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

Microwave-assisted synthesis of ZnO structures for effective degradation of methylene blue dye under solar light illumination

The presence of dyes in wastewater poses a high risk to both human health and the environment due to their potential toxicity and ecological impacts. Zinc(ii) oxide is a low-cost, non-toxic material that can serve as a sustainable and effective solution to the global water pollution crisis. In this study, we propose a facile one-step synthesis of various ZnO structures by microwave irradiation. The primary goal of this study was to explore the morphology-dependent photocatalytic activity of various ZnO structures, as well as the impact of interfering anions on the Methylene Blue (MB) photodegradation under solar light illumination. Photocatalytic activity studies show that the sample denoted as 0.56 M-ZnO with a sheet-like structure has remarkable catalytic activity under solar light illumination, reaching ∼96.6% degradation of 30 mL MB solution (3 × 10-5 M) within 40 minutes. The BET specific surface area and band gap of the optimal 0.56 M-ZnO sample were observed to be 12.42 m2 g-1 and 2.89 eV, respectively. It was shown that the presence of anions like Cl-, NO3-, and HCO3- can reduce the catalytic activity of 0.56 M-ZnO structure to some extent, although more than 70% MB degradation can still be obtained under neutral pH conditions. The superior catalytic efficacy observed in the 0.56 M-ZnO photocatalyst can be attributed to its improved crystallinity, large surface area, and enhanced production of hydroxyl radicals. The low-cost synthesis, combined with high photocatalytic activity collectively underscores the efficiency and practical usability of produced ZnO photocatalysts for dye degradation.

Hierarchical 0D CuO Wrapped by Petal-like 2D ZnO: A Strategic Approach of Superhydrophobic Melamine Sponge toward Wastewater Treatment

In addressing the pressing environmental challenges posed by frequent oil spills, this work presents a novel approach of synthesizing a superhydrophobic three-dimensional (3D) porous melamine sponge (MS). CuO and ZnO nanoparticles were grown on the MS via a hydrothermal method to create MS/CuO/ZnO with multiscale hierarchical nanostructures. The resulting material exhibited a stable water contact angle of 155° through various tests. MS/CuO/ZnO demonstrated exceptional oil absorption capacities (40-145 g/g and 0.83-0.99 mL.cm-3), surpassing 98% efficiency in oil separation, and retained reusability for 10 cycles. Impressively, the sponge achieved successful separation of oil/water emulsions with a permeation flux of 14870 L m-2 h-1. The composite sponge, distinguished by its high photodegradation ability, can degrade both water- and oil-targeted pollutants under visible light irradiation from light-emitting diode (LED). With its remarkable attributes including superior oil absorption, excellent oil/water separation, mechanical resistance, and excellent photocatalytic ability, it exhibits considerable potential for applications in both wastewater treatment and large-scale marine oil spill response. The easily prepared MS/CuO/ZnO emerges as a versatile solution capable of addressing pressing challenges and marking a significant leap toward sustainable and impactful environmental remediation.

Enhanced photoreduction efficiency of Cr(VI) driven by visible light in a new Zr-based metal-organic framework modified by hydroxyl groups

While metal-organic framework (MOF) photocatalysts have demonstrated a unique Cr(VI) photoreduction capability in recent decades, their performance is still insufficient for practical applications because of their low Cr(VI) uptake and poor visible light response. To cope with these drawbacks, a new OH-modified Zr-based MOF, termed HCMUE-1, was successfully prepared via a solvothermal method in this work. The complete characterization of HCMUE-1 was performed through various techniques, including powder X-ray diffraction (PXRD), Raman spectroscopy, Fourier transform infrared (FT-IR), thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), scanning electron microscopy combined with energy-dispersive X-ray (SEM-EDX), and X-ray photoelectron spectroscopy (XPS). The obtained data exhibited the excellent Cr(VI) photoreduction efficiency of HCMUE-1, reaching up to 98% after 90 min and almost 100% after 120 min under visible light illumination in a low acidic medium. Noteworthily, HCMUE-1 retained the same Cr(VI) removal rate for at least seven cycles without considerable loss. Further experimental investigations demonstrated that the structural stability and surface morphology of HCMUE-1 were retained after photoreduction. Moreover, the photocatalytic reduction mechanism of Cr(VI) to Cr(III) was interpreted through a series of systematic experimental measurements. These results indicate that HCMUE-1 possesses potential as an efficient photocatalyst for reducing toxic Cr(VI) species from wastewater in real-life conditions.

Phase controlled green synthesis of wurtzite (P63mc) ZnO nanoparticles: interplay of green ligands with precursor anions, anisotropy and photocatalysis

Green approaches for nanosynthesis often lack the precise control of synthetic outcomes, which is primarily due to the poorly defined reaction protocols. Herein, we investigated the use of lignocellulosic agro-waste, sugarcane press mud (PM), for the synthesis of ZnO nanoparticles using three different precursor salts and their further application in the photocatalytic degradation of rhodamine dyes. This approach resulted in the formation of ZnO nanoparticles with two different morphologies, i.e., sheet-like structure from the zinc sulphate and nitrate precursors, whereas sphere-like structures from zinc acetate. In all three cases, the wurtzite phase (P63mc) of ZnO nanoparticles remained consistent. Also, the ZnO nanoparticles were found to be positively charged ("ζ" = +8.81 to +9.22 mv) and nearly monodispersed, with a size and band gap in the range of ∼14-20 nm and 3.78-4.1 eV, respectively. Further, the potential photocatalytic activity of these nanoparticles was investigated under direct sunlight. At the same photocatalyst dose of 0.1 g L-1, the three ZnO nanoparticles showed varying efficiencies due to their shape anisotropy. The ZnO NPs from acetate salt (∼20 nm, sheet like) showed the highest dye degradation efficiency (90.03%) in 4.0 hours, indicating the role of the catalyst-dye interface in designing efficient photocatalysts.

Graphene-based strontium niobate-zinc oxide heterojunction photocatalyst for effective reduction of hexavalent chromium

A hydrothermal technique was employed to synthesize a Sr₂Nb₂O₇-rGO-ZnO (SNRZ) ternary nanocatalyst, in which ZnO and Sr₂Nb₂O₇ were deposited on reduced graphene oxide (rGO) sheets. The surface morphologies, optical properties, and chemical states, of the photocatalysts were characterized to understand their properties. The SNRZ ternary photocatalyst was superior over the reduction of Cr (VI) to harmless Cr (III) compared to the efficiencies obtained using bare, binary, and composite catalysts. The effects of various parameters, including the solution pH and weight ratio, on the photocatalytic reduction of Cr (VI) were investigated. The highest photocatalytic reduction performance (97.6%) was achieved at pH 4 and a reaction time of 70 min. Photoluminescence emission measurements were used to confirm efficient charge migration and separation across the SNRZ, which improved the reduction of Cr (VI). A feasible reduction mechanism for the SNRZ photocatalyst is proposed. This study presents an effective, inexpensive, non-toxic, and stable catalyst, for the reduction of Cr (VI) to Cr (III) using SNRZ ternary nanocatalysts.

Hydrogen Bonds as Stability-Controlling Elements of Spherical Aggregates of ZnO Nanoparticles: A Joint Experimental and Theoretical Approach

The effects of various organic additives, such as diethanolamine (DEA) and ethanolamine (EA), and variations in aging times on the formation and stability mechanisms of spherical aggregates of ZnO nanoparticles (NPs) prepared by using solvothermal synthesis were studied. The experimental results of the structural, morphological and optical properties monitored by using X-ray diffraction, field-emission scanning electron microscopy (FE-SEM) and UV-Vis spectroscopy were supported by quantum chemical calculations at the level of density functional theory (DFT). Understanding the mechanism of spherical ZnO aggregate formation and its stability by mimicking the processes at the computer level was achieved through theoretical simulations of the ZnO surface/additive interactions using (ZnO)₃₆-DEA and (ZnO)₃₆-EA models. The fine-tuned spherical aggregation of ZnO nanoparticles was driven by various interactions, in particular, strong O-H∙∙∙O and weak N-H∙∙∙O hydrogen bonds as controlling interactions. The calculated negative free release energy, ∆G*_INT, indicates that the ZnO surface/additive interaction in diethanolamine media is a spontaneous exergonic process (∆G*_INT = -7.73 kcal mol^-1), whereas, in ethanolamine media, it is an unfavorable, slightly endergonic process (∆G*_INT > 0). The presence of two strong O-H∙∙∙O hydrogen bonds and, at the same time, a weaker N-H∙∙∙O hydrogen bond is the key factor for the very good and long-term aggregate stability of ZnO NPs in DEA media. This integrated experimental-theoretical study highlights the stability and compactness of spherical ZnO aggregates of ZnO NPs, prepared in the presence of diethanolamine compared to ethanolamine media, and provides a promising method and flexible design of ZnO nanomaterials to improve their adsorptive and optical properties.

Photocatalytic degradation of dyes by novel electrospun nanofibers: A review

Textile industry is a significant contributor of wastewater, which contains pollutants including dye and other chemical substances. The release of thousands of tons of dye used in textile processing and finishing into natural streams and aquatic bodies present dire harm to the environment. In response to environmental concerns, a number of research have been done using low-cost technology to produce absorbents that can remove dyes from water bodies. Distinct techniques such as adsorption, enzymatic and photocatalytic degradation, etc. have been employed to remove dyes. In the last few decades, photocatalysis, a simple and green strategy, has emerged as the most valuable and recent principle that deals with wastewater treatment, using uniquely fabricated nanomaterials. Among them, rapid and versatile electrospinning methods have been used for the construction of a large surface area, hierarchical and reusable nanofibers for environmental remediation. As a flexible and fast fabrication method, reviewing the use of electrospun photocatalytic nanofibers, influential parameters in electrospinning and their effectiveness in the generation of oxidizing agents are a promising platform for the fabrication of novel nanomaterials in photocatalytic degradation of dyes. This review discusses techniques for dye removal, electrospun nanofibers, their fabrication and application in photocatalysis; mechanism of photocatalytic degradation, and challenges and suggested remedies for electrospun nanofibers in photocatalysis.

Silver nanoparticles modified ZnO nanocatalysts for effective degradation of ceftiofur sodium under UV-vis light illumination

Light-induced photocatalytic degradation of ceftiofur sodium (CFS) has been assessed in the presence of plasmonic zinc oxide nanostructures (ZnONSTs), like, ZnO nanoparticles, ZnO nanorods (ZnONRs) and ZnO nanoflowers (ZnONFs). Silver nanoparticles (Ag NPs) loaded ZnO nanostructures (Ag-ZnONSTs) are obtained through seed-assisted chemical reaction followed by chemical reduction of silver. The surface modification of ZnO nanostructures by Ag NPs effectually altered their optical properties. Further, the surface plasmonic effect of Ag NPs facilitates visible light absorption by ZnONSTs and improved the photogenerated electron and hole separation, which makes the ZnONSTs a more active photocatalyst than TiO₂ (P25) nanoparticles. Especially, Ag-ZnONRs showed higher CFS oxidation rate constant (k' = 4.6 × 10^-4 s^-1) when compared to Ag-ZnONFs (k' = 2.8 × 10^-4 s^-1) and Ag-ZnONPs (k' = 2.5 × 10^-4 s^-1), owing to their high aspect ratio (60:1). The unidirectional transport of photogenerated charge carriers on the Ag-ZnONRs may be accountable for the observed high photocatalytic oxidation of CFS. The photocatalytic oxidation of CFS mainly proceeds through ^•OH radicals generated on the Ag-ZnONRs surface under light illumination. In addition, heterogeneous activation of peroxymonosulfate by Ag-ZnONRs accelerates the rate of photocatalytic mineralization of CFS. The quantification of oxidative radicals supports the proposed CFS oxidation mechanism. Stability studies of plasmonic Ag-ZnONSTs strongly suggests that it could be useful to clean large volume of pharmaceutical wastewater under direct solar light irradiation.

Photocatalytic Performance of Undoped and Al-Doped ZnO Nanoparticles in the Degradation of Rhodamine B under UV-Visible Light:The Role of Defects and Morphology

Quasi-spherical undoped ZnO and Al-doped ZnO nanoparticles with different aluminum content, ranging from 0.5 to 5 at% of Al with respect to Zn, were synthesized. These nanoparticles were evaluated as photocatalysts in the photodegradation of the Rhodamine B (RhB) dye aqueous solution under UV-visible light irradiation. The undoped ZnO nanopowder annealed at 400 °C resulted in the highest degradation efficiency of ca. 81% after 4 h under green light irradiation (525 nm), in the presence of 5 mg of catalyst. The samples were characterized using ICP-OES, PXRD, TEM, FT-IR, ²⁷Al-MAS NMR, UV-Vis and steady-state PL. The effect of Al-doping on the phase structure, shape and particle size was also investigated. Additional information arose from the annealed nanomaterials under dynamic N₂ at different temperatures (400 and 550 °C). The position of aluminum in the ZnO lattice was identified by means of ²⁷Al-MAS NMR. FT-IR gave further information about the type of tetrahedral sites occupied by aluminum. Photoluminescence showed that the insertion of dopant increases the oxygen vacancies reducing the peroxide-like species responsible for photocatalysis. The annealing temperature helps increase the number of red-emitting centers up to 400 °C, while at 550 °C, the photocatalytic performance drops due to the aggregation tendency.

Development of chitosan decorated Fe3O4 nanospheres for potential enhancement of photocatalytic degradation of Congo red dye molecules

Fe₃O₄ nanospheres (Nsps) and chitosan (Cts)/Fe₃O₄ Nsps were prepared using a one-pot hydrothermal method and subsequently used as photocatalysts against the degradation of Congo red (CR) dye molecules. The sphere-shaped Fe₃O₄ nanoparticles were heterogeneously decorated by the Cts matrix, which was confirmed by powder X-ray diffraction, scanning and transmission electron microscopies. The Cts/Fe₃O₄ Nsps demonstrated 98% efficient photocatalytic activity against CR dye molecules upon 60 min exposure to visible light compared to Fe₃O₄ Nsps (77% for 60 min). When compared to Fe₃O₄ Nsps, the visible light photocatalytic efficiency of Cts/Fe₃O₄ Nsps against CR dye molecules was significantly improved.

Insight into the Growth Mechanism and Photocatalytic Behavior of Tubular Hierarchical ZnO Structures: An Integrated Experimental and Theoretical Approach

The hydrothermal synthesis of zinc oxide (ZnO) particles from zinc acetylacetonate monohydrate in "pure" aqueous solution and in aqueous NaOH solution at 90 °C is reported. The structural and morphological properties of ZnO particles were investigated by powder X-ray diffraction, X-ray absorption spectroscopy (XAS), field emission scanning electron microscopy, and transmission electron microscopy. The effect of NaOH on the growth mechanism and photocatalytic performance of hierarchical ZnO structures was investigated. The experimental findings, supported by results of quantum chemical calculations at the level of density functional theory, were used to propose the mechanism of nucleation and preferential growth of finely tuned hollow and nonhollow ZnO structures and their effects on the photocatalytic activity. The calculations indicate that the process of ZnO nucleation in "pure" aqueous solution mainly proceeds by the reaction of small monomers, while tetramers play a crucial role in aqueous NaOH solution. Both the preferred ZnO nanostructure and microstructure growth processes are driven by O-H···O hydrogen bonds as controlling elements. The calculated values of the E_O···H interaction indicate a stronger interaction via O-H···O hydrogen bonds in "pure" aqueous media (E_O···H = -11.73 kcal mol^-1) compared to those obtained in aqueous NaOH solution (E_O···H = -8.41 kcal mol^-1). The specific structural motif of the (ZnO-H₂O)₁₂ dodecamers with calculated negative ΔG*_INT free release energy indicates that the formation of anisotropic nanocrystalline ZnO with the c-axis as the primary growth direction is spontaneous and accelerated exclusively in "pure" aqueous solution, whereas it is an unfavorable endergonic process in aqueous NaOH solution (ΔG*_INT > 0). Efforts have been made to determine the photocatalytic efficiency of the ZnO samples based on the XAS measurements. ZnO particles obtained in "pure" aqueous solution show the highest photocatalytic activity due to the presence of a larger amount of oxygen vacancies.

One-pot synthesis of Ag-Cu-Cu2O/C nanocomposites derived from a metal-organic framework as a photocatalyst for borylation of aryl halide

Mixed metal-metal oxide/C (Ag-Cu-Cu2O/C) nanocomposites were synthesized by the heat treatment of a metal-organic framework under a N2 flow using the one-pot synthesis method. The as-prepared nanocomposites were characterized using a range of techniques, such as TEM, elemental mapping, XRD, N2 sorption, UV-Vis DRS, and XPS. The nanoparticles were successfully formed with high dispersion in porous carbon materials and high crystallinity based on the analysis results. The Ag-Cu-Cu2O/C nanocomposites (35 nm) showed high photocatalytic activity and good recyclability toward the borylation of aryl halides under a xenon arc lamp. This result can enhance the interest in photocatalysis for various applications, particularly in organic reactions, using a simple and efficient synthesis method.

Saussurea lappa plant rhizome extract-based zinc oxide nanoparticles: synthesis, characterization and its antibacterial, antifungal activities and cytotoxic studies against Chinese Hamster Ovary (CHO) cell lines

The plant extracts are known for their anti-inflammatory, antifungal, antiviral and antibacterial properties. The use of plant extracts in the preparation of bio-materials increases their biological application. In this concern, herein reporting an eco-friendly procedure which is also a simple and cost effective, for the synthesis of Zinc Oxide nanoparticles (ZnONPs) using Saussurea lappa plant root (rhizome) extract as a fuel. The prepared nanoparticles were confirmed using various characterization techniques. The Dynamic light scattering data showed 123.5 nm particle size with -99.9 mv zeta potential which indicates excellent stability of the particles. The peak at 541 cm-1 in the IR spectrum is assigned to the stretching frequency of the zinc-binding to oxygen. The X-ray diffraction peaks confirm the close association with JCPDS Data Card No: 36-1451. The FESEM data revealed a hexagonal wurtzite structure with a hexagonal shape of synthesized ZnO nanoparticles. The antibacterial studies indicate the gram-negative strains showed better inhibition activity than gram-positive strains. Among Fungal strains, Aspergillus niger and flavus, Fusarium oxysporum, and Rhizopus oryzae showed good inhibition activity at higher concentrations. The cytotoxic data indicates the 5 μg/mL of the ZnO particles showed cytotoxicity on the CHO cell line and with IC50 value 3.164 ± 0.8956 μg/mL.