Imtiaz S. Mulla

Enhanced supercapacitance of multiwalled carbon nanotubes functionalized with ruthenium oxide

In this study we report the pseudocapacitive behavior of multiwalled carbon nanotubes (MWNTs) functionalized with hydrous ruthenium oxide in 1 M sulfuric acid for supercapacitor applications. The entangled network of nanotubes which forms open mesopores and their chemical stability with a basal geometry makes them suitable for such applications. Oxidative treatment to generate oxygenated functional groups on the tube-ends and along the sidewalls enables facile derivatization by hydrous ruthenium oxide to enhance the inherent capacitance. A specific capacitance of 80 F/g is obtained after ruthenium oxide functionalization, which is significantly greater than that of pristine MWNTs (30 F/g) in the same medium.

A highly selective ammonia gas sensor using surface-ruthenated zinc oxide

Preparation and characterisation of a highly selective surface-ruthenated zinc oxide sensor for ammonia in trace levels is demonstrated. The introduction of some RuO linkages on the surface of ZnO leads to a dramatic enhancement in the sensitivity (S) to 1000 ppm level of ammonia at 300°C (S=440) as compared to the similar response obtained for pure ZnO (S=1.5). A systematic study using various characterisation techniques such as EDAX, SEM, CV and XPS indicates the key role played by the amount and distribution of Ru species on the surface.

Field emission studies of novel ZnO nanostructures in high and low field regions

A study of the field emission characteristics of novel structures of ZnO, namely marigolds, multipods and microbelts, has been carried out in both the close proximity configuration and the conventional field emission microscope. The use of a conventional field emission microscope overcomes the drawback of arc formation at high field values. The nonlinearity in the Fowler-Nordheim (F-N) plot, a characteristic feature of semiconductors has been observed and explained on the basis of electron emission from both the conduction and the valence bands. The current stability exhibited by these structures is also promising for future device applications.

Ultralow threshold field emission from a single multipod structure of ZnO

The field emission of individual ZnO multipods and a single arm of a multipod structure grown by a vapor deposition were carried out. A current of 1 nA with an ultralow onset voltage of 40 V was observed repeatedly for the single multipod as well as for the arm. The nonlinearity observed in the Fowler–Nordheim plots have been interpreted on the basis of the theory of electron emission from semiconductors and a scheme explaining the field emission behavior in both the high- and low-field regions owing to the very high geometrical factor has been picturized.

Selective hydrogen sensing properties of surface functionalized tin oxide

A simple way of controlling the selectivity of tin oxide based hydrogen sensors by surface functionalization is demonstrated by introducing some misfit regions on tin oxide surface using covalent attachment of Ru, Pd and Ag species. The sensitivity and the selectivity of tin oxide is found to be dramatically enhanced in comparison with pure tin oxide. In addition, synergistic sensitivity effects are observed by simultaneously introducing several noble metal species on the surface of tin oxide, whereby the operating temperature for maximum sensitivity towards hydrogen is found to be drastically reduced. For example, pure tin oxide and surface ruthenated tin oxide shows highest sensitivity (8 and 120, respectively) towards hydrogen at 300°C, while tin oxide functionalized with Ru/Pd and Ru/Pd/Ag shows higher sensitivity (1350 and 360, respectively) at significantly lower temperatures (250 and 150°C, respectively). The amount and distribution of these heterospecies on the surface (affecting the surface electron states of tin oxide) are the most important parameters to be controlled to obtain optimum sensitivity and selectivity as a function of operating temperature.

The Fowler−Nordheim Plot Behavior and Mechanism of Field Electron Emission from ZnO Tetrapod Structures

Field emission measurements of current-voltage characteristics are reported for tetrapod structures of ZnO. The nonlinear Fowler-Nordheim (FN) plot is analyzed according to a model of calculation based on saturation of conduction band current and predominance of valence band current at high-field values. The simulated FN plot exhibits similar features to those observed experimentally. The model of calculation suggests that the slope variation of the FN plot, in the high-field and low-field regions, does not depend on the magnitude of saturation. Instead, it is a characteristic of the energy band structure and voltage-to-barrier-field conversion factor of the emitting material.

Controlled synthesis of ZnO from nanospheres to micro-rods and its gas sensing studies

1D ZnO rods are synthesized using less explored hydrazine method. Here we find, besides being combustible hydrazine can also be used as a structure-directing agent. The ratio of zinc nitrate (ZN) to hydrazine is found to control the morphology of ZnO. At lower concentration of ZN as compared with hydrazine the morphology of ZnO is found to be spherical. As we increase the hydrazine content the morphology changes from spherical (diameter approximately 100 nm) to the elongated structures including shapes like Y, T as well dumbbell (diameter approximately 40 nm and length approximately 150 nm). Interestingly for more than 50% of hydrazine ZnO micro-rods are formed. Such rods are of diameter approximately 120 nm having length of about 1 microm for ZN to hydrazine ratio of 1:9, isolated as well as bundle of rods are seen in scanning electron microscopy (SEM). The X-ray diffraction (XRD) reveals the phase formation with average particle size of 37 nm as calculated using Scherrers formula. The high-resolution transmission electron microscopy (HRTEM) is also done to confirm the d-spacing in ZnO. Gas sensing study for these samples shows high efficiency and selectivity towards LPG at all operating temperatures. Photoluminescence (PL) study for these samples is performed at room temperature to find potential application as photoelectric material.

Comparative studies of doped and surface modified tin oxide towards hydrogen sensing: Synergistic effects of Pd and Ru

Abstract A surface functionalised gas sensing material capable of giving better sensitivity and selectivity to hydrogen is demonstrated by grafting few Ru–O and Pd–O linkages in the surface of tin oxide. A systematic evaluation of the surface coverage, morphology and thermal stability using different techniques like EDAX, Cyclic voltammetry, SEM and TGA indicates the importance of an optimum distribution of Ru/Pd ratio (1.28) in dictating both sensitivity and selectivity. A comparison of the optimum performance of conventionally doped (225) and surface modified (1350) tin oxide to 1000 ppm hydrogen at 250°C illustrates the clear advantages offered by surface modified tin oxide in enhancing the sensitivity. Moreover, the synergistic effect by surface functionalisation with both Ru and Pd species is emphasised in comparison to individually modified tin oxide sensors.

Humidity sensing properties of surface functionalised polyethylene and polypropylene films

The use of different types of surface modified polymers such as polyethylene (PE) and polypropylene (PP) as humidity sensors has been investigated. Some of these surface functionalised polymers after controlled sulphonation have promising humidity-dependent resistance changes (10 9 -10 6 Ω with a change in the relative humidity (RH) from 30 to 95%) and also other favourable characteristics such as linearity and short response time. A systematic study of the humidity sensing behaviour with and without different types of surface functionalisation (sulphonation, lithiation and gold nanoclusters attachment) reveals that the sensitivity is controlled by surface structure and the extent of functionalisation, causing a change in carrier concentration and the mobility of protons and counter ions.

Self-assembled monolayers of diphenyl disulphide: a novel cathode material for rechargeable lithium batteries

The use of self-assembled monolayers (SAM) of organic disulfide as novel cathode materials for high specific energy, rechargeable, lithium batteries is demonstrated for the first time. The suitability of monolayer films of diphenyl disulfide (DDS) as cathode materials for facilitating reversible insertion and de-insertion of Li + ions is examined by means of cyclic voltammetry (CV), infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and charge-discharge measurements. The SAM-based cathodes on coupling with Li-metal anodes in 0.1 M LiClO 4 and tetrahydrofuran (THF) show good thermodynamic feasibility along with an open-circuit voltage of 2.9 V. The electrochemical capacity obtained is found, however, to fade during continuous cycling. This indicates a loss of electroactivity concomitant with the destruction of the monolayer functionalized cathode. The reasons for the coulombic efficiency of these rechargeable SAM-based cathodes are explained in terms of two different mechanistic modes of interaction of Li + ions with the monomolecular film.