Arnab Kanti Giri

A rapid and green synthetic approach for hierarchically assembled porous ZnO nanoflakes with enhanced catalytic activity

Three dimensionally (3D) assembled hierarchical porous ZnO structures are of key importance for their applications in sensors, lithium-ion batteries, solar cells and in catalysis. Here, the controlled synthesis of 3D hierarchically porous ZnO architectures constructed of two dimensional (2D) nano-sheets through the calcination of a hydrozincite [Zn5(CO3)2(OH)6] intermediate is presented. The intermediate 3D hierarchical hydrozincite has been synthesized by a novel organic surfactant and solvent free aqueous protocol at room temperature using an aqueous solution of ammonium carbonate and laboratory grade bulk ZnO in a short time (20–30 min). The amount of carbonate and the reaction temperature play a crucial role in the formation of the 3D hierarchical morphology and on the basis of the experimental results a probable reaction mechanism is proposed. On calcination, the synthesized 3D hierarchical hydrozincite resulted in ZnO with an almost identical morphology to the parental hydrozincite. On decomposition a porous structure having a surface area of 44 m2 g−1 is obtained. The synthesized hierarchical ZnO morphology exhibits an improved catalytic activity for the synthesis of 5-substituted-1H-tetrazoles with different nitriles and sodium azide than that of nanocrystalline ZnO and bulk ZnO, as well as other developed solid catalysts. The catalyst is easily recyclable without a significant loss in catalytic activity.

Hierarchically order porous lotus shaped nano-structured MnO2 through MnCO3: chelate mediated growth and shape dependent improved catalytic activity

Design of hierarchical nanostructures towards a specific morphology is an important research area due to their shape dependent properties. Here, 3D hierarchically assembled lotus shaped porous MnO2 is synthesized using a simple aqueous solution based chelating agent (citric acid) mediated growth of MnCO3 followed by calcination at 350 °C. MnCO3 in other shapes, such as rods, spheres and nano-aggregates, is also synthesized just by varying the chelating agents. It is observed that the geometry and strength of the chelating ligands has a crucial role in the controlled shape selective synthesis and based on this a probable chelating agent driven formation mechanism is discussed. The synthesized porous MnO2 shapes exhibit excellent shape dependent catalytic oxidation of α-pinene to verbenone using molecular oxygen as the oxidant. The lotus shaped porous MnO2 shows superior activity, with 94% conversion of α-pinene and 87% selectivity of verbenone, to that of other MnO2 shapes. The activity is reasonably high compared to heterogeneous as well as homogeneous catalysts reported in the literature and bulk MnO2 with respect to both their conversion and selectivity. The synthesized lotus shaped MnO2 also showed good catalytic activity towards oxidation of allylic compounds to corresponding ene–ones using molecular oxygen as oxidant and is reusable.

Porous ZnO microtubes with excellent cholesterol sensing and catalytic properties

The controlled formation of porous ZnO microtubes via the formation of tubular hydrozincite under ambient conditions from bulk ZnO followed by calcination at 500 °C for 2 h is presented. The tubular structure is a hierarchical assembly of ZnO flowers to form uniform tubes, with ∼30 μm length, ∼2 to 7 μm width and ∼400 to 500 nm wall thickness, where the flowers are made of 3D assembled porous ZnO flakes. The surface area of the tubular ZnO structure is quite good (58 m2 g−1). The developed synthetic procedure is quite flexible, and we have also synthesized nanostructured ZnO of varying morphologies from bulk ZnO just by changing the synthetic conditions. The developed ZnO microtubes showed excellent microstructure-based sensing and catalytic properties. A novel biosensor based on the synthesized porous tubular ZnO exhibited high sensitivity (54.5 mA M−1 cm−2) and low LOD (limit of detection, 0.2 mM (S/N = 3)) of cholesterol at room temperature, superior to that of sensors made of other porous ZnO shapes synthesized by varying the conditions, as well as other sensors reported in the literature. It is superior even in comparison with a nano gold modified sensor. The tubular ZnO structure also showed superior catalytic activity (92%) for the synthesis of 5-benzyl-1H-tetrazole to that of other reported solid catalysts. Thus, it is expected that the developed porous tubular ZnO should find potential industrial application in the sensor as well as the catalysis field. Moreover, the synthesis from bulk ZnO makes the procedure cost effective.

Morphology-mediated tailoring of the performance of porous nanostructured Mn2O3 as an anode material

Tailoring of functional properties by varying the size and shape of porous nanostructured materials is an important frontier area of research. Herein, we report the successful synthesis of nanostructured Mn2O3 with desired 3D architectures such as porous hollow spheres, lotus shapes and tubular shapes, as well as aggregated nanoparticles, through the calcination of corresponding MnCO3 with the same architecture. Porous structures were formed upon evolution of CO2 during decomposition of the carbonate intermediate, and hollow structures were formed through a nonequilibrium interdiffusion process, i.e., the Kirkendall effect. The bare MnCO3 structures were synthesized using the chelating agents citric acid (CA), tartaric acid (TA), oxalic acid (OA), and ethylenediaminetetraacetic acid (EDTA), which mediated the growth of these MnCO3 structures by hydrothermal treatment of a precursor solution containing MnCl2, ammonium carbonate and chelating agent. A systematic evaluation of the effect of the morphology of the synthesized Mn2O3 on its performance as an anode material in Li-ion batteries reveals that the shape and the nature of pores of Mn2O3 strongly influence its Li-ion storage capacity. A superior specific capacity of 478 mAh g−1 is obtained for hollow spheres with 38% retention after 30 cycles compared to other shapes due its high accessible surface area and inner hollow architecture.

An amperometric cholesterol biosensor with excellent sensitivity and limit of detection based on an enzyme-immobilized microtubular ZnO@ZnS heterostructure

The controlled synthesis of porous nanostructure materials and their use as active materials for cholesterol sensing are of key importance in current research due to their exceptionally high surface area, their optical and electrical properties, and their good electron transport characteristics. In the present study we have synthesized a microtubular ZnO@ZnS heterostructure from the corresponding ZnO microtubes by a simple aqueous chemical sulphidation process. ZnS microtubes have also been synthesized by removal of ZnO from the ZnO@ZnS heterostructure using acetic acid. Both ZnO@ZnS and ZnS microtubes have a high surface area (56 and 68 m2 g−1, respectively) and a modified electronic structure. A ZnO@ZnS heterostructure-modified electrode has excellent amperometric cholesterol-sensing performance, with sensitivity 52.67 mA M−1 cm−2, signal to noise (S/N) ratio = 15, and limit of detection (LOD) 0.02 mM with S/N = 3. The sensing performance of the ZnO@ZnS heterostructure, including both sensitivity and LOD, is superior to that reported either for ZnO-based or Au- or Pt-modified sensors. Its superior performance originates both from its high microstructure-based surface area and its modified electronic structure, which facilitate electron transport to the electrode.

Heterogeneously Porous γ-MnO2-Catalyzed Direct Oxidative Amination of Benzoxazole through CH Activation in the Presence of O2

Oxidative amination of azoles through catalytic C-H bond activation is a very important reaction due to the presence of 2-aminoazoles in several biologically active compounds. However, most of the reported methods are performed under homogeneous reaction conditions using excess reagents and additives. Herein, we report the heterogeneous, porous γ-MnO2-catalyzed direct amination of benzoxazole with wide range of primary and secondary amines. The amination was carried under mild reaction conditions and using molecular oxygen as a green oxidant, without any additives. The catalyst can easily be separated by filtration and reused several times without a significant loss of its catalytic performance. Of note, the reaction tolerates a functional group such as alcohol, thus indicating the broad applicability of this reaction.

Rectangular ZnO porous nano-plate assembly with excellent acetone sensing performance and catalytic activity

The controlled synthesis of a hierarchically assembled porous rectangular ZnO plate (2.5–3.5 μm length, 1.5–2.5 μm width and 100–150 nm thickness) from bulk ZnO without using any organic substrates, such as solvents/surfactants/structure-directing agents, is presented. The synthesized ZnO plates are single crystalline with exposed (100) facets on the flat surface, porous and formed through the calcination of a hydrozincite [Zn5(CO3)2(OH)6] intermediate. A gas sensor based on the synthesized porous ZnO architecture exhibited high sensitivity towards acetone even in low concentration (S = 3.4 in 1 ppm acetone) with good selectivity. The ZnO nanostructured material as a heterogeneous catalyst also showed excellent catalytic activity for the synthesis of 5-substituted-1H-tetrazoles (yield = 94%). Both the activities are superior than those of other reported ZnO based acetone sensors and heterogeneous catalysts. We believe that the improved properties of the synthesized ZnO nanostructure is due to the exposed (100) facets, and its porous and assembled structure, which provides a reasonably large accessible surface area, and facilitates diffusion and mass transport of gas or substrate molecules.

Porous cesium impregnated MgO (Cs–MgO) nanoflakes with excellent catalytic activity for highly selective rapid synthesis of flavanone

Magnesium oxide (MgO) is an excellent base catalyst and its performance is well controlled by its morphology, surface area and surface structures. Here, a simple methodology for the synthesis of porous cesium impregnated MgO (Cs–MgO) nano flakes, with enhanced surface area (156 m2 g−1), basic properties and improved catalytic activity for flavanone synthesis, is presented. The synthesis of Cs–MgO nano flakes is performed through impregnation of CsNO3 on a nesquehonite [Mg(HCO3)OH·2H2O] rod, followed by calcination. During impregnation the metastable nesquehonite rod rehabilitated to hydromagnesite [4 MgCO3. Mg(OH)2·4H2O] flakes. The flakes were porous, constructed by building blocks of small nanoparticles (10–25 nm) with a large number of edges and corners, step edges and step corners and numerous base sites of various strength (surface hydroxyl groups, low coordinate O2− sites). It is observed that the amount of cesium in the MgO surface has a strong effect on its properties as well as its activity. The synthesized Cs–MgO nanoflakes showed significant improvement in the yield of flavanone through the Claisen–Schmidt condensation. A substantial increase in the reaction rate was also observed when DMF was used as a solvent without catalyst deactivation. As much as ∼90% conversion of 2′-hydroxyacetophenone with ∼81% selectivity of flavanone was observed in just 15–20 min using the synthesized 0.5% Cs loaded MgO nanoflakes as a catalyst and DMF as a solvent. The improved catalytic activity of Cs–MgO as a catalyst and the promotion effect of DMF is discussed by studying the interaction of the substrate and the solvent on the catalyst surface and identification of intermediates formed on the catalyst surface under the reaction conditions using FT-IR.