Ultrasonics Sonochemistry 21 (2014) 1194–1199
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
CuGaS2 hollow spheres from Ga–CuS core–shell nanoparticles Ji-Hyun Cha, Duk-Young Jung ⇑ Department of Chemistry, Center for Human Interface Nanotechnology, SKKU Advanced Institute of Nanotechnology, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
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Article history: Received 21 October 2013 Received in revised form 2 December 2013 Accepted 5 December 2013 Available online 13 December 2013 Keywords: Core–shell nanoparticles Ultrasound synthesis Gallium colloidal CuGaS2 compounds Hollow sphere Kirkendall effect
a b s t r a c t A liquid gallium emulsion was prepared as a starting material using ultrasound treatment in ethylene glycol. Core–shell particles of Ga@CuS were successfully synthesized by deposition of a CuS layer on gallium droplets through sonochemical deposition of copper ions and thiourea in an alcohol media. The core and shell of Ga@CuS products were composed of amorphous gallium metal and covellite phase CuS, which transformed into chalcopyrite CuGaS2 hollow spheres after sulfurization at 450 °C, which was the lowest crystallization temperature. The formation of hollow nanostructures was ascribed to the Kirkendall mechanism, in which liquid gallium particles play an important role as reactive templates. In conclusion, we obtained CuGaS2 hollow spheres with a 430 nm outer diameter and 120 nm shell thickness that had the same crystal structure and electrical properties as bulk CuGaS2. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite compounds, particularly Cu(In1–xGax)(S2–ySey) (CIGSSe), are well known compound semiconductors and lightabsorbing materials for thin film photovoltaic cells and have received increasing attention as alternatives to single crystalline silicon cells due to their direct band gap, high absorption coefficient, tunability of optical band gap energies, and long-term stability [1]. Recently, CIGSSe-based cells have been reported with light conversion efficiencies greater than 20% and close to that of crystalline silicon performance [2]. CuGaS2 is one of the most promising absorber materials for generating high voltages because of its large direct band gap of 2.49 eV [3,4]. The synthesis and characterization of nanosized CuGaS2 materials are interesting in many ways, including control of chemical composition, crystal phase, and morphology [5,6]. Likewise, while nanosized CuGaS2 materials can be prepared by various synthetic routes [5–8], there remains considerable room to study synthetic methods for new multicomponent nanostructures such as Core–shells consisting of hollow spheres of CuGaS2 compounds or precursors. Recent developments in sonochemical syntheses have allowed for preparation of various nanostructures such as Core–shells [9,10] and hollow structures [11–13]. Specifically, irradiation of various liquid systems with ultrasonic energy induces acoustic cavitation that disperses liquid metal into small droplets [14,15]
⇑ Corresponding author. Tel.: +82 31 290 7074; fax: +82 31 290 7075. E-mail address: [email protected] (D.-Y. Jung). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.12.004
resulting in the synthesis of metal sulfides and metal oxides, etc [16,17]. An important advantage of sonochemical synthesis methods compared with other approaches is the ability to produce phase pure compounds. In previous reports, metal and alloy microspheres were produced by ultrasonic cavitation in silicon oil [15]. However, gallium spheres prepared in silicon oil have sub-micron diameters ranging from a fraction of a micrometer to 100 lm. In the present study, we obtained nanosized gallium metal spheres as a result of the low surface tension and high viscosity of ethylene glycol. Dispersion of gallium nanoparticles in polar solvents using ultrasound has not been previously reported. However, a dispersion method of gallium nanoparticles was reported by Meléndrez et al. [18] using chemical liquid deposition (CLD) that included physical vapor deposition of metallic gallium in organic solvents. Although gallium nanoparticles approximately 10 nm in size were synthesized by CLD, this method is inefficient due to the extreme synthetic conditions consisting of an inert atmosphere at 77 K. Here, we describe a facile route for generating Ga@CuS Core– shell particles by employing an ultrasound process and transformation with novel hollow-structured chalcopyrite compounds. First, Ga metal nanoparticles were prepared by dispersing a liquid metal drop with ultrasound irradiation. Second, CuS-coated Ga nanoparticles were synthesized using Ga nanoparticles, CuSO4, and thiourea by a sonochemical process. Finally, Ga@CuS particles were transformed into hollow CuGaS2 spheres by sulfurization at 450 °C.
J.-H. Cha, D.-Y. Jung / Ultrasonics Sonochemistry 21 (2014) 1194–1199
2. Experimental section 2.1. Materials Copper sulfate pentahydrate (CuSO45H2O, Aldrich, P98%), gallium metal (9Digit Co., Ltd, 99.9999%), thiourea (Aldrich, P99.0%), ethylene glycol (Aldrich, 99%) and 1-hexanol (Aldrich) were used as received without further purification. 2.2. Synthesis of Ga@CuS core shell nanoparticles A gallium metallic emulsion was prepared by adding gallium metal (2.2 mmol) to 30 ml of ethylene glycol (EG), which was then dispersed with a high–power ultrasonic generator (20 kHz, 500KW, Sonics & Materials Inc. VC-505) for 30 min. CuSO45H2O (2.2 mmol) was added to the gallium emulsion and mixed under ultrasound for 30 min. Finally, thiourea (4.4 mmol) was dissolved in 10 ml of the EG solvent, after which the thiourea solution was injected into the mixture under ultrasound for 1 hour at 90 °C. The resulting reaction mixture became greenish-black in color, and was cooled down to room temperature. The resulting precipitates were separated by filtration. 2.3. Transformation into CuGaS2 hollow spheres Ga@CuS nanoparticles were deposited onto glass substrates by dropping the colloidal solution in 1-hexanol with the particles at a concentration of 10 mg/mL. The as-deposited films were then dried in a vacuum chamber overnight. The sulfurization process was conducted in a rapid thermal annealing system for 1 hour under a H2S/Argon mixed atmosphere with 1% H2S (Scheme 1). 2.4. Characterization X-ray diffraction (XRD) data were obtained with a Rigaku Ultima IV X-ray diffractometer at 40KV and 30 mA with Cu-Ka radiation (k = 1.5405 Å). The morphologies and size distributions of the samples were studied by scanning electron microscopy (SEM, Philips, XL30) and a dynamic light scattering size analyzer (DLS, Malven, Zetasizer 1000HS). Elemental analysis and mapping was performed by X-ray fluorescence (XRF) and energy dispersive X-ray spectroscopy (EDS) attached to a high-resolution transmission electron microscope (HR-TEM, JEOL, JEM ARM 200F), respectively. Raman spectroscopy measurements were carried out with a WITec ALPHA300 instrument using a 532 nm laser source. Diffuse reflectance spectroscopy was performed with a UV-visNIR spectrophotometer (Shimadze, UV-3600) equipped with an IRS-300 integrating sphere attachment. 3. Results and discussion After ultrasound treatment, the ethylene glycol solution of gallium metal turned opaque with a gray color because of good dispersion of the gallium metal. The resulting suspension was stable for a few hours without precipitation. As shown in Fig. 1,
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SEM images demonstrated the spherical shape and irregular size of the prepared gallium particles. We noted that the sample preparation steps for SEM measurement, which consisted of solvent drying and a high vacuum process, may have modified the morphology of the particles. The size distribution of gallium particles measured by dynamic light scattering also revealed that the prepared colloidal solution contained gallium particles smaller than 1 micrometer, the majority of which were in the range of 200–600 nm. The spherical particles of gallium had an average diameter of 344(5) nm and the largest population of particle sizes was 310(5) nm. The gallium emulsion of 344 nm was exclusively prepared by sonication. Although gallium metal melts, the emulsion could not be produced by other methods because of its high surface tension and viscosity. The ultrasound treatments should be applied in order to form spherical Ga nanoparticles in liquid medium. Under ultrasound waves, cavitation and implosion play important roles in the formation of low melting point metal colloidal solutions. Indeed, the dispersion of liquid metal in the form of a small droplet in the solvent initiates the process of cavitation erosion. Specifically, the erosion of metal results from asymmetric collapse of single bubbles attached to, or near, the surface of the metal, which produces a shockwave and liquid jet, the physical force of which deforms and breaks up the metal surface [19]. Therefore, ultrasound energy breaks up gallium metal into smaller spheres with a broad size distribution, which is followed by quenching the liquid mixture at a temperature lower than the melting point of gallium to produce solid particles. When the prepared colloidal solutions were cooled below the melting point of gallium, the size of gallium nanoparticles was stable over a period of three days. When CuSO45H2O powder was mixed with gallium emulsion under ultrasound, color change and other precipitations were not observed. The addition of a thiourea solution with CuSO4-Ga in ethylene glycol under ultrasound at 90 °C changed the color of the solution from gray to greenish-black, which was ascribed to the formation of a copper sulfide (CuS) phase. When CuSO4 reacts with thiourea, not only ultrasound but also thermal treatment higher than 90 °C was needed. If the reaction temperature was lower than 90 °C, CuS was not synthesized under irradiation of ultrasound. The ultrasound treatments should be applied in order to form spherical Ga nanoparticles in liquid medium. Fig. 2(a) shows deposits of copper sulfide approximately 90 nm thick that were deposited on the 500 nm diameter gallium droplets, which were denoted as Ga@CuS. The elemental map for the Ga@CuS nanoparticles shown in Fig. 2(b–d) clearly demonstrated a Core–shell structure with gallium in the core region and copper sulfide (copper and sulfur atom) in the shell. The SEM images of product with and without ultrasound treatments are shown in Fig. 3. The nanoparticles synthesized with ultrasound has good uniformity of core shell structure and dispersibility in ethanol. However, the products prepared without ultrasound treatments show aggregation of Ga and CuS and poor dispersibility in ethanol. Without ultrasound, first 150 °C was the minimum temperature for the reaction of CuSO4 and thiourea, second the core shell structure of Ga@CuS was not prepared in homogeneous manner.
Scheme 1. Schematic process of synthesis of Ga@CuS Core–shell nanoparticles and hollow CuGaS2 particles.
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Fig. 1. (a) SEM image and (b) size distribution data of as-prepared gallium colloidal.
Fig. 2. (a) TEM image and (b–d) elemental maps of Ga@CuS nanoparticles.
When CuSO45H2O and the Ga emulsion were mixed, Cu2+ ions bound to the surface of Ga because Ga particles carry a negative surface charge. Specifically, the average zeta potential value of Ga dispersed in EG was measured as 28.6 mV. When a thiourea solution was added to the mixture, the Cu2+ ions reacted with S2 ions, which resulted in the release from decomposition of thiourea on the surface of Ga droplets. No other compounds were observed at the interface of gallium and copper sulfide, such as those that may have formed from incorporation of gallium, copper and sulfur, including Cu-Ga alloys and GaS compounds. In addition, neither copper oxide nor gallium oxide was detected despite performing the synthesis under ambient conditions. Mixtures of digenite Cu1.8S (rhombohedral) and covellite CuS (hexagonal) have been synthesized previously from CuSO45H2O and thiourea in ethylene glycol [20]. Further, covellite CuS compounds are predominantly
obtained when the oxidizing power of the counter anion is increased. Thus, we obtained monophasic covellite CuS because the gallium metal increased the oxidation power of our reaction system. The powder XRD patterns of gallium colloidal and Ga@CuS Core–shell nanoparticles are shown in Fig. 4. The gallium colloidal exhibited a broad peak indicating an amorphous structure; however, the XRD peaks of the Core–shell particles showed distinct peaks related to covellite CuS [JCPDS 06-0464] and amorphous gallium. All of the diffraction peaks related with CuS could be indexed with the hexagonal covellite structure (JCPDS 06-0464) with a = 3.810(3) Å and c = 16.40(2) Å, which were calculated by the least squares method. The gallium droplets did not combine with copper and sulfur, and the products demonstrated a clear Ga@CuS Core–shell structure. The average chemical composition of Ga@CuS
J.-H. Cha, D.-Y. Jung / Ultrasonics Sonochemistry 21 (2014) 1194–1199
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Fig. 3. SEM images of Ga@CuS nanoparticles (a) with ultrasound and (b) without ultrasound. The inset images are digital photographs of the products dispersed in ethanol after 5 min.
Fig. 4. The powder X-ray diffraction patterns of (a) gallium colloidal and (b) Ga@CuS Core–shell nanoparticles. (CuS data came from JCPDS #06-0464.)
analyzed by XRF was 34.6% Cu, 32.6% Ga, and 32.5% S. While the starting materials had a 1:1:2 molar ratio of Cu:Ga:S, the chemical formula of products was suggested to be [email protected]. We next characterized the optical properties of Ga@CuS nanocomposites by photoluminescence (PL) spectroscopy. The typical emission band of covellite CuS appears to range from 388 nm to 440 nm in PL spectra [20]; however, the Ga@CuS sample did not exhibit an emission band. We ascribed this effect to conventional PL quenching, which appears frequently with increasing quantity of core metal. The PL quenching observed in our study may have arisen from significant electron transfer from the excited CuS shell to the Ga metal core [21]. Fig. 5 shows the XRD patterns of Ga@CuS samples sulfurized under an H2S/Ar gas atmosphere in a rapid thermal annealing system at different temperatures. When the Ga@CuS samples were heated at 250 °C (Fig. 5(a)), the XRD patterns remained the same as pristine Ga@CuS. After sulfurization at 300 °C and 400 °C for 60 min (Fig. 5(b and c)), the Ga@CuS nanoparticles were transformed into mixed phases of CuGaS2 (tetragonal) and tetragonal CuGa2 alloy (JCPDS 65-1636). Although the sample was heated at 400 °C for 6 hours, XRD peaks associated with the CuGa2 phase were still observed, indicating that the oxidation reaction of gallium cores to gallium sulfide was thermodynamically unfavorable and a rate-determining step. Fig. 5(d) shows the XRD pattern for CuGaS2 obtained after sulfurization at 450 °C, which was similar to that of single-phase CuInSe2 chalcopyrite [22]. The lattice
Fig. 5. The powder X-ray diffraction patterns of Ga@CuS were sulfurized at (a) 250, (b) 300, (c) 400, (d) 450 and (e) 500 °C in H2S/Ar atmosphere for 60 min.
parameters for the as-prepared CuGaS2 were refined by the least square method, a = b = 5.35(3) Å and c = 10.54(1) Å, and were in good agreement with values reported in the literature for CuGaS2 (a = b = 5.52279(7) Å and c = 10.47429(6) Å) [23]. After heat treatment above 450 °C, the XRD peaks corresponding to CuGa2 phase disappeared and the sample completely transformed into a chalcopyrite CuGaS2 phase without detectable side products (Fig. 5(e)). Although Ga@CuS began to transform into CuGaS2 by sulfurization starting at 300 °C, samples treated below 450 °C involved the CuGa2 alloy compound. The Core–shell structure developed a phase separation of two layers, namely CuGa2 alloy in the core and CuGaS2 in the outer layer due to diffusion of the gas phase sulfur source. Ga@CuS films were prepared by drop-casting an ink solution of Ga@CuS dispersed in 1-hexanol on soda-lime glass substrates followed by drying as shown in Fig. 6(a). The average thickness of the Ga@CuS layer was about 2.5 lm. The as-prepared Ga@CuS particles consisted of spherical shapes approximately 400 nm in size, the surfaces of which were coated by a CuS phase with a size of approximately 100 nm as shown in Fig. 6(c). Interestingly, a hollow structure of the CuGaS2 particles was obtained by sulfurization at 450 °C as shown in Figs. 6(b) and (d). The hollow particles manifested as a uniform shell thickness, as shown in Fig. 6(d). The products consisted of spherical particles with a diameter of 430 nm and shell thickness of approximately 120 nm with a void volume of about 7.6% v/v. The average ratio (R1/R2) of hollow spheres for
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Fig. 6. The SEM images of (a and c) as-prepared Ga@CuS nanoparticle and (b and d) annealed samples at 450 °C.
the inner radius (R1) and outer radius (R2) was 0.461. When average sized Ga metal nanoparticles (344 nm) were transformed entirely into chalcopyrite CuGaS2 hollow spheres with an R1/R2 ratio of 0.461, the diameter of the hollow sphere and shell thickness was calculated as 454 nm and 123 nm, respectively, which were consistent with measured values. Thus, CuGaS2 hollow spheres were obtained after consuming all of the gallium metal in Ga@CuS. The central void was formed as a result of Ga out-diffusion. The size of Ga nanoparticles is closely related with the diameter of the void. The 344 nm Ga nanoparticles transformed 190 nm void, which is smaller than Ga nanoparticles. The volume of the unit cell expended to 192% in the transformation of CuS into CuGaS2. The gallium colloidal particles served as reactive templates, which were consumed completely in the sulfurization process at high temperature as a result of the Kirkendall effect. The use of
reactive templates is a general synthetic method of metal sulfides and hollow oxide spheres because it does require a template removal step [24]. In our study, the central void was formed as a result of a discrepancy in diffusion rates between the Ga core and CuS shell. Likewise, the outward flow of Ga to the CuS shell was due to a coupled reaction-diffusion phenomena at the CuS/Ga and CuS/H2S interface, leading to synthesis of hollow CuGaS2 spheres [25]. Previously, Cu@In2Se3 nanocomposites prepared by a sonochemical method were transformed into CuInSe2 nanoparticles without distinguishing the nanostructure after thermal treatment [26]. The difference between this observation and our results was attributed to the higher rate of diffusion of Cu compared with Ga. The Raman spectra of (a) as-prepared Ga@CuS and (b) CuGaS2 hollow samples are shown in Fig. 7. The Raman spectra of the
Fig. 7. Raman spectra of (a) Ga@CuS and (b) hollow CuGaS2 particles prepared at 450 °C.
Fig. 8. Tauc plot for the CuGaS2 compounds on photon energy plotted as (htF(R1))2 versus photon energy ht. The inset is diffuse reflectance UV-vis spectrum of CuGaS2 compounds.
J.-H. Cha, D.-Y. Jung / Ultrasonics Sonochemistry 21 (2014) 1194–1199
as-prepared Ga@CuS showed a single peak at 479.1 cm1, which is the characteristic vibration mode of the covellite CuS phase [27]. Peaks of solid gallium and other binary compounds were not observed [28]. The Raman spectra of the samples treated at 450 °C revealed the presence of chalcopyrite CuGaS2 compounds. Likewise, the peaks at the 313.2 and 388.7 cm1 were related to the A1 and E vibration modes from the chalcopyrite structure [29]. The band gap energy of the prepared CuGaS2 compounds at 450 °C were measured using a diffuse reflectance UV-vis spectrometer equipped with an integrating sphere, as shown in Fig. 8. The band gap energy determined from Tauc plots was 2.49 eV, which was consistent with the energy of corresponding bulk compounds [30]. This result indicated that the phase transformation from Ga@CuS Core–shell nanoparticles into single-phase CuGaS2 compounds occurred by H2S/Ar atmosphere annealing at 450 °C. 4. Conclusions We used an ultrasound method to prepare gallium colloidal and Ga@CuS nanomaterials. Irradiation with ultrasound induced formation of a gallium emulsion with good stability and particles approximately 300 nm in size. In particular, because of the surface tension and viscosity of ethylene glycol, we were able to synthesize a small sized gallium colloidal mixture without the need for an organic surfactant. Sonochemical reaction of copper and sulfur ions at the surface of gallium droplets allowed for the synthesis of Core–shell structured materials with a Ga metal core and CuS outer shell. Upon sulfurization at 450 °C, the gallium metal moved to the outer surface of particles, resulting in the formation of hollowstructured chalcopyrite CuGaS2 compounds. Owing to the formation of CuGa2 below 450°, heat treatment higher than this temperature was required to obtain single phase CuGaS2. In conclusion, preparing Core–shell particles with a low-melting point metal core by ultrasound may be widely applicable to hollowstructured chalcopyrite compounds of various compositions. Acknowledgements This work was supported by the Basic Science Research Program (NRF-2009-0094023 and NRF-2009-0083540), the ‘‘National Agenda Project’’ program of Korea Research Council of Fundamental Science & Technology (2N36870-13-164, KRCF) and the Industrial Strategic Technology Development Program (10033436, KEIT). References [1] S. Hegedus, Thin film solar modules: the low cost, high throughput and versatile alternative to Si wafers, Prog. Photovoltaics Res. Appl. 14 (2006) 393–411. [2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, New world record efficiency for Cu(In, Ga)Se2 thin-film solar cells beyond 20%, Prog. Photovoltaics Res. Appl. 19 (2011) 894–897. [3] J.Q. Hu, B. Deng, C.R. Wang, K.B. Tang, Y.T. Qian, Hydrothermal preparation of CuGaS2 crystallites with different morphologies, Solid State Commun. 121 (2002) 493–496. [4] S.K. Kim, J.P. Park, M.K. Kim, K.M. Ok, I. Shim, Preparation of CuGaS2 thin films by two-stage MOCVD method, Sol. Energy Mater. Sol. Cells 92 (2008) 1311–1314.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
CuGaS2 hollow spheres from Ga–CuS core–shell nanoparticles Ji-Hyun Cha, Duk-Young Jung ⇑ Department of Chemistry, Center for Human Interface Nanotechnology, SKKU Advanced Institute of Nanotechnology, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 2 December 2013 Accepted 5 December 2013 Available online 13 December 2013 Keywords: Core–shell nanoparticles Ultrasound synthesis Gallium colloidal CuGaS2 compounds Hollow sphere Kirkendall effect
a b s t r a c t A liquid gallium emulsion was prepared as a starting material using ultrasound treatment in ethylene glycol. Core–shell particles of Ga@CuS were successfully synthesized by deposition of a CuS layer on gallium droplets through sonochemical deposition of copper ions and thiourea in an alcohol media. The core and shell of Ga@CuS products were composed of amorphous gallium metal and covellite phase CuS, which transformed into chalcopyrite CuGaS2 hollow spheres after sulfurization at 450 °C, which was the lowest crystallization temperature. The formation of hollow nanostructures was ascribed to the Kirkendall mechanism, in which liquid gallium particles play an important role as reactive templates. In conclusion, we obtained CuGaS2 hollow spheres with a 430 nm outer diameter and 120 nm shell thickness that had the same crystal structure and electrical properties as bulk CuGaS2. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite compounds, particularly Cu(In1–xGax)(S2–ySey) (CIGSSe), are well known compound semiconductors and lightabsorbing materials for thin film photovoltaic cells and have received increasing attention as alternatives to single crystalline silicon cells due to their direct band gap, high absorption coefficient, tunability of optical band gap energies, and long-term stability [1]. Recently, CIGSSe-based cells have been reported with light conversion efficiencies greater than 20% and close to that of crystalline silicon performance [2]. CuGaS2 is one of the most promising absorber materials for generating high voltages because of its large direct band gap of 2.49 eV [3,4]. The synthesis and characterization of nanosized CuGaS2 materials are interesting in many ways, including control of chemical composition, crystal phase, and morphology [5,6]. Likewise, while nanosized CuGaS2 materials can be prepared by various synthetic routes [5–8], there remains considerable room to study synthetic methods for new multicomponent nanostructures such as Core–shells consisting of hollow spheres of CuGaS2 compounds or precursors. Recent developments in sonochemical syntheses have allowed for preparation of various nanostructures such as Core–shells [9,10] and hollow structures [11–13]. Specifically, irradiation of various liquid systems with ultrasonic energy induces acoustic cavitation that disperses liquid metal into small droplets [14,15]
⇑ Corresponding author. Tel.: +82 31 290 7074; fax: +82 31 290 7075. E-mail address: [email protected] (D.-Y. Jung). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.12.004
resulting in the synthesis of metal sulfides and metal oxides, etc [16,17]. An important advantage of sonochemical synthesis methods compared with other approaches is the ability to produce phase pure compounds. In previous reports, metal and alloy microspheres were produced by ultrasonic cavitation in silicon oil [15]. However, gallium spheres prepared in silicon oil have sub-micron diameters ranging from a fraction of a micrometer to 100 lm. In the present study, we obtained nanosized gallium metal spheres as a result of the low surface tension and high viscosity of ethylene glycol. Dispersion of gallium nanoparticles in polar solvents using ultrasound has not been previously reported. However, a dispersion method of gallium nanoparticles was reported by Meléndrez et al. [18] using chemical liquid deposition (CLD) that included physical vapor deposition of metallic gallium in organic solvents. Although gallium nanoparticles approximately 10 nm in size were synthesized by CLD, this method is inefficient due to the extreme synthetic conditions consisting of an inert atmosphere at 77 K. Here, we describe a facile route for generating Ga@CuS Core– shell particles by employing an ultrasound process and transformation with novel hollow-structured chalcopyrite compounds. First, Ga metal nanoparticles were prepared by dispersing a liquid metal drop with ultrasound irradiation. Second, CuS-coated Ga nanoparticles were synthesized using Ga nanoparticles, CuSO4, and thiourea by a sonochemical process. Finally, Ga@CuS particles were transformed into hollow CuGaS2 spheres by sulfurization at 450 °C.
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2. Experimental section 2.1. Materials Copper sulfate pentahydrate (CuSO45H2O, Aldrich, P98%), gallium metal (9Digit Co., Ltd, 99.9999%), thiourea (Aldrich, P99.0%), ethylene glycol (Aldrich, 99%) and 1-hexanol (Aldrich) were used as received without further purification. 2.2. Synthesis of Ga@CuS core shell nanoparticles A gallium metallic emulsion was prepared by adding gallium metal (2.2 mmol) to 30 ml of ethylene glycol (EG), which was then dispersed with a high–power ultrasonic generator (20 kHz, 500KW, Sonics & Materials Inc. VC-505) for 30 min. CuSO45H2O (2.2 mmol) was added to the gallium emulsion and mixed under ultrasound for 30 min. Finally, thiourea (4.4 mmol) was dissolved in 10 ml of the EG solvent, after which the thiourea solution was injected into the mixture under ultrasound for 1 hour at 90 °C. The resulting reaction mixture became greenish-black in color, and was cooled down to room temperature. The resulting precipitates were separated by filtration. 2.3. Transformation into CuGaS2 hollow spheres Ga@CuS nanoparticles were deposited onto glass substrates by dropping the colloidal solution in 1-hexanol with the particles at a concentration of 10 mg/mL. The as-deposited films were then dried in a vacuum chamber overnight. The sulfurization process was conducted in a rapid thermal annealing system for 1 hour under a H2S/Argon mixed atmosphere with 1% H2S (Scheme 1). 2.4. Characterization X-ray diffraction (XRD) data were obtained with a Rigaku Ultima IV X-ray diffractometer at 40KV and 30 mA with Cu-Ka radiation (k = 1.5405 Å). The morphologies and size distributions of the samples were studied by scanning electron microscopy (SEM, Philips, XL30) and a dynamic light scattering size analyzer (DLS, Malven, Zetasizer 1000HS). Elemental analysis and mapping was performed by X-ray fluorescence (XRF) and energy dispersive X-ray spectroscopy (EDS) attached to a high-resolution transmission electron microscope (HR-TEM, JEOL, JEM ARM 200F), respectively. Raman spectroscopy measurements were carried out with a WITec ALPHA300 instrument using a 532 nm laser source. Diffuse reflectance spectroscopy was performed with a UV-visNIR spectrophotometer (Shimadze, UV-3600) equipped with an IRS-300 integrating sphere attachment. 3. Results and discussion After ultrasound treatment, the ethylene glycol solution of gallium metal turned opaque with a gray color because of good dispersion of the gallium metal. The resulting suspension was stable for a few hours without precipitation. As shown in Fig. 1,
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SEM images demonstrated the spherical shape and irregular size of the prepared gallium particles. We noted that the sample preparation steps for SEM measurement, which consisted of solvent drying and a high vacuum process, may have modified the morphology of the particles. The size distribution of gallium particles measured by dynamic light scattering also revealed that the prepared colloidal solution contained gallium particles smaller than 1 micrometer, the majority of which were in the range of 200–600 nm. The spherical particles of gallium had an average diameter of 344(5) nm and the largest population of particle sizes was 310(5) nm. The gallium emulsion of 344 nm was exclusively prepared by sonication. Although gallium metal melts, the emulsion could not be produced by other methods because of its high surface tension and viscosity. The ultrasound treatments should be applied in order to form spherical Ga nanoparticles in liquid medium. Under ultrasound waves, cavitation and implosion play important roles in the formation of low melting point metal colloidal solutions. Indeed, the dispersion of liquid metal in the form of a small droplet in the solvent initiates the process of cavitation erosion. Specifically, the erosion of metal results from asymmetric collapse of single bubbles attached to, or near, the surface of the metal, which produces a shockwave and liquid jet, the physical force of which deforms and breaks up the metal surface [19]. Therefore, ultrasound energy breaks up gallium metal into smaller spheres with a broad size distribution, which is followed by quenching the liquid mixture at a temperature lower than the melting point of gallium to produce solid particles. When the prepared colloidal solutions were cooled below the melting point of gallium, the size of gallium nanoparticles was stable over a period of three days. When CuSO45H2O powder was mixed with gallium emulsion under ultrasound, color change and other precipitations were not observed. The addition of a thiourea solution with CuSO4-Ga in ethylene glycol under ultrasound at 90 °C changed the color of the solution from gray to greenish-black, which was ascribed to the formation of a copper sulfide (CuS) phase. When CuSO4 reacts with thiourea, not only ultrasound but also thermal treatment higher than 90 °C was needed. If the reaction temperature was lower than 90 °C, CuS was not synthesized under irradiation of ultrasound. The ultrasound treatments should be applied in order to form spherical Ga nanoparticles in liquid medium. Fig. 2(a) shows deposits of copper sulfide approximately 90 nm thick that were deposited on the 500 nm diameter gallium droplets, which were denoted as Ga@CuS. The elemental map for the Ga@CuS nanoparticles shown in Fig. 2(b–d) clearly demonstrated a Core–shell structure with gallium in the core region and copper sulfide (copper and sulfur atom) in the shell. The SEM images of product with and without ultrasound treatments are shown in Fig. 3. The nanoparticles synthesized with ultrasound has good uniformity of core shell structure and dispersibility in ethanol. However, the products prepared without ultrasound treatments show aggregation of Ga and CuS and poor dispersibility in ethanol. Without ultrasound, first 150 °C was the minimum temperature for the reaction of CuSO4 and thiourea, second the core shell structure of Ga@CuS was not prepared in homogeneous manner.
Scheme 1. Schematic process of synthesis of Ga@CuS Core–shell nanoparticles and hollow CuGaS2 particles.
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Fig. 1. (a) SEM image and (b) size distribution data of as-prepared gallium colloidal.
Fig. 2. (a) TEM image and (b–d) elemental maps of Ga@CuS nanoparticles.
When CuSO45H2O and the Ga emulsion were mixed, Cu2+ ions bound to the surface of Ga because Ga particles carry a negative surface charge. Specifically, the average zeta potential value of Ga dispersed in EG was measured as 28.6 mV. When a thiourea solution was added to the mixture, the Cu2+ ions reacted with S2 ions, which resulted in the release from decomposition of thiourea on the surface of Ga droplets. No other compounds were observed at the interface of gallium and copper sulfide, such as those that may have formed from incorporation of gallium, copper and sulfur, including Cu-Ga alloys and GaS compounds. In addition, neither copper oxide nor gallium oxide was detected despite performing the synthesis under ambient conditions. Mixtures of digenite Cu1.8S (rhombohedral) and covellite CuS (hexagonal) have been synthesized previously from CuSO45H2O and thiourea in ethylene glycol [20]. Further, covellite CuS compounds are predominantly
obtained when the oxidizing power of the counter anion is increased. Thus, we obtained monophasic covellite CuS because the gallium metal increased the oxidation power of our reaction system. The powder XRD patterns of gallium colloidal and Ga@CuS Core–shell nanoparticles are shown in Fig. 4. The gallium colloidal exhibited a broad peak indicating an amorphous structure; however, the XRD peaks of the Core–shell particles showed distinct peaks related to covellite CuS [JCPDS 06-0464] and amorphous gallium. All of the diffraction peaks related with CuS could be indexed with the hexagonal covellite structure (JCPDS 06-0464) with a = 3.810(3) Å and c = 16.40(2) Å, which were calculated by the least squares method. The gallium droplets did not combine with copper and sulfur, and the products demonstrated a clear Ga@CuS Core–shell structure. The average chemical composition of Ga@CuS
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Fig. 3. SEM images of Ga@CuS nanoparticles (a) with ultrasound and (b) without ultrasound. The inset images are digital photographs of the products dispersed in ethanol after 5 min.
Fig. 4. The powder X-ray diffraction patterns of (a) gallium colloidal and (b) Ga@CuS Core–shell nanoparticles. (CuS data came from JCPDS #06-0464.)
analyzed by XRF was 34.6% Cu, 32.6% Ga, and 32.5% S. While the starting materials had a 1:1:2 molar ratio of Cu:Ga:S, the chemical formula of products was suggested to be [email protected]. We next characterized the optical properties of Ga@CuS nanocomposites by photoluminescence (PL) spectroscopy. The typical emission band of covellite CuS appears to range from 388 nm to 440 nm in PL spectra [20]; however, the Ga@CuS sample did not exhibit an emission band. We ascribed this effect to conventional PL quenching, which appears frequently with increasing quantity of core metal. The PL quenching observed in our study may have arisen from significant electron transfer from the excited CuS shell to the Ga metal core [21]. Fig. 5 shows the XRD patterns of Ga@CuS samples sulfurized under an H2S/Ar gas atmosphere in a rapid thermal annealing system at different temperatures. When the Ga@CuS samples were heated at 250 °C (Fig. 5(a)), the XRD patterns remained the same as pristine Ga@CuS. After sulfurization at 300 °C and 400 °C for 60 min (Fig. 5(b and c)), the Ga@CuS nanoparticles were transformed into mixed phases of CuGaS2 (tetragonal) and tetragonal CuGa2 alloy (JCPDS 65-1636). Although the sample was heated at 400 °C for 6 hours, XRD peaks associated with the CuGa2 phase were still observed, indicating that the oxidation reaction of gallium cores to gallium sulfide was thermodynamically unfavorable and a rate-determining step. Fig. 5(d) shows the XRD pattern for CuGaS2 obtained after sulfurization at 450 °C, which was similar to that of single-phase CuInSe2 chalcopyrite [22]. The lattice
Fig. 5. The powder X-ray diffraction patterns of Ga@CuS were sulfurized at (a) 250, (b) 300, (c) 400, (d) 450 and (e) 500 °C in H2S/Ar atmosphere for 60 min.
parameters for the as-prepared CuGaS2 were refined by the least square method, a = b = 5.35(3) Å and c = 10.54(1) Å, and were in good agreement with values reported in the literature for CuGaS2 (a = b = 5.52279(7) Å and c = 10.47429(6) Å) [23]. After heat treatment above 450 °C, the XRD peaks corresponding to CuGa2 phase disappeared and the sample completely transformed into a chalcopyrite CuGaS2 phase without detectable side products (Fig. 5(e)). Although Ga@CuS began to transform into CuGaS2 by sulfurization starting at 300 °C, samples treated below 450 °C involved the CuGa2 alloy compound. The Core–shell structure developed a phase separation of two layers, namely CuGa2 alloy in the core and CuGaS2 in the outer layer due to diffusion of the gas phase sulfur source. Ga@CuS films were prepared by drop-casting an ink solution of Ga@CuS dispersed in 1-hexanol on soda-lime glass substrates followed by drying as shown in Fig. 6(a). The average thickness of the Ga@CuS layer was about 2.5 lm. The as-prepared Ga@CuS particles consisted of spherical shapes approximately 400 nm in size, the surfaces of which were coated by a CuS phase with a size of approximately 100 nm as shown in Fig. 6(c). Interestingly, a hollow structure of the CuGaS2 particles was obtained by sulfurization at 450 °C as shown in Figs. 6(b) and (d). The hollow particles manifested as a uniform shell thickness, as shown in Fig. 6(d). The products consisted of spherical particles with a diameter of 430 nm and shell thickness of approximately 120 nm with a void volume of about 7.6% v/v. The average ratio (R1/R2) of hollow spheres for
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Fig. 6. The SEM images of (a and c) as-prepared Ga@CuS nanoparticle and (b and d) annealed samples at 450 °C.
the inner radius (R1) and outer radius (R2) was 0.461. When average sized Ga metal nanoparticles (344 nm) were transformed entirely into chalcopyrite CuGaS2 hollow spheres with an R1/R2 ratio of 0.461, the diameter of the hollow sphere and shell thickness was calculated as 454 nm and 123 nm, respectively, which were consistent with measured values. Thus, CuGaS2 hollow spheres were obtained after consuming all of the gallium metal in Ga@CuS. The central void was formed as a result of Ga out-diffusion. The size of Ga nanoparticles is closely related with the diameter of the void. The 344 nm Ga nanoparticles transformed 190 nm void, which is smaller than Ga nanoparticles. The volume of the unit cell expended to 192% in the transformation of CuS into CuGaS2. The gallium colloidal particles served as reactive templates, which were consumed completely in the sulfurization process at high temperature as a result of the Kirkendall effect. The use of
reactive templates is a general synthetic method of metal sulfides and hollow oxide spheres because it does require a template removal step [24]. In our study, the central void was formed as a result of a discrepancy in diffusion rates between the Ga core and CuS shell. Likewise, the outward flow of Ga to the CuS shell was due to a coupled reaction-diffusion phenomena at the CuS/Ga and CuS/H2S interface, leading to synthesis of hollow CuGaS2 spheres [25]. Previously, Cu@In2Se3 nanocomposites prepared by a sonochemical method were transformed into CuInSe2 nanoparticles without distinguishing the nanostructure after thermal treatment [26]. The difference between this observation and our results was attributed to the higher rate of diffusion of Cu compared with Ga. The Raman spectra of (a) as-prepared Ga@CuS and (b) CuGaS2 hollow samples are shown in Fig. 7. The Raman spectra of the
Fig. 7. Raman spectra of (a) Ga@CuS and (b) hollow CuGaS2 particles prepared at 450 °C.
Fig. 8. Tauc plot for the CuGaS2 compounds on photon energy plotted as (htF(R1))2 versus photon energy ht. The inset is diffuse reflectance UV-vis spectrum of CuGaS2 compounds.
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as-prepared Ga@CuS showed a single peak at 479.1 cm1, which is the characteristic vibration mode of the covellite CuS phase [27]. Peaks of solid gallium and other binary compounds were not observed [28]. The Raman spectra of the samples treated at 450 °C revealed the presence of chalcopyrite CuGaS2 compounds. Likewise, the peaks at the 313.2 and 388.7 cm1 were related to the A1 and E vibration modes from the chalcopyrite structure [29]. The band gap energy of the prepared CuGaS2 compounds at 450 °C were measured using a diffuse reflectance UV-vis spectrometer equipped with an integrating sphere, as shown in Fig. 8. The band gap energy determined from Tauc plots was 2.49 eV, which was consistent with the energy of corresponding bulk compounds [30]. This result indicated that the phase transformation from Ga@CuS Core–shell nanoparticles into single-phase CuGaS2 compounds occurred by H2S/Ar atmosphere annealing at 450 °C. 4. Conclusions We used an ultrasound method to prepare gallium colloidal and Ga@CuS nanomaterials. Irradiation with ultrasound induced formation of a gallium emulsion with good stability and particles approximately 300 nm in size. In particular, because of the surface tension and viscosity of ethylene glycol, we were able to synthesize a small sized gallium colloidal mixture without the need for an organic surfactant. Sonochemical reaction of copper and sulfur ions at the surface of gallium droplets allowed for the synthesis of Core–shell structured materials with a Ga metal core and CuS outer shell. Upon sulfurization at 450 °C, the gallium metal moved to the outer surface of particles, resulting in the formation of hollowstructured chalcopyrite CuGaS2 compounds. Owing to the formation of CuGa2 below 450°, heat treatment higher than this temperature was required to obtain single phase CuGaS2. In conclusion, preparing Core–shell particles with a low-melting point metal core by ultrasound may be widely applicable to hollowstructured chalcopyrite compounds of various compositions. Acknowledgements This work was supported by the Basic Science Research Program (NRF-2009-0094023 and NRF-2009-0083540), the ‘‘National Agenda Project’’ program of Korea Research Council of Fundamental Science & Technology (2N36870-13-164, KRCF) and the Industrial Strategic Technology Development Program (10033436, KEIT). References [1] S. Hegedus, Thin film solar modules: the low cost, high throughput and versatile alternative to Si wafers, Prog. Photovoltaics Res. Appl. 14 (2006) 393–411. [2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, New world record efficiency for Cu(In, Ga)Se2 thin-film solar cells beyond 20%, Prog. Photovoltaics Res. Appl. 19 (2011) 894–897. [3] J.Q. Hu, B. Deng, C.R. Wang, K.B. Tang, Y.T. Qian, Hydrothermal preparation of CuGaS2 crystallites with different morphologies, Solid State Commun. 121 (2002) 493–496. [4] S.K. Kim, J.P. Park, M.K. Kim, K.M. Ok, I. Shim, Preparation of CuGaS2 thin films by two-stage MOCVD method, Sol. Energy Mater. Sol. Cells 92 (2008) 1311–1314.
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