Amorphous Surface Layer Research Articles

Electrochemically Doped and Hydrogen Peroxide–Treated TiO2 Nanotube Arrays as an Electrode for Supercapacitor with Excellent Cycling Stability

Ordered anodic TiO2 nanotube arrays hold great potential as electrode materials for high-performance supercapacitors. However, their poor cycle stability restricts practical applications. Here we develop a simple and successful strategy to improve their cycle stability. The TiO2 nanotube arrays are electrochemically doped in a basic solution, followed by soaking them in a hydrogen peroxide solution for a short time. The as-prepared TiO2 nanotube arrays possess a core–shell structure with an amorphous surface layer of 2–3 nm thickness. The supercapacitors made by the as-prepared TiO2 nanotube arrays exhibit an excellent cycling stability with a capacitance retention of 94.1% after 10000 cycles. The presented results make TiO2 an attractive candidate as electrode materials in high-performance supercapacitors.

Organization of Bone Mineral: The Role of Mineral–Water Interactions

The mechanism (s) that drive the organization of bone mineral throughout the bone extracellular matrix remain unclear. The long-standing theory implicates the organic matrix, namely specific non-collagenous proteins and/or collagen fibrils, while a recent theory proposes a self-assembly mechanism. Applying a combination of spectroscopic and microscopic techniques in wet and dry conditions to bone-like hydroxyapatite nanoparticles that were used as a proxy for bone mineral, we confirm that mature bone mineral particles have the capacity to self-assemble into organized structures. A large quantity of water is present at the surface of bone mineral due to the presence of a hydrophilic, amorphous surface layer that coats bone mineral nanoparticles. These water molecules must not only be strongly bound to the surface of bone mineral in the form of a rigid hydration shell, but they must also be trapped within the amorphous surface layer. Cohesive forces between these water molecules present at the mineral–mineral interface not only hold the mature bone mineral particles together, but also promote their oriented stacking. This intrinsic ability of mature bone mineral particles to organize themselves without recourse to the organic matrix forms the foundation for the development of the next generation of orthopedic biomaterials.

Thermal pretreatment of diamond wire sawn multi-crystalline silicon wafers for wet acid texturization of solar cells

Diamond wire sawn (DWS) multi-crystalline (mc) silicon wafers for solar cells are known to be difficult to texturize with the conventional wet acidic etching technology. Thermal annealing as pretreatment for wet acid texturization of the DWS mc-Si wafers has been studied. Improvement in morphology and reduction of the light reflectivity of the texture has been obtained with the pretreatment. Solar cells fabricated with the pre-treated DWS mc-Si wafers have been found to yield an ∼0.15% absolute efficiency gain in average, as compared with the solar cells fabricated in the same line with the same batch of wafers without the pretreatment. The thermal pretreatment was found to crystallize the amorphous surface layers on DWS mc-Si wafers. This is believed to be the reason for the above improvement. The present thermal pretreatment provides a promising low cost technology for texturization of DWS mc-Si wafers in photovoltaic industry.

Selective Oxidation of n‐Butane over Vanadium‐Phosphorus Oxide: Oxygen Activation and Dynamics

AbstractThe complex microkinetics of the selective oxidation of n‐butane to maleic anhydride (MAN) over vanadyl pyrophosphate‐based (VPP) catalysts has been widely investigated so far, however, they are still under debate. We present novel results of kinetic analyses and isotope labeling to shed light on the activation of gas‐phase oxygen and the dynamics of oxygen species on the catalyst surface and in the bulk. We suggest the existence of at least two catalytically relevant surface oxygen species responsible for selective and unselective activation of n‐butane, which are formed by adsorption and consecutive dissociation of O2 at the active site. The formation of selective [O] from unselective [O2] is likely promoted by co‐fed H2O, which increases the MAN selectivity. Regarding the VPP bulk serving as a reservoir for active [O] species it is concluded that the dynamic exchange is limited to the amorphous surface layer forming under reaction conditions, whereas the crystalline bulk is not involved into the reaction. Labelling studies are heavily influenced by vital 16O/18O scrambling of MAN isotopes with the reaction product H2O.

Formation and properties of a Zr-based amorphous coating by laser cladding

(Zr0.53Al0.1Ni0.05Cu0.3Ti0.02)99Y1 (at%) coating with amorphous layer of about 180 μm thick was prepared on a steel substrate by using laser cladding method. The coating is compact and shows good metallurgical bonding with substrate. The microstructure, microhardness and corrosion behavior along the depth from the coating surface to the substrate were investigated. It is found that a gradient structure consisted of amorphous surface layer, amorphous–crystalline transitional layer and substrate is formed after the laser cladding. The microhardness and corrosion behavior exhibit variation with the microstructural evolution at different depths from the coating surface. The microhardness and corrosion resistance in 3.5 wt% NaCl solution of the amorphous surface layer are comparable to those of the as-cast Zr-based BMG with the same composition, and higher than those of the steel substrate.

Thickness and Stacking Sequence Determination of Exfoliated Dichalcogenides (1T-TaS2, 2H-MoS2) Using Scanning Transmission Electron Microscopy.

Layered transition metal dichalcogenides (TMDs) have attracted interest due to their promise for future electronic and optoelectronic technologies. As one approaches the two-dimensional (2D) limit, thickness and local topology can greatly influence the macroscopic properties of a material. To understand the unique behavior of TMDs it is therefore important to identify the number of atomic layers and their stacking in a sample. The goal of this work is to extract the thickness and stacking sequence of TMDs directly by matching experimentally recorded high-angle annular dark-field scanning transmission electron microscope images and convergent-beam electron diffraction (CBED) patterns to quantum mechanical, multislice scattering simulations. Advantageously, CBED approaches do not require a resolved lattice in real space and are capable of neglecting the thickness contribution of amorphous surface layers. Here we demonstrate the crystal thickness can be determined from CBED in exfoliated 1T-TaS2 and 2H-MoS2 to within a single layer for ultrathin ≲9 layers and ±1 atomic layer (or better) in thicker specimens while also revealing information about stacking order-even when the crystal structure is unresolved in real space.

Deposition of Ni nanoparticles on black TiO2 nanowire arrays for photoelectrochemical water splitting by atomic layer deposition

Ni nanoparticles were deposited on to black TiO2 nanowire arrays by atomic layer deposition (ALD) using Ni(dmap)2 (dmp = dimethylamino-2-propoxide) as precursors. The size of the Ni nanoparticles was well controlled by the number of ALD cycles. The growth rate of the nanoparticles on the black TiO2 nanowire arrays was 0.5 Å/cycle, which was comparable with that on the pristine samples, even though a surface amorphous layer was formed. When the Ni ALD was applied to the pristine and black TiO2 nanowire arrays, both samples exhibited an enhanced absorption in the visible light range. Interestingly, the maximum absorption was observed for the black TiO2 nanowire arrays due to the higher polarization that induced the stronger surface plasmon resonance (SPR) of the Ni nanoparticles. The wavelength of the absorption red shifted from 440 to 500 nm when the cycle number increased from 25 to 100. The highest intensity of the absorption was obtained for 50 cycles of Ni ALD. This was attributed to the inter-particle interaction related to the size and loading density of the Ni nanoparticles. 50 cycles of Ni ALD on the black TiO2 nanowire arrays exhibited the most effective photoresponse in the visible light range, resulting in a higher carrier concentration. As a consequence, a superior photoelectrochemical water splitting property was obtained.

Hydrous Iridium-Metal Oxide Nanoarchitectures for High Activity Oxygen Evolution Electrocatalysts

To further the development of proton-exchange membrane (PEM) electrolyzers, oxygen evolution electrocatalysts with high activity, extended durability, and lower costs are needed. Although iridium oxide (IrOx) electrocatalysts have shown high activity and reasonable stability for the oxygen evolution reaction (OER) in acidic conditions, the high costs and limited supply of iridium have motivated our group and other groups to explore approaches that increase the activity and stability of iridium-based catalysts. We have recently demonstrated bimetallic nickel-platinum two-dimensional (2D) nanoframes that function as oxygen reduction electrocatalysts with high specific activity and improved stability.1 Using this approach, we created self-supported metallic Ni-Ir and Co-Ir alloy nanoarchitectures and evaluated their performance as oxygen evolution electrocatalysts. The Ir-M (M=Ni, Co) alloy 2D nanoframes were synthesized by thermal reduction of iridium-decorated metal hydroxide nanosheets followed by chemical leaching. The interaction of iridium with nickel and cobalt was investigated to tune the surface atomic and electronic structure and increase the OER activity. The nanoframes have a self-supported, carbon-free three-dimensional matrix that allows molecular access to the catalytically active surface sites. After electrochemical conditioning within the OER potential range, the surface is predominantly transformed from metallic to a metal oxide/hydroxide, and the metallic alloy phase is retained below the amorphous surface layer. Oxygen evolution activities were determined using rotating disk electrode configuration. The OER mass activities of hydrous iridium-metal oxide nanoarchitectures were significantly higher than those of commercial IrOx. Different temperature treatments were determined to significantly alter the atomic-level structure and influence the electrochemical activity and stability. Additional efforts are aimed at understanding the effect of altering the non-noble transition metal and treatment conditions on the structure and OER activity and stability. The ability to combine highly catalytically active surfaces within a carbon-free 3D nanoarchitecture provides the opportunity to design OER catalysts with improved activity and stability. References Godínez-Salomón, F.; Mendoza-Cruz, R; Arellano-Jimenez, M.J., Jose-Yacaman, M.; Rhodes, C.P; Metallic Two-dimensional Nanoframes: Design of Carbon-free Hierarchical Nickel-Platinum Alloy Electrocatalyst Nanoarchitecture with Enhanced Oxygen Reduction Activity and Stability, ACS Applied Materials & Interfaces, 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043

Time-dependent feldspar dissolution rates resulting from surface passivation: Experimental evidence and geochemical implications

To which extent does the apparent negative correlation reported between silicate weathering rates and time result from the spontaneous physicochemical evolution of the fluid–mineral interface? To address this question, labradorite powders inserted in nylon bags and buried into two different topsoil horizons for four years were subjected to nanoscale characterization of their near-surface regions using transmission electron microscopy. These characterizations revealed the occurrence of a 30 to 70 nm-thick discontinuous amorphous silica-rich surface layer (ASSL) with a sharp crystalline-amorphous interfacial boundary between labradorite and the layer. Dissolution experiments conducted in mixed-flow reactors at ambient temperature and acidic pH demonstrated that the reactivity of fresh and naturally weathered labradorite powders decreased with time, with the dissolution rate of fresh powders remaining systematically greater than that of naturally weathered powders, all over the duration of the experiments (3 weeks). In addition, the dissolution rate of all labradorite batches was noticeably lower in solutions containing elevated concentrations of SiO2(aq), which we attributed to the passivating effect of the ASSLs. This suggestion was confirmed with a simple passivation model, which enabled to capture (i) the greater reactivity of fresh powders; (ii) the dependence of the dissolution rate on [SiO2(aq)]; (iii) the gradual decline of powder dissolution rate with time and (iv) the discontinuous occurrence of ASSLs. The model further supports that surface passivation could be one of the (non-exclusive) mechanisms that could account for the so-called kinetic “field-lab discrepancy”. The geochemical implications of the recognition of the passivation mechanism are broad, ranging from the need to revisit the kinetic rate laws implemented in geochemical codes to the questioning of the formalism used for determining weathering rates from the study of U-series nuclides in soils and weathering profiles.

The Chemistry of Eolian Quartz Dust and the Origin of Chert

Abstract Among the numerous models that have been suggested for the primary and predominant source of silica for chert, we suggest that eolian dust is worthy of further considerations. Such considerations are supported by the common association of Phanerozoic chert with evaporites, limestones, dolomites, or other strata that were deposited within or near arid paleoclimates. This association suggests a direct or indirect causal link between aridity and chert formation. In addition, eolian processes export large quantities of quartz-rich dust from arid climate ergs or loess lands. The chemistry of abraded quartz particles derived therefrom is consistent with chert formation. Abrasion of quartz particles produces an amorphous surface layer and underlying lattice disorder. An inverse relation between particle size distribution (< 64 μm) and the enthalpy of solution of abraded quartz is indicative of the degree of lattice disorder; as particle size decreases, the enthalpy of solution increases. The amorphous surface layer and underlying lattice disorder enhance both the rate and amount of dissolution of quartz dust particles; the solubilized silica can then be reprecipitated and diagenetically altered to the various silica polymorphs that occur in chert. An eolian supply and the chemistry of eolian abraded quartz particles may account for chert formation in disparate depositional environments that encompass deep-sea chert, chert in epicontinental seas including shallow shelves, and chert that formed in continental eolinites, lakes, and soils (arid climate silcretes).

Preferential Cation Vacancies in Perovskite Hydroxide for the Oxygen Evolution Reaction

AbstractThe oxygen evolution reaction (OER) is an ideal model to study the relationship between the activity and the surface properties of catalysts. Defect engineering has been extensively developed to tune the electrocatalytic activity for OER. Compared to the anion vacancies in metal oxides, cation vacancies are more challenging to selectively generate, and the insight into the structure and activity of cation vacancies‐rich catalysts are lacked. Herein, using SnCoFe perovskite hydroxide as a precursor, abundant Sn vacancies on the surface were preferentially produced by Ar plasma. Sn vacancies could be preferentially produced as confirmed by the X‐ray absorption spectra, probably owing to the lower lattice energy and weaker chemical bonds of Sn(OH)4. The Sn vacancies promoted the exposure of active CoFe sites, resulting in an amorphous surface layer, modulated the conductivity, and thus enhanced the OER performance.

Preferential Cation Vacancies in Perovskite Hydroxide for the Oxygen Evolution Reaction.

The oxygen evolution reaction (OER) is an ideal model to study the relationship between the activity and the surface properties of catalysts. Defect engineering has been extensively developed to tune the electrocatalytic activity for OER. Compared to the anion vacancies in metal oxides, cation vacancies are more challenging to selectively generate, and the insight into the structure and activity of cation vacancies-rich catalysts are lacked. Herein, using SnCoFe perovskite hydroxide as a precursor, abundant Sn vacancies on the surface were preferentially produced by Ar plasma. Sn vacancies could be preferentially produced as confirmed by the X-ray absorption spectra, probably owing to the lower lattice energy and weaker chemical bonds of Sn(OH)4 . The Sn vacancies promoted the exposure of active CoFe sites, resulting in an amorphous surface layer, modulated the conductivity, and thus enhanced the OER performance.

Dynamics of self-reorganization explains passivation of silicate glasses

Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a challenge with major implications in geoscience and industry. One of the main uncertainties limiting the development of predictive models lies in the formation of an amorphous surface layer––called gel––that can in some circumstances control the reactivity of the buried interface. Here, we report experimental and simulation results deciphering the mechanisms by which the gel becomes passivating. The study conducted on a six-oxide borosilicate glass shows that gel reorganization involving high exchange rate of oxygen and low exchange rate of silicon is the key mechanism accounting for extremely low apparent water diffusivity (∼10−21 m2 s−1), which could be rate-limiting for the overall reaction. These findings could be used to improve kinetic models, and inspire the development of new molecular sieve materials with tailored properties as well as highly durable glass for application in extreme environments.

Impact of alkali on the passivation of silicate glass

Amorphous silica-rich surface layers, also called gels, can passivate silicate glass and minerals depending on environmental conditions. However, several uncertainties remain on the mechanisms controlling the formation of these layers. In this paper, the influence of exogenous ions supplied by solutions is studied, both on the formation and on the properties of the gel formed on international simple glass (ISG). ISG was altered at 90 °C, pH90 °C 7, in silica-saturated solutions containing various alkaline cations separately (Li+, Na+, K+, and Cs+). The alteration kinetics observed with Li and Na in the solution is similar to that observed with no ions, while K and Cs in the solution tend to decrease glass alteration. Furthermore, for K or Cs ions, the kinetics decreases as the ionic strength of the solution increases. The passivation layer formed in these solutions shows a selectivity toward cations following the series K > Cs > Na >> Li. These alkalis replace Ca from pristine glass in the altered structures, leading to differences in [AlO4]− units charge compensation. Importantly, exchange between Ca and alkali also affects the total quantity of water inside each gel and this effect is well correlated with the observed drop in glass alteration.

Surface Nanocrystallization and Amorphization of Dual-Phase TC11 Titanium Alloys under Laser Induced Ultrahigh Strain-Rate Plastic Deformation.

As an innovative surface technology for ultrahigh strain-rate plastic deformation, laser shock peening (LSP) was applied to the dual-phase TC11 titanium alloy to fabricate an amorphous and nanocrystalline surface layer at room temperature. X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy (HRTEM) were used to investigate the microstructural evolution, and the deformation mechanism was discussed. The results showed that a surface nanostructured surface layer was synthesized after LSP treatment with adequate laser parameters. Simultaneously, the behavior of dislocations was also studied for different laser parameters. The rapid slipping, accumulation, annihilation, and rearrangement of dislocations under the laser-induced shock waves contributed greatly to the surface nanocrystallization. In addition, a 10 nm-thick amorphous structure layer was found through HRTEM in the top surface and the formation mechanism was attributed to the local temperature rising to the melting point, followed by its subsequent fast cooling.

Статистична теоретична модель динамічної Бреґґової дифракції в двошаровій кристалічній системі з аморфним поверхневим шаром

Статистична теоретична модель динамічної Бреґґової дифракції в двошаровій кристалічній системі з аморфним поверхневим шаром

The effects of sodium content and hydrogenation of TiO2 nanotubes on photocatalytic activity

The effects of sodium content and hydrogenation of TiO2 nanotubes on their photocatalytic activity were investigated. TiO2 nanotubes with different contents of sodium were prepared by an alkaline reflux method, followed by washing with different concentrations of acid. When a hydrogen thermal treatment was applied to the nanotubes, the white nanotubes with and without sodium would transform into blue polyhedral and light yellow chain-like structures, respectively. UV–vis absorption spectra indicated that a higher absorption in the visible light range was obtained for the blue polyhedral structure. Interestingly, an amorphous surface layer and Ti3+ species appeared and became more pronounced when the sodium was removed completely. This led to the formation of a defect energy band belt, which not only narrowed the band gap but lowered the rate of recombination of photo electrons and holes. As a consequence, a higher rate of photocatalytic decomposition of methanol for hydrogen evolution under visible light illumination were obtained.

Toward an understanding of surface layer formation, growth, and transformation at the glass–fluid interface

Silicate glass is a metastable and durable solid that has application to a number of energy and environmental challenges (e.g., microelectronics, fiber optics, and nuclear waste storage). If allowed to react with water over time silicate glass develops an altered layer at the solid-fluid interface. In this study, we used borosilicate glass (LAWB45) as a model material to develop a robust understanding of altered layerformation (i.e., amorphous hydrated surface layer and crystalline reaction products). Experiments were conducted at high surface area-to-volume ratio (∼200,000 m−1) and 90 °C in the pressurized unsaturated flow (PUF) apparatus for 1.5-years to facilitate the formation of thick altered layers and allow for the effluent solution chemistry to be monitored continuously. A variety of microscopy techniques were used to characterize reacted grains and suggest the average altered layer thickness is 13.2 ± 8.3 μm with the hydrated and clay layer representing 74.8% and 25.2% of the total altered layer, respectively. The estimate of hydrated layer thickness is within the experimental error of the value estimated from the B release rate data (∼10 ± 1 μm/yr) over the 1.5-year duration. PeakForce® quantitative nanomechanical mapping results suggest the hydrated layer has a modulus that ranges between ∼20 and 40 GPa, which is in the range of porous silica that contains from ∼20 to ∼50% porosity, yet significantly lower than dense silica (∼70–80 GPa). Scanning transmission electron microscopy (STEM) images confirm the presence of pores and an analysis of a higher resolution image provides a qualitative estimate of ≥22% porosity in the hydrated layer with variations in void volume with increasing distance from the unaltered glass. Chemical composition analyses, based on a combination of time-of-flight secondary-ion mass spectrometry (ToF-SIMS), scanning electron microscopy with X-ray energy dispersive spectroscopy (EDS), and STEM-EDS, clearly show that the altered layer is mainly composed of Al, H, Si, and O with the clay layer being enriched in Li, Zn, Fe, and Mg. The amorphous hydrated layer is enriched in Ca, H, and Zr with a minor amount of K. Furthermore, ToF-SIMS results also suggest the B profile is anti-correlated with the H profile in the hydrated layer. Our selected-area electron diffraction results suggest the structure of the hydrated layer closely resembles opal-AG (amorphous gel-like) with an average crystallite size of ∼0.7 nm which is smaller than the critical nucleus for silica nanoparticles (i.e., 1.4–3 nm). These results suggest the hydrated layer is more consistent with a polymeric gel rather than a colloidal gel and is comprised of molecular units (<1 nm in size) that result from the difficult to hydrolyze bonds, such as SiOZr units, during the glass corrosion process. The size of individual particles or molecular units is a function of formation conditions (e.g., pH, ionic strength, nano-confinement, solute composition) in the hydrated layer.

In-situ transformation into MoSe2/MoO3 heterogeneous nanostructures with enhanced electrochemical performance as anode material for sodium ion battery

As anode materials, the electrochemical performance of layered transition metal dichalcogenides (TMDs), can be improved with surface layer protection. In this study, flower-like MoSe2 nanostructures are easily obtained from a solvothermal reaction followed by an annealed process in inert atmosphere. Then the MoSe2 nanostructures are further treated in air at 350 °C, and amorphous MoO3 can be formed on MoSe2 surfaces, resulting into MoSe2/MoO3 heterogeneous nanostructures. The MoSe2/MoO3 composite, as anode materials for sodium ion batteries (SIBs), shows better performance than those of pristine MoSe2, which includes charge/discharge capacity, cycling stability and rate capability. At low current of 50 mA g−1, the discharge capacity can reach as high as 729 mAh g−1. In addition, even at high current of 10 A g−1, the discharge capacity maintains at 314 mAh g−1. At high currents of 1000 and 2000 mA g−1, after 200 cycles, the capacity retention can be kept at 101.9% and 99.5%, respectively. The enhanced performance of MoSe2/MoO3 heterogeneous nanostructures can be attributed to the amorphous MoO3 surface layer, which prevents selenium dissolution and subsequently improves the cycle performance and increases sodiation/desodiation kinetics because of the amorphous nature.

Creation and Annihilation of Charge Traps in Silicon Nanocrystals: Experimental Visualization and Spectroscopy

Recent studies have shown the presence of an amorphous surface layer in nominally crystalline silicon nanocrystals (SiNCs) produced by some of the most common synthetic techniques. The amorphous surface layer can serve as a source of deep charge traps, which can dramatically affect the electronic and photophysical properties of SiNCs. We present results of a scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) study of individual intragap states observed on the surfaces of hydrogen-passivated SiNCs deposited on the Au(111) surface. STS measurements show that intragap states can be formed reversibly when appropriate voltage-current pulses are applied to individual SiNCs. Analysis of STS spectra suggests that the observed intragap states are formed via self-trapping of charge carriers injected into SiNCs from the STM tip. Our results provide a direct visualization of the charge trap formation in individual SiNCs, a level of detail which until now had been achieved only in theoretical studies.