Suneel Kumar

Synergetic effect of MoS2–RGO doping to enhance the photocatalytic performance of ZnO nanoparticles

The sunlight driven photocatalytic activity of semiconductor based nanostructures has attracted widespread attention in recent years for environmental remediation and energy applications. Numerous good semiconductors, including ZnO, have wide bandgaps and are active only under ultraviolet light, which comprises only 5% of sunlight. Several strategies, such as noble metal doping, non-metal doping, etc., have been adapted to make ZnO heterostructures active in the visible light region. One other strategy is to dope ZnO with narrow bandgap semiconductors like MoS2. In addition, co-doping with graphene as a support material can enhance pollutant adsorption and can aid in electron transport, thereby leading to pollutant degradation. In this work, we report our investigations on the synergetic role played by MoS2–RGO doping to enhance the photocatalytic activity of ZnO nanoparticles, and especially to utilize both the UV and visible light regions of the solar spectrum. The ZnO–MoS2–RGO heterostructures, having different levels of doping, were prepared by a facile hydrothermal method and were characterized thoroughly using different spectroscopy and microscopy techniques. The photocatalytic performance was evaluated by studying the degradation of methylene blue, a model dye pollutant, and carbendazim, a colorless hazardous fungicide, under natural sunlight irradiation. The results reveal that doping of ZnO nanoparticles with 1 wt% MoS2–RGO was optimal and possessed the highest photocatalytic activity among all the investigated samples. A possible mechanism is proposed and discussed in detail.

N-doped ZnO–MoS2 binary heterojunctions: the dual role of 2D MoS2 in the enhancement of photostability and photocatalytic activity under visible light irradiation for tetracycline degradation

In this work, we report the fabrication of binary semiconductor heterojunctions comprising N-doped ZnO nanorods loaded with two-dimensional MoS2 nanoflowers in varying amounts, using a facile hydrothermal synthesis method. These semiconductor heterojunctions have been demonstrated to be highly efficient photocatalysts with enhanced performance under visible light irradiation for the degradation of a pharmaceutical pollutant, tetracycline. The superior photocatalytic activity of the heterojunctions can be attributed to the synergistic effect of N-doping of ZnO and loading of MoS2 leading to higher absorption of visible light, efficient separation of photogenerated charge carriers and rapid charge transfer to reaction sites, as per the conduction band potentials of both N-doped ZnO and MoS2. In addition, the two-dimensional nanoflower morphology of MoS2 provides more reaction sites for the adsorption of pollutants, due to its large surface area. Furthermore, the transfer of holes from the valence band of N-doped ZnO to the valence band of MoS2 prevents the photocorrosion of N-doped ZnO resulting in enhanced photostability of the catalyst during the reaction.

Efficient Electron Transfer across ZnO-MoS2-RGO Heterojunction for Remarkably Enhanced Sunlight Driven Photocatalytic Hydrogen Evolution

The development of noble metal-free catalysts for hydrogen evolution is required for energy applications. In this regard, ternary heterojunction nanocomposites consisting of ZnO nanoparticles anchored on MoS2 -RGO (RGO=reduced graphene oxide) nanosheets as heterogeneous catalysts show highly efficient photocatalytic H2 evolution. In the photocatalytic process, the catalyst dispersed in an electrolytic solution (S2- and SO32- ions) exhibits an enhanced rate of H2 evolution, and optimization experiments reveal that ZnO with 4.0u2005wtu2009% of MoS2 -RGO nanosheets gives the highest photocatalytic H2 production of 28.616u2005mmolu2009h-1 u2009gcat-1 under sunlight irradiation; approximately 56 times higher than that on bare ZnO and several times higher than those of other ternary photocatalysts. The superior catalytic activity can be attributed to the inu2005situ generation of ZnS, which leads to improved interfacial charge transfer to the MoS2 cocatalyst and RGO, which has plenty of active sites available for photocatalytic reactions. Recycling experiments also proved the stability of the optimized photocatalyst. In addition, the ternary nanocomposite displayed multifunctional properties for hydrogen evolution activity under electrocatalytic and photoelectrocatalytic conditions owing to the high electrode-electrolyte contact area. Thus, the present work provides very useful insights for the development of inexpensive, multifunctional catalysts without noble metal loading to achieve a high rate of H2 generation.

Two-dimensional carbon-based nanocomposites for photocatalytic energy generation and environmental remediation applications

In the pursuit towards the use of sunlight as a sustainable source for energy generation and environmental remediation, photocatalytic water splitting and photocatalytic pollutant degradation have recently gained significant importance. Research in this field is aimed at solving the global energy crisis and environmental issues in an ecologically-friendly way by using two of the most abundant natural resources, namely sunlight and water. Over the past few years, carbon-based nanocomposites, particularly graphene and graphitic carbon nitride, have attracted much attention as interesting materials in this field. Due to their unique chemical and physical properties, carbon-based nanocomposites have made a substantial contribution towards the generation of clean, renewable and viable forms of energy from light-based water splitting and pollutant removal. This review article provides a comprehensive overview of the recent research progress in the field of energy generation and environmental remediation using two-dimensional carbon-based nanocomposites. It begins with a brief introduction to the field, basic principles of photocatalytic water splitting for energy generation and environmental remediation, followed by the properties of carbon-based nanocomposites. Then, the development of various graphene-based nanocomposites for the above-mentioned applications is presented, wherein graphene plays different roles, including electron acceptor/transporter, cocatalyst, photocatalyst and photosensitizer. Subsequently, the development of different graphitic carbon nitride-based nanocomposites as photocatalysts for energy and environmental applications is discussed in detail. This review concludes by highlighting the advantages and challenges involved in the use of two-dimensional carbon-based nanocomposites for photocatalysis. Finally, the future perspectives of research in this field are also briefly mentioned.

Role of RGO support and irradiation source on the photocatalytic activity of CdS–ZnO semiconductor nanostructures

Photocatalytic activity of semiconductor nanostructures is gaining much importance in recent years in both energy and environmental applications. However, several parameters play a crucial role in enhancing or suppressing the photocatalytic activity through, for example, modifying the band gap energy positions, influencing the generation and transport of charge carriers and altering the recombination rate. In this regard, physical parameters such as the support material and the irradiation source can also have significant effect on the activity of the photocatalysts. In this work, we have investigated the role of reduced graphene oxide (RGO) support and the irradiation source on mixed metal chalcogenide semiconductor (CdS–ZnO) nanostructures. The photocatalyst material was synthesized using a facile hydrothermal method and thoroughly characterized using different spectroscopic and microscopic techniques. The photocatalytic activity was evaluated by studying the degradation of a model dye (methyl orange, MO) under visible light (only) irradiation and under natural sunlight. The results reveal that the RGO-supported CdS–ZnO photocatalyst performs considerably better than the unsupported CdS–ZnO nanostructures. In addition, both the catalysts perform significantly better under natural sunlight than under visible light (only) irradiation. In essence, this work paves way for tailoring the photocatalytic activity of semiconductor nanostructures.

Bioinspired Functional Surfaces for Technological Applications

Biological matters have been in continuous encounter with extreme environmental conditions leading to their evolution over millions of years. The fittest have survived through continuous evolution, an ongoing process. Biological surfaces are the important active interfaces between biological matters and the environment, and have been evolving over time to a higher state of intelligent functionality. Bioinspired surfaces with special functionalities have grabbed attention in materials research in the recent times. The microstructures and mechanisms behind these functional biological surfaces with interesting properties have inspired scientists to create artificial materials and surfaces which possess the properties equivalent to their counterparts. In this review, we have described the interplay between unique multiscale (micro- and nano-scale) structures of biological surfaces with intrinsic material properties which have inspired researchers to achieve the desired wettability and functionalities. Inspired by naturally occurring surfaces, researchers have designed and fabricated novel interfacial materials with versatile functionalities and wettability, such as superantiwetting surfaces (superhydrophobic and superoleophobic), omniphobic, switching wettability and water collecting surfaces. These strategies collectively enable functional surfaces to be utilized in different applications such as fog harvesting, surface-enhanced Raman spectroscopy (SERS), catalysis, sensing and biological applications. This paper delivers a critical review of such inspiring biological surfaces and artificial bioinspired surfaces utilized in different applications, where material science and engineering have merged by taking inspiration from the natural systems.

Plant leaves as natural green scaffolds for palladium catalyzed Suzuki–Miyaura coupling reactions

This work presents a novel approach of using natural plant leaf surfaces having intricate hierarchical structures as scaffolds for Pd nanoparticles and demonstrated it as a Green dip catalyst for Suzuki-Miyaura coupling reactions in water. The influence of the topographical texture of the plant leaves on the deposition and catalytic properties of Pd nanoparticles are presented and discussed. The catalytic activity can be correlated to the surface texture of the leaves, wherein it has been found that the micro/nanostructures present on the surface strongly influence the assembly and entrapment of the nanoparticles, and thereby control aggregation and leaching of the catalysts. This approach can provide insights for the future design and fabrication of bioinspired supports for catalysis, based on replication of leaf surfaces.

Recyclable, bifunctional composites of perovskite type N-CaTiO3 and reduced graphene oxide as an efficient adsorptive photocatalyst for environmental remediation

Graphene–semiconductor photocatalysts are of utmost interest for achieving visible light activity but there is ambiguity about the role played by graphene in these photocatalysts. Such photocatalysts exhibit both adsorptive and photocatalytic activities towards pollutant degradation. Herein, we report nitrogen doped CaTiO3 (NCT) perovskite coupled with reduced graphene oxide (RGO), RGO–NCT, composites for the first time and utilize them for environmental remediation as an efficient adsorptive photocatalyst. The adsorption and photocatalytic activity were evaluated by investigating the adsorption/degradation of a model dye, methylene blue (MB). In comparison with bare CT and NCT, the RGO–NCT composites exhibit enhanced photocatalytic activity, which could be attributed to the synergistic effect of the adsorption of dye molecules on the RGO surface followed by their degradation under visible light irradiation. In the degradation mechanism, we propose that the degradation of MB is not only due to the dye photosensitization process but also involves the true photocatalytic action of the RGO–NCT composites. The clear role of graphene as an electron acceptor is demonstrated by photoluminescence spectroscopic study. Also, the degradation of a colorless pollutant thiabendazole (TBZ) under visible light irradiation supports our hypothesis. In addition, the commendable stability and recyclability of RGO–NCT photocatalysts demonstrates that these materials can be used as potential, viable and stable photocatalysts for environmental remediation. In principle, we anticipate that our work paves the way for tailoring the photocatalytic activity of perovskite-type semiconductor materials by coupling with graphene for the design of recyclable bifunctional photocatalysts.

Perovskite-structured CaTiO3 coupled with g-C3N4 as a heterojunction photocatalyst for organic pollutant degradation

A novel graphitic carbon nitride (g-C3N4)–CaTiO3 (CTCN) organic–inorganic heterojunction photocatalyst was synthesized by a facile mixing method, resulting in the deposition of CaTiO3 (CT) nanoflakes onto the surface of g-C3N4 nanosheets. The photocatalytic activity of the as-synthesized heterojunction (along with the controls) was evaluated by studying the degradation of an aqueous solution of rhodamine B (RhB) under UV, visible and natural sunlight irradiation. The CTCN heterojunction with 1:1 ratio of g-C3N4/CT showed the highest photocatalytic activity under sunlight irradiation and was also demonstrated to be effective for the degradation of a colorless, non-photosensitizing pollutant, bisphenol A (BPA). The superior photocatalytic performance of the CTCN heterojunction could be attributed to the appropriate band positions, close interfacial contact between the constituents and extended light absorption (both UV and visible region), all of which greatly facilitate the transfer of photogenerated charges across the heterojunction and inhibit their fast recombination. In addition, the two-dimensional (2D) morphology of g-C3N4nanosheets and CT nanoflakes provides enough reaction sites due to their larger surface area and enhances the overall photocatalytic activity. Furthermore, the active species trapping experiments validate the major role played by superoxide radicals (O2 −•) in the degradation of pollutants. Based on scavenger studies and theoretically calculated band positions, a plausible mechanism for the photocatalytic degradation of pollutants has been proposed and discussed.

Amorphous titania matrix impregnated with Ag nanoparticles as a highly efficient visible- and sunlight-active photocatalyst material

Abstract Amorphous titania matrix impregnated with Ag nanoparticles has been synthesised based on a facile colloidal synthesis route and has been explored for visible- and sunlight-activated photocatalytic applications. The photocatalytic activity of the material under visible light and sunlight irradiation was demonstrated by investigations on the degradation of a common pollutant, nitrobenzene. In comparison to other TiO2-based nanomaterials, this amorphous titania impregnated with Ag nanoparticles was found to be highly efficient for visible light as well as sunlight-active photocatalytic applications. The superior catalytic activity could be attributed to the efficient charge separation and lessened recombination of the photo-generated electrons and holes at the Ag–titania interface and the presence of multiple metal–metal oxide interfaces due to dispersed impregnation of Ag nanoparticles on amorphous titania matrix. Also, it was found that the photocatalytic activity of the prepared nanocomposites had better efficiency under the sunlight irradiation, which may be attributed to the plasmonic heating of the Ag nanoparticles in the sunlight. Fundamentally, this work illustrates that the dispersed impregnation of noble metal nanoparticles on semiconductor metal oxide matrix can be used to tune the photophysical properties of the final material to enhance its photocatalytic performance.