1 Introduction

In the recent years, scientists all around the world have been making efforts in producing nanophosphor materials as these are considered as very valuable asset in the field of optical applications such as solid-state laser devices, optical amplifiers, light emitting diodes (LED’s), solar cells, X-ray medical radiography, cathode ray-tubes, sensor technology, optical markers, and plasma display panels [1,2,3,4]. The motive is to design and develop a near-ultraviolet (NUV) excited but eco-friendly phosphor material with better luminescence, long emission lifetime, high thermal stability, narrow emissions bands and with reasonable product reliability [5,6,7,8,9]. In this context, rare-earth trivalent ions play an important role when incorporated in a suitable host matrix; which is obviously due to their outstanding luminescent properties, arising from the intra-configurational 4f–4f and 4f–5d transitions [10]. Furthermore, it is a quite well-known fact that Tb3+ ions can act as activators for green-emitting nanophosphor in various suitable host matrixes. Many researchers are paying attention to grow the Tb3+-based green-emitting phosphor crystals. The reason being for the high popularity of Tb3+ ions as an activator is a dominant peak at around 545 nm (5D0 → 7F5 transition) which can be tweaked to a great extent due to the large electric-dipole character [11,12,13,14]. Nonetheless, other transitions originating from the 5DJ excited state are also of great significance as far as visible emission is concerned.

Though there are some reports on the single Tb3+-doped Ba3GdP3O12compositions via conventional solid-state method [15, 16]; however, to the best of the author knowledge, concentration-dependent PL properties, Rietveld analysis and synthesis via other chemical routes are unavailable to date. This paper covers the fabrication of a series of Ba3Gd(1−x)P3O12: xTb3+ (x = 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08) via urea-based solution combustion method for the first time. The structural prototype, phase purity, surface morphology, emission dynamics and optical properties of the synthesized nanophosphor were characterized by X-ray diffraction technique, Rietveld refinement using FULLPROF program, transmission electron microscopy, decay time and photoluminescence spectroscopy, respectively. The corresponding critical distance (Rc) for energy transfer was determined experimentally and then employed to demonstrate the mechanism responsible for the concentration quenching arising from energy-migration among the neighboring Tb3+ ions. The assessed outcomes display that the solution-combustion approach for the synthesis of a blue–green tunable nanophosphor provided a fast and economic technique which diminished the sintering temperature as well as the duration of the heat treatment. The synthesized phosphor samples were found with high purity, single-phased crystalline nanostructure, better luminescent intensity than the sample prepared from the solid-state method. So, the obtained nanophosphors can be utilized as a solid-state lighting nanomaterial.

2 Experimental setup

2.1 Material and synthesis

A series of Ba3Gd(1−x)TbxP3O12 (x = 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08) nanophosphors was prepared from urea-assisted solution combustion technique. The initial materials used were Gd(NO3)3·6H2O, Tb(NO3)3·5H2O, Ba(NO3)2, (NH4)2HPO4 and NH2CONH2 were of analytical grade. In a typical procedure, all the nitrates in stoichiometric quantities were dissolved in a minimal amount of deionized water. The quantity of urea used as a fuel for the propellant reaction was calculated by using total oxidizing and reducing valencies of oxidizer and fuel [17]. After obtaining the homogenous solution, the final solution was placed in a preheated furnace kept at 500 °C for 15–20 min. Initially, the solution boils with the liberation of huge amounts of gases. These gases burnt and ignited with a blaze yielding a foamy solid. Further, the obtained voluminous sample was effortlessly milled to get fine precursor powder by using an agate mortar and annealed at 1200 °C for 5 h in the air to attain better crystallinity.

2.2 Materials characterization

Powder X-ray diffraction (PXRD) patterns of all the samples were evaluated by a Rigaku make (Ultima-IV) powder X-ray diffractometer from 2θ = 10°–80° at a scanning rate of 2°/min using Cu Kα as radiation source, while the tube voltage and tube current were kept constant at 40 kV and 40 mA, correspondingly. Furthermore, the phase analysis and refinement of parameters of the synthesized nanophosphor sample was achieved with the Rietveld method in FULLPROF software package. The isotropic displacement and fractional coordinates of all the mixed atoms were constrained during the refinement. Technai-G2 transmission electron microscope (TEM) was used to study the surface morphology and crystallite size. The Diffuse Reflectance Spectrum (DRS) of all the powder samples were recorded in the wavelength ranging from 200 to 800 nm using Shimadzu UV-3600 accessorized with an integrated sphere and BaSO4 as reference standard was used. Edinburgh fluorescence spectrophotometer (FLS-980) equipped with 450 W Xenon lamp as the excitation source was used to study the photoluminescence spectra, color coordinates and decay curves of terbium doped Ba3GdP3O12 nanophosphors. The photo-multiplier tube (PMT) voltage was set at 400 V while the excitation-emission bandwidths were fixed at 2.5 nm. The whole process was performed at room temperature and atmospheric pressure.

3 Result and discussion

3.1 Structural analysis

The analysis of the crystal structure and corresponding phase purity for all the samples was performed by powder X-ray diffraction (PXRD) technique. Figure 1 displays the XRD patterns of the complete Ba3Gd(1−x)TbxP3O12 (x = 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08) nanophosphor series along with the standard JCPDS card No. 29-0163. It can be seen that all the peaks of the XRD patterns are well-matched with the standard data. The cubic phase with a space group of I-43d without any other impurity peak indicates that Tb3+ ions substitute the Gd3+ ions without disturbing the crystal structure prototype. In support of the above-mentioned statement, Rietveld refinement in FULLPROF software was achieved for Ba3Gd0.93Tb0.07P3O12 nanocrystal using its PXRD profile. Figure 2 displays the calculated (red line) and observed (black line) patterns; the deviation of experimental and theoretical data (blue); and the corresponding Bragg positions (pink). The refinement converged to a reasonable fit with Rp = 9.03(%), Rwp = 11.7(%), Rexp = 8.43(%) and χ2 = 1.94. The refinement process yielded various structural parameters such as a = b = c = 10.445 Å, V = 1111 Å3 and Z = 4. Table 1 shows the comparative study of the crystal structure data of the host and of Ba3Gd0.93Tb0.07P3O12 system. The comparison clarifies that there is a slight decrease in the unit cell volume from 1151.009 to 1148.95 Å3 upon substitution of Gd3+ ions by Tb3+ ions, which can be explained on the account of smaller ionic radius of Tb3+ (92.3 pm) than Gd3+ (93.8 pm), and hence decreases the density of the doped nanophosphor [18]. The refined atomic positions and their occupancies are tabularized in Table 2; whilst Table 3 displays the several interatomic distances (Å) in Ba3Gd0.93Tb0.07P3O12 nanocrystal.

Fig. 1
figure 1

XRD profile of Ba3Gd1−xTbxP3O12 (x = 0.02–0.08) nanophosphors along with standard JCPDS data of Ba3GdP3O12 (Color figure online)

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Fig. 2
figure 2

Rietveld refinement of Ba3Gd0.93Tb0.07P3O12 nanocrystals, χ2 = 1.94, Rp = 9.03(%), Rwp = 11.7(%) and Rexp = 8.43(%) (Color figure online)

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Table 1 Comparison of crystal structure data of Ba3Gd0.93Tb0.07P3O12 nanophosphors with standard Ba3GdP3O12
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Table 2 Refined positions of all atoms and occupancy data for the Ba3Gd0.93Tb0.07P3O12 nanocrystals
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Table 3 Interatomic distance (Å) of all designated cations of the particular Ba3Gd0.93Tb0.07P3O12 nanophosphor
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Moreover, the average crystallite size was evaluated by using the full-width at half-maxima (FWHM) of the most prominent diffraction peak for Ba3Gd0.93Tb0.07P3O12 nanophosphor from Scherrer’s equation [19, 20]:

$$D = \frac{0.941\lambda }{{\left( {B_{O}^{2} (2\theta } \right) - B_{Si}^{2} \left( {2\theta } \right))^{{\frac{1}{2}}} Cos\left( \theta \right)}}$$
(1)

where D is the average crystallite size, λ the X-ray wavelength (0.15406 nm), θ is the diffraction angle; βSi(2θ) and β0(2θ) are the full-width at half-maximum (FWHM, in radian) of the standard silicon and of the synthesized sample, correspondingly. The results of the particle size suggested a range of 40–70 nm. Additionally, to study the surface morphology along with particle size of as-prepared Ba3Gd1−xTbxP3O12 nanophosphor samples Transmission Electron Microscopy (TEM) was carried out. Figure 3 displays the TEM micrograph of the Ba3Gd0.93Tb0.07P3O12 nanophosphor, showing well-dispersed particles with minute agglomeration magnified at 100 nm. Therefore, it is quite safe to say that the results from Scherrer’s equation and TEM micrography, are found to be in good agreement with each other. Later on, the Strain (\(\varepsilon\)) and Dislocation density (\(\delta\)) were calculated for all the synthesized nanophosphors and are reported in Table 4 utilizing the following equations [21, 22]:

$$\varepsilon = \frac{\beta Cos\theta }{4}$$
(2)
$$\delta = \frac{1}{{D^{2 } }}$$
(3)
Fig. 3
figure 3

TEM micrographs of Ba3Gd0.93Tb0.07P3O12 powder calcined at 1100 °C

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Table 4 The strain and dislocation density parameters in Ba3GdxTb(1−x)P3O12 nanophosphor series
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Furthermore, the variance of the absorption coefficient against photon energy, along with the reliable diffuse reflectance spectra (inset), for the Ba3GdP3O12 host is shown in Fig. 4. The Kubelka–Munk function is utilized to evaluate the magnitude of the optical band gap (Eg) for solids. So, the values of various kinds of band-gap can be implicated by using the below relation [23]:

Fig. 4
figure 4

The relationship between the absorption coefficient and photon energy for Ba3GdP3O12 host matrix and (inset) corresponding diffuse reflectance spectra

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$$\left[ {F(R_{\infty } h\nu } \right]^{n} = C\left( {h\nu - E_{g} } \right)$$
(4)

where C is a proportionality constant, hν denotes the energy of the photon and Eg signifies the band gap (eV). The symbol n is 0.5, 2, 1.5 and 3 for allowed indirect, allowed direct, forbidden direct and forbidden indirect transitions, respectively. F(R∞) implies the Kubelka–Munk function whose standards is well-defined as [24]:

$$F\left( {R_{\infty } } \right) = \frac{{\left( {1 - R_{\infty } } \right)^{2} }}{{2R_{\infty } }} = \frac{K}{S}$$
(5)

where K and S are the absorption and scattering coefficients, correspondingly; while R∞ represents the ratio of Rsample to Rstandard. The extrapolation of the line with F(R∞) hν = 0 exposed a band-gap of 4.88 eV.

3.2 Optical properties

Figure 5 displays the photoluminescence excitation (PLE) spectrum of Tb3+ ions doped Ba3GdP3O12 nanophosphor obtained when monitored at λem of 545 nm due to (5D4 → 7F5) transition. The PLE revealed that the spectrum consists of a broad band ranging from 200 to 240 nm (centered at 224 nm), which obviously arises due to the transition from ground state 4f8 to the excited state 4f7 5d1 (f–d inter-configurational transition) in trivalent terbium ions [25, 26]. Along with this, weak absorptions at 273 nm and 311 nm are also found and are assigned as the intra-configurational 4f–4f transitions of the Tb3+ ions in Ba3GdP3O12 host. Among these peaks, the broad-band at 224 nm has been chosen to evaluate the emission spectra of Ba3GdP3O12: Tb3+ (2–8 mol%).

Fig. 5
figure 5

Excitation spectra of Ba3Gd1−xTbxP3O12 (x = 0.01–0.08) nanophosphor studied at λem = 545 nm (Color figure online)

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The photoluminescence emission spectrum of Tb3+ doped nanophosphors mainly consists of several prominent peaks in the wavelength region of 500–700 nm. The PL emission spectra shows the peaks at 415 nm, 436 nm and 456 nm are arising from 5D3 → 7FJ (J = 5, 4, 3) transitions, whereas the transitions at 487 nm, 545 nm, 582 nm and 620 nm appear from 5D4 → 7FJ (J = 6, 5, 4, 3), respectively [27,28,29,30]. Among these peaks, transitions from 5D3 are responsible for the blue color of the nanophosphors while transitions from the 5D4 state are responsible for the characteristic green emission in the case of trivalent terbium ions. It can be seen from Fig. 6, that the most intense peak located at 545 nm (5D4 → 7F5) will be primarily responsible for the emission color of the nanophosphors. Furthermore, the PL emission intensity of the peaks from the 5D4 excited state shows remarkable enhancement on increase in activator concentration in the host matrix up to 7 mol% but decreases afterward as shown in Fig. 7. This decrease in the PL intensity for 5D4 → 7FJ transitions can be ascribed to the concentration quenching effect. The reason for the concentration quenching in PL spectra can be explained on the basis of decreasing distance between the neighboring Tb3+ ions in the host matrix, resulting in a considerable increase in the energy transfer via non-radiative process [31, 32].

Fig. 6
figure 6

Emission spectra of Ba3Gd1−xTbxP3O12 (x = 0.02–0.08) nanophosphor excited at λex = 224 nm (Color figure online)

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Fig. 7
figure 7

Variation of emission intensity for Ba3Gd1−xTbxP3O12 nanophosphors (x = 0.02–0.08) at 545 nm as a function of Tb3+ concentration, sintered at 1200 °C

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Besides this, it is also worthy to note that the intensity of the emission peaks arising from 5D3 → 7FJ transitions decreases with the increase in the Tb3+ ions concentration as shown in Fig. 8. This leads to the shift in the emission color from light blue to light green. The energy level plot for the Tb3+-doped Ba3GdP3O12 also includes the DC (down-conversion) energy transfer mechanism through radiative as well as non-radiative paths. Upon excitation, the Tb3+ ions get excited from the 7F6 ground state to higher energy state (224 nm); which in turn gets depopulated non-radiatively to enrich the lower energy levels (5D3, 5D4) of Tb3+ ions. The tunability of the emission color for terbium ions may be attributed to the cross-relaxation phenomenon. The same energy gap between the excited energy states 5D3–5D4 and ground state 7F6–7F0 levels elucidates the cross-relaxation via resonant energy transfer as shown in schematic energy diagram (Fig. 9) [33].

Fig. 8
figure 8

Normalized Emission spectrum of Ba3Gd1−xTbxP3O12 (x = 0.02–0.07) nanophosphor scrutinized at λex = 224 nm (Color figure online)

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Fig. 9
figure 9

Energy transfer mechanism for Tb3+ ions in Ba3Gd1−xTbxP3O12 (x = 0.02–0.08) nanophosphor

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Generally, the transfer of the resonant energies between the two adjacent Tb3+ ions can occur via three mechanisms i.e., exchange interaction, electric multipolar interaction and radiation re-absorption. Out of these three possibilities, radiation re-absorption can be discarded as there is no spectral overlapping of excitation and emission bands. Therefore, it is necessary to figure out the critical distance (Rc) to assess the potential mechanism for energy transfer. So, for the 5D4 emission in the Ba3Gd0.93Tb0.07P3O12 (critical concentration) nanophosphor Rc can be evaluated by using the equation reported by Blasse and Grabmaier and expressed as [34, 35]:

$$R_{c} = 2\left[ {\frac{3V}{{4\prod x_{c} N}}} \right]^{1/3}$$
(6)

where V is the volume of the unit cell (Å3), N is the number of replaceable cations in the unit cell and xc is the critical concentration. After putting N = 4, xc = 0.07 and V = 1148.95 Å3 for the Rc is measured to be 19.87 Å; suggesting that the resonant energy transfer occurs via multipolar interaction among the adjacent trivalent terbium ions in this case. Furthermore, dominating electric multipolar interactions are categorized as dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q) interactions. So, the relationship between the dopant ions concentrations (x) and luminescence intensity (I) can be expressed as [36, 37]:

$$log\left( {I/x} \right) = - \frac{Q}{3}logx + A$$
(7)

where A is a constant, x is the concentration of activator ion greater than the critical concentration, Q denotes the type of multipolar interaction value of which varies as 6, 8 or 10 for dipole–dipole (d–d), dipole-quadrupole (d-q) and quadrupole–quadrupole (q–q) interactions, respectively. As the graph demonstrated in Fig. 10 shows the relationship between log(I/x) and log(x) with the slope value − 2.2568 ± 0.11516 results in the value of Q ≈ 6; indicating that the dipole–dipole interactions are the main cause for the cross-relaxation energy transfer in Tb3+ doped Ba3GdP3O12 nanophosphors.

Fig. 10
figure 10

Plot of log (I/x) as a function of log(x) in Ba3Gd1−xTbxP3O12 nanophosphors

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Also, the decay behavior of 5D4 state in Ba3GdP3O12:Tb3+, at λex = 224 nm and λem = 545 nm, was evaluated. The obtained curves for all Ba3Gd1-xTbxP3O12 nanophosphors are well fitted by the first exponential decay by utilizing the equation [38]:

$$I = I_{0} \,{ \exp }\left( {{{ - t} \mathord{\left/ {\vphantom {{ - t} \tau }} \right. \kern-0pt} \tau }} \right)$$
(8)

where I and Io are the luminescence intensity at time t and 0 respectively, τ is the fluorescence lifetime and A is a constant. The values of photoluminescent lifetime for different Tb3+-doped samples reveal that the value of τ declines with the increase in the activated concentration (Table 5). Figure 11 represents the single exponential decay of the Ba3Gd0.93Tb0.07P3O12 which can be viewed in terms of homogenous dispersal of all terbium ions in the host matrix with similar local environment returning at the same rate.

Table 5 CIE1931 chromaticity index and life-time for Ba3GdxTb(1−x)P3O12 (2–8 mol %) nanophosphors
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Fig. 11
figure 11

The luminescence decay curve for 545 nm (5D4 → 7F5 of Tb3+) emission of Ba3Gd0.93Tb0.07P3O12 nanophosphors

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The Commission Internationale de I’Eclairage (CIE) chromaticity coordinates depicts the emission color of the nanophosphors. Figure 12 shows the calculated chromaticity coordinates (x, y) of the Ba3Gd1−xP3O12: xTb3+ (x = 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08) on the CIE 1931 color space chromaticity diagram obtained from emission spectra under the excitation of 224 nm wavelength and charted in Table 5. The chromaticity coordinates for all the synthesized nanophosphors clearly show the tunability from blue to green region on the intensification of Tb3+ ions in the host lattice. The luminescent tunability in Ba3GdP3O12: Tb3+ (2–8 mol%) finds its use in the wider range of display devices or in solid state lighting.

Fig. 12
figure 12

CIE chromaticity diagram for Ba3Gd1−xTbxP3O12 (x = 0.02–0.08) nanophosphors

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4 Conclusion

A series of blue–green Ba3Gd(1−x)P3O12: xTb3+ nanocrystals has been efficaciously synthesized via urea-assisted solution-combustion technique. Its structural morphology, energy-transfer mechanism, photoluminescent (PL) excitation-emission and decay time behavior were examined in detail by means of powder X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Diffuse Reflectance (DR) and PL spectroscopy. The Rietveld investigation shows the cubic phase formation of all the nanophosphors with I-43d (220) space group and concludes that Gd3+ ions can be well replaced by Tb3+ ions without any major modification in the crystal structure of the host matrix. The optical band-gap of the host was assessed to be 4.9 eV, offering high potential as a host for lanthanide activators. Upon excitation at λex = 224 nm, the photoluminescent emission spectra showed the two main characteristic peaks at 545 nm and 478 nm as a result of 5D4 → 7F5 (green and magnetic-dipole) and 5D4 → 7F6 (blue and electric-dipole) transitions, respectively. The decay analysis exhibited that the activator resides in a single crystallographic site, a fact that is also supported by the Rietveld refinement. The Rc of the energy transfer (19.87 Å) combined with Dexter’s modeling provides information about energy migration via dipole–dipole interactions. The PL result displayed that the blue-to-green tunable emission can be attained simply via varying the activator concentration, with 7 mol% as the optimal concentration for standard CIE coordinates of green emission. All the results propose that Ba3Gd(1−x)P3O12: xTb3+ crystals can find their usage as a green phosphor component in display gadgets and solid-state lighting.