PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Morphology-controlled 2D ordered arrays by heating-induced deformation of 2D colloidal monolayer Yue Li, Weiping Cai,* Bingqiang Cao, Guotao Duan, Cuncheng Li, Fengqiang Sun and Haibo Zeng Received 14th September 2005, Accepted 16th November 2005 First published as an Advance Article on the web 8th December 2005 DOI: 10.1039/b513050f We develop a strategy to fabricate morphology-controlled two dimensional (2D) ordered arrays by solution-dipping sintered colloidal monolayer template. By heating colloid monolayer templates for different times, the morphology of ordered arrays can be controlled effectively, which is valuable to investigate the morphology-dependent optical, magnetic, electrochemical, catalytic properties of arrays. With increase of the heating time for templates, 2D ordered arrays with different morphologies can be fabricated in turn, such as, spherical pore array, honeycomb-shaped nanowall array, nanopillar array and regular network. Two kinds of morphology-controlled 2D periodic arrays, ferric oxide and silica, have been fabricated successfully by this way. This route is universal for synthesis of other compounds’ ordered arrays with controlled morphology. This strategy has expanded the applications of the colloidal monolayers as templates to prepare ordered nanostructured functional arrays.
Introduction Two dimensional ordered nanostructured arrays have many applications in the fields of gas sensors,1 cell culture,2 photonic crystals,3 optical waveguides,4 biotechnology5 etc. Traditional fabricating methods mainly focus on lithographical technology, such as optical,6 e-beam,7 X-ray8 etc. Due to its low sample throughput and high cost, only few laboratories can afford them. Several alternative techniques have also been developed based on self-assemblying principles.9 Among these methods, 2D colloidal crystal template method has attracted much attention in fabrication of versatile ordered nanostructured arrays, such as, ordered pore films (by electrodeposition,10 the sol–gel techniques11,12 and solution-dipping template synthesis strategy13), the periodic metal, semiconductor and polymer nanoparticle arrays,14–17 nanoring arrays,13–15 nanochain, nanogap, nano overgap arrays (by metal evaporating deposition14) and the nanowell arrays (by CF4 plasma striking18). By two-step replication, the polymer hollow sphere and solid sphere arrays can be obtained.19 In our previous work, the morphology of pore array can be easily controlled by changing the concentration of precursor solution via solution-dipping template synthesis strategy.13 In this study, we develop another a new route of morphologycontrolled growth of ordered array by heat-induced deformation of 2D colloidal monolayer, which is of great benefit to the research of morphology-dependent properties. However, the morphologies of the ordered arrays obtained by heat-induced deformation of 2D colloidal monolayer are very different from that we previously reported.13 In this method, the 2D colloidal crystals were heated above its glass transition temperature for different time, which can Key Laboratory of Material Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, Anhui, P. R. China. E-mail:
[email protected]; Fax: +86 551 5591434
This journal is ß The Royal Society of Chemistry 2006
lead to their deformation with different extent. Using them as templates, the morphology of ordered arrays can be tuning from pore, gradually to honeycomb-shape with nanowall and then to nanopillar, finally to regular network. Taking Fe2O3 as example, the fabrication process is described in detail in this paper.
Experimental Monodispersed polystyrene spheres (PSs) (1000 nm, 500 nm, 350 nm in diameter, respectively) suspensions (2.5 wt% in water, surfactant-free) were purchased from Alfa Aesar Company (standard deviation of the diameter of PSs is less than 5%). Fe(NO3)3, methylene chloride (CH2Cl2), tetraethyl orthosilicate, nitric acid and alcohol were of analytical grade and were used as bought from Shanghai Chemical reagent factory without further purification. Deionized water (18 MV) was used as the solvent in our study. Large-scale monolayer colloidal crystals with PSs of different diameters were firstly prepared on the glass substrates, which were ultrasonically cleaned in acetone and then in ethanol for 1 h, by spin-coating method, as our previously described.19,20 Secondly, the as-prepared monolayer colloidal crystals on the glass slides were put into an airtight-oven and kept at 120 uC for a certain time, and then taken them out and cooled naturally to room temperature. Subsequently, ca. 10 mL of 0.5 M Fe(NO3)3 solution was dripped onto the heat-treated monolayer colloidal crystals with a quantitative pipette and it could infiltrate into the space between the substrate and colloid monolayer. And then the samples were filled with precursor solution at room temperature. Finally, the samples were dried at 110 uC for 1 h and were annealed at 400 uC for 2 h to make the precursor decompose completely and the PSs template be burnt out. Two dimensional ordered arrays of other different materials were also synthesized using the procedures similar to those J. Mater. Chem., 2006, 16, 609–612 | 609
presented above.13 For example, for fabricating the silica 2D ordered array, the silica precursor solution tetraethyl orthosilicate, alcohol and distilled water (molar ratio, 1 : 4 : 20) was firstly prepared by stirring and its PH value was controlled to ca. 1.0 by adding a small quantity of nitric acid. Then, a drop of precursor with 10 mL was added on the colloid templates and the sample full of precursor solution was put into a beaker sealed with cover to gel at room temperature for about 7 days. Finally, the the sample was immersed in the (CH2Cl2) by ultrasonic method to remove the PSs templates. The as-prepared samples were characterized by field emission scanning electronic microscopy (FE-SEM) (JEOL 6700). X-Ray diffraction (XRD) spectra and energy disperse spectrum of X-ray (EDS) were measured on Philips X’Pert using Cu Ka line (0.15419 nm) and Oxford INCA analysis system, respectively.
Results and discussion By spin-coating method, large-scale monolayer colloidal crystals were successfully fabricated by a self-assembly process, as showed in Fig. 1a, which is a typical image obtained with 1000 nm PSs. We can see that the spherical PSs take on hexagonal close-packed lattice structures and contact with each other by quasi-point style. The colloid crystals (1000 nm PS in diameter) were sintered at 120 uC for different time, their morphologies were different, as displayed in Fig. 1a–g. It is well known that the glass transition of polystyrene (Tg) is ca. 100 uC,21,22 that is, if it is sintered at the temperature above Tg, it will be deformed. In our case, different morphologies of colloid crystals were produced by such deformation at 120 uC for different time. Compared with the colloidal monolayer without sintering (Fig. 1a), the deformation caused by heating leads to contact evolution between neighboring PSs changing gradually from quasi-point contact (see Fig. 1a) to facet contact (see Figs. 1b–e). Correspondingly, the interstices among the PSs in colloidal monolayer become smaller and smaller from top view. If over-heating the colloidal monolayer (say, for 25 min), the interstices among PSs will almost disappear due to the excessive deformation, as displayed in Fig. 1f. If the heating time is up to 30 min, the PSs are almost melted, as shown in Fig. 1g. Similarly, the different morphology of the monolayer with other different diameters (say 500 and 350 nm) can also be obtained based on the heating deformation strategy, the results are similar to Fig. 1 (not show here). Obviously, through controlling the heating time, the morphology of the colloidal crystal can be well tuned. Based on these deformed templates with different degree, we got the ordered patterns with different morphologies, as illustrated in Fig. 2, which correspond to the colloidal templates shown in Fig. 1, respectively. The corresponding X-ray diffraction peaks are in agreement with the standard values of bulk a-Fe2O3, indicating that the ordered structures consist of crystal a-Fe2O3, which comes from decomposition of FeO(OH), as shown in Fig. 3. Using the template without heating, we can get the ordered pore array with circular pore morphology (Fig. 3a), which is in agreement with that reported previously.13 After heating the monolayer for 7 min, the morphology of array takes on honeycomb shape (Fig. 2b), 610 | J. Mater. Chem., 2006, 16, 609–612
Fig. 1 FE-SEM images of the 2D colloidal monolayer (1000 nm PS in diameter) after sintering at 120 uC for different time: a–g: 0, 7, 10, 15, 20, 25, 30 min, respectively. The scale bar is 1 mm in each image.
each unit exhibits a hollow hexagonal prism with nanowalls as their sides and there is a small pore in almost whole walls, which comes from the increasing contact area between neighboring PSs due to the heating deformation. Further increasing the heating time, the small pillar with regular triangle prism shape appears at the node of the network, these pillars grow higher and the corresponding aspect ratio becomes larger with increase of heating time (Fig. 2b–g). The reason the formation of nanopillar array is that, due to heating deformation of colloidal spheres in the monolayer, the contact between neighboring PSs changes from point to facet; meanwhile, the PSs are deformed so much by heating that the triangular prism channels are formed at the interstices among the adjacent colloidal spheres. With increase of heating time, the contact extent becomes larger and the channels become higher and thinner gradually. In our case, on increasing the heating time from 10 min to 20 min, the aspect ratio increased from about 1 : 1 (the side length of the regular triangle shaped cross section and the height of the typical prism are This journal is ß The Royal Society of Chemistry 2006
Fig. 3 XRD spectra of the Fe(NO3)3 solution after dried at 110 uC for 1 h (the sample for XRD measurement was obtained by ultrasonically washing in methylene chloride (CH2Cl2) for 2 min to remove the colloidal template) (a) and subsequently annealed at 400 uC for 2 h (b). The peaks in curves (a) and (b) are indexed by FeO(OH) and a-Fe2O3.
If the heating time of template is more than this (say, 30 min, Fig. 1g), the melting of PSs lead that there is no space between the substrate and PS film, so the template has lost it function and nothing will be obtained. Our results indicate that, by controlling the heating time of colloid templates, the morphologies of ordered array can be controlled effectively. In addition, for the smaller diameter (say, 500 nm and 350 nm) of PSs in the monolayer colloidal template, its morphologies can be also controlled via the same method. The similar morphology-controlled ordered array can be also fabricated. For instance, if the heating time is appropriate (10 min and 6 min, respectively), Fe2O3 nanopillar arrays with higher density can be also prepared, as illustrated in Fig. 4.
Fig. 2 FE-SEM images of ordered patterns fabricated by different deformed colloidal monolayers. Images a, b, c, d, f and h were obtained by the templates shown in Fig. 1a, b, c, d, e, f, respectively. The number at the top left in each image corresponds to sintering time of different template. Image e is the local magnification of area of black frame in image d and image g is the feature picture of image f.
175 nm and 170 nm, respectively, see Fig. 2c) to 2 : 1 (the corresponding side length and height are 97 nm and 200 nm, respectively, see Fig. 2f,g), as shown in Fig. 2c–g. These arrays also exhibit hexagonal alignment. Here it should be mentioned that the nanopillars do not grow directly on the substrate, but on the nodes of the Fe2O3 skeleton network surrounding the previous bottoms of the sintered PSs. Such nanopillar arrays with small aspect ratio have many applications in sensor arrays, piezoelectric antenna arrays, and optoelectronic devices.23 Very interestingly, if the template is heated for too long a time (say, 25 min), the pillars will vanish due to no channels in the over-heated template and the morphology takes on a regular network, as shown in Fig. 2g, which is different from the result (Fig. 2a) obtained by the template without heating. This journal is ß The Royal Society of Chemistry 2006
Fig. 4 FE-SEM images of periodic Fe2O3 nanopillar arrays fabricated by the sintered (at 120 uC) colloidal templates with PSs diameter: (a) 500 nm (heating: 10 min), (c) 350 nm (heating: 6 min). (b) and (d) are the local magnified images corresponding to (a) and (c) respectively.
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devices,26 especially in next-generation integrated nanophotonic devices, bimolecular labelling and identification.13
Acknowledgements This work was cosupported by the National Natural Science Foundation of China (grant number: 10474099), Anhui Natural Science Foundation (Grant No. 050440902) and National Project for Basic Research (Grant No. 2006CB300402).
References
Fig. 5 Morphology of periodic silica nanopillar array formed by sol– gel based on the sintered template. The inset is corresponding local magnification.
Similarly, for other materials, this method is also suitable, for example, silica. If adding the silica precursor solution onto the template (1000 nm PSs in diameter) sintered for 15 min, and puting it into a beaker sealed with cover (solvent can evaporate very slowly) to gel (and evaporate) at room temperature, after removing the PSs, we can also obtain the periodic silica nanopillar arrays, as indicated in Fig. 5. Here, the slow evaporation is important, otherwise, the silica gel will easily broken, due to surface stress, as extensively reported.24
Conclusions In conclusion, a new strategy of solution-dipping sintered monolayer colloidal template is presented to fabricate morphology-controlled 2D ordered arrays. In this method, through controlling the sintering time, the colloidal monolayer template can be deformed with different extent. With increase of the heating time of templates, spherical pore array, honeycomb shape array, nanopillar array, regular network can, in turn, be obtained. Additionally, the unit size in arrays is adjustable by changing the diameter of PSs in the templates. Moreover, this strategy is universal for many materials beyond the Fe2O3 and silica illustrated in this paper. Such morphology-controlled arrays may have potential practical applications in waveguide ring laser,25 energy storage or conversion, gas sensor, catalyse, field emission, and biomedical
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