Strong tribocatalysis of strontium titanate nanofibers through harvesting friction energy for dye decomposition

Friction is a common clean energy and can be harvested and converted into electricity energy via triboelectricity, which can electrochemically drive dye decomposition in theory. In this work, the tribocatalytic Rhodamine B dye decomposition has been experimentally realized in strontium titanate (SrTiO₃) nanofibers, which are synthesized via a hydrothermal method. In the tribocatalytic dye decomposition process, the friction is exerted in the interface between catalyst surface and a polytetrafluoroethylene (PTFE) Teflon rod setup with the different stirring speed. The RhB dye decomposition ratios of SrTiO₃ nanofibers at these stirring speeds of 200 rpm, 400 rpm, 600 rpm, and 800 rpm are respectively 24.2%, 51.8%, 73.9% and 88.6%, yielding to these reaction rate constants of ～0.0112 h^?1, ～0.0260 h^?1, ～0.0562 h^?1 and ～0.0877 h^?1. The main active species, which play an important role in tribocatalytic process, are the superoxide radicals and holes on basis of the active species quenching experiment results. The excellent tribocatalysis activity makes SrTiO₃ nanofibers potential for application in dye wastewater treatment through utilizing the environmental friction energy.     

Synergetic piezo-photocatalytic effect in SbSI for highly efficient degradation of methyl orange

SbSI nanowires (SbSI NWs) and SbSI microrods (SbSI MRs) were synthesized by simple sonochemical and hydrothermal methods, respectively. The SbSI NWs possess better catalytic performance due to the smaller size and larger specific surface area. Meanwhile, the piezoelectric effect and photocatalytic performance of SbSI were combined for the first time. The degradation rate of methyl orange (MO) solution (30 mg/L) by SbSI NWs reaches 97.1% within 2 min under the synergistic effect of visible light and ultrasonic vibration, which is 4.83 times of photocatalysis and 14.22 times of piezocatalysis. The reactive oxide species (ROS) such as hydroxyl radicals (?OH) and superoxide radicals (?O₂^?) were examined by radical trapping experiments to demonstrate their critical role in degrading MO molecule. Furthermore, a possible mechanism of the piezo-photocatalytic process was proposed and degradation pathway of MO dyes by SbSI NWs was suggested based on the HPLC-MS results. It shows that the degradation process of MO includes the breakage of azo double bond, the removal of sulfonic group, the breakage of carbon nitrogen bond, and finally the formation of NO₃^?, SO₄^2?, CO₂ and H₂O. This work provides a promising case for the practical application in dye wastewater treatment of piezoelectric semiconductors.     

Photocatalytic activity of nanocrystalline mesoporous-assembled TiO2 photocatalyst for degradation of methyl orange monoazo dye in aqueous wastewater

In this work, nanocrystalline mesoporous-assembled TiO₂ photocatalyst was synthesized by a sol–gel process with the aid of a structure-directing surfactant and employed for the photocatalytic degradation of methyl orange azo dye (monoazo dye), as compared to various commercially available non-mesoporous-assembled TiO₂ powders. The experimental results showed that the synthesized mesoporous-assembled TiO₂ nanocrystal calcined at 500 °C provided superior decolorization and degradation performance to the non-mesoporous-assembled commercial TiO₂ powders. In addition, several operational parameters affecting the decolorization and degradation of methyl orange, namely photocatalyst dosage, initial dye concentration, H₂O₂ concentration, and initial solution pH, were systematically investigated, using the mesoporous-assembled TiO₂ nanocrystal. The optimum conditions were a photocatalyst dosage of 7 g/l, an initial dye concentration of 5 mg/l, a H₂O₂ concentration of 0.5 M, and an initial solution pH of 4.7, exhibiting the highest decolorization rate of methyl orange.     

A novel approach of synthesizing nano Y2O3 powders for the fabrication of submicron IR transparent ceramics

A facile methodology of synthesizing highly reactive, round-edged, Sulfur–free nano Y₂O₃ powders to fabricate submicron IR transparent yttria ceramics having a unique combination of superior optical and mechanical properties are reported for the first time. Dispersion of yttrium hydroxide into aqueous sol and addition of seed particles produced near-spherical yttria powders having non – aggregated particles with narrow size distribution. The powder exhibited excellent sinterability reaching near-theoretical density at temperatures around 1400 °C in air. Effective inter-particle coordination and traces of Al additives assisted achieving superior densification. Sintered specimens showed average grain sizes closer to 700 nm. Post-sinter hot isostatic pressing eliminated the residual porosity from the sintered samples leading to exhibit IR transmissions up to 84% in the 2.0–9.0 μm regions, equivalent to single crystal Y₂O₃. Achieving densification through solid-state sintering and retaining the sintered grain sizes in the submicron regions significantly enhanced the mechanical properties. Sintered and HIPed Y₂O₃ specimens were further characterized for their thermal properties at temperature regions between ambient to 950 °C.     

Surfactant-free synthesis of a novel octahedral ZnFe2O4/graphene composite with high adsorption and good photocatalytic activity for efficient treatment of dye wastewater

Recently, significant effort has been made toward the development of graphene-based visible-ligh-responsive photocatalysts and their application to dye wastewater treatment. Herein, a series of octahedral ZnFe₂O₄/graphene (ZnFe₂O₄-G) nanocomposites were synthesized using a one-pot solvothermal reaction without the need of a surfactant as novel bifunctional materials exhibiting both high adsorption and good visible-light-responsive photocatalyst properties. The crystal structure, morphology and photocatalytic degradation properties, as well as adsorption behavior, of the octahedral ZnFe₂O₄/graphene composites were investigated in detail. The adsorption capacity and UV–vis spectrometry results indicate that the dye removal efficiency over the samples followed the order of: methylene blue (MB) > rhodamine B (RhB) > methyl orange (MO). The ZnFe₂O₄-G materials exhited enhanced photocatalytic degradation properties for cationic dyes (MB and RhB) compared to those for the anionic dye (MO). In addition, the experimental results indicate that the ZnFe₂O₄-G materials can decompose H₂O₂ in the visible-light photocatalytic process to form hydroxyl radicals (•OH), which are mainly responsible for the photodegradation of the organic contaminants.     

Synthesis of non-agglomerating submicron/nano-Yb2Si2O7 powders by a carbon-coated coprecipitation method

We herein report a novel carbon-coated coprecipitation method to synthesize non-agglomerating submicron/nano-β-Yb₂Si₂O₇ powders. Yb(C₂H₃O₂)₃·4H₂O and silica sol (alkaline) were chosen as the ytterbium source and silicon source, respectively. A carbon surface layer is processed using sucrose as carbon source to hinder the contact and sintering of β-Yb₂Si₂O₇ particles during heat treatment under a protective atmosphere, and then the carbon layer could be completely removed by inletting air at 900°C during cooling process. The as-synthesized Yb₂Si₂O₇ powders demonstrate an average size of 271 nm with a narrow distribution of 142–531 nm. The results show that the carbon layer could successfully solve the severe agglomeration or sintering of nanometer β-Yb₂Si₂O₇ particles during the calcination process. The as-developed carbon-coated coprecipitation technique is an effective method to fabricate high purity submicron/nano-Yb₂Si₂O₇ powders.     

Photocatalytic performance of β-Ga2O3 microcubes towards efficient degradation of malachite green

In this work, the photocatalytic degradation of malachite green (MG) solutions by β-Ga₂O₃ was investigated. The latter was synthetized by reacting gallium nitrate with concentrated formic acid, which produced gallium formate. The thermal decomposition of this compound at 850 °C produced single-phase β-Ga₂O₃. The resulting morphology corresponds to non-agglomerated microcubes, with a size in the range of 0.8 and 2.3 μm. The surface chemical composition and bandgap energy of this oxide were determined by X-ray photoelectron spectroscopy (XPS) and the Tauc method, respectively. The photodegradation of MG was carried out under violet light (λ = 405 nm), at room temperature, using the as-prepared powder. The results revealed a fast degradation of the dye during the first 20 min, which attenuates over time. The rate of photodegradation depends on the amount of β-Ga₂O₃ used and can be fitted by an exponential equation. The role of free hydroxyl radicals and reactive oxygen species in photocatalysis was addressed by using analytic techniques (FTIR and XPS).     

Synthesis of magnetically recyclable porous CoFe2O4/Rh composites with large BET surface area for enhanced photocatalytic degradation of organic pollutant

The organic pollutants in water have been a great environment challenges to human beings, and photocatalytic degradation is an effective method to solve this problem. In this paper, the Rh-loaded cobalt ferrite CoFe₂O₄ (CFO) nanoparticles have been successfully synthesized by in situ photodeposition of Rh nanoparticles onto the porous CFO particles as the photocatalysts. After incorporating Rh nanoparticles, the CFO/Rh composite has a higher specific surface area and is more efficient in charge separation than the bare CFO. The photocatalytic efficiency of decomposing Malachite Green (MG) is improved from 70% over the bare CFO to 97% over the optimized CFO/Rh in 60 min. The CFO/Rh sample also demonstrates its durability for the degradation of MG in 5 photocatalytic reaction cycles. Additionally, hydroxyl radicals (?OH) and superoxide radicals (?O₂^?) are proved to be the crucial reactive species during the photocatalytic degradation of MG with CFO/Rh, evidenced by the active species capture experiments. This work provides a useful approach to enhance the photocatalytic activity of semiconductors for degrading organic dyes.     

Rapid synthesis of YAG phosphor by facile mechanical method

In this study, synthesis of yttrium aluminum garnet (YAG):Ce³⁺ phosphor powders for white light emitting diodes was investigated by mechanical method using the attrition-type mill with no external heating and no flux in dry phase. High mechanical energy input to the starting powder mixture of Y₂O₃, Al₂O₃ and CeO₂ achieved the synthesis of YAG:Ce³⁺ without any flux materials. X-ray diffraction patterns of the processed powders after 5 min processing revealed the peaks of YAG were clearly identified. The maximum temperature of the mill chamber during the processing was 240℃. The YAG phosphor obtained by the mechanical method revealed the internal quantum yield of 65% in the case of the sample mechanically processed under a reducing atmosphere. The synthesized powder showed granule structure consisting of submicron size of YAG particles, which is better handling for the fabrication of light emitting diode devices.     

Effect of grain boundary state and grain size on the microstructure and mechanical properties of alumina obtained by SPS: A case of the amorphous layer on particle surface

The work was aimed at the investigation of kinetics of Spark Plasma Sintering (SPS) of the α-Al₂O₃ particles with amorphous surface layers and investigation of the effect of the amorphous layers on the grain growth and on the mechanical properties of alumina. The objects of investigations comprised:(i) submicron α-Al₂O₃ powder, (ii) submicron α-Al₂O₃ powder with the amorphous layers on the particles' surfaces, and (iii) the fine-grained α-Al₂O₃ powder. The submicron powders (i) and (ii) were used to analyze the effect of the amorphous layers on the sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle sizes on the shrinkage kinetics. The effect of the temperature regime and of the rate (V_h) on the shrinkage kinetics of the submicron and fine alumina powders has been studied. The shrinkage curves were analyzed using the Young–Cutler and Coble models. The sintering kinetics was shown to be determined by the intensity of grain boundary diffusion for the submicron powders and by simultaneous lattice diffusion and grain boundary one for the fine powders. The amorphous layers on the surfaces of the submicron α-Al₂O₃ particles were found to affect the grain boundary migration rate and the Coble equation parameters at the final stages of SPS. The abnormal characteristics of the alumina ceramics sintered from the submicron powder with the amorphous layers on the particles’ surfaces were suggested to originate from the increased concentration of the defects and of the excess free volume at the grain boundaries formed during crystallization of the amorphous layers.     

Structural and magnetic properties of Co/Al2O3 cermet synthesized by mechanical ball milling

Cobalt nanoparticles in the alumina matrix were synthesized using high energy mechanical ball milling of Co₃O₄ and Al powders mixture. The effect of ball mill time of 1 up to 12 h on the phase formation and crystalline lattice of the samples was investigated by the fitting of the X-ray diffraction patterns with Fullprof software and Rietveld method. The results show that 6 h milling of the primary powders yields a nanocomposite of Co/Al₂O₃ cermet. The formation of Co/Al₂O₃ nanocomposite was confirmed by a morphological study using scanning electron microscopy and transmission electron microscopy. The prepared nanocomposite by 12 h ball mill time has ferromagnetic properties with a high saturation magnetization value of 118 emu/g. Also, using Henkel plot analysis, it was shown that there are strong dipole-dipole magnetic interactions between the prepared cobalt nanoparticles in the Al₂O₃ matrix.     

Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends

Samarium borate (SmBO₃) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm₂O₃) and boron oxide (B₂O₃) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B₂O₃ phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700–1250 °C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV–vis) techniques. Fine-grained and pure SmBO₃ powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO₃ powders were synthesized after mechanical activation for 1 h and annealing at 700 °C for 2 h. The polymorphic transformation temperature of SmBO₃ powders from triclinic to hexagonal is about 1080 °C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO₃ phase and its transformation to hexagonal form were found to take place at ∼50–100 °C lower temperatures than those reported in other methods. Mainly hexagonal SmBO₃ powders were obtained after annealing at 1150 °C in the presence of a very small amount of triclinic SmBO₃. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO₃ phase) and 0.68% (hexagonal SmBO₃ phase) depending on their crystal structures.     

Efficient α/β-Bi2O3 composite for the sequential photodegradation of two-dyes mixture

A mixture of α/β-Bi₂O₃ and α-Bi₂O₃ powders were obtained by a simple solid state reaction–annealing route at 550 °C. The structure, optical properties and surface area of the commercial α and β-Bi₂O₃ and the synthesized α-phase and α/β-composite were well characterized by X-ray diffraction, diffuse reflectance spectra and N₂ physisorption. The annealed sample at 550 °C showed 20% of β-phase, forming a heterojunction of α/β-Bi₂O₃ whereas annealing at elevated temperature (650 °C) lead to the α-phase. Optical properties showed that the presence of the β-phase is mainly responsible for narrowing the energy band gap. The photocatalytic activity of the commercial α and β-Bi₂O₃ and the synthesized α-phase and α/β-composite were investigated in degradation of single dyes, Indigo Carmine (IC) and Rhodamine-B (RhB) under both UV and visible light-induced photocatalysis. For the best photocatalyst, the photodegradation in a two-dye mixture solution was systematically studied considering the type of dye, the adsorption capacity of the samples and the behavior of dye photodegradation. The photocatalytic performance of α/β-Bi₂O₃ was comparatively much higher than the commercial α and β-Bi₂O₃, indicating that better performance of efficient charge separation and transfer across α/β-Bi₂O₃ composite was obtained. Possible mechanism of the single dye and two-dye mixture degradation was given by using α/β-Bi₂O₃ composite.     

Nb2O5 as efficient and recyclable photocatalyst for indigo carmine degradation

Heterogeneous photocatalysis is a significant green technology for application in water purification. The application of Nb₂O₅ catalyst for the photodegradation of contaminants is few reported in the literature. Thus, the Nb₂O₅ catalyst was characterized by SEM, FTIR, surface area and charge surface density. This catalyst was applied to degrade indigo carmine dye, which was compared with degradation catalyzed by TiO₂ and ZnO. Almost 100% of dye degradation occurred at 20, 45 and 90 min for TiO₂, ZnO and Nb₂O₅, respectively. The effect of Nb₂O₅ catalyst concentration, pH and ionic strength (μ) was investigated. The Nb₂O₅ activity increased at 0.7 g/L and for higher catalyst concentrations the degradation was kept constant. Degradation of indigo carmine dye catalyzed by Nb₂O₅ was improved at pH < 4.0 and μ = 0.05 mol/L. TiO₂, ZnO and Nb₂O₅ were recovered and re-applied in other nine reaction cycles. While TiO₂ and ZnO have an abrupt loss of their catalytic activity, Nb₂O₅ maintained 85% of catalytic activity after 10 reaction cycles.     

Photocatalytic decolourisation and degradation of Reactive Orange 4 by TiO2-UV process

The photocatalytic decolourisation and degradation of an azo dye Reactive Orange 4 (RO4) in aqueous solution with TiO₂-P25 (Degussa) as photocatalyst in slurry form has been carried out using UV-A light (365 nm). There is a significant difference in adsorption of dye on TiO₂ surface with the change in the solution pH. The effect of various parameters such as catalyst loading, pH and initial concentration of the dye on decolourisation and degradation have been determined. The dye is decolourised in 80 min and completely degraded in 180 min under optimum conditions. The degradation was strongly enhanced in the presence of electron acceptors such as H₂O₂, (NH₄)₂S₂O₈ and KBrO₃. The photodecolourisation and degradation kinetics are discussed in terms of Langmuir–Hinshelwood kinetic model. The degradation intermediates were analysed by GC–MS technique.     

Effect of Y2O3 addition on tribological properties of plasma sprayed Al2O3-13% TiO2 coating

The effect of Y₂O₃ addition on structure, mechanical properties and tribological properties of Al₂O₃-13 wt% TiO₂ coating was investigated. The addition of 20 wt% Y₂O₃ resulted in better densification, stabilization of alpha (α) alumina phase and improvement in fracture toughness of Al₂O₃-13 wt% TiO₂ coating. Abrasive wear tests were performed over a range of loads and sliding speeds. The stabilization of α alumina phase further increased with an increase in severity of wear test conditions, as noted from X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) analysis of worn coatings. Al₂O₃-13 wt% TiO₂-20 wt% Y₂O₃ coating displayed lower friction coefficient and lower abrasive wear rate than Al₂O₃-13 wt% TiO₂ coating, which was due to synergistic effect of α alumina phase and formation of magneli phase oxide of titanium; Ti₂O₃. Friction energy map was used to rationalize observed wear rates, to identify different regimes of wear and degradation modes of coatings.     

Effect of high energy milling on microstructure and mechanical properties of Al2O3-10 wt% Co composites consolidated by spark plasma sintering (SPS)

Al₂O₃-10 wt% Co composites were prepared by high energy milling in the presence of ethyl alcohol and with subsequent spark plasma sintering (SPS). The powders milled for 5 and 30 h were sintered by SPS at 1350 °C for 5 min. The effect of milling time on the sinterability and mechanical properties was studied. The morphology and structure of milled powders were investigated by scanning electron microscopy, dynamic light scattering and X-ray diffraction. The Co phase forms plate-like particles of different sizes, while finely fragmented Al₂O₃ particles are incorporated in the Co phase, forming composite particles. The average size of the composite particles decreases with increasing milling time, achieving 1.33 μm after 30 h. Crystallite size and micro-strain are inversely proportional. Overall, all the samples display homogeneous microstructures, high density (85.29–91.60%) and microhardness in the range 11.41–14.37 GPa.     

Integrated piezo-photocatalysis of electrospun Bi4Ti3O12 nanostructures by bi-harvesting visible light and ultrasonic energies

Co-utilization of solar and mechanical energies via the piezo-phototronic effect is a new-emerging strategy for the implementation of catalysis. Herein, a coupling among piezoelectricity, semiconductor, and photoexcitation of Bi₄Ti₃O₁₂ nanostructures (BiTO NSs) to enable a high piezo-photocatalytic activity is demonstrated. Under the advantages of improving carrier density and suppressing the carrier recombination, the electrospun BiTO NSs calcined at 600 °C exhibits a superior piezo-photocatalytic performance with a Rhodamine B degradation rate of 0.071 min^?1 that is 2.5-fold and 6.7-fold for the piezocatalytic and photocatalytic solos, respectively. The high piezo-photocatalytic performance is comprehensively ascribed to several properties, including high surface area, small crystal size, suitable energy band, large piezoelectric polarization, and rich oxygen vacancy. Furthermore, by bi-harvesting the visible light and ultrasonic energies, BiTO NSs can efficiently produce superoxide and hydroxyl radicals that are responsible for the dye degradation. This work provides a new strategy for developing high-performance catalysts and sheds new insights into the piezo-photocatalysis.     

Studies on the activation of hydrogen peroxide for color removal in the presence of a new Cu(II)-polyampholyte heterogeneous catalyst

In this work we describe the application of a new non-soluble and non-porous complex with copper ion based on ethylene glycol diglycidyl ether (EGDE), methacrylic acid (MAA) and 2-methylimidazole (2MI) in the decolorization of an azo dye Methyl Orange (MO) as a model pollutant at room temperature.The complex with copper ion was studied by ESR and SEM and was tested as a heterogeneous catalyst for H₂O₂ activation. A possible mechanism of interaction involves the production of hydroxyl radicals (confirmed by ESR), dioxygen and water.The Cu(II)-polyampholyte/H₂O₂ system acted efficiently in the color removal of MO. The adsorption and oxidative degradation of the azo-based dye followed pseudo-first-order kinetic profiles, and the rate constant for degradation had a second-order dependence on copper ion content in the mixture.A removal of MO higher than 90% was achieved in 20 min at pH 7.0, combining 0.8 mM of complexed copper ions in the mixture with 24 mM hydrogen peroxide.The dye adsorbed on the polyampholyte following a L4-type isotherm with 4.9 μmol g⁻¹ maximum loading capacity and 3.1 μM dissociation constant for the first monolayer.     

A novel bio-mimetic approach for the fabrication of Bi2O3 nanoflakes from rambutan (Nephelium lappaceum L.) peel extract and their photocatalytic activity

This paper describes a highly efficient and robust solar photocatalytic treatment for the degradation of methyl orange dye over bio-synthesized Bi₂O₃ nanoflakes. Bio-mimetic way is adapted to synthesis Bi₂O₃ nanoflakes from the plant source of rambutan (Nephelium lappaceum L.) fruit peel extract. The bio-synthesized nanoflakes were characterized using X-ray diffraction studies, ultra-violet-visible diffuse reflectance spectra, field emission scanning electron microscope, high resolution transmittance electron microscope, energy dispersive X-ray spectroscopy, BET Surface area, ultra-violet-visible spectrophotometer and FT-IR spectroscopy. The photocatalytic activity of Bi₂O₃ nanoflakes were investigated using methyl orange dye under direct sunlight illumination in open atmosphere. The result shows that Bi₂O₃ nanoflakes were effectively degrading the dye to about 94.66% at 10 h of exposure time. The decreases in chemical oxygen demand values from 88.8 mg/l to 16.2 mg/l, shows the mineralization of methyl orange dye along with color removal.