Surface Disordered Research Articles

Bifunctional urea surface-modified high voltage LiNi0.5Mn1.5O4 cathode for enhanced electrochemical performance

Spinel LiNi0.5Mn1.5O4 (LNMO) cathode material has high specific energy density due to its high working voltage (4.7 V vs. Li/Li+). However, high voltage inevitably brings instability of the surface and interface layer properties and ultimately electrolyte decomposition and irreversible side reactions, which incurs structural degradation, capacity decline, and poor kinetics. Herein, a simple and novel surface modification strategy is proposed to obtain a bifunctional modification cathode material with carbon–nitrogen nanocoating and surface disorder phase transition, which is prepared through gas–solid heat treatment of pristine material and urea in a closed reactor. The surface disordered phase transition introduces favorable oxygen vacancies and synchronously constructs the fast diffusion channel of lithium ions, while the carbon–nitrogen nanocoating effectively alleviates the impedance growth of the electrode/electrolyte interface. Therefore, the surface modified U-LMNO exhibits an initial discharge capacity of 130.1 mAh g−1 at 0.2 C and a capacity retention rate of 95.1% after 200 cycles at 1 C, significantly superior to pristine P-LMNO. These results highlight the essentiality of high-voltage LNMO cathode material surface interface design and facilitate the development and commercialization of high-energy-density LNMO-based lithium-ion batteries.

A high-efficiency spin polarizer based on edge and surface disordered silicene nanoribbons

Using the tight-binding formalism, we explore the effect of weak disorder upon the conductance of zigzag edge silicene nanoribbons (SiNRs), in the limit of phase-coherent transport. We find that the fashion of the conductance varies with disorder, and depends strongly on the type of disorder. Conductance dips are observed at the Van Hove singularities, owing to quasilocalized states existing in surface disordered SiNRs. A conductance gap is observed around the Fermi energy for both edge and surface disordered SiNRs, because edge states are localized. The average conductance of the disordered SiNRs decreases exponentially with the increase of disorder, and finally tends to disappear. The near-perfect spin polarization can be realized in SiNRs with a weak edge or surface disorder, and also can be attained by both the local electric field and the exchange field.

Rayleigh waves, surface disorder, and phonon localization in nanostructures

We introduce a technique to calculate thermal conductivity in disordered nanostructures: a finite-difference time-domain (FDTD) solution of the elastic wave equation combined with the Green-Kubo formula. The technique captures phonon wave behavior and scales well to nanostructures that are too large or too surface disordered to simulate with many other techniques. We investigate the role of Rayleigh waves and surface disorder on thermal transport by studying graphenelike nanoribbons with free edges (allowing Rayleigh waves) and fixed edges (prohibiting Rayleigh waves). We find that free edges result in a significantly lower thermal conductivity than fixed ones. Free edges both introduce Rayleigh waves and cause all low-frequency modes (bulk and surface) to become more localized. Increasing surface disorder on free edges draws energy away from the center of the ribbon and toward the disordered edges, where it gets trapped in localized surface modes. These effects are not seen in ribbons with fixed boundary conditions and illustrate the importance of phonon surface modes in nanostructures.

Island-like mesoporous amorphous Fe2O3 layer: surface disorder engineering for enhanced lithium-storage performance

Molybdenum trioxide (MoO3) nanobelt is very attractive anode electrode for LIBs because of their high theoretical capacity. To enhance the capacity and cyclic performance of MoO3-based electrode materials of lithium ion battery (LIB), here, as a proof of concept, we report in this work a novel surface disordered engineering strategy of fabrication of island-like mesoporous amorphous Fe2O3 layer on MoO3 nanobelts (a-Fe2O3@MoO3). The island-like mesoporous amorphous Fe2O3 layer is obtained by direct hydrolysis of FeCl3.6H2O on MoO3 nanobelts assisted by low-temperature heat treatment. Here, the FeCl3.6H2O plays a multifunctional role of the formation of amorphous Fe2O3 layer, disordering MoO3 nanobelts and increasing the specific surface area and porosity of MoO3 nanobelts. The as-formed amorphous Fe2O3 layer is demonstrated to significantly improve the kinetics behavior of lithium-ion diffusion and electronic transport due to its isotropic feature during cycling. As a result, the designed anode exhibits dramatically enhanced electrochemical properties compared with individual MoO3 nanobelts and physical mixture of Fe2O3 powdes and MoO3 nanobelts: a high initial discharge capacity of 1523mAhg⿿1 at 50mAg⿿1, remarkable rate capability (386mAhg⿿1 at 500mAg⿿1) and outstanding cycling performance. Our results reveal new possibilities of designing amorphous oxides layer of anode electrodes by surface disorder engineering on achieving enhanced LIBs performance.

Synergistic Effect of Titanate-Anatase Heterostructure and Hydrogenation-Induced Surface Disorder on Photocatalytic Water Splitting

Black TiO2 obtained by hydrogenation has attracted enormous attention due to its unusual photocatalytic activity. In this contribution, a novel photocatalyst containing both a titanate–anatase heterostructure and a surface disordered shell was in situ synthesized by using a one-step hydrogenation treatment of titanate nanowires at ambient pressure, which exhibited remarkably improved photocatalytic activity for water splitting under simulated solar light. The as-hydrogenated catalyst with a heterostructure and a surface disordered shell displayed a high hydrogen production rate of 216.5 μmol·h–1, which is ∼20 times higher than the Pt-loaded titanate nanowires lacking of such unique structure. The in situ-generated heterostructure and hydrogenation-induced surface disorder can efficiently promote the separation and transfer of photoexcited electron–hole pairs, inhibiting the fast recombination of the generated charge carriers. A general synergistic effect of the heterostructure and the surface disordered s...

Surface Disordered Ge–Si Core–Shell Nanowires as Efficient Thermoelectric Materials

Ge-Si core-shell nanowires with surface disorder are shown to be very promising candidates for thermoelectric applications. In atomistic calculations we find that surface roughness decreases the phonon thermal conductance significantly. On the contrary, the hole states are confined to the Ge core and are thereby shielded from the surface disorder, resulting in large electronic conductance values even in the presence of surface disorder. This decoupling of the electronic and phonon transport is very favorable for thermoelectric purposes, giving rise to promising room temperature figure of merits ZT > 2. It is also found that the Ge-Si core-shell wires perform better than pure Si nanowires.

Kinetic Control of MnWO4 Nanoparticles for Tailored Structural Properties

This work reports on the kinetic control of MnWO4 nanoparticles with an aim to achieve the tailored structural properties. Using citric acid as the capping agent, MnWO4 nanocrystals were prepared to show particle sizes ranging from 8 to 29 nm under hydrothermal conditions. The grain-growth kinetics for MnWO4 nanoparticles was determined to quantitatively follow the equation, D5 = (4.59 × 1020)te−17.0/T, where D is the particle size at given reaction time, t, and reaction temperature, T. Systematic sample characterization using combined techniques of X-ray diffraction, transmission electron microscope, selected area electron diffraction, Barret−Emmett−Teller technique, Fourier transformed infrared spectra, UV−visible diffuse reflectance spectra, and Raman spectra indicates that particle size reduction led to an apparent lattice expansion, which is followed by lowering of lattice symmetry and band gap broadening. Different from the bulk counterparts, new vibration modes were observed in both Infrared and Raman spectra at about 913 and 930 cm−1, respectively, which intensified monotonously with particle size reduction, leading to a picture that MnWO4 nanoparticles were terminated by surface disordered layers. All these size-dependent physical properties were closely related to the surface disorder and the relevant absorbates.

Optics of surface disordered systems

No surface is perfectly flat at every length scale. However, all natural and man-made surfaces show some degree of roughness. Therefore, it is imperative to know how surface disorder may affect physical process at, or in the immediate vicinity, of the surface. In this review, we focus on the long-standing problem of electromagnetic wave scattering from randomly rough surfaces. This topic has implications and practical applications in fields of science and engineering ranging from observational astronomy to the electronic and medical industry. How randomly rough surfaces can be described statistically is outlined, and we introduce the theoretical and computational methods most frequently used in the study of light scattering from randomly rough metal or dielectric surfaces. A large part of the review is devoted to the description and the discussion of the physical origin behind various multiple scattering phenomena that can exist when light interacts with a random surface. Some of the addressed phenomena are; the enhanced backscattering and satellite peak phenomena; forward scattering (specular peak) enhancement; coherent effects in the angular intensity correlation functions and the second harmonic generated light (nonlinear effect).

Electron and phonon transport in silicon nanowires: Atomistic approach to thermoelectric properties

We compute both electron and phonon transmissions in thin disordered silicon nanowires (SiNWs). Our atomistic approach is based on tight-binding and empirical potential descriptions of the electronic and phononic systems, respectively. Surface disorder is modeled by introducing surface silicon vacancies. It is shown that the average phonon and electron transmissions through long SiNWs containing many vacancies can be accurately estimated from the scattering properties of the isolated vacancies using a recently proposed averaging method [Markussen et al., Phys. Rev. Lett. 99, 076803 (2007)]. We apply this averaging method to surface disordered SiNWs in the diameter range of 1--3 nm to compute the thermoelectric figure of merit ZT. It is found that the phonon transmission is affected more by the vacancies than the electronic transmission leading to an increased thermoelectric performance of disordered wires, in qualitative agreement with recent experiments. The largest $\text{ZT}>3$ is found in strongly disordered $⟨111⟩$-oriented wires with a diameter of 2 nm.

Transport regimes in surface disordered graphene sheets

The conductance $G$ of graphene stripes (of width $W$ and length $L$) with surface disorder and near the Dirac point is investigated numerically. Incoherent metallic leads are attached to the sample ends across its width. In samples of $W∕L\ensuremath{\geqslant}1$, the system behaves diffusively with $G\ensuremath{\rightarrow}(2∕\ensuremath{\pi})(W∕L)\ifmmode\times\else\texttimes\fi{}2{e}^{2}∕h$, for $L\ensuremath{\rightarrow}\ensuremath{\infty}$, as found in the clean limit, although with a sharply reduced shape dependence. The conductance in elongated samples, $L⪢W$, decays exponentially with $L$, indicating localization. A similar behavior is found in clean elongated systems due to the existence of minigaps near the Dirac point. Average decay lengths are larger in the presence of disorder. A magnetic field does not appreciably change the conductance unless the flux per unit cell is significant. The distribution of conductances is almost flat between lower and upper cutoffs. Away from the Dirac point, we find the standard ballistic behavior characteristic of systems with rough edges.

Investigation of microstructural and optical properties in SnO2 nanocluster-assembled solids

Abstract Nanoclusters of SnO 2 are prepared by the hydrothermal method. The micro-structure and physical properties of SnO 2 nanocluster-assembled solids (NAS) with different average particle diameters (4–80 nm) have been systematically studied by using X-ray diffraction (XRD), photoacoustic spectroscopy (PAS) and differential scanning calorimetry (DSC). The lattice of NAS is significantly distorted and is consistent with the crystallization of an amorphous phase (or a surface disordered phase), which is present in some samples when the diameters of the clusters are less than 4 nm. The specific heat (C p ) curve of NAS exhibits an appreciable structural relaxation in the range of 180–340 °C, and a grain growth peak (accompanied by a transition from the surface disorder phase to the order phase) when the temperature is in the range of 350–400 °C. At the same time, PAS shows a broad absorption peak in the range of 350–495 nm. With the nanocluster average size decreasing, there is a blueshift of the PAS absorption peak. With increasing annealing temperature, the grains grow faster and the absorption peak decreases gradually.

Impurity Distribution Profiles and Surface Disorder after Laser Induced Diffusion.

ABSTRACTA model for laser induced diffusion is proposed, assuming the melting of Si surface under pulsed laser irradiation and the diffusion ,in liquid phase, of a thin filmof impurity deposited according to the “limited source” conditions. Depending on the thickness of the film, it results in a dopant distribution profile with a surface disordered layer induced either by segregation effects or precipitation of the dopant in excess of the solubility limit achieved by laser annealing. Experimental results, obtained for a ruby laser irradiation of thin films of different impurities of group III and V, like antimony, bismuth, gallium and indium deposited on silicon substrates are in good agreement with the model. Their effective segregation coefficients have been deduced by fitting the experimental amount of dopant precipitated in the disordered surface layer with the numerical calculations. A cellular structure is seen on surface.