Optimizing the structure and yield of vanadium oxide nanotubes by periodic 2D layer scrolling.

Metal oxide nanotubes with wide interlayer van der Waals spaces are important materials for a range of applications from energy storage to catalysis, and from energy efficient catalysts and metal–insulator systems to smart window technologies. Controlling the crystalline quality is critical for the material's physical properties on the nanoscale. We report a systematic investigation into the optimization of structural quality and yield of vanadium oxide nanotubes (VONTs) synthesized by hydrothermal treatment. Usually, interdigitation of alkyl-amine chains occurs between V2O5 lamina, a stitching process that allows scrolling of 2D crystalline sheets into nanotubes with consistently high quality. Through detailed microscopy and spectroscopy examination, we demonstrate that two amine molecules per V2O5 unit optimizes the structure, quality and yield of the VONTs, and that uniform coverage of the juxtaposed V2O5 surfaces in the interlayer spacing minimizes non-uniformities and defects. This observation is consistent for a range of primary amine lengths (hexylamine to hexadecylamine). Through statistical investigation of hundreds of VONTs under each condition, we uncover the effect of amine chain length of V2O5 2D sheet thickness, and mechanism for optimum VONT quality. Finally, we summarize non-uniformities during VONT synthesis including, bending, spiraling and twisting of the scrolled crystalline layers

Vanadium oxide polycrystalline nanorods and ion-exchanged nanotubes for enhanced lithium intercalation.

In this work we investigate two alternative methods to remove amine molecules from as-synthesized vanadium oxide nanotubes (VONTs). Thermal treatment results in the formation of polycrystalline nanorods (poly-NRs) and ion exchange reactions with NaCl result in the formation of Na-VONTs. The removal of amine molecules is confirmed by monitoring the inorganic and organic phase changes and decomposition, respectively, using electron microscopy, IR spectroscopy and X-ray diffraction analyses. We compare the electrochemical performance of as-synthesized VONTs, poly-NRs and Na-VONTs. This work demonstrates that the presence of amine molecules within the layers of vanadium oxide impedes the intercalation of lithium ions, and that their removal results in a significant improvement in electrochemical characteristics. Out of the three vanadium oxide nanostructures investigated, poly- NRs exhibit the most promising results for practical use as a cathode material

A study of the oxygen electrochemistry of ruthenium and iridium

This thesis is concerned with an investigation of the anodic behaviour of ruthenium and iridium in aqueous solution and particularly of oxygen evolution on these metals. The latter process is of major interest in the large-scale production of hydrogen gas by the electrolysis of water. The presence of low levels of ruthenium trichloride ca. 10-4 mol dm-3 in acid solution give a considerable increase in the rate of oxygen evolution from platinum and gold, but not graphite, anodes. The mechanism of this catalytic effect was investigated using potential step and a.c. impedance technique. Earlier suggestions that the effect is due to catalysis by metal ions in solution were proved to be incorrect and it was shown that ruthenium species were incorporated into the surface oxide film. Changes in the oxidation state of these ruthenium species is probably responsible for the lowering of the oxygen overvoltage. Both the theoretical and practical aspects of the reaction were complicated by the fact that at constant potential the rates of both the catalysed and the uncatalysed oxygen evolution processes exhibit an appreciable, continuous decrease with either time or degree of oxidation of the substrate. The anodic behaviour of iridium in the oxide layer region has been investigated using conventional electrochemical techniques such as cyclic voltammetry. Applying a triangular voltage sweep at 10 Hz, 0.01 to 1.50V increases the amount of electric charge which the surface can store in the oxide region. This activation effect and the mechanism of charge storage is discussed in terms of both an expanded lattice theory for oxide growth on noble metals and a more recent theory of irreversible oxide formation with subsequent stoichiometry changes. The lack of hysteresis between the anodic and cathodic peaks at ca. 0.9 V suggests that the process involved here is proton migration in a relatively thick surface layer, i.e. that the reaction involved is some type of oxide-hydroxide transition. Lack of chloride ion inhibition in the anodic region also supports the irreversible oxide formation theory; however, to account for the hydrogen region of the potential sweep a compromise theory involving partial reduction of the outer regions of iridium oxide film is proposed. The loss of charge storage capacity when the activated iridium surface is anodized for a short time above ca. 1.60 V is attributed to loss by corrosion of the outer active layer from the metal surface. The behaviour of iridium at higher anodic potentials in acid solution was investigated. Current-time curves at constant potential and Tafel plots suggested that a change in the mechanism of the oxygen evolution reaction occurs at ca. 1.8 V. Above this potential, corrosion of the metal occurred, giving rise to an absorbance in the visible spectrum of the electrolyte (λ max = 455 nm). It is suggested that the species involved was Ir(O2)2+. A similar investigation in the case of alkaline electrolyte gave no evidence for a change in mechanism at 1.8 V and corrosion of the iridium was not observed. Oxygen evolution overpotentials were much lower for iridium than for platinum in both acidic and alkaline solutions

Electrochemical formation of nanoporosity in n-InP anodes in KOH

We review our recent work on anodic formation of nanoporosity in n-InP in aqueous KOH. Typically, a nanoporous sub-surface region is formed beneath a thin, dense near-surface layer. Atomic force microscopy (AFM) shows pit formation on the surface in the earlier stages of etching, and transmission electron microscopy (TEM) shows individual nanoporous domains separated from the surface by a thin InP layer. Each domain develops from a surface pit. We developed a model based on propagation of pores along the A directions. The model predicts porous domains with a truncated tetrahedral shape and this was confirmed by scanning electron microscopy (SEM) and TEM. Pores are cylindrical and have well-developed facets only near their tips. No porous layers are observed at a KOH concentration of 1.1 mol dm -2 or lower. Linear sweep voltammograms (LSVs) show a pronounced anodic peak corresponding to the formation of the porous region. We describe a technique to deconvolute the effects of potential and time in LSVs and explain their shape and their relationship to porous layer formation

Anodic formation and characterization of nanoporous InP in aqueous KOH electrolytes

The anodic behavior of highly doped (> 1018 cm-3) n-InP in aqueous KOH was investigated. Electrodes anodized in the absence of light in 2- 5 mol dm-3 KOH at a constant potential of 0.5- 0.75 V (SCE), or subjected to linear potential sweeps to potentials in this range, were shown to exhibit the formation of a nanoporous subsurface region. Both linear sweep voltammograms and current-time curves at constant potential showed a characteristic anodic peak, corresponding to formation of the nanoporous region. No porous region was formed during anodization in 1 mol dm-3 KOH. The nanoporous region was examined using transmission electron microscopy and found to have a thickness of some 1- 3 μm depending on the anodization conditions and to be located beneath a thin (typically ∼40 nm), dense, near-surface layer. The pores varied in width from 25 to 75 nm and both the pore width and porous region thickness were found to decrease with increasing KOH concentration. The porosity was approximately 35%. The porous layer structure is shown to form by the localized penetration of surface pits into the InP, and the dense, near-surface layer is consistent with the effect of electron depletion at the surface of the semiconductor

Deconvolution of the potential and time dependence of electrochemical porous semiconductor formation

A layer of porous InP is grown beneath a thin dense surface layer when n-InP electrodes are anodized to sufficiently high potentials in aqueous KOH solutions. The shape of the linear sweep (LSV) or the cyclic voltammogram (CV) is dependent on carrier concentration. A technique is presented to deconvolute the effects of potential and time on a CV. The results obtained from this technique are used to explain the shape of the anodic current response and its relation to porous layer formation. The accuracy of the deconvolution technique is then tested by comparison to experimental results

Formation of nanoporous InP by electrochemical anodization

Porous InP layers can be formed electrochemically on (100) oriented n- InP substrates in aqueous KOH. A nanoporous layer is obtained underneath a dense near-surface layer and the pores appear to propagate from holes through the near-surface layer. In the early stages of the anodization transmission electron microscopy (TEM) clearly shows individual porous domains which appear to have a square-based pyramidal shape. Each domain appears to develop from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain. When the domains grow, the current density increases correspondingly. Eventually, the domains meet forming a continuous porous layer, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Numerical models of this process have been developed. Current-time curves at constant potential exhibit a peak and porous layers are observed to form beneath the electrode surface. The density of pits formed on the surface increases with time and approaches a plateau value

A mechanistic study of anodic formation of porous InP

When porous InP is anodically formed in KOH electrolytes, a thin layer ~40 nm in thickness, close to the surface, appears to be unmodified. We have investigated the earlier stages of the anodic formation of porous InP in 5 mol dm-3 KOH. TEM clearly shows individual porous domains which appear triangular in cross-section and square in plan view. The crosssections also show that the domains are separated from the surface by a ~40 nm thick, dense InP layer. It is concluded that the porous domains have a square-based pyramidal shape and that each one develops from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain, and these domains eventually form a continuous porous layer. This implies that the development of porous domains beneath the surface is also progressive in nature. Evidence for this was seen in plan view TEM images. Merging of domains continues to occur at potentials more anodic than the peak potential, where the current is observed to decrease. When the domains grow, the current density increases correspondingly. Eventually, domains meet, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Quantitative models of this process are being developed

Numerical simulation of the anodic formation of nanoporous InP

Anodic etching of n-type InP in KOH electrolytes under suitable conditions leads to the formation of a nanoporous region beneath a ~40 nm dense near-surface layer [1]. The early stages of the process involve the formation of square-based pyramidal porous domains [2] and a mechanism is proposed based on directional selectivity of pore growth along the directions. A numerical model of this mechanism is described in this paper. In the algorithm used the growth is limited to the directions and the probability of growth at any pore tip is controlled by the potential and the concentration of electrolyte at the pore tip as well as the suitability of the pore tip to support further growth. The simulated porous structures and their corresponding current versus time curves are in good agreement with experimental data. The results of the simulation also suggest that, after an initial increase in current caused by the spreading out of the porous domains from their origins, growth is limited by the diffusion rate of electrolyte along the pores with the final fall-off in current being caused by irreversible processes such as the formation of a passivating film at the tips or some other modification of the state of the pore tip

Mechanism that dictates pore width and <111>a pore propagation in InP

We report a mechanism for pore growth and propagation based on a three-step charge transfer model. The study is supported by electron microscopy analysis of highly doped n-InP samples anodised in aqueous KOH. The model and experimental data are used to explain propagation of pores of characteristic diameter preferentially along the A directions. We also show evidence for deviation of pore growth from the A directions and explain why such deviations should occur. The model is self-consistent and predicts how carrier concentration affects the internal dimensions of the porous structures