We demonstrate a water-based etching strategy for converting solid silica shells into porous ones with controllable permeability. It overcomes the challenges of the alkaline-based surface-protected etching process that we previously developed for the production of porous and hollow silica nanostructures. Mild etching around the boiling point of water partially breaks the imperfectly condensed silica network and forms soluble monosilicic acid, eventually producing mesoscale pores in the silica structures. With the surface protection from poly(vinyl pyrrolidone) (PVP), it is possible to maintain the overall shape of the silica structures while at the same time to create porosity inside. By using bulky PVP molecules which only protect the near-surface region, we are able to completely remove the interior silica and produce hollow particles. Because the etching is mild and controllable, this process is particularly useful for treating small silica particles or core-shell particles with very thin silica shells for which the alkaline-based etching method has been difficult to control. We demonstrated the precise control of the permeation of the chemical species through the porous silica shells by using a model reaction which involves the etching of Ag encapsulated inside Ag@SiO(2) by a halocarbon. It is expected that the water-based surface-protected etching method can be conveniently extended to the production of various porous silica shells containing functional materials whose diffusion to outside and/or reaction with outside species can be easily controlled.
Mesoporous nanocrystal clusters of anatase TiO2 with large surface area and enhanced photocatalytic activity have been successfully synthesized. The synthesis involves the self-assembly of hydrophobic TiO2 nanocrystals into submicron clusters, coating of these clusters with a silica layer, thermal treatment to remove organic ligands and improve the crystallinity of the clusters, and finally removing silica to expose the mesoporous catalysts. With the help of the silica coating, the clusters not only maintain their small grain size but also keep their mesoporous structure after calcination at high temperatures (with BET surface area as high as 277 m2/g). The etching of SiO2 also results in the clusters having high dispersity in water. We have been able to identify the optimal calcination temperature to produce TiO2 nanocrystal clusters that possess both high crystallinity and large surface area, and therefore show excellent catalytic efficiency in the decomposition of organic molecules under illumination by UV light. Convenient doping with nitrogen converts these nanocrystal clusters into active photocatalysts in both visible light and natural sunlight. The strategy of forming well-defined mesoporous clusters using nanocrystals promises a versatile and useful method for designing photocatalysts with enhanced activity and stability.
Magnetically responsive photonic crystal structures have been produced by self-assembling superparamagnetic colloidal particles under the balance of repulsive and attractive interactions. The optical properties can be tuned rapidly, widely, and reversibly in the entire visible spectrum. By taking advantage of many unique features of the field responsive systems, we demonstrate in this article some of their niche applications for various display purposes, including antifraud devices, rewritable photonic papers, and full colour high resolution printing systems.
Abstract Nanoparticles of transition metals, particularly noble metals, are widely used in catalysis. However, enhancing their stability during catalytic reactions has been a challenge that has limited the full use of the benefits associated with their small size. In this Feature Article, a general “encapsulation and etching” strategy for the fabrication of nanocatalyst systems is introduced in which catalyst nanoparticles are protected within porous shells. The novelty of this approach lies in the use of chemical etching to assist the creation of mesopores in a protective oxide shell to promote efficient mass transfer to encapsulated metal nanoparticles. The etching process allows for the direct transformation of dense silica coatings into porous shells so that chemical species can reach the catalyst surface to participate in reactions while the shells act as physical barriers against aggregation of the catalyst particles. By using the surface‐protected etching process, both yolk–shell and core–satellite type nanoreactors are synthesized and their utilization in liquid‐ and gas‐phase catalysis is demonstrated. The thermal and chemical stability of the metallic cores during catalytic reactions is also investigated, and further work is carried out to enhance recyclability via the introduction of superparamagnetic components into the nanoreactor framework.
Mesoporous hollow TiO2 shells with controllable crystallinity have been successfully synthesized by using a novel partial etching and re-calcination process. This method involves several sequential preparation steps as follows: 1) Synthesis of SiO2@TiO2@SiO2 colloidal composites through sol–gel processes and crystallization by calcination, 2) partial etching to preferentially remove portions of the SiO2 layers contacting the TiO2 surface, and 3) re-calcination to crystallize the TiO2 and finally etching of the inner and outer SiO2 to produce mesoporous anatase TiO2 shells. The partial etching step produces a small gap between SiO2 and TiO2 layers which allows space for the TiO2 to further grow into large crystal grains. The re-calcination process leads to well developed crystalline TiO2 which maintains the mesoporous shell structure due to the protection of the partially etched outer silica layer. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, the as-prepared mesoporous TiO2 shells show significantly enhanced catalytic activity. In particular, TiO2 shells synthesized with optimal crystallinity by using this approach show higher performance than commercial P25 TiO2.
Advanced oxidation processes (AOPs) based on sulfate radicals (SO4•-) are superior route for water treatment comparing to traditional AOPs, owing to their higher selectivity, longer half-life, and tolerance to wider pH range. To find an efficient source of SO4•-, peroxymonosulfate (PMS) molecules are widely used, which could be activated by the catalytic Fe(II) ions in traditional AOPs. However, this SO4•- -based catalytic activation method suffers from low conversion rate of Fe(III) to Fe(II), requires a large amount of catalytic Fe(II) ions, and produces a large amount of iron sludge as waste, which significantly limit its practical application for pollutants treatment. Herein, we show that by using molybdenum dioxide (MoO2) as a co-catalyst, the rate of Fe(III)/Fe(II) cycling reactions in the PMS system accelerated significantly, with a reaction rate constant 48 times that of conventional PMS/Fe(II) system. Our results showed outstanding removal efficiency (98%) of organic pollutant in 10 min with extremely low concentration of Fe(II) (0.036 mM), outperforming most reported SO4•--based AOPs systems. Additionally, MoO2 showed excellent stability and efficiency for wide range of pH values, recyclability for multiple activation cycles, practicality for removal of other organic compounds such as phenol and methylene blue. Surface chemical analysis combined with density functional theory (DFT) calculation demonstrated that both Fe(III)/Fe(II) cycling and PMS activation occurred on the (110) crystal plane of MoO2, while the exposed Mo4+ on the MoO2 surface are responsible for the co-catalyzing of iron ions to activate the PMS radicals. Considering its performance, low cost, and non-toxicity, using MoO2 as a co-catalyst in SO4•--based AOPs is a promising technique for large-scale practical environmental remediation.
Tuning crystal: Superparamagnetic nanocrystal clusters can self-assemble into colloidal photonic crystals in solution, whose stop bands can be magnetically tuned across the entire visible spectrum. Owing to the high magnetization and the highly charged polyacrylate-capped surface of each cluster, the colloidal photonic crystals show a rapid, reversible, and widely tunable optical response to external magnetic fields.
We have successfully assembled superparamagnetic colloids into ordered structures with magnetically tunable photonic properties in nonpolar solvents by establishing long-range electrostatic repulsive forces using charge control agents. Reverse micelles resulted from the introduction of charge control agents such as AOT molecules can enhance the charge separation on the surfaces of n-octadecyltrimethoxysilane modified Fe3O4@SiO2 particles. The significantly improved long-range electrostatic repulsion can counterbalance the magnetically induced attraction and therefore allow ordering of superparamagnetic colloids in nonpolar solvents. This system possesses a fast and fully reversible optical response to the external magnetic fields, long-term stability in performance, and good diffraction intensity.
In this perspective, several examples of work from our laboratory are reported where colloidal or self-assembly chemistry has been used to design new catalysts with specific properties. In the first, platinum nanoparticles with well-defined shapes have been dispersed on a high-surface-area silica support in order to take advantage of the structure sensitivity exhibited by the interconversion between the cis and trans isomers of olefins. The second case involves the use of dendrimers as scaffolding structures to prepare catalysts with small platinum nanoparticles of well-defined size. Reduced sintering of metal nanoparticles on supported catalysts is accomplished in our third example via their encapsulation inside a layer of mesoporous silica deposited on top, after metal dispersion, and etched using a newly developed surface-protection process. The final project refers to the use of yolk@shell metal-oxide systems as nanoreactors for photocatalysis. In all those examples, new synthetic nanotechnology has been directed to address a specific issue in catalysis previously identified by surface-science studies.
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