Scanning transmission electron microscopy and atom probe tomography analysis of non-stoichiometry long-period-stacking-ordered structures in Mg-Ni-Y/Sm alloys

The long-period-stacking-ordered (LPSO) structure affects the mechanical, corrosion and hydrolysis properties of Mg alloys. The current work employs high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and atom probe tomography (APT) to investigate the structural and local chemical information of LPSO phases formed in Mg-Ni-Y/Sm ternary alloys after extended isothermal annealing. Depending on the alloying elements and their concentrations, Mg-Ni-Y/Sm develops a two-phase LPSO + α-Mg structure in which the LPSO phase contains defects, hybrid LPSO structure, and Mg insertions. HAADF-STEM and APT indicate non-stoichiometric LPSO with incomplete Ni6(Y/Sm)8 clusters. In addition, the APT quantitatively determines the local composition of LPSO and confirms the presence of Ni within the Mg bonding layers. These results provide insight into a better understanding of the structure and hydrolysis properties of LPSO-Mg alloys

Enabling Solar Water Oxidation by BiVO₄ Photoanodes in Basic Media

Titanium dioxide (TiO₂) deposited by atomic layer deposition (ALD) has been the most commonly used protection layer to enhance chemical and photoelectrochemical stabilities of photoelectrodes. In this study, we report a new electrochemical deposition method that can place a thin, conformal TiO₂ coating layer on a photoelectrode. This method takes <1 min and may serve as a practical alternative to ALD for the deposition of a TiO₂ layer. The uniform quality of the TiO₂ protection layer was confirmed by demonstrating the chemical stability of the BiVO₄/TiO₂ electrode in strongly basic media (pH 12 and 13) where BiVO₄ readily dissolves. More importantly, the high-quality TiO₂ protection layer made it possible to comparatively investigate photoelectrochemical properties and stabilities of the BiVO₄ and BiVO₄/TiO₂ electrodes, which was critical to elucidate the effect that the chemical instability of BiVO₄ in basic media has on the rate of photocorrosion. Systematic photoelectrochemical studies for sulfite oxidation and water oxidation provided a coherent understanding of how the interplay among the relative rates of interfacial charge transfer, surface recombination, and photocorrosion affects the photocurrent generation and photostability of BiVO₄. On the basis of this understanding, stable photocurrent generation for water oxidation could be achieved at pH 12 over 20 h using a BiVO₄/TiO₂/FeOOH/NiOOH electrode where FeOOH/NiOOH served as oxygen evolution catalyst. The results and discussion contained in this study provide new insights into the understanding of photocurrent decay caused by photocorrosion involving dissolution, enabling the development of effective strategies to achieve stable photocurrent generation

Evolution of Hollow TiO₂ Nanostructures via the Kirkendall Effect Driven by Cation Exchange with Enhanced Photoelectrochemical Performance

Hollow nanostructures are promising building blocks for electrode scaffolds and catalyst carriers in energy-related systems. In this paper, we report a discovery of hollow TiO₂ nanostructure evolution in a vapor–solid deposition system. By introducing TiCl₄ vapor pulses to ZnO nanowire templates, we obtained TiO₂ tubular nanostructures with well-preserved dimensions and morphology. This process involved the cation exchange reaction between TiCl₄ vapor and ZnO solid and the diffusion of reactants and products in their vapor or solid phases, which was likely a manifestation of the Kirkendall effect. The characteristic morphologies and the evolution phenomena of the hollow nanostructures from this vapor–solid system were in a good agreement with the Kirkendall effect discovered in solution systems. Complex hollow TiO₂ nanostructures were successfully acquired by replicating various ZnO nanomorphologies, suggesting that this unique cation exchange process could also be a versatile tool for nanostructure replication in vapor–solid growth systems. The evolution of TiO₂ nanotubes from ZnO NW scaffolds was seamlessly integrated with TiO₂ NR branch growth and thus realized a pure TiO₂-phased 3D NW architecture. Because of the significantly enlarged surface area and the trace amount of Zn left in the TiO₂ crystals, such 3D TiO₂ nanoforests demonstrated enhanced photoelectrochemical performance particularly under AM (air mass) 1.5G illumination, offering a new route for hierarchical functional nanomaterial assembly and application

Structure, morphology and chemical composition of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

WS2 and Cr–WS2 nanocomposite coatings were deposited at different Cr contents (approximately 15–50 at. %) on silicon and mild steel substrates using an unbalanced magnetron sputtering system. X-ray diffraction (XRD) was used to study the structure of Cr–WS2 coatings and the bonding structure of the coatings was studied using X-ray photoelectron spectroscopy (XPS). The characterization of different phases present in Cr–WS2 coatings was carried out using micro-Raman spectroscopy. The XPS and Raman data indicated the formation of a thin layer of WO3 on the surface of Cr–WS2 coatings and the intensity of the oxide phase decreased with an increase in the Cr content, which was also confirmed using energy-dispersive X-ray analysis results. The surface morphologies of WS2 and Cr–WS2 coatings were examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy. It has been demonstrated that incorporation of Cr in WS2 strongly influences the structure and morphology of Cr–WS2 coatings. The XRD and FESEM results suggested that increase in the Cr content of Cr–WS2 coatings resulted in a structural transition from a mixture of nanocrystalline and amorphous phases to a complete amorphous phase. The cross-sectional FESEM data of WS2 coating showed a porous and columnar microstructure. For the Cr–WS2 coatings, a mixture of columnar and featureless microstructure was observed at low Cr ontents (≤23 at.%),whereas, a dense and featureless microstructure was observed at high Cr contents. Detailed cross-sectional transmission electron microscopy (TEM) studies of Cr–WS2 coatings prepared at Cr content ≤23 at.% indicated the presence of both nanocrystalline (near the interface) and amorphous phases (near the surface). Furthermore, high-resolution TEM data obtained from the nanocrystalline region showed inclusion of traces of amorphous phase in the nanocrystalline WS2 phase. Potentiodynamic polarization measurements indicated that the corrosion resistance of Cr–WS2 coatings was superior to that of the uncoated mild steel substrate and the corrosion rate decreased with an increase in the Cr content

Nitrogen Doped 3D Titanium Dioxide Nanorods Architecture with Significantly Enhanced Visible Light Photoactivity

Surface-reaction-limited pulsed chemical vapor deposition (SPCVD) is able to create 3D TiO₂-based hierarchical nanowire (NW) architecture with superhigh surface area and good electronic transport properties for high-performance photoelectrode development. However, how to intentionally dope the nanorods (NRs) through the SPCVD process to improve their electronic properties and light absorption behavior is still unexplored. In this paper, a comprehensive study of doping TiO₂ NRs with nitrogen through the SPCVD technique is reported for the first time. The high-density nitrogen doped TiO₂ NR branches with controlled doping concentrations were synthesized on dense Si NW forest by introducing designed number of TiN cycles to TiO₂ growth cycles. Microscopic studies revealed the influence of nitrogen doping on the crystal growth behavior and NR morphology, as well as the elements distribution inside the lattices. Nitrogen doping lowered the band gap of TiO₂ NRs and effectively activated visible light photoactivity. It also largely improved the incident-photon-to-current-conversion efficiency in the UV range. Successful synthesis of N doped TiO₂ NRs by the SPCVD method introduces a strong new capability to this novel and powerful 3D NR growth technique. It enables composition and optoelectronic property control of the novel 3D NR structures, allowing performance enhancement or creating new functionality