La₂O₂CO₃:Tb³⁺ one-dimensional nanorod with green persistent luminescence

Abstract

Trivalent terbium-doped oxycarbonate (La₂O₂CO₃:1%Tb³⁺) one-dimensional nanorods are synthesized via a facile precipitation method. The average length of La₂O₂CO₃:1%Tb³⁺ nanorods is 184.5 nm. Doping Tb³⁺ ions led to several visible emission peaks at 486 nm, 542 nm, and 587 nm under excitation of 258 nm wavelength light. The green afterglow at 542 nm can be detected almost 600 s after ceasing the UV-light irradiation. It can be calculated that the La₂O₂CO₃:1%Tb³⁺ sample has one shallow trap depth (E = 0.848 eV) by measuring the thermoluminescence. All the results indicate that a simple precipitation method can synthesize a one-dimensional nanorod with green persistent luminescence.

Trivalent terbium-doped oxycarbonate (La₂O₂CO₃:1%Tb³⁺) one-dimensional nanorods are synthesized via a facile precipitation method.

Introduction

Persistent luminescent nanoparticles (PLNPs) are types of optical materials that have plenty of crystal defects as the charge-trappers. PLNPs can store charge carriers (electrons and holes) under excitation with ultraviolet (UV) radiation and then release subsequently for several seconds, minutes and even hours.^1–4 PLNPs are playing an important role in multi-disciplinary fields because of their broad applications, such as in safety signage, night-vision surveillance, latent fingerprint images, decoration and biomedicine.^5–9

Nowadays, only a few terbium doped bulk-phosphors such as Lu₂O₃:Tb³⁺,¹⁰ SrAl₂O₄:Tb³⁺etc,¹¹ have demonstrated their green afterglow. The synthesis method of these materials is a solid-state method, yet almost no successful chemical route can be traced to perfect the size, morphology and dispersion for PLNPs.^10–14 However, the effective combination of nanostructure and rare earth doped persistent luminescent materials are the valid ways to expand the development of PLNPs in the multi-disciplinary fields.^15–17 For example, it had been reported many PLNPs with excellent morphology and size properties synthesized via effectively chemical methods, which have shown great application potential in the field of biology.^2,18–22 Therefore, it is necessary and meaningful to find a method of synthesizing a green PLNPs with controllability morphology and uniform dispersity. Furthermore, the inorganic compounds: La₂O₂CO₃ are used to be host materials. Because rare-earth doping La₂O₂CO₃ can be substituted for La³⁺ ions homogeneously and cannot change host crystal structures, as well as bringing good luminescent properties and high chemical stability.^12,23

In this research, the green La₂O₂CO₃:1%Tb³⁺ PLNPs are prepared via a facile precipitation method which appear to be uniformly rod-shaped (length: 50–200 nm, uniform and regular). The La₂O₂CO₃:1%Tb³⁺ nanorods exhibit a visible emission at 486 nm, 542 nm, and 587 nm under 258 nm excitation. The green afterglow at 542 nm can be detected almost 600 s after ceasing the UV-light irradiation. The thermoluminescence (ThL) spectrum reveals that La₂O₂CO₃:1%Tb³⁺ nanorods have a shallow trap depth at 0.848 eV.

Experiment methods

Sample preparation

Chemicals and materials

The initial rare-earth nitrates, including La(NO)₃·6H₂O (99.99%) and Tb(NO₃)₃·6H₂O (99.99%) were purchased from Shanghai Aladdin Biochemical Technology Company, and NH₄OH (25 wt%) was purchased from Tianjin Zhiyuan Chemical Reagent Company, and anhydrous ethanol was purchased from Sinopharm Chemical Reagent Company. All chemicals were of analytical-grade reagents and were used directly without further purification.

Preparation of La₂O₂CO₃:1%Tb³⁺

The La₂O₂CO₃:1%Tb³⁺ nanorods were compounded by a precipitation method.^23,24 Firstly, 0.01 mol of La(NO₃)₃·6H₂O and 0.0001 mol of Tb(NO₃)₃·6H₂O were added to 100 mL of deionized water followed through vigorous stirring for 10 min. Secondly, NH₄OH (25 wt%) was added to the mixture to reach the pH = 10. Finally, the mixture in a beaker, wrapped with a polyethylene film, was heated in a water bath at 90 °C for 2 h and heavily stirred and then cooled to room temperature. The precursor products La(OH)₃:1%Tb³⁺ nanorods were obtained by washing the resulting precipitates with deionized water and anhydrous ethanol respectively for three times, and then dried at 80 °C for 12 h in the oven. The final La₂O₂CO₃:1%Tb³⁺ nanorods were collected after annealing the precursor products at 600 °C for 3 h in the muffle furnace. The carbon atom maybe come from carbon dioxide in air atmosphere.

Characterization

The X-ray diffraction (XRD) (Cu/Kα) patterns of samples were recorded on a PANAlytical X'Pert PRO X-ray polycrystal diffractometer. Transmission electron microscopy (TEM) images and scanning electron microscopy (SEM) images were obtained on a Thermo-Talos F200S and Hitach-SU8220, respectively. Dynamic light scattering spectrum of samples was performed on a Malvern Instruments-Zetasizer Nano ZS at 25 °C. The dispersant is ethanol. Room temperature photoluminescence (PL), PL excitation (PLE) spectra, afterglow spectrum and decay curves of samples were monitored on a high-resolution spectrofluorometer (UK, Edinburgh Instruments, FLS 980) equipped with a 500 W Xenon lamp as an excitation source, with a Hamamatsu R928P visible photomultiplier (250–850 nm). Thermoluminescence (ThL) spectrum of samples was measured on a FJ-427A TL meter (Beijing, China).

Results and discussions

Phase and crystal structure

The X-ray diffraction (XRD) patterns of samples are described in Fig. 1a. The XRD patterns of the host La₂O₂CO₃ are in good agreement with the hexagonal La₂O₂CO₃ phase (JCPDS No. 37-0804) and no impurity peak was detected. When small amount of Tb³⁺ doped in the host, the peak position remains unchanged but the main peak shape of the XRD patterns becomes wider. In La₂O₂CO₃, the La site (ionic radius: 0.127 nm, CN = 10) bonds to three category O atoms (three O sites: O₁, O₂, O₃), thus offering the applicable decahedron coordination LaO₁₀ to attach the rare-earth emitter Tb³⁺ (ionic radius: 0.110 nm, CN = 9).²⁵ Tb³⁺ may replace La³⁺ lattice site in this architecture based on the ionic radius and LaO₁₀ coordination configuration (Fig. 1b). It is possible that the peak of the doped sample is broadening owing to the substitution of the La³⁺ by the relatively size Tb³⁺.

Fig. 1. XRD patterns (a) and crystal structure (b) of La₂O₂CO₃:1%Tb³⁺ sample.

Morphology and texture

The size and morphology of La₂O₂CO₃:1%Tb³⁺ nanorods are investigated via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). As revealed in Fig. 2a, La₂O₂CO₃:1%Tb³⁺ sample is a kind of one-dimension rod-like nanomaterial with uniform dispersion. Additionally, the hydrodynamic size of La₂O₂CO₃:1%Tb³⁺ nanorods is measured via dynamic light scattering (DLS). It confirms that size distribution follows a normal distribution with average length 184.5 nm. The polydispersity index (PdI) value (0.248) shows that the sample is moderately polydisperse (Fig. 2b).²⁶ These results indicate that the precipitation method can successful compound well-dispersed PLNPs.

Fig. 2. SEM image (a) and dynamic light scattering (DLS) spectrum (b) of La₂O₂CO₃:1%Tb³⁺ sample.

For further revealing the information on morphology and structural features of La₂O₂CO₃:1%Tb³⁺ nanorods, the TEM and HRTEM also be measured. Fig. 3a and b display the typical TEM images of La₂O₂CO₃:1%Tb³⁺ nanorods. It demonstrates that the diameter of nanorods is 15–20 nm and the length are 50–200 nm, which is in keeping with DLS result. Fig. 3c displays the HRTEM image of La₂O₂CO₃:1%Tb³⁺ nanorods. The lattice planes of nanorod are counted to be 2.0388 nm and 1.3299 nm, which belong to (110) and (211) crystallographic planes of La₂O₂CO₃, respectively. It indicates that La₂O₂CO₃:1%Tb³⁺ nanorod is polycrystalline while the different crystal orientations can be found distinctly. The presence of indiscernible lattice fringes in La₂O₂CO₃ nanorod due to the formation of defects.²⁷ The crystal diffraction spot image of a single nanorod (Fig. 3d) insinuates that nanorod is polycrystalline, which matches greatly with the HRTEM results.

Fig. 3. TEM images (a and b), HRTEM image (c) and crystal diffraction spot image (d) of La₂O₂CO₃:1%Tb³⁺ sample.

Photoluminescent properties

The excitation and emission spectra are used to illustrate the photoluminescent property of La₂O₂CO₃:1%Tb³⁺ nanorods (the doping content of 1% has been reported to be a suitable value for preparing Tb³⁺-doped luminescent phosphors),^5,23,28,29 as shown in Fig. 4a. An evident excitation peak at 258 nm can be measured when monitoring at 542 nm emission at room temperature. It can be assigned to the spin allowed inter-configurational 4f⁸–4f⁷5d¹ transition of Tb³⁺.^30–33 The emission spectrum of La₂O₂CO₃:1%Tb³⁺ covers a wide wavelength range between 400 nm and 600 nm. Peaks at 486 nm, 542 nm and 587 nm are observed, which can be ascribed by the 4f–4f transitions from ⁵D₄ to ⁷F_J (J = 6, 5, 4). The blue emission peaks in 400–470 nm are attributed to the ⁵D₃ to ⁷F_J (J = 5, 4, 3) transitions, respectively (Fig. 4b).^9,32,34–37 The bright green-light emitting belongs to the strongest transition ⁵D₄ to ⁷F₅ of largest probability for both forced electric dipole and magnetic dipole induced transitions.^13,36

Fig. 4. The excitation and emission spectrum (a) and the energy levels image (b) of the La₂O₂CO₃:1%Tb³⁺ sample.

Persistent luminescence property

The afterglow decay curve of La₂O₂CO₃:1%Tb³⁺ sample is presented in Fig. 5a. It clearly reveals that the decay process includes a fast decay part and a slow decay process. A strong afterglow at 542 nm appear at the initial stage of fast decay while a long afterglow duration almost 600 s after ceasing the UV-light irradiation support the existing slow decay. The color of the sample is yellow-color possibly due to the existence of extra tetravalent terbium ions (Tb⁴⁺).³⁸ Particularly, no feature fluorescence of Tb⁴⁺ are found in Fig. 4a, because it may play the role of ionic impurities. It is well-known that the generation of long persistent luminescence is primarily attributed to the capture of excited electrons via creating traps in the host lattice.¹ The ThL curve can provide vital information about the number, depth and density of traps,³⁹ which are advantageous to reveal the possible afterglow mechanism. Therefore, ThL curve of La₂O₂CO₃:1%Tb³⁺ sample is measured (Fig. 5b). Samples were excited at T_exc by 254 nm light for 20 min, the heating rate of 2 K s⁻¹. ThL spectrum consists a broad band locating at 424 K. The depths and densities of traps can be estimated by the following equations:⁴⁰E = T_m/500 (where E is the thermo-active energy of trap depths (in eV) denoting the energy gap between the electron trap and the conduction band, and T_m is the ThL peak temperature in kelvin). The trap depth of La₂O₂CO₃:1%Tb³⁺ is 0.848 eV.

Fig. 5. The persistent luminescence spectrum (the duration time is 5 s between the turn-off of the excitation light and the beginning of the measurements, and measurement apparatus) (a), thermoluminescence spectrum (the scan rate is 2 K s⁻¹, and the duration time is 10 s between the turn-off of the excitation light and the beginning of the measurements for the TL measurements) (b), EPR spectrum (c) and the afterglow mechanism (d) of the La₂O₂CO₃:1%Tb³⁺ sample.

Electron spin resonance (ESR) is an effective and direct characterization to analysis varied behaviours of defects.^13,26Fig. 5c exhibits the ESR test result of La₂O₂CO₃:1%Tb³⁺ sample. It can be seen an ESR signal at g = 2.0, which is close to free electrons generating from oxygen vacancies ^26,41–43 is a common electron-trap in oxide long afterglow phosphors.⁴⁴

The terbium ions in the La₂O₂CO₃ host lattice would substitute the La³⁺. Hence, when most of Tb³⁺ ions entered La³⁺ site, the neutral defect sites will be given as eqn (1):^36,44

1

On the other hand, the Tb⁴⁺ impurities also have chance to substitute the La³⁺, the unbalanced substitution occurred as eqn (2):^36,45

2

The Tb ions with a positive net charge acted as electron-traps. Meanwhile, a charge compensation is coming out owing the unequally charges possibly.⁴⁶ It means a simultaneous creation of a La³⁺ vacancy which acted as hole-traps.

Based on experimental results of afterglow properties, doping Tb³⁺ ions not only acted as an activator but also formed point defects The Tb⁴⁺ impurities not only changed the sample's color, but also created electron-traps and hole-traps A possible diagrammatic drawing of La₂O₂CO₃:1%Tb³⁺ afterglow mechanism is shown in Fig. 5d. To explain, the Tb³⁺ energy levels are putted between the conduction band (CB) and valence band (VB). The band gap of La₂O₂CO₃ is 5.83 ev.²⁴ During ultraviolet (UV) light irradiation (①), most of the excited electrons and holes are transferred through the CB and VB to the luminescence centers of Tb³⁺ respectively (②), and then causing the characteristic emissions of Tb³⁺ (⑤). However, some of the excited electrons and holes will be captured by electron-traps from CB or hole-traps from VB through non-radiative transition respectively (③). Then, the stored electrons and holes can escape from the traps by the proper thermal activation, and the released energy be transferred to Tb³⁺ ions via the VB and CB (④). Then the afterglow of Tb³⁺ (⑤) is appearing.

Conclusions

In this paper, the La₂O₂CO₃:1%Tb³⁺ PLNPs have been synthesized via a facile precipitation method. The morphology and texture of La₂O₂CO₃:1%Tb³⁺ sample display uniform dispersion and rod-like shape. The strongest emission peak locates at 542 nm which attributes to ⁵D₄ to ⁷F₅ transition. The green afterglow at 542 nm can be detected almost 600 s after ceasing the UV-light irradiation. ThL measurement reveals that La₂O₂CO₃:1%Tb³⁺ sample possesses one shallow trap depth calculated to be 0.848 eV. The results show that the one-dimensional nanorods with green long afterglow can be successfully synthesized via a simple precipitation method.

Conflicts of interest

There are no conflicts to declare.