Highly Ordered Self-Assemblies of Submicrometer Cu2O Spheres and

Mar 1, 2010 - Synthetic Architecture of Multiple Core–Shell and Yolk–Shell Structures of (Cu2O@)nCu2O (n = 1–4) with Centricity and Eccentricity...
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Highly Ordered Self-Assemblies of Submicrometer Cu2O Spheres and Their Hollow Chalcogenide Derivatives Maolin Pang and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, KAUST-NUS GCR Program, and Minerals, Metals, and Materials Technology Center, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received November 12, 2009. Revised Manuscript Received February 19, 2010 Highly ordered superlattices assembled from transition metal oxide/sulfide submicrometer particles are difficult to prepare due to lack of monodisperse primary building blocks. In this work, we have successfully synthesized monodisperse Cu2O spheres with diameters in the submicrometer regime of 130-135 nm. Using the as-prepared Cu2O spheres as solid precursor, uniform hollow CuS and CuSe derivatives have also been synthesized in solution media. More importantly, a range of two-dimensional and three-dimensional superlattices of Cu2O, CuS, and CuSe solid/hollow spheres have been assembled for the first time. Without assistance of conventional sacrificing solid templates, the degree of ordering achieved in these superlattices is comparable to those reported for well-studied silica and polystyrene beads. The realization of these self-assembled superlattices may provide a new way of thin film design and fabrication for this class of photosensitive semiconducting materials using their prefabricated building blocks.

Introduction Accompanied with development of nanotechnology, synthesis and self-assembly of monodisperse nanoparticles have received paramount attention over the past two decades due to a major paradigm shift to bottom-up strategies.1-8 In addition to various two-dimensional (2D) arrays and superlattices,1-3 for instance, highly ordered three-dimensional (3D) assemblages and supercrystals organized from size-controlled nanoparticles (i.e., primary building blocks) have been reported in recent years.4,5 In particular, well-established methods for preparation of ultrauniform silica and polystyrene beads in submicrometer (i.e., several hundred nanometers in diameter) or micrometer range have been reported widely in the literature which leads to tremendous success of photonic crystal fabrications.1,2 Compared to selfassemblies of nanoparticles with the sizes small than 100 nm and the silica and polymeric beads, large-scale superlattices assembled by larger freestanding particles in the submicrometer range have been less successful because of difficulties in obtaining uniform particles via direct synthesis, especially for preparations of transition metal oxide building blocks. In order to prepare ordered superlattices of transition metal oxides with building block size in the submicrometer range, certain model cases must be established. In this work, therefore, we choose cuprous oxide *Corresponding author. E-mail: [email protected]. (1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (2) Pileni, M. P. Nat. Mater. 2003, 2, 145–150. (3) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444–446. (4) Fan, H. Y.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H. F.; Lopez, G. P.; Brinker, C. J. Science 2004, 304, 567–571. (5) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55–59. (6) (a) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348– 351. (b) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006–1009. (7) (a) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969–971. (b) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930–5933. (c) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2005, 127, 18262–18268. (8) Li, J.; Tang, S. B.; Lu, L.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 9401– 9409.

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(Cu2O) as a model functional material for this topical investigation. Considering their cubic crystal system, Cu2O crystallites may possibly be aggregated into highly symmetric building blocks such as spheres for self-assembly. Cu2O is an important transition metal oxide with potential applications across a number of technological fields. Being a p-type oxide semiconductor with a direct band gap of 2.17 eV, Cu2O has been found in applications of solar energy utilization, chemical and biosensing, photon-activated water splitting, lowtemperature CO oxidation, photodegradation of organic pollutants, micro- and nanoelectronics, and magnetic storage devices, etc.9-11 Because of its technological importance, shape and size controlled synthesis of Cu2O micro- and nanostructures has received significant attention in recent years. Using various solution-based approaches, for example, nanoparticles, nanoplates, nanocubes, octahedra, spheres, nanocages, nanowires, and nanorods of Cu2O have been synthesized.9-21 Closely related to the above research, copper chalcogenides such as copper sulfide (CuS, covellite) and copper selenide (CuSe, klockmannite) have also been investigated extensively owing to their important optical (9) Siegfried, M. J.; Choi, K. S. Adv. Mater. 2004, 16, 1743–1746. (10) Siegfried, M. J.; Choi, K. S. J. Am. Chem. Soc. 2006, 128, 10356–10357. (11) Kuo, C. H.; Huang, M. H. J. Am. Chem. Soc. 2008, 130, 12815–12820. (12) Yin, M.; Wu, C. K.; Lou, Y. B.; Burda, C.; Koberstein, J. T.; Zhu, Y. M.; O’Brien, S. J. Am. Chem. Soc. 2005, 127, 9506–9511. (13) (a) Chang, Y.; Teo, J. J.; Zeng, H. C. Langmuir 2005, 21, 1074–1079. (b) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369–7377. (14) (a) Gou, L. F.; Murphy, C. J. Nano Lett. 2003, 3, 231–234. (b) Gou, L. F.; Murphy, C. J. J. Mater. Chem. 2004, 14, 735–738. (15) Zhang, H. G.; Zhu, Q. S.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. Adv. Funct. Mater. 2007, 17, 2766–2771. (16) Xu, Y. Y.; Chen, D. R.; Jiao, X. L.; Xue, K. Y. J. Phys. Chem. C 2007, 111, 16284–16289. (17) Zhang, J. T.; Liu, J. F.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2006, 18, 867–871. (18) Jiao, S. H.; Xu, L. F.; Jiang, K.; Xu, D. S. Adv. Mater. 2006, 18, 1174–1177. (19) Lu, C. H.; Qi, L. M.; Yang, J. H.; Wang, X. Y.; Zhang, D. Y.; Xie, J. L.; Ma, J. M. Adv. Mater. 2005, 17, 2562–2567. (20) Wang, W. Z.; Wang, G. H.; Wang, X. S.; Zhan, Y. J.; Liu, Y. K.; Zheng, C. L. Adv. Mater. 2002, 14, 67–69. (21) Tan, Y. W.; Xue, X. Y.; Peng, Q.; Zhao, H.; Wang, T. H.; Li, Y. D. Nano Lett. 2007, 7, 3723–3728.

Published on Web 03/01/2010

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Figure 1. TEM images of Cu2O solid spheres: (a, b) single layer of hexagonal close-packed superlattices, (c, d) double layers of hexagonal close-packed AB superlattices, (e, f) triple layers of hexagonal close-packed ABA superlattices, (g, h) triple layers of hexagonal close-packed ABC superlattices, (i) double layers of square superlattices, and (j) triple layers of square superlattices. Overlapping effects of these superlattices in the TEM images are illustrated with purple spheres (the first layer), green spheres (the second layer), and blue spheres (the third layer).

and electrical properties.18,22-31 Different methods have been devised to prepare CuS and CuSe into various morphologies, such (22) Feng, X. P.; Li, Y. X.; Liu, H. B.; Li, Y. L.; Cui, S.; Wang, N.; Jiang, L.; Liu, X. F.; Yuan, M. J. Nanotechnology 2007, 18, 145706–145711. (23) Zhang, H.; Zhang, Y. Q.; Yu, J. X.; Yang, D. R. J. Phys. Chem. C 2008, 112, 13390–13394.

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as nanocrystals, nanoplates, nanotubes, nanorods, nanowires, mesocages, and spheres for CuS as well as nanotubes, microparticles, nanoplates, and spheres for CuSe.18,22-31 (24) Lim, W. P.; Wong, C. T.; Ang, S. L.; Low, H. Y.; Chin, W. S. Chem. Mater. 2006, 18, 6170–6177.

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Using cuprous oxide as a starting solid to prepare copper sulfide or copper selenide hollow nanomaterials has been reported by several research groups.18,29-31 For example, Cu2O micro- and nanomaterials with different shapes can be converted into Cu7S4 and Cu2-xSe hollow structures.30,31 By chemical transformation of an in situ formed sacrificial CuBr, Cu2O and CuS hollow spheres with a shell thickness of about 20-25 nm have been prepared.29 Nevertheless, highly ordered superlattices of these materials have not been realized due to lack of uniform spherical building blocks. Since a spherical object possesses the highest symmetry and is an ideal primary building block or “artificial atom” for supercrystal construction, it remains as a paramount challenge for one to prepare highly monodisperse spheres for copper oxides and chalcogenides, concerning superlattice architecture of this class of materials through self-assembly. Herein, as a first step toward this goal, we have devised a facile method to synthesize highly monodisperse Cu2O spheres at room temperature. Using the as-prepared submicrometer Cu2O spheres as starting a solid precursor, we have also prepared monodisperse phase-pure hollow spheres of CuS and CuSe. More importantly, we have successfully prepared a variety of 2D and 3D superlattices of Cu2O, CuS, and CuSe solid/hollow spheres. It should be mentioned that the realization of this type of self-assembled superlattices may be considered as a bottom-up approach of thin-film design and fabrication for the three semiconducting materials.

Experimental Section Cu2O nanospheres were prepared by adding hydrazine hydrate (N2H4 3 H2O) into a solution of Cu2þ and NaOH (in 2-propanol) using polyvinylpyrrolidone (PVP) as capping agents. In a typical synthesis, Cu(NO3)2 3 3H2O (0.1 mmol, Merck) was dissolved in 60 mL of 2-propanol (Tedia), followed by the addition of 0.2 g of PVP (K30, MW = 40 000, Fluka) and 1 mmol of NaOH solution (in 2-propanol). After stirring for 10 min, 0.4 mL of N2H4 3 H2O (35 wt %, Aldrich) was added to the above mixture dropwise and stirred for another 10 min. Upon the addition of N2H4 3 H2O, orange Cu2O precipitates were formed immediately. The Cu2O products were collected by centrifugation and washed with deionized water and anhydrous ethanol three times. All of the experiments were performed at room temperature. In order to obtain phase-pure CuS hollow spheres, a stoichiometric amount of ammonium sulfide [(NH4)2S, 20 wt % aqueous solution, Strem Chemical] was added into a well-dispersed Cu2O nanosphere suspension (aqueous), and the reaction system was stirred for 1 h at room temperature. The products were also collected by centrifugation and washed with deionized water and anhydrous ethanol three times. In the preparation of CuSe hollow spheres, on the other hand, 0.1 mmol of Se powder was first transformed into divalent Se2- with 0.1 mmol of sodium borohydrate (NaBH4) in 10 mL of deionized water, and the solution was stirred for 10 min at room temperature.32 And then a stoichiometric amount of the resultant Se2- solution was added into the (25) Lou, W. J.; Chen, M.; Wang, X. B.; Liu, W. M. J. Phys. Chem. C 2007, 111, 9658–9663. (26) Larsen, T. H.; Sigman, M.; Ghezelbash, A.; Doty, R. C.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 5638–5639. (27) Mao, G. Z.; Dong, W. F.; Kurth, D. G.; M€ohwald, H. Nano Lett. 2004, 4, 249–252. (28) Wang, W. Z.; Geng, Y.; Yan, P.; Liu, F. Y.; Xie, Y.; Qian, Y. T. J. Am. Chem. Soc. 1999, 121, 4062–4063. (29) Gao, J. N.; Li, Q. S.; Zhao, H. B.; Li, L. S.; Liu, C. L.; Gong, Q. H.; Qi, L. M. Chem. Mater. 2008, 20, 6263–6269. (30) Cao, H. L.; Qian, X. F.; Wang, C.; Ma, X. D.; Yin, J.; Zhu, Z. K. J. Am. Chem. Soc. 2005, 127, 16024–16025. (31) Cao, H. L.; Qian, X. F.; Zai, J. T.; Yin, J.; Zhu, Z. K. Chem. Commun. 2006, 4548–4550. (32) Klayman, D. L.; Griffin, T. S. J. Am. Chem. Soc. 1973, 95, 197–199.

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above Cu2O nanosphere suspension and stirred for 1 h at room temperature to get phase-pure CuSe. In order to prepare 2D or 3D superlattices, 0.1-0.2 mL of Cu2O (or similarly CuS and CuSe products) suspension (redispersed in ethanol after preparation, and the concentration is about 2.5 mg/mL) was diluted by adding 2 mL of ethanol, and then 0.1-0.3 mL of polyoxyethylene sorbitan trioleate [Tween85: C60H108O8 3 (C2H4O)n] was added into the above suspension. The mixture was stirred at room temperature for 30 min and then washed with ethanol for one time. At last the resultant product was redispersed in 0.05 mL of ethanol and 0.02 mL of deionized water. For the self-assembly process, a carbon-coated TEM copper grid was placed into a glass vial containing the above solution. The vial was tilted by 45° and stood still for about 1836 h at room temperature (21 °C). The crystallographic information on the prepared samples was established by powder X-ray diffraction (XRD, Shimadzu, model XRD-6000, Cu KR radiation λ = 1.5406 A˚). The crystallite size can be estimated by the XRD according to the Scherrer equation Dhkl = 0.941λ/β cos θ, where Dhkl is the average grain size, λ is the X-ray wavelength (0.15406 nm), and θ and β are the diffraction angle and full width at half-maximum (fwhm) of an observed peak (hkl), respectively. Morphological investigation was carried out with field-emission scanning electron microscopy (FESEM), transmission electron microscopy, and selected area electron diffraction (TEM/SAED, JEM-2010, 200 kV). Surface analysis for the samples was performed with X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical). The spectra of all interested elements were referenced to the C 1s peak arising from adventitious carbon (its binding energy was set at 284.6 eV). The optical band gaps of the above prepared nanostructured samples were determined with a UV-vis spectrophotometer (Shimadzu UV-2450).

Results and Discussion Monodisperse Cu2O nanospheres with uniform diameters in the range of 130-135 nm were first synthesized in our experiments. Figure 1a,b shows two typical transmission microscopy (TEM) images of the sphere products obtained at room temperature. On the basis of our TEM and electron diffraction (ED) investigation, the Cu2O nanospheres are comprised of many small oriented crystallites, and thus they are essentially polycrystalline despite the presence of large oriented domains (Supporting Information, SI-1). The resultant Cu2O nanospheres could be used as a sacrificial solid precursor for the synthesis of CuS or CuSe hollow spheres by adding an appropriate sulfide or selenide source into the Cu2O aqueous suspension. Upon the addition of a (NH4)2S aqueous solution or a mixture of NaBH4 and Se powder at room temperature,32 the initial orange color of the Cu2O nanospheres suspension changes to black immediately. As shown in Figure 2, after reacting with the sulfide or selenide sources, the solid Cu2O was converted exclusively into CuS or CuSe hollow spheres at room temperature. The thus-obtained CuS and CuSe hollow spheres were also quite uniform and nearly monodisperse. It should be noted that the morphologies of both CuS and CuSe hollow spheres are similar to that of the original Cu2O solid spheres, but the mean outer diameter of the resultant hollow spheres increases to 170-180 nm for the CuS and 220-240 nm for the CuSe. The shell thickness is about 20-25 nm for the CuS and 35-40 nm for the CuSe, and the inner diameter is around 130-140 nm for the CuS and 150-160 nm for the CuSe accordingly. The difference in size can be largely attributed to the different sizes of anions in the two copper chalcogenides [ionic radii: r(S2-) = 1.84 A˚ and r(Se2-) = 1.98 A˚], noting that they both have an identical crystal structure. It should also be mentioned that in order to obtain CuS and CuSe hollow spheres with DOI: 10.1021/la904292t

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Figure 2. TEM images of CuS hollow spheres: (a-c) single layer of hexagonal close-packed superlattices; (d) double layers of hexagonal close-packed AB superlattices; (e) double layers of square superlattices and TEM images of CuSe hollow spheres: (f, g) single layer of hexagonal close-packed superlattices; (h) double layers of square superlattices; and (i) triple layers of square superlattices. Overlapping effects of these superlattices in the TEM images can also be referred to the color illustrations of Figure 1.

Figure 3. XRD patterns of (a) Cu2O solid spheres, (b) CuO þ CuS

hollow spheres, (c) CuS hollow spheres, (d) CuO þ CuSe hollow spheres, and (e) CuSe hollow spheres.

smooth surface, a certain amount of PVP (e.g., 0.01 g in 2 mL of deionized water) was added into the Cu2O suspension before adding S2- or Se2- source. The inner diameters of those hollow spheres may be a little bit larger than the original Cu2O solid spheres after reactive phase transformations. This hollowing (33) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714.

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process could be explained by the Kirkendall diffusion,33,34 which will be further addressed shortly. To the best of our knowledge, the highly ordered superlattices in Figures 1 and 2 are the first results obtained for the three functional materials in this spatial range without using sacrificial templates (such as silica or PS beads). Figure 3 reports the X-ray diffraction (XRD) patterns for our Cu2O, CuS, and CuSe samples (Supporting Information SI-2). All the diffraction peaks of the as-prepared Cu2O nanospheres agree perfectly with those of cubic Cu2O (space group Pn3m, JCPDS card no. 34-1354), confirming the formation of phasepure Cu2O. According to the previous literature results, there are at least five stable phases for the CuxS system (i.e., covellite CuS, anilite Cu1.75S, digenite Cu1.8S, djurlite Cu1.95S, and chalcocite Cu2S).18,29 In the present study, nevertheless, we are able to obtain a phase-pure covellite (CuS, space group P63/mmc, JCPDS no. 06-0464) polycrystalline hollow spheres from Cu2O by adding a stoichiometric amount of (NH4)2S, excluding all other copper sulfide phases (Figure 3c). Our EDX analysis for the CuS hollow spheres indicates that the molar ratio of Cu and S is indeed nearly 1:1 (Supporting Information SI-3). In fact, the complete conversion from Cu2O to CuS mainly depends on the amount of sulfide source added into the Cu2O aqueous suspension. If less (NH4)2S was used, only a mixture of CuO and CuS could be obtained (34) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744–16746.

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Figure 4. Representative XPS spectra of (a, b) Cu2O solid spheres, (c, d) CuS hollow spheres, and (e, f) CuSe hollow spheres.

(Figure 3b), which suggests that CuO is also an intermediate phase in addition to Cu2O. As for the CuxSe system, phase-pure polycrystalline hollow spheres of klockmannite (CuSe, space group P63/mmc, JCPDS no. 86-1240) could also be synthesized by adding a stoichiometric amount of Se2- anions (Figure 3e), while a mixture of CuO and CuSe was produced when the Se2anions used in synthesis were below stoichiometry (Figure 3d). Taking the account of our XRD results, it is clear that the Cu2O could also be oxidized to CuO solid intermediate by dissolved air from ambience, and the oxygen anions of CuO were then replaced by chalcogenide anions. The following three chemical reaction equations are thus proposed for the above processes (where E = S and Se), noting that nominal eq 3 is also an overall reaction of (1) and (2): 1 Cu2 OðsÞ þ O2ðdissovledÞ ¼ 2CuOðsÞ 2

ð1Þ

CuOðsÞ þ E2 - ðaqÞ þ H2 O ¼ CuEðsÞ þ 2OH - ðaqÞ

ð2Þ

1 Cu2 OðsÞ þ 2E2 - ðaqÞ þ 2H2 O þ O2ðdissovledÞ 2 ¼ 2CuEðsÞ þ 4OH - ðaqÞ

ð3Þ

To support the above proposed reactions, our further experiments reveal that Cu- or Cu2O-based nanocrystallites could be easily oxidized, and the Cu2O spheres obtained with our present solution method at room temperature were not very stable. For Langmuir 2010, 26(8), 5963–5970

example, if they were stored in deionized water other than ethanol solvent or just exposed to laboratory air, the solid Cu2O spheres would be oxidized quickly, resulting in bluish powder products. The observed chemical process should be ascribed to reactions with the dissolved oxygen and/or hydration with moisture of the air. In our preparation of CuS hollow spheres from the Cu2O precursor, we also tried to conduct the same conversion experiment in ethanol solvent instead of in water medium while keeping other conditions unaltered. Nevertheless, no obvious change could be observed and CuS phase could not be obtained in the ethanol solution. This observation, together with our XRD evidence in Figure 3, clearly demonstrates that the Cuþ can be easily oxidized to Cu2þ in the aqueous solution by the dissolved oxygen to form the intermediate product CuO prior to the CuS phase. Since chemical conversion starts from surface, we also employed the surface-sensitive technique X-ray photoelectron spectroscopy (XPS) in our present study. Figure 4 gives some XPS spectra for representative elements of our samples. Deconvolution of Cu 2p3/2 spectrum of Cu2O nanospheres (Figure 4a) reveals a main peak, which is accompanied by two small peaks on the high binding energy side. The main peak located at 931.6 eV should be ascribed to Cuþ and the two small peaks at 933.3 and 934.7 eV to Cu2þ states (e.g., CuO and Cu(OH)2, respectively, due to a small degree of surface oxidation and hydration) which were not detectable by XRD.12,13,35,36 The O 1s spectrum in (35) Ghijsen, J.; Tjeng, L. H.; Elp, J. V.; Eskes, H.; Westerink, J.; Sawatzky, G. A.; Czyzyk, M. T. Phys. Rev. B 1988, 38, 11322–11330. (36) Chawla, S. K.; Sankarraman, N.; Payer, J. H. J. Electron Spectrosc. Relat. Phenom. 1992, 61, 1–18.

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Figure 6. FTIR spectra of Tween-85-treated samples: Cu2O solid spheres (see Figure 1), CuS hollow spheres, and CuSe hollow spheres (see Figure 2).

Figure 5. A yolk-shell intermediate of (Cu2O þ CuO)@CuSe during the chemical conversion of Cu2O solid spheres to CuSe hollow spheres (TEM image) and related XRD patterns of (a) the sample in the above TEM image, (b) the Cu2O standard, (c) the CuO standard, and (d) the CuSe standard.

Figure 4b can be resolved into four peaks. The O 1s peak at 529.4 eV is due to the presence of surface CuO on the Cu2O nanospheres,37,38 the main peak at 530.9 eV is attributed to Cu2O,39 and the small components at 532.1 and 533.3 eV are ascribable to hydroxyl groups and molecular water adsorbed on the surface, respectively.13,40 Combining the above XPS and XRD results, it can be concluded that the Cu2O nanospheres has been successfully prepared by our present solution method at room temperature. For the Cu 2p3/2 XPS spectra of CuS (Figure 4c), the two peaks located at around 931.4 and 932.7 eV are assigned to lattice Cu2þ and its satellite shakeup, respectively, and the S 2p doublet at around 162.1 and 163.2 eV (Figure 4d) corresponds to S 2p3/2 and S 2p1/2 of the CuS phase. The weaker peaks located at 164.2 and 165.2 eV should be attributable to the presence S-O bond due to surface oxidation.39 On the other hand, the small peak at 161.2 eV can be thought of as the presence of a small amount of surface unbound S2- species. Similarly, in the XPS spectra of CuSe, the Cu 2p3/2 peaks at 931.1 and 932.3 eV can be assigned to Cu2þ and its shakeup satellite of CuSe (Figure 4e). The Se 3d doublet peaks (Figure 4f) are located at 54.0 (3d5/2) and 54.8 eV (3d3/2), respectively,41 while a small component at 56.3 eV is (37) Wang, W. Z.; Zhan, Y. J.; Wang, X. S.; Liu, Y. K.; Zheng, C. L.; Wang, G. H. Mater. Res. Bull. 2002, 37, 1093–1100. (38) Xu, J. F.; Ji, W.; Shen, Z. X.; Tang, S. H.; Ye, X. R.; Jia, D. Z.; Xin, X. Q. J. Solid State Chem. 1999, 147, 516–519. (39) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. E.; Muilenber, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, Physical Electronics Division: Waltham, MA, 1979. (40) Xu, R.; Zeng, H. C. Langmuir 2004, 20, 9780–9790. (41) Jiang, Y.; Wu, Y.; Xie, B.; Zhang, S. Y.; Qian, Y. T. Nanotechnology 2004, 15, 283–286.

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thought to be some surface oxidized Se species during the our sample handling.39 Thus, our XPS study also compositionally confirms that the Cu2O nanospheres have been transformed into CuS and CuSe hollow spheres using this low-temperature approach. As mentioned earlier, the conversion of Cu2O nanospheres to their chalcogenide derivatives CuS and CuSe is believed to be based on Kirkendall diffusion. This underlying mechanism has been widely demonstrated for the formation of hollow structures in recent years.30,31,33,34 On the basis of the above XPS results, we understand that there was a small amount of CuO on the surface of Cu2O nanosphere. Because of an extremely small Ksp (6.3  10-36, 18 °C) of CuS, for instance, this thin surface oxide could be transformed to CuS when there are S2- anions in contact with it, which then served as starting interface for the nucleation and subsequent growth of CuS phase. Because of presence of the ambient dissolved oxygen, surface Cuþ was continuously oxidized to Cu2þ and deposited back at the CuS shell, forming CuS hollow spheres. Since the atomic ratio of Cu and S in our synthesis was stoichiometric, phase-pure CuS hollow spheres can be eventually obtained. A similar discussion could also be given to the formation of CuSe hollow spheres, noting that Ksp of CuSe is even smaller (7.94  10-49). It is believed that this modified Kirkendall process also involved dissolution-precipitation as it took place in solution media. In this agreement, fortunately, we have been able to arrest (Cu2O þ CuO)@CuSe yolk-shell intermediates. As directly evidenced in the TEM image of Figure 5, during the formation of CuSe hollow spheres, the solid cores (a mixed phase of Cu2O and CuO, according to eqs 1-3) gradually reduced their size, releasing Cu2þ cations through surface oxidation and redepositing them back onto the CuSe shells. Furthermore, direct dissolution of cuprous oxide was also possible since Cu2O has a higher solubility than CuO (i.e., the reaction proceeds according to eq 3). As a result, the dissolving solid mixture (i.e., Cu2O þ CuO) turned into movable cores (the TEM image of Figure 5), and they disappeared finally as have been reported in Figure 2. Consistent with the TEM findings, all the discussed phases were detected in an XRD study shown in Figure 5. With low-concentration Cu2O suspensions, slow evaporation of solvent at room temperature leads to formations of different types of superlattices from the Cu2O “artificial atoms”. Because of their uniform sizes, the Cu2O nanospheres were able to selfassemble into a long-range two-dimensional periodic closepacked array (monolayer, Figure 1a,b) on the major part of Langmuir 2010, 26(8), 5963–5970

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Figure 7. (a) UV-vis spectra (absorbance) and (b) plots of (REphoton)2 versus Ephoton with the direct transition for the as-prepared Cu2O solid spheres, CuS hollow spheres, and CuSe hollow spheres.

copper grid with the assistance of Tween-85 surfactant and the capillary force of solvent, especially at the concentration of 0.1 mL of Cu2O suspension (see Experimental Section for more details). Three-dimensional stacking of Cu2O nanospheres (i.e., double layers or triple layers) with {111} surface terminations can also be observed at a higher concentration of 0.2 mL. For the triple-layer arrangements, both hexagonal close-packed (hcp; Figure 1e,f) and face-centered cubic (fcc; Figure 1g,h) supercrystalline structures can be found. It should be noted that the “artificial atoms” Cu2O spheres used in this self-assembly process are all the same, and the only difference between the two types of superstructures is their stacking sequence but not the Cu2O building blocks. The hcp stacking cycle is between the two equivalent shifted positions (i.e., 3 3 3 ABAB 3 3 3 arrangement), whereas the fcc stacking cycle is among three layer registrations (i.e., 3 3 3 ABCABC 3 3 3 arrangement). In relation to the fcc stacking, additionally, square supercrystalline assemblages with double- or triple-layered configurations can also be obtained (Figure 1i,j), giving rise to the {100} surface terminations of the same fcc supercrystalline system. Similarly, 2D and 3D superlattices of CuS and CuSe hollow spheres can also be attained via slow evaporation of dilute suspensions. Figure 2 displays some representative supercrystalline assemblies of these hollow “artificial atoms”. However, because of less uniformity in size, longrange order becomes more difficult to attain for the hollow sphere assemblages, compared to the Cu2O solid spheres shown in Figure 1. It should be mentioned that in order to get crack-free 2D or 3D (g2 layers) superlattices of Cu2O spheres, precise control of the concentration is the most important (e0.2 mL). Furthermore, surfactant polyoxyethylene sorbitan trioleate (Tween-85) is indispensable in this self-assembly process. Figure 6 displays some representative FTIR spectra of our samples. The peaks at 628 and 620 cm-1 are assigned to Cu-O and Cu-X (X = S and Se) bond vibrations in the Cu2O solid spheres and CuX hollow spheres, respectively. In agreement with the XPS finding, a small band at 3326 cm-1 is due to hydroxylation (Cu-O-H) on the surface of Cu2O solid spheres.40 The Tween-85 fingerprint absorption peaks over the wavenumber range of 1000-3000 cm-1 affirm the presence of this nonionic surfactant and thus possible oleate functional groups on the surface of these samples,40 which contribute to the self-assembly, although there are less surface organics on the CuS and CuSe hollow spheres. The concentration of Tween-85 used in the experiments seems not so important, as any excess surfactant will be washed away by ethanol (once). If they are washed twice, however, the Cu2O spheres will aggregate Langmuir 2010, 26(8), 5963–5970

randomly. Evaporation speed of the solvent is another crucial parameter for controlling the formation of superlattices. Ethanol and deionized water mixed solvent works better than the pure ethanol alone, since a lower evaporation rate can be attained from this cosolvent. We have also investigated the effect of temperature on the above self-assembly processes. Crack-free 2D ordered array of Cu2O spheres cannot be easily obtained at either low (0 °C) or high temperatures (30-50 °C). The UV-vis absorption spectra of our Cu2O, CuS, and CuSe samples are displayed in Figure 7a. A broad band in the range of 300-500 nm can be observed for Cu2O solid spheres. The absorption centered at 370 nm indicates a clear blue shift from what normally observed at around 570 nm for bulk Cu2O, which can be ascribed to the quantum confinement effect.12,13,19 The UV-vis absorption spectra of the CuS and CuSe hollow spheres show broad absorption peaks centered at about 420 and 523 nm, respectively. Both samples also exhibit an increasing absorption band in the near-IR region, which may reflect an electronacceptor state lying within the band gap or scattering by the particles in the dispersion system.16,42 In order to estimate the band-gap energies for the three types of samples, a classical Tauc approach was further employed using the equation of REphoton = K(Ephoton- Egap)1/2, where R is the absorption coefficient, K is a constant, Ephoton is the discrete photon energy, and Egap is the optical band-gap energy.43 Based on the direct transition, three representative plots of (REphoton)2 versus Ephoton are reported in Figure 7b. The extrapolated value of Ephoton at R = 0 gives an absorption edge energy corresponding to Egap = 2.49 eV for Cu2O solid spheres, 2.26 eV for CuS hollow spheres, and 1.90 eV for CuSe hollow spheres. It should be pointed out that although the apparent sizes of these spheres are in the submicrometer regime of 130-240 nm, the crystallites within the spheres must be much smaller. According to our simple estimations with the Scherrer equation (see Experimental Section), the size of primary crystallites comprised of Cu2O submicrometer spheres is about 6.8 nm (with the 111 peak), and the sizes of crystallites in the derived CuS and CuSe hollow spheres are 7.3 nm (with the 110 peak) and 1.8-3.0 nm (with the 110 peak and 006 peak), respectively. The found the crystallite size range is quite comparable with the Bohr exciton diameters of these materials.44 In view of polycrystalline nature and nanocrystallinity in the spheres and (42) Yu, X. L.; Cao, C. B.; Zhu, H. S.; Li, Q. S.; Liu, C. L.; Gong, Q. H. Adv. Funct. Mater. 2007, 17, 1397–1401. (43) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318–1321. (44) Borgohain, K.; Murase, N.; Mahamuni, S. J. Appl. Phys. 2002, 92, 1292– 1297.

DOI: 10.1021/la904292t

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Pang and Zeng

shell structures, the observed blue shifts in Egap of the three samples with respect to bulk Cu2O (2.17 eV), CuS (1.2 eV), and CuSe (1.05 eV) can be indeed attributable to the quantum confinement effects.13,45-47 Apart from the small crystallite sizes in these solid/hollow spheres, however, the actual shapes of the crystallites and the ways of organization of crystallites in the final structures also affect the observed quantum confinement. All these influencing causes will be interesting topics for future research.

Conclusions In summary, we have developed a solution-based method to synthesize monodisperse Cu2O spheres with diameters in the submicrometer regime of 130-135 nm. With the as-prepared submicrometer Cu2O spheres as solid precursor, we have further prepared CuS and CuSe hollow spheres using wet chemical approaches. More importantly, we have successfully (45) Wong, E. M.; Searson, P. C. Appl. Phys. Lett. 1999, 74, 2939–2941. (46) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (47) Malik, M. A.; O’Brien, P.; Revaprasadu, N. Adv. Mater. 1999, 11, 1441– 1444.

5970 DOI: 10.1021/la904292t

organized the as-prepared Cu2O solid spheres and their derived CuS and CuSe hollow building blocks into various 2D and 3D superlattices for the first time. Without using sacrificial solid templates, the degree of ordering achieved in these superlattices is comparable to those reported for silica and polystyrene beads. Starting with the Cu2O spheres as solid precursor, furthermore, we have also examined mechanistic aspects of Kirkendall processes in the formation of CuS and CuSe hollow spheres and measured optical band-gap energies for all the copper-containing solids studied in this work. The realization of these self-assembled superlattices provides us a bottom-up approach for rationale design and fabrication of thin films for this class of photosensitive semiconducting materials. Acknowledgment. The authors gratefully acknowledge the financial support of Ministry of Education, Singapore and King Abdullah University of Science and Technology, Saudi Arabia. Supporting Information Available: Experimental details, TEM/SAED and EDX results. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(8), 5963–5970