Covalent functionalization has been effectively employed to attach benzene functionalities to MoS and MoSe nanosheets by the reaction with para-substituted iodobenzenes bearing −OCH, −H, and −NO as the substituents, where the electron-donating and electron-withdrawing power of the para substituent varies significantly. The functionalization is based on the formation of a C−S or C−Se linkage at the expense of the C−I bond on reaction of the iodobenzene with electron-rich 1T-MoS or 1T-MoSe. The degree of functionalization is in the range 4–24 % range, the value increases with the electron-withdrawing power of the para substituent. Semiconducting 2H-MoS and 2H-MoSe nanosheets can also be functionalized with iodobenzene by carrying out the reaction in the presence of a Pd catalyst. We have also carried out functionalization of 1T-MoS with pyrene, coumarin, and porphyrin derivatives. By using first-principles density functional calculations, we show that the bonding of the functional groups with the 1T phase is stronger than with the 2H phase. This is reflected in notable changes in the electronic structure of the former upon functionalization; a gap opens up in the electronic spectrum of the 1T phase. Functionalization with para-substituted benzenes leads to a change in the work function.

Vishnoi Pratap    Sampath Archana    Umesh vasudeo Waghmare    C. Rao    

Chemistry - A European Journal

2017

Ultrathin epitaxial films (10-90 nm thick) of VO have been grown on c-AlO by atomic layer deposition using vanadyl acetylacetonate as the vanadium precursor along with oxygen plasma. Various process parameters have been optimized for the purpose, and excellent crystalline films could be obtained below 200 °C, without the need for post-heat treatment. With a moderate temperature window, the process yields a growth rate of 0.45 A/ cycle. The films have been characterized by electron microscopy, atomic force microscopy, Raman spectroscopy, and other means. The films exhibit a (001) preferred orientation with respect to c-AlO and undergo compressive strain at the initial few monolayer growth to adjust epitaxially with the substrate. Heterojunction diodes based on TiO (p)-(n)VO as well as a humidity sensor have been fabricated using the VO films.

Sreedhara    Ghatak    Bharath B.    C. Rao    

ACS Applied Materials and Interfaces

2017

Substitution of aliovalent anions in metal oxides and chalcogenides significantly affects the electronic structure and properties of the materials. Thus, substitution of P and Cl in CdS decreases the band gap and favorably influences the photocatalytic activity. Complete substitution of a trivalent (A) and a monovalent (B) anions in a cadmium chalcogenides, CdX, should give rise to a material of the composition Cd AB or CdAB, but a compound with the composition CdPCl (or CdPCl) is obtained in the case of CdS. We have investigated the analogous compounds, CdAsBr and CdSbI, wherein the anions in CdSe and CdTe are substituted by As, Br and Sb, I respectively. These compounds are direct band gap semiconductors with a band gap of 1.8–1.9 eV and a photoluminescence band in the visible region. First-principles calculations show both CdAsBr and CdSbI to be direct band gap semiconductors. The arsenic bromide is predicted to be photochemically more active for HER than the antimony iodide.

Roy Anand    Suchitra    Manjunath Krishnappa    Ahmad Tokeer    Umesh vasudeo Waghmare    C. Rao    

Solid State Communications

2017

Generation of hydrogen by photochemical, electrochemical, and other means is a vital area of research today, and a variety of materials have been explored as catalysts for this purpose. CN, MoS, and nitrogenated RGO (NRGO) are some of the important catalytic materials investigated for the hydrogen evolution reaction (HER) reaction, but the observed catalytic activities are somewhat marginal. Prompted by preliminary reports that covalent cross-linking of 2D materials to generate heteroassemblies or nanocomposites may have beneficial effect on the catalytic activity, we have synthesized nanocomposites wherein CN is covalently bonded to MoS or NRGO nanosheets. The photochemical HER activity of the CN-MoS nanocomposite is found to be remarkable with an activity of 12778 μmol h g and a turnover frequency of 2.35 h. The physical mixture of CN and MoS, on the other hand, does not exhibit notable catalytic activity. Encouraged by this result, we have studied electrochemical HER activity of these composites as well. CN-MoS shows superior activity relative to a physical mixture of MoS and CN. Density functional theory calculations have been carried out to understand the HER activity of the nanocomposites. Charge-transfer between the components and greater planarity of cross-linked layers are important causes of the superior catalytic activity of the nanocomposites. Covalent linking of such 2D materials appears to be a worthwhile strategy for catalysis and other applications.

Pramoda    Gupta Uttam    Chhetri Manjeet    Bandyopadhyay A.    Swapan Kumar Pati    C. Rao    

ACS Applied Materials and Interfaces

2017

Borocarbonitrides, BCN, constitute a new family of layered two-dimensional materials and can be considered to be derived from graphene. They can be simple composites containing graphene and BN domains or more complex materials possessing B-C and C-N bonds besides B-N and C-C bonds. Properties of these materials depend on the composition, and the method of synthesis, wherein one can traverse from the insulating end (BN) to the conducting end (graphene). In this article, we present an up-to-date review of the various aspects of borocarbonitrides including synthesis, characterization and properties. Some of the properties have potential applications, typical of them being in gas adsorption and energy devices such as supercapacitors, fuel cells and batteries. Performance of borocarbonitrides as catalysts in the electrochemical hydrogen evolution reaction is impressive. It is noteworthy that with certain compositions on borocarbonitrides, field-effect transistors can be fabricated.

C. Rao     Gopalakrishnan K.   

ACS Applied Materials and Interfaces

2017

Generation of H from water has emerged to be an important alternative for replacing fossil fuels. In this context, a variety of materials has been investigated as electrocatalysts or photocatalysts for the reduction of water. Of these, transition metal dichalcogenides have emerged to be effective catalysts yielding good hydrogen evolution activity. This is particularly true of photocatalysis. MoS in the form of nanosheets exhibits particularly good activity in its metallic form and also in covalently linked composites with CN and such two-dimensional (2D) materials. In this article, we present both electrochemical and photochemical catalysis by 2D-transition metal dichalcogenides in the hydrogen evolution reaction.

Gupta Uttam    C. Rao    

Nano Energy

2017

Graphene, MoS and similar layered materials are emerging with a variety of potential applications in energy devices. In particular, they perform well as catalysts in supercapacitors as well as in the Oxygen Reduction Reaction (ORR) of fuel cells. This chapter presents a brief survey of supercapacitors and ORR catalysts based on graphene, MoS and related materials.

Gopalakrishnan K.    C. Rao    

Two-Dimensional Nanostructures for Energy-Related Applications

2017

Metal–organic frameworks (MOFs) are exceptional as gas adsorbents but their mechanical properties are poor. We present a successful strategy to improve the mechanical properties along with gas adsorption characteristics, wherein graphene (Gr) is covalently bonded with M/DOBDC (M=Mg, Ni, or Co, DOBDC=2,5-dioxido-1,4-benzene dicarboxylate) MOFs. The surface area of the graphene–MOF composites increases up to 200–300 m g whereas the CO uptake increases by ca. 3–5 wt % at 0.15 atm and by 6–10 wt % at 1 atm. What is significant is that the composites exhibit improved mechanical properties. In the case of Mg/DOBDC, a three-fold increase in both the elastic modulus and hardness with 5 wt % graphene reinforcement is observed. Improvement in both the mechanical properties and gas adsorption characteristics of porous MOFs on linking them to graphene is a novel observation and suggests a new avenue for the design and synthesis of porous materials.

Kumar Ram    Raut Devaraj    Upadrasta Ramamurty    C. Rao    

Angewandte Chemie - International Edition

2016

Understanding the mechanical properties of ultra-thin films formed by the self-assembly of molecules/nanoparticles/colloids at fluid-fluid interfaces is central to many technological applications. Here, we have carried out interfacial rheology measurements to systematically investigate the concentration dependent viscoelastic response of 2D films of Fullerene C at the air-water interface. With increasing C concentration, amplitude sweep measurements show that the films undergo a transition from viscoelastic liquid-like to viscoelastic solid-like behaviour. Interestingly, for high C60 concentrations's, the loss modulus G″ reaches a maximum before the onset of power-law shear-thinning in G′, the storage modulus, and G″. The power-law exponents have a ratio 2. This response is typical of systems that show soft glassy behaviour. We also observe a power-law increase in G′ and G″ at low frequencies in the frequency response measurements and a transition from Newtonian to shear-thinning behaviour, with increasing shear rate, in steady shear measurements. Our results are in qualitative agreement with the phenomenological soft glassy rheology model.

Santhosh V.    Voggu Rakesh    Chaturbedy Piyush    Ganapathy Rajesh    C. Rao    

Carbon

2016

We report superior hydrogen evolution activity of metal-free borocarbonitride (BCN) catalysts. The highly positive onset potential (-56 mV vs. RHE) and the current density of 10 mA cm at an overpotential of 70 mV exhibited by a carbon-rich BCN with the composition BCN demonstrates the extraordinary electrocatalytic activity at par with Pt. Theoretical studies throw light on the cause of high activity of this composition. The high activity and good stability of BCNs surpass the characteristics of other metal-free catalysts reported in recent literature.

Chhetri Manjeet    Maitra Somak    Chakraborty Himanshu    Umesh vasudeo Waghmare    C. Rao    

Energy and Environmental Science

2016