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Direct formation of small Cu2O nanocubes, octahedra, and octapods for efficient synthesis of triazoles† Ya-Huei Tsai, Kaushik Chanda, Yi-Ting Chu, Chun-Ya Chiu and Michael H. Huang* In most studies describing the preparation of Cu2O crystals of various morphologies, the particle sizes are normally hundreds of nanometers to micrometers due to rapid particle growth, so they are not exactly nanocrystals. Here we report surfactant-free formation of sub-100 nm Cu2O nanocrystals with systematic shape evolution from cubic to octahedral structures by preparing an aqueous mixture of Cu(OAc)2, NaOH, and N2H4 solution. Adjustment of the hydrazine volume enables the particle shape control. Uniform nanocubes and octahedra were synthesized with edge lengths of 37 and 67 nm, respectively. Novel Cu2O octapods with an edge length of 135 nm were also produced by mixing CuCl2 solution, SDS surfactant, NaOH solution, and NH2OH$HCl reductant solution. All of them are nearly the smallest Cu2O nanocrystals of the same shapes ever reported. These small cubes, octahedra, and octapods were employed as catalysts in the direct synthesis of 1,2,3-triazoles from the reaction of
Received 17th April 2014 Accepted 14th May 2014
alkynes, organic halides, and NaN3 at 55 C. All of them displayed high product yields in short reaction times. The octahedra enclosed by the {111} facets are the best catalysts, and can catalyze this cycloaddition reaction with high yields in just 2 h when different alkynes were used to make diverse
DOI: 10.1039/c4nr02076f
triazole products. Hence, the small Cu2O particles provide time-saving, energy-efficient, and high
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product yield benefits to organocatalysis.
Introduction Cu2O is an excellent material for the facile fabrication of nanocrystals with a series of well-dened particle shapes for facet-dependent property investigation.1–10 For example, Cu2O nanocubes bound by the {100} facets are practically inactive toward the photodegradation of methyl orange, while rhombic dodecahedra bound by the {110} faces are highly active for this reaction.11–13 A single Cu2O octahedron bound by the {111} surfaces are electrically conductive when a pair of tungsten probes are in contact with the particle for electrical conductivity measurements, but a nanocube is essentially non-conductive below an applied voltage of 3 V.14 Cu2O rhombic dodecahedra are far more catalytically active than cubes and octahedra in the cycloaddition reactions to generate triazoles and isoxazoles.15,16 While various approaches and conditions have been used to make nice Cu2O polyhedra, the particle dimensions have mostly been in the range of over 100 nm to a few micrometers.17–21 Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: [email protected] † Electronic supplementary information (ESI) available: SEM images of Cu2O nanocrystals with shape evolution, XRD patterns, calculations for the determination of volumes needed for the catalysis experiment, spectral characterization of the triazole products synthesized and their NMR spectra. See DOI: 10.1039/c4nr02076f
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Crystals with sizes of a few hundreds of nanometers are quite typical. To facilitate the examination of additional facetdependent properties of Cu2O crystals and enhance their catalytic efficiency, it should be highly desirable to drastically reduce the particle size. Cu2O nanocubes with sizes below 50 nm synthesized in the presence of polyethylene glycol and octahedra with sizes below 100 nm produced by g-irradiation are known.22,23 Diluting the reagent concentrations in an aqueous mixture of CuCl2, NaOH, and N2H4 has been shown to reduce the size of Cu2O cubes from 350 nm to 50 nm.7 Recently, sub-100 nm Cu2O cubes have been obtained by mixing CuSO4, trisodium citrate and NaOH.24 However, it is still quite challenging to make Cu2O nanocrystals of a series of shapes with sizes below 100 nm, especially for octahedral particles. Another interesting and exotic particle morphology is the octapodal shape. Both Cu2O and Ag2O octapods derived from facedepression of the cubic structure have been reported.7,25–29 O2-assisted etching of Cu2O cubes in the presence of sodium ascorbate has been used to produce the etched cubes or octapods.7 Again the octapods synthesized typically have an edge length in the range of 500 nm to over 10 micrometers. It should be nice to signicantly reduce their size. In this study, we have used a surfactant-free method to grow sub-100 nm Cu2O nanocrystals with shape evolution from cubic to octahedral structures by adding hydrazine (N2H4) as the reducing agent. To make small Cu2O octapods with an edge of
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just 135 nm, a solution condition similar to that used previously to grow large Cu2O cubes to hexapods was adopted, but the reagent concentrations have been adjusted.11 The small Cu2O cubes, octahedra, and octapods have been employed as efficient catalysts in a three-component click reaction for the synthesis of regioselective 1,2,3-triazoles. The octahedra are highly efficient catalysts giving excellent yields in the formation of other 1,4disubstituted 1,2,3-triazoles, demonstrating the importance of making exceptionally small Cu2O nanocrystals to enhance their activity in catalysis.
Experimental section Chemicals Copper acetate (Cu(OAc)2, 98%, J. T. Baker), hydrazine hydrate (N2H4$H2O, 99.8%, Alfa Aesar), anhydrous copper(II) chloride (CuCl2, 97%, Aldrich), hydroxylamine hydrochloride (NH2OH$HCl, 99%, Aldrich), sodium dodecyl sulfate (SDS, 100%, J. T. Baker), and sodium hydroxide (NaOH, 98%, Aldrich) were used as received. Ultrapure distilled and deionized water (18.2 MU) was used for all solution preparations. Synthesis of sub-100 nm cubic and octahedral Cu2O nanocrystals To grow the small Cu2O nanocubes and octahedra, 9.7 and 7.0 mL of deionized water were respectively added to a sample vial. The vials were kept in a water bath set at 50 C. Then 0.1 mL of 0.1 M Cu(OAc)2 solution and 0.1 mL of 0.1 M NaOH solution were introduced with vigorous stirring. The resulting solution turned light blue, indicating the formation of a Cu(OH)2 precipitate. Then, 0.1 mL and 2.8 mL of 0.2 M N2H4 solution were quickly injected into each vial with vigorous stirring for the synthesis of Cu2O cubes and octahedra, respectively. The solution turned light yellow immediately. The total solution volume in each vial is 10 mL. The solution was aged in the water bath for 30 min for nanocrystal growth and centrifuged at 5500 rpm for 5 min. Aer the top solution was decanted, the precipitate was washed with 10 mL of 1 : 1 volume ratio of water and ethanol. The precipitate was centrifuged and washed twice more to remove unreacted chemicals. The nal washing step used 5 mL of ethanol, and the precipitate was dispersed in 0.5 mL of ethanol for storage and analysis. Synthesis of Cu2O octapods First, 72.6 mL of deionized water was added to a sample vial. The vial was placed in a water bath set at 32–34 C. Then 0.8 mL of 12.5 mM CuCl2 solution and 0.087 g of SDS powder were added to the vial with vigorous stirring, and the resulting solution was le undisturbed for 5 min. Then, 1.6 mL of 0.125 M NaOH solution was introduced, and the solution turned light blue indicating the formation of a Cu(OH)2 precipitate. Finally, 5 mL of 25 mM NH2OH$HCl solution was quickly injected into the vial with stirring for 20 s. The solution turned yellow within seconds. The total solution volume is 80 mL. Aer aging the solution in the water bath for 2 h for nanocrystal growth, the solution was centrifuged at 5000 rpm
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for 3 min. The same sample purication steps were used as described above. Cu2O nanocrystal-catalyzed click reactions Benzyl bromide (0.0428 g, 0.25 mmol, 1.0 equiv.) and NaN3 (0.026 g, 0.40 mmol, 1.6 equiv.) in 5 mL of ethanol were introduced into a 25 mL round bottomed ask containing a stirrer bar. Aer the mixture was stirred for 10 min at room temperature, phenyl acetylene (0.0255 g, 0.25 mmol, 1.0 equiv.) was added into the solution. A xed amount of small Cu2O nanocrystals was immediately added to the reaction mixture (1.00 mL of nanocubes, 0.81 mL of octahedra, or 2.30 mL of octapods with a calculated total particle surface area of 87 cm2). Subsequently, the reaction mixture was heated to 55 C with stirring. The progress of the reaction was monitored by thin-layer chromatography (TLC). Aer completion of reaction (2–5.5 h), the reaction mixture was centrifuged at 5000 rpm for 3 min to remove the nanocrystals. The solvent was then removed under reduced pressure to obtain the crude compound which was isolated without column chromatography to obtain the corresponding triazole as the product. To evaluate the extent of versatility of the ultrasmall octahedral nanocrystal catalysts, the same cycloaddition reaction was repeated with alkynes having different substituents. Instrumentation Transmission electron microscopy (TEM) characterization was performed using a JEOL JEM-2100 electron microscope with an operating voltage of 200 kV. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-7000F electron microscope. X-ray diffraction (XRD) patterns were collected with the use of a Shimadzu XRD-6000 diffractometer with Cu Ka ˚ UV-vis spectra were acquired using a radiation (l ¼ 1.5418 A). JASCO V-670 spectrophotometer.
Results and discussion To make sub-100 nm Cu2O nanocrystals, a mixed solution of Cu(OAc)2 and NaOH placed in a 50 C water bath was prepared to form a blue Cu(OH)2 precipitate rst, followed by the introduction of various volumes of N2H4 reductant solution to tune the particle shape. Here 0.1 and 2.8 mL of 0.2 M N2H4 solution were added to yield cubic and octahedral Cu2O nanocrystals, respectively. No surfactant was used, so the particles have clean surfaces favorable for catalytic applications. The solution turned from light yellow to bright yellow in 5 min aer the addition of 0.1 mL of N2H4 solution in the formation of Cu2O nanocubes. In contrast, the solution color turned from yellow to orange within 30 s aer the addition of 2.8 mL of N2H4 solution. The solution became less transparent aer 5–10 min, indicating more octahedra were formed. The results show that the Cu2O octahedra are formed more quickly than cubes, and particle shape control can be achieved by tuning their growth kinetics. Cu2O particles should form through the following reaction. 4Cu(OH)2 + N2H4 4 2Cu2O + N2 + 6H2O
(1)
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Since an equal molar amount of Cu(OAc)2 and NaOH was used, not all copper source has turned into Cu(OH)2. Cu2+ can be reduced directly to Cu2O via the reaction given below.
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4Cu2+ + N2H4 + 2H2O 4 2Cu2O + N2 + 8H+
(2)
Eqn (2) suggests that the solution pH can be acidic. In fact, the solution pH in the formation of Cu2O nanocubes is 5.40. Since N2H4 is a Lewis base, addition of a large amount of hydrazine can make the solution basic. The solution prepared for the growth of Cu2O octahedra was measured to have a pH value of 9.54. To synthesize Cu2O octapods, water, CuCl2 solution, SDS surfactant, NaOH solution, and NH2OH$HCl reductant solution were mixed at 32–34 C. The reaction conditions are similar to those described previously, but the reagent concentrations have been nely adjusted.11 Because more NaOH was added, the solution pH is 10.43. The control of the Cu2O crystal shape by tuning the solution NaOH concentration has been demonstrated.4 Fig. 1 shows SEM images of the synthesized Cu2O nanocubes, octahedra, and octapods. The particles all have good size and shape uniformity with a standard deviation of about 10% (see Table S1, ESI† for the standard deviations of the particle sizes). The nanocubes and octahedra are particularly small with average edge lengths of 37 and 67 nm, respectively. If the corner-to-opposite corner distance is used, the average particle size is 98 nm for octahedra. These particle sizes are considered as very small when compared to those reported for Cu2O nano- and microcrystals in most studies. A possible reason very small Cu2O nanocrystals can be synthesized is due to the use of hydrazine, which is a strong reducing agent, to generate many nuclei and this reduces the nal particle size. The octapod-shaped particles are also quite small with an edge length of 135 nm. An octapod can be considered as a structure derived from a cube with a crossed depression on each face, so the {100} face should account for a signicant portion of the particle surface. Additional SEM images of the octapods viewed from different directions are shown in Fig. S2, ESI.† If the volume of 0.2 M N2H4 solution added was 0.8 and 1.8 mL, small cuboctahedra and truncated octahedra can be obtained (see Fig. S2, ESI†). This work also shows that Cu2O nanocrystal shape control can be achieved by tuning the reduction rate.12 The fact that particles of various shapes can be synthesized without adding the surfactant also implies that face-selective molecular capping is not the mechanism by which the nanocrystal shape is tuned. Only the Cu2O nanocubes, octahedra, and octapods are further characterized, because these particles are more homogeneous for better property examination. Fig. S3, ESI† presents XRD patterns of the Cu2O nanocubes, octahedra, and octapods. Octahedra show an exceptionally strong (111) reection peak as expected. Cubes display a stronger (200) peak compared to that seen for octahedra due to their {100} faces. Octapods give equal intensity for the (111) and (200) peaks, reecting their structural relationship with a cube. Fig. 2 shows TEM characterization of a single cube and an octahedron. The (200) lattice planes are aligned parallel to the cubic face. The selected-area electron
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Fig. 1 SEM images of the ultrasmall Cu2O (a) nanocubes, (b) octahedra, and (c) octapods.
diffraction (SAED) pattern of the cube recorded along the [002] zone axis gives the expected diffraction pattern, which is consistent with the orientations of the lattice fringes seen in the high-resolution TEM images of the same cube. For the octahedron, (111) lattice planes are aligned parallel to the {111} facets. The SAED pattern recorded is also consistent with the orientation of the octahedron. Fig. 3 shows the TEM images and SAED patterns of a single octapod viewed along the [100] and [110] directions. A square diffraction pattern was obtained when viewed along the [100] direction. The (200) and (110) lattice fringes are aligned parallel to the edges and a corner of the octapod, conrming the structural relationship of an octapod with a cube. When viewed along the [110] direction, the (111) diffraction spots and lattice fringes are aligned toward the corners of the octapod.
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Fig. 2 (a and d) TEM images and (b and e) the corresponding SAED patterns of ultrasmall cubic and octahedral Cu2O nanocrystals viewed along zone axes. (c and f) High-resolution TEM images of the square regions revealing the (110), (111), and (200) planes of Cu2O. the [002] and [022]
Fig. 3 (a and d) TEM images and (b and e) the corresponding SAED patterns of a single Cu2O octapod viewed from different directions. (c and f) High-resolution TEM images of the square regions in panels a and d.
Fig. 4 shows UV-vis absorption spectra of Cu2O cubes, octahedra, and octapods and photographs of the particle solutions. The solution of Cu2O cubes is yellow, while the larger octahedra and octapods display golden yellow and orange solution colors, respectively. The cubes show two absorption bands at 365 and 455 nm. The major absorption band for the octahedra is located at 460 nm. The absorption band becomes more red-shied with increasing particle dimensions, and the octapods exhibit an absorption band centered at 535 nm. Previously we have employed Cu2O cubes, octahedra, and rhombic dodecahedra with average sizes of 240 to 430 nm for the multicomponent direct synthesis of 1,2,3-triazoles from the reaction of alkynes, organic halides, and NaN3.15 The same crystals were also used as catalysts for the [3 + 2] cycloaddition reaction for the regioselective synthesis of 3,5-disubstituted isoxazoles.16 Rhombic dodecahedra were found to be the most active catalysts giving highest yields in both studies, followed by octahedra and the least active cubes. Differences in the number and extent of fully exposed surface copper atoms on the three
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Fig. 4 UV-vis absorption spectra of Cu2O cubes, octahedra, and octapods.
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surface planes were used to explain the observed catalytic results. The catalytic activity has been conrmed by the results from reactions taking place on the particle surfaces, because Cu(I) ions were not detectable in the solution.15 We believe that the small Cu2O nanocrystals should act as more efficient catalysts because they have a larger surface area than bigger crystals of the same mass. For proper comparison of their catalytic activities, the total particle surface area should be approximately the same. The dried particles were weighted to determine the number of particles present in 1 mL of the nanocrystal solution (see Table S2, ESI† for the calculations). Here 1.00 mL of nanocubes, 0.81 mL of octahedra, and 2.30 mL of octapods with a calculated total particle surface area of 87 cm2 were used for all the catalytic reactions in this study. For the [3 + 2] cycloaddition reactions by click chemistry, azides are typically obtained from the reaction of organic halides and NaN3. The synthesized azides subsequently react with aliphatic or aromatic alkynes to give the nal products. In this work, we found that a one-pot multicomponent click reaction can still be successfully carried out using the small Cu2O nanocrystal catalysts, so a single reaction directly gives the cycloaddition product.
Table 1
For the initial reaction, benzyl bromide and NaN3 were subjected to a [3 + 2] cycloaddition reaction with phenyl acetylene for the one-pot synthesis of 1-benzyl-4-phenyl-1H-1,2,3triazoles in ethanol at 55 C under a nitrogen atmosphere (see Table 1). Upon completion of the reaction, 1H NMR spectra of the as-synthesized product indicate the regioselective formation of pure 1,4-disubstituted triazoles without any byproducts. Cu2O nanocubes gave a yield of 89% aer 5.5 h of reaction, while octahedra delivered a 98% yield in 2 h and octapods gave a yield of 90% in 4.3 h. Octahedra are clearly the best and most efficient catalysts among these three nanocrystal morphologies. The results are consistent with our previous report showing that the {111} facets of Cu2O nanocrystals are more catalytically active than the {100} facets. This is because some of the surface copper atoms are fully exposed on the {111} plane for more facile Cu-acetylide formation, but the {100} surface has only partially exposed surface copper atoms and is less active. Octapods enclosed by signicant {100} surfaces are more catalytically efficient than the cubes because of the presence of other more active faces. Previously, a total particle surface area of 28 cm2 was used to catalyze the same amounts of reagents, and
Comparison of catalytic activity of different ultrasmall Cu2O nanocrystals for the following 1,3-dipolar cycloaddition reaction
Particle shape
Volume needed to have a total surface area of 87 cm2 (mL)
Reaction time (h)
Yield
Cubes Octahedra Octapods
1.00 mL 0.81 mL 2.30 mL
5.5 2 4.3
89% 98% 90%
Table 2
A list of three-component 1,3-dipolar cycloaddition reactions catalyzed by small Cu2O octahedra using organic halides, alkynes, and
NaN3a
Massb
Time (h)
Yieldc%
1
235
2
98
2
189
2
98
3
265
2
93
Entry
a c
Alkyne
Oragnic halides
Products
Reagents and conditions:1 (0.25 mmol), 2 (0.25 mmol), NaN3 (0.38 mmol) in EtOH (3 mL) at 55 C. b LRMS detected with an EI ionization source. Isolated yields.
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the product yields were 90% aer 4.5 h of reaction for the large octahedra and 88% aer 7 h of reaction for the large cubes.15 Although the total particle surface area of the small Cu2O nanocrystals used in this study is 3.1 times more than that used before and direct comparison is not possible, the fact a higher yield was obtained in a signicantly less reaction time, especially for octahedra, still demonstrates the benet of making small Cu2O nanocrystals for time-saving and energy-efficient organocatalysis. The Cu2O octahedra were tested for recyclability. Aer two runs of reaction, the particle shape looks the same as before (see Fig. S4, ESI† for a SEM image of the recycled octahedra). The product yield for the second cycle is close to that in the rst run. To show octahedra as consistently excellent catalysts, two more reactions using quite different alkynes as reagents were performed (entries 2 and 3, Table 2). Very high product yields were also obtained in just 2 h of reaction, so that the small octahedra are highly effective at catalyzing this cycloaddition reaction with the use of a variety of substrates to generate diverse triazoles.
Conclusion A surfactant-free synthetic approach to make a series of small Cu2O nanocrystals with shape evolution from cubic to octahedral structures and sizes of less than 100 nm has been developed by mixing aqueous Cu(OAc)2, NaOH, and N2H4 solutions at 50 C. Adjusting the volume of N2H4 reductant solution introduced enables the particle shape evolution. Small Cu2O octapods with an average edge length of 135 nm were also synthesized. The Cu2O cubes, octahedra, and octapods all can act as highly active catalysts in the [3 + 2] cycloaddition reaction via click chemistry for the synthesis of 1,2,3-triazoles. In particular, the octahedra bound by the {111} facets are the best catalysts giving excellent yields in just 2 h of reaction with the use of a range of alkynes to produce diverse 1,4-disubstituted triazole products. The small Cu2O nanocrystals have the desired sizes for examination of truly nanoscale facet-dependent properties of Cu2O and should enhance the efficiency in other organocatalytic and photocatalytic reactions.
Acknowledgements We thank the National Science Council of Taiwan for the nancial support of this work (NSC 101-2113-M-007-018-MY3 and NSC 102-2811-M-007-003).
References 1 C.-H. Kuo and M. H. Huang, Nano Today, 2010, 5, 106–116. 2 M. H. Huang, S. Rej and S.-C. Hsu, Chem. Commun., 2014, 50, 1634–1644. 3 C.-Y. Chiu and M. H. Huang, J. Mater. Chem. A, 2013, 1, 8081– 8092. 4 M. Leng, C. Yu and C. Wang, CrystEngComm, 2012, 14, 8454– 8461.
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Paper
5 Q. Hua, T. Cao, H. Bao, Z. Jiang and W. Huang, ChemSusChem, 2013, 6, 1966–1972. 6 Q. Hua, K. Chen, S. Chang, Y. Ma and W. Huang, J. Phys. Chem. C, 2011, 115, 20618–20627. 7 Y. Xu, H. Wang, Y. Yu, L. Tian, W. Zhao and B. Zhang, J. Phys. Chem. C, 2011, 115, 15288–15296. 8 Y.-H. Tsai, C.-Y. Chiu and M. H. Huang, J. Phys. Chem. C, 2013, 117, 24611–24617. 9 C.-H. Kuo and M. H. Huang, J. Phys. Chem. C, 2008, 112, 18355–18360. 10 Y.-C. Yang, H.-J. Wang, J. Whang, J.-S. Huang, L.-M. Lyu, P.-H. Lin, S. Gwo and M. H. Huang, Nanoscale, 2014, 6, 4316–4324. 11 J.-Y. Ho and M. H. Huang, J. Phys. Chem. C, 2009, 113, 14159– 14164. 12 W.-C. Huang, L.-M. Lyu, Y.-C. Yang and M. H. Huang, J. Am. Chem. Soc., 2012, 134, 1261–1267. 13 W.-C. Wang, L.-M. Lyu and M. H. Huang, Chem. Mater., 2011, 23, 2677–2684. 14 C.-H. Kuo, Y.-C. Yang, S. Gwo and M. H. Huang, J. Am. Chem. Soc., 2011, 133, 1052–1057. 15 K. Chanda, S. Rej and M. H. Huang, Chem.–Eur. J., 2013, 18, 16036–16043. 16 K. Chanda, S. Rej and M. H. Huang, Nanoscale, 2013, 5, 12494–12501. 17 K. X. Yao, X. M. Yin, T. H. Wang and H. C. Zeng, J. Am. Chem. Soc., 2010, 132, 6131–6144. 18 M. D. Susman, Y. Feldman, A. Vaskevich and I. Rubinstein, ACS Nano, 2014, 8, 162–174. 19 C.-H. Kuo and M. H. Huang, Adv. Funct. Mater., 2007, 17, 3773–3780. 20 X. Liang, L. Gao, S. Yang and J. Sun, Adv. Mater., 2009, 21, 2068–2071. 21 Y. Sui, W. Fu, H. Yang, Y. Zeng, Y. Zhang, Q. Zhao, Y. Li, X. Zhou, Y. Leng, M. Li and G. Zou, Cryst. Growth Des., 2010, 10, 99–108. 22 L. Guo and C. J. Murphy, J. Mater. Chem., 2004, 14, 735–738. 23 P. He, X. Shen and H. Gao, J. Colloid Interface Sci., 2005, 284, 510–515. 24 I.-C. Chang, P.-C. Chen, M.-C. Tsai, T.-T. Chen, M.-H. Yang, H.-T. Chiu and C.-Y. Lee, CrystEngComm, 2013, 15, 2363– 2366. 25 M.-J. Kim, Y.-S. Cho, S.-H. Park and Y.-D. Huh, Cryst. Growth Des., 2012, 12, 4180–4185. 26 L.-M. Lyu, W.-C. Wang and M. H. Huang, Chem.–Eur. J., 2010, 16, 14167–14174. 27 H. Y. Zhao, Y. F. Wang and J. H. Zeng, Cryst. Growth Des., 2008, 8, 3731–3734. 28 H. Shi, K. Yu, Y. Wang, Q. Wang and Z. Zhu, Appl. Phys. A: Mater. Sci. Process., 2012, 108, 709–717. 29 D. Wang, M. Mo, D. Yu, L. Xu, F. Li and Y. Qian, Cryst. Growth Des., 2003, 3, 717–720.
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Direct formation of small Cu2O nanocubes, octahedra, and octapods for efficient synthesis of triazoles† Ya-Huei Tsai, Kaushik Chanda, Yi-Ting Chu, Chun-Ya Chiu and Michael H. Huang* In most studies describing the preparation of Cu2O crystals of various morphologies, the particle sizes are normally hundreds of nanometers to micrometers due to rapid particle growth, so they are not exactly nanocrystals. Here we report surfactant-free formation of sub-100 nm Cu2O nanocrystals with systematic shape evolution from cubic to octahedral structures by preparing an aqueous mixture of Cu(OAc)2, NaOH, and N2H4 solution. Adjustment of the hydrazine volume enables the particle shape control. Uniform nanocubes and octahedra were synthesized with edge lengths of 37 and 67 nm, respectively. Novel Cu2O octapods with an edge length of 135 nm were also produced by mixing CuCl2 solution, SDS surfactant, NaOH solution, and NH2OH$HCl reductant solution. All of them are nearly the smallest Cu2O nanocrystals of the same shapes ever reported. These small cubes, octahedra, and octapods were employed as catalysts in the direct synthesis of 1,2,3-triazoles from the reaction of
Received 17th April 2014 Accepted 14th May 2014
alkynes, organic halides, and NaN3 at 55 C. All of them displayed high product yields in short reaction times. The octahedra enclosed by the {111} facets are the best catalysts, and can catalyze this cycloaddition reaction with high yields in just 2 h when different alkynes were used to make diverse
DOI: 10.1039/c4nr02076f
triazole products. Hence, the small Cu2O particles provide time-saving, energy-efficient, and high
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product yield benefits to organocatalysis.
Introduction Cu2O is an excellent material for the facile fabrication of nanocrystals with a series of well-dened particle shapes for facet-dependent property investigation.1–10 For example, Cu2O nanocubes bound by the {100} facets are practically inactive toward the photodegradation of methyl orange, while rhombic dodecahedra bound by the {110} faces are highly active for this reaction.11–13 A single Cu2O octahedron bound by the {111} surfaces are electrically conductive when a pair of tungsten probes are in contact with the particle for electrical conductivity measurements, but a nanocube is essentially non-conductive below an applied voltage of 3 V.14 Cu2O rhombic dodecahedra are far more catalytically active than cubes and octahedra in the cycloaddition reactions to generate triazoles and isoxazoles.15,16 While various approaches and conditions have been used to make nice Cu2O polyhedra, the particle dimensions have mostly been in the range of over 100 nm to a few micrometers.17–21 Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: [email protected] † Electronic supplementary information (ESI) available: SEM images of Cu2O nanocrystals with shape evolution, XRD patterns, calculations for the determination of volumes needed for the catalysis experiment, spectral characterization of the triazole products synthesized and their NMR spectra. See DOI: 10.1039/c4nr02076f
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Crystals with sizes of a few hundreds of nanometers are quite typical. To facilitate the examination of additional facetdependent properties of Cu2O crystals and enhance their catalytic efficiency, it should be highly desirable to drastically reduce the particle size. Cu2O nanocubes with sizes below 50 nm synthesized in the presence of polyethylene glycol and octahedra with sizes below 100 nm produced by g-irradiation are known.22,23 Diluting the reagent concentrations in an aqueous mixture of CuCl2, NaOH, and N2H4 has been shown to reduce the size of Cu2O cubes from 350 nm to 50 nm.7 Recently, sub-100 nm Cu2O cubes have been obtained by mixing CuSO4, trisodium citrate and NaOH.24 However, it is still quite challenging to make Cu2O nanocrystals of a series of shapes with sizes below 100 nm, especially for octahedral particles. Another interesting and exotic particle morphology is the octapodal shape. Both Cu2O and Ag2O octapods derived from facedepression of the cubic structure have been reported.7,25–29 O2-assisted etching of Cu2O cubes in the presence of sodium ascorbate has been used to produce the etched cubes or octapods.7 Again the octapods synthesized typically have an edge length in the range of 500 nm to over 10 micrometers. It should be nice to signicantly reduce their size. In this study, we have used a surfactant-free method to grow sub-100 nm Cu2O nanocrystals with shape evolution from cubic to octahedral structures by adding hydrazine (N2H4) as the reducing agent. To make small Cu2O octapods with an edge of
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just 135 nm, a solution condition similar to that used previously to grow large Cu2O cubes to hexapods was adopted, but the reagent concentrations have been adjusted.11 The small Cu2O cubes, octahedra, and octapods have been employed as efficient catalysts in a three-component click reaction for the synthesis of regioselective 1,2,3-triazoles. The octahedra are highly efficient catalysts giving excellent yields in the formation of other 1,4disubstituted 1,2,3-triazoles, demonstrating the importance of making exceptionally small Cu2O nanocrystals to enhance their activity in catalysis.
Experimental section Chemicals Copper acetate (Cu(OAc)2, 98%, J. T. Baker), hydrazine hydrate (N2H4$H2O, 99.8%, Alfa Aesar), anhydrous copper(II) chloride (CuCl2, 97%, Aldrich), hydroxylamine hydrochloride (NH2OH$HCl, 99%, Aldrich), sodium dodecyl sulfate (SDS, 100%, J. T. Baker), and sodium hydroxide (NaOH, 98%, Aldrich) were used as received. Ultrapure distilled and deionized water (18.2 MU) was used for all solution preparations. Synthesis of sub-100 nm cubic and octahedral Cu2O nanocrystals To grow the small Cu2O nanocubes and octahedra, 9.7 and 7.0 mL of deionized water were respectively added to a sample vial. The vials were kept in a water bath set at 50 C. Then 0.1 mL of 0.1 M Cu(OAc)2 solution and 0.1 mL of 0.1 M NaOH solution were introduced with vigorous stirring. The resulting solution turned light blue, indicating the formation of a Cu(OH)2 precipitate. Then, 0.1 mL and 2.8 mL of 0.2 M N2H4 solution were quickly injected into each vial with vigorous stirring for the synthesis of Cu2O cubes and octahedra, respectively. The solution turned light yellow immediately. The total solution volume in each vial is 10 mL. The solution was aged in the water bath for 30 min for nanocrystal growth and centrifuged at 5500 rpm for 5 min. Aer the top solution was decanted, the precipitate was washed with 10 mL of 1 : 1 volume ratio of water and ethanol. The precipitate was centrifuged and washed twice more to remove unreacted chemicals. The nal washing step used 5 mL of ethanol, and the precipitate was dispersed in 0.5 mL of ethanol for storage and analysis. Synthesis of Cu2O octapods First, 72.6 mL of deionized water was added to a sample vial. The vial was placed in a water bath set at 32–34 C. Then 0.8 mL of 12.5 mM CuCl2 solution and 0.087 g of SDS powder were added to the vial with vigorous stirring, and the resulting solution was le undisturbed for 5 min. Then, 1.6 mL of 0.125 M NaOH solution was introduced, and the solution turned light blue indicating the formation of a Cu(OH)2 precipitate. Finally, 5 mL of 25 mM NH2OH$HCl solution was quickly injected into the vial with stirring for 20 s. The solution turned yellow within seconds. The total solution volume is 80 mL. Aer aging the solution in the water bath for 2 h for nanocrystal growth, the solution was centrifuged at 5000 rpm
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for 3 min. The same sample purication steps were used as described above. Cu2O nanocrystal-catalyzed click reactions Benzyl bromide (0.0428 g, 0.25 mmol, 1.0 equiv.) and NaN3 (0.026 g, 0.40 mmol, 1.6 equiv.) in 5 mL of ethanol were introduced into a 25 mL round bottomed ask containing a stirrer bar. Aer the mixture was stirred for 10 min at room temperature, phenyl acetylene (0.0255 g, 0.25 mmol, 1.0 equiv.) was added into the solution. A xed amount of small Cu2O nanocrystals was immediately added to the reaction mixture (1.00 mL of nanocubes, 0.81 mL of octahedra, or 2.30 mL of octapods with a calculated total particle surface area of 87 cm2). Subsequently, the reaction mixture was heated to 55 C with stirring. The progress of the reaction was monitored by thin-layer chromatography (TLC). Aer completion of reaction (2–5.5 h), the reaction mixture was centrifuged at 5000 rpm for 3 min to remove the nanocrystals. The solvent was then removed under reduced pressure to obtain the crude compound which was isolated without column chromatography to obtain the corresponding triazole as the product. To evaluate the extent of versatility of the ultrasmall octahedral nanocrystal catalysts, the same cycloaddition reaction was repeated with alkynes having different substituents. Instrumentation Transmission electron microscopy (TEM) characterization was performed using a JEOL JEM-2100 electron microscope with an operating voltage of 200 kV. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-7000F electron microscope. X-ray diffraction (XRD) patterns were collected with the use of a Shimadzu XRD-6000 diffractometer with Cu Ka ˚ UV-vis spectra were acquired using a radiation (l ¼ 1.5418 A). JASCO V-670 spectrophotometer.
Results and discussion To make sub-100 nm Cu2O nanocrystals, a mixed solution of Cu(OAc)2 and NaOH placed in a 50 C water bath was prepared to form a blue Cu(OH)2 precipitate rst, followed by the introduction of various volumes of N2H4 reductant solution to tune the particle shape. Here 0.1 and 2.8 mL of 0.2 M N2H4 solution were added to yield cubic and octahedral Cu2O nanocrystals, respectively. No surfactant was used, so the particles have clean surfaces favorable for catalytic applications. The solution turned from light yellow to bright yellow in 5 min aer the addition of 0.1 mL of N2H4 solution in the formation of Cu2O nanocubes. In contrast, the solution color turned from yellow to orange within 30 s aer the addition of 2.8 mL of N2H4 solution. The solution became less transparent aer 5–10 min, indicating more octahedra were formed. The results show that the Cu2O octahedra are formed more quickly than cubes, and particle shape control can be achieved by tuning their growth kinetics. Cu2O particles should form through the following reaction. 4Cu(OH)2 + N2H4 4 2Cu2O + N2 + 6H2O
(1)
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Since an equal molar amount of Cu(OAc)2 and NaOH was used, not all copper source has turned into Cu(OH)2. Cu2+ can be reduced directly to Cu2O via the reaction given below.
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4Cu2+ + N2H4 + 2H2O 4 2Cu2O + N2 + 8H+
(2)
Eqn (2) suggests that the solution pH can be acidic. In fact, the solution pH in the formation of Cu2O nanocubes is 5.40. Since N2H4 is a Lewis base, addition of a large amount of hydrazine can make the solution basic. The solution prepared for the growth of Cu2O octahedra was measured to have a pH value of 9.54. To synthesize Cu2O octapods, water, CuCl2 solution, SDS surfactant, NaOH solution, and NH2OH$HCl reductant solution were mixed at 32–34 C. The reaction conditions are similar to those described previously, but the reagent concentrations have been nely adjusted.11 Because more NaOH was added, the solution pH is 10.43. The control of the Cu2O crystal shape by tuning the solution NaOH concentration has been demonstrated.4 Fig. 1 shows SEM images of the synthesized Cu2O nanocubes, octahedra, and octapods. The particles all have good size and shape uniformity with a standard deviation of about 10% (see Table S1, ESI† for the standard deviations of the particle sizes). The nanocubes and octahedra are particularly small with average edge lengths of 37 and 67 nm, respectively. If the corner-to-opposite corner distance is used, the average particle size is 98 nm for octahedra. These particle sizes are considered as very small when compared to those reported for Cu2O nano- and microcrystals in most studies. A possible reason very small Cu2O nanocrystals can be synthesized is due to the use of hydrazine, which is a strong reducing agent, to generate many nuclei and this reduces the nal particle size. The octapod-shaped particles are also quite small with an edge length of 135 nm. An octapod can be considered as a structure derived from a cube with a crossed depression on each face, so the {100} face should account for a signicant portion of the particle surface. Additional SEM images of the octapods viewed from different directions are shown in Fig. S2, ESI.† If the volume of 0.2 M N2H4 solution added was 0.8 and 1.8 mL, small cuboctahedra and truncated octahedra can be obtained (see Fig. S2, ESI†). This work also shows that Cu2O nanocrystal shape control can be achieved by tuning the reduction rate.12 The fact that particles of various shapes can be synthesized without adding the surfactant also implies that face-selective molecular capping is not the mechanism by which the nanocrystal shape is tuned. Only the Cu2O nanocubes, octahedra, and octapods are further characterized, because these particles are more homogeneous for better property examination. Fig. S3, ESI† presents XRD patterns of the Cu2O nanocubes, octahedra, and octapods. Octahedra show an exceptionally strong (111) reection peak as expected. Cubes display a stronger (200) peak compared to that seen for octahedra due to their {100} faces. Octapods give equal intensity for the (111) and (200) peaks, reecting their structural relationship with a cube. Fig. 2 shows TEM characterization of a single cube and an octahedron. The (200) lattice planes are aligned parallel to the cubic face. The selected-area electron
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Fig. 1 SEM images of the ultrasmall Cu2O (a) nanocubes, (b) octahedra, and (c) octapods.
diffraction (SAED) pattern of the cube recorded along the [002] zone axis gives the expected diffraction pattern, which is consistent with the orientations of the lattice fringes seen in the high-resolution TEM images of the same cube. For the octahedron, (111) lattice planes are aligned parallel to the {111} facets. The SAED pattern recorded is also consistent with the orientation of the octahedron. Fig. 3 shows the TEM images and SAED patterns of a single octapod viewed along the [100] and [110] directions. A square diffraction pattern was obtained when viewed along the [100] direction. The (200) and (110) lattice fringes are aligned parallel to the edges and a corner of the octapod, conrming the structural relationship of an octapod with a cube. When viewed along the [110] direction, the (111) diffraction spots and lattice fringes are aligned toward the corners of the octapod.
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Fig. 2 (a and d) TEM images and (b and e) the corresponding SAED patterns of ultrasmall cubic and octahedral Cu2O nanocrystals viewed along zone axes. (c and f) High-resolution TEM images of the square regions revealing the (110), (111), and (200) planes of Cu2O. the [002] and [022]
Fig. 3 (a and d) TEM images and (b and e) the corresponding SAED patterns of a single Cu2O octapod viewed from different directions. (c and f) High-resolution TEM images of the square regions in panels a and d.
Fig. 4 shows UV-vis absorption spectra of Cu2O cubes, octahedra, and octapods and photographs of the particle solutions. The solution of Cu2O cubes is yellow, while the larger octahedra and octapods display golden yellow and orange solution colors, respectively. The cubes show two absorption bands at 365 and 455 nm. The major absorption band for the octahedra is located at 460 nm. The absorption band becomes more red-shied with increasing particle dimensions, and the octapods exhibit an absorption band centered at 535 nm. Previously we have employed Cu2O cubes, octahedra, and rhombic dodecahedra with average sizes of 240 to 430 nm for the multicomponent direct synthesis of 1,2,3-triazoles from the reaction of alkynes, organic halides, and NaN3.15 The same crystals were also used as catalysts for the [3 + 2] cycloaddition reaction for the regioselective synthesis of 3,5-disubstituted isoxazoles.16 Rhombic dodecahedra were found to be the most active catalysts giving highest yields in both studies, followed by octahedra and the least active cubes. Differences in the number and extent of fully exposed surface copper atoms on the three
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Fig. 4 UV-vis absorption spectra of Cu2O cubes, octahedra, and octapods.
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surface planes were used to explain the observed catalytic results. The catalytic activity has been conrmed by the results from reactions taking place on the particle surfaces, because Cu(I) ions were not detectable in the solution.15 We believe that the small Cu2O nanocrystals should act as more efficient catalysts because they have a larger surface area than bigger crystals of the same mass. For proper comparison of their catalytic activities, the total particle surface area should be approximately the same. The dried particles were weighted to determine the number of particles present in 1 mL of the nanocrystal solution (see Table S2, ESI† for the calculations). Here 1.00 mL of nanocubes, 0.81 mL of octahedra, and 2.30 mL of octapods with a calculated total particle surface area of 87 cm2 were used for all the catalytic reactions in this study. For the [3 + 2] cycloaddition reactions by click chemistry, azides are typically obtained from the reaction of organic halides and NaN3. The synthesized azides subsequently react with aliphatic or aromatic alkynes to give the nal products. In this work, we found that a one-pot multicomponent click reaction can still be successfully carried out using the small Cu2O nanocrystal catalysts, so a single reaction directly gives the cycloaddition product.
Table 1
For the initial reaction, benzyl bromide and NaN3 were subjected to a [3 + 2] cycloaddition reaction with phenyl acetylene for the one-pot synthesis of 1-benzyl-4-phenyl-1H-1,2,3triazoles in ethanol at 55 C under a nitrogen atmosphere (see Table 1). Upon completion of the reaction, 1H NMR spectra of the as-synthesized product indicate the regioselective formation of pure 1,4-disubstituted triazoles without any byproducts. Cu2O nanocubes gave a yield of 89% aer 5.5 h of reaction, while octahedra delivered a 98% yield in 2 h and octapods gave a yield of 90% in 4.3 h. Octahedra are clearly the best and most efficient catalysts among these three nanocrystal morphologies. The results are consistent with our previous report showing that the {111} facets of Cu2O nanocrystals are more catalytically active than the {100} facets. This is because some of the surface copper atoms are fully exposed on the {111} plane for more facile Cu-acetylide formation, but the {100} surface has only partially exposed surface copper atoms and is less active. Octapods enclosed by signicant {100} surfaces are more catalytically efficient than the cubes because of the presence of other more active faces. Previously, a total particle surface area of 28 cm2 was used to catalyze the same amounts of reagents, and
Comparison of catalytic activity of different ultrasmall Cu2O nanocrystals for the following 1,3-dipolar cycloaddition reaction
Particle shape
Volume needed to have a total surface area of 87 cm2 (mL)
Reaction time (h)
Yield
Cubes Octahedra Octapods
1.00 mL 0.81 mL 2.30 mL
5.5 2 4.3
89% 98% 90%
Table 2
A list of three-component 1,3-dipolar cycloaddition reactions catalyzed by small Cu2O octahedra using organic halides, alkynes, and
NaN3a
Massb
Time (h)
Yieldc%
1
235
2
98
2
189
2
98
3
265
2
93
Entry
a c
Alkyne
Oragnic halides
Products
Reagents and conditions:1 (0.25 mmol), 2 (0.25 mmol), NaN3 (0.38 mmol) in EtOH (3 mL) at 55 C. b LRMS detected with an EI ionization source. Isolated yields.
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the product yields were 90% aer 4.5 h of reaction for the large octahedra and 88% aer 7 h of reaction for the large cubes.15 Although the total particle surface area of the small Cu2O nanocrystals used in this study is 3.1 times more than that used before and direct comparison is not possible, the fact a higher yield was obtained in a signicantly less reaction time, especially for octahedra, still demonstrates the benet of making small Cu2O nanocrystals for time-saving and energy-efficient organocatalysis. The Cu2O octahedra were tested for recyclability. Aer two runs of reaction, the particle shape looks the same as before (see Fig. S4, ESI† for a SEM image of the recycled octahedra). The product yield for the second cycle is close to that in the rst run. To show octahedra as consistently excellent catalysts, two more reactions using quite different alkynes as reagents were performed (entries 2 and 3, Table 2). Very high product yields were also obtained in just 2 h of reaction, so that the small octahedra are highly effective at catalyzing this cycloaddition reaction with the use of a variety of substrates to generate diverse triazoles.
Conclusion A surfactant-free synthetic approach to make a series of small Cu2O nanocrystals with shape evolution from cubic to octahedral structures and sizes of less than 100 nm has been developed by mixing aqueous Cu(OAc)2, NaOH, and N2H4 solutions at 50 C. Adjusting the volume of N2H4 reductant solution introduced enables the particle shape evolution. Small Cu2O octapods with an average edge length of 135 nm were also synthesized. The Cu2O cubes, octahedra, and octapods all can act as highly active catalysts in the [3 + 2] cycloaddition reaction via click chemistry for the synthesis of 1,2,3-triazoles. In particular, the octahedra bound by the {111} facets are the best catalysts giving excellent yields in just 2 h of reaction with the use of a range of alkynes to produce diverse 1,4-disubstituted triazole products. The small Cu2O nanocrystals have the desired sizes for examination of truly nanoscale facet-dependent properties of Cu2O and should enhance the efficiency in other organocatalytic and photocatalytic reactions.
Acknowledgements We thank the National Science Council of Taiwan for the nancial support of this work (NSC 101-2113-M-007-018-MY3 and NSC 102-2811-M-007-003).
References 1 C.-H. Kuo and M. H. Huang, Nano Today, 2010, 5, 106–116. 2 M. H. Huang, S. Rej and S.-C. Hsu, Chem. Commun., 2014, 50, 1634–1644. 3 C.-Y. Chiu and M. H. Huang, J. Mater. Chem. A, 2013, 1, 8081– 8092. 4 M. Leng, C. Yu and C. Wang, CrystEngComm, 2012, 14, 8454– 8461.
Nanoscale
Paper
5 Q. Hua, T. Cao, H. Bao, Z. Jiang and W. Huang, ChemSusChem, 2013, 6, 1966–1972. 6 Q. Hua, K. Chen, S. Chang, Y. Ma and W. Huang, J. Phys. Chem. C, 2011, 115, 20618–20627. 7 Y. Xu, H. Wang, Y. Yu, L. Tian, W. Zhao and B. Zhang, J. Phys. Chem. C, 2011, 115, 15288–15296. 8 Y.-H. Tsai, C.-Y. Chiu and M. H. Huang, J. Phys. Chem. C, 2013, 117, 24611–24617. 9 C.-H. Kuo and M. H. Huang, J. Phys. Chem. C, 2008, 112, 18355–18360. 10 Y.-C. Yang, H.-J. Wang, J. Whang, J.-S. Huang, L.-M. Lyu, P.-H. Lin, S. Gwo and M. H. Huang, Nanoscale, 2014, 6, 4316–4324. 11 J.-Y. Ho and M. H. Huang, J. Phys. Chem. C, 2009, 113, 14159– 14164. 12 W.-C. Huang, L.-M. Lyu, Y.-C. Yang and M. H. Huang, J. Am. Chem. Soc., 2012, 134, 1261–1267. 13 W.-C. Wang, L.-M. Lyu and M. H. Huang, Chem. Mater., 2011, 23, 2677–2684. 14 C.-H. Kuo, Y.-C. Yang, S. Gwo and M. H. Huang, J. Am. Chem. Soc., 2011, 133, 1052–1057. 15 K. Chanda, S. Rej and M. H. Huang, Chem.–Eur. J., 2013, 18, 16036–16043. 16 K. Chanda, S. Rej and M. H. Huang, Nanoscale, 2013, 5, 12494–12501. 17 K. X. Yao, X. M. Yin, T. H. Wang and H. C. Zeng, J. Am. Chem. Soc., 2010, 132, 6131–6144. 18 M. D. Susman, Y. Feldman, A. Vaskevich and I. Rubinstein, ACS Nano, 2014, 8, 162–174. 19 C.-H. Kuo and M. H. Huang, Adv. Funct. Mater., 2007, 17, 3773–3780. 20 X. Liang, L. Gao, S. Yang and J. Sun, Adv. Mater., 2009, 21, 2068–2071. 21 Y. Sui, W. Fu, H. Yang, Y. Zeng, Y. Zhang, Q. Zhao, Y. Li, X. Zhou, Y. Leng, M. Li and G. Zou, Cryst. Growth Des., 2010, 10, 99–108. 22 L. Guo and C. J. Murphy, J. Mater. Chem., 2004, 14, 735–738. 23 P. He, X. Shen and H. Gao, J. Colloid Interface Sci., 2005, 284, 510–515. 24 I.-C. Chang, P.-C. Chen, M.-C. Tsai, T.-T. Chen, M.-H. Yang, H.-T. Chiu and C.-Y. Lee, CrystEngComm, 2013, 15, 2363– 2366. 25 M.-J. Kim, Y.-S. Cho, S.-H. Park and Y.-D. Huh, Cryst. Growth Des., 2012, 12, 4180–4185. 26 L.-M. Lyu, W.-C. Wang and M. H. Huang, Chem.–Eur. J., 2010, 16, 14167–14174. 27 H. Y. Zhao, Y. F. Wang and J. H. Zeng, Cryst. Growth Des., 2008, 8, 3731–3734. 28 H. Shi, K. Yu, Y. Wang, Q. Wang and Z. Zhu, Appl. Phys. A: Mater. Sci. Process., 2012, 108, 709–717. 29 D. Wang, M. Mo, D. Yu, L. Xu, F. Li and Y. Qian, Cryst. Growth Des., 2003, 3, 717–720.
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