Dual-functional persistent luminescent nanoparticles with enhanced persistent luminescence and photocatalytic activity

Abstract

Dual-functional nanoparticles with near-infrared (NIR) persistent luminescence and sun-light photocatalytic activity are highly desired for medical diagnosis and environmental protection. Here, we report a facile one-step method for simultaneous enhancement of persistent luminescence and photocatalytic activity of the dual-functional persistent luminescent nanoparticles (PLNPs). The Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs, which were less than 10 nm in size, were synthesized by an ethylene glycol-assisted hydrothermal method. The persistent luminescence and the photocatalytic activity of the PLNPs were significantly and simultaneously improved via additional doping of Bi³⁺ in ZnGa₂O₄:Cr. The prepared dual-functional PLNPs have great potential in pollutant photo-degradation and long-term imaging in vivo.

NIR persistent luminescence and photocatalytic activity of the PLNPs were significantly and simultaneously improved via additional doping of Bi³⁺.

1. Introduction

The long-lasting phosphorescence (or afterglow) of persistent luminescent materials is an optical phenomenon whereby luminescence can remain for a long time (hours to days) after the excitation source is removed.^1–3 In recent years, persistent luminescent materials have been drawing increasing attention in many areas, such as emergency signage, traffic signs, lighting, photocatalysis, and medicine.^4–7 Molecular probes based on persistent luminescent nanoparticles (PLNPs) exhibit a super-long afterglow time and do not need in situ excitation during biomedical imaging, so they completely avoid tissue auto-fluorescence, light scattering, and photo-toxicity originating from the excitation light.^8–10 PLNPs, alone or in combination with other photocatalysts such as TiO₂, have great potential in photocatalysis owing to their potentially increased solar light absorption during day time and subsequent increased solar energy release at night or after the removal of illumination source.^3,11 Therefore, dual-functional PLNPs with near-infrared (NIR) persistent luminescence and photocatalytic activity have great potential in medical diagnosis and environmental protection.

Zinc gallate (ZnGa₂O₄) is a cubic normal AB₂O₄ spinel crystal semiconductor with the Fd3m space group.¹² It is one of the excellent blue-emitting phosphors with wide band gaps (4.4 eV for ZnGa₂O₄) and has great potential applications in displays, owing to its strong blue emission, better chemical and thermal stability, and excellent cathodoluminescence properties.¹³ ZnGa₂O₄ also acts as an excellent photo-catalyst due to its hybridized orbitals of Zn4s4p, Ga4s4p, and band gap, which can improve the mobility of photo-generated electrons and the absorption efficiency of ultraviolet (UV) light.^5,14

ZnGa₂O₄ is also an excellent photoluminescence host material when doped with transition metals, such as Mn-doped ZnGa₂O₄ for green emission, and Cr³⁺-doped ZnGa₂O₄ (ZnGa₂O₄:Cr) for red or near-infrared (NIR) emission.¹⁵ In particular, ZnGa₂O₄:Cr is a bright NIR-emission persistent luminescence phosphor, which is considered a promising optical nanoprobe for in vivo bio-imaging.^16–18 Co-doping of ZnGa₂O₄ or zinc gallogermanate systems with Cr³⁺ and rare earth ions, such as Pr³⁺ and Dy³⁺, can significantly improve the persistent luminescence intensity and afterglow time of PLNPs, because trivalent lanthanide ions with special 4f electron configurations can provide more efficient trap density and depth.^9,19,20 However, afterglow time of the Cr³⁺-doped ZnGa₂O₄ is still not long enough for long-term in vivo imaging. Furthermore, the Cr³⁺-doped ZnGa₂O₄ is hardly used on photocatalytic degradation owing to its lower photocatalytic activity, compared to un-doped ZnGa₂O₄. Therefore, the development of an effective one-step methodology to simultaneously improve the persistent luminescence and the photocatalytic activity of dual-functional PLNPs are highly desired for pollutant photo-degradation and long-term imaging in vivo. Nevertheless, to the best of our knowledge, no work on simultaneous enhancement of persistent luminescence and photocatalytic activity for Cr³⁺ co-doped ZnGa₂O₄ have been reported so far.

Here, we report a facile one-step method for simultaneous enhancement of persistent luminescence and photocatalytic activity of the dual-functional PLNPs. The Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs, which were less than 10 nm in size, were synthesized by an ethylene glycol-assisted hydrothermal method. The NIR persistent luminescence and photocatalytic activity of Cr³⁺ co-doped ZnGa₂O₄ are significantly, and simultaneously, improved by additional doping with Bi³⁺. The prepared dual-functional PLNPs are promising for in vivo bioimaging and photocatalytic application.

2. Experimental

2.1. Sample preparation

Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs were prepared by an ethylene glycol assisted hydrothermal method. To prepare the ZnGa_1.97O₄:Cr_0.01/Bi_0.02, 10.0 mL of ethylene glycol, 3.94 mL of Ga³⁺ (0.2 mol L⁻¹), 1 mL of Zn²⁺ (0.4 mol L⁻¹), 0.4 mL of Cr³⁺ (0.01 mol L⁻¹), and 0.8 mL of Bi³⁺ (0.01 mol L⁻¹) were mixed and stirred at room temperature for 30 min. The pH of the mixture solution was then adjusted to 9 with aqueous ammonia (15 wt%) and stirred at room temperature for 1 h. The resulting mixture was transferred to a 20 mL Teflon-lined stainless-steel autoclave in ambient environment and heated at 170 °C for 24 h. The autoclave was then allowed to cool to room temperature. The purified products were separated by centrifugation and the NPs were precipitated and washed with distilled water and ethanol and dried in a vacuum oven at 60 °C for 12 h. Finally, the PLNPs were annealed in air at 1000 °C for 3 h.

2.2. Characterization

X-ray diffraction (XRD) patterns were recorded on a D/max-2500 diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) and high-resolution TEM images were obtained on JEOL-100CX II and JEM-2100F field emission transmission electron microscopes (JEOL, Japan), respectively. The elemental composition of samples was analyzed with an EDAX Genesis XM2 attached to the S-4800 SEM (Hitachi, Japan). X-ray photoelectron spectra (XPS) were recorded on Escalab 250Xi (Thermo Fisher Scientific, USA) X-ray photoelectron spectrometer. The excitation and emission spectra and the long-lasting luminescence decay curves were obtained at room temperature using a F-4500 fluorescence spectrophotometer (Hitachi, Japan). The UV-Vis diffuse reflectance spectra were obtained using a UV-2600 UV-Vis Spectrophotometer (Shimadzu, Japan).

3. Results and discussion

3.1. Structure and morphology

Fig. 1 shows the XRD patterns of ZnGa₂O₄, ZnGa₂O₄:Cr, and ZnGa₂O₄:Cr/Bi. All the peaks are assigned to the ZnGa₂O₄ sample with 2θ at 18.45, 30.28, 35.7, 37.4, 43.58, 57.56, 63.19, 65.82, and 74.72, which were indexed to the (111), (220), (311), (222), (400), (422), (511), (440), and (531) crystal planes, respectively.²⁰ All diffraction peaks are consistent with the ZnGa₂O₄ spinel phase (JCPDS no. 38-1240). When the contents of Bi are 0.01 and 0.02, no characteristic peaks of other phases, such as ZnO, Ga₂O₃ and B₂O₃, were observed, indicating the formation of a pure spinel phase zinc gallate solid solution. When the content of Bi³⁺ in ZnGa₂O₄:Cr/Bi was increased from 0.02 to 0.03, we observed a strong peak at 28.01°, which was indexed to the rhombohedral Bi structure. The result indicates that the increasing content of Bi³⁺ influences the crystal structure of ZnGa₂O₄.

Fig. 1. XRD patterns of ZnGa₂O₄, ZnGa₂O₄:Cr and ZnGa₂O₄:Cr/Bi.

Fig. 2a shows TEM image of the as-synthesized ZnGa_1.97O₄:Cr_0.01/Bi_0.02 PLNPs and reveals that the PLNPs are less than 10 nm in size. The high-resolution TEM analysis shown in Fig. 2b indicates that the particles are single crystalline and the distance between the lattice fringes is 4.81 Å, which corresponds to the d-spacing of the spinel ZnGa₂O₄ (111) lattice planes. The selected-area electron diffraction (SAED) pattern further confirmed that the crystal of as-synthesized ZnGa_1.97O₄:Cr_0.01/Bi_0.02 PLNPs is a pure spinel single crystal (Fig. 2c). The results of EDX analysis are demonstrated that the ZnGa_1.97O₄:Cr_0.01/Bi_0.02 PLNPs were composed of only Zn, Ga, Cr, Bi, and O (Fig. 2d). These results revealed that the ZnGa₂O₄ crystal structure was successfully doped with Bi and Cr.

Fig. 2. (a–c) TEM, HR-TEM, SAED images and (d) EDX patterns of ZnGa_1.97O₄:Cr_0.01/Bi_0.02.

3.2. Persistent luminescence of PLNPs

Fig. 3 shows the phosphorescence emission spectra of ZnGa₂O₄:Cr and ZnGa₂O₄:Cr/Bi PLNPs at room temperature at an excitation wavelength of 254 nm. ZnGa₂O₄:Cr produced an intensive NIR emission band peaking at 698 nm (assigned to ²E → ⁴A₂ transition of Cr³⁺ ions) that superimposes on a broad emission band at 600–800 nm (assigned to ⁴T₂ → ⁴A₂ transition of disordered Cr³⁺ ions).² The emission spectra of ZnGa₂O₄:Cr/Bi are similar to ZnGa₂O₄:Cr, but the emission peaks of ZnGa₂O₄:Cr/Bi were slightly red-shifted due to the crystal field strength of Cr³⁺ centres decreased by additional doping of Bi³⁺ in ZnGa₂O₄:Cr.²¹ Fig. S1 (ESI†) shows the phosphorescence excitation spectra of ZnGa₂O₄:Cr and ZnGa₂O₄:Cr/Bi. There are four main absorption bands. The strong band peak around 260 nm is probably the combination of the ZnGa₂O₄ host excitation band and the O–Cr charge transfer band.⁹ The bands peaked at approximately 411 nm, 471 nm, and 554 nm and originated from the ⁴A₂ → ⁴T₁ (te²) transition, the ⁴A₂ → ⁴T₁ (t²e) transition, and the ⁴A₂ → ⁴T₂ transition originated from the 3d intrashell transitions of Cr³⁺, respectively.²

Fig. 3. Phosphorescence emission spectra of ZnGa₂O₄:Cr and ZnGa₂O₄:Cr/Bi at an excitation wavelength of 254 nm.

Fig. 4 shows the persistent luminescence emission decay curves of ZnGa₂O₄:Cr and ZnGa₂O₄:Cr/Bi PLNPs, monitored at 698 nm after being excited by 254 nm UV light for 5 min. The data represent the persistent luminescence intensity (I) as a function of time (t) and the data recording lasted for 20 min. The persistent luminescence intensity increased as the content of Bi increased from 0 to 0.02, but decreased with a further increase from 0.02 to 0.03. The most intense persistent luminescence was obtained for ZnGa_1.97O₄:Cr_0.01/Bi_0.02. The decay curves of the persistent luminescence can be well fitted by the three-exponential eqn (1) as follows:⁹

1

where I(t) is the intensity of persistent luminescence at time t; I₀ is the initial intensity of persistent luminescence; A₁, A₂, and A₃ are constants; τ₁, τ₂, and τ₃ are derived lifetimes for the exponential components indicating the three different decay processes. The fitting results according to the above formula are presented in Table S1 (ESI†). These average photoluminescence lifetimes (τ_av) indicate that the ZnGa_1.97O₄:Cr_0.01/Bi_0.02 PLNPs produced a much longer photoluminescence lifetime (32.41 s) than the single Cr³⁺-doped ZnGa₂O₄ PLNPs (9.35 s). The above experimental results indicated that the additional doping of Bi³⁺ cannot cause remarkable changes in the excitation and emission spectra of ZnGa₂O₄:Cr but can significantly improve the intensity and time of persistent luminescence for ZnGa₂O₄:Cr PLNPs. Previous work reported that additional doping with Bi³⁺ in ZnGa₂O₄:Cr can also enhance the persistent luminescence of PLNPs.^22,23 The Cr³⁺ prefers to substitute for Ga³⁺ (with a similar ionic radius) in distorted octahedral coordination in ZnGa₂O₄:Cr. The Bi³⁺ substituting for Ga³⁺ can cause a higher periodic lattice distortion in ZnGa₂O₄:Cr, because the radius of Bi³⁺ (103 pm) is bigger than that of Ga³⁺ (62 pm). The higher periodic lattice distortion could generate more defects in the ZnGa₂O₄ host.²⁴ Doping with Bi³⁺ probably increases the afterglow time of ZnGa₂O₄:Cr PLNPs by producing more hole traps.¹⁹

Fig. 4. Afterglow decay curve of ZnGa₂O₄:Cr and ZnGa₂O₄:Cr/Bi after irradiation with a 254 nm UV lamp for 5 min.

3.3. Photocatalytic activity of PLNPs

Fig. 5 shows the UV-Vis diffuse reflectance spectra of ZnGa₂O₄, ZnGa₂O₄:Cr, and ZnGa_1.97O₄:Cr_0.01/Bi_0.02. The Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs have a broad absorption spectra from UV to visible light (670 nm), and the UV-Vis light absorption is remarkably enhanced, compared to ZnGa₂O₄ and ZnGa₂O₄:Cr. The broad absorption band of the Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs at 450 nm is associated with the charge transfer band from Bi³⁺ 6s² to Bi⁵⁺ 6s⁰.^25,26 We carried out the XPS experiments to verify the existence of Bi⁵⁺. The Bi 4f XPS spectrum shows two distinct peaks which were fitted into three peaks at 158.87 eV, 160.23 eV and 163.92 eV, respectively (Fig. S2 (ESI†)). The peaks of Bi 4f_7/2 at 158.87 eV and Bi 4f_5/2 at 163.92 eV are corresponding to Bi(iii). The peak of Bi 4f_7/2 at 160.23 eV is ascribed to Bi(v), which indicates existence of the Bi⁵⁺ in the ZnGa_1.97O₄:Cr_0.01/Bi_0.02.^27,28 Band gaps (E_g) of ZnGa₂O₄, ZnGa₂O₄:Cr, and ZnGa_1.97O₄:Cr_0.01/Bi_0.02 were calculated to be approximately 4.38 eV, 4.45 eV, and 2.75 eV, respectively (Fig. S3 (ESI†)), according to the following eqn (2):²⁹

αhv = A(hv − E_g)² 2

where α is the absorption coefficient, hv is the photon energy, and A is a constant. The experimental results shows that band gap of the ZnGa₂O₄ was slightly increased by doping Cr³⁺, probably due to the self-reduction effect on Cr³⁺ ions reduce into lower valence states and the absence of localized states in the band gap.^30,31 In contrast, the additional doping of ZnGa₂O₄:Cr with Bi³⁺ can significantly decrease the band gap of ZnGa₂O₄, owing to the electronic structure of Bi with a unique 6s orbital,^27,32 which has the benefit of increasing solar light absorption in applications of photocatalysis degradation.

Fig. 5. UV-Vis diffuse reflectance spectra of ZnGa₂O₄, ZnGa₂O₄:Cr and ZnGa_1.97O₄:Cr_0.01/Bi_0.02.

Rhodamine B (RhB) was used as the probe molecule to evaluate the photocatalytic performance of the Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs. The photo-degradations efficiency of ZnGa₂O₄, ZnGa₂O₄:Cr, and ZnGa₂O₄:Cr/Bi for RhB are shown in Fig. 6. The as-prepared samples were analysed first, to establish an adsorption–desorption equilibrium between the catalyst and the dye molecules. The suspension was stirred in darkness for 40 min, and the samples were then placed under UV light irradiation. The ZnGa_1.97O₄:Cr_0.01/Bi_0.02 PLNPs exhibited an excellent photocatalytic activity with a degradation efficiency of 99.2% after 100 min irradiation with UV light. Experimental results indicate that the photocatalytic activity of the pure ZnGa₂O₄ is lower than that of the Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ and higher than that of the single Cr³⁺ doped ZnGa₂O₄ (Fig. 6). The photocatalytic activity of ZnGa₂O₄ was suppressed by the doping with Cr³⁺, because the Cr³⁺ ions in ZnGa₂O₄ act as recombination centres, which can reduce the number and lifetime of the electron–hole pairs.³³ By contrast, additional doping of Bi³⁺ in ZnGa₂O₄:Cr not only enhanced the UV-Vis light absorption of ZnGa₂O₄, but also produced more hole trap energy levels, which resulted in more photo-generated electron–holes being captured by traps, thus inhibiting the recombination efficiency of the photo-generated electron–holes.²⁴ As a consequence, the lifetime of the photo-generated electron–holes and the afterglow time were prolonged.

Fig. 6. The photo-degradation efficiency of ZnGa₂O₄, ZnGa₂O₄:Cr and ZnGa₂O₄:Cr/Bi for RhB.

To investigated the role of reactive oxygen species (ROS) in the degradation process, we carried out the ROS scavenging experiments with different scavengers, such as p-benzoquinone (BQ) as O₂˙⁻ scavenger, isopropyl alcohol (IPA) as HO˙ scavenger, and ammonium oxalate (AO) as the hole scavenger.³⁴ The experimental results shows that the photo-degradation of RhB is dramatically decreased by adding AO in the RhB solution which indicates that the holes are the most dominant species in the photocatalytic degradation process (Fig. S4 (ESI†)). Compared to AO and BQ, the addition of IPA result in slight decrease for degradation of RhB, which indicates the HO˙ radicals are not main contributor for degradation process. The electrons and holes are generated on the surface of PLNPs under light UV radiation. The photogenerated electron–hole pairs reacted with oxygen and water to generate peroxide (O₂˙⁻) and hydroxyl radicals (HO˙).³⁵ The resulting radicals cause degradation of the organic molecules. This photocatalytic reaction can be represented as follows:

PLNPs + hv → h_VB⁺ + e_CB⁻ 3

e_CB⁻ + O₂ → O₂˙⁻ 4

h_VB⁺ + H₂O → HO˙ + H⁺ 5

Dye + (O₂˙⁻/HO˙) → degradation 6

4. Conclusions

The Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs were synthesized by an ethylene glycol assisted hydrothermal method. The particle size of the as-prepared PLNPs was less than 10 nm. The photodegradation efficiency of Bi³⁺, Cr³⁺ co-doped ZnGa₂O₄ PLNPs for RhB was 99.2% after 100 min irradiation with UV light. Compared to ZnGa₂O₄:Cr and un-doped ZnGa₂O₄, the additional doping of Bi³⁺ in ZnGa₂O₄:Cr not only significantly enhanced the NIR persistence luminescence of the PLNPs, but also remarkably improved the photocatalytic degradation efficiency of RhB. Addition of Bi³⁺ in ZnGa₂O₄:Cr may produce more trap energy levels, which prolongs the lifetime of the photo-generated electron–holes and the afterglow time. The novel dual-functional PLNPs are promising for in vivo bioimaging and photocatalytic applications.

Conflicts of interest

The authors declare no conflict of interest.