Arijit Sen

Morphology and phase tuning of α- and β-MnO2 nanocacti evolved at varying modes of acid count for their well-coordinated energy storage and visible-light-driven photocatalytic behaviour

A simple hydrothermal method is developed to synthesize two different phases, α and β of MnO2 nanocacti (comprising nanowires with 1–10 nm diameter self assembled by ultrathin sheets) as well as MnO2 nanorods (10–40 nm diameter) without any seed or template. Sudden addition of concentrated H2SO4 (0.3–0.4 μL) results in the formation of nanocacti while gradual addition (dropwise) of H2SO4 solution (0.3–0.4 M) results in nanorods. Besides, the α phase of MnO2 exists at relatively high acidic strength (4 pH) compared to the β phase, which is consistent at 5 pH. Thus this could be the first report exploring the possibilities of tuning morphology as well as the phase of MnO2 through simple optimizations in acidic content. We find that polymorphic MnO2 nanocacti exhibit superior photocatalytic activity and high energy capacity as an anode in Li-ion batteries than polymorphic MnO2 nanorods. The α phase of MnO2 performs better than the β phase. α-MnO2 nanocacti demonstrate high visible light driven photocatalytic activity by degrading >90% of congo red and methyl orange dyes in 40 mg L−1 organic dye aqueous solution with 0.1 g of the as-prepared sample within 25 and 70 min, respectively. We highlight the differences between the photocatalytic activities of different phases, α and β of MnO2 nanostructures, depending on the charge transport through different dimensions of the same pristine MnO2. The constant cycling stability of α-MnO2 nanocacti with capacities as low as 300 mA h g−1 at 1C rate after 50 cycles as an anode makes it a promising material for energy storage applications. We attribute the high electro- and photo-chemical activity for α-MnO2 nanocacti to their highly mesoporous structure making this one of the highest specific surface areas (271 m2 g−1) possibly ever reported for pristine MnO2.

Facile size-controllable synthesis of single crystalline β-MnO2 nanorods under varying acidic strengths

A simple one-pot hydrothermal synthesis of single crystalline β-MnO2 nanorods with diameters in the range of 10–40 nm is reported. During the synthesis process, the acid molarities were varied from 1.1 M down to 0.2 M in steps of 0.3 M while keeping the other reaction parameters constant, resulting in gradual transformation of the size of β-MnO2 from micro to the nanoscale dimension. The as synthesized nanorods exhibit soft ferromagnetic behavior and possess a high catalytic activity with an onset potential of −0.17 V in facilitating the oxygen reduction reaction (ORR).

Enhanced pseudocapacitance from finely ordered pristine α- MnO 2 nanorods at favourably high current density using redox additive

Abstract A flexible technique is developed using hydrochloric acid to modify the redox reaction between potassium permanganate and sodium nitrite in order to grow ultrafine α-MnO2 nanorods, hydrothermally. The nanorods grown were 10–40 nm diameters in range. Not any crack, fissure, imperfection or dislocation is observed in the nanorods suggesting it to be finely ordered. Structure, phase and purity of as developed nanorods were determined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Energy-dispersive X-ray spectroscopy. Peseudocapacitance of α-MnO2 nanorods was tested using a three electrode system. Considerably very high pseudocapacitance value of 643.5 F/g at 15 A/g current density was calculated from the galvanostatic discharge current measurement. Also excellent cyclability is observed with high retention of 90.5% after 4000 cycles. Highly uniform and confined morphology of the nanorods helps smooth the electron dynamics between electrode/electrolyte interfaces resulting in superior performance. Most importantly, the use of potassium ferricyanide as redox additive to KOH electrolyte was proved to be quite effective as it provides extra redox couple [Fe(CN)6]3−/[Fe(CN)6]4− which helps in further smoothening of electron transition thereby resulting in considerably superior pseudocapacitive performance.

Precise control of morphology of ultrafine LiMn2O4 nanorods as a supercapacitor electrode via a two-step hydrothermal method

We report three different synthesis routes, which maintain similar reaction conditions, to find an effective way to precisely control the growth of ultrafine one-dimensional (1D) LiMn2O4 in the form of nanorods. We developed a novel method of mixing the precursors through a hydrothermal technique, yielding low dimensional precursors for an effective solid state reaction to synthesize the nanorods. However, to achieve these, highly uniform β-MnO2 nanorods were initially grown as one of the main precursors. The uniformity observed in the as grown β-MnO2 nanorods using the hydrothermal technique helps to attract minute LiOH particles upon mixing over their highly confined nano-regime surfaces. This facilitated the solid state reaction between MnO2 and LiOH to develop one of the finest LiMn2O4 nanorods with diameters of 10–80 nm, possessing a high surface area of 88.294 m2 g−1. We find superior charge storage behaviour for these finely ordered 1D nanostructures as supercapacitor electrodes in KOH with K3Fe(CN)6 as an electrolyte, in contrast to Li2SO4. A high pseudo-capacitance of 653.5 F g−1 at 15 A g−1 is observed using a galvanostatic discharge time with a high retention capacity of 93% after 4000 cycles. The enhanced charge storage property may arise from the redox couple [Fe(CN)6]3−/[Fe(CN)6]4− and K+ ions of the electrolyte. To the best of our knowledge, we demonstrate for the first time the effectiveness of a two-step hydrothermal method in tuning the supercapacitive behaviour of 1D LiMn2O4 in a redox additive electrolyte.

One-pot synthesis and first-principles elasticity analysis of polymorphic MnO2 nanorods for tribological assessment as friction modifiers

One-pot synthesis of single-crystalline α and β-MnO2 nanorods was carried out by selectively varying the acidic concentrations. Ultrafine one dimensional nanorods with diameters of about 10–40 nm are achieved. The respective phases of the nanorods were then altered through simple optimization in the molar concentration of H2SO4. Morphological transition from microstructure to nanostructure is also examined by changing the acid concentration from high to low. Elastic and tribological properties of these nanomaterials were subsequently explored, with a view to their possible applications as nanoadditives in green lubricants. While β-MnO2 nanorods showed a reduction in the coefficient of friction by about 15%, α-MnO2 nanorods turned out to be even better nanoadditives yielding a reduction of as high as 30%. Moreover, both the polymorphs of MnO2 nanostructures led to lower roughness when used as nanoadditives in the base oil. Our analysis suggests that such enhancement of antiwear properties originates primarily from the mutual interplay between the rolling action and the protective layer formation by respective polymorphs of quasi-1D MnO2. From the first-principles analysis, we envisage that α-MnO2 nanorods may potentially serve as efficient nanoadditives in comparison with β-MnO2 nanorods due to superior elastic properties of the former.

Enhancement of anisotropic magnetoresistance in zigzag graphene nanodevices

We theoretically investigate the anisotropic charge transport behavior of zigzag graphene nanodots (zGNDs) connected to ferromagnetic nanoelectrodes. It turns out that when a zGND is attached to Ni-electrodes through S-linkers, the spin-polarized conductance exhibits oscillatory behavior with a maximum peak around θ ≈ 18°. A small change in the anisotropic spin polarization can thus cause a sudden jump in the device conductance. The two states of electronic conductance obtained by simply tuning the electrode magnetization may well serve as 1s and 0s to find potential utilities in designing spintronic logic circuits at the nanoscale.

Charge transport in a zigzag silicene nanoribbon

Nanoscale transport properties of a zigzag silicene nanoribbon (zSiNR) are studied using first-principles calculations based on the non-equilibrium Green’s function approach. Our theoretical analysis demonstrates how the scattering wavefunctions in the device region can shed light on the conductance behavior of a nanoelectronic device, made up of 3-zSiNR, spanning the width of three hexagons. The lowering of conductance at 100 mV bias is due mainly to the dominant character of the lowest unoccupied molecular orbital (LUMO) in the transmission profile. A zSiNR, having higher conductance than germanene, can thus be a potential candidate for silicon-based nanoelectronic devices due to its rich optoelectronic properties.

Asymmetric Coulomb oscillation and giant anisotropic magnetoresistance in doped graphene nanojunctions

Abstract We report here the charge transport behavior in graphene nanojunctions in which graphene nanodots, with relatively long relaxation time, are interfaced with ferromagnetic electrodes. Subsequently we explore the effect of substitutional doping of transition metal atoms in zigzag graphene nanodots (z-GNDs) on the charge transport under non-collinear magnetization. Only substitutional doping of transition metal atoms in z-GNDs at certain sites demonstrates the spin filtering effect with a large tunnelling magnetoresistance as high as 700%, making it actually suitable for spintronic applications. From the electrical field simulation around the junction area within the electrostatic physics model, we find that the value of electric field strength increases especially with doped graphene nanodots, as the gap between the gate electrode and tip axis is reduced from 3 nm to 1 nm. Our detailed analysis further suggests the onset of asymmetric Coulomb oscillations with varying amplitudes in graphene nanodots, on being doped with magnetic ions. Such kind of tunability in the electronic conductance can potentially be exploited in designing spintronic logic gates at nanoscale.

Charge transport behavior of 1D gold chiral nanojunctions

Abstract Understanding the process of electron tunneling in chirality-induced single-molecule junctions is imperative for the development of nanoscale switching and artificial nanomotors. Based on the combined non-equilibrium Greens functions formalism and the ground-state density functional theory, we present here the charge transport behavior of chiral gold (7,3) nanowires (NWs) in comparison with various other chiral and achiral 1D gold nanostructures as the principal leads to form stable single-molecule junctions. For σ-saturated alkane chains, we find that the contact potential barriers vary widely with the achiral leads but not with the chiral ones, although a close resemblance exists in the tunneling constants. Lower energy gaps for single-molecule junctions with Au(7,3)NWs ensure better electronic conductance even after allowing for the low thermal loss, due mainly to the close-packed arrangements of atoms with minimum wire tension. Our first-principles quantum transport analysis further suggests that chiral Au(7,3)NWs render higher electronic conductance than chiral gold (5,3) nanotubes (NTs), once bridged by either σ-saturated or π-conjugated molecular moieties. It, however, turns out that asymmetricity in the characteristics of channel formation at the lead-molecule contact remains often associated with chiral Au(7,3)NWs only.

Elastic and inelastic electron tunneling in doped gold wire nanojunctions

Control and design of thermodynamically stable nanojunctions constitute to pose major challenges in realizing truly nanoelectronic devices as complex hybridization of atomic orbitals, especially at the junction edges, often determines the tunability of electronic conductance along with its thermopower. Using first-principles nonequilibrium analysis, we probe here how the doping of transition metal ions may control the charge transport in Au-chain nanojunctions. We find that electronic conductance of a pure gold-chain break junction is reduced considerably once it is thiolated on either side of the contact regions. Additional doping of Ni or Co ions, however, try to compensate it by yielding lower junction resistance while Mn ions behave differently as selective dopant. Onset of phonon modes further shed light on how the inelastic electron tunneling spectra can sense the chemical signature of various doped nanojunctions.