Rajendra C. Pawar

Gold nanoparticle modified graphitic carbon nitride/multi-walled carbon nanotube (g-C3N4/CNTs/Au) hybrid photocatalysts for effective water splitting and degradation

Gold nanoparticles (Au) used for stable plasmonic photocatalysts in hybrids of Au, graphitic carbon nitride (g-C3N4), and carbon nanotubes (CNTs), were evaluated for effective photodegradation of organic pollutants and photoelectrochemical (PEC) water splitting. These hybrids are formed at room temperature using sonication, and were shown to be effective for photodegradation of Rhodamine B (RhB) under irradiation with visible light. The hybrid samples resulted in a significant increase in photocatalytic activity compared with single-component samples of g-C3N4. In particular, the g-C3N4/CNTs/Au hybrids exhibited an exponential increase in the photocatalytic activity by a factor of almost 40. Structural and compositional analyses show the successful formation of ternary g-C3N4/CNTs/Au hybrids. The SPR due to the Au nanoparticles led to high optical absorbance, and the inclusion of the CNTs led to effective separation of photogenerated charge carriers, resulting in substantial improvement of the photocatalytic properties. PEC measurements indicate effective use of charge carriers, and open-circuit voltage decay measurements demonstrated increased lifetime of the photogenerated charge carriers in the hybrid samples. The ternary g-C3N4/CNTs/Au sample resulted in a large specific surface area, providing a large number of active sites for the adsorption of organic molecules. Therefore, a facile and room temperature fabrication method was shown to introduce Au and CNTs in the hybrid for substantial improvement of photocatalytic activities and effective water splitting.

Room-temperature synthesis of nanoporous 1D microrods of graphitic carbon nitride (g-C3N4) with highly enhanced photocatalytic activity and stability.

A one-dimensional (1D) nanostructure having a porous network is an exceptional photocatalytic material to generate hydrogen (H2) and decontaminate wastewater using solar energy. In this report, we synthesized nanoporous 1D microrods of graphitic carbon nitride (g-C3N4) via a facile and template-free chemical approach at room temperature. The use of concentrated acids induced etching and lift-off because of strong oxidation and protonation. Compared with the bulk g-C3N4, the porous 1D microrod structure showed five times higher photocatalytic degradation performance toward methylene blue dye (MB) under visible light irradiation. The photocatalytic H2 evolution of the 1D nanostructure (34 μmol g−1) was almost 26 times higher than that of the bulk g-C3N4 structure (1.26 μmol g−1). Additionally, the photocurrent stability of this nanoporous 1D morphology over 24 h indicated remarkable photocorrosion resistance. The improved photocatalytic activities were attributed to prolonged carrier lifetime because of its quantum confinement effect, effective separation and transport of charge carriers, and increased number of active sites from interconnected nanopores throughout the microrods. The present 1D nanostructure would be highly suited for photocatalytic water purification as well as water splitting devices. Finally, this facile and room temperature strategy to fabricate the nanostructures is very cost-effective.

Size-controlled BiOCl–RGO composites having enhanced photodegradative properties

Visible light-active bismuth oxychloride–reduced graphene oxide (BiOCl–RGO) composite photocatalysts were synthesised using a hydrothermal method at low temperature, and at a low cost. This approach reduced the recombination of electron–hole pairs and thereby provided more efficient photocatalysts. The size of BiOCl structure was controlled by polyvinylpyrrolidone (PVP) addition. Furthermore, formation of nanosized BiOCl sheets and BiOCl–RGO composites were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Fabricated BiOCl–RGO composite with PVP exhibited better photocatalytic activity than pure BiOCl grown with and without PVP towards degradation of Rhodamine B (RhB). It was found that the composite photocatalyst degrades RhB completely within 310 min as compared with several hours for pure BiOCl. The improved photocatalytic performance of BiOCl–RGO composite was attributed to its high specific surface area (22.074 m2 g−1 and existence of polar surfaces, compared with 9.831 m2 g−1 for pure BiOCl). The analyses indicated that RGO helped to reduce recombination losses and improve electron transport. It also showed that presence of polar surfaces improved photocatalytic activity of BiOCl. Hence, BiOCl–RGO composite is a promising catalyst for the degradation of organic pollutants under visible light and could be used in applications such as water purification devices.

Evaluation of a multi-dimensional hybrid photocatalyst for enrichment of H2 evolution and elimination of dye/non-dye pollutants

A unique ZnTiO3/g-C3N4 (ZNTCN) heterogeneous photocatalyst was fabricated using an electrospinning method combined with a sonication process. Initially, 1-dimensional (1D) nanofibers of ZnTiO3 (ca. 160 nm in diameter) were obtained by electrospinning and then combined with 2-dimensional (2D) nanosheets of g-C3N4via a facile sonication approach. The hybrid ZNTCN exhibited a considerable enhancement of the photocatalytic H2 evolution and degradation of methylene blue under visible-light irradiation. The rate of H2 evolution with the ZNTCN sample (295.88 μmol g−1) was six-fold and three-fold higher than those of pure ZnTiO3 (56.72 μmol g−1) and g-C3N4 (106.22 μmol g−1), respectively. In addition, the optimal ZNT : CN ratio (40 : 60 w/w) had shown excellent photodegradation performance towards removal of methylene blue, phenol, 4-chlorophenol, and 4-nitrophenol contaminants compared to those of bare ZnTiO3 nanofibers and g-C3N4 sheets. Clearly, the g-C3N4 nanosheets interacted synergistically with the ZnTiO3 nanofibers. The improvement in photocatalytic activity was mainly attributed to rapid charge transportation, increased optical absorbance and efficient separation of photoelectrons from the barrier potential produced at the interface. Moreover, dramatic suppression of recombination losses of the hybrid photocatalyst was confirmed through room-temperature photoluminescence, photocurrent response, and electrochemical impedance spectroscopy. These novel heterogeneous photocatalysts were shown to be very promising in green technology as well as for degradation of phenolic/non-phenolic pollutants.

Ultra-thin coating of g-C3N4 on an aligned ZnO nanorod film for rapid charge separation and improved photodegradation performance

Type II heterogeneous films with one dimensional (1D) zinc oxide (ZnO) nanorods coated with a graphitic carbon nitride (g-C3N4) layer (1D ZnO/gC3N4) were fabricated by a simple reflux and thermal vapor condensation process. The grown 1D ZnO/gC3N4 films were used to degrade methylene blue (MB) dye under visible-light irradiation. Additionally, photoelectrochemical (PEC) measurements were conducted to explore charge separation and transportation processes. The fabricated films had a photocurrent density of 0.12 mA cm−2, which is 3.7-times higher than that of bare ZnO nanorods, and had good stability over 5 h. Moreover, the photocatalytic activities of ZnO with the g-C3N4 films performed well over multiple cycles without requiring a complex washing process for the photocatalytic recovery step. The improved performance stemmed from direct coating of an ultra-thin g-C3N4 layer (<10 nm thick) over ZnO nanorods, which induced high optical absorbance in the visible range, effective charge separation and transportation and low interfacial charge transfer resistance. A photodegradation mechanism was proposed based on the generation of OH˙ and hole radicals during MB dye degradation; these radicals were verified using tert-butanol and EDTA-2Na scavengers. The fabricated core–shell films are very promising components for PEC devices for water purification applications.

Stable and magnetically reusable nanoporous magnetite micro/nanospheres for rapid extraction of carcinogenic contaminants from water

We report the growth of porous magnetite microspheres via a template-free hydrothermal route at relatively low temperatures. The growth parameters including the concentration of reactant, reaction temperature and reaction time were examined for the formation of nanopores in the microspheres. Ascorbic acid played a crucial role in the formation of nanoporous microspheres, and also acts as a reducing agent, resulting from Fe3+ conversion into Fe2+ iron during the hydrothermal process. The resulting samples were used to remove acid orange 7 (AO7) and Cr(VI) from water at room temperature via adsorption. The maximum AO7 adsorption capacity calculated from the Langmuir–Freundlich model was found to be 69.13 mg g−1 for an optimum porous structure, which is significantly greater than that of structures formed without ascorbic acid (i.e., 4.61 mg g−1). Simultaneously, the Cr(VI) removal performance of the porous structure (i.e., 96.98 mg g−1) was found to be superior than that of pristine structure (i.e., 10.67 mg g−1). The enhanced removal activity was attributed to the existence of a large number of nanopores, resulting in a large specific surface area of 87.36 m2 g−1. Recycling tests were carried using a magnetic field to separate the powder samples from water, and the samples were found to be stable for up to three cycles. These results show that these porous microspheres have potential applications in cost-effective wastewater treatment.

Decoration of Au nanoparticles onto BiOCl sheets for enhanced photocatalytic performance under visible irradiation for the degradation of RhB dye

ABSTRACT Plasmonic photocatalysts are promising candidates for use in the degradation of pollutants. Their ability to degrade a wide range of organic pollutants stems from key properties such as high visible light absorption, the ability to generate hot electrons and the formation of a Schottky barrier that facilitates effective separation of charge carriers. In the present work, we synthesised bismuth oxychloride sensitised with gold nanoparticles (NPs, 20–50 nm) via a two-step chemical process at low temperature. The fabricated Au/BiOCl powder was evaluated in the degradation of Rhodamine B (RhB) dye under visible light irradiation. The photocatalytic performance of the Au/BiOCl hybrid was almost double that of pristine BiOCl. This enhanced performance was attributed to electron transfer from BiOCl to Au via the formation of heterojunctions at the BiOCl/Au interface. Additionally, the surface plasmon resonance effect of the Au NPs provided high optical absorbance in the visible spectrum. TEM (transmission electron microscopy) analysis indicated the presence of polar (010) facets on the BiOCl sheets, which also contributed to dramatically improving their photocatalytic performance. The degradation time of the Au/BiOCl hybrid was 200 min compared with 320 min for pure BiOCl.

Dielectric properties of sol–gel synthesized SrTiO3/(Ba0.7Sr0.3)TiO3 and SrTiO3/Ba(Zr0.3Ti0.7)O3 thin film heterostructures

The paper presents synthesis of Ba0.7Sr0.3TiO3 (BST), BaZr0.3Ti0.7O3 (BZT) and SrTiO3 (ST) thin films and their heterostructures using modified Pechini method. The La0.7Sr0.3MnO3 has been used as a conducting bottom layer to form metal ferroelectric metal capacitor. The thin films are spin coated on SiO2/n-Si(100) substrates. The thin films thus deposited are characterized for crystal structure, morphology, dielectric, complex impedance and admittance properties. Deposition of surface layer ST is observed to reduce loss tangent tan δ of BST and BZT thin films, still maintaining equivalent magnitude of figure of merit γ. The results on dielectric properties are analyzed in terms of the Maxwell–Wagner model and Koop’s phenomenological theory.

Oxidation Prevention Properties of Reduced Graphene Oxide Mixed with 1-Octanethiol-Coated Copper Nanopowder Composites

1-Octanethiol-coated Cu nanoparticles were mixed with reduced graphene oxide rGO to fabricate Cu nanoinks with enhanced oxidation prevention. Graphene oxide GO was synthesized using modified Hummer’s method and rGO was reduced from GO using hydrazine hydrate. Copper nanoinks were fabricated with varying concentrations of rGO Cu : rGO ratios of 100 : 1, 500 : 1, and 1000 : 1 wt.%. The coating layers on the copper nanoparticles and rGO were observed using transmission electron microscopy and characterized by X-ray photoemission spectroscopy, X-ray diffraction, and Raman spectroscopy. It was observed that surface roughness increased as the concentration of rGO in Cu patterns increased, and an optimized Cu : rGO weight ratio of 1,000 : 1 was established. After sintering, the electrical properties and corrosion resistance of copper patterns both with and without rGO were measured and monitored for 200 days. The copper pattern with rGO Cu : rGO = 1,000 : 1 was found to maintain its initial resistivity 1.63 × 10−7 Ω·m for 150 days. Corrosion tests were conducted to confirm the oxidation prohibition of rGO. The resistance polarization Rp of the copper pattern was measured to be 1.5 times higher than that of the copper pattern without rGO. Thus, rGO was shown to prevent oxidation and improve the conductivity of copper patterns.

Basics of Photocatalysis

Presently, the most challenging issue is the elimination of contaminants existing in polluted water due to their adverse effect on human health and environment. Photocatalysis is one of the best, renewable, and green approach that remedies the water pollution problems. It has gained momentum since water splitting by Fujishima and Honda under UV irradiation, and remarkable progress has been made with the assistance of nanotechnology to resolve energy and pollution problems. Interestingly, its simplicity and low cost make it suitable and affordable for practical application in water purification. Chapter 1 describes the basic mechanism of heterogenous photocatalysis, degradation of toxic pollutants, and principles of semiconductor-based heterogeneous photocatalysis. Further, reactions, that is, generation of charge carriers, recombination losses, and degradation of pollutants, under illumination of solar light have been described in detail. The parameters required for efficient photocatalysts, such as optimum band gap energy, rapid transportation of photoelectrons, suitable band positions, and intrinsic electron mobility, have been summarized. Hence, this chapter consists of fundamental details about heterogeneous photocatalysis.