Sonochemical versus hydrothermal synthesis of bismuth tungstate nanostructures: Photocatalytic, sonocatalytic and sonophotocatalytic activities

Ultrasonic reactor set-ups and applications: A review

Sonochemistry contributes to green science as it uses less hazardous solvents and methods to carry out a reaction. In this review, different reactor designs are discussed in detail providing the necessary knowledge for implementing various processes. The main characteristics of ultrasonic batch systems are their low cost and enhanced mixing; however, they still have immense drawbacks such as their scalability. Continuous flow reactors offer enhanced production yields as the limited cognition which governs the design of these sonoreactors, renders them unusable in industry. In addition, microstructured sonoreactors show improved heat and mass transfer phenomena due to their small size but suffer though from clogging. The optimisation of various conditions of regulations, such as temperature, frequency of ultrasound, intensity of irradiation, sonication time, pressure amplitude and reactor design, it is also discussed to maximise the production rates and yields of reactions taking place in sonoreactors. The optimisation of operating parameters and the selection of the reactor system must be considered to each application's requirements. A plethora of different applications that ultrasound waves can be implemented are in the biochemical and petrochemical engineering, the chemical synthesis of materials, the crystallisation of organic and inorganic substances, the wastewater treatment, the extraction processes and in medicine. Sonochemistry must overcome challenges that consider the scalability of processes and its embodiment into commercial applications, through extensive studies for understanding the designs and the development of computational tools to implement timesaving and efficient theoretical studies.

Harnessing ultrasound in photocatalysis: Synthesis and piezo-enhanced effect: A review

The photocatalytic technique has drawn far-ranging interests in addressing the current issues; however, its property suffers from the limited visible light response and rapid recombination of carriers. To address these issues, two specific approaches have been proposed to enhance the photocatalytic activity: (1) ultrasound-assisted synthesis has been utilized to prepare photocatalysts, resulting in refined grain size, increased specific surface area, and reduced photogenerated carrier recombination; (2) sonophotocatalysis and piezoelectric enhanced photocatalysis have been developed to accelerate the reaction, which utilizes the synergism between ultrasound and light. On one side, sonophotocatalysis generates cavitation bubbles which induce more reactive radicals for redox reactions. On the other side, ultrasound induces deformation of the piezoelectric material structure, which changes the internal piezoelectric potential and improves the photocatalytic performance. Currently, intensive efforts have been devoted to related research and great progress has been reached with applications in pollutant degradation, new energy production, and other fields. This work starts by elucidating the fundamental concept of ultrasound-assisted photocatalyst synthesis and photocatalysis. Then, the synergistic behavior between ultrasonic and light in ultrasonic-assisted photocatalysis has been thoroughly discussed, including pollutant degradation, water splitting, and bacterial sterilization. Finally, the challenge and outlook are investigated and proposed.

Sonophotocatalysis—Limits and Possibilities for Synergistic Effects

Advanced oxidation processes are promising techniques for water remediation and degradation of micropollutants in aqueous systems. Since single processes such as sonolysis and photocatalysis exhibit limitations, combined AOP systems can enhance degradation efficiency. The present work addresses the synergistic intensification potential of an ultrasound-assisted photocatalysis (sonophotocatalysis) for bisphenol A degradation with a low-frequency sonotrode (f = 20 kHz) in a batch-system. The effects of energy input and suspended photocatalyst dosage (TiO2-nanoparticle, m = 0–0.5 g/L) were investigated. To understand the synergistic effects, the sonication characteristics were investigated by bubble-field analysis, hydrophone measurements, and chemiluminescence of luminol to identify cavitation areas due to the generation of hydroxyl radicals. Comparing the sonophotocatalysis with sonolysis and photocatalysis (incl. mechanical stirring), synergies up to 295% and degradation rates of up to 1.35 min−1 were achieved. Besides the proof of synergistic intensification, the investigation of energy efficiency for a degradation degree of 80% shows that a process optimization can be realized. Thus, it could be demonstrated that there is an effective limit of energy input depending on the TiO2 dosage.

Synergistic impacts of sonolysis aided photocatalytic degradation of water pollutant over perovskite-type CeNiO3 nanospheres

The current study reports on the preparation of perovskite-type CeNiO3 nanostructures as a sonophotocatalyst via a facile hydrothermal approach followed by annealing at 800 °C.

Sonoprocessing: From Concepts to Large-Scale Reactors

Intensification of ultrasonic processes for diversified applications, including environmental remediation, extractions, food processes, and synthesis of materials, has received attention from the scientific community and industry. The mechanistic pathways involved in intensification of ultrasonic processes that include the ultrasonic generation of cavitation bubbles, radical formation upon their collapse, and the possibility of fine-tuning operating parameters for specific applications are all well documented in the literature. However, the scale-up of ultrasonic processes with large-scale sonochemical reactors for industrial applications remains a challenge. In this context, this review provides a complete overview of the current understanding of the role of operating parameters and reactor configuration on the sonochemical processes. Experimental and theoretical techniques to characterize the intensity and distribution of cavitation activity within sonoreactors are compared. Classes of laboratory and large-scale sonoreactors are reviewed, highlighting recent advances in batch and flow-through reactors. Finally, examples of large-scale sonoprocessing applications have been reviewed, discussing the major scale-up and sustainability challenges.

Synergistic effect of sonication on photocatalytic oxidation of pharmaceutical drug carbamazepine

Photocatalytic, sono-photocatalytic oxidation of pharmaceutical drug of carbamazepine was successfully carried out using Ag/AgCl supported BiVO4 catalyst. For this purpose, firstly, photocatalytic oxidation was optimized by central composite design methodology and then synergistic effect of sonication was investigated. Low frequency (20 kHz) probe type and high frequency (850 kHz) plate type sonication at pulse and continuous mode were studied to degrade the carbamazepine (CBZ) containing wastewater. Pulse duties of 1:5 and 5:1 (on : off) were tested using the high frequency sonication system in the sono-photocatalytic oxidation of CBZ. The effects of frequency, power density measured from calorimetry by changing amplitudes were discussed in the sono-photocatalytic oxidation of CBZ. Complete carbamazepine removal was achieved at the optimum conditions of 5 ppm CBZ initial concentration with 1.5 g/L of catalysts loading and at an alkaline pH of 10 at the end of 4 h of photocatalytic reaction under visible LED light irradiation. Both low frequency and high frequency sonication systems caused an increase in photocatalytic efficiency in a shorter treatment time of 60 min. CBZ removal increased from 44% to 65.42% in low frequency sonication of 20 kHz at the amplitude of 20% (0.15 W/mL power density). In the case of high frequency ultrasonic system (850 kHz), CBZ removal increased significantly from 44% to 89.5 % at 75% amplitude (0.12 W/mL power density) within 60 min of reaction. Continuous mode sonication was observed to be more effective than that of pulse mode sonication not only for degradation efficiency and also for electrical energy consumption needed to degrade CBZ. Sono-catalytic oxidation was also conducted with simulated wastewater that contains SO42-, CO32-, NO3-, Cl- anions and natural organic component of fulvic acid. The CBZ degradation was inhibited slightly in the presence of NO3- and Cl-, and fulvic acid, however, the existence of SO42- and CO32- increased the degradation degree of CBZ. Toxicity tests were performed to determine the toxicity of untreated CBZ, and treated CBZ by photocatalytic, and sono-photocatalytic oxidations.

Degradation of amoxicillin with sono, photo, and sonophotocatalytic oxidation under low-frequency ultrasound and visible light

The presence of pharmaceutically active compounds in aquatic bodies is a global concern, and suitable treatment technologies are required. In this study, the efficacy of photocatalytic, sonocatalytic, and sonophotocatalytic oxidation processes for the degradation of amoxicillin (AMX) was investigated using visible light with N doped TiO₂ (N-TiO₂) nanoparticles as the catalyst and low-frequency ultrasound in a novel multifrequency reactor. The influence of different operational parameters on the extent of AMX degradation was studied. Sonophotocatalytic oxidation was found more efficient for AMX degradation when compared to photocatalysis or sonocatalysis alone, and may be due to the reduced bandgap of the catalyst, enhanced cavitation effect due to the presence of the solid catalyst, and improved mass transfer of pollutants. AMX degradation during sono, photo, and sonophotocatalytic oxidation processes was in good agreement with pseudo-first-order kinetics. Empirical kinetic models were also developed using multiple linear regression for predicting the degradation efficiency accounting for the operational parameters. Scavenger experiments suggested that ^•OH radicals largely contributed to AMX degradation, and a plausible mechanism for degradation was proposed. Further, possible degradation pathways for all three treatment processes are also proposed after identifying the degradation products.

Degradation synergism between sonolysis and photocatalysis for organic pollutants with different hydrophobicity: A perspective of mechanism and application for high mineralization efficiency

Despite extensive studies, the fundamental understanding of synergistic mechanisms between sonolysis and photocatalysis for the abatement of persistent organic pollutants (POPs) remains uncertain. As different phases formed under ultrasound irradiation, hydrophilic POPs, sulfamethoxazole (SMX, K_ow: 0.89), predominantly resides in bulk liquid and is ineffectively degraded by sonolysis (k_US = 3.33 × 10^-3 min^-1) since <10% of hydroxyl radicals (·OH) formed at the gas-liquid interface of cavitation is diffused into the bulk, whereas the other fraction rapidly recombines into hydrogen peroxide (H₂O₂). This study provides a proof-of-concept for the mechanism by presenting various analytical results, endorsing the synergistic role of photoexcited electrons in splitting sonolysis-induced H₂O₂ into ·OH, particularly in the bulk phase. In a sonophotocatalytic system, the hydrophobic POPs such as bisphenol A (BPA) and atrazine (ATZ) were mainly degraded in gas-liquid interface indicated by the low synergistic values correlation compared to SMX [i.e., SMX has a higher synergistic factor, f_syn (3.26) than BPA (1.30) and ATZ (1.35)]. Also, f_syn was found linearly correlated with the contribution factor of photocatalysis to split H₂O₂. Three times of consecutive kinetics using an effluent of municipal (MP) wastewater spiked by POPs presented >98% POPs and >96% total organic carbon (TOC) removal.

Preparation of MgTi2O5 nanoparticles for sonophotocatalytic degradation of triphenylmethane dyes

MgTi2O5 (magnesium dititanate) nanoparticles were prepared by a simple hydrothermal assisted post-annealing method and characterized with various analytical techniques. The catalytic properties (sonocatalytic, photocatalytic and sonophotocatalytic activity) were evaluated using the degradation of triphenylmethane dyes (crystal violet, basic fuchsin, and acid fuchsin). The sonophotocatalytic activity of MgTi2O5 nanoparticles towards crystal violet was found to be ~2.9 times higher than the photocatalytic activity and ~20 times higher than that of the sonocatalytic processes. In addition, the sonophotocatalytic efficiency of MgTi2O5 nanoparticles was found to be remarkable for the degradation of basic fuchsin (cationic dye) and acid fuchsin (anionic dye). The mechanism of these catalytic activities has been discussed in detail.

A review on recent advances in hydrogen energy, fuel cell, biofuel and fuel refining via ultrasound process intensification

Hydrogen energy is one of the most suitable green substitutes for harmful fossil fuels and has been investigated widely. This review extensively compiles and compares various methodologies used in the production, storage and usage of hydrogen. Sonochemistry is an emerging synthesis process and intensification technique adapted for the synthesis of novel materials. It manifests acoustic cavitation phenomena caused by ultrasound where higher rates of reactions occur locally. The review discusses the effectiveness of sonochemical routes in developing fuel cell catalysts, fuel refining, biofuel production, chemical processes for hydrogen production and the physical, chemical and electrochemical hydrogen storage techniques. The operational parameters and environmental conditions used during ultrasonication also influence the production rates, which have been elucidated in detail. Hence, this review's major focus addresses sonochemical methods that can contribute to the technical challenges involved in hydrogen usage for energy.

Ultrasound assisted deposition of highly stable self-assembled Bi2MoO6 nanoplates with selective crystal facet engineering as photoanode

The use of crystal facets of photocatalysts is well known as a promising strategy for the design of new photocatalysts with interesting physicochemical features for energy production applications. In this work, Bi₂MoO₆ thin films were synthesized by two methods, electrodeposition and sonoelectrodeposition. Preferential growth orientation depended on synthesis method. Results suggested that sonoelectrodeposition led to dominate the crystal facet {1 0 0} growth with self-assembled nanoplate morphologies while growth orientation in the {0 1 0} facet was dominant in electrodeposition in the absence of ultrasonic waves. As a highlight result, the {1 0 0} facet shows a smaller band gap, higher photocatalytic water splitting than the {0 1 0} facet. Efficient separation of charge pairs and long life time of photogenerated electrons was observed to be intrinsic features of the {1 0 0} facets. The higher charge transfer was confirmed by a higher photocurrent from linear sweep voltammetry and a smaller Nyquist radius arc. Ultrasound plays a key role in growth orientation and led to a production of homogeneous films with nanoplates which self-assembled together to form a flower-like structure. While in the absence of ultrasound the film has coral-like structure. Highly stable sonoelectrodeposited films exhibited incident photon-to-electron conversion efficiency (IPCE) of 22.4% at the specific wavelength of 500 nm. The sonoelectrodeposition method could act as a promising method for forming new films with specific crystal facet selection and developing as highly efficient photoanodes for PEC water splitting.

Ultrasonication construction of the nano-petal NiCoFe-layered double hydroxide: An excellent water oxidation electrocatalyst

Unlike other preparation methods of NiCoFe-layered double hydroxides, the present study provides a facile ultrasound method for synthesis of the nano-petal NiCoFe-layered double hydroxide (LDH) prepared under intensification frequency of 40 kHz and ultrasonic power of 305 W. The effect of time reaction on the morphology of NiCoFe-LDH was investigated using the Field Emission-Scanning Electron Microscopy images. The results show that time reaction can affect the morphology and it also showed that the optimal time for synthesis of nano-petal NiCoFe-LDH was 60 min. Then, the effect of nano-petal NiCoFe-LDH on oxygen evaluation reaction activity was studied and compared with NiCoFe-LDH-c nano paricles. Also, in order to study the effect of Co²⁺ of nano-petal NiCoFe-LDH at water oxidation, the activity of NiFe-LDH synthesized in the same conditions was investigated. The results show that nano-petal NiCoFe-LDH has low onset potential (0.46 V vs. SCE), overpotential (~227 mV) and Tafel slope (234 mV per decade) in comparison with other NiCoFe-LDH nanoparticles (synthesis using co-precipitation method and ultrasonication method within 30 and 120 min), and NiFe-LDH. Based on the obtained results, the nano-petal NiCoFe-LDH can be as a suitable electrocatalyst with good stability for water oxidation reaction in the present 0.1 M KOH media.

Bi2MoO6 nanofilms on the stainless steel mesh by PS-PED method: Photocatalytic degradation of diclofenac sodium as a pharmaceutical pollutant

For the first time, Bi₂MoO₆ nanofilms were successfully synthesized by simultaneous pulse sonication-pulse electrodeposition (PS-PED) on the stainless steel mesh surface. Bismuth molybdate films were formed under various combinations of electrodeposition and sonication (sono-electrodeposition) in continuous and pulse modes. Porous Bi₂MoO₆ films synthesized by PS-PED method and showed the highest efficiency in photocatalytic degradation in comparison with other films. Bi₂MoO₆ film obtained from PS-PED had a thickness of 13.78 nm while, the thickness for the electrodeposition method was 39.52 nm. The high photocatalytic efficiency is attributed to the high surface roughness and low thickness of film synthesized by PS-PED method. Indeed, ultrasound played a key role in the synthesis of films with high surface roughness. On the other hand, shock waves and micro-jets could be dissolved diffusion problems and reduced the dendrite like structures in deposition process. Simultaneous application of pulse modes for both combined methods led to more growth of crystallographic planes. This is due to reaction of ions on the surface in interval relaxation times and produce more nuclei for growth. In order to obtain a high efficiency, response surface methodology was used for optimization of effective variable parameters (t_on, t_off and sonication amplitude) in film preparation.

Ultrasound-promoted synthesis of γ-graphyne for supercapacitor and photoelectrochemical applications

As a novel type of carbon materials, graphynes possesses the merits of high carrier mobility and large surface areas, etc. However, to date, the main research of graphynes is focused on theoretical calculation whereas few strategies have been reported for the fabrication of graphynes. In this work, a facile method named ultrasound-promoted synthesis was developed to fabricate γ-graphyne using PhBr₆ and CaC₂ as the raw materials. The reaction system in argon atmosphere ultrasonically vibrated for 24 h in the ultrasonic bath at a power of 180 W and frequency of 53 kHz. The structure, morphology, and component of the obtained samples were detected by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, FT-IR spectra, scanning electron microscopy, transmission electron microscopy, and the selected area electron diffraction. The electrochemical and photoelectrochemical measurements indicate that γ-graphyne can be used as superior electrode mateirals in supercapacitor and photoelectrochemical catalysis. From the results of galvanostatic charge/discharge measurements, the γ-graphyne supercapacitor delivers a maximum specific capacitance of 81 F/g at 0.2 A/g and a capacitance retention rate of 87.5% after 5000 cycles at 3 A/g. Moreover, UV-vis light photoelectrochemical response and high carrier density are observed for γ-graphyne. It is also demonstrated that the charge-transfer resistance is low by Tafel slopes and Nyquist plots. This work puts forward a new and facile strategy for the fabrication of γ-graphyne and explores its application in the field of solar energy conversion and storage.

Sono-electrodeposition of novel bismuth sulfide films on the stainless steel mesh: Photocatalytic reduction of Cr (VI)

In this work, for the first time, bismuth sulfide (Bi₂S₃) film formed on the stainless steel mesh surface as a suitable substrate. Different films were synthesized by various combinations of the two methods (sonochemistry and electrochemistry) in continuous and pulse modes. The Bi₂S₃ films characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Atomic Force Microscope (AFM). To reach the best film for photocatalytic reduction of Cr (VI), different films were deposited on the substrate by changing three independent variables including pulse times (t_on, t_off) and sonication amplitude. Response Surface Methodology (RSM) applied for optimization of independent variables by using Central Composite Design (CCD). Here, the films prepared by sono-electrodeposition in pulse modes led to high photocatalytic efficiency in comparison with other films. The results confirmed that ultrasound affected the morphology of film due to the production of cavitation, micro jets and acoustic streaming. On the other hand, ultrasound decreased double layer thickness and dissolved diffusion problems. Sono-electrodeposition in pulse modes produced films with pumice and fractal like structures. The high photocatalytic activity attributed to special morphologies that have key roles in separation of hole/electron pairs and light multi-scattering.

Novel method of square wave voltammetry for deposition of Bi2S3 thin film: Photocatalytic reduction of hexavalent Cr in single and binary mixtures

In this work a new method of square‒wave voltammetry was performed for electrodepositing of Bi₂S₃ film on the stainless steel mesh surface as low-cost and effective substrate in visible light harvesting. First, the obtained porous film was characterized by X‒ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), and Raman analysis. In following, photocatalytic activity of film for reduction of hexavalent Cr was evaluated under sunlight irradiation. Results suggested that the presence of voids having several nanometers to 2 μm diameters on the film surface plays key role in photocatalytic processes. Indeed, surface voids as trapping cites and producing routs leads to multiple scattering of visible light. It can be noted that morphology of thin film, substrate structure, and diameters of voids are the main parameters to achieving the high reduction efficiency. Cr(VI) was completely reduced within 100  min under sunlight irradiation. Photoreduction mechanistic studies induced by Bi₂S₃ film suggested that active oxygen species such as HOO^• and O₂ have significant role in photocatalytic reaction. Finally, the evaluation of simultaneous photocatalytic process of binary mixture (Cr(VI) and Rhodamin B) was shown synergistic improvement of Cr(VI) and Rhodamin B degradation due to rapid surface reactions.

Effect of pH in the Hydrothermal Preparation of Bi2WO6 Nanostructures

In this study, Bi2WO6 was prepared by the hydrothermal method. The effects of reaction temperature (150/170/200 °C) and reaction time (6/12/24 h) were investigated. The role of strongly acidic pH (1 >) and the full range between 0.3 and 13.5 were studied first. Every sample was studied by XRD and SEM; furthermore, the Bi2WO6 samples prepared at different temperatures were examined in detail by EDX and TEM, as well as FT-IR, Raman and UV-vis spectroscopies. It was found that changing the temperature and time slightly influenced the crystallinity and morphology of the products. The most crystallized product formed at 200 °C, 24 h. The pure, sheet-like Bi2WO6, prepared at 200 °C, 24 h, and 0.3 pH, gradually transformed into a mixture of Bi2WO6 and Bi3.84W0.16O6.24 with increasing pH. The nanosheets turned into a morphology of mixed shapes in the acidic range (fibers, sheets, irregular forms), and became homogenous cube- and octahedral-like shapes in the alkaline range. Their band gaps were calculated and were found to vary between 2.66 and 2.59 eV as the temperature increased. The specific surface area measurements revealed that reducing the temperature favors the formation of a larger surface area (35.8/26/21.6 m2/g belonging to 150/170/200 °C, respectively).