Persistent luminescent and photocatalytic properties of Zn_xGa₂O_3+x (0.8 ≤ x ≤ 1) phosphors

Abstract

The Zn_xGa₂O_3+x (x = 1, 0.95, 0.9, 0.85, 0.8) phosphors were synthesised successfully via high temperature solid-state reaction. The photoluminescent, persistent luminescent and photocatalytic properties of the phosphors had been studied systematically. The results indicated that their excitation and emission spectra were similar to those of ZnGa₂O₄ phosphors and all of them have excellent persistent luminescent and photocatalytic properties. The optical properties were changed with the ratio of the Zn²⁺/Ga³⁺ and the Zn_0.85Ga₂O_3.85 phosphor showed the best persistent luminescent and photocatalytic properties. The Zn_0.85Ga₂O_3.85 phosphor can be effectively activated by an ultraviolet lamp and ultraviolet excitation can lead to 10 min of persistent green emission. Moreover, the Zn_0.85Ga₂O_3.85 could provide more defect energy levels which can act as photogenerated electron traps and maintain the electron–hole pairs for a longer period, enhancing the persistent luminescent and photocatalytic performance.

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1. Introduction

Persistent luminescence is an optical phenomenon, whereby a material is excited with radiation and resulting in luminescent emission remains visible for a long time (seconds to hours) after the excitation has stopped.^1–3 These materials are drawing more and more attention in recent years due to the great potential applications in many fields, such as emergency signage, traffic signs and in vivo bio-imaging.

Recently, zinc gallate (ZnGa₂O₄) phosphor has attracted much attention for use in field emission display (FED) and thin film electroluminescence device (TFED), because ZnGa₂O₄ phosphor shows higher chemical and thermal stability than sulfide phosphors.^1,2 ZnGa₂O₄ crystallizes in the normal spinel structure (Fd3m) with Ga³⁺ ions occupying octahedral sites and Zn²⁺ ions occupying tetrahedral sites. Its band gap is about 4.4 eV, and it exhibits a strong blue emission for material not doped with impurities.² ZnGa₂O₄ can also act as a host material for multicolor emitting phosphor when doped with transition metals: Cr-doped ZnGa₂O₄ (ZnGa₂O₄:Cr³⁺) for red emission³ and Co-doped ZnGa₂O₄ (ZnGa₂O₄:Co³⁺) for reddish orange emission.⁴ In addition, ZnGa₂O₄ has excellent performance in air-pollution control. Because ZnGa₂O₄ has hybridized orbitals of Zn4s4p, Ga4s4p and the wide band gap (4.4 eV), it can improve the mobility of photogenerated electrons and the absorption efficiency in ultraviolet (UV) lamps.⁵ Some papers reported that high temperature reaction triggered the vaporization of zinc ions and formed intrinsic defect in the hosts.^4,5 Thus, ZnGa₂O₄ exhibits the persistent luminescent characteristics. However, the influence of zinc deficiency on the persistent luminescent and photocatalytic performance of ZnGa₂O₄ phosphor is rarely reported.

In this study, we synthesised the ZnGa₂O₄ phosphor with slightly Zn-deficient composition successfully and investigated their effects on the luminescent, persistent luminescent and photocatalytic properties. The mechanism for enhancement of persistent luminescent and photocatalytic properties was also discussed.

2. Experimental

The ZnGa₂O₄ phosphors were synthesised via high temperature solid-state reaction method. ZnO powder (99.9%), Ga₂O₃ powder (99.9%) were milled for about 30 min in an agate mortar. The stoichiometric proportions of raw materials were weighed according to the nominal compositions of ZnGa₂O₄ (S₁), Zn_0.95Ga₂O_3.95 (S₂), Zn_0.9Ga₂O_3.9 (S₃), Zn_0.85Ga₂O_3.85 (S₄), Zn_0.8Ga₂O_3.8 (S₅), respectively. Then the mixtures were moved into a corundum crucible and sintered at 1300 °C for 5 hours in an air ambient.

The X-ray powder diffraction (XRD) verified the phase structure of all samples by using a diffractometer with Cu Kα irradiation (λ = 1.5406 Å) at 36 KV tube voltage and 20 mA tube current. We set the scanning range from 10° to 70°. The excitation, emission spectra and afterglow decay were measured by a Hitachi F-7000 Florescence Spectrophotometer. Prior to the afterglow decay measurements, the samples were excited by an UV lamp (λ_ex = 254 nm) for 5 min. Then measurement was in a dark place without excitation. The thermoluminescent (TL) curves were recorded by a FJ427A1 thermoluminescent dosimeter. The heating rate was 1 °C s⁻¹ for all samples. Prior to the TL measurement, each sample was first exposed to an UV lamp (λ_ex = 254 nm) for 2 min and then put in dark waiting for 3 min.

The photocatalytic degradation tests were carried out in photoreactor (BL-GHX-V), using 50 mg S₁, S₂, S₃, S₄ and S₅ powder dispersed in 200 mL Rhodamine B (RhB) aqueous solution (10 mg L⁻¹), respectively. The mixture aqueous solution was put into a beaker with magnetic stirring for 30 min. The light source was a 500 W UV lamp and continuous magnetic stirring should be maintained the suspension of powder in the RhB solution. Then 5 mL of the suspension was gathered for every 10 min and measured via an Ultraviolet-visible Light Spectrometer by measuring its absorbance at a wavelength of 550 nm.

3. Results and discussion

The XRD patterns of five samples are shown in Fig. 1. The diffraction peaks of S₁ can be indexed as a pure cubic spinel (space group Fd3m) phase ZnGa₂O₄, and all diffraction peaks are in good agreement with the standard data of ZnGa₂O₄. For S₄ and S₅, extra peaks are appeared in addition to the peaks of ZnGa₂O₄, which correspond to the phase of β-Ga₂O_3. It reveals that the obtained products are the mixture of ZnGa₂O₄, β-Ga₂O₃ and Zn_xGa₂O_3+x. Compared with the standard data of JCPDS no. 38-1240, there is no apparent shift can be found in all samples.

Fig. 1 XRD patterns of all samples.

As a kind of self-activated phosphor, ZnGa₂O₄ emits blue emission. Fig. 2a shows the excitation and emission spectra of Zn_xGa₂O_3+x (x = 1, 0.95, 0.9, 0.85, 0.8) phosphors. The excitation spectrum monitored at 430 nm shows a broad band from 230 nm to 280 nm with the peak position at 254 nm, which can be attributed to the charge transfer of Ga–O.² Under excitation with 254 nm UV, the emission spectrum of all samples consists of a broad band ranging from 390 nm to 520 nm with a maximum at 430 nm, which is originated from the self-activation center of the octahedral Ga–O group in the ZnGa₂O₄ lattices.^2,5 Moreover, there is a weak emission peak at 500 nm which can be attributed to the ²E_A → ⁴A₂ transition of Ga³⁺ in the distorted octahedral.^2,5 The red shift phenomenon in the emission with the decrease of Zn²⁺ concentration is observed. Because the ionic radii of the Ga³⁺ is greater than Zn²⁺, the radii of the ions are as follows: R(Zn²⁺) = 0.060 nm, R(Ga³⁺) = 0.062 nm.⁶ The Zn²⁺ substituted by the Ga³⁺ increases the lattice constant of ZnGa₂O₄. Thus, the peak of emission shifts toward long wavelength. The decay curves of five samples recorded within 0–600 s are presented in Fig. 2b. As can be seen, all samples exhibit similar decay processes which contain a rapid decay at beginning and a slow decay process afterward.^7,8 The sequence of the initial intensity for different Zn²⁺ concentration is x = 0.85 > x = 0.9 > x = 0.8 > x = 0.95 > x = 1.0. In our study, the decay curves can be well fitted by a double-exponential equation and as follows:

where I(t) is phosphorescence intensity; I₁ and I₂ are constants which represent the rapid and slow initial luminescent intensity at t = 0 s; τ₁ and τ₂ are the decay constants, which decide rapid decay and slow decay components, respectively, and the decay duration depends on the second exponential.⁹ According to the above formulas, the fitting results are listed in Fig. 2c. These parameters indicate that Zn_0.85Ga₂O_3.85 is the optimal value to produce the persistent luminescence, and the duration is about 10 min (>0.35 mcd m⁻²). The persistent luminescence of five samples is visually evaluated using a fluorescence microscopy in the dark environment. The inset of Fig. 2b shows the changes of emission “brightness” with a decay time up to 10 min after exposure to an UV lamp (λ_ex = 254 nm) for 5 min. It clearly shows that S₄ can be effectively activated by an UV lamp and UV excitation can lead to 10 min persistent green emission.

Fig. 2 (a) Excitation and emission spectra of all samples; (b) persistent luminescent decay curve after irradiation by 254 nm and images of all samples taken at different persistent luminescence times; (c) the fitting results of decay curves. The phosphors were irradiated for 5 min.

As already known, thermoluminescence (TL) measurement provides an efficient way to reveal the information of traps. In general, the trap concentration is approximately proportional to the TL intensity and the maximum temperature of the TL peak provides information on the depth of the carrier trap,^9–11 and the optimal TL peak is usually located at the temperature range of 50–120 °C for the excellent persistent luminescent properties.^9,11 As shown in Fig. 3, S₁ shows a quite weak TL band centered at 75 °C which confirms the poor persistent luminescence. The TL curves of S₂, S₃, S₄ and S₅ have the same peak location and similar shape to that of S₁, but their intensity are largely enhanced, and it indicates a higher defect concentration. It is well known that the ZnO is volatilized during ZnGa₂O₄ synthesis due to the high vapor pressure. One vacancy defect of with 2 positive charges and one vacancy defect of with 2 negative charges would be created in the host. Then the Ga³⁺ ions would occupy the defect of and from Ga_Zn⁺, which act as the electron trap. And the defect of would transform into the and act as the hole trap. Moreover, more Ga³⁺ ions occupy the sites with Zn²⁺ deficient, which will raise the concentration of the Ga_Zn⁺ (electron trap). Therefore, the persistent luminescence enhances firstly. However, the distance of Ga–Ga would be shortened as the concentration of Ga_Zn⁺ continues to raise with Zn²⁺ deficient, the concentration quenching occurs. Thus, the persistent luminescence begins to decrease.

Fig. 3 TL curves of all samples.

Fig. 4 shows photocatalytic activity as a function of irradiation time with different Zn²⁺ ion concentration. It can be seen that with the decrease of Zn²⁺ concentration, the photocatalytic activities of Zn_xGa₂O_3+x (x = 1, 0.95, 0.9, 0.85, 0.8) first increase and then decrease. The Zn_0.85Ga₂O_3.85 phosphor exhibits the highest photocatalytic activity, and the decomposition rate for the S₄/Rhodamine B (RhB) composites reached 98.5% after 90 min. As we known, the high density of surface hydroxyls, proper band positions, broad hybridized orbitals, and large surface area all would improve the high photocatalytic activity of ZnGa₂O₄.¹² And ZnGa₂O₄ could provide more defect energy levels which can act as photogenerated electrons trap and maintain the electron–hole pairs for a while, enhancing the photocatalytic performance.^13,14 In our case, a large numbers of oxygen vacancies have been found in ZnGa₂O₄ phosphors as evident from TL results. The oxygen vacancies act as an electron traps, which implies a decreased recombination rate of the electrons and holes. The large numbers of oxygen vacancies generate more hydroxyl radicals on the surface, and the strong redox ability of photogenerated electron–hole pairs may also improve the photocatalytic performance. The UV-Vis diffuse reflectance spectra of Zn_xGa₂O_3+x are displayed in Fig. 5a. And the band gap is determined based on the UV-Vis spectra and is displayed in Fig. 5b. It can be seen that with Zn²⁺ ion concentration decreases, E_g (band gap energy) of Zn_xGa₂O_3+x decreases, increasing higher UV utility efficiency. Thus, photocatalytic activity is improved. However, the reducing of E_g would make E_VB (valence bands) and E_CB (conduction band) of Zn_xGa₂O_3+x phosphors shift to the Fermi level and decrease the redox ability of photogenerated electron–hole pairs.^15,16 Thus, further decrease of Zn²⁺ concentration may lead to decrease in photocatalytic performance of ZnGa₂O₄.

Fig. 4 Degradation of RhB in the presence of all samples. The light source was a 500 W UV lamp.

Fig. 5 (a) The UV-Vis diffuse reflectance spectra; (b) optical band gap E_g of all samples.

4. Conclusion

In summary, the Zn_xGa₂O_3+x (x = 1, 0.95, 0.9, 0.85, 0.8) phosphors were synthesised via high temperature solid-state reaction successfully. The influence of Zn²⁺ ions concentration on PL, decay curves, TL and photocatalytic activity were studied in detail. The emission intensity decreases and the peak of emission shifts toward long wavelength with the decrease of Zn²⁺ ion concentration. The Zn_0.85Ga₂O_3.85 phosphor can be effectively activated by an UV lamp and the UV excitation can lead to 10 min of persistent green emission. According to the analysis of TL curves, it can be supposed that more Ga³⁺ ions occupy the sites with Zn²⁺ deficient, which will raise the concentration of electron trap. However, the distance of Ga–Ga would be shortened as the concentration of Ga_Zn⁺ continues to raise, the concentration quenching occurs. Studies of the ZnGa₂O₄ phosphor photocatalytic activity have indicated that with Zn²⁺ ion concentration decreases, E_g (band gap energy) of Zn_xGa₂O_3+x decreases, increasing higher UV utility efficiency. And photocatalytic activity is improved. But the reducing of E_g would make E_VB and E_CB of Zn_xGa₂O_3+x phosphors shift to the Fermi level and decrease the redox ability of photogenerated electron–hole pairs. Therefore, photocatalytic performance first increases and then decreases at the same condition.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (no. 21271048) and State Key Laboratory of Rare Earth Resource Utilization Open Projects (RERU2013007).

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