Formation Of Nanocavities Research Articles

Formation of intragranular nanocavities in the Cr2O3 layer during the corrosion of Cr3C2+Cr coated Zirlo™

Intragranular nanocavities were observed in the Cr2O3 layer of a Cr3C2+Cr coated Zirlo™ specimen corroded in steam at 1200 ℃ for 1 h. By comparing the coated specimen that was quenched from 1200 ℃ with the one that was not, we considered the root cause of the intragranular nanocavities in Cr2O3 was that the nanocavities that formed in the original coating were trapped into the Cr2O3 grains during the corrosion process. This explanation may also apply to the formation of intragranular nanocavities in single metal Cr-coated zirconium alloys corroded at high temperatures.

Oxygen-Loss-Induced Structural Degradation in ε-LiVOPO4

The ε-LiVOPO4 cathode for Li-ion batteries has attracted wide attention with its multivalent electronic states and improved discharge capacity of over 300 mAh/g. Oxygen loss stands as a potential cause for structural degradations of the ε-LiVOPO4 cathode and its derivatives but has been barely studied. Through in situ environmental transmission electron microscopy, we probe lattice oxygen loss and the associated structural degradations by spatially and temporally resolving the atomic-scale structural dynamics and phase transformation pathways in ε-LiVOPO4. We demonstrate that the mild oxygen loss at 400 °C induces a topotactic phase transformation of ε-LiVOPO4 → α-Li3V2(PO4)3 in the particle surface via a nucleation and growth mechanism, leading to the formation of a core-shell configuration. The phase transformation can be reversed by switching to an oxidizing environment, in which the α-Li3V2(PO4)3 is reoxidized to ε-LiVOPO4. By contrast, oxygen loss at higher temperatures of 500 and 600 °C results in a high concentration of oxygen vacancies that subsequently induces irreversible structural damages including lattice amorphization and formation of nanocavities. This work illustrates the fundamental mechanisms governing the structural failure of oxide cathodes and underlines possible strategies to overcome such issues by exploiting environmental constraints.

Damage in Xe-implanted 4H-SiC under severe conditions

Xenon ions were implanted in 4H-SiC at 400 and 700 °C with fluences of 1 × 1016 and 3 × 1016 cm−2, respectively. Transmission Electron Microscopy (TEM) experiments were carried out to study the resulting microstructure and its evolution after a 1400 °C annealing. Observations show that the temperature of implantation plays an active role in the formation of as-generated defects. The implantation temperature of 400 °C is too low to allow the formation of visible defects and only a strongly damaged band is observed in the ion stopping range. The post-implantation annealing results in the formation of nanometric cavities along the ion path. In the buried ion stopping region, xenon solid cavities or nano-precipitates of about 10 nm in diameter are present. Other types of defects were also formed during annealing, a high density of stacking faults is observed among the solid cavities and black spot defects are observed beyond the implanted area. At a temperature of 700 °C, the vacancies become mobile enough to cause the formation of nanocavities all along the ions path, i.e. voids in the near surface region and full-gas cavities in the xenon stopping range. At this temperature stacking faults were also formed as well as the defect band at the back. The annealing only causes a slight growth of the solid cavities.

Orientation-dependent superelasticity and fatigue of CuAlMn alloy under in situ micromechanical tensile characterization

The in situ micromechanical tensile tests are conducted to characterize the superelastic behaviors for [223], [1214], [325] and [205] oriented Cu67Al24Mn9 micro-slats. The stress-induced martensitic transformations are captured in all textures corresponding to strong crystallographic anisotropy. We propose a one-dimensional constitutive model considering the directional anisotropy of elastic modulus for cubic symmetry and the crystallographic compatibility of twinned martensite. The modeled mechanical behaviors agree with the micromechanical tensile tests well, which suggests that the formation of compatible twins is the primary deformation mechanism. Particularly in the [205] texture, we observed formation of nano-cavities (<100nm) on the lateral surface of the micro-slat. Based on the analysis of compatible martensite twin laminates and fcc slip systems, we theorize that the massive normal elongation and little lateral shear cause the formation, stretching and growth of nano-cavities at nano scales to accommodate the external loads. As a result, the structural and functional fatigue resistance is improved compared to other textures. The experimental and theoretical results in this paper are potentially useful to guide the texture design of Cu-based shape memory alloy for high transformation strain and low functional fatigue.

Laser-Induced Interfacial Spallation for Controllable and Versatile Delamination of Flexible Electronics.

The control of interface status is greatly critical to release large-area, ultrathin flexible electronics from the donor wafer to achieve mechanical flexibility. This paper discovers a laser-induced interfacial spallation process for controllable and versatile delamination of polyimide (PI) films from transparent substrates and makes a comprehensive mechanism study of the controllability of interfacial delamination after laser irradiations. Microscopic observations show that backside irradiations will result in the formation of nanocavities around the PI-glass interface, enabling a significant decrease in interface adhesion. Theoretical calculations indicate that gas products generated from thermal decomposition of PI will cause hydrodynamic spallation of molten PI around the interface. The controllable spallation behavior benefits the formation/elimination of fibrous microconnections between the PI film and glass substrate. A substantial regulation of interfacial micromorphologies can achieve precise control of interface adhesion, mass production of functional nanostructures, and nondestructive peeling of ultrathin flexible devices. The results could be useful for the fabrication of flexible electronics and biomimetic surfaces.

Insights into the fatigue cracking of Alloy 690TT subjected to fretting wear under partial slip conditions

Fatigue crack initiation and propagation is one of the failure modes of Alloy 690TT when subjected to fretting wear under partial slip conditions. Detail characterization was performed for the fatigue cracks by using transmission electron microscopy (TEM). The results indicated that slip bands, subgrain boundaries, and oxide film contributed to the fatigue crack initiation and propagation. Intergranular and intragranular oxidation accelerated the crack propagation and changed the propagation path. The dislocation interactions and the synergy of oxidation and amorphization were responsible for the formation of nano-cavities. Amorphous structure was formed by the oxidation at crack inner and surrounding the nano-cavities due to severe plastic deformation. The combination of oxidation and amorphization along with the chemical and structural disorder accelerated the crack propagation.

Enhanced Photocurrent Generation in Proton‐Irradiated “Giant” CdSe/CdS Core/Shell Quantum Dots

AbstractGroup II–VI quantum dots (QDs) possess tunable electrical and optical properties that make them very attractive for high‐tech applications and power generation. The effects of proton irradiation on both the structural and physical properties of “giant” CdSe/CdS core–shell QDs (g‐CS QDs) are investigated. These experiments shed light on photoelectron delocalization in g‐CS QDs, where current linkages and strong variations in optical emission result from the spatial extension of the photoelectron wavefunctions over the conduction bands of CdSe and CdS. Monte Carlo simulations of ion–matter interactions show that the damaging rates can be set from the energy of impinging protons to promote the formation of structural defects in the core or shell. The formation of nanocavities is demonstrated after irradiation doses higher than ≈1017 H+ cm−2, while a continuous decrease in luminescence intensity is observed for increasing proton fluencies. This feature is accompanied by a concomitant lifetime decrease marking the rise of nonradiative phenomena and the occurrence of greater photocarrier transfers between CdS and CdSe. Current‐to‐voltage characterizations evidence that proton implantation can be implemented to enhance the photocurrent generation in g‐CS QDs. This increase is attributed to the delocalization of photoelectrons in the CdS shell, whose improvement is found to promote electron–hole pair separation.

Nickel vanadate microspheres with numerous nanocavities synthesized by spray drying process as an anode material for Li-ion batteries

For use in next-generation energy storage applications, including electric vehicles, capacity and cycle life of lithium ion batteries need further improvement. Moreover, to achieve fast lithiation kinetics of the electrode materials, high power density and quick charging ability are necessary. Nickel vanadate (Ni3V2O8) microsphere with tens of nanocavities is one of candidates for anode materials suitable for lithium ion batteries. The synthesis of microspheres is possible by a pilot-scale spray drying process and facile one-step oxidation heat treatment. Dextrin, which is present in the microspheres after spray drying process, plays a key role in the formation of nanocavities. Oxidation at different temperatures yields carbon composite microspheres with nanocavities and hierarchical Ni3V2O8 microspheres with nanocavities. The nanocavities facilitate electrolyte contact with the electrode material and alleviate volume change during lithiation/delithiation. The merits of the nanocavities in the Ni3V2O8 microspheres enable a high discharge capacity of 1045 mA h g−1 for the 2nd cycle at 1 A g−1 and long cycle life. Furthermore, Ni3V2O8 microspheres deliver a high discharge capacity of 612 mA h g−1 at a high current density of 6 A g−1.

Fabrication of Suspended III-V Nanofoils by Inverse Metal-Assisted Chemical Etching of In0.49Ga0.51P/GaAs Heteroepitaxial Films.

Metal-assisted chemical etching (MacEtch) has been established as a low-cost, benchtop, and versatile method for large-scale fabrication of semiconductor nanostructures and has been heralded as an alternative to conventional top-down approaches such as reactive-ion etching. However, extension of this technique to ternary III-V compound semiconductor alloys and heteroepitaxial systems has remained relatively unexplored. Here, Au-assisted and inverse-progression MacEtch (I-MacEtch) of the heteroepitaxial In0.49Ga0.51P/GaAs material system is demonstrated, along with a method for fabricating suspended InGaP nanofoils of tunable thickness in solutions of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). A comparison between Au- and Cr-patterned samples is used to demonstrate the catalytic role of Au in the observed etching behavior. Vertical etch rates for nominally undoped, p-type, and n-type InGaP are determined to be ∼9.7, ∼8.7, and ∼8.8 nm/min, respectively. The evolution of I-MacEtch in the InGaP/GaAs system is tracked, leading to the formation of nanocavities located at the center of off-metal windows. Upon nanocavity formation, additional localized mass-transport pathways to the underlying GaAs substrate permit its rapid dissolution. Differential etch rates between the epilayer and substrate are exploited in the fabrication of InGaP nanofoils that are suspended over micro-trenches formed in the GaAs substrate. A model is provided for the observed I-MacEtch mechanism, based on an overlap of neighboring injected hole distribution profiles. The nanofabrication methodology shown here can be applied to various heteroepitaxial III-V systems and can directly impact the conventional processing of device applications in photonics, optoelectronics, photovoltaics, and nanoelectronics.

UDM enhanced physical and mechanical properties through the formation of nanocavities in an epoxy matrix

The matrix modification of relatively low viscous epoxy based polymer treated under ultrasonic mixing (UM) and ultrasonic mixing with simultaneous stirring by a rotating impeller, referred to as ultrasonic dual mixing (UDM), and the effect of processing techniques has been investigated in terms of the formation of nanocavities in the epoxy matrix. Nanocavities of size 42±8nm have been formed uniformly in the epoxy matrix by UDM. The effect of a change in matrix morphology on the viscoelastic, tensile and thermal properties of the cured epoxy resin has been studied. The UDM processed cured epoxy matrix showed 18.26% and 88.34% improvement in tensile strength and toughness as compared to unprocessed epoxy. Thermal gravimetric analysis (TGA) of UDM processed epoxy showed significant enhancement in the thermal stability of the epoxy matrix.

Effect of nanocavities on Ge nanoclustering and out-diffusion in SiO2

Germanium nanocrystals (Ge-ncs) were synthesized by implantation of Ge+ ions into the fused silica, followed by a thermal annealing at 1000 °C. High-resolution transmission electron microscopy was employed to characterize both the morphology of the formed Ge-ncs and the evolution of their depth-distribution as a function of annealing durations. The formation of nanocavities in the vicinity of nanocrystal/SiO2 interface is evidenced, as well as their influence on the release of the compressive stress exerted on Ge-ncs by surrounding SiO2. Some Ge-ncs are found inside nanocavities, and can move into the implanted layer through a nanocavity-assisted diffusion mechanism. This finding sheds light on a new process that can explain the non-uniformity of the Ge-nanocrystal spatial distribution.

Synergistic effect of nanocavities in anatase TiO2 nanobelts for photocatalytic degradation of methyl orange dye in aqueous solution

Nanocavities are empty voids exposed on the surface of one dimensional TiO2 nanostructured material. Often, they exhibited beneficial optical and electrical properties that leads to efficient photocatalytic reactions. This study reports formation of nanocavities on anatase TiO2 nanobelts (TNB) through dehydroxylation of surface hydroxyl groups during calcination process (350–600°C). The morphological and crystal structure analysis of TNB-500, -550 and -600 displayed the nanobelts shape with high density of nano-size cavities and increase in average diameter with calcination temperature. The SAED patterns confirm the anatase TiO2 phase. The enhanced light absorption properties of biphasic anatase/TiO2-B and anatase TiO2 than H2Ti3O7 are attributed to transformation of crystal structure upon calcination process. The catalytic activity was evaluated for degradation of methyl orange dye in aqueous solution under solar light irradiation. The reaction variables such as calcination temperature, amount of catalyst and pH of the methyl orange dye solution were studied and discussed in detail. Under optimal experimental conditions TNB-550 photocatalyst displayed highest degradation performance about 8 folds higher than H2Ti3O7. The high performance is explained as due to synergistic properties of one dimensional anatase TiO2 with high density of nanocavities leading to one dimensional transfer of electrons and high absorption co-efficient in UV-A spectrum are suitable for efficient red-ox reactions.

GaN Nano-Cones Prepared by Jet-Printed Nanospheres Arrays

Site and dimension controllable nanocone arrays have been fabricated by a combination of inkjet-printing and nanosphere lithography. Each printed array is of well-formed circular geometry with diameters of about 20 μm. The nanocones with base diameters of 500 nanometers and sharp tips are formed due to etch isotropy. Photoluminescence has been carried out to study the optical properties of the Gallium Nitride nanostructures. The approach is demonstrated to be effective for the enhancement of light extraction, as well as for the formation of site-controlled nanocavities.

Controlling the Formation of Nanocavities in Kirkendall Nanoobjects through Sequential Thermal Ex Situ Oxidation and In Situ Reduction Reactions.

Controlling the porosity, the shape, and the morphology of Kirkendall hollow nanostructures is the key factor to tune the properties of these tailor-made nanomaterials which allow in turn broadening their applications. It is shown that by applying a continuous oxidation to copper nanowires following a temperature ramp protocol, one can synthesize cuprous oxide nanotubes containing periodic copper nanoparticles. A further oxidation of such nanoobjects allows obtaining cupric oxide nanotubes with a bamboo-like structure. On the other hand, by applying a sequential oxidation and reduction reactions to copper nanowires, one can synthesize hollow nanoobjects with complex shapes and morphologies that cannot be obtained using the Kirkendall effect alone, such as necklace-like cuprous oxide nanotubes, periodic solid copper nanoparticles or hollow cuprous oxide nanospheres interconnected with single crystal cuprous oxide nanorods, and aligned and periodic hollow nanospheres embedded in a cuprous oxide nanotube. The strategy demonstrated in this study opens new avenues for the engineering of hollow nanostructures with potential applications in gas sensing, catalysis, and energy storage.

Fabrication of porous TiO2 nanorod array photoelectrodes with enhanced photoelectrochemical water splitting by helium ion implantation.

Porous photoelectrodes show high efficiency in hydrogen production by water splitting. However, fabrication of porous nanorods is usually difficult. Here, we report a simple approach to fabricate a kind of novel porous rutile titanium dioxide nanorod array by an advanced ion implantation method using multiple-energy helium ion implantation and subsequent annealing. The porous nanostructure enhances the photoelectrochemical performance of the titanium dioxide nanorod array photoelectrodes under Uv-visible light illumination, where the highest photocurrent density was relatively about 10 times higher than that of the pristine titanium dioxide nanorod array. The formation of nanocavities mainly contributes to the enhancement of the photocurrent density by trapping holes inside to separate the charge carriers. The study demonstrates that ion implantation could be an effective approach to develop novel porous nanostructural photoelectrodes for the application of hydrogen production.

In situ observation and investigation on the formation mechanism of nanocavities in TiO2nanofibers

In this work, the formation and evolution mechanism of nanocavities generated on the surface of TiO2 nanofibers during the phase transformation were investigated through ex situ and in situ transmission electron microscopy and X-ray diffraction. The results indicate that the calcination atmosphere plays an important role in the morphology of the nanocavities. For the TiO2 nanofibers calcined in air, the nanocavities show a hexagonal prism morphology, which is related to the phase transformation from TiO2 (B) to anatase. Meanwhile, under vacuum conditions, the nanocavities exhibit an irregular shape due to the occurrence of phase transition from TiO2 (B) to Ti3O5. The difference in the morphologies of these nanocavities leads to the different surface areas of the nanofibers and further affects the photocatalytic activity. This work also illustrates that the phase transformation from anatase to rutile is not only related to the activation temperature, but also to the calcination atmosphere. Ultra-high vacuum may retard the transformation.

Inhibitive formation of nanocavities by introduction of Si atoms in Ge nanocrystals produced by ion implantation

Germanium nanocrystals (Ge-nc) were successfully synthesized by co-implantation of Si and Ge ions into a SiO2 film thermally grown on (100) Si substrate and fused silica (pure SiO2), respectively, followed by subsequent annealing at 1150 °C for 1 h. Transmission electron microscopy (TEM) examinations show that nanocavities only exist in the fused silica sample but not in the SiO2 film on a Si substrate. From the analysis of the high-resolution TEM images and electron energy-loss spectroscopy spectra, it is revealed that the absence of nanocavities in the SiO2 film/Si substrate is attributed to the presence of Si atoms inside the formed Ge-nc. Because the energy of Si-Ge bonds (301 kJ·mol−1) are greater than that of Ge-Ge bonds (264 kJ·mol−1), the introduction of the Si-Ge bonds inside the Ge-nc can inhibit the diffusion of Ge from the Ge-nc during the annealing process. However, for the fused silica sample, no crystalline Si-Ge bonds are detected within the Ge-nc, where strong Ge outdiffusion effects produce a great number of nanocavities. Our results can shed light on the formation mechanism of nanocavities and provide a good way to avoid nanocavities during the process of ion implantation.

Rationally Designed Nitrogen-Rich Metal–Organic Cube Material: An Efficient CO2 Adsorbent and H2 Confiner

Metal–organic frameworks (MOFs) have been postulated for years as industrially achievable materials for CO2 capture and H2 storage. However, a great leap forward their real applications is still pending. This article details the design of a MOF material, including the reasoned choices of metal ion, organic linker, and even the structural subunits, as efficient adsorbent of both CO2 and H2. In particular, it was planned (i) to raise the polarizability of the framework by using a highly N-rich organic linker and (ii) to favor the formation of nanocavities with suitable size to confine small molecules. The resultant tetrazole-imidazole-based ITF-1 material containing nanocubes certainly achieved all these aimed premises. Nevertheless, such structural and compositional distinctiveness was reflected in some noteworthy adsorption features. CO2 adsorption was highly remarkable, and it was characterized by a wide hysteresis loop along the whole studied range of pressures. H2 molecules totally filled the nanocavities under only 0.1 bar at 77 K, and at 273 K, H2 uptake became tens of times higher than expected according to their textural properties. These unique adsorption results, together with the discussed relationship between structure/composition of ITF-1 and their adsorption features, underline the importance of a tailored MOF materials for particular applications.

On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3.

Hexagonal (W,V)O3-x oxides of high thermal stability have been synthesized hydrothermally through the intermediate products Nax(W,V)O3·zH2O and (NH4)0.33-x(W,V)O3-y. The obtained crystals show nanostructured surface via the formation of a dense population of polyhedral nanocavities self-distributed along particular crystallographic directions. Nanocavities present a regular size that ranges from 5 to 10 nm in both length and width. The synthesis process involves a significant topotactic relationship between the as-synthesized product and the desired final product and this relationship is suggested as the origin of the observed surface nanostructure. The comparison of our results with observations in different solids has allowed us to suggest that the formation of nanocavities is an extensive spontaneous process when materials are obtained by the chemical reactions of solids leading to products with defined crystallographic orientation with respect to the original compound. The characterization provides evidence regarding the potential relevance of nanocavities in the functional properties of the resulting solids.

Correction: On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3

Correction for ‘On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3’ by E. Garcia-Gonzalez et al., Dalton Trans., 2014, 43, 14644–14652.