Two Substrate-Confined Sol–Gel Coassembled Ordered Macroporous

Article pubs.acs.org/Langmuir

Two Substrate-Confined Sol−Gel Coassembled Ordered Macroporous Silica Structures with an Open Surface Wenhua Guo,†,‡ Ming Wang,*,† Wei Xia,† and Lihua Dai† †

School of Physics and Technology, Nanjing Normal University, Key Laboratory on Opto-Electronic Technology of Jiangsu Province, Nanjing 210046, China ‡ Jiangsu Laboratory of Advanced Functional Materials, Department of Physics and Electronic Engineering, Changshu Institute of Technology, Changshu 215500, China ABSTRACT: A sol−gel cooperative assembly method was demonstrated for the fabrication of inverse opal films with an open surface. In this method, a sol−gel silicate precursor was cooperatively assembled into the interstitial spaces of microspheres at the same time when polystyrene templates formed in between two desired substrates. Silica inverse opals with a threedimensional ordered macroporous (3DOM) structure were obtained after selective removing the colloidal templates by calcination. The open surfaces with a high degree of interconnected porosity and extremely uniform pore size were characterized by scanning electron microscope (SEM). Optical transmission spectra reveals the existence of considerable deep band gaps of up to 70% and steep band edges of up to 6%/nm in the [111] directions of the 3DOM silica samples. A little shrinkage confirmed by transmission spectra is not larger than 3%, in consistent with the results measured by SEM, which revealing the sufficient and compact infiltration into the interstitial spaces by our confined sol−gel coassembly method. With different incidence angles, the positions of pseudogaps can be easily tuned in the wide range from 720 nm to 887 nm, agreed well with the calculated values by the Bragg law. All the results prove that the sol−gel coassembly method with two substrates confinement is a simple, low cost, convenient and versatile method for the fabrication of silica inverse opals without overlayers in large domains.

1. INTRODUCTION Recently, a great deal of attention has been paid to the fabrication of inverse opals, or three-dimensional ordered macroporous (3DOM) materials owing to their distinct structural features. Such 3DOM materials have high degree of interconnected porosity with extremely uniform size, and periodic distributions of pores, which have the wide range of possible applications in many fields, such as photonic crystals, catalysts, sensors and various other applications.1−8 The most frequently used method of preparing such ordered structures is based on colloidal crystal templating method, which typically requires three sequential steps.5−8 First, form a colloidal crystal template of close-packed, uniformly sized latex or silica spheres, then fill the interstitial spaces with a fluid precursor capable of solidification, and at last selectively remove the template to obtain a porous inverse replica. Thus, the fabrication of a high quality template is the crucial step to obtain final ordered porous structure. Although a variety of assembly methods9−16 have been developed, the creation of high quality colloidal crystal films without the formation of cracks, domain boundaries, spherical vacancies, and other defects in large area is still an intellectually challenging problem. The next two problems to be solved are effective infiltration of the voids of the colloidal template and minimization of the shrinkage after © 2013 American Chemical Society

selective removal of the template. To address these challenges, various approaches have been developed.17−23 For example, diluting the precursor in ethanol can lower the viscosity and improve the wetting properties of the sol precursor.17 However, this approach typically required multiple infiltration steps in order to achieve a mechanically robust structure. Recently, an elegant method employed “lift-off/turn over” strategy was developed to reduce the overlayer on the top of the 3DOM structure.19 However, this method involves multiple steps, and the formation of overlayers cannot be completely avoided. It is worthy of note that the assembly of 3DOM structure with nanocrystalline particles by a cooperatively assembly method20−23 have been reported. But, this method requires the nanoparticles in considerable small size and easily leads to the jamming effect which can make it difficult to obtain the high quality colloidal composite. In a very recent study, Hatton et al.24 have demonstrated the evaporative coassembly of a sacrificial colloidal template with a matrix material to successfully synthesize highly ordered inverse opal films over centimeter length scales. However, the ratio of the matrix Received: April 7, 2012 Revised: March 28, 2013 Published: April 24, 2013 5944

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when PS spheres are deposited on the substrate from a hydrolyzed tetraethoxy silane (TEOS) solution, the sol−gel silicates assemble themselves in the voids of the colloidal template at the same time. Therefore, the voids can be completely filled by the silica gel matrix material in a single step. The coassembly method insures the sufficient infiltration and deposition of silica gel during the formation of colloidal composite simultaneously. Moreover, the overlayer of the macroporous structure resulted from excessive infiltration and deposition by conventional methods can be availably eliminated with the confinement of two substrates, as well as surface bumpiness of the colloidal composite.25After removal of the colloidal template and the polymeric top substrate, 3DOM structure with an open surface in large domain can be obtained.

precursor to colloidal suspension should be controlled precisely which leads the experiment difficult to be repeated. Additionally, the surface of the inverse opal was not thoroughly smooth along the direction of crystal growth and cracks were occurred every 50−100 μm domain.24,25 Therefore, a simple and effective method is urgent to be proposed which can meet all these challenges and obtain 3DOM structure with open surface as desired. In this paper, we demonstrate the cooperatively evaporative assembly method of a sacrificial polystyrene (PS) colloidal template with a sol−gel precursor in a single step to yield a colloidal composite, then selective removing the template to yield silica inverse opals with an open surface. SEM images reveal that highly ordered silica porous structures without overlayers in large areas have been fabricated successfully using this simple coassembly method. Transmission spectra with deep photonic band gaps and steep photonic band edges have confirmed the high quality of the 3DOM structures, which can be applied in photonic crystals, optical switching, and sensing.1−8

3. EXPERIMENTAL SECTION In our experiments, the monodispersed polystyrene spheres (the range of diameter was from 300 to 700 nm, approximately 5% standard deviation) were purchased from Bangs Lab. Inc. The substrates used were polymethylmethacrylate (PMMA) and silica glass slides cleaved in two pieces lengthwise (∼38 mm × 13 mm). Prior to use, all substrates and glass vials were soaked in a chromic sulfuric acid cleaning solution overnight, rinsed thoroughly with ultrapure water, and dried in a stream of nitrogen. The standard TEOS solution consisted of 1:1:1.5 ratio by weight of TEOS (98% Aldrich), 0.10 M HCl, and ethanol (100%), respectively, stirred at room temperature for 1 h prior to use.24 Then the hydrolyzed TEOS solution (0.1 mL) and PS colloidal dispersion (1.0 mL 1.5 vol %) were added to 5 mL of deionized water. After the full ultrasonic dispersion of the mixture slurry, two clean substrates were vertically dipped into the prepared slurry in the glass vial, leading the slurry sucked in-between of the two substrates by capillary forces.12−15 All the setup was placed into a constant temperature oven and the temperature was set to 50 °C. The solvent content was evaporated slowly over a period of 12 h. The number of layers (thickness) of the 3DOM film can be controlled by varying the concentration of PS dispersion (the optimal TEOS-tocolloidal ratio was kept all the time), as well as the space between the two close substrates. The distance between the two substrates is separated by two 2D translation stages. The substrate is fixed on each stage, which can be adjusted from micrometer to millimeter in the two dimensions. For the alignment, the two stages were positioned coaxially on the optical bench. One of the stages is fixed, and the other can be moved to a desired distance as needed and then fixed. The resolution of movable length of the translation stages is 1 μm, so we can ajust the distance from micrometer to millimeter precisely. Similar to the colloidal crystal opals,12−15 the mechanical stability of 3DOM structures is also enhanced by the confinement in between two close substrates. After selective removal of the colloidal crystal template and the polymeric top substrate by calcination in air at 500 °C for 1 h (the colloidal composite was slowly heated to 500 °C in 7 h), a high quality and mechanical stability 3DOM structure with a planar open surface was finally obtained. The prepared silica inverse opals were characterized by scanning electron microscopy (SEM, JSM-5610) for structural feature and by transmission spectra at normal incidence using a vis-near-IR spectrophotometer (VARIAN Cary-5000). Transmission spectra for different oblique incidence angles were collected by an optical spectrum analyzer (OSA, AQ6370), and the light for detection was from a white light source (AQ4305).

2. MECHANISM OF THE COASSEMBLY METHOD WITH TWO-SUBSTRATE CONFINEMENT The major advantage of this method is that the coassembly of a colloidal composite avoids the need for liquid infiltration into a preassembled colloidal template. Figure 1 shows the schematic

Figure 1. (a) Schematic of sol−gel coassembly of colloidal composites in-between two desired substrates. (b) Dried colloids/silica gel composite structure with a polymeric substrate on top. (c) Silica porous film with a planar open surface. The top substrate (red) is polymethylmethacrylate (PMMA) and the other is silica glass slide.

of the simple fabrication method used, which combines the vertical deposition with capillarity between two desired substrates. Previously, the vertical deposition with two confined substrates was used to assemble opal structure films and also was aid to enhance the robustness and mechanically stability of the resulting opals.12−15 However, in our sol−gel coassembly method, the main advantage of the confined substrates is to eliminate the overlayers of sol−gel matrix material over the porous structure surface. Two substrates (one is polymeric slide) are dipped into a slurry which contains PS colloidal dispersion and a sol−gel precursor solution. Then the mixture of the suspension drove by capillary forces is penetrated into the limited space confined by two substrates. In particular,

4. RESULTS AND DISCUSSION Figure 2 shows the SEM images of the 3DOM silica inverse structures made from 390 nm PS spheres. In these samples, the absence of as-grown defects in the largely observed area is apparent and perfect hexagonal close packing of air spheres can be observed, which reveal that our two-substrate coassembly method produces highly 3DOM structures with a planar open 5945

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Figure 2. SEM images of the silica inverse opals made using 390 nm PS sphere templates. (a), (b), and inset are the top view of the sample with different magnifications. (c) Large area view of the cross section and inset are the magnified images indicated by the white squares. (d) Side view of the silica inverse opal. The thickness is about 18 μm.

surface and single crystalline domains of hundreds of micrometers. Figure 2a shows a low-magnification SEM image of the (111) surface of a silica inverse opal using the sol−gel coassembly method. The parallel lines are known as Moiré fringes and are induced by interactions between a scanning line pattern of an electron microscope and hexagonal lattices of colloidal spheres at a specific magnificaiton.26 The well-defined, uniform, large-area linear Morié pattern exhibits the highly ordered hexagonal surface packing in single domains (with dimensions of at least 0.2 × 0.2 mm2 in Figure 2a) Cracks formed during the drying process were significantly reduced in these samples, with intact films spreading over several millimeters. This excellent ordering structure with no cracks is attributed to the added sol−gel precursor and the confined top substrate. During the coassembly process, the sol−gel matrix that undergoes polycondensation at the time of colloidal assembly acts not only as an infiltrated precursor but as a glue to assembling spheres. The interconnected silicate network in the coassembled composite films inhibits the formation/ propagation of cracks for film thickness up to approximately 18 μm (Figure 2c). However, the thickness of our case is much larger than that of the 20 layer inverse opal film (approximately 5 μm) without any confinement in a previously published work.24 We suggest that the thicker film resulted from the limited space inside two substrates consists of dense sol−gel precursor and are robust and stable enough in the drying and sintering stages. Additionally, the top substrate also provides sites for the relaxation of tensile stresses during the gelation process. Beneath each air sphere in the first layer, a regular triangular pattern of three dark regions connecting another

three air spheres of the underlying layer is shown in Figure 2b, indicating the highly ordered structure of the silica inverse porous films. Furthermore, the excellent uniformity of structural details such as wall thickness and interconnecting windows can also be observed. The wall thickness is observed to be about 40 nm by averaging the detected values by SEM analysis from several different areas, as shown in the inset of Figure 2b with high magnifications. The sizes of the air spheres are observed to be ∼381 nm, which is about 2.6% less than that of the original colloidal spheres with diameters of 390 nm. Such little shrinkage should attribute to that the sol−gel precursor cooperatively penetrated into the voids of PS templates during our assembly process, which could simultaneously compact the whole structure and the solid network. The confinement of two close substrates is also responsible for the little shrinkage and stability of the resulting silica inverse opals. Similar results have been observed in the preparation of other three samples and the diameters of air pores are 478, 565, and 672 nm, respectively (the templates were made of PS spheres with different diameters of 490, 580, and 690 nm, respectively). The lattice constants of these three samples are close to those of the PS spheres used as templates with little shrinkage less than 3% to the overall structures, which indicates that the large volume shrinkage which usually occurs in the common sol−gel infiltration process18−25 has been avoided during our sol−gel coassembly method. The little shrinkage further illuminates the dense sol−gel precursor assembly into the voids of spheres. This makes the high degree of order of the resulting 3DOM structures extend to tens of micrometers with no cracks and 5946

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Figure 3. SEM images of the silica inverse opal coassembled from 580 nm PS spheres. (a) An interior (111) layer. (b) Some representative defects filled with more silica gel.

Figure 4. SEM images of the silica inverse opals obtained from 690 nm PS templates with/without two substrate confinement. (a) Cross section and (b) fracture of the sample fabricated by coassembly method without a top PMMA substrate confinement. (c) Sol−gel coassembly method with two substrates confinement with the distance about 8 μm. The inset bar is 1 μm.

indicated by a circle is surrounded by more sol−gel precursor, as well as the smaller PS spheres in the sacrificial template indicated by arrows. This is favorable for the subsequent assembly of the template and can also improve the quality of the final porous structure. Therefore, the sample remains high quality with ordered porous structure. Although the same kind of phenomena can also been found in the samples fabricated by the conventional two-steps, which fabricate the template first and then infiltrate precursor into the voids of the template, they are formed by different processes, and the effects on the final porous structure are different.20−23 In our coassembly process, it is the immediate mending of these defects that can avoid the template from collapsing and greatly enhance the robustness and stability of the final porous structure after heating and calcining process.24 For comparison, the sample fabricated under the same condition only with one substrate for deposition (without top confined substrate) is observed by SEM analysis. In Figure 4a and b, the open surface is slightly covered with a silica gel overlayer, though the ratio of the concentration of PS colloidal dispersion to that of silica sol−gel precursor is remain optimizing. The ordered macropores with hexagonal closepacked structure on the (111) surface can dimly be seen through the slight silica overlayers, as well as the high order of cross section in the inset of Figure 4a. Because of the differences in mechanical properties between the bulk silica layer and the porous silica structure, stresses introduced during

also enhances the robustness and stability of the inverse opal structures.21−23 The large area view of top surface and cross section is shown in Figure 2c. The formation of overlayers is eliminated completely through the whole area. The smooth surface with open pores over large area up to hundreds of micrometers (Figure 2a) can be distinctly observed, which benefits from the application of the confinement of two substrates in the sol−gel coassembly method. The magnified SEM fracture images in the inset of Figure 2c shows the detailed structure of the tridimensional framework on the top and an interior (111) layer of the silica inverse opal film in the bottom, which further manifests the highly ordered face-centered cubic (FCC) structure inside. Figure 2d shows the side view of the sample. The terrace structure which belong either to {100} or {111} facets indicates that the high quality of the silica porous films possess FCC structure. In our sol−gel coassembly method with two-substrate confinement, the inverse silica opals can exhibit a high degree of interconnected porosity with extremely uniform size (in the range of 300−700 nm) and periodic distributions of pores from the interior to the top (Figure 3a). The result further manifests the highly ordered FCC structures inside of the samples. Furthermore, spherical vacancies, which are common defects in samples fabricated using the self-assembly method,9−13 can be immediately filled by sol−gel precursor when the template is being assembled, which is shown in Figure 3b. A small hole 5947

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Figure 5. (a) Normal transmission spectra of silica porous structures. The diameters of the air pore were measured directly from the magnified SEM images of 3DOM structures. (b) Linear relation between the PBG positions and air-sphere sizes.

Figure 6. Schematic of the OSA setup. SMF: single-mode fiber.

guished discrepancy can be attributed to the two substrates applied for confining the colloidal template, as well as the sol− gel coassembled into the voids of microspheres simultaneously. The confinement of two substrates can not only produce thicker porous films, but also improve the mechanic stability of the final porous structures.12−15 Furthermore, after selectively removal of the two substrates (by calcinaing the polymer slide or chemical etching the silica slide), a free-standing ordered macroporous structure with controlled thickness and a uniformly flat and open surface can be obtained accordingly. Thus, the resultant samples with high porosity and specific surface area can meet the practical applications such as in photocatalysis,5−8 solar cells,27 and bio- or chemosensing.28 The optical properties of the 3DOM films were characterized by vis-near-IR transmittance spectra with the incident light normal to the surface of glass substrates coated with silica inverse opal films. Spectra were then taken and compared from different parts of the 3DOM structures to account for local inhomogeneities (local defects, cracks, etc.).19−23 For all of the samples, the depths of the photonic band gaps (PBGs) exceed 60%, which reveals the high crystalline quality of the inverse opals over a large area (Figure 5a). The largest slope of the photonic band edge (PBE) reaches up to 6%/nm, which is promising to be applied as optical switchers and sensors.28−31 Transmission dips are centered at 701, 887, 1028, and 1215 nm, respectively. Figure 5b shows the linear relationship of the PBG position to the pore size. Alternatively, from the slope of the curve in Figure 5b, the refractive index of the silica gel-filled void area (calculated with Bragg’s law) is 1.43, consistent with the reported value of the reactive index of the dried silica gel,32 which shows that the voids of the PS colloidal template are

the drying and calcination steps often lead to severe structural distortions such as cracking in the final porous structure, as shown in Figure 4b. Additionally, the thick bulk layers tend to fracture randomly, resulting in the (111) crystal face lying at highly oblique angles from normal.19,23−25 Such structural deviations from the planar structure have a profound effect on the resulting photonic properties, especially when collecting optical data at normal incidence to the film top surface.14 However, with the confinement of two desired substrates, all these disadvantages of the overlayer on the surface of the inverse silica opals and cracks can be effectively solved in sol− gel coassembly method. In our confined coassembly experiment, the PS spheres can flow into the limited space and arrange to hexagonal packed structure by capillary forces, together with the sol−gel silicate precursor assembled themselves in the voids of the particles spontaneously. The PS colloidal template is in touch with the substrates and the excess silica sol on top of the 3D colloidal template can be thoroughly eliminated, which leads to the resulting inverse opal with an open surface. Figure 4c shows a side-view of the porous structure prepared from a 690 nm PS colloidal template by using our coassembly method, in which the high degree of vertical ordering was well maintained throughout the whole thickness of nearly 8 μm. The magnified image in the inset of Figure 4c shows in detail the regular packing of air spheres in the cross section of the sample, which revealing the FCC structure of the silica inverse opal. In a single deposition, the thickness of the 3DOM structures can be altered from a single monolayer to nearly 100 layers (Figure 2c), which is several times greater than the maximal thickness of the opals obtained by other convective self-assembly method.8−13 This distin5948

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Figure 7. (a) Transmission spectra for different incidence angles, θ, with respect to the surface normal of the silica inverse opal made of 490 nm PS spheres. From top to bottom: θ = 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, and 40°. Spectra have been vertically shifted for the sake of clarity. (b) The relationship between the PBG positions (λmax2) and the sinusoid of incidence angles (sin2 θ). The straight line is a linear fit by the Bragg formula.

completely filled with sol−gel precursor during cooperative assembly process. The result further demonstrates that a highly ordered porous structure exists in three dimensions of the silica inverse opal and confirms the sufficient infiltration of silica gel in the voids of the PS colloidal template. The interesting optical property of these 3DOM structures with deep PBG and steep PBE may find applications in photonic crystals, optical switching and bio- or chemosensing.1−8,28−31 The silica inverse opals do not produce complete PBGs due to the low refractive index of silica.33 However, through infiltration of matrix materials with higher dielectric constants such as silicon,1,34 3DOM film process complete PBGs where omnidirectional light of any polarization can be reflected. This unique feature is promising to be applied in the areas of optical communication, waveguides with sharp corners, and even Si bandgap devices incorporated into microelctronic integrated circuits.34−36 In order to quantitatively characterize the silica inverse opals, transmission spectra were investigated by our setup whose diagram is shown in Figure 6.37−39 The relationship between the incidence angles and the center positions of the transmission dip has been studied. An ANDO AQ4305 white light source and AQ6307 optical spectrum analyzer (OSA) were used. Two cleaved single mode fibers (SMF-28) were coupled to the source and OSA to illuminate the samples and collect the transversely transmitted light. In our case, two lenses were used to focus incident light to a single spot (∼100 μm diameter) in one side and the other collect the transmitted light, then transport to the detected fiber. For the alignment, the two SMFs were positioned coaxially of longitudinal separation by two 3D translation stages. The silica inverse opal sample was then inserted in-between. Figure 7 shows the transmission spectra for different incidence angles with respect to the surface normal of the silica inverse opal coassembled from a 490-nm-diameter PS colloidal template. The incidence angles indicated in Figure 7a changed from 0° to 40°, and the spectra were recorded every 5°. The transmission spectra were normalized to that insertion of a clean glass slide without a silica inverse opal. All of the optical transmission spectra clearly appear considerable PBGs in the curves, which originated in the intensive Bragg diffractions from ordered structures. Obviously, at the angles deviated from normal incidence, a wider and shallow stop band occurs in every curve. The peak positions in the spectra depend

on the incidence angles and they shift to shorter wavelengths with increasing detection angles. Figure 7b shows the square of peak positions related to the sinusoidal function of incidence angles, which scaled well linearly. Provided the FCC structure of the silica inverse opal, in the calculation of the pore diameters, the position of stop bands (band gaps) can be estimated using a modified Bragg law34−38 which takes into account refraction of light in the composite structure and the incident angle of the incoming light λmax = 1.633D navg 2 − sin 2 θ

where navg is the average refractive index of the photonic structure, θ is the angle between the incident light and the surface normal of the sample, and D is the air pore size of silica inverse opal. For our silica inverse opal in FCC structure, navg = nsilica fsilica + nair fair, where nsilica = 1.43, nair = 1.00, and fsilica and fair are the volume fractions occupied by silica gel and air in the structure (generally taken as 26% and 74%, respectively, for a FCC lattice). Experimentally, D and navg are determined by measuring the position of the reflectance maximum (λmax) at different angles of incidence, and plotting λmax2 against sin2 θ . Thus, from the plot of fitted linear curve with λmax2 versus sin2 θ, the diameter of air pore D and the refractive index of silica framework nsilica can be estimated from the straight fitted line with slope = −(1.633D) 2 and y-axis intercept = (1.633D)2navg2.40 The calculated value of air pore size is 478 nm, which is agreed well with the result measured by SEM observation. This further demonstrates that a highly ordered porous structure exists in three dimensions of the silica inverse opal and confirms the sufficient infiltration of silica gel in the voids of the PS template. Similar results can be obtained from the other three silica inverse opals with different air pores. The interesting optical properties of silica inverse opals undoubtedly reveal the success and efficiency of our sol−gel coassembly method with two-substrate confinement, which is promising to make tunable photonic crystals applied in the field of optical switching and sensing.28−31,34−40 Our results clearly indicate, that besides the reduction of the synthetic steps involved in the fabrication of inverse opals, the sol−gel coassembly described here offers significant improvement of the quality of the films without of cracking and overlayer. Also, the thickness of the inverse opals can be dramatically improved up to tens of micrometer with the 5949

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Langmuir confinement of top substrate, which is much larger than the published work.21−25 The most remarkable advantage of the sol−gel coassembled method confined by two substrates is considerably little shrinkage of no more than 3%, which has been confirmed by the SEM images and transmission spectra. First, sol−gel coassembly method makes the gel precursor infiltrate the interstitial voids immediately, which ensures the sufficient and complete infiltration. Thus, the density of TEOS between colloidal spheres interstitial voids was so compact to retain the shrinkage of the diameter of the air spheres and form solid frame structure. In addition, the immediate mending of the defects such as vacancies can avoid the template from collapsing and cracking, which greatly enhances the robustness and stability of the final porous structure after heating and calcining process. Second, with the confinement of the top polymer substrate, the capillary force between the voids of PS spheres is improved, especially to the top layer of colloidal crystal. Thus, enhanced capillary flow can be induced, which can draft more gel precursor through the interstitial voids and reinforce the density of the solid frame structure. These two discrepancies from conventional “template-infiltration” method can not only reduce the cracks during the formation of the template and the drying progress, but also inhibit the shrinkage of the diameter of the air spheres in these samples. Furthermore, during the gelation process, the interfaces between the assembling colloidal spheres and the polymerizing sol−gel solution may provide sites for the relaxation of tensile stresses. Controlled solvent release during the polycondensation reaction can also occur at these interfaces and be channeled through the interconnected porous network to evaporate at the surface. The controllable release not only prevents the silica inverse opals from cracking but also restrains the large shrinkage due to the dense assembled colloidal spheres and sol−gel matrix.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Grant No. 61178044, 91123015), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20113207110004), the Key Program of the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 10KJA510024), the General Program of the Natural Science Foundation of Jiangsu Higher Education Institutions of China (Grant No. 12KJB140001), the Open Project of Jiangsu Laboratory of Advanced Functional Materials (Grant No. 12KFJJ003), and the Young Researcher Fund of Changshu Institute of Technology (Grant No. QZ1113).

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5. CONCLUSIONS A sol−gel coassembly method with the confinement by two close substrates was developed to fabricate large area silica inverse opals with open surface pores and without overlayers. Using this simple, fast, and relatively inexpensive method, we have successfully fabricated large-area, high ordered 3DOM structures with different pore sizes. The mechanical stability and little shrinkage (approximately