Porous TiO2 microspheres with tunable properties for photocatalytic air purification

Black titania by sonochemistry: A critical evaluation of existing methods

In the field of photocatalysis, the fabrication of black titania is a booming topic, as it offers a system with improved solar light harvesting properties and increased overall efficiency. The darkening of white TiO2 powders can be ascribed to surface hydroxylation, oxygen vacancies, Ti3+ centres, or a combination thereof. A handful of studies suggests these defects can be conveniently introduced by acoustic cavitation, generated during sonochemical treatment of pristine TiO2 powders. In reproducing these studies, P25 TiO2 samples were ultrasonicated for various hours with a power density of 8000 W/L, resulting in powders that indeed became gradually darker with increasing sonication time. However, HAADF-STEM revealed that extensive erosion of the sonotrode tip took place and contaminated the samples, which appeared to be the primary reason for the observed colour change. This was confirmed by UV-Vis DRS and DRIFTS, that showed no significant alteration of the catalyst surface after sonication. EPR measurements showed that only an insignificant fraction of Ti3+ centres were produced, far less than in a TiO2 sample that was chemically reduced with NaBH4. No evidence of the presence oxygen vacancies could be found. The enhanced photocatalytic activities of ultrasonicated materials reported in literature can therefore not be ascribed to the synthesis of actual black (defected) TiO2, but rather to specific changes in morphology as a result of acoustic cavitation. Also, this study underlines the importance of considering probe erosion in sonochemical catalyst synthesis, which is an unavoidable side effect that can have an important impact on the catalyst appearance, properties and performance.

Synthesis and Characterization of Anatase TiO2 Microspheres Self-Assembled by Ultrathin Nanosheets

In this paper, we report a novel and simple method for synthesizing the microspheres self-assembled from ultrathin anatase TiO₂ nanosheets with a high percentage of (001) facets via the hydrolysis process of the single-reagent (potassium fluorotitanate). We then used optical microscopy, scanning electron microscopy, and high-resolution confocal laser Raman spectroscopy to characterize the microspheres generated under different conditions. The study found that the size of the anatase TiO₂ microspheres synthesized was 0.5–3 μm. As the synthesis time increased, the corroded surface of the microspheres gradually increased, resulting in the gradual disappearance of the edges and corners of the anatase nanosheets. The exposure percentage of the (001) facets of ultrathin anatase nanosheets synthesized for 2 h at 180–200 °C are close to 100%. The microsphere whose surface is completely covered by these anatase nanosheets also has nearly 100% exposed (001) facets. This new anatase nanosheet-based self-assembled microsphere will have great application potential in pollution prevention, environmental protection, and energy fields.

Advances in the synthesis and design of nanostructured materials by aerosol spray processes for efficient energy storage

The increasing demand for energy storage has motivated the search for highly efficient electrode materials for use in rechargeable batteries with enhanced energy density and longer cycle life. One of the most promising strategies for achieving improved battery performance is altering the architecture of nanostructured materials employed as electrode materials in the energy storage field. Among numerous synthetic methods suggested for the fabrication of nanostructured materials, aerosol spray techniques such as spray pyrolysis, spray drying, and flame spray pyrolysis are reliable, as they are facile, cost-effective, and continuous processes that enable the synthesis of nanostructured electrode materials with desired morphologies and compositions with controlled stoichiometry. The post-treatment of spray-processed powders enables the fabrication of oxide, sulfide, and selenide nanostructures hybridized with carbonaceous materials including amorphous carbon, reduced graphene oxide, carbon nanotubes, etc. In this article, recent progress in the synthesis of nanostructured electrode materials by spray processes and their general formation mechanisms are discussed in detail. A brief introduction to the working principles of each spray process is given first, and synthetic strategies for the design of electrode materials for lithium-ion, sodium-ion, lithium-sulfur, lithium-selenium, and lithium-oxygen batteries are discussed along with some examples. This analysis sheds light on the synthesis of nanostructured materials by spray processes and paves the way toward the design of other novel and advanced nanostructured materials for high performance electrodes in rechargeable batteries of the future.

Ultrasound assisted synthesis of Ag-decorated TiO2 active in visible light

Titanium dioxide is the most popular photocatalyst to degrade organic pollutants in air, as well as in water. The principal drawback preventing its commercial application lies in its limited absorption of the visible light (400-700nm), while it is active under UV irradiation (≤387nm). Supporting noble metals in the form of nanoparticles on TiO₂ increases its activity in the visible range. However, both the synthesis of noble metal nanoparticles and their deposition on TiO₂ are multi-step processes that often require organic solvents. Here, we deposit Ag nanoparticles from AgNO₃ on the surface of micrometric TiO₂ with H₂O as a solvent and under ultrasound irradiation at 30Wcm^-2. Ultrasound increases the surface amount of Ag on TiO₂ with heterogeneous size distribution of Ag nanoparticles, which are bigger and overlaid (1-20nm vs. 0.5-3nm) compared to the sample obtained in traditional conditions (TEM images). While this change in morphology had no effect on acetone photodegradation under UV light, the 5%, 10%, and 20% Ag-TiO₂ degraded 17%, 20% and 24% acetone under visible light, respectively. The 10% by weight Ag-TiO₂ sample obtained in absence of ultrasound only degraded 14% acetone in 6h, while the bare TiO₂ was not active.

Ultrasound-Assisted Synthesis of Nanostructured Oxide Materials

This chapter is focused on the use of high intensity ultrasound for the preparation of nanostructured materials with an emphasis on recent prominent examples of the production of dense or porous metal oxides through sonochemical and ultrasonic spray pyrolysis routes. Sonochemistry enables the synthesis of oxides that are often unachievable by traditional methods or affords known materials with shape, size, and nano/microstructure control under fast reaction conditions. The fundamental principles of acoustic cavitation, as well as the main ultrasonic parameters affecting the cavitation phenomenon, are first summarized. Next, the applications of ultrasound in the synthesis of nanostructured oxide materials following both preparation methods are reviewed. Particular focus is given to the ultrasound-assisted synthesis of metal oxide nanoparticles for energy applications.

Electrospun metal and metal alloy decorated TiO2 nanofiber photocatalysts for hydrogen generation

Photocatalytic hydrogen generation by electrospun TiO₂ nanofibers decorated with various co-catalysts (Pt₂Pd, PtCu, Cu, Pt, Pd) was explored.

Copper NPs decorated titania: A novel synthesis by high energy US with a study of the photocatalytic activity under visible light

The most important drawback of the use of TiO2 as photocatalyst is its lack of activity under visible light. To overcome this problem, the surface modification of commercial micro-sized TiO2 by means of high-energy ultrasound (US), employing CuCl2 as precursor molecule to obtain both metallic copper as well as copper oxides species at the TiO2 surface, is here. We have prepared samples with different copper content, in order to evaluate its impact on the photocatalytic performances of the semiconductor, and studied in particular the photodegradation in the gas phase of some volatile organic molecules (VOCs), namely acetone and acetaldehyde. We used a LED lamp in order to have only the contribution of the visible wavelengths to the TiO2 activation (typical LED lights have no emission in the UV region). We employed several techniques (i.e., HR-TEM, XRD, FT-IR and UV-Vis) in order to characterize the prepared samples, thus evidencing different sample morphologies as a function of the various copper content, with a coherent correlation between them and the photocatalytic results. Firstly, we demonstrated the possibility to use US to modify the TiO2, even when it is commercial and micro-sized as well; secondly, by avoiding completely the UV irradiation, we confirmed that pure TiO2 is not activated by visible light. On the other hand, we showed that copper metal and metal oxides nanoparticles strongly and positively affect its photocatalytic activity.

In Situ Formation of Disorder-Engineered TiO2(B)-Anatase Heterophase Junction for Enhanced Photocatalytic Hydrogen Evolution

Hydrogenation of semiconductors is an efficient way to increase their photocatalytic activity by forming disorder-engineered structures. Herein, we report a facile hydrogenation process of TiO2(B) nanobelts to in situ generate TiO2(B)-anatase heterophase junction with a disordered surface shell. The catalyst exhibits an excellent performance for photocatalytic hydrogen evolution under the simulated solar light irradiation (∼580 μmol h(-1), 0.02 g photocatalyst). The atomically well-matched heterophase junction, along with the disorder-engineered surface shell, promotes the separation of electron-hole and inhibits their recombination. This strategy can be further employed to design other disorder-engineered composite photocatalysts for solar energy utilization.

Three-dimensional Gd-doped TiO2 fibrous photoelectrodes for efficient visible light-driven photocatalytic performance

Demonstration of the role of electrode geometry on in situ Gd-doping and their effects in the photocatalytic activity.