Preparation of Inverse Opal Zirconia

Inverse opal zirconia is useful in many ways because of their ability to combine several chemical and physical properties. In this research, polystyrene template was fabricated by self-assembly method and inverse opal zirconia was prepared by colloidal crystal-templating method. The process of preparation of inverse opal zirconia as well as effects on morphology and phase of as-prepared inverse opal zirconia were studied. The results showed precursor ratio of zirconium acetate and methanol, mass ratio of polystyrene templates and precursor and dipping times had remarkable influence on morphology of inverse opal zirconia. When the precursor ratio was 1:1; the mass ratio was 1:15 and dipped once, much better morphology of inverse opal zirconia was obtained. The mass ratio, sintering temperature and holding time had significant effect on crystallization of zirconia. Pure phase zirconia could be obtained when sintered at 600 C, holding time was 2h and the mass ratio was 1:1. A distinguished single stop band in the visible region of the spectrum and unique structural color were observed in inverse opal zirconia, which will make this material promising candidate for novel pigment.


Introduction
Photonic crystals are an artificial structure with periodic spatial variation in their refraction index that can control the propagation and spontaneous emission of light in some or all directions of space [1][2][3].Photonic crystals, namely, colloidal crystals, have emerged as attractive optical materials for controlling and manipulating the transmission of light based on the property of photonic band gaps.If the photonic band gap falls into the visible light range between 380 and 780 nm, visible light of specific wavelengths is not allowed to propagate in the photonic crystal structure, thus being selectively reflected [4][5][6].
Photonic crystals have traditionally been fabricated using either top-down (microlithography) or bottom-up (self-assembly) approaches [7][8][9].One of the simplest ways to make photonic crystals structures is the self-assembly technique, which assemblies monodisperse colloidal spheres [10].Polystyrene (PS) photonic crystals based on selfassembly method was widely used to generate a photonic crystal and due to the low fabrication costs, easily obtained raw materials and mild processing conditions [11][12][13].
Inverse opal, photonic crystals materials have attracted much attention and applied extensively in the field of photonics, tissue engineering, sensing, and catalysis, owing to their distinct structural features with three-dimensionally periodic arrays of air pores within a matrix of some functional materials [14].People are interested in inverse opal because of their ability to combine several chemical and physical properties that are useful in many ways [15][16][17][18].So far, although the well-ordered colloidal crystal templates provide an excellent route for preparation of a wide variety of inverse opal inorganic oxides, such as zirconia, the optimal process of preparation of inverse opal zirconia are less reported.
In this article, PS template was fabricated by self-assembly method and inverse opal zirconia was also prepared by colloidal crystal-templating method.Various effects on phase and morphology of as-prepared inverse opal zirconia and influence rules were investigated.The optimal process of preparation of pure phase inverse opal zirconia will be promising.

Experimental
PS microspheres with the size range of 340 ± 10 nm were synthesized by emulsifierfree emulsion polymerization method with styrene monomer as raw materials.PS colloidal crystals were obtained by evaporation deposition self-assembly method in a certain environment.After self-assembled, PS colloidal crystals were used as template to fabricate inverse opal zirconia according to the following steps.Zirconium acetate was mixed with methanol as sol of zirconia precursor with various ratios (1:0.6,1:0.8, 1:1 and 1:1.2).Then, kept the mass ratio of PS template and zirconia precursor (1:5, 1:10, 1:15 and 1:20), PS templates were dipped into sol of zirconia precursor and infiltrated various times (once, two, three and four times).Finally, the templates with zirconia precursor were sintered at various temperature(350 °C, 450 °C, 600 °C or 700 °C), controlling heating rate for 2 °C/min and holding time for (2, 3 and 4 h) to remove PS templates.
Morphologies of PS templates and inverse opal zirconia samples were characterized by scanning electron microscopy (S4800, Japan).Moreover, reflectance spectra in the visible range of samples were measured by spectrometer (1050, Lambda), and X-ray diffraction (D/max2200PC, Japan) was used to characterize phase and purity of zirconia.With increased the precursor ratio further to 1:1.2, three-dimensional ordered pores were observed with some defects after removal of PS template as shown in Fig. 2 d.Meanwhile, parts of PS templates were damaged and the periodic fashion of the closedpacked structure decreased.When the precursor ratio increased to 1:1, as-prepared zirconia even copied the structure of PS template with much less defects shown in Fig. 2 c, because PS template was well filled with sol of zirconia precursor with suitable permeability and viscosity.Therefore, the suitable precursor ratio of zirconium acetate and methanol for preparation of inverse opal zirconia was 1:1.

Effect of the mass ratio on morphology of inverse opal zirconia
When the precursor ratio was 1:1, morphologies of as-prepared zirconia obtained from various mass ratios of PS templates and precursor were shown in Fig. 3.When the mass ratio was 1:5, incomplete filling of PS template occurred, since the amount of sol of zirconia precursor was too small to enter the void of PS template, leading to incomplete threedimensional pore structure with some defects.
When the mass ratio increased to 1:20, too much amounts of sol of zirconia precursor resulted in many blocked pore channel and agglomerated inorganic material among the inverse opal structure as shown in Fig. 3 d.Inverse opal zirconia with partial deformation and distortion was observed in Fig. 3 b when the mass ratio increased to 1:10.Further increased the mass ratio to 1:15, PS template was completely filled with sol of zirconia precursor and the template was removed entirely.Complete three-dimensional pore structure with uniform wall thickness was obtained as shown in Fig. 3 c.The pores were arranged according with FCC structure of the template and connected with small windows.Therefore, the suitable mass ratio of PS templates and precursor for preparation of inverse opal zirconia was 1:15, which is promising to be optimal further.

Effect of dipping times on morphology of inverse opal zirconia
Fig. 4 shows SEM images of as-prepared inverse opal zirconia with various dipping times.By comparison, it was observed that entire three-dimensional pore structure was obtained after dipped once.
With increased dipping times, the periodic fashion of three-dimensional pore structure decreased continuously and some of the pores were covered by inorganic substance transformed from the excess sol.Hence, PS template was filled sufficiently with sol of zirconia precursor after dipped once.

Effect of holding time on phase of as-prepared zirconia
Fig. 6 showed XRD patterns of zirconia sintered at 600 o C with various holding time.When holding time was 2 h, main diffraction peaks were relatively sharp.Increased holding time to 3 h, except main crystal phase, there was a weak diffraction peak in (012) and (024) crystal faces, which belonged to six square system ZrO 2 .Further increased holding time to 4 h, diffraction intensity of each crystal face was weakened obviously, indicating that longer sintering time could destroy crystal structures of zirconia.It was also found that pure tetragonal phase zirconia was obtained when holding time was 2 h.

Effect of the precursor ratio on phase of as-prepared zirconia
Fig. 7 shows XRD patterns of zirconia obtained at different precursor ratios of zirconium acetate and methanol.When the precursor ratio was 1:0.6, cubic phase zirconia appeared on (200) crystal face and monoclinic phase zirconia was observed on (-112) crystal face, which showed the product was prone to multiphase.When the precursor ratio increased to 1:0.8~1:1, phase of zirconia was quite pure.Increased the precursor ratio further to 1:1.2, diffraction intensity of the main peak improved, while, there was new phases formed.Six square phases Zr 3 O 1-x appeared on (012) crystal face, and monoclinic phases of zirconia were found on (201) crystal face.Therefore, the precursor ratio of 1:1 was beneficial to the formation of pure phase zirconia.

Optical properties of inverse opal zirconia
Fig. 8 shows optical reflectance spectra and photograph of inverse opal zirconia samples produced by the optimal process using colloidal crystal templating.A distinguished single stop band in the visible region of the spectrum and brilliant colors were observed (Fig. 8).This inverse opal zirconia with a lower dielectric contrast formed an incomplete band gap, where propagation was only partially inhibited.The width of the stop band was typical for this inverse opal zirconia obtained by colloidal crystal templating, in which boundaries between ordered domains and surface defects on each crystal particle contributed to broadening of stop bands.It could be concluded long-range structural order, as well as the periodic arrangement of features of pore walls of inverse opal zirconia was important to produce stop bands.

Conclusion
In summary, inverse opal zirconia was successfully prepared by colloidal crystaltemplating method.Precursor ratio of zirconium acetate and methanol, mass ratio of PS templates and precursor and dipping times had remarkable influence on morphology of inverse opal zirconia.When the precursor ratio was 1:1; the mass ratio was 1:15 and dipped once, much better morphology of inverse opal zirconia was obtained.Inappropriate precursor ratio of zirconium acetate and methanol resulted in PS template poor filling because of unsuitable permeability and viscosity of sol of zirconia precursor.Improper mass ratio of PS templates and precursor would lead to excessive filling or deficient filling.Dipping more times led to the periodic fashion of three-dimensional pore structure decreased continuously and destroyed the inverse opal structure.Meanwhile, it was found that the mass ratio, sintering temperature and holding time had significant effect on crystallization of zirconia.Pure phase zirconia could be obtained when sintered at 600 o C, holding time was 2 h and the mass ratio was 1:1.Three-dimensionally ordered inverse opal zirconia materials prepared by colloidal crystal templating formed an incomplete band gap and exhibited unique structural color.The inverse opal zirconia will be promising in many applications, such as structural color, sensor and advertisement etc.

Fig. 1a
Fig. 1a shows PS microspheres were spherical and monodisperse with uniform size of 340 ± 10 nm.The top-view images in Fig. 1b demonstrated clearly ordered-packing and facecentered cubic arrangement of PS template.

Fig. 2
Fig.2depicted SEM images of inverse opal zirconia obtained at various precursor ratios of zirconium acetate and methanol.The low precursor ratio (1:0.6)led to insufficient filling of PS template and absence from large amounts of pore structure because of the destruction of the original face-centered cubic arrangement of PS template as shown in Fig.2a.When precursor ratio of zirconium acetate and methanol increased to 1:0.8 as shown in Fig.2 b, PS template was still incompletely filled with sol of zirconia precursor, which resulted in the ordered-packing structure of PS template disappearing and inorganic pore wall of zirconia becoming much thinner after sintered.With increased the precursor ratio further to 1:1.2, three-dimensional ordered pores were observed with some defects after removal of PS template as shown in Fig.2 d.Meanwhile, parts of PS templates were damaged and the periodic fashion of the closedpacked structure decreased.When the precursor ratio increased to 1:1, as-prepared zirconia even copied the structure of PS template with much less defects shown in Fig.2 c, because PS template was well filled with sol of zirconia precursor with suitable permeability and viscosity.Therefore, the suitable precursor ratio of zirconium acetate and methanol for preparation of inverse opal zirconia was 1:1.

Fig. 5
Fig. 5 illustrated XRD patterns of as-prepared inverse opal zirconia sintered at various temperature.Tetragonal phase zirconia was formed when sintered at 350 o C to 700 o C, and pure tetragonal phase zirconia was obtained when sintered at 350 o C and 450 o C. With increased sintering temperature from 350 o C to 600 o C, diffraction intensity of (011) (110) (600) (020) (013) crystal face increased gradually, and the diffraction peak became sharper, which showed crystalline of zirconia became better.When sintered at 700 o C, the main phase began to obscure, and more monoclinic zirconia appeared.Diffraction intensity of monoclinic zirconia at (011) (-110) (111) crystal face became larger, while, diffraction intensity of main crystal phase became smaller.Thus, pure phase zirconia could be obtained when sintered at 600 o C.

Fig. 6 .
Fig. 6.XRD patterns of inverse opal zirconia sintered at 600 o C for various holding time.