Morphological Evolution of Cu2O Nanocrystals in an Acid Solution

Dec 15, 2010 - Jaya Pal , Chanchal Mondal , Anup Kumar Sasmal , Mainak Ganguly , Yuichi Negishi , and Tarasankar Pal. ACS Applied Materials & Interfac...
0 downloads 0 Views 4MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Morphological Evolution of Cu2O Nanocrystals in an Acid Solution: Stability of Different Crystal Planes Qing Hua,†,‡,§ Daili Shang,§ Wenhua Zhang,‡,^ Kai Chen,‡,^ Sujie Chang, Yunsheng Ma,§ Zhiquan Jiang,† Jinlong Yang,†,§ and Weixin Huang*,†,‡,§ †,‡,§

† Hefei National Laboratory for Physical Sciences at the Microscale, ‡CAS Key Laboratory of Materials for Energy Conversion, §Department of Chemical Physics, and ^Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China

Received November 9, 2010. Revised Manuscript Received December 3, 2010 The morphological evolution of uniform Cu2O nanocrystals with different morphologies in a weak acetic acid solution (pH=3.5) has been studied for cubic, octahedral, rhombic dodecahedral, {100} truncated octahedral, and {110} truncated octahedral nanocrystals. Cu2O nanocrystals undergo oxidative dissolution in weak acid solution, but their morphological changes depend on the exposed crystal planes. We found that the stability of Cu2O crystal planes in weak acid solution follows the order of {100} . {111}>{110} and determines how the morphology of Cu2O nanocrystals evolves. The stable {100} crystal planes remain, and new {100} facets form at the expense of the less stable {111} and {110} crystal planes on the surface of Cu2O nanocrystals. Density functional theory calculations reveal that the Cu-O bond on Cu2O(100) surface has the shortest bond length. These results clearly exemplify that the morphology of inorganic crystals will evolve with the change of local chemical environment, shedding light on fundamentally understanding the morphological evolution of natural minerals and providing novel insights into the geomimetic synthesis of inorganic materials in the laboratory.

1. Introduction Minerals in nature have rich compositions and morphologies, but the evolution of their composition and morphology is not well understood, particularly with respect to how they are influenced by the local chemical environment. Recently, the activity of human beings has exerted dramatic influence on the environment. The acidification of oceans has been found to accelerate greatly in the first decade of the 21st century.1-3 It is thus of broad interest and great importance to investigate the influence of the acidification of water resources in nature (ocean, lake, river, rain, etc.) on the composition and morphology of minerals. On one hand, this might serve as an indicator or provide a prediction of how human beings’ activity will affect the natural environment; on the other hand, this could also shed light on fundamentally understanding the evolution of composition and morphology of minerals in nature and then provide novel insights into the geomimetic synthesis of inorganic materials in the laboratory. Cuprite is an oxide mineral composed of Cu2O and usually appears as cubic, octahedral, or dodecahedral forms, or in combinations. Numerous Cu2O nano- and microstructures with wellcontrolled uniform morphologies have been recently synthesized, including cubes,4-6 octahedra,7 {100} truncated octahedra,8 *To whom all correspondence should be addressed: Fax þ 86-551-3600437; e-mail [email protected]. (1) Caldeira, K.; Wickett, M. E. Nature 2003, 425, 365. (2) Orr, J. C.; et al. Nature 2005, 437, 681. (3) Hall-Spencer, J. M.; Rodolfo-Metalpa, R.; Martin, S.; Ransome, E.; Fine, M.; Turner, S. M.; Rowley, S. J.; Tedesco, D.; Buia, M.-C. Nature 2008, 454, 96. (4) Gou, L. F.; Murphy, C. J. Nano Lett. 2003, 3, 231. (5) Li, X. D.; Gao, H. S.; Murphy, C. J.; Gou, L. F. Nano Lett. 2004, 4, 1903. (6) Kuo, C. H.; Chen, C. H.; Huang, M. H. Adv. Funct. Mater. 2007, 17, 3773. (7) Kuo, C. H.; Huang, M. H. J. Phys. Chem. C 2008, 112, 18355. (8) Zhang, D. F.; Zhang, H.; Guo, L.; Zheng, K.; Han, X. D.; Zhang, Z. J. Mater. Chem. 2009, 19, 5220. (9) Liang, X. D.; Gao, L.; Yang, S. W.; Sun, J. Adv. Mater. 2009, 21, 2068. (10) Yao, K. X.; Yin, X. M.; Wang, T. H.; Zeng, H. C. J. Am. Chem. Soc. 2010, 132, 6131.

Langmuir 2011, 27(2), 665–671

rhombic dodecahedra,9,10 {110} truncated octahedra,9 {100} truncated rhombic dodecahedra,11 26-facet polyhedral,12 nanowires,13-16 nanoplates,17 nanocages,18,19 and branched and hollow structures.20-23 The morphologies of as synthesized Cu2O nanocrystals have been observed to evolve with their local chemical environment. Octahedral Cu2O nanocrystals were reported to transform into octahedral Cu2O nanocages and corneretched Cu2O octahedra in Fehling’s solutions containing glucose and PdCl2 without and with the presence of polyvinylpyrrolidone (PVP), respectively.19 Sui et al.18 reported that polyhedral Cu2O nanoparticles exposed to air at room temperature could undergo oxidative etching and transform to Cu2O nanoframes and nanocages with single-crystal walls. Kuo and Huang11 reported the morphological transformation from {100} truncated rhombic dodecahedral Cu2O nanocages to Cu2O nanoframes with elliptical pores on the {110} faces in an aqueous HCl solution with the addition of ethanol. Choi’s group has systematically investigated the fabrication and morphological transformation of Cu2O nanocrystals in an electrochemical environment.24-26 (11) Kuo, C. H.; Huang, M. H. J. Am. Chem. Soc. 2008, 130, 12815. (12) Zhang, Y.; Deng, B.; Zhang, T. R.; Gao, D. M.; Xu, A. W. J. Phys. Chem. C 2010, 114, 5073. (13) Wang, W. Z.; Wang, G. H.; Wang, X. S.; Zhan, Y. J.; Liu, Y. K.; Zheng, C. L. Adv. Mater. 2002, 14, 67. (14) Hong, X.; Wang, G.; Zhu, W.; Shen, X.; Wang, Y. J. Phys. Chem. C 2009, 113, 14172. (15) Singh, D. P.; Neti, N. R.; Sinha, A. S. K.; Srivastava, O. N. J. Phys. Chem. C 2007, 111, 1638. (16) Tan, Y. W.; Xue, X. Y.; Peng, Q.; Zhao, H.; Wang, T. H.; Li, Y. D. Nano Lett. 2007, 7, 3723. (17) Kuo, C. H.; Hua, T. E.; Huang, M. H. J. Am. Chem. Soc. 2009, 131, 17871. (18) Sui, Y. M.; Fu, W. Y.; Zeng, Y.; Yang, H. B.; Zhang, Y. Y.; Chen, H.; Li, Y. X.; Li, M. H.; Zou, G. T. Angew. Chem., Int. Ed. 2010, 49, 4282. (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. (20) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 273. (21) Chang, Y.; Teo, J. J.; Zeng, H. C. Langmuir 2005, 21, 1074. (22) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369. (23) Ho, J. Y.; Huang, M. H. J. Phys. Chem. C 2009, 113, 14159.

Published on Web 12/15/2010

DOI: 10.1021/la104475s

665

Article

Hua et al.

Uniform Cu2O nanocrystals with various morphologies offer a model system for a comprehensive study of the influence of the acidification of water on the morphology of minerals in nature. In this article, we report the morphological evolution of different types of Cu2O nanocrystals including cubes exposing {100} crystal planes, octahedra exposing {111} crystal planes, rhombic dodecahedra exposing {110} crystal planes, {100} truncated Cu2O octahedra exposing {100} and {111} crystal planes, and {110} truncated Cu2O octahedra exposing {110} and {111} crystal planes in a weak acid solution. The stability of different crystal faces in a weak acid solution has been successfully established to follow the order of {100} . {111} > {110} and plays a decisive role in the morphological evolution process of various Cu2O nanocrystals.

2. Experimental Section 2.1. Synthesis of Cu2O Nanocrystals. In this work, five types of Cu2O nanocrystals were prepared. All the chemicals were analytical grade reagents and used as received without further purification. The synthesis of cubic, octahedral, and {100} truncated octahedral Cu2O nanocrystals followed Zhang et al.’s recipe.8 In a typical procedure, 5.0 mL NaOH aqueous solution (2.0 mol/L) was added dropwise into 50 mL of CuCl2 aqueous solution (0.01 mol/L) containing different amounts of polyvinylpyrrolidone (PVP, MW=30 000) (cubic: 0 g; octahedral: 4.44 g; {100} truncated octahedral: 1.67 g) at 55 C. After adequately stirring for 0.5 h, 5.0 mL of ascorbic acid aqueous solution (0.6 mol/L) was added dropwise into the solution. The mixed solution was adequately stirred for different time (cubic: 5 h; octahedral: 3 h; {100} truncated octahedral: 3 h) at 55 C. The resulting precipitate was collected by centrifugation and decanting, then washed with distilled water and absolute ethanol, and finally dried in vacuum at RT for 12 h. The synthesis of rhombic dodecahedral and {110} truncated octahedral Cu2O nanocrystals followed Liang et al.’s recipe.9 In a typical procedure, 1 mmol of CuSO4 was dissolved in 40 mL of water to form a clear solution into which different amounts of oleic acid (rhombic dodecahedral: 4 mL; {110} truncated octahedral: 3.5 mL) and 20 mL of absolute ethanol were added successively with vigorous stirring. When the mixture was heated to 100 C, 10 mL of NaOH solution (8 mmol) was added. After 5 min, a 30 mL aqueous solution containing 3.42 g D-(þ)-glucose was added under constant stirring. The mixture reacted for another 60 min, and a brick-red color gradually appeared. The resulting precipitate was collected by centrifugation and decanting, then washed with distilled water and absolute ethanol, and finally dried in vacuum at RT for 12 h. 2.2. Etching of Cu2O Nanocrystals in an Acid Solution. 25 mg of Cu2O nanocrystals was dispersed into 40 mL of acetic acid solution (pH = 3.5). The mixture was stirred with a magnetic stirrer at 500 rpm. At desirable reaction times, Cu2O nanocrystals were collected by centrifugation and decanting, then washed with distilled water and absolute ethanol, and finally stored in absolute ethanol. 2.3. Structural Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert PRO diffractometer using a nickel-filtered Cu KR (wavelength: 0.154 18 nm) radiation source with the operation voltage and operation current being 40 kV and 50 mA, respectively. UV-vis absorption spectra were measured on a Shimadzu UV-2450 UV-vis scanning spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer using monochromatized Al KR (hν = 1486.7 eV) as the excitation source. The likely charging of samples was corrected by setting the binding energy of the adventitious (24) Siegfried, M. J.; Choi, K. S. Adv. Mater. 2004, 16, 1743. (25) Siegfried, M. J.; Choi, K. S. Angew. Chem., Int. Ed. 2005, 44, 3218. (26) Siegfried, M. J.; Choi, K. S. J. Am. Chem. Soc. 2006, 128, 10356.

666 DOI: 10.1021/la104475s

carbon (C 1s) to 284.8 eV. Scanning electron microscope (SEM) experiments were performed on a FEI Sirion200 field emission scanning electron microscope operated at beam energy of 5.0 kV. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) experiments were preformed on a JEOL-2010 high-resolution transmission electron microscope with an electron acceleration energy of 200 kV. The optimized structures of various Cu2O surfaces were calculated within the spin-polarized density functional theory implemented in the DMol package, whose details were reported previously.27

3. Results and Discussion Figure 1 shows the SEM images of cubic (a1 and a2), octahedral (d1 and d2), and rhombic dodecahedral (g1 and g2) Cu2O nanocrystals. All three types of Cu2O nanocrystals have uniform morphology, and cubic Cu2O nanocrystals are 400-700 nm, octahedral Cu2O nanocrystals are about 300 nm, and rhombic dodecahedral Cu2O nanocrystals are 600-900 nm. The XRD patterns of these samples (left panel in Figure 2) confirm their crystal phase to be cubic fcc Cu2O structure (JCPDS card no. 78-2076; space group Pn3m; lattice constant a = 4.27 A˚). The (200) diffraction peak in the XRD pattern of cubic Cu2O nanocrystals exhibits a much stronger intensity over other diffraction peaks, indicating that cubic Cu2O nanocrystals exclusively expose {100} planes. Similar results were also observed for the (111) diffraction peak in the XRD pattern of octahedral Cu2O nanocrystals exclusively exposing {111} planes and the (220) diffraction peak in the XRD pattern of rhombic dodecahedral Cu2O nanocrystals exclusively exposing {110} planes. The Cu 2p XPS spectra of these samples (right panel in Figure 2) only show a single component with a Cu 2p3/2 binding energy at 932.2 eV corresponding to Cu(I) in Cu2O, demonstrating that their surface composition remain as Cu2O. No difference was observed in the XRD pattern and Cu 2p XPS spectra (Figure 2) after these Cu2O nanocrystals were etched in weak acetic acid solution with pH=3.5; however, the formation of Cu(II) in the solution was clearly identified by UV-vis absorption spectroscopy (Figure S1) of the aqueous solutions after the same amount of various Cu2O nanocrystals was etched in the weak acid solution for 150 min. The UV-vis absorption peak at 789 nm arising from Cu(II) in the aqueous solution clearly indicates the occurrence of oxidative dissolution of Cu2O following the equation Cu2O þ O2 þ 8Hþ f4Cu2þ þ 4H2O. Interestingly, SEM results reveal that the morphological evolution of different Cu2O nanocrystals depends sensitively on their exposed crystal facets during the course of the oxidative dissolution process (etching process). Cubic Cu2O nanocrystals roughly keep their original shape (Figure 1, b1, b2, c1, and c2), although their surfaces become rough with the corners becoming sharper after etching in the weak acid solution. Some pinholes are clearly visible on the surfaces of cubic Cu2O nanocrystals after etching for 150 min (Figure 1, c1 and c2). These results are further confirmed by TEM observations (Figure 3a-c). The SAED patterns of original cubic Cu2O nanocrystals and cubic Cu2O nanocrystals etched for 150 min (insets in Figure 3a,b) demonstrate that both expose {100} planes and the view direction is along [001]. Lamellar overlayers were clearly observed in the TEM image of cubic Cu2O nanocrystals etched for 150 min (Figure 3c), and the HRTEM image of the corresponding lamellar structure (Figure 3d) reveals two vertical sets of lattice fringes of 0.30 nm, i.e., the lattice fringe of the Cu2O (110) plane. These results clearly demonstrate that the lamellar overlayers on etched cubic Cu2O nanocrystals still expose {100} (27) Bao, H. Z.; Zhang, W. H.; Shang, D. L.; Hua, Q.; Ma, Y. S.; Jiang, Z. Q.; Yang, J. L.; Huang, W. X. J. Phys. Chem. C 2010, 114, 6676.

Langmuir 2011, 27(2), 665–671

Hua et al.

Article

Figure 1. SEM images of Cu2O nanocrystals etched for different times: cubic Cu2O nanocrystals etched for 0 min (a1 and a2), 10 min (b1 and b2), 150 min (c1 and c2); octahedral Cu2O nanocrystals etched for 0 min (d1 and d2), 10 min (e1 and e2), 150 min (f1 and f2); rhombic dodecahedral Cu2O nanocrystals etched for 0 min (g1 and g2), 10 min (h1 and h2), 150 min (i1 and i2). The insets in (g1), (h1), and (i1) show the corresponding single Cu2O nanocrystal.

Figure 2. XRD patterns and Cu 2p XPS spectra of as synthesized cubic (a), octahedral (c), rhombic dodecahedral (e) Cu2O nanocrystals and cubic (b), octahedral (d), and rhombic dodecahedral (f) Cu2O nanocrystals etched for 150 min. Langmuir 2011, 27(2), 665–671

planes. Therefore, it can be adequately concluded that cubic Cu2O nanocrystals always exclusively expose the {100} planes during the course of oxidative dissolution in weak acid solution. The SEM images (Figure 1, e1 and e2) show that the edges of octahedral Cu2O nanocrystals exclusively exposing {111} planes become jagged after etching in the weak acid solution for 10 min while their faces do not show any obvious changes. With prolonged etching time (150 min), stepped parallel layers of square facets with different sizes whose corners locate at the edges of original octahedron and whose normal directions are the same as that of the vertices of the original octahedra become clearly visible (Figure 1, f1 and f2), although the etched Cu2O nanocrystals are still octahedral. According to the symmetry analysis, the normal direction of the vertices of the octahedron is along [100]; therefore, the exposed crystal plane of facets should be {100}. Parts a and b of Figure 4 respectively show the TEM images of original octahedral Cu2O nanocrystals and Cu2O nanocrystals etched for 150 min in DOI: 10.1021/la104475s

667

Article

Figure 3. TEM images of cubic Cu2O nanocrystals etched for 0 min (a) and 150 min (b, c) and HRTEM image of cubic Cu2O nanocrystals etched for 150 min (d). The insets in (a) and (b) show the corresponding SEAD patterns.

Figure 4. TEM images of octahedral Cu2O nanocrystals with (111) surface toward the outside of paper etched for 0 min (a) and for 150 min (b, c) and HRTEM image of cubic Cu2O nanocrystals etched for 150 min (d). The insets in (a) and (b) show the corresponding SEAD patterns.

which the view direction is inferred from the SAED patterns to be along [111]. It can be clearly seen that the edges of the octahedral Cu2O nanocrystal become rough after etching for 150 min. The HRTEM image of one edge of an etched octahedral Cu2O nanocrystal reveals three sets of {110} planes with 60 as the included angle but fails to provide any novel information since the view direction is along [111]. We successfully prepared a specimen of octahedral Cu2O nanocrystals for TEM imaging with an unusual 668 DOI: 10.1021/la104475s

Hua et al.

Figure 5. TEM images of octahedral Cu2O nanocrystals with (100) surface toward the outside of paper etched for 0 min (a) and for 150 min (b, c) and HRTEM image of cubic Cu2O nanocrystals etched for 150 min (d). The insets in (a) and (b) show the corresponding SEAD patterns. The inset in (d) enlarges the image of lattice fringes.

view direction along [100]. Parts a and b of Figure 5 respectively show the TEM images of the original octahedral Cu2O nanocrystals and Cu2O nanocrystals etched for 150 min in which the view direction is inferred from the SAED patterns to be along [100]. The projection of octahedral Cu2O nanocrystals without etching (Figure 5a) along the [100] view direction is square, demonstrating that their edges are flat. After etching for 150 min (Figure 5b), the edges of etched octahedral Cu2O nanocrystals have zigzag structures arising from the projection of the square facets on etched octahedral Cu2O nanocrystals shown in the corresponding SEM image (Figure 1, f2). The zoom-in TEM image (Figure 5c) clearly shows that the corner of zigzag structures is about 90, and the HRTEM image (Figure 5d) reveals two vertical sets of lattice fringes of 0.30 nm, i.e., the lattice fringe of Cu2O (110) planes, demonstrating that the square facets on etched octahedral Cu2O nanocrystals exclusively expose {100} planes. Therefore, oxidative dissolution of octahedral Cu2O nanocrystals exclusively exposing {111} planes in the weak acid solution leads to the formation of facets exposing {100} planes, evidencing that the Cu2O {100} planes are more stable than the Cu2O {111} planes. The faces of rhombic dodecahedral Cu2O nanocrystals exclusively exposing {110} planes obviously become rough after etching in the weak acid solution for 10 min (Figure 1, h1 and h2). After etching for 150 min, stepped layers of square facets with different sizes substantially develop although the etched Cu2O nanocrystals are still with the rhombic dodecahedral shape (Figure 1, i1 and i2). Figure 6 shows the TEM and HRTEM images of original rhombic dodecahedral Cu2O nanocrystals and rhombic dodecahedral Cu2O nanocrystals etched for 150 min. The SAED patterns (insets in Figure 6a,b) demonstrate that the view direction is along [110]. The edges of original rhombic dodecahedral Cu2O nanocrystal are flat (Figure 6a), but those etched for 150 min have clearly visible zigzag structures with a 90 vertex (Figure 6b,c). The HRTEM image of the zigzag structure (Figure 6d) reveals a lattice fringe of 0.43 nm corresponding to the Langmuir 2011, 27(2), 665–671

Hua et al.

lattice fringe of Cu2O (100) planes, demonstrating that their normal direction is along [100]. Therefore, oxidative dissolution of rhombic dodecahedral Cu2O nanocrystals exclusively exposing {110} planes in the weak acid solution leads to the formation of facets exposing {100} planes, evidencing that the Cu2O {100} planes are more stable than the Cu2O {110} planes. The above results clearly demonstrate that Cu2O {100} planes are more stable than Cu2O {111} and {110} planes in weak acid solution with pH=3.5. We have changed the pH value of the acetic acid solution and the concentration of acetate anion and found that the shape evolution of different Cu2O nanocrystals was obviously influenced by the pH value, but not by the concentration of acetate anion. Therefore, it is unlikely that acetate anion plays an important role in the shape evolution of Cu2O nanocrystals in the acetic acid solution. It is the stability of Cu2O {100}, {111}, and {110} planes in the weak acid solution that determines the morphological evolution of Cu2O nanocrystals during the course of the oxidative dissolution reaction, in which exposed {100} planes

Figure 6. TEM images of rhombic dodecahedral Cu2O nanocrystals etched for 0 min (a) and for 150 min (b, c) and HRTEM image of cubic Cu2O nanocrystals etched for 150 min (d). The insets in (a) and (b) show the corresponding SEAD patterns. The inset in (d) enlarges the image of lattice fringes.

Article

are preserved and new {100} planes form at the expense of exposed {111} and {110} planes. We believe that this conclusion also holds for other acid solutions. Kuo and Huang11 previously studied the oxidative dissolution of {100} truncated rhombic dodecahedral Cu2O nanocages exposing {100} and {110} planes in aqueous HCl solution with the addition of ethanol and observed the selective etching of {110} planes that eventually forms Cu2O nanoframes with elliptical pores on the {110} faces. Their results also demonstrate that Cu2O {100} planes are more stable than Cu2O {110} planes in the weak acid solution. Selective etching of a specific crystal plane for nanocrystals in a chemical reaction due to the different stability of exposed crystal planes has also been reported for several other systems. Im et al.28 employed HCl to selectively etch silver twin nanocrystals, leading to the formation of uniform cubic silver nanocrystals. Mulvihill et al.29 reported selective etching of {100} facets on {100} truncated octahedral silver nanocrystals by ammonia. Selective chemical etching of magnetite microoctahedra in NaBH4 aqueous solution was reported to form magnetite (111) single crystal microplates.30 Fabrication of large area aligned silicon nanowire arrays has also been reported by metal-induced selective etching of silicon substrates in an oxidizing HF solution.31-36 The relative stability of Cu2O {100}, {111}, and {110} planes the weak acid solution (pH = 3.5) can be further manifested by the morphology evolution of {100} truncated octahedral Cu2O nanocrystals exposing both {100} and {111} planes and {110} truncated octahedral Cu2O nanocrystals exposing both {110} facets and {111} planes, whose SEM results are presented in Figure 7. Both types of Cu2O nanocrystals are uniform, with {100} truncated octahedral Cu2O nanocrystals (Figure 7, a1 and a2) having a size of about 300 nm while {110} truncated octahedral Cu2O nanocrystals (Figure 7, c1 and c2) have a size of about 500-700 nm. After {100} truncated octahedral Cu2O nanocrystals were etched for 10 min, the {111} planes disappear while the {100} planes remain (Figure 7, b1 and b2), directly proving that Cu2O {100} planes are much more stable than Cu2O {111} planes. The stability difference between {111} and {110} planes is not as large as that between {111} and {100} planes. After etching for 10 min, the square {100} facets obviously form on the {110} planes of {110} truncated octahedral Cu2O nanocrystals and extend to the {111} planes; however, the {111} planes are still visible. This infers that Cu2O {111} planes are much more stable than Cu2O {110} planes. Therefore, the order of the stability of low-indexed crystal planes of Cu2O nanocrystals in weak acid solution follows {100} . {111} > {110}. This could also be inferred from UV-vis absorption spectra (Figure S1). The concentration of Cu(II) in the aqueous solutions as the product of oxidative dissolution of Cu2O

Figure 7. SEM images of Cu2O nanocrystals etched for different times:{100} truncated octahedral Cu2O nanocrystals etched for 0 min (a1, a2), 10 min (b1, b2); {110} truncated octahedral Cu2O nanocrystals etched for 0 min (c1, c2), 10 min (d1, d2). Langmuir 2011, 27(2), 665–671

DOI: 10.1021/la104475s

669

Article

Hua et al.

Figure 8. Optimized structures of Cu2O (111) and (100) surfaces. The red, brick red, and green balls represent oxygen, coordinated saturated copper (CuCSA), and coordinated unsaturated copper (CuCus) atoms, respectively. The distance between the nearest-neighboring Cu(I) and O on different surfaces is also indicated.

Figure 9. Schematic illustrations of cubic, octahedral, and rhombic dodecahedral Cu2O nanocrystals (upper panel) and the proposed possible oxidative dissolution mechanisms of Cu2O nanocrystals in the weak acid solution (lower panel).

after the same amount of various Cu2O nanocrystals was etched in the weak acid solution for 150 min follows the order rhombic dodecahedral > octahedral . cubic, consistent with the order of their stability. (28) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2154. (29) Mulvihill, M. J.; Ling, X. Y.; Henzie, J.; Yang, P. D. J. Am. Chem. Soc. 2010, 132, 268. (30) Hua, Q.; Huang, W. X. J. Mater. Chem. 2008, 18, 4286. (31) Peng, K. Q.; Lu, A. J.; Zhang, R. Q.; Lee, S. T. Adv. Funct. Mater. 2008, 18, 3026–3035. (32) Chen, C. Y.; Wu, C. S.; Chou, C. J.; Yen, T. J. Adv. Mater. 2008, 20, 3811– 3815. (33) Peng, K. Q.; Zhang, M. L.; Lu, A. J.; Wong, N. B.; Zhang, R. Q.; Lee, S. T. Appl. Phys. Lett. 2007, 90, 163123–163125. (34) Clark, I. T.; Aldinger, B. S.; Gupta, A.; Hines, M. A. J. Phys. Chem. C 2010, 114, 423–428. (35) Faggin, M. F.; Green, S. K.; Clark, I. T.; Queeney, K. T.; Hines, M. A. J. Am. Chem. Soc. 2006, 128, 11455–11462. (36) Huang, Z. P.; Shimizu, T.; Senz, S.; Zhang, Z.; Zhang, X. X.; Lee, W.; Geyer, N.; Gosele, U. Nano Lett. 2009, 9, 2519–2525.

670 DOI: 10.1021/la104475s

The oxidative dissolution of Cu2O nanocrystals always initiates from the surface. Figure 8 illustrates the optimized structures of Cu2O (100), (111), and (110) surfaces. On Cu2O (100), the first layer consists of coordinated unsaturated O (OCUS) and the second layer consists of coordinated saturated Cu (CuCSA). The distance between the nearest-neighboring OCUS and CuCSA (d(CuCSAOCUS)) is 1.76 A˚. On Cu2O(111), the first layer consists of (OCUS) and the second layer consists of CuCSA and coordinated unsaturated Cu (CuCUS) with a distance along z direction of 0.23 A˚. The distance between the nearest-neighboring OCUS and CuCSA is 1.83 A˚, and that between the nearest-neighboring CuCUS and coordinated saturated O (OCSA) (d(CuCUS-OCSA)) is 1.91 A˚. On Cu2O(110), the first layer consists of both OCUS and CuCSA and the second layer consists of CuCSA. The distance between the nearestneighboring OCUS and CuCSA (d(CuCSA-OCUS)) is 1.82 A˚. Since the Cu-O bond on the surface must be broken for the oxidative dissolution of Cu2O in the weak acid solution, the strength of Cu-O bond on Cu2O surface should play a decisive role. Langmuir 2011, 27(2), 665–671

Hua et al.

Inferred from the optimized structures of Cu2O (100), (111), and (110) surfaces, the Cu-O bond on Cu2O (100) is shortest and thus strongest; therefore, in the view of Cu-O bond strength, Cu2O(100) surface is most stable in the weak acid solution, agreeing with our experimental results. On the basis of the above results and discussion, we propose a likely mechanism for the morphological evolution of various Cu2O nanocrystals in weak acid solution, as schematically illustrated in Figure 9. Since Cu2O (100) is most stable, the morphological evolution always tends to expose the {100} planes on Cu2O nanocrystals as much as possible, no matter which crystal planes the original Cu2O nanocrystals expose. Cubic Cu2O nanocrystals already exclusively exposing the {100} planes retain their shape during the course of oxidative dissolution (type I mechanism in Figure 9). For rhombic dodecahedral Cu2O nanocrystals exclusively exposing {110} planes, their morphological evolution follows the type II mechanism in Figure 9. The {100} planes will remain and form as long as they are forming by the likely dissolution of some ions on the Cu2O {110} planes as indicated by the dashed lines in type II mechanism. This will form square facets exposing the {100} planes on rhombic dodecahedral Cu2O nanocrystals, as experimentally observed by SEM and TEM. Experimentally, the morphological change of octahedral Cu2O nanocrystals exclusively exposing {111} planes initiates from the edge of octahedron whose normal direction is along [110]. Therefore, the morphological evolution of octahedral Cu2O nanocrystals exclusively exposing {111} planes also follows the type II mechanism, agreeing with the experimental observations that the {100} planes develop from the edge of octahedral Cu2O nanocrystals. This argument also applies in the case of {110} truncated octahedral Cu2O nanocrystals. The morphological evolution of various Cu2O nanocrystals during the course of oxidative dissolution in weak acid solution indicates that the acidification of water resources in nature will definitely result in morphological changes of natural minerals. This could be extended to the change of the local chemical environment that the minerals experience. The relative stability of crystal planes exposed on the mineral crystal surface changes with the variation in the local chemical environment: new stable

Langmuir 2011, 27(2), 665–671

Article

crystal planes will form at the expense of less stable crystal planes on the surface, which determines their morphological evolution. Therefore, the morphology that the natural mineral exhibits might undergo a long-term evolution from the original morphology of the mineral due to the ongoing changing of the local chemical environment. In this view, chemical etching of inorganic crystals might provide a suitable approach for both the geomimetic synthesis of inorganic materials and the fabrication of inorganic materials with novel structures and properties.

4. Conclusions We have comprehensively investigated the morphological evolution of various Cu2O nanocrystals in weak acid solution and established that the stability of Cu2O crystal planes follow the order {100} . {111}>{110}. The relative stability of Cu2O {100}, {111}, and {110} crystal planes determines how the morphology of Cu2O nanocrystals evolves. The stable {100} crystal planes remain, and new stable {100} facets form at the expense of the less stable {111} and {110} crystal planes on the surface of Cu2O nanocrystals. These results clearly demonstrate that the variation of chemical environment caused by human beings’ activity will definitely affect the natural environment; meanwhile, they shed light on fundamentally understanding the morphological evolution of natural minerals and provide novel insights into the geomimetic synthesis of inorganic materials. Acknowledgment. This work was financially supported by National Natural Science Foundation of China (grant 20773113), the solar energy project of Chinese Academy of Sciences, the Ministry of Science and Technology of China (2010CB923302), the MOE program for PCSIRT (IRT0756), the Fundamental Research Funds for the Central Universities (WK2060030005), and the MPG-CAS partner group. Supporting Information Available: Figure S1 and the complete list of ref 2. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la104475s

671