Hydrothermal Growth of CuS Nanowires from Cu-Dithiooxamide, a Novel Single-Source Precursor Poulomi Roy and Suneel K. Srivastava* Inorganic Materials and Nanocomposite Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1921-1926
ReceiVed March 11, 2006; ReVised Manuscript ReceiVed May 19, 2006
ABSTRACT: CuS nanowires have been successfully prepared from Cu-dithiooxamide, a novel single-source precursor, by a hydrothermal method at 120 °C for 24 h. The nanowires are 40-80 nm in diameter and up to a few microns long, and a possible reaction mechanism of their formation is proposed. The effects of reaction temperature, duration, and solvents also were studied. X-ray diffraction patterns showed the formation of a covellite form of CuS having a hexagonal phase under hydrothermal conditions. The morphology of the products was studied by scanning electron microscopy and transmission electron microscopy. The composition and purity of products were examined by energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. Optical studies of the products also were carried out. Introduction Semiconductor metal sulfides, especially MoS2,1,2 Bi2S3,3 CuS,4 CdS,5 ZnS,6 and PbS7 and A2S3-M2S3-M′S (A ) Ga, In; M ) trivalent metal; M′ ) divalent metal),8,9 have attracted ever increasing attention in the past few years, due to their excellent physical as well as chemical properties. But very recently, a considerable amount of interest has been focused on copper sulfides owing to their variations in stoichiometric composition, valence states, nanocrystal morphologies, complex structures, and different unique properties.10,11 The stoichiometric composition of copper sulfide varies in a wide range from Cu2S at the copper-rich side to CuS2 at the copper-deficient side, such as CuS, Cu1.96S, Cu1.94S, Cu1.8S, Cu7S4, and Cu2S.10 Copper sulfides find their potential applications in numerous fields, such as in photo thermal conversion, as p-type semiconductors in solar cell devices, as coatings for microwave shields in the form of thin films, and as super ionic materials, optical filters, room-temperature ammonia gas sensors,12-17 etc. Green copper sulfide (covellite) is of special interest owing to its application as a cathode material in lithium rechargeable batteries.18 It shows metallic conductivity and becomes a superconductor at 1.6 K.19 Very recently, attempts have been focused on the synthesis of copper sulfides in the form of nanoparticles,20 nanorods,21-25 nanotubes,26,27 nanowires,28,29 nanodisks,4 flower-like structures,11 and various preparative methods such as thermolysis,27 template-assisted growth,21,23,26,28,29 microwave irradiation,24 hydrothermal or solvothermal methods,11,27 and chemical vapor deposition,20 have been reported. Interestingly, among these various forms, one-dimensional (1D) growth has received tremendous attention in recent years due to its promising applications in electrical, optical, and magnetic nanodevices.30,31 The synthesis of such 1D nanostructures of copper sulfides involved mostly template or surfactant-assisted pathways. Mao et al.21 synthesized nanorod arrays of copper sulfide using spincoated monolayers of arachidic acid assembled on graphite as 1D nanostripes with bilayer periodicity as a new molecular template. Gao and co-workers23 reported surfactant-assisted growth of nanorods and proposed the mechanism related to their formation. Liao and his group24 demonstrated a route for the * Corresponding author: E-mail:
[email protected]; tel: +913222-283334; fax: +91-3222-255303.
preparation of copper sulfide nanorods of diameter 5-10 nm and length 30-50 nm via microwave-induced heating in aqueous solution under ambient air. Synthesis of Cu2S nanorods in well-aligned large arrays on a copper surface by a gas-solid reaction under ambient temperature also was reported.25 Wu et al.29 developed the methodology for the growth of Cu2S nanowires using anodic aluminum oxide as a hard template. Another interesting route to synthesize these nanosized metal chalcogenides involves the growth of products directly from an efficient single-source precursor. Although some reports are available on the preparation of nanosized CdS,32-34 Bi2S3,35 Sb2S336 using a single-source precursor, to the best of our knowledge only one report is available on copper sulfide in which Larsen and co-workers22 synthesized Cu2S nanorods by a solventless thermolysis of a copper alkylthiolate precursor. Encouraged by this, we report for the first time, successful synthesis of copper sulfide nanowires with a high aspect ratio from a copper-dithiooxamide (DTO) complex used as a singlesource precursor in a comparatively simple and easy hydrothermal pathway. The Cu-DTO complex used in the present work is easily synthesizable and air-stable. In addition, no capping agent has been used for the preparation of CuS nanowires. The effect of different solvents also has been discussed. Experimental Section Chemicals. Cupric chloride, CuCl2‚2H2O (Merck), and DTO (Merck) were used as the main reagents. Ethanol (Bengal chemicals, India) and ammonium hydroxide (Bengal chemicals, India) were also used for the preparation of the Cu-DTO complex. All the reagents were used without any further purification. Synthesis. The Cu-DTO complex was prepared according to the method reported by Abboudi et al.37 A total of 0.5 g of CuCl2‚2H2O and 0.3 g of DTO were dissolved separately in 50 mL of ethanol, and the solutions were stirred for 2 h to form a homogeneous solution. The DTO solution was added slowly to the metal salt solution under stirring conditions followed by the addition of 25 mL of ammonium hydroxide solution. The stirring of the resultant solution was continued for another 2 h, which resulted in complete precipitation of the black colored Cu-DTO complex. The precipitate of Cu-DTO was filtered and washed with distilled water and ethanol and finally dried at 40 °C for 4 h. The hydrothermal synthesis of copper sulfide from Cu-DTO as a single-source precursor was carried out by taking 0.3 g of this complex dispersed in 35 mL of distilled water in a Teflon-lined stainless steel
10.1021/cg060134+ CCC: $33.50 © 2006 American Chemical Society Published on Web 06/29/2006
1922 Crystal Growth & Design, Vol. 6, No. 8, 2006 autoclave and maintained at 120 °C for 24 h. After completion of the reaction, the reactor was allowed to cool to room temperature naturally. The black product obtained was filtered, washed thoroughly using distilled water and ethanol, and finally dried in a vacuum at 60 °C for 4 h and characterized. Instruments and Characterization. X-ray diffraction (XRD) patterns of the products were recorded on a Philips PW-1710 X-ray diffractrometer (40 kV, 20 mA) using Cu KR radiation (λ ) 1.5418 Å) in the 2θ range of 20°-60°. Fourier transform infrared (FTIR) spectra of the samples were recorded on a Thermonicolet/Nexus 870 FTIR spectrometer in the range of 4000-500 cm-1. Scanning electron microscopy (SEM) photographs of gold-coated products were carried out on a JEOL JSEM-5800 at an acceleration voltage of 20 kV. Transmission electron microscopy (TEM) photographs and electron diffraction (ED) were performed on a Philips CM 200 (200 KV) transmission electron microscope. Energy-dispersive X-ray analysis (OXFORD ISIS-300 model) was used to determine the chemical composition of the products. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Rigaku XPS 7000 spectrometer using nonmonochromatic Mg K (alpha) (1253.6 eV) at a source of 200 W. Chamber pressure during the measurement was about 10-7 Pa. The binding energies were referred to the adventitious C 1s peak at 284.6 eV. A UV/Vis spectrophotometer Perkin-Elmer Lambda 20 was used to carry out the optical measurements of the sample dispersed in ethanol. The photoluminescence measurements were performed on a PerkinElmer LS55 luminescence spectrophotometer.
Roy and Srivastava Scheme 1. Decomposition of Cu-DTO Complex under Hydrothermal Conditions
Results and Discussion Single-Source Precursor. Abboudi et al.37,38 have performed studies on the structural chemistry of the DTO complex of copper. According to them, the ligand-to-metal ratio in CuDTO is ≈ 1 and its proposed molecular formula is Cu(C2S2N2H2)(H2O).38 The DTO ligand is quasi-planar and in a trans conformation; the coordination with Cu-metal is effective by two sulfur and two nitrogen atoms, giving rise to the form of a polymeric chain. The elemental analysis of the Cu-DTO complex reported by Abboudi and co-workers37 is well matched with our product. The complex precursor is amorphous in nature as confirmed by the absence of sharp peaks in the XRD pattern. Formation of Copper Sulfide. According to Abboudi and co-workers,37 the complex decomposes completely at very high temperature (680 °C), whereas in our case under hydrothermal conditions the complex decomposes to CuS at 120 °C. The formation of CuS is anticipated to proceed according to Scheme 1, in which the water molecule is coordinated to copper according to the model as proposed by Abboudi et al.38 Under hydrothermal conditions, the Cu-N and C-S bonds break, leading to the formation of [CuS2] and glyoxal (compound C) as byproducts, which remain in the solution. [CuS2] is metastable at normal temperature and pressure,39 and thereby it tends to be transformed into the most stable states either as CuS or as Cu2S. Our proposed view in Scheme 1 is further confirmed by FTIR studies of pure DTO, Cu-DTO, and the solution product collected from a hydrothermal reaction mixture and is shown in Figure 1a-c. The peaks at 3464, 3295, and 3213 cm-1 in pure DTO can be assigned undoubtedly to the -NH2 stretching vibration. In Cu-DTO, peaks shift to lower frequencies, i.e., at 3433, 3243 cm-1, indicating the formation of a M-N bond in the complex.11,41 The peak at 1199 cm-1 in pure DTO is due to the stretching vibration for the CdS bond, which appears as a doublet in the Cu-DTO complex at 1113 and 1056 cm-1 due to formation of the M-S bond.40,41 The sharp peak at 1588 cm-1 in DTO can be attributed due to the C-N stretching vibration, which shifted to 1510 cm-1 for the Cu-DTO complex, and in either case, peaks in the range of 1300-1450 cm-1 are due to a δ(NH) vibration.39 In Figure 1c, the broad peak at 3453 cm-1 may be due to the presence of a hydrogen-
bonded -OH stretching vibration41 in the intermediate compound formed during the conversion of compound B to C in Scheme 1, which is more likely to hide some N-H vibrations. The sharp peak around 1650 cm-1 is attributed the hydrogenbonded CdO stretching vibration41 for the formation of compound C. We also carried out the reaction by using different polar solvents, such as ethanol, ethylenediamine, and nonpolar solvent, e.g., toluene. However, the yield under hydrothermal conditions is much greater compared to that under solvothermal conditions, suggesting that the reaction is more favorable in a water medium. Figure 2 represents the XRD pattern of as-synthesized products under hydrothermal treatment at 120 °C/24 h as well as at 175 °C/24 h. Both of these diffraction patterns are similar and can be perfectly indexed as hexagonal CuS (space group: P63/mmc) with lattice parameters a ) 3.794 Å and c ) 16.35 Å, which are well matched with the standard values (JCPDS 78-0876). The absence of any other peaks indicates the high purity of products. The broad diffraction pattern in Figure 2a indicates the formation of a nanosized product at 120 °C. However, on increasing the reaction temperature, the intensity of peaks increases, indicating the formation of a highly crystalline product at higher temperature. Figure 3 shows XRD patterns of solvothermal products using ethanol, ethylenediamine, and toluene as solvents under similar conditions. It is noticed that CuS is obtained in ethanol and toluene medium. However, in ethylendiamine, it leads to the formation of Cu2S [JCPDS 84-1770] due to the reducing property.11 The Cu-DTO complex also was treated thermally at 680 ( 5 °C in argon atmosphere, and the final temperature was maintained for 4 h and the XRD pattern of this pyrolyzed product corresponds to monoclinic Cu2S [JCPDS 83-1462]. The morphology of products was studied by SEM and TEM analysis, and it was found that the morphology of the products depends on duration, reaction temperature, and the solvent used. It also has been noted that the growth of the nanowires did not take place even after a reaction duration of 12 h. On continuing the reaction up to 16 h, growth of few nanowires along with a
Hydrothermal Growth of CuS Nanowires
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Figure 1. FT-IR spectra of (a) pure DTO, (b) Cu-DTO complex, and (c) the solution product obtained after the hydrothermal reaction.
Figure 2. XRD pattern of the hydrothermal product obtained at (a) 120 °C/24 h and (b) 170 °C/24 h.
few particles was observed and upon further increase in the reaction duration, the formation of nanowires tended to dominate. All this suggests that the slow reaction rate at low reaction temperature influences the formation of nanowire-type morphology. As a consequence, more nanowires formed as the reaction was prolonged. The optimum reaction duration is 24 h, in which formation of nanowires is observed along with some nanoparticles as shown in Figure 4a. On increasing the temperature from 120 to 175 °C, the rate of reaction increases, and as a result the reaction is completed in a very short time. This clearly suggests that the product remained in the growth process for most of the time.42 As a result, the morphology of product obtained at higher temperature changes from nanowires to microrods with a diameter of 1-2 µm and a length of up to 10 µm as shown in Figure 4b. The nanowires are curved, but the microrods are straight and smoother in nature, which also may indicate the more crystalline nature of the product at high temperature.43 The nanowire morphology obtained at 120 °C after 24 h was further studied in detail by TEM and is shown in Figure 5. The
Figure 3. XRD pattern of the product obtained at 120 °C/24 h in (a) ethanol, (b) ethylenediamine, and (c) toluene.
TEM image in Figure 5a indicates the presence of many nanowires with irregular shape, size, and rough surface, which is seen to be curved and nonuniformly distributed and also
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Roy and Srivastava
Figure 4. SEM images of products obtained hydrothermally at (a) 120 °C and (b) 175 °C for 24 h.
Figure 6. SEM images of products obtained at 120 °C/24 h in (a) ethanol, (b) ethylenediamine, and (c) toluene as solvents.
Figure 5. TEM images of as-prepared CuS nanowires prepared by a hydrothermal treatment at 120 °C/24 h: (a) lots of nanowires; (b) a single nanowire in high magnification; inset: SAED pattern.
accompanied by a few nanoparticles. This curved nature of nanowires provides a favorable condition for the application in
nanodevices mainly for the development of the nanoelectronic and photoelectronic industries.44 Figure 5b is the high magnification TEM images of nanowire, indicating a diameter of 4080 nm. The inset in Figure 5b shows a selected area electron diffraction (SAED) pattern. The clear rings consisting of a parallel distribution of spots indicate the single crystalline nature of the product. From the center to the edge, the rings indicate the diffraction of (102), (110), and (203) planes of the hexagonal phase of CuS. Figure 6a-c reveals the SEM images of solvothermal products at 120 °C after 24 h. The product obtained using ethanol as solvent also comprises nanowires of low
Hydrothermal Growth of CuS Nanowires
Crystal Growth & Design, Vol. 6, No. 8, 2006 1925
Figure 8. UV absorption spectra of (a) CuS nanowires at 120 °C/24 h and (b) CuS microrods at 175 °C/24 h.
Figure 7. SEM images of pyrolysis products (a) microbelts and (b) its porous surface.
dimensional compared to that under hydrothermal treatment along with a few long nanowires, as shown in Figure 6a. This may be due to a poor decomposition rate of the complex in ethanol rather than in water, which is also supported by the low yield of product in ethanol. Ethylenediamine is a very strong bidentate ligand and when used as solvent leads to the breaking of Cu-DTO as well as to the formation of a new Cuethylenediamine complex, and both these processes competes with one other. The randomization of the reaction in all probability might be responsible for the formation of a hexagonal-shaped particle in the size range of few nanometers to 1 µm as shown in Figure 6b. When toluene was used as solvent, nanoparticles of CuS were obtained with a size in the nanometer range. It may be mentioned that as toluene is a noncoordinating solvent, copper sulfide nanoparticles formed during the solvothermal reaction do not have a capped surface, which can facilitate the agglomeration of nanoparticles as observed in Figure 6c.45 The change in morphology of products using different solvents also was confirmed by TEM analysis (not shown here). The morphology of the product (Figure 7) obtained after the pyrolysis of the Cu-DTO complex at high temperature also was studied. It shows that the product is composed of microbelts that are 10 µm in width and several micrometers in length, and the surface is porous in nature as shown in Figure 7, panels a and b, respectively. It is believed that the porous nature of the surface is due to the collapse in the porous areas induced by a different surface tension in the decomposition process.46
XPS analysis of CuS nanowires (120 °C/24 h) was carried out to determine the composition of the product. The full spectrum (not shown here) indicates the presence of Cu, S, C, and O peaks where the C and O peaks were due to the absorption of air on the surface of nanowires. The high-resolution spectra in the Cu 2p region revealed the presence of two strong peaks at 932.1 and 952.1 eV for Cu 2p3/2 and Cu 2p1/2, respectively, separated by 20.0 eV. In addition, a small chemical shift (0.3 eV) occurred compared to the elemental copper. Another closeup survey in the S 2p region showed the presence of a doublet peak at 162.1 eV. All these peak positions are well matched with reported values.47 The Cu/S ratio of CuS nanowires was calculated from the peak areas of Cu and S-cores, and the value was found to be 1.02, closely matched with the Cu/S value (0.98) estimated from EDX analysis. Figure 8a,b represents the absorption spectrum in the range 350-800 nm of products obtained at 120 °C/24 h and 175 °C/ 24 h, respectively. The product obtained at lower temperature with nanowire morphology shows a small absorption peak at ≈400 nm. However, no such peak is observed for the product obtained at higher temperature containing some microrods. Very recently, similar findings were also reported by Tan et al.26 for CuS nanotubes and by Ji et al.48 for CuS nanorods. Further studies on the absorption spectrum of various phases of copper sulfides were reported by Haram and co-workers,20,49 who proposed the presence of a characteristic broad absorption band in near-IR region for covellite copper sulfide (CuS), which decreases on increasing sulfur content (i.e., from covellite to digenite (Cu1.8S) to djurleite (Cu1.96S)) and is absent in the chalcocite phase (Cu2S), and also suggested that overall absorption spectrum is not affected by the size and shape of the products. We also observed the similar broad absorption band in the near-IR region, which further confirmed the formation of covellite CuS. Figure 9a,b shows the room-temperature photoluminescence spectra of CuS nanowires (120 °C/24 h) and microtubes (175 °C/24 h), respectively, and the samples prepared by dispersing in water/alcohol (1:1) solvent. Both of these emission spectra were obtained in the excitation wavelength of 370 nm. The spectrum of the product with microrod morphology shows a broad peak at 462 nm, whereas the spectrum of CuS nanowires shows a peak at 450 nm with a shoulder peak at 420 nm. The small blue shift may be due to the nanostructure of CuS at 120 °C. Although Jiang and co-workers10 reported that there is no emission peak for CuS in the range of 400-800 nm, our result is very consistent with the PL result reported by Ou et al.43 According to them, various morphologies of copper sulfide may be responsible for this different phenomenon of PL.
1926 Crystal Growth & Design, Vol. 6, No. 8, 2006
Figure 9. PL spectra of (a) CuS nanowires at 120 °C/24 h and (b) CuS microrods at 175 °C/24 h.
Conclusion In the present work, we synthesized CuS nanowires using an easily synthesizable and environmentally friendly singlesource precursor, Cu-DTO, by a simple hydrothermal method at 120 °C. A detailed study on this complex and the possible reaction mechanism for the formation of CuS was outlined. The XRD pattern indicates the formation of a covellite nature of CuS with a hexagonal phase under hydrothermal conditions. The morphology of the product was studied by SEM and TEM. It also was observed that the reaction temperature, duration, and the solvents had notable effects on the morphology of products. At low temperature (120 °C), we synthesized CuS nanowires that were 40-80 nm in diameter and up to few microns long, while at a high temperature (175 °C) we obtained microrods that were 1-2 µm in diameter and ≈10 µm long. The composition and purity of product were confirmed by energy dispersive X-ray analysis (EDX) and XPS analysis. The optical properties of the products also were investigated in detail, and the PL spectrum shows a small blue shift of the CuS nanowires due to its nanostructure. Acknowledgment. S. K. Srivastava and P. Roy are thankful to CSIR and Indian Institute of Technology, Kharagpur, respectively, for financial support. The authors greatly acknowledge the valuable help from Professor C. N. R. Rao and Usha Govind Tumkurkar, JNCASR, Bangalore, for TEM analysis of samples. References (1) Albu-Yaron, A.; Le´vy-Cle´ment, C.; Katty, A.; Bastide, S.; Tenne, R. Thin Solid Films 2000, 361-362, 223. (2) Ota, J. R.; Srivastava, S. K. J. Nano Sci. Nanotech. 2006, 6, 168. (3) Ota, J. R.; Srivastava, S. K. Nanotechnology 2005, 16, 2415. (4) Sigman, M. B.; Ghezelbash, A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050. (5) Roy, P.; Srivastava, S. K. Mater. Chem. Phys. 2006, 95, 235. (6) Chen, X.; Xu, H.; Xu, N.; Zhao, F.; Lin, W.; Lin, G.; Fu, Y.; Huang, Z.; Wang, H.; Wu, M. Inorg. Chem. 2003, 42, 3100. (7) Meldrum, F. C.; Flath, J.; Knoll, W. Langmuir 1997, 13, 2033.
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