Mohammad Razaul Karim

Proton Conductivities of Graphene Oxide Nanosheets: Single, Multilayer, and Modified Nanosheets

Proton conductivities of layered solid electrolytes can be improved by minimizing strain along the conduction path. It is shown that the conductivities (σ) of multilayer graphene oxide (GO) films (assembled by the drop-cast method) are larger than those of single-layer GO (prepared by either the drop-cast or the Langmuir-Blodgett (LB) method). At 60% relative humidity (RH), the σ value increases from 1×10(-6) S cm(-1) in single-layer GO to 1×10(-4) and 4×10(-4) S cm(-1) for 60 and 200 nm thick multilayer films, respectively. A sudden decrease in conductivity was observed for with ethylenediamine (EDA) modified GO (enGO), which is due to the blocking of epoxy groups. This experiment confirmed that the epoxide groups are the major contributor to the efficient proton transport. Because of a gradual improvement of the conduction path and an increase in the water content, σ values increase with the thickness of the multilayer films. The reported methods might be applicable to the optimization of the proton conductivity in other layered solid electrolytes.

Graphene oxide and reduced graphene oxide hybrids with spin crossover iron(III) complexes

Graphene (rGO) based hybrid materials exhibiting electrical conductivity and spin crossover (SCO) behavior are reported. The non-conductive [Fe(qnal)2]nGO (1·GO) and [Fe(qsal)2]nGO (2·GO) hybrids have been prepared by employing the electrostatic interaction between the negatively charged graphene oxide (GO) nanosheet and the respective iron(III) complex cations in [Fe(qnal)2]+Cl− and [Fe(qsal)2]+Cl−. The conductive [Fe(qnal)2]nrGO (1·rGO) and [Fe(qsal)2]nrGO (2·rGO) hybrids were obtained by thermal reduction of 1·GO and 2·GO. 1·GO and 1·rGO exhibit SCO behavior, and 1·rGO also shows a light-induced excited spin state trapping (LIESST) effect. Thus, in 1·rGO the electrical conductivity of rGO and the SCO behavior of [Fe(qnal)2]+ coexist in a single structure. We propose that the observed cooperativity for the rGO nanosheet-bound iron(III) [Fe(qnal)2]+ SCO material occurs through formation of large domains via π–π stacking between the graphene skeleton and the [Fe(qnal)2]+ cations.

In situ oxygenous functionalization of a graphite electrode for enhanced affinity towards charged species and a reduced graphene oxide mediator

The weak affinity of bare carbon based electrodes for biological molecules or charged species is a major drawback for their direct application in analytical electrochemistry. We observed that the surfaces of graphite rods and glassy carbon (GC) ring electrodes can be modified by oxygenated functional groups through controlled electrochemical oxidation in aqueous media. Study of their cyclic voltammetry, surface conductivity, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, infrared spectroscopy (IR) and powder X-ray diffraction (PXRD) spectroscopy confirmed the chemical change. In the electrodynamics study, the modified GC ring in a rotating ring-disk electrode (RRDE) assembly showed better electron transfer efficiency than the virgin electrode when evaluated using a Ru(bpy)32+/3+ couple. In comparison to the virgin electrode, the modified graphite rod exhibited better affinity toward polyvinylpyrrolidone protected reduced graphene oxide (PVP-rGO) attached to glucose oxidase enzymes (GOx), and showed the direct attachment of mediators to the oxygenated electrode surface. These observations imply that a small oxygenated layer on the carbon electrode surface can significantly increase its activity. The present evidence indicates the possibility for the oxygenous functionalization of carbon based electrodes for applications in areas like electrokinetics studies and biosensing, where strong analyte–electrode interactions are useful.

Proton conductors produced by chemical modifications of carbon allotropes, perovskites and metal organic frameworks

Proton conductors are distinct from electronic conductors by virtue of their low conductivity values and the necessity for the presence of a third material as a carrier to transport protons. Proton conductors have applications in fuel cells, hydrogen separating phases, steam electrolysis, sensors and biological transport systems. Though naturally occurring proton conductors show very poor conductance, designing superionic conductors is possible nowadays due to the improvements in porosity and interlayer oriented proton conduction tracks, alignment of particle conduction sites, hydration dynamics, water uptake capacity and stability. Insight into all the possible ways for improving the proton conductivity values of materials needs to be considered for designing new types of superionic conductors. Based on this necessity, herein, we have reviewed the gradual trend in proton conductivities in ancient type perovskites and ceramics, metal organic frameworks and the most recently developed oxidized carbon allotropes. As the strategies for improving the conduction tracks and proton migration behaviours of these three classes are not the same, they are discussed in different sections, in light of their intrinsic properties, scope of modification and structures. By consideration of a detailed list of updated articles, reports associated with strategies for obtaining gradually increased proton conductivities, including the improvement of conduction tracks, assembly of particles and increase in water content, have been focused on for detailed discussion. Furthermore, an in-depth discussion on the function of proton conductors in energy applications and biological transport systems has been reviewed concisely.

Superionic conductivity in hybrid of 3‐hydroxypropanesulfonic acid and graphene oxide

Insertion of 3-hydroxypropanesulfonicacid (HPS) in the graphene oxide (GO) interlayer results in high proton conductivity (10-2 -10-1  S cm-1 ), owing to an improvement in oxygen content, interlayer distance and water absorbing capacity. This result indicates that hydroxyalkylsulfonicacids can be perfect guest molecules for improving the proton conductivity of carbon materials.

Surface Modification of the ZnO Nanoparticles with γ-Aminopropyltriethoxysilane and Study of Their Photocatalytic Activity, Optical Properties and Antibacterial Activities

Abstract Surface modification of Zinc oxide nanoparticles (ZnO) with γ-aminopropyltriethoxy silane (APTES) was investigated. Successful surface modification of the nanoparticles was confirmed experimentally by X-ray Photoelectron Spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The effect of the surface modifier concentration on the grafting density and surface area was studied by CHN elemental analysis and Brunauer–Emmett–Teller (BET) analysis. The photocatalytic activity and UV shielding ability of the surface-modified particles prepared in water-ethanol solvent in the presence of the surface modifiers were compared to those of non-modified particles. As a case study, It was observed by methylene blue (MB) dye degradation experiment that the photocatalytic activity in the presence of modified nanoparticles was lower than that observed with non-modified ZnO nanoparticles. Dispersion stability tests visually showed that APTES grafted nanoparticles had acquired better stability than non-modified ZnO nanoparticles in aqueous solution.

Role of hydrophilic groups in acid intercalated graphene oxide as a superionic conductor

Hybrid materials that are obtained from intercalation of different polar and hydrophilic acid guest molecules within graphene oxide (GO) have been found to exhibit high proton conductivity. The accommodation of these guest precursors within the GO interlayer takes place through weak physical forces. PXRD patterns with enhanced interlayer distance confirmed successful intercalation processes. Raman and IR spectral data reveal the absence of any covalent bond between the functional groups of the guest and the GO host. TGA data confirms improved water adsorbing capacity of the hybrids. At ambient conditions and 90% relative humidity (RH), a RH dependent impedance study shows that intercalation of formic acid (FA) and phosphoric acid (PA) within GO results in ∼7 times higher proton conductivity compared with that for the pristine GO sample. The low activation energy values suggest that proton conduction in the samples is aided by the Grotthuss mechanism. Improvement in the water adsorbing capacity is primarily responsible for such high proton conductivity. The current study suggests that cheap and environmentally friendly GO-based intercalated hybrid materials, such as GO/PA and GO/FA with enhanced proton conductivity, can be appropriate replacements for the expensive Nafion-based solid electrolytes.

Reduced graphene oxide–transition metal hybrids as p-type semiconductors for acetaldehyde sensing

Acetaldehyde gas sensing using hybrids of reduced graphene oxide (rGO) and transition metal elements, rGO–M (M = oxide/hydroxide of Mn, Fe, Co and Ni) has been investigated. Thin films of GO, rGO and rGO–M on conductive glass were deposited through simple and affordable techniques and characterized using Raman spectroscopy, powder X-ray diffraction patterns, field emission scanning electron microscopy and electrical conductivity measurements. Concentration dependent resistances during the oxidation of acetaldehyde gas in air (10 to 50%) were investigated by using a sensing probe devised from rGO, GO–M and the rGO–M hybrids, of which rGO–Ni exhibited the maximum sensitivity. In rGO–Ni, in combination with rGO the NiOOH precursor can capture electrons (generated from acetaldehyde oxidation) through the holes, while indicating rGO–Ni to be a p-type semiconductor. This report implies the possibility of developing inexpensive graphene based p-type semiconductors for sensing other gases as well.

Reduced graphene oxide-transition metal hybrids for hydrogen generation by photocatalytic water splitting

We report the photocatalytic activities of graphene oxide-metal ion hybrid namely rGO–M (rGO is reduced graphene oxide and M = Mn2+, Fe3+, Ni2+, Co2+ and Al3+). The hybrids exhibit photocatalytic behavior to split water and generate hydrogen gas under UV light irradiation at room temperature.

Proton Conductivity of Graphene Oxide on Aging

The aging effect on the proton conductivity of graphene oxide (GO) is investigated. Characterizations of oxygenated functional groups and measurement of the proton conductivity have been performed using freshly prepared GO and the same sample after preserving for three years under ambient conditions. Although GO retains its layered structure, a slight deviation in its powder X-ray diffraction (PXRD) pattern and Raman spectra upon aging implies some changes in the interlayer distance and functional groups. Decomposition of epoxy groups on aging has been recognised by X-ray photoelectron spectroscopy (XPS) analysis. The proton conductivity was found to be reduced by 25 % after three years of aging.