Controlled Hydrothermal Synthesis and Photoluminescence of Nanocrystalline ZnGa2O4:Cr3+ Monospheres
© The Author(s). 2017
Received: 7 February 2017
Accepted: 13 March 2017
Published: 23 March 2017
The hydrothermal synthesis of nanocrystalline ZnGa2O4:Cr3+ (ZGC) red phosphor monospheres was accomplished in this work, and the effects of system pH, reactant content, reaction time, and citrate anions (Cit3−) on the phase and morphology evolution of the product were systematically studied. Under the optimized conditions of Cit3−/M = 1.0 molar ratio (M = total cations), pH = 5.0, and 0.2 mmol of Zn2+, well-dispersed ZGC monospheres with an average diameter of ~454 ± 56 nm (average crystallite size ~15 nm) were successfully obtained via hydrothermal reaction at 180 °C for 18 h. Cit3+ ions were demonstrated to be crucial to the formation of monospheres and substantially affect the pathway of phase formation. The ZGC monospheres calcined at 800 °C (average diameter ~353 ± 59 nm; average crystallite size ~30 nm) have an intensity ~6 times that of the original phosphor for the 700 nm red emission of Cr3+ (the 2E → 4A2 transition) under excitation with the O2− → Ga3+ charge transfer band at 250 nm. Fluorescence decay analysis found that the 700 nm emission has lifetime values of ~5 ms for the ZGC phosphors.
The zinc gallate compound of ZnGa2O4 belongs to the group of cubic-structured AB2O4 normal spinels (space group: Fd-3m), in which the Zn2+ ions occupy the tetrahedrally coordinated A sites and the Ga3+ ions reside at the octahedrally coordinated B sites. The compound has been drawing increasing attention for wide applications in the fields of lighting, display, and optical imaging for biology, owing to its excellent thermal and chemical stability and wide bandgap (~4.4–4.7 eV) . ZnGa2O4 is also known as a type of self-activated phosphors and may emit blue light under short UV or electron beam irradiation, owing to the occurrence of O-Ga charge transfer . As a phosphor host, the Mn2+, Eu3+, and Cr3+ activator ions doped into the ZnGa2O4 lattice and residing at the Ga3+ sites are known to emit bright green, red, and red luminescence under proper excitations, respectively . It is also worth noting that the transition metal ion of Cr3+ may emit near-infrared persistent luminescence when the chemical composition and lattice defects of ZnGa2O4 are properly manipulated, which allows the material to have potential applications in the optical imaging of vascularization, tumor, and grafted cells [3–5]. It is widely accepted that phosphor particles with a spherical shape may exhibit superior luminescence and have advantages in practical application over other morphologies, owing to the fact that the spherical shape may minimize the light scattering on particle surfaces and a denser luminescence layer can be constructed via close packing of the spheres [6, 7]. For these, developing a technique to synthesize Cr3+-doped ZnGa2O4 (ZnGa2O4:Cr) phosphor spheres is of practical importance. Various synthetic approaches have been established up to date for ZnGa2O4-based phosphors, typically including solid state reaction, thermal evaporation of ZnO-Ga powders, pulverizing single crystals grown by the flux method, sol-gel, electrospin, hydrothermal reaction, and chemical precipitation [8–13]. Morphology control of the product, however, yet remains an issue needed to address. We introduced in this work a hydrothermal strategy to produce well-defined ZnGa2O4:Cr3+ monospheres, and the effects of citrate (Cit3−) anions, system pH, and reactant content on the phase structure and morphology evolution were demonstrated in detail. In the following sections, we report the synthesis and photoluminescence properties of the nanostructured ZnGa2O4:Cr3+ monospheres.
The stock solutions of Cr3+ (0.002 M) and Zn2+ (0.1 M) were obtained by dissolving the corresponding metal nitrates in distilled water, and the Ga3+ solution (0.2 M) was prepared by dissolving Ga2O3 in nitric acid (HNO3) via hydrothermal treatment at 100 °C. Proper amounts of the above solutions were then mixed together according to the intended chemical formula of Zn (Ga1.995Cr0.005) O4. Whenever needed, a certain amount of trisodium citrate (Cit3−) was added into the solution, followed by dilution with distilled water to a total volume of 75 mL. Under magnetic stirring, a proper amount of HNO3 (63 wt%) or ammonium hydroxide solution (NH4OH, 28 wt%) was then added to adjust the mixture to a certain pH value. After homogenizing for 30 min, the as-obtained mixture was transferred to a Teflon-lined stainless steel autoclave, which was then put into an air oven preheated to 180 °C for a certain period of hydrothermal reaction. After natural cooling to room temperature, the hydrothermal product was collected via centrifugation and washed three times with deionized water and once with ethanol, followed by drying in an air oven at 60 °C for 12 h. Calcination of the hydrothermal product was performed in the air at 800 °C for 2 h. The hydrothermal product will hereafter be referred to as nZGC, where n is the amount of Zn2+ (in mmol) in the hydrothermal reaction system for the synthesis of Zn (Ga1.995Cr0.005) O4 phosphors.
Phase identification was made via X-ray diffractometry (XRD, Model PW3040/60, Philips, Eindhoven, The Netherlands) operated at 40 kV/40 mA, using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm) and a scanning rate of 5°/min in the 2θ range of 10°–70°. The morphology and microstructure of the products were analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo, Japan) under an acceleration voltage of 15 kV. Thermogravimetry of the sample was made in the air on a Model Thermo Plus TG8120 equipment (Rigaku, Tokyo), using a heating rate of 10 °C/min. Fourier transform infrared spectroscopy (FT-IR, Spectrum RXI, PerkinElmer, Shelton, CT, USA) was performed by the standard KBr method. Photoluminescence properties of the phosphors, including excitation, emission, and fluorescence decay, were measured at room temperature using an LS-55 fluorospectrophotometer (PerkinElmer).
Results and Discussion
Samples Synthesized Without Citrate Anions
Optimization of the Synthesis Parameters to Yield ZGC Monospheres
Nanocrystalline ZnGa2O4:Cr3+ (ZGC) monospheres were synthesized in this work via hydrothermal reaction at 180 °C and in the presence of Cit3− ions, which emit red emission at 700 nm (the 2E → 4A2 transition of Cr3+) upon short UV excitation with the O2− → Ga3+ charge transfer band at 250 nm. The optimal processing parameters were determined to be Cit3−/M = 1.0 molar ratio (M = total cations), pH = 5.0, 0.2 mmol of Zn2+, and a reaction time of 18 h. Calcining the as-synthesized ZGC monospheres at 800 °C for 2 h brought about an ~6-fold intensity increment for the 700 nm emission, owing to dehydration, removal of organic residues, and crystallinity improvement. The phosphor monospheres were analyzed to have lifetime values of ~5 ms for the 700 nm red emission.
This work was financially supported by the National Training Program of Innovation and Entrepreneurship for undergraduates (201610145430).
TL, JHL, and XXY carried out the experiments; JGL and TL were involved in the results discussion and drafted the manuscript. All the authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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