Ranjit Thapa

Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst.

Electrocatalysts for oxygen reduction are a critical component that may dramatically enhance the performance of fuel cells and metal-air batteries, which may provide the power for future electric vehicles. Here we report a novel bio-inspired composite electrocatalyst, iron phthalocyanine with an axial ligand anchored on single-walled carbon nanotubes, demonstrating higher electrocatalytic activity for oxygen reduction than the state-of-the-art Pt/C catalyst as well as exceptional durability during cycling in alkaline media. Theoretical calculations suggest that the rehybridization of Fe 3d orbitals with the ligand orbitals coordinated from the axial direction results in a significant change in electronic and geometric structure, which greatly increases the rate of oxygen reduction reaction. Our results demonstrate a new strategy to rationally design inexpensive and durable electrochemical oxygen reduction catalysts for metal-air batteries and fuel cells.

Amino-functionalized graphene quantum dots: origin of tunable heterogeneous photoluminescence

Graphene quantum dots are known to exhibit tunable photoluminescence (PL) through manipulation of edge functionality under various synthesis conditions. Here, we report observation of excitation dependent anomalous m-n type fingerprint PL transition in synthesized amino functionalized graphene quantum dots (5-7 nm). The effect of band-to-band π*-π and interstate to band n-π induced transitions led to effective multicolor emission under changeable excitation wavelength in the functionalized system. A reasonable assertion that equi-coupling of π*-π and n-π transitions activated the heterogeneous dual mode cyan emission was made upon observation of the PL spectra. Furthermore, investigation of incremented dimensional scaling through facile synthesis of amino functionalized quantum graphene flakes (20-30 nm) revealed it had negligible effect on the modulated PL pattern. Moreover, an effort was made to trace the origin of excitation dependent tunable heterogeneous photoluminescence through the framework of energy band diagram hypothesis and first principles analysis. Ab initio results suggested formation of an interband state as a manifestation of p orbital hybridization between C-N atoms at the edge sites. Therefore comprehensive theoretical and experimental analysis revealed that newly created energy levels can exist as an interband within the energy gap in functionalized graphene quantum structures yielding excitation dependent tunable PL for optoelectronic applications.

Field emission properties of ZnO nanosheet arrays

Electron emission properties of electrodeposited ZnO nanosheet arrays grown on Indium tin oxide coated glass substrates have been studied. Influence of oxygen vacancies on electronic structures and field emission properties of ZnO nanosheets are investigated using density functional theory. The oxygen vacancies produce unshared d electrons which form an impurity energy state; this causes shifting of Fermi level towards the vacuum, and so the barrier energy for electron extraction reduces. The ZnO nanosheet arrays exhibit a low turn-on field of 2.4 V/μm at 0.1 μA/cm2 and current density of 50.1 μA/cm2 at an applied field of 6.4 V/μm with field enhancement factor, β = 5812 and good field emission current stability. The nanosheet arrays grown by a facile electrodeposition process have great potential as robust high performance vertical structure electron emitters for future flat panel displays and vacuum electronic device applications.

Doped h-BN monolayer as efficient noble metal-free catalysts for CO oxidation: the role of dopant and water in activity and catalytic de-poisoning

Using a first principles approach, we investigated the catalytic activity of a noble metal-free n-doped (C → B, O → N) hexagonal boron nitride (h-BN) monolayer for CO oxidation. The CO adsorption ability and hence the preferred Eiley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanism for CO oxidation is dopant-dependent: CO is chemisorbed on O-doped h-BN (OBN) while it physically interacts with the C-doped h-BN (CBN) surface. Even though both C and O doping create similar donor states below the Fermi level (Ef), O doping results in a larger bond length of O–B1 (one of the nearest B atom), out-of-plane displacement of the B1 atom, and less positive charge on the B1 atom, synergistically contributing to higher atomic activity. The presence of a pre-adsorbed O2 molecule on both types of surfaces eliminates any chances of CO poisoning of the surface, and CO oxidation prefers to proceed via the ER mechanism with a small activation barrier. The high values of Sabatier activities suggest that the doped h-BN surface is superior to Au55and Pt55nanoclusters. In case of CO oxidation by means of the LH mechanism, a stable O2⋯CO intermediate is produced, which requires a high barrier energy to break the O–O bond. However, the presence of a H2O molecule increases the activity of the catalyst and helps in catalytic CO de-poisoning.

Rules of Boron–Nitrogen Doping in Defect Graphene Sheets: A First‐Principles Investigation of Band‐Gap Tuning and Oxygen Reduction Reaction Catalysis Capabilities

Introduction of defects and nitrogen doping are two of the most pursued methods to tailor the properties of graphene for better suitability to applications such as catalysis and energy conversion. Doping nitrogen atoms at defect sites of graphene and codoping them along with boron atoms can further increase the efficiency of such systems due to better stability of nitrogen at defect sites and stabilization provided by B-N bonding. Systematic exploration of the possible doping/codoping configurations reflecting defect regions of graphene presents a prevalent doping site for nitrogen-rich BN clusters and they are also highly suitable for modulating (0.2-0.9 eV) the band gap of defect graphene. Such codoped systems perform significantly better than the platinum surface, undoped defect graphene, and the single nitrogen or boron atom doped defect graphene system for dioxygen adsorption. Significant stretching of the O-O bond indicates a lowering of the bond breakage barrier, which is advantageous for applications in the oxygen reduction reaction.

Field emission properties of spinel ZnCo2O4 microflowers

ZnCo2O4 microflowers were synthesized by a simple low temperature hydrothermal route. A single three-dimensional microflower consists of hundreds of self-assembled petals, with a thickness of several nanometers. These microflowers have exceptionally thin edges with a few petal layers. The ZnCo2O4 microflowers appeared to be stable and good field emitters.

Flexible cold cathode with ultralow threshold field designed through wet chemical route.

A flexible cold cathode based on a uniform array of ZnO nanowires over carbon fabrics was designed via a simple wet chemical route. The structural parameters of the nanowires (i.e. length, diameter) as well as their arrangement over the carbon fibers were tailored by adjusting nutrient solution composition and growth duration. The optimized arrays of ZnO nanowires exhibit excellent electron emission performance with ultralow turn-on as well as threshold fields of 0.27 and 0.56 V µm(-1). This threshold field value is the lowest compared to any of the previous zinc-oxide-based cold cathodes realized through either chemical or vapor phase processes. In addition, the current density can reach an exceptionally high value of ∼ 11 mA cm(-2) at an applied electric field of only 0.8 V µm(-1). Flexible electronic devices based on a field emitter cold cathode may thus be realized through chemical processing at low budget but having high efficiency.

Facile synthesis of Ag nanowire–rGO composites and their promising field emission performance

Crystalline, ultra long silver nanowires (Ag NWs), few-layered rGO (reduced graphene oxide) and their rGO–Ag NW nanocomposite have been synthesized using a polyol reflux technique under optimized experimental conditions. The field emission performance of the rGO–Ag NW nanocomposite, rGO and Ag NW emitters was investigated. The turn on field required to draw an emission current density of ∼1 μA cm−2 was found to be ∼5.00, 3.92 and 2.40 V μm−1 for the Ag NW, rGO and rGO–Ag NW nanocomposite emitters, respectively. The combined contribution of the sharp edges of the thin graphene sheets and high aspect ratio of the Ag nanowires, and their synergetic effect in the rGO–Ag NW nanocomposite, are responsible for the enhanced field emission behavior. First-principles density functional calculations show that the enhanced field emission may also be due to the overlapping of the electronic structures of the Ag NWs and rGO nanosheets.

First-Principles Identification of Iodine Exchange Mechanism in Iodide Ionic Liquid

We investigated the microscopic mechanism of ion transport in iodide ionic liquid, using first-principles calculations. We show that the desorption barrier of polyiodides (I3(-) or I5(-)) from the cation is in a similar energy range as or higher than the barrier for the bond dissociation and ensued desorption of neutral iodine (I2). This suggests that, instead of the physical diffusion of such a negatively charged multiatomic species, the exchange of neutral iodine (I2) between the polyiodides can be an easier channel for the movement of polyiodide. For the transport of the monoiodide anion (I(-)), we suggest the contribution of the Grotthuss-type ion exchange through the intermediately formed even-member anion (I2n(-)), in addition to drift and diffusion. As a result, we suggest that, instead of the commonly cited diffusion of the triiodide/iodide (I3(-)/I(-)) redox couple, the exchange of neutral iodine (I2) and the Grotthuss-type transport (I(-)) constitute the dominant ion transport mechanism.

First principles guide to tune h-BN nanostructures as superior light-element-based hydrogen storage materials: role of the bond exchange spillover mechanism

We investigate the interaction of molecular hydrogen with light-element-based n-doped hexagonal boron nitride (h-BN) nanostructures and moreover explore the bond exchange mechanism for spillover of atomic hydrogen using dispersion-corrected density functional theory (DFT-D) calculations. A number of doped configurations were tested and it has been found that co-doping of C and O on h-BN sheet significantly increases the adsorption energy of molecular H2. The charge transfer from the n-doped h-BN surface to H2 is found to be the reason for the higher interactions that boosted the binding energy. In addition, the doped h-BN surfaces act as catalysts and dissociate the H2 molecule with a very low activation barrier, but the migration of the resulting H atoms on the surface requires high energy. In order to facilitate easy and fast migration of H atoms, we introduce the bond exchange mechanism using external mediators i.e. borane (BH3) and gallane (GaH3) molecules which serve as secondary catalysts and help in lowering the migration barrier, leading to the formation of a hydrogenated surface. The partially hydrogenated surface in turn can also act as a hydrogen storage material, with a higher propensity to adsorb hydrogen molecules when compared to the unhydrogenated surface. Hence the surface proposed in this work can be used to store a substantial quantity of hydrogen as an energy source with easy adsorption and desorption kinetics.