ABSTRACT
Anodization of transition metals, particularly the valve metals (V, W, Ti, Ta, Hf, Nb, and Zr) and their alloys, has emerged as a powerful tool for controlling the morphology, purity, and thickness of oxide nanostructures. The present review is focused on the advances in the synthesis of micro/nanostructures of anodic tantalum oxides (ATO) in inorganic, organic, and mixed inorganic-organic type electrolytes with critically highlighting anodization parameters, such as applied voltage, current, time, and electrolyte temperature. Particularly, the growth of ATO nanostructures in fluoride containing electrolytes and their applications are briefly covered. The details of the current- or voltage-time transient and its relation to the growth of the anodic oxide films are presented systematically. The main discussion revolves around the incorporation of various electrolyte species into the surface of ATO structures and its effects on their physicochemical properties. The latest progress in understanding the growth mechanism of nanoporous/nanotubular ATO structures is outlined. Additionally, the impact of annealing temperature (ranging from 400-1000 °C) and atmosphere on the crystalline structure, morphology, impurity content, and physical properties of the ATOs is briefly described. The common modification methods, such as decorating with other transition metal/metal oxide, heteroatom doping, or generating defects in the ATO structures, are discussed. Besides, the review also covers the most promising applications of these materials in the fields of capacitors, supercapacitors, memristive devices, corrosion protection, photocatalysis, photoelectrochemical (PEC) water splitting, and biomaterials. Finally, future research directions for designing ATO-based nanomaterials and their utilities are indicated.
ABSTRACT
In this work, we systematically followed the growth of MnO x nanostructures on trimesic acid (TMA)/benzoic acid (BA) functionalised nitrogen doped graphene (NG) and studied their electrocatalytic activity towards oxygen reduction reaction (ORR). In these hybrid materials the MnO x phase, their morphology and Mn surface valency were guided by the functional molecules, their concentration and the duration of reaction, which in turn significantly affected the ORR activity. During the growth in the presence of TMA, agglomerated nanostructures were formed at 2 h reaction, which transformed to well dispersed 4-7 nm particles at 6 h over a large area of NG. However, in the presence of BA, MnOOH nano-flecks were formed at 2 h and transformed to MnOOH nanowires and oval shaped Mn3O4 particles at 8 h of reaction. The valency of surface Mn on the different MnO x nanostructures was ascertained by X-ray photoelectron spectroscopy (XPS). The ORR activity of samples were studied by cyclic voltammetry (CV) and rotating disc electrode (RDE) in alkaline medium. Among all the studied samples, the highest ORR activity with most efficient 4e- transfer process is observed for TMA modified NG-MnO X obtained at 6 h of reaction, which is due to its well dispersed nanostructure morphology.
ABSTRACT
Molecular doping on graphene, through noncovalent functionalization offers a great opportunity to tune charge density on graphene for catalytic applications. Although enhanced oxygen reduction activity has been reported in heteroatom doped graphene, the synergistic advantage of molecular and heteroatom co-doping has not yet been studied. Here, we report the remarkably enhanced catalytic activity of benzoate or 1-pyrenebutyrate functionalized N-doped graphene (BA-NrGO/PB-NrGO) towards the oxygen reduction reaction (ORR) in alkaline medium. An efficient 4e(-) reduction process with a more positive onset potential (Eonset = 0.85 V vs. RHE) and high ORR activity (Jk = 3.16 mA cm(-2) at 0.65 V) has been observed in BA-NrGO. DFT studies show that the stimulated ORR activity is due to functionalisation induced increased charge density on active sites.
