PAM-PAA microgel inverse opal photonic crystal and pH response

The colloidal crystal template or opal with a closed-packed face-centered cubic (fcc) lattice was prepared from monodisperse polystyrene (PS) spheres by vertical sedimentation.The template provided void space for infiltration of monomer precursor composed of acrylate acid,acrylamide and ammonium persulfate,as well as microgel from the subsequent copolymerization.The sample was immersed in dimethylbenzene for completely removing PS spheres to form PAM inverse opal hydrogels (IOH_(PAM)) or PAM/PAA inverse opal hydrogels (IOH_(PAM/PAA)) photonic crystals.The PS spheres were replaced by air spheres,which interconnected each other through the windows.The study of responses to pH show that there are two peaks for both IOH_(PAM) and IOH_(PAM/PAA) films,but the IOH(PAM/PAA) peaks shift to higher pH,and the peaks are independent with the AA content. (?)2007 Xiao Dong Wang.Published by Elsevier B.V.on behalf of Chinese Chemical Society.All rights reserved.     

Directional fluorescence spectra of laser dye in opal and inverse opal photonic crystals

We have investigated the fluorescence from R6G dye molecules embedded in fcc photonic crystals with a large range of lattice parameters. Both polystyrene opals and alumina inverse opals are studied, allowing us to compare direct and inverted structures. We observe clear stop bands in the fluorescence spectra, whose center positions, widths, and depths are analyzed and compared to stop bands from reflectivity measurements. In the frequency range of first-order stop gaps, the measured stop band centers and widths agree well with theoretical predictions. The depths are interpreted in terms of the mean free path (disorder) and the Bragg attenuation length (order). We observe intriguing enhanced emission at the blue side of the stop bands, which is attributed to the escape of diffuse light from the photonic crystal (related to both order and disorder). We perform the first experiments in the range of second-order stop gaps, which is the regime where the photonic band gap is anticipated. We observe complex multiple-Bragg features that correlate favorably with reflectivity peaks.     

Opal and inverse opal photonic crystals: Fabrication and characterization

Three-dimensional photonic crystals made of close-packed polymethylmethacrylate (PMMA) spheres or air spheres in silica, titania and ceria matrices have been fabricated and characterized using SEM, XRD, Raman spectroscopy and UV–Vis transmittance measurements. The PMMA colloidal crystals (opals) were grown by self-assembly from aqueous suspensions of monodisperse PMMA spheres with diameters between 280 and 415 nm. SEM confirmed the PMMA spheres crystallized uniformly in a face-centred cubic (fcc) array, and UV–Vis measurements show that the colloidal crystals possess pseudo photonic band gaps in the visible and near-IR regions. Inverse opals were prepared by depositing silica (SiO₂), titania (TiO₂) or ceria (CeO₂) in the voids of the PMMA colloidal crystals using sol-gel procedures, then calcining the resulting structure at 550 °C to remove the polymer template. The resulting macroporous materials showed fcc ordering of air spheres separated by thin frameworks of amorphous silica, nanocrystalline titania or nanocrystalline ceria particles, respectively. Optical measurements confirmed the photonic nature of the inverse opal arrays. UV–Vis data collected for the opals and inverse opals obeyed a modified Bragg’s law expression that considers both diffraction and refraction of light by the photonic crystal architectures. The versatility of the colloidal crystal template approach for the fabrication of macroporous oxide structures is demonstrated.     

Clickable inverse opal: a useful platform for fabrication of stimuli-responsive photonic materials

Based on azide-containing clickable inverse opal, a strategy for efficiently fabricating functional photonic materials was developed. By using three types of ethynylated compounds as model molecules, it is found that different functional groups can be facilely introduced into the prepared inverse opal via click reaction to access various inverse opaline materials.     

Tunable photonic band gap crystals based on a liquid crystal-infiltrated inverse opal structure

Composite materials comprised of nematic liquid crystals (LCs) and SiO(2) inverse opal films were fabricated. Their optical properties were quite different from those of inverse opal films without the LCs. The optical properties could be controlled by changing the refractive indices of the LCs, which vary with orientation, phase, and temperature. In particular, the optical properties were drastically changed by thermal or photoinduced isothermal phase transitions of the LCs. This means that the photonic band structure could be controlled, and tunable photonic crystals have been achieved, based on the inverse opal structure. The mechanism of this change was investigated by the evaluation of the effective refractive indices. As a result, it was found that the change in optical properties was derived from the orientation of the LC molecules in the voids in the inverse opal film. Furthermore, once the mechanism was understood, it was also possible to control the position of the reflection peak by changing the alignment of the LCs. Such materials have the possibility for practical use in optical devices and fundamental research systems.     

Ni-NiO core-shell inverse opal electrodes for supercapacitors

A general template-assisted electrochemical approach was used to synthesize three-dimensional ordered Ni core-NiO shell inverse opals (IOs) as electrodes for supercapacitors. The Ni-NiO IO electrodes displayed pseudo-capacitor behavior, good rate capability and cycling performance.     

Transformation of hydrogel-based inverse opal photonic sensors from FCC to L1(1) during swelling

The structural evolution of Bragg diffracting inverse opal hydrogel sensors during swelling is directly observed by two-photon laser scanning fluorescence microscopy and compared to predictions from finite element analysis. A fluorescently labeled pH-sensitive hydrogel is UV-polymerized in a dried polystyrene colloidal crystal template, which is etched to yield an inverse opal. Fluorescence imaging of the hydrogel at different pH values reveals an inhomogeneous deformation of the FCC array of aqueous pores. The pores elongate along the sample normal direction and collapse along the sample parallel directions, consistent with the Bragg response, which indicates a 1-D increase in the interlayer distance. Interconnects between the pores serve as anchor points during hydrogel expansion into the pores. Pinning of the hydrogel to the substrate causes a change of the hydrogel lattice symmetry during deformation, from FCC (ABC stacking) to L1(1) (ABCA'B'C' stacking). Reconstructed cross-sections confirm that a 1-D increase in the interlayer distance along the substrate normal direction is responsible for the diffraction response of an inverse opal hydrogel sensor. Comparison with predictions from finite element analysis shows qualitative agreement, although the experimental mesostructure is significantly more deformed than the calculated data, due to buckling in the experimental system that is not captured by the model.     

Carbon and carbon-silicon carbide nanocomposites with inverse opal structure

Synthesis, morphology, and structural characteristics of carbon and SiC/C nanocomposites with a lattice of inverse opal were investigated. Porous structure characteristics were determined by gas adsorption-desorption. Photoluminescence of SiC/C nanocomposites, induced by implantation of helium ions, and their structure revealed by high-resolution transmission electron microscopy were studied.     

Hierarchical twin-scale inverse opal TiO2 electrodes for dye-sensitized solar cells

We describe the preparation of three-dimensional hierarchical twin-scale inverse opal (ts-IO) electrodes for dye-sensitized solar cells (DSSCs). The ts-IO TiO(2) structure was obtained from a template fabricated via the assembly of mesoscale colloidal particles (40-80 nm in diameter) in the confined geometry of a macroporous IO structure. The photovoltaic properties of ts-IO electrodes were optimized by varying the layer thickness or the size of mesopores in the mesoscale colloidal assembly. Electron transport was investigated using impedance spectroscopy. The result showed that due to the competing effects of recombination and dye adsorption, the maximum efficiency was observed at an electrode thickness of 12 μm. The electrodes of smaller mesopores diameters yielded the higher photocurrent density due to the decrease in the electron transport resistance at the TiO(2)/dye interface. A maximum efficiency of 6.90% was obtained using an electrode 12 μm thick and a mesopore diameter of 35 nm.     

Bilayer inverse opal TiO2 electrodes for dye-sensitized solar cells via post-treatment

We investigated the formation of bilayer inverse opal TiO(2) (io-TiO(2)) structures via post-treatment with a TiO(2) precursor solution and characterized the photovoltaic performances of the resulting electrodes for use in dye-sensitized solar cells. The post-treatment of TiO(2) inverse opals in a precursor solution grew rutile TiO(2) nanoparticles on anatase crystalline phase io-TiO(2) surfaces, resulting in anatase/rutile bilayer structures. We achieved a maximum photovoltaic conversion efficiency of 4.6% using a 25 μm thick electrode formed with the post-treated io-TiO(2) under simulated AM 1.5 light. This efficiency represents a 183% improvement over the non-post-treated io-TiO(2) electrodes. The shell thickness was controlled by the post-treatment time. The effects of shell thickness on photovoltaic performance were investigated by measuring the morphologies and electrochemical impedance of the post-treated io-TiO(2). We found that post-treatment up to a certain period of time increased the surface area and electron lifetime, but further treatment resulted in decreased area and saturated lifetimes. The optimal post-treatment time was identified, and the optimal io-TiO(2) electrodes were characterized.     

Facile synthesis of TiO2 inverse opal electrodes for dye-sensitized solar cells

Engineering of TiO(2) electrode layers is critical to guaranteeing the photoconversion efficiency of dye-sensitized solar cells (DSSCs). Recently, a novel approach has been introduced for producing TiO(2) electrodes using the inverted structures of colloidal crystals. This paper describes a facile route to producing ordered macroporous electrodes from colloidal crystal templates for DSSCs. Using concentrated colloids dispersed in a volatile medium, the colloidal crystal templates were obtained within a few minutes, and the thickness of the template was easily controlled by changing the quantity of colloidal solution deposited. Here, the effects of the structural properties of the inverse opal TiO(2) electrodes on the photovoltaic parameters of DSSCs were investigated. The photovoltaic parameters were measured as a function of pore ordering and electrode film thickness. Moreover, DSSC applications that used either liquid or viscous polymer electrolyte solutions were investigated to reveal the effects of pore size on performance of an inverse opal TiO(2) electrode.     

Opal and inverse opal fabricated with a flow-controlled vertical deposition method

In this work, an improved vertical deposition method, namely, a flow-controlled vertical deposition (FCVD) method, was used to grow colloidal crystals with large spherical colloids in water solvent and to infiltrate the colloidal crystals. Using the FCVD method, latex spheres as large as 2 microm can be fabricated into colloidal crystals in water. In addition, the method works very well for controlling surface morphologies of silica-infiltrated opals. Furthermore, fabrication of colloidal crystal heterostructures was demonstrated.     

Fabrication and characterization of cerium-doped barium titanate inverse opal by sol-gel method

Cerium-doped barium titanate inverted opal was synthesized from barium acetate contained cerous acetate and tetrabutyl titanate in the interstitial spaces of a polystyrene (PS) opal. This procedure involves infiltration of precursors into the interstices of the PS opal template followed by hydrolytic polycondensation of the precursors to amorphous barium titanate and removal of the PS opal by calcination. The morphologies of opal and inverse opal were characterized by scanning electron microscope (SEM). The pores were characterized by mercury intrusion porosimetry (MIP). X-ray photoelectron spectroscopy (XPS) investigation showed the doping structure of cerium, barium and titanium. And powder X-ray diffraction allows one to observe the influence of doping degree on the grain size. The lattice parameters, crystal size and lattice strain were calculated by the Rietveld refinement method. The synthesis of cerium-doped barium titanate inverted opals provides an opportunity to electrically and optically engineer the photonic band structure and the possibility of developing tunable three-dimensional photonic crystal devices.     

Thermoresponsive inverse opal films fabricated with liquid-crystal elastomers and nematic liquid crystals

Liquid-crystal elastomers together with nematic liquid crystals have been used as inverse opal materials to fabricate thermoresponsive photonic crystal directly. In the vicinity of the phase-transition point of the mixture, the photonic band gaps of such inverse opal films exhibited a strong temperature dependence. As the molar ratio of liquid-crystal elastomers and nematic liquid crystals changed, the character of their PBGs also changed with increasing temperature. Such a temperature-tuning effect in the photonic band gap should be of great interest in thermal switches and thermal sensors.     

Tunable multicolor pattern and stop-band shift based on inverse opal hydrogel heterostructure

The inverse opal hydrogel heterostructure (polyacrylamide (PAAm) (left side)-polyacrylic acid (PAA)/PAAm interpenetrating polymer network (IPN) (right side) is created by colloidal crystal templating. The two parts, PAAm and IPN, appear different structural colors due to the varied lattice constants and solvent response behaviors. The IPN part keeps red color and PAAm part shows different colors when the composition of the mixed solvent (ethanol and water) and crosslinking degree are changed. For example, as the ethanol content in the mixed solvents increases from 0% to 70%, the PAAm part color changes from red, yellow, green to blue when the PAAm crosslinking degree is 5 mol% or 1 mol%. Meanwhile, a large blue shift of about 200 nm can be realized covering almost the entire wavelength of visible light due to the decreased lattice spacing induced by the PAAm shrinkage. Thus, multi two-color patterns can be realized by changing the color of PAAm and the color of IPN remain red as background for contrast. Moreover, the IPN part can change from red to green by reducing the PAAm infiltration time in IPN part, which can realize the change from two-color pattern to one-color pattern at green region.     

丙烯酰胺反蛋白石凝胶膜的制备及pH响应性能研究

以聚苯乙烯(PS)胶体晶体为模板,采用"三明治"法得到收缩率较低、表面无覆盖层、三维长程有序的聚丙稀铣胺(PAM)反蛋白石凝胶膜。实验发现,粒径较小的模板有利于得到结构规整的凝胶膜。反蛋白石凝胶膜有明显的pH响应性,随着pH增大,PAM反蛋白石凝胶膜的最大衍射波长红移,且在pH=8.49时,达到最大;pH继续增大时,最大衍射波长表现为蓝移趋势。并且可重复响应而不破坏其结构,表明该凝胶膜具有较大的实用价值。     

A novel tailored bimodal porous silica with well-defined inverse opal microstructure and super-microporous lamellar nanostructure

A novel tailored bimodal porous silica with well-defined inverse opal microstructure and super-microporous lamellar nanostructure has been successfully synthesized by simultaneous application of three-dimensional order polystyrene (PS) beads and an amphiphilic ionic liquid (AIL) as templates.     

Highly crystalline inverse opal transition metal oxides via a combined assembly of soft and hard chemistries

A combined assembly of soft and hard chemistries is employed to generate highly crystalline three-dimensionally ordered macroporous (3DOM) niobia (Nb2O5) and titania (TiO2) structures by colloidal crystal templating. Polystyrene spheres with sp2 hybridized carbon are used in a reverse-template infiltration technique based on the aqueous liquid phase deposition of the metal oxide in the interstitial spaces of a colloidal assembly. Heating under inert atmosphere as high as 900 degrees C converts the polymer into sturdy carbon that acts as a scaffold and keeps the macropores open while the oxides crystallize. Using X-ray diffraction it is demonstrated that for both oxides this approach leads to highly crystalline materials while heat treatments to lower temperatures commonly used for polymer colloidal templating, in particular for niobia, results in only weakly crystallized materials. Furthermore it is demonstrated that heat treatment directly to higher temperatures without generating the carbon scaffold leads to a collapse of the macrostructure. The approach should in principle be applicable to other 3DOM materials that require heat treatments to higher temperatures.     

Tantalum(V) nitride inverse opals as photonic structures for visible wavelengths

The development of materials with a complete photonic band gap at visible wavelengths is believed to have the potential to lead to new control over long-lived emissive excited states, single molecule lasers, and nearly lossless nanoscale waveguides. In this work we move toward that goal with the synthesis of an inverse opal thin film of Ta3N5 produced through atomic layer deposition. The highly regular architecture achievable by atomic layer deposition is combined with an unusually high refractive index and transparency in at least part of the visible spectrum. The result is a material that represents the closest example to date of a photonic crystal with a band gap at optical wavelengths.     

Thermoresponsive assembly of charged gold nanoparticles and their reversible tuning of plasmon coupling

Charged colloidal gold nanoparticles (AuNPs) can be assembled and disassembled in an aqueous solution in response to temperature change and display reversible thermoresponsive tuning of plasmon coupling. The reversible tuning was made possible by manipulating the electrostatic interaction through the temperature-dependent zeta potential of the charged AuNPs (see the extinction spectra of a typical AuNP dispersion).