Zhongyu Cai

Fabrication of TiO2 Binary Inverse Opals without Overlayers via the Sandwich-Vacuum Infiltration of Precursor

A sandwich-vacuum method was demonstrated for the fabrication of titania (TiO(2)) binary inverse opals with an open surface. In this method, a moisture-stable TiO(2) precursor was backfilled into the interstitial spaces of polystyrene binary colloidal crystals (PS bCCs), which served as a template. Removal of the template by calcination yielded TiO(2) binary inverse opals with a 3D-ordered macroporous (3DOM) structure. Optical reflectance spectra revealed the existence of a pseudostop band gap in the 3DOM TiO(2) samples. The position of the pseudostop band gap shifted to the low-wavelength region as the number ratio of small over large PS spheres was increased in the template. The sandwich-vacuum method proved to be simple and rapid for the fabrication of TiO(2) binary inverse opals without overlayers in large domains. The 3DOM TiO(2) materials were used as a photocatalyst for the degradation of benzoic acid. Results showed that in comparison to TiO(2) nanoparticles prepared under the same sintering conditions, the 3DOM TiO(2) materials displayed enhanced photocatalytic activity.

Binary colloidal crystals fabricated with a horizontal deposition method

We describe the use of a horizontal deposition method to prepare large-area binary colloidal crystals (bCCs). Two different sets of binary polystyrene spheres were employed to demonstrate the validity of this method. By varying the number ratios of small spheres with respect to large spheres, the stoichiometric configuration of the bCCs can be altered. Stable corresponding replica structures of the bCCs were also prepared, and the cross-sectional images of the binary inverse opals were obtained. Optical characterization demonstrated the presence of pseudostop bands, which were in agreement with the compositions of the material. The formation of the bCC by such a simple self-assembly method was attributed to the cooperative effect of interparticle electrostatic interactions and geometrical constrictions. This facile fabrication method further enhances the application potential of the bCCs and their inverse porous replicas with a binary pore system in the fields of photonics, solar cells, separations, catalysis, and biosensing.

In situ gold-loaded titania photonic crystals with enhanced photocatalytic activity

A facile two-step method is developed to fabricate three-dimensional ordered macroporous (3DOM) gold-loaded titania (TiO2) photonic crystals with enhanced photocatalytic activities. Firstly, a mixed solution of polystyrene (PS) colloids, chloroauric acid and titanium(IV)-bis-lactato-bisammonium dihydroxide (TiBALDH) sol was co-assembled into PS/TiO2 colloidal crystal films. Subsequent calcination of the samples led to the removal of PS and transformed amorphous TiO2 into the anatase phase. The resultant 3DOM inverse TiO2 opals (i-TiO2-o) and gold-loaded i-TiO2-o (i-Au-TiO2-o) show centimeter-scale long-range ordering. Photocatalytic characterization of the i-TiO2-o and i-Au-TiO2-o showed activities two- and five-fold higher than nanocrystalline TiO2, respectively. This enhanced photocatalytic performance can be attributed to the synergetic effect of slow photons near the absorption edge of anatase TiO2 nanocrystals and chemically amplified photochemistry. The present method is much more simple and straightforward than conventional colloidal crystal templating methods. In particular, the gold nanoparticles can be in situ loaded into the inverse opal structure with controllable size and content. Furthermore, both i-TiO2-o and i-Au-TiO2-o films fabricated by this method show a highly ordered structure without overlayers in a large area, which facilitates the adsorption of target pollutants and ultimate utilization of solar energy.

Fabrication of Large Domain Crack-Free Colloidal Crystal Heterostructures with Superposition Bandgaps Using Hydrophobic Polystyrene Spheres

An improved convective self-assembly method was developed to fabricate crack-free colloidal crystal heterostructure over a relatively large area. A composite opaline heterostructure composed of polystyrene (PS) colloids was first fabricated. Subsequent calcination of the opaline heterostructure led to the formation of inverse opaline heterostructure composed of SiO(2) or TiO(2). Both opaline and inverse opaline heterostructures demonstrated long-range ordering in a relatively large domain (>100 × 100 μm(2)). Optical reflection measurements of the inverse opaline heterostructures showed dual stop bands as a consequence of the superposition of the stop bands from the individual compositional colloidal crystals (CCs). In addition, the relative position of the two stop bands can be adjusted by varying the size of the colloidal spheres in the original CCs template. Both types of colloidal crystal heterostructures can be used for optical filters, high-efficiency back-reflectors or electrodes in solar cells, differential drug release, and protein patterning.

An improved convective self-assembly method for the fabrication of binary colloidal crystals and inverse structures

We report an improved convective self-assembly method for the fabrication of highly ordered, crack-free binary colloidal crystals (BCCs) and the associated inverse structures in large domains at length scales of several centimeters. With this method, BCCs can be fabricated in a non-close packed pattern and binary inverse opal films can be obtained over a centimeter scale. The presence of tetraethyl orthosilicate (TEOS) sol in the self-assembly system was found to play a significant role in the resultant structures. The pseudostop band positions are adjustable via varying the number ratio of small to large polystyrene (PS) spheres. At a given TEOS-to-PS ratio, the binary inverse opal film thickness was controllable by varying the colloidal volume fraction with an upper thickness threshold (>16 layers).

Highly ordered and gap controllable two-dimensional non-close-packed colloidal crystals and plasmonic–photonic crystals with enhanced optical transmission

We present a facile one-step co-self-assembly method for the fabrication of two-dimensional (2D) non-close-packed (ncp) colloidal crystals (CCs) using polystyrene (PS) colloidal spheres and tetraethylorthosilicate (TEOS) sol. The resultant 2D ncp CCs demonstrated long-range ordering at the centimeter-scale without cracks due to the addition of TEOS sol. The inter-particle gap of the ncp CCs can be easily controlled by varying the amount of TEOS sol added during the assembly process. The crack-free 2D CCs were employed to form hybrid plasmonic–photonic crystals by depositing a thin film of gold. Experimental and simulated transmission spectra of the hybrid plasmonic–photonic crystals were found to be in good agreement. This simple and low-cost method provides a platform for the fabrication of high-quality 2D ncp CCs and may facilitate the development of various applications of ncp CCs.

Fabrication of well-ordered binary colloidal crystals with extended size ratios for broadband reflectance.

Binary colloidal crystals (BCCs) possess great potentials in tuning material properties by controlling the size ratio of small to large colloidal spheres (γS/L). In this paper, we present a method for the fabrication of BCCs with much more extended size ratios than those obtained in conventional convective self-assembly method. It is found that γS/L can be extended to 0.376 by adding TEOS sol into the colloidal suspension. The resulting polystyrene/silica (PS/SiO2) BCCs show distinctive reflections, indicating their well-ordered structure. The extended size ratios render more flexibility in engineering the photonic bandgap structures of BCCs and hence provide a better platform for developing a range of applications such as photonics, spintronics, sensing and bioseparation.

Sandwich-structured Fe2O3@SiO2@Au nanoparticles with magnetoplasmonic responses

We report a method for the fabrication of relatively uniform sandwich-like core-interlayer-shell nanostructures by using γ-Fe2O3 as the inner core, SiO2 as the interlayer, and relatively uniform gold (Au) as the outer shell. The resulting novel hybrid nanoparticle combines the intense local fields of nanorods with the highly tunable plasmon resonances of nanoshells. The length and diameter of the resulting nanoparticles can be tuned by the aspect ratio of α-Fe2O3, the interlayer of SiO2 and the outer layer of Au. After calcination under H2 and then exposure to air, α-Fe2O3 was transformed into γ-Fe2O3, which provides the hybrid particle magnetic tunability. This metal oxide (γ-Fe2O3) dielectric core, the SiO2 interlayer and the Au shell spindle nanoparticle resemble a grain of Au nanorice (γ-Fe2O3@SiO2@Au ellipsoids). The core-interlayer-shell geometry possesses greater structural and magnetic tunability than a nanorod or a nanoshell. The plasmon resonance of this novel γ-Fe2O3@SiO2@Au geometry is believed to arise from a hybridization of the primitive plasmons of an ellipsoidal cavity inside a continuous Au shell. The unique magnetoplasmonic properties of this γ-Fe2O3@SiO2@Au nanostructure are highly attractive for applications such as surface plasmon resonance sensing because of the dipole resonance of the resultant nanostructure and recyclable catalysts arising from the outer Au layer and the inner magnetic γ-Fe2O3 core.