General Synthesis of Large-Scale Arrays of One-Dimensional

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DOI: 10.1021/cg9001835

General Synthesis of Large-Scale Arrays of One-Dimensional Nanostructured Co3O4 Directly on Heterogeneous Substrates

2010, Vol. 10 70–75

J. Jiang,* J. P. Liu, X. T. Huang,* Y. Y. Li, R. M. Ding, X. X. Ji, Y. Y. Hu, Q. B. Chi, and Z. H. Zhu Department of Physics, Central China Normal University, Wuhan 430079, P. R. China Received February 13, 2009; Revised Manuscript Received October 29, 2009

ABSTRACT: We have developed a general two-step synthesis of large-scale arrays of one-dimensional (1D) nanostructured Co3O4 directly on various substrates. Throughout a controllable hydrothermal process using urea as mineralizer and hereafter with a postcalcination process under air atmosphere, Co3O4 1D nanostructure arrays have been grown firmly on insulating substrates, such as glass slides and ceramics, which is quite convenient for the construction of gas sensor devices without any extra electrode preparation process. Furthermore, this direct-growth approach can be readily extended to conductive substrates (ITO, Ti, Fe-Co-Ni alloy), and meanwhile due to the robust mechanical adhesion and one-dimensional carrier transportation architecture firmly contacted to the metal, the metal substrate-supported Co3O4 arrays could act as a promising electrode material and be straightforwardly integrated into electronic and electrochemical nanodevices.

Introduction As a technologically important p-type magnetic versatile semiconductor, spinel oxide Co3O4 has been widely utilized in gas sensors,1,2 Li-ion batteries,3-7 electrochromic devices,8 heterogeneous catalysts,9,10 and field emission.11,12 Since transition-metal oxides MOx (where M is Co, Ni, Fe, or Cu) were first discovered for Li-ion batteries’ negative electrode materials in 2000,13 Co3O4 has been reported to be a promising alternative material for the next generation of Li-ion batteries because it exhibits superior specific capacity (896 mA h g-1 in theory) and capacity retention ability. Besides, based on its specific arrangement manner of surface atoms and the unique catalytic property, Co3O4 nanocrystals with different shapes can be compounded with rare earth metal oxides and serve as a substitute for the traditional noble catalyst in the activation of hydrocarbon.9 Meanwhile, since Co3O4 is a highly sensitive material, its nanostructure film has great potential to be exclusively applied in the detection of environmental deleterious gases14 in the future. Over the past few years, 1D ZnO, SnO2, etc. nanoarrays have been studied extensively and demonstrated to be the optimized architecture for the electronic/electrochemical electrodes and chemical sensors.15 In this regard, the novel structures directly grown on the substrates have much better electron transportation capability compared to the bulk counterparts and the traditional powder materials. However, Co3O4 nanostructured arrays have seldom been reported,5 and large-scale arrays of 1D metal oxides directly grown on various substrates are also hard to obtain, owing to the lack of novel synthetic routes. Generally, although the templatebased method has been greatly studied to prepare the metal oxide arrays by employing porous membranes (AAO), surfactants (soft template), or even viruses,16 large-area arrays with high quality are still difficult to realize in view of drawbacks originating from the poor template area, the post-treatment *To whom correspondence should be addressed. Fax: þ86-027-67861185. E-mail: [email protected] (X.T.H.); [email protected]. edu.cn (J.J.). pubs.acs.org/crystal

Published on Web 12/16/2009

(removal) of templates, and, more importantly, unavoidable impurities introduced during the synthesis process. In addition, this preparation method cannot be popularized to commercial manufactures on a large scale for its own disadvantage of high consumption and low efficiency. Previously, a great number of well-designed spinel Co3O4 nanostructures in the powder form (nanowires/nanorods,17,18 nanotubes,19 nanospheres,20 and nanowalls21) have attracted remarkable attention for their high surface area and superior electrochemical reactivity. However, the general synthesis of large-scale arrays of 1D Co3O4 directly on various substrates was never reported. In this paper, a general two-step template-free approach for large-area directed growth of Co3O4 1D nanostructured arrays on bulk insulating substrates (glass slides and ceramics) is demonstrated. Significantly, we found that, with the introduction of F-, the precursor can be firmly grown on the smooth glass slide and ceramics which were reported previously not suitable for the growth of metal oxides.24 The presence of F- is considered to play an important role in reducing the nucleation rate and activating substrates,27 leading to a robust mechanical adhesion between the final arrays and substrate. It is further noteworthy that the direct-growth approach with F- can be easily extended to conductive substrates including ITO (indium tin oxide) and Fe-Co-Ni alloy (nanorod). Besides, without the assistance of F- at even a lower temperature, it is interesting to find that metal substrates (Ti and Fe-Co-Ni alloy (nanowire)) are still effective for the growth of a high-density Co3O4 1D nanostructure. Compared with the template-based method, our general synthesis route is of high efficiency and low expenditure, which holds potential promise in the scale-up fabrication. Experimental Section Synthesis in the presence of F- (glass slide, ceramic substrate, ITO, and Fe-Co-Ni alloy): In a typical synthesis, 5 mmol of Co(NO3)2 3 6H2O, 10 mmol of NH4F, and 25 mmol of CO(NH2)2 were dissolved in 50 mL of water under stirring, respectively. After 10 min r 2009 American Chemical Society

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Figure 1. XRD patterns of (a) the precursor and (b) the as-made 1D Co3O4 nanoarrays, respectively. of slight stirring, the homogeneous solution prepared above was transferred into Teflon-lined stainless steel autoclaves. Then, a piece of clean substrate was immersed into the reaction solution. Hereafter, the autoclave was sealed and maintained at 120 °C for 5 h and allowed to cool down to room temperature spontaneously. After the reaction, the substrate was collected and rinsed with distilled water several times in order to remove the free nanoparticle debris and the residual reactant. Finally, the substrate was calcinated at 400 °C in air for 4 h and the as-synthesized products were attained. Synthesis without F- (only Ti and Fe-Co-Ni alloy): The preparation of Co3O4 nanorods/nanowires on the corresponding substrates is quite similar to that described above but with a few modifications. Without adding NH4F, 5 mmol of Co(NO3)2 3 6H2O and 25 mmol of CO(NH2)2 were dissolved in 50 mL of water under stirring, respectively. After 10 min of slight stirring, the homogeneous solution was transferred into Teflon-lined stainless steel autoclaves and Ti or Fe-Co-Ni alloy (Fe/Co/Ni= 52.23:18.07:29.7) substrate was introduced into the reaction solution. Then, the autoclave was sealed and maintained at 95 °C for 8 h and allowed to cool down to room temperature spontaneously. After the reaction and the essential cleanness, the substrate was calcinated at 400 °C under air atmosphere for 4 h. It is worthy to point out here that various substrates with the same tailored size (25  50 mm2) and shapes were employed in our experiment. Regardless of their size or shape, the experiment result showed that alignment of the one-dimensional Co3O4 nanostructures could occur on all the clean substrates. The nanostructured products grown on the substrate were directly subjected to powder X-ray diffraction (XRD, Cu KR radiation; λ = 1.5418 A˚) measurement and scanning electron microscopy (SEM, JSM-6700F; 5 kV) characterization. For the transmission electron microscopy (TEM and HRTEM, JEM-2010FEF; 200 kV) observations, the Co3O4 was scraped from the substrate and sonicated in ethanol, and the suspension was further dropped onto a Cu grid, followed by evaporation of the solvent in the ambient environment.

Results and Discussion Parts a and b of Figure 1 show the XRD patterns of the product obtained after 5 h of reaction and the final product, respectively. In our synthesis, the overall preparation process contains two crucial steps. First, by using urea as the mineralizer and undergoing an easily controllable hydrothermal

process, a thin film of cobalt-hydroxide-carbonate has been successfully attained on various substrates. Second, after the adequate pyrolysis of cobalt-hydroxide-carbonate samples at 400 °C in air for 4 h, Co3O4 1D nanoarrays can be obtained with a robust mechanical adhesion to the heterogeneous substrates. In accordance with standard XRD patterns, Figure 1a shows the XRD result of the precursor cobalthydroxide-carbonate (JCPDS Card No. 38-0547), and no peaks of impurities are observed in this pattern. Whereas, different from the literature,22 the highest peak is for the (412) plane, indicating a substantial texture effect in accordance with the crystal shape anisotropy and orientation. Figure 1b reveals that all the diffraction peaks can be indexed to cubic phase Co3O4 with the lattice constant a = 8.083 A˚, which is consistent with the value in the standard card (JCPDS Card No. 42-1467). No other peaks of impurities are observed. Figure 2a displays a typical photograph of large-scale uniform 1D Co3O4 arrays on a glass slide substrate after calcination at 400 °C for 4 h, and the inset is the corresponding optical image of the precursor cobalt-hydroxide-carbonate before the annealing process. In the literature, as an alternative precursor of spinel oxide Co3O4, cobalt-hydroxide-carbonate has been proved to be a desirable material for its dominated crystal growth and controllable morphologies.22 After the calcination treatment for a long time, the original purple thin film of cobalt-hydroxide-carbonate can be completely converted to black spinel Co3O4 and no toxic byproducts occur during the entire thermal decomposition process. Figure 2b and c show representative SEM images of Co3O4 nanorod arrays directly grown on a glass slide substrate. From the SEM images, gathered Co3O4 nanorod arrays with a relatively high density are obviously observed. With a sharp tip, each nanorod is of around 100 nm in diameter. Figure 2d and e display TEM images of Co3O4 nanorods, from which we can find that, with a mean diameter of 100 nm, the single nanorod is about 2 μm along the length direction. Figure 2f shows a high resolution TEM (HRTEM) image taken from the tip of the nanorod, which indicates a lattice spacing of

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Figure 2. (a) Photograph of large-scale uniform 1D Co3O4 arrays on a glass slide after calcination; the inset is the corresponding optical image of cobalt-hydroxide-carbonate before the annealing process. (b, c) SEM images and (d, e) TEM images of a Co3O4 nanorod directly grown on a glass slide substrate. (f ) HRTEM image of an as-prepared Co3O4 nanorod. (g) SAED pattern of an as-prepared Co3O4 nanorod.

0.24 nm, corresponding to the (311) crystal planes of spinel Co3O4, in good agreement with the original XRD pattern in Figure 1b. According to Figure 2g, the SAED pattern confirms a high crystallinity of the Co3O4 nanorod. Enlightened by the previous work24 in terms of the investigations between the preferential growth of metal oxides and selective substrates, we have found that the typical synthesis of 1D nanostructured Co3O4 arrays is easily extended to other substrates, such as ceramics, ITO, and Fe-Co-Ni alloy (nanorod); this growth could be related to the presence of F-. Figure 3a displays a general SEM image of as-obtained Co3O4 nanorod arrays directly grown on Fe-Co-Ni alloy. From Figure 3b and c, we can obviously observe that several nanorods with a mean diameter of less than 100 nm are bundled up to form a nanocornlike structure. Besides, the magnified image indicates that each nanorod is assembled by a cluster of individual units, which is quite consistent with the previous research about the morphology of the nanopowder reported before.23 Figure 3d shows a SEM image of asprepared Co3O4 nanorod arrays on ITO. It is apparent that well-aligned Co3O4 nanorods are distributing uniformly. From Figure 3e and f, an interesting nanostructure has been noted in which the tips of several smooth nanorods with an average diameter of 80 nm are coupled up markedly during the formation process. Herein, from the experimental results above we consider that with the introduction of the additive F-, the as-made Co3O4 1D nanostructured arrays are prone to gather together during the total process of crystal growth, which is extremely similar to the “coalescence growth” mechanism of ZnO.24 Unlike the case of the glass slide substrate (or ceramic substrate, ITO, and Fe-Co-Ni alloy (nanorod)), it is quite easier for the thin film of cobalt-hydroxide-carbonate to grow on metal substrates (Ti and Fe-Co-Ni alloy (nanowire)) directly without a high temperature and the assistance of F-. After a few adjustments of the typical synthesis, the general method can be easily extended to Ti substrate and Fe-Co-Ni

alloy (nanowire). Figure 4a displays a SEM image of Co3O4 nanorod arrays directly grown on Ti foil with a good orientation. Figure 4b reveals that the surfaces of Co3O4 nanorods with a mean diameter of 100 nm are permeated with a large quantity of mesoporous structures. The reason may be ascribed to the successive release and loss of CO2 and H2O during the thermal decomposition of precursor. As manifested in Figure 4c and d, large-scale uniform Co3O4 nanowire arrays with a smaller diameter of 80 nm can be evidently observed. Compared to the Co3O4 nanorod arrays on Fe-Co-Ni alloy substrate in Figure 3b, it indicates that, without adding F-, the 1D nanostructures are prone to grow individually instead of being coupled up markedly. With respect to the formation mechanism of cobalt-hydroxide-carbonate on a glass slide substrate (or ceramic substrate, ITO, and Fe-Co-Ni alloy) with the assistance of F-, the chemical reactions involved in the preparation process can be expressed with the following formulas:22,23,26 Co2þ þ xF - f CoFx ðx -2Þ H2 NCONH2 þ H2 O f 2NH3 þ CO2 CO2 þ H2 O f CO3 2 - þ 2Hþ NH3 3 H2 O f NH4 þ þ OH CoFx ðx -2Þ - þ 0:5ð2 -yÞCO3 2 - þ yOH - þ nH2 O f CoðOHÞy ðCO3 Þ0:5ð2 -yÞ 3 nH2 O þ xF 3CoðOHÞy ðCO3 Þ0:5ð2 -yÞ 3 nH2 O þ 0:5O2 f Co3 O4 þ ð3n þ 1:5yÞH2 O þ 1:5ð2 - yÞCO2 On the basis of the previous research related to cobalthydroxide-carbonate nanoparticles,21,22 we propose a fourstep growth mechanism concerning the growth of Co3O4 1D

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Figure 3. SEM images of a Co3O4 nanorod directly grown on (a-c) an Fe-Co-Ni alloy substrate and (d-f ) ITO.

Figure 4. SEM images of a Co3O4 nanorod directly grown on (a, b) Ti substrate and (c, d) Fe-Co-Ni alloy (nanowire) without F-.

nanostructure arrays on substrates, as schematically shown in Figure 5. Step I: At the beginning, bivalent Co2þ ions were fully coordinated with F- to form CoFx(x-2)- in the asprepared homogeneous solution. As the temperature of the reactant solution ramped to 120 °C in the oven, the hydrolysis-precipitation process of urea took place around 70 °C and a number of CO32- and OH- anions was formed gradually, which could help to release Co2þ ions slowly from CoFx(x-2)- in the solution. Step II: When the concentration of anions (CO32-, OH-) in the as-prepared reactant solution increased, the further reaction led to the formation of a

nucleus. With the choice of suitable substrates in the chemical reaction system, it was observed that the nucleus was prone to form on the substrates’ surface rather than in aqueous solution, which was highly demonstrated in the previous work,23,24 related to the general direct-growth mechanism of metal oxide MOx (where M is Zn, Sn). Here, it is important to mention that F- in the solution has played a crucial role throughout the preparation process. Without the assistance of the additive NH4F, reproducible experimental results reveal the fact that the precursor cobalt-hydroxide-carbonate cannot directly grow on smooth substrates (glass slide, ceramic substrate,

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Figure 5. Schematic mechanism for the direct-growth process of one-dimensional Co3O4 arrays on the backside of substrates.

or ITO). After moderate F- anions were introduced and the coordinated CoFx(x-2)- formed first, Co2þ ions were released slowly into the reaction system and gradually promoted the formation of a nucleus on substrates. Instead of the rapid nucleus formation, we speculate that the slow-down process is greatly helpful for the substantial contact between nucleus and the smooth substrates, which are not appropriate for the growth of metal oxides. In addition, we consider this could also be associated with the interaction between substrates and hydrofluoric acid (HF) formed during the complex reaction process. As reported in the literature,27 this substrate preactivation can be favorable for the formation of nuclei on substrates, ensuring the good contact between metal oxide and substrates. Aimed to demonstrate the function of F-, 10 mmol of NH4Cl has been used as the substitution of NH4F for comparative study. Different from the previous experimental result, the as-made purple thin film of precursor on a glass slide (or ceramic substrate) was easy to rinse away entirely during the essential cleanness process, which evidently reveals the fact that F- is largely associated with the adhesion between substrates and the nanoarrays. With regard to Ti substrate, it is not necessary to employ F- because of the sufficient substrate-surface roughness, which is well demonstrated for the nucleus formation and metal oxides’ growth.24 Step III: Due to the continuously proceeding reaction, the growing cobalt-hydroxide-carbonate nuclei were beginning to impinge on other neighboring crystals and assemble along the specific orientation preferentially. However, it is interesting to find the phenomenon that the uniform thin film of cobalthydroxide-carbonate with novel 1D architecture can merely grow on the backside of the substrates, while for the upside only the coarse film covered with lots of accumulated particles can be observed. In addition, after the final annealing process, the products on the upside were easy to peel off, which cannot be used in various applications because of their poor adhesion. The reason was supposed that the crystal growth on the upside was severely hindered because a large quantity of precipitations was formed rapidly in the solution and directly wreathed the nucleus on the substrate in view of gravity, hindering the formation of a 1D structure. Step IV: After annealing for 4 h in air, cobalt-hydroxide-carbonate decomposed gradually and black Co3O4 1D nanostructured arrays on substrates were obtained. Herein, it is worthwhile to point out that the cobalt oxide grown on different substrates shows different structures. To the best of our abilities, we consider that the yield of distinguishing structures could be attributed to the intrinsic properties of substrates. Different interfacial chemistry could affect the size of nuclei formed on the substrates and the further crystal growth, leading to the distinguishing

morphologies. However, substrates can affect the structures of films to some extent but not alter the general morphology of 1D nanoarrays. Furthermore, to demonstrate the robust mechanical adhesion of final products, a typical cross-sectional image of 1D nanostructured Co3O4 arrays on a glass slide has been shown (please see the Supporting Information). As displayed in Figure S1, good contact between the film and substrate can be observed. Conclusions In summary, a general two-step template-free synthesis of large-scale one-dimensional Co3O4 arrays directly on various substrates was developed by a controllable hydrothermal process, and meanwhile we found that, with the introduction of F-, the precursor could be firmly grown on the smooth glass slide and ceramics, which were reported previously not suitable for the growth of metal oxides. Significantly, this preparation is quite convenient for the construction of gas sensor devices without any extra electrode preparation process. Besides, the direct-growth approach can be easily extended to conductive substrates (ITO, Ti, Fe-Co-Ni alloy). Compared with the template-based method, our general synthesis route is of high efficiency and low expenditure. Acknowledgment. We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 50872039; No.50802032). Supporting Information Available: Cross-sectional SEM image of 1D nanostructured Co3O4 arrays on a glass slide. This material is available free of charge via the Internet at http://pubs.acs.org.

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