Synthesis and characterization of TiO2 powders by the double-nozzle electrospray pyrolysis method. Part 2. Material evaluation

Here we report the synthesis and characterization of anatase TiO₂ powders by the double-nozzle electrospray pyrolysis method. Titanium(IV) bis(ammonium lactato) dihydroxide (TALH) aqueous solution (2.0 wt%) and pure H₂O were separately injected into capillaries by using two syringe pumps, and were electrosprayed by using positive (+4 kV) and negative (−4 kV) DC voltage, respectively. Under a stream of dry clean air, the droplets were carried to the stainless steel tube heated with a tubular furnace at 350–450 °C. Thanks to the neutralization of droplets by the double-nozzle electrospray, the final TiO₂ powder yield after pyrolysis was much improved from 6% (by single nozzle) to 55.4% (in this study), although the particle size distribution became wider due to the electrical neutralization and coalescence of the droplets. Photocatalytic activity for H₂ evolution was studied.     

Visible LED light photoelectrochemical sensor for detection of L-Dopa based on oxygen reduction on TiO2 sensitized with iron phthalocyanine

The present work describes the development of a new strategy to photoelectrochemical detection of L-Dopa at low potential based on oxygen reduction on TiO₂ sensitized with iron phthalocyanine (FePc/TiO₂). The FePc/TiO₂ composite shows a photocurrent 10-fold higher than that of pure TiO₂ nanoparticles and it was 4-fold higher than that of FePc exploiting visible light. The band gaps of pure TiO₂ nanoparticles, FePc and FePc/TiO₂, calculated according to the Kubelka–Munk equation, were 3.22 eV, 3.11 eV and 2.82 eV, respectively. The FePc/TiO₂ composite showed a low charge transfer resistance in comparison to the photoelectrode modified with FePc or TiO₂. Under optimized conditions, the photoelectrochemical sensor shows a linear response range from 20 up to 190 μmol L^− 1 with a sensitivity of 31.8 μA L mmol^− 1 and limit of detection of 1.5 μmol L^− 1 for the detection of L-Dopa.     

Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles

Ag and Au nanoparticles were found to significantly enhance the photocatalytic activity of self-organized TiO₂ nanotubular structures. The catalyst systems are demonstrated to be highly efficient for the UV-light induced photocatalytic decomposition of a model organic pollutant – Acid Orange 7. The metallic nanoparticles with a diameter of ∼10 ± 2 nm (Ag) and ∼28 ± 3 nm (Au) were attached to a nanotubular TiO₂ layer that consists of individual tubes of ∼100 nm of diameter, ∼2 μm in length and approx. 15 nm of wall thickness. Both metal particle catalyst systems enhance the photocatalytic decomposition significantly more on the nanotubes support than placed on a compact TiO₂ surface.     

Preparation and photocatalytic activity of Cu-deposited TiO2 film with high transparency

Cu-deposited TiO₂ films were prepared by photoreduction of Cu(II) in the presence of sodium formate. With the initial Cu(II) concentrations more than 100 mg L^?1, induction periods were observed before the transmittance decreased. Scanning electron microscopy indicated that Cu particles of 2.6 ± 0.5 μm were deposited isolatedly with much open space in the induction periods. The films prepared by changing the irradiation time within the induction periods showed a higher photocatalytic activity than a pure TiO₂ for the degradation of methylene blue under the reaction condition without purging air.     

Fast formation of thick and transparent titania nanotubular films from sputtered Ti

Ti films sputtered on transparent fluorine-doped tin oxide glass substrates were anodized in fluoride-containing organic electrolyte in the presence of H₂O. In this work, anodic TiO₂ nanotubes (ATNs) as long as 9.2 ± 0.3 μm were obtained with high growth rate of 0.64 ± 0.3 μm min^?1. We demonstrated the optimum anodization conditions for ATN growth on foreign substrates, were within the range of 0.3–0.5% (wt) NH₄F, with 3–5% (vol) H₂O at 60 V. XPS and ICP-MS were utilized to elucidate the increase of thickness and volume expansion obtained from the sputtered Ti film to their ATN forms. The ATN films exhibited excellent uniformity and adhesion to the substrates.     

A novel approach for determining total titanium from titanium dioxide nanoparticles suspended in water and biosolids by digestion with ammonium persulfate

Titanium dioxide (i.e. TiO₂) in nano-form is a constituent of many nanomaterials that are used in sunscreens, cosmetics, industrial products and in biomedical applications. Quantification of TiO₂ nanoparticles in various matrixes is a topic of great interest for researchers studying the potential health and environmental impacts of nanoparticles. However, analysis of TiO₂ as Ti⁴⁺ is difficult because current digestion techniques require use of strong acids that may be a health and safety risk in the laboratory. To overcome this problem, we developed a new method to digest TiO₂ nanoparticles using ammonium persulfate as a fusing reagent. The digestion technique requires short times to completion and optimally requires only 1 g of fusing reagent. The fusion method showed >95% recovery of Ti⁴⁺ from 6 μg mL^?1 aqueous suspensions prepared from 10 μg mL^?1 suspension of different forms of TiO_2, including anatase, rutile and mixed nanosized crystals, and amorphous particles. These recoveries were greater than open hot-plate digestion with a tri-acid solution and comparable to microwave digestion with a tri-acid solution. Cations and anions commonly found in natural waters showed no significant interferences when added to samples in amounts of 10 ng to 110 mg, which is a much broader range of these ions than expected in environmental samples. Using ICP-MS for analysis, the method detection limit (MDL) was determined to be 0.06 ng mL^?1, and the limit of quantification (LOQ) was 0.20 ng mL^?1. Analysis of samples of untreated and treated wastewater and biosolids collected from wastewater treatment plants yielded concentrations of TiO₂ of 1.8 and 1.6 ng mL^?1 for the wastewater samples, respectively, and 317.4 ng mg^?1 dry weights for the biosolids. The reactions between persulfate ions and TiO₂ were evaluated using stoichiometric methods and FTIR and XRD analysis. A formula for the fusing reaction is proposed that involves the formation of sulfate radicals.     

Photocurrent generated on a carotenoid-sensitized TiO2 nanocrystalline mesoporous electrode

Photocurrent was observed upon monochromatic illumination of an ITO electrode coated with a TiO₂ nanocrystalline mesoporous membrane with carotenoid 8′-apo-β-caroten-8′-oic acid (ACOA) deposited as a sensitizer (illuminated area 0.25 cm²) and immersed in an aqueous 10 mM hydroquinone (H₂Q), 0.1 M NaH₂PO₄ solution (pH = 7.4) purged with argon, using a platinum flag counter electrode (area 3.3 cm²) and a SCE reference electrode. The carotenoid-sensitized short-circuit photocurrent reached 4.6 μA/cm² upon a 40 μW/cm² incident light beam at 426 nm, with an IPCE (%, incident monochromatic photon-to-photocurrent conversion efficiency) as high as 34%. The short-circuit photocurrent was stable during 1 h of continuous illumination with only a 10% decrease. An open-circuit voltage of 0.15 V was obtained (upon 426 nm, 40 μW/cm² illumination) which remained at a constant value for hours. The observed open-circuit voltage is close to the theoretical value (0.22 V) expected in such a system. The action spectrum resembled the absorption spectrum of ACOA bound on the TiO₂ membrane with a maximum near 426 nm. No decay of the ACOA on the TiO₂ surface was observed after 12 h, presumably because of rapid regeneration of ACOA from ACOA⁺ at the surface by electron transfer from H₂Q.     

Demetallation of methemoglobin in cellulose nanofibril–TiO2 nanoparticle composite membrane electrodes

Porous composite films containing cellulose nanofibrils (from sisal) and TiO₂ nanoparticles (ca. 6 nm diameter) are obtained in a layer-by-layer assembly process. Each layer consists of ca. 0.18 μg cellulose nanofibrils and ca. 0.72 μg TiO₂ (determined by QCMB) and adds a thickness of ca. 16 nm (by AFM) to the uniform deposit. The TiO₂ nanophase is creating conducting pathways for electrons in a relatively open cellulose structure (ca. 82% open pores) potentially suitable for the immobilization of large redox proteins such as methemoglobin.Methemoglobin is shown to readily adsorb into the cellulose–TiO₂ film. However, electrochemical responses for the immobilized methemoglobin in aqueous 0.1 M phosphate buffer at pH 5.5 suggest that facile demetallation occurs. Experiments with Fe³⁺ in the absence of protein result in voltammetric responses indistinguishable from those observed for immobilized methemoglobin. In the presence of ethylenediamine tetraacetic acid (EDTA) the voltammetric signals for the Fe³⁺ immediately disappear. Complementary experiments conducted in 0.1 M acetate buffer at pH 5.5 demonstrate that methemoglobin can indeed be immobilized in electrochemically active form and without demetallation loss of the voltammetric signal in the presence of EDTA. Demetallation appears to occur (i) in the presence of phosphate, (ii) at pH 5.5, (iii) and in the presence of a charged surface.     

Photocatalytic activity of nitrogen and copper doped TiO2 nanoparticles prepared by microwave-assisted sol-gel process

Cu and N-doped TiO₂ photocatalysts were synthesized from titanium (IV) isopropoxide via a microwave-assisted sol-gel method. The synthesized materials were characterized by X-ray diffraction, UV-vis diffuse reflectance, photoluminescence (PL) spectroscopy, SEM, TEM, FT-IR, Raman spectroscopy, photocurrent measurement technique, and nitrogen adsorption–desorption isotherms. Raman spectra and XRD showed an anatase phase structure. The SEM and TEM images revealed the formation of an almost spheroid mono disperse TiO₂ with particle sizes in the range of 9-17 nm. Analysis of N₂ isotherm measurements showed that all investigated TiO₂ samples have mesoporous structures with high surface areas. The optical absorption edge for the doped TiO₂ was significantly shifted to the visible light region. The photocurrent and photocatalytic activity of pure and doped TiO₂ were evaluated with the degradation of methyl orange (MO) and methylene blue (MB) solution under both UV and visible light illumination. The doped TiO₂ nanoparticles exhibit higher catalytic activity under each of visible light and UV irradiation in contrast to pure TiO₂. The photocatalytic activity and photocurrent ability of TiO₂ have been enhanced by doping of the titania in the following order: (Cu, N) - codoped TiO₂ > N-doped TiO₂ > Cu-doped TiO₂ > TiO₂. COD result for (Cu, N)-codoped TiO₂ reveals ∼92% mineralization of the MO dye on six h of visible light irradiation.     

Sensitive liquid chromatography–tandem mass spectrometry method for the determination of pantoprazole sodium in human urine

A sensitive and selective liquid chromatographic–tandem mass spectrometric (LC–MS–MS) method was developed to determine pantoprazole sodium (PNT) in human urine. After solid-phase extraction with SPE cartridge, the urine sample was analysed on a C₁₈ column (symmetry 3.5 μm; 75 mm × 4.6 mm i.d) interfaced with a triple quadrupole tandem mass spectrometer. Positive electrospray ionization was employed as the ionization source. The mobile phase consisted of acetonitrile–water (90:10, v/v). The method was linear over a concentration range of 1–100 ng mL^?1. The lower limit of quantitation was 1 ng mL^?1. The intra-day and inter-day relative standard deviation across three validation runs over the entire concentration range was <10.5%. The accuracy determined at three concentrations (8.0, 50.0 and 85.0 ng mL^?1 PNT) was within ±1.25% in terms of relative errors.     

In situ synthesis of composite MTiO3–Al2O3 coatings via laser zone melting

Laser zone melting was employed in this work to prepare MTiO₃ based coatings over commercial, polycrystalline Al₂O₃ substrates, using the corresponding mixtures of powdered alkaline earth carbonates and TiO₂ as starting materials. In situ synthesis of the series M = Ca, Sr, Ba was studied using a CO₂ laser as the heating source, emitting at 10.6 μm, following substrate preheating to a temperature of 750 °C and sample displacement speed of 500 mm h⁻¹. Microstructure (SEM) and phase composition (XRD) demonstrated in situ formation of crystalline MTiO₃ perovskite (M = Ca, Sr), MAl₂O₄ (M = Ca), MAl₁₂O₁₉ magnetoplumbite type (M = Sr) and MAl₁₄O₂₂ β-alumina type (M = Ba) phases. Substantial interaction with the substrate resulted in stable, 50–150 μm thick, composite coatings.     

Molecular dynamics simulations of adsorption of hydrophobic 1,2,4-trichlorobenzene (TCB) on hydrophilic TiO2 in surfactant emulsions and experimental process efficiencies of photo-degradation and -dechlorination

Molecular dynamics simulations have been carried out to assess event details in the adsorption of the hydrophobic substrate 1,2,4-trichlorobenzene (TCB) on the hydrophilic TiO₂ particle surface in the presence of the biosurfactant 4-O-(4′,6′-di-O-acetyl-2′,3′-di-O-alkanoyl-β-d-mannopyranosyl)-d-erythritol (MEL) and the SDS (sodium dodecylsulfate) surfactant in aqueous media at ambient temperature with interaction forces between organic molecules and TiO₂ calculated from Lennard–Jones and Coulomb potential models. The surfactant and the TCB substrate were arranged in the vicinity of TiO₂ using 768 water molecules in the simulations. Results indicate that the MEL molecules adsorb on the TiO₂ surface in ca. 5 ps (k = 2 × 10¹¹ s^?1) through the oxygen atoms of the hydrophilic functions. Contact distance between the MEL and the TiO₂ surface shortened further after 10 ps with adsorption controlled mostly by van der Waals’ forces. The TCB molecules are trapped within the alkyl chains of the MEL system, which subsumes TCB to a greater extent by the repulsion of water, thereby facilitating the TCB molecules to approach the highly hydrophilic and positively charged TiO₂ particle surface. Along with the simulations, also examined was the photodegradation of this hydrophobic TCB substrate that takes place at the TiO₂/water interface in the presence of the MEL biosurfactant. For comparison, the advantages of the MEL in this task and of the commonly used SDS surfactant were determined under otherwise identical experimental conditions (200 mg L^?1 of surfactants) by examining the dynamics of the photo-induced degradation and dechlorination of TCB. The photodegradation of TCB was nearly quantitative in MEL/TiO₂ in contrast to only ca. 22% complete in SDS/TiO₂ dispersions. It is deduced that wastewater treatments with the highly hydrophilic TiO₂ are seriously limited in their photodegradation of hydrophobic pollutants, an issue easily resolvable by the presence of biodegradable surfactants.     

Atomic layer deposition of TiO2 nanotubes and its improved electrostatic capacitance

Titanium dioxide (TiO₂) nanotubes are fabricated into anodic aluminum oxide (AAO) membrane via atomic layer deposition (ALD). For the ALD of TiO₂, gaseous precursors, titanium (IV) isopropoxide and water are sequentially applied and chemically reacted with each other. A thickness of nanotubes is precisely controlled by the applied cycle numbers of ALD and the morphology of nanostructures is investigated by SEM and TEM. The amorphous property of TiO₂ nanostructures is revealed by XRD and the composition of nanotubes is measured by TEM–EDX. The impurity contents and binding structure of the nanostructures are analyzed by XPS. The electrostatic capacitance of TiO₂ nanotubes into AAO is 480 μF/cm² and it is about 3 times higher compared with AAO membrane (172 μF/cm²).     

Formation of porous TiOx biomaterials in H3PO4 electrolytes

In this work, formation of porous TiO_x layers and theirs corrosion behavior were studied. Application of H₃PO₄ electrolytes results in porous TiO_x formation. The process is enhanced by small amount of HF content in the electrolyte. The HF results in higher current density, enhancing dissolution. Small 0.5% HF concentration results in nanopores formation, with pore diameter of about 45 nm. Increase of HF concentration up to 10% results in pores with average diameter of about 5.2 μm. An increase of etching time results in larger pore diameter, but between large 2–5 μm diameter pores smallest ones were observed with diameter below 200 nm. In the initial etching process a remnants of the flat surface are presents with initial cracks in the surface, indicating places for growth of the pores.The TiO_x layers can be used as a biomaterial. The corrosion behavior of the layer investigated in Ringer’s solution, revealed an excellent corrosion resistance, with respect to pure Ti.     

Enhanced electrochemical performance of carbon-coated TiO2 nanobarbed fibers as anode material for lithium-ion batteries

We report the electrochemical performance of carbon-coated TiO₂ nanobarbed fibers (TiO₂@C NBFs) as anode material for lithium-ion batteries. The TiO₂@C NBFs are composed of TiO₂ nanorods grown on TiO₂ nanofibers as a core, coated with a carbon shell. These nanostructures form a conductive network showing high capacity and C-rate performance due to fast lithium-ion diffusion and effective electron transfer. The TiO₂@C NBFs show a specific reversible capacity of approximately 170 mAh g^− 1 after 200 cycles at a 0.5 A g^− 1 current density, and exhibit a discharge rate capability of 4 A g^− 1 while retaining a capacity of about 70 mAh g^− 1. The uniformly coated amorphous carbon layer plays an important role to improve the electrical conductivity during the lithiation–delithiation process.     

Effects of electron beam irradiation on the photoelectrochemical properties of TiO2 film for DSSCs

TiO₂ has been widely utilized for various industrial applications such as photochemical cells, photocatalysts, and electrochromic devices. The crystallinity and morphology of TiO₂ films play a significant role in determining the overall efficiency of dye-sensitized solar cells (DSSCs). In this study, the preparation of nanostructured TiO₂ films by electron beam irradiation and their characterization were investigated for the application of DSSCs. TiO₂ films were exposed to 20–100 kGy of electron beam irradiation using 1.14 MeV energy acceleration with a 7.46 mA beam current and 10 kGy/pass dose rates. These samples were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) analysis. After irradiation, each TiO₂ film was tested as a DSSC. At low doses of electron beam irradiation (20 kGy), the energy conversion efficiency of the film was approximately 4.0% under illumination of simulated sunlight with AM 1.5 G (100 mW/cm²). We found that electron beam irradiation resulted in surface modification of the TiO₂ films, which could explain the observed increase in the conversion efficiency in irradiated versus non-irradiated films.     

Photocatalytic degradation of paraquat and genotoxicity of its intermediate products

The photocatalytic degradation of paraquat (1,1-dimethyl-4,4′-bipyridylium dichloride) aqueous solutions in the presence of polycrystalline TiO₂ Degussa P25 irradiated by near-UV light was investigated. The substrate and total organic carbon concentrations were monitored by UV spectroscopy and TOC measurements, respectively: the complete photocatalytic mineralization of paraquat (20 ppm) was achieved after ca. 3 h of irradiation by using 0.4 g l⁻¹ of catalyst amount at natural pH (ca 5.8). On the contrary no significant photodegradation of paraquat was observed in the absence of TiO₂ under similar experimental conditions. To evaluate the genotoxicity of paraquat and its intermediates produced during heterogeneous photocatalytic treatment, in vitro tests such as Ames test, with and without rat liver microsomal fractions (S9 mix), and micronucleus test, were used. Results obtained with Salmonella typhimurium (strain TA100) showed that paraquat and photocatalytic products were unable to induce gene mutations when photocatalysis was used in the presence of the optimum amount of TiO₂, i.e. 0.4 g l⁻¹, whereas an increase of revertants his⁺ per plate was observed after 300 min irradiation in the presence of very low amount of TiO₂ (0.04 g l⁻¹). The negative results from micronucleus test suggest that mutagenic, but non-clastogenic, late intermediates of paraquat photo-oxidation were formed when the photocatalytic runs of paraquat degradation were carried out by using 0.04 g l⁻¹ of photocatalyst.     

Synthesis and magnetoelectric studies on Na0.5Bi0.5TiO3–BiFeO3 solid solution ceramics

Polycrystalline ceramics of 1 ? x[Na_0.5Bi_0.5TiO₃] ? x[BiFeO₃] (NBT–BFO) were synthesized by the modified Pechini's method to study their magnetic and magnetoelectric properties. A series of solid solutions exhibiting magnetoelectric output were formed when two iso-structural compounds Na_0.5Bi_0.5TiO₃ (NBT) and BiFeO₃ (BFO) were combined. Polarization-electric field hysteresis loops revealed that the maximum polarization (～23 μC/cm² for x = 0.1) decreased continuously with the increase of BFO content, following a hard doped effect. Piezoelectric charge coefficient (d₃₃) = 41 pC/N was obtained for the ceramics with x = 0.1 and the value continues to decrease with the composition. Magnetic hysteresis loops represent the canted antiferromagnetic nature for x ≤ 0.6 and ferromagnetic-like behavior for the BFO-rich compositions. Magnetoelectric coupling was determined by measuring the magnetoelectric voltage coefficient which is ～12.4 mV/cm-Oe at an ac magnetic field of 10 Oe (1 kHz), for x = 0.1 sample.     

Facile synthesis of thick ordered mesoporous TiO2 film for dye-sensitized solar cell use

Crack-free thick ordered mesoporous TiO₂ films with excellent optical quality have been synthesized by combination of “Doctor Blade” technique and a two-step evaporation induced self-assembly (EISA) method. By employing the as-synthesized mesoporous film with the thickness of 7 μm as the photoanode in dye-sensitized solar cell (DSC), a solar conversion efficiency of 6.53% has been obtained at 30 mW cm⁻² light intensity.