Search Results (25)

The metal-based/ceramic interface structure is a key research focus in science, and addressing the stability of the interface has significant scientific importance as well as economic value. In this project, the work of adhesion, heat of segregation, electronic structure, charge density, and density of states for doped-M (M = Ti, Mg, Cu, Zn, Si, Mn, or Al) Ni (111)/Al₂O₃ (0001) interface structures are studied using first-principle calculation methods. The calculation results demonstrate that doping Ti and Mg can increase the bonding strength of the Ni–Al₂O₃ interface by factors of 3.4 and 1.5, respectively. However, other dopants, such as Si, Mn, and Al, have a negative effect on the bonding of the Ni–Al₂O₃ interface. As a result, the alloying elements may be beneficial to the bonding of the Ni–Al₂O₃ interface, but they may also play an opposite role. Moreover, the Ti and Mg dopants segregate from the matrix and move to the middle position of the Ni–Al₂O₃ interface during relaxation, while other dopants exhibit a slight segregation and solid solution in the matrix. Most remarkably, the segregation behavior of Ti and Mg induced electron transfer to the middle of the interface, thereby increasing the charge density of the Ni–Al₂O₃ interface. For the optimal doped-Ti Ni–Al₂O₃ interface, bonds of Ti–O and Ti–Ni are found, which indicates that the dopant Ti generates stable compounds in the interface region, acting as a stabilizer for the interface. Consequently, selecting Ti as an additive in the fabrication of metal-based ceramic Ni–Al₂O₃ composites will contribute to prolonging the service lifetime of the composite. Full article

Ionic liquids (ILs) have been explored as a way of improving the performance of ZnO-based optoelectronic devices; however, there are few fundamental studies of the IL/ZnO interface. Here, the adsorption of the IL 1-octyl-3-methylimidazolium tetrafluoroborate [C₈C₁Im][BF₄] on ZnO (0001) and ZnO ($10\overline{1}0$) has been studied using synchrotron-based soft X-ray photoelectron spectroscopy. The results indicate that [C₈C₁Im][BF₄] is deposited intact on the ZnO (0001) surface; however, there is some dissociation of [BF₄]⁻ anions, resulting in boron atoms attaching to the oxygen atoms in the ZnO surface and forming B₂O₃. In contrast, the deposition of [C₈C₁Im][BF₄] on the ZnO ($10\overline{1}0$) surface at −150 °C results in the appearance of more chemical environments in the spectra. We propose that the high temperature of the IL evaporator causes some conversion of [C₈C₁Im][BF₄] to a carbene–borane adduct, resulting in the deposition of both the IL and adduct onto the ZnO surface. The adsorption and desorption of the analogous IL 1-butyl-3-methylimidazolium tetrafluoroborate [C₄C₁Im][BF₄] was investigated on ZnO (0001) using synchrotron-based soft X-ray photoelectron spectroscopy. The results indicate that [C₄C₁Im][BF₄] is deposited largely intact at −150 °C and forms islands when heated to room temperature. When heated to over 80 °C, it begins to react with the ZnO surface and decomposes. This is a much lower temperature than the long-term thermal stability of the pure IL, quoted in the literature as ~400 °C, and of IL on powdered ZnO, quoted in the literature as ~300 °C. This indicates that the ZnO surface may catalyse the thermal decomposition of [C₄C₁Im][BF₄] at lower temperatures. This is likely to have a negative impact on the potential use of ILs in ZnO-based photovoltaic applications, where operating temperatures can routinely reach 80 °C. Full article

AZO-coated ZnO core–shell nanorods were successfully fabricated using the mist chemical vapor deposition method. The influence of coating time on the structural, optical, and photocatalytic properties of zinc oxide nanorods was investigated. It was observed that the surface area of AZO-coated ZnO core–shell nanorods increased with an increase in coating time. The growth orientation along the (0001) crystal plane of the AZO thin film coating was the same as that of zinc oxide nanorods. The crystallinity of AZO-coated ZnO core–shell nanorods was significantly improved as well. The optical transmittance of AZO-coated ZnO core–shell nanorods was greater than 55% in the visible region. The degradation efficiency for methyl red dye solution increased with an increase in coating time. The highest degradation efficiency was achieved by AZO-coated ZnO core–shell nanorods with a coating duration of 20 min, exhibiting a degradation rate of 0.0053 min⁻¹. The photodegradation mechanism of AZO-coated ZnO core–shell nanorods under ultraviolet irradiation was revealed. Full article

The purpose of this study is to synthesize and explore the relationship between the optical properties and gas-sensing performance of ZnO nanowires (NWs). Well-aligned ZnO nanowire (NW) arrays were synthesized on a silicon substrate using the thermal evaporation method without any catalyst or additive. The structures, surface morphologies, chemical compositions, and optical properties of the products were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) together with energy-dispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy, and their gas-sensing properties for NO₂ were examined. The results showed that single-crystalline ZnO NWs with high density grow uniformly and vertically on a Si substrate. The FESEM and TEM images indicate that ZnO NWs have an average diameter of roughly 135–160 nm with an average length of roughly 3.5 μm. The results from XRD confirm that the ZnO NWs have a hexagonal wurtzite structure with high crystalline quality and are highly oriented in the [0001] direction (i.e., along the c-axis). The deconvoluted O 1s peak at ~531.6 eV (29.4%) is assigned to the oxygen deficiency, indicating that the ZnO NWs contain very few oxygen vacancies. This observation is further confirmed by the PL analysis, which showed a sharp and high-intensity peak of ultraviolet (UV) emission with a suppressed deep-level (DL) emission (very high: I_UV/I_DL > 70), indicating the excellent crystalline quality and good optical properties of the grown NWs. In addition, the gas-sensing properties of the as-prepared ZnO NWs were investigated. The results indicated that under an operating temperature of 200 °C, the sensor based on ZnO NWs is able to detect the lowest concentration of 1.57 ppm of NO₂ gas. Full article

The first-principle calculation method based on the density functional theory (DFT) in combination with the LDA+U algorithm is employed to study the electronic structure and magnetic properties of Co/Mn co-doped ZnO nanowires. Special attention is paid to the optimal geometric replacement position, the coupling mechanism, and the magnetic origin of Co/Mn atoms. According to the simulation data, Co/Mn co-doped ZnO nanowires of all configurations exhibit ferromagnetism, and substitution of Co/Mn atoms for Zn in the (0001) inner layer brings nanowires to the ground state. In the magnetic coupling state, the obvious spin splitting is detected near the Fermi level, and strong hybridization effects are observed between the Co/Mn 3d and O 2p states. Moreover, the ferromagnetic ordering forming Co²⁺-O²⁻-Mn²⁺ magnetic path is established. In addition, the calculation results suggest that the magnetic moment mainly takes its origin from the Co/Mn 3d orbital electrons, and the size of the magnetic moment is related to the electronic configurations of Co/Mn atoms. Therefore, a realistic description of the electronic structure of Co/Mn co-doped ZnO nanowires, obtained via LDA+U method, shows their potential for diluted magnetic semiconductor materials. Full article

Mechanical characterization of quasi one-dimensional nanostructures is essential for the design of novel nanoelectromechanical systems. However, the results obtained on basic mechanical quantities, such as Young’s modulus and fracture strength, show significant standard deviation in the literature. This is partly because of diversity in the quality of the nanowire, and partly because of inappropriately performed mechanical tests and simplified mechanical models. Here we present orientation-controlled bending and fracture studies on wet chemically grown vertical ZnO nanowires, using lateral force microscopy. The lateral force signal of the atomic force microscope was calibrated by a diamagnetic levitation spring system. By acquiring the bending curves of 14 nanowires, and applying a two-segment mechanical model, an average bending modulus of 108 ± 17 GPa was obtained, which was 23% lower than the Young’s modulus of bulk ZnO in the [0001] direction. It was also found that the average fracture strain and stress inside the nanowire was above 3.1 ± 0.3 % and 3.3 ± 0.3 GPa, respectively. However, the fracture of the nanowires was governed by the quality of the nanowire/substrate interface. The demonstrated technique is a relatively simple and productive way for the accurate mechanical characterization of vertical nanowire arrays. Full article

This study is devoted to the luminescence and stimulated emission properties of the ZnO hybrid structure, which is vertically aligned microcrystals with the [0001] crystallographic orientation and a pronounced hexagonal shape formed on a continuous layer of micron thickness. These microcrystals are up to 10 µm high and up to 8 µm in diameter and form the main part of the structure’s thickness. The structure was synthesized on the M($10\overline{1}0$) plane of sapphire using the magnetron sputtering method. Luminescence of the structure, represented only by conventional near-UV and green components under low-intensity continuous photoexcitation, confirms its high structural and optical quality. Under pulsed photoexcitation with relatively high intensity, stimulated emission (SE) was observed from the structure in the near-UV region at room temperature. The threshold power density for SE was 0.1–0.2 MW/cm². Exceeding the threshold leads to a significant increase in the emission intensity compared to the control film without [0001] microcrystals, also grown on M($10\overline{1}0$) sapphire. It was assumed that the optical gain is provided by the whispering gallery modes of individual [0001] microcrystals as a result of inelastic exciton–electron scattering, at least at near-threshold excitation intensities. Full article

ZnO nano- and microstructures doped with K were grown by the Vapor–Solid method. Wires and needles are the main morphology observed, although some structures in the form of ribbons and triangular plates were also obtained. Besides these, ball-shaped structures which grow around a central wire were also detected. Raman and cathodoluminescence investigations suggest that variations in morphology, crystalline quality and luminescence emissions are related to the different lattice positions that K occupies depending on its concentration in the structures. When the amount is low, K ions mainly incorporate as interstitials (K_i), whereas K occupies substitutional positions of Zn (K_Zn) when the amount of K is increased. Electron Backscattered Diffraction shows that ribbons and triangular plates are oriented in the (0001) direction, which indicates that the growth of this type of morphologies is related to distortions introduced by the K_i since this position favors the growth in the (0001) plane. In the case of the ball-shaped structures, the compositional analysis and Raman spectra show that they consist of K₂SO₄. Finally, the capability of the elongated structures to act as waveguides and optical resonators was investigated. Due to the size of the K ion, practically double that of the Zn, and the different positions it can adopt within the ZnO lattice (K_i or K_Zn), high distortions are introduced that compromise the resonators performance. Despite this, quality factor (Q) and fineness (F) show acceptable values (80 and 10 at 544 nm, respectively), although smaller than those reported for doping with smaller size alkali, such as Li. Full article

Engineered nanoparticle–support interaction is an effective strategy for tuning the structures and performance of engineered nanoparticles. Here, we show that tuning the dehydroxylation of kaolinite nanoclay as the support could induce zinc oxide–kaolinite interactions. We used free energy theory, electron microscopy, and X-ray photoemission spectroscopy to identify interaction strengths between metal oxides and the underlying nanoclay induced by dehydroxylation. Desirable exposure of nanoparticle sites and the geometrical and crystal structure were obtained by tuning the interface interactions between ZnO nanoparticles and nanoclay. The surface free energy of zinc oxide–nanoclay results in different interfacial interactions, and the properties of the surface free energy electron-donating (γ⁻) and electron-accepting (γ⁺) parameters have significant effects on the electron acceptor. This could, in turn, promote stronger interactions between zinc oxide and the kaolinite surface, which produce more active (0001) Zn-polar surfaces with promoting zinc oxide nanoparticles growing along the <0001> direction. Reactive oxygen species, leached zinc ions, and electron transfer can modulate the antibacterial activities of the samples as a function of interface free energy. This further demonstrates the interfacial interactions induced by dehydroxylation. This work has new application potential in biomedicine and materials science. Full article

Temperature-, excitation wavelength-, and excitation power-dependent photoluminescence (PL) spectroscopy have been utilized to investigate the orientation-modulated near band edge emission (NBE) and deep level emission (DLE) of ZnO single crystals (SCs). The near-band-edge emission of ZnO SC with <0001> orientation exhibits strong and sharp emission intensity with suppressed deep level defects (mostly caused by oxygen vacancies V_o). Furthermore, Raman analysis reveals that <0001> orientation has dominant E₂ (high) and E₂ (low) modes, indicating that this direction has better crystallinity. At low temperature, the neutral donor-to-bound exciton (D^oX) transition dominates, regardless of the orientation, according to the temperature-dependent PL spectra. Moreover, free-exciton (FX) transition emerges at higher temperatures in all orientations. The PL intensity dependence on the excitation power has been described in terms of power-law (I~L^α). Our results demonstrate that the α for <0001>, <1120>, and <1010> is (1.148), (1.180), and (1.184) respectively. In short, the comprehensive PL analysis suggests that D^oX transitions are dominant in the NBE region, whereas oxygen vacancies (V_o) are the dominant deep levels in ZnO. In addition, the <0001> orientation contains fewer V_o-related defects with intense excitonic emission in the near band edge region than other counterparts, even at high temperature (~543 K). These results indicate that <0001> growth direction is favorable for fabricating ZnO-based highly efficient optoelectronic devices. Full article

A Mn-ZnO nanowire microsphere was prepared by using the hydrothermal method. The effects of Mn doping concentration and hydrothermal growth conditions on the crystal structures, morphologies, magnetic and optical properties of ZnO nanowire microsphere were studied. The characterization results showed Mn-ZnO nanowire microsphere with uniform and dense distributions along the [0001] direction with a hexagonal wurtzite structure. No impurity phases were detected in microsphere specimens. The room-temperature ferromagnetism of the Mn-ZnO nanowire microsphere was detected, with the saturation magnetization of 2.4 × 10⁻¹ emu/g and a coercive field of 369 Oe. Furthermore, with the increase of Mn²⁺ ions doping concentration, the luminescence intensity of the sample decreases in both UV and visible regions, and slight blueshift in the visible light regions was observed. The theoretical results presented obvious spin polarization near the Fermi level, with strong Mn 3d and O 2p hybridization effects. The magnetic moments were mainly generated by Mn 3d and partial contribution of O 2p orbital electrons. Therefore, the Mn-ZnO nanowire microsphere can be used as a potential magneto-optical material. Full article

First-principles calculations based on density functional theory (DFT) were carried out to study the energetic stability and electronic properties of a bimetallic-doped α-Fe₂O₃ photoanode surface with (Zn, Ti) and (Zn, Zr) pairs for enhanced PEC water splitting. The doped systems showed negative formation energies under both O-rich and Fe-rich conditions which make them thermodynamically stable and possible to be synthesised. It is found that in a bimetallic (Zn, Ti)-doped system, at a doping concentration of 4.20% of Ti, the bandgap decreases from 2.1 eV to 1.80 eV without the formation of impurity states in the bandgap. This is favourable for increased photon absorption and efficient movement of charges from the valance band maximum (VBM) to the conduction band minimum (CBM). In addition, the CBM becomes wavy and delocalised, suggesting a decrease in the charge carrier mass, enabling electron–holes to successfully diffuse to the surface, where they are needed for water oxidation. Interestingly, with single doping of Zr at the third layer (L3) of Fe atoms of the {0001} α-Fe₂O₃ surface, impurity levels do not appear in the bandgap, at both concentrations of 2.10% and 4.20%. Furthermore, at 2.10% doping concentration of α-Fe₂O₃ with Zr, CBM becomes delocalised, suggesting improved carrier mobility, while the bandgap is altered from 2.1 eV to 1.73 eV, allowing more light absorption in the visible region. Moreover, the photocatalytic activities of Zr-doped hematite could be improved further by codoping it with Zn because Zr is capable of increasing the conductivity of hematite by the substitution of Fe³⁺ with Zr⁴⁺, while Zn can foster the surface reaction and reduce quick recombination of the electron–hole pairs. Full article

Zinc oxide (ZnO) possesses superior chemical and physical properties so that it can occupy an essential position in the application of nanostructures. In this paper, ZnO nano-rod arrays were synthesized by a simple one-step hydrothermal approach with the assistance of cetyl trimethyl ammonium bromide (CTAB). Exposure of the {0001} facets could be controlled by adjusting the amount of CTAB and the maximum exposure of the {0001} facets of ZnO nanorods is obtained at 1.2 g of CTAB. The photocurrent, EIS, and PL measurements support the facile charge transfer with minimum recombination of the photogenerated excitons of the ZnO nano-rod arrays obtained at 1.2 g of CTAB. Consequently, the obtained ZnO nano-rod arrays at the optimal CTAB of 1.2 g exhibit an excellent photocatalytic degradation rate of 99.7% for rhodamine B (RhB), while the degradation rate of RhB by the ZnO obtained without CTAB is only 35%. Full article

In the present research, a Mg–4Zn–1.2Y–0.8Nd (wt.%) alloy was heat treated and hot extruded with different passes. XRD, SEM, TEM and tensile testing were employed to characterize the microstructure evolution and mechanical properties. The results exhibited that the semi-continuously distributed W-Mg₃Zn₃Y₂ phases formed the skeleton structure which separated the α-Mg matrix into a dual-size grain structure. In addition, the Mg₂₄Y₅, Mg₄₁Nd₅ and Y₂O₃ phase was also observed in the heat-treated alloy. Moreover, it was found that the Mg24Y5 phase had an orientation relationship with the α-Mg matrix of α${[111]}_{\mathrm{Mg}24\mathrm{Y}5}//{[0001]}_{\mathsf{\alpha }-\mathrm{Mg}}$ and α${(10\overline{1})}_{\mathrm{Mg}24\mathrm{Y}5}//{(10\overline{1}0)}_{\mathsf{\alpha }-\mathrm{Mg}}$, and the Mg₄₁Nd₅ phase had an orientation relationship with the α-Mg matrix of α${[001]}_{\mathrm{Mg}41\mathrm{Nd}5}//{[0001]}_{\mathsf{\alpha }-\mathrm{Mg}}$. The one-pass hot extrusion segmented the secondary phases into small ones and refined the α-Mg matrix. Due to the partly recrystallization and crystal orientation difference, the coarse elongated grain surrounded by fine recrystallized grain and secondary phase was the main feature of the one-pass hot extruded alloy. Furthermore, the secondary phases exhibited the linear distribution along the direction of hot extrusion. The two-pass hot extrusion refined the secondary phase and matrix further, which produced the ultrafine α-Mg matrix with uniform grain size and a well redistributed secondary phase. Due to the microstructure optimization by the multi-pass hot extrusion, the ductility and strength of the Mg–Zn–Y–Nd alloy were well improved, especially the two-pass hot extruded alloy which was significant improved in ductility and strength simultaneously. Full article

The growth of high-quality ZnO layers with optical properties congruent to those of bulk ZnO is still a great challenge. Here, for the first time, we systematically study the morphology and optical properties of ZnO layers grown on SiC substrates with off-cut angles ranging from 0° to 8° by using the atmospheric pressure meta–organic chemical vapor deposition (APMOCVD) technique. Morphology analysis revealed that the formation of the ZnO films on vicinal surfaces with small off-axis angles (1.4°–3.5°) follows the mixed growth mode: from one side, ZnO nucleation still occurs on wide (0001) terraces, but from another side, step-flow growth becomes more apparent with the off-cut angle increasing. We show for the first time that the off-cut angle of 8° provides conditions for step-flow growth of ZnO, resulting in highly improved growth morphology, respectively structural quality. Temperature-dependent photoluminescence (PL) measurements showed a strong dependence of the excitonic emission on the off-cut angle. The dependences of peak parameters for bound exciton and free exciton emissions on temperature were analyzed. The present results provide a correlation between the structural and optical properties of ZnO on vicinal surfaces and can be utilized for controllable ZnO heteroepitaxy on SiC toward device-quality ZnO epitaxial layers with potential applications in nano-optoelectronics. Full article