1 Introduction

White-light emitting diode (WLED) has been considered as a next generation of lighting source in recent years due to considerable merits like high energy saving, easy maintenance, long durability, efficient conversion of electrical energy to light, eco-friendly, less electrical consumption [1,2,3]. In general, the commercially available WLEDs can be obtained by GaN blue LED and Y3Al5O12:Ce3+ yellow phosphors combination [4,5,6]. In this type of WLEDs, due to red emission deficiency it exhibits poor colour rending index (CRI) of less than 80% and high correlated colour temperature (CCT) of above certain temperature [7,8]. To fulfill these issues, many researchers were constantly working to prepare a novel red, green and blue (RGB) emission in a single-component material and it may be the direction of solid-state lighting development.

So far, considerable interest in inorganic materials doped with trivalent lanthanide shows an excellent luminescent properties. Basically, lanthanide ion exhibits the transitions within the partially filled 4fn inner shell. These parity forbidden transitions were attributed the low molar absorption coefficient and efficient luminescent lifetime [9,10,11]. Various attempts show that Eu3+ ion exhibits an intense red emission in the visible region due to 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transitions [12,13]. Similarly, Dy3+ ion also exhibits three emission colours in the visible region of blue 4F9/2 → 6H15/2, yellow 4F9/2 → 6H13/2 and red 4F9/2 → 6H11/2 emissions [14,15].

Many studies on Eu3+/Dy3+ co-doped phosphors with diverse host materials have been published to date. Chang Chengkang et al [16] studied the photoluminescence (PL) properties of Eu3+/Dy3+ co-doped Sr3Al2O6 phosphor and synthesized it. Meza-Rocha et al [17] created/fabricated a multicolour luminescence of potassium-zinc phosphate glasses activated with Eu3+, Dy3+ and Dy3+/Eu3+. Nannan Yao and colleagues [18] produced ZnO: Eu3+, Dy3+ material for solar cell applications and improved efficiency of the of dye-sensitized solar cells, about 212 and 245% higher than pure TiO2 and about 91.4 and 105% higher than with TiO2/graphene (G) structure, respectively.

Sun Xin-Yuan et al [19] used a solid-state process to successfully synthesize Dy3+, Eu3+- doped, and Eu3+/Dy3+-co-doped SrGd2O4 scintillating phosphors and investigated its luminescence properties. Zhai Yongqing et al [20] studied the luminous characteristics of ZnWO4:Eu3+, Dy3+ materials, which they synthesized using a hydrothermal technique for WLED applications. The PL properties of Eu3+, Dy3+-activated Ca3La(VO4)3 phosphors for WLED were produced and examined by Vengala Rao et al [21]. Wang Huayu et al [22] have investigated PL properties, energy transfer and synthesized Eu3+/Dy3+-co-doped NaGd(MoO4)2 phosphors for solid-state lighting application [22]. The above all, it is inferred that the desired wavelength of the light is attained by adjusting/modifing the ratio of Dy3+ ions co-doping concentration with Eu3+ ions [14,15].

Inorganic luminescent materials, such as alkaline earth metal vanadates containing Ln3+ ions, have recently become a popular choice for WLED applications. These materials have a significant potential for usage as a single-component WLED material because of their excellent luminescence [23,24,25]. Among various alkaline earth metal vanadates, M2V2O7 (M = Ca, Sr, Ba) phosphors were widely used in filter, antenna, solid-state lighting [26,27,28] and other applications. In the previous work, the self-activated Ba2V2O7 phosphor was found to be an effective and excellent luminescent material for ultra-violet (UV)-chip-excited WLED. This phosphor has a broad absorbance in the UV region and a broad emission in the visible region [29]. The drawback found in Ba2V2O7 phosphor material was the insufficient red emission in the visible spectrum. To overcome this problem, an attempt was made to synthesize Ba2–xEu0.03DyxV2O7 (BEDVO) phosphor doped with the fixed concentration of europium (Eu3+) ion (x = 0.03) and varied the concentration of dysprosium (Dy3+) ion as a co-doped. In this present investigation, Ba2–xEu0.03DyxV2O7 (x = 0.01, 0.02, 0.03, 0.04 and 0.05) phosphor was synthesized by hydrothermal method. The structural, optical and photoluminescent properties were investigated for WLED applications.

2 Experimental

2.1 Materials

Barium nitrate (Ba(NO3)2), sodium metavanadate (NaVO3), europium oxide (Eu2O3), dysprosium oxide (Dy2O3) and ammonia solution (NH4) purchased from LOBA chemicals (analytical grade), India, with high purity (99%) were used without any further purification. Distilled water was used as the solvent throughout the synthesis protocol.

2.2 Preparation of BEDVO phosphor

Ba2–xEu0.03DyxV2O7 (x = 0.01, 0.02, 0.03, 0.04 and 0.05) phosphor was prepared by using the hydrothermal method. Ba(NO3)2 (8.19 g) and NaVO3 (1.95 g) were dissolved in 40-ml distilled water separately and the mixed solutions were stirred for 15 min. Eu2O3 (0.085 g) was dissolved in 2 ml of nitric acid and they were mildly heated to obtain europium nitrate. Similarly, 0.029 g of Dy2O3 was dissolved in 2 ml of nitric acid and they were mildly heated to obtain dysprosium nitrate. These dysprosium nitrate and europium nitrate mixtures were then added to the mixed source materials under vigorous stirring. The pH level was adjusted to about 12 by using the ammonia solution with continuous stirring. After adjusting pH, the solution was stirred for 1 h. The mixed solution was transferred into 100 ml Teflon-lined autoclave and it was tightly sealed. The solution in the autoclave was heated at 180°C for 48 h in a muffle furnace, which was then cooled to room temperature naturally. The precipitate was filtered and washed several times with distilled water. The final white colour powder was obtained after drying at 60°C on a hot plate for few hours.

2.3 Characterization techniques

PANalytical Xpert-Pro diffractometer with Cu-K radiation in the scan range of 10°–60° is used to analyse X-ray diffraction (XRD) investigations for the BEDVO phosphors. The diffused reflectance spectra were recorded from 200 to 1200 nm range using a UV-2600 SHIMADZU spectrophotometer with an aid of BaSO4 as a non-absorbing standard reference. A Shimadzu RF-5301PC spectrofluorometer is used to record PL spectra. The chromaticity colour coordinates of the Commission International de l'Eclairage (CIE) were determined using the GOCIE Version 2 CIE-1931 plot programme. At room temperature, all measurements were taken.

3 Results and discussion

3.1 Structural analysis

Powder X-ray diffraction (PXRD) is a versatile, non-destructive analytical method for identification and quantitative determination of various crystalline forms in the phosphors. Figure 1a shows the PXRD patterns of the as-synthesized phosphors. All the PXRD profiles were well indexed to triclinic crystal structure [JCPDS card no.: 76-0612] of the parent element Ba2V2O7 phosphor. The additional peaks or traces corresponding to Eu3+ and Dy3+ ions are absent in figure 1a. Further, it was clearly evident that, as concentration of Dy3+ dopant increases, the diffraction peaks were slightly shifted towards the lower angle side, as shown in figure 1b. This peak shifting and line broadening were due to the partial replacement of the higher ionic radii (Ba2+; r = 0.14 nm) ions by lower ionic radii (Eu3+; r = 0.095 nm and Dy3+; r = 0.108 nm) ions [30,31]. This observation obeys the Vegard’s law [32]. Similar observations were reported by Taniguchi Kouta et al [33], Kazuki Sakoda and Masanori Hirano [34] and Fang Mu-Huai et al [35] for the samples Sr1–xEuxGa2S4, Zn(AlxGa1–x)2O4 and SrLi(Al1−xGax)3N4:Eu2+, respectively. Further, it can be marked from the increase in relative intensity of (102) XRD peak that the increase in Dy3+ doping concentration leads to an increase in the crystalline nature of the samples.

Figure 1
figure 1

(a) Powder XRD patterns of BEDVO phosphors. (b) The enlarged view of peaks in the 2θ range (26–27°).

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The acceptable percentage difference (Dr) between radii of Eu3+/Dy3+ dopant ion and Ba2+ substituted ion should not be more than 30%, which was obtained by equation (1):

$$ D_{{\text{r}}} = \frac{{R_{{\text{s}}} - R_{{\text{d}}} }}{{R_{{\text{s}}} }} \times 100\% , $$
(1)

where Rs is the radius of substituted ion and Rd is the radius of dopant ion [36]. The estimated value of Dr for Eu3+/Dy3+ dopant phosphor was found to be 28 and 22%, respectively. Thus, it was confirmed that the Eu3+/Dy3+ ions have been successfully incorporated into the Ba2+ lattice site in the Ba2V2O7 host without any structural change.

3.2 UV–visible-NIR spectral analysis

Figure 2a shows the diffused reflectance spectra of the as-synthesized BEDVO phosphors from UV to NIR region, i.e., 200–1200 nm. The absorption peak centred at 215 nm was due to the fraction of electronic charge transferred between donor and acceptor (CT) in O2− → Eu3+/Dy3+ ions [37]. The broadband observed in the range of 241 to 350 nm centred at 296 nm was due to the CT between 1A1 → 1T1 and 1A1 → 1T2 in the VO4 tetrahedral group [38,39]. Further, small peaks were also observed in the visible to NIR region, i.e., 400–1200 nm [40]. The enlarged view of this region is shown in figure 2b. The appearance of the peaks at 469, 537 and 574 nm was attributed to 7F0 → 5D2, 7F1 → 5D1 and 7F0 → 5D0 transitions of Eu3+ ion, respectively. Also, the peaks appeared at 802, 909 and 1108 nm were corresponding to 6H15/2 → 6F5/2, 6H15/2 → 6F7/2 and 6H15/2 → 6F7/2, 9/2 transitions of Dy3+ ion, respectively [41,42,43]. It was confirmed that the formation of the meta-stable state between the valence band and the conduction band is due to the substitution of Eu3+/Dy3+ ions.

Figure 2
figure 2

(a) The diffuse reflectance spectra and (b) the enlarged view of peaks in the range 400–1200 nm of BEDVO phosphors.

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Further, the absorption coefficient of BEDVO phosphors can be achieved using Kubelka–Munk function, expressed as in equation (2):

$$ F\left( R \right) = \frac{K}{S} = \frac{{\left( {1 - R} \right)^{2} }}{2R}, $$
(2)

where K, S and R represent the absorption coefficient, scattering coefficient and observed reflectivity, respectively [37,44].

The absorption coefficient (F(R)) and the optical bandgap (Eg) of the as-prepared BEDVO phosphors can be related by Tauc relation, expressed as in equation (3):

$$ \left( {h\upsilon F\left( R \right)} \right)^{1/n} = A\left( {h\upsilon - E_{{\text{g}}} } \right), $$
(3)

where F(R) is the absorption coefficient, A the proportionality constant, hυ is the photon energy, Eg the bandgap energy. The value of n is ½ for direct-type transition and 2 for an indirect-type transition. The estimated bandgap values of the as-synthesized phosphors are shown in figure 3. The estimated values of the bandgap energy BEDVO phosphors were 3.57, 3.42, 3.38, 3.32 and 3.27 eV, respectively, for x = 0.01, 0.02, 0.03, 0.04 and 0.05. There were two significant reasons for the decrease in the bandgap (indirect bandgap) values; one is the formation of meta-stable state between the valence band and the conduction band (i.e., in between direct bandgap) and the other is the formation of the distorted VO4 tetrahedral group due to the influence of the dopant ion [45,46].

Figure 3
figure 3

The Tauc plot for determining the bandgap of BEDVO phosphors.

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3.3 PL spectral analysis

Figure 4 shows the excitation spectra of the as-prepared BEDVO phosphors monitored under 574 nm excitation wavelength. All the excitation spectra consist of the intense broad band peak in the range of 300 to 390 nm, which was due to the CT of O2− → V5+ bond in the host matrix. When the concentration of Dy3+ increased, the sharp peak at 286 nm for x = 0.02 concentration reachs maximum, which was due to the second-harmonic generation [47,48,49]. This indicates that these phosphors could be excited by the near-UV light along with the charge transfer band, which might find application in solid-state lighting.

Figure 4
figure 4

PLE spectra of BEDVO phosphors monitored at 574 nm emission.

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The emission spectra of the as-synthesized BEDVO phosphors were recorded with UV excitation wavelength at 286 nm, depicted in figure 5. The intense broad band peak was obtained in the range of 400 to 568 nm for all the phosphors. Figure 6 shows the emission spectra of the as-prepared BEDVO phosphors excited at 344, 335, 346, 346 and 339 nm, respectively. Here, the broad and few intense peaks were obtained in the emission spectra. The broad peak centred at 493 nm was due to the localized energy transfer between V5+ and O2– in the VO4 tetrahedral group. The observed peaks in figures 4 and 5 are listed in table 1, along with its CT and bandgap measured [50,51,52,53,54,55,56,57].

Figure 5
figure 5

The PL spectra of BEDVO phosphors monitored at 286 nm.

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Figure 6
figure 6

The PL spectra of BEDVO phosphors monitored at 344, 335, 346, 346 and 339 nm excitations.

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Table 1 Emission spectra of the as-synthesized BEDVO phosphors [50,51,52,53,54,55,56,57].
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Figure 6 shows that when the Eu3+ doping concentration increased, the broad emission peak intensity rised to a maximum of x = 0.02 concentration, after which the peak intensity declined. The peak intensity of the dopant transition peaks was found to increase maximum up to x = 0.02 concentration, however as the doping concentration was increased further, the peak intensity was found to decrease due to the concentration quenching effect. Similarly, the transition 5D0 → 7F4 was pushed into the high energy region and the peak broadening also decreases, which was due to the lowering of the bond distance between Eu3+ and O2–. The luminescence property of the co-doped rare-earth ion is found to be dependent on the excitation wavelength, as a result of this finding [58].

According to Blasse’s theory [59], the critical distance (Rc) of BEDVO phosphors were studied. The critical distance (Rc) is defined as the distance for which the probability of energy transfer equals to the probability of radiation emission of energy donator by the [VO4]3– in the system. For BEDVO phosphor, the value of N, Xc and V were 8, 0.02 and 377.841 Å3, respectively. The critical energy transfer distance was estimated using the Blasse’s theory to 10.44 Å, which was greater than the critical distance, Rc = 5 Å. Therefore, the as-prepared BEDVO phosphors belongs to multipolar energy transfer mechanisms.

There are two types of energy transfer methods that depend on the critical distance between the sensitizer and activator ions, according to Dexter’s reports [60]. They were exchanging interactions and multipolar interactions, if the value of the critical distance was about 3–4 Å. The exchange interaction may take place if the sensitizer and activator orbits overlapped. Thus, the multipolar interactions may dominate. It is concluded from the critical distance that the type of energy transfer mechanism between the sensitizer [VO4]3– and the activator Eu3+ ion in BEDVO phosphors is thus classified as multipolar interactions. Dexture’s reports reveal that the multipolar interaction takes place during the interaction between sensitizer and sensitizer or sensitizer and activator. This multipolar interaction will cause a non-radiative energy transfer. The following equation (4) determines it:

$$ \frac{1}{x} = \frac{K}{{1 + \beta x^{Q/3} }},$$
(4)

where K and β are the constants, x is the doping concentration of Dy3+ ions. The value of Q = 3, 6, 8 and 10, which represents the exchange interaction, dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively [61]. Assuming \(\beta x^{{{\text{Q}}/{3}}} \gg {1}\) [62], then the above equation (4) can be simply rearranged as follows:

$$ {\text{log}}\left( \frac{1}{x} \right) = A - \frac{Q}{3}{\text{log}}x, $$
(5)

where A = logK – log β. Figure 7 was plotted using equation (5) for log(1/x) vs. record (x) for Dy3+-doped Ba2V2O7 phosphor. From figure 7, the slope of the curves were obtained to be –1.45 for the emission spectra excited at 268 nm. Its corresponding Q-value was determined as 2.1025. From this result, it shows that the concentration quenching mechanism of Dy3+ ion in the phosphors host was due to dipole–dipole exchange interactions.

Figure 7
figure 7

Dependence of log(I/x) on log(x) of BEDVO phosphors under 268 nm excitation.

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3.4 Photometric studies

Figure 8a and b shows the chromaticity CIE diagram of the as-synthesized BEDVO phosphors. The photographic images of the as-prepared Eu3+/Dy3+ co-doped Ba2V2O7 phosphors were taken in the daylight and irradiated under the UV light 365 nm. All the samples appeared to be white colour in daylight. The as-prepared BEDVO phosphors irradiated under UV light at 365 nm shows glaring white to bluish white colour, with the increase of Dy3+ concentration as shown in figure 8c. The CIE chromaticity coordinates, CRI and CCT values of the BEDVO phosphors were tabulated in Table 1. The CRI and CCT values were calculated from the PL emission spectra of the as-prepared BEDVO phosphors. The CCT values for the visible emission for these phosphors were calculated using McCamy’s empirical [63] formula (equation (6)):

$$ {\text{CCT}} = - 499n^{3} + 3525n^{2} - 6823n + 5520.33.$$
(6)
Figure 8
figure 8

The CIE chromaticity diagram showing the emission colours of BEDVO phosphors excited at (a) 286, (b) 344, 335, 346, 346 and 339 nm excitations, and (c) images of the phosphors excited by 365 nm under a UV lamp and their corresponding daylight images.

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Figure 8a shows the as-synthesized phosphors excited at 286 nm, which emits bluish white colour, whereas the CCT values were slightly changed due to the doping concentration of Dy3+ ion. Figure 8b shows the as-prepared BEDVO phosphors excited under 344, 335, 346, 346 and 339 nm, respectively. The bluish white emission colour for all the BEDVO phosphors were observed, whereas the CCT values were changed when Dy3+ concentration increases. The summarized results are shown in table 2 for easy comparison. The above all, it is well demonstrated that the as-prepared BEDVO phosphors can be applicable for UV and near-UV phosphor converted WLEDs.

Table 2 Comparison of CIE coordinates, CRI, CCT and their corresponding emission colours of the Ba2–xV2O7:0.03Eu3+, xDy3+ (x = 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors excited at (a) 286 and (b) 344, 335, 346, 346 and 339 nm excitations.
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4 Conclusion

The hydrothermal approach was used to synthesize a series of single-phase Ba1.97–xEu0.03DyxV2O7 (x = 0.01, 0.02, 0.03, 0.04 and 0.05). The PXRD patterns for all the BEDVO phosphors show that the dopant is substituted on the host lattice without changing the crystal system. The creation of a meta-stable state in the energy gap is confirmed by the diffused reflectance spectra, which is owing to the substitution of the Dy3+ ions in the as-prepared BEDVO phosphors. The PL spectra further demonstrate the doping of Dy3+ ion. The PL spectra obtained at 268, 344, 335, 346, 346 and 339 nm radiated a blue white colour with insignificant fluctuations in CIE coordinates. The white colour emission of BEDVO phosphors is confirmed by the irradiation under the UV light 365 nm for WLED applications. In summary, it is concluded that BEDVO phosphors have the potential ability for application in the UV and near-UV phosphor converted WLEDs.