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Ga2O3:Cr3+, In3+ phosphors, synthesized via a high temperature solid state reaction, exhibit photocatalytic activity and persistent luminescence. With substituting In3+ for Ga3+ in Ga2O3:Cr3+, the duration of near-infrared (NIR) persistent luminescence was prolonged obviously at room temperature under 254 nm ultraviolet (UV) excitation and the photocatalytic activity was highly improved. The emission and excitation spectra indicated that In3+ doping has no obvious effect on peak positions of Ga2O3:Cr3+. The thermoluminescence (TL) curves showed that a new suitable trap was introduced into Ga2O3:Cr3+ by In3+ doping. It was considered that photocatalytic activity and persistent luminescence properties are highly associated. What’s more, the new trap plays an important role for capturing photo-generated electrons or holes, which is highly responsible for the high separation of photo-generated electron-hole pairs and could improve the persistent luminescence and photocatalytic properties of Ga2O3:Cr3+ effectively.
© 2016 Optical Society of America
1. Introduction
β-Ga2O3 is a promising III semiconductor, which exhibits a wide band gap (Eg = 4.9 eV), has excellent thermal and chemical stability, unique conduction and tunable optical properties [1,2]. The β-Ga2O3 single crystal belongs to the monoclinic system (C2/m), and the lattice parameters are a = 12.214Å, b = 3.0371Å, c = 5.7981Å, β = 103.83° [3]. Due to the outstanding luminescence properties, β-Ga2O3 has attracted considerable attention for its application in flat panel displays, solar energy conversion devices, UV emitters, high temperature stable gas sensors, emergency lighting, imaging storage and identification taggants, etc [4–8]. It can radiate various photoluminescence spectra from visible to NIR light when doped with different impurities [9–11] Recently, NIR persistent luminescence phosphors have been widely researched as the vast potential for future development in real-time optical imaging [8,12], because the NIR emission possess maximum radiation penetration through tissue and can minimize background autofluorescence [13]. Among those, trivalent chromium ion (Cr3+) activated gallium oxide phosphors have become widely reported because of their excellent chemical stability, excellent ability of Cr3+ ions to substitute for Ga3+ ions in distorted octahedral sites, suitable host crystal field strength around Cr3+ ions for intense NIR persistent luminescence and suitable emission range [14–16]. Meanwhile, β-Ga2O3 is also an environmental friendly material. Hou et al. and Girija et al. recently studied the photocatalytic decomposition of benzene by β-Ga2O3 powder under ambient conditions and found that it exhibits high and stable photocatalytic activity than commercial TiO2 [7, 17]. The band gap of β-Ga2O3 (4.9 eV) is wider than that of TiO2 (3.2 eV), and the position of its conduction band (ECB = −2.95 eV) is higher than that of TiO2 (ECB = −4.21 eV) relative to the vacuum energy level [18]. Thus, theoretically, photo-generated electrons in the conductive band of β-Ga2O3 have much higher reductive capability than that of TiO2 [19]. Basically, photo-generated electrons/holes played an important role in photocatalysis of the semiconductors [20, 21]. Upon absorption of UV light that have energy equal to or higher than its band gap, amounts of electrons (e-) and holes (h+) are generated and then migrated to the surface of the catalyst to undergo redox reaction with the adsorbed reactants. The high oxidative potential of the holes leads to the oxidation of the toxic organic compounds at the surface and the surrounding environment of the catalyst to non-toxic substances [22]. Therefore, photocatalysis is a promising approach for the degradation of toxic organic compounds with high efficiency, ease of operation, and cost effectiveness [23–26].
To further improve the photocatalytic activity of β-Ga2O3, various modifications such as metal doping, morphology controlling and semiconductor coupling have been used [27–29]. In our previous studies, Zn2+ ions co-doping in Ga2O3:Cr3+0.01 could prolong the persistent luminescence and photocatalytic properties [30]. However, it is not equivalent for Zn2+ ions substituting Ga3+ ions. In this paper, In3+ ions were selected to dope in Ga2O3:Cr3+0.01 to study the persistent luminescence and photocatalytic properties for an equivalent doping. It is interesting to research the relationship between the persistent luminescence and photocatalytic properties because trap centers in phosphor materials could affect the separation and lifetime of photo-generated electrons and holes, and thus affect the photocatalytic properties [31]. what’s more, it is a promising way to develop the photocatalytic activity of photocatalysts via controlling the luminescence properties.
2. Experimental
2.1 Materials
The samples of Ga1.94O3: Cr0.01, In0.05 (S1) and Ga1.99O3: Cr0.01 (S2) were synthesized via the high temperature solid-state reaction method. Different stoichiometric amount of Ga2O3 (99.99%), In2O3 (99.99%) and Cr2O3 (99%) powders were weighed as raw powders. The raw powders were mixed for about 1 h in an agate mortar to form a homogeneous fine powder for sintering. Then the mixed powders were moved into corundum crucibles and sintered at 1300 °C for 4 h in the air atmosphere. When the samples were cooled to room temperature, pestle them again respectively with the agate mortar.
2.2 Characterization
The crystal structure of the phosphors was analyzed by X-ray powder diffraction (XRD) using Cu Kα (λ = 1.5418 Å) irradiation operating at 36 kV and 20 mA. Data were collected between 10° and 70° (2θ) at room temperature with a 0.02° step size. The emission, excitation spectra and persistent luminescence decay curves of the samples were recorded at room temperature using a Hitachi F-7000 fluorescence spectrophotometer. Appropriate optical filters were used to avoid interference from the exciting light among spectral measurements. The exciting light was generated from an xenon lamp which was equipped in the fluorescence spectrophotometer. A FJ27A1 dosimeter was used to record TL curves with the heating rate of 1 °C/s after the samples were exposed to radiation from a UV lamp for 5 min and placed in dark room for 3 min.
2.3 Photocatalytic activity test
The photocatalytic activities were characterized by the photocatalytic degradation of Rhodamine B (RhB). A 500 W mercury lamp with 5 A operating current was used as light source, the mercury lamp was positioned in a cylindrical Pyrex vessel and cooled by circulating water to control the reaction temperature at about 27 °C when irradiation was performed. The 0.01 g of S1 and S2 powder were dispersed in two glass tubes with 40 ml RhB aqueous solution (4 × 10−5 mol/l). Then the suspensions were placed in the dark for 30 min to achieve an adsorption/desorption equilibrium between the photocatalyst and RhB aqueous solution, during which vigorous magnetic stirring was maintained to keep S1 and S2 particles suspended in the RhB aqueous solution. After that, about 5 mL suspensions were taken out and centrifuged for 5 min at 3000 r/min to separate the suspended solid and then measure the absorbance of the centrifugal solution as the initial values. Then turn on the mercury lamp and make irradiation performed. In order to obtain the changes of the RhB concentration, the absorbance of the centrifugal solution was measured every 10 min at a wavelength of 552 nm, corresponding to the maximum absorption of RhB. After each measurement, the suspended solid and clear supernatant were poured back into glass tubes respectively to make sure there is always a comparable amount of S1 and S2 in the RhB aqueous solution. The percentage of degradation was recorded as C/C0, in which C is the absorbance of RhB solution at certain irradiated time intervals (10 min) and C0 is the initial value.
3. Results and discussion
The structure of the samples was studied by the XRD patterns. As shown in Fig. 1, all the diffraction peaks are consistent with the standard phase of β-Ga2O3 (JCPDS No.41-1103) and no characteristic peaks of the dopants have been observed, which indicates that the doping of 1% mol chromium and 5% mol indium have no significant influence on the crystal structure of β-Ga2O3.
Fig. 1 XRD patterns of the synthesized samples (S1: Ga1.94O3: Cr0.01, In0.05; S2: Ga1.99O3: Cr0.01) and the pattern of JCPDS No.41-1103 for β-Ga2O3.
Figure 2 shows the excitation and emission spectra of S1 and S2. In emission spectra, the emission at 690 nm is characteristic of Cr3+ ions and can be attributed to the spin-forbidden 2E-4A2 transition [32]. In excitation spectra, there are three main absorption bands and the band peaked at about 465 nm is the matrix absorption band. The broad band peaked at about 320 nm is the charge transfer band which is corresponding to electrons transfer from oxygen 2p orbital to empty 4s4p orbital of gallium. The bands peaked at about 465 nm and 580 nm are originating from the 4A2-T1 transition and 4A2-4T2 transition of Cr3+, respectively [8, 33, 34]. It can be seen from Fig. 2 that the substitution of In3+ causes no apparent changes in excitation and emission spectra.
Fig. 2 Emission (a) and excitation (b) spectra of S1: Ga1.94O3: Cr0.01, In0.05 and S2: Ga1.99O3: Cr0.01 phosphors at room temperature. The emission spectra were under 320 nm excitation and the excitation spectra were monitored at 690 nm.
Figure 3 shows the persistent luminescence decay curves of S1 and S2 phosphors monitored at 690 nm after excited by 254 nm light for 5min. The data were recorded as a function of persistent luminescence intensity (I) versus time (t) and the record lasted for 20 min. After 20 min, the persistent luminescence still can be seen by naked eye. As can be seen from Fig. 3, as-prepared samples exhibit similar decay processes and the persistent luminescence intensity of S1 and S2 phosphors decrease quickly in the first several minutes and then decay slowly. The persistent luminescence characteristics can be evaluated by the fitting curve, because the decay process of the phosphor contains the rapid decaying process and the slow decaying process [35,36]. The double exponential function is applied to fit into the decay curves in this paper. The formation of the function is as follows:
Fig. 3 The persistent luminescence decay curves of S1: Ga1.94O3: Cr0.01, In0.05 and S2: Ga1.99O3: Cr0.01 phosphors monitored at 690 nm after excited by the 254 nm light for 5 min. The fitting curves are shown as green empty circle and triangle, respectively.
Where t is the decay time; I is the intensity of persistent luminescence at t time; I0 is a constant from the offset; I1 and I2 are constants which depend on the rapid and slow initial luminescent intensity at t = 0; τ1 and τ2 are the time constant [37]. The fitting results are shown in Table 1. It can be seen that S2 shows a much better persistent luminescence properties than that of S1, indicating that the co-doping with In3+ ions can obviously prolong the afterglow duration of the sample.
Table 1. Fitted results for the persistent luminescence curves of S1 and S2.
Persistent luminescence is considered to be generated by the de-trapped charge carriers which recombine in the luminescence centers under thermal disturbance. Therefore, the TL curve is a good method to measure the trapping property of defects and trap levels of the persistent luminescence phosphors [38]. In Fig. 4, the TL curve of S1 can be divided into two peaks, one located at about 354 K and the other one located at about 391 K. In contrast with the TL curve of S2, the first peak at about 354 K of S1 can be considered as the same one as the peak of S2 which located at about 352 K. Therefore, the peak at 354 K comes from the lattice defects of S1 and the peak located at 391 K is induced by In3+ doping. To the best of our knowledge, the thermal activation energy E (eV) could be estimated by the formula of E = Tm (K)/500 [39]. So the trap depths of S1 can be calculated as 0.70 eV and 0.78 eV, respectively. As a kind of native defect material, Ga2O3 possess many lattice defects such as the oxygen vacancy and the gallium-oxygen vacancy pairs [40]. Although they are all equivalent for Cr3+ and In3+ to substitute Ga3+, In3+ doping may cause a higher periodic lattice distortion since the radius of In3+ (0.80 Å) is bigger than that of Ga3+ (0.62 Å) but the radius of Cr3+ (0.615 Å) is almost the same. Higher periodic lattice distortion could generate more defects in the host, In3+ substituting for Ga3+ could form InGa × which act as hole traps, since Ga3+ is more electronegative than In3+. It is easier for Ga3+ ions to get electrons than In3+ ions and it is easier for In3+ ions to get holes than Ga3+ ions [41]. InGa × traps could be related to the TL peak which located at about 391 K of S2. These new lattice distortion defects and InGa × traps will catch more photo-generated carriers to prolong the duration of persistent luminescence. That may be the cause of S2 shows a much better persistent luminescence properties than that of S1.
Fig. 4 TL curves of S1: Ga1.94O3: Cr0.01, In0.05 and S2: Ga1.99O3: Cr0.01 at the region of 310 K-450 K. The samples were irradiated for 5 min by a UV lamp and then placed in the dark room for 3 min before the measurement. The fitting curve of S1 was shown as the red dash line, which can be divided into two peaks and were shown as green and blue dash lines, respectively. The inset shows the TL curve of S2 for comparing.
To evaluate the photocatalytic activity of S1 and S2, photo-degradations of RhB were measured as a function of irradiation time shown in Fig. 5. It can be seen that S1 and S2 all showed obvious photocatalytic activity for RhB degradation. It needs 180 min of UV irradiation to decompose 90% of RhB with S2, while it costs only 70 min with S1. Thus, the photocatalytic activity of S1 is 2.57 times as that of S2, which indicated that the doping of In3+ can highly improve the photocatalytic activity of Ga2O3: Cr3+. It is widely accepted that the photo-generated carries play important roles in photocatalysis, the amount and lifetime of photo-generated electrons can highly influence the photocatalytic activity of photocatalyst. Under UV light irradiation, a mass of electrons and holes are created in the sample. Most of these electrons and holes would recombine at recombination centers immediately after the stoppage of excitation and a little amount would move to the surface of photocatalyst. The little amount of photo-generated carriers on the catalyst surface would react with adsorbed dissolve oxygen and water molecules to form active ·OH radicals and H2O2 which would be highly useful for the degradation of organics [42]. In the persistent luminescence phosphors, the recombination rate of electron-hole pairs could be decreased by the presence of traps which can maintain the photo-generated electrons/holes for a longer period and thus in favor of increasing photocatalytic activity. So the photocatalytic activity and persistent luminescence properties are highly associated [31]. Since a new hole trap is formed in Ga1.94O3: Cr3+0.01, In0.05, more holes would be captured than that of Ga1.99O3: Cr3+0.01 to decrease the recombination rate of electrons and holes. Then more carriers would move to the surface of compound to degrade organics. Moreover, the new traps could store the energy of carriers and release slowly in the form of persistent luminescence. That’s the reason why the persistent luminescence and photocatalytic properties of Ga1.95O3:Cr3+0.01, In0.05 are more excellent than that of Ga1.99O3:Cr3+0.01.
Fig. 5 Photocatalytic activity of S1: Ga1.94O3: Cr0.01, In0.05 and S2: Ga1.99O3: Cr0.01 for degradation of RhB under mercury lamp, the percentage of degradation is recorded at 10 min irradiated time intervals.
Figure 6 shows a possible mechanism of persistent luminescence and photocatalysis of Ga1.94O3:Cr3+0.01, In0.05. Under the excitation of UV light, a mass of electrons coupled with the excitation energy can be promoted from valence band (VB) to conduction band (CB) (progress (1)). In the CB, most of electrons will be transferred directly to the luminescence centers via the lattice (progress (2)), followed by the 2E-4A2 emission as photoluminescence (progress (3)). Meanwhile, the other part of carriers will be trapped by lattice defects and InGa × instead of moving to the ground state (progress (4)). After the stoppage of excitation, with the thermal disturbance at proper temperature, these electrons and holes trapped by lattice defects and InGa × will be released from the traps and transferred to the luminescence center and then followed by the characteristic Cr3+ emissions as persistent luminescence (progress (5)). The electrons trapped in lattice defects such as oxygen vacancies and the holes trapped in InGa × will be released slowly and move to catalyst surface. The electrons and holes on the catalyst surface will oxidate dissolve oxygen to O2- (progress I) and reduce hydroxyl ion and water molecules to •OH (progress II) [43]. If the traps have suitable depth, the carriers will be released easily and at suitable rate for long time to prolong the duration of persistent luminescence and improve photocatalytic properties of the samples. That means the persistent luminescence properties and photocatalytic properties are associated with each other by the suitable traps in the samples, therefore excellent persistent luminescence properties will lead to excellent photocatalytic properties.
4. Conclusions
In the present study, Ga2O3:Cr3+ phosphors were prepared by the high temperature solid state reaction in air atmosphere and exhibit excellent persistent luminescence and photocatalytic properties. When doped with In3+, the persistent luminescence and photocatalytic properties are obviously improved, which is due to the formation of new deep hole traps InGa × and higher distorted lattice structure in the crystal. In the persistent luminescence phosphors, the recombination rate of electron-hole pairs could be decreased by the presence of traps, which can maintain the photo-generated electrons/holes for a longer period and thus in favor of increasing photocatalytic activity. Therefore the persistent luminescence and photocatalytic properties are highly associated. It is promising to design new photocatalyst with high photocatalytic activity by controlling photoluminescence properties of phosphors.
Acknowledgments
This work is supported by the National Nature Science Foundation of China (No. 21271048).
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