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Facile synthesis of nearly monodispersed copper sulfide nanocrystals
Electrochimica Acta 53 (2007) 213–217
Facile synthesis of nearly monodispersed copper sulfide nanocrystals Toshihiro Kuzuya a,∗ , Keiichi Itoh b , Minoru Ichidate a , Takahide Wakamatsu a , Yasuhiro Fukunaka c , Kenji Sumiyama b b
a Faculty of Urban Science, Meijo University, Kani 509-0261, Japan Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan c Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan
Received 2 December 2006; received in revised form 7 June 2007; accepted 7 June 2007 Available online 19 June 2007
Abstract We synthesized monodispersed copper sulfide (Cux S) NCs at nearly ambient temperature. Alkan thiol and amphiphilic amine mixture (ligand mixture) enable us to use relatively low toxic, stable and low cost materials as metal and sulfur source. Furthermore, oleylamine (OA) and dodecanethiol (DT) play important roles to control Cu/S composition on NCs. This compositional change of Cux S NCs was attributed to the introducing of S–S bonding (Sn 2− ) into NCs. © 2007 Elsevier Ltd. All rights reserved. Keywords: Cu2 S; CuS; Monodispersed nanocrystals; Dodecanethiol; Oleylamine
1. Introduction Copper sulfide (Cux S) is a narrow band gap and p-type semiconductor. The optoelectrical properties of Cux S NCs have been investigated to apply to nonlinear optical materials [1], NC sensitizer or p-type semiconductor material for hetero-junction solar cell [2]. Recent reports also indicate that its high ionic conductivity enabled Cu2 S and Ag2 S thin films to serves as a nano-switch for programmable IC and logic circuits [3]. These attractive applications encourage the fabrication of nanostructured copper sulfides. The control of size and shape of NCs is an important issue to fabricate nanostructured materials, because of size and shape dependence of physicochemical and optoelectrical properties. In copper sulfide system, Cux S involves various compounds such as Cu2 S ␥, -chalcocite), Cu1.96 S (djurleite), Cu1.8 S (digenite) and CuS (covellite) [4]. They exhibit stoichiometrydependent optoelectric properties: Cu1.96 S, Cu1.9 S, Cu1.8 S and CuS have interband absorption peaks, while Cu2 S does not. Cux S is also characterized by an increase in the band gap with the increase of the x value (Eg = 1.2 eV for x = 2, 1. 5eV for x = 1.8 and ∼2.0 eV for x = 1) [1]. There∗
Corresponding author. Tel.: +81 52 735 5258; fax: +81 52 735 5258. E-mail address: [email protected] (T. Kuzuya).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.06.033
fore, the composition control of copper sulfide NCs is an alternative way to tune the optoelectrical properties of NC devices. Recently, we synthesized monodispersed copper sulfide (Cux S) NCs at nearly ambient temperature. Alkan thiol and amphiphilic amine mixture (ligand mixture) enable us to use metal acetates and sulfur powder, which have the advantages of relatively low toxicity, stability and low cost, as metal and sulfur source. In this report, the control of the chemical composition and size of NCs has been examined with various experimental conditions. 2. Experimental procedures 2.1. Synthesis of Cux S NCs A typical experimental procedure is summarized as follows. All reagents were used as received. In order to prepare copper stock solution, 0.18 g (1 mmol) of copper(II) acetate (97%, Wako pure chemicals) was mixed with 5 cm3 of technical grade oleylamine (70%, Wako pure chemicals; OA) (Cu/OA solution). About 0.065 g (1 mmol) of sulfur powder (98%, Wako pure chemicals) was mixed with 5 cm3 of DT (97%, Wako pure chemicals). This mixture solution was heated up to ∼373 K. Then 20 cm3 of hexane was added to this solution. Copper salt pow-
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Table 1 Experimental conditions Sample name
Sa.1 Sa.2 Sa.3 Sa.4
Cu-oleylamine solution
S/DT or S/OA solution
Cu(ace)2 (mg)
OA (ml)
187.3 (1.0 mmol)
5 2.5 2.5 5
Duration time (ks)
S (mg)
DT or OA (ml)
65.5
5 (DT) 10 (DT) 2.5 (OA) 5 (OA)
0.3a , 86.4b 7.2c 7.2d 7.2e
Note. Reaction temperature 303 K. a Fig. 1a. b Fig. 1b–e. c Figs. 1f and 4(b). d Fig. 2(a) and (c). e Figs. 2b, 2d, 3, 4(a).
der or copper stock solution was injected with vigorous agitation into this mixture solution at 303 K. Experimental conditions are summarized in Table 1.
absorption spectra were observed between 200 and 1700 nm (JASCO, V-570 equipped with PbS detector). 3. Results
2.2. Purification When ethanol was mixed with reaction solution, precipitates were formed. They were separated by centrifuging the colloidal solution to remove excess reaction agents and redispersed in hexane. This precipitate/redispersion procedure was repeated several times to purify the precipitates. 2.3. Analysis A drop of the hexane solution of NCs was placed on a carboncoated micro grid for transmission electron microscope (TEM) observation, and examined with field emission TEM (Hitachi, HF-2000) operating at 200 kV with a point-to-point resolution of 0.23 nm. The crystal structures of Cux S NCs were identified by X-ray diffraction (XRD) (Mac Science, M18XCE). UV–vis
When Cu/OA solution was directly added to sulfur source solution (S/OA or S/DT), the solution color rapidly turned dark brown, indicating formation of copper sulfide NCs. These precipitation reactions were conducted at ambient temperature (∼303 K). The brown (S/DT system) and bluish brown (S/OA system) precipitates were easily recovered by a polar-solvent precipitation method. Figs. 1 and 2 show TEM image and XRD patterns of brown and bluish brown precipitates. These results indicate that DT and OA play important role to control the morphology and Cu/S composition of copper sulfide NCs. 3.1. S/DT reaction system TEM image of copper sulfide NCs synthesized using S/DT solution as a sulfur source (DT:OA = 1:1) are shown in Fig. 1(a)
Fig. 1. Copper sulfide NCs obtained by using S/DT solution. (a) and (b) represent the time evolution of copper sulfide NCs. Increased DT:OA volume ratio gave relatively monodispersed copper sulfide NCs (σ/dave ∼ 7%). (a) DT:OA = 1:1, duration time = 0.3 ks, (b) 86.4 ks, (c) XRD pattern of copper sulfide NCs shown in (b), (d) and (e) HRTEM image of coinage copper sulfide NCs shown in (b), (f) DT:OA = 4:1(vol), duration time = 7.2 ks.
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Fig. 2. Copper sulfide NCs obtained using S/OA solution. These were conducted at various OA concentrations, while keeping all the other conditions the same. (a and c) [OA] = 5 cm3 , (b and d) = 10.0 cm3 .
and (b). They revealed that rod-like NCs (indicated by the arrow shown in Fig. 1(a) coexist with spherical-like NCs. Fig. 1(a) and (b) represent the time evolution of copper sulfide NCs, indicating that NCs grew uniformly with time. Fig. 1(c) shows the powder XRD pattern of copper sulfide NCs synthesized using S/DT solution. Characteristic diffraction peaks of a Cu2 S chalcocite phase are evidently observed. While the diffraction peaks of the non-stoichiometric copper sulfide phases ((0 1 20) of Cu1.8 S and (2 0 1c) of Cu1.96 S) were not detected. In Fig. 1(d), the lattice image of spherical-like NCs is 0.19 nm, being ascribed to the {1 1 0} plane of the same hexagonal Cu2 S. The lattice image shown in Fig. 1(e) indicates that the lattice spacing of these rodlike NCs is 0.34 nm, which corresponds to that between {1 0 0} planes of the hexagonal Cu2 S. These results provide the evidence of the formation of Cu2 S NCs with coinage morphology [5,6]. However, the TEM image cannot give detailed information concerning the three-dimensional shape of copper sulfide NCs. The high DT: OA volume ratio (4:1) can provide nearly monodispersed coinage Cu2 S NCs (d = 5.2 ± 0.36 nm, t = 3.2 nm; see Fig. 1(f).
XPS spectra of S2p revealed that higher binding energy components (162–164 eV) of CuS NCs were evidently stronger than that of Cu2 S. These components can be allotted to a set of 2p1/2 and 2p2/3 of Sn 2− or Sn 0 . Assuming that a spin-orbit splitting (2p3/2−/1/2 ) and an area ratio of 2p1/2 to 3/2 of sulfur species are 1.24 eV and 1:2, respectively [9], Gaussian–Lorentz (GL) fitting analysis was applied to S2p spectra. XPS analysis give the atomic fractions of sulfur species, which is proportional to the area intensities of S-2p3/2 peaks and originated from S of thiol group, S2 2− S(I) and S2− S(II) in Cux S NCs. One-third of sulfur
3.2. S/OA reaction system Fig. 2(a) and (b) show the morphological variations of copper sulfide NCs, synthesized by using S/OA solution. The higher oleylamine concentration leads to the formation of nearly monodispersed copper sulfide NCs (d = 4.9 ± 0.35 nm). However, in the room temperature synthesis, S/OA solution cannot give high quality NCs with smooth surfaces and regular morphology. The XRD patterns, shown in Fig. 2(c) and (d), indicate the formation of covellite (CuS, hexagonal) NCs. CuS “covellite” takes a complex layer structure. It is considered that 2/3 of copper atoms are tetrahedrally coordinated Cu(I) and the remaining 1/3 are trigonally coordinated Cu(II) [7,8]. However, XPS spectra of CuS NCs (Cu2p ; Fig. 3(a)) did not exhibit characteristic satellite peaks (∼942 eV) which is assigned to Cu(II) [8].
Fig. 3. XPS spectra of copper sulfide NCs obtained using S/OA solution (TEM and XRD are shown in Fig. 2(b) and (d)): (a) Cu2p spectra; (b) S2p spectra. Arrow in (a) indicates the peak position of Cu2+ satellite peak.
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Fig. 4. UV–vis spectra of copper sulfide NCs obtained using S/OA (Sa.4) (a) and S/DT (Sa.2) (b) solution (upper); Tauc’s plots of spectra (a) and (b) (lower).
atom in CuS NCs (Sa.4) are oxidation state mono-valent sulfur S(I) (S2 2− ). 3.3. UV–vis spectra of Cux S NCs A feature of UV–vis spectrum sensitively depends on Cu/S composition (that is amount of “Cu-defect”) of copper sulfide NCs. UV–vis spectrum of CuS show a characteristic absorption band in near-IR region (Fig. 4(a)). Fig. 4(b) indicates that Cu2 S NCs evidently have the absorption peak, which was weaker and exhibit a large red shift from that of CuS NCs. These near-IR absorptions are due to interband-transitions (absorptions) from valence states to unoccupied states [10]. Peak position (Emax ) of near IR absorption represents gap energy between valence state and unoccupied state. Tauc’s plots of (a) and (b) are show in Fig. 4 (lower): α0.5 versus hν (indirect transition) and (αhν)0.5 versus hν (direct transition). Band gap energy Eg of CuS sphere NCs (dave = 4.9 nm) and coinage Cu2 S NCs (dave = 5.2 ± 0.36 nm, t = 3.2 nm) were estimated to be 1.63 eV(direct) and 1.37 (indirect; 1.78 (direct)), respectively. Emax values of CuS and Cu2 S NCs obtained from Gaussian fitting are 1.14 and 0.61 eV.
4. Discussion Based upon the above-mentioned results, we obtained the following reaction procedures; when the synthesis of copper sulfide NCs were conducted using the S/DT solution, the brown precipitates “chalcocite Cu2 S” were obtained. Sulfur was reduced by DT to form H2 S, which was solvated by DT molecules. Schemes are summarized as follows: 2RSH + S ⇔ (RSH)2 S ⇔ H2 S + RSSR
(1)
This conclusion is confirmed by the fact that hydrogen sulfide smell was slightly perceived in any sulfur solution after addition of DT. Then, Cu(II) ions, which were solvated by ligands, reacted with H2 S to form copper sulfide NCs. The formation of Cu2 S phase indicates that the precipitation of copper sulfide NCs was accompanied with the reduction processes of Cu(II) ion, which are represented as follows [8,11]: 2Cu2+ + 2RSH = 2Cu+ + RSSR + 2H+
(2)
2Cu2+ + 2S2− = Cu2 S + S0
(3)
T. Kuzuya et al. / Electrochimica Acta 53 (2007) 213–217
In the case of S/OA system, the formation of sulfur anion is considered to be supplied by the decomposition of S–OA complex. XPS analysis revealed that CuS “covellite” contains the oxidation sulfur S2 2− component “Cu-defect”, which may cause the surface roughening and the formation of irregular shape NCs. Indeed, the reduction of S0 to S5 2− : 5S + 2e = S5 2− (−0.34 eV) is energetically more favorable than to S2− (−0.48 eV). Ligands (such as DT and OA), which have an electron donor character, serve as a reduction agent for sulfur. In the case of alkylamine, it is found that sulfur reduction requires higher reaction temperature. The proportion of sulfur species (S0 , S2− , Sn 2− , etc.) in reaction system depends upon reaction temperature and a reduction ability of ligand or its mixture. These results indicate that DT has the nature to be stronger reduction agent for sulfur. The ligands mixture plays alternative important roles to control the sulfuration rate. The stability and sulfuration rate of copper sulfide(Cun Sm ) NCs represent a product of activities of copper and sulfur ions (e.g. the solubility product: Ksp = [Cu+ ]n [S2− ]m ). Since DT and OA serve as masking agents to copper ion, the sulfuration rate depends strongly on the strength of ligand-Cu ion interaction. OA reacts with DT to form an DT-OA association and serve as a de-proton agent for H2 S. Actually, the sulfuration reaction without oleylamine proceeded very slowly and formed irregular shape Cu2 S NCs. These effects lead to the decrease in the activity of DT (strong ligand) and the increase in the reactivity of sulfur source. UV–vis spectra exhibit the little change in the UV-blue spectra feature. However, it is found that interband structure of Cux S NCs is affected by Cu/S composition. In CuS NCs, the optical absorption peak is observed in the near IR region, corresponding to the interband transition. The XPS results suggest that the interband is attributed to the introduction of the impurity state of S2 2− . Silvester et al. reported that CuS (covellite) NCs had absorption band whose absorption maxima Emax was at ∼920 nm (bulk material 1200 nm), and it was not affected by its composition [8]. However, Emax of Cu2 S NCs (Sa.2) was quite different from that of CuS (Sa.4). It is reasonable that Emax depends on the crystalline structure, because the different crystal structures and lattice constants of Cu2 S and CuS give rise to the different electrostatic field in which electrons move. 5. Conclusion We synthesized monodispersed Cu2 S and CuS NCs at nearly ambient temperature. Dodecanethiol and oleylamine mixture (ligand mixture) enable us to use low toxic, stable and low
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cost material, as metal and sulfur source. Ligand mixture also plays important roles to control Cu/S composition on NCs. Using S/DT solution led to the precipitation of Cu2 S “Chalcocite” phase. When the synthesis of Cux S NCs was conducted using S/OA solution, CuS NCs were formed. This compositional change of Cux S NCs was attributed to the introducing of Sn 2− species into NCs. Therefore, the proportion of Cu-defect in Cux S NCs may be controlled by an appropriate choice of ligand. Acknowledgements This work was supported by a grant form the NITECH 21st Century COE Program, “World Ceramics Center for Environmental Harmony”, Meijo University Open Research Center project and Intellectual Cluster Project of Aichi-Nagoya Area given by the Ministry of Education, Science, Culture and Sports, Japan. References [1] (a) V.I. Klimov, P. Haring-Bolivar, H. Kurz, V.A. Karavanskii, Superlattice Microstruct. 20 (1996) 395; (b) V. Klimov, P. Haring Bolivar, H. Kurz, V. Karavanskii, V. Krasovskii, Yu. Korkishko, Appl. Phys. Lett. 67 (1995) 653. [2] (a) L. Reijnen, B. Meester, A. Goossens, J. Schoonman, Mater. Sci. Eng. 19 (2002) 311; (b) Y. Lou, A.C.S. Samia, J. Cowen, K. Banger, X. Chen, H. Lee, C. Burda, Phys. Chem. Chem. Phys. 5 (2003) 1091. [3] (a) http://pc.watch.impress.co.jp/docs/2004/0216/nano.htm; (b) K. Terabe, T. Hasegawa, T. Nakayama, M. Aono, Mater. Jpn. 44 (2005) 757, and references therein. [4] R.V. Gains, H.C.W. Skinner, E.E. Foord, B. Mason, A. Rosenzweig, W.T. King, E. Gowty, Dana’s New Mineralogy, Wiley, New York, 1997, p. 1819. [5] (a) H. Zhang, G. Wu, X. Chang, Langmuir 21 (2005) 4281; (b) A. Ghezelbash, B.A. Korgel, Langmuir 21 (2005) 9451. [6] (a) T.H. Larsen, M. Sigman, A. Ghezelbash, C. Doty, B.A. Korgel, J. Am. Chem. Soc. 125 (2003) 5638; (b) M.B. Sigman, A. Ghezelbash, T. Hanras, A.E. Saunders, F. Lee, B.A. Korgel, J. Am. Chem. Soc. 125 (2003) 16050. [7] H.T. Evans Jr., J.A. Konnert, Am. Mineral. 61 (1976) 996. [8] E.J. Silvester, F. Grieser, B.A. Sexton, T.W. Healy, Langmuir 7 (1991) 2917. [9] E.Z. Kurmaev, J. van EK, D.L. Ederer, L. Zhou, T.A. Callcott, R.C.C. Perera, V.M. Cherkashenko, S.N. Shamin, V.A. Trofimova, S. Bartkowski, M. Neumann, A. Fujimori, V.P. Moloshang, J. Phys. Condens. Matter 10 (1998) 1687. [10] (a) S.A. Zolotovskaya, V.G. Savitski, P.V. Prokoshin, K.V. Yumashev, J. Opt. Soc. Am. B 23 (2006) 1268; (b) M.C. Brelle, C.L. Torres-Martinez, J.C. McNulty, R.K. Mehra, J.Z. Zhang, Pure Appl. Chem. 72 (2000) 101; (c) S.K. Haram, A.R. Mahadeshwar, S.G. Dixit, J. Phys. Chem. 100 (1996) 5868. [11] Y. Negishi, H. Murayama, T. Tsukuda, Chem. Phys. Lett. 366 (2002) 561, and references therein.
Facile synthesis of nearly monodispersed copper sulfide nanocrystals Toshihiro Kuzuya a,∗ , Keiichi Itoh b , Minoru Ichidate a , Takahide Wakamatsu a , Yasuhiro Fukunaka c , Kenji Sumiyama b b
a Faculty of Urban Science, Meijo University, Kani 509-0261, Japan Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan c Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan
Received 2 December 2006; received in revised form 7 June 2007; accepted 7 June 2007 Available online 19 June 2007
Abstract We synthesized monodispersed copper sulfide (Cux S) NCs at nearly ambient temperature. Alkan thiol and amphiphilic amine mixture (ligand mixture) enable us to use relatively low toxic, stable and low cost materials as metal and sulfur source. Furthermore, oleylamine (OA) and dodecanethiol (DT) play important roles to control Cu/S composition on NCs. This compositional change of Cux S NCs was attributed to the introducing of S–S bonding (Sn 2− ) into NCs. © 2007 Elsevier Ltd. All rights reserved. Keywords: Cu2 S; CuS; Monodispersed nanocrystals; Dodecanethiol; Oleylamine
1. Introduction Copper sulfide (Cux S) is a narrow band gap and p-type semiconductor. The optoelectrical properties of Cux S NCs have been investigated to apply to nonlinear optical materials [1], NC sensitizer or p-type semiconductor material for hetero-junction solar cell [2]. Recent reports also indicate that its high ionic conductivity enabled Cu2 S and Ag2 S thin films to serves as a nano-switch for programmable IC and logic circuits [3]. These attractive applications encourage the fabrication of nanostructured copper sulfides. The control of size and shape of NCs is an important issue to fabricate nanostructured materials, because of size and shape dependence of physicochemical and optoelectrical properties. In copper sulfide system, Cux S involves various compounds such as Cu2 S ␥, -chalcocite), Cu1.96 S (djurleite), Cu1.8 S (digenite) and CuS (covellite) [4]. They exhibit stoichiometrydependent optoelectric properties: Cu1.96 S, Cu1.9 S, Cu1.8 S and CuS have interband absorption peaks, while Cu2 S does not. Cux S is also characterized by an increase in the band gap with the increase of the x value (Eg = 1.2 eV for x = 2, 1. 5eV for x = 1.8 and ∼2.0 eV for x = 1) [1]. There∗
Corresponding author. Tel.: +81 52 735 5258; fax: +81 52 735 5258. E-mail address: [email protected] (T. Kuzuya).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.06.033
fore, the composition control of copper sulfide NCs is an alternative way to tune the optoelectrical properties of NC devices. Recently, we synthesized monodispersed copper sulfide (Cux S) NCs at nearly ambient temperature. Alkan thiol and amphiphilic amine mixture (ligand mixture) enable us to use metal acetates and sulfur powder, which have the advantages of relatively low toxicity, stability and low cost, as metal and sulfur source. In this report, the control of the chemical composition and size of NCs has been examined with various experimental conditions. 2. Experimental procedures 2.1. Synthesis of Cux S NCs A typical experimental procedure is summarized as follows. All reagents were used as received. In order to prepare copper stock solution, 0.18 g (1 mmol) of copper(II) acetate (97%, Wako pure chemicals) was mixed with 5 cm3 of technical grade oleylamine (70%, Wako pure chemicals; OA) (Cu/OA solution). About 0.065 g (1 mmol) of sulfur powder (98%, Wako pure chemicals) was mixed with 5 cm3 of DT (97%, Wako pure chemicals). This mixture solution was heated up to ∼373 K. Then 20 cm3 of hexane was added to this solution. Copper salt pow-
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Table 1 Experimental conditions Sample name
Sa.1 Sa.2 Sa.3 Sa.4
Cu-oleylamine solution
S/DT or S/OA solution
Cu(ace)2 (mg)
OA (ml)
187.3 (1.0 mmol)
5 2.5 2.5 5
Duration time (ks)
S (mg)
DT or OA (ml)
65.5
5 (DT) 10 (DT) 2.5 (OA) 5 (OA)
0.3a , 86.4b 7.2c 7.2d 7.2e
Note. Reaction temperature 303 K. a Fig. 1a. b Fig. 1b–e. c Figs. 1f and 4(b). d Fig. 2(a) and (c). e Figs. 2b, 2d, 3, 4(a).
der or copper stock solution was injected with vigorous agitation into this mixture solution at 303 K. Experimental conditions are summarized in Table 1.
absorption spectra were observed between 200 and 1700 nm (JASCO, V-570 equipped with PbS detector). 3. Results
2.2. Purification When ethanol was mixed with reaction solution, precipitates were formed. They were separated by centrifuging the colloidal solution to remove excess reaction agents and redispersed in hexane. This precipitate/redispersion procedure was repeated several times to purify the precipitates. 2.3. Analysis A drop of the hexane solution of NCs was placed on a carboncoated micro grid for transmission electron microscope (TEM) observation, and examined with field emission TEM (Hitachi, HF-2000) operating at 200 kV with a point-to-point resolution of 0.23 nm. The crystal structures of Cux S NCs were identified by X-ray diffraction (XRD) (Mac Science, M18XCE). UV–vis
When Cu/OA solution was directly added to sulfur source solution (S/OA or S/DT), the solution color rapidly turned dark brown, indicating formation of copper sulfide NCs. These precipitation reactions were conducted at ambient temperature (∼303 K). The brown (S/DT system) and bluish brown (S/OA system) precipitates were easily recovered by a polar-solvent precipitation method. Figs. 1 and 2 show TEM image and XRD patterns of brown and bluish brown precipitates. These results indicate that DT and OA play important role to control the morphology and Cu/S composition of copper sulfide NCs. 3.1. S/DT reaction system TEM image of copper sulfide NCs synthesized using S/DT solution as a sulfur source (DT:OA = 1:1) are shown in Fig. 1(a)
Fig. 1. Copper sulfide NCs obtained by using S/DT solution. (a) and (b) represent the time evolution of copper sulfide NCs. Increased DT:OA volume ratio gave relatively monodispersed copper sulfide NCs (σ/dave ∼ 7%). (a) DT:OA = 1:1, duration time = 0.3 ks, (b) 86.4 ks, (c) XRD pattern of copper sulfide NCs shown in (b), (d) and (e) HRTEM image of coinage copper sulfide NCs shown in (b), (f) DT:OA = 4:1(vol), duration time = 7.2 ks.
T. Kuzuya et al. / Electrochimica Acta 53 (2007) 213–217
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Fig. 2. Copper sulfide NCs obtained using S/OA solution. These were conducted at various OA concentrations, while keeping all the other conditions the same. (a and c) [OA] = 5 cm3 , (b and d) = 10.0 cm3 .
and (b). They revealed that rod-like NCs (indicated by the arrow shown in Fig. 1(a) coexist with spherical-like NCs. Fig. 1(a) and (b) represent the time evolution of copper sulfide NCs, indicating that NCs grew uniformly with time. Fig. 1(c) shows the powder XRD pattern of copper sulfide NCs synthesized using S/DT solution. Characteristic diffraction peaks of a Cu2 S chalcocite phase are evidently observed. While the diffraction peaks of the non-stoichiometric copper sulfide phases ((0 1 20) of Cu1.8 S and (2 0 1c) of Cu1.96 S) were not detected. In Fig. 1(d), the lattice image of spherical-like NCs is 0.19 nm, being ascribed to the {1 1 0} plane of the same hexagonal Cu2 S. The lattice image shown in Fig. 1(e) indicates that the lattice spacing of these rodlike NCs is 0.34 nm, which corresponds to that between {1 0 0} planes of the hexagonal Cu2 S. These results provide the evidence of the formation of Cu2 S NCs with coinage morphology [5,6]. However, the TEM image cannot give detailed information concerning the three-dimensional shape of copper sulfide NCs. The high DT: OA volume ratio (4:1) can provide nearly monodispersed coinage Cu2 S NCs (d = 5.2 ± 0.36 nm, t = 3.2 nm; see Fig. 1(f).
XPS spectra of S2p revealed that higher binding energy components (162–164 eV) of CuS NCs were evidently stronger than that of Cu2 S. These components can be allotted to a set of 2p1/2 and 2p2/3 of Sn 2− or Sn 0 . Assuming that a spin-orbit splitting (2p3/2−/1/2 ) and an area ratio of 2p1/2 to 3/2 of sulfur species are 1.24 eV and 1:2, respectively [9], Gaussian–Lorentz (GL) fitting analysis was applied to S2p spectra. XPS analysis give the atomic fractions of sulfur species, which is proportional to the area intensities of S-2p3/2 peaks and originated from S of thiol group, S2 2− S(I) and S2− S(II) in Cux S NCs. One-third of sulfur
3.2. S/OA reaction system Fig. 2(a) and (b) show the morphological variations of copper sulfide NCs, synthesized by using S/OA solution. The higher oleylamine concentration leads to the formation of nearly monodispersed copper sulfide NCs (d = 4.9 ± 0.35 nm). However, in the room temperature synthesis, S/OA solution cannot give high quality NCs with smooth surfaces and regular morphology. The XRD patterns, shown in Fig. 2(c) and (d), indicate the formation of covellite (CuS, hexagonal) NCs. CuS “covellite” takes a complex layer structure. It is considered that 2/3 of copper atoms are tetrahedrally coordinated Cu(I) and the remaining 1/3 are trigonally coordinated Cu(II) [7,8]. However, XPS spectra of CuS NCs (Cu2p ; Fig. 3(a)) did not exhibit characteristic satellite peaks (∼942 eV) which is assigned to Cu(II) [8].
Fig. 3. XPS spectra of copper sulfide NCs obtained using S/OA solution (TEM and XRD are shown in Fig. 2(b) and (d)): (a) Cu2p spectra; (b) S2p spectra. Arrow in (a) indicates the peak position of Cu2+ satellite peak.
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Fig. 4. UV–vis spectra of copper sulfide NCs obtained using S/OA (Sa.4) (a) and S/DT (Sa.2) (b) solution (upper); Tauc’s plots of spectra (a) and (b) (lower).
atom in CuS NCs (Sa.4) are oxidation state mono-valent sulfur S(I) (S2 2− ). 3.3. UV–vis spectra of Cux S NCs A feature of UV–vis spectrum sensitively depends on Cu/S composition (that is amount of “Cu-defect”) of copper sulfide NCs. UV–vis spectrum of CuS show a characteristic absorption band in near-IR region (Fig. 4(a)). Fig. 4(b) indicates that Cu2 S NCs evidently have the absorption peak, which was weaker and exhibit a large red shift from that of CuS NCs. These near-IR absorptions are due to interband-transitions (absorptions) from valence states to unoccupied states [10]. Peak position (Emax ) of near IR absorption represents gap energy between valence state and unoccupied state. Tauc’s plots of (a) and (b) are show in Fig. 4 (lower): α0.5 versus hν (indirect transition) and (αhν)0.5 versus hν (direct transition). Band gap energy Eg of CuS sphere NCs (dave = 4.9 nm) and coinage Cu2 S NCs (dave = 5.2 ± 0.36 nm, t = 3.2 nm) were estimated to be 1.63 eV(direct) and 1.37 (indirect; 1.78 (direct)), respectively. Emax values of CuS and Cu2 S NCs obtained from Gaussian fitting are 1.14 and 0.61 eV.
4. Discussion Based upon the above-mentioned results, we obtained the following reaction procedures; when the synthesis of copper sulfide NCs were conducted using the S/DT solution, the brown precipitates “chalcocite Cu2 S” were obtained. Sulfur was reduced by DT to form H2 S, which was solvated by DT molecules. Schemes are summarized as follows: 2RSH + S ⇔ (RSH)2 S ⇔ H2 S + RSSR
(1)
This conclusion is confirmed by the fact that hydrogen sulfide smell was slightly perceived in any sulfur solution after addition of DT. Then, Cu(II) ions, which were solvated by ligands, reacted with H2 S to form copper sulfide NCs. The formation of Cu2 S phase indicates that the precipitation of copper sulfide NCs was accompanied with the reduction processes of Cu(II) ion, which are represented as follows [8,11]: 2Cu2+ + 2RSH = 2Cu+ + RSSR + 2H+
(2)
2Cu2+ + 2S2− = Cu2 S + S0
(3)
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In the case of S/OA system, the formation of sulfur anion is considered to be supplied by the decomposition of S–OA complex. XPS analysis revealed that CuS “covellite” contains the oxidation sulfur S2 2− component “Cu-defect”, which may cause the surface roughening and the formation of irregular shape NCs. Indeed, the reduction of S0 to S5 2− : 5S + 2e = S5 2− (−0.34 eV) is energetically more favorable than to S2− (−0.48 eV). Ligands (such as DT and OA), which have an electron donor character, serve as a reduction agent for sulfur. In the case of alkylamine, it is found that sulfur reduction requires higher reaction temperature. The proportion of sulfur species (S0 , S2− , Sn 2− , etc.) in reaction system depends upon reaction temperature and a reduction ability of ligand or its mixture. These results indicate that DT has the nature to be stronger reduction agent for sulfur. The ligands mixture plays alternative important roles to control the sulfuration rate. The stability and sulfuration rate of copper sulfide(Cun Sm ) NCs represent a product of activities of copper and sulfur ions (e.g. the solubility product: Ksp = [Cu+ ]n [S2− ]m ). Since DT and OA serve as masking agents to copper ion, the sulfuration rate depends strongly on the strength of ligand-Cu ion interaction. OA reacts with DT to form an DT-OA association and serve as a de-proton agent for H2 S. Actually, the sulfuration reaction without oleylamine proceeded very slowly and formed irregular shape Cu2 S NCs. These effects lead to the decrease in the activity of DT (strong ligand) and the increase in the reactivity of sulfur source. UV–vis spectra exhibit the little change in the UV-blue spectra feature. However, it is found that interband structure of Cux S NCs is affected by Cu/S composition. In CuS NCs, the optical absorption peak is observed in the near IR region, corresponding to the interband transition. The XPS results suggest that the interband is attributed to the introduction of the impurity state of S2 2− . Silvester et al. reported that CuS (covellite) NCs had absorption band whose absorption maxima Emax was at ∼920 nm (bulk material 1200 nm), and it was not affected by its composition [8]. However, Emax of Cu2 S NCs (Sa.2) was quite different from that of CuS (Sa.4). It is reasonable that Emax depends on the crystalline structure, because the different crystal structures and lattice constants of Cu2 S and CuS give rise to the different electrostatic field in which electrons move. 5. Conclusion We synthesized monodispersed Cu2 S and CuS NCs at nearly ambient temperature. Dodecanethiol and oleylamine mixture (ligand mixture) enable us to use low toxic, stable and low
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cost material, as metal and sulfur source. Ligand mixture also plays important roles to control Cu/S composition on NCs. Using S/DT solution led to the precipitation of Cu2 S “Chalcocite” phase. When the synthesis of Cux S NCs was conducted using S/OA solution, CuS NCs were formed. This compositional change of Cux S NCs was attributed to the introducing of Sn 2− species into NCs. Therefore, the proportion of Cu-defect in Cux S NCs may be controlled by an appropriate choice of ligand. Acknowledgements This work was supported by a grant form the NITECH 21st Century COE Program, “World Ceramics Center for Environmental Harmony”, Meijo University Open Research Center project and Intellectual Cluster Project of Aichi-Nagoya Area given by the Ministry of Education, Science, Culture and Sports, Japan. References [1] (a) V.I. Klimov, P. Haring-Bolivar, H. Kurz, V.A. Karavanskii, Superlattice Microstruct. 20 (1996) 395; (b) V. Klimov, P. Haring Bolivar, H. Kurz, V. Karavanskii, V. Krasovskii, Yu. Korkishko, Appl. Phys. Lett. 67 (1995) 653. [2] (a) L. Reijnen, B. Meester, A. Goossens, J. Schoonman, Mater. Sci. Eng. 19 (2002) 311; (b) Y. Lou, A.C.S. Samia, J. Cowen, K. Banger, X. Chen, H. Lee, C. Burda, Phys. Chem. Chem. Phys. 5 (2003) 1091. [3] (a) http://pc.watch.impress.co.jp/docs/2004/0216/nano.htm; (b) K. Terabe, T. Hasegawa, T. Nakayama, M. Aono, Mater. Jpn. 44 (2005) 757, and references therein. [4] R.V. Gains, H.C.W. Skinner, E.E. Foord, B. Mason, A. Rosenzweig, W.T. King, E. Gowty, Dana’s New Mineralogy, Wiley, New York, 1997, p. 1819. [5] (a) H. Zhang, G. Wu, X. Chang, Langmuir 21 (2005) 4281; (b) A. Ghezelbash, B.A. Korgel, Langmuir 21 (2005) 9451. [6] (a) T.H. Larsen, M. Sigman, A. Ghezelbash, C. Doty, B.A. Korgel, J. Am. Chem. Soc. 125 (2003) 5638; (b) M.B. Sigman, A. Ghezelbash, T. Hanras, A.E. Saunders, F. Lee, B.A. Korgel, J. Am. Chem. Soc. 125 (2003) 16050. [7] H.T. Evans Jr., J.A. Konnert, Am. Mineral. 61 (1976) 996. [8] E.J. Silvester, F. Grieser, B.A. Sexton, T.W. Healy, Langmuir 7 (1991) 2917. [9] E.Z. Kurmaev, J. van EK, D.L. Ederer, L. Zhou, T.A. Callcott, R.C.C. Perera, V.M. Cherkashenko, S.N. Shamin, V.A. Trofimova, S. Bartkowski, M. Neumann, A. Fujimori, V.P. Moloshang, J. Phys. Condens. Matter 10 (1998) 1687. [10] (a) S.A. Zolotovskaya, V.G. Savitski, P.V. Prokoshin, K.V. Yumashev, J. Opt. Soc. Am. B 23 (2006) 1268; (b) M.C. Brelle, C.L. Torres-Martinez, J.C. McNulty, R.K. Mehra, J.Z. Zhang, Pure Appl. Chem. 72 (2000) 101; (c) S.K. Haram, A.R. Mahadeshwar, S.G. Dixit, J. Phys. Chem. 100 (1996) 5868. [11] Y. Negishi, H. Murayama, T. Tsukuda, Chem. Phys. Lett. 366 (2002) 561, and references therein.