Abstract
We demonstrate the fabrication of free-standing inverse opals with gradient pores via a combination of electrophoresis and electroplating techniques. Our processing scheme starts with the preparation of multilayer colloidal crystals by conducting sequential electrophoresis with polystyrene (PS) microspheres in different sizes (300, 600, and 1000 nm). The critical factors affecting the stacking of individual colloidal crystals are discussed and relevant electrophoresis parameters are identified so the larger PS microspheres are assembled successively atop of smaller ones in an orderly manner. In total, we construct multilayer colloidal crystals with vertical stacking of microspheres in 300/600, 300/1000, and 300/600/1000 nm sequences. The inverse opals with gradient pores are produced by galvanostatic plating of Ni, followed by the selective removal of colloidal template. Images from scanning electron microscopy exhibit ideal multilayer close-packed structures with well-defined boundaries among different layers. Results from porometer analysis reveal the size of bottlenecks consistent with those of interconnected pore channels from inverse opals of smallest PS microspheres. Mechanical properties determined by nanoindentation tests indicate significant improvements for multilayer inverse opals as compared to those of conventional single-layer inverse opals.
Highlights
The synthesis of materials with tailored porosity and pore size has attracted considerable attention for their promising potentials in electrocatalysis, sensing, scaffolds, and capacitors [1,2,3,4,5,6]
Inverse opals consisting of metals, oxides, and polymers in a wide range of pore sizes have been demonstrated in studies [8,9,10,11]
The fabrication of inverse opals starts with the construction of colloidal crystals serving as the template to allow the filling of selective materials into the interstitial voids among the close-packed microspheres, followed by the removal of the colloidal template
Summary
The synthesis of materials with tailored porosity and pore size has attracted considerable attention for their promising potentials in electrocatalysis, sensing, scaffolds, and capacitors [1,2,3,4,5,6]. The resulting single-layer colloidal crystals adopt a polycrystalline structure whose grain sizes are relatively limited and for which crystallographic defects—such as vacancies, grain boundaries, and voids—are inevitable These physical constraints are detrimental in fabricating inverse opals with desirable microstructures. After identifying relevant processing parameters, we were able to demonstrate colloidal crystals in either planar and cylindrical form with precise colloidal layers, and their surfaces were rather uniform [29,30] This allowed us to fabricate single-layer inverse opals with ideal hexagonal honeycombs and controlled thickness [31,32]. Because of multilayer colloidal templates without defects, the resulting multilayer inverse opals were removed from the substrates, forming free-standing inverse opaline membranes In nanoindentation tests, these multilayer inverse opals revealed improved mechanical properties compared with their single-layer counterparts
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