D.M. Tobaldi

Calcium phosphate-based materials of natural origin showing photocatalytic activity

Calcium phosphate based materials of natural origin with photocatalytic properties were produced. Bones of Atlantic cod fish were treated in appropriate solutions (either Ca- or Ti-containing salts) and successively annealed. Results showed multiphasic materials (hydroxyapatite, β-tricalcium phosphate and anatase titania) with excellent photocatalytic performance under both UV and visible light, with an anatase concentration of only about 2 mol%. Results with such a low amount of anatase have never been reported before for a calcium phosphate-based material; this is attributed to the presence of anatase being mainly on the surface. Single-phase hydroxyapatite (Ca10(PO4)6(OH)2 or HAp) also showed some photocatalytic properties and antibacterial activity.

Nano-titania doped with europium and neodymium showing simultaneous photoluminescent and photocatalytic behaviour

Titanium dioxide (TiO2) nanomaterials are attracting increasing interest, mostly because of their superior photocatalytic and antibacterial properties. In this work, we report the synthesis, by a “green” aqueous sol–gel route, of TiO2 doped with Eu and Nd (1 and 5 mol%), in order to extend its photocatalytic activity to the visible range, and also take advantage of the luminescent features of the rare earth metals (REs). Gels were dried in an oven at 75 °C, and then thermally treated at 450 °C in a static air flow furnace. Semi-quantitative phase composition (QPA) and microstructure (crystalline domain shape and size distribution) of the synthetic powders were studied by means of advanced X-ray methods: Rietveld refinements, and the whole powder pattern modelling (WPPM) technique, respectively. From these X-ray techniques, it was seen that the unit cell volume of RE-modified titania underwent a huge expansion, thus suggesting the incorporation of RE into the TiO2 lattice, accompanied by a decrease in the average crystalline domain diameters of anatase, rutile and brookite. The optical properties were also investigated by diffuse reflectance spectroscopy (DRS). Photoluminescent behaviour was assessed in the visible and near infra-red (NIR) spectral ranges. Photocatalytic activity of the samples was assessed both in liquid–solid and in gas–solid phases. The influence of the lighting conditions (UV- and visible-light exposure) was also taken into account for evaluating the photocatalytic activity. Nd–TiO2 was shown to be a bifunctional material, having both photocatalytic activity and NIR photoluminescent emission induced by the same excitation source, an unprecedented result.

Sensing properties and photochromism of Ag–TiO2 nano-heterostructures

Achieving advanced multifunctional materials displaying several coexisting properties is currently one of the most exciting and innovative research topics. In this study, we report the engineering of a multifunctional material exhibiting, at the same time, tuneable photochromic behaviour and gas-sensing properties for acetone detection. The photochromic property of silver modified (1–10 mol% Ag) titanium dioxide (titania, TiO2) NPs was monitored under consecutive UVA-light exposure times, and the change in colour was thoroughly investigated with both spectroscopic and colourimetric analyses. All Ag modified samples exhibited a significant change in colour and visible spectra after only 15 seconds of exposure, and this increased with further exposure. It was shown that both the silver molar content in the Ag–TiO2 nano-heterostructure, as well as the UVA-light irradiation time, governed the tunability of the photochromic behaviour (the colour changed from pale yellow to dark blue in Ag-modified specimens, while it remained white in unmodified TiO2). The same nano-heterostructures were also tested as sensing materials for resistive metal oxide gas sensors (MOS). These Ag–TiO2 nano-heterostructures proved to be highly sensitive for the detection of acetone vapours at low concentrations (<1 ppm), superior to the best TiO2-based sensors so far reported. This is the first thorough study to qualitatively monitor, in real-time, the growth of Ag0 NPs on a TiO2 matrix, assessing both optical spectroscopy and colourimetric CIEL*a*b* analysis (e.g. what meets the eye), and to also demonstrate the superior acetone gas-sensing properties of such nano-heterostructures.

A hydroxyapatite–Fe2O3 based material of natural origin as an active sunscreen filter

The use of sunscreens as protective barriers against skin damage and cancer, by absorbing harmful UVA and UVB rays, is becoming an increasingly important issue. Such products are usually based on TiO2 or ZnO, although both Fe2O3 and hydroxyapatite (Ca10(PO4)6(OH)2, HAp) doped with metal ions have been reported as being ultraviolet (UV) absorbing materials. HAp is the main component of bone; it is, therefore, highly biocompatible. In the present work, an iron-doped HAp-based material, containing both Fe ions substituted into the HAp structure and iron oxide in hematite (α-Fe2O3) form, was successfully developed from waste cod fish bones. This was achieved through a simple process of treating the bones in a Fe(ii) containing solution, followed by heating at 700 °C. The material showed good absorption in the whole UV range and did not form radicals when irradiated. The sunscreen cream formulated with this material could be used as a broad sunscreen protector (λcrit > 370 nm), showing high absorption both in the UVA and UVB ranges. Because of its absorption properties it would be classified as 5 star protection according to the Boots UVA star rating system. The cream is also photostable, and does not cause irritation or erythema formation when in contact with the human skin. These results show that a food by-product such as fish bones could be converted into a valuable product, with potential applications in health care and cosmetics. This is the first time a HAp-based sunscreen cream has been developed and validated as a proof of concept.

Dielectrical Properties of CeO2 Nanoparticles at Different Temperatures.

A template-free precipitation method was used as a simple and low cost method for preparation of CeO2 nanoparticles. The structure and morphology of the prepared nanoparticle samples were studied in detail using X-ray diffraction, Raman spectroscopy and Scanning Electron Microscopy (SEM) measurements. The whole powder pattern modelling (WPPM) method was applied on XRD data to accurately measure the crystalline domain size and their size distribution. The average crystalline domain diameter was found to be 5.2 nm, with a very narrow size distribution. UV-visible absorbance spectrum was used to calculate the optical energy band gap of the prepared CeO2 nanoparticles. The FT-IR spectrum of prepared CeO2 nanoparticles showed absorption bands at 400 cm-1 to 450 cm-1 regime, which correspond to CeO2 stretching vibration. The dielectric constant (εr) and dielectric loss (tan δ) values of sintered CeO2 compact consolidated from prepared nanoparticles were measured at different temperatures in the range from 298 K (room temperature) to 623 K, and at different frequencies from 1 kHz to 1 MHz.

Effect of Fe-doping on the structure and magnetoelectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 synthesized by a chemical route

B-site Fe-doped (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 was synthesized by a facile chemical route to study the effect of doping on its physical properties. Detailed analysis of X-ray diffraction and Raman spectroscopy data revealed an increased lattice strain and thereby deviation from the morphotropic phase boundary with the progressive doping of Fe from 1 to 5 mol%. Such structural changes have resulted in the weakening of the energy band gap as well as deterioration of the ferroelectric polar nature which was evidenced by a shift of tetragonal to cubic transitions towards room temperature and hard doping effects in ferroelectric hysteresis. The doped samples exhibited room temperature ferromagnetism. Combined Mossbauer and X-ray photoelectron spectroscopic studies suggest that oxygen vacancies and defect complexes induced by Fe doping play a major role in magnetic properties. Local piezoresponse measurements illustrated imprint characteristics of ferroelectric domains in undoped and doped samples at the nanoscale. Room temperature magnetoelectric (ME) measurements revealed that 1 mol% Fe doped sample, having higher ferroelectric polarization and moderate magnetization, exhibits a strong ME response with a coefficient of 12.8 mV cm−1 Oe−1. The present study on Fe-doping effects on the structure and related ME properties of this important lead-free material is useful to tailor multiferroic applications in electronics.

Non-aqueous sol–gel synthesis through a low-temperature solvothermal process of anatase showing visible-light photocatalytic activity

A novel, facile method based on a non-aqueous sol–gel solvothermal process has been developed to synthesise spherical TiO2 nanoparticles (NPs) in one pot. The reaction between titanium(IV) tert-butoxide (Ti[OC(CH3)3]4) and benzyl alcohol was a simple process, which resulted in the formation of highly crystalline titania NPs with a small size of only 6 nm, and with a correspondingly high surface area. The chemical formation mechanism of the metal oxide NPs has been proposed, and the degree of surface hydroxyls (–OH groups) has been examined. The products of the synthesis were characterised by X-ray powder diffraction (XRPD) using the advanced whole powder pattern modelling (WPPM) method, high-resolution transmission electron microscopy (HR-TEM), thermo-gravimetric analysis (TGA), UV-visible diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NMR) spectroscopy. The photocatalytic activity (PCA) was evaluated in both the liquid–solid phase, by monitoring the degradation of an organic dye (methylene blue (MB)) under UV-light irradiation, and in the gas–solid phase, by following the degradation of 2-propanol under UV- and visible-light exposures. The synthesized titania powders not only exhibited excellent photocatalysis in the liquid–solid phase (under UV irradiation), but also possessed a superior PCA in the gas–solid phase under a visible-light exposure. The effects on the PCA of the very small crystalline domain size, surface composition and the presence of organic molecules due to the synthesis process of the TiO2 NPs were shown to account for this behaviour.

Influence of sol counter-ions on the visible light induced photocatalytic behaviour of TiO2 nanoparticles

Titanium dioxide (TiO2) nanoparticles are attracting increasing interest because of their superior photocatalytic and antibacterial properties. Here, aqueous titanium oxy-hydroxide sols were made, using a green synthesis method, from the controlled hydrolysis/peptisation of titanium isopropoxide. Three different mineral acids were used to peptise the sol (HNO3, HBr and HCl), and provide counter-ions. The influence of nitrate or halide sol counter-ions on size distributions of the starting sols were measured via photon correlation spectroscopy (PCS). Semi-quantitative phase composition analysis (QPA), on the gels thermally treated at 450 and 600 °C, was carried out via Rietveld refinement of the X-ray powder diffraction (XRD) patterns. Photocatalytic activity of the prepared samples was also assessed, in the gas–solid phase, monitoring NOx degradation using both solar and white lamps (artificial indoor lightning). Both halides (chlorine or bromine) encouraged the anatase-to-rutile phase transition (ART), resulting in powders containing up to 77 wt% rutile and only 5 wt% brookite after heating to only 450 °C, with particle sizes ~50 nm, and these produced 100% rutile at 600 °C. Photocatalytic tests in the gas phase, using a white lamp, showed that the halide-stabilised sols, thermally treated at 450 °C, gave titania with the highest NOx conversion rate – twice that of Degussa P25.

Quantitative XRD characterisation and gas-phase photocatalytic activity testing for visible-light (indoor applications) of KRONOClean 7000®

Carbon-modified commercial anatase (KRONOClean 7000®) was quantitatively characterised with XRD for the first time – full phase composition (both crystalline and amorphous content) and microstructure. The material was found to be bimodal anatase, mostly ∼4 nm diameter, but with a small amount of a larger fraction between ∼12 and 15 nm. Absorption in the visible range was confirmed by UV-Vis analysis, whilst XPS showed that an aromatic carbon compound is at the origin of that absorption. Also, the photocatalytic activity of this commercial nano-TiO2 was assessed, monitoring the abatement of NOx using a white LED lamp, irradiating exclusively in the visible region. Experiments simulated an indoor environment, highlighting the potential of this nano-TiO2 for adoption as a standard for visible-light photocatalytic activity, i.e. applications for innovative interior eco-building materials.

Study of far infrared optical properties and, photocatalytic activity of ZnO/ZnS hetero-nanocomposite structure

ZnO nanoplates/ZnS nanoparticles as a hetero-nanocomposite structure was prepared by a single pot precipitation method without using any capping ligands or other additives. The morphology of the prepared nanocomposite was studied by scanning electron microscopy (SEM). Optical properties of the prepared samples were studied by UV-visible reflectance and Raman spectroscopy. The FT-IR spectroscopy along with the Kramers–Kronig (k–k) method and classical dispersion theory were utilized to calculate far-infrared optical constants of the prepared samples. The photocatalytic activity of the prepared nanoheterostructure was also assessed in the gas–solid phase, by monitoring the NOx abatement using a white-lamp to re-create an indoor environment.