Progress in Graphene/Metal Oxide Composite Photocatalysts for Degradation of Organic Pollutants

Graphene oxide as an emerging sole adsorbent and photocatalyst: Chemistry of synthesis and tailoring properties for removal of emerging contaminants

Contaminants of emerging concern (CEC) contain a wide range of compounds, such as pharmaceutical waste, pesticides, herbicides, industrial chemicals, organic dyes, etc. Their presence in the surrounding has extensive and multifaceted effects on human health as they have the potential to persist in the environment, accumulate in biota, and disrupt ecosystems. In this regard, various remediation methods involving different kind of functional nanomaterials with unique properties have been developed. The functional nanomaterials can provide several mechanisms for water pollutant removal, such as adsorption, catalysis, and disinfection, in a single platform. Graphene oxide (GO) is a two-dimensional carbon-based material that has an extremely large surface area and a large number of active sites. Recent advances in synthesising GO have shown great progress in tailoring its various physiochemical, optical, surface, structural properties etc., making it better adsorbent and photocatalysts. In this review, sole adsorbent and standalone photocatalytic performances of GO for the removal of CEC have been discussed in light of tailoring its adsorption and photocatalytic properties through novel synthesis routes and optimizing synthesis parameters. This review also examines various models describing the structure of GO and its surface/structural modifications for improved adsorption and photocatalytic properties. The article provides valuable information for the production of efficient and cost-effective GO-based sole adsorbents and photocatalysts as compared to the traditional materials. Furthermore, future prospective and challenges for sole GO nanostructures to compete with traditional adsorbents and photocatalysts have been discussed providing interesting avenues for future research.

Design and fabrication of self-suspending aluminum-plastic/semiconductor photocatalyst devices for solar energy conversion

The design and synthesis of self-suspending photocatalyst device with easy recyclability is important for practical application. Here, this work utilizes aluminum-plastic package waste as raw material to prepare an aluminum-plastic supported TiO2 (AP-TiO2) photocatalyst device through 3D printing design and surface deposition method. A series of characterizations were carried out to explore the structure, morphology and performance of the AP-TiO2 device. Under UV light illumination, the AP-TiO2-50 efficiently degrade 93.6% tetracycline hydrochloride (THC) after 4 hr, which increases by 8.3% compared with that of TiO2 powder suspension system with the same catalyst amount. Based on it, AP-ZnO, AP-CdS, AP-g-C3N4 and AP-Pt-TiO2 are also fabricated, and applied in photocatalytic degradation and hydrogen evolution, which all exhibit higher photoactivities than powder suspension systems. This work provides a new avenue for the fabrication of advanced recyclable photocatalyst device. Moreover, the work offers a novel sight for the high-value utilization of aluminum-plastic package waste, which has positive implications for environmental protection.

Simultaneous removal of Cr(Ⅵ) and tetracycline from wastewater by dielectric barrier discharge plasma coupled with TiO2/rGO/Cu2O composites: Performance and mechanism

In this study, dielectric barrier discharge (DBD) plasma combined with titanium dioxide/reduced graphene oxide/copper oxide (TiO2/rGO/Cu2O) composites for simultaneous removal of hexavalent chromium (Cr(Ⅵ)) and tetracycline (TC) from wastewater were explored systematically. The TiO2/rGO/Cu2O composites were successfully prepared to improve the specific surface area and charge carrier separation rate. When Cr(Ⅵ) and TC coexist, the two pollutants have better removal efficiency without changing the initial pH. Moreover, the removal efficiency of Cr(Ⅵ) and TC could be further improved in the DBD-TiO2/rGO/Cu2O system, indicating that the TiO2/rGO/Cu2O composites also exhibited good synergistic effects with the DBD plasma. The mechanism exploration showed that the TiO2/rGO/Cu2O composite catalyst could be activated in DBD system to produce various active species by photocatalytic reaction, among which photo-generated electrons and •O2- could significantly enhance Cr(Ⅵ) reduction, while photo-generated holes and •OH could improve TC degradation. More importantly, the intermediate products obtained from TC degradation can be oxidized to •CO2- by photo-generated holes, which can also facilitate the reduction of Cr(Ⅵ). This study shows that DBD combined with TiO2/rGO/Cu2O composites are capable of simultaneous Cr(Ⅵ) reduction and TC degradation, which would provide novel ideas for practical wastewater remediation.

Morphology Controlled Deposition of Vanadium Oxide (VOx) Nanoparticles on the Surface of Highly Reduced Graphene Oxide for the Photocatalytic Degradation of Hazardous Organic Dyes

Semiconducting nanomaterials based heterogeneous photocatalysis represent a low-cost, versatile technique for environmental remediation, including pollution mitigation, energy management and other environmental aspects. Herein, we demonstrate the syntheses of various heterogeneous photocatalysts based on highly reduced graphene oxide (HRG) and vanadium oxide (VOx)-based nanocomposites (HRG-VOx). Different shapes (rod, sheet and urchin forms) of VOx nanoparticles were successfully fabricated on the surface of HRG under solvo-/hydrothermal conditions by varying the amount of water and ethanol. The high concentration of water in the mixture resulted in the formation of rod-shaped VOx nanoparticles, whereas increasing the amount of ethanol led to the production of VOx sheets. The solvothermal condition using pure ethanol as solvent produced VOx nano-urchins on the surface of HRG. The as-prepared hybrid materials were characterized using various spectroscopic and microscopic techniques, including X-ray diffraction, UV-vis, FTIR, SEM and TEM analyses. The photocatalytic activities of different HRG-VOx nanocomposites were investigated for the photodegradation of methylene blue (MB) and methyl orange (MO). The experimental data revealed that all HRG-VOx composite-based photocatalysts demonstrated excellent performance toward the photocatalytic degradation of the organic dyes. Among all photocatalysts studied, the HRG-VOx nanocomposite consisting of urchin-shaped VOx nanoparticles (HRG-VOx-U) demonstrated superior photocatalytic properties towards the degradation of dyes.

Development of ZnO/SnO2/rGO hybrid nanocomposites for effective photocatalytic degradation of toxic dye pollutants from aquatic ecosystems

The impact of ZnO/SnO₂/reduced graphene oxide nanocomposites (ZnO/SnO₂/rGO NCs) for improved photocatalytic degradation of organic dye pollution is examined in this study. The developed ternary nanocomposites had a variety of characteristics that were detected, such as crystallinity, recombination of photogenerated charge carriers, energy gap, and surface morphologies. When rGO was added to the mixture, the optical band gap energy of ZnO/SnO₂ was lowered, which improved the photocatalytic activity. Additionally, in comparison to ZnO, ZnO/rGO, SnO₂/rGO samples, the ZnO/SnO₂/rGO nanocomposites demonstrated exceptional photocatalytic effectiveness for the destruction of orange II (99.8%) and reactive red 120 dye (97.02%), respectively after 120 min exposure to sunlight. The high electron transport properties of the rGO layers, which make it feasible to efficiently separate electron-hole pairs, are attributed to the enhanced photocatalytic activity of the ZnO/SnO₂/rGO nanocomposites. According to the results, synthesized ZnO/SnO₂/rGO nanocomposites are a cost-efficient option for removing dye pollutants from an aqueous ecosystem. Studies show that ZnO/SnO₂/rGO nanocomposites are effective photocatalysts and may one day serve as the ideal material to reduce water pollution.

Research progress of photocatalysis for algae killing and inhibition: a review

The healthy development of biodiversity has been threatened frequent water eutrophication. In recent years, photocatalytic technology, which has attracted researchers' attention, not only showed increasing potential in the field of organic pollutant degradation, but also many kinds of photocatalysts were used in the field of red tide pollution control at present, which showed superior ability to inactivate harmful algae and degrade algal toxins. Researches have also explored the mechanisms of photocatalytic algae inhibition. In this study, the current research progress in the field of photocatalytic algae inhibition was systematically discussed from several aspects, such as common types of photocatalysts, modification methods of photocatalysts, types of tested algae for photocatalytic algae inhibition, and action mechanism of inactivated algae cells, so as to provide a certain theoretical basis for further application research of photocatalysts in the field of algae removal in the later period.

Highly efficient, bioactive, and bifunctional sorbent p-n-p visible light heterogeneous photocatalyst utilizing ultra-fine ZnS nanoparticles embedded in a polymeric nanocomposite

This study reports the successful synthesis of a ZnS@GO@Pani polymeric nanocomposite (NC) via chemical polymerization. The product was used for simultaneous photocatalytic degradation-adsorption of malachite green (MG), a carcinogenic and widely used dye. The physicochemical properties of the prepared NC were characterized by various techniques. Morphological and XRD results confirmed the fine size of ZnS nanoparticles (NPs) with an approximate mean size of 5 nm, uniformly distributed within the polymeric matrix. For comparative purposes, photocatalytic dye degradation-adsorption of this nanohybrid was explored both in the dark and under natural light. It was observed that 0.1 g of the ternary NC in MG aqueous solution (20 ppm) leads to dye adsorption within 15 minutes with an efficiency of 70% under dark conditions. Also, MG removal efficiency of up to 90% was achieved in 15 minutes under natural light owing to integrated photocatalytic degradation-adsorption mechanisms. Adsorption isotherm studies were performed considering Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) models. The results showed that the Freundlich isotherm with R 2 = 0.988 is well consistent with the experimental data. Integrated photocatalytic degradation-adsorption kinetics were modeled with pseudo-first-order (PFO) and pseudo-second-order (PSO) models where PSO with R 2 = 0.999 best fitted the data, implying the predominant role of chemical adsorption in the dye removal process. Antibacterial tests revealed superior antibacterial activity of the prepared ZnS@GO@Pani NC against both Gram-negative and Gram-positive bacteria, demonstrating the remarkable synergistic effect of ZnS NPs embedded in the GO@Pani matrix. Accordingly, the prepared NC could be regarded as a promising candidate for wastewater treatment applications. The leaching and regeneration studies also confirmed that the prepared NC is a non-toxic dye removal agent with good reusability.

An efficient CuO/rGO/TiO2 photocatalyst for the synthesis of benzopyranopyrimidine compounds under visible light irradiation

This study reports the synthesis of CuO/rGO/TiO2 in coupling reaction under visible light irradiation. Its photocatalytic performance was explored in a pseudo 4-component and a domino reaction for the synthesis of benzopyranopyrimidine compounds. It can be recovered and recycled for 5 runs.

Facile fabrication of novel Fe3O4-SnO2-gC3N4 ternary nanocomposites and their photocatalytic properties towards the degradation of carbofuran

Herein, Fe₃O₄-SnO₂ nanoheterojunction has been synthesized and successfully encapsulated in gC₃N₄ matrix using a novel hydrothermal technique. The synthesized material was characterized using sophisticated analytical methods like XRD, TEM, BET, UV-Vis, VSM and XPS to evaluate structural, morphological, optical, magnetic and surface chemical properties. The hybrid nanostructure Fe₃O₄-SnO₂-gC₃N₄ has been utilized for the LED light-induced photocatalytic degradation of carbofuran. The catalyst exhibited notable photocatalytic performance under visible light with an efficiency of ~89% and pseudo first order rate constant of 0.015 min^-1. The result of change in variables like catalyst dose, pollutant concentration, pH and contact time on the photodegradation efficiency and degradation kinetics was studied. The incorporation of Fe₃O₄ improved the magnetic separation of the catalyst after several cycles of operation, thereby improving the practical utility of the catalyst system to tackle organic pollutants.

An improved plasmonic Au–Ag/TiO2/rGO photocatalyst through entire visible range absorption, charge separation and high adsorption ability

Plasmonic nanohybrids for visible light absorption: the combination of small alloyed AuAg NPs, TiO₂ NPs and rGO nanosheets provides wide visible light absorption improving the photocatalytic efficiency towards water pollutant remediation.

ZnO nanoparticles as photodegradation agent controlled by morphology and boron doping

ZnO nanoparticles with different morphology and doping possess different atomic planes at their interfaces. This changed their catalytic efficiency during degradation experiments with dyes, significantly dependent also on used dopant concentrations.

Synthesis, Characterization, and Photocatalytic Performance of ZnO–Graphene Nanocomposites: A Review

ZnO is an exciting material for photocatalysis applications due to its high activity, easy accessibility of raw materials, low production costs, and nontoxic. Several ZnO nano and microstructures can be obtained, such as nanoparticles, nanorods, micro flowers, microspheres, among others, depending on the preparation method and conditions. ZnO is a wide bandgap semiconductor presenting massive recombination of the generated charge carriers, limiting its photocatalytic efficiency and stability. It is common to mix it with metal, metal oxide, sulfides, polymers, and nanocarbon-based materials to improve its photocatalytic behavior. Therefore, ZnO–nanocarbon composites formation has been a viable alternative that leads to new, more active, and stable photocatalytic systems. Mainly, graphene is a well-known two-dimensional material, which could be an excellent candidate to hybridize with ZnO due to its excellent physical and chemical properties (e.g., high specific surface area, optical transmittance, and thermal conductivity, among others). This review analyses ZnO–graphene nanocomposites’ recent advances, addressing the synthesis methods and the resulting structural, morphological, optical, and electronic properties. Moreover, we examine the ZnO–graphene composites’ role in the photocatalytic degradation of organic/inorganic pollutants.