Photocatalytic Crystalline and Amorphous TiO2 Nanotubes Prepared by Electrospinning and Atomic Layer Deposition

In this work core/shell composite polymer/TiO₂ nanofibers and from those TiO₂ nanotubes were prepared. First, poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP) fibers were synthetized by electrospinning. They were covered with a 100 nm thick amorphous TiO₂ layer by atomic layer deposition at 50 °C. Later the polymer core was removed by two different methods: dissolution and annealing. In the case of dissolution in water, the as-prepared TiO₂ nanotubes remained amorphous, while when annealing was used to remove the polymers, the TiO₂ crystallized in anatase form. Due to this, the properties of amorphous and crystalline TiO₂ nanotubes with exactly the same structure and morphology could be compared. The samples were investigated by SEM-EDX, ATR-IR, UV-Vis, XRD and TG/DTA-MS. Finally, the photocatalytic properties of the TiO₂ nanotubes were studied by decomposing methyl-orange dye under UV light. According to the results, crystalline anatase TiO₂ nanotubes reached the photocatalytic performance of P25, while amorphous TiO₂ nanotubes had observable photocatalytic activity.

Antibiofilm effect of mesoporous titania coatings on Pseudomonas aeruginosa biofilms

Activation of photocatalytic titania by ultraviolet-A (UVA) radiation has been proposed as a good approach for combating bacteria. Titania powder, in solution or immobilized on a surface, has excellent UVA-assisted killing properties on several microorganisms. However, these properties could not be demonstrated in biofilms of Pseudomonas aeruginosa, a resistant opportunistic human pathogen that can cause severe complications in patients who are immunocompromised or have burn wounds or cystic fibrosis. P. aeruginosa biofilms have detrimental effects on health and industry, causing serious economic damage. In this study, the effect of titania photocatalysis for controlling P. aeruginosa biofilms was investigated by employing different coatings obtained through sol-gel and evaporation-induced self-assembly. Biofilms were grown on non-mesoporous and mesoporous titania surfaces with different pore sizes, which were achieved based on the use of surfactants Brij-58 and Pluronics-F127. In addition, two structural forms of titania were assayed: amorphous and anatase. As well as inhibiting biofilm formation, these coatings significantly enhanced the bactericidal effect of UVA on P. aeruginosa biofilms. The most efficient surface with regard to total antibacterial effect was the mesoporous Brij-58-templated anatase film, which, compared to control biofilms, decreased the number of viable bacteria by about 5 orders, demonstrating the efficacy of this methodology as a disinfection system.

New insights into the atomic structure of amorphous TiO2 using tight-binding molecular dynamics

Amorphous TiO₂ (a-TiO₂) could offer an attractive alternative to conventional crystalline TiO₂ phases for photocatalytic applications. However, the atomic structure of a-TiO₂ remains poorly understood with respect to that of its crystalline counterparts. Here, we conduct some classical molecular dynamics simulations of a-TiO₂ based on a selection of empirical potentials. We show that, on account of its ability to dynamically assign the charge of each atom based on its local environment, the second-moment tight-binding charge equilibration potential yields an unprecedented agreement with available experimental data. Based on these simulations, we investigate the degree of order and disorder in a-TiO₂. Overall, the results suggest that a-TiO₂ features a large flexibility in its local topology, which may explain the high sensitivity of its structure to the synthesis method being used.

Photonic Band Gap and Bactericide Performance of Amorphous Sol-Gel Titania: An Alternative to Crystalline TiO₂

In addition to its traditional application in white pigments, nanocrystalline titania (TiO₂) has optoelectronic and photocatalytic properties (strongly dependent on crystallinity, particle size, and surface structure) that grant this naturally occurring oxide new technological applications. Sol-gel is one of the most widely used methods to synthesize TiO₂ films and NPs, but the products obtained (mostly oxy-hydrated amorphous phases) require severe heat-treatments to promote crystallization, in which control over size and shape is difficult to achieve. In this work, we obtained new photocatalytic materials based on amorphous titania and measured their electronic band gap. Two case studies are reported that show the enormous potential of amorphous titania as bactericide or photocatalyst. In the first, amorphous sol-gel TiO₂ thin films doped with N (TiO2−xNx, x = 0.75) were designed to exhibit a photonic band gap in the visible region. The identification of Ti-O-N and N-Ti-O bindings was achieved by XPS. The photonic band gaps were found to be 3.18 eV for a-TiO₂ and 2.99 eV for N-doped a-TiO₂. In the second study, amorphous titania and amine-functionalized amorphous titania nanoparticles were synthetized using a novel base-catalysed sol-gel methodology. All the synthesized amorphous TiO₂ nanoparticles exhibit bactericide performance (E. coli, ASTME 2149-13).

Intrinsic charge trapping in amorphous oxide films: status and challenges

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states stemming from the disorder of amorphous metal oxide networks. We start from presenting the results for amorphous (a) HfO₂, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy measurements and theoretical calculations using density functional theory shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO₂. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modeling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO₂ and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO₂, a-Al₂O₃, a-TiO₂. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO₂ and a-SiO₂ weakens Hf(Si)-O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O^2- ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions.

Visible-Light-Activated Black Organotitanias: How Synthetic Conditions Influence Their Structure and Photocatalytic Activity

A series of low-temperature, visible-light-activated black organotitanias were synthesised through a sol-gel strategy that allowed the in situ incorporation of p-phenylenediamine (PPD) into the framework of anatase nanoparticles. The effect of the synthetic conditions on the crystalline structure and photocatalytic activity of these materials was assessed by several characterisation techniques, which revealed a small crystalline domain size (4.6-5.5 nm), effective incorporation of PPD inside the nanoparticles, and a significant reduction in the band gap of these materials (from 3.2 to 2.7-2.9 eV). A systematic study of the synthetic parameters also allowed a significant reduction of the solvent used for the preparation of these black organotitanias (20-fold), as well as the crystallisation time, without compromising the structural properties and photocatalytic activity of these materials. The organotitanias with the highest PPD content and high crystallinity result in the best performing materials in the photocatalytic degradation of rhodamine 6G under both UV- and visible-light irradiation.