Urmimala Maitra

Graphene analogues of inorganic layered materials.

The discovery of graphene has created a great sensation in chemistry, physics, materials science, and related areas. The unusual properties of graphene have aroused interest in other layered materials, such as molybdenum sulfide and boron nitride. In the last few years, single- as well as few-layer as well as chalcogenides and other inorganic materials have been prepared and characterized by a variety of methods. These materials possess interesting properties, and some have potential applications. This Review provides an up-to-date account of these emerging two-dimensional nanomaterials. Not only are the synthesis and characterization covered, but also important aspects such as spectroscopic and optical properties, magnetic and electrical properties, as well as applications. Salient features of the composites formed from the layered inorganic structures with graphene and polymers are presented along with a brief description of borocarbonitrides.

Highly Effective Visible-Light-Induced H2Generation by Single-Layer 1T-MoS2and a Nanocomposite of Few-Layer 2H-MoS2with Heavily Nitrogenated Graphene

Two sorts of MoS2 : A single-layer, metallic form of MoS2 (1T-MoS2 ) and a nanocomposite of a second form of MoS2 (few-layer 2H-MoS2 ) with heavily nitrogenated reduced graphene oxide (NRGO; N content ca. 15 %) show outstanding performance in the production of H2 under visible-light illumination.

Extraordinary synergy in the mechanical properties of polymer matrix composites reinforced with 2 nanocarbons

One of the applications of nanomaterials is as reinforcements in composites, wherein small additions of nanomaterials lead to large enhancements in mechanical properties. There have been extensive studies in the literature on composites where a polymer matrix is reinforced by a single nanomaterial such as carbon nanotubes. In this article, we examine the significant synergistic effects observed when 2 different types of nanocarbons are incorporated in a polymer matrix. Thus, binary combinations of nanodiamond, few-layer graphene, and single-walled nanotubes have been used to reinforce polyvinyl alcohol. The mechanical properties of the resulting composites, evaluated by the nanoindentation technique, show extraordinary synergy, improving the stiffness and hardness by as much as 400% compared to those obtained with single nanocarbon reinforcements. These results suggest a way of designing advanced materials with extraordinary mechanical properties by incorporating small amounts of 2 nanomaterials such as graphene plus nanodiamond or nanodiamond plus carbon nanotube.

Chemical storage of hydrogen in few-layer graphene

Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing up to 5 wt % of hydrogen. Spectroscopic studies reveal the presence of sp3 C-H bonds in the hydrogenated graphenes. They, however, decompose readily on heating to 500 °C or on irradiation with UV or laser radiation releasing all the hydrogen, thereby demonstrating the possible use of few-layer graphene for chemical storage of hydrogen. First-principles calculations throw light on the mechanism of dehydrogenation that appears to involve a significant reconstruction and relaxation of the lattice.

Unusual magnetic properties of graphene and related materials

High-temperature ferromagnetism in graphene and other graphite-derived materials reported by several workers has attracted considerable interest. Magnetism in graphene and graphene nanoribbons is ascribed to defects and edge states, the latter being an essential feature of these materials. Room-temperature ferromagnetism in graphene is affected by the adsorption of molecules, especially hydrogen. Inorganic graphene analogues formed by layered materials such as BN and MoS2 also show such ferromagnetic behaviour. Magnetoresistance observed in graphene and graphene nanoribbons is of significance because of the potential applications.

Importance of trivalency and the e(g)(1) configuration in the photocatalytic oxidation of water by Mn and Co oxides.

Prompted by the early results on the catalytic activity of LiMn2O4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn2O3, LaMnO3, and MgMn2O4 are found to be very good catalysts with turnover frequencies of 5 × 10−4 s−1, 4.8 × 10−4 s−1, and 0.8 ×10−4 s−1, respectively. Among the cobalt oxides, Li2Co2O4 and LaCoO3—especially the latter—exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10−4 s−1 and 1.4 × 10−3 s−1, respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co3+ in the Co oxides is in the intermediate t2g5eg1 state whereas Mn3+ is in the t2g3eg1 state. The presence of the eg1 electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.

Characterization of few-layer 1T-MoSe2 and its superior performance in the visible-light induced hydrogen evolution reaction

Based on earlier results on the photocatalytic properties of MoS2, the 1T form of MoSe2, prepared by lithium intercalation and exfoliation of bulk MoSe2, has been employed for the visible-light induced generation of hydrogen. 1T-MoSe2 is found to be superior to both 2H and 1T MoS2 as well as 2H-MoSe2 in producing hydrogen from water, the yield being in the 60–75 mmol h−1 g−1 range with a turn over frequency of 15–19 h−1. First principles calculations reveal that 1T-MoSe2 has a lower work function than 2H-MoSe2 as well as 1T and 2H-MoS2, making it easier to transfer an electron from 1T-MoSe2 for the production of H2.

Photoluminescence, white light emitting properties and related aspects of ZnO nanoparticles admixed with graphene and GaN

ZnO nanoparticles exhibit a broad band centred around 530 nm in the photoluminescence (PL) spectrum due to the presence of oxygen vacancies. Composites of ZnO nanoparticles with graphenes show marked changes in the PL spectrum with broad bands covering the entire visible region, making them candidates for solid state lighting, while graphene prepared by arc discharge of graphite in a hydrogen atmosphere (HG) containing 2-3 layers as well as boron-doped (BHG) and nitrogen-doped (NHG) samples of HG give white light when admixed with ZnO nanoparticles; excellent results are obtained with the addition of just 7 wt% of BHG to the ZnO nanoparticles. Mixtures of ZnO and GaN nanoparticles also exhibit white light emission. The quantum yields of these ZnO nanoparticle based white light sources are in the 4-6% range. Photoconductivity characteristics of ZnO nanoparticles are affected by the addition of even a small amount of graphene (<0.5 wt%).

Synthesis, characterization, photocatalysis, and varied properties of TiO2 cosubstituted with nitrogen and fluorine.

TiO2 (anatase) codoped with nitrogen and fluorine, synthesized by a simple solid state route, using urea and ammonium fluoride as sources of nitrogen and fluorine, respectively, as well as by decomposition of (NH4)2TiF6 for comparison, has been characterized by various techniques. XPS analysis shows the composition to be TiO1.7N0.18F0.12 for urea-based method (N, F-TiO2-urea) and TiO1.9N0.04F0.06 for complex decomposition method (N, F-TiO2-complex). Both the materials are defect-free as revealed by photoluminescence spectroscopy. Thus, N, F-TiO2-urea exhibits smaller defect-induced magnetization compared to the nitrogen-doped sample. Cosubstitution of N and F is accompanied with an enhancement of the absorption of light in the visible region giving rise to yellow color and with a band gap of ∼2.2 eV in the case of N, F-TiO2-urea. It exhibits enhanced photocatalytic activity and also significant hydrogen evolution (400 μmol/g) on interaction with visible light in the absence of any cocatalyst, which is much higher compared to N, F-TiO2-complex and N-TiO2. First-principles calculations show significant local distortions on codoping TiO2 with N and F and a lowering of energy by 1.93 eV per N, F pair. With virtual negative and positive charges on nitrogen and fluorine, respectively, the dopants prefer pairwise clustering. Our calculations predict a reduction in the band gap in TiO2 cosubstituted with nitrogen and fluorine. The calculated band structure shows that nitrogen 2p states form a separate subband just above the valence band which is enhanced on incorporation of fluorine. Our calculations also indicate anomalous Born effective charges in N, F-TiO2 and predict enhanced photocatalytic activity on codoping of TiO2 by N and F.

Strategies for the Synthesis of Graphene, Graphene Nanoribbons, Nanoscrolls and Related Materials

Single-layer graphene (SLG), the 3.4 Å thick two-dimensional sheet of sp(2) carbon atoms, was first prepared in 2004 by mechanical exfoliation of graphite crystals using the scotch tape technique. Since then, SLG has been prepared by other physical methods such as laser irradiation or ultrasonication of graphite in liquid media. Chemical methods of synthesis of SLG are more commonly used; the most popular involves preparation of single-layer graphene oxide followed by reduction with a stable reagent, often assisted by microwave heating. This method yields single-layer reduced graphene oxide. Other methods for preparing SLG include chemical vapour deposition over surfaces of transition metals such as Ni and Cu. Large-area SLG has also been prepared by epitaxial growth over SiC. Few-layer graphene (FLG) is prepared by several methods; arc discharge of graphite in hydrogen atmosphere being the most convenient. Several other methods for preparing FLG include exfoliation of graphite oxide by rapid heating, ultrasonication or laser irradiation of graphite in liquid media, reduction of few-layer graphene oxide, alkali metal intercalation followed by exfoliation. Graphene nanoribbons, which are rectangular strips of graphene, are best prepared by the unzipping of carbon nanotubes by chemical oxidation or laser irradiation. Many graphene analogues of inorganic materials such as MoS(2), MoSe(2) and BN have been prepared by mechanical exfoliation, ultrasonication and by chemical methods involving high-temperature or hydrothermal reactions and intercalation of alkali metals followed by exfoliation. Scrolls of graphene are prepared by potassium intercalation in graphite or by microwave irradiation of graphite immersed in liquid nitrogen.