CuO films and their hydrophobic property

Chemical Engineering Journal 167 (2011) 388–396

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Effect of sodium borohydride on growth process of controlled flower-like nanostructured Cu2 O/CuO films and their hydrophobic property Guoli Fan, Feng Li ∗ State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, PR China

a r t i c l e

i n f o

Article history: Received 4 September 2010 Received in revised form 19 December 2010 Accepted 29 December 2010 Keywords: CuO film Architecture Nanostructure Hydrophobicity

a b s t r a c t In this paper, we report on the three-dimensional nanostructured Cu2 O/CuO films with controlled flower-like shapes using a direct crystallization approach in the presence of sodium borohydride without using any performed template, surfactant or oxidant. The microstructures and shapes of Cu2 O/CuO architectures were investigated by field emission scanning electron microscopy, X-ray diffraction and transmission electron microscopy. Three types of chrysanthemum-like, candock-like and dandelionlike CuO microstructures consisting of densely packed building blocks of nanobelts or nanoribbons were achieved by governing the concentrations of NaBH4 . Possible growth mechanisms for the controlled organization of primary building units into three-dimensional flower-like architectures were proposed. After simple surface modification with sodium laurate, the resulting films displayed hydrophobic and even superhydrophobic properties owing to their special surface nano-/microstructures. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Since the morphological diversity of inorganic nanomaterials has a significant impact on their functional diversification and potential applications, control of the unusual shapes and size of materials is stimulating worldwide interests in areas of materials science [1–8]. In particular, over the past decade, one- and twodimensional (1D and 2D) nanostructures (nanowires, nanoribbon, nanorods, nanopellets and nanobelts) have been a focus of extensive research, due to their unique optical, electrical and mechanical properties [9–20]. Very lately, tremendous efforts have been dedicated to investigate the self-assembly of 1D or 2D building blocks into special three-dimensional (3D) hierarchical architectures with the use of surfactants or templates, because such configurations may not only provide the needs of many novel technologies based on nanoscale machines and devices, but also give insight into the construction of micro-/nanoscale devices [21–24]. Cupric oxide, a p-type semiconductor with a narrow band gap of 1.2 eV [25], is used in a wide range of applications such as solar energy transformation [26], electrode materials for rechargeable lithium-ion batteries [27], gas sensors [28] and catalysts [29,30]. Historically, there were a number of research works to prepare nanostructured CuO including nanorods, nanowires, nanoribbons, nanobundles, nanourchins, nanoplates, peachstones,

∗ Corresponding author. Tel.: +86 10 64451226; fax: +86 10 64425385. E-mail address: lifeng [email protected] (F. Li). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.12.090

shuttles and hallow in the form of powder [31–39]. In comparison to CuO microparticles, nanostructured 1D or 2D CuO possesses high surface-to-volume ratio, larger surface area, more electron transfer passages, lower charge transfer resistance, and more accessibility to reaction sites, which can lead to the enhanced efficiency in the electrocatalytic performance [40–43]. More importantly, complex 3D CuO nanostructures also are highly desirable in current nanomaterials synthesis and applications [44–46]. For example, 3D flower-like CuO nanostructures can exhibit excellent performance in the fields of catalysis, electrochemistry and field emission [45,46], because high-density nanoscale “petals” not only provide large surface areas and structural defects, but also avoid the internal resistance aroused by the high aspect ratio. Typically, aligned and ordered more complex CuO architectures onto a substrate are obtained by a variety of masking/patterning techniques [47–51], which generally require the complicated processing and/or special equipments. Owing to the great chemical flexibility and synthetic tenability, the solution-based approaches provide a simple process toward 3D CuO nanostructures at low temperatures [45,46,52]. In most cases, however, the addition of oxidants or surfactants usually brought out expensive costs as well as environment pollution. Considering that the nucleation and growth processes for the formation of inorganic films have significant effects on their microstructure and shape, it remains a big challenge to develop more effective, facile and readily amenable to scale-up protocols for the synthesis of highly ordered nanostructures as building units of CuO architectures on substrates. Therefore, in this present

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Fig. 1. (a) Low-magnification and (b and c) high-magnification FESEM micrographs of candock-like CuO architectures grown on Cu foil; (d) TEM micrograph of an individual candock-like CuO microstructure and (e and f) HRTEM micrographs of an individual CuO nanobelt; (g) SEAD pattern of an individual CuO nanobelt; (h) XRD patterns of candock-like CuO architectures; (i) EDS spectrum of candock-like CuO architectures.

work, we established an environmentally benign template-free solution-based approach to direct fabricate three-dimensional CuO architectures with various flower-like shapes on copper foils in the presence of sodium borohydride. Herein, NaBH4 acted as both alkaline reagent and reductant to induce the growth of different CuO nanostructures, thus favoring the formation of controlled bionic hierarchical configurations. Tunable hydrophobic/superhydrophobic surfaces of as-fabricated CuO architectures were realized based on their special surface nano-/microstructures. To the best of our knowledge, there is no report on direct solution-based approach to fabricate the controlled flower-like CuO microstructures on copper foils using NaBH4 acted as alkaline reagent and reductant. The findings in this work are attractive for their merits such as simplicity, safety, environmentally benign, commercial feasibility, and good potential for scale-up. 2. Experimental 2.1. Materials The copper foils (purity >99.5%) with the thickness of 0.08 mm were purchased from Shanghai Jing Xi Chemical Technology Co.,

Ltd., NaBH4 (99.7%), sodium laurate (99.0%), ethanol (99.7%), acetone (99.5%) and other chemicals (analytical grade) were purchased from Beijing Chemical Reagent Co., Ltd., and used as received without further purification.

2.2. Synthesis of CuO films In a typical synthesis, 3.04 g NaBH4 was dissolved in 80 mL of deionized water to form a clear solution of 1.0 M. Two pieces of copper foils (20 mm × 20 mm × 0.08 mm) were ultrasonically cleaned in ethanol and then acetone before being placed vertically in a Teflon-lined stainless steel autoclave, which was placed in a conventional oven at 70 ◦ C for 72 h. The obtained black CuO films were then taken out from the solution, rinsed with deionized water, and dried at 60 ◦ C for 2 h. The growth of CuO films were also carried out by applying different concentration of NaBH4 (0.5 and 2.0 M), reaction time (24, 48 and 96 h) and alkaline reagent (NaOH) under an identical procedure. The surface of as-fabricated CuO films was modified by immersion in 0.05 M sodium laurate aqueous solution at 70 ◦ C for 7 h. After immersion, the modified CuO films were rinsed with ethanol and dried at room temperature.

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Fig. 2. (A) XRD patterns of candock-like CuO architectures with the reaction time; (B) FESEM micrograph of film obtained after 24 h. Insets are FESEM (left) and TEM (right) micrographs of interlaced nanobelts at the edges of polyhedrons; (C) FESEM micrograph of film obtained after 48 h. Inset is TEM micrograph of an individual underdeveloped candock-like CuO microstructure; (D) FESEM micrograph of film obtained after 96 h. Inset is TEM micrograph of an individual candock-like CuO microstructure.

2.3. Characterization

3. Results and discussion

Powder X-ray diffraction (XRD) patterns were collected on a Shimadzu XRD-6000 diffractometer, using Cu-Ka radiation (40 kV, ˚ between 20◦ and 80◦ with a scanning 30 mA and K␣ = 1.54178 A) rate of 5◦ min−1 . Field emission scanning electron microscopy (FESEM) was carried out on a Zeiss Supra 55 instrument. The accelerating voltage applied was 20 kV, combined with energy dispersive X-ray spectroscopy (EDS) for the determination of metal composition. All the samples were sputtered with gold. Transmission electron microscopy (TEM) was carried out on a Hitachi H-800 transmission electron microscope with an accelerating voltage of 100 kV. For TEM analysis, powders were scraped form the surface of the substrate and dispersed in ethanol, a droplet of the ultrasonically dispersed sample was placed onto an amorphous carbon-coated copper grid and then dried at atmospheric ambient. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEOL-2100 high-resolution transmission electron microscope at an accelerating voltage of 200 kV. The low-temperature N2 adsorption–desorption experiments of CuO particles scraped from substrate of CuO films were carried out using a Quantachrome Autosorb-1C-VP system. Specific surface areas of samples were calculated from the adsorption isotherm according to the Brunauer–Emmett–Teller (BET) method. Static water contact angles were measured on a commercial drop-shape analyses system (DSA 100, KüRSS GmbH, Germany) at ambient temperature. The equilibrium water contact angle (CA) was measured with a fixed needle supplying a water drop of 5 ␮L while the drop-shape analysis system determined the contact angle. Five different points on each sample were investigated. The sliding angle was measured by tilting the sample stage slowly until the drop began to roll.

3.1. Synthesis and characterization of Cu films The morphologies and microstructures of as-fabricated CuO films were examined by FESEM micrographs with different magnifications. Fig. 1a illustrated a whole FESEM micrograph of CuO product grown on a copper foil using 1.0 M aqueous NaBH4 solution at 70 ◦ C for 72 h. It was obviously seen that a large number of microscale papillae with an average diameter of about 2.3–3.5 ␮m were compactly and uniformly grown on the substrate. A closeup FESEM micrograph observation revealed that the single papilla possessed a superstructure of elegant flower-like appearance, similar to candocks in nature (Fig. 1b). Interestingly, the individual candock with densely-packed “petals”, in fact, was constructed by a large number of separated nanobelts. These nanobelts, which pointed outward from a common center, seemed to be aligned perpendicularly to hemisphere-like surface. Meanwhile, many spacing among these “petals” in the flower-like superstructure may favor the diffusion of small molecules in practical catalysis application. Further detailed FESEM micrograph observation from the “petals” demonstrated that these broad and flexible nanobelts were about 20–25 nm in thickness and narrowed progressively from the basal portions to the cuspidal tips (Fig. 1c). Further information about CuO films was obtained from TEM and HRTEM micrographs. As shown in Fig. 1d, numerous belt-like petals were closely packed together in a high density arrangement and orderly self-assembled into individual 3D hierarchical microstructure, consistent with FESEM observation. From the enlarged TEM micrograph of an individual “petal” (Fig. 1e), the dark/light contrasts were clearly observed along the axial direction of “petal”, which suggested that such belt-like nanostructure was composed of tiny nanoparticles. HRTEM micrograph of the

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Fig. 3. (a) Low-magnification FESEM, (b) high-magnification FESEM and (c) TEM micrographs of chrysanthemum-like CuO architectures grown on copper foil; (d) lowmagnification FESEM, (e) high-magnification FESEM and (f) TEM micrographs of dandelion-like CuO architectures grown on copper foil.

nanobelts (Fig. 1f) further revealed the presence of an interplanar distance of about 0.253 nm that was characteristic of [0 0 2] plane of monoclinic CuO phase (JCPDS 05-0661). Selected area electron diffraction (SEAD) pattern taken from an individual “petal” (Fig. 1g) illustrated that there were four discernible diffraction rings corresponding to CuO phase, suggestive of the polycrystalline nature of the nanobelts. The phase purity of CuO film was determined by XRD. As shown in Fig. 1h, a typical XRD pattern of CuO film presented two characteristic reflections attributed to [0 0 2] and [1 1 1] planes of monoclinic CuO phase, except for the characteristic reflections originating from the Cu substrate. No other crystalline phases, such as Cu(OH)2 and Cu2 O, were detected. In addition, EDS analysis (Fig. 1i) revealed that only the Cu and O elements were contained in the product and the Cu/O atomic ratio was very close to 1:1, confirming the formation of pure CuO phase on the substrate. To obtain a better understanding of the growth process of the hierarchical candock-like CuO architecture, the evolution of the microstructure and morphology of product was further investigated by taking XRD patterns, FESEM and TEM micrographs at different growth stages. In the initial stage (24 h), the reflection peaks of Cu2 O phase (JCPDS 05-0667) were observed in the XRD patterns of the sample (Fig. 2A). At the same time, as shown in Fig. 2B, some irregular Cu2 O polyhedrons were randomly grown on copper foil, and small interlaced mat-like nanobelts coexisted at the edges of the polyhedrons. After 48 h, monoclinic CuO peaks were observed in the sample, except for a small amount of Cu2 O. Here, we found that nanobelts grew more and more and selfassembled into many underdeveloped flower-like clusters on the polyhedrons (Fig. 2C). With the reaction time increasing to 72 h, only the characteristic reflections of CuO phase were detected. Consequently, candock-like microstructures were formed on the substrate (Fig. 1a). Finally, as the reaction time was prolonged to 96 h, the reflection intensity of CuO phase was further enhanced and larger candock-like CuO architectures covered densely and compactly on the substrate (Fig. 2D). It was proposed from the above results that CuO crystals developed progressively from small

nanobelts to flower-like clusters and, finally, 3D candock-like aggregates by simply manipulating the reaction time. Control experiments in lower or higher NaBH4 concentrations under the same condition were also carried out in order to investigate the influence on the morphology and microstructure of the formed CuO films. When the NaBH4 concentration was decreased to 0.5 M, obtained CuO product presented a kind of chrysanthemumlike shape over the whole area of substrate (Fig. 3a). The close-up FESEM micrograph demonstrated that each of CuO microstructures was built from numerous curly nanoribbons, which scrolled themselves up at the tips (Fig. 3b). The TEM micrograph (Fig. 3c) of an individual CuO chrysanthemum further revealed that tender nanoribbons were tangled together to aggregate into the flowerlike geometry. With the NaBH4 concentration increasing to 2.0 M, dense CuO papillae with the diameters ranging from 3 to 5 ␮m were distributed on the substrate (Fig. 3d). The enlarged FESEM and TEM micrographs (Fig. 3e and f) of an individual papilla showed that a large quantity of subuliform nanobelts grew radically from the center to form a perfect dandelion-like conformation. The growth process of the dandelion-like architecture is similar to that of candock-like one. At the beginning of reaction (24 h), numerous regular Cu2 O octahedrons were generated on the substrate at the higher NaBH4 concentration (Fig. 4a). With the reaction time, CuO nanobelts were formed gradually and located vertically at the tips or edges of octahedrons (Fig. 4b and c). At last, subuliform nanobelts aggregated into well-aligned dandelion-like superstructures covering compactly on the substrate (Fig. 4d). According to FESEM micrographs, the particle size distributions of flower-like CuO particles were determined from no less than 100 particles (Fig. 5). It was observed that the average particle size increased gradually from about 1.8 for chrysanthemum-like CuO to about 2.9 for candock-like CuO, and about 4.4 ␮m for dandelionlike CuO. The measured BET surface areas of chrysanthemum-like, candock-like, and dandelion-like CuO particles scraped from substrate of CuO films were about 8.3, 11.6 and 12.9 m2 /g, respectively. The smaller values of specific surface area further confirmed the presence of larger flower-like CuO microparticles on substrates.

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Fig. 4. The time-dependant FESEM micrographs of dandelion-like CuO architectures: (a) 24 h, (b) 48 h, (c) 72 h and (d) 96 h.

Additionally, to investigate the reproducibility of the growth of flower-like CuO, CuO nanostructures were repeatedly synthesized under identical conditions for three times. It was found that the morphologies and shapes of CuO nanostructures obtained kept almost the same by FESEM observation (see supplementary data), indicating that the growth of such CuO nanostructures was wellreproducible. On the other side, the use of aqueous NaOH solution as alkaline reagent was investigated to obtain CuO film on copper foil under otherwise the same condition. However, in this case, we found that only side-by-side closely-packed CuO nanoplatelets were vertically arranged on the substrate (Fig. 6). It implied that alkaline reagent added should be a key factor on the growth of the CuO nanostructures. 3.2. Growth mechanism for flower-like CuO architectures In the present synthesis system, NaBH4 may act as both alkaline reagent and reductant to control the growth of flower-like CuO architectures on copper foil. The chemical reactions involved were believed to proceed as follows (Eqs. (1)–(4)): BH4 − (aq) + 4H2 O → H3 BO3(aq) + 4H2 + OH− (aq) 2Cu(s) + O2 + 2H2 O + 4OH

−

(aq)

→ 2[Cu(OH)4 ]

2−

(1) (aq)

(2)

8[Cu(OH)4 ]2− (aq) + BH4 − (aq) → 4Cu2 O(s) + H3 BO3(aq) + 8H2 O + 17OH − (aq) 2Cu2 O(s) + O2 → 4CuO(s)

(3) (4)

At the beginning of reaction, added NaBH4 could react slowly with water to generate OH− under hydrothermal conditions (Eq. (1)), thus raising the pH value of aqueous reaction solution. It is

well-known that copper in air or under humid conditions can be oxidized by oxygen and the reaction is very slowly due to the formation of an oxide passivation layer on the surface. However, the oxidation may be accelerated intensely in the presence of alkali solution with oxidants such as (NH4 )2 S2 O8 and K2 S2 O8 [46,52–54]. Once Cu2+ cations are generated, they can quickly enter the solution in the form of [Cu(NH3 )n ]2+ or [Cu(OH)4 ]2− complex. In our case, due to the high OH− concentration in the solution, trace amount of O2 in the solution could play the role of oxidant. Therefore, copper foil continuously released Cu2+ ions into the aqueous solution to immediately form complex [Cu(OH)4 ]2− with the OH− ions (Eq. (2)) [45,53–55]. In the presence of NaBH4 , these complex [Cu(OH)4 ]2− units could be further reduced fast into Cu2 O nuclei (Eq. (3)). Then, the initially formed Cu2 O nuclei were deposited on the substrate and coalesced to from thermodynamically favored large octahedral Cu2 O [55], which could provide many high-energy sites for the growth of crystals. With the increasing reaction time, the growth rate of Cu2 O crystals decreased gradually due to the continuous consumption of NaBH4 . Finally, through a slow oxidation by the trace amount of dissolved oxygen, Cu2 O crystals were transformed into CuO crystals (Eq. (4)). In our system, the higher initial NaBH4 concentration could lead to the formation of larger amounts of octahedral Cu2 O nanostructures on copper substrate, because more Cu2 O nuclei could generate in the higher basicity solution as a result of fast supersaturation and then deposit on the substrate to form Cu2 O crystals. Further, the high intrinsic anisotropic property of octahedral Cu2 O crystals could benefit the growth of CuO nanostructures at the expense of previously formed amphoteric Cu2 O through a dissolution–recrystallization process followed by an oxidation in a basic solution [56]. Moreover, it is believed that higher chemical potential conditions determined by the higher pH value and solute concentrations of the solutions in the reaction system would be advantageous for the nanostructure growth [57]. Thus, with the decreasing concentration of OH− ions due to the hydroly-

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Fig. 6. FESEM micrograph of plate-like CuO film formed in aqueous NaOH solution. Inset is high-magnification FESEM micrograph of CuO nanoplates.

Fig. 5. The particles size distributions of three flower-like CuO architectures: (a) chrysanthemum-like CuO, (b) candock-like CuO, and (c) dandelion-like CuO.

sis of NaBH4 during the growth stages, most octahedral Cu2 O crystals gradually disappeared, while more new CuO nanostructures with different morphologies were obtained. At the low initial NaBH4 concentration of 0.5 M, the nucleation rate decreased significantly with the reaction time. Slow nucleation and growth rate of CuO crystals would favor the formation of long and thin nanoribbons. When the initial NaBH4 concentration was 1.0 M, the proper basicity of aqueous solution led to the formation of belt-like “petals”. With the initial NaBH4 concentration increasing to 2.0 M, numerous Cu2 O crystals grown on the substrate were transformed into uniform subuliform nanobelts. Finally, diversified 3D bionic CuO architectures were achieved via oriented attachment of CuO nanostructures (nanoribbons, nanobelts, subuliform nanobelts), the driving force for the growth of flower-like architectures possibly originates from the coalescing and aggregation of surface particles by Van der Waals forces. As a result, flower-like CuO microstructures could be obtained through three-step processes including the formation of Cu2 O crystals on the substrate, the growth of CuO nanostructures and the coalescing and aggregation of CuO nanostructures. Scheme 1 illustrated schematically the growth mechanism of three-dimensional hierarchical CuO configurations at different NaBH4 concentrations. However, further theoretical and experimental works need to be carried out in order to determine the exact nature of the growth mechanism for these unique and interesting CuO architectures.

Scheme 1. Schematic illustration of the growth mechanism of three-dimensional hierarchical flower-like CuO architectures prepared at different concentrations of NaBH4 .

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Fig. 7. FESEM micrographs of different CuO films after hydrophobization with sodium laurate: (a) chrysanthemum-like CuO film; (b) candock-like CuO film; (c) dandelion-like CuO film; (d) platelet-like CuO film prepared in NaOH solution. Insets on right top are shapes of water droplet on the surface of films. Inset on the left top of (c) shows sliding behavior of water droplet on the surface of film.

3.3. Interfacial properties of CuO films Metal oxide surfaces are commonly high-energy hydrophilic surfaces, on which a drop of liquid can spread rapidly. Lately, copper-based film materials with different surface patterns have also been used to prepare superhydrophobic surfaces [58]. It is expected that as-fabricated flower-like CuO architectures with special nano-/microstructures may result in a particular wettability. We found that when the water droplet was dropped onto the flower-like CuO films, it spread very quickly over the surface with a low water CA of about 8◦ , suggestive of the surface hydrophilicity of CuO films. To achieve the surface hydrophobicity, CuO films was modified by immersion in an aqueous solution of sodium laurate. Fig. 7a–c shows the captured micrographs of water droplet on the surface of modified CuO films. The water CA was found to increase from 145 ± 2◦ for chrysanthemum-like CuO film to 153 ± 2◦ for candock-like one and 156 ± 2◦ for dandelion-like one, indicative of the surface hydrophobicity or superhydrophobicity. Moreover, the tilt angle of CuO film with dandelion-like surface pattern was lower than 3◦ , which suggested that the surface of film was a typical self-cleaning surface [59]. Compared to those of the flower-like CuO films, however, the water CA of the plateletlike CuO film formed in the NaOH solution was only 138 ± 2◦ after the same modification (Fig. 7d). The aforementioned results verified that the surface wettability of CuO films was converted from hydrophilicity to hydrophobicity by the simple surface modification, because sodium laurate could easily self-assemble onto the surface and decrease the surface free energy [60]. Moreover, special surface combination of nano-/microstructures of CuO configurations could trap air in the spacing among particles. This led to the significant decrease in the contact area between water and the surface protrusions of particles, resulting in hydrophobicity or even superhydrophobicity in terms of the Cassie–Baxter equation

Scheme 2. Schematic illustration of the shape evolution of water droplets on different flower-like CuO architectures prepared at different concentrations of NaBH4 .

[61]. Scheme 2 illustrated the shape evolution of water droplets on different flower-like CuO architectures. The increasing CA from chrysanthemum to candock and dandelion was mainly ascribed to the increasing air surface fraction with water on the films. 4. Conclusions In summary, we developed a new green synthetic route to three types of special flower-like CuO architectures on copper foils by

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a facile template-free solution-based approach. The results indicated that NaBH4 acted as both alkaline agent and reductant for the growth of unusual CuO nanostructures. The morphology of CuO configurations on the substrate was diversified by governing the NaBH4 concentrations. With the increasing NaBH4 concentration, the shape of CuO particles changed progressively from threedimensional chrysanthemum-like aggregates to candock-like and dandelion-like CuO ones. The average particle size increased gradually from about 1.8 for chrysanthemum-like CuO to about 2.9 for candock-like one, and about 4.4 ␮m for dandelion-like one. Possible growth mechanisms were proposed. The surface hydrophobicity/superhydrophobicity was achieved on the modified CuO films due to the combination of uniform surface nano-/microstructure. This synthesis route presented here manifests the advantages of ease, benignancy to environment, flexibility, versatility, manufacturability for fabricating hierarchical CuO architectures. It is expected that this promising approach can be applied to the morphosynthesis of functional CuO-based materials for potential applications as advanced catalysts, clean surface and sensors. Acknowledgements We gratefully thank the financial support from the National Natural Science Foundation of China and 973 Program (2011CBA00506). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2010.12.090. References [1] J.S. Son, X.D. Wen, J. Joo, J. Chae, S. Baek, K. Park, J.H. Kim, K. An, J.H. Yu, S.G. Kwon, S.H. Choi, Z. Wang, Y.W. Kim, Y. Kuk, R. Hoffmann, T. Hyeon, Large-scale soft colloidal template synthesis of 1.4 nm thick CdSe nanosheets, Angew. Chem. Int. Ed. 48 (2009) 6861–6864. [2] C. Chen, W. Chen, J. Lu, D. Chu, Z. Huo, Q. Peng, Y. Li, Transition-metal phosphate colloidal spheres, Angew. Chem. Int. Ed. 48 (2009) 4816–4819. [3] D. Portehault, C. Sophie, N. Nassif, E. Baudrin, J.P. Jolivet, A core-corona hierarchical manganese oxide and its formation by an aqueous soft chemistry mechanism, Angew. Chem. Int. Ed. 47 (2008) 6441–6444. [4] C. Jeong, Y.B. Kim, S.H. Lee, J.H. Kim, Preparation of born-doped a-SiC:H thin films by ICP-CVD method and to the application of large-area heterojunction solar cells, J. Nanosci. Nanotechnol. 10 (2010) 3321–3325. [5] D. Walsh, S. Mann, Fabrication of hollow porous shells of calcium carbonate from self-organizing media, Nature 377 (1995) 320–323. [6] G. Neri, Non-conventional sol–gel routes to nanosized metal oxides for gas sensing: from materials to applications, Sci. Adv. Mater. 2 (2010) 3–15. [7] B. Liu, H.C. Zeng, Fabrication of ZnO “dandelions” via a modified Kirkendall process, J. Am. Chem. Soc. 126 (2004) 16744–16746. [8] L. Qi, H. Cölfen, M. Antonietti, Crystal design of barium sulfate using doublehydrophilic block copolymers, Angew. Chem. Int. Ed. 39 (2000) 604–607. [9] Y. Wang, H. Yang, Synthesis of iron oxide nanorods and nanocubes in an imidazolium ionic liquid, Chem. Eng. J. 147 (2009) 71–78. [10] R. Selvin, H.L. Hsu, N.S. Arul, S. Mathew, Comparison of photo-catalytic efficiency of various metal oxide photo-catalysts for the degradation of methyl orange, Sci. Adv. Mater. 2 (2010) 58–63. [11] Y.W. Tan, X.Y. Xue, Q. Peng, H. Zhao, T.H. Wang, Y.D. Li, Controllable fabrication and electrical performance of single crystalline Cu2 O nanowires with high aspect ratios, Nano Lett. 7 (2007) 3723–3728. [12] Y.F. Gao, M. Nagai, T.-C. Chang, J.-J. Shyue, Solution-derived ZnO nanowire array film as photoelectrode in dye-sensitized solar cells, Cryst. Growth Des. 7 (2007) 2467–2471. [13] W.S. Chiu, P.S. Khiewa, D. Isaa, M. Cloke, S. Radiman, R. Abd-Shukor, M.H. Abdullahb, N.M. Huang, Synthesis of two-dimensional ZnO nanopellets by pyrolysis of zinc oleate, Chem. Eng. J. 142 (2008) 334–343. [14] C.K. Chan, H. Peng, R.D. Twesten, K. Jarausch, X.F. Zhang, Y. Cui, Fast, completely reversible Li insertion in vanadium pentoxide nanoribbons, Nano Lett. 7 (2007) 490–495. [15] G.Z. Shen, D. Chen, 1-D hetero-nanostructures: from growth to devices, Sci. Adv. Mater. 1 (2009) 213–226. [16] N.Q. Wu, J. Wang, D.N. Tafen, H. Wang, J.-G. Zheng, J.P. Lewis, X.G. Liu, S.S. Leonard, A. Manivannan, Shape-enhanced photocatalytic activity of singlecrystalline anatase TiO2 (1 0 1) nanobelts, J. Am. Chem. Soc. 132 (2010) 6679–6685.

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