Shalini Tripathi

Synthesis of Hollow Nanotubes of Zn2SiO4 or SiO2: Mechanistic Understanding and Uranium Adsorption Behavior

We report a facile synthesis of Zn2SiO4 nanotubes using a two-step process consisting of a wet-chemical synthesis of core-shell ZnO@SiO2 nanorods followed by thermal annealing. While annealing in air leads to the formation of hollow Zn2SiO4, annealing under reducing atmosphere leads to the formation of SiO2 nanotubes. We rationalize the formation of the silicate phase at temperatures much lower than the temperatures reported in the literature based on the porous nature of the silica shell on the ZnO nanorods. We present results from in situ transmission electron microscopy experiments to clearly show void nucleation at the interface between ZnO and the silica shell and the growth of the silicate phase by the Kirkendall effect. The porous nature of the silica shell is also responsible for the etching of the ZnO leading to the formation of silica nanotubes under reducing conditions. Both the hollow silica and silicate nanotubes exhibit good uranium sorption at different ranges of pH making them possible candidates for nuclear waste management.

High-index faceted Au nanocrystals with highly controllable optical properties and electro-catalytic activity

We introduce a new and naturally abundant mild reducing agent, tannic acid, to improve the seed-mediated growth method for the synthesis of elongated tetrahexahedral Au nanocrystals enclosed with high-index (730) planes, at room-temperature. The control of the dimensions, plasmonics and electro-catalysis of such high-index faceted nanocrystals is remarkable.

Insights into nucleation, growth and phase selection of WO3: morphology control and electrochromic properties

Electrochromic application of nanoscale WO3 demands stringent control in terms of phase-purity and morphology. Here, we show that two different phases of WO3 with distinct morphologies (viz. 2D plates of orthorhombic phase and 1D rods of hexagonal phase) can be obtained by tuning the solvothermal reaction conditions. Control experiments along with density functional theory based ab initio calculations show that the reaction pathway critically depends on the capping agent used in the reaction. Using this concept in conjunction with crystallographic arguments, we rationalize the morphology evolution of the two phases. Furthermore, the synthesized phases exhibit very different electrochromic properties in terms of H+ diffusion, which can be rationalized by the calculated trend in the H+-intercalation energies.

A comprehensive analysis and rational designing of efficient Fe-based oxygen electrocatalysts for metal–air batteries

Precious metal based electrocatalysts are considered as the most efficient ones to drive kinetically sluggish oxygen evolution/reduction reactions (OER/ORR) for metal–air batteries and fuel-cells. However, their monofunctionality in addition to their exorbitant cost has stimulated the quest for economically viable bifunctional electrocatalysts for use in next generation electrochemical energy devices. Here, we report Fe–Fe3C encapsulated in Fe–Nx enriched spheres of N-doped carbon nanotubes (FCMR, M = 3, 10, 25, 40 represents the ramping rate of temperature during synthesis) as a potentially enduring, cost effective, highly efficient bifunctional electrocatalyst for total oxygen electrochemistry (ORR and OER) and a comprehensive study to elucidate the role of various Fe moieties. In addition to the improved OER/ORR activities as evident from the better onset potential, lower Tafel slopes and high current densities over commercially available RuO2/Pt–C electrocatalysts and several recently reported state-of-the-art bi-functional electrocatalysts, FC10R shows a current retention value of ∼93 and ∼98% after the accelerated cyclic stability test for the OER and ORR, respectively. The preferable 4e− pathways and suppressed peroxide generation in the ORR by FC10R further ensure maximum electrochemical energy harvesting. Remarkably, the complete oxygen electrochemistry of FC10R in alkaline medium as evaluated from ΔE (=Ej(OER) = 10 − E1/2(ORR) = 0.758 V) is significantly lower than that of commercially available/recently reported electrocatalysts and advocates the minimum cyclic loss. The overall study elucidates the synergistic effect of Fe–Nx coordination and Fe3C moieties on oxygen electrochemistry, and FC10R has shown its potential to serve as a non-precious metal based bifunctional electrocatalyst for next generation electrochemical energy conversion and storage devices. Finally, a prototype Al–air battery arrangement using FC10R as an air-cathode for powering a green light emitting diode has been demonstrated.

Manipulation of Optoelectronic Properties and Band Structure Engineering of Ultrathin Te Nanowires by Chemical Adsorption.

Band structure engineering is a powerful technique both for the design of new semiconductor materials and for imparting new functionalities to existing ones. In this article, we present a novel and versatile technique to achieve this by surface adsorption on low dimensional systems. As a specific example, we demonstrate, through detailed experiments and ab initio simulations, the controlled modification of band structure in ultrathin Te nanowires due to NO2 adsorption. Measurements of the temperature dependence of resistivity of single ultrathin Te nanowire field-effect transistor (FET) devices exposed to increasing amounts of NO2 reveal a gradual transition from a semiconducting to a metallic state. Gradual quenching of vibrational Raman modes of Te with increasing concentration of NO2 supports the appearance of a metallic state in NO2 adsorbed Te. Ab initio simulations attribute these observations to the appearance of midgap states in NO2 adsorbed Te nanowires. Our results provide fundamental insights into the effects of ambient on the electronic structures of low-dimensional materials and can be exploited for designing novel chemical sensors.

Mechanistic study of the reduction of MoO 2 to Mo 2 C under methane pulse conditions

Molybdenum carbide (Mo2C), an interstitial transition metal carbide, has been used in a myriad of industrial applications due to its refractory nature, extreme hardness and strength, and high electrical and thermal conductivity. It also possesses catalytic activity for many chemical processes such as hydrodeoxygenation, reforming, water–gas shift, and the Fischer–Tropsch reaction. Among the current synthesis methods available to produce β-Mo2C, temperature-programmed reduction yields materials with the highest specific surface areas. The objective of the present work is to perform a detailed investigation of the carburization process and to determine the key intermediate phases that are formed during reduction. To achieve this objective, we performed the carburization process under pulse conditions wherein a small amount of CH4 in each pulse was reacted with a packed bed of MoO2. Our XRD and TEM results demonstrate that the solid-phase transformation from MoO2 to β-Mo2C follows a “plum-pudding” mechanism where Mo metal crystallites are constantly formed as the key intermediate phase throughout the matrix.

PtBi Alloy Nanoparticles on Reduced Graphitic Oxide Support for Electrocatalysis

Pt nanoparticles on suitable supports are commonly employed as electrocatalysts to improve the slow kinetics of methanol oxidation [1]. However, it is not cost-effective and suffers from two serious limitations. Firstly, in the course of methanol oxidation, the evolution of CO gas poisons the Pt-catalyst leading to its reduced cyclic stability and secondly, Pt nanoparticles (NPs) undergo coarsening at the operative voltage in fuel cell, gradually losing its activity. In this context, alloying Pt with a non-noble element along with achieving a dispersion of the catalyst on a conducting support can solve these problems. We have demonstrated earlier that dispersion of ultrafine Pt NPs on a reduced graphitic oxide (rGO) is achievable through a microwave (MW) assisted synthesis route [2], as shown in Figure 1 (a, e,f). However, achieving an alloy/intermetallic nanoparticle catalyst still remains a challenge owing to the difficulties associated with the co-reduction of two metals with widely different reduction potentials. In this work, we report a MW-based wet chemical approach for alloying Pt NPs with Bi on rGO support that enhances the stability of the catalyst. In the MW-assisted ultrafast route, we were able to synthesize Pt-Bi intermetallic nanoparticles on rGO support upon reaction. X-ray diffraction (XRD) shows the formation of PtBi alloy phases with two distinct intermetallic compositions (Figure 1 (g)), hexagonal PtBi phase, along with rhombohedral PtBi2 phase. Figure 1 (c) shows high resolution transmission electron (HRTEM) image of PtBi intermetallic nanoparticle showing fringes corresponding to (100) planes of the hexagonal structure. Scherrer analysis from the XRD pattern and detailed scanning transmission electron microscopy (STEM) coupled with EDS of the particles reveal that the larger particles have PtBi2 stoichiometry, and the smaller ones have a PtBi stoichiometry (Figure 2).

Wet-chemical Synthesis of Electrochromic WO3 and WxMo1-xO3 Nanomaterials with Phase and Morphology Control

Nanoscale WO3 has emerged as a multifunctional material as it has found various applications in electrochromic devices [1], gas sensing [2] and photocatalysis [3]. A wealth of literature is available on synthesis of different phases of WO3 with distinct morphologies. However, a thorough understanding of the growth mechanism of the material is still lacking. Furthermore, owing to the comparable ionic radii of W and Mo, WO3 phases can be alloyed with MoO3 under same synthetic conditions, leading to new mixed oxide phases. The electrochomic efficiency depends on the ability to control the phases and the morphology in these systems.

Electrochromic tungsten molybdenum oxide: synthesis with phase and morphology control

Presence of a myriad of WO3 phases demands a stringent control over the microstructure and phase to attain the desired tailoring of the properties. Among several applications, such as electrochromicity, photocatalysis and gas sensing, the electrochromic behaviour of this material has gained significant attention. Here, we thoroughly investigate the growth mechanism of the different WO3 phases under solvothermal reaction conditions, along with their electrochromic behaviour. Under the same synthetic conditions, we explore the effect of Mo-doping into the WO3 lattice, leading to the formation of new mixed oxide phases. Experiments involving the growth mechanism of different phases for WO3 and Wx Mo1-x O3 show that WO3 can form two different phases, i.e. hexagonal and orthorhombic, directed by the presence of oxalic acid in the reaction medium. Interestingly, when oxalic acid is used as a capping reagent, a plate-like 2-D morphology of the orthorhombic WO3 phase is obtained, whereas absence of capping agent yields to a 1-D rod-like morphology corresponding to a hexagonal phase of WO3 . DFTbased calculation of oxalate binding strength on different WO3 surfaces clearly shows that binding energy of the oxalate is higher on the orthorhombic {002} surface than on hexagonal {11-20} surface. Moreover, our experiments show that when a Mo precursor is introduced in the same reaction medium, formation of a 2-D plate-like morphology was observed, but the structure is closely related to the hexagonal WO3 phase. STEM-EDS profiling of the elemental composition shows that the flakes have a W0.5 Mo0.5 O3 composition. In terms of electronic properties, the orthorhombic WO3 shows significant presence of reduced W5+ species, indicating a difference in the reducibility. All these factors, viz. phase, morphology and electronic property affect the electrochromic efficiency of the material. In this spirit, we explored the intercalation kinetics of H+ in the two phases of WO3 . Our electrochromicity experiments show that the hexagonal phase has a faster kinetics of H+ diffusion. This trend is also supported by ab initio calculations, which shows a higher intercalation energy in the orthorhombic phase compared to that in hexagonal one, indicating towards a slower proton intercalation. Electrochemically measured diffusion coefficient values also reinforce this observation. We have further investigated the electrochromic property of the mixed oxide phase, illustrating the effect of Mo incorporation into the WO3 lattice.