Using of sonochemically prepared components for vapor phase growing of SbI3·3S8

Ultrasonics Sonochemistry 17 (2010) 892–901

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Using of sonochemically prepared components for vapor phase growing of SbI33S8 M. Nowak a,*, M. Kotyczka-Moran´ska a, P. Szperlich a, Ł. Bober a, M. Jesionek a, M. Ke˛pin´ska a, D. Stróz_ b, J. Kusz c, J. Szala d, G. Moskal d, T. Rzychon´ d, J. Młyn´czak e, K. Kopczyn´ski e a

´ skiego 8, 40-019 Katowice, Poland Solid State Physics Section, Institute of Physics, Silesian University of Technology, Krasin Institute of Material Science, University of Silesia, Bankowa 12, 40-007 Katowice, Poland c Institute of Physics, University of Silesia, Bankowa 14, 40-007 Katowice, Poland d ´ skiego 8, 40-019 Katowice, Poland Department of Materials Science, Silesian University of Technology, Krasin e Institute of Optoelectronics, Military University of Technology, Gen. Sylwestra Kaliskiego 2, 00-908 Warsaw, Poland b

a r t i c l e

i n f o

Article history: Received 3 December 2009 Received in revised form 19 January 2010 Accepted 19 January 2010 Available online 28 January 2010 Keyword: Sonochemistry Additive compounds Antimony triiodide–sulfur

a b s t r a c t The using of sonochemically prepared components for growth of SbI33S8 single crystals from the vapor phase is presented for the first time. The good optical quality of the obtained crystals is important because this material is valuable for optoelectronics due to its non-linear optical properties. The products were characterized by using techniques such as X-ray crystallography, powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, high-resolution transmission electron microscopy, selected area electron diffraction, optical diffuse reflection spectroscopy and optical transmittance spectroscopy. The direct and indirect forbidden energy gaps of SbI33S8 illuminated with plane polarized light with electric field parallel and perpendicular to the c-axis of the crystal have been determined. The second harmonic generation of light in the grown crystals was observed. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Over the last years the non-linear optical (NLO) materials have been receiving a lot of attention because of their enormous potential for many useful technological applications, e.g. second harmonic generation (SHG) and parametric generation of light. High values of the second-order non-linear susceptibility and wide optical transparency range are both required for such applications. Having these features, the optically non-linear molecular compounds which crystallize in non-centrosymmetric space groups are of special interest [1–3]. Among them is the family of polar crystals of tetrahedral triiodide adducts of trigonal symmetry, e.g. the SbI33S8. This material is known as: antimony triiodide–sulfur [4], antimony triiodide–sulfur crystalline complex [1], the 1:3 complex between antimony triiodide and sulfur [1,2], adduct (complex) of antimony triiodide with sulfur SbI33S8 [3,5], addition compound SbI33S8 [6–8] or antimony iodide sulfide [9]. The crystalline SbI33S8 exhibits second-order non-linear properties that are quite strong in magnitude [1]. It is optically anisotropic and uniaxial negative [3]. The c-axis of this crystal is parallel to its optical axis [1]. The ordinary refractive index (no = n11 = n22) is bigger in the ab plane (the plane of iodine atoms) than the extraordinary index (ne = n33) along the crystal trigonal c-axis [3].

* 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 Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2010.01.008

The formation of solid addition compounds between antimony iodide and sulfur was reported as early as 1908 [4]. Up to now the SbI33S8 crystals were grown by slow evaporation of the solvent from a carbon disulphide solution containing antimony triiodide and sulfur [1,4,7,8,10,11]. Benzene and carbon tetrachloride were used as solvents [1], too. Unfortunately NLO measurements need good-quality crystals that are not obtained with these procedures [1]. The aim of this paper is to present a new procedure of growing SbI33S8 single crystals from sonochemically prepared intermediate product. This approach is based on the following. Obviously, the good-quality single crystals of many materials are obtained from vapor phase, e.g. by sublimation. Therefore, we decided to use this method to grow the single crystals of SbI33S8. To avoid the easy decomposition of additive compound, we had to perform the sublimation in relatively low temperature. At sufficiently small dimensions, the sublimation rate increases as predicted from classical sublimation theory. Because the decreased temperature of sublimation is one of the features of nanomaterials [12,13], one should use SbI33S8 nanoparticles as the source for sublimation. Up to now there is no information on such nanomaterial in literature, but recently [14] the sonochemical method was used for direct preparation of nanocrystalline antimony sulfoiodide (SbSI). Therefore, we applied a similar sonochemical procedure to obtain the SbI33S8. One should notice that the sublimation temperature of small-molecule semiconductors can be reduced by sonocrystallization [15]. This is possible through the increase of crystal lattice

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energy by the introduction of crystal defects, poor crystallinity, or the formation of metastable polymorphs [15].

2. Experiment The intermediate product, for the growth of SbI33S8 single crystals by sublimation, was prepared sonochemically from the constituents (the elements: antimony, sulfur and iodine), weighed in the stoichiometric ratio for SbI33S8. 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.) were purchased from POCH S.A. (Gliwice, Poland). In a typical procedure, the elemental mixture with stoichiometric ratio of e.g. 0.421 g Sb, 2.662 g S and 1.317 g I, was immersed in 40 ml absolute ethanol at room temperature and under ambient pressure, which was contained in a glass cylinder of 40 mm internal diameter. The vessel was closed during the experiment to prevent volatilization of the precipitant in long time tests. It was partly submerged in water in an ultrasonic reactor (InterSonic IS-UZP-2, frequency 35 kHz, with 80 W peak electrical power and 2 W/cm2 power density guaranteed by the manufacturer). The sonolysis was carried out at 323 K. Using the CP-401 pH-meter (Elmetron) with ERPt13 and ERH-11S electrodes (Hydromet), we have measured the Eh = 0.25 V and pH = 1.2 of the Sb–S–I–ethanol sol after 10 min of sonication. Although the used experimental set up and the applied procedure were the same as the applied for sonochemical preparation of SbSI ethanogel [14], in contrary to the former case, even after 10 h of sonication of the mixture of reagents (Sb:I:S with atomic ratio 1:3:24) a yellow sol was obtained. It was observed that after a few hours some precipitates settled down. The sols were four times centrifuged to extract the products using the MPW-223e centrifuge, MPW Med. Instruments (Poland). Each time the liquid above the sediment was replaced with pure ethanol to wash the precipitates. At the end the yellow centrifuged product was covered by colorless ethanol. As in the cases of sonochemical preparation of other nanocrystalline products, e.g. SbSI [14], the products were dried at room temperature (Fig. 1a). To check the differences between sublimations of the dried product and powdered single crystals of SbI33S8, these two materials have been entered into thermisil ampoules connected with OERLIKON PT50 turbomolecular drag pumping station (Leybold Vacuum). The pressure in the heated ampoules was measured using ACC1009 gauge (Alcatel). The temperature of the investigated material was measured during heating of the ampoules in oil bath using 211 Temperature Monitor (Lake Shore) with Pt103 sensor. The heating rate was about 1.7 K/min. Fig. 2 presents temperature dependences of pressures during two cycles of heating of the investigated materials. One can see that in contrary to

Fig. 2. Vapor pressures of the dried product of Sb, I and S (with atomic ratio 1:3:24) sonication in ethanol (A, B) and of the powdered single crystals of SbI33S8 (C, D) (A, C – first heating, B, D – second heating).

the case of powdered SbI33S8 single crystals the pressure in an ampoule with the product of Sb, I and S sonication increased with heating from 315 to 376 K, attained maximum of a broad peak (from 376 to 382 K), and then decreased in the temperature range from 382 to 403 K. In the second cycle the temperature dependence of the pressure in the same ampoule did not show any special effect. To obtain SbI33S8 single crystals from the vapor phase, the growing was performed in closed ampoules in vertical furnace, which was composed of two zones that temperatures could be controlled independently. The sonochemically prepared material was put into thermisil ampoules of length 13.5 cm and diameter 2.8 cm. Any seed crystal was not prepared. After evacuation (p = 0.1 Pa) the ampoules were sealed. The lower part of the ampoule was wrapped with a sheet of aluminum foil in order to obtain a homogeneous temperature distribution in the source zone. If there is a place where the temperature is not uniform in the source zone, nucleation tends to occur from undesired positions such as on the side wall of the ampoule or even on the top of the source materials. The aluminum foil restricts the position of nucleation only at the top of the ampoule and allows the growth of large crystals. The source temperature was established T1 = 383(1) K and the seed temperature was T2 = 353(1) K. The typical time of growing the SbI33S8 single crystal was 168 h. The typical size of the crystal grown by this method was about 0.5  1  20 mm3. It had goodlooking surfaces and no hollow was observed in it (Fig. 1b). Characterization of the investigated materials was accomplished using different techniques, such as powder X-ray diffraction (XRD), X-ray crystallography, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), high-resolution

Fig. 1. Images of dried product (a) of the sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol and (b) SbI33S8 single crystal grown from the vapor phase of the sonochemical product.

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transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), optical diffuse reflection spectroscopy (DRS) and optical transmittance spectroscopy. The powder XRD measurements were performed on a JEOL JDX-7S X-ray diffractometer with graphite-monochromatized CuKa 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°. The crystal structure of the SbI33S8 single crystals was characterized using an Oxford Diffraction KM4 kappa diffractometer with Sapphire3 CCD detector and graphite-monochromat0 A). The crystal was mounted on a ed Mo Ka radiation (0.71073 Å quartz glass capillary and cooled by a cold dry nitrogen gas stream (Oxford Cryosystems equipment). The temperature stability of the instrument was ±0.2 K. Accurate cell parameters were determined after refinement using the program CrysAlis CCD [16]. For the integration of the collected data the program CrysAlis RED was used [17]. The structure was first solved using the direct method with SHELXS-97 software, and then was refined with SHELXL-97 [18]. Scanning electron micrograph and EDAX patterns were taken on a Hitachi S-4200 scanning electron microscope with Noran Instruments EDS Voyager 3500 spectrometer. The size and structure of the products of sonication of Sb, S and I were further characterized with HRTEM on a JEOL-JEM 3010 microscope, working at 300 kV accelerating voltage. The point-to-point resolution was 0.17 nm, and lattice resolution was 0.14 nm. The SAED investigations were also conducted using a JEOL-JEM 3010 microscope. The DRS measurements were carried out on a spectrophotometer PC2000 (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 proportional to the absorption coefficient [19].

F KM ðRd Þ ¼

ð1  Rd Þ2 a 2Rd

ð1Þ

where Rd describes the coefficient of diffuse reflectance and a is the absorption coefficient of light in the investigated material. For the optical transmittance measurements the SbI33S8 single crystals were mounted in 1.33 Pa vacuum in an optical D2209 chamber (MMR Technologies, Inc.). This chamber was equipped with R2205 Cryogenic Microminiature Refrigeration II-B System and K7701 temperature controller (MMR Technologies, Inc.). Investigations reported in this paper were performed at 293 K. The optical transmittance was measured using PC2000 (Ocean Optics Inc.) spectrophotometer with master and slave cards with 600 lines grating (blazed at 500 and 400 nm, respectively). The spectrophotometer was equipped with appropriate waveguide cables and the deuterium–halogen light source DH2000-FHS from Ocean Optics Inc. The multiple averaged spectral characteristics containing 2048 data points for various wavelengths were registered using the OOI-Base program from Ocean Optics Inc. at constant temperature of the sample. The presented measurements were carried out  0) plane of the SbI33S8 single for normal illumination of the (0 1 crystals. The incoming light beam was linear polarized with the electric vector parallel (||) and perpendicular (?) to the c-axis of the crystal. The optical transmittance (To) data were transformed in the so called absorbance

A ¼ a d ¼ log

  1 To

3 ns pulse duration, 14 lJ pulse energy) pumped with laser-diode LIMO25-F100-LD808 was used in these tests. The non-linear signal was detected by spectrometer Avantes AvaSpec-3648 after spectral separation from the fundamental beam by spectral filtering. The time-dependence characteristics of the 1064 nm radiation laser pulse were measured with InGaAs ET-3000 photodiode by an Agilent Technologies MSO7104A oscilloscope. The data were collected by PC. The detailed analysis of the SHG characteristics of the presented material will be reported in near future. 3. Results and discussion The powder XRD pattern of the product of sonication of Sb, I and S is shown in Fig. 3. The well-defined, sharp diffraction lines suggest the well-crystallized substance. Unfortunately, the identification of these lines (Table 1) shows that the investigated material is a mixture of many substances: SbI33S8, SbI3, SbSI, Sb3I and sulfur. Fig. 4a presents the EDAX spectrum of the product of Sb, I and S sonication in ethanol. Only characteristic peaks for antimony, iodine and sulfur are observed (Fig. 4a). The ratios of the Sb, I and S components determined by EDAX investigations (Table 2) are different from theoretical composition of SbI33S8. It is in agreement with XRD data: the product of sonication is a mixture of different components. The SEM micrographs of this product also present its complicated microstructure (Fig. 5a). Fig. 6 shows typical TEM images of different nanoparticles found in this product. The HRTEM images (Fig. 7) reveal that they exhibit good crystallinity and allow determining what kind of material they represent. One can distinguish three systems of clear lattice fringes (Fig. 7a) of the nanoplatelet from the dendritic pattern observed in Fig. 6a. The fringe spacings of (1) 345.7(10) pm, (2) 402.5(9) pm, and (3) 331.0(20) pm correspond to the interplanar distances 342.24,  1 2), and (0 3 1) or 399.60, and 332.95 pm between the (2 0 0), (1 (0 2 2) planes of P21/c SbI3 crystal [22], respectively. Fig. 7b shows structure of the nanowires observed in Fig. 6a. The observed fringe spacings of 331.7(13) pm correspond to the interplanar distance 324.94 pm between the (2 2 0) planes of SbSI crystal [24]. Fig. 7c presents microstructure of a nanoparticle observed in Fig. 6c. The fringe spacings equal (1) 315(3) pm, (2) 258(3) pm, and (3) 313(4) pm. The first and latter ones may correspond to the interplanar distances 317(17) pm observed in antimony subiodide (Sb3I) [26]. The second fringe spacing corresponds to the interplanar distance 268(16) pm also observed in Sb3I [26]. It should be underlined that the interplanar spacings correspond very well with the main X-ray diffraction lines presented in Tables 3–5. Unfortunately, the nanowires observed in Fig. 6b were too thick to obtain the HRTEM images. However, they gave

ð2Þ

where d is absorption path length. The SHG of k = 532 nm (2.33 eV) radiation was observed on powered SbI33S8 crystals placed in slide glass container. The laser system consisted of a Nd:YAG/Cr:YAG microlaser (k = 1064 nm;

Fig. 3. The powder XRD pattern of dried product of Sb, I and S (with atomic ratio 1:3:24) sonication in ethanol ( , , , – peaks appropriate for SbI33S8 [7,20], S8 [21], SbI3 [22,23] and SbSI [24,25], respectively; details are presented in Table 1).

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Table 1 Comparison of interplanar spacings determined by powder XRD of the product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol with literature data for S8, SbI33S8, SbI3, SbSI and Sb3I; (Irel – relative intensities of the observed X-ray diffraction peaks). Sonochemical product

S8 Ref. [18]

d (pm)

Irel

d (pm)

1242.1 578.0 409.8 385.6 357.7 354.0 344.6 338.0 333.2 321.8

820 390 650 1000 560 410 800 290 370 620

312.0 309.2 302.0 299.4 292.1 284.7 276.3 268.0 263.2 252.4 248.9 242.2 237.1 229.3 226.1 214.9 211.4

790 930 310 340 330 370 270 370 370 230 260 260 200 210 190 210 270

577.07 386.27

SbI33S8 Ref. [17] hkl

SbI3 Ref. [19]

d (pm)

hkl

1240.8

110

409.39

021

358.20

600

344.15

520

113

SbSI Ref. [21]

d (pm)

hkl

579.70

110

342.24 337.35 332.95 320.97

200  122

311.63

 212

301.52

112

d (pm)

Sb3I Ref. [23] hkl

d (pm)

222 353.9

345.66

026

334.08 322.49

311 117 206

311.78

313

285.83 268.31 263.23

243.08 237.46 229.48

212.09

308.42

051

299.31 290.96

241 511

276.17

431

263.43 252.30

161 701

225.84

271

031022 130

044 242 137

300.32

121

285.03

211

252.50 246.87

040 320

306.4

265.4 252.49

221

214.53

 33 123 2

317 335 021

062 319

good SAED patterns presented in Fig. 8. It should be noted that the nanowires observed in Fig. 6b are probably the SbI33S8 nanocrystals. They gave the appropriate electron diffraction pattern (Fig. 8) and the interplanar spacings determined using SAED (see Table 6) are comparable with those obtained from XRD and reported in literature [20]. Figs. 9 and 10 present the SAED patterns and the simulated diagrams for the SbI3 and Sb3I that are observed in Fig. 6a and c. The interplanar spacings determined using SAED diffraction patterns (see Tables 3 and 5) are only little different from values obtained in XRD and HRTEM investigations as well as from the data reported in literature [22,26]. To establish the uncertainties of the d-spacings determined using the XRD, the instrumental parameters were evaluated using the NIST Alumina plate. Peak positions were defined by Pearson VII function. The uncertainties of the d-spacings determined using HRTEM have been estimated as the standard deviation of the average value calculated from series of results. The SAED d-spacings were determined from the electron diffraction patterns using the DigitalMicrograph software of GATAN Company. In general, the standard accuracy of the d-spacing determination from the X-ray spectra is much higher than the accuracy for the TEM methods, which is at most 1 pm. Additionally it should be noted that it is difficult to maintain the same sample temperature in our XRD and HRTEM measurements. Taking the above into account it can be claimed that the results obtained from different methods are in good agreement. Fig. 11 presents comparison of the diffuse reflectance spectra of the dried product (curve A) of sonication of antimony, iodide and sulfur (with atomic ratio 1:3:24) in ethanol with data published for SbI33S8 single crystal [3], antimony subiodide [27], SbSI ethanogel [14], as well as sulfur and SbI3 in ethanol. To obtain more detailed information about the product of Sb, I and S sonication, the deconvolution [28] of the spectrum of Kubelka–Munk function

was done (Fig. 12). The best results were obtained for FK–M = 9.4(1)FK–M(SbI33S8) + 6.56(3)FK–M(Sb3I) + 0.713(4)FK–M(SbI3) + 0.117(2)FK–M(SbSI) + 0.01(13)FK–M(S). One can see that the product consists of SbI3, SbSI, Sb3I, SbI33S8 and sulfur. It is in agreement with the results of XRD, HRTEM and SAED investigations. The presented results are qualitative. The quantitative ones may be obtained in future investigations on the influence of the concentration of the components on Kubelka–Munk function. In summary, the sonochemical formation of SbI33S8 nanowires is accompanied with the formation of other species, e.g. SbI3 [29], Sb3I [26], SbSI [14], also iodine can be sonochemically oxidized to  triiodide (I 3 ) by OH radicals produced during cavitation [30,31]. However, the oxidative power of the I 3 ion is lower than that of iodine molecules, I2, itself [32]. So, the I 3 seems to be not essential for the presented sonochemical preparation of SbI33S8. The investigated Sb–S–I–C2H5OH system is somewhat similar to the Sb–S–I– H2O system used in hydrothermal synthesis of bulk SbSI-type crystals. The useful yield of different single crystals produced by hydrothermal synthesis depends on concentration and composition of the solvents, ratio of the initial components in the charge, Eh and pH of the medium, and temperature [33]. Eh–pH diagrams have become a standard method of illustrating equilibrium relationships between dissolved and solid species. Fig. 13 presents comparison of the measured by us Eh and pH values after 10 min of sonochemical preparation of SbI33S8, SbI3, Sb3I and SbSI in ethanol as well as the data obtained after 10 min sonication of powdered single crystals of SbI3 in ethanol. According to [34] the ðSbI3 Þ complexes are durable for pH < 3. Area between dashed lines a and b in Fig. 13 represents the reported in [33] Eh and pH ranges in which all Sb, S, and I elements should be in the appropriate valence states for Sb3+S2I1 synthesis, i.e. for the hydrothermal synthesis of bulk SbSI in Sb–S–I–H2O system. One can notice that the registered by us values of Eh and pH in the case of sonochemically prepared components for vapor phase growing of SbI33S8 are little different

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M. Nowak et al. / Ultrasonics Sonochemistry 17 (2010) 892–901 Table 2 Comparison of the experimentally determined chemical contents of dried product of the sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol and SbI33S8 single crystal grown from the vapor phase of the sonochemical product with the theoretical content of SbI33S8. Material

Concentration of elements determined by EDAX (at.%) Sb

S

I

Sonochemical product Single crystal Theoretical composition

7(1) 4(1) 3.6

88(1) 86(1) 85.7

5(1) 10(1) 10.7

of ethanol under ultrasonic irradiation can be summarized as follows: (1) iodine, I2, dissolved in ethanol reacts with antimony and forms the antimony triiodide, SbI3, also dissolved in ethanol

2Sb þ 3I2 ! 2SbI3

Fig. 4. EDAX spectra of dried product (a) of the sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol and (b) SbI33S8 single crystal grown from the vapor phase of the sonochemical product.

from the measured in the case of sonochemical preparation of SbSI and Sb3I (Fig. 13). They are also little above the upper limit (line b) calculated in [33] as appropriate for the SbSI synthesis, i.e. the upper limit of stability of the sulfur ions S2. Therefore, the coexistence of the SbSI, Sb3I and SbI33S8 species in the sonicated solution is justified. The sonochemical mechanisms of preparation of the SbSI and Sb3I were postulated in [35,26], respectively. The probable reaction route of SbI33S8 synthesis and the mechanism of formation of its nanowires using elemental Sb, S and I in the presence

ð3Þ

(2) the solid structure of sulfur is definitely molecular and contains S8 molecules which are symmetrical puckered rings with S–S distance 0.12 nm and bond angle a = 105° [36]. Chemical evidence suggests that the molecule is S8 even in the melt at temperatures not too far above the melting point [36]. When a cavitation bubble collapses violently near a solid surface, liquid jets are produced and high-speed jets of liquid are driven into the surface of a solid. These jets and shock waves cause removal of small particles from the surface. So, the ultrasonic irradiation facilitates the cleavage of solid sulfur into S8 molecules. (3) the ultrasonically produced small nanoparticles have a higher reactivity toward the formation of new compounds. Hence the released S8 react with SbI3 to yield SbI33S8 molecules

SbI3 þ 3S8 ! SbI3  3S8

ð4Þ

in which charge transfer bonds are formed between the three iodine atoms of a particular pyramidal SbI3 molecule (the electron acceptor; the Lewis acid) and three sulfur atoms belonging to its three partner S8 rings (the electron donors; the Lewis or Bronsted base) [7,37,38]. (4) the created SbI33S8 molecules, under the microjets and shockwaves formed at the collapse of the bubbles are pushed towards each other and are held by chemical forces. Therefore, the nuclei of SbI33S8 are formed as a result of the interparticle collisions (see e.g. [39]).

Fig. 5. Typical SEM micrographs of (a) dried product of Sb, I and S (with atomic ratio 1:3:24) sonication in ethanol and (b) SbI33S8 single crystal grown from the vapor phase of the sonochemical product.

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Fig. 6. Typical TEM images of individual nanoparticles observed in the product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol (a – SbSI nanowires and dendritic patterns observed on the platelets of SbI3; b – nanowires of SbI33S8 and agglomerates of nanoparticles of Sb3I; and c – nanoparticles of Sb3I).

Fig. 7. Typical HRTEM images of individual nanoparticles observed in the product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol (a – SbI3 nanoplatelet from the dendritic pattern observed in Fig. 6a. The fringe spacings of (1) 345.7(10) pm, (2) 402.5(9) pm and (3) 331(2) pm correspond to the interplanar distances 342.24, 399.60  12), and (031) or (022) planes of P21/c SbI3 crystal [22], respectively; b – SbSI nanocrystal observed in Fig. 6a. The fringe spacing of and 332.95 pm between the (200), (1 331.7(13) pm corresponds to the interplanar distance 324.94 pm between the (220) planes of SbSI crystal [24]; and c – nanoparticle of Sb3I observed in Fig. 6c. The fringe spacings equal (1) 315(3) pm, (2) 258(3) pm, and (3) 313(4) pm correspond to the interplanar distances observed in Sb3I [26]).

Table 3 Comparison of interplanar spacings determined by powder XRD, SAED and HRTEM of SbI3 observed in the product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol. Results of the XRD

Results of the SAED

Results of the HRTEM

Literature Ref. [19]

dhkl (pm)

Circle

Sign

dhkl (pm)

hkl

dhkl (pm)

2 1

402.5(9) 345.7(10)

3

331.0(20)

 12 1 200  22 1 031 022 130  212 212 112 122 221  33 123 2  04 3 302 115

399.60 342.24 337.35 332.95

dhkl (pm)

344.6 338.0 333.2 321.8 312.0 3

306.1

1

268.6

4 5 2

190.6 176.3 150.3

302.0 252.4 214.9

Results of the XRD

Results of the SAED

Results of the HRTEM

Literature Ref. [23]

dhkl (pm)

Circle

dhkl (pm)

Sign

Method

dhkl (pm)

5 6

537.8 381.8 XRD SAED XRD HRTEM

353.9 345 306.4 317(17)

SAED

298

XRD HRTEM SAED

265.4(42) 268(16) 180

Results of the XRD

Results of the HRTEM

Literature Ref. [21]

dhkl (pm)

Mark

dhkl (pm)

Plane

dhkl (pm)

1

331.7(13)

220 121 211 040 320

324.94 300.32 285.03 252.50 246.87

dhkl (pm)

354.0 302.0

320.97 311.63 311.63 301.52 271.50 252.50 214.53 190.28 178.60 150.26

Table 4 Comparison of interplanar spacings determined by powder XRD, SAED and HRTEM of SbSI observed in the product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol.

302.0 284.7 252.4 248.9

Table 5 Comparison of interplanar spacings determined by powder XRD, SAED and HRTEM of Sb3I observed in the product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol.

1 3 2

1

315.(3)

3

313.(4)

289.3 278.5 273.0

263.2

2 4

190.6

258.(3)

(5) the freshly formed nuclei in the solution are unstable and have the tendency to grow into nanowires along the c-axis parallel to the unshared pair of electrons of the Sb atom in SbI3 molecule (see Fig. 14). Thus, this solid material has a tendency to form highly anisotropic, 1D structures. Local turbulent flow associated with cavitation and acoustic streaming greatly accelerates mass transport in the liquid phase; (6) the aggregated SbI33S8 nanowires produce larger species; during the sonication time, the surface state of the nanowires might change: the dangling bonds, defects, or traps decrease gradually, and the species grow until the surface

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M. Nowak et al. / Ultrasonics Sonochemistry 17 (2010) 892–901

Fig. 8. Electron diffraction pattern of an individual SbI33S8 nanowire observed in Fig. 6b and its simulated diagram (the determined interplanar distances are presented in Table 6).

Table 6 Comparison of interplanar spacings determined by powder XRD, SAED and HRTEM of SbI33S8 observed in the product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol. Results of the XRD

Results of the SAED

Literature Ref. [17]

dhkl (pm)

Circle

Plane

dhkl (pm)

110 021 600 520 051 241 511 431 161 701 271 854 912 13,1,0

1240.8 409.39 358.20 344.15 308.42 299.31 290.96 276.17 263.43 252.30 225.84 0.18922 0.15791 0.15887

1242.1 409.8 357.7 344.6 309.2 299.4 292.1 276.3 263.2 252.4 226.1

dhkl (pm)

6 2 4

306.0 299.0 290.6

1 3 5

0.1892 0.1579 0.1587

Fig. 10. Electron diffraction pattern of an individual nanoparticle of Sb3I observed in Fig. 6c (the determined interplanar distances are presented in Table 5).

Fig. 11. Diffuse reflectance spectra of the dried product (A) of Sb, I and S (with atomic ratio 1:3:24) sonication in ethanol, j – SbI33S8 single crystal [3], (B) antimony subiodide [27], (C) SbSI ethanogel [14], (D) sulfur in ethanol, (E) SbI3 in ethanol, and (F) powdered crystals of SbI33S8 grown from the vapor phase of the sonochemical product. The last characteristic was normalized to 80% of the maximum reflectivity, as it was done in Ref. [3]).

Fig. 9. Electron diffraction pattern of an individual SbI3 nanocrystal from the dendritic pattern observed in Fig. 6a and its simulated diagram (the determined interplanar distances are presented in Table 3).

state becomes stable; surface corrosion and fragmentation by ultrasound irradiation, results in the formation of regular nanowires. Compounds of low volatility, which are unlikely to enter cavitation bubbles, experience a high-energy environment resulting from the pressure changes associated with the propagation of the acoustic wave or with shock waves; or they can react with radical species generated by sonolysis of the solvent [30]. In the presented

Fig. 12. Spectrum of Kubelka–Munk function (j) calculated for the diffuse reflectance of the dried product of Sb, I and S sonication presented by curve A in Fig. 11. Dashed curve represents the fitted spectral dependence of the following sum of Kubelka–Munk functions: 0.713(4)FK–M(SbI3) + 0.117(2)FK–M(SbSI) + 6.56(3)FK–M(Sb3I) + 9.4(1)FK–M(SbI33S8) + 0.01(13)FK–M(S) calculated for the diffuse reflectance data presented in Fig. 11. The solid curves show the appropriate Kubelka–Munk functions of SbI3 (A), SbSI (B), antimony subiodide (C), SbI33S8 (D), and sulfur (E) in ethanol (description in the text).

M. Nowak et al. / Ultrasonics Sonochemistry 17 (2010) 892–901

Fig. 13. Eh–pH diagram of results obtained after 10 min of sonochemical preparation of SbI33S8 ( ), SbI3 ( ), Sb3I ( ) and SbSI (d) in ethanol as well as the data obtained after 10 min sonication of powdered single crystals of SbI3 ( ) in ethanol (description of the experiments in Table 7; area between dashed lines a and b represents the ranges of pH and Eh values established in Ref. [33] as necessary for hydrothermal synthesis of bulk SbSI).

case, the reagents Sb and S are much less volatile than the ethanol and the iodine, so they stay in the interfacial region of the cavitation bubbles to yield SbI33S8 nanocrystals. The fine crystallinity of the products, which was confirmed by the SAED results (Fig. 8) of an individual SbI33S8 nanowire observed in Fig. 6b, strongly supports this hypothesis. It should be underlined that the SbI3 molecules deform appreciably in the crystalline state [6] and antimony triiodide does not form molecular crystals. On the other hand, the SbI33S8 has a single molecular structure. The unit cell of SbI33S8 contains one molecule of the adduct in which the antimony triiodide molecule is placed on the trigonal axis of the crystal, with its molecular dipole moment parallel to it, and the sulfur S8 rings are situated within the mirror planes and with their chief axes nearly parallel to the chief axis of the crystal [7]. A lengthening of the Sb–I distance from 271.9 to 274.7 pm due to the formation of a bond between iodine and sulfur was observed [5]. At the same time the I–I distance is decreased from 413.8 to 410.1 pm, corresponding to a decrease in the I–Sb–I angle from 99.1° to 96.6° [5]. There is therefore a change in the antimony triiodide molecule due to the formation of the addition complex. Each iodine atom is attached to a sulfur atom of a particular S8 molecule with an I–S distance of 360 pm and an angle Sb–I–S of 169.4° [7]. The sulfur ring is in SbI33S8, within the probable limits of error, identical with that found in

899

the orthorhombic modification of sulfur (the mean S–S bond distance is 204.6 pm (in sulfur 204.8 pm) and the S–S–S angle is 107°510 (in sulfur 107°550 ) [7]). Additional short intermolecular distances also occur: each antimony atom has three iodine neighbors at distances of 385 pm, all belonging to the nearest antimony triiodide molecule situated on the same trigonal axis [7]. These Sb–I distances are certainly shorter than the van der Waals radius sum and indicate a comparatively strong interaction between the atoms [7]. It would actually appear possible that the antimony atom in the triiodide molecule may have acceptor properties sufficiently strong to result in the formation of addition compounds depending on charge transfer bonds between antimony atoms and donor atoms belonging to partner molecules [7]. Further, each iodine atom has in addition to the sulfur atom mentioned above (at a distance of 360 pm) four sulfur neighbors all belonging to different sulfur rings with I–S separations of 378 pm resp. 388 pm. Here, contrary to the finding in the former case, the angles Sb–I–S (72.2° and 122.9°) are far from approaching 180° [7]. So, it appears very probable that the great stability of the crystalline SbI33S8 compound depends not only on the shortest I–S charge transfer bond but also to some extent on the just mentioned interactions between antimony and iodine and between iodine and sulfur atoms [7]. In this connection the fact should not be forgotten that there is rather strong evidence of 1:3 complexes (e.g. between iodoform and quinoline) being present even in dilute solution of the analogous addition compound (see Ref. in [7]). It should be underlined that antimony, sulfur and iodine can form onium ions [40]. These ions are formed when an unshared pair of electrons on the central atom with zero formal charge is used to form an additional covalent bond [40]. Onium ions are considered to be the positively charged highervalency (higher-coordination) compounds [40]. The higher pressure of gases above the nanocrystalline product of Sb, I and S sonication than above the single crystals of SbI33S8 (Fig. 2) is in agreement with the results reported in [12,13,15]. One should notice that evaporation/sublimation of the sonochemically prepared SbI33S8 is most efficient in temperatures from 376 to 382 K while the melting point of bulk SbI33S8 crystals is 389– 391 K [7]. Bulk SbSI melts without decomposing at temperature of about 673 K [41]. The temperatures of melting of bulk SbI3 and sulfur are 443.6 K [42] and 392 K [36], respectively. The sublimation pressure of SbI3 at temperatures lower than 403 K is very low [43]. The Sb3I melts above 423 K. The results of EDAX of the vapor grown SbI33S8 single crystals (Fig. 4b) are well comparable with the theoretical composition of SbI33S8 (Table 2). In contrast with the sonochemical product the grown crystal was homogeneous and had mirror like surfaces (Figs. 1 and 5). The X-ray analysis of the grown crystal confirmed its

Fig. 14. Unit cell packing diagrams for SbI33S8 adduct crystal at 290.0(2) K down crystallographic c-axis (a) and down a-axis (b). The dotted lines indicate the closest contacts between iodine I and sulfur S atoms in the adduct molecules. The cell constants are a = b = 2483.57(11) pm, c = 442.44(2) pm.

900

M. Nowak et al. / Ultrasonics Sonochemistry 17 (2010) 892–901 Table 8 Comparison of direct (EgDf) and indirect (EgIf) energy gaps and other absorption parameters obtained from the fittings of the absorbance spectra of SbI33S8 single crystal illuminated with plane polarized light with electric field parallel (||) and perpendicular (?) to the c-axis of the crystal (T = 293 K).

Fig. 15. Fitting of the absorbance spectra of SbI33S8 single crystal illuminated with plane polarized light with electric field parallel (h) and perpendicular (j) to the c-axis of the crystal (T = 293 K). Solid curves represents the least square fitted theoretical dependences for the sum of constant absorption term as well as direct and indirect forbidden absorptions (description in the text; values of the fitted parameters are given in Table 8).

trigonal rhombohedral structure with the space group R3 m. The unit cell contains three molecules of the adduct (Z = 3) in which the antimony triiodide molecule is placed on the trigonal axis c3 of the crystal, with its molecular dipole moment parallel to it, and the sulfur S8 molecules have three mirror planes. The molecular structure of SbI33S8 with the numbering scheme and displacement ellipsoids at 290.0(2) K is presented in Fig. 14. Fig. 14a shows the unit cell packing diagram viewed normal to the (0 0 1) plane, i.e., down the crystal 3-fold c-axis of SbI33S8. Fig. 14b shows the (0 1 0) plane of the unit cell, where the molecules can be seen in the direction normal to the crystal c-axis. The dotted lines indicate the directions of the possible intermolecular charge transfer interactions between iodine and sulfur atoms in the adduct molecules. The cell constants are a = b = 2483.57(11) pm, c = 442.44(2) pm. Values of these parameters are comparable with the data published in Ref. [7]. The more detail multitemperature X-ray analysis of the SbI33S8 crystals will be presented in near future. Fig. 15 presents the spectral characteristics of absorbance calculated from optical transmittance of SbI33S8 single crystal. Applying the method of simultaneous fitting of many mechanisms of absorption to the spectral dependence of absorbance (see Ref. [19]), these data were used to determine the optical energy gap of the SbI33S8 single crystal. In this method the following least square function has been minimized:

v2 ¼

n h X

ðF K—M ðhmi Þ  B

X

aj ðhmi Þ

i2

ð5Þ

j

i¼1

where i represents photons of different energy, aj describes various mechanisms of light absorption, and B is the proportionality factor. Fig. 15 presents the spectrum of FK–M of the investigated SbI33S8 single crystal and the best fitted theoretical dependence

Parameters

Values of the fitted parameters ||

?

EgDf (eV) EgIf (eV) A3 (1/[(eV)1/2 m]) A60 (1/[(eV)3 m]) A0 (1/m)

2.71 2.45 211.1 85.54 3.035

2.63 2.35 185.4 63.12 2.807

appropriate for the sum of indirect forbidden absorption without excitons and phonon statistics (a1), direct forbidden absorption without excitons (a2), and constant absorption term (a3) (see Ref. cited in Refs. [19,44]):

a1 ¼ A60 ðhm  E3gIf Þ for hm > EgIf A3 ðhm  EgDf Þ3=2 hm a3 ¼ A0

a2 ¼

for hm > EgDf

ð6aÞ ð6bÞ ð6cÞ

where EgIf represents the indirect forbidden energy gap, EgDf is the direct forbidden energy gap, A60, A3 are constant parameters. The constant absorption term A0 is an attenuation coefficient that is considered as the sum of the scattering and absorption independent of photon energy (hm) near the absorption edge. The fitting presented in Fig. 15 is rather good. Values of the fitted parameters are given in Table 8. One can notice in Fig. 15 that, as it was reported in [3], the SbI33S8 adduct absorbs light very little at wavelength 532 nm (the wavelength of radiation effectively generated by SHG using the Nd:YAG/Cr:YAG microlaser). 4. Conclusions The product of sonication of Sb, I and S (with atomic ratio 1:3:24) in ethanol is a mixture of SbI3, Sb3I, SbSI, SbI33S8 and sulfur nanoparticles. The SbI33S8 nanowires are effectively grown via a solid-solution–solid pathway under the ultrasonic irradiation. They can be successfully used as raw material for growth of SbI33S8 single crystals from the vapor phase. The temperature of the source zone can be equal T1 = 383 K while the seed temperature can be T2 = 353 K. The grown single crystals of SbI33S8 have good optical properties. They are suitable for generation of second harmonic of light. For the first time the optical energy gap of the SbI33S8 has been determined. At 293 K this material has direct and indirect energy gaps EgDf = 2.71 eV and EgIf = 2.45 eV for plane polarized radiation with the electric vector parallel to the c-axis of the crystal. At the same temperature the optical energy gaps for plane polarized radiation with the electric vector perpendicular to the c-axis of the crystal are equal EgDf = 2.63 eV and EgIf = 2.35 eV. Acknowledgement

Table 7 Masses of reagents used in Eh–pH investigations of sonochemically prepared various nanomaterials and the powdered SbI3 crystals. Weight of reagents Sb (g) S (g) I (g) Total weight (g) C2H5OH (ml)

Sonochemically prepared nanomaterials SbI33S8

SbI3

Sb3I

SbSI

0.1052 0.6655 0.3292 1.0999 10

0.1053 – 0.3296 0.4349 10

0.249 – 0.796 1.045 10

0.9472 0.25 0.987 2.1842 10

Powdered SbI3 crystals – – – 0.4344 10

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