Microwave-Driven In-liquid Plasma in Chemical and Environmental Applications. III. Examination of Optimum Microwave Pulse Conditions for Prolongation of Electrode Lifetime,and Application to Dye-Contaminated Wastewater

Generating in-liquid plasma using continuous microwave radiation has proven problematic as the surface of the electrode undergoes significant deterioration because of the generated plasma. This article describes a method by which this problem can be resolved by the utilization of pulsed microwave radiation from a magnetron microwave generator and presents results in the search for optimal pulsed microwave irradiation conditions; these would avoid damage to the electrode and would afford reduced power consumption. Results show that continuous generation of in-liquid plasma that avoids electrode (antenna) damage requires strict and very limited pulsed oscillation conditions. Evaluation of this device was investigated by the discoloration of a rhodamine-B (RhB) dye-contaminated wastewater, for which it was shown that higher treatment efficiency can be obtained compared to more traditional methods such as the UV photolysis (UV), the UV-assisted photocatalytic TiO₂ method (UV/TiO₂), and the NaClO methodology (NaClO). The energy consumed during the 3 min needed to discolor 50 mL of a 0.10 mM aqueous RhB dye solution was 6.3?×?10^?3 kWh per mg of RhB; complete mineralization of the dye solution by the in-liquid plasma occurred within 15 min (loss of TOC).

     

Enhanced visible-light-driven photodegradation of Acid Orange 7 azo dye in aqueous solution using Fe-N co-doped TiO2

FeN -co-doped TiO₂ photocatalysts are prepared by sol–gel method using titanium tetraisopropoxide, urea and iron(II) acetylacetonate as precursors of titania, nitrogen and iron, respectively. The prepared samples are analysed from chemical-physical point of view by X-ray diffraction (XRD), Raman spectroscopy, UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), specific surface area measurements and scanning electron microscopy (FESEM). UV–Vis DRS spectra evidence that the co-doping of TiO₂ with N and Fe leads to the narrowing of the band gap value (2.7 eV) with respect to Fe-doped TiO₂ (2.8 eV) and N-doped TiO₂ (2.9–3 eV). XRD patterns show that photocatalysts are mainly in anatase phase and Fe and N ions are successfully incorporated into the TiO₂ lattice. The average crystallite size of Fe-N co-doped TiO₂ is slightly lower than the other samples and equal to about 7 nm and the specific surface area of the co-doped sample results to be 117 m² g⁻¹. Photocatalytic performances of all prepared samples are evaluated by analysing the degradation of Acid Orange 7 azo dye under visible light irradiation. Photocatalytic efficiency obtained using FeN co-doped TiO₂ strongly increases compared to undoped TiO₂, N-doped TiO₂ and Fe-doped TiO₂ photocatalysts. In detail, using the co-doped photocatalyst, dye discoloration and mineralization result equal to about 90 and 83% after 60 min of LEDs visible light irradiation, underlining the best performances of the FeN co-doped TiO₂ photocatalyst both in terms of treatment time and electric energy consumption.     

Thermodynamic study of direct reduction of high-chromium vanadium–titanium magnetite (HCVTM) based on phase equilibrium calculation model

High-chromium vanadium–titanium magnetite (HCVTM) is a good valuable resource with high iron content in the form of complex iron ore which contains various valuable metal elements such as iron, vanadium, titanium, chromium. Direct reduction of HCVTM is studied based on thermodynamic analysis. Combined TG experimental verification and equilibrium calculation model was used to analyze the reaction sequence and equilibrium amount in this paper. The contents in HCVTM reduction system are simplified as 18 kinds of chemical compositions. Reductions of Fe₃O₄ and FeO·TiO₂ are the main reduction reactions and are mainly reduced by C. The reduction reaction sequence of FeO·TiO₂ is FeO·TiO₂, TiO₂, TiC, and Ti; the reduction reaction sequence of Fe₃O₄ is Fe₃O₄, FeO, and Fe. The minimum reduction temperature of HCVTM is 860 °C. The reduction of Cr is difficult to implement, and the minimum reduction temperature of V is above 700 °C. The gas phase in this system is mainly CO when the temperature is above 1000 °C. CO partial pressure curve of gasification reaction is in the shape of ‘S’ with increase of temperature. When the temperature is 1350 °C, C/O is 1.0 and reduction time is 30 min, HCVTM can be reduced thoroughly and the reduction degree can reach to 0.98. When C/O is lower than 1.0, FeTi₂O₅ is the reduction intermediate products from FeO·TiO₂. When C/O is 1.0, diffraction peaks of Fe₃O₄ and FeO·TiO₂ disappear, and they are reduced to Fe and TiO₂.

     

Tio2@Sn core–shell nanotubes for fast and high density Li-ion storage material

TiO₂@Sn core–shell nanotube material prepared by thermal decomposition of SnCl₄ on TiO₂ nanotubes at 300 °C has been demonstrated superior Li-ion storage capability of 176 mA h/g even at high current rate of 4000 mA/g (charge and discharge of all TiO₂ within 5 min) in spite of using low carbon content (5 wt%). This value corresponds to volumetric energy densities of 317 mA h/cm³, and its value was 3.5-fold larger than that of the bare TiO₂ nanotubes.     

Reduction of TiO2 with Hydrogen Plasma

Experiments were performed in a thermal plasma furnace to examine the reduction of TiO₂ by hydrogen plasma. The plasma gas composition was 50% hydrogen–50% argon, the plasma torch input power was 13 kW, the TiO₂ initial mass was 10 g and the processing time was varied from 10 to 90 min. The reaction product contained between 67% and 73% titanium by mass for all tests. This level of reduction was consistent with chemical equilibrium modeling. There was no detectable enhancement of reduction due to the presence of atomic hydrogen. The Ti-O microstructures produced were characterized using quantitative SEM analysis. The microstructures showed a number of similarities with structures described by previous researchers.     

Etching Characteristics and Mechanisms of TiO<Subscript>2</Subscript> Thin Films in CF<Subscript>4</Subscript> + Ar,Cl<Subscript>2</Subscript> + Ar and HBr + Ar Inductively Coupled Plasmas

The comparative study of etching characteristics and mechanisms for TiO₂ thin films in CF₄ + Ar, Cl₂ + Ar and HBr + Ar inductively coupled plasmas was carried out. The etching rates for TiO₂, Si and photoresist were measured as functions of gas mixing ratios at fixed gas pressure (10 mTorr), input power (800 W) and bias power (300 W). It was found that the maximum TiO₂ etching rate of ~130 nm/min correspond to pure CF₄ plasma while an increase in Ar fraction in a feed gas results in the monotonic non-linear decrease in the TiO₂ etching rates in all three gas mixtures. Plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling supplied the data on the densities of plasma actives specie as well as on particle and energy fluxes to the etched surface. It was concluded that, under the given set of experimental conditions, the TiO₂ etching kinetics in all gas systems correspond to the ion-assisted chemical reaction with a domination of the chemical etching pathway. It was found also that the differences in the absolute TiO₂ etching rates correlate with the energy thresholds for TiO₂ + F, Cl or Br reaction, and the reaction probabilities for F, Cl and Br atoms exhibit the different changes with the ion energy flux according to the volatility of corresponding etching products.     

Composite of natural bamboo (Dendrocalamus strictus) and TiO2: Its photocatalytic potential in the degradation of methylene blue under the direct irradiation of solar light

Composites of visible-light-active bamboo charcoal powder (BCP)/TiO₂ were fabricated by the straight calcination method by using TiCl₄ as the source of titanium and natural bamboo as the carbon source. The dispersion of TiO₂ nanoparticles was observed onto the surface of the BCP. The introduction of microstructure sizes of the bamboo powder played an important role in enhancing the optical properties of BCP/TiO₂ composites. The composites of BCP/TiO₂ showed the photocatalytic activities both under visible-light irradiation and UV irradiation. The methylene blue dye was used as the experimental check-up. The photodegradation reactions followed zero-order and pseudo-first-order kinetics. In the time duration of 100 min, about 58.31% and 95% of methylene blue were degraded by TiO₂ and BCP/TiO_2, respectively; 60 min more time was required by TiO₂ to achieve 93% of degradation. This suggests that addition of BCP helps in the reduction in time and also shows high durability after up to four cycles having the degradation efficiencies of 95%, 94.1%, 92.7% and 86.6%, respectively. Hence, this research could overlay in the domain of green energy.

     

Effect of RF and microwave oxygen plasma on the performance of Pd gate MOS sensor for hydrogen

The combined effect of microwave and RF oxygen plasma treatment of SiO₂ surface on the hydrogen sensitivity of Pd gate MOS sensor has been studied. Nine different samples of thermally grown SiO₂ surface have been taken and treated with oxygen plasma of different microwave power (100 W, 150 W and 200 W respectively) while keeping RF power fixed (20 W) for different durations (5 min, 10 min and 15 min). Pd gate MOS sensors with these plasma treated SiO₂ surface as dielectric have been fabricated and tested for different concentrations (500–3500 ppm) of hydrogen at room temperature. It is observed that the sensitivity of the sensor increases for higher duration of plasma exposure and also with microwave power but decreases when the sensor is treated with 200 W microwave power for 10 min and 15 min durations. The sensor treated with oxygen plasma of 200 W microwave power for 5 min duration exhibited the highest hydrogen sensitivity (74.4%). Fixed oxide charge density has also been evaluated as a function of exposure time for varying microwave power. Surface morphology of plasma treated SiO₂ surfaces was studied by AFM to have the estimation of porosity. The high sensitivity can be attributed to the fact that oxygen plasma treatment provides the availability of higher number of adsorption sites and modification in the surface state density i.e. surface state density increases for plasma treated sensors.     

Photocatalytic hydrogen evolution over mesoporous TiO2/metal nanocomposites

Na⁺ complex with the dibenzo-18-crown-6 ester was used as a template to synthesize mesoporous titanium dioxide with the specific surface area 130–140 m²/g, pore diameter 5–9 nm and anatase content 70–90%. The mesoporous TiO₂ samples prepared were found to have photocatalytic activity in Cu^II, Ni^II and Ag^I reduction by aliphatic alcohols. The resulting metal–semiconductor nanostructures have remarkable photocatalytic activity in hydrogen evolution from water–alcohol mixtures, their efficiency being 50–60% greater than that of the metal-containing nano-composites based on TiO₂ Degussa P25.The effects of the thermal treatment of mesoporous TiO₂ upon its photocatalytic activity in hydrogen production were studied. The anatase content and pore size were found to be the basic parameters determining the photoreaction rate. The growth of the quantum yield of hydrogen evolution from TiO₂/Ag⁰ to TiO₂/Ni⁰ to TiO₂/Cu⁰ was interpreted in terms of differences in the electronic interaction between metal nanoparticles and the semiconductor surface. It was found that there is an optimal metal concentration range where the quantum yield of hydrogen production is maximal. A decrease in the photoreaction rate at further increment in the metal content was supposed to be connected with the enlargement of metal nanoparticles and deterioration of the intimate electron interaction between the components of the metal–semiconductor nanocomposites.     

Highly photocatalytic activity of metallic hydroxide/titanium dioxide nanoparticles prepared via a modified wet precipitation process

Surface modification is one important approach to increase the photocatalytic activity of TiO₂. By using a modified wet precipitation process, novel M(OH)_x/TiO₂ nanoparticles were synthesized, where M(OH)_x represents ferric or cupric hydroxide. The prepared M(OH)_x/TiO₂ powders were characterized with XRD, FT-IR, BET, UV–vis DRS, and TGA, and were observed to yield high photocatalytic ability by using methyl orange (MO) as a model compound of organic pollutants to be degraded. Due to the accelerating effects of the new photocatalyst, the half-time of MO during its photocatalytic degradation at pH 6.0 over M(OH)_x/TiO₂ was decreased from 332 min for unmodified neat TiO₂ to 63 min for Fe(OH)₃/TiO₂ and 65 min for Cu(OH)₂/TiO₂, respectively. The enhancing effects of M(OH)_x/TiO₂ was further observed in a wide composition range with various M/Ti atomic ratios in the photocatalysts and in a wide pH range of the MO solution from 3 to 7. This enhancing effect is mainly attributed to the increased trapping of the photogenerated electron by the higher valence sites (Fe(III) or Cu(II)) in the hydroxide layer near the M(OH)_x/TiO₂ interface and the enriched surface hydroxyl groups which accept photogenerated holes to yield more hydroxyl radicals.     

Rheology and crystallization behavior of PLLA/TiO2‐g‐PDLA composites

Poly(L‐lactide) (PLLA) composites with TiO₂‐g‐poly(D‐lactide) (PDLA), which was synthesized by surface‐initiated opening ring polymerization with TiO₂ as initiator and Sn(Oct)₂ as catalyst, were prepared by solution casting. The synthesized TiO₂‐g‐PDLA was characterized by transmission electron microscope (TEM) and dynamic laser scattering (DLS), showing larger size corresponding to that of TiO₂. Fourier transform infrared spectroscopy (FT‐IR) and X‐ray photoelectron spectroscopy (XPS) measurements were further carried out and indicated that PDLA was grafted onto TiO₂ through covalent bond. For PLLA/TiO₂‐g‐PDLA composites, the stereocomplex crystallites were formed between PDLA grafted on the surface of TiO₂ and the PLLA matrix, which was determined by FT‐IR, differential scanning calorimetry (DSC), and X‐ray diffractometer (XRD). The influence of stereocomplex crystallites on the rheological behavior of PLLA/TiO₂‐g‐PDLA was investigated by rheometer, which showed greater improvement of rheological properties compared to that of PLLA/TiO₂ composites especially with a percolation content of TiO₂‐g‐PDLA between 3 wt%–5 wt%. The crystallization and melting behavior of PLLA/TiO₂‐g‐PDLA composites were studied by DSC under different thermal treatment conditions. The formed PLA stereocomplex network acted as nucleating agents and a special interphase on the functional surface of TiO₂, which resulted in imperfect PLLA crystal with lower melting temperature. When the thermal treatment was close to the melting temperature of PLA stereocomplex, the crystallinity approached to the maximum. The isothermal crystallization study by polarizing microscope (POM) indicated that stereocomplex network presented stronger nucleation capacity than TiO₂‐g‐PDLA. Copyright © 2015 John Wiley & Sons, Ltd.     

The effect of ball-milling on the thermal behavior of anatase-doped hematite ceramic system

High energy ball-milling methods were employed in the synthesis of anatase-doped hematite xTiO₂(a) · (1−x)α-Fe₂O₃ (x = 0.1, 0.5, and 0.9) ceramic system. The thermal behavior of as obtained ceramic system was characterized by simultaneous DSC–TG. The pure anatase phase was found to be stable below 800 °C, but there is a 10.36% mass loss due to the water content. Two exothermic peaks on DSC curves of pure anatase indicate the different crystallization rates. The pure hematite partially decomposed upon heating under argon atmosphere. Ball-milling has a strong effect on the thermal behaviors of both anatase and hematite phases. For x = 0.1 and 0.5, there is gradual Ti substitution of Fe in hematite lattice, and the decomposition of hematite is enhanced due to the smaller particle size after ball-milling. The crystallization of hematite was suppressed as the enthalpy values decreased due to the anatase-hematite solid–solid interaction. For x = 0.9, most of the anatase phase converted to rutile phase after long milling time. The thermal behavior of xTiO₂(a) · (1−x)α-Fe₂O₃ showed smaller enthalpy value of the hematite transformation to magnetite and anatase crystallization due to the small fraction of hematite phase in the system and hematite–anatase interaction, while the mass loss upon heating increased as a function of milling time due to more water content absorbed by the smaller particle size.     

Catalytic Properties of TiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles in Plasma Chemical Treatment

We proposed here a new process coupling dielectric barrier discharge (DBD) plasma with magnetic photocatalytic material nanoparticles for improving yield in DBD degradation of methyl orange (MO). TiO₂ doped Fe₃O₄ (TiO₂/Fe₃O₄) was prepared by the sol-gel method and used as a new type of magnetic photocatalyst in DBD system. It was found that the introduction of TiO₂/Fe₃O₄ in DBD system could effectively make use of the energy generated in DBD process and improve hydroxyl radical contributed by the main surface Fenton reaction, photocatalytic reaction and catalytic decomposition of dissolved ozone. Most part of MO (88%) was degraded during 30 min at peak voltage of 13 kV and TiO₂/Fe₃O₄ load of 100 mg/L, with a rate constant of 0.0731 min^?1 and a degradation yield of 7.23 g/(kW h). The coupled system showed higher degradation efficiency for MO removal.     

Etching Characteristics and Mechanisms of TiO2 Thin Films in HBr/Ar and Cl2/Ar Inductively-Coupled Plasmas

The TiO₂ etching characteristics and mechanisms in HBr/Ar and Cl₂/Ar inductively-coupled plasmas were investigated under fixed gas-mixing ratio and bias power conditions. It was found that in both systems, an increase in gas pressure from 4 to 10 mTorr results in a non-monotonic TiO₂ etching rate, while a variation of input power in the range 500–800 W causes a faster-than-linear acceleration of the etching process. Plasma diagnostics performed by Langmuir probes and zero-dimensional plasma modeling provided data on plasma parameters, steady-state densities, and fluxes of the active species on the etched surface. The model-based analysis of the etching mechanism showed that for the given set of processing parameters, the TiO₂ etch kinetics correspond to the transitional regime of ion-assisted chemical reaction in which a chemical-etch pathway dominates.     

Studies on the kinetics of synthesis of TiC by calciothermic reduction of TiO<Subscript>2</Subscript> in presence of carbon

Reaction kinetics of the formation of TiC by calciothermic reduction of TiO₂ in presence of carbon have been investigated using thermal analysis (TG-DTA) of a powder mixture of TiO₂, Ca, and C in argon atmosphere at different heating rates. Both the reaction initiation and the peak temperatures are found to increase with heating rates. The appearance of exothermic peaks in the DTA plots after Ca melting indicates the reduction of TiO₂ by liquid calcium and formation of TiC by in-situ reaction of Ti with C. The apparent activation energy of the process has been found to be 170.8±0.5 kJ mol^–1.     

Short time deposition of TiO2 nanoparticles on SiC as photocatalysts for the degradation of organic dyes

The deposition of TiO₂ nanoparticles on SiC was carried out by mechanical milling under different conditions. SiC–TiO₂ samples were used as photocatalysts for the degradation of organic dyes such as methylene blue and rhodamine B. A short time deposition of TiO₂ nanoparticles was observed during mechanical milling (2 min at 200 rpm) to cover the SiC particles. The presence of SiC and TiO₂ (anatase and rutile) was confirmed by means of X-ray diffraction after thermal treatment at 450 °C. The deposition of TiO₂ on SiC was corroborated by scanning electron microscopy analysis; the thickness of the thin layer of TiO₂ deposited on SiC increases as the proportion of TiO₂ increases. The energy band gap values obtained for these compounds were around 3.0 eV. SiC–TiO₂ photocatalysts prepared by mechanical milling exhibited better activity under UV-light irradiation for the degradation of methylene blue and rhodamine B than commercial TiO₂ powder (titania P25).     

Thermal and hydrothermal stability of flame hydrolytically synthesized SiO2/TiO2 mixed oxides

The phase stability of the two TiO₂ modifications (anatase and rutile) in fumed SiO₂/TiO₂ nano-composites (0–24.8 wt-% silica) under thermal and hydrothermal conditions was investigated by X-ray powder diffraction, transmission electron microscopy (TEM) and gas adsorption methods (BET). The results show that the phase transformation from anatase to rutile type of structure and the growth of anatase crystallites are significantly retarded by mixing small amounts of SiO₂ into TiO₂, while the specific surface area is maintained. The SiO₂/TiO₂-composites reveal a remarkable shift in the anatase to rutile transformation temperature from approx. 500 °C (pure TiO₂) to approx. 1000 °C (samples with SiO₂ contents of more than 10%). The rate of phase transformation from anatase to rutile is enhanced under hydrothermal conditions compared to conventional thermal treatment, e.g. pure titania (AEROXIDE^® TiO₂ P25) annealed under hydrothermal conditions (100 g/m³ absolute humidity, 4 h at 600 °C) had a rutile content of 85%, while the same specimens annealed in absence of humidity contained only 46% rutile. However, the difference in rate of phase transformation became less pronounced when the silica content in SiO₂/TiO₂-composites was further increased.TEM results showed that the surface of the anatase crystallites was covered with silica. This averts coalescence of anatase crystallites and keeps them under a critical size during the annealing process. When the crystal domains grew larger, a rapid conversion to rutile took place. The critical size of anatase crystallites for the phase transformation was estimated to be 15–20 nm.     

Preparation,characterization and photocatalytic properties of TiO<Subscript>2</Subscript> nanostructured spheres synthesized by the Sol–Gel method modified with ethylene glycol

Monodispersed nanostructured TiO₂ spheres were obtained by the Sol–Gel method modified with ethylene glycol. The sample morphology and surface textural properties were characterized by X-ray diffraction (XRD), N₂-physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and diffuse reflectance spectroscopy (DRS). The SEM image showed spheres with sizes ranging from 600 to 700 nm. In addition, HRTEM micrographs reveal hexagonal grains slightly elongated (20 nm). The powders present a BET surface area of 116 m² g⁻¹. Samples without thermal treatment and those treated at 400 °C both showed characteristic reflections of the anatase phase. The photocatalytic activity of the prepared TiO₂ spheres was determined by degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution. Kinetics parameters have displayed than the nanostructured material present a reaction half-life time of 30 min and it was two times faster than commercial TiO₂ (P25).     

Rheological and structural details of biocidal iPP-TiO2 nanocomposites

Nanocomposites obtained from incorporation of TiO₂ nanoparticles in different amounts, ranging from 0.5 to 5 wt.%, into an isotactic polypropylene (iPP) matrix are achieved via a straightforward and cost-effective melting process. These materials exhibit a powerful germicide capability over a wide variety of regular bacteria and other microorganisms widely present in the environment that cause infections and serious illness. The iPP-TiO₂ nanocomposites show similar or improved structural characteristics than those of the pure iPP matrix and aspects as important as processability and final mechanical performance seem to be not affected because of the incorporation of these TiO₂ nanoparticles. Validation of time–temperature superposition of the molten polymers is observed within the temperature range analyzed. On the other hand, the α polymorph is the one primarily attained for these specimens. Crystallinity and most probable crystallite size are slightly dependent on TiO₂ content, both increasing as oxide composition is enlarged.     

Evaluation of the fuel cell performances of TiO2/PAN electrospun carbon-based electrodes

Electrocatalytic effect of the untreated and TiO₂+polyacrylonitrile (PAN) modified discarded battery coal (DBC) and pencil graphite electrodes (PGE) were evaluated in fuel cell (FC) applications. TiO₂+PAN solution is coated on PGE and DBC electrodes by electrospinning. According to the FESEM and EDS characterizations, TiO₂ and PAN nanofibers are found to be approximately 40 and 240 nm in size. TiO₂+PAN/PGE showed the best FC performances with 2.00 A cm^–2 current density and 5.05 W cm^–2 power density values, whereas TiO₂+PAN/DBC showed 0.68 A cm^–2 current density and 0.62 W cm^–2 power density values. Electrochemical characterizations of PGE and TiO₂+PAN/PGE electrodes were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. Finally, long-term FC measurement results of developed electrodes exhibited very reasonable recovery values. Along with the comparison of the electrode performances, the recovery of DBCs as electrodes for renewable energy production has been achieved.