Highly Ordered, Accessible and Nanocrystalline Mesoporous TiO₂ Thin Films on Transparent Conductive Substrates

Highly porous (<i>V</i>_mesopore = 25–50%) and ordered mesoporous titania thin films (MTTF) were prepared on ITO (indium tin oxide)-covered glass by a fast two-step method. The effects of substrate surface modification and thermal treatment on pore order, accessibility and crystallinity of the MTTF were systematically studied for MTTF deposited onto bare and titania-modified ITO. MTTF exposed briefly to 550 °C resulted in highly ordered films with grid-like structures, enlarged pore size, and increased accessible pore volume when prepared onto the modified ITO substrate. Mesostructure collapse and no significant change in pore volume were observed for MTTF deposited on bare ITO substrates. Highly crystalline anatase was obtained for MTTF prepared on the modified-ITO treated at high temperatures, establishing the relationship between grid-like structures and titania crystallization. Photocatalytic activity was maximized for samples with increased crystallization and high accessible pore volume. In this manner, a simple way of designing materials with optimized characteristics for optoelectronic applications was achieved through the modification of the ITO surface and a controlled thermal treatment

2D-SAXS In Situ Measurements as a Tool To Study Elusive Mesoporous Phases: The Case of <i>p</i>6<i>mm</i> TiO₂

Mesoporous titania thin films (MTTF) with <i>p</i>6<i>mm</i> pore array are currently attracting a great interest because of their 2D-hexagonal arrangement of close-packed cylindrical pores, but they are difficult to synthesize. This work seeks to understand such apparent elusiveness of the <i>p</i>6<i>mm</i> phase for MTTF. To that end, a wide variety of MTTF were prepared using different precursors, templates, aging and stabilization conditions, according to procedures reported in the literature and several variations thereof. Initially, the obtained MTTF were cross-characterized by ex situ transmission electron microscopy, two-dimensional small-angle X-ray scattering (2D-SAXS), and grazing incidence SAXS, with no clear evidence of a well-organized <i>p</i>6<i>mm</i> pore structure in the final system. In view of the difficulty of obtaining the desired pore array, the mesophase evolution was monitored during the synthesis, aging, stabilization, and calcination stages, resorting to in situ 2D-SAXS experiments. The performed experiments indicate that the key parameters that affect the ordering in these systems are the humidity of conservation but, mainly, the thermal treatment. Among the different strategies rehearsed, the desired mesophase was finally obtained by a combination of high humidity aging, exposure to NH₃ vapors, and a thermal treatment at low temperature, followed by ethanol extraction of the template. However, even resorting to such mild treatments, the obtained MTTF presented an ordered <i>p</i>6<i>mm</i> structure combined with disordered regions, as determined by electron microscopy and porosimetry measurements. Thus, the obtained results stress that considering the mesophase obtained only at initial or intermediate synthesis stages as the one that survives in the template-free system is not a safe assumption. Moreover, complementary experimental techniques should be used to determine mesoporous structures in the final systems. Finally, this work shows the strength of in situ 2D-SAXS measurements as a technique to design new synthetic procedures, in order to obtain specific properties of the final material

Heterogeneous Catalytic Activity of Platinum Nanoparticles Hosted in Mesoporous Silica Thin Films Modified with Polyelectrolyte Brushes

Platinum nanoparticles of 3 nm diameter were included in mesoporous silica thin films by controlling the mesopore surface charge with a short polymer brush. This metal–polymer–mesopore nanocomposite presents high catalytic activity toward ammonia oxidation at low temperatures with 4.5% weight platinum loading. An anomalous partial selectivity toward nitrous oxide is observed for the first time, which can be traced back to the synergy of the particles and modified surface. This effect opens a path toward the design of nanocomposite catalysts with highly controlled environments, in which the size- and function-controlled cavities can be tuned in order to lower the reaction barriers

Nanochemistry in Confined Environments: Polyelectrolyte Brush-Assisted Synthesis of Gold Nanoparticles inside Ordered Mesoporous Thin Films

A robust and straightforward strategy allowing the controlled confinement of metal nanoparticles within the 3D framework of mesoporous films is presented. The chemical methodology is based on the inner surface modification of mesoporous silica films with polyelectrolyte brushes. We demonstrate that the macromolecular building blocks significantly enhance the site-selective preconcentration of nanoparticle precursors in the inner environment of the mesoporous film. Then, chemical reduction of the preconcentrated precursors led to the formation of metal nanoparticles locally addressed in the mesoporous structure. We show that the synergy taking place between two versatile functional nanobuilding blocks (ordered mesocavities and polymer brushes) can produce stable embedded nanoparticles with tuned optical properties in a very simple manner. As a general framework, the strategy can be easily adapted to different sets of polymer brushes and mesoporous films in order to regulate the monomer−precursor interactions and, consequently, manipulate the site-selective character of the different chemistries taking place in the film. We consider that the “integrative chemistry” approach described in this work provides new pathways to manipulate the physicochemical characteristics of hybrid organic−inorganic advanced functional assemblies based on the rational design of chemistry and topology in confined environments

Silver Nanoparticle-Mesoporous Oxide Nanocomposite Thin Films: A Platform for Spatially Homogeneous SERS-Active Substrates with Enhanced Stability

We introduce a nanoparticle-mesoporous oxide thin film composite (NP-MOTF) as low-cost and straightforward sensing platforms for surface-enhanced Raman Spectroscopy (SERS). Titania, zirconia, and silica mesoporous matrices templated with Pluronics F-127 were synthesized via evaporation-induced self-assembly and loaded with homogeneously dispersed Ag nanoparticles by soft reduction or photoreduction. Both methods give rise to uniform and reproducible Raman signals using 4-mercaptopyridine as a probe molecule. Details on stability and reproducibility of the Raman enhancement are discussed. Extensions in the design of these composite structures were explored including detection of nonthiolated molecules, such as rhodamine 6-G or salicylic acid, patterning techniques for locating the enhancement regions and bilayered mesoporous structures to provide additional control on the environment, and potential size-selective filtration. These inorganic oxide–metal composites stand as extremely simple, reproducible, and versatile platforms for Raman spectroscopy analysis