Band gap tuning and surface modification of carbon dots for sustainable environmental remediation and photocatalytic hydrogen production - A review

Carbon Dots-Decorated Carbon-Based Metal-Free Catalysts for Electrochemical Energy Storage

In the past ten years, carbon dots-decorated, carbon-based, metal-free catalysts (CDs-C-MFCs) have become the fastest-growing branch in the metal-free materials for energy storage field. However, the further development of CDs-C-MFCs needs to clear up the electronic transmission mechanism rather than primarily relying on trial-and-error approaches. This review presents systematically and comprehensively for the first time the latest advances of CDs-C-MFCs in supercapacitors and metal-air batteries. The structure-performance relationship of these materials is carefully discussed. It is indicated that carbon dots (CDs) can act as the electron-rich regions in CDs-C-MFCs owing to their unique properties, such as quantum confinement effects, abundant defects, countless functional groups, etc. More importantly, specific doping can effectively modify the charge/spin distribution and then facilitate electron transfer. In addition, present challenges and future prospects of the CDs-C-MFCs are also given.

One-step synthesis of high photocatalytic graphitic carbon nitride porous nanosheets

As a metal-free photocatalyst, graphitic carbon nitride (g-C₃N₄) has attracted tremendous attention. Preparation of porous few-layer g-C₃N₄ nanosheets has been proven to be an effective strategy to obtain high photocatalytic performance. At present, most methods are expensive, time-consuming or complicated. Here, a low-cost, facile and environment-friendly one-step synthesis method of porous few-layer g-C₃N₄ nanosheets is designed by introducing water in the precursor. Straightforward calcination of the precursor, which decomposes to form ammonia, can produce g-C₃N₄ nanosheets with the assistance of water. Under the visible light (>400 nm), the photocatalytic H₂ evolution performance of the so-obtained nanosheets is 3214 μmol · g^-1 · h^-1, which is 17.3 times of the original bulk g-C₃N₄. The apparent quantum yield is 27% under the 380 nm monochromatic light irradiation.

Sustainable environmental management and related biofuel technologies

Environmental sustainability criteria and rising energy demands, exhaustion of conventional resources of energy followed by environmental degradation due to abrupt climate changes have shifted the attention of scientists to seek renewable sources of green and clean energy for sustainable development. Bioenergy is an excellent alternative since it can be applied for several energy-requirements after utilizing suitable conversion methodology. This review elucidates all aspects of biofuels (bioethanol, biodiesel, and butanol) and their sustainability criteria. The principal focus is on the latest developments in biofuel production chiefly stressing on the role of nanotechnology. A plethora of investigations regarding the emerging techniques for process improvement like integration methods, less energy-intensive distillation techniques, and bioengineering of microorganisms are discussed. This can assist in making biofuel-production in a real-world market more economically and environmentally viable.

Z-scheme binary 1D ZnWO4 nanorods decorated 2D NiFe2O4 nanoplates as photocatalysts for high efficiency photocatalytic degradation of toxic organic pollutants from wastewater

In this study, dimensionally coupled Z-scheme binary nanocomposites from two-dimensional (2D) NiFe₂O₄ nanoplates and one-dimensional (1D) ZnWO₄ nanorods are prepared for efficient degradation of an antibiotic tetracycline (TC) and organic dye rhodamine B (RhB) under solar illumination. NiFe₂O₄/ZnWO₄ nanocomposites were synthesized by a simple and ecological in-situ hydrothermal method without the use of surfactants. Structural and morphological studies revealed the formation of heterostructure and 1D ZnWO₄ nanorods were uniformly distributed over the surface of NiFe₂O₄ nanoplates. Light-harvesting capability was improved and optimized by loading with different amounts of ZnWO₄. Photoluminescence analysis demonstrated inhibited nature of the recombination of photo-excited charge carriers in the nanocomposites. Photocatalytic experiments revealed that the nanocomposite exhibited improved Z-scheme electron-transfer for the degradation of TC under solar illumination. In particular, NFZW-20 nanocomposite demonstrated superior photocatalytic degradation of TC of approximately 98% within 105 min. Furthermore, their photocatalytic performance was investigated by RhB dye under the solar irradiation to achieve 98% of degradation of RhB in 70 min. Improved photocatalytic activities are attributed to the Z-scheme electron-transfer mechanism, which could enhance the superior ability of light absorption and reduced recombination rate of the photogenerated charge carriers.

Molecular biohydrogen production by dark and photo fermentation from wastes containing starch: recent advancement and future perspective

Changing lifestyle is increasing the energy demand. Fossil fuel is unable to deliver such huge energy. Clean energy from renewable source can solve this problem. Hydrogen is a clean and energy-efficient fuel and used for electricity generation by fuel cells or can be used in combustion engine. Easy availability of starch wastes from different industrial food processing wastes makes it a potential source for hydrogen (H₂) generation. Among various processes such as steam reforming, electrolysis, biophotolysis of water and anaerobic fermentation, anaerobic fermentation technique is environmentally friendly and requires less external energy, making it a preferred process for H₂ generation. Dark fermentation process can use wide range of substrates including agricultural and industrial starchy waste with low level of undesirable compounds. Application of both anaerobic dark and photofermentation can improve H₂ yield and production rate. H₂ production from wastes containing starch serves dual benefit of waste reduction and energy generation. As starch is a polymer and all hydrogen-producing bacteria cannot produce amylase to hydrolyze it, a pretreatment step is required to convert starch into glucose and maltose. In this present review paper, we have summarized: (i) potential of various types of starch-containing wastes as feedstock, (ii) various fermentation techniques, (iii) optimization of external process parameter, (iv) application of bioreactor and simulation in fermentation technique and (v) advancement in H₂ production from starchy wastes.

Naturally available diatomite and their surface modification for the removal of hazardous dye and metal ions: A review

The presence of toxic pollutants such as dyes and metal ions at higher concentrations in water is very harmful to the environment. Removal of these pollutants using diatomaceous earth or diatomite (DE) and surface-modified DE has been extensively explored due to their excellent physio-chemical properties and low cost. Therefore, naturally available DE being inexpensive, their surface modified adsorbents could be one of the potential candidates for the wastewater treatment in the future. In this context, the current review has been summarized for the removal of both pollutants i.e., dyes and metal ions by surface-modified DE using the facile adsorption process. In addition, this review is prominently focused on the various modification process of DE, their cost-effectiveness; the physio-chemical characteristics and their maximum adsorption capacity. Further, real-time scenarios of reported adsorbents were tabulated based on the cost of the process along with the adsorption capacity of these adsorbents.

ZnO nanosheets-decorated Bi2WO6 nanolayers as efficient photocatalysts for the removal of toxic environmental pollutants and photoelectrochemical solar water oxidation

Herein we report the fabrication of novel Bi₂WO₆/ZnO heterostructured hybrids for organic contaminant degradation from wastewater and photoelectrochemical (PEC) water splitting upon solar illumination. The Bi₂WO₆/ZnO photocatalysts were synthesized using a simple and eco-friendly hydrothermal process without the support of any surfactants. From the photocatalytic experiments, heterostructured Bi₂WO₆/ZnO nanohybrid catalysts exhibited considerably better photocatalytic performance for rhodamine B (RhB) degradation under solar illumination. The BWZ-20 nanocomposite demonstrated superior photodegradation of RhB dye up to 99% in about 50 min. Furthermore, BWZ-20 photoelectrode showeda lower charge-transfer resistance than other samples prepared, suggesting its suitability for PEC water splitting. The photocurrent densities of Bi₂WO₆/ZnO photoelectrodes were evaluated under the solar irradiation. The BWZ-20 photoelectrode exhibited a significant photocurrent density (0.45 × 10^-3A/cm²) at +0.3 V vs. Ag/AgCl, which was~1036-times higher than that of pure Bi₂WO₆, and ~4.8-times greater than the pure ZnO. Such improved photocatalytic and PEC activities are mainly attributed to the formation of an interface between ZnO and Bi₂WO₆, superior light absorption ability, low charge-transfer resistance, remarkable production of charge carriers, easy migration of charges, and suppression of the recombination of photogenerated charge carriers.

Facile synthesis of TiO2/Ag3PO4 composites with co-exposed high-energy facets for efficient photodegradation of rhodamine B solution under visible light irradiation

In this study, TiO₂/Ag₃PO₄ composites based on anatase TiO₂ nanocrystals with co-exposed {101}, {010}/{100}, {001} and [111]-facets and Ag₃PO₄ microcrystals with irregular and cubic-like polyhedron morphologies were successfully synthesized by combining hydrothermal and ion-exchange methods. The anatase TiO₂ nanocrystals with different high-energy facets were controllably prepared via hydrothermal treatment of the exfoliated [Ti₄O₉]²⁻/[Ti₂O₅]²⁻ nanosheet solutions at desired pH values. The Ag₃PO₄ microcrystal with different morphologies was prepared via the ion-exchange method in the presence of AgNO₃ and NH₄H₂PO₄ at room temperature, which was used as a substrate to load the as-prepared anatase TiO₂ nanocrystals on its surface and to form TiO₂/Ag₃PO₄ heterostructures. The apparent rate constant of the pH 3.5-TiO₂/Ag₃PO₄ composite was the highest at 12.0 × 10⁻³ min⁻¹, which was approximately 1.1, 1.2, 1.4, 1.6, 13.3, and 24.0 fold higher than that of pH 0.5-TiO₂/Ag₃PO₄ (10.5 × 10⁻³ min⁻¹), pH 7.5-TiO₂/Ag₃PO₄ (10.2 × 10⁻³ min⁻¹), pH 11.5-TiO₂ (8.8 × 10⁻³ min⁻¹), Ag₃PO₄ (7.7 × 10⁻³ min⁻¹), blank sample (0.9 × 10⁻³ min⁻¹), and the commercial TiO₂ (0.5 × 10⁻³ min⁻¹), respectively. The pH 3.5-TiO₂/Ag₃PO₄ composite exhibited the highest visible-light photocatalytic activity which can be attributed to the synergistic effects of its heterostructure, relatively small crystal size, large specific surface area, good crystallinity, and co-exposed high-energy {001} and [111]-facets. The as-prepared TiO₂/Ag₃PO₄ composites still exhibited good photocatalytic activity after three successive experimental runs, indicating that they had remarkable stability. This study provides a new way for the preparation of TiO₂/Ag₃PO₄ composite semiconductor photocatalysts with high energy crystal surfaces and high photocatalytic activity.

TiO₂/Ag₃PO₄ composites with co-exposed {101}, {010}/{100}, {001} and [111]-facets were successfully synthesized by combining hydrothermal and ion-exchange methods.

Biosynthesis of Copper Oxide Nanoparticles with Potential Biomedical Applications

Introduction

In recent years, the use of cost-effective, multifunctional, environmentally friendly and simple prepared nanomaterials/nanoparticles have been emerged considerably. In this manner, different synthesizing methods were reported and optimized, but there is still lack of a comprehensive method with multifunctional properties.

Materials and Methods

In this study, we aim to synthesis the copper oxide nanoparticles using Achillea millefolium leaf extracts for the first time. Catalytic activity was investigated by in situ azide alkyne cycloaddition click and also A³ coupling reaction, and optimized in terms of temperature, solvent, and time of the reaction. Furthermore, the photocatalytic activity of the synthesized nanoparticles was screened in terms of degradation methylene blue dye. Biological activity of the synthesized nanoparticles was evaluated in terms of antibacterial and anti-fungal assessments against Staphylococcus aureus, M. tuberculosis, E. coli, K. pneumoniae, P. mirabili, C. diphtheriae and S. pyogenes bacteria's and G. albicans, A. flavus, M. canis and G. glabrata fungus. In the next step, the biosynthesized CuO-NPs were screened by MTT and NTU assays.

Results

Based on our knowledge, this is a comprehensive study on the catalytic and biological activity of copper oxide nanoparticles synthesizing from Achillea millefolium, which presents great and significant results (in both catalytic and biological activities) based on a simple and green procedure.

Conclusion

Comprehensive biomedical and catalytic investigation of the biosynthesized CuO-NPs showed the mentioned method leads to synthesis of more eco-friendly nanoparticles. The in vitro studies showed promising and considerable results, and due to the great stability of these nanoparticles in a green media, effective biological activity considered as an advantageous.

Recent advancements in the applications of carbon nanodots: exploring the rising star of nanotechnology

Nanoparticles possess fascinating properties and applications, and there has been increasing critical consideration of their use. Because carbon is a component with immaterial cytotoxicity and extensive biocompatibility with different components, carbon nanomaterials have a wide scope of potential uses. Carbon nanodots are a type of carbon nanoparticle that is increasingly being researched because of their astounding properties such as extraordinary luminescence, simplicity of amalgamation and surface functionalization, and biocompatibility. Because of these properties, carbon nanodots can be used as material sensors, as indicators in fluorescent tests, and as nanomaterials for biomedical applications. In this review, we report on the ongoing and noteworthy utilization of carbon quantum dots such as bioimaging tests and photocatalytic applications. In addition, the extension and future components of these materials, which can be investigated for new potential applications, are discussed.

Waste-to-energy nexus for circular economy and environmental protection: Recent trends in hydrogen energy

The energy demand has increased exponentially worldwide owing to the continuously growing population and urbanization. The conventional fossil fuels are unable to satiate this requirement causing price inflation and significant environmental damage due to unrestrained emission of greenhouse gases. The focus now has shifted towards alternative, economical, renewable and green sources of energy such as hydrogen to deal with this bottle-neck. Hydrogen is a clean energy-source having high energy content (122 kJ/g). Recently, biological methods for the hydrogen production have attracted much attention because traditional methods are expensive, energy-exhaustive and not eco-friendly. The employment of biological methods promises utilization of waste or low-value materials for producing energy and building waste-to-energy nexus. Around 94% of the waste is discarded precariously in India and waste generation is growing at an alarming rate of 1.3% per year. The "waste-to-energy" techniques follow 'Reuse, Reduce, Recycle, Recovery and Reclamation' system solving three subjects at once; waste-management, energy-demand and environmental concern. Moreover, these methods have easy operability, cost-effectiveness and they help to shift from linear to circular model of economy for sustainable development. Biological processing of waste materials like agricultural discard (lignocellulosic biomass), food-waste and industrial discharge can be used for biohydrogen production. Dark and photo fermentation are the chief biological processes for the transformation of organic substrates to hydrogen. Dark fermentation is the acidogenic fermentation of carbohydrate-rich materials without light and oxygen. Clostridia, Enterobacter and Bacillus spp. are appropriate heterotrophic bacteria for dark fermentation. Various pretreatment methods like heat treatment, acid or base treatment, ultrasonication, aeration, electroporation, etc., can be applied on inoculums to increase H₂ producing bacteria eventually improving the hydrogen yield. However, only around 33% of COD in organic materials is transformed to H₂ by this method. Photofermentation by the photosynthetic non-sulfur bacteria (PNS) converts organic substrate to H₂ and CO₂ in the presence of nitrogenase enzyme in ammonium-limited and anoxygenic conditions. Rhodobacter or Rhodopseudomonas strains have been widely examined in this regard. But these methods are only able to produce H₂ with a poor yield. Combining dark and photofermentation is a noteworthy alternative for procuring enhanced hydrogen yields. Two-stage sequential method utilizes volatile fatty acids accumulated as byproducts after dark fermentation (in the first stage) for photofermentation by suitable bacteria (in the second stage). A proper investigation of the dark fermenter effluents is required before using them as a substrate for photo-fermentation. In a single-stage dark and photofermentation, co-culture of anaerobic and PNS bacteria in a single reactor is carried out for obtaining improved yield. The single stage system is comparatively inexpensive and less laborious; moreover, a limited requirement for an intermediate dilution stage is necessary. Economic analysis of hydrogen production showed that H₂ production by the present methods, save pyrolysis, is reasonably higher than the conventional approaches of fuel production. Probable routes to make H₂ production more cost-effective are reducing the cost of photobioreactor, installing proper storage system, etc. A constructive effort in the area of research and development of biological approaches of H₂ production technologies is vital. The commercial viability of biohydrogen production is imperative for accomplishment of circular economy system and sustainable development.

Recent Developments in Synthesis and Photocatalytic Applications of Carbon Dots

The tunable photoluminescent and photocatalytic properties of carbon dots (CDs) via chemical surface modification have drawn increased attention to this emerging class of carbon nanomaterials. Herein, we summarize the advances in CD synthesis and modification, with a focus on surface functionalization, element doping, passivation, and nanocomposite formation with metal oxides, transition metal chalcogenides, or graphitic carbon nitrides. The effects of CD size and functionalization on photocatalytic properties are discussed, along with the photocatalytic applications of CDs in energy conversion, water splitting, hydrogen evolution, water treatment, and chemical degradation. In particular, the enzyme-mimetic and photodynamic applications of CDs for bio-related uses are thoroughly reviewed.

Progress in the functional modification of graphene/graphene oxide: a review

Graphene and graphene oxide have attracted tremendous interest over the past decade due to their unique and excellent electronic, optical, mechanical, and chemical properties. This review focuses on the functional modification of graphene and graphene oxide. First, the basic structure, preparation methods and properties of graphene and graphene oxide are briefly described. Subsequently, the methods for the reduction of graphene oxide are introduced. Next, the functionalization of graphene and graphene oxide is mainly divided into covalent binding modification, non-covalent binding modification and elemental doping. Then, the properties and application prospects of the modified products are summarized. Finally, the current challenges and future research directions are presented in terms of surface functional modification for graphene and graphene oxide.

Graphene and graphene oxide have attracted tremendous interest over the past decade due to their unique and excellent electronic, optical, mechanical, and chemical properties.