Size-Controlled Synthesis of Anatase in a Mesoporous Silica, SBA-15

Formation of BiOX (X = Cl and Br) in a mesoporous silica by the infiltration of Bi salts and the subsequent reaction with HX vapor

The formation of BiOX (X = Br and Cl) nanoparticles in a mesoporous silica (SBA-15) was found by the reaction of the infiltrated bismuth oxo species with HX vapor at room temperature. The cylindrical pores of SBA-15 led to the directional growth of BiOX nanorods and control of the particle diameter.

Heterostructural transformation of mesoporous silica-titania hybrids

Mesoporous silica (SBA-15 with the BJH pore size of 8 nm) containing anatase nanoparticles in the pore with two different titania contents (28 and 65 mass%), which were prepared by the infiltration of the amorphous precursor derived from tetraisopropyl orthotitanate into the pore, were heat treated in air to investigate the structural changes (both mesostructure of the SBA-15 and the phase and size of the anatase in the pore). The mesostructure of the mesoporous silica and the particle size of anatase unchanged by the heat treatment up to 800 °C. The heat treatment at the temperature higher than 1000 °C resulted in the collapse of the mesostructure and the growth of anatase nanoparticles as well as the transformation to rutile, while the transformation of anatase to rutile was suppressed especially for the sample with the lower titania content (28 mass%). The resulting mesoporous silica-anatase hybrids exhibited higher benzene adsorption capacity (adsorption from water) over those heated at lower temperature, probably due to the dehydroxylation of the silanol group on the pore surface. The photocatalytic decomposition of benzene in water by the present hybrid heated at 1100 °C was efficient as that by P25, a benchmark photocatalyst.

2,4-Dichlorophenoxyacetic acid (2,4-D) photodegradation on WO3-TiO2-SBA-15 nanostructured composite

A current environmental problem is the uncontrolled use of various pesticides that are harmful to the environment and public health. The herbicide 2,4-D is widely used, making it a vector of contamination for aquatic bodies, air, soil, and biomass. In recent decades, researchers have studied remediation of this compound in the environment. In this work, WO₃ and TiO₂ were supported on SBA-15 molecular sieve by the in situ anchoring (ISA) method, with different molar percentages of WO₃ in relation to the oxide content: X = 25%, 50%, and 75%. The W-Ti-S (X) samples were characterized by XDR, XRF, Raman, FTIR, diffuse reflectance of UV-vis, and adsorption and desorption of N₂. SBA-15 mesoporous structure was not destroyed even after the incorporation of the oxides. XRD analyses associated with Raman result found a predominance of the anatase phase for titanium oxide, and the FRX showed low incorporation of nanoparticles. Photocatalytic tests indicated that the catalytic activity depends on WO₃ and TiO₂ content, although all W-Ti-S (X) samples exhibited similar TOF value. The W-Ti-S (25) sample had the highest photocatalytic activity, 76% herbicide photodegradation under ultraviolet irradiation, at 270 min. The analysis of the catalytic cycles indicated that W-Ti-S (25) keeps out 70% of photocatalytic activity in the fourth catalytic cycle. In addition, the W-Ti-S (25) catalytic activity under direct sunlight irradiation was similar to that under artificial UV irradiation.

Crystallization of well-defined anatase nanoparticles in SBA-15 for the photocatalytic decomposition of acetic acid

Anatase nanoparticles with a size of ca. 5 nm were prepared in mesoporous silica (SBA-15 with the pore diameter of 6 nm) by impregnation of the precursor derived from titanium tetraisopropoxide and subsequent heat treatment in air. The mesoporous structure of the anatase–silica hybrid and the size of the anatase particles were kept unchanged during the crystallization of anatase at 200–600 °C. The hybrids were applied as a photocatalyst for the decomposition of acetic acid in water under UV irradiation to find the heat treatment over 400 °C led to higher efficiency of the reaction (45–55 μmol h⁻¹ of carbon-dioxide production) over the samples heated at temperatures lower than 300 °C (3–14 μmol h⁻¹ of carbon-dioxide production).

Crystallization of well-defined anatase nanoparticles (5 nm) in the mesopore (6 nm) of SBA-15 over 400 °C, results the high photocatalytic activity for decomposition of acetic acid compared with other commercial titanium dioxides.

Template Synthesis of Well-Defined Rutile Nanoparticles by Solid-State Reaction at Room Temperature

Well-defined nanoparticles of rutile (with the size of 5 nm) were successfully prepared by the unusual solid-state transformation of an amorphous precursor in well-defined nanospace of a mesoporous silica template (SBA-15) at room temperature. An aqueous colloidal suspension of the rutile nanoparticles was successfully obtained by dissolution of SBA-15 and subsequent pH adjustment. The isolated rutile nanoparticles were used for H₂ evolution from an aqueous methanol solution by UV irradiation.

Preferential immobilization of size-controlled anatase nanoparticles in mesopores

Precise structural design of a host-guest complex was carried out from the aspects of the size and the location of the guest (anatase particles), and the remaining open space of the host (mesoporous silica). The size of the anatase particles was successfully controlled (3, 5 and 8 nm) during the preparation, and the size-controlled nanoparticles were preferentially encapsulated into the mesopores with a diameter of 8 nm. Due to the precise control of the anatase particles, size dependent photoluminescence of the anatase quantum dots was observed for the first time. The change in the porosity of the mesoporous silica by the immobilization of the anatase in the pore was followed to find a systematic variation of the porosity corresponding to the loaded anatase amount. This correlation can be useful to estimate the location of the guest in/on the host for the host-guest hybrids.

Photoactive nanoarchitectures based on clays incorporating TiO2 and ZnO nanoparticles

Thought as raw materials clay minerals are often disregarded in the development of advanced materials. However, clays of natural and synthetic origin constitute excellent platforms for developing nanostructured functional materials for numerous applications. They can be easily assembled to diverse types of nanoparticles provided with magnetic, electronic, photoactive or bioactive properties, allowing to overcome drawbacks of other types of substrates in the design of functional nanoarchitectures. Within this scope, clays can be of special relevance in the production of photoactive materials as they offer an advantageous way for the stabilization and immobilization of diverse metal-oxide nanoparticles. The controlled assembly under mild conditions of titanium dioxide and zinc oxide nanoparticles with clay minerals to give diverse clay-semiconductor nanoarchitectures are summarized and critically discussed in this review article. The possibility to use clay minerals as starting components showing different morphologies, such as layered, fibrous, or tubular morphologies, to immobilize these types of nanoparticles mainly plays a role in i) the control of their size and size distribution on the solid surface, ii) the mitigation or suppression of the nanoparticle aggregation, and iii) the hierarchical design for selectivity enhancements in the catalytic transformation and for improved overall reaction efficiency. This article tries also to present new steps towards more sophisticated but efficient and highly selective functional nanoarchitectures incorporating photosensitizer elements for tuning the semiconductor-clay photoactivity.

Pore shape-reflecting morphosynthesis of lithium niobium oxide via mixed chloride flux growth in the presence of mesoporous silica

A new synthesis method, "chloride flux growth in the rigid nanospace of mesoporous silica", was developed to obtain lithium niobium oxide anisotropic nanoparticles. The morphologies reflect the pore size and shape of the used mesoporous silicas. This method has great potential for synthesizing size-tuned anisotropic nanoparticles of other complex metal oxides.