Synthesis of Ag-ZnO with multiple rods (multipods) morphology and its application in the simultaneous photo-catalytic degradation of methyl orange and methylene blue

Theoretical and experimental study of the photodegradation of methyl orange in the presence of different morphologies of Au-ZnO using Monte Carlo dynamic simulation

Herein, a simple approach was formed based on synthesizing different morphologies of ZnO and Au-ZnO as photocatalyst. In this study, ZnO and Au-ZnO were synthesized via a co-precipitation method and fully characterized via scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). Three different ratios of Zn2+:OH- (1:2, 1:3, and 1:5) controlled the morphology of samples, which were made into spindle, star, and flower structures, respectively. Then, the photocatalytic activity was studied and compared. Their comparison showed that the flower morphology for ZnO and Au-ZnO was more effective in photocatalytic degradation and decolorization of methyl orange dye. Also, quantum and Monte Carlo (MC) calculations were carried out to investigate the adsorption of methyl orange (MO) molecules on ZnO(111) surface in the presence of Au or without Au in aqueous conditions by Monte Carlo adsorption locator simulations in the Materials Studio 2017 software. Au created a tendency to form a relatively strong interaction of MO with the ZnO(111) surface. The adsorption of MO on Au-ZnO(111) in the presence of Au was more significant than that of MO on ZnO(111), suggesting Au could significantly improve the reactivity of the methyl orange toward the ZnO(111).

Synthesis of Vanadium Doped Lanthanum Bismuthate Nanorods for Enhanced Photocatalytic Activity

Vanadium doped lanthanum bismuthate nanorods with vanadium ratio of 1%, 3%, 5% and 10 wt.% were fabricated through the hydrothermal method using sodium orthovanadate as vanadium source. Vanadium doped lanthanum bismuthate nanorod products were analyzed by scanning electron microscopy, X-ray diffraction pattern and diffuse reflection spectrum. X-ray diffraction patterns show that vanadium in the vanadium doped lanthanum bismuthate nanorods exists as triclinic Bi₂₃V₄O_44.5 and monoclinic LaVO₄ phases. Scanning electron microscopy observations show that the size and micro-morphology of the vanadium doped products are closely relative to the vanadium mass ratio. The length of the vanadium doped nanorods decreases and the morphology changes from nanorods to irregular nanoparticles with increasing the vanadium mass ratio. Solid UV-vis diffuse reflectance measurement shows that the bandgap value of the doped lanthanum bismuthate nanorods is narrowed from 2.37 eV to 2.25 eV after the vanadium doping ratio is increased from 1% to 10%. The doped lanthanum bismuthate nanorods illustrate enhanced photocatalytic performance for methylene orange (MO) removal with the irradiation of sunlight. The catalytic performance for MO removal depends on the irradiation time, vanadium content and dosage of the nanorods. The doped lanthanum bismuthate nanorods with the vanadium mass ratio of 10% possess the best MO catalytic degradation performance.

Excellent UV-Light Triggered Photocatalytic Performance of ZnO.SiO2 Nanocomposite for Water Pollutant Compound Methyl Orange Dye

The photocatalytic activity of eco-friendly zinc oxide doped silica nanocomposites, synthesized via a co-precipitation method followed by heat-treatment at 300, 600, and 900 °C is investigated. The samples have been characterized by employing X-ray diffraction method, and further analyzed using the Rietveld Refinement method. The samples show a space group P63mc with hexagonal structure. The prepared composites are tested for their photocatalytic activities for the degradation of methyl orange-based water pollutants under ultra-violet (UV) irradiation using a 125 W mercury lamp. A systematic analysis of parameters such as the irradiation time, pH value, annealing temperatures, and the concentration of sodium hydroxide impacting the degradation of the methyl orange (MO) is carried out using UV-visible spectroscopy. The ZnO.SiO2 nanocomposite annealed at 300 °C at a pH value of seven shows a maximum photo-degradation ability (~98.1%) towards methyl orange, while the photo-degradation ability of ZnO.SiO2 nanocomposites decreases with annealing temperature (i.e., for 600 and 900 °C) due to the aspect ratio. Moreover, it is seen that with increment in the concentration of the NaOH (i.e., from 1 to 3 g), the photo-degradation of the dye component is enhanced from 20.9 to 53.8%, whereas a reverse trend of degradation ability is observed for higher concentrations.

One-Step Microwave-Assisted Synthesis and Visible-Light Photocatalytic Activity Enhancement of BiOBr/RGO Nanocomposites for Degradation of Methylene Blue

In this study, bismuth oxybromide/reduced graphene oxide (BiOBr/RGO), i.e. BiOBr-G nanocomposites, were synthesized using a one-step microwave-assisted method. The structure of the synthesized nanocomposites was characterized using Raman spectroscopy, X-ray diffractometry (XRD), photoluminescence (PL) emission spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and ultraviolet-visible diffuse reflection spectroscopy (DRS). In addition, the ability of the nanocomposite to degrade methylene blue (MB) under visible light irradiation was investigated. The synthesized nanocomposite achieved an MB degradation rate of above 96% within 75 min of continuous visible light irradiation. In addition, the synthesized BiOBr-G nanocomposite exhibited significantly enhanced photocatalytic activity for the degradation of MB. Furthermore, the results revealed that the separation of the photogenerated electron-hole pairs in the BiOBr-G nanocomposite enhanced the ability of the nanocomposite to absorb visible light, thus improving the photocatalytic properties of the nanocomposites. Lastly, the MB photo-degradation mechanism of BiOBr-G was investigated, and the results revealed that the BiOBr-G nanocomposites exhibited good photocatalytic activity.

Efficient photocatalytic degradation of dyes using photo-deposited Ag nanoparticles on ZnO structures: simple morphological control of ZnO

In this work, morphology-controlled ZnO structures were prepared via a hydrothermal method by simple adjustments in the NaOH concentration. The NaOH concentration variation from 0.2 to 1.2 M resulted in the formation of ZnO structures in shapes such as walnut, spherical flower, flower, rod, and urchin-like. The extent of OH- ions is the main factor influencing the growth of ZnO structures. Well-defined morphologies, good crystallinity, and optical properties were obtained for all ZnO structures. Among these ZnO structures, ZnOsf (spherical flower-like) structure showed a greater percentage of photodegradation of methyl orange and rhodamine B dyes. Surface plasmon resonance was achieved by modifying the surface of ZnO with Ag nanoparticles. ZnOsf was loaded with Ag nanoparticles by a facile photo-deposition method. Ag-ZnOsf showed superior photoactivity and recyclability for the degradation of methyl orange and rhodamine B. Therefore, modification of different ZnO structures can help realize potential catalysts for future environmental applications.

Starch, cellulose, pectin, gum, alginate, chitin and chitosan derived (nano)materials for sustainable water treatment: A review

Natural biopolymers, polymeric organic molecules produced by living organisms and/or renewable resources, are considered greener, sustainable, and eco-friendly materials. Natural polysaccharides comprising cellulose, chitin/chitosan, starch, gum, alginate, and pectin are sustainable materials owing to their outstanding structural features, abundant availability, and nontoxicity, ease of modification, biocompatibility, and promissing potentials. Plentiful polysaccharides have been utilized for making assorted (nano)catalysts in recent years; fabrication of polysaccharides-supported metal/metal oxide (nano)materials is one of the effective strategies in nanotechnology. Water is one of the world's foremost environmental stress concerns. Nanomaterial-adorned polysaccharides-based entities have functioned as novel and more efficient (nano)catalysts or sorbents in eliminating an array of aqueous pollutants and contaminants, including ionic metals and organic/inorganic pollutants from wastewater. This review encompasses recent advancements, trends and challenges for natural biopolymers assembled from renewable resources for exploitation in the production of starch, cellulose, pectin, gum, alginate, chitin and chitosan-derived (nano)materials.

Comparison of Ag and AgI-Modified ZnO as Heterogeneous Photocatalysts for Simulated Sunlight Driven Photodegradation of Metronidazole

Ag and AgI-modified ZnO composites (Ag/ZnO and AgI/ZnO) were synthesized in facile ways. The photocatalysts were used for the photodegradation of metronidazole (MNZ) under the irradiation of simulated sunlight. The results of experiments showed that both Ag/ZnO and AgI/ZnO had a specific molar ratio to reach the best performance. Ag/ZnO performed better in the photodegradation of MNZ than AgI/ZnO under the same conditions. The reaction rate constant of AgI/ZnO was less affected by the variation of initial concentration of MNZ or pH values. The main reactive oxygen species of the photocatalytic process are OH, O2− and h+, but the free radicals which play the most critical part differ in Ag/ZnO and AgI/ZnO. Several intermediates were revealed by LC–MS/MS analysis. The stability of the photocatalysts was evaluated by a series of repeated MNZ photodegradation experiments. The results showed that AgI/ZnO had better stability than Ag/ZnO.

Laser-assisted preparation of Pd nanoparticles on carbon cloth for the degradation of environmental pollutants in aqueous medium

Laser ablation in liquid (LAL), one of the attractive methods for fabrication of nanoparticles, was used for the modification of carbon cloth (CC) by deposition of palladium nanoparticles (Pd NPs); a simple stirring method was deployed to deposit Pd NPs on the CC surface. Characterization techniques viz X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscope (SEM), energy-dispersive X-ray spectrometry (EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were applied to study the surface of the ensuing samples which confirmed that LAL technique managed to fabricate and deposit the Pd NPs on the surface of CC. In addition, the catalytic prowess of the carbon cloth-Pd NPs (CC/Pd NPs) was investigated in the NaBH₄- or HCOOH-assisted reduction of assorted environmental pollutants in aqueous medium namely hexavalent chromium [Cr(VI)], 4-nitrophenol (4-NP), congo red (CR) and methylene blue (MB). The CC/Pd NPs system has advantages such as high stability/sustainability, high catalytic performance and easy reusability.

A hierarchical Ca/TiO2/NH2-MIL-125 nanocomposite photocatalyst for solar visible light induced photodegradation of organic dye pollutants in water

In this study, for the first time, the Ca/TiO₂/NH₂-MIL-125 nanocomposite photocatalyst was synthesized for the purpose of photodegradation of Methyl Orange (MO) and Rhodamine B (RhB) dyes under visible light irradiation. The structural and chemical properties of the nanocomposite photocatalyst were characterized through FTIR, XRD, TGA, PL, XPS, ICP-OES and UV-DRS. For the photodegradation efficiency analysis, the effect of pH (3, 5, 7, 9, and 11), photocatalyst dosage (0.1, 0.2, 0.4, 0.6, and 0.8 g L⁻¹), dye concentration (1–40 mg L⁻¹), and contact time (10–120 min) was precisely evaluated. The largest photodegradation efficiency for RhB and MO dye models was 82.87% and 86.22%, respectively, that was obtained under optimal conditions in terms of pH and photocatalyst dosage and for Ca(30%)/TiO₂/NH₂-MIL-125. The photodegradation process of the dyes complied well with the first-order kinetic model. Moreover, the nanocomposite photocatalyst showed consistent photodegradation efficiency and after 6 successive cycles with fresh dye solutions, it could still perform comparably well. Taken together, Ca/TiO₂/NH₂-MIL-125 photocatalyst is able to show a high photodegradation efficiency for dye pollutants and optimum stability and reusability.

In this study, for the first time, the Ca/TiO₂/NH₂-MIL-125 nanocomposite photocatalyst was synthesized for the purpose of photodegradation of Methyl Orange (MO) and Rhodamine B (RhB) dyes under visible light irradiation.

Rate-Limiting Steps of Dye Degradation over Titania-Silica Core-Shell Photocatalysts

In this work, we employed a step-by-step sol-gel process to controllably deposit ultra-thin layers of SiO2 on anatase nanoparticles in the range between 0 and 1 nm. The deposition was confirmed by TEM, EDX, and ATR-FTIR (e.g., Ti-O-Si band at 960 cm-1). Zeta potential measurements unravelled a continuous change in surface charge density with increasing silica shell thickness. The photocatalysts were evaluated towards adsorption and degradation of positively-charged and negatively-charged dyes (methylene blue, methyl orange) under UV illumination. The growth mechanism follows the Stranski–Krastanov model with three thickness regimes: (a) Flat islands (first step), (b) mono/bilayers (second/third step), and (c) regular thick films (fourth/fifth step). The results suggest different rate limiting processes for these regimes: (a) For the thinnest scenario, acidic triple-phase boundaries (TPBs) increase the activity for both dyes with their accessibility being the rate limiting step; (b) for continuous mono/bilayers, dye adsorption on the negatively-charged SiO2 shells becomes the rate liming step, which leads to a stark increase in activity for the positively-charged MB and a decrease for MO; (c) for thicker shells, the activity decreases for both dyes and is limited by the charge transport through the isolating shells.

Evaluation of MnO2-templated iron oxide-coated diatomites for their catalytic performance in heterogeneous photo Fenton-like system

Herein, iron oxide-coated diatomites were prepared through hydrothermal synthesis and sacrificial template redox etching reaction. The microstructure characterization results revealed that the Fe₂O₃ nanorods were uniformly distributed on the surface of diatomite. The effects of diverse synthetic parameters on morphology of as-synthesized Fe₂O₃@diatomite were investigated. When the reaction time was prolonged from 12h to 24h, the morphology of FeOOH would change from nanorods into nanoflowers. Two different crystal phases of Fe₂O₃@diatomite were obtained from the FeOOH@diatomite samples calcined under different atmospheres. The catalytic activity of α-Fe₂O₃@diatomite was evaluated by the heterogeneous photo Fenton-like system through degradation of methylene blue (MB, 10 - 40mgL^-1) in the presence of hydrogen peroxide (H₂O₂, 9mM - 120mM) under UV light irradiation. It was found that α-Fe₂O₃@diatomite composites showed very excellent degradation efficiency, which was about 99% within 120min under UV irradiation. This catalyst extended the range of pH values of homogeneous Fenton reaction, in which the MB removal rate was maintained over 80.8%. Moreover, the α-Fe₂O₃@diatomite catalyst still showed sound reusability after 5 rounds of degradation of MB dye. In principles, a possible photo-catalytic mechanism was proposed to testify metal oxides composites for heterogeneous photo Fenton-like reaction.

Modulation of valence band maximum edge and photocatalytic activity of BiOX by incorporation of halides

To better know the photocatalytic performance of bismuth oxyhalides (BiOX, X = Cl, Br, I) regulated by incorporation of halides within nanostructures, BiOX nanosheets were synthesized through morphology controllable solvothermal method and characterized systematically. The organic structural property greatly influences the photocatalytic activity of BiOX: 1) as for neutral molecular phenol, BiOX shows photocatalytic activity in the order of BiOCl > BiOBr > BiOI under simulated sun light irradiation, and the photo-oxidation kinetics follow Eley-Rideal mechanism; and 2) for adsorbed anionic orange II (OII) and cationic methylene blue (MB), BiOX shows photocatalytic activity in the order of BiOCl > BiOBr > BiOI, and the photo-oxidation kinetics follow Langmuir-Hinshelwood mechanism. The crystal structure of the catalyst also greatly influences the photocatalytic activity of BiOX: 1) The relative photo-oxidation power of O₂^•- radicals or HO radicals involved in this study were different which were quantitatively detected using typical radical trapping agent, separately; 2) The relative oxidation power of photogenerated holes (h⁺) in this study were in the order of BiOCl > BiOBr > BiOI, which may be ascribed to lowering the valence band maximum edge of BiOX through incorporation of halides as the atomic number of halides decreased. This study provides novel explanation for fabricating BiOX heterojunctions with tunable photocatalytic reactivity via regulating the halides ratio.

Low temperature one-pot synthesis of Cu-doped ZnO/Al2O3 composite by a facile rout for rapid methyl orange degradation

ZnO/Al₂O₃ composite modified by 2.5-10wt% Cu were prepared using a set of wet chemical techniques (solvothermal, co-precipitation, ultrasonic and reflux) and characterized. All processes were done under the atmospheric condition and without any calcination step. The product was characterized by PXRD, FE-SEM, FT-IR and UV-Vis spectroscopy. The microscopy images clearly showed lamellar particles morphology in nano regime. X-ray diffraction studies revealed the wurtzite-type ZnO and γ-Al₂O₃ crystallinity. The Band gap varied from 3.20eV for the un-doped Al₂O₃/ZnO sample to 2.18eV for the Al₂O₃/ZnO:Cu with Cu content of 10%. Formed Al₂O₃/ZnO:Cu composite exhibited a supreme photocatalytic performance upon the degradation of methyl orange (MO) under visible-light irradiation. The optimum catalyst indicated completely remove of MO dye with the 15mgL^-1 concentration from the water after 50min photocatalytic reaction time, where for the optimum sample the removal efficiency was 77% after only 10min reaction. Results showed that the inserting of Cu in composite improves the photocatalytic activity of ZnO under visible light. The degradation rate of MO for optimum Cu-doped sample was almost 20 times the un-doped sample.

The role of graphene oxide and graphene oxide-based nanomaterials in the removal of pharmaceuticals from aqueous media: a review

In this review paper, the ill effects of pharmaceuticals (PhAs) on the environment and their adsorption on graphene oxide (GO) and graphene oxide-based (GO-based) nanomaterials have been summarised and discussed. The adsorption of prominent PhAs discussed herein includes beta-blockers (atenolol and propranolol), antibiotics (tetracycline, ciprofloxacin and sulfamethoxazole), pharmaceutically active compounds (carbamazepine) and analgesics such as diclofenac. The adsorption of PhAs strictly depends upon the experimental conditions such as pH, adsorbent and adsorbate concentrations, temperature, ionic strength, etc. To understand the adsorption mechanism and feasibility of the adsorption process, the adsorption isotherms, thermodynamics and kinetic studies were also considered. Except for some cases, GO and its derivatives show excellent adsorption capacities for PhAs, which is crucial for their applications in the environmental pollution cleanup.

Rapid degradation of dyes in water by magnetic Fe(0)/Fe3O4/graphene composites

Magnetic Fe(0)/Fe3O4/graphene has been successfully synthesized by a one-step reduction method and investigated in rapid degradation of dyes in this work. The material was characterized by N2 sorption-desorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), vibrating-sample magnetometer (VSM) measurements and X-ray photoelectron spectroscopy (XPS). The results indicated that Fe(0)/Fe3O4/graphene had a layered structure with Fe crystals highly dispersed in the interlayers of graphene, which could enhance the mass transfer process between Fe(0)/Fe3O4/graphene and pollutants. Fe(0)/Fe3O4/graphene exhibited ferromagnetism and could be easily separated and re-dispersed for reuse in water. Typical dyes, such as Methyl Orange, Methylene Blue and Crystal Violet, could be decolorized by Fe(0)/Fe3O4/graphene rapidly. After 20min, the decolorization efficiencies of methyl orange, methylene blue and crystal violet were 94.78%, 91.60% and 89.07%, respectively. The reaction mechanism of Fe(0)/Fe3O4/graphene with dyes mainly included adsorption and enhanced reduction by the composite. Thus, Fe(0)/Fe3O4/graphene prepared by the one-step reduction method has excellent performance in removal of dyes in water.

Review on ZnO hybrid photocatalyst: impact on photocatalytic activities of water pollutant degradation

Zinc oxide (ZnO) is one of the most widely used benchmark standard photocatalysts in the field of environmental applications. However, the large band gap of ZnO and the massive recombination of photogenerated charge carriers, especially in its nanosize, limit the overall photocatalytic efficiency. This can be further overcome by modifying the electronic band structure of ZnO by hybridization with a narrow band gap material, including metal, metal oxide, carbon based, and polymeric based. Indeed, ZnO hybridization with the respective materials contributed to its sensitizer by shifting the absorption wavelength to the visible region of the spectrum. This review encompasses several advancements made in the mentioned aspects, and also some of the new physical insights related to the charge transfer events, such as charge carrier generation, trapping, detrapping, and their transfer to surface, are discussed for each strategy of the hybrid ZnO. The synergistic effects in the mixed polymorphs of ZnO and also the theories proposed for their enhanced activity are reported. The review also highlights the potential application of ZnO hybrid for different kinds of pollutants from different wastewater sources.

Continuous synthesis of hedgehog-like Ag–ZnO nanoparticles in a two-stage microfluidic system

Hedgehog-like Ag–ZnO nanoparticles were successfully prepared in a continuous microfluidic system and showed a superior photocatalytic activity in the degradation of methyl orange.

Heterogeneous electro-Fenton oxidation of azo dye methyl orange catalyzed by magnetic Fe3O4 nanoparticles

Azo dye methyl orange (MO) degradation by heterogeneous electro-Fenton (EF) with a magnetic nano-Fe3O4 catalyst was investigated. In this study, Fe3O4 was synthesized by a coprecipitation method and characterized by X-ray diffraction and scanning electron microscopy. The influences of the main operating parameters such as current density, pH, catalyst dosage and aeration rate were studied. The results revealed that higher current density, catalyst dosage and aeration rate facilitated the degradation of MO, whereas the degradation efficiency of MO was decreased with an increase in the initial pH. After 90 min EF process, the volume of 500 mL, the initial concentration of 50 mg L(-1) MO solution could be degraded by 86.6% with the addition of 1 g L(-1) Fe3O4 under the current density of 10 mA cm(-2) and pH 3, compared with 69.5% for the electrolytic process alone. Meanwhile, a total organic carbon removal of 32% was obtained, up to 35.5 mg L(-1) accumulated H2O2 and less than 3.5 mg L(-1) Fe leaching were detected. Moreover, based on the results of natural bond orbital (at B3LYP/6-311G (d, p)) and liquid chromatograph-triple quadrupole mass spectrometer analysis, nine intermediates were identified and the proposed degradation pathways were investigated.