ARTICLE pubs.acs.org/JPCC
CuO Barrier Limited Corrosion of Solid Cu2O Leading to Preferential Transport of Cu(I) Ion for Hollow Cu7S4 Cube Formation Mrinmoyee Basu,† Arun Kumar Sinha,† Mukul Pradhan,† Sougata Sarkar,† Govind,‡ and Tarasankar Pal*,† ‡ †
National Physical Laboratory, New Delhi, India Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
bS Supporting Information ABSTRACT: Highly ordered, uniform Cu7S4 hollow cubes have been successfully synthesized in a mild, low-temperature condition from freshly prepared solid Cu2O cubes. Cu2O cubes have been synthesized at ∼80 °C, exploiting the water-soluble Cu(II)EDTA complex (λmax = 730 nm) as precursor and glucose as reducing agent under alkaline conditions. In the synthetic pathway, Cu2O solid cubes act as corrosion-prone, sacrificial templates. Kinetic parameters describe the corrosion of Cu2O solid cubes in the presence of sulfide ions, which is the product of hydrolysis of thioacetamide. Corrosion results in a nonstoichiometric hollow Cu7S4 structure like a solid cubic template. Strong affinity of Cu(I) toward sulfide (“soft”“soft” interaction) fetches Cu(I) from the central region of the solid Cu2O template, making hollow cubes of Cu7S4. Mechanistically, the thin film of the oxidized surface layer on Cu2O cubes protects the template. Then the oxidized layer offers resistance to the passage of sulfide ions for its inward transportation. Conversely, softsoft affinity fetches Cu(I) ions from inside. Finally, hollow Cu7S4 cubes are formed at the solidliquid interface. The transformation process has been further examined and confirmed from UVvisible spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectra, and impedance measurement. Hollow Cu7S4 cubes with increased surface area are generated from solid Cu2O cubes via Kirkendall diffusion.
’ INTRODUCTION To materials scientists, an important goal has been to focus on the development of different ways for fabricating specific morphologies with uniform size because of the morphology and size-dependent various properties of different materials.1 Hollow structures, whatever may be the size, with nanometer or micrometer dimensions represent an important class of materials. Shells of hollow structures may be constructed with the help of various materials of high scientific and technological importance. As a unique nanostructure, hollow nanoparticles exhibit many outstanding features, such as low density, high surface-to-volume ratio, and the effect of void space.2 Hollow structures are currently of great interest because of their wide range of applicability in fillers; catalysts; photonic crystals; artificial cells; delivery vehicle systems; containers for encapsulation; photonic crystals; protection of biologically active agents such as proteins, enzymes, or DNA; and controlled release capsules for drugs.3 Due to having such a wide range of applicability, various methods have been developed for the fabrication of hollow structures of a diverse class of inorganic materials. Among those widely used methods are liquid droplets, latex templates, polymer templates, inorganic nanoparticles, liquid crystals, hard templates, micelles, microemulsion, interface-mineralizing, and in situ-generated bubbles.4 Generally, all those methods where hard templates are used require template materials to build different architectures, r 2011 American Chemical Society
and the templates need to be removed later. This problem comes to an end when various hollow structures are synthesized exploiting different physical/chemical processes based on the Kirkendall effect, Ostwald ripening, or chemically induced selftransformation.5 Among them, the very successful approach is the Kirkendall effect, which has now been established as an effective way of aiming to form various hollow and coreshell type structures through chemical reaction.6 In the course of the reaction time, the core is removed, which occurs through Kirkendall diffusion at the same time; therefore, no modification of the template surface and no special process for removing the template core has been needed. The Alivisatos group have reported an example of the Kirkendall mechanism-mediated synthesis route for the synthesis of hollow nanoparticles in a one-pot approach.7 Recently, semiconducting metal chalcogenides have been receiving considerable attention due to their remarkable properties and brilliant application prospects.8 Most commonly prepared metal chalcogenides are ZnS, CdS, CuS, CoS, PbS, etc. due to their potential applicability.9 Copper sulfide, one of the most important primitive p-type semiconductors, transforms into a superconductor at low temperature. It finds numerous potential Received: February 28, 2011 Revised: May 25, 2011 Published: May 26, 2011 12275
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Scheme 1. Schematic Representation of the Formation of Cu2O from the Cu(II)EDTA Complex
thioacetamide (TAA) as the sulfur source. To the best of our knowledge, it is the first example for the successful synthesis of Cu7S4 hollow cubes from Cu2O cubes.
Figure 1. UVvis spectral information to study the progress of the reaction, indicating the formation of Cu2O from the Cu(II)EDTA complex.
applications as semiconductor, sensor, solar energy converter, cathode material, optical filter, and nonlinear optical material applications in catalysis, thermoelectric cooling materials, etc.10 Copper sulfide possesses five stable phases at room temperature, which includes covellite, anilite, digenite, djurlite, and chalcocite.11 Cu7S4, anilite being a nonstoichiometric copper sulfide, has a Cu/S ratio of 1.75. In 1969, Morimoto et al. discovered the low-temperature mineral phase anilite.12 Synthetic approaches to anilite have been carried out by the reaction of an aqueous suspension of Cu2O with H2S. There are very few reports for the synthesis of anilite via wet chemical synthetic procedures. Zhang et al. have reported synthesis of Cu7S4 nanotubes on copper substrate.13 They have synthesized Cu(OH)2 nanowire as the first step on a copper substrate, and finally, they obtained Cu7S4 nanotubes out of the reaction of Cu(OH)2 and Na2S as the sulfer source. Qian and Wang et al. have synthesized hollow Cu7S4 nanocages. They have synthesized Cu2O first using ascorbic acid as reducing agent and then introducing thiourea as a sulfur source at high temperature.14 Wang et al. have synthesized Cu7S4 hollow nanoparticles using nanocube Cu2O.15 In that method, they have prepared monodispersed Cu2O cubes via a modified solution route as reported by Cao et al. and after that, the desired products have been fabricated introducing Na2S into the aqueous suspension of Cu2O. Here, we have found a facile, mild, wet chemical route for the synthesis of Cu7S4 hollow cubes at room temperature exploiting uniform cubic Cu2O as sacrificial templates. Cu7S4 is developed through a fast Kirkendall process between the releases of Cu(I) from the solid core toward the shell, where sulfide ion is present. For the first template, Cu2O cubes have been synthesized exploiting the complex Cu(II)EDTA as precursor and glucose as the reducing agent. After that, Cu7S4 hollow cubes have been synthesized with the help of the Kirkendall effect exploiting
’ EXPERIMENTAL SECTION All the reagents were of analytical grade and were used without further purification. Double-distilled water was used throughout the experiment. CuSO4 was purchased from S. d. Fine-Chem. Ltd.; EDTA and NaOH were purchased from Hi Media Laboratory Pvt. Ltd. Glucose and thioacetamide were purchased from LOBA Chemie. Synthesis of Solid Cu2O Cube Templates. First, 10 mL of 0.01 M CuSO4 solution was added to 10 mL of 0.01 M EDTA solution. The blue color of the CuSO4 solution became deep, indicating the formation of the Cu(II)EDTA complex. Cu(II)EDTA complex formation can be easily monitored by UVvis spectroscopy because it shows a sharp peak at 730 nm, which matches well with the literature.16 Then, to this solution, 1 mL of 0.1 M glucose solution was added, and the solution was then stirred well for 15 min. After that, 1 mL of 1 M NaOH was added to it, and the solution was heated in a water bath for 30 min. The solution color changed from blue to green to red (Scheme 1). When the reaction mixture was heated in a water bath during the UVvis study, it was observed that the absorbance maxima of the Cu(II)EDTA complex decreases with time, which is indicative of a forward reaction (Figure 1). A suspension of Cu2O particles is obtained. The red particles were collected, washed thoroughly with ethanol, and dried well in air. Synthesis of Solid Cu7S4 Hollow Cubes. A 0.5 g portion of Cu2O was added to 10 mL of water to make a suspension. Then to this solution 10 mL of 0.1 M thioacetamide was added. After the addition of thioacetamide to the Cu2O solution, the color changes from red to gray to black (Scheme 2). The mixture was stirred at room temperature. After an additional 1 h of reaction under magnetic stirring; centrifuging at 3000 rpm for 5 min; and after several washings with water and ethanol, respectively; and finally, drying at room temperature, a black powder was obtained. Characterizations. Absorption spectra of the Cu(II)EDTA complex were recorded on a Shimadzu UV-160 spectrophotometer (Kyoto, Japan), with the solutions in a 1 cm quartz cuvette, whereas electronic absorption spectra of solid Cu2O and Cu7S4 using diffuse reflectance spectra mode were recorded with a Cary model 5000 UVvis/NIR spectrophotometer. FTIR spectral characteristics of the samples were collected in reflectance mode with a Nexus 870 Thermo-Nicolet instrument coupled with a Thermo-Nicolet Continuum FTIR microscope. The phase and purity of the products Cu2O and Cu7S4 were determined by X-ray powder diffraction (XRD) using an X-ray diffractometer with Cu KR radiation (λ = 1.5418 Å). Scans were collected on dry nanomaterials in the range of 2080°. XPS 12276
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Scheme 2. Schematic Representation of Stepwise Transformation: Red Cu2O to Black Cu7S4
Figure 2. UVvis spectra of (a) as-synthesized solid, red Cu2O cubes having a peak at 550 nm; (b) compound from the intermediate step to show the transformation from Cu2O to Cu7S4, which also shows a peak at 550 nm, but with decreased intensity; (c) final product Cu7S4 without the signature peak at 550 nm.
analysis was performed on ESCA-Perkin Elmer (PHI-1257) system using MgKa x-ray source. The particle size, shape, and morphology of nanoparticles were observed with a field emission scanning electron microscope (Supra 40, Carl ZEISS Pvt. Ltd.), and an EDS machine (Oxford link and ISIS 300) attached to the instrument was used to obtain the nanocrystal composition. TEM and HR-TEM measurements of the metal oxide and sulfide sols were performed on a Hitachi H-9000 NAR instrument on samples prepared by placing a drop of fresh metal oxide and sulfide sols on copper grids precoated with carbon films, followed by solvent evaporation under vacuum. Nitrogen adsorptiondesorption measurements were performed at 77.3 K by using a quantachrome instrument utilizing the BET model for the calculation of surface areas. The complex impedance measurement at room temperature was carried out by a precision impedance analyzer (model Agilent-4294A) using a sinusoidal voltage signal having an amplitude of 500 mV and frequencies in the range of 100 Hz110 MHz.
’ RESULTS AND DISCUSSIONS The optical properties of the as-prepared Cu2O cubes and Cu7S4 hollow cubes were ascertained with the help of UVvisible spectroscopy to resolve the excitonic or interband transitions. Red Cu2O and black Cu7S4 were exploited for a UVvis study in solid form. With the utmost care, we can observe the transformation of Cu2O cubes into Cu7S4 hollow cubes. The electronic spectral information clearly indicates that there is a sharp absorbance band at 550 nm of Cu2O, which is good in agreement with the reported data.17 With the increase in the reaction time (i.e., with the progress of the transformation of Cu2O to Cu7S4), the peak intensity at 550 decreases. Finally, the end product shows complete diminution of the 550 nm peak for Cu2O (Figure 2). So with the help of UVvis spectrophotometry, we can successfully demonstrate complete transformation of Cu2O to Cu7S4. Figure 3 shows the full range FTIR spectrum of the asobtained Cu2O nanostructures, and the inset data show only 12277
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Figure 3. FTIR spectra of the (a) as-synthesized solid, red Cu2O cubes having a peak at 627 cm1 and (b) hollow Cu7S4 cubes without any characteristic peak.
Figure 4. XRD patterns of (a) as-synthesized Cu2O cubes, (b) the intermediate product showing peaks of both Cu2O and Cu7S4, and (c) the final product (i.e., Cu7S4 of the transformation process).
one peak: at 627 cm1. According to the standard data of Cu2O, the nanostructure peak at 629 cm1 has been attributed to the CuO vibration of the Cu2O nanocrystal.18 Because Figure 3 contains only one peak at 627 cm1, the presence of any impurity can be ignored in the FTIR spectrum. It is also clear that the asprepared red particles are nothing but Cu2O nanostructure. The FTIR spectrum of the prepared Cu7S4 structure does not show any characteristic peak. This observation gives a clear idea for the complete transformation from Cu2O to Cu7S4.
X-ray diffraction analysis has been carried out to investigate the phase purity of both samples that are shown in Figure 4. In Figure 4a, all the diffraction peaks in the XRD pattern can be indexed as the cubic symmetry of cuprite (Cu2O, JCPDS No. 050667),19 confirming that the product is phase-pure cubic Cu2O. There are no other peaks of impurity due to metallic Cu and CuO. Figure 4c shows all the diffraction peaks of Cu7S4 which have been indexed to pure Cu7S4 (Cu7S4, JCPDS No. 230958).20 XRD analysis also was carried out in the intermediate 12278
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Figure 8. TEM images of highly symmetrical Cu7S4 hollow cubes under (a) low and (b) high magnification.
Figure 5. FESEM images of Cu2O cubes with symmetrical structure with tight size distribution with smoothed faces: (a) low magnification, (b) medium magnification, and (c, d) high magnification.
Figure 6. FESEM images of Cu7S4 hollow cubes show a highly symmetrical structure with uniform size but quite a rough surface in (a) low magnification, (b, c) medium magnification, and (d) high magnification.
Figure 7. TEM images of highly symmetrical Cu2O cubes under (a) low and (b) high magnification.
step to note the chemical transformation that is shown in Figure 4b. Figure 4b demonstrates diffraction peaks of both Cu2O and Cu7S4. From this XRD pattern, it is clear that there is a decrease in intensity in the planes due to Cu2O, and new peaks grow up. In Figure 4c, we find complete disappearance of the peak due to Cu2O, so XRD spectra help us to conclude that there is chemical reaction and, eventually, the transformation of solid Cu2O to Cu7S4. Finally, we
see that the hollow cubes become composed of pure Cu7S4, and there is no remaining impurity of Cu2O. The surface properties of the as-synthesized Cu2O cubes were critically examined with the help of XPS analysis. Because XPS is a very powerful technique to study the electronic state, we present an interesting fact for the Cu2O to Cu7S4 transformation. XPS spectra of the freshly prepared Cu2O show the peak fit of Cu 2p3/2 peak revealed a main peak at 933.5 eV, which accompanied a series of satellites on the high-binding-energy side 940.3, and another at 943.1 eV (Figure S1 in the Supporting Information). The shakeup satellite peaks correspond to the Cu2þ state of CuO, which could not be accounted for from XRD analysis. Therefore, this shows that although the XRD study indicates that the prepared cubes are of pure Cu2O, the XPS spectra unequivocally prove the presence of a thin CuO layer on the Cu2O surface. However, the as-prepared CuO film covering the Cu2O cubes evolved exclusively Cu7S4 cubes without any trace of copper oxide matrix, as revealed from EDS analysis. SEM micrographs display the shape and size of the as-synthesized Cu2O, which is shown in Figure 5. Low magnification SEM images present the well-defined, uniform, solid, cubic morphology of the as-prepared Cu2O. The SEM image of an individual Cu2O nanocube shows the smoothed faces and regular shapes having a high degree of symmetry in its structure. The average edge length of each cubes remains in the micrometer region, so from the size calculation of the cubes from the SEM images, it is better to say microcubes rather than nanocubes. These solid Cu2O cubes serve as the sacrificial template and the shape controller and react with S2 generated from the hydrolysis of TAA under alkaline conditions at room temperature. SEM images of the finally prepared sample are shown in Figure 6. Low-magnification SEM images show that a huge number of hollow cubes have been synthesized. This information also suggests that there is a complete transformation of Cu2O solid cubes to Cu7S4 hollow cubes. With the increase in magnification in the FESEM study, it is clear that the hollow cubes are in the micrometer region, well-defined, uniform, and have a high degree in symmetry, but there is a major difference in the morphology of Cu2O and Cu7S4. Faces of Cu2O cubes are smooth whereas Cu7S4 is very rough, which is clearly shown in Figure 6. TEM images of both the samples Cu2O and Cu7S4 are in good agreement with the SEM images. TEM images of Cu2O show the cubic nature having high symmetry in its structure with smooth faces (Figure 7). On the other hand, the TEM image of Cu7S4 clearly demonstrates the hollow nature of the synthesized cubes (Figure 8). The outer diameter (o.d.), inner diameter (i.d.), and shell thickness of Cu7S4 hollow spheres are 730, 610, and 60 nm, respectively (Figure S2 in the Supporting Information). In the TEM image of a single hollow cube, the contrast between the dark edge and the pale center provides convincing evidence of the hollow structure (Figure 9). 12279
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Figure 9. Dark field TEM images of highly symmetrical (a) Cu2O solid cubes and (b) Cu7S4 hollow cubes.
Figure 10. HR-TEM images of (a) Cu2O cubes and (b) Cu7S4 hollow cubes.
Figure 10a shows the HR-TEM image of a Cu2O cube. From the HRTEM image, the lattice spacing has been calculated, which is 0.30 nm corresponding to the (110) crystal plane of Cu2O.21 The HR-TEM image of Cu7S4 shows lattice spacing of 0.18 nm, which corresponds to the (886) crystal plane of Cu7S4 (Figure 10b).13 EDS analysis has confirmed the presence of the elements Cu and O in the solid Cu2O cubes and the presence of Cu and S in the hollow cubes (Figure 11); therefore, EDS analysis also deliberately supports the complete transformation of Cu2O to Cu7S4. The hollow nature of the Cu7S4 cubes has been confirmed from the measurement of the pore size distribution, which was obtained by the nitrogen adsorptiondesorption isotherm and BarrettJaynerHalender (BJH) method. From the BJH isotherm, a distinct hysterisis loop has been observed. From the experiment, surface area was calculated: 32.40 m2/g. This shows that the obtained Cu7S4 sample is a kind of porous material obtained from the Cu2O cube, which is solid, devoid of any porosity, and has a surface area of 3.2 m2/g, confirmed from the BJH isotherm (Figure 12). Impedance Measurement. The impedance analysis is an important technique for its simplicity and clarity in describing the electrical processes occurring in any two-electrode device configurations on applying an alternating voltage signal. The complex impedance measurements on any two-electrode device configuration can give a wide range of information about the dynamic and dielectric properties of the device material itself. The technique involves the analysis of alternating responses of a two-electrode configuration subjected to a small sinusoidal electrical signal of variable frequency and then determining the frequency dependence of the impedance over a wide range of frequencies of the signal. The method enables the evaluation and separation of the contributions due to the bulk material, grain boundaries, and bulk-electrode interfacial phenomena, if any, to the overall electrical properties in the frequency domain studies.22
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Figure 11. EDS spectra of the (a) Cu2O solid cubes and (b) Cu7S4 hollow cubes. The inset shows the position on which the EDS studies were done.
Keeping the importance of impedance measurement in mind, a complex impedance spectrum has been obtained for our samples. The Nyquist plots show variation of the real and imaginary parts of the sample impedance with the variation of frequency of the alternating voltage signal of small amplitude on the complex plane. Figure 13 shows the typical complex impedance spectra obtained for (a) Cu2O and (b) Cu7S4. The appearance of a single semicircular arc in Figure 13b indicates the presence of a single relaxation process, and the low frequency intercept of the semicircular arc on the real axis gives an estimate of the dc resistance (i.e., the frequency-independent resistance) for the sample Cu7S4. Further, it also provides an indication that the electrical processes occurring in Cu7S4 are due to only the copper vacancies in the lattice, which is also responsible for their use in optoelectronics. From the earlier reports, we come to know that the Cu7S4 QD film turned out to be a p-type semiconducting film.23 In the present case, the estimated dc resistance turns out to be 220 Ω for Cu7S4, which is prepared from a high resistive starting material Cu2O (Figure 13a). Formation Mechanism of the Cu7S4 Hollow Cubes. The well-known Cu(II)EDTA complex with 1:1 stoichiometry has been prepared by the age-old spectrophotometric titration (λmax 730 nm).16 The complex remains water-soluble and becomes a precursor for the synthesis of well-defined Cu2O cubes. The Cu(II)EDTA complex has been chosen, keeping an eye to its very high stability constant 18.8 (logK1), leaving aside other wellknown, weak, bidentate Cu(II) chelated complexes of tartrate, citrate, etc. In this context, it is pertinent to mention that the corresponding Cu(II) tartrate chelate has a stability constant value 3.2 (log K1), and that of citrate is 6.1 (log K1). Thus, in the present experimental protocol, the hexadentate ligand EDTA chelate of Cu(II) becomes responsible for the controlled release of Cu(II) ions, leading to the habitual cubic morphology of Cu2O with smoothed faces. Otherwise, corrugated cubic, octahedral, or polyhedral shapes emerge for Cu2O. Hence, monodispersed template free Cu2O cubes with uniform diameters in the range of 11.1 μm were first synthesized in our experiment exploiting a simple wet chemical method in which glucose acts as a reducing agent under alkaline conditions. Here, the simple reaction is reported to obtain cubic Cu2O as a fact of mass action where a kinetic factor controls the release of Cu(II) ions. Solid Cu2O formation drives the reaction in the forward direction. CuðIIÞ EDTA þ C5 H11 O5 CHO f
Cu2 O V þ C5 H11 O5 COOH red
In this Cu2O formation process, no heterogeneity is involved. The resultant Cu2O cubes were used as a sacrificial solid 12280
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Figure 12. Nitrogen adsorptiondesorption isotherm of (a) as-synthesized solid Cu2O cubes and (b) hollow Cu7S4 cubes.
Scheme 3. Schematic Representation of the Formation of Cu7S4 Hollow Cube from Cu2O Cube with CuO Barrier
Figure 13. Typical complex impedance spectra for (a) Cu2O and (b) Cu7S4.
precursor for the synthesis of Cu7S4 hollow cubes by adding an appropriate amount of sulfide source into the Cu2O aqueous suspension. Upon the addition of TAA into the aqueous solution at room temperature, the initial red color of the Cu2O suspension immediately changes to gray. CH3 CSNH2 þ OH f CH3 COO þ S2 þ NH3 þ H2 O Cu2 OðSÞ þ S2 f Cu7 S4 ðSÞ After reacting with the sulfide sources, the solid Cu2O is converted exclusively to hollow Cu7S4 cubes at room temperature after 1 h. Thus, the as-obtained Cu7S4 hollow cubes become uniform and nearly monodispersed. It is noted that the morphology of Cu7S4 hollow cubes is similar to that of the original Cu2O solid cubes, but the mean outer diameter of the resultant hollow cubes is increased to 100120 nm for the Cu7S4 (Figure S3 in the Supporting Information). The inner diameters of those hollow cubes also become slightly larger than the original Cu2O solid cubes after reactive phase transformations. This hollowing process could be explained by the Kirkendall diffusion. In the reaction process, solid Cu2O cubes were synthesized first via wet chemical synthetic procedure. Cu2O cubes are covered instantly
with a very thin layer of CuO, which is detectable only by XPS. This thin layer remains as a barrier over the Cu2O cubes in the solidliquid interface and protects the Cu2O cubes from the outside. When TAA has been added and kept for some time, sulfide ions are released from the TAA upon its hydrolysis in alkaline condition. Therefore, a direct chemical reaction between sulfide ions with Cu2O is hindered. However, the reaction progresses because of the diffusion of copper(I) ions through the newly formed CuO barrier at the solidliquid interface. The specific Cu2O structure allows diffusion of copper(I) ions from the central region to the surface and Cu7S4 is produced there at first. Finally, CuO in the barrier layer is also converted into sulfide, and no trace of oxide remains behind with Cu7S4 structure. Thus, it makes the conversion quantitative. It has been assumed that on the surface of the Cu2O cubes, a very thin layer of Cu7S4 is formed first.14 Due to the bigger size of the sulfide ions, its diffusion is presumably hindered. The formation of void space within the cube is due to gradual outward movement of copper(I) ions from within; as a result, Cu7S4 particles tend to grow in size during the corrosion process (Scheme 3). The evidence of preferential binding of copper(I) with sulfide emerges from the facile reaction of sulfide ion with Cu2O, leaving aside CuO as maintained earlier that the hollow Cu7S4 cube formation takes place under room temperature and stirring conditions. This process follows fast kinetics. Exploiting the Kirkendall effect, several interesting structures have recently been produced: spherical shells, nanotubes, and other structures with controlled dimensions and geometries.8b,24 The literature reports that hollow structures are generated from solid structures following the Kirkendall effect via a coreshell mechanism.25 But in the present case, the coreshell structure has not been observed for Kirkendall diffusion. Critical analysis of 12281
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The Journal of Physical Chemistry C the TEM and SEM observation and also the time-dependent SEM images show that at the very beginning, a few hollow Cu7S4 structures are produced, and most of the Cu2O cubes remained intact as solid cubes (Figure S4 in the Supporting Information). With the lapse of time, all the solid Cu2O cubes are transformed into hollow Cu7S4. Introduction of excess TAA (>0.2 M) into the reaction mixture produced hollow cubes in no time. With a high sulfide concentration in the solution, the rate of diffusion of the Cu(I) from the innermost Cu2O region to the surface becomes quite high, so the rough surface structure becomes inevitable for the cubes of Cu7S4 (Figure S5 in the Supporting Information). Conversely, for lower TAA conditions (0.01 M), hollow structures with quite smooth surfaces become obvious. Under these circumstances, a longer time is needed to obtain the hollow cubes.
’ CONCLUSIONS Highly symmetrical, uniform Cu7S4 hollow cubes were successfully synthesized exploiting symmetrical smooth Cu2O cubes as sacrificial templates at room temperature and from alkaline solution. A stable Cu(II)EDTA complex was chosen for Cu2O formation, and then for Cu2O to Cu7S4 transformation, TAA was used as the sulfide source. The presence of a Cu(II) layer on the smooth Cu2O cube and the softsoft interaction related higher diffusion of Cu(I) ion drive the exclusive evolution of hollow cubes of Cu7S4. ’ ASSOCIATED CONTENT
bS
Supporting Information. (1) XPS of Cu2O cube; (2) FESEM of a single hollow cube showing the inner, outer wall thickness; (3) FESEM of both Cu2O and Cu7S4 showing the comparative sizes; (4) FESEM image at the intermediate stage of the transformation process: Cu2O to Cu7S4; (5) FESEM image of the prepared Cu7S4 in the presence of a high concentration of sulfide source. This information is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors are thankful to the UGC, DST, NST and CSIR, New Delhi, and the IIT Kharagpur. ’ REFERENCES (1) MacLachlan, M. J.; Manners, I.; Ozin, G. A Adv. Mater. 2000, 12, 675. (2) (a) Lou, X. W.; Archer., L. A.; Yang, Z. C. Adv. Mater. 2008, 20, 3987. (b) Niu, K. Y.; Yang, J.; Kulinich, S. A.; Sun, J.; Du, X. W. Langmuir 2010, 26, 16652. (3) (a) Lim, B.; Jiang, M. J.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X. M.; Zhu, Y. M.; Xia, Y. N. Science 2009, 324, 1302. (b) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (c) Hu, Y.; Jiang, X. Q.; Ding, Y.; Chen, Q.; Yang, C. Z. Adv. Mater. 2004, 16, 933. (d) Im, S. H.; Jeong, U.; Xia, Y. Nat. Mater. 2005, 4, 671. (e) Sun, Y. G.; Mayers, B.; Xia, Y. N. Adv. Mater. 2003, 15, 641. (f) Caruso, F. Adv. Mater. 2001, 13, 11. (g) Sukhorukov, G. B.; Rogach, A. L.; Garstka, M.; Springer, S.; Parak, W. J.; Munoz-Javier, A.; Kreft, O.;
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