Uday Narayan Maiti

Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm(2) current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far.

25th Anniversary Article: Chemically Modified/Doped Carbon Nanotubes & Graphene for Optimized Nanostructures & Nanodevices

Outstanding pristine properties of carbon nanotubes and graphene have limited the scope for real-life applications without precise controllability of the material structures and properties. This invited article to celebrate the 25th anniversary of Advanced Materials reviews the current research status in the chemical modification/doping of carbon nanotubes and graphene and their relevant applications with optimized structures and properties. A broad aspect of specific correlations between chemical modification/doping schemes of the graphitic carbons with their novel tunable material properties is summarized. An overview of the practical benefits from chemical modification/doping, including the controllability of electronic energy level, charge carrier density, surface energy and surface reactivity for diverse advanced applications is presented, namely flexible electronics/optoelectronics, energy conversion/storage, nanocomposites, and environmental remediation, with a particular emphasis on their optimized interfacial structures and properties. Future research direction is also proposed to surpass existing technological bottlenecks and realize idealized graphitic carbon applications.

Three‐Dimensional Shape Engineered, Interfacial Gelation of Reduced Graphene Oxide for High Rate, Large Capacity Supercapacitors

DOI: 10.1002/adma.201303503 Assembly of graphene into functional macroscopic objects, such as fi lms, [ 1 ] sheets, [ 2 ] fi bers, [ 3 ] foams, [ 4,5 ] and other complex architectures, [ 6 ] is of enormous research interest. How to attain desired structures in a cost effective and manufacturable manner is crucial for energy harvest/storage, catalysis, sensors and so on. Unlike fullerene or carbon nanotubes, whose assembly generally relies on weak van der Walls force or chemical modifi cation, two-dimensional graphene may straightforwardly exploit strong interlayer π – π stacking. Unfortunately, such a strong and directional interaction frequently results in graphitic stacking with minimal surface area. [ 7,8 ]

Liquid crystal size selection of large-size graphene oxide for size-dependent N-doping and oxygen reduction catalysis.

Graphene oxide (GO) is aqueous-dispersible oxygenated graphene, which shows colloidal discotic liquid crystallinity. Many properties of GO-based materials, including electrical conductivity and mechanical properties, are limited by the small flake size of GO. Unfortunately, typical sonochemical exfoliation of GO from graphite generally leads to a broad size and shape distribution. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction catalysis. In the biphasic GO dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously concentrated within the liquid crystalline phase. N-Doping and reduction of the size-selected GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO demonstrates quaternary dominant N-doping and the lowest onset potential (-0.08 V) for oxygen reduction catalysis, signifying that quaternary N-dopants serve as principal catalytic sites in N-doped graphene.

Low temperature solution processed ZnO/CuO heterojunction photocatalyst for visible light induced photo-degradation of organic pollutants

The possibility of integrating manifold functionalities, coupled with various associated noble interface phenomena in the hierarchical nanoforms, either comprised of geometrical intricacies or achieved via the rational coupling of several components, has made them immensely pertinent from both research and technological aspects. Here, an oxide based nanostructure hybrid has been realized by integrating low bandgap copper oxide nanosheet with high bandgap one dimensional zinc oxide nanowires on a flexible carbon cloth as well as on a flat substrate. These bandgap modulated hybrid nanostructures are generated for the efficient absorption of visible light, targeting their possible use in waste water management. Our work presents a novel ambient condition protocol for morphological tuning in the nanoscale or their organization in a hierarchical structure. Environmental remediation through catalytic activity under the visible light irradiation of the synthesized samples was inspected using both anionic and cationic dyes (methyl orange and Rhodamine B, respectively) as the model contaminants, where the optimized heterostructure exhibits significantly better performance than the mono component oxides. Such enhanced performance could be explained by the formation of favorable staggered gap multiple p–n junctions at ZnO/CuO interface, which in turn retards the photogenerated electron–hole pair recombination within the heterostructure. The signature of successful p–n junction formation at ZnO nanorod/CuO nanosheet interface has been identified via current–voltage measurements with a conducting tip AFM in contact mode. The creative designing of novel heterojunctions adopting this protocol will pave the way for the utilization of the entire visible light range: thus, offering potential in solar energy conversion devices.

Selective and Regenerative Carbon Dioxide Capture by Highly Polarizing Porous Carbon Nitride

Energy-efficient CO2 capture is a stringent demand for green and sustainable energy supply. Strong adsorption is desirable for high capacity and selective capture at ambient conditions but unfavorable for regeneration of adsorbents by a simple pressure control process. Here we present highly regenerative and selective CO2 capture by carbon nitride functionalized porous reduced graphene oxide aerogel surface. The resultant structure demonstrates large CO2 adsorption capacity at ambient conditions (0.43 mmol·g(-1)) and high CO2 selectivity against N2 yet retains regenerability to desorb 98% CO2 by simple pressure swing. First-principles thermodynamics calculations revealed that microporous edges of graphitic carbon nitride offer the optimal CO2 adsorption by induced dipole interaction and allows excellent CO2 selectivity as well as facile regenerability. This work identifies a customized route to reversible gas capture using metal-free, two-dimensional carbonaceous materials, which can be extended to other useful applications.

Ultra-thin graphene edges at the nanowire tips: a cascade cold cathode with two-stage field amplification

A multistage field emitter based on graphene-linked ZnO nanowire array is realized by means of spin-coating a graphene dispersion (reduced graphene oxide) over a nanostructured platform followed by plasma modification. Spin-coating leads to interlinking of graphene sheets between the neighboring nanowires whereas plasma etching in the subsequent step generates numerous ultra-sharp graphene edges at the nanowire tips. The inherent tendency of graphene to lay flat over a plane substrate can easily be bypassed through the currently presented nanostructure platform based technique. The turn-on and threshold field significantly downshifted compared to the individual components in the cascade emitter. Through the facile electron transfer from nanowires to graphene due to band bending at the ZnO-graphene interface together with multistage geometrical field enhancement at both the nanowire and graphene edges remain behind this enriched field emission from the composite cold cathode. This strategy will open up a new direction to integrate the functionalities of both the graphene array and several other inorganic nanostructure array for practical electronic devices.

Dopant-specific unzipping of carbon nanotubes for intact crystalline graphene nanostructures.

Atomic level engineering of graphene-based materials is in high demand to enable customize structures and properties for different applications. Unzipping of the graphene plane is a potential means to this end, but uncontrollable damage of the two-dimensional crystalline framework during harsh unzipping reaction has remained a key challenge. Here we present heteroatom dopant-specific unzipping of carbon nanotubes as a reliable and controllable route to customized intact crystalline graphene-based nanostructures. Substitutional pyridinic nitrogen dopant sites at carbon nanotubes can selectively initiate the unzipping of graphene side walls at a relatively low electrochemical potential (0.6 V). The resultant nanostructures consisting of unzipped graphene nanoribbons wrapping around carbon nanotube cores maintain the intact two-dimensional crystallinity with well-defined atomic configuration at the unzipped edges. Large surface area and robust electrical connectivity of the synergistic nanostructure demonstrate ultrahigh-power supercapacitor performance, which can serve for AC filtering with the record high rate capability of −85° of phase angle at 120 Hz.

Complementary p- and n-type polymer doping for ambient stable graphene inverter.

Graphene offers great promise to complement the inherent limitations of silicon electronics. To date, considerable research efforts have been devoted to complementary p- and n-type doping of graphene as a fundamental requirement for graphene-based electronics. Unfortunately, previous efforts suffer from undesired defect formation, poor controllability of doping level, and subtle environmental sensitivity. Here we present that graphene can be complementary p- and n-doped by simple polymer coating with different dipolar characteristics. Significantly, spontaneous vertical ordering of dipolar pyridine side groups of poly(4-vinylpyridine) at graphene surface can stabilize n-type doping at room-temperature ambient condition. The dipole field also enhances and balances the charge mobility by screening the impurity charge effect from the bottom substrate. We successfully demonstrate ambient stable inverters by integrating p- and n-type graphene transistors, which demonstrated clear voltage inversion with a gain of 0.17 at a 3.3 V input voltage. This straightforward polymer doping offers diverse opportunities for graphene-based electronics, including logic circuits, particularly in mechanically flexible form.

Room temperature deposition of ultra sharp ZnO nanospike arrays on metallic, non-metallic and flexible carbon fabrics: Efficient field emitters

Synthesis of large scale arrays of ZnO nanospikes with ultra sharp tips (<10 nm) is achieved through a newly developed chemical methodology. This is the first room temperature report of growing 1D ZnO nanostructures on heterogeneous substrates. The deposition was performed on various substrates including planar (silicon, conducting glass, copper foil), cylindrical (metallic wire) and flexible carbon cloth. The deposition technique is a universal one in the sense that it is compatible with complex shape substrates as well as independent of the nature of the substrate material. The arrangement of the nanospikes over the substrates may be altered from a dense array to a distributed flower like structure depending on the substrate pretreatment. The nanospikes exhibit excellent field emission performance caused by their emission beneficial geometry with turn-on and threshold fields showing significant dependence on the growth substrate. The turn-on field and the field enhancement factor are found to be the lowest and the highest for the ZnO deposited on carbon cloth substrates having values of 0.7 V μm−1 and 2.6 × 104 respectively. These are the best values compared to all reported chemically synthesized ZnO nanostructures and comparable to the best results of vapor phase deposited ZnO nanostructures. The synthesis method reported here will open-up a new door for future development of nano-based flexible electronics.