ZnGa2O4 Nanocrystals Research Articles

Sintering of lixiviated nano glass-ceramics: An original route to elaborate transparent ceramics

We report the elaboration of translucent ZnGa2O4 ceramics by an innovative process, the sintering by Spark Plasma Sintering (SPS) of spinel ceramic powder obtained by the lixiviation of glass-ceramics. Indeed, a selective dissolution of the silica-rich glass matrix allows isolating ZnGa2O4 nanocrystals embedded in the silicate glass-ceramic. This glass-ceramic is elaborated from the parent glass of composition 55 SiO2 - 5 Na2O - 17 ZnO - 23 Ga2O3 (%mol) that undergoes a subsequent heat-treatment at 1100 °C under air leading to the crystallization of zinc gallate spinel nanocrystals with size in the 20–60 nm range. Lixiviated nanocrystals are collected and sintered by SPS to get a bulk ceramic. While the transmittance of this ceramic is only 60% at 4.5 µm, which is lower than the theoretical transmittance (82.5%), these first results are still promising and pave the way to new transparent ceramic processing route. Indeed, glass crystallization combined with selective lixiviation process is a powerful, non-conventional elaboration process that can further allow the stabilization and isolation of metastable crystalline phases with interesting functional properties.

Synthesis and Nanostructure Investigation of Hybrid β-Ga2 O3 /ZnGa2 O4 Nanocomposite Networks with Narrow-Band Green Luminescence and High Initial Electrochemical Capacity.

The material design of functional "aero"-networks offers a facile approach to optical, catalytical, or and electrochemical applications based on multiscale morphologies, high large reactive area, and prominent material diversity. Here in this paper, the synthesis and structural characterization of a hybrid β-Ga2 O3 /ZnGa2 O4 nanocomposite aero-network are presented. The nanocomposite networks are studied on multiscale with respect to their micro- and nanostructure by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and are characterized for their photoluminescent response to UV light excitation and their electrochemical performance with Li-ion conversion reaction. The structural investigations reveal the simultaneous transformation of the precursor aero-GaN(ZnO) network into hollow architectures composed of β-Ga2 O3 and ZnGa2 O4 nanocrystals with a phase ratio of ≈1:2. The photoluminescence of hybrid aero-β-Ga2 O3 /ZnGa2 O4 nanocomposite networks demonstrates narrow band (λem = 504nm) green light emission of ZnGa2 O4 under UV light excitation (λex = 300nm). The evaluation of the metal-oxide network performance for electrochemical application for Li-ion batteries shows high initial capacities of ≈714 mAh g-1 at 100mA g-1 paired with exceptional rate performance even at high current densities of 4 A g-1 with 347 mAh g-1 . This study provides is an exciting showcase example of novel networked materials and demonstrates the opportunities of tailored micro-/nanostructures for diverse applications a diversity of possible applications.

Crystallization kinetics and optical properties of transparent glass-ceramics embedding ZnGa2O4 nanocrystals with enhanced defect luminescence

In this study, the transparent glass-ceramics containing spinel ZnGa2O4 nanocrystals were obtained by controlling crystallization. XRD, SEM, and TEM results show that after heat treatment at 750–950 °C, ZnGa2O4 nanocrystals with spinel structure can be precipitated in the glass matrix. As the heat treatment temperature increases, the nanocrystal size of ZnGa2O4 in the glass matrix shows an increasing trend, which results in a slight decrease in the transmittance of the glass-ceramics. However, it still maintains high optical transparency in the visible-light range. Under the excitation of 281 and 355 nm ultraviolet light, the transparent glass-ceramics embedding ZnGa2O4 nanocrystals show wide emission bands. Moreover, with the increase of heat-treatment temperatures, the CIE chromaticity coordinates of the glass-ceramics gradually move from the green-light region towards the blue-light region, which indicates that the transparent ZnGa2O4 glass-ceramics have potential applications in blue-green laser and color display.

Structure and Luminescence Properties of Transparent Germanate Glass-Ceramics Co-Doped with Ni2+/Er3+ for Near-Infrared Optical Fiber Application.

An investigation of the structural and luminescent properties of the transparent germanate glass-ceramics co-doped with Ni2+/Er3+ for near-infrared optical fiber applications was presented. Modification of germanate glasses with 10–20 ZnO (mol.%) was focused to propose the additional heat treatment process controlled at 650 °C to obtain transparent glass-ceramics. The formation of 11 nm ZnGa2O4 nanocrystals was confirmed by the X-ray diffraction (XRD) method. It followed the glass network changes analyzed in detail (MIR—Mid Infrared spectroscopy) with an increasing heating time of precursor glass. The broadband 1000–1650 nm luminescence (λexc = 808 nm) was obtained as a result of Ni2+: 3T2(3F) → 3A2(3F) octahedral Ni2+ ions and Er3+: 4I13/2 → 4I15/2 radiative transitions and energy transfer from Ni2+ to Er3+ with the efficiency of 19%. Elaborated glass–nanocrystalline material is a very promising candidate for use as a core of broadband luminescence optical fibers.

Highly stabile ZnGa2O4:Eu nanocrystals as a fluorescence probe for bio-imaging

Acetylacetonate (acac) passivated and europium incorporated ZnGa2O4 nanocrystals (ZGO:Eu NCs) were successfully prepared via a facile one-step non-injection process in oleylamine at a relatively low reaction temperature of 300 °C under N2 ambient. The resultant ZGO:Eu NCs exhibited a well-dispersed morphology with an average particle size of 4 nm. The enhanced and broad absorption band from 250 to 450 nm is attributed to the synergistic effect between the acac as an antenna and the coexistence of Eu3+ and Eu2+. Under 405 nm excitation, a full spectrum emission covering from 400 to 720 nm was observed. The sharp red emission coming from Eu(III) 5D0 → 7FJ (J = 1–4) transitions can be enhanced by adjusting Eu concentrations and reaches the optimum at 20 at% Eu. Subsequently, the as-prepared NCs were transferred into an aqueous media via a ligand exchange by polyethyleneimine (PEI) to obtain the hydrophilic ZGO:Eu@PEI NCs. The initial fluorescent properties are preserved well. Finally, the strong red fluorescence emission, suitable excitation window, high photo-stability, excellent water solubility, and prominent bio-compatibility make ZGO:Eu@PEI NCs as high quality bio-imaging probes successfully identify living KB cells. The method mentioned in this paper overcomes the bottle-neck of the inorganic bulk phosphors as probes in bio-imaging, and as a result, expands their applications in optical detection and medical diagnosis fields.

Persistent Luminescence of ZnGa2O4:Cr3+ Transparent Glass Ceramics: Effects of Excitation Wavelength and Excitation Power

Highly transparent glass ceramics, synthesized from a glass precursor with a nominal composition of SiO2/ZnO/Ga2O3/Na2O/Cr2O3 (65:17:23:5:0.125), were prepared by glass crystallization. This method enables the synthesis of up to 50 wt.‐% of size‐controllable Cr3+‐doped ZnGa2O4 nanocrystals embedded in an SiO2‐rich glass matrix after crystallization at 900–1000 °C for 10 min. Rietveld refinements and transmission electron microscopy imaging showed that the nanoparticle sizes are ca. 16 and 33 nm for the glass ceramics annealed at 900 and 1000 °C, respectively. Photoluminescence measurements of these glass ceramics revealed intense deep red (λ = 695 nm) persistent emission that is detectable for up to 10 h, similar to that previously observed for ZnGa2O4 powder. As the glass ceramics exhibit good transparency, the persistent glow arises from the entire sample volume, which is of interest for outdoor applications and for more detailed studies of the mechanism for the persistent luminescence. Thermoluminescence measurements showed a wide distribution of trap depths centered at ca. 325 K. The traps can be charged not only under UV light but also under visible‐light excitation from λ = 365 to 625 nm. Electron paramagnetic resonance spectroscopy measurements of the Cr3+ ions showed clearly that the ZnGa2O4 nanocrystals in the glass ceramics exhibit the same types of defects and disorder as those in microcrystalline ZnGa2O4 powder, and this suggests that antisite defects are the dominant traps responsible for the persistent luminescence. It is proposed that the ability to charge ZnGa2O4 particles with visible light can be attributed to the combination of a two‐photon absorption and a local electric field owing to antisite defects in the vicinities of some of the Cr3+ ions.

Control of Elemental Distribution in the Nanoscale Solid-State Reaction That Produces (Ga1-xZnx)(N1-xOx) Nanocrystals.

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga1-xZnx)(N1-xOx) nanocrystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa2O4 NCs. (Ga1-xZnx)(N1-xOx) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga1-xZnx)(N1-xOx) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (i.e., x values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (Ga1-xZnx)(N1-xOx) NCs in the solid state that involves phase transformation of cubic spinel ZnGa2O4 to wurtzite (Ga1-xZnx)(N1-xOx) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga1-xZnx)(N1-xOx) NCs varies with composition and elemental distribution.

Super broadband near-infrared luminescence in Ni2 +-Er3 + co-doped transparent glass ceramics

Ni2+-Er3+ co-doped glass ceramics containing ZnGa2O4 nanocrystals were fabricated by in situ controlled crystallization method. The adverse energy transfer between Ni2+ and Er3+ could be suppressed in the ZnGa2O4 glass ceramics. And the super-broadband infrared luminescence with full width at half maximum (FWHM) of about 400nm covering 1.1–1.7μm wavelength region was observed in Ni2+-Er3+ co-doped glass ceramics. The two lifetimes of 1NE-GC950 revealed that two luminescence mechanism involved in the luminescence process, Ni2+: 3T2g (3F)→3A2g (3F) and Er3+: 4I13/2→4I15/2 respectively. Compared to that in the Ni2+/Er3+ co-doped glass ceramic containing γ-Ga2O3 and LaF3 nanocrystals, the nanocrystals in the present paper is just only one, ZnGa2O4 nanocrystals, and it could also present super-broadband luminescence.

Cr3+-doped gallium-based transparent bulk glass ceramics for optical temperature sensing

A series of Cr3+-doped transparent bulk glass ceramics embedded with LiGa5O8, Ga2O3 or ZnGa2O4 nanocrystals were fabricated as the alternatives for Cr3+-doped single-crystals to explore their possible application in fluorescence lifetime based temperature sensing. X-ray diffraction, transmission electron microscopy and optical spectroscopy were adopted to investigate their microstructure and luminescent behaviors. Impressively, the decay lifetimes of these glass ceramic samples were found to be highly temperature-sensitive, attributing to the competition of radiation transitions from the thermally coupled Cr3+2E state with a long lifetime and 4T2 one with a short lifetime. Finally, such temperature-dependent luminescence of Cr3+ was successfully interpret by a two-level kinetic model, which gives a highest temperature sensitivity of 0.76%K−1 (at 295K), 0.46%K−1 (at 370K) and 0.58%K−1 (at 420K) for the Cr3+-doped LiGa5O8, Ga2O3 and ZnGa2O4 glass ceramics, respectively.

Rapid Synthesis of Nanocrystalline ZnGa2O4 Phosphor at Low Temperature

ABSTRACTA novel microwave-assisted synthesis technique was used for the rapid preparation of nanocrystalline ZnGa2O4 at two different temperatures. The crystalline spinel oxide is formed at temperatures as low as 100 oC within few minutes, at a high yield of 96%, requiring no post-synthesis annealing. The as-prepared samples are polycrystalline and phase-pure as verified by XRD, with a crystallite size of ∼5 nm. Polycrystalline ZnGa2O4 substituted with Mn2+, Cr3+, Cu2+, and Co2+ was also similarly prepared. All samples are highly monodispersed, as measured by TEM. The ZnGa2O4 nanocrystals without further surface modification can be readily dispersed in chloroform to form a fully transparent colloidal solution, using which the bandgap of ZnGa2O4 was determined to be ~4.5 eV. The entire synthesis procedure, including solution preparation, microwave irradiation, and centrifugation takes about 30 minutes, which is faster than any procedure reported for a complex oxide like ZnGa2O4, as well as one with a small thermal budget. Photoluminescence shows a broad emission extending from 330 nm to 800 nm, which is surmised to be due to the defect structure in the oxide produced.

ZnxGa2O3+x(0 ≤ x ≤ 1) solid solution nanocrystals: tunable composition and optical properties

Homogenous ZnxGa2O3+x (0 ≤ x ≤ 1) solid solution nanocrystals were successfully prepared by a solvothermal approach and characterized by X-ray powder diffraction (XRD), X-ray spectra in fluorescence (XRF), transmission electron microscopy (TEM), UV-vis spectroscopy, and X-ray photoelectron spectrum (XPS). XRD pattern analysis revealed that the lattice parameters of ZnxGa2O3+x (0 ≤ x ≤ 1) solid solution nanocrystals increased linearly with the increase of ZnGa2O4 ratio according to Vegard's law, which confirmed the homogenous structures of these solid solution nanocrystals. UV-vis absorption spectra demonstrated that the band gap of the obtained ZnxGa2O3+x solid solution nanocrystals could be precisely tuned in the range of 4.43 to 3.70 eV by elaborately controlling the content of zinc. The emission of ZnxGa2O3+x solid solution nanocrystals strengthened and red-shifted gradually with the increase of x value, and ZnGa2O4 nanocrystals showed an intense blue emission. XPS results revealed that ZnxGa2O3+x binding energy of O2− 1s increased with the increase of the x value, indicating the decrease of the oxygen vacancies (Vo*) of Oh sites and the increase of the symmetry of the Ga–O octahedron, which resulted in the emission red-shift of ZnxGa2O3+x solid solution.

Nonaqueous Synthesis of Colloidal ZnGa2O4Nanocrystals and Their Photoluminescence Properties

Colloidal ZnGa2O4 nanocrystals with a mean size of about 7 nm were successfully synthesized by using a non-aqueous sol–gel method.The nanocrystals can easily be dispersed in chloroform to form stable colloids without the use of any additional stabilizers. Strong blue fluorescence was observed from the clear colloidal solution of the nanocrystals under UV light.

Synthesis and optical properties of transparentZnO–Ga2O3–SiO2 glass-ceramics embedded with cobalt-doped nanocrystals

Transparent ZnO–Ga2O3–SiO2 glass-ceramics embedded with Co2+:ZnGa2O4 nanocrystals were fabricated by the sol–gel method. X-ray diffraction, infrared spectroscopyand high-resolution transmission electron microscopy showed that the resultingmaterial is composed of an amorphous silicate network and single-crystallineZnGa2O4 particles. Analysis of the absorption and luminescence properties indicates that Coions are trapped in the crystalline phase and located in the tetrahedral sites inZnGa2O4 nanocrystals. Thecrystal field parameter Dq of 418 cm−1 and the Racahparameters B of 761 cm−1 andC of3424 cm−1 were determinedfor tetrahedral Co2+ ions.