Hierarchical zinc aluminate 3D nanostructures, synthesized by bio-inspired ultrasound assisted sonochemical route: Display and dosimetry applications

1 Introduction

Thermoluminescence dosimetry (TLD) has an application in personnel and environmental dosimetry, radiation therapy, ageing of archaeological and geological samples, etc. Thermoluminescence (TL) property plays an vital role in acquiring the data of the kinetic parameters such as trap depth (E), frequency factor (s) and order of kinetics of the traps present in the solids (Kitis et al., 1994; Kortov et al., 1994). Further, TL depends on the numerous parameters such as the energy band gap, fabrication route, crystallite size, lattice defects and largely the effects of impurities/dopants present in the materials (Premkumar et al., 2013a). To control the TL response, different types of materials both in bulk and nano were prepared and discussed in the literature (Premkumar et al., 2014; Sunitha et al., 2014, 2013; Burda et al., 2005). Presently, nanophosphors with different morphologies have attracted several researches in material science, particularly in the field of luminescence. Recent literature evident that TL studies of various luminescent nanomaterials showed better potentiality when compared to the conventional microcrystalline phosphors towards the dosimetry of ionizing radiations (Shivaram et al., 2013; Premkumar et al., 2013b; Hari Krishna et al., 2014). Therefore, in order to obtain efficient TLD phosphor, synthesis method plays a key role in engineering the crystallite size, shape and surface area of the phosphors (Vossmeyer et al., 1994; Dimitrijevic et al., 2007).

Till date numerous synthesis routes have been used to prepare ZnAl₂O₄ nanopowders, such as solid-state, co-precipitation, hydrothermal, sol-gel and solution combustion methods (Rani, 2017; Yang et al., 2017; Kumari and Dwivedi, 2016; Premkumar et al., 2013b; Kumar and Gupta, 2015). However, the majority of these methods needs sophisticated equipment and complicated experimental procedures. Therefore, it is still a challenge to research community to explore a simple, fast, environmental friendly and cost effective synthesis method to avoid highly toxic chemicals and utilization of more energy (Lou and Hao, 2004; Strek et al., 2000; Cheng et al., 2006; Rusu et al., 2009). Ultrasound assisted sonochemical synthesis method using plant extracts as surfactants have showed the better path towards the same (Venkataravanappa et al., 2016; Skorb and Andreevac, 2013). The main advantage of this method was to reduce the calcination temperature, selection of new solvents and vary the experimental condtions to get the required shape, size and surface area of the product and in shorter duration. When ultrasound was passed through a liquid, successive rarefaction and compression of waves at a very high temperature and pressure will create disintegration and collapsing of bubbles which was sufficient to modify the materials properties (Bang and Suslick, 2010).

As a green revolution, use of naturally existing contents as surfactants for the synthesis of nanomaterials is of more advantageous. Therefore, the bio- inspired synthesis of nanostructures is highly proficient and cost-effective (Sankar et al., 2014). Recently, several extracts from the plants were used to fabricate superstructures of oxides, silicates, sulphides, fluorides, etc. (Suresh et al., 2018; Venkataravanappa et al., 2016; Amith Yadav et al., 2017; Darshan et al., 2016; Dhanalakshmi et al., 2017).

Aloe Vera (A.V.) was an eternal juicy plant grows in dry and hot climatic conditions has been used for multifunctional applications such as antiprotozoal, UV protective, anti-inflammatory and wound-healing properties (Reynolds and Dweck, 1999; Shin et al., 1997; Umano et al., 1999; Saccu et al., 2001). In addition, it has been reported that the employ of A.V. gel was found to be highly useful for the synthesis of various shapes of nano/micro structures because of its environment friendly, non-polluting compatible solvent system. It is also eco-friendly reducing agent and a non-hazardous gelling agent for stabilizing the nanostructures (Chandran et al., 2006; Maensiri et al., 2008; Phumying et al., 2013; Klinkaewnarong et al., 2010; Laokul and Maensiri, 2009; Laokul et al., 2011; Visinescu et al., 2011; Varma, 2012; Boudreau and Beland, 2006).

Over the past few decades, many researchers have been focused on the fabrication of efficient luminescent nanophosphors due to its wide range of applications in display, solar cells, catalysts, sensors, etc. (Thinesh Kumar et al., 2012; Ciupina et al., 2004; Van der Laag et al., 2004). Among them, nano zinc aluminate (ZnAl₂O₄) has been found to be an efficient host material with a wide optical band gap (3.8 eV), high thermal stability and possibility of generating broad band emission. Due to its transparent and electro conductive properties, it can be used for ultraviolet (UV) photo electronic devices (Ravikumar et al., 2014a).

To the best of our knowledge, this is the first report on the synthesis of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors using bio-sacrificial A.V. gel assisted ultrasonication method. Influence of various experimental parameters on the morphologies of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors was systematically investigated. The TL, photoluminescence (PL) and photometric characteristics of fabricated phosphors were thoroughly studied and discussed.

2 Experimental

3 Results and discussion

Fig. 1 shows the SEM micrographs of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors synthesized with different ultrasound irradiation time (1–6 h). When ultrasound irradiation time was at ∼1 h, spike-like structures begin on the surface was observed (Fig. 1(a)). When it was increased, small spikes undergo self assembly and grow to form well-defined clear spike like structured with typical widths of ∼20–45 μm. The physical phenomenon in this process involves acoustic cavitation. Due to ultrasound irradiation, the formation, growth and collapse of the bubbles happens in the reaction mixture. The solute vapour diffuses volume of the bubble which leads to bubble growth. In addition, evaporated water molecules and gas molecules were also incorporated in the bubbles. When the bubbles reaches to unstable size, collapse of bubbles will takes place in the reaction mixture. This collapse can generate high temperature ∼5000 °C and high pressure ∼2000 atm in the reaction environment (Suslick and Doktycz, 1990). The generation of shock waves can also originated due to bubble collapse. Moreover, the bubble collapse happened in a very short time, and was favored to very high cooling rate at ∼10¹¹ K/s. Hinder to crystallization and assembly of the product was due to this high cooling rate. This may be the reason for the formation of hierarchical morphology of materials in the presence of ultrasound irradiation at low temperature. The schematic representation to demonstrate the nucleation, growth and collapse of a bubble stimulated by ultrasound irradiation was shown in Fig. S2.

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Fig. 1 SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor synthesized with different sonication time (1, 2, 3, 4, 5 and 6 h) with 20 ml A.V. gel concentration.

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Fig. 2 shows the SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors with different concentration of A.V. (5, 10, 15, 20, 25 and 30 ml) and ultrasonic irradiation time of about 5 h. It was evident that, as the concentration of A.V. gel increases, the layer formation in structures gradually reduces, finally a conversion of layered to bud like structures were obtained. To know the establishment of the layered like morphology, a methodical broad range (from acidic to basic) of pH controlled (5, 7, 9, 11, 12 and 13) experiments were done on ZnAl₂O₄:Tb³⁺ (3 mol%) and its influence on the structure/microstructure of prepared superstructures were investigated with 5 h of ultrasonic irradiation time (Fig. 3). SEM studies revealed that the morphology of the products varied significantly by changing pH values from 3 to 8 during mixing of the reactants. When pH is equal to 3 and 4, resulted in the self-assembly of spherical structures, whereas, when pH is equal to 5, resulted into the cactus like morphology. We also found that increasing of pH after a certain threshold value (for example pH 9) resulted in total collapse of the layered-like morphology. Fig. 4. SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor with different sonication power (20, 22, 24 and 26 kHz) and ultrasonic irradiation time of about 5 h. The above results evident that the concentration of A.V. gel, pH and sonication power plays an important role in tuning the 3D-micro/superstructures of ZnAl₂O₄ nanophosphor.

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Fig. 2 SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor fabricated with different concentration of A.V. gel (5, 10, 15, 20, 25 and 30 ml) with 5 h of ultrasonic irradiation time.

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Fig. 3 SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor synthesized with various pH values (5, 7, 9, 11, 12 and 13) with 5 h of ultrasonic irradiation time and 25 ml of A.V. gel.

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Fig. 4 SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor synthesized with different sonication power (20, 22, 24 and 26 kHz) with 5 h of ultrasonic irradiation time and 25 ml of A.V. gel.

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The effect of temperature on morphology of the prepared sample was systematically examined for temperatures 50 ​°C, 60 °C and 70 °C in the presence of ultrasound. It was evident from the images that the orderly stacked flakes side by side to form nacre-like structure when temperature is 50 °C. When temperature was increased to 60 °C, groove-like structure was observed. Further, flower-like morphology was observed when temperature was increased to 70 °C (Fig. 5). To know the effect of ultrasound during the synthesis, for the reaction mixture prepared with 30 ml A.V. gel, mechanical stirring of various time period (1 h, 2 h and 3 h) was applied instead of ultrasound. Interestingly large agglomerated, irregular particles were formed when the stirring time was 1 h (Fig. 6). In addition, self assembled particles were observed after 3 h of stirring. The stirring up to 3 h was required to complete the precipitation. This shows that ultrasound assisted fabrication method requires extremely less time to prepare nanostructures and also it supports the effect of cavitation for the formation of well-ordered crystallized nanostructures. A.V. gel contains many varieties of polysaccharides wrapped with protein chains. At the time of stirring, these networks of polysaccharides reassembles in a complex structured form and then during dissociation it leaves behind the trapped nanopartilces. This can be explained by egg-box model as reported elsewhere (Lakshmesha et al., 2014; Kavyashree et al., 2015; Basavaraj et al., 2017).

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Fig. 5 SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor prepared with different temperature (50, 60 and 70 °C) with 5 h of ultrasonic irradiation time.

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Fig. 6 SEM images of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor obtained with mechanical stirring by different time (1, 2, and 3 h,) with 30 ml of A.V. gel.

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Depending upon the nature of solvent, reactants (e.g., volatile and non volatile precursors) and vapour pressure, normally the reaction takes place in three different regions. Initially the reaction process may occur in interior part of the cavitation bubbles and bulk solution in case of volatile precursors (e.g., metal carboxylates, water etc.) whereas in non-volatile precursors (metal acetates and metal chlorides), the reaction takes place at interface of cavitation bubbles and the bulk solution. The reaction takes place between the reactant molecules and surviving OH^∗ or H^∗ at room temperature in third region. In ultrasound process water pyrolyzed into OH^∗and H^∗ radicals as given below.

The reaction mechanism for the synthesis of ZnAl₂O₄:Tb³⁺ nanophosphor was as below:

TEM, HRTEM images and SAED patterns of ZnAl₂O₄:Tb³⁺ (3 and 5 mol%) nanophosphors synthesized with 5 h sonication time was shown in Fig. S3. TEM images shows thin sheet like nanostructures of ZnAl₂O₄:Tb³⁺ nanophosphor. The HRTEM image shows the well - defined crystal planes with an average spacing of ∼0.28 nm and 0.32 nm (Fig. S3(c & d)). The SAED patterns were well matched with the (h k l) values corresponding to the prominent peaks of the PXRD profiles which is discussed in the subsequent section.

The PXRD patterns of undoped and ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors was shown in Fig. 7(a). The obtained patterns exhibits cubic phase and were well indexed with JCPDS No. 82-1043 (Ravikumar et al., 2014b). As the concentration of dopant Tb³⁺ increases, no appreciable shift in diffraction peaks was observed which indicate the Tb³⁺ ions were successfully occupied in the host sites. The average crystallite size (D) of the prepared samples was calculated from both W-H plots (Fig. 7(b)) and Scherrer’s relation (Darshan et al., 2016) and tabulated in Table 1. It was evident that the values of “D” was found to decrease with increase of dopant concentration. The PXRD profile of the synthesized sample under ultrasound method exhibits more intense peak and appreciable line broadening were observed as compared to samples synthesized by using conventional stirring method (shown in blue rectangle in Fig. 8). These results were evident for the smaller crystallite size of the prepared sample using ultrasound method as compared to mechanical stirring method. In addition, PXRD pattern of prepared phosphor using stirrer method exhibits the presence of traces of by-products due to incomplete reaction of solution (traces shown with star mark in Fig. 8).

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Fig. 7 (a) PXRD patterns and (b) W–H plots of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphor.

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Fig. 8 PXRD patterns of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor synthesized with and without ultrasonic irradiation.

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To investigate the various structural parameters, Rietveld refinement of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors has been performed using FullProf software (Rodriguez-Carvajal, 2010). The Rietveld refined parameters namely occupancy, atomic functional positions of the product was listed in Table 2. It was observed that the prepared samples exhibit cubic crystal structure with lattice parameter a = 8.0707 Å, V = 525.696 ų and space group = Fd $\overline{3}$ m. The refined parameters elucidate that the ZnAl₂O₄ belong to the Oh₇ group with a tetrahedral coordination for the Zn²⁺ ions at 8a positions and a trigonal distorted octahedron surrounding the Al³⁺ site at 16d positions. In normal spinels, all the octahedral positions were occupied by Zn²⁺ ions. If the spinel was partially inverse, some Zn²⁺ ions occupy 16d positions and some Al³⁺ ions occupy 8a positions (Ravikumar et al., 2014b). The refined XRD profile of ZnAl₂O₄:Tb³⁺ (3 mol%) was shown in Fig. S4(a). Further, refined parameter was used as inputs for the construction of packing diagram using Diamond software and was depicted in Fig. S4(b).

Fig. S5 shows the FTIR spectra of undoped and Tb³⁺ (0.5, 1 and 3 mol %) doped ZnAl₂O₄ nanophosphors. The bands at ∼3443 and 1645 cm⁻¹ were attributed to —OH stretching vibrations and deformative water molecules. Further, the band at ∼1524 cm⁻¹ was attributed to Al-O stretching vibration. The band at ∼489, 574 and 680 cm⁻¹ was due the stretching vibrations (tetrahedral and octahedral) in the host lattice (Ravikumar et al., 2014b).

Fig. 9(a) shows the TL glow curves of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors irradiated with 5 kGy γ - dose. A symmetric peak at 194 °C was recorded for all doped samples. It was observed that the intensity increases up to 3 mol% and then decreases. The traps of the ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors acquire the maximum capacity to trap charge carriers. Further, the TL glow curves of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor synthesized with and without ultrasound irradiation under same irradiation dose (5 kGy) was shown in Fig. 9(b). It was observed that, TL intensity found to be more in ultrasound irradiation when compared to normal stirring. Fig. 9(c) depicts the TL response of the optimum ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor was investigated over the different gamma dose range from 0.1 to 6 kGy at a heating rate of 2.5 °C s⁻¹. A unique intense peak at 197 °C was observed for the entire range of samples. The integrated area of the curve (197 °C) increases up to 5 kGy gamma dose after that it diminishes (Fig. 9c). This was mainly due to concentration quenching phenomena and was commonly observed for all the systems. As the dopant ions concentration increases, ions may act as self-quencher by undergone non-radiative cross transitions resulting in quenching of luminescence (Premkumar et al., 2012a). Further, it was observed that with increase in the γ-dose, the peak positions were slightly shifted towards higher temperature side. This was due to luminescent centers/traps as well as the site symmetry of the dopant ions in the host material [29]. Fig. 9d witnessed the good linearity in the lower gamma doses (5 kGy), while for higher γ-doses (>5 kGy) the sample display sub-linear behavior. This linearity behavior was extremely beneficial in radiation dosimetry for the measurement of high γ-doses (Sunitha et al., 2012).

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Fig. 9 (a) TL glow curves of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors irradiated with 5 kGy γ - dose, (b) TL glow curves of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor with and without ultrasound irradiation, (c) TL glow curves of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor irradiated to different γ - dose and (d) variation of TL glow peak intensity at 197 °C with different γ - doses (0.1–6 k Gy).

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The sub-linear behavior of TL intensity with various γ-doses can be described with the help of Track interaction model (TIM) (Horowitz et al., 1996). The luminescent centers produced through γ-doses depend on the length and cross-section of the tracks in the ZnAl₂O₄ host matrix. For smaller sized particles especially in nanostructured materials, the size of the track was very much less therefore, for lower γ-irradiated samples the trap centre/luminescent centre (TC/LC) will be very small due to smaller sized particles which may miss during γ-exposures. On the other hand, with increase in γ exposures, the particles which were missed previously will now start generating TCs/LCs resulting in increase of TL intensity. Still further increase in γ exposures, the overlapping of the defect centre (tracks) takes place due to which the production of TCs/LCs results in saturation or decrease in TL intensity (Manjunatha et al., 2012).

Fig. 10 shows the effect of various heating rates (2, 3 and 5 °C s⁻¹) on TL peaks of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors γ - irradiated with 5 kGy irradiation. It was observed that with the increase in heating rate, TL peak shifted towards higher temperature side. Further, increase in heating rate, the TL intensity diminishes due to thermal quenching phenomena. This non - radiative thermal quenching phenomenon was due to discharge of charge carriers from traps recombining at recombination centers and non-radiative transitions were responsible for various transformation energies (Umesh et al., 2014). Fading characteristics of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors was presented in inset of Fig. 10.

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Fig. 10 TL glow curves of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor recorded for different heating rates (γ dose = 5 kGy) [Inset: Fading characteristics of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor].

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A phosphor to show good dosimetric applications, the glow curve should be finely resolved unique peak TL glow curves along with simple trap distribution. TL peaks were related to the trap levels which were present in the band gap of solid materials. These trap levels were considered by estimating diverse kinetic parameters (E, b, and s) related with these levels. Kinetic parameters were discussed in literature (Ravikumar et al., 2014b). The dosimetric nature of prepared samples was discussed based on these kinetic parameters. Consequently, various methods were used to estimate the kinetic parameters of trap levels. In the present studies, four different methods were employed to estimate the kinetic parameters.

3.1

3.1 Estimation of kinetic parameters by various methods

3.1.1

3.1.1 Chen’s method

The activation energy ‘E’ was calculated by employing the Chen’s set of empirical equations as discussed in the literature (Chen and Kirish, 1981). The form factor (µ_g) was estimated and found to be in the range 0.42–0.47. Generally, for first and second order kinetics (µ_g) close to 0.42 and 0.52 respectively (Manjunatha et al., 2012). Therefore, in the present case order of kinetics was found to be unity. The frequency factor (s) was estimated by using the following equation;

(12)

$$s=\frac{\beta E}{{\mathit{kT}}_m^2}\exp \left(\frac{E}{{\mathit{kT}}_m}\right){\left[1+(b-1){\mathrm{\Delta }}_m\right]}^{-1}$$ where $\beta =\frac{\mathit{dT}}{\mathit{dt}}$ , ${\mathrm{\Delta }}_m=\frac{2{\mathit{kT}}_m}{E}$ and $\mathrm{\Delta }=\frac{2\mathit{kT}}{E}$ .

3.1.2

3.1.2 Grosweiner method

‘E’ and ‘s’ values were estimated as per literature reported elsewhere (Hari Krishna et al., 2014) and tabulated in Table 3.

Table 3 Estimated kinetic parameters of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor γ-irradiated in the dose range 0.1–6 kGy.

γ-dose (kGy)	Tm (°C)	b (μg)	Activation energy E (eV)			Frequency factor s (Hz)
			Chen	Grosweiner	Luschiks	Chen	Grosweiner	Luschiks
0.1	188	1(0.46)	0.640	0.657	0.690	1.49 × 107	1.07 × 107	1.99 × 107
0.25	193	1(0.42)	0.568	0.558	0.566	1.17 × 107	1.93 × 107	1.28 × 107
0.5	194	1(0.41)	0.542	0.555	0.541	8.06 × 106	8.64 × 106	8.43 × 106
1	195	1(0.43)	0.556	0.554	0.552	5.26 × 107	5.00 × 107	5.87 × 107
2	195	1(0.43)	0.566	0.565	0.556	9.71 × 107	9.86 × 107	9.85 × 107
3	196	1(0.44)	0.647	0.609	0.671	3.81 × 107	3.27 × 107	3.36 × 107
4	197	1(0.43)	0.554	0.551	0.540	5.22 × 107	5.77 × 107	5.71 × 107
5	197	1(0.50)	0.694	0.596	0.537	2.94 × 107	2.20 × 107	2.53 × 107
6	198	1(0.42)	0.559	0.556	0.551	8.00 × 106	8.35 × 106	8.34 × 106

3.1.3

3.1.3 Luschiks method

‘E’ and ‘s’ values estimated using the below mentioned relation (Premkumar et al., 2012b);

(13)

$$E=0.978\frac{{\mathit{kT}}_m^2}{\delta }$$

(14)

$$s=0.976\left(\frac{\beta }{\delta }\right)\exp \left(0.976\frac{T_m}{\delta }\right)$$ where δ = T₂–T_m, β; the heating rate, T_m; maximum temperature peak.

The values of ‘E’ and ‘s’ for ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors was estimated from above mentioned methods and tabulated in Table 3. Further, fading characteristics of prepared nanophosphors was monitored at RT. The observed fading was ∼25% over a period of 30 days is high enough to be considered as significant TLD phosphor in dosimetry.

Fig. 11 shows the PL excitation spectra of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors monitored at 543 nm emission. The spectra exhibits a series of peaks at ∼317 nm, 350 nm and 379 nm corresponds to 4f–4f transitions of dopant Tb³⁺ ions (Darshan et al., 2016c). Fig. 12(a) depicts the PL emission spectra of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors excited at 379 nm at RT. The spectra consists of characteristic peaks at ∼489 nm, 543 nm, 586 nm and 621 nm due to ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄ and ⁵D₄ → ⁷F₃ transitions respectively (Darshan et al., 2016c). The peaks at ∼543 nm (green emission) and 489 nm (blue emission) were due to magnetic dipole (MD) and electric dipole (ED) transitions of Tb³⁺ ions respectively (Darshan et al., 2016c). The PL spectra of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphors with and without ultrasound was shown in Fig. 12(b). Figure clearly shows that the both the curves exhibit similar behavior but difference in PL intensity. Further, asymmetry ratio (A₂₁) was used to determine the degree of distortion from the inversion symmetry of the local environment of the Tb³⁺ ions in host matrix with varying Tb³⁺ concentration (Venkatachalaiah et al., 2017a).

(15)

$$A_{21}=\frac{\int I_2\left({}^5{\text{D}}_4\to {}^7{\text{F}}_6\right)d\lambda }{\int I_1\left({}^5{D}_4\to {}^7{F}_5\right)d\lambda }$$ where I₁ and I₂; intensities of MD and ED transition respectively. Variation of A₂₁ with varying Tb³⁺ concentration in ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphor was tabulated in Table 4. Fig. 12(c) depicts the effect of various Tb³⁺ concentrations on PL emission intensity of the host. It was observed that the PL intensity was increased up to 3 mol% of Tb³⁺ concentration and afterwards decreases was due to concentration quenching phenomena resulting from energy transfer between Tb³⁺ ions.

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Fig. 11 Excitation spectra of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphor at λ_Emi = 543 nm.

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Fig. 12 (a) Emission spectra of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphor, (b) emission spectra of ZnAl₂O₄:Tb³⁺ (3 mol%) nanophosphor with and without ultrasound irradiation, (c) variation of PL intensity with concentration of Tb³⁺ ions and (d) logarithmic plot of (x) V/s (I/x).

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Table 4 Judd-Ofelt intensity parameters (Ω₂, Ω₄), Emission peak wavelengths (λ_p in nm), radiative transition probability (A_T), calculated radiative (τ_rad) lifetime, branching ratio (β_R) and asymmetric ratio (A₂₁) of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphor (λ_ex = 379 nm).

ZnAl2O4:Tb3+conc. (mol%)	Judd-Ofelt intensity parameters (×10−20 cm2)		Emission peak wavelength λp in nm	AT (s−1)	τrad (ms)	βR	A21
	Ω2	Ω4
0.25	6.23	1.08	549	50.7	17.3	0.9	0.25
0.5	6.48	1.24	543	56.8	20.3	0.9	0.69
1	6.70	1.45	544	53.9	25.2	0.99	0.67
3	6.99	1.89	544	60.7	14.7	0.9	0.40
5	7.2	2.01	543	58.5	27.4	0.9	0.21

To know the type of interaction involved in energy transfer phenomena one can estimate using Van Uitert theory (Van Uitert, 1971). The emission intensity (I) per activator ion is as below:

(16)

$$\frac{I}{X}=K{\left(1+\beta {(X)}^{Q/3}\right)}^{-1}$$ where $X$ ; Tb³⁺ ions concentration, K and β; constants for a given excitation (λ = 379 nm) and Q; a constant of multi-polar interaction (dipole – dipole (d–d); dipole – quadrupole (d–q) and quadrupole – quadrupole (q–q) interactions when Q is equal to 6, 8 and 10 respectively)

(17)

$$\mathit{Log}\frac{I}{\chi }=A-\frac{Q}{3}\log \chi$$ where $\left(A=\log k-\log \beta \right)$ . Fig. 12(d) shows the curve of log I/χ v/s. log χ for ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors and which clears that the relation was approximately linear and the slope of the line was ∼−1.926. The calculated value of Q was found to be 6.64 which was almost equal 6 shows that the concentration quenching phenomena was due to d-d transition.

The J-O intensity parameters (Ω₂ & Ω₄), radiative transition probability, lifetime, branching and asymmetric ratio were evaluated and tabulated in Table 4. The intensity of the hyper sensitive electric dipole (⁵D₄ → ⁷F₅) transition depends on the local symmetry of Tb³⁺ ions in the host. The value Ω₂ was due to structural changes or covalency of Tb³⁺ ions and also depends on short range effects. The change in values of Ω₂ observed in Table 4 was attributed to distortions present around the dopant Tb³⁺ ion sites. As the concentration of Tb³⁺ ions increases, the value of Ω₂ was also increased which show that high covalence of the metal-ligand bonds of Tb³⁺ ion sites in the host matrix. The parameter Ω₄ was due to the long range effects and symmetry of Tb³⁺ ions. Its value increases with increase in the dopant concentration, results to decrease in the electron density of the ligands. The measured branching ratio of ZnAl₂O₄:Tb³⁺ nanophosphors were found to be ∼0.9 > 0.50. Therefore, the present phosphor was suitable for green color emitting display devices.

Fig. 13(a) shows the CIE (International Commission on Illumination) (Darshan et al., 2016d; Venkatachalaiah et al., 2017b) chromaticity diagram of the prepared ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors. CIE color co-ordinates (x, y) were calculated and listed in Table 5. CIE co-ordinates for the prepared samples were located in ‘green’ region. Further, the correlated color temperature (CCT) was assessed by Planckian locus, which was only a small portion of the (x, y) chromaticity diagram and there exist many operating points outside the Planckian locus (Basavaraj et al., 2017; Venkataravanappa et al., 2017). The CCT diagram of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors was shown in Fig. 13(b) and the obtained values were summarized in Table 5. Based on the obtained results, it was clear that the present phosphor was found to be highly useful in solid-state lighting and display devices fabricated with UV (InGaN) or blue (GaN) chips.

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Fig. 13 (a) CIE and (b) CCT diagram of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphor.

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Table 5 Photometric characteristics of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphor.

Tb3+ concentration (mol %)	X	Y	CP (%)	CCT (K)
0.25	0.3259	0.4698	85	2855
0.5	0.3029	0.4474	84	4255
1	0.3207	0.4717	88	4905
3	0.3207	0.5053	90	5028
5	0.3170	0.5240	81	3829

4 Conclusions

A series of ZnAl₂O₄:Tb³⁺ (0.25–5 mol%) nanophosphors were synthesized by self-sacrificial A.V. gel assisted ultrasonication method. This method offered more advantages including fast and simple process with reproducibility of nanostructures and their possible applications. Various structural evolutions of present samples were systematically investigated by SEM studies. Gamma induced TL studies in the range 100 Gy to 1 kGy exhibited a well resolved glow peak at 197 °C. The intensity at the glow peak temperature increases linearly up to 5 kGy and then it decreases with further increase of γ-dose. Up to 5 kGy, the phosphor was quite useful for TL dosimetry. The PL spectra evidence that present phosphor emits bright green color under near ultra violet excitation (379 nm). Photometric characteristics of the synthesized phosphors were estimated by using PL data. The present work explores new possibilities to the design of new engineered nanostructures for display devices and dosimetry applications.