Ji Bong Joo

Core-shell nanostructured catalysts.

Novel nanotechnologies have allowed great improvements in the syn-thesis of catalysts with well-controlled size, shape, and surface properties. Transition metal nanostructures with specific sizes and shapes, for instance, have shown great promise as catalysts with high selectivities and relative ease of recycling. Researchers have already demonstrated new selective catalysis with solution-dispersed or supported-metal nanocatalysts, in some cases applied to new types of reactions. Several challenges remain, however, particularly in improving the structural stability of the catalytic active phase. Core-shell nanostructures are nanoparticles encapsulated and protected by an outer shell that isolates the nanoparticles and prevents their migration and coalescence during the catalytic reactions. The synthesis and characterization of effective core-shell catalysts has been at the center of our research efforts and is the focus of this Account. Efficient core-shell catalysts require porous shells that allow free access of chemical species from the outside to the surface of nanocatalysts. For this purpose, we have developed a surface-protected etching process to prepare mesoporous silica and titania shells with controllable porosity. In certain cases, we can tune catalytic reaction rates by adjusting the porosity of the outer shell. We also designed and successfully applied a silica-protected calcination method to prepare crystalline shells with high surface area, using anatase titania as a model system. We achieved a high degree of control over the crystallinity and porosity of the anatase shells, allowing for the systematic optimization of their photocatalytic activity. Core-shell nanostructures also provide a great opportunity for controlling the interaction among the different components in ways that might boost structural stability or catalytic activity. For example, we fabricated a SiO₂/Au/N-doped TiO₂ core-shell photocatalyst with a sandwich structure that showed excellent catalytic activity for the oxidation of organic compounds under UV, visible, and direct sunlight. The enhanced photocatalytic efficiency of this nanostructure resulted from an added interfacial nonmetal doping, which improved visible light absorption, and from plasmonic metal decoration that enhanced light harvesting and charge separation. In addition to our synthetic efforts, we have developed ways to evaluate the accessibility of reactants to the metal cores and to characterize the catalytic properties of the core-shell samples we have synthesized. We have adapted infrared absorption spectroscopy and titration experiments using carbon monoxide and other molecules as probes to study adsorption on the surface of metal cores in metal oxide-shell structures in situ in both gas and liquid phases. In particular, the experiments in solution have provided insights into the ease of diffusion of molecules of different sizes in and out of the shells in these catalysts.

A Yolk@Shell Nanoarchitecture for Au/TiO2 Catalysts†

Traditionally, gold has been considered a fairly inert metal. However, Bond and Haruta challenged that view a few decades ago by showing that, when dispersed as small nanoparticles, gold can catalyze several hydrogenation and oxidation reactions. Many reports have been published since on the usefulness of gold catalysis for the promotion of a wide variety of reactions. 5] Unfortunately, this has not translated into many practical applications. One of the main limitations has been that supported gold catalysts are often unstable under reaction conditions: they tend to sinter and grow into larger particles, and that leads to the loss of the unique properties seen in the original nanoparticles. Although several schemes have been advanced to address this shortcoming, no satisfactory solution has been found yet. This is particularly true for the case of gold catalysts dispersed on titania, a support of interest because of its synergistic effect in further facilitating the promotion of oxidation 8] and photocatalytic reactions. Here we report on the preparation and characterization of a Au@TiO2 catalyst with a new yolk@shell nanostructure. The new catalyst shows an activity comparable to that of other more conventional Au/TiO2 catalysts but an increased stability against sintering. The growth of titania hollow shells encapsulating metal nanoparticles is more difficult than with silica, the material with which most reported yolk@shell nanostructures have been made to date. However, we have now accomplished this by following a sol–gel-based templating protocol where citrate-stabilized gold nanoparticles are first coated with a sacrificial layer of silica using tetraethyl orthosilicate (TEOS). An outer shell of titania is then grown on top using tetrabutyl titanate (TBOT), and the silica etched away using an aqueous solution of NaOH. 13] The size of the void space inside the titania shells can be controlled by the amount of TEOS used during the growth of the silica layer, and the thickness of the shell by repeating the TBOT sol–gel step multiple times. Figure 1a,c displays typical transmission electron microscopy (TEM) images of the Au@TiO2 yolk@ shell nanostructure used herein, which consist of gold nano-

Control of the nanoscale crystallinity in mesoporous TiO2 shells for enhanced photocatalytic activity

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.

Tailored synthesis of mesoporous TiO2 hollow nanostructures for catalytic applications

Nanostructured TiO2 has attracted significant attention due to its advantageous properties for practical catalytic applications. TiO2 hollow nanostructures consisting of nanoscale porous shells are highly desirable because they possess high active surface area, reduced diffusion resistance, and improved accessibility, which provide many new opportunities to the design of highly active nanostructured catalysts. Although much has been explored, tailored synthesis of TiO2-based hollow nanostructures towards practical catalytic applications has been very limited. In this article, we first introduce the general synthetic strategies for preparing TiO2 hollow nanostructures, and then focus our discussion on the novel synthetic strategies developed in our group with emphasis on controlling the crystallinity as well as physical characteristics of TiO2 hollow nanostructures. We further discuss several catalytic applications of TiO2-based hollow shells and metal@TiO2 yolk–shell nanostructures for photocatalytic dye degradation, H2 production and gas-phase CO oxidation. Finally, we conclude with our personal perspective on the future research efforts for addressing several remaining challenges in the design of TiO2-based catalysts.

Crystallinity control of TiO2 hollow shells through resin-protected calcination for enhanced photocatalytic activity

We report a novel resin-protected calcination process for preparing hollow TiO2 nanoshells with controllable crystallinity and phase. This method involves coating a template core with TiO2 and then a protection layer through sol–gel processes and then crystallization of the TiO2 shell by calcination. Through a systematic study on the crystallization behaviour of the TiO2 hollow shells with variation in core template and outer protection layer, we find that the grain growth and phase transformation of TiO2 is determined by several parameters such as the protection material, core composition, and calcination conditions. In particular, the use of a crosslinked polymer as the protection layer for calcination, enables the production of TiO2 shells with high crystallinity and controllable anatase–rutile mixed phases, which show significantly enhanced photocatalytic activity compared to those produced by SiO2-protected calcination. The photocatalytic activity could be further improved by improving the water dispersity of TiO2 shells through base treatment. With the ease of achieving photocatalytic activity comparable to commercial Degussa-P25 TiO2, it is expected that the TiO2 shells with controllable crystallinity and phase could be further engineered by incorporating more active components for producing highly active composite photocatalysts.

Diffusion through the Shells of Yolk–Shell and Core–Shell Nanostructures in the Liquid Phase

Recent advances in synthetic materials sciences such as the incorporation of self-assembly, sol‐gel, and layer-by-layer deposition chemistry have afforded the development of many interesting and potentially useful nanostructures. Among those, nanoarchitectures in which a core nanoparticle, often a transition metal, is encapsulated by an outer layer of a second material, typically a porous oxide, have gained much popularity recently. The shells in these so-called core‐shell nanostructures can offer a number of functionalities, including protection of the core from the outside environment, help in maintaining its compositional and structural integrity, prevention of the core from aggregating or sintering into larger particles, selective percolation of molecules in and out of the interior of the shell, increases in solubility and/or biocompatibility, and addition of new physical or chemical properties. [1‐3] More sophisticated nanostructures can also be synthesized with a void space, in a yolk‐shell or rattle-type nanoarchitecture, to create individualized nanoreactors around the core nanoparticles. [4‐6] The new properties of these nanostructures have already been exploited in sensing, [7,8] drug delivery and biomedical imaging, [9‐11] catalysis and electrocatalysis (including fuel cells), [12‐14] and the development of batteries [15] and energy storage devices. [16]

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H2

Significance The use of catalysis assisted by light is a promising route to convert solar energy into chemical fuels such as H2. Photon absorption promotes electrons in semiconductor catalysts from the valence to the conduction band of those solids, and the excited electrons may be used to reduce protons in water to produce hydrogen gas. Unfortunately, those electrons tend to rapidly return to their ground state instead. The addition of metals has been shown to enhance photocatalytic activity, and that has been explained by their ability to trap the excited electrons and quench the electron–hole recombination process that neutralizes the photoexcitation. Here we challenge the validity of this model and provide an alternative explanation for the enhancement. A new thinking about this mechanism may result in new searches for appropriate cocatalysts. The production of hydrogen from water with semiconductor photocatalysts can be promoted by adding small amounts of metals to their surfaces. The resulting enhancement in photocatalytic activity is commonly attributed to a fast transfer of the excited electrons generated by photon absorption from the semiconductor to the metal, a step that prevents deexcitation back to the ground electronic state. Here we provide experimental evidence that suggests an alternative pathway that does not involve electron transfer to the metal but requires it to act as a catalyst for the recombination of the hydrogen atoms made via the reduction of protons on the surface of the semiconductor instead.

Porous tubular carbon nanorods with excellent electrochemical properties

We report on the preparation of porous tubular carbon nanorods with a high surface area and excellent electrochemical properties through a template-mediated process. The synthesis involves the preparation of rod-like nickel–hydrazine complexes in a reverse micelle, sequential coating of the nanorods with a layer of phenolic resin and then silica, carbonization at a high temperature in an inert atmosphere, and finally release of tubular carbon nanorods through sequential etching in NaOH and HCl solutions. The silica layer protects the nanorods from aggregation during carbonization and its subsequent removal renders the tubular carbon nanorods a hydrophilic surface, which significantly improves its dispersity in aqueous media. When used as active electrode materials for supercapacitors, the as-prepared tubular carbon nanorods show superior electrochemical performance to commercial graphite and carbon nanotubes.

A Sulfated ZrO2 Hollow Nanostructure as an Acid Catalyst in the Dehydration of Fructose to 5‐Hydroxymethylfurfural

Mesoporous hollow colloidal particles with well-defined characteristics have potential use in many applications. In liquid-phase catalysis, in particular, they can provide a large active surface area, reduced diffusion resistance, improved accessibility to reactants, and excellent dispersity in reaction media. Herein, we report the tailored synthesis of sulfated ZrO2 hollow nanostructures and their catalytic applications in the dehydration of fructose. ZrO2 hollow nanoshells with controllable thickness were first synthesized through a robust sol-gel process. Acidic functional groups were further introduced to the surface of hollow ZrO2 shells by sulfuric acid treatment followed by calcination. The resulting sulfated ZrO2 hollow particles showed advantageous properties for liquid-phase catalysis, such as well-maintained structural integrity, good dispersity, favorable mesoporosity, and a strongly acidic surface. By controlling the synthesis and calcination conditions and optimizing the properties of sulfated ZrO2 hollow shells, we have been able to design superacid catalysts with superior performance in the dehydration of fructose to 5-hydroxymethyfurfural than the solid sulfated ZrO2 nanocatalyst.

Thermal Synthesis of Silver Nanoplates Revisited: A Modified Photochemical Process

The well-known photochemical and thermal methods for silver nanoplate synthesis have been generally regarded as two parallel processes without strong connections. Here we report a surprising finding that both visible light and ambient O2, which are critically important in the photochemical process, also play determining roles in the thermal synthesis. By designing a series of control experiments, we reveal that the typical thermal synthesis is essentially a modified photochemical synthesis coupled with the unique redox properties of H2O2. Light irradiation and dissolved O2 are found to be essential for initiating the formation of nanoplates, but the continued growth of nanoplates is supported by the oxidative etching and subsequent reduction of Ag due to H2O2. O2 resulting from the catalytic decomposition of H2O2 etches small nanoparticles to produce Ag(+) ions, which are then reduced back to Ag(0) by anions of H2O2 to support the growth of nanoplate seeds. The involvement of H2O2 in the reaction significantly speeds up the nanoplate formation process. These findings not only greatly improve our understanding of the unique functions of H2O2 in the thermal synthesis, but also bridge the two well established synthesis processes with a unified mechanism, and significantly enhance the reproducibility of the thermal synthesis of Ag nanoplates by identifying the critical importance of ambient light and O2.