Michael Dahl

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.

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.

Control of the crystallinity in TiO2 microspheres through silica impregnation

The crystal grain size as well as the crystallinity in nanoscale TiO2 have been determined to be of importance to the photocatalytic activity, however it is difficult to control while maintaining a high surface area. Here we demonstrate the synthesis of anatase titania microspheres with controllable grain sizes, which are obtained by impregnating the porous networks of amorphous titania microspheres with silicate oligomers through the addition and hydrolysis of the precursor tetraethyl orthosilicate (TEOS) and calcining the obtained composite. Varying the amount of TEOS added controls to the degree of silicate impregnation, which restricts the anatase grain growth during calcination and enables the tuning of the grain size from 4–25 nm while maintaining a spherical morphology. The silica is then removed through base etching in order to utilize the microspheres. The photocatalytic performance of microspheres with enhanced crystallinity is tested using the model system of the degradation of Rhodamine B under UV irradiation and the rate constants determined are compared against the surface area and grain size of the microspheres to determine optimal conditions.

Photocatalytic Surface-Initiated Polymerization on TiO2 toward Well-Defined Composite Nanostructures

We demonstrate the use of TiO2 nanospheres as the photoinitiator for photocatalytic surface-initiated polymerization for the synthesis of various inorganic/polymer nanocomposites with well-defined structures. The excitation of TiO2 by UV-light irradiation produces electrons and holes which drive the free radical polymerization near its surface, producing core/shell composite nanospheres with eccentric or concentric structures that can be tuned by controlling the surface compatibility between the polymer and the TiO2. When highly porous TiO2 nanospheres were employed as the photoinitiator, polymerization could disintegrate the mesoporous framework and give rise to nanocomposites with multiple TiO2 nanoparticles evenly distributed in the polymer spheres. Thanks to the well-developed sol-gel chemistry of titania, this synthesis is well-extendable to the coating of the polymers on many other substrates of interest such as silica and ZnS by simply premodifying their surface with a thin layer of titania. In addition, this strategy could be easily applied to coating of different types of polymers such as polystyrene, poly(methyl methacrylate), and poly(N-isopropylacrylamide). We expect this photocatalytic surface-initiated polymerization process could provide a platform for the synthesis of various inorganic/polymer hybrid nanocomposites for many interesting applications.

Dual‐pore Carbon Shells for Efficient Removal of Humic Acid from Water

A template-mediated process for the preparation of mesoporous carbon shells with high surface area, dual-pore structure, and excellent performance in the adsorption of humic acid is reported. Their synthesis involves templating phenolic resin against wrinkled silica nanospheres, subsequent carbonization under Ar atmosphere, and final release of dual-pore mesoporous carbon shells by etching the silica templates. An additional silica layer was used to protect the phenolic resin from aggregation during carbonization, and its subsequent removal gives the carbon shells a hydrophilic surface, which significantly improves their dispersity in aqueous media. When used as adsorbents for humic acid removal, the as-prepared dual-pore mesoporous carbon shells show superior adsorption performance to activated carbon.