Subsequent Annealing In Oxygen Research Articles

Photocatalytic active ZnO1−x S x @CNTs heteronanostructures

Herein, we report on the use of vertically aligned multiwall carbon nanotubes (CNTs) films as support for ZnO/ZnS photocatalytic active nanostructures. The CNTs were synthetized via a hot-filament chemical vapor deposition (HfCVD), using Fe catalyst on top of Al2O3 buffer layer. Controlled point defects in the CNTs outer walls were created by exposure to a low pressure nonthermal water vapors diffusive plasma and acted as seeds for subsequent pulsed-electrodeposition of Zn nanoparticles. This was to achieve a direct and improved contact between the nanoparticles and CNTs. To obtain ZnO, ZnS and mix phase of ZnO/ZnS spread on CNTs, the oxidation, sulfurization and 2 steps subsequent annealing in oxygen and sulfur rich atmospheres were applied. High resolution transmission electron microscopy with energy dispersive x-rays spectroscopy in scanning mode, provided the chemical mapping of the structures. X-ray diffraction (XRD) analyses proved the hexagonal phase of ZnO and ZnS, obtained after oxidation in H2O and S vapors, respectively. In the case of the samples obtained by the 2 steps subsequent annealing, XRD showed mainly the presence of ZnO and a small amount of ZnS. The benefit of the secondary annealing in S vapor was seen as an absorption enhancement of the ZnO1−x S x @CNTs sample having the absorption edge at 417 nm, whereas the absorption edge of ZnO@CNTs was 408 nm and of ZnS@CNTs 360 nm. For all the samples, compared to the bare ZnO and ZnS, the absorption red shift was observed which is attributed to the CNTs involvement. Therefore, this study showed the double sides benefit to induce the absorption of ZnO of the visible light, one from S doping and second of CNTs involvement. The absorption enhancement had a positive impact on photocatalytic degradation of methyl blue dye, showing that ZnO1−x Sx@CNTs heteronanostructure was the best photocatalyst among the studied samples.

Structural and optoelectronic characteristics of β-Ga2O3 epitaxial films with Zn alloying and subsequent oxygen annealing

In this article, we have reported the fabrication of ∼7.5 at% Zn alloyed β-Ga2O3 epitaxial films and their high performance solar-blind photodetectors.

A Ternary Molten Salt Approach for Direct Regeneration of LiNi0.5 Co0.2 Mn0.3 O2 Cathode.

Recycling spent lithium-ion batteries (LIBs) is an urgent task in view of the resource shortage and environmental concerns. Here, a facile ternary molten salt approach is presented for efficiently regenerating the LiNi0.5 Co0.2 Mn0.3 O2 (NCM523) cathode of spent LIBs. Such an approach involves the treatment of spent cathode powder in the ternary molten salt at a moderate temperature (400°C) and subsequent annealing in oxygen. The Li loss and degraded phases in spent NCM that cause the capacity decay can be fully remedied after the regeneration process. As a result, the regenerated cathode delivers a reversible capacity of 160 mAh g-1 at 0.5 C with retention of 93.7% after 100 cycles and maintains a high capacity of 132 mAh g-1 at a high rate of 5 C. The electrochemical performance of regenerated NCM cathode is compared favorably to the fresh NCM cathode, which demonstrates the feasibility of the molten salt approach to directly regenerate spent NCM cathode.

Porous titania photoelectrodes built on a Ti-web of microfibers for polymeric electrolyte membrane photoelectrochemical (PEM-PEC) cell applications

Conventional photoelectrochemical (PEC) cells are based on planar photoelectrodes supported on glass substrates and liquid electrolytes. Only few recent studies have examined an alternative PEC design which is robust and scalable, where the key elements are polymeric electrolyte membranes and porous photoelectrodes. This work aims to give further insights on the operation of such cells utilizing titania photoelectrodes and proton and hydroxide ion conducting membranes. Two families of photoelectrodes were developed on Ti porous substrate; TiO2 nanotubes grown by anodization and subsequent oxygen annealing, and TiO2 layers developed under oxygen annealing. Initial screening of the photoanodes for water splitting and (poly)alcohol photo-oxidation took place in conventional PEC cells. We found that the annealing temperature affects the performance of the photoanodes, evidenced by a monotonic increase in the activity for water photo-oxidation with increasing annealing temperature. Moreover it was demonstrated that anatase phase is predominantly active for the (poly)alcohol electro-oxidation, while there is a synergy between rutile and anatase which is beneficial for water splitting. In addition, the most promising photoanodes for water splitting were evaluated in our polymeric electrolyte membrane photoelectrochemical(PEM-PEC) cell during gas phase operation. It was found that PEM-PEC operation is more efficient when OH- conducting membranes are used, while the nature of the carrier gas does not significantly influence the activity. Overall, PEM-PEC operation is more promising than conventional PEC in both acidic and alkaline media, since comparable (or even at some cases higher) photocurrents were obtained while liquid pumping systems are not required for PEM-PEC devices.

Atomic structure of a metal-supported two-dimensional germania film

Author(s): Lewandowski, AL; Schlexer, P; Buchner, C; Davis, EM; Burrall, H; Burson, KM; Schneider, WD; Heyde, M; Pacchioni, G; Freund, HJ | Abstract: The growth and microscopic characterization of two-dimensional germania films is presented. Germanium oxide monolayer films were grown on Ru(0001) by physical vapor deposition and subsequent annealing in oxygen. We obtain a comprehensive image of the germania film structure by combining intensity-voltage low-energy electron diffraction (I/V-LEED) and ab initio density functional theory (DFT) analysis with atomic-resolution scanning tunneling microscopy (STM) imaging. For benchmarking purposes, the bare Ru(0001) substrate and the (2×2)3O covered Ru(0001) were analyzed with I/V-LEED with respect to previous reports. STM topographic images of the germania film reveal a hexagonal network where the oxygen and germanium atom positions appear in different imaging contrasts. For quantitative LEED, the best agreement has been achieved with DFT structures where the germanium atoms are located preferentially on the top and fcc hollow sites of the Ru(0001) substrate. Moreover, in these atomically flat germania films, local site geometries, i.e., tetrahedral building blocks, ring structures, and domain boundaries, have been identified, indicating possible pathways towards two-dimensional amorphous networks.

Preparation and characterization of RF magnetron sputtered CuO/CaTi4O9 thin films with enhanced third-order nonlinear optical properties

The titanate, is a material of interest for various energy applications, including photovoltaics, catalysts, and high-rate energy storage devices. Herein, its related materials, CuO/CaTi4O9 [CCTO] thin films, were successfully fabricated on SrTiO3 (100) substrates by RF magnetron sputtering assisted with subsequent oxygen annealing. This obtained CCTO thin films were then systemically studied by X-ray powder diffraction (XRD), atomic force microscopy (AFM), scan electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). It was found that CuO and CaTi4O9 (001) particles were closely accumulated together on the surface of the substrate in the annealing process after comparing with that of the as-prepared thin film, which was verified by SEM and AFM results. Furthermore, we investigated the third-order nonlinear optical (NLO) properties of the as-prepared and annealed CCTO thin film by means of the Z-scan technique using 650nm femtosecond laser pulse. Post-deposition oxygen annealing was found to modify the morphological characteristics of the films, resulting in enhancing their NLO properties. The observation of NLO performance of annealed CCTO thin film indicates that RF magnetron sputtering is a feasible method for the fabrication of optical thin films, which can be expanded to fabricate other NLO materials from the corresponding dispersions. Naturally, we concluded that the CCTO thin film occupy a better NLO property, and thus enlarge its application in nonlinear optics.

Characterization of Cost Effective PbTiO<sub>3</sub> Nanofibres by Electrospinning Technique

This research aims to synthesize and characterize the PbTiO3 nanofibres by electrospinning technique. PbTiO3 nanofibre was fabricated onto Al-substrate by home-made electrospinning setup. The spinning or running time was set to 20 min. After spinning, subsequent annealing in oxygen and atmospheric ambient was performed at 600°C for 1 h and followed to form the PbTiO3 nanofibres. Surface morphology and fibres quality of PbTiO3 nanofibres were examined by Field Emission Scanning Electron Microscopy (FESEM). From FESEM image, the fibre diameter was measured to be (150-180 nm) with different collector distances. The experimental data resulted from this research showed that the home-made device compiled and growth chemistry was technically simple and easily adaptable.

Cation stoichiometry and electrical transport properties of the NdGaO3/(0 0 1)SrTiO3 interface

The interface formed between two wide band-gap insulators, NdGaO3 and SrTiO3 renders metallic behavior, similar to the LaAlO3/SrTiO3 interface. The interface conductivity depends strongly upon oxygen pressure during growth of the NdGaO3 film and subsequent annealing in oxygen. The conductivity of a (10 uc) NdGaO3/SrTiO3 film, pulsed laser deposited at low (pO2 = 10−4 mbar) oxygen pressure, vanishes after annealing at 600 °C in oxygen atmosphere. For a similar interface formed at high oxygen pressure (pO2 = 0.3 mbar), the metallic conductivity remains also after post annealing. Medium energy ion spectroscopy (MEIS) in random (non-channeling) direction showed that a substantial part of Ga is missing in films deposited at low pressure, while optimal stoichiometry is approached in films deposited at high pressure. Aligned (channeling) MEIS likewise show that the Ga/Nd ratio approaches the stoichiometric value as the pressure is increased from 10−4 to 0.3 mbar. This is interpreted as due to gallium desorption from a growing film at high temperature and low oxygen pressure while the re-evaporation of gallium is considerably suppressed at higher pressure. We discuss the possible role of stoichiometry on electrical transport properties.

Growth and surface properties of epitaxial SnO2

The surface potentials of SnO2 films grown epitaxially by magnetron sputtering on TiO2 and Al2O3 substrates with (110), (001), (101), and (100) SnO2 surface orientations are determined using in situ photoelectron spectroscopy. Epitaxial growth is verified using X-ray diffraction and low energy electron diffraction. The emphasis lies on the determination of work functions and ionization potentials of epitaxial SnO2 surfaces. SnO2 films prepared under chemically reducing conditions exhibit work functions φ of 4.25–4.48 eV and ionization potentials IP of 7.54–8.11 eV. It is furthermore demonstrated that a subsequent annealing in oxygen alters the surface dipole, visible through a large increase of ionization potential. This is due to a change of the surface termination from a reduced to a stoichiometric surface which exhibits Sn in the +IV oxidation state. The observed increase of IP varies from 0.48 eV for the (110) SnO2 surface to 1.05 eV for the (101) SnO2 surface.

Room-temperature ferromagnetism in nanocrystalline Cu/Cu2O core-shell structures prepared by magnetron sputtering

Cu/Cu2O core-shell nanoparticles with diameters around 8–9 nm have been fabricated by magnetron sputtering pure Cu targets with subsequent annealing in oxygen. Room-temperature ferromagnetism (FM) was observed in the samples annealed at 150 °C for 10–120 min. The maximum of saturated magnetization is as high as 19.8 emu/cc. The photoluminescence spectra show solid evidence that the FM originates from Cu vacancies in the Cu2O shell of the Cu/Cu2O core-shell nanoparticles. Furthermore, the FM can be modulated by the amount of Cu vacancies through the Cu/Cu2O core-shell interface engineering. Fundamentally, the FM can be understood by the charge-transfer ferromagnetism model based on Stoner theory.

Effects of Melting Diffusion and Annealing in Oxygen on Superconducting Characteristics of GdBCO Coated Conductors: Preliminary Results

This study examined the superconducting properties of GdBCO coated conductor tapes before and after melting diffusion and subsequent annealing in oxygen. Such investigation is necessary in order to determine the optimal parameters of these processes for the future development of a superconducting joint between coated conductors. The melting diffusion process under a reduced oxygen partial pressure (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O2</sub> ) at 850 °C for 60 s enabled the maintenance of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis orientation of Gd123 with few secondary phases such as CuO and BaCuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , resulting from partially melted GdBCO. To restore the oxygen lost during the melting diffusion process, an additional oxygenation annealing process was conducted. The critical current of the GdBCO CC tape oxygenation annealed at 600 °C for 50 h and then at 500 °C for 200 h sequentially was 108 A, indicating that the superconducting properties were considerably recovered. Based on the optimal joining parameters obtained in this study, future studies on the superconducting joint of the GdBCO CC will be carried out.

Strain relaxation in single crystal SrTiO3 grown on Si (001) by molecular beam epitaxy

An epitaxial layer of SrTiO3 grown directly on Si may be used as a pseudo-substrate for the integration of perovskite oxides onto silicon. When SrTiO3 is initially grown on Si (001), it is nominally compressively strained. However, by subsequent annealing in oxygen at elevated temperature, an SiOx interlayer can be formed which alters the strain state of SrTiO3. We report a study of strain relaxation in SrTiO3 films grown on Si by molecular beam epitaxy as a function of annealing time and oxygen partial pressure. Using a combination of x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy, we describe the process of interfacial oxidation and strain relaxation of SrTiO3 on Si (001). Understanding the process of strain relaxation of SrTiO3 on silicon will be useful for controlling the SrTiO3 lattice constant for lattice matching with functional oxide overlayers.

Investigation of epitaxial Nd1.85Ce0.15CuO4 − y film surface by low energy electron diffractometry

The surface of epitaxial Nd1.85Ce0.15CuO4 − y (001) (NCCO) film has been studied by low energy electron diffractometry (LEED) and photoelectron spectroscopy. Ar+ ion etching of a surface with subsequent annealing in oxygen at atmosphere pressure has been found to lead to the ordered structure restoration of surface layers with the symmetry type and lattice parameters corresponding to the NCCO phase. Annealing in vacuum at temperatures close to the boundary of thermodynamic phase stability results in the formation of epitaxial Ce0.5Nd0.5O1.75 phase on a surface that is indicated in the LEED pattern as additional spots corresponding to the surface lattice (√2 × √2)R45°.

Effects of Defect Control on the Polarization Properties in Bi2WO6-Based Single Crystals

Leakage-current and polarization-switching properties of defect-controlled Bi 2 WO 6 single crystals have been investigated along the crystallographic a axis at 25°C. High-pressure oxygen annealing decreased leakage current, indicating that electron is a detrimental carrier of the leakage-current property. 2-mol% Mn substitution at the W site is shown to be effective for suppressing leakage current density to the order of 108 A/cm2. Mn-doped crystals annealed under high-pressure oxygen atmosphere showed a well saturated polarization hysteresis loop with a remanent polarization of 50 μC/cm2 and a coercive field of 39 kV/cm. It is found that high-pressure oxygen annealing is effective for decreasing coercive field of Mn-doped crystals due to less oxygen vacancies. It is concluded that Mn substitution and the subsequent oxygen annealing enhance polarization switching as well as reduce leakage current in the BW system.

Structural and photoluminescence properties of ZnO nanoparticles on silicon oxide

Isolated, self assembled ZnO nanoparticles are grown in two steps: by the electron beam evaporation of Zn on oxidised silicon wafers, during which isolated Zn nanodots are grown, and a subsequent annealing in oxygen that results in the desired ZnO nanodots. Low temperature PL measurements of the ZnO nanodots show that the near band edge part of the spectra is dominated by a zero phonon line near 3.36 eV which is an overlap of two emitting lines near 3.363 eV and 3.367 eV. Characterization by TEM and EELS shows that the nanoparticles are zinc oxide single crystals grown with their c-axis perpendicular to the substrate; their distribution, size and crystallinity depend on the deposition parameters of zinc and the growth substrate. We discuss the effect of these parameters on the morphology of the resulting material. Our approach demonstrates a simple method for the growth of high purity isolated ZnO nanodots of similar sizes, distributed uniformly on a large surface.

Fabrication, characterization and studies of annealing effects on ferromagnetism inZn1−xCoxO nanowires

Diluted magnetic semiconductorZn1−xCoxO (x≤0.11) nanowires withaverage diameter of ∼40 nm were prepared by thermal evaporation, followed by high-energy Co ionimplantation. Bombardment by Co ions produced a good number of structuraldefects (stacking faults and orientational variations) in the nanowires. Theas-implanted nanowires were paramagnetic. We performed two types ofthermal annealing, one in 1 atm argon flow and the other in a high vacuum, at600 °C, and studied the effects of annealing on the magnetic properties of these nanowires. Argonannealing removed structural defects in the nanowires and the nanowires then revealedferromagnetic ordering. This result suggests that structure defects are harmful to theoccurrence of ferromagnetism in the Co-implanted ZnO. The structure of the as-implantedand the annealed nanowires was inspected in detail by using scanning electron microscopy,energy dispersive x-ray spectroscopy, maps of electron energy loss spectra, x-ray diffraction,and high-resolution transmission electron microscopy. Taken together, these studiessuggested that no second phase existed on the scale down to the spatial resolution of∼0.5 nm. Noticeably, the nanowires even displayed largely enhanced ferromagnetism afterannealing in a high vacuum. A subsequent annealing in oxygen has also beenperformed on those vacuum-annealed nanowires to study the roles played by the Ovacancies in determining the ferromagnetic properties of the nanowires. Our resultsindicate that both the improved structural quality and the increased number ofO vacancies are key factors for the occurrence of ferromagnetic ordering in theZn1−xCoxO nanowires.

Zinc oxide nanoparticles on silicon

Self-assembled ZnO nanoparticles are grown by electron beam evaporation of Zn on silicon oxide and silicon substrates and subsequent annealing in oxygen. Characterization by TEM and EELS shows that the nanoparticles can be zinc oxide single crystals grown with their c -axis perpendicular to the substrate; their distribution, size and crystallinity depend on the deposition parameters of zinc and the growth substrate. We discuss the effect of these parameters on the morphology of the resulting material.

Self-assembled zinc oxide nanodots on silicon oxide

Self assembled ZnO nanodots are grown by electron beam evaporation of Zn on thermally oxidized silicon substrates and subsequent annealing in oxygen. Characterization by TEM and EELS shows that the quantum dots are indeed zinc oxide single crystals grown with their c-axis perpendicular to the substrate; their distribution and size depends on the deposition parameters of zinc onto the substrates.

Enhanced residual secondary phase dissolution by atmosphere control in Bi-2212 superconductors

For power applications, where high current carrying capabilities are required and therefore thick film and bulk material are called for, the Bi 2Sr 2Ca 1Cu 2O 8− d (Bi-2212) compound has evolved as one of the most promising. During partial melt processing the crystallization from the partial melt state upon cooling is incomplete, due to the stability of 0 14 x 24 and sterically hindered diffusion and redistribution of melt originating from the highly anisotropic crystallizing Bi-2212 platelets. Therefore considerable amounts of residual 0 14 x 24 and “4413”, the mixed state of Bi-2212 and 2201, and residual melt are present in the microstructures at 850 °C. The subsequent annealing in oxygen leads to a limited dissolution of 0 14 x 24 phase and the conversion of “4413” to Bi-2212. Thermodynamic calculations of BiO 3/2–Sr 2/3Ca 1/3O–CuO cross-sections at 850 °C at different pO 2 by the CALPHAD method show that the stability range of 0 14 x 24 depends highly on the oxygen partial pressure ( pO 2). These results correlate perfectly with the experimental observations. By annealing at low pO 2 (0.1 bar) after crystallization, the amount of residual phases is reduced by more than 50% and the critical current density of the fully processed material increases by 65% compared to material annealed at high pO 2 (1 bar). This increase is attributed to a gain of current carrying cross-section as a consequence of enhanced 0 14 x 24 dissolution.

Doping and characterization of YBa2Cu3O7-δ thin filmsprepared by pulsed laser deposition

High-quality, smooth, optimally doped, c-axis thin films ofYBa2Cu3O7-δ were grown on (001) MgO substrates by pulsed laserdeposition and subsequent oxygen annealing. They were characterizedstructurally by θ-2θ and four-axis x-ray diffraction and,electrically, by a self-inductance measurement. Through subsequent annealingin reduced pressure oxygen environments, underdoped films were produced andagain characterized using the same techniques. The doping is reversible andreproducible. A monotonic decrease of Tc and an increase of the c-axislength were found with a decrease of the oxygen partial pressure during theannealing. From these data corresponding values of δ were determined.The width of the rocking curve of the (005) peak was found to be substantiallyunchanged, at 0.28-0.29°, between the optimally doped and theunderdoped films. The relative peak intensities showed a systematic variationwith δ in agreement with model calculations by Ye and Nakamura. Thegrain alignment is exclusively along the MgO high symmetry axes throughout thefilm. Resistance-temperature characteristics allowed the pseudogap onsettemperature, T*, to be determined as a function of doping.