 0.651 ± 0.004 nm correspond to the interplanar distances between the (1 1 0) planes of SbSI crystal. The surface of the nanorod shows an amorphous layer.

nanorods exhibit a single-crystalline structure with a preferred growth oriented along the [0 0 1] direction. The diffraction patterns simulated for SbSI fit well the experimental ones (Fig. 7b). The diffuse reflectance spectrum of the dried SbSI gel is compared in Fig. 8 with the spectrum registered for a bulk powder of SbSI single crystals grown in our laboratory from the vapour phase by the sublimation–condensation process. It is obvious that the diffuse reflectance descends sharply around 660 nm in the case of sonochemically produced SbSI gel (Fig. 8). The diffuse reflectance spectrum of bulk powder of SbSI single crystals is very close to that of SbSI gel for the wavelength shorter than 625 nm. The long wavelength edge of the diffuse reflectance of powder of SbSI sin-

gle crystals has a smoother slope (Fig. 8). This disturbance is probable due to the large amount of crystal defects and additional electron states in the energy gap of the powdered SbSI. Fig. 9 presents the spectrum of FK–M and the best least square fitted theoretical dependence appropriate for the sum of indirect forbidden absorption without excitons and phonon statistics (a1) [67], Urbach ruled absorption (a2) [68] and constant absorption term (a3) [69]: a1 ¼ 0

for hm 6 EgIf 3

a1 ¼ A60 ðhm  EgIf Þ   hm a2 ¼ AU exp EU a3 ¼ A0

for hm > EgIf

ð2Þ ð3Þ ð4Þ

where EgIf represents the indirect forbidden energy gap, EU is the Urbach energy, A0, A60, AU are constant parameters. The determined values of these parameters for the sonochemically produced SbSI gel and the powdered SbSI

Fig. 7. Electron diffraction pattern of individual nanorod from sonochemically prepared SbSI ethanogel in the orientation close to the [1 1 0] zone axis (a) and its simulated diagram (b).

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Fig. 9. The spectrum of the Kubelka–Munk function calculated for the diffuse reflectance of the dried SbSI gel presented in Fig. 8. Solid curve represents the least square fitted theoretical dependence for the sum of indirect forbidden absorption without excitons and phonon statistics, Urbach ruled absorption, and constant absorption term (description in the text; values of the fitted parameters are given in Table 1).

Table 1 The values of indirect forbidden energy gap EgIf, the Urbach energy EU and the other parameters of sonochemically produced SbSI gel and the powdered SbSI single crystals that were determined from the fitting of the spectrum of Kubelka–Munk function evaluated from the measured diffuse reflectance Fitted parameters

SbSI gel

Powdered SbSI single crystals

A60, 1/(eV3 m) AU, 1/m EU, eV A0, 1/m EgIf, eV

54.83(14) 0.3570(19) · 109 0.1031(16) 0.0321(13) 1.829(27)

35.20(27) 0.2523(67) · 109 0.1027(43) 0.0141(17) 1.790(31)

single crystals are given in Table 1. The determined value of the energy band gap of SbSI gel is close to the value evaluated for powdered SbSI single crystals. It is also well compared to the bulk value of band gap of SbSI reported in the literature [10–15]. 4. Discussion In this study, we applied ultrasound irradiation to induce the 1D growth of the SbSI nanocrystals. The transient high temperature and high pressure field produced during ultrasound irradiation provide a favorable environment for the 1D growth of the SbSI nanocrystals, though the bulk solution surrounding the collapsing bubbles is at ambient temperature and atmospheric pressure. The crystal habit is determined by the relative specific surface energies associated with its facets. Hence, the shape of a single-crystalline nanostructure often reflects the intrinsic symmetry of the corresponding lattice. Of course this influences the crystal growth kinetics, by which the

fastest growing planes should disappear to leave behind the slowest growing planes as the facets of the product. It seems that SbSI belongs to the many solid materials [48] that naturally grow into 1D nanostructures, and this habit is determined by the highly anisotropic bonding in the crystallographic structure. The observed rod type morphology of the product (Fig. 6) is possibly due to the inherent chain type structure and growth habit of SbSI. The SbSI crystals are needle-like (column-shaped; long pillars) along the [0 0 1] axis of the orthorhombic cell, coinciding with the ferroelectric (polar) c-axis [10–15]. Thus, the major faces of the plates generally terminate as {1 1 0} planes, parallel to the ferroelectric polar c-axis. The SbSI crystal growths in this direction due to the presence of double chains [(SbSI)1]2 consisting of two chains related by a two-fold screw axis and linked together by a short and strong Sb– S bonds [6]. Weak van der Waals-type bonds bind the double chains. The structure leads to the growth rate anisotropy, for example, Molnar et al. [70] have estimated the growth rate in the c-direction is two orders of magnitude greater than growth perpendicular to it. The unusual chain-type structure of SbSI played a critical role in the formation of the nanowiskers. Once amorphous SbSI began to crystallize, the in situ-generated SbSI nuclei would connect with each other and self-assemble to form chain-type structures. High temperature and high pressure produced by acoustic cavitation is favorable for the 1D self-assembly of these nuclei. A nanocrystalline product is expected if the reaction takes place at the interface [57,58]. Ultrasound can also promote chemical reaction and crystal growth by mixing heterogeneous phases involving the dispersion of an insoluble solid reactant, e.g. SbSI, in a liquid medium. The observed faster gelation of SbSI with the increase of temperature (Fig. 1) is consistent with the general observation in sonochemistry that raising the solution temperature increases the degree of crystallinity. This effect cannot be attributed to the sonochemical effect, but could be due to an enhanced thermal effect, because sonochemistry provides an enormous cooling rate (>1010 K s1) sufficient to cause solidification before crystallizing can occur [71]. The density of single crystal SbSI is equal 5.275 g/cm3 [14]. When the sonificated SbSI gel had the volume of 17 cm3 and its mass was equal 4.24897 g, it means that the SbSI nanocrystals amounts only 4.7% (0.805 cm3) of the xerogel volume. It is consistent with the SEM observations (Fig. 4). Taking into account the average lateral dimensions and length of the nanocrystals measured by SEM and HRTEM, the surface areas of the sonochemically prepared SbSI can be estimated as about 75 m2/g. The extremely high surface-to-volume ratios associated with these nanostructures make their electrical properties extremely sensitive to species adsorbed on surfaces [48]. It should be underlined, that one of the major applications for 1D nanostructures is likely related to the sensing of important molecules, either for medical, environmental or security-checking purposes.

M. Nowak et al. / Ultrasonics Sonochemistry 15 (2008) 709–716

5. Conclusions In summary, we described a novel, very simple, sonochemical synthesis of nanophase, high-surface-area SbSI ethanogel at low temperature. It is a convenient, fast, mild, efficient and environmentally friendly route for producing SbSI nanorods in a single step. In contrary to the other procedures of SbSI preparation the new process is free of any explosion hazard [72]. SbSI nanorods with lengths up to several micrometers and lateral dimensions in the range of 10–50 nm can be routinely prepared via a sonochemical route from an ethanol solution of the elements antimony, sulphur and iodine. The composition, morphology, dimensions, microstructures, and optical properties of the new form of SbSI were characterized. Microstructural analysis reveals that the SbSI nanorods crystallize in an orthorhombic structure and predominantly grow along the [0 0 1] direction. The XRD, HRTEM and SAED patterns show that the as-prepared particles are well crystallized. It should be underlined that SbSI nanowires were synthesized using the sonochemical route with a yield approaching 100%. Since the sonochemical process was carried out at ambient temperature and pressure, it may be predicted that upscaling of this method will lead to large quantities of nanosized SbSI rods (nanowhiskers) with uniform morphology and high purity. It can also be extended to the preparation of some other AVBVICVII semiconductors. Usually the resulting nanocrystalline products, e.g. Bi2S3 [61], Sb2S3 [62,63], after sonication are separated by centrifugation, washed with appropriate liquids and further dried. The difference between these reactions and the presented in this paper is that sonochemically SbSI gel was formed. The presented results show that ultrasound irradiation is favorable for the formation of SbSI nanorods with uniform shape and high crystallinity. Further studies on the properties of the sonochemically prepared SbSI are underway. Ferroelectric SbSI nanowires should provide promising materials for fundamental investigations on nanoscale ferroelectricity [73], and they may also be useful in nanoscale nonvolatile memory applications [74]. Nonvolatile polarization domains as small as 100 nm2 in size can be induced on BaTiO3 nanowires, suggesting that ferroelectric nanowires may be used to fabricate nonvolatile memory devices with an integration density approaching 1 terabit/cm2 [74]. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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