Ga2O3 Phase Research Articles

Pd2Ga-Based Colloids as Highly Active Catalysts for the Hydrogenation of CO2 to Methanol

Colloidal Pd2Ga-based catalysts are shown to catalyze efficiently the hydrogenation of CO2 to methanol. The catalysts are produced by the simple thermal decomposition of Pd(II) acetate in the presence of Ga(III) stearate, which leads to Pd0 nanoparticles (ca. 3 nm), and the subsequent Pd-mediated reduction of Ga(III) species at temperatures ranging from 210 to 290 °C. The resulting colloidal Pd2Ga-based catalysts are applied in the liquid-phase hydrogenation of carbon dioxide to methanol at high pressure (50 bar). The intrinsic activity is around 2-fold higher than that obtained for the commercial Cu-ZnO-Al2O3 (60.3 and 37.2 × 10–9 molMeOH m–2 s–1), respectively, and 4-fold higher on a Cu or Pd molar basis (3330 and 910 μmol mmolPd or Cu–1 h–1). Detailed characterization data (HR-TEM, STEM/EDX, XPS, and XRD) indicate that the catalyst contains Pd2Ga nanoparticles, of average diameters 5–6 nm, associated with a network of amorphous Ga2O3 species. The proportion of this Ga2O3 phase can be easily tuned by ad...

Synthesis and thermoelectric properties of composite oxides in the pseudobinary system ZnO-Ga2O3

Phase relations in the pseudobinary system ZnO-Ga2O3 were investigated at 1723 K. The homologous phase Ga2O3(ZnO)m was formed in a region 9 ≦ n ≦ 38 for ZnO: Ga2O3 = n: 1, and Ga-doped ZnO (wurtzite structure) was in n ≧ 398. In between the two regions (i.e. 38 < n < 398) a composite of Ga-doped ZnO and the homologous phase Ga2O3(ZnO)mmax (mmax ≈ 38) was formed. Thermoelectric properties of the composite were examined along with Ga-doped ZnO and the homologous phase Ga2O3(ZnO)33. In the composite, the homologous phase appears to be very effective in lowering the thermal conductivity. As a result of a trade-off relationship between the power factor and the thermal resistivity, thermoelectric performance happened to be roughly the same for the composite and Ga-doped ZnO samples.

Synthesis and characterization of Ga2O3 nanosheets on 3C-SiC-on-Si by low pressure chemical vapor deposition

This study presents the synthesis of single crystalline β-Ga2O3 nanosheets on SiC by low pressure chemical vapor deposition. High purity gallium (Ga) metal and oxygen as source materials and argon as carrier gas were utilized for the synthesis of the nanosheets on a 3C-SiC-on-Si substrate. These single-crystal Ga2O3 nanosheets are free-standing 2D extrusions from their 1D rods, typically 1.5–7 μm in lateral size and 20–140 nm in thickness, featuring aspect ratios ranging from ∼10 to 350. Structural studies based on transmission electron microscopy and Raman spectroscopy revealed the monoclinic phase of Ga2O3 with a single crystalline nature. High resolution transmission electron microscopy with a selected area electron diffraction pattern recorded on a single β-Ga2O3 nanosheet further confirmed their single crystalline nature, with a growth direction perpendicular to (111) crystallographic plane. The growth process governing the formation of these nanosheets is a vapor-solid growth mechanism since no metal catalyst was used. These β-Ga2O3 nanosheets exhibit new possibilities and potential for future functional nanodevices that would benefit from their extremely large surface area to volume ratios.

High-level damage saturation below amorphisation in ion implanted β-Ga2O3

Ion implantation induced effects were studied in single crystalline 〈010〉 oriented bulk β-Ga2O3 at room temperature using P, Ar and Sn ions with ion fluences ranging from 1×1011 up to 2×1015cm−2. Rutherford backscattering spectrometry in channelling configuration (RBS) using He ions of various ion energies was applied for damage analysis. Clear damage peaks are visible in the RBS spectra. The concentration of displaced lattice atoms in the maximum of the distribution (as deduced from the channelling spectra) increases with increasing ion fluence up to a saturation value of about 90%. Once this level is reached, further implantation only leads to a broadening of the distribution, while the concentration remains at 90%. The ion fluence dependence of maximum damage concentration is represented by a common model assuming two types of defects: point defects (which can recombine with those already existing from previous ion impacts) and non-recombinable damage clusters. The damage produced dominantly consists of randomly displaced lattice atoms, which indicates point defects and point defect complexes. For higher damage levels also a contribution of correlated displaced lattice atoms can be identified. This suggests that the damage clusters are not amorphous. A possible explanation of the observed results could be the formation of another phase of Ga2O3.

Synthesis of Ion Conducting Sodium Zirconium Gallate + Yttria-Stabilized Zirconia by a Vapor Phase Process

Two phase samples containing ∼70 vol.% Ga2O3 and ∼30 vol.% yttria-stabilized zirconia (YSZ) were fabricated by sintering powder compacts in air at 1500°C for 1 h and at 1600°C for 1 h and 3 h. Single phase samples of Ga2O3 were fabricated by sintering at 1500°C for 1 h. Sintered samples were packed in Na-β″-alumina powder of composition ∼8.85 wt% Na2O, 0.75 wt% Li2O and 90.4 wt% Al2O3, and were heat treated at 1250°C for up to 20 h. The two phase Ga2O3 + YSZ samples converted into a sodium zirconium gallate of composition Na0.7Ga4.7Zr0.3O8 and balance yttria-stabilized zirconia. The reaction of single phase Ga2O3 was limited to a thin surface layer in which Na-β″-gallate formed. Rapid conversion of Ga2O3 + YSZ samples into Na0.7Ga4.7Zr0.3O8 + YSZ occurred by coupled diffusion of O2− ions through YSZ and of Na+ ions through the formed Na0.7Ga4.7Zr0.3O8, similar to the conversion of α-Al2O3 + YSZ into Na-β″-alumina + YSZ. The phase Na0.7Ga4.7Zr0.3O8 is isostructural with Na0.7Ga4.7Ti0.3O8 which is a one dimensional sodium ion conductor. The conductivity of two phase Na0.7Ga4.7Zr0.3O8 + YSZ composite was measured by Electrochemical Impedance Spectroscopy.

Phase Tuning of Nanostructured Gallium Oxide via Hybridization with Reduced Graphene Oxide for Superior Anode Performance in Li-Ion Battery: An Experimental and Theoretical Study.

The crystal phase of nanostructured metal oxide can be effectively controlled by the hybridization of gallium oxide with reduced graphene oxide (rGO) at variable concentrations. The change of the ratio of Ga2O3/rGO is quite effective in tailoring the crystal structure and morphology of nanostructured gallium oxide hybridized with rGO. This is the first example of the phase control of metal oxide through a change of the content of rGO hybridized. The calculations based on density functional theory (DFT) clearly demonstrate that the different surface formation energy and Ga local symmetry of Ga2O3 phases are responsible for the phase transition induced by the change of rGO content. The resulting Ga2O3-rGO nanocomposites show promising electrode performance for lithium ion batteries. The intermediate Li-Ga alloy phases formed during the electrochemical cycling are identified with the DFT calculations. Among the present Ga2O3-rGO nanocomposites, the material with mixed α-Ga2O3/β-Ga2O3/γ-Ga2O3 phase can deliver the largest discharge capacity with the best cyclability and rate characteristics, highlighting the importance of the control of Ga2O3/rGO ratio in optimizing the electrode activity of the composite materials. The present study underscores the usefulness of the phase-control of nanostructured metal oxides achieved by the change of rGO content in exploring novel functional nanocomposite materials.

Effects of structural characteristics on microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)Ti1−xGa4x/3O3 ceramics

Structural features and microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)Ti1−xGa4x/3O3 (SCNTGx, 0.2≤x≤0.6) samples were studied. The X-ray patterns revealed that a single phase with orthorhombic perovskite-like structure was obtained in the range of x=0.2–0.4. A certain amount of Ga2O3 phase was detected at x=0.5 from the XRD data. However, some unknown phase was formed as a secondary phase when x increased up to 0.6. Moreover, as Ga3+ concentration increased, the ϵr decreased due to the smaller ionic polarization of Ga3+ than that of Ti4+, the Q×f value increased firstly and then decreased because of the existence of the unknown secondary phase. And the τf values changed from a positive to negative value, arising from a reduction in tolerance factor (t) for tilted region (t<1) in perovskite structure. Furthermore, an extra Raman band centered at about 240/236cm−1 was detected in SCNTGx ceramics at x=0.5, 0.6, which was caused by the appearance of the Ga2O3 phase. The SCNTGx (x=0.5) ceramic sintered at 1350°C for 4h showed good microwave dielectric properties with ϵr=43.7, Q×f=60,100GHz (at 4.71GHz) and τf=8.3ppm/°C. Obviously, the introduction of Ga2O3 could efficiently improve Q×f value of the SCNTGx specimens.

Mesoporous mixed-phase Ga2O3: Green synthesis and enhanced photocatalytic activity

Mesoporous mixed-phase Ga2O3 was synthesized by calcining the GaOOH precursor. The composition, crystal phase and microstructures of Ga2O3 were characterized in detail. The phase composition of the as-prepared Ga2O3 depended on the calcination temperature and the mixed-phase Ga2O3 was obtained at 600–700°C. As compared to the pure-phase α-Ga2O3 and β-Ga2O3, the mixed-phase Ga2O3 exhibited an enhanced photocatalytic property for the degradation of metronidazole solution. The heterojunction in the mixed-phase Ga2O3 was beneficial to the separation of photogenerated electrons and holes. Moreover, the mixed-phase Ga2O3 possessed mesopore structure, which increased more reaction sites and was in favor of the contact of metronidazole molecules with reaction sites. The recycling experiments show that the mixed-phase Ga2O3 has good stability and can be separated easily from the reaction system.

Study of Growth and Characterization of Fe-Catalyzed β-Ga2O3 Nanowires

ABSTRACTIn the present study, Fe as a catalyst was used to grow single crystalline monoclinic gallium oxide (β-Ga2O3) nanowires using chemical-vapor-deposition method. The morphology, structure and luminescence properties of the as-grown β-Ga2O3 nanowires were investigated using various characterization techniques. The diameter of the as-grown nanowires was in the range of 50 to 100 nm, and the lengths up to tens of micrometers. The structural investigation of the nanowires was carried out using X-ray diffraction that showed monoclinic phase of Ga2O3. Further, the transmission electron microscope (TEM) investigations along with selected area diffraction pattern revealing single crystalline nature of the nanowires. The as-grown β-Ga2O3 nanowires had preferred orientation along [1-1-1] direction. The high resolution TEM image showed regular arrangement of atoms and the lattice spacing between (1-1-1) planes was around 0.266 nm. The luminescence properties of the as-grown nanowires were measured using cathodoluminescence (CL) spectroscopy. The CL measurements of β-Ga2O3 nanowires revealed a strong broad UV-blue emission band and a weak red emission band.

Study of iron-catalysed growth of β-Ga2O3 nanowires and their detailed characterization using TEM, Raman and cathodoluminescence techniques

In this paper, we demonstrate a new catalyst (Fe) to grow single crystalline beta-gallium oxide (β-Ga2O3) nanowires (NWs) via the vapour–liquid–solid mechanism using the chemical vapour deposition technique. The structural studies of these NWs showed the highly crystalline monoclinic phase of Ga2O3. This was confirmed by detailed scanning transmission electron microscope investigations demonstrating the NW to be single crystalline β-Ga2O3, growing along the normal of the plane. We also compared Raman and cathodoluminescence (CL) properties of the as-grown β-Ga2O3 NWs with a bulk Ga2O3 single crystal grown by the Czochralski method. It was observed that Raman peak positions of a single β-Ga2O3 NW had a red frequency shift of about 0.3–1.4 cm−1 as compared to a bulk Ga2O3 single crystal, which was in fact quite small. In addition, the CL measurements of β-Ga2O3 NWs and the bulk Ga2O3 single crystal exhibited similar spectra, having a strong broad UV–blue emission band and a weak red emission band. Moreover, the structural, morphological and optical properties of Fe-catalysed β-Ga2O3 NWs were comparable to those of Au-catalysed β-Ga2O3 NWs.

Self‐Catalytic Growth of β‐Ga2O3 Nanostructures by Chemical Vapor Deposition

In this work, we have studied the synthesis of single crystalline self‐catalyzed beta gallium oxide (β‐Ga2O3) nanostructures by chemical vapor deposition technique. We have adopted a new approach to grow the nanostructures instead of using conventional foreign metal nano‐catalyst based‐approaches. The as‐grown nanostructures including nanowires and nanosheets (NSHs) were grown on spin‐coated Ga2O3 films. The structural studies such as X‐ray diffraction, Raman and transmission electron microscope (TEM) investigations on the nanostructures showed monoclinic phase of Ga2O3 and single crystalline structure. Furthermore, high‐resolution TEM with a selected area electron diffraction pattern recorded on a single β‐Ga2O3 nanowire and nanosheet verified their single crystalline nature, having [001] as favorable growth direction. The energy dispersive X‐ray spectroscopy‐elemental mapping of as‐grown nanostructures indicated uniform distribution of Ga and O. Cathodoluminescence imaging and spectrum revealed excellent luminescence characteristics of nanostructures with a broad UV‐blue emission band (1.80–4.20 eV nm) centered at 2.64 eV. This study also highlights the growth mechanism of NSHs. These β‐Ga2O3 nanostructures have great potential in nanofunctional devices.

InxGa1−xN performance as a band-gap-tunable photo-electrode in acidic and basic solutions

Performance of InxGa1−xN photo-electrodes at concentrations of In ranging from 0 to 100 % was investigated in basic and acidic solutions under 1 Sun illumination. Photocorrosive effects of InxGa1−xN samples in aqueous solutions are revealed and strategies for a more efficient use of these electrodes are discussed. Formation of Ga2O3 phase and N2 under photoanodic conditions can explain the photo-corrosive effect. It is shown that the product of charge carrier density and mobility, n×μ, scales with the photo-current density in GaN.

Influence of post-deposition annealing in air and vacuum on the properties of thermally evaporated gallium oxide films

Oxidative annealing in air or reductive annealing in vacuum around 773K of thermally evaporated gallium oxide films produces monoclinic β-Ga2O3 films of distinctly different compositional, optical, morphological and electrical properties. The pristine films prepared by the evaporation of Ga2O3 powders are oxygen deficient, amorphous and absorbing in UV–visible region. The air annealed films are transparent (band gap ∼4.9eV), display nanometric granular morphology and are characterized by <1.0eV extrinsic and 1.2–1.6eV intrinsic activation energies in the Arrhenius plots of electrical conductivity. The growth of Ga2O3 phase on vacuum annealing takes place through the decomposition of Ga2O, one of the constituents of the pristine films. The vacuum annealed films exhibit comparatively lower transparency (band gap <4.5eV), comprise micron-sized dendrites or fibres and have <1.0eV extrinsic and 1.7–2.0eV intrinsic activation energies. The incorporation of these properties results from compositional changes in films induced by annealing in air or vacuum ambient.

Chemical bonding, optical constants, and electrical resistivity of sputter-deposited gallium oxide thin films

Gallium oxide (Ga2O3) thin films were made by sputter deposition employing a Ga2O3 ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (Ts), from 25 to 600 °C. The effect of Ts on the chemical bonding, surface morphological characteristics, optical constants, and electrical properties of the grown films was evaluated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the Ga 2p doublet, i.e., the Ga 2p3/2 and Ga 2p1/2 peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga3+) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE ∼ 531 eV, which is also characteristic of Ga-O bonds in the Ga2O3 phase. The granular morphology of the nanocrystalline Ga2O3 films was evident from AFM measurements, which also indicate that the surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing Ts. The SE analyses indicate that the index of refraction (n) of Ga2O3 films increases with increasing Ts due to improved structural quality and packing density of the films. The n(λ) of all the Ga2O3 films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (∼200 Ω-cm) for amorphous Ga2O3 films grown at Ts = RT-300 °C and decreased to ∼1 Ω-cm for nanocrystalline Ga2O3 films grown at Ts ≥ 500–600 °C. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga2O3 films is derived.

Ga掺入对立方相MgZnO薄膜晶体结构的影响

The influence of gallium (Ga) doping on the structure of cubic MgZnO was studied by changing the doping level. Cubic Mg0.56Zn0.44O films with various Ga-doping levels were grown by metal-organic chemical vapor deposition method. The mole fraction of Ga in this experiment was 0, 2.8%, 3.1%, 3.3%, and 4.5% in MgZnO. Ga2O3, ZnO and ZnGa2O4 phase separations took place subsequently with Ga mole fraction increasing to 3.1%, 3.3%, and 4.5%. The Ga2O3 and ZnGa2O4 phase separations can be attributed to their solubilities in MgZnO, and the formation of ZnO comes from the introduction of Ga which breaks the metastable structure of cubic MgZnO.

Structural Evolution and Growth Mechanism of Self-Assembled Wurtzite Gallium Nitride (GaN) Nanostructures by Chemical Vapor Deposition

We present the fabrication of self-assembled wurtzite gallium nitride (GaN) nanostructures with manifold morphologies through chemical vapor deposition under controlled deposition conditions. The different nanostructures including vertical standing whiskered nanowires (NWs), entangled NWs, nanorods, micro/nanotowers, highly transparent ultrathin nanosheets, and hexagonal microcrystals were evolved by the direct reaction of metal Ga with NH3 using a self-catalytic process by varying the important growth parameters such as temperature, source to substrate distance, and the reactor pressure. The growth mechanism of GaN nanostructures with manifold morphologies was interpreted with a surface diffusion model by accounting the direct impingement and surface migration of adatoms. Electron microscopy studies combined with a selected area electron diffraction pattern recorded on the NWs show wurtzite structure with preferential growth direction of (0001). X-ray diffraction studies on different nanostructures show that the hexagonal GaN contains neither cubic GaN nor Ga2O3 phases. Room temperature photoluminescence spectra reveal high optical quality of the nanostructures grown under either equimolar ratio or slightly nitrogen-rich regime, and, interestingly, GaN microcrystals grown under Ga-rich conditions were dominated by the defect induced green and yellow luminescence.

Investigation of opto-dielectric properties of Ti-doped Ga2O3 thin films

Several Ti-incorporated Ga-oxide (Ga2O3) thin films with different amounts of Ti contents have been prepared by vacuum evaporation method on glass and silicon substrates. The Ti incorporation level was measured with energy dispersion X-ray spectroscopy (EDX) method. The crystalline structure of the prepared films was determined with X-ray diffraction method. Experimental data indicate that Ti4+ ions doped in host Ga2O3 form solid solutions (SS) even with so large Ti% content ∼10.4% wt. All the prepared solid solutions have the known orthorhombic (ɛ-phase) phase of Ga2O3. The doping controls the optical and electrical properties of the host Ga2O3. It was found that the bandgap of the prepared undoped Ga oxide is 5.23 eV, which was decreased monotonically with increasing of Ti doping level so that it is possible to engineer the bandgap. Furthermore, the electrical measurements show that with Ti doping, it is possible to turn the high-k Ga oxide into low-k dielectric material. The optical sensitivity of the capacitance, dissipation factor, and ac-conductance of the Ga2O3:Ti films grown on Si was studied as a function of Ti-doping level. It was observed that the prepared Ga2O3:Ti film with 6–10% doping level has the highest photosensitivity among the other samples.

Band gap broadening and photoluminescence properties investigation in Ga2O3 polycrystal

Ga2O3 thin films were deposited on c-plane Al 2O3 substrates by electron beam evaporation equipment. The effects of post anneal treatment on structure and optical properties of Ga 2O3 were investigated. The X-ray diffraction (XRD) results of the as-grown and the annealed samples indicated the films consisted with the mix of $$\beta$$ -phase polycrystalline and amorphous Ga2O3. The electron diffraction pattern confirmed the existence of the nanocrystal grains. AFM images revealed that the anneal treatment promoted the film crystallization. Both Ga2O 3 fims exhibited high transparency from visible light to near infrared region. An obvious band gap broadening phenomenon was observed for the annealed sample comparing with the as-grown sample. The optical band gap of the annealed sample was as large as 5.68 eV, which was inconsistent with the bulk $$\beta$$ -phase Ga2O3. Meanwhile, the center of ultraviolet emission peak blue shifted about 0.42 eV for the annealed samples. The mechanism of the band gap broadening effect and ultraviolet emission peak blue shift were discussed. 2013 Springer Science+Business Media New York.

Low-temperature growth of InxGa1−xN films by radio-frequency magnetron sputtering

The low-temperature growth of InxGa1−xN films on quartz glass substrates utilizing radio-frequency magnetron sputtering is investigated. In the InxGa1−xN films prepared using an In–Ga alloy target, grazing incidence X-ray diffraction (GIXRD) peaks corresponding to wurtzite structure were observed. X-ray photoelectron spectroscopy (XPS) was applied to study the extent of oxygen contamination and chemical states, and secondary ion mass spectrometry (SIMS) was used to evaluate the distribution profiles of oxygen impurity in the as-grown InxGa1−xN thin films. XPS and SIMS analysis indicate that the entire thin films have oxide phases. However, no evidence of In2O3, Ga2O3, or indium oxynitride phases was shown in XRD studies. It may be predicted that the oxygen impurities formed amorphous oxide phases embedded in InxGa1−xN matrix. According to our findings, indium is a major phase in the InxGa1−xN thin films which suggests that a significant amount of indium remains un-reacted with N2. The optical transmittance spectra of the as-grown films show interference fringe patterns. The indium fraction x of the as-deposited InxGa1−xN thin films can be calculated out by the transmittance data.

Microstructure and micro-Raman studies of nitridation and structure transition of gallium oxide nanowires

Abstract Here we present a detailed study on nitridation and structure transition in monoclinic gallium oxide (β-Ga2O3) nanowires grown on Si substrates with chemical vapor phase epitaxy. The nanowires were systematically nitridated at different temperatures. Their morphologies and microstructures were precisely characterized using field-emission scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and confocal micro-Raman spectroscopy. It is found that heat treatment of Ga2O3 nanowires in the gas of ammonia results in rich substructures including the Ga2O3 phase, the crystalline GaN phase, and other meta structures. The identification of these structures helps to understand some interesting phenomena observed in nanostructures, such as the microstructural origin of the unknown Raman lines in GaN nanowires.