Muthana Ali, Shane P. Meaney, Md. Joynul Abedin, Phillip Holt, Mainak Majumder, Rico F. Tabor, “Graphene oxide–silica hybrid capsules for sustained fragrance release”, J. Colloid Interface Sci., 2019, 552, pp 528-539
Encapsulation of active or valuable cargoes has become one of the most important methods for controlled delivery and release. However, many existing capsule technologies suffer from scalability issues, and capsules from surfactant- or polymer-stabilised emulsions tend to have weak shells or limited stability. Here we present a robust and scalable method for the surfactant-free preparation of silica hybrid capsules templated from Pickering emulsions stabilised by graphene oxide. These capsules are produced using a single step, undemanding formulation process with cheap and scalable precursors. The mechanical and chemical stability provided by the silica shell grown around these droplets is explored using surface pressure measurements and atomic force microscopy, demonstrating that a rigid and robust capsule is produced from higher loadings of silica precursor. In order to demonstrate the utility of these capsules, the sustained release of a fragrance molecule (vanillin) from the capsules is monitored, and compared to release from unencapsulated vanilla oil. It is seen that the capsules retain the fragrance for multiple weeks, offering new pathways for scalable encapsulation systems for the delivery of valuable actives.
Tanesh D. Gamot, Arup R. Bhattacharyya
,Tam Sridhar, Alex J. Fulcher, Fiona Beach, Rico F. Tabor, Mainak Majumder “Enhanced Thermal Conductivity of High Internal Phase Emulsions with Ultra-Low Volume Fraction of Graphene Oxide” , : Langmuir 2019, 35, 2738−2746
Thermal conductivity enhancement in a multiphase fluid such as water-in-oil emulsion can substantially improve efficacies in a broad range of applications. However, nanoparticle additives that are often used to do so can catastrophically destabilize a delicate emulsion system, in our case, a high internal phase emulsion (HIPE), whereas
Zhifeng Yi, Andrea Merenda, Lingxue Kong, Aleksandra Radenovic, Mainak Majumder, Ludovic F. Dumée,“Single step synthesis of Schottky-like hybrid graphene – titania interfaces for efficient photocatalysis”, : Scientific reports, 2018, 8, 8154
The development of 2D nanomaterial coatings across metal surfaces is a challenge due to the mismatch between the metal microstructure and the nanoscale materials. The naturally occurring thin oxidative layer present across all metal surfaces, may lead to low adherence and connectivity. In this paper, graphene/titania/Titanium hybrid flms were for the frst time fabricated by a single step chemical vapour deposition process across Titanium foils. The presence of graphene as a dopant was found to enhance the photocatalytic performance of the fnal products, applied to the degradation of organic molecules and to lead to Schottky-like junction formation at the metal/oxide interface. These Schottky junctions, where vacancies are present across the titania material due to the graphene doping and where Ti3+ ions are predominantly located, yield enhanced catalytic performance. The highest degradation rate was found to be 9.66×10−6min−1, achieved by the sample grown at 700°C for 5min, which was 62% higher than the sample just treated at that temperature without graphene growth. This work provides evidence that graphene may be grown across pure Titanium metal and opens new avenues in biomedical devices design, tribological or separation applications.
Abozar Akbari, Sally E. Meragawi, Samuel T. Martin, Ben Corry, Ezzatollah Shamsaei, Christopher D. Easton, Dibakar Bhattacharyya, Mainak Majumder,“ Solvent Transport Behavior of Shear Aligned Graphene Oxide Membranes and Implications in Organic Solvent Nanofiltration
”, ACS Appl. Mater. Interfaces 2018, 10, 2067−2074
Solvent transport in membranes composed of stacked sheets of graphene oxide (GO) with molecular scale channels and a complex arrangement of hydrophobic and hydrophilic domains is not well understood. Here, we observe that the interlayer space between GO sheets expands in different solvents without disturbing the membrane integrity and is typically larger in aqueous media compared to nonaqueous media. However, the membranes have a tighter molecule sieving feature in aqueous media as demonstrated by lower permeance and higher solute rejection arising from interfacial water layers “sticking” to charged polar groups. As a result of this polar interaction, the permeance of polar solvents in GO membrane scales inversely to the polarity of the solvent, which is contrary to other polymeric and ceramic hydrophilic membranes and also scales inversely to the viscosity of solvents as per continuum expectations. We highlight the extended solvent-handling space of GO membranes, such as in polar protic, polar aprotic, and nonpolar solvents, demonstrating versatility over a commercial nanofiltration membrane, and we predict exciting new applications in advanced separation engineering.
M. Shaibani, S.J.D. Smith, P. Chakraborty Banerjee, K. Konstas, A. Zafari, D.E Lobo, M. Nazari, A. F Hollenkamp, M R Hill, M. Majumder “Framework-mediated synthesis of highly microporous onion-like carbon: energy enhancement in supercapacitors without compromising power”, J. Mater. Chem. A, 2017, Advance Article
High power technologies require the delivery of large amounts of energy in brief time periods – a demand that cannot be met by the current materials used in supercapacitors because they do not store charge efficiently at high discharge rates. To address this unmet challenge, high surface area graphitic carbons with a large fraction of micropores, for increased capacity, and high pore connectivity in addition to high conductivity, for fast current-switching response, are needed. Typically, the high temperatures required for the transformation of carbonaceous precursors into graphitic networks collapses the micropores, so that even the most promising morphology in a precursor is lost during carbonisation. Here we demonstrate that a high surface area diamondoid framework (PAF-1) under moderate temperatures (≤900 °C) undergoes topology-preserved conversion into a onion-like carbon (OLC) material which uniquely exhibits high specific surface area (1084 m2 g−1), narrow, predominantly 5 Å pores and high electrical conductivity (2.8 S cm−1) – we term this as microporous onion-like carbon (MOLC). We measure capacitance of 211 F g−1 at 5 mV s−1, and unprecedented 52% capacitance retention at ultrafast 2000 mV s−1 – this property is a direct consequence of the curved graphitic planes of the OLCs incorporating microporosity from the precursor structure. Despite the fact that the micropores in our OLC are small and narrow, we show that optimal matching of the electrolyte (on the basis of solvated ionic radii) to the pore size enables us to achieve high power and energy densities simultaneously (—e.g., 3.85 W h kg−1 can be extracted at 27.7 kW kg−1). Also, MOLC electrodes with high-mass loading (8 mg cm−2) demonstrate remarkable areal capacitances (≈3 F cm−2) over a varied range of high discharge currents, which is among the highest reported for supercapacitor electrodes.
Parama Chakraborty Banerjee, Derrek E. Lobo, Tim Williams, Mahdokht Shaibani, Matthew R. Hill, Mainak Majumder, “Graphitic carbon nanofiber growth from catalytic-metal organic frameworks & their electrochemical double layer properties”, J. Mater. Chem. A, 2017,5, 25338-25349
Metal–organic frameworks have been widely studied as a template to nanoporous or well-structured carbon materials; however, utilizing the catalytic properties of MOFs to grow graphitic one dimensional nanostructures has been rarely reported. We have studied the evolution of carbon structures produced by transition metal doping (Ni in our case) in a Zn-based MOF and subjecting the MOF to a typical chemical vapour deposition process. Structures ranging from fine to thick graphitized carbon nanofibers are produced systematically at a relatively low temperature of 650 °C by increasing the Ni concentration in the MOF. A general trend of an increase in the nanofiber diameter with an increase in the Ni concentration and the formation of plate-like graphitic structures suggest that the catalyst particles undergo coalescence and impact the morphology of carbon structures produced. Without any further modification, the carbon nanofibers show modest electrical double layer (18 F g−1) behaviour in 1 M Na2SO4, but quite interestingly, the finely divided Ni particles undergo faradaic redox reactions in 1 M KOH and boost the capacitance to 348 F g−1. Our work suggests that our approach of utilizing catalytic MOFs for the growth of well-defined carbon nanostructures can serve more than one purpose towards the synthesis of hybrid materials for energy storage.
Mahdokht Shaibani, Anthony F Hollenkamp, Matthew R Hill, Mainak Majumder “Permselective membranes in lithium–sulfur batteries”, Current Opinion in Chemical Engineering 2017,16:31–38
Integration of permselective membranes – as a subset of the separator systems – in the configuration of lithium–sulfur battery, is a relatively simple solution to tackle the issue of polysulfide dissolution into the electrolyte. Cation-selective materials such as graphene oxide (GO) and Nafion have demonstrated to serve effectively as electrostatic shields for polysulfide anions, retarding their diffusion to the anode side of the battery. Looking into the future, the introducing of these materials in the configuration/composition of multifunctional/multilayered separator systems will promote the high efficiency cycling of high content sulfur cathodes. Enhanced performance metrics and careful minimisation of the mass/volume of these extra components should result in enhanced volumetric/gravimetric energy densities on a cell level for lithium–sulfur battery.
Mainak Majumder , Alessandro Siria , Lydéric Bocquet “Flows in one-dimensional and
two-dimensional carbon nanochannels: Fast and curious”, MRS Bulletin, 42, 2017, 278-282
Carbon materials exist in a large number of allotropic forms and exhibit a wide range of physical and chemical properties. From the perspective of fluidics, particularly within the confines of the nanoscale afforded by one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene structures, many unique properties have been discovered. However, other questions, such as the link between electronic states and hydrodynamics and accurate model predictions of transport, remain unanswered. Theoretical studies, experiments in large-scale ensembles of CNTs and stacked graphene sheets, and precise measurements at the single-pore and single-molecule level have helped in our understanding. These activities have led to explosive growth in the field, now known as carbon nanofluidics. The ability to produce membranes and devices from fluid phases of graphene oxide, which retain these special properties in molecular-scale flow channels, promises realization of applications in the near term.
Muthana Ali, Thomas M. McCoy, Ian R. McKinnon, Mainak Majumder, Rico F. Tabor” Synthesis and Characterization of Graphene Oxide–Polystyrene Composite Capsules with Aqueous Cargo via a Water–Oil–Water Multiple Emulsion Templating Route”, ACS Appl. Mater. Interfaces, 2017, 9 (21), pp 18187–18198
Graphene oxide/polystyrene (GO/PS) nanocomposite capsules containing a two-compartment cargo have been successfully fabricated using a Pickering emulsion strategy. Highly purified GO sheets with typically micrometer-scale lateral dimensions and amphiphilic characteristics were prepared from the oxidation reaction of graphite with concomitant exfoliation of the graphite structure. These GO sheets were employed as a stabilizer for oil-in-water emulsions where the oil phase comprised toluene or olive oil. The stability and morphology of the emulsions were extensively studied as a function of different parameters including GO concentration, aqueous phase pH, ultrasonication time, effects of added electrolytes and stability to dilution. In selected conditions, the olive oil emulsions showed spontaneous formation of multiple w/o/w emulsions with high stability, whereas toluene formed simple o/w emulsions of lower overall stability. Olive oil emulsions were therefore used to prepare capsules templated from emulsion droplets by surrounding the oil phase with a GO/PS shell. The GO sheets, emulsions and composite capsules were characterized using a variety of physical and spectroscopic techniques in order to unravel the interactions responsible for capsule formation. The ability of the capsules to control the release of a model active agent in the form of a hydrophilic dye was explored, and release kinetics were monitored using UV–visible spectroscopy to obtain rate parameters. The composite capsules showed promising sustained release properties, with release rates 11× lower than the precursor GO-stabilized multiple emulsion droplets.
Tanesh D. Gamot, Arup Ranjan Bhattacharyya, Tam Sridhar, Fiona Beach, Rico F. Tabor, Mainak Majumder” Synthesis and Stability of Water-in-Oil Emulsion Using Partially Reduced Graphene Oxide as a Tailored Surfactant”, Langmuir, 2017, 33 (39),10311–10321
Graphene oxide (GO) is widely known as an amphiphile having hydrophilic oxygen functionality and unoxidized graphitic patches as the hydrophobic domains. Exploiting this amphiphilicity, GO serves as a surfactant to stabilize oil–water interfaces. While there are numerous reports on GO as a surfactant, most of these reports concern oil-in-water (O/W) emulsions, and there are very few on the formation of water-in-oil (W/O) emulsions. We prepared W/O emulsions using partially reduced graphene oxide (prGO) as a surfactant. The partial reduction introduces a subtle hydrophilic–lipophilic balance (HLB), which favors the formation of the W/O emulsion. The morphological features and rheological characteristics of the W/O emulsion with 75:25 water-to-oil ratio were investigated and analyzed in detail. The W/O emulsion was found to have polydispersity with wide range of droplet sizes varying between 2 to 500 μm. Using confocal microscopy, the role of parameters such as extent of reduction, continuous phase volume fraction and the concentration of GO on the stability, microstructure and variation of droplet size distribution of the W/O emulsion were carefully monitored. With prGO concentration as large as 0.05% (w/w), highly concentrated emulsion will form, and are stable up to 20 days from formation; destabilization occurred from sedimentation and subsequent coalescence as the partially reduced GO was limited by its dispersion ability in the oil-phase studied here. Understanding the mechanisms behind the transient stability will enable the development of novel emulsion compositions containing GO as a multifunctional additive.
P. Kumar, P.S Gandhi and M. Majumder, “Interfacing 3D micro/nanochannels with a branch-shaped reservoir enhances fluid and mass transport”, J. Micromech. Microeng. 27 (2017) 015026 (11pp)
Three-dimensional (3D) micro/nanofluidic devices can accelerate progress in numerous fields such as tissue engineering, drug delivery, self-healing and cooling devices. However, efficient connections between networks of micro/nanochannels and external fluidic ports are key to successful applications of 3D micro/nanofluidic devices. Therefore, in this work, the extent of the role of reservoir geometry in interfacing with vascular (micro/nanochannel) networks, and in the enabling of connections with external fluidic ports while maintaining the compactness of devices, has been experimentally and theoretically investigated. A statistical modelling suggested that a branch-shaped reservoir demonstrates enhanced interfacing with vascular networks when compared to other regular geometries of reservoirs. Time-lapse dye flow experiments by capillary action through fabricated 3D micro/nanofluidic devices confirmed the connectivity of branch-shaped reservoirs with micro/nanochannel networks in fluidic devices. This demonstrated a ~2.2-fold enhancement of the volumetric flow rate in micro/ nanofluidic networks when interfaced to branch-shaped reservoirs over rectangular reservoirs. The enhancement is due to a ~2.8-fold increase in the perimeter of the reservoirs. In addition, the mass transfer experiments exhibited a ~1.7-fold enhancement in solute flux across 3D micro/nanofluidic devices that interfaced with branch-shaped reservoirs when compared to rectangular reservoirs. The fabrication of 3D micro/nanofluidic devices and their efficient interfacing through branch-shaped reservoirs to an external fluidic port can potentially enable their use in complex applications, in which enhanced surface-to-volume interactions are desirable.
S.T. Martin, A. Akbari, P. Chakraborty Banerjee, A. Neild and M.Majumder “The inside-out supercapacitor: induced charge storage in reduced graphene oxide “, Phys. Chem. Chem. Phys., 2016,18, 32185-32191
Iontronic circuits are built using components which are analogous to those used in electronic circuits, however they involve the movement of ions in an electrolyte rather than electrons in a metal or semiconductor. Developments in these circuits’ performance have led to applications in biological sensing, interfacing and drug delivery. While transistors, diodes and elementary logic circuits have been demonstrated for ionic circuits if more complex circuits are to be realized, the precident set by electrical circuits suggests that a component which is analogous to an electrical capacitor is required. Herein, an ionic supercapacitor is reported, our experiments show that charge may be stored in a conductive porous reduced graphene oxide film that is contacted by two isolated aqueous solutions and that this concept extends to an arbitrary polarizable sample. Parametric studies indicate that the conductivity and porosity of this film play important roles in the resultant device’s performance. This ionic capacitor has a specific capacitance of 8.6 F cm−3 at 1 mV s−1 and demonstrates the ability to filter and smooth signals in an electrolyte at a variety of low frequencies. The device has the same interfaces as a supercapacitor but their arrangement is changed, hence the name inside-out supercapacitor
SP. Pachfule, D.Shinde, M. Majumder & Q.Xu “Fabrication of carbon nanorods and graphene nanoribbons from a metal–organic framework, Nature Chemistry 8, 718–724 (2016) ,
One- and two-dimensional carbon nanomaterials are attracting considerable attention because of their extraordinary electrical, mechanical and thermal properties, which could lead to a range of important potential applications. Synthetic processes associated with making these materials can be quite complex and also consume large amounts of energy, so a major challenge is to develop simple and efficient methods to produce them. Here, we present a self-templated, catalyst-free strategy for the synthesis of one-dimensional carbon nanorods by morphology-preserved thermal transformation of rod-shaped metal–organic frameworks. The as-synthesized non-hollow (solid) carbon nanorods can be transformed into two- to six-layered graphene nanoribbons through sonochemical treatment followed by chemical activation. The performance of these metal–organic framework-derived carbon nanorods and graphene nanoribbons in supercapacitor electrodes demonstrates that this synthetic approach can produce functionally useful materials. Moreover, this approach is readily scalable and could be used to produce carbon nanorods and graphene nanoribbons on industrial levels.
D.B. Shinde, J. Brenker, C. D. Easton, R. F. Tabo , A. Neild, and M. Majumder,”Shear Assisted Electrochemical Exfoliation of Graphite to Graphene”, Langmuir, 2016, 32 (14), pp 3552–3559
The exfoliation characteristics of graphite as a function of applied anodic potential (1–10 V) in combination with shear field (400–74 400 s–1) have been studied in a custom-designed microfluidic reactor. Systematic investigation by atomic force microscopy (AFM) indicates that at higher potentials thicker and more fragmented graphene sheets are obtained, while at potentials as low as 1 V, pronounced exfoliation is triggered by the influence of shear. The shear-assisted electrochemical exfoliation process yields large (∼10 μm) graphene flakes with a high proportion of single, bilayer, and trilayer graphene and small ID/IG ratio (0.21–0.32) with only a small contribution from carbon–oxygen species as demonstrated by X-ray photoelectron spectroscopy measurements. This method comprises intercalation of sulfate ions followed by exfoliation using shear induced by a flowing electrolyte. Our findings on the crucial role of hydrodynamics in accentuating the exfoliation efficiency suggest a safer, greener, and more automated method for production of high quality graphene from graphite..
M. B. Coskun, A. Akbari, D.T. H. Lai, A. Neild, M. Majumder, T. Alan “Ultrasensitive Strain Sensor Produced by Direct Patterning of Liquid Crystals of Graphene Oxide on a Flexible Substrate”, ACSAppl.Mater.Interfaces 2016, 8, 22501 − 22505
Ultrasensitive flexible strain sensors were developed through the combination of shear alignment of a high concentration graphene oxide (GO) dispersion with fast and precise patterning of multiple rectangular features on a flexible substrate. Resistive changes in the reduced GO films were investigated under various uniaxial strain cycles ranging from 0.025 to 2%, controlled with a motorized nanopositioning stage. The devices uniquely combine a very small detection limit (0.025%) and a high gauge factor with a rapid fabrication process conducive to batch production.
M.Shaibani, A.Akbari, P.Sheath, C.D. Easton, P.Chakraborty-Banerjee, K.Konstas, A. Fakhfouri, M. Barghamadi, M. M. Musameh, A.S. Best, T.Ruther, P.J. Mahon, M.R.Hill, A.F. Hollenkamp, M.Majumder, “Supressed Polysulfide Crossover in Li-S Batteries through a High-Flux Graphene Oxide Membrane Supported on a Sulfur Cathode”, ACS Nano, 2016, 10 (8), pp 7768–7779
Utilization of permselective membranes holds tremendous promise for retention of the electrode-active material in electrochemical devices that suffer from electrode instability issues. In a rechargeable Li–S battery—a strong contender to outperform the Li-ion technology—migration of lithium polysulfides from the sulfur cathode has been linked to rapid capacity fading and lower Coulombic efficiency. However, the current approaches for configuring Li–S cells with permselective membranes suffer from large ohmic polarization, resulting in low capacity and poor rate capability. To overcome these issues, we report the facile fabrication of a high-flux graphene oxide membrane directly onto the sulfur cathode by shear alignment of discotic nematic liquid crystals of graphene oxide (GO). In conjunction with a carbon-coated separator, the highly ordered structure of the thin (∼0.75 μm) membrane and its inherent surface charge retain a majority of the polysulfides, enabling the cells to deliver very high initial discharge capacities of 1063 and 1182 mAh gelectrode–1 for electrodes with 70 and 80% sulfur content, respectively, at the practical 0.5 C rate. The very high sulfur utilization and impressive capacity retentions of the high sulfur content electrodes result in some of the highest performance metrics in the literature of Li–S (e.g., electrode capacity of 835 mAh gelectrode–1 after 100 cycles at 0.5 C with a sulfur content of 80%). We show that the structural order of the shear-aligned GO membrane is key in maintaining good kinetics of the charge transfer processes in Li–S batteries.
A.Akbari, P.Sheath, D.B. Shinde, M.Shaibani, P.C. Banerjee, R.Tkacz, D.Bhattacharyya, M.Majumder, “Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide”, Nature Communications, 7, Article number 10891, doi:10.1038/ncomms10891
Graphene-based membranes demonstrating ultrafast water transport, precise molecular sieving of gas and solvated molecules shows great promise as novel separation platforms; however, scale-up of these membranes to large-areas remains an unresolved problem. Here we demonstrate that the discotic nematic phase of graphene oxide (GO) can be shear aligned to form highly ordered, continuous, thin films of multi-layered GO on a support membrane by an industrially adaptable method to produce large-area membranes (13 × 14 cm2) in <5 s. Pressure driven transport data demonstrate high retention (>90%) for charged and uncharged organic probe molecules with a hydrated radius above 5 Å as well as modest (30–40%) retention of monovalent and divalent salts. The highly ordered graphene sheets in the plane of the membrane make organized channels and enhance the permeability (71±5 l m−2 hr−1 bar−1 for 150±15 nm thick membranes)..
S.J.Chen, W.Wang, K.S. Crentsii, F.Collins, X.L. Zhao, M.Majumder, W.H. Duan, “Distribution of carbon nanotubes in fresh ordinary Portland cement pastes: understanding from a two-phase perspective”, RSC Adv., 2016,6, 5745-5753,
Significant research advances have been made in the field of carbon nanotube (CNT) reinforced ordinary Portland cement (OPC) paste composites in recent years. However, the distribution of CNTs in fresh OPC paste is yet to be fully researched and quantified, thereby creating a technical barrier to CNT utilization in concrete construction. In this study, fresh OPC paste was treated as a two-phase material containing solid particles (cement grains) and liquid solutions (pore solutions). A centrifugation-based technique was proposed to separate these two phases and the presence of CNTs in each phase was quantified. UV-Vis spectrometry showed that the degree of dispersion can achieve above 90 wt% using polycarboxylate superplasticizer. The results suggested an upper limit of 0.26 wt% for CNT addition into water before mixing with OPC, and the dispersion was found to be stable for at least 4 hours. Based on scanning electron imaging, the adsorption phenomenon of CNTs on OPC grains with size less than 4 μm was discovered. Energy-dispersive X-ray spectroscopy indicated these adsorptive particles have lower Ca to Si ratio. It was observed that about 0.5 mg of CNTs per gram of OPC grains was adsorbed in solid OPC grains in typical fresh OPC pastes. On the basis of these results, a conceptual model was proposed for the distribution of CNTs in fresh OPC paste where about 33 wt% of the CNTs stay in pore solution and 65 wt% of CNTs are adsorbed on OPC grains.
P.Sheath & M.Majumder, “Flux accentuation and improved rejection in graphene-based filtration membranes produced by capillary-force-assisted self-assembly, “, Phil.Trans.R.Soc.A, 2015, 374: 20150028
Separation and flux performance were compared in graphene-based membranes that differed only in the method of deposition of reduced graphene oxide platelets. Membranes with higher degree of order were produced by evaporation-induced capillary-force self-assembly, which showed higher steric rejection properties while simultaneously accentuating water permeance compared to membranes produced by the traditional vacuum filtration technique. These studies attempt to establish structure–property correlations in graphene-based membranes.
D.E. Lobo, P.C. Banerjee-Chakraborty, CD. Easton, M.Majumder, ” Miniaturized Supercapacitors: Focused Ion Beam Reduced Graphene Oxide with Enhanced performance Metrics”, Adv. Energy Mater. 2015, 5, 1500665
Miniaturization of energy storage devices with enhanced performance metrics can reduce the footprint of microdevices being used in our daily life. Micro-supercapacitor architectures with planar geometry provides several advantages, such as, the ability to control and reduce the distances ions travel between two electrodes, easy integration to microdevices, and offer the potential of being extended into 3D without compromising the interelectrode distances. Here, focused ion beam (FIB) technology is used to directly write miniaturized planar electrode systems of reduced graphene oxide (FIB-rGO) on films of graphene oxide. Using optimized ion beam irradiation, interdigitated FIB-rGO electrode designs with 40 μm long and 3.5 μm wide fingers with ultrasmall interelectrode spacing of 1 μm demonstrate a large capacitance (102 mF cm−2), ultrasmall time response (0.03 ms), low equivalent series resistance (0.35 mΩ cm2), and retain 95% of the capacitance after 1000 cycles at an ultrahigh current density of 45 mA cm−2. These performance metrics show remarkable improvements on several counts of supercapacitor performance over existing reports due to the miniaturized electrode dimensions and minimal damage to the graphene sheets. It is believed that these results can provide avenues for large-scale fabrication of arrayed, planar, high-performance micro-supercapacitors with a small environmental footprint.
Above: Cover Image in Advanced Energy Materials
A.W. Thornton, A.Ahmed, S.K. Kannam, B.D. Todd, M.Majumder, A.J. Hill, ” Analytical Diffusion Mechanism (ADiM) model combining specular, Knudsen and surface diffusion”, J.Memb.Sci., 485, 1-9, 2015
We present a unified transport model that combines specular, Knudsen and surface diffusion mechanisms, termed the Analytical Diffusion Mechanism (ADiM) model. The ADiM model uniquely describes the transport behaviour of the bulk gas and adsorbed phase taking place in rough and smooth nanopores. Experiments and molecular simulations of nitrogen flow through aligned nanotube-based membranes are used to verify the model. In addition, we explore entrance effects using a suction energy mechanism that is compatible with ADiM and can accelerate gas permeance by an order of magnitude. Finally, ADiM is used to assess the effect of tube size on post-combustion carbon dioxide separation from fossil fuel plants.
L.Dumee, L.He, Z.Wang, P.Sheath, J. Xiong, C.Feng, M.Y. Tan, F.She, M.Duke, S. Gray, A.Pacheco, P.Hodgson, M.Majumder, L. Kong, “Growth of nano-textured graphene coatings across highly porous stainless steel supports towards corrosion resistant coatings” Carbon, 87, 395-408, 2015
In this paper, we demonstrated for the first time the growth of 3D networks of graphene nano-flakes across porous stainless steel substrates of micron sized metal fibres, and its anti-corrosion properties. The controlled formation of graphene-grade coatings in the form of single sheets to complex and homogeneously distributed 2–4 μm long nano-pillars is demonstrated by Scanning Electron Microscopy. The morphology and stability of these structures are shown to be particularly related to the temperature and feed gas flow rate during the growth. The number of layers across the graphene materials was calculated from the Raman spectra and is shown to range between 3 and more than 15 depending on the growth conditions and to be particularly related to the time and flow rate of the experiment. The presence of the graphene was shown to massively enhance the specific surface area of the material and to contribute to the increased corrosion resistance and electrical conductivity of the material without compromising the properties or structure of the native stainless steel materials. This new approach opens up a new route to the facile fabrication of advanced surface coatings with potential applications in developing new thermal exchangers, separation and bio-compatible materials.
R. Khare, D.B. Shinde, S.Bansode, M.A. More, M.Majumder, V.K. Pillai, D.J. Late, “Graphene nanoribbons as prospective field emitter” Appl. Phys. Lett. 106, 023111, 2015
Field emission characteristics of graphene nanoribbons (GNRs) synthesized by unzipping of multiwall carbon nanotubes using a facile hydrothermal route have been investigated at a base pressure of 1 × 10−8 mbar. The values of turn-on field, required to draw an emission current densities of 1 and 10 μA/cm2, are found to be 2.8 and 5.8 V/μm, respectively, and a maximum emission current density of 500 μA/cm2has been drawn at an applied field of 9.8 V/μm. The emission current stability of the GNRs emitter was studied at preset values of 1 and 10 μA over a period of 3 h, and is found to be excellent. The field emission results demonstrated herein suggest that GNRs based field emitters can open up many opportunities for their potential utilization as large area field emitters in various vacuum micro-nanoelectronic devices such as flexible field emission displays, portable X-ray, and microwave tubes.
P.Chakraborty-Banerjee, Derrek Lobo, R. Middag, W.K. Ng, Mahdokht E. Shaibani and Mainak Majumder , “Electrochemical Capacitance of Ni-doped Metal Organic Framework and Reduced Graphene Oxide Composites: More than the Sum of its Parts” ACS Appl. Mater. Interfaces, 2015, 7 (6), pp 3655–3664
Composites of a Ni-doped metal organic framework (MOF) with reduced graphene oxide (rGO) are synthesized in bulk (gram scale) quantities. The composites are composed of rGO sheets, which avoid restacking from the physical presence of MOF crystals. At larger concentration of rGO, the MOF crystals are distributed on the overlapping and continuous rGO sheets. Ni in Ni-doped MOF is found to engage in a two-electron, reversible, efficient, redox reaction shuttling between Ni and Ni(OH)2 in aqueous potassium hydroxide (KOH) electrolyte. The reaction is rather unique as Ni-based supercapacitors use a one-electron transfer Faradaic redox reaction between Ni(OH)2 and NiO(OH). Employing electrochemical impedance spectroscopy, we determined the charge transfer resistance to be 184 mΩ for MOF, 74 mΩ for a Ni-doped MOF and 6 mΩ for a rGO–Ni-doped MOF composite, but these modifications do not affect the mass transfer resistance. This novel redox reaction in conjunction with the lowered charge transfer resistance from the introduction of rGO underpins the synergy that dramatically increases the capacitance to 758 F/g in the rGO–Ni-doped MOF composite, when the parent MOF could store only 100 F/g and a physical composite of rGO and Ni-doped MOF could algebraically achieve about 240 F/g. A generic approach of doping MOFs with a redox active metal and forming a composite with rGO transforms an electro-inactive MOF to high capacity energy storage material with energy density of 37.8 Wh/kg at a power density of 227 W/kg. These results can promote the development of high-performance energy storage materials from the wide family of MOFs available.
M.Miansari, J.R.Friend. P.Chakraborty-Banerjee, M.Majumder, L.Y.Yeo, “Graphene-based planar nanofluidic rectifier”, J. Phys. Chem. C, 2014, 118 (38), pp 21856–21865
Structurally symmetric two-dimensional multilayered graphene oxide films, which facilitate ion transport through ‘nanochannels’ comprising the interstitial spaces between each stacked sheet within the film, are for the first time shown to exhibit peculiar ion current rectification and nonlinear current-voltage characteristics below a critical electrolyte concentration when the interstitial spacing becomes comparable to the Debye screening length such that the film becomes permselective. We attribute the unexpected rectification behavior to the fore-aft asymmetry that arises in the diffusion boundary layer on both sides of the millimeter long film upon reversal between the high resistance positive bias state to the low resistance negative bias state; the asymmetry being primarily a consequence of the trapping and release of counter-ions within the film, compounded by the nonuniform electric field that occurs in the tortuous nanochannels within the film. In addition to elucidating the influence of the electrolyte concentration and applied bias voltage, we demonstrate the possibility of tuning the ion selectivity and hence the rectification behavior through the solution pH. These first graphene-based nanofluidic rectifiers, which are easily synthesized, therefore offer a flexible, robust, low cost and facile large-scale alternative to conventional nanochannels that require elaborate and sophisticated nanofabrication.
S.Martin, A.Neild, M.Majumder,”Graphene-based ion rectifier using macroscale geometric asymmetry”, APL Mat. 2, 092803, (2014) – Special Topic in 2D Materials
Ion rectification is the asymmetrical conduction of ions through a system under different polarities of applied potentials. In this article we report the finding of a novel form of ion rectification in graphene oxide (GO) and reduced graphene oxide (RGO) films which act as an ensemble array of nanochannels. Rectification is imparted by introducing geometric asymmetry in fluidic inlets to the counter-ion selective nanochannels of GO/RGO which creates asymmetry in the enrichment/depletion effects at the macro-/nano-interface. The devices are made simply by cutting a GO or RGO film into a trapezoid and sealing the film within a Polydimethylsiloxane (PDMS) block so that fluid may only enter through one of two inlets. These devices exhibit rectification ratios larger than 20 (in 1 mM NaCl) while operating at modest voltages [-1V,+1V].
S.J. Chen, B.Zou, F.Collins, X.L. Zhao, M.Majumder, W.H.Duan,”Predicting the influence of ultrasonication energy on the reinforcing efficiency of carbon nanotubes”, Carbon, 2014
Ultrasonication is extensively used in the fabrication of carbon nanotube (CNT) reinforced composites. However, there has been very limited research into predicting the effect of ultrasonication treatment on the reinforcing efficiency of CNTs. In this study, a theoretical framework with supporting experiments is developed to address this issue. The distribution of CNT lengths and the concentration of dispersed CNTs are characterized using scanning electron microscopy images and UV-vis spectra. After ultrasonication, the length of CNT is found to follow log-normal distributions which show a shortening effect. The concentration of dispersed CNT increases with ultrasonication energy but reaches a plateau after about 250 J/ml. The distribution of CNT lengths and the concentration of dispersed CNTs are incorporated into a micromechanics-based model to simulate the crack bridging behavior of CNTs. Results show that the distribution of CNT lengths leads to better estimation of reinforcing effect than does the average length. Furthermore, for unit volume of dispersed CNTs, the reinforcing efficiency decreases monotonically with increased ultrasonication. Based on the proposed model, the predicted optimal ultrasonication energy (89 J/ml) for reinforcing is found before the dispersion plateau is reached.
R.Tkacz, R.Oldenbourg, S.B. Mehta, A. Verma M.Miansari, M.Majumder,”pH Dependent Isotropic to Nematic Phase Transitions in Graphene Oxide Dispersions Reveal Droplet Liquid Crystalline Phases”, Chem.Commun, 2014
Size fractionation, amplified by the surface charge density of graphene oxide (GO) sheets, broadens the pH dependent isotropic (I) to nematic (N) phase transition in aqueous dispersions of graphene oxide (GO). In this biphasic region, a highly organized droplet nematic phase of uniform size (20 ± 2.8 μm diameter) with an isotropic interior is observed.
D.B. Shinde, M.Majumder, V.K. Pillai,”Counter-Ion Dependent Longitudinal Unzipping of Multi-Walled Carbon Nanotubes to Highly Conductive and Transparent Carbon Nanoribbons”, Scientific Reports 4, Article # 4363
Here we report for the first time, a simple hydrothermal approach for the bulk production of highly conductive and transparent graphene nanoribbons (GNRs) using several counter ions from K2SO4, KNO3, KOH and H2SO4 in aqueous media, where, selective intercalation followed by exfoliation gives highly conducting GNRs with over 80% yield. In these experiments, sulfate and nitrate ions act as a co-intercalant along with potassium ions resulting into exfoliation of multi-walled carbon nanotubes (MWCNTs) in an effective manner. The striking similarity of experimental results in KOH and H2SO4 that demonstrates partially damaged MWCNTs, implies that no individual K+, SO42− ion plays a key role in unwrapping of MWCNTs, rather this process is largely effective in the presence of both cations and anions working in a cooperative manner. The GNRs can be used for preparing conductive 16 kΩsq−1, transparent (82%) and flexible thin films using low cost fabrication method.
R.Tkacz, R.Oldenbourg, A.Fulcher, M.Miansari, M.Majumder,”Capillary-Force Assisted Self-Assembly (CAS) of highly Ordered and Anisotropic Graphene-Based Thin Films”, J.Phys.Chem.C, 2014, 118 (1), 259–267
We report capillary-force-assisted self-assembly (CAS) as a method for preparation of thin films of chemically reduced graphene oxide (rGO) with unidirectional organization of rGO sheets. The films were initiated at the contact line of the air–liquid–solid interface and form directly on solid substrates dipped in an isotropic colloidal suspension of rGO. Assisted by capillary forces at the contact line, the suspension undergoes an isotropic-to-anisotropic phase transition and becomes aligned with the film growth direction as the contact line moves across the substrate surface. We determined the degree of order in rGO films and assemblies by birefringence and diattenuation imaging. The slow axis of the rGO platelets within the CAS films displayed a narrow angular distribution (±3°) within a film area of 1 mm2, resulting in the highest possible order parameter (S) of 1 with 8-fold enhancement of electrical conductivity compared to films formed by traditional techniques such as filtration. Our straightforward film fabrication technique is scalable to produce large areas of films, and by controlling the rates of convective to diffusive mass transport, films with varying degree of order can be produced.
A.W.K. Ma, J.Nam, N.Behabtu, F.Mirri, C.C. Young, B.Dan, D. Tsentalovich, M.Majumder, L.Song, Y.Cohen, P.M. Ajayan, M.Pasquali, “Scalable Formation of Carbon Nanotube Films Containing Highly Aligned Whiskerlike Crystallites”, Ind. Eng. Chem. Res., 2013, 52 (26), pp 8705–8713
We report the creation of carbon nanotube films from superacids by a scalable process, where film morphology is controlled by initial fluid phases. These films were formed by dip-coating biphasic (isotropic + liquid crystalline) carbon nanotube (CNT) chlorosulfonic acid solutions. Chlorosulfonic acid has low volatility and is therefore removed by solvent extraction instead of conventional drying processes. At intermediate concentrations, the solutions contain liquid crystalline domains which stretch and align streamwise during dip-coating. These elongated domains further act as “nucleating sites” for large, aligned whiskerlike crystallites during subsequent solvent extraction. The final films contain highly aligned CNT crystallites embedded in a mesh of randomly oriented CNTs.
M.Majumder, P.Sheath, J.I. Mardel, T.G. Harvey, A.W. Thornton, A.Gonzago, D.F. Kennedy, I.Madsen, J.W. Taylor, D.R. Turner, M.R. Hill “Aqueous Molecular Sieving and Strong Gas Adsorption in Highly Porous MOFs with a Facile Synthesis”, Chem. Mater. 2012, 24 (24), pp 4647–4652
Aqueous molecular sieving is demonstrated in a new series of isostructural metal organic frameworks based on the perylene tetracarboxylate (PTC) ligand. The frameworks can be formed in water at room temperature with Mg, Ni, and other first row transition metal ions and adopt a highly porous topology that results in predicted surface areas of over 2000 m2 g–1 and periodic channels of around 6 Å in diameter. Unusually, the M-PTC MOFs are highly resistant to moisture and can be readily synthesized on multigram scales. The frameworks have been shown to exhibit molecular sieving in the absorption from mixtures of organic molecules at low aqueous concentrations, with an application demonstrated on a dangerous water-borne herbicide, Paraquat. Ni-PTC also exhibits a structural flexibility that leads to strong and selective gas adsorption characteristics, with an IAST selectivity of 300 for carbon dioxide being adsorbed over nitrogen. Binding enthalpies for hydrogen and carbon dioxide are also very strong in comparison to other MOFs, at 10.75 and 52.50 kJ/mol respectively.
D. Lobo, J.Fu, T. Gengenbach, and M. Majumder “Localized Deoxygenation and Direct Patterning of Graphene Oxide Films by Focussed Ion Beams”, Langmuir, 2012, 28, 41,14815–14821
Exposure to controlled doses(~4.65x10-3 to 2.79x10-2 nC/µm2 ion fluence) of Ga ions via a focused ion beam (FIB) deoxygenates graphene oxide (GO) and increases the electrical conductivity in 100 × 100 μm patches by several orders of magnitudes compared to the unexposed GO. Raman spectra and carbon/oxygen ratio in exposed areas are indicative of chemically reduced graphene oxide (rGO). This novel FIB-induced conversion technique is harnessed for direct imprinting of complex micron-scale shapes and sub-20 nm lines of rGO in insulating films and flakes of GO establishing the capability for generating features varying in sizes from ~10’s nm to ~100’s of μm, in a mask-less and efficient manner..
M. Majumder, Clint s Rendall, J. Alexander Eukel, James Y.L Wang, Natnael Behabtu, Cary Pint, Tzu-Yu Liu, Alvin W Orbaek, Francesca Mirri, Jaewook Nam, Andrew R Barron, Robert H. Hauge, Howard K. Schmidt, and Matteo Pasquali “Overcoming the ‘Coffee-Stain Effect by Compositional Marangoni Flow Assisted Drop-Drying”, J.Phys.Chem.B, 2012, 116 (22), pp 6536–6542
Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the “coffee-stain” effect, due to convective macroscopic flow into the contact line. Here we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon®. Visualization of droplet shape and internal flow shows initial droplet spreading and strong re-circulating flow during spreading and shrinkage. The initial spreading is due to diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient which drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles.
R.K. Singh Raman, P. Chakraborty Banerjee, Derrek E. Lobo, Hemtej Gullapalli, Madusha Sumandasa, Ashwin Kumar, Lokesh Choudhary, Rachel Tkacz, Pulickel M. Ajayan, M.Majumder “Protecting Copper from Electrochemical Degradation by Graphene Coating ” Carbon, 2012, 50,11,4040–4045
Graphene coating on copper (Cu) is shown to increase the resistance of the metal to electrochemical degradation by one and half orders of magnitude. Detailed electrochemical characterization in aggressive chloride environment shows the impedance of Cu increasing dramatically and the anodic and cathodic current densities of the coated Cu becoming nearly 1–2 orders of magnitude smaller when coated with graphene. The observations are counterintuitive as graphite in contact with metals increases metallic corrosion. The results can bring paradigm changes in the development of anti-corrosion coatings using conformal, ultra thin graphene films.
Ji Wu, Karen Gerstandt, M.Majumder, Xin Zhan and Bruce J. Hinds “Highly efficient electroosmotic flow through functionalized carbon nanotube membranes ” Nanoscale, 2011, 3, 3321-3328
Carbon nanotube membranes with inner diameter ranging from 1.5–7 nm were examined for enhanced electroosmotic flow. After functionalization via electrochemical diazonium grafting and carbodiimide coupling reaction, it was found that neutral caffeine molecules can be efficiently pumped via electroosmosis. An electroosmotic velocity as high as 0.16 cm s−1 V−1 has been observed. Power efficiencies were 25–110 fold improved compared to related nanoporous materials, which has important applications in chemical separations and compact medical devices. Nearly ideal electroosmotic flow was seen in the case where the mobile cation diameter nearly matched the inner diameter of the single-walled carbon nanotube resulting in a condition of using one ion is to pump one neutral molecule at equivalent concentrations.
W.Gao, M. Majumder, L. Alemany, T. Narayanan, M. Ibarra, B.K. Pradhan, P.M. Ajayan “Engineered Graphite Oxide Materials for Application in Water Purification” ACS Appl. Mater. Interfaces, 2011, 3 (6), pp 1821-1826
Retaining the inherent hydrophilic character of GO (graphite-oxide) nanosheets, sp2 domains on GO are covalently modified with thiol groups by diazonium chemistry. The surface modified GO adsorbs 6-fold higher concentration of aqueous mercuric ions than the unmodified GO. “Core-shell” adsorbent granules, readily useable in filtration columns, are synthesized by assembling aqueous GO over sand granules. The nanostructured GO-coated sand retains at least 5-fold higher concentration of heavy metal and organic dye than pure sand. The research results could open avenues for developing low-cost water purification materials for the developing economies.
M.Majumder and B.Corry, “Anomalous Decline of Water Transport in Covalently Modified Carbon Nanotube Membranes”, Chem. Commun., 2011, 47, 7683-7685
Carbon nanotube membranes have been shown to rapidly transport liquids; but progressive hydrophilic modification-contrary to expectations-induces a drastic reduction of water flow. Enhanced electrostatic interaction and the disruption of the mechanically smooth graphitic walls is the determinant of this behavior. These results have critical implications in the design of nanofluidic devices.
M.Majumder and P.M. Ajayan “Carbon Nanotube Membranes: A new frontier in membrane science” in: Enrico Drioli and Lidietta Giorno Comprehensive Membrane Science and Engineering (Elsevier Science)
M. Majumder, N.Chopra, B.J. Hinds, “Mass Transport through Carbon Nanotube Membranes in Three Different Regimes: Ionic Diffusion and Gas and Liquid Flow” ACS Nano, 2011, 5, 3867-78
Transport phenomena through the hollow conduits of carbon nanotubes (CNTs) are subjects of intense theoretical and experimental research. We have studied molecular transport over the large spectrum of ionic diffusion to pressure-driven gaseous and liquid flow. Plasma oxidation during the fabrication of the membrane introduces carboxylic acid groups at the CNT entrance, which provides electrostatic “gatekeeper” effects on ionic transport. Diffusive transport of ions of different charge and size through the core of the CNT is close to bulk diffusion expectations and allows estimation of the number of open pores or porosity of the membrane. Flux of gases such as N2, CO2, Ar, H2, and CH4 scaled inversely with their molecular weight by an exponent of 0.4, close to expected kinetic theory velocity expectations. However, the magnitude of the fluxes was ~15- to 30-fold higher than predicted from Knudsen diffusion kinetics and consistent with specular momentum reflection inside smooth pores. Polar liquids such as water, ethanol, and isopropyl alcohol and nonpolar liquids such as hexane and decane were dramatically enhanced, with water flow over 4 orders of magnitude larger than “no-slip” hydrodynamic flow predictions. As direct experimental proof for the mechanism of near perfect slip conditions within CNT cores, a stepwise hydrophilic functionalization of CNT membranes from as-produced, tip-functionalized, and core-functionalized was performed. Pressure-driven water flow through the membrane was reduced from 5 × 104 to 2 × 102 to less than a factor of 5 enhancements over conventional Newtonian flow, while retaining nearly the same pore area.
M.J. Green, C.C. Young, A.N.G. Parra-Vasquez, M.Majumder, V.Juloori, N. Behabtu, C. L. Pint, J. Schmidt, E.Kesselman, R.H. Hauge, Y.Cohen, Y.Talmon and M.Pasquali “Direct imaging of carbon nanotubes spontaneously filled with solvent” Chem. Commun., 2011, 47,4,1228–1230
For the first time, cryo-TEM imaging is used to directly show spontaneous filling of carbon nanotubes immersed in a solvent in the native state at ambient conditions. Multi-walled carbon nanotubes are dissolved in chlorosulfonic acid, and the high contrast between the acid and the carbon shows the difference between filled and unfilled nanotubes.
M. Majumder, A. Stinchcomb, B.J. Hinds, “Towards Mimicking Natural Protein Channels with Aligned Carbon Nanotube Membranes for Active Drug Delivery” Life Sciences, 86, 15-16, 2010, 563-68
Aims: Carbon nanotube (CNT) membranes offer an exciting opportunity to mimic natural protein channels due to 1) a mechanism of dramatically enhanced fluid flow 2) ability to place ‘gatekeeper’ chemistry at the entrance to pores 3) the ability for biochemical reactions to occur on gatekeeper molecules and 4) an ability to chemically functionalize each side of the membrane independently. Main methods: Aligned CNT membranes were fabricated and CNT pore entrances modified with gatekeeper chemistry. Pressure driven fluid flow and diffusion experiments were performed to study the mechanisms of transport through CNTs. Key findings: The transport mechanism through CNT membranes is primarily 1) ionic diffusion near bulk expectation 2) gas flow enhanced 1–2 orders of magnitude primarily due to specular reflection 3) fluid flow 4–5 orders of magnitude faster than conventional materials due to a nearly ideal slip-boundary interface. The transport can be modulated by ‘gatekeeper’ chemistry at the pore entrance using steric hindrance, electrostatic attraction/repulsion, or biochemical state. The conformation of charged tethered molecules can be modulated by applied bias setting the stage for programmable drug release devices. Significance The membrane structure is mechanically far more robust than lipid bilayer films, allowing for large-scale chemical separations, delivery or sensing based on the principles of protein channels. The performance of protein channels is several orders of magnitude faster than conventional membrane materials. The fundamental requirements of mimicking protein channels are present in the CNT membrane system.
M. Majumder, C. Rendall, M. Li, A. Eukel, H.K. Schmidt, M. Pasquali, “Insight into the physics of spray-coating of SWNT films” Chem.Engg.Sci.., 65, 6, 2010, 2000-08
Spray coating is a scalable and high-throughput process for fabrication of transparent and conducting coatings (TCCs) composed of single-walled carbon nanotubes (SWNTs). Presently the fundamentals of this process are not well understood. We show that suppression of coalescence of spray droplets by sufficiently rapid heat- and mass-transfer yields homogeneous SWNT films by preventing the formation of ‘coffee stains’ of larger length scale. Such heat and mass transfer is driven by differential evaporation between the top and edges of the drops, whereas thermal and compositional effects on surface tension and buoyancy are weak. Ultrasonic spraying ensures that the droplets are deposited without significant splashing, and delayed splashing at higher Weber number is evidenced. We find that the performance of spray-coated TCCs made from HiPCO SWNTs is limited by bundle diameter rather than length of the constituent SWNTs and bundes. Vapor acid doping with concentrated sulfuric acid roughly doubles the conductivity of the TCCs
M. Majumder, K. Keis, J. Cole, C. Meadows, X. Zhan, B.J. Hinds, “Enhanced Electrostatic Modulation of Transport Through CNT Membrane by Diazonium Grafting Chemistry” J. Memb. Sci., 2008, 316, 1-2, 89-96.
A membrane structure consisting of an aligned array of open ended carbon nanotubes (~7 nm i.d.) spanning across an inert polymer matrix allows the diffusive transport of aqueous ionic species through CNT cores. The plasma oxidation process that opens CNTs tips inherently introduces carboxylic acid groups at the CNT tips, which allows for a limited amount of chemical functional at the CNT pore entrance. However for numerous applications, it is important to increase the density of carboxylic acid groups at the pore entrance for effective separation processes. Aqueous diazonium-based electrochemistry significantly increases the functional density of carboxylic acid groups. pH dependent dye adsorption–desorption and interfacial capacitance measurements indicate 5–6 times increase in functional density. To further control the spatial location of the functional chemistry, a fast flowing inert liquid column inside the CNT core is found to restrict the diazonium grafting to the CNT tips only. This is confirmed by the increased flux of positively charged Ru(bipy)+23 with anionic functionality. The electrostatic enhancement of ion diffusion is readily screened in 0.1 M electrolyte solution consistent with the membrane pore geometry and increased functional density.
M. Majumder, X. Zhan, R. Andrews, B.J. Hinds “Voltage Gated Carbon Nanotube Membranes” Langmuir , 2007, 23, 8624-8631.
Membranes composed of an array of aligned carbon nanotubes, functionalized with charged molecular tethers, show voltage gated control of ionic transport through the cores of carbon nanotubes. The functional density of tethered charge molecules is substantially increased by the use of electrochemical grafting of diazonium salts. Functionality can be forced to occur at the CNT tip entrances by fast fluid flow of an inert solvent through the core during electrochemical functionalization. The selectivity between Ru(bi-pyridine)32+ and methyl viologen2+ flux is found to be as high as 23 with -130 mV bias applied to the membrane as the working electrode. Changes in the flux and selectivity support a model where charged tethered molecules at the tips are drawn into the CNT core at positive bias. For molecules grafted along the CNT core, negative bias extends the tethered molecules into the core. Electrostatically actuated tethers induce steric hindrance in the CNT core to mimic voltage gated ion channels in a robust large area platform.
M. Majumder, N. Chopra, R. Andrews, B.J. Hinds, “Nanoscale Hydrodynamics: Enhanced Flow in Carbon Nanotubes”, Nature, 2005, 438, 3, 44
Nanoscale structures that could mimic the selective transport and extraordinarily fast flow possible in biological cellular channels would have a wide range of potential applications. Here we show that liquid flow through a membrane composed of an array of aligned carbon nanotubes is four to five orders of magnitude faster than would be predicted from conventional fluid-flow theory. This high fluid velocity results from an almost frictionless interface at the carbon-nanotube wall.
M. Majumder, N. Chopra, B.J. Hinds, “Effect of Tip Functionalization on Transport through Vertically Oriented Carbon Nanotube Membranes”, J. Amer. Chem. Soc., 2005,127, 9062-70.
Ionic flux through a composite membrane structure, containing vertically aligned carbon nanotubes crossing a polystyrene matrix film, was studied as a function of chemical end groups at the entrance to carbon nanotubes’ (CNTs) cores. Plasma oxidation during the membrane fabrication process introduced carboxylic acid groups on the CNTs’ tips that were modified using carbodiimide mediated coupling between the carboxylic acid and an accessible amine group of the functional molecule. Functionalization molecules included straight chain alkanes, anionically charged dye molecules, and an aliphatic amine elongated by polypeptide spacers. Functionalization was confirmed by FTIR spectroscopy, and areal functional density was estimated by transmission electron microscopy studies of thiol terminated sites decorated by nanocrystalline gold. The transport through the membrane of two different sized but equally charged molecules (ruthenium bipyridine [Ru-(bipy)32+] and methyl viologen [MV2+]) was quantified in a U-tube permeation cell by UV−vis spectroscopy. Relative selectivity of the permeates varied from 1.7 to 3.6 as a function of tip-functionalization chemistry. Anionic charged functional groups sharply increased the flux of the cationic permeates. This effect was reduced at higher solution ionic strength consistent with shorter Debye screening length. The observed selectivities were consistent with a hindered diffusion model with functionalization at the CNT tip and not along the length of the CNT core.
N. Chopra, M. Majumder, B.J. Hinds, “Bi-functional Carbon Nanotubes by Sidewall Protection”, Adv. Funct.Mat., 2005, 15,858-64.
A vertically aligned array of multiwalled carbon nanotubes (MWCNTs) impregnated with polystyrene is utilized to independently functionalize each end of the MWCNTs. The presence of the polystyrene matrix prevents sidewall oxidation of the CNTs, resulting in carboxylate deri vatization at the CNT tips during processing via plasma oxidation. The membrane is subsequently dissolved in toluene, resulting in a suspension of CNTs with carboxylate-derivatized tips. The CNT tips are further functionalized using carbodiimide-mediated linking of carboxylate at the CNT tips with an amine of 2-aminoethanethiol. This treatment results in thiol functionality and Fourier-transform infrared (FT-IR) studies confirm amide-bond formation. Gold nanoparticles that are readily observed using transmisison electron microscopy (TEM) are then covalently linked to the thiol functional groups. Estimates of the average nanoparticle density are observed to decrease from ~526 particles μm-1 near the CNT tips to negligible values (< 7 particles μm–1) at locations beyond 700 nm from the CNT tips. This is consistent with a membrane geometry where CNTs tips are above the polystyrene surface owing to differing oxidation rates. Bifunctional CNTs (with different chemical functionality at either end of each CNT) is achieved by thiol functionalization on only one side of the oxidized CNT membrane floating on top of a 2-aminoethanethiol functionalization reaction solution. After dissolution of the polystyrene matrix, TEM analysis shows gold-nanoparticle decoration at the thiol-functionalized end of the CNT
M. Majumder, S. Mukhopadhyay, O.Parkash, D.Kumar, “Sintering and Crystallization Behavior of Chemically Prepared Cordierite for Application in Electronic Packaging”, Ceram. Inter., 2004, 30, 1067-1070.
Synthesis and sintering of dense cordierite ceramics, prepared from magnesium and aluminium ions precipitated in dispersed silica have been reported and phase evolution at 1200 °C been studied by XRD. Densification of compacted precursor powder at 1200 °C has been substantiated by SEM and it was found that the dielectric properties of cordierite ceramics prepared by this route are good enough for substrate application.