Emergence and Evolution of Crystallization in TiO2 Thin Films: A Structural and Morphological Study

Application of Anodic Titanium Oxide Modified with Silver Nanoparticles as a Substrate for Surface-Enhanced Raman Spectroscopy

The formation of nanostructured anodic titanium oxide (ATO) layers was explored on pure titanium by conventional anodizing under two different operating conditions to form nanotube and nanopore morphologies. The ATO layers were successfully developed and showed optimal structural integrity after the annealing process conducted in the air atmosphere at 450 °C. The ATO nanopore film was thinner (1.2 +/- 0.3 μm) than the ATO nanotube layer (3.3 +/- 0.6 μm). Differences in internal pore diameter were also noticeable, i.e., 88 +/- 9 nm and 64 +/- 7 nm for ATO nanopore and nanotube morphology, respectively. The silver deposition on ATO was successfully carried out on both ATO morphologies by silver electrodeposition and Ag colloid deposition. The most homogeneous silver deposit was prepared by Ag electrodeposition on the ATO nanopores. Therefore, these samples were selected as potential surface-enhanced Raman spectroscopy (SERS) substrate, and evaluation using pyridine (aq.) as a testing analyte was conducted. The results revealed that the most intense SERS signal was registered for nanopore ATO/Ag substrate obtained by electrodeposition of silver on ATO by 2.5 min at 1 V from 0.05M AgNO3 (aq.) (analytical enhancement factor, AEF ~5.3 × 104) and 0.025 M AgNO3 (aq.) (AEF ~2.7 × 102). The current findings reveal a low-complexity and inexpensive synthesis of efficient SERS substrates, which allows modification of the substrate morphology by selecting the parameters of the synthesis process.

Solution Precursor Plasma Spraying of TiO2 Coatings Using a Catalyst-Free Precursor

The microstructural characteristics and phase composition of solution precursor plasma-sprayed (SPPS) titania-based coatings using a catalyst-free precursor are reported in this work. An ethanol-based solution containing titanium isopropoxide was used to deposit TiO₂ coatings. The thermal behavior of the solution precursor changed as its phase transformation temperature increased when the molar concentration was increased from 0.3 M to 0.6 M. Scanning electron micrographs showed that the surface of the coatings was composed of nano- and submicron-sized spherical particles (<1 μm) with sintered and melted particles. The cross-sections showed a porous structure using lower concentrations and dense coating formation with micropores using higher concentrations, with thicknesses of about 5 µm-8 µm. Moreover, the coatings when the number of spray passes was increased were 16 µm-20 µm thick, giving an average layer thickness of 0.6 µm deposited per spray pass in all cases. Phase analysis revealed the presence of both the anatase and rutile phases of TiO₂ in coatings sprayed with various concentrations at various stand-off distances. More detailed discussion is presented with respect to the effects of the solution concentration, stand-off distance, and number of spray passes on the coating's phase composition and microstructure.

Low-Temperature Synthesis of Cu-Doped Anatase TiO2 Nanostructures via Liquid Phase Deposition Method for Enhanced Photocatalysis

Titanium dioxide (TiO₂) photocatalysis can harness the energy from sunlight, providing a solution to many green- and energy-related problems. In this study, we aimed to produce Cu doped TiO₂ (Cu-TiO₂) structures at a low temperature (~70 °C) under atmospheric pressure based on liquid phase deposition. The products prepared with Cu nitrate exhibited anatase-phase TiO₂ with the presence of Cu, and the particles showed a waxberry-like structure. Changing the Cu nitrate concentration allowed control of the atomic concentration; we confirmed ~1.3 atm.% of Cu ions in the product when we applied 10 mM in the precursor solution. By doping Cu, the light absorption edge shifted to 440 nm (~2.9 eV), and we proved the photocatalytic reaction through action spectral measurement. We observed the decomposition of acetaldehyde into CO₂ on Cu-TiO₂ photocatalysts, which produced optimized improvements in photocatalytic activity at Cu dopant levels between 0.2 and 0.4 atm.%. This study demonstrates that the liquid phase deposition technique can be used for doping metallic ions into TiO₂, which shows promise for preparing novel and unique nanomaterials as visible light photocatalysts.

Meta-Heuristics Optimization of Mirrors for Gravitational Wave Detectors: Cryogenic Case

In this paper, we explore the behavior of several optimization methods for reducing coating Brownian noise in the mirrors of gravitational wave detectors. We will refer to cryogenic operating temperatures, where the low refractive index material has mechanical losses higher than those of the high refractive index material. This situation is the exact opposite of that which occurs at room temperature, which is already widely known. The optimal design of the dielectric mirror (without a priori assumptions on thicknesses) can be obtained through the combined multi-objective optimization of transmittance and thermal noise. In the following, we apply several multi-objective meta-heuristics to compute the Pareto front related to the optimization problem of dielectric mirror thicknesses made of two materials (binary coatings). This approach gives us more certainty about the structure of the final result. We find strong evidence that all meta-heuristics converge to the same solution. The final result can be interpreted with simple physical considerations, providing useful rules to simplify the thicknesses of the optimization algorithm.

Facile Fabrication of Single-Walled Carbon Nanotube/Anatase Composite Thin Film on Quartz Glass Substrate for Translucent Conductive Photoelectrode

A single-walled carbon nanotube/anatase (SWCNT/anatase) composite thin film with a transmittance of over 70% in the visible-light region was fabricated on a quartz glass substrate by heat treating a precursor film at 500 °C in air. The precursor film was formed by spin coating a mixed solution of the titania molecular precursor and well-dispersed SWCNTs (0.075 mass%) in ethanol. The anatase crystals and Ti3+ ions in the composite thin films were determined by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The effect of the heating process on the SWCNTs was analyzed using Raman spectroscopy. The composite film showed an even surface with a scratch resistance of 4H pencil hardness, as observed using field-emission scanning electron microscopy and atomic force microscopy. The electrical resistivity and optical bandgap energy of the composite thin film with a thickness of 100 nm were 6.6 × 10-2 Ω cm and 3.4 eV, respectively, when the SWCNT content in the composite thin film was 2.9 mass%. An anodic photocurrent density of 4.2 μA cm-2 was observed under ultraviolet light irradiation (16 mW cm-2 at 365 nm) onto the composite thin film, thus showing excellent properties as a photoelectrode without conductive substrates.

Synthesis and Photochemical Properties of Monolithic TiO2 Nanowires Diode

In this paper, the structural and photochemical properties of a monolithic photochemical diode are discussed. The present structure is composed, from the top to the bottom, of a TiO₂ nanowire layer, a TiO₂ film, a Ti foil, and a porous layer made of Pt nanoparticles. The synthesis of the nanowires was simply carried out by Au-catalysed-assisted process; the effects of the annealing temperature and time were deeply investigated. Morphological and structural characterizations were performed by scanning electron microscopy and Raman spectroscopy. The analyses showed the rutile structure of the TiO₂ nanowires. The photocatalytic properties were studied through the degradation of methylene blue (MB) dye under UV light irradiation. The nanowires induced an enhancement of the photo-degradation rate, compared to TiO₂ in a bulk form, due to an increase in the surface area. Moreover, the presence of a nano-porous Pt layer deposited on the rear side of the samples provided a further increase in the MB degradation rate, related to the scavenging effect of Pt nanoparticles. The overall increment of the photo-activity, due to the nano-structuration of the TiO₂ and to the presence of the Pt layer, resulted a factor 7, compared to the bulk reference. In addition, photovoltage measurements allowed to assess the effects of TiO₂ nano-structuration and Pt nanoparticles on the electron accumulation.