UV excited green luminescence of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor

Abstract

Eu, Dy co-doped strontium aluminate nanophosphors were prepared by the combustion synthesis method. Their structure and morphology were investigated by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy. According to the XRD and the TEM analysis, the average crystallite size was found to be in the nanometer range. The phase structure of the prepared nanophosphor is consistent with a standard monoclinic phase with a space group P2₁. The prepared SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor emitted green light with a peak at 510 nm showing blue shift, which is due to the reduction in the particle size. Two distinct peaks were observed in the ML intensity versus time curve. The two peaks in ML indicate the presence of charge transfer in an ML process.

Introduction

Phosphors are the solid material that emits light, when exposed to radiation such as ultraviolet light or an electron beam. Phosphors display the property of luminescence. Phosphors are made from a variety of materials and different combinations are appropriate for different applications. Hundreds of thousands of phosphors have been synthesized, each one having its own characteristic colour of emission and period of time during which light is emitted after excitation ceases. The alkaline earth aluminate, SrAl₂O₄ is one of the most important persistent luminescent materials (phosphor). Their high initial luminescent intensity and low-dimensional long afterglow property makes the strontium aluminate phosphors an ideal substance to be widely used in many fields such as luminescent paint, luminescent plastic, luminescent ceramic, etc., and may lead to future nanoscale display devices [1–3]. Their afterglow lifetime and intensity can be enhanced by co-doping with some of the rare earth ions [4–6]. Rare earth ions co-doped strontium aluminate (SrAl₂O₄:Eu²⁺, Dy³⁺) phosphors are one of the most efficient long afterglow materials with excellent persistent luminescence in the green region. Compared with sulfide phosphorescent phosphors, SrAl₂O₄:Eu, Dy phosphor possesses safer, chemically stable, very bright, and long-lasting photoluminescence (PL) with no radiation, which results in an unexpectedly large field of applications, such as luminous paints in highways, airports, buildings, and ceramics products, as well as in textiles, the dial plates of glow watches, warning signs, and escape routines, etc. [7–12].

The grain size of phosphor powders prepared through the solid-state reaction method is in several tens of micrometers. Various approaches, such as chemical synthesis techniques, co-precipitation [8], sol–gel [13], and combustion synthesis methods [14], have been applied to prepare SrAl₂O₄ and/or its phosphors. These methods can produce small particles at low temperatures; however, the phase purity remains a problem. Phosphors of small particles must be obtained by grinding the larger phosphor particles. Those processes easily introduce additional defects and greatly reduce luminescence efficiency [15]. The combustion process to prepare the sample, however, is very facile and only takes a few minutes, which has been extensively applied to the preparation of various nanoscale oxide materials. This synthesis technique makes use of the heat energy liberated by the redox exothermic reaction at a relative low igniting temperature between metal nitrates and urea or other fuels. Furthermore, the process is also safe, instantaneous, and energy saving. Therefore, the combustion synthesis appears to hold promise for the preparation of nanosized aluminate phosphors.

The luminescence induced by any mechanical action on solids is known as mechanoluminescence (ML). The light emissions induced by the elastic deformation, plastic deformation, and fracture of solids are called elastico ML, plastico ML and fracto ML, respectively, and the light induced by rubbing of solids or separation of two solids in contact is known as tribo ML or triboluminescence [16, 17]. The ML phenomenon has been known for centuries; however, it is only in recent years the intense elastico and fracto mechanoluminescent materials have been found to have the potential forbeing the stress sensors, fracture sensors, damage sensors, and impact sensors. The phosphor are used for construction materials in detecting damage in a bridge or high buildings, as well as in display and decorative materials in hotels, the plaza, etc.

The present paper reports the SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphors prepared by the combustion route. H₃BO₃ is used as the flux and the urea is used as the fuel. The nanophosphors were characterized using X-ray diffraction (XRD), scanning electron microcopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectra (FTIR) analysis. FTIR spectra were recorded to investigate the qualitative analysis of the elements present in the prepared phosphors on the basis of different vibration bands. Studies of luminescent properties were also been made on the basis of the PL and ML phenomenon.

Experimental

For preparing powder samples of SrAl₂O₄:Eu²⁺, Dy³⁺, stoichiometric amounts of strontium nitrate [Sr(NO₃)₂ (99.90 %)], aluminum nitrate [Al(NO₃)₃·9H₂O (99.90 %)], and urea [NH₂CONH₂ (99.99 %)] were used as raw materials. In addition to these, 2 % europium oxide [Eu₂O₃ (99.99 %)] as activators and 4 % Dy₂O₃ as co-activator were dissolved in concentrated nitric acid (HNO₃) (99.90 %) before transferring them to a silica crucible. Boric acid [H₃BO₃ (99.90 %)] is used as the flux while the urea (NH₂CONH₂) is used as the fuel. After the solution is transferred into the crucible with comparatively larger volume, it is placed into a furnace already maintained at a temperature of 600 °C. Initially, a dehydration process was performed by boiling the solution, followed by evolving decomposition under the presence of large amounts of gases (oxides of carbon, nitrogen, and ammonia). Soon the reagent is spontaneously ignited and undergoes combustion to produce foamy and voluminous ash. The mixture froths and swells forming foam, which ruptures with a flame and glows to incandescence. This continues for the next few seconds and when it is done, the crucible is taken out of the furnace and was kept in the open to allow cooling in an open atmosphere. Upon cooling, we get a fluffy form of material, which is then crushed using agate pestle mortar to get the material in the powder form. The whole process was completed within less than 5 min.

The samples were prepared in a digital furnace maintained at 600 °C temperature. The XRD pattern has been obtained from PANalytical 3 kW X’pert Powder XRD using CuKα radiation, and the data were collected over the 2θ range 15°–50°. The sample required for TEM analysis were prepared by dispersing the SrAl₂O₄:Eu²⁺ phosphor in methanol using an ultrasound bath technique. A drop of this dispersed suspension was put onto a 200-mesh carbon-coated copper grid and then dried in the air. The surface morphology was determined by the field emission scanning electron microscopy (FESEM). The sample was coated with a thin layer of gold (Au) and then the surface morphology of the prepared phosphor was observed by a SEM; (JEOL–JSM-5600) operated at the acceleration voltage of 20 kV attached with the EDX. Energy dispersive X-ray spectroscopy (EDX) was used for the elemental analysis of the prepared phosphor. FTIR spectrum were recorded with the help of IR Affinity-1 by SHIMADZU for investigating the finger print region as well as the functional groups of prepared phosphor in the middle infrared region (4000–400 cm⁻¹) by mixing the sample with potassium bromide (KBr, IR grade). The PL spectra were recorded by a Perkin-Elmer LS 45 Fluorescence Spectrometer using the xenon lamp as the excitation source. The ML was monitored by a homemade setup having a RCA 931 photomultiplier tube positioned below the lucite plate and connected to a storage oscilloscope (Scientific 300 MHz, SM 340). Loads of constant mass (400 g) were dropped from different heights. All measurements were carried out at room temperature.

Results and discussions

XRD analysis

XRD patterns of the SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor are shown in Fig. 1. The prepared sample gives much broader and less intense peaks, indicating smaller grain size and lower crystallinity degrees. From the diffraction peaks of XRD spectra, the lattice constant of the sample is estimated to be a = 0.8401, b = 0.8718, and c = 0.5119 nm. This is less than the bulk counterpart. It indicates that not only the lattice constants contract but also the lattice distortion changes when the size of this complex structure is reduced to nanometre scales. SrAl₂O₄ has two phases, a high-temperature hexagonal phase (β-phase) and a low temperature monoclinic phase (α-phase). It could be observed that only pure SrAl₂O₄ phase exists in the samples; this phase is monoclinic with a space group P2₁. The other impurity phases such as β-SrAl₂O₄, Sr₃Al₂O₆ and Sr₄Al₁₄O₂₅ were not observed. The obtained XRD pattern confirms the existence of a predominant monoclinic SrAl₂O₄ phase, which is estimated to be around 89 %. This was determined through the relative intensity ratio match technique using the ICDD PDF 00-024-1187 and JCPDS No. 34-0379 files. The XRD patterns were similar for all the prepared samples and, hence, are not reported to avoid repetition. No peak other than those from the parent is resolved in the XRD pattern, which suggests the formation of single phase material. It is to be noted that the small amount of doped rare-earth active ions Eu²⁺ and Dy³⁺ have almost no effect on the basic crystal structure of SrAl₂O₄. The principle peaks were identified as the (011), ($\bar{2}$11), (220), (211), and (031) planes that characterize the crystal line SrAl₂O₄ structure. The average crystallite size was determined from the XRD pattern parameters according to the Debye–Scherrer equation D = kλ/βcosθ, where D is the average crystallite size, k is Scherrer’s constant equal to 0.90, β is the full-width at half-maximum (FWHM), and θ is the diffraction angle. The size of the particle computed by using the Debye–Scherrer formula was 25 nm [18]. SrAl₂O₄ undergoes a phase transition from monoclinic to hexagonal at 800 °C. The synthesis temperature at 600 °C is lower than that used normally in sol–gel and solid-state reaction methods, and at this temperature most fractions of the particles are in monoclinic phase.

Fig. 1

XRD patterns of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphors

Transmission electron microscopy (TEM)

Transmission electron microscope image of the prepared SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor is shown in Fig. 2. It reveals that the nanophosphors are nearly spheres of very small size and the grain sizes were distributed in the range of ~3–7 nm, which is less than the mean crystallite size deduced from XRD results. Some of the particles show agglomeration and the agglomerate sizes are <12 nm.

Fig. 2

TEM image of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor

Scanning electron microscopy (SEM)

Figure 3 shows the SEM micrographs of SrAl₂O₄:Eu²⁺, Dy³ powder samples. In order to determine the morphology of the sample, typical SEM measurements were carried out. The morphology shows that the grains are multi-sized with a number of distinct micro-structural features due to agglomeration of primary nanoparticles. Some of the particles could be considered in the nano range. The average particle sizes obtained from SEM images were found to be in the range 0.1–2 µm. The particles possess foamy-like morphology formed from highly agglomerated crystallites. The surfaces of the foams show lots of cracks voids and pores, which were formed by the escaping gases during the combustion reaction. The escaping gases provide an irregular heat dissipation condition and the temperatures to account for the asymmetrical material sintering formation. The SEM image shows clearly that the particles are not exactly spherical.

Fig. 3

SEM image of nano SrAl₂O₄:Eu²⁺, Dy³

Energy dispersive X-ray spectroscopy (EDX)

The composition (qualitative and quantitative) of the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor has been measured using EDX spectrum. Energy dispersive X-ray spectroscopy is a standard procedure for identifying and quantifying elemental composition of a sample area as small as a few nanometers. The existence of europium (Eu) and dysprosium (Dy) in the phosphor is clear in their corresponding EDX spectra. There appeared to be no other emissions apart from strontium (Sr), aluminum (Al), oxygen (O), europium (Eu), and dysprosium (Dy) in SrAl₂O₄:Eu²⁺, Dy³⁺ in the EDX spectrum. All the expected elements with their relative ratios are presented in the Table 1, which confirms the formation of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor (Fig. 4).

Table 1 Composite element of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor

Fig. 4

EDX of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor

Fourier transform infrared (FTIR)

FTIR spectroscopy is a quick and precise measurement technique for qualitative species measurement. The infrared spectrum of an inorganic compound represents its physical properties. Figure 5 shows the Fourier transform infrared spectra of SrAl₂O₄: Eu²⁺, Dy³⁺ nanophosphor. The spectrum exhibits a broad band at ~3500 cm⁻¹ due to the O–H stretching vibrations of the hydroxyl groups, which might be due to the presence of moisture through the environment. The strong peak at ~2900 and 2850 cm⁻¹, and is assigned as due to the N–H anti-symmetric band. The sample showed a weak peak at ~1700 cm⁻¹, and is assigned as due to the C–O anti-symmetric band [19].The appearance of a very strong band at ~1500–1400 and 1200 cm⁻¹ is due to the symmetric stretching vibrations of the N–O group, which might have resulted from the nitrate of the raw material [Sr(NO₃)₂ and Al(NO₃)₃·9H₂O] [20]. The metal oxygen stretching frequencies in the range ~900–700 cm⁻¹ are associated with the vibrations of Al–O and Sr–O–Al bonds. The anti-symmetric stretching bands between ~600, 540, and 500 cm⁻¹ are attributed to the Sr–O vibrations. The symmetric bonding of O–Al–O appeared at around ~460 cm⁻¹. The FTIR spectra confirm the formation of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor [21].

Fig. 5

FTIR spectra of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor

Photoluminescence (PL)

The PL excitation spectra of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor is shown in Fig. 6. The excitation spectra were observed in the range of 220–420 nm. From the excitation spectra, a broad band with excitation peak at 349 nm could be seen. The emission wave length for this excitation spectra was 510 nm. Figure 7 shows the emission spectra of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor. The emission spectra were recorded in the range of 400–625 nm. Under the ultra-violet excitation of 350 nm, the broad emission band was observed. The prepared SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor would emit green light with peak at 510 nm. The emission peak of the phosphor is attributed to the typical emission of Eu²⁺ due to the 4f ⁶5d ¹ → 4f ⁷ transition. Compared with the other researchers, the blue shift in the excitation and emission peak was found, which is due to the reduction in the particle size of the prepared samples in the nanometer range [22, 23]. For the nano SrAl₂O₄, the unit cell volume becomes small and the width of the bandgap becomes large. Thus, the energy of the 5d configuration ascends correspondingly since the 5d energy level has a closer relation to the conduction band of the compound. In our PL measurement, the special Dy³⁺emission spectra found at the 486 nm (bluish-green region) and 577 nm (yellow region) and special Eu³⁺ emission spectrum found at the 590 and 616 nm (orange region) were absent. There is no other emission due to Dy³⁺ and Eu³⁺ that was recorded, which implies that Eu³⁺ ions have been reduced as Eu²⁺ completely, and the co-doped Dy³⁺ did not emit and transferred the absorbed energy to Eu²⁺ ions in the crystal lattice matrices [24, 25]. When Dy³⁺ is doped in phosphors, the Dy³⁺ ions have an influence on the excitation and emission mechanism. Electrons are transferred from the Dy³⁺ ions to the excited Eu²⁺ ions [26, 27]. The interaction between Dy³⁺ and Eu²⁺ ions changes the structure of the energy level of Eu²⁺. The Dy³⁺ ions play the role of inducing the formation of the holes' trap levels and in prolonging the afterglow. The electrons, captured by traps donated by Dy³⁺, can be released slowly and recombined with the excited state of Eu²⁺ under thermal re-excitation, and then return to the Eu²⁺ ground state accompanied with light emission. The trap–transport–detrap process results in the long afterglow phenomenon of the phosphor. Thus, in SrAl₂O₄:Eu²⁺ samples with Dy³⁺, creation of more and more hole-trap levels takes place leading to greater PL intensities. However, the position of the emission peak in the phosphorescence curve shows negligible change, regardless of the varied amount of the Dy³⁺ ions doping.

Fig. 6

Excitation spectra SrAl₂O₄:Eu²⁺, Dy³⁺

Fig. 7

Emission spectra of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor

CIE chromaticity coordinate

In general, the color of any phosphor material is represented by means of color coordinates. The emission spectrum of the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor was converted to the CIE 1931 chromaticity using the photoluminescent data and the interactive CIE software (CIE coordinate calculator) diagram as shown in Fig. 8. The calculated chromaticity coordinates for the green light emitted from SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor is given by (x = 0.2381, y = 0.4502).

Fig. 8

CIE chromaticity diagram of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor

Mechanoluminescence (ML)

In the ML studies, an impulsive deformation technique has been used for measurements. The experiment was carried out for a fixed moving piston (400 g) dropped with different heights of 25, 35, 45, and 55 cm. For every time for a ML measurement, the quantity of the powder sample was kept constant. The samples were not radiated by any of the sources such as UV, laser, X-ray, or γ radiation. Figure 9 shows the characteristic curve between ML intensity versus time for different heights. When a load is applied to SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor light is emitted due to a fracture produced in the prepared nanophosphor, also called a fraco-mehanoluminescence (FML). The FML may occur due to the piezoelectric effect as well as due to an electrostatic field near the charged dislocations; however, for the crystals of micrometer size and the bigger size, the moving dislocations are the origin of FML. In the case of nanoparticles, the dislocation segment is very small and; hence, the bending in the dislocation segment may not be significant to cause the appreciable interaction between the bending segment of dislocation and filled electron traps in the nanocrystals [16]. It was observed that, initially the ML intensity increases with time, attains a peak value and then it decreases with time. Two distinct peaks were observed in the ML intensity versus time curve. The first peak is the transient response of the pressure exerted on the sample whereas the second peak is related to the electrons captured in the shallow traps. In these ML measurements, maximum ML intensity has been obtained for the 55 cm dropping height.

Fig. 9

ML intensity versus time of SrAl₂O₄:Eu²⁺, Dy³⁺ nanophosphor

Figure 10 shows the time dependence of the ML intensity of the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor for different impact velocities. It is seen that ML intensity increases linearly with increasing impact velocity [$\upsilon_{0} = \sqrt {2gh}$, (where h, is the different heights)] of the moving piston. The ML intensity of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor increases linearly with increasing the mechanical stress, that is, the ML intensity increases with the increasing impact energy. When a mechanical stress is applied on the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor, one of its newly created surfaces gets positively charged and the other surface of the crack gets negatively charged. Thus, an intense electric field of the order of 10⁶–10⁷ V/cm⁻¹ is produced. Under such order of electric field, the ejected electrons from the negatively charged surface may be accelerated and, subsequently, their impact on the positively charged surfaces may excite the luminescence centre. Thus, depending on the prevailing conditions, recombination luminescence may be produced. For the impact velocity υ ₀, the impact pressure P ₀ will be equal to, P ₀ = Zυ ₀, where Z is a constant. The application of pressure produces a local piezoelectric field in the phosphor whereby the piezoelectric field near the activator ions (Eu²⁺) may be high due to the change in the local structure. The local piezoelectric field may reduce the trap-depth of the carriers, or it may cause the band bending. In the case of decrease in trap depth of the carriers, thermal detrapping of the carriers may be produced. In the case of band bending, the trapped charge carriers may tunnel to the respective band. Subsequently, the electrons moving in the conduction band may be captured in the excited state of the activator ions located adjacent to the bottom of the conduction band whereby excited ions, for example, excited Eu²⁺ ions are produced. The de-excitation of excited activator ions may give rise to the light emission characteristic of the excited ions.

Fig. 10

ML intensity versus impact velocity of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor

Figure 11 shows that the ML spectrum of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor. The small difference of the peak location between the ML spectra (515 nm) and PL (510 nm) can be ascribed to the different measuring devices. The spectrum consists of a broad emission band peaking at 515 nm, which is similar to the spectra of PL spectra of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor. This implies that ML is emitted from the same emitting center of Eu²⁺ ions as PL, which is also produced by the transition of Eu²⁺ ions between the excited states ($4f^{6}5d^{1}$) to ground state (4f ⁷) and is responsible for the green ML emission.

Fig. 11

ML spectra of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor

Conclusions

The SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors were synthesized by the combustion method. According to the XRD analysis, the average crystallite size was found to be approximately 25 nm. The phase structure of the prepared nanophosphor is consistent with standard tetragonal crystallography. From the TEM analysis, average particle size of the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor was ~5 nm. This confirms that the prepared samples are in the nano range. Under t ultra-violet excitation, the prepared SrAl₂O₄:Eu²⁺ nanophosphor emitted green light with a peak at 510 nm and was confirmed from the calculated CIE coordinates, which were found to be very close to the standard green light for human eyes. The blue shift in the emission peak was observed, which is due to the reduction in the particle size of the prepared samples in the nanometer range. It is worthy to note that the dependence between ML intensity of the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor and the load is close to linearity. Based on the above analysis this nanophosphor could also be used as sensors to detect the stress on an object. Thus, the combustion synthesis method furnishes a simple method for preparing an aluminate- based nanophosphor.