Journal of Crystal Growth 233 (2001) 287–291

Template-assisted synthesis of Sb8O10(OH)2I2 tubular crystalsunder hydrothermal conditions

Qing Yanga,b, Kaibin Tanga,b,*, Chunrui Wanga,b, Fuping Lic, Bin Haib,Guozhen Shenb, Changhua Anb, Weichao Yua,b, Yitai Qiana,b

aStructure Research Laboratory, University of Science and Technology, Hefei, Anhui 230026, People’s Republic of ChinabDepartment of Chemistry, University of Science and Technology, Hefei, Anhui 230026, People’s Republic of China

cDepartment of Polymer Science and Engineering, University of Science and Technology, Hefei, Anhui 230026,

People’s Republic of China

Received 16 November 2000; accepted 28 May 2001

Communicated by K. Nakajima

Abstract

Mesoscale octaantimony decaoxide dihydroxide iodide (Sb8O10(OH)2I2) tubular crystals have been successfullysynthesized for the first time through a hydrothermal route from Sb2S3, NaI and NaOH at 2001C. In the process, theSb2S3 prismatic crystals serve as templates and play an important role in the formation of the Sb8O10(OH)2I2 tubular

crystals in alkali aqueous solution. The products are characterized by X-ray powder diffraction and scanning electronmicroscopy. The sample has tubular prismatic structure. A typical tubule is 6–8mm in length, 30–50 mm in diameter,and 5 mm in thickness. r 2001 Elsevier Science B.V. All rights reserved.

PACS: 61.10.N; 61.66.F; 61.72

Keywords: A1. X-ray diffraction; A2. Hydrothermal crystal growth; B1. Inorganic compounds

The synthesis and characterization of tubularmaterials have attracted much attention over thelast few years. Most studies in the field have beenfocused on nanotubes [1–9]. Recently, the forma-tion and growth mechanism of micro and mesoscales have attracted a lot of research interest [10–19]. However, investigations on millimeter-sizedtubules have been largely restricted because the

preparation is not always readily available. Up tonow, the synthesis of mesoscale crystals with openstructure has been a challenge for materialscientists. In our previous works, we have suc-ceeded in synthesizing antimony sulfide [20], andsilver selenide tubular crystals [21]. Octaantimonydecaoxide dihydroxide iodide (Sb8O10(OH)2I2)was firstly synthesized in sealed Pyrex (gold tubes)by M. Edstrand in 1955 [22], but Sb8O10(OH)2I2tubular crystals have not been reported so far.Here, we report on the mesoscale Sb8O10(OH)2I2tubular crystals synthesized, for the first time,using Sb2S3 prismatic crystals as templates in

*Corresponding author. Department of Chemistry, Univer-

sity of Science and Technology, Hefei, Anhui 230026, People’s

Republic of China. Fax: +86-551-360-1600.

E-mail address: kbtang@ustc.edu.cn (K. Tang).

0022-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 5 0 4 - 4

alkali aqueous solution under hydrothermal con-ditions at 2001C.In the process, Sb8O10(OH)2I2 tubular crystals

were obtained in the Teflon-lined stainless steelautoclave, based on the overall reaction:

8Sb2S3 þ 22NaOHþ 2NaI-

Sb8O10ðOHÞ2I2 þ 10H2Oþ 8Na3SbS3: ð1Þ

In our experiments, the reagents used are ofanalytical purity. There are two steps in theprocess. The first step is to synthesize templateSb2S3 crystals. They are synthesized from anti-mony trichloride and sodium thiosulfate in anautoclave at 1601C for 8–10 h according to thefollowing reaction [23]:

2SbCl3 þ 9Na2S2O3-

Sb2S3 þ 6NaClþ 3Na2S4O6 þ 3Na2SO3: ð2Þ

Secondly, an appropriate amount of as-preparedSb2S3 crystals, with NaI, and NaOH (with a molarratio of 1 : 5 : 2.75) is added into another autoclave,which is filled with distilled water up to 90% of thetotal volume (50ml). The autoclave is maintainedat 2001C for 20 h after being sealed, and then it iscooled to room temperature naturally. The pre-cipitates are washed with distilled water andabsolute alcohol to remove the impurities. Afterdrying in a vacuum at 60–701C for 4 h, the crystalswith a high reflective luster are obtained.The formation of the tubular crystals Sb8O10

(OH)2I2 could attribute to reactions (3) [24] and(4):

Sb2S3 þ 6OH�-SbO3�3 þ SbS3�3 þ 3H2O; ð3Þ

8SbO3�3 þ 14H2Oþ 2I�-

Sb8O10ðOHÞ2I2 þ 26OH�: ð4Þ

In the alkali solution, the Sb2S3 crystals could becompletely or partially dissolved according toreaction (3) in different feedstock between Sb2S3and NaOH. Since Sb2S3 prismatic crystals areserved as the morphological templates, the feed-stock of NaOH should be properly controlled.X-ray diffraction (XRD) patterns of the product

are recorded on a Rigaku D/max-gA rotationanode X-ray diffractometer with CuKa radiation(l=1.54178 (AA). XRD patterns show that Sb2S3 isan orthorhombic phase with lattice parameters

a=11.216, b=11.325, and c=3.827 (AA, whichare close to the reported data (JCPDS Card File,6-474). Fig. 1 shows the XRD patterns of thepolycrystalline powders ground from the finalcrystals. All the reflections can be indexed to themonoclinic phase Sb8O10(OH)2I2 with latticeparameters a=19.312, b=4.141, c=10.933 (AA,and b=109.621, which are in good agreement withthe reported data for Sb8O10(OH)2I2 [15]. Nocharacteristic peaks of impurities such as Sb2S3,Sb2O3, or Na3SbS3 are observed.The morphology and the size of the samples are

observed by scanning electron microscopy (SEM),which is performed on an X-650 scanning electronmicroanalyzer. SEM images of the as-grownSb8O10(OH)2I2 crystals are showed in Fig. 2a, b.The Sb8O10(OH)2I2 crystals are tubular morphol-ogies (Fig. 2a). Fig. 2b shows a typical tabularprismatic crystal with 6–10mm in length, 3–60 mmin width, and 3–5 mm in thickness. Fig. 2c indicatesthe SEM image of the Sb2S3 crystals synthesized atthe first step and served as templates. Thetemplates are prismatic crystals. The morphologiesof Sb8O10(OH)2I2 are similar to those of templates.However, it is of interest that the average diameterof Sb8O10(OH)2I2 tubules is 30–50 times as that ofSb2S3 crystals, and the average length is 600–1000times as that of Sb2S3 crystals. Fig. 2d is the SEMimage of the samples produced in the secondprocedure after 1 h and it shows that Sb2S3 crystalsare partially dissolved according to reaction (3).When the reaction is carried out for 6 h, someSb8O10(OH)2I2 tubular crystals are obtained viareaction (4), shown in Fig. 2e.

Fig. 1. XRD patterns of the Sb8O10(OH)2I2 tubular crystals.

Q. Yang et al. / Journal of Crystal Growth 233 (2001) 287–291288

Fig. 2. SEM images of the produced Sb8O10(OH)2I2 tubular crystals and Sb2S3 templates: (a) the as-produced Sb8O10(OH)2I2 tubular

crystals; (b) a typical Sb8O10(OH)2I2 tubular crystal; (c) Sb2S3 prismatic crystals that served as templates; (d) the sample obtained after

1 h; (e) the sample obtained after 6 h.

Q. Yang et al. / Journal of Crystal Growth 233 (2001) 287–291 289

The growth mechanism of the tubular crystalscould be proposed as follows. Firstly, Sb2S3crystals start to dissolve in alkali solution. Mean-while, antimony (III) hydrolyzes in alkali solutionunder hydrothermal conditions, which promotethe formation of Sb–O–Sb–O chains on the surfaceof Sb2S3 [15]. Then, hollow crystal nuclei couldform from the ends of the Sb2S3 crystals (Fig. 2d).From the hollow ends, the epitaxial growth alongaxial direction further gives endless sheets ofcompound Sb8O10(OH)2I2 [15] in reaction (4)and forms tubular crystals. With the proceedingreactions (3) and (4), the remaining Sb2S3 crystalsat the ends could change into Sb8O10(OH)2I2gradually. The different sizes between Sb8O10

(OH)2I2 tubules and Sb2S3 templates could beeasily understood, because the orthorhombicSb2S3 prismatic crystals are pyramids, while theSb8O10(OH)2I2 crystals could grow large and longin the epitaxial growth from the hollow nuclei (asthe Fig. 2e).The feedstock of the reactants is investigated in

the process. The molar ratio between Sb2S3 andNaOH plays an important role in the formation ofSb8O10(OH)2I2. When Sb2S3 is excessive, the finalproducts contain some unreacted Sb2S3. Thefeedstock of NaI has no obvious effects onthe formation of the tubular crystals. The appro-priate molar ratio of Sb2S3, NaI, and NaOH is1 : 0.5–6 : 2.5–3.0. The pH value of the startingmaterials in the system in 10 and it decreases whilethe reaction proceeds. When pHo8 at the begin-ning, Sb2S3 is difficult to dissolve. When pH>12,Sb2S3 is dissolved rapidly and completely. Afterthe reaction, the pH in the solution should be noless than 7, and Sb2S3 may be produced fromSbS3

3� if the solution is acidic.In order to obtain well-crystallized tubular

crystals, the processing temperature should be noless than 1801C and the processing period shouldbe more than 6 h [15]. Otherwise, the products willcontain some amount of amorphous antimonialcompound. The temperature should be higherthan 1901C and reaction period should be longerthan 10 h.In summary, prismatic Sb8O10(OH)2I2 tubular

crystals have been successfully synthesized throughan alkali-dissolving method under hydrothermal

conditions from SbCl3, NaI, and NaOH at 2001Cfor the first time. The templates and the feedstock ofthe reactants play an important role in the forma-tion of the Sb8O10(OH)2I2 tubular crystals.Although the growth mechanism of the tubules isproposed, a clear understanding of the detailedmechanism and reaction kinetics of the growth areneeded. This method may be employed to synthesizeSb8O10(OH)2Br2, SbSI, and other tubular crystals.

Acknowledgements

Financial support of this work by the NationalNature Science Foundation of China and the 973Projects of China is gratefully acknowledged. Weare indebted to Profs. Guien Zhou, Fanqing Liand Dr. Hao Cheng who have helped us withcharacterization.

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