Microwave-assisted Hydrothermal Process Research Articles

Exposed (040) facets of BiVO4 modified with Pr3+ and Eu3+: Photoluminescence emission and enhanced photocatalytic performance

BiVO4 materials are promising candidates for photocatalytic applications due to their optical and electronic properties, and among several methodologies, the doping of these materials has shown good results for applications. In this work, we prepared samples of BiVO4 in tetragonal zirconia and monoclinic scheelite phase doped with Europium and Praseodymium through the co-precipitation method followed by microwave-assisted hydrothermal processing. It was observed that doping favored the orientation of the material along the 040 planes, which presents a better photocatalytic response, in addition to doping modifying the morphology and size of the materials. According to photoluminescence analysis, doping favored charge transfer, which is an essential factor for the photocatalytic properties of the material. The photocatalytic performance was tested with pollutants such as dyes (methylene blue and rhodamine B) and a hormone (17a-ethinylestradiol), showing that doping potentiated the degradation of these pollutants, and the material doping with Pr3+ presented the best result for all the contaminants tested, indicating that it is possible to use these materials for this application.

A novel synergistic effect of TiO2 and ZnO incorporation in PES-based thin-film nanocomposite nanofiltration membrane for treatment of textile wastewater.

A novel synergistic effect of TiO2 and ZnO incorporation in the PES-based thin-film nanocomposite nanofiltration membranes was developed for the treatment of common effluent treatment plant (CETP) textile wastewater. PES@TiO2 membranes were developed by phase inversion via the immersion precipitation method followed by the addition of zinc oxide nanoparticles prepared by the rapid microwave-assisted hydrothermal process via interfacial polymerization. p-Phenylenediamine was used as a monomer for the IP process that was coated on the PES@TiO2 support layer. Various techniques have been applied to characterize the developed thin-film nanocomposite membranes such as Fourier transform infrared (FTIR) microscopy, field emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), and contact angle measurement to examine the presence of vibrational modes, surface morphology, the crystal structure of nanoparticles, and hydrophilicity of the membrane, respectively. Membrane properties include porosity, salt rejection, mean pore radius, pure water flux, and industrial effluent rejection efficiency that were studied. The thin-film nanocomposite membrane T5-PES@TiO2(2%)-ZnO(0.3%) was prepared with a combination of 17 wt% PES, 78 wt% DMF, 3 wt% PVP K30, 2% TiO2, 2.5 wt% PPD, 0.3 wt% ZnO, and 1.0 wt% TMC that exhibited high water permeability, porosity, higher industrial effluent rejection, and salt rejection efficiency compared to the neat PES membrane.

Microwave-assisted synthesis of MnO2 nanosorbent for adsorptive removal of Cs(I) and Sr(II) from water solutions.

In this study, a flower-like porous δ-MnO2 nanostructure was synthesized by a microwave-assisted hydrothermal process for adsorptive removal of strontium (Sr(II)) and cesium (Cs(I)) from wastewater. The prepared δ-MnO2 nanosorbent exhibited superior affinity for Sr(II) over Cs(I) in the single-solute system, with partition coefficient (PC) values of 10.2 and 2.3L/g, respectively, at pH 6.0. In the two-solute system, the flower-like δ-MnO2 also adsorbed Sr(II) (PC=3.81L/g) more selectively than Cs(I) (PC 1.15L/g). Further, their adsorption capacities decreased by 12 and 16%, respectively, relative to the single-solute system. In contrast, adsorption of the ions onto δ-MnO2 was affected less sensitively in dual than in single system when changes occurred in environmental variables such as pH (2-8) and ionic strength (1-100mM). Adsorption kinetics, thermodynamics, and isotherm studies demonstrated the pivotal role of the monolayer surface active sites of endothermic δ-MnO2 (e.g., a complexation interaction with Mn-OH). Furthermore, the δ-MnO2 nanosorbent exhibited good regenerability, retaining more than 80% of its adsorption capacity when tested over four reuse cycles. The overall results of this study are expected to help establish strategies to effectively remove metal contaminants from wastewater using a green and low-cost hierarchical nanosorbent.

Regulation preferred crystal plane and oxygen vacancy of CoWO4 with morphology remolding to boost electrochemical performances for battery-supercapacitor hybrid device electrode

Crystal plane engineering and defect engineering are feasible approaches to tune the electrochemical performance of nanocmaterials. Herein, a carambola-like CoWO4 microsphere with preferred crystal planes and oxygen vacancies is synthesized by a microwave-assisted hydrothermal process together with post annealing treatment. The in-depth observation on microstructure and electrochemical tests suggest that it is the new preferred crystal planes and oxygen vacancies in CoWO4 that improves conductivity and reaction activity to lead to a much higher capacity. Further density functional theory (DFT) analysis reveals that tuning the preferred crystal plane and introduction of oxygen vacancy into CoWO4 not only effectively enhance electronic conductivity, but also promote OH− adsorption/desorption, and reduce electronic transmission barrier. As a result, the as-obtained carambola-like CoWO4 microsphere with preferred crystal planes and oxygen vacancies delivers high specific capacity (493.7 C g−1/137.1 mA h g−1 at 1 A g−1) with superior rate capability (148.6 C g−1/41.3 mA h g−1 at 15 A g−1). Moreover, the cycling-induced morphology evolution lead to unconventional capacity increasing during cycling. The as-fabricated A-CoWO4//6 M KOH//activated carbon (AC) BSH device exhibits a maximum energy density of 27.5 Wh kg−1 at 1031.4 W kg−1 and 95.7% capacity retention after 12000 cycles. This work provides a possible way to improve the electrochemical performances of other metal oxide electrodes with low-capacity and irreversibility.

The fabrication of hierarchical porous nano-SnO2@carbon@humic acid ternary composite for enhanced capacity and stability as anode material for lithium ion battery

High performance of SnO2 @C@HA ternary anode material was fabricated by microwave-assisted hydrothermal and subsequent calcination process in the presence of humic acid (HA) and super P. In the calcination process, tin hydroxide is converted to tin oxide and then be crystallized. For comparison purpose, two other composites including SnO2 @C and SnO2 @HA are also prepared, and the structures, morphology and electrochemical performances of these composites are investigated in detail. The results showed that the addition of HA improved the dispersibility of the SnO2 nanoparticles and the specific surface area of the composite. Electrochemical measurement showed that the SnO2 @C@HA anode exhibited high retention capacity during charge-discharge cycling as compared to that of the binary systems, which was attributed to the porous hierarchical structure and high conductivity of SnO2 @C@HA. The reversible specific capacity of SnO2 @C@HA anode can be maintained at 612.0 mA h g−1 after 500 cycles at a current density of 500 mA g−1.

Valence-mixed iron phthalocyanines/(1 0 0) Bi2MoO6 nanosheet Z-scheme heterojunction catalysts for efficient visible-light degradation of 2-chlorophenol via preferential dechlorination

It is vital for selectively preferential dechlorination by visible-light photocatalysis to eliminate high-risk 2-chlorophenol (2-CP) contaminant. However, selective adsorption and efficient charge separation remain challenging. Herein, ultrathin (100) facet-exposed Bi2MoO6 (BMO) nanosheets (∼6 nm thickness) with enhanced selective adsorption for 2-CP due to the Bi-Cl interaction and highly visible-light activity for 2-CP degradation have been synthesized via a pH-controlled microwave-assisted hydrothermal process. The visible-light activity could be improved by modifying iron phthalocyanines (FePc) via H-bond induced assembly and subsequent H2 thermal treatment. The visible-light activity of as-optimized H2-treated heterojunction (H-FePc/BMO) was 4.5 and 2.5-time higher than BMO and FePc/BMO. Raman spectra, X-ray photoelectron spectrum, electron paramagnetic resonance, photoelectrochemical measurements, radical trapping experiment and ion chromatography, confirmed that the improved photoactivity of H-FePc/BMO could be attributed to the selective adsorption of 2-CP inducing photogenerated-holes as dominating active species with nearly 100% preferential dechlorination, the enhanced charge separation via Z-scheme transfer and the promoted O2 activation from valence-mixed Fe0/Fe2+ species in FePc after H2 treatment. This study may open up a new way to the high-efficiency degradation of chlorophenols using bismuth-containing oxide-based photocatalysts.

Novel Co(OH)F/Zn(OH)F heterostructures for acetone gas sensor applications: Materials synthesis, characterization, and sensor performance evaluation

The detection of acetone gas using a gas sensor provides a non-invasive means for monitoring diabetes patients. Various Co-based materials have been developed as acetone-sensing materials. In this work, we developed novel heterostructured materials consisting of metal fluoride hydroxides, Co(OH)F, and Zn(OH)F using a microwave-assisted hydrothermal synthesis process. The ratio between the Co(OH)F microflowers and Zn(OH)F nanorods (3:1) and the mixing conditions (120 °C for 20 min) were optimized. A gas sensor based on the heterostructured material prepared under the optimized conditions demonstrated a significant enhancement in gas response compared with the sensor based on pristine Co(OH)F microflowers. The optimized gas sensor exhibited a very high response of 7.6 toward 5 ppm of acetone gas at an operating temperature of 150 °C, with good selectivity and stability. Considering the energy band diagrams confirmed that the formation of p–n junctions at the interface led to an improvement in the gas response compared with that of the pristine Co(OH)F microflowers.

Photoresponse of high quality epitaxial BiFeO3 films grown through hydrothermal method and rapid microwave assisted method

High quality BiFeO3 film with enhanced remanent polarization and magnetization attracts much interest for its great application potential in information storage and photoelectric devices. Here we grew single-crystal epitaxial BiFeO3 film on SrTiO3 substrate through hydrothermal method at low temperature, the film has a quality higher than any reported BiFeO3 films grown through hydrothermal method. Furthermore, BiFeO3 film was grown through a 25 min rapid microwave-assisted hydrothermal process. The two BiFeO3 films exhibited obvious light response to periodically switching on and off of a LED light, and the film grown through microwave-assisted hydrothermal method produced a 4.7V open-circuit voltage exceeding the band gap of BiFeO3 originating from the ferroelectric photovoltaic effect. These results demonstrate that hydrothermal and microwave-assisted hydrothermal method are simple, inexpensive and rapid to grow high quality epitaxial BiFeO3 film with potential application in photoelectric detection and photovoltaic.

Comparative study on fuel characteristics and pyrolysis kinetics of corn residue-based hydrochar produced via microwave hydrothermal carbonization

Corn residues are an important source of bioenergy. Due to their highly diverse lignocellulosic structures, the hydrochar produced from microwave-assisted carbonization of different corn residues may have distinct fuel properties and pyrolysis kinetics. This study comprehensively investigated the effect of processing temperature on the basic fuel properties of hydrochar and examined the pyrolysis behavior of hydrochar as a precursor through kinetic analysis. The results indicate that the fuel quality of corn straw hydrochar prepared by microwave-assisted hydrothermal carbonization at 230 °C was significantly improved over that of its feedstock, with a higher heating value of approximately 20.7 MJ/kg. Hydrochar prepared by microwave-assisted hydrothermal carbonization of corn cob at 230 °C presents noticeable environmental advantages because it contains the lowest ash and nitrogen contents (0.5% and 0.5%, respectively) and lower sulfur content (0.05%). Moreover, regarding the kinetic modeling, the Doyle and Coats-Redfern models, which are both first-order and single-step kinetic models, were identified as satisfactory in interpreting the key pyrolysis kinetic parameters. Additionally, the microwave-assisted hydrothermal process increased the apparent activation energy of hydrochar due to the increase in crystallinity and the increase in the number of CC and CO bonds.

Preparation of Silicon Hydroxyapatite Nanopowders under Microwave-Assisted Hydrothermal Method.

The synthesis of partially substituted silicon hydroxyapatite (Si-HAp) nanopowders was systematically investigated via the microwave-assisted hydrothermal process. The experiments were conducted at 150 °C for 1 h using TMAS (C4H13NO5Si2) as a Si4+ precursor. To improve the Si4+ uptake in the hexagonal structure, the Si precursor was supplied above the stoichiometric molar ratio (0.2 M). The concentration of the TMAS aqueous solutions used varied between 0.3 and 1.8 M, corresponding to saturation levels of 1.5–9.0-fold. Rietveld refinement analyses indicated that Si incorporation occurred in the HAp lattice by replacing phosphate groups (PO43−) with the silicate (SiO4−) group. FT-IR and XPS analyses also confirmed the gradual uptake of SiO4− units in the HAp, as the saturation of Si4+ reached 1.8 M. TEM observations confirmed that Si-HAp agglomerates had a high crystallinity and are constituted by tiny rod-shaped particles with single-crystal habit. Furthermore, a reduction in the particle growth process took place by increasing the Si4+ excess content up to 1.8 M, and the excess of Si4+ triggered the fine rod-shaped particles self-assembly to form agglomerates. The agglomerate size that occurred with intermediate (0.99 mol%) and large (12.16 mol%) Si contents varied between 233.1 and 315.1 nm, respectively. The excess of Si in the hydrothermal medium might trigger the formation of the Si-HAp agglomerates prepared under fast kinetic reaction conditions assisted by the microwave heating. Consequently, the use of microwave heating-assisted hydrothermal conditions has delivered high processing efficiency to crystallize Si-HAp with a broad content of Si4+.

Physicochemical properties and photocatalytic de-NOx performance of TiO2 nanostructures via microwave hydrothermal strategy

TiO2 nanostructures have been prepared via microwave hydrothermal process using different NaOH concentrations. The physicochemical properties of the products were systematically investigated by using UV–vis Diffuse Reflectance Spectra, X-ray Photoelectron Spectra, Photoluminescence Spectra, Raman, N2 adsorption-desorption, and Transmission Electron Microscope analysis. The results indicated a close correlation between the NaOH concentration and different types of nanostructured titania products obtained from the microwave-assisted hydrothermal process. At low NaOH concentrations (4 M and 6 M), the precursor TiO2 nanoparticles partially converted into a mixture of one dimensional (1D) nanostructures (e.g., nanorods or nanowires) and 2D nanostructures such as nanosheets. As the NaOH concentration is increased to 8 M and 10 M, the obtained TiO2 products contain nanosheet and nanotube-like structures, respectively, with a significantly larger specific surface area. The photocatalytic performance of the synthesized TiO2 products, prepared under different NaOH concentrations, were evaluated through the toxic NOx gas removal efficiencies. The nanostructured TiO2 samples prepared at a higher NaOH concentration have shown improved de-NOx efficiencies than the TiO2–P25 precursor.

Electrochemical performance of SnO2 rods and SnO2/rGO, SnO2/MWCNTs composite materials as an anode for lithium-ion battery application-A comparative study

Surfactant and organic solvents-based SnO2 rods and SnO2/rGO, SnO2/MWCNTs composite materials were synthesized by microwave-assisted hydrothermal process. Powder X-ray diffraction analysis revealed the rutile phase formation. Surface morphology of the prepared samples and their chemical compositions were investigated by SEM and EDS, respectively. Lithium ion batteries (LIBs) were fabricated from synthesized SnO2 rods and composites SnO2/rGO, SnO2/MWCNTs as anode materials and it revealed promising initial discharge capacity of 1426 mAh g−1 and 1575 mAh g−1, 1189 mAh g−1 respectively. Electrochemical studies showed that the discharge capacities retained even after 100th cycle were found to be 171 mAh g−1, 351 mAh g−1 and 214 mAh g−1 even at a high current density of 500 mA g−1, with high coulombic efficiency for SnO2 rods and SnO2/rGO, SnO2/MWCNTs composites, respectively. These findings are better than the commercially used graphite as anode material. Further, electrochemical impedance spectra of the fabricated LIBs having SnO2/rGO and SnO2/MWCNTs composites used as anode material showed less charge transfer resistance as compared to bare SnO2 rods. Due to low charge transfer resistance, improved electrical conductivity and the large surface area of rGO nanosheets, the SnO2/rGO composite exhibited better electrochemical performance when compared with the bare SnO2 rods and SnO2/MWCNTs composite.

Boosting the electrochemical properties of CoCo2O4 porous nanowire arrays by microwave-assisted synthesis for battery–supercapacitor hybrid devices

Cobalt oxide (CoCo2O4) porous nanowires are synthesized by a microwave assisted hydrothermal process (MHP) followed by annealing.

Zn(OH)F Nanorods for Highly Sensitive NO2 Gas Sensor Applications

In this study, Zn(OH)F nanorods were synthesized via a microwave-assisted hydrothermal process and employed for NO2 gas sensor, for the first time. Without adding NH4F in the synthesis process, caltrop-like ZnO structure was formed. The characteristics of Zn(OH)F nanorods were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), and compared to those of caltrop-like ZnO. Additionally, the Zn(OH)F nanorods were further systematically studied by annealing them at 250, 350, and 450 °C to examine thermal stability. The Zn(OH)F nanorods start to transform after annealing at 350 °C. The sensor based on the Zn(OH)F nanorods showed very high response of 63.4 toward 1.3 ppm of NO2 gas at 100 °C, which is 8.6 times higher than the response of the sensor based on the caltrop-like ZnO. In addition, the sensor based on Zn(OH)F nanorods showed an excellent selectivity toward H2 (5 ppm), C2H5OH (10.6 ppm), NH3 (10.6 ppm), C6H5CH3 (36.2 ppm), and CH3COCH3 (2.8 ppm).

A strategy for designing low-cost, environment-friendly, high energy and power density sodium-ion full cells: Effect of extrinsic pseudocapacitance

In this work, we demonstrate the advantage of introducing extrinsic pseudocapacitance behavior in the active material for designing low-cost, high energy, and power density sodium-ion full cells. Here, tin and antimony based alloy nanopowder is synthesized via microwave assisted hydrothermal process (MAHP) for use as a negative electrode. Ex-situ x-ray diffraction studies reveal that the alloy nanopowder stores Na+ via a diffusion-controlled alloying mechanism. Extrinsic pseudocapacitance behavior is subsequently introduced by sandwiching the alloy nanopowder between nitrogen doped reduced graphene oxide nanosheets via MAHP. When coupled with a diffusion-controlled positive electrode comprising of an iron-based prussian blue analog, the resulting full cell demonstrates a high energy density of 262.5 Wh kg−1 at 1C rate with 85.7% capacity retention at the end of 100 cycles. Detailed cyclic voltammetry and electrochemical impedance studies provide insight into the effect of extrinsic pseudocapacitance in improving the electrochemical performance of the full cell.

Two-step synthesis of B and N co-doped porous carbon composites by microwave-assisted hydrothermal and pyrolysis process for supercapacitor application

A series of nitrogen and boron co-doped porous carbon composites (BNCs) were obtained using quick, easy and environmentally microwave-assisted hydrothermal treatment combined with a pyrolysis process subsequent. Biomass waste of fir bark is used as the carbon precursor, while ammonium pentaborate tetrahydrate is used as both boron (B) and nitrogen (N) source. The synthesis process and characteristics of the developed BNCs are investigated, and the results show that the N and B is successfully doped, generating a synergetic effect on the structure, surface characteristics and electrochemical properties. The BNCs achieves specific surface area up to 955 m2 g−1 with mainly microporosity, and possess 10–13 % of B and 15~20 % of N content. The gravimetric capacitance values for the electrodes containing BNC-20 active materials is 188 F g−1 at 0.5 A g−1, 2 times that of non-doped carbon, while maintaining 90% capacitance retention capability after 10,000 charge/discharge cycles. Furthermore, the assembled symmetric cell yielded high performance of volumetric capacitive of 91 F cm−3 at 0.5 A g−1, superior energy density of 9.33 Wh kg−1 at 400 W kg−1 with 91% capacitance retention after 10,000 charge/discharge cycles at 1 A g−1. Our work offers a potential relatively low-cost, effective and environmentally design of heteroatoms doped electrode materials with enhanced supercapacitors performance.

Magnetic and Intercalation Properties of BaRu2O6 and SrRu2O6

Ruthenium oxides have been extensively studied for their attractive magnetic properties. For instance, the honeycomb-layered ruthenate SrRu2O6 has attracted a lot of attention in the last years for its unusually high Néel temperature (565 K), which is among the highest for 4d transition-metal oxides. Motivated by this intriguing feature, several authors suggested to tune the magnetic properties via atomic substitutions. However, in spite of intense efforts, this could not happen because of the instability of the targeted phases at high temperature. We unlocked this bottleneck and succeeded in synthetizing the Ba-based analogue BaRu2O6 by a microwave-assisted hydrothermal process. We describe herein for the first time its structure and physico-chemical properties. Synchrotron X-ray, neutron powder diffraction, and transmission electron microscopy confirm this new compound to be structurally related to SrRu2O6 with however a different oxygen layer stacking. Thereafter, BaRu2O6 is shown to be an antiferromagnet with a high Néel temperature (525 K), and this behavior is discussed in light of its crystal structure. Finally, both SrRu2O6 and BaRu2O6 can electrochemically insert lithium, and the interlayer alkali-earth cations act as pillars, which strongly affect the structural evolution through lithiation. Our results offer new keys toward a better understanding of the magnetic and electrochemical properties of complex ruthenium-based layered oxides.

Microwave-assisted preparation and improvement mechanism of carbon nanotube@NiMn2O4 core-shell nanocomposite for high performance asymmetric supercapacitors

A carbon nanotube (CNT)@NiMn2O4 core-shell nanocomposite is successfully prepared by a facile microwave-assisted hydrothermal process. The as-obtained nanocomposite exhibits low charge transfer resistance (2.19 Ω), high specific capacitance (up to 915.6 F g−1 measured by charge-discharge at 1 A g−1) and excellent cycling stability (93.0% capacity retention after 5000 cycles at 5 A g−1). It is found that the weaker crystallinity induced by microwave treatment, synergism between CNTs and NiMn2O4 in the core-shell heterostructure and more defects and vacancies in CNTs caused by acidifying pretreatment can all devote to improving the electrochemical performance of the as-obtained nanocomposite. An asymmetric supercapacitor device using the as-obtained CNT@NiMn2O4 as positive electrode is also successfully fabricated, which exhibits the maximum energy density of 36.5 Wh kg−1 at a power density of 800 W kg−1 and outstanding cycling stability with 82.8% retains capacitance after 10000 cycles at 5 A g−1. The excellent performance indicates that as-fabricated CNT@NiMn2O4 is one of the competitive electrode materials for energy storage devices.

Microwave-assisted hydrothermal synthesis of MoS2–Ag3PO4 nanocomposites as visible light photocatalyst for the degradation of tetracycline hydrochloride

ABSTRACT In the modern era, industrialization has facilitated the human life but produced several severe pollutants as well that are hazardous in nature. Thus the degradation of these hazardous material has drawn considerable attention. This study deals with the synthesis of MoS2/Ag3PO4 heterojunction nanocomposite with 1–50% wt. using a microwave-assisted hydrothermal process as well as photocatalytic activity of tetracycline hydrochloride (TCH) degradation has been analysed. The compositional properties of nanocomposite catalysts have been studied through X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy as well as structural and morphological studies were conducted through scanning electron microscopy, transmission electron microscopy, Brunauer–Emmet–Teller, photoluminescence, N2 physical adsorption, UV–vis diffuse reflectance spectroscopy. This provides an excellent and efficient mechanism for the remediation of residual organic contaminants under visible light that could be used to decontaminate the atmosphere.

Synthesis of silver-cerium titanate nanotubes and their surface properties and antibacterial applications.

Nano-heterostructures of titanate nanotubes were synthesized and they revealed a complex structure with the formation of TiO2 (anatase), CeO2, Ag2O and metallic silver nanoparticles on the outer walls and intercalation of Ce4+ and Ag+ into the interlayer spaces of the nanotubes by microwave-assisted hydrothermal process and subjected to ion exchange reactions. To the best of our knowledge, this is the first reported silver and cerium co-exchanged titanate nanotubes for bio-applications. The co-ion exchange processes preserved the original tubular structure of titanate nanotubes with significant changes of the superficial as well as interlamellar environment. This study opens up possibility of synthesizing complex, functional nano-heterostructure with the scope of modification of the final structure, especially the amount and oxidation state of the intercalated cation (Ce4+, Ce3+ and Ag+) as well as the quantity and variety of the decorating nanoparticles (CeO2, Ag2O or metallic Ag). The interplay of which, in turn, can lead to important biological properties and applications, owing to their ion-liberation capacity. The samples were tested in antibacterial activity with two different kind of bacteria (gram positive and negative), cell cytotoxicity and adhesion, and it was found that the nano-heterostructure formed shows high antibacterial activity with low cytotoxicity and high cell adhesion, which makes it a promising material for further health applications.