Integrated Nanocatalysts

Despite significant advancements in catalysis research, the prevailing catalyst technology remains largely an art rather than a science. Rapid development in the fields of nanotechnology and materials chemistry in the past few decades, however, provides us with a new capacity to re-examine existing catalyst design and processing methods. In recent years, “nanocatalysts” has become a term often used by the materials chemistry and catalysis community. It refers to heterogeneous catalysts at nanoscale dimensions. Similar to homogeneous catalysts, freestanding (unsupported) nanocatalysts are difficult to separate after use. Because of their small sizes, they are also likely to be cytotoxic and pose a threat to the environment and therefore may not be practical for industrial use. On the other hand, if they are supported on ordinary catalyst carriers, the nanocatalysts would then revert to act as conventional heterogeneous catalysts, since chemists have known active metal clusters or oxide particles in the nanoscale regime long before the nanotechnology era. To resolve this problem, we need new research directions and synthetic strategies.Important advancements in catalysis research now allow chemists to prepare catalytic materials with greater precision. By controlling particle composition, structure, shape, and dimension, researchers can move into the next phase of catalyst development if they can bridge these old and new technologies. In this regard, one way seems to be to integrate active nanostructured catalysts with boundary-defined catalyst supports that are “not-so-nano” in dimension. However, these supports still have available hierarchical pores and cavity spaces. In principle, these devices keep the essence of traditional “catalyst-plus-support” type systems. They also have the advantages of nanoscale engineering, which involves both high level design and integration processes in their fabrication. Besides this, the active components in these devices are small and are easy to integrate into other systems. For these reasons, we refer to the final catalytic devices as integrated nanocatalysts (INCs).In this Account, we describe the current status of nanocatalyst research and introduce the various possible forms of design and types of integration for INC fabrication with increasing compositional and structural complexities. In addition, we discuss present difficulties and urgent issues of this research and propose the integration of the INCs into even more complex “supracatalysts” for future research

Synthetic Architectures of TiO₂/H₂Ti₅O₁₁·H₂O, ZnO/H₂Ti₅O₁₁·H₂O, ZnO/TiO₂/H₂Ti₅O₁₁·H₂O, and ZnO/TiO₂ Nanocomposites

Although synthetic investigations of inorganic nanomaterials had been carried out extensively over the past decade, few of them have been devoted to fabrication of complex nanostructures that comprise multicomponents/phases (i.e., composite nanobuilding blocks), especially in the area of structural/morphological architecture. In this work, nanobelts of a protonated pentatitanate (H2Ti5O11·H2O) were synthesized hydrothermally for the first time. Two technologically important transition-metal-oxides TiO2 and ZnO were then grown respectively or sequentially onto the surface of the as-prepared nanobelts in aqueous mediums. With a main emphasis on organizational manipulation, the present investigation examines general issues of morphological complexity, synthetic interconvertibility, and material combinability related to fabrication of inorganic nanocomposites. Using this model material system, we demonstrate that complex binary and tertiary composite building blocks of TiO2/H2Ti5O11·H2O, ZnO/H2Ti5O11·H2O, ZnO/TiO2/H2Ti5O11·H2O, and ZnO/TiO2 can be architected stepwise in solution. Structural features of these nanocomposites have also been addressed

Self-Aligned Growth of Hexagonal TiO₂ Nanosphere Arrays on α-MoO₃ (010) Surface

At present there are two basic types of self-assembly of inorganic nanomaterials:  surfactant-assisted organization and patterned-substrate deposition. In this paper, we report a novel assembled scheme for fabrication of hexagonal superlattice arrays of anatase TiO2 nanospheres on the (010) crystal plane of α-MoO3 without the assistance of organic surfactants or substrate prefabrication. The growth and self-assembly of TiO2 nanospheres take place simultaneously under hydrothermal conditions at 150−200 °C according to a continuous self-aligned process in which the localized enrichment of hydrolysis product HF (TiF4 is the precursor for TiO2) is believed to play a crucial role in initiating the self-alignment. This “growth-cum-assembly” mechanism has also been validated with our various growth experiments and materials characterization. Key synthetic parameters have been identified and future explorations have been suggested for this ordered scheme

Manipulative Synthesis of Multipod Frameworks for Self-Organization and Self-Amplification of Cu₂O Microcrystals

A full range of novel multipod frameworks of Cu2O microcrystals has been prepared in solution phase via manipulating synthetic parameters. More importantly, a new organization scheme for three-dimensional crystal aggregates has been elucidated in this work; faceted microcrystal subunits (6, 8, and 12 pieces) with simple cubic or face-centered cubic lattices have been organized with space instruction of the formed frameworks. This organization scheme can be divided into two discrete steps:  (i) fractal growth of multipod framework from a nucleation center (i.e., space-definition), and (ii) attachment of crystal subunits according to the space locations assigned in (i). The microcrystal stacks also provide a base for creation of intracrystal porosity and crystal self-amplification. A remarkable correlation among the various crystal morphologies of Cu2O to their respective multipod frameworks has been revealed for the first time

Carbon Nanotubes Supported Mesoporous Mesocrystals of Anatase TiO₂

Carbon nanotubes (CNTs) and titanium dioxides (TiO2) are among the most studied functional materials in recent years. In this work, we have developed a one-pot chemical approach to prepare mesocrystals of anatase TiO2 onto multiwalled CNTs with controllable surface coverage, surface area, crystal orientation, and TiO2/CNTs ratio. Under mild reaction conditions (at 60 °C and slightly higher than 1 atm), [001]-oriented petal-like TiO2 mesocrystals have been evenly grown onto CNTs in aqueous solution. The overlayer metal-oxide phase has crystallite sizes of 2−4 nm and a very uniform size of mesopores centered at 2.5 nm. Some possible mechanisms for pore formation under this surfactant-free condition are discussed. Our investigation also indicates that the CNTs supported mesocrystals of anatase TiO2 can be used as composite catalysts, and they have been proved to be highly active and robust for photocatalytic degradation of methyl orange, without any additional treatments

Fabrication of ZnO “Dandelions” via a Modified Kirkendall Process

We report that in addition to the fabrication of hollow nanomaterials, the Kirkendall-type diffusion can also be utilized in synthetic nanoarchitecture, through which low-dimensional nanobuilding blocks can be designed and organized chemically into complex geometrical conformations. Our approach may provide a new chemical alternative to materials self-organization. In principle, a great variety of inorganic “dandelions” and their nanocomposites can be tailored and fabricated through this type of total synthetic architecture

Hollow ZnO Microspheres with Complex Nanobuilding Units

Hollow ZnO Microspheres with Complex Nanobuilding Unit

Self-Generation of Tiered Surfactant Superstructures for One-Pot Synthesis of Co₃O₄ Nanocubes and Their Close- and Non-Close-Packed Organizations

Self-generation of ionic organic capping from nonionic surfactant polyoxyethylene (20) sorbitan trioleate (Tween-85) has been realized for the controlled synthesis of single crystalline Co3O4 quantum dots (3.0−5.7 nm) in cubic morphology from related layered hydroxide precursors at 80−95 °C. With chemical modification of hydrophobic functional groups on the surface of Co3O4 nanocubes; furthermore, various nanocube-containing micellar superstructures can be further assembled through hydrophobic interactions between Tween-85 molecules and the surface coating under “one-pot” conditions. In particular, square arrangements, spherical domains, and line-assemblies of the prepared Co3O4 nanocubes and their inter-transformations have been attained for the first time by manipulating intersurfactant-interactions. Hydrolysis of Tween-85 and the resultant tiered surfactant superstructures have been investigated with FTIR/UV−vis/EA/TGA/DTA/XPS methods, and the capping species has been identified to be alkylated oleic carboxylate anions derived from Tween-85. Pronounced quantum confinement effects have been observed with the prepared Co3O4 nanocubes, and the optical band gap energies determined are 3.95 and 2.13 eV, respectively, for O2-→ Co2+ and O2-→ Co3+ charge-transfer processes