Suppression of photocatalysis and long-lasting luminescence in ZnGa₂O₄ by Cr³⁺ doping

Abstract

ZnGa₂O₄ powder, synthesized by the solid state method, exhibited efficient photocatalytic activity for rhodamine B (RhB) degradation under mercury lamp illumination. However, the photocatalytic activity of ZnGa₂O₄ was highly suppressed when doped with Cr³⁺ ions. We discussed the mechanism of photocatalysis based on the photoluminescence properties of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺, and the blue fluorescence lifetimes of host ZnGa₂O₄ powders with different Cr³⁺ concentrations were also measured. The results indicated that the doped Cr³⁺ ions act as recombination centers that can highly reduce the amount and lifetime of the electron–hole pairs, thus reducing the photocatalytic activity of ZnGa₂O₄. The thermoluminescence (TL) curves of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ showed that the amount of trapped electrons/holes in ZnGa₂O₄ is almost seven times higher than that of ZnGa₂O₄:Cr³⁺. The suppressed long-lasting luminescence intensity and photocatalytic activity of ZnGa₂O₄:Cr³⁺ were supposed to have come from the decrease of trapped electrons/holes and shortened lifetimes of electron–hole pairs. Possible mechanisms for long-lasting luminescence and photocatalysis of ZnGa₂O₄ coupled with photoluminescence mechanisms of ZnGa₂O₄:Cr³⁺ were also proposed.

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Introduction

Long-lasting luminescence is an optical phenomenon whereby luminescence can remain visible for a long time (seconds to hours) after the excitation has stopped.^1–3 These materials have long been of great interest and are drawing more and more attention in recent years due to the great potential for applications in many fields such as emergency signage, traffic signs and in vivo bio-imaging.⁴

As a type of wide band gap (4.5 eV) semiconductor, ZnGa₂O₄ phosphor has attracted enormous attentions due to its many possible applications for field emission displays and electroluminescent devices due to its chemically and mechanically stable structure, which may be sustained under harsh environments.^5,6 ZnGa₂O₄ phosphor has a cubic normal AB₂O₄ spinel crystal structure with an Fd3m space group⁷ in which Zn²⁺ ions, surrounded by 4 oxygen atoms, occupy the tetrahedral A sites and Ga³⁺ ions, surrounded by 6 oxygen atoms, occupy the octahedral B sites. With this spinel structure, ZnGa₂O₄ can be described in terms of a close packed cubic arrangement of 32 oxygen anions. There are 64 tetrahedral sites and 32 octahedral sites in which only a half of the octahedral gaps and one-eighth of the tetrahedral gaps are filled with cations.⁸ Thus, ZnGa₂O₄ is an ideal host lattice for doping with transition metals or rare earth elements to form luminescence centers. ZnGa₂O₄ shows blue emission attributed to self-activated centers when undoped,^9,10 intense green emission when doped with Mn²⁺ and near-infrared (NIR) luminescence when doped with Cr³⁺.^11,12

Recently, ZnGa₂O₄ is also reported to exhibit excellent performance in water splitting¹³ and air purification^14,15 due to its high photocatalytic activity. The highly dispersed LUMO (bottom of conduction band), composed of hybridized orbitals of Ga 4s4p and Zn 4s4p atomic orbitals^16,17 is considered to promote the mobility of photo-generated electrons and benefit the high photocatalytic activity of ZnGa₂O₄.¹⁸ In the work of Sun et al.,¹⁹ ZnGa₂O₄ synthesized by a rapid microwave hydrothermal method exhibits efficient photocatalytic activities, which are even better than those of commercial TiO₂ P25. Nowadays, inspired by the methods to improve the photocatalytic activities of TiO₂, many approaches have been proposed to improve the photocatalytic activity of ZnGa₂O₄ (controlling morphology, calcining temperature, pH value, trace element doping, chemical ratio).¹⁸ In the literature about photocatalysis of TiO₂, Cr³⁺ ions doping has attracted interest in the study of visible light-induced photocatalytic activities due to its extension of the spectral response in the visible range. Although there are many articles about Cr³⁺ ions doped into TiO₂ (ref. 25–27) and even though Cr³⁺ doped ZnGa₂O₄ can increase the disorder of the spinel structure of ZnGa₂O₄ and thus induce more defects that may be favorable for photocatalytic activity, the photocatalytic activity of ZnGa₂O₄:Cr³⁺ has never been discussed.

Herein, ZnGa₂O₄ host and Cr³⁺ doped ZnGa₂O₄ samples are prepared by the traditional high temperature solid state method. Photocatalytic activity tests show that the photo-degradation efficiency of ZnGa₂O₄ is highly suppressed via doping with Cr³⁺, which is unexpected, and thus this triggered us to discuss the mechanism of the photocatalysis of ZnGa₂O₄:Cr³⁺ in detail. It is generally considered that peroxide (˙O₂⁻) and hydroxyl (˙OH) radicals, which are generated by the reaction of photo-generated electrons with O₂ and H₂O, are highly responsible for the photocatalytic degradation of pollutants and can oxidize organic pollutants into CO₂ and H₂O.²² Thus, the photo-generated electrons play important roles in photocatalysis and the amount and lifetime of photo-generated electrons can highly influence the activity of a photocatalyst. It is known that the photo-generated electrons and holes recombine quickly at recombination centers after the stoppage of excitation. The recombination rate could be decreased by the presence of ions that can act as electron or hole traps and maintain the photo-generated electrons/holes for a longer period, thus favoring the increase in photocatalytic activity. On the other hand, the recombination rate could be increased by the presence of ions that act as recombination centers such as defects or multiphases,²⁸ which can reduce the amount and lifetime of electron–hole pairs. Therefore, the relative efficiency of a metal ion dopant depends on whether it serves as a mediator of interfacial charge transfer or as a recombination center.²⁹ It is well known that photoluminescence results from the recombination of photo-generated electrons and holes, and the recombination rate determines the luminescence intensity. Therefore, there is a competitive mechanism between luminescence intensity and photocatalytic activity. That is to say, the lower the recombination rate of photo-generated electrons and holes, the lower the photoluminescence intensity and the higher the photocatalytic activity. The suppression of photocatalytic activity in ZnGa₂O₄ by Cr³⁺ doping can be investigated based on the luminescence properties of ZnGa₂O₄:Cr³⁺. It is promising to discuss the photocatalytic activity accompanied with luminescence properties, and a promising approach via controlling the luminescence properties can be proposed to develop the photocatalytic activity of photocatalysts.

Experimental

Materials

Chemical reagents ZnO (99%), Ga₂O₃ (99.99%), and Cr₂O₃ (99%) were used as starting materials. Chromium was nominally doped with 0.1%, 0.3%, 0.5%, 0.7%, 1.0% and 1.5% mol relative to gallium. The appropriate amount of starting materials with different stoichiometric ratios were mixed and carefully ground in an agate mortar for about 1 h to ensure homogeneity, and then the mixed powders were placed into a corundum crucible and calcined at 1300 °C for 4 h in an air atmosphere. When the samples cooled to room temperature, they were reground in the agate mortar.

Characterization

The crystal structure of the powder phosphors were analyzed using an X-ray diffractometer at room temperature using Cu K_α (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 step size of 0.02°. The surface morphologies were observed using a scanning electron microscopy (SEM, S-3400N-II, Japan). The particle sizes of the powder phosphors were measured by a Laser Particle Size Analyzer (JL-1197). The excitation and emission spectra, long-lasting luminescence decay curves and fluorescence decay curves were obtained at room temperature using a Hitachi F-7000 fluorescence spectrophotometer. An FJ27A1 TL dosimeter was used to measure TL curves at a heating rate of 1 °C s⁻¹ after the samples were exposed to radiation from a UV lamp for 5 min and placed in a dark room for 3 min.

Photocatalytic activity test

Photocatalytic activities were characterized by the photo-degradation of RhB. A 500 W mercury lamp with a 5 A operating current was used as the 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 during irradiation. The as-prepared ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ particles (0.01 g) were first dispersed in two glass tubes with 40 ml RhB solution (4 × 10⁻⁵ mol l⁻¹), and another glass tube containing only 40 ml RhB solution was used as the blank. Then, the suspensions were stirred in the dark for 30 min to achieve an adsorption/desorption equilibrium between the photocatalyst and RhB solution, during which vigorous magnetic stirring was maintained to keep ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ particles suspended in the RhB solution. Then, 5 ml of the suspensions was added into centrifuge tubes and centrifuged for 3 min at 3000 rpm to eliminate the solid particles. The clear supernatants were used to measure the changes in the RhB concentration (the solid particles and clear supernatant were poured back into respective glass tubes to ensure there is always a comparable amount ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ in the RhB solution). The concentration of RhB was monitored at the maximum absorbance wavelength of 553 nm. The percentage of degradation was recorded as C/C₀, where C is the absorbance of RhB solution at certain irradiated time intervals (10 min) and C₀ is the absorbance of the initial RhB solution. The blank solution was also tested under same experimental conditions for comparison.

Results and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ calcined at 1300 °C for 4 h. All the peaks are assigned to the ZnGa₂O₄ spinel phase (JCPDS no. 38-1240) and no characteristic peaks for the dopants have been observed. Nine distinctive peaks match well with the (111), (220), (311), (222), (400), (422), (511), (440), and (531) crystal planes of ZnGa₂O₄, respectively. This result indicates that the pure spinel phase of ZnGa₂O₄ was formed in the investigated samples. Therefore, the doping of 0.5% mol transition metal chromium relative to gallium has no significant influence on the crystal structure of ZnGa₂O₄. The morphologies of the as-synthesized ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ are demonstrated by the SEM images (shown in Fig. 2). As shown in Fig. 2, the as-prepared ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ samples show same irregular shapes. Particles are partly agglomerated, and the particles of ZnGa₂O₄:Cr³⁺ are agglomerated more significantly. The actual particle sizes of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ are about 0.75 μm and 0.70 μm, respectively. Fig. 3 shows the distribution of particle sizes in powder phosphors. As shown in Fig. 3, the average particle sizes of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ are about 0.78 μm and 0.80 μm, respectively. These results are consistent with the actual particle sizes and agglomeration shown in Fig. 2. The results relating to crystal structure, morphology, particle size and size distribution indicated that ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ possess almost the same surface areas, and the same amount of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ powders are comparable in the photocatalytic reaction.

Fig. 1 XRD patterns of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺.

Fig. 2 The morphologies of the as-synthesized ZnGa₂O₄ (a) and ZnGa₂O₄:Cr³⁺ (b).

Fig. 3 Frequency distribution of particle sizes for ZnGa₂O₄ (a) and ZnGa₂O₄:Cr³⁺ (b).

The photo-degradation of RhB was measured as a function of irradiation time, as shown in Fig. 4. The degradation rate of RhB under irradiation was very slow when no photocatalyst was added (blank in Fig. 4). Moreover, the photocatalytic activity of ZnGa₂O₄:Cr³⁺ can be ignored. On the contrary, RhB concentration underwent an obvious decline after 110 min irradiation in the presence of ZnGa₂O₄ with 60% RhB degradation.

Fig. 4 Photocatalytic activity of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ for the degradation of RhB under mercury lamp; the percentage of degradation was recorded at intervals of 10 min.

An appropriate electronic structure is essential for the photocatalytic activity. Recently, Sun et al.¹⁹ reported that the band-edge positions of the conduction and valence bands (E_CB and E_VB, respectively) can be calculated by the following empirical equations:³⁰

E_CB = −χ + 1/2E_g (1)

E_VB = −χ − 1/2E_g (2)

where E_g is the band gap energy and χ is the Mulliken electronegativity of a pristine semiconductor. This method has achieved success in calculating band positions and photoelectric thresholds for many materials.^31–33 For the ZnGa₂O₄ catalyst, using the empirical equations described above, E_VB was determined to be 3.35 V versus the normal hydrogen electrode (NHE), which is more positive than that of E(˙OH/OH⁻) (2.38 V, NHE), and E_CB was −1.45 V (NHE), which is more negative than that of E(O₂/˙O₂⁻) (−0.33 V, NHE).³⁴ The higher valence band position and lower conduction band position means ˙OH can be generated by UV-irradiated ZnGa₂O₄ in an RhB solution, thus exhibiting photocatalytic activity. From Fig. 4, it is obvious that the photo-degradation of RhB with ZnGa₂O₄ is highly suppressed when doped with Cr³⁺. As mentioned above, there is a competitive mechanism between luminescence intensity and photocatalytic activity; the reason why doping with Cr³⁺ could suppress the photocatalytic activity of ZnGa₂O₄ will be discussed in detail mainly on the basis of the luminescence properties of ZnGa₂O₄:Cr³⁺.

Fig. 5 shows the emission spectra of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ at room temperature under excitation with 254 nm wavelength light. The self-activated photoluminescence of the ZnGa₂O₄ host exhibited a broad-band emission peaking at 340 nm, a narrow-band emission with the maximum emission at 505 nm, and a weak band emission peaking at 680 nm. The emission band at 505 nm (²E_A–⁴A₂) appears likely due to the electron and hole recombination between the native defects (oxygen defect, zinc defect, and gallium–oxygen vacancy pair).^35–37 The emission band at 340 nm is similar to the band at 360 nm reported by Kim et al.,^38,39 which is the blue-shift behavior of the emission band from 430 nm to 340 nm. The blue-shift behavior results from the oxygen vacancy () defects, which distort the symmetry of the O_h site, leading to the shift of Ga–O transition emission of regular octahedral sites at 430 nm to 340 nm (Ga–O transition emission of distorted octahedral sites).^40,41 Moreover, 698 nm emission accompanying the 340 nm emission is identified as the transition from the state to the O²⁻ state.³⁹ In contrast, ZnGa₂O₄:Cr³⁺ gave an intense NIR emission band at 698 nm due to the ²E–⁴A₂ transition of distorted Cr³⁺ ions in ZnGa₂O₄.⁴² The host emission at 505 nm is highly suppressed after doping Cr³⁺ into ZnGa₂O₄ due to an effective non-radiative energy transfer between the host emission and the absorption of Cr³⁺ (⁴A₂–⁴T₁ (te²), ⁴A₂–⁴T₁ (t²e) and ⁴A₂–⁴T₂ transitions), resulting from their large spectral overlap.⁴² The result is similar to the energy transfer between the blue emission of ZnGa₂O₄ host and the absorption of Cr³⁺.^42,43 The excitation spectra of ZnGa₂O₄:Cr³⁺ has three main bands (inset of Fig. 5); the excitation peak at 260 nm results from the combination of the ZnGa₂O₄ host excitation band and the O–Cr charge transfer band.⁴² The other two bands at 410 nm (⁴A₂–⁴T₁ (te²) transition) and 550 nm (⁴A₂–⁴T₂ transition) originate from the 3d intra-shell transitions of Cr³⁺.⁴⁴

Fig. 5 Emission spectra of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ at room temperature under excitation with 254 nm wavelength light. The inset shows the excitation spectra of ZnGa₂O₄:Cr³⁺ monitored at 689 nm.

As we mentioned above, the recombination rate of photo-generated electrons and holes determines the photoluminescence intensity. Hence, the intense emission intensity of ZnGa₂O₄:Cr³⁺ at 698 nm compared with that of ZnGa₂O₄ at 505 nm indicates that the doped Cr³⁺ ions may act as recombination centers. The incorporated recombination centers, which can highly reduce the amount and lifetime of photo-generated electron–hole pairs, could be the cause of the suppressed photocatalytic activity of ZnGa₂O₄:Cr³⁺.

To prove that Cr³⁺ ions act as recombination centers, the blue fluorescence lifetimes (at 505 nm) of ZnGa₂O₄ were measured with different Cr³⁺ doping concentrations (x = 0.0%, 0.1%, 0.3%, 0.5%, 0.7%, 1.0%, 1.5%). As showed in Fig. 6, the fluorescence decay curves can be fitted by a single exponential function and the fitting results of the lifetimes are shown in the inset. It is clearly observed that the lifetimes of ZnGa₂O₄ at 505 nm become shorter with the increase of Cr³⁺ concentration as a result of energy transfer from ZnGa₂O₄ to Cr³⁺.⁴⁵ The fluorescence lifetime reflects the average residence time of photo-generated electrons in the excited state. The shortened fluorescence lifetime of ZnGa₂O₄ means that the energy transfer becomes quicker with the increase of Cr³⁺ concentration, and the average residence time of photo-generated electrons in the excited state is shortened faster. Therefore, more channels are formed, benefiting the recombination of electrons and holes to transfer energy to Cr³⁺. Thus, Cr³⁺ ions act as recombination centers in ZnGa₂O₄, which accelerate the recombination of electrons and holes.

Fig. 6 The fluorescence decay curves of ZnGa₂O₄ (505 nm emission) with different Cr³⁺ doping concentration (x = 0.0%, 0.1%, 0.3%, 0.5%, 0.7%, 1.0%, 1.5%). The inset shows the fitting results of the lifetimes.

As it is generally accepted, long-lasting luminescence is a phenomenon whereby luminescence can last for hours after the stoppage of the excitation. The long-lasting luminescence of the phosphors is generated by the recombination of trapped electrons and holes that are released slowly through thermal motion after the stoppage of excitation. We measured the afterglow decay curves of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ phosphors monitored at 505 nm and 698 nm after 5 min irradiation with a 254 nm UV lamp, respectively. As can be seen from Fig. 7, the as-prepared samples exhibited similar decay processes that contain a rapid decay phase at the beginning and a slow decay process afterward.^46,47 In our study, the decay curves can be well fitted by a double exponential equation as follows:

where t is the decay time; I represents the phosphorescent intensity at the t time; I₁ and I₂ are constants that depend on the rapid and slow initial luminescent intensity at t = 0, while τ₁ and τ₂ are decay constants that decide the rate for the rapid and slow exponential decay components, respectively. The parameters τ₁ and τ₂ are calculated and the fitting results are shown in Table 1. These parameters indicate that ZnGa₂O₄ exhibits better long-lasting luminescence properties than that of ZnGa₂O₄:Cr³⁺. After 5 min decay time, the long-lasting luminescence intensity of ZnGa₂O₄ is still high. The inset of Fig. 7 displays the afterglow brightness of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ recorded at different decay times after irradiation with 254 nm wavelength light for 5 min. The results pertaining to long-lasting luminescence are consistent with those of photocatalytic performance exhibited in Fig. 4. We assume that the doped Cr³⁺ ions act as recombination centers, but not as trap centers and the suppression of the long-lasting luminescence intensity of ZnGa₂O₄:Cr³⁺ comes from less trapped electrons/holes in ZnGa₂O₄:Cr³⁺.

Fig. 7 Afterglow decay curves of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ phosphors monitored at 505 nm and 698 nm after 5 min irradiation with a 254 nm UV lamp, respectively. The fitting curves are shown as the green line with stars and the red line with circles. The inset shows the digital photos of afterglow brightness of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ recorded at different decay times after irradiation with 254 nm wavelength light for 5 min.

Table 1 The fitting results of decay curves

Samples	A1/(a.u.)	τ1/(s)	A2/(a.u.)	τ2/(s)
ZnGa2O4	31.00	6.96	11.49	63.24
ZnGa2O4:Cr^3+	20.77	3.00	6.93	68.77

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Because a TL curve can reflect the trapping property of defects as well as the relevance of the long-lasting luminescence and the trap energy levels, we measured the TL curves of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ in the range of 40–260 °C. In Fig. 8, under the same experimental conditions (same sample volumes are irradiated for 5 min by UV lamp, and then placed in dark room for 3 min before the measurement), the maximum TL intensity of ZnGa₂O₄ is almost seven times higher than that of ZnGa₂O₄:Cr³⁺ under thermal disturbance. This means that after the stoppage of irradiation from the UV lamp, the amount of trapped electrons/holes in ZnGa₂O₄ is almost seven times higher than that of ZnGa₂O₄:Cr³⁺. When heating, the captured electrons and holes will be released from traps and will recombine with each other in the form of TL. Fewer trapped electrons/holes mean fewer trap centers. The results of these TL curves confirmed that fewer trap centers are formed in ZnGa₂O₄ when doped with Cr³⁺.

Fig. 8 TL curves of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ in the range of 40–260 °C. The sample was irradiated for 5 min by UV lamp, and then placed in dark room for 3 min before the measurement.

In ZnGa₂O₄, abundant photo-generated electrons and holes were produced with an irradiation energy that was larger than the band gap energy of ZnGa₂O₄;⁴⁸ one portion of the photo-generated electrons and holes recombined at recombination centers quickly to release energy in the form of photoluminescence; the other portion can be trapped by lattice defects, impurities, or co-dopants that act as traps in the material.^49,50 The trapped photo-generated electrons/holes can delay the recombination of electrons and holes, which promotes the separation of electrons and holes and enhances charge transfer, thus effectively improving the photocatalytic activity. After stopping the radiation and with the thermal disturbance at proper temperatures, these trapped electrons and holes will be released from the traps, which can recombine with each other, followed by the emission of light as long-lasting luminescence. Hence, photocatalysis and long-lasting luminescence are significantly correlated. That is to say, the more trapped electrons and holes, the stronger the long-lasting luminescence intensity, and the higher the photocatalytic activity will be.

Based on the discussion above, a possible schematic illustration of the long-lasting luminescence and photocatalysis for ZnGa₂O₄ was proposed, and it is depicted in Fig. 9a. Upon 254 nm UV excitation, the incident photons are absorbed by the ZnGa₂O₄ host and the electrons are promoted from the valence band of ZnGa₂O₄ to the conduction band (progress (1)). Most of the electrons are transferred via the lattice directly to the luminescence centers (progress (2)), followed by the emissions ²E_A–⁴A₂ and ²E_B–⁴A₂ as photoluminescence (progress (3) and (4)), then a part of the excitation energy associated with the excited free electrons are captured by native defects via non-radiative relaxation (progress (5)). After UV irradiation is stopped, with the thermal disturbance at proper temperatures, these carriers will be released from the traps and transferred via the host to the luminescence centers (progress (6)), where they recombine with the opposite charge to produce the ²E_A–⁴A₂ emission as long-lasting luminescence. When excited free electrons are captured by electron traps, the photo-generated electrons and holes can be effectively separated, which could prevent the recombination of electrons and holes and lead to a longer lifetime of the photo-generated electrons and holes. The O₂ near the photocatalyst/liquid interface could be then reduced to the superoxide radical anion ˙O₂⁻ and hydrogen peroxide H₂O₂ (eqn (3) and (4)) by the photo-generated electrons. The ˙O₂⁻ and H₂O₂ can further interact to produce hydroxyl radical ˙OH (eqn (6)), which is a powerful oxidizing species that can decompose most organic pollutants in water.⁵¹

e⁻ + O₂ → ˙O₂⁻ (3)

O₂ + 2H⁺ + 2e⁻ → H₂O₂ (4)

H₂O₂ + e⁻ → ˙OH + OH⁻ (5)

H₂O₂ + ˙O₂⁻ → ˙OH + OH⁻ + O₂ (6)

H₂O₂ → 2˙OH (7)

Fig. 9 (a) Schematic illustration of the long-lasting luminescence and photocatalysis of ZnGa₂O₄. (b) Mechanism illustration of the persistent energy transfer between ZnGa₂O₄ host and Cr³⁺ ions.

Fig. 9b presents a possible mechanism of photoluminescence for ZnGa₂O₄:Cr³⁺. After UV irradiation is stopped, the photo-generated electrons and holes captured by traps can be released slowly through thermal motion. Instead of recombining with the opposite charge to produce the ²E_A–⁴A₂ emission as long-lasting luminescence, the energy associated with electrons of the host is persistently transferred to the Cr³⁺ ion slowly via an effective non-radiative energy transfer (progress (7)).⁵² The persistent energy transfer leads to the promotion of the 3d electrons of Cr³⁺ from ground-state (⁴A₂) to the excited-states ⁴T₂ (progress (8)). The transitions between ⁴T₂ and ²E are just non-radiative reactions. Then, the ²E–⁴A₂ transition gives a NIR emission band at 698 nm to produce the persistent luminescence (progress (9)) that comes from the persistent energy transfer from the host.

Conclusions

In summary, ZnGa₂O₄ presents high photocatalytic activity to degrade RhB and excellent long-lasting luminescence properties under UV irradiation. The favorable edge position of E_CB and E_VB results in the strong redox ability of ZnGa₂O₄. When doped with Cr³⁺, its photocatalytic activity and long-lasting luminescence intensity are highly suppressed, and there is an effective persistent energy transfer between the host emission and the absorption of Cr³⁺, resulting in the NIR long-lasting luminescence at 698 nm. The surface areas of the as-synthesized ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ samples are almost same and the average particle sizes are about 0.78 μm and 0.80 μm, respectively. These results indicated that the same amount of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ powders are comparable in the photocatalytic reaction. From luminescence properties, fluorescence decay curves and TL curves, it can be concluded that doped Cr³⁺ ions act as recombination centers but not as trap centers, and that the amount of trapped electrons/holes in ZnGa₂O₄ is almost seven times higher than that of ZnGa₂O₄:Cr³⁺. Therefore, it is proposed that the suppressed photocatalytic activity and long-lasting luminescence intensity in ZnGa₂O₄:Cr³⁺ come from the decrease in trapped electrons/holes and the shortened lifetime of the electron–hole pairs. Long-lasting luminescence intensity and photocatalytic activity are highly correlated. The photoluminescence properties can provide a firm theoretical foundation for designing new photocatalysts with high photocatalytic activity, as well as for evaluating the photocatalytic activities of some photocatalysts.

Acknowledgements

This study is supported by the National Nature Science Foundation of China (no. 21271048).

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