ZnO and Ni-doped ZnO photocatalysts: Synthesis, characterization and improved visible light driven photocatalytic degradation of methylene blue

Green-route synthesis and ab-initio studies of a highly efficient nano photocatalyst:Ce/zinc-oxide nanopetals

In the present work, Zinc-oxide nanostructures and Ce/Zinc-oxide nanopetals were synthesized by a new environmentally friendly green synthesis method using the Withania coagulans plant. Cerium nitrate Ce(NO3)3 and zinc nitrate Zn(NO3)2 were used as precursors. The prepared nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet spectroscopy (UV-vis). Crystal planes (100), (002), (101), (102), (110), (103), (200), (112) and (201) at 2θ 31.75°, 34.35°, 36.2°, 47.55°, 56.6°, 62.75°, 66.3°, 67.9°, and 69.09° respectively confirmed the hexagonal wurtzite crystal structure of Zinc-oxide. Angular shifts for Ce1% doped Zinc-oxide and Ce3% doped Zinc-oxide nanopetal nanostructures were observed in the (100) and (101) planes of the crystal. More specifically, using Scherrer's equation, the crystallite sizes of Zinc-oxide, Ce1% doped Zinc-oxide nanopetals, Ce3% doped Zinc-oxide nanopetals, and Ce5% doped Zinc-oxide nanopetals were 16.48 ± 02 nm, 17.8 ± 2 nm, 18.8 ± 2 nm, and 18.87 ± 2 nm, respectively. The pure Zinc-oxide grain had the appearance of a nanoflower. On the other hand, the nanopetal structure of Ce5% doped Zinc-oxide nanopetals had oval-shaped nanopetal morphology. The absorption peaks were observed at 373, 376.4, 377, and 378 nm for Zinc-oxide, Ce1% doped Zinc-oxide nanopetals, Ce3% doped Zinc-oxide nanopetals, and Ce5% doped Zinc-oxide nanopetals, respectively, which results in a progressive redshift. The gap energies of Zinc-oxide, Ce1% doped Zinc-oxide nanopetals, Ce3% doped Zinc-oxide nanopetals, and Ce5% doped Zinc-oxide nanopetals were 2.796, 2.645, 2.534, and 2.448 eV, respectively. Photodegradation under visible light (>400 nm) indicates the high efficiency of the photocatalyst based on Ce5% doped Zinc-oxide nanopetals. DFT calculations, structural changes, charge analysis, and electronic band structures were carried out to confirm the experiment.

Enhancing the efficiency of magnetically driven carbon nitride-based nanocomposites with magnetic nanoflowers for the removal of methylene blue dye at neutral pH

The present study focuses on the elaboration of magnetic nanocomposites by the in situ incorporation of magnetite (Fe3O4) nanoparticles (NPs) with spherical and nanoflower-like morphologies in graphitic carbon nitride (g-C3N4) sheets using two different synthetic routes. Nanomaterials are characterized by TEM, SEM, XRD, FTIR, BET, zetametry, vibrating sample magnetometry, and UV-vis absorption spectroscopy. The decoration of the carbon nitride matrix with the magnetic NPs enhanced optical and textural properties. The influence of the morphology of the magnetic NPs on the adsorptive and photocatalytic properties of the nanocomposites under different pH conditions (4.5, 6.9, and 10.6) was assessed from batch tests to remove methylene blue (MB) from aqueous solutions. In extreme pH conditions, the nanocomposites exhibited lower or equivalent MB removal capacity compared to the pure g-C3N4. However, at neutral medium, the nanocomposite with incorporated Fe3O4 nanoflowers showed a significantly higher removal efficiency (80.7%) due to the combination of a high adsorption capacity and a good photocatalytic activity in this pH region. The proposed nanocomposite is a promising alternative to remove cationic dyes from water by magnetic assistance, since no pH adjustment of the polluted effluent is required, reducing costs and environmental impact in the dyeing industry.

Facile Synthesis of Ni-Doped ZnO Nanoparticles Using Cashew Gum: Investigation of the Structural, Optical, and Photocatalytic Properties

This work adopted a green synthesis route using cashew tree gum as a mediating agent to obtain Ni-doped ZnO nanoparticles through the sol-gel method. Structural analysis confirmed the formation of the hexagonal wurtzite phase and distortions in the crystal lattice due to the inclusion of Ni cations, which increased the average crystallite size from 61.9 nm to 81.6 nm. These distortions resulted in the growth of point defects in the structure, which influenced the samples' optical properties, causing slight reductions in the band gaps and significant increases in the Urbach energy. The fitting of the photoluminescence spectra confirmed an increase in the concentration of zinc vacancy defects (VZn) and monovacancies (Vo) as Zn cations were replaced by Ni cations in the ZnO structure. The percentage of VZn defects for the pure compound was 11%, increasing to 40% and 47% for the samples doped with 1% and 3% of Ni cations, respectively. In contrast, the highest percentage of VO defects is recorded for the material with the lowest Ni ions concentration, comprising about 60%. The influence of dopant concentration was also reflected in the photocatalytic performance. Among the samples tested, the Zn0.99Ni0.01O compound presented the best result in MB degradation, reaching an efficiency of 98.4%. Thus, the recovered material underwent reuse tests, revealing an efficiency of 98.2% in dye degradation, confirming the stability of the photocatalyst. Furthermore, the use of different inhibitors indicated that •OH radicals are the main ones involved in removing the pollutant. This work is valuable because it presents an ecological synthesis using cashew gum, a natural polysaccharide that has been little explored in the literature.

Degradation of enrofloxacin by a novel Fe-N-C@ZnO material in freshwater and seawater: Performance and mechanism

In this study, we investigated the doping of Fe-N-C with ZnO (Fe-N-C@ZnO) to enhance its performance in the reduction of biological toxicity and degradation of enrofloxacin (ENR) in seawater. The steady-state/transient fluorescence analysis and free radical quenching test indicated an extremely low electron-hole recombination rate and the generation of reactive oxygen species in Fe-N-C@ZnO, leading to an improvement in the energy efficiency. We compared the ENR degradation efficiencies of Fe-N-C@ZnO and ZnO using both freshwater and seawater. In freshwater, Fe-N-C@ZnO exhibited a slightly higher degradation efficiency (95.00%) than ZnO (90.30%). However, the performance of Fe-N-C@ZnO was significantly improved in seawater compared to that of ZnO. The ENR degradation efficiency of Fe-N-C@ZnO (58.87%) in seawater was 68.39% higher than that of ZnO (34.96%). Furthermore, the reaction rate constant for ENR degradation by Fe-N-C@ZnO in seawater (7.31 × 10-3 min-1) was more than twice that of ZnO (3.58 × 10-3 min-1). Response surface analysis showed that the optimal reaction conditions were a pH of 7.42, a photocatalyst amount of 1.26 g L-1, and an initial ENR concentration of 6.56 mg L-1. Fe-N-C@ZnO prepared at a hydrothermal temperature of 128 °C and heating temperature of 300 °C exhibited the optimal performance for the photocatalytic degradation of ENR. Based on liquid chromatography-mass spectrometry analysis, the degradation processes of ENR were proposed as three pathways: two piperazine routes and one quinolone route.

Fabrication of a P-Si/ZnO heterojunction based on galvanic cell driven and the complete degradation of RhB via fast charge transfer

Semiconductor heterojunctions can significantly enhance photocatalytic degradation efficiency by facilitating rapid interfacial charge transfer. This article is based on the galvanic-cell driven principle; porous silicon (P-Si) was prepared by the carbon-catalytic etching method, and ZnO was loaded on its surface via electroless chemical deposition technology to form a P-Si/ZnO heterojunction, which was applied to the degradation of Rhodamine B (RhB). At a deposition temperature of 90 °C, a flawless 1D hexagonal prism structure of ZnO was formed, allowing the P-Si/ZnO heterojunction to completely degrade RhB within 2 hours with a degradation rate of 100%. Compared with a single P-Si material, the degradation performance is improved by 1.7 times. The formation of the built-in electric field and the rapid charge transfer at the heterojunction interface realized the complete degradation of RhB organic pollutants. After 20 cycles of use, the photocatalytic degradation rate remains above 70%, demonstrating excellent stability and recyclability.

Theoretical and Experimental Study of the Photocatalytic Properties of ZnO Semiconductor Nanoparticles Synthesized by Prosopis laevigata

In this work, the photocatalytic activity of nanoparticles (NPs) of zinc oxide synthetized by Prosopis laevigata as a stabilizing agent was evaluated in the degradation of methylene blue (MB) dye under UV radiation. The theoretical study of the photocatalytic degradation process was carried out by a Langmuir-Hinshelwood-Hougen-Watson (LHHW) model. Zinc oxide nanoparticles were synthesized by varying the concentration of natural extract of Prosopis laevigata from 1, 2, and 4% (weight/volume), identifying the samples as ZnO_PL1%, ZnO_PL2%, and ZnO_PL4%, respectively. The characterization of the nanoparticles was carried out by Fourier transform infrared spectroscopy (FT-IR), where the absorption band for the Zn-O vibration at 400 cm-1 was presented; by ultraviolet-visible spectroscopy (UV-vis) the value of the band gap was calculated, resulting in 2.80, 2.74 and 2.63 eV for the samples ZnO_PL1%, ZnO_PL2%, and ZnO_PL4%, respectively; XRD analysis indicated that the nanoparticles have a hexagonal zincite crystal structure with an average crystal size of 55, 50, and 49 in the sample ZnO_PL1%, ZnO_PL2%, and ZnO_PL4%, respectively. The morphology observed by TEM showed that the nanoparticles had a hemispherical shape, and the ZnO_PL4% sample presented sizes ranging between 29 and 45 nm. The photocatalytic study showed a total degradation of the MB in 150, 120, and 60 min for the samples ZnO_PL1%, ZnO_PL2%, and ZnO_PL4%, respectively. Also, the model explains the experimental observation of the first-order kinetic model in the limit of low concentrations of dye, indicating the influence of the mass transfer processes.

Treatment of diluted palm oil mill effluent (POME) synchronous with electricity production in a persulfate oxidant-promoted photocatalytic fuel cell

Attributable to the prosperous production growth of palm oil in Malaysia, the generated palm oil mill effluent (POME) poses a high threat owing to its highly polluted characteristic. Urged by the escalating concern of environmental conservation, POME pollution abatement and potential energy recovery from the effluent are flagged up as a research topic of interest. In this study, a cutting-edge photocatalytic fuel cell (PFC) system with employment of ZnO/Zn nanorod array (NRA) photoanode, CuO/Cu cathode, and persulfate (PS) oxidant was successfully designed to improve the treatment of POME and simultaneous energy production. The photoelectrodes were fabricated and characterized by field emission scanning electron microscopy with energy (FESEM), X-ray diffraction (XRD), energy-dispersive X-ray (EDX), and Brunauer, Emmett, and Teller analysis (BET). Owing to the properties of strong oxidant of PS, the proposed PFC/PS system has exhibited exceptional performance, attaining chemical oxygen demand (COD) removal efficiency of 96.2%, open circuit voltage (Voc) of 740.0 mV, short circuit current density (Jsc) of 146.7 μA cm-2, and power density (Pmax) of 35.6 μW cm-2. The pre-eminent PFC/PS system performance was yielded under optimal conditions of 2.5 mM of persulfate oxidant, POME dilution factor of 1:20, and natural solution pH of 8.51. Subsequently, the postulated photoelectrocatalytic POME treatment mechanism was elucidated by the radical scavenging study and Mott-Schottky (M-S) analysis. The following recycling test affirmed the stability and durability of the photoanode after four continuous repetition usages while the assessed electrical energy efficiency revealed the economic viability of PFC system serving as a post-treatment for abatement of POME. These findings contributed toward enhancing the sustainability criteria and economic viability of palm oil by adopting sustainable and efficient POME post-treatment technology.

Organic waste peel-assisted synthesis of ZnSe nanoparticles for solar-driven photocatalytic degradation of cationic and anionic dye

The environment and public health are currently being threatened by the water pollution caused by dyes. Finding eco-friendly and economically viable photocatalysts has been a hot issue in recent years, as photocatalytic dye degradation is essential for eliminating dye from contaminated water as compared to other methods because of the cost factor and efficiency in removing organic contaminants. Using un-doped ZnSe for degrading activity has very seldom been attempted up to this point. Therefore, the current research focuses on the use of zinc selenide nanomaterials, which are produced via a green synthesis process from the organic waste peels of orange and potato using the hydrothermal method, and utilizes them as photocatalysts for the degradation of dyes using sunlight as a natural source of light. The crystal structure, bandgap, and surface morphology and analysis of the synthesized materials serve as indicators of their characteristics. Citrate in orange peel-mediated synthesis assists in forming a particle size of 1.85 nm and a large surface area of 17.078 m2/g enabling more surface-active sites resulting in degradation efficiency of 97.16% and 93.61% for methylene blue and Congo red dye, respectively, which outperforms commercial ZnSe in the dye degradation. The presented work maintains overall sustainability in real-practical applications by utilizing sunlight in photocatalytic degradation activity instead of sophisticated equipment and using waste peels as a capping and stabilizing agent in the green synthesis method for the preparation of photocatalysts.

Rational design of α-MnO2/HT-GCN nanocomposite for effective photocatalytic degradation of ciprofloxacin and pernicious activity

The present study is mainly concerned with the development of cost-efficient composite material utilized to produce one-dimensional manganese oxide (α-MnO2) nanoparticles coated on two-dimensional graphitic carbon nitrides (HT-GCN) as nanocomposite (α-MnO2/HT-GCN) for highly efficient CIP degradation. The α-MnO2 nanoparticles (NPs) were prepared by a simple hydrothermal technique before being decorated on HT-GCN (H denotes protonation and T represents thermal-decomposition-graphitic carbon nitride). Tauc plots were used to calculate the band gap values of the photocatalysts α-MnO2 (1.74 eV), GCN (2.84 eV), HT-GCN (2.63 eV), and α-MnO2/HT-GCN (2.31 eV). The mechanism was investigated by various scavengers, particularly isopropanol (•OH) makes a significant role in the photodegradation process. The degradation percentage for ciprofloxacin was 89.2% and the rate of reaction R2 = 0.9913. This study demonstrates a unique method for developing a heterojunction-based nanocomposite of α-MnO2/HT-GCN, which exhibit better light absorption performance.

Characterization and application of natural Moroccan material for methyl violet 2B dye removal from aqueous solution

In this paper, a natural Moroccan material from the Nador area in the north east of Morocco was studied as an adsorbent to remove methyl violet 2B dye from aqueous solutions. This material has never been studied before in this region, and it will be used in its raw state. It was collected and characterized by X-ray diffraction, FTIR spectroscopy, scanning electron microscopy, X-ray fluorescence, thermal analysis, N₂ gas adsorption-desorption, pH_PZC, and Brunauer-Emmett-Teller (BET). The studies are realized with a 500-µm grain size and 182m²/g BET surface area. XRD showed the presence of significant peaks belonging to natural zeolite type clinoptilolite-Ca and minor phases. Several parameters were studied such as contact time, adsorbent mass, initial dye concentration, initial pH solution, the particle size of the material, and temperature. Out of the three isotherm models investigated after 60 min of contact time in the experiments, the Langmuir model gave the best fit to the experimental data (R² = 0.99). The results of kinetic and thermodynamic studies revealed that the adsorption process obeyed pseudo-second-order, spontaneous (ΔG° < 0), endothermic (ΔS° > 0). The adsorption of methyl violet 2B dye is chemisorptions and physisorption. The maximum theoretical adsorption capacity was 30.30 mg·g^-1 at 23 °C for a particle diameter of 500 µm. The desorption study shows that the material can be desorbed using solvents. The reuse study indicates that the same amount of natural zeolite can be used several times which makes the process efficient and sustainable. The obtained results indicate that the country of Morocco has natural zeolite among its resources and that it can be used as an efficient adsorbent for the removal of dyes.

Enhanced photocatalytic activity of Cu and Ni-doped ZnO nanostructures: A comparative study of methyl orange dye degradation in aqueous solution

Heterogeneous photocatalysis has been considered one of the most effective and efficient techniques to remove organic contaminants from wastewater. The present work was designed to examine the photocatalytic performance of metal (Cu and Ni) doped ZnO nanocomposites in methyl orange (MO) dye degradation under UV light illumination. The wurtzite hexagonal structure was observed for both undoped/doped ZnO and a crystalline size ranging between 8.84 ± 0.71 to 12.91 ± 0.84 nm by X-ray diffraction (XRD) analysis. The scanning electron microscope (SEM) and energy dispersive X-ray (EDX) revealed the irregular spherical shape with particle diameter (34.43 ± 6.03 to 26.43 ± 4.14 nm) and ensured the purity of the individual elemental composition respectively. The chemical bonds (O-H group) and binding energy (1021.8 eV) were identified by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results respectively. The bandgap energy was decreased from 3.44 to 3.16 eV when Ni dopant was added to the ZnO lattice. The comparative photocatalytic activity was observed in undoped and doped nanocomposites and found to be 76.31%, 81.95%, 89.30%, and 83.39% for ZnO, Cu/ZnO, Ni/ZnO, and Cu/Ni/ZnO photocatalysts, respectively, for a particular dose (0.210 g) and dye concentration (10 mg L^-1) after 180 min illumination of UV light. The photocatalytic performance was increased up to 94.40% with the increase of pH (12.0) whereas reduced (35.12%) with an increase in initial dye concentration (40 mg L^-1) using Ni/ZnO nanocomposite. The Ni/ZnO nanocomposite showed excellent reusability and was found 81% after four consecutive cycles. The best-fitted reaction kinetics was followed by pseudo-first-order and found reaction rate constant (0.0117 min^-1) using Ni/ZnO nanocomposite. The enhanced photodegradation efficiency was observed due to decreases in bandgap energy and the crystalline size of the photocatalyst. Therefore, Ni/ZnO nanocomposite could be used as an emerging photocatalyst to degrade bio-persistent organic dye compounds from textile wastewater.

Recent progress on plant extract-mediated biosynthesis of ZnO-based nanocatalysts for environmental remediation: Challenges and future outlooks

The plant extract mediated green synthesis of nanomaterials has attracts enormous interest due to its cost-effectiveness, greener, and environmentally friendly. It is also considered as an alternative and facile method in which the phytochemicals can be used as a natural capping and reducing agents and helped to produce nanomaterials with high surface area, different sizes, and shapes. One of the materials fabricated using green methods is zinc oxide (ZnO) semiconductor due to its enormous applications in different field areas. In this review, an overview of recent progress on green synthesized ZnO-based catalysts and various modification methods for the purpose of enhancing the catalytic activity of ZnO and the corresponding structural-activity and interactions towards the removal of pollutants are highlighted. Particularly, the plant extract mediated ZnO-based photocatalysts application for the removal of pollutants via photocatalytic degradation, reduction reaction, and adsorption mechanism are demonstrated. Besides, the opportunities, challenges, and future outlooks of ZnO-based materials for environmental remediation with green and sustainable methods are also included. We believe that this review is a timely and comprehensive review on the recent progress related to plant extract mediated ZnO-based nanocatalysts synthesis and applications for environmental remediation.

Computer chip-inspired design of nanocellulose/carbon dots hydrogel as superior intensifier of nano-sized photocatalyst for effective Cr(VI) removal

Hydrogel, a common carrier of photocatalyst that suffers from compromised catalytic efficiency, is still far from practical application. Herein, based on "computer chip-inspired design", a novel nanocellulose/carbon dots hydrogel (NCH) was fabricated as superior intensifier instead of common carrier of sodium titanate nanofibre (STN), where carbon dots (CDs) enhanced amino group-induced adsorption for Cr(VI), promoted photocatalytic properties of STN via transferring the photogenerated electron-hole pairs and improved amino group-induced desorption for reduced product (Cr(III)) via electrostatic repulsion, showing an efficiency of 1 + 1 > 2. Adsorption and photocatalysis experiments demonstrated superior removal performance of the NCH incorporating STN, as shown by theoretical maximum adsorption capacity of 425.74 mg/g and kinetic constant of 0.0374 min^-1 in the photocatalytic process, which was nearly 6.6 and 7.3 times of STN. A series of experiments was conducted to confirm the novel mechanism of CDs-enhanced adsorption-photocatalysis-desorption synergy. This work not only provides new insights into the fabrication of a superior intensifier for nanosized photocatalyst, but also proposes one new mechanism of CDs-enhanced adsorption-photocatalysis-desorption synergy, which is helpful for designing and optimizing nanosized photocatalyst.

Removal of the Pigment Congo Red from Synthetic Wastewater with a Novel and Inexpensive Adsorbent Generated from Powdered Foeniculum Vulgare Seeds

In this research, powdered Foeniculum vulgare seed (FVSP) was treated separately with H2C2O4, ZnCl2, and a mixture of ZnCl2-CuS. The characteristics of the treated and untreated FVSP samples, as well as their abilities to eliminate Congo red (CR) from solutions, were investigated. The influences of the empirical circumstances on CR adsorption by the ideal adsorbent were studied. The thermodynamic, isothermal, and dynamic constants of this adsorption were also inspected. The ideal adsorbent was found to be the FVSP sample treated with a ZnCl2-CuS mixture, which eliminated 96.80% of the CR dye. The empirical outcomes proved that this adsorption was significantly affected by the empirical circumstances, and the second-order dynamic model as well as the Langmuir isotherm model fit the empirical data better than the first-order model and the Freundlich model. The values of Ea (15.3 kJ/mol) and ∆Ho (32.767 kJ/mol ≤ ∆Ho ≤ 35.495 kJ/mol) evidence that CR anions were endothermally adsorbed on Zn/Cu-FVSP via the ionic exchange mechanism. The superior Qmax values (434.78, 625.00, 833.33 mg/g), along with the cheapness and stability of the adsorbent used in this work, are evidence to confirm that this adsorbent will receive special interest in the field of contaminated water purification.

Noval green sodium alginate/gellan gum aerogel with 3D hierarchical porous structure for highly efficient and selective removal of Congo red from water

Rational design of adsorbed materials with three-dimensional (3D) hierarchical porous structure, sustainable, high adsorption capacity, and excellent selective is of great significance in practical applications. Herein, a novel aerogel adsorbed material with 3D hierarchical porous architecture was fabricated by employing naturally abundant sodium alginate (SA)/gellan gum (GG) as basic construction blocks to achieve sustainability as well as applying polyethyleneimine (PEI) as functional material for highly efficient and selective capture of Congo red (CR). The aerogel sorbent exhibited strong microstructure, numerous active adsorption sites and being ultralight. The resulting aerogel adsorbent showed high adsorption capacity (3017.23 mg/g) toward CR, exceedingly most previously reported sorbents. Furthermore, the aerogel adsorbent was accompanied by outstanding selectivity for CR in four binary dye systems. Meanwhile, after 3 cycles, the adsorption capacity decreased by 14.8 %, but still maintained the adsorption capacity of 559.79 mg/g. Therefore, excellent adsorption performance, and superb selectivity prefigures its great prospects for wastewater purification.