Preparation, structural characterisation and antibacterial properties of Ga-doped sol–gel phosphate-based glass

A sol–gel preparation of Ga-doped phosphate-based glass with potential application in antimicrobial devices has been developed. Samples of composition (CaO)_0.30(Na₂O)_0.20−x (Ga₂O₃) _x (P₂O₅)_0.50 where x = 0 and 0.03 were prepared, and the structure and properties of the gallium-doped sample compared with those of the sample containing no gallium. Analysis of the ³¹P MAS NMR data demonstrated that addition of gallium to the sol–gel reaction increases the connectivity of the phosphate network at the expense of hydroxyl groups. This premise is supported by the results of the elemental analysis, which showed that the gallium-free sample contains significantly more hydrogen and by FTIR spectroscopy, which revealed a higher concentration of –OH groups in that sample. Ga K-edge extended X-ray absorption fine structure and X-ray absorption near-edge structure data revealed that the gallium ions are coordinated by six oxygen atoms. In agreement with the X-ray absorption data, the high-energy XRD results also suggest that the Ga³⁺ ions are octahedrally coordinated with respect to oxygen. Antimicrobial studies demonstrated that the sample containing Ga³⁺ ions had significant activity against Staphylococcus aureus compared to the control.     

Phase separation in oxygen deficient gallium oxide films grown by pulsed-laser deposition

The oxygen pressure and the substrate temperature during pulsed-laser deposition play a major role on the nature and properties of gallium oxide films. At moderate substrate temperature (673 K) and under high vacuum (10⁻⁷ mbar) a nanocomposite film composed of Ga metallic clusters embedded in a stoichiometric Ga₂O₃ matrix may be obtained without postdeposition annealing. The growth of such films is due to a phase separation of largely oxygen deficient metastable gallium oxide films Ga₂O_x (x = 2.3) into the most stable phases (Ga and Ga₂O₃) and occurs for particular growth conditions. The composition and the surface morphology of films as well as their electrical behaviour are interpreted according to the effects of the parameters governing this phase separation (oxygen deficiency and temperature). It is suggested that the initial step in the disproportionation reaction is the formation of stoichiometric Ga₂O₃ nanocrystallites in the metastable sub-oxide Ga₂O_x phase. The crystallization of such nanosize particles is governed by the local distribution of oxygen and gallium species impinging the substrate during the growth and allowing nucleation centre with the Ga₂O₃ composition.     

Preparation and characterisation of nanostructural TiO<Subscript>2</Subscript>–Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> binary oxides with high surface area derived form particulate sol–gel route

Nanostructured and nanoporous TiO₂–Ga₂O₃ films and powders with various Ti:Ga atomic ratios and high specific surface area (SSA) have been prepared by a new straightforward particulate sol–gel route. Titanium isopropoxide and gallium (III) nitrate hydrate were used as precursors and hydroxypropyl cellulose (HPC) was used as a polymeric fugitive agent (PFA) in order to increase the SSA. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) revealed that powders contained both rhombohedral α-Ga₂O₃ and monoclinic β-Ga₂O₃ phases, as well as anatase and rutile. It was observed that the Ga₂O₃ formed from the nitrate precursor retarded anatase-to-rutile transformation. Furthermore, transmission electron microscope (TEM) analysis also showed that Ga₂O₃ hindered the crystallisation and crystal growth of powders. SSA of powders, as measured by Brunauer–Emmett–Teller (BET) analysis, was enhanced by introducing Ga₂O₃. Ti:Ga = 50:50 (at%/at%) binary oxide annealed at 500 °C produced the smallest crystallite size (2 nm), the smallest grain size (18 nm), the highest SSA (327.8 m²/g) and the highest roughness. Ti:Ga = 25:75 (at%/at%) annealed at 800 °C showed the smallest crystallite size (2.4 nm) with 32 nm average grain size and 40.8 m²/g surface area. Ti:Ga = 75:25 (at%/at%) annealed at 800 °C had the highest SSA (57.4 m²/g) with 4.4 nm average crystallite size and 32 nm average grain size. One of the smallest crystallite size and one of the highest SSA reported in the literature is obtained, and they can be used in many applications in areas from optical electronics to gas sensors.     

Preparation and characterization of bulk ZnGa2O4

High-quality bulk ZnGa₂O₄ has been synthesized from equimolar mixtures of ZnO and Ga₂O₃ by the conventional solid-state method. For the first time, the sample has been characterized in detail to confirm the formation of pure single phase of spinel ZnGa₂O₄. The formation of ZnGa₂O₄ has been confirmed by sintering the mixtures of ZnO and Ga₂O₃ at different temperatures, ranging from 900–1200 °C. It is observed that the single phase of ZnGa₂O₄ has been formed at and above 1000 °C sintering temperature for 24 h. The crystallinity and phase formation of this single phase has been confirmed by X-ray diffraction. X-ray photoelectron spectroscopic studies have been carried out for bulk ZnGa₂O₄ sintered at 1000 °C for 24 h which showed 14% Zn, 28% Ga and 58% O, indicating stoichiometric ZnGa₂O₄. A new parameter, the energetic separation between the Zn 2p_3/2 and Ga 2p_3/2 peaks, has been used as a sensitive tool to distinguish between a complete formation of ZnGa₂O₄ compound and a mixture of ZnO and Ga₂O₃ powders. Surface morphology studies by scanning electron microscopy reveal that the formation of ZnGa₂O₄ takes place in mosaic rod-like structure. The purity of the compound has also been checked by the energy dispersive X-ray method, indicating the absence of foreign ions and the ratio of zinc to gallium has been calculated and found to be 1 : 2, indicating stoichiometric ZnGa₂O₄.     

Synthesis of ultrafine β-Ga2O3 nanopowder via hydrothermal approach: A strong UV “excimer-like” emission

Interest in nano-oxides has emerged from their technological applications in fields like microelectronics, catalysis, coatings, energy storage, and environment protection and remediation. There are various available methods for preparation and production of materials, but developing new routes to nanocrystalline materials is a challenging task for materials scientists. An innovative newly developed pathway to the synthesis of gallium oxide nanopowders, so-called “high-productivity/high-yield” process is presented here. The utilized method is simple, fast and environmental friendly. Ultrafine nanopowders of gallium oxide is prepared by addition of hydrogen peroxide (H₂O₂) to metal gallium (Ga) via a hydrothermal route at a low temperature (100 °C) and without any surfactant. β-Ga₂O₃ nanopowders are achieved directly in an autoclave when the initial molar ratio of Ga to O is 1:4. Combination of X-ray diffraction (XRD) and fluorescence analysis are employed to characterize the resulting nanopowders. Detailed results are discussed.     

Discrepancy of room temperature ferromagnetism in Mo-doped In 2 O 3

Molybdenum-doped indium oxide nanopowders were synthesized via mechanical alloying with subsequent annealing at a relatively low temperature of 600  $^{\circ}$ C. The morphologies and crystal structures of the synthesized nanopowders were examined by using scanning electron microscopy (SEM) and X-ray diffraction patterns. X-ray diffraction pattern of the milled mixture shows the presence of both In₂O₃ phase and Mo element. The presence of broad peaks in the pattern confirms that the synthesized powders are nanosized. The X-ray diffraction of annealed samples at 600  $^{\circ}$ C shows the absence of Mo peaks revealing that the Mo was incorporated into the crystal lattices of In₂O₃. Interestingly, it was observed that the diffraction peaks were still broad in the annealed samples indicating the single phase at the nanoscale. From the XRD pattern, the calculated crystallite sizes were in the range of 12–18 nm. Magnetic properties of the synthesized Mo-doped In₂O₃ nanopowders were examined and it was found that the obtained nanopowders possess diamagnetic properties.     

Room-temperature synthesis of γ-Ga2O3 nanoparticles from gallium metal via ultrasound irradiation

In this study, gallium oxide (Ga₂O₃) nanoparticles were synthesized by sonochemical techniques at room temperature. We investigate the ultrasound irradiation conditions and the solvent type to synthesize Ga₂O₃ nanoparticles. γ-Ga₂O₃ nanoparticles were synthesized by irradiating gallium metal with ultrasound in hydrazine monohydrate solvent. The irradiation of hydrazine with ultasound suppresses the generation of ·OH, and GaOOH was not formed, and gallium metal directly oxidized. When the synthesized γ-Ga₂O₃ nanoparticles were heat-treated, a transition to β-Ga₂O₃ was observed. The heat-treated Ga₂O₃ nanoparticles showed excellent photocatalytic activity for the decomposition of RhB under UV irradiation.     

Single-step growth dynamics of core–shell GaN on Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> freestanding nanoprotruded microbelts

Freestanding wurtzite GaN nanoprotruded microbelts with Ga₂O₃ core, with typical thickness 1–10 μm, and length of few millimeters are synthesized by thermal annealing of Ga metal and subsequent reaction with ammonia at a low flow rate. They are of distinctive rectangular shape with a typical width of 10–100 μm. Thickness of the belt is about 1/10th of the width and length up to a few millimeters. The GaN, Ga₂O₃ layers and the GaN–Ga₂O₃ interface are characterized by high-resolution transmission electron microscopy after focused ion beam sectioning of the belt. Initially, Ga₂O₃ nucleates after reaction with the O₂ available in the environment, and subsequent reaction with NH₃ results in the formation of core–shell structure in the catalyst-free vapor–solid growth process. Having a low-symmetry phase, Ga₂O₃ can grow only in certain preferred directions thus controlling the final morphology of the belt. Nanoscale protrusions ~50–100 nm found on the surface of the belts could be an ideal system for building functional devices.     

Chemical transport ofβ-Ga2O3 using chlorine as a transporting agent

The chemical transport of Ga₂O₃ with chlorine as a transporting agent has been investigated. Thermochemical properties of gallium chlorides have been considered on the basis of literature data. Thermodynamic analysis of transport reactions at 600 to 1500 K has been performed and GaCl₃ was found the predominant gallium chloride in the vapour phase. The crystallization temperature of about 1150 to 1250 K proved to be the best conditions for the crystal growth ofβ-Ga₂O₃.     

Bicrystalline gallium oxide nanobelts

We have synthesized gallium oxide (Ga₂O₃) nanobelts by heating GaN powders in the conventional furnace. The nanobelts exhibited a unique bicrystalline structure that consisted of two single-crystalline monoclinic Ga₂O₃ nanobelts, which split along the twin boundary that exists at the centerline. Energy dispersive X-ray spectroscopy and electron energy loss spectroscopy spectra coincidentally indicated the presence of nitrogen in the Ga₂O₃ nanobelts. Photoluminescence spectra exhibited the visible light emission. We discussed the possible emission mechanisms, including the effect of the nitrogen dopant.     

Effect of Mn doping on the structural,morphological, optical and magnetic properties of indium tin oxide films

We report on the preparation and characterization of high purity manganese (3–9 wt.%) doped indium tin oxide (ITO, In:Sn = 90:10) films deposited by sol–gel mediated dip coating. X-ray diffraction and selected area electron diffraction showed high phase purity cubic In₂O₃ and indicated a contraction of the lattice with Mn doping. High-resolution transmission electron microscopy depicted a uniform distribution of ∼20 nm sized independent particles and particle induced x-ray emission studies confirmed the actual Mn ion concentration. UV-Vis diffuse reflectance measurements showed band gap energy of 3.75 eV and a high degree of optical transparency (90%) in the 100–500 nm thick ITO films. X-ray photoelectron spectroscopy core level binding energies for In 3d_5/2 (443.6 eV), Sn 3d_5/2 (485.6 eV) and Mn 2p_3/2 (640.2 eV) indicated the In³⁺, Sn⁴⁺ and Mn²⁺ oxidation states. Magnetic hysteresis loops recorded at 300 K yield a coercivity H_c ∼ 80 Oe and saturation magnetization M_s ∼ 0.39 μ_B/Mn²⁺ ion. High-temperature magnetometry showed a Curie temperature T _c > 600 K for the 3.2% Mn doped ITO film.     

Optical properties and structure of R2O–Ga2O3–SiO2 and RO–Ga2O3–SiO2 glasses

Refractive index and molar refraction of Li₂O–, Na₂O–, CaO–, and BaO–Ga₂O₃–SiO₂ glasses have been used to test the validity of a structural model of silicate glasses containing Ga₂O₃ glasses. Ga₂O₃ enters these types of glass in a similar manner as Al₂O₃. It is assumed that, for (SiO₂/Ga₂O₃) >1 and (Ga₂O₃/R₂O) ≤1, Ga₂O₃ associates primarily with modifier oxides to form GaO₄ units. The rest of modifier oxide forms silicate units with non-bridging oxygen ions. Silicate structural units have the same factors as found for binary alkali- and alkaline earth silicate glasses. Differences between experimental and model values suggest another structure for (Ga₂O₃/SiO₂) ≥1.     

X-ray study of gadolinium gallium garnet epitaxial layers containing divalent Co ions

Structural perfection of gadolinium gallium garnet (GGG; Gd₃Ga₅O₁₂) epitaxial layers with incorporated divalent Co ions has been studied by means of high resolution X-ray diffraction, X-ray topography, transmission electron microscopy and optical spectroscopy. Epitaxial layers were grown by liquid phase epitaxy from super-cooled high temperature solution with different concentration of Co₃O₄ and GeO₂ on both sides of the polished < 111> oriented GGG substrates. In order to facilitate incorporation of Co²⁺ ions into the garnet lattice optically inert Ge⁴⁺ ions have to be introduced first. High structural perfection is a prerequisite to obtain absorption spectra required for the use GGG epitaxial layers as tunable infrared absorbers.     

Structural,optical, and opto-dielectric properties of W-doped Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> thin films

A layer-by-layer deposition technique of Ga₂O₃ and WO₃ by vacuum evaporation method on glass and silicon substrates and subsequent annealing in oxygen atmosphere to form W-doped Ga₂O₃ (or Ga₂O₃:W) films was attempted here. The W doping level was measured by the energy dispersive X-ray fluorescence radiographic analysis. The crystalline structure of Ga₂O₃:W films was determined by the X-ray diffraction method. Experimental data indicate that W⁶⁺ ions doped in host Ga₂O₃ forming solid solutions (SS), in which the molar ratio (r) of W to Ga is 9.6, 13.4, 18.2, 22.7 and 30.4%. All the prepared SS have the known β-Ga₂O₃ crystalline structure. This doping controls the optical and electrical properties of the host Ga₂O₃. The optical properties of the prepared Ga₂O₃:W films were studied by UV–VIS–NIR absorption spectroscopy method from which the bandgap was determined. In general, it was found that the prepared Ga₂O₃:W films are wide-bandgap semiconductors with bandgap 4.69–4.47 eV and have dielectric properties. The optical sensitivity of the capacitance, dissipation factor and ac-conductance of the Ga₂O₃:W films grown on Si was studied as a function of W-doping level. It was observed that the prepared Ga₂O₃:W film of r = 22.7% has the highest photosensitivity amongst the other samples.     

X-ray diffraction and57Fe Mössbauer spectra of the system Fe2O3−Ga2O3

The influence of gallium substitution on the chemical and structural properties of haematite, α-Fe₂O₃, has been studied using X-ray diffraction and⁵⁷Fe Mössbauer spectroscopy. The presence of only α-(Ga _x Fe_1?x )₂O₃ phase is detected for the compositions withx between 0.01 and 0.90. A gradual decrease of the unit-cell parameters of α-(Ga _x Fe_1?x )₂O₃ with the increase of gallium substitution is measured.⁵⁷Fe Mössbauer spectra showed that the value of the magnetic hyperfine field of pure α-Fe₂O₃ decreases with increasing gallium for iron substitution. The hyperfine magnetic structure, which is observed for α-(Ga _x Fe_1?x )₂O₃ at room temperature, collapsed for the composition withx?0.50. The changes in the⁵⁷Fe Mössbauer spectra of the α-(Ga _x Fe_1?x )₂O₃ phase are discussed in the sense of the electronic relaxation and the superparamagnetic effects.     

Synthesis and characterization of pyrochlore-type yttrium titanate nanoparticles by modified sol–gel method

Pyrochlore-type yttrium titanate (Y₂Ti₂O₇) nanoparticles were successfully synthesized by a simple soft-chemistry technique viz. citric acid sol–gel method (CAM). The preparation process was monitored by X-ray diffraction, thermogravimetric–differential thermal analysis and Fourier transform–infrared experiments and the microstructures and average size of as-prepared products were characterized by transmission electron microscopy and high resolution transmission electron microscopy images. It was found that compared with traditional solid state reaction (SSR), Y₂Ti₂O₇ nanopowders were synthesized at a relatively low temperature (750°C) for shortened reaction time. Detailed analysis showed that the as-prepared Y₂Ti₂O₇ with good dispersibility and narrow size distribution were quasi-spherical; the average size was about 20–30 nm, also, the obtained products had higher BET surface area (50 m²/g). These properties are very helpful for a photocatalyst to achieve excellent activity and may result in better behaviour in hydrogen storage.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the crystallization behavior of SiO<Subscript>2</Subscript>–MgO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A series of glass comprising of SiO₂–MgO–B₂O₃–Y₂O₃–Al₂O₃ in different mole ratio has been synthesized. The crystallization kinetics of these glasses was investigated using various characterization techniques such as differential thermal analysis (DTA), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Crystallization behavior of these glasses was markedly influenced by the addition of Y₂O₃ instead of Al₂O₃. Addition of Y₂O₃ increases the transition temperature, T _g, crystallization temperature, T _c and stability of the glasses. Also, it suppresses the formation of cordierite phase, which is very prominent and detrimental in MgO-based glasses. The results are discussed on the basis of the structural and chemical role of Y³⁺ and Al³⁺ ions in the present glasses.     

Effects of glass components on crystallization and dielectric properties of BST glass–ceramics

Crystallization behavior was studied for glass powders in which some portions of AlF₃ in the net composition of 60(Ba_0.7Sr_0.3)TiO₃–25SiO₂–15AlF₃ were replaced with Ga₂O₃ or Bi₂O₃. The replacement with Ga₂O₃ resulted in a progressive increase in crystallization temperature, which effectively assisted the viscous sintering of glass powders to produce densified BST glass–ceramics at relatively lower temperatures. For the Bi₂O₃-replaced glass powders, an increasing amount of Bi₂O₃ replacement lowered the crystallization temperature and yielded less densified glass–ceramics containing a considerable amount of glassy phase. The temperature dependence of permittivity was estimated for the Ga₂O₃- and Bi₂O₃-replaced glass–ceramics as a function of sintering conditions and the amount of replacement, respectively.     

Crystal structure and electrical properties of CaNaBi<Subscript>2</Subscript>Nb<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics

Monophasic CaNaBi₂Nb₃O₁₂ powders were synthesized via the conventional solid-state reaction route. Rietveld refinement of the X-ray powder diffraction (XRD) data and selected area electron diffraction (SAED) studies confirmed the phase to be a three-layer Aurivillius oxide associated with an orthorhombic B2cb space group. The dielectric properties of the ceramics have been studied in the 300–800 K temperature range at various frequencies (1 kHz to 1 MHz). A dielectric anomaly was observed at 676 K for all the frequencies corresponding to the ferroelectric to paraelectric phase transition as it was also corroborated by the high temperature X-ray diffraction studies. The incidence of the polarization–electric field (P vs. E) hysteresis loop demonstrated CaNaBi₂Nb₃O₁₂ to be ferroelectric.     

Synthesis of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowders

A procedure has been developed for the synthesis of MgAl₂O₄ nanopowders with a characteristic particle size of 10–40 nm. Translucent hydrous xerogels have been synthesized as precursors to MgAl₂O₄. The synthesized magnesium aluminum spinel nanopowders are promising for the fabrication of optical ceramics.