Progress in Nanoporous Templates: Beyond Anodic Aluminum Oxide and Towards Functional Complex Materials

Successful synthesis of ordered porous, multi-component complex materials requires a series of coordinated processes, typically including fabrication of a master template, deposition of materials within the pores to form a negative structure, and a third deposition or etching process to create the final, functional template. Translating the utility and the simplicity of the ordered nanoporous geometry of binary oxide templates to those comprising complex functional oxides used in energy, electronic, and biology applications has been met with numerous critical challenges. This review surveys the current state of commonly used complex material nanoporous template synthesis techniques derived from the base anodic aluminum oxide (AAO) geometry

Graphene-Based Nanostructures in Electrocatalytic Oxygen Reduction

Application of graphene-type materials in electrocatalysis is a topic of growing scientific and technological interest. A tremendous amount of research has been carried out in the field of oxygen electroreduction, particularly with respect to potential applications in the fuel cell research also with use of graphene-type catalytic components. This work addresses fundamental aspects and potential applications of graphene structures in the oxygen reduction electrocatalysis. Special attention will be paid to creation of catalytically active sites by using non-metallic heteroatoms as dopants, formation of hierarchical nanostructured electrocatalysts, their long-term stability, and application as supports for dispersed metals (activating interactions)

RESISTIVE SWITCHING CHARACTERISTICS OF NANOSTRUCTURED AND SOLUTION-PROCESSED COMPLEX OXIDE ASSEMBLIES

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction into a conductive state in metal oxide insulators has been known for almost 50 years, the fundamental resistive switching mechanism is a complex phenomenon that is still long-debated, complex process. Further improvements to existing memristor performance require a complete understanding of memristive properties under various operation conditions. Additional technical issues also remain, such as the development of facile, low-cost fabrication methods as an alternative to expensive, ultra-high vacuum (UHV) deposition methods. This collection of work explores resistive switching within metal oxide-based memristive material assemblies by analyzing the fundamental physical insulating material properties. Chapter 3 aims to translate the utility and simplicity of the highly ordered anodic aluminum oxide (AAO) template structure to complex, yet more functional (memristive) materials. Functional oxides possessing ordered, scalable nanoporous arrays and nanocapacitor arrays over a large area is of interest to both the fields of next-generation electronics and energy storing/harvesting devices. Here their switching performance will be evaluated using conductive atomic force microscopy (C-AFM). Chapter 4 demonstrates a convective self-assembly fabrication method that effectively enables the synthesis of a low-cost solution processed memristor comprising binary oxide and perovskite ABO3 nanocrystals of varying diameter. Chapter 5 systematically compares the influence of inter-nanoparticle distance on the threshold switching SET voltage of hafnium oxide (HfO2) memristors. Utilizing shorter phosphonic acid ligands with higher binding affinity on the nanocrystal surface enabled a record-low SET voltage to be achieved. Chapter 6 extends the scope to the fine tuning of solution processed memristors with two types of perovskites nanocrystals. The primary advantage of nanocrystal memristors is the ability to draw from additional degrees of freedom by tuning the constituent nanocrystal material properties. Recent advancement of solution phase techniques enables a high degree of controllability over the nanocrystal size and structure. Thus, this work found in this dissertation aims to understand and decouple the effects of the geometric size and substitutional nanocrystal parameters on resistive switching

Layer-structured niobium oxides and their analogues for advanced hybrid capacitors

© 2019 Elsevier B.V. Niobium-based oxides including niobium oxide (Nb2O5) and their analogues with quasi-2D network of open and stable Wadsley-Roth shear crystal structure, have gained great interest for advanced hybrid supercapacitors due to their outstanding rate capability derived from the intercalation pseudocapacitive kinetics. To realize their full potential as battery-type anode electrodes for supercapacitor, various strategies have been effectively implemented to overcome the drawbacks especially the poor intrinsic electrical conductivity, including structure design, surface modification, conductivity enhancement, and electrode engineering. Here, we provide a comprehensive overview of the latest progress of Nb-based oxides for high-rate hybrid supercapacitors in the aspects of structure-performance relationship, performance-optimizing strategies, and energy storage mechanisms. We will also present our insights into the challenges and perspectives for future development and industrial applications

Unconventional and Exotic Magnetism in Carbon-Based Structures and Related Materials

The detailed analysis of the problem of possible magnetic behavior of the carbon-based structures was fulfilled to elucidate and resolve (at least partially) some unclear issues. It was the purpose of the present paper to look somewhat more critically into some conjectures which have been made and to the peculiar and contradictory experimental results in this rather indistinct and disputable field. Firstly the basic physics of magnetism was briefly addressed. Then a few basic questions were thoroughly analyzed and critically reconsidered to elucidate the possible relevant mechanism (if any) which may be responsible for observed peculiarities of the "magnetic" behavior in these systems. The arguments supporting the existence of the intrinsic magnetism in carbon-based materials, including pure graphene were analyzed critically. It was concluded that recently published works have shown clearly that the results of the previous studies, where the "ferromagnetism" was detected in pure graphene, were incorrect. Rather, graphene is strongly diamagnetic, similar to graphite. Thus the possible traces of a quasi-magnetic behavior which some authors observed in their samples may be attributed rather to induced magnetism due to the impurities, defects, etc. On the basis of the present analysis the conclusion was made that the thorough and detailed experimental studies of these problems only may shed light on the very complicated problem of the magnetism of carbon-based materials. Lastly the peculiarities of the magnetic behavior of some related materials and the trends for future developments were mentioned.Comment: 40 pages, 5 tables, 221 Reference

Novel Design And Synthesis Of Transition Metal Hydroxides And Oxides For Energy Storage Device Applications

Supercapacitors (SCs) and Li-ion batteries (LIBs) are two types of important electrical energy storage devices with high power density and high energy density respectively. However, to satisfy the increasing demand of high-performance energy storage devices, the energy density of SCs and power/energy densities of LIBs have to be further improved. The exploration, research, and development of electrode materials with high-performance for applications in SCs and LIBs are still needed to meet the ever-increasing demand on energy and power densities. Herein, the amorphous Ni-Co-Mo ternary hydroxides nanoflakes for SCs and oxides nanoflakes for LIBs with ultrathin stature, abundant open spaces, and interconnecting mesoporous were prepared via electrodeposition method and further annealing process, respectively. The as-obtained materials with unique hierarchical structures offer a large electrochemical active area, resulting in a fast ion transportation (OH- in SCs and Li+ in LIBs) electrolyte immersion, as well as provide effective pathways for electron transport. Thus, the as-prepared Ni-Mo-Co triple hydroxides and oxides electrodes exhibit a high specific capacitance /capacity (3074 F g-1 at 2 A g-1 in SCs and 1132.31 mA h g-1 at 0.2 A g-1 in LIBs), remarkable rate performance, as well as long-term cyclability in SCs and LIBs, respectively. Also, the effect of composition of trimetallic hydroxides on SCs performance have been studied, and the performance have been optimized by tuning the feeding ratio of Ni, Mo, and Co. It is found that supreme performance was achieved when feeding ratio Ni/Mo/Co (1/1/0.4)

Correlation between geometrical and structural properties of mixed oxide ultrathin nanotubes and their solar water splitting performance

The objective of this study was to study the effect of Nb alloying with Ti on the photoelectrochemical performance of the resulted oxide upon anodization. In this regard, nanotubes were grown on Ti-Nb alloy via electrochemical anodization and their corresponding photocatalytic behavior was investigated and compared with those grown on an ordinary Ti substrate. After preparing and optimizing the nanotubes dimensions for the required geometrical structure, the as formed tubes were annealed at different temperatures and in air), then characterized with respect to their morphological, structural, and photoelectrochemical properties. From the morphological and structural point of view, optimized and well aligned ultra-thin wall nanotubes were successfully synthesized on the surface of Ti-Nb alloy. To the best of our knowledge, these dimensions have not been reported before. One of the challenges was that the oxide layer formed on the surface of the alloy was not precisely identified in literature, where some authors reported the formation of combination of individual oxides (TiO2 and Nb2O5), whereas, others claimed it was a mixed oxide TiNbOx. Raman and X-ray diffraction test results confirmed the formation of individual anatase and monoclinic Nb2O5 phases. Detailed XRD analysis was performed and the crystallite size as well as microstrain were calculated and found to be minimal indicating negligible effect of lattice induced tension or compression. It is worth mentioning that insignificant structural changes are favorable to maintain good electron mobility. Hence, point defect equations were deduced and it was found that that oxygen vacancies were the prevailing ionic defects rather than electronic Nb compensation. From the aforementioned results, ultrathin wall nanotubes formed on TiNb alloy were achieved, for the first time, with clear representation of the oxide layer composition. Such oxide layer showed better stability upon annealing at high temperatures. Although, UV-Vis test results showed small or negligible enhancement in the absorption, profile the photo-electrochemical measurements showed much higher photocurrent for Ti-Nb oxide alloy than bare TiO2 prepared at the same conditions for the sake of comparison. In conclusion, the Ti-Nb NTs showed enhanced stability over a wide range of temperatures, where the transition from anatase to rutile was shifted to higher temperature in addition to an increase in the photoconversion capability, resulting in a more efficient water splitting process

A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond

Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene