Synthesis of Sb2Se3 Nanowires via a Solvothermal Route from the Single Source Precursor Sb[Se2P(OiPr)2]3 Hao-Wei Chang, Bijay Sarkar, and C. W. Liu* Department of Chemistry, National Dong Hwa UniVersity, Hualien 97401, Taiwan
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2691–2695
ReceiVed December 28, 2006; ReVised Manuscript ReceiVed September 19, 2007
ABSTRACT: A solvothermal route was employed for the syntheses of nanocrystalline Sb2Se3 from the single source precursor Sb[Se2P(OiPr)2]3 at different temperatures. The products were shown to be nanowires by electron microscopy, and the composition was confirmed as Sb2Se3 by powder X-ray diffraction and energy dispersive spectroscopy. The diameter and length of the nanowires were extended at 150 °C, as compared to 100 °C in the solvothermal process. The direct band gap energy of the nanowires was estimated by absorption spectroscopy to be 1.49 eV. Structural elucidation of the precursor Sb[Se2P(OiPr)2]3 was completed by single crystal X-ray diffraction. Introduction Research on nanotechnology is interdisciplinary in nature and concerns the fabrication of nanomaterials and their applications in a variety of areas involving scientists from biology,1 physics,2 and chemistry.3 It has emerged as a fast growing field in the last decade because the properties of nanomaterials are quite different compared to those of bulk materials. In particular, 1-D nanostructures such as nanotubes,4 nanowires,5 nanorods,6 and nanoribbons7 have attracted considerable interest due to their potential use in electronic, optical, magnetic, or mechanical devices. Group V-VI binary compounds, which are highly anisotropic semiconductors and crystallize as layered structures parallel to the growth direction, have attracted attention due to their photovoltaic and thermoelectric properties. Sb2Se3 is a member of the V2VI3 group of compounds with a layer structure that adopts orthorhombic symmetry and exhibits a high thermoelectric figure of merit (ZT). Furthermore, theoretical studies8 suggest that ZT can be improved by reduction to the size of nanoparticles. Fabrication of nanocrystalline Sb2Se3 has been demonstrated using spray-deposition9/electrodeposition of thin films,10 solvo-/ hydrothermal synthesis of one-dimensional nanocrystals,11 a solvent-relief-self-seeding (SRSS) process,4 the successive ionic layer adsorption and reaction (SILAR) method,12 chemical bath deposition,13 chemical vapor deposition,14 and other methods.15 To our knowledge, the synthesis of Sb2Se3 nanocrystals by a single source precursor method has not been reported. The potential advantages include ease of separation of nanocrystalline material and better control over the size and shape, as fewer parameters (viz. temperature, solvent type) need to be manipulated. By taking advantage of P-Se bond cleavage in diselenophosphates,16 nonstoichiometric Cu2-xSe nanowires were prepared from [Cu4{Se2P(OiPr)2}4] under CVD conditions.17 Here we report the fabrication of Sb2Se3 nanowires from Sb[Se2P(OPri)2]3 under solvothermal conditions as a function of temperature. The synthesis of the Sb[Se2P(OiPr)2]3 precursor is also reported along with structural characterization through single-crystal X-ray diffraction. Sb2Se3 nanowires were investigated by powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and * Corresponding author. E-mail:
[email protected]. Fax: +886-38633570.
energy dispersive spectroscopy. Solid-state optical absorption spectroscopy was also conducted at room temperature. Experimental Section Materials. All the solvents (Mallinckrodt Chemicals, LR grade) were purified prior to use following standard procedures.18 Reagent grade phosphorus powder (red, 99%), selenium powder (99.5%), and Sb(CH3COO)3 (99.99%) were purchased from Aldrich. The compound NH4[Se2P(OiPr)2] was synthesized according to a literature procedure.19 Characterizations. Elemental analyses were performed on a Heraeus Vario EL III analyzer. NMR spectra were recorded with a Bruker Advance DPX300 FT-NMR spectrometer. The 31P{1H} and 77Se{1H} NMR spectra were referenced externally against 85% H3PO4 (δ ) 0 ppm) and PhSeSePh (δ ) 463 ppm), respectively. Single-crystal X-ray diffraction analysis was performed on a Bruker SMART APEX II CCD diffractometer at T ) 293 K and with Mo KR radiation (λ ) 0.71073 Å). The data were collected using the 2θ - ω scan technique. Data reduction was performed with SAINT,20 and absorption correction was performed using SADABS. The structure was solved by direct methods and the refinement performed by a least-squares method on F2 with the SHELXL-97 package,21 incorporated in SHELXTL/PC V5.10.22 The powder X-ray diffraction (XRD) patterns of the nanowires were recorded using a Bruker D8 diffractometer with Cu KR radiation (λ ) 1.5406 Å) at 25 °C. Samples for electron microscopy were prepared by dispersing the as-prepared samples in methanol, under ultrasonication. A drop of the dispersion was deposited onto the carbon coated copper grid. The methanol was allowed to evaporate naturally. These uncoated samples were used to record images with electron microscopes. The morphologies of the crystals were observed with a field emission scanning electron microscope (FE-SEM), using a JEOL JSM6500F, with an accelerating voltage of 15 kV in secondary electron mode. Transmission electron microscopy (TEM) images and the selected area electron diffraction (SAED) patterns were recorded with a JEOL JEM-3010 transmission electron microscope, using an accelerating voltage of 300 kV. Energy dispersive spectroscopy (EDS) was recorded using an Oxford energy dispersive X-ray analyzer, attached to the JEOL JSM-6500F scanning electron microscope for microarea composition analysis. Thermal gravimetric analysis (TGA) was carried out with a Mettler Toledo TGA851 instrument using a 10 °C/ min scanning rate. Solid-state UV–visible spectra were recorded with a Hitachi U3010 spectrometer. Synthesis of Sb[Se2P(OiPr)2]3 (I). The synthesis of Sb[Se2P(OiPr)2]3 (Scheme 1) followed the modified procedure reported by Zingaro et al. that had been used to prepare Sb[Se2P(OEt)2]3.23 A suspension of NH4[Se2P(OiPr)2] (1.00 g, 3.076 mmol) in 40 mL of dichloromethane was added to Sb(OAc)3 (0.306 g, 1.025 mmol), and the resulting mixture was stirred for 4 h under N2. The reaction mixture was filtered through Celite under a N2 atmosphere; the yellow filtrate was collected and evaporated to dryness using a rotary evaporator under reduced pressure. The resulting solid was dissolved in hexane (20 mL), filtered through Celite, and evaporated to dryness. After evaporation of the solvent,
10.1021/cg060954m CCC: $37.00 2007 American Chemical Society Published on Web 11/09/2007
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Chang et al. Scheme 1
the yellow powder, Sb[Se2P(OiPr)2]3, was collected. A single crystal suitable for X-ray crystallography was grown from dichloromethane layered with hexane. Yield: 92% (0.984 g). Anal. Calcd for C18H42O6SbSe6P3: C, 20.73; H, 4.06. Found: C, 20.74; H, 3.99. 31P {1H} NMR (121 MHz, CDCl3, 25 °C): 69.9 ppm [s, JP-Se ) 638 Hz, 3P]. 1H NMR (300 MHz, CDCl3, 25 °C, δ in ppm): 1.37 [d, J ) 6.2 Hz, 36H, CH(CH3)2], 4.89 [m, 6H, CH(CH3)2]. 77Se NMR (38 MHz, CDCl3, 25 °C): 162.2 ppm (d, JSe-P ) 642 Hz, 6Se). Formation of Sb2Se3 Nanowires. The nanowires were prepared by a solvothermal method from the single-source precursor Sb[Se2P(OiPr)2]3 (Scheme 1). Sb[Se2P(OiPr)2]3 (I) (0.8g, 0.767 mmol)
was added to 20 mL of methanol and heated in a Teflon coated stainless steel autoclave, at either 100 or 150 °C for 12 h. After cooling to room temperature, the product was a black solid. The solid was washed with methanol (1 mL × 3 times) and dried at 70 °C for 5 h. The composition of the solid was found by powder XRD and EDS to be Sb2Se3.
Results and Discussion The reaction of NH4[Se2P(OiPr)2] with Sb(CH3COO)3 in dichloromethane at room temperature resulted in the antimony
Figure 2. Intermolecular Se · · · Se interactions in Sb[Se2P(OiPr)2]3 with isopropyl groups omitted for clarity. Se1C · · · Se1D ) 3.886 Å. Figure 1. Thermal ellipsoid drawing (50%) of Sb[Se2P(OiPr)2]3. Selected bond lengths (Å) and angles (deg): Sb(1)-Se(1), 2.7714(6); Sb(1)-Se(2), 2.9173(7); Se(1)-P(1), 2.169(1); Sb(2A)-P(1), 2.140(2); Se(2A)-Sb(1)-Se(1), 77.19(2); Se(2A)-P(1)-Se(1), 110.95(6). [Symmetry code: A ) -y + 1, x - y, z; B ) -x + y + 1, -x + 1, z] Table 1. Selected Crystallographic Data for Sb[Se2P(OiPr)2]3 formula
C18H42O6P3SbSe6
space group a, Å c, Å R, deg β, deg γ, deg V, Å3 Z Fcalcd, g/cm3 F(000) θ, deg reflections collected independent reflections R1a wR2b
R(–)3 22.9851(9) 12.4864(10) 90 90 120 5712.9(6) 6 1.819 2988 1.77–25.05 20279 2257 [R(int) ) 0.0417] 0.0341 0.0866
R1 ) Σ|Fo| - |Fc|/Σ|Fo|. [w(Fo2)2]}1/2. a
b
wR2 ) {Σ[w(Fo2 - Fc2)2]/Σ
Figure 3. XRD pattern of the product (top); standard pattern from the orthorhombic phase Sb2Se3 (JCPDS 15-0861) (bottom).
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Figure 4. SEM images of Sb2Se3 nanowires: (a) low magnification; (b) high magnification at 100 °C; (c) high magnification at 150 °C. (d) EDS result that indicates the nanowires are composed of Sb2Se3.
Figure 5. (a) TEM image, (b) HRTEM image, and (c) SAED pattern of a single Sb2Se3 nanowire.
salt of the selenium compound Sb[Se2P(OiPr)2]3 (I), in 92% yield. A single resonance with a set of selenium satellites was exhibited in 31P NMR, and a doublet peak arising from the coupling of 77Se and 31P nuclei was identified in 77Se NMR. These results implied that all dsep ligands in I were equivalent in solution at NMR time scales at room temperature. Though Zingaro and co-workers reported the synthesis of the Sb[Se2P(OEt)2]3 in 1969,23 there was no structural validation of the compound. Here, single crystal X-ray diffraction has allowed elaboration of the structural properties of the isopropoxy analogue Sb[Se2P(OiPr)2] (I) crystallizing in the trigonal space group R3j with Z ) 6. Six Se atoms from three dsep ligands were found to be coordinated to antimony in a distorted octahedral geometry. The two Sb-Se bond distances for each
Figure 6. Solid-state UV–visible absorption spectrum of nanocrystalline Sb2Se3.
dsep ligand [Se2P(OiPr)2-, abbreviated as dsep], i.e., Sb(1)-Se(1) and Sb(1)-Se(2), were unequal [2.7714(6) and 2.9173(7) Å, respectively]. The bite angles of Se-Sb-Se and Se-P-Se within a chelated dsep ligand are 77.19(2)° and 110.95(6)°, respectively. The thermal ellipsoid diagram of compound I is shown in Figure 1. The crystal data are summarized in Table 1, and selected bond lengths and angles are given in the figure legends. Interestingly, several secondary Se · · · Se interactions, observed between the selenium atoms of the dsep ligands of the neighboring molecules, form a loosely bound dimer, as
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Figure 7. TGA diagram of Sb[Se2P(OiPr)2]3.
Scheme 2
represented by dashed lines in Figure 2. Those Se · · · Se distances (3.886 Å) are less than the sum of the van der Waals radii of two selenium atoms (4.0 Å).24 The XRD patterns of the nanowires synthesized at both the temperatures can be indexed with the orthorhombic crystal system with space group Pnma, and the cell parameters a ) 11.63 Å, b ) 3.99 Å, and c ) 11.78 Å are well matched with the standard values (JCPDS 15-0861) (Figure 3). Three secondary electron images recorded at different magnifications are displayed in Figure 4a-c. Figure 4a shows that the product contains a large quantity of wirelike material. The lengths of the product are in the range of several microns. Higher magnification images of wires fabricated at 100 and 150 °C, shown in parts b and c, respectively, of Figure 4 reveal that the diameters of the products were in the nanometer region and that wires formed at 150 °C are larger (diameter ∼ 70–80 nm, length ∼ 3–5 µm), as compared to those formed at 100 °C (diameter ∼ 30–50 nm, length ∼ 2–3 µm). EDS analysis shows that the Sb/Se atomic ratio is 39.9:60.0 (close to 2:3), giving the nanowire a possible composition of Sb2Se3. (The presence of copper peaks is attributable to the copper grid used to hold the sample.) A transmission electron microscopy (TEM) lattice image and selected area electron diffraction (SAED) provide further insight
into the microstructural details of wires. A single Sb2Se3 nanowire was chosen (Figure 5a) that at higher magnification yielded interference fringes of regular spacing (Figure 5b). The lattice spacing of 0.52 nm can be assigned to (120), and the preferential growth direction along [001] is indicated with an arrow. The discrete SAED spots (Figure 5c) confirm the nanowire is a single crystal and can be indexed according to the orthorhombic antimony selenide structure. The solid-state UV–visible absorption spectrum of the Sb2Se3 nanowires shows a broad absorption band in the range 400–860 nm as represented in Figure 6. The value of the band gap is determined by the intersection point of the tangent of the absorption edges. Extrapolation of this broad band around 750–850 nm affords an edge at 833 nm that corresponds to an energy of direct band gap absorption of 1.49 eV, close to the reported value of 1.46 eV.11d This value indicates that it is a promising candidate for applications in thin film solar cells because an ideal material should have a value of ∼1.5 eV.25 Three major weight losses were observed in a TGA diagram (Figure 7). The initial weight loss of approximately 38% in the temperature range 180–220 °C is possibly attributable to the loss of isopropoxy groups from the precursor, while the weight loss of 8% from 220 to 500 °C might be related to the vaporization of phosphorous. Finally, a weight loss of ca. 22%
Synthesis of Sb2Se3 Nanowires
recorded between 500 and 640 °C accounts for the loss of half of the selenium atoms to form the species SbSe3. The calculated weight losses (%) of these three steps are 34.0, 8.9, and 22.7%, respectively. In solvothermal synthesis, the precursor also might have been broken into SbSe3, that might have converted to Sb2Se3 nanostructures via the intermolecular Se · · · Se interactions under high pressure and high temperature solvothermal conditions. To better understand the probable mechanism of solvothermal synthesis of nanowires, the stepwise thermal decomposition is represented in Scheme 2.
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Conclusions A simple solvothermal synthesis of Sb2Se3 nanowires from a single source precursor Sb[Se2P(OiPr)2]3 has been demonstrated. The structural elucidation of Sb[Se2P(OiPr)2]3, by single crystal X-ray diffraction, has also been described. The dimensions of the nanowires can be regulated by changing the reaction temperature during the solvothermal process. The Sb2Se3 nanostructure is expected to show novel optical and electrical properties, and future work will focus on the fabrication of various metal selenide nanomaterials from a single source precursor of the type M[Se2P(OR)2]n via solvothermal methods. Acknowledgment. Financial support from the National Science Council of Taiwan (NSC 95-2119-M-259-001) is greatly acknowledged. Supporting Information Available: X-ray crystallographic files in CIF format for compound I. This material is available free of charge via the Internet at http://pubs.acs.org.
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