Introduction

Inorganic luminescent materials have more excellent property such as low photobleaching, longer luminescent lifetimes and narrow emission bands than organic fluorescent dyes and semiconductor quantum dots [1,2,3,4]. Among various materials reported, rare-earth (RE)-doped molybdates have attracted considerable attention and been a significant research topic not only for the basic scientific interest but also for their remarkable photoelectronic performances in fields such as negative thermal expansion materials, photocatalysis, phosphors, solid-state lasers and catalysis [5, 6]. As a fascinating group of molybdates, double alkaline rare-earth molybdates ARE(MoO4)2 (A = Na, K; RE = trivalent rare-earth cations) with tetragonal and monoclinic symmetries have been widely reported owing to their high chemical durability, large rare-earth ions admittance and large absorption cross sections for luminescent hosts [7,8,9]. Particularly, they have a relatively low lattice phonon energy which would be conducive to prevent concentration quenching effect and increase the possibility of radiative transitions. It is beneficial to a high quantum yield of down/up conversion (DC/UC) process. Therefore, great endeavors have been devoted to prepare tetragonal NaRE(MoO4)2 (RE = Y, Gd, La, Eu, Ce) nano-/microstructured host materials which share the scheelite-like (CaMoO4) iso-structure [2, 7,8,9,10,11]. However, lanthanide ion-doped NaRE(MoO4)2 with monoclinic phase have rarely been reported in previous work and there is no systematic research about the luminescent properties of monoclinic NaScMo2O8:RE3+ [12].

Rare-earth elements played an important role in modern lighting and display fields due to their special electronic structure and profuse energy levels (4fn5s25p6, 0 ≤ n ≤14) [13, 14]. Among various RE3+ ions, Eu3+ is an important activator for red emission because of its intrinsic characteristic transition (5D0 → 7F2 at around 615 nm) [15, 16], which has already started commercial applications as red phosphors for decades (like Y2O3:Eu3+), but it has weak-line absorption of f–f transitions in the near-ultraviolet (NUV) region [17, 18]. It is very significant to improve the luminescence efficiency of Eu3+ and the ratio of red emission (610–620 nm, 5D0 → 7F2) to orange emission (590–600 nm, 5D0 → 7F1) in Eu3+. The energy transfer (ET) from sensitizers to activators in a proper host is an effective way to solve the above problem [19, 20]. Tb3+ ion, as a good sensitizer for the Eu3+ ion, not only enhance the luminescence efficiency of Eu3+ ions but also broaden the absorption region in NaY(MoO4)2 [12], Na3Gd(PO4)2 [21], SrMg2LaW2O12 [22] and CaYAlO4 [23] phosphors owing to introduction of more impurity energy levels in the host. Furthermore, the emission color of phosphors can be regulated by changing the ratio of Tb3+ to Eu3+ ions. The UC emission is anti-Stokes emission process which has been the focus of much research due to the merits of the high photochemical stability, large anti-Stokes shifts, partially filled 4f orbitals, the absence of autofluorescence of biotissues and sharp emission lines [24]. Therefore, they were applied to the fields of biotechnologies, three-dimensional displays, optical temperature sensors, solar cells and optical amplifiers [25,26,27]. The UC process can be divided into three broad classes: excited-state absorption (ESA), energy transfer (ET) and photon avalanche (PA) [24]. The efficient UC phosphors is usually doped with rare-earth ions which has a unique set of energy levels and generally exhibits a set of sharp emission peaks with distinguishable spectroscopic fingerprints [28]. Yb3+ ions have a much larger near-infrared (NIR) absorption cross section, which are often co-doped as excellent sensitizers along with Er3+ or Ho3+ to yield strong red or green UC emissions [29]. The ET between RE3+ ions via the nonradiative process would result in tunable multicolor emissions.

In this paper, NaScMo2O8 phosphors have been proven to be an excellent host matrix for the luminescence of RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho), which were successfully synthesized by surfactant-free hydrothermal method and subsequent calcination at 800 °C. The obtained phosphors exhibited good emission properties when activated with RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho). It was found that multicolor emissions (green → yellow → red) were acquired due to the effective ET of MoO4 2− → Tb3+ → Eu3+. Furthermore, the energy transfer from Tb3+ to Eu3+ was dominant by dipole–dipole interaction. The efficient ET from Yb3+ to Er3+ (Ho3+) also took place in NaScMo2O8 host to get UC luminescence.

Experimental section

Synthesis

In a typical process, 1 mmol of ScCl3, appropriate stoichiometric RECl3 and 35 ml deionized water were added into a 100-ml beaker firstly. After vigorous agitation for 10 min, 6 mmol Na2MoO4·2H2O was dissolved in the above solution with strong magnetic stirring; then, a white colloidal suspension was obtained. The pH value was subsequently adjusted to 7 by dropwise adding NaOH solution. Under strong stirring for 30 min, the suspension was transferred into a 50-ml Teflon-lined autoclave sealed in a stainless steel vessel and maintained at 180 °C for 24 h. After natural cooling, the hydrothermal products were washed with distilled water and alcohol several times and then dried at 60 °C for 12 h. Finally, the final products were collected after further calcination at 800 °C for 2 h (Fig. S1 in Supporting Information).

Characterization

Powder X-ray diffraction (XRD) was performed on a Purkinje General Instrument MSALXD3 using Cu Kα radiation (λ = 0.15406 nm) with a scanning rate of 10° min−1 in the 2θ range from 10° to 60° at 20 mA and 36 kV. The morphologies and energy-dispersive spectrometry (EDS) spectra of the samples were observed by means of a field emission-scanning electron microscope (FESEM, XL30, Philips) operated at an accelerating voltage of 10 kV. The DC fluorescence spectra were obtained using a Hitachi F-7000 spectrophotometer equipped with a 150-W xenon lamp as the excitation source, and the lifetime decays were measured on FLSP920 fluorescence spectrophotometer and Shimidazu R9287 photomultiplier (200–900 nm) equipped with a liquid-nitrogen-cooled InGaAs (800–1700 nm) as detector. The UC luminescence spectra were measured using a 980-nm laser with MDL-N-980-8 W as the excitation source and detected using a LS 55 (PerkinElmer) from 400 to 750 nm. All the measurements were performed at room temperature.

Results and discussion

Phase and morphology

The XRD pattern of the sodium scandium molybdate precursor sample is shown in Fig. S2 (Supporting Information). Compared with all the standard XRD patterns in JCPDS cards, the precursor cannot be indexed to a certain compound because the product produced by the hydrothermal method probably contains some hydrous compounds from the solution medium [6]. Figure 1 displays the XRD patterns of NaScMo2O8 and NaScMo2O8:RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho) samples annealed at 800 °C as well as the JCPDS card (No. 32-1150) for NaScMo2O8, respectively. All the patterns match well with the pure monoclinic phase (JCPDS#32-1150), and no additional peaks or other phases can be found, revealing that the doped RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho) ions have been effectively dissolved in the NaScMo2O8 host matrix. Notably, when the Sc3+ was substituted by the RE3+ with bigger radius, the corresponding XRD peaks shift to lower angle direction due to the Vegard law [1]. The relative peak intensity of [001] for the samples doped with RE3+ is visibly enhanced, implying that the prismatic structures of the NaScMo2O8:RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho) samples grow preferentially along the [001] direction. The strong and sharp diffraction peaks indicate good crystallinity of the as-prepared samples, which is good for luminescence.

Figure 1
figure 1

XRD patterns of the NaScMo2O8 and NaScMo2O8:RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho) samples. The standard data for NaScMo2O8 (JCPDS#32-1150) are also presented in the figure for comparison

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The morphology and chemical element of the sodium scandium molybdate precursor samples (Fig. 2a, c) and the corresponding NaScMo2O8 samples annealed at 800 °C for 2 h (Fig. 2b, d) were inspected by using SEM and EDS test, respectively. As shown in Fig. 2a, the SEM image of sodium scandium molybdate precursor sample consists of nonuniform rectangular sheets with size in micron level. After annealing at 800 °C for 2 h (Fig. 2b), the obtained NaScMo2O8 sample becomes irregular blocks with larger size due to the decomposition of the sodium scandium molybdate precursor sample during calcination process. This phenomenon can be proved by the EDS of the precursor in Fig. 2c, and the sodium scandium molybdate precursor may contain elements of C, Na, Sc, Mo and O (Table S1 in Supporting Information). The EDS in Fig. 2d suggests that the calcined products are composed of Na, Sc, Mo and O elements with corresponding atomic ratio of 1.06:1.00:2.16:7.75, which is similar to the theoretical value (1:1:2:8) of the NaScMo2O8 crystals without considering the instrument error (Table S2 in Supporting Information).

Figure 2
figure 2

SEM images and EDS spectrum of the sodium scandium molybdate precursor sample (a, c) and the corresponding NaScMo2O8 sample annealed at 800 °C for 2 h (b, d)

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Downconversion luminescence and energy transfer

The excitation and emission spectra of the NaSc(1-x)Mo2O8:xTb3+ (x = 0.01–0.15) and NaSc(1-x)Mo2O8:xEu3+ (x = 0.01–0.15) samples are shown in Fig. 3, respectively. The Tb3+ ions could be used as an activator for green-emitting materials owing to their 5D4 → 7F5 transition, and the Eu3+ ions can be investigated as a red-emitting conversion phosphor due to their 5D0 → 7F2 transition, respectively. As shown in Fig. 3a, upon excitation into the MoO4 2− at 292 nm (Fig. S3 in Supporting Information), the emission spectrum (right) of Tb3+ consists of 5D4 → 7F6 (492 nm) in the blue region, 5D4 → 7F5 (549 nm, strongest peak) in the green region and 5D4 → 7F4 (589 nm)/5D4 → 7F3 (625 nm) in the red region, which are much stronger than that of MoO4 2− [6]. When monitored at 549 nm (5D4 → 7F5 of Tb3+), there are two parts in the range of 200–500 nm, which are ascribed to the charge-transfer (C–T) transitions of Mo6+–O2− from 200 to 350 nm [9, 30,31,32] and the typical intraconfigurational f–f transitions of Tb3+ ions from 350 to 500 nm, respectively. Obviously, the characteristic excitation spectrum of Tb3+ becomes unapparent compared to that of MoO4 2−. The above results illustrate that the energy transfer from MoO4 2− to Tb3+ took place [33, 34]. Figure 3b displays the excitation spectrum (left) monitored at 618 nm (5D0 → 7F2 of Eu3+), which consists of a broad and strong charge-transfer band of MoO4 2− ranging from 200 to 350 nm with a maximum at around 292 nm and a series of typical f–f transitions of Eu3+ ions at 367 nm (7F0 → 5D4), 382 nm (7F0 → 5L7), 395 nm (7F0 → 5L6), 417 nm (7F0 → 5D3) and 465 nm (7F0 → 5D2), respectively. It is worth noting that the emission of Eu3+ under 292 nm excitation was mainly composed of the 5D0 → 7F1 (596 nm) magnetic dipole transition and the 5D0 → 7F2 (618 nm) electric dipole transition, respectively. The asymmetry ratio of I(5D0 → 7F2)/I(5D0 → 7F1) was equal to 7.8 which indicates that the Eu3+ ions have no inversion center in NaScMo2O4 host and the NaScMo2O4:Eu3+ product is beneficial to improve the color purity of the red phosphor. The energy transfer from MoO4 2− to Eu3+ is similar to that from MoO4 2− to Tb3+. The doping concentration of the luminescent center has a significant impact on the performance of phosphors [35]. With increasing concentration of RE3+ ions, the resonance energy transfer is allowed because the distance of the luminescent centers becomes short enough to bring about concentration quenching of RE3+. So, it is very important to find the optimum doping concentration. Obviously, the quenching concentration of Tb3+ or Eu3+ in NaScMo2O8 host both is 10% (Fig. 3), indicating that NaScMo2O8 is a good matrix for luminescent materials like GdY(MoO4)3:RE3+ (RE = Eu, Dy, Sm, Tb) [30], NaGd(MoO4)2:Eu3+,Tb3+ [31], CaMoO4:Eu3+ [32], AgRE(WO4)2:Ln3+ (RE = Y, La, Gd, Lu; Ln = Eu, Tb, Sm, Dy, Yb/Er, Yb/Tm) [36] and Gd2(WO4)3:Tb3+/Eu3+ [37] phosphors with excitation wavelength at around 292 nm.

Figure 3
figure 3

Excitation (left) and emission spectra (right) of the NaSc(1-x)Mo2O8:xTb3+ (x = 0.01–0.15) samples (a) and the NaSc(1-x)Mo2O8:xEu3+ (x = 0.01–0.15) samples (b)

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The ET from Tb3+ to Eu3+ in molybdates has been reported in previous studies [2, 6, 33]. In this paper, a series of experiments were done to demonstrate the ET process between Tb3+ and Eu3+ in the NaScMo2O8 host. Figure 4 shows the excitation (a) and emission (b) spectra of the NaSc(0.975-y)Mo2O8:0.025Eu3+, yTb3+ (y = 0–0.075) sample. The excitation spectra recorded at 618 nm (5D0 → 7F2) of Eu3+ are all composed of a broad and strong charge-transfer band of MoO4 2− with peak at 292 nm and a series of typical f–f transitions of Eu3+ and Tb3+ ions at 395 nm (7F0 → 5L6 of Eu3+), 465 nm (7F0 → 5D2 of Eu3+) and 488 nm (7F6 → 5D4 of Tb3+), except for the excitation intensity (Fig. 4a). The presence of the excitation peak of Tb3+ (488 nm, 7F6 → 5D4) in the excitation spectrum monitored with Eu3+ emission (618 nm, 5D0 → 7F2) clearly indicates that an energy transfer has occurred from Tb3+ to Eu3+ in the NaScMo2O8 host. As shown in Fig. 4b, under 292 nm excitation (the characteristic peak of MoO4 2−), the emission intensity of Eu3+ at 618 nm in NaSc0.965Mo2O8:0.025Eu3+, 0.01 Tb3+ (blue line) is much stronger than that of single-doped Eu3+ in NaSc0.975Mo2O8:0.025Eu3+ (black line). With increase in Tb3+ concentrations, the emission intensity of Eu3+ at 618 nm increases gradually and then slowly decreases due to concentration quenching [6]. The emission spectra in Fig. 4b directly support that the ET of MoO4 2− → Tb3+ → Eu3+ can take place in the NaScMo2O8 host. Also taking into account the excitation spectrum in Fig. 4a, if Tb3+ ion was excited from 7F6 → 5D4 (488 nm), there should be energy transfer from the excited 5D4 level of Tb3+ to the 5D0 level of Eu3+ in the NaSc(0.975-y)Mo2O8:0.025Eu3+, yTb3+ (y = 0.01–0.075) samples. Upon excitation into the 7F6 → 5D4 transition of Tb3+ at 488 nm, the emission spectra of the NaSc0.9Mo2O8:0.05Tb3+, 0.05Eu3+, NaSc0.95Mo2O8:0.05Tb3+ and NaSc0.95Mo2O8:0.05Eu3+ are presented in Fig. 4c for comparison. The emission spectrum of NaSc0.95Mo2O8:0.05Tb3+ (green line) exhibits the f → f transitions of Tb3+ with a strong peak at 549 nm (5D4 → 7F5); there is no obvious emission peak in the NaSc0.95Mo2O8:0.05Eu3+ sample (black one), but the emission spectrum of NaSc0.9Mo2O8:0.05Tb3+, 0.05Eu3+ presents much stronger characteristic peaks of Eu3+ at 596 nm (5D0 → 7F1) and 618 nm (5D0 → 7F2) than that of Tb3+ at 549 nm (5D4 → 7F5). This phenomenon further demonstrates that the energy can be effectively transferred from Tb3+ to Eu3+.

Figure 4
figure 4

Excitation (a) and emission (b) spectra of the NaSc0.975Mo2O8:0.025Eu3+, yTb3+ (y = 0–0.075) samples. c Emission spectra of the NaSc0.9Mo2O8:0.05Tb3+, 0.05Eu3+, NaSc0.95Mo2O8:0.05Tb3+ and NaSc0.95Mo2O8:0.05Eu3+ samples upon excitation to the 7F6 → 5D4 transition of Tb3+ at 488 nm, respectively

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Figure 5a shows the multicolor luminescence based on the ET of MoO4 2− → Tb3+ → Eu3+ by changing Eu3+ concentration in the NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+. Upon excitation into the MoO4 2− at 292 nm, the NaSc0.95Mo2O8:0.05Tb3+ only emits its characteristic emissions without doping Eu3+; when doped with a small amount of Eu3+ (x = 0.001), the characteristic emission of Eu3+ can be observed apart from Tb3+ emission. With increase in Eu3+ doping concentrations, the emission intensity of Tb3+ at 549 nm (5D4 → 7F5) gradually decreases and that of Eu3+ at 618 nm (5D0 → 7F2) and 596 nm (5D0 → 7F1) increases simultaneously because of the energy transfer from Tb3+ to Eu3+. And then, the emission spectra of Eu3+ gradually become the dominant one with maximum value at x = 0.1; with further increasing Eu3+, the emission of Eu3+ decreases due to the Eu3+–Eu3+ internal concentration quenching effect. The results above further reflect that the energy of the red emission of Eu3+ is derived from Tb3+ and the multicolor luminescence can be tuned by adjusting the relative ratio of Tb3+ to Eu3+. The CIE chromaticity coordinates (Fig. 5b) of NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ vary from green region (0.27, 0.58) to red region (0.62, 0.34) via yellow region (0.42, 0.47) by altering Eu3+ concentration, which can be seen clearly from the corresponding luminescence photographs excited by a 254-nm UV lamp (inset in Fig. 5b).

Figure 5
figure 5

a Emission spectra of NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ (x = 0–0.15); b the CIE chromaticity coordinates of NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ (x = 0–0.025) phosphors under 292 nm excitation and the corresponding luminescence photographs excited by a 254-nm UV lamp

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The energy transfer also can be confirmed by the luminescence decay curves of Tb3+, as shown in Fig. 6a. According to Dexter theory [38], the nonradiative energy transfer can shorten the lifetime of Tb3+. The decay time of Tb3+ (excited at 292 nm and monitored at 549 nm) as a function of the Eu3+ doping concentrations in NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ can be obtained from equation:

$$ I_{t} = I_{0} \exp ( - t/\tau ) $$
(1)

where I t and I 0 are the luminescence intensities at time t and t = 0, respectively, and τ is the decay time. The values of Tb3+ for NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ are determined to be 0.433 and 0.065 µs at x = 0 and 2.5, respectively. We can conclude that with the increase in Eu3+ concentration, the fluorescence lifetime of Tb3+ (5D4) state decreases due to the strong energy transfer from Tb3+ to Eu3+ via nonradiative process in NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ phosphors.

Figure 6
figure 6

a Decay curves of Tb3+ for the NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ (x = 0 and 0.025) phosphors. b The dependence of I S0/I S of Tb3+ on \(\hbox{C}_{({\rm Tb}^{3+}+{\rm Eu}^{3+})^{n/3}}\)

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In general, there are two ways for the energy transfer from Tb3+ to Eu3+ in a phosphor: One is exchange interaction, and the other is electric multipolar interaction. If the critical distance (R c) between Tb3+ and Eu3+ is less than 4 Å, the energy transfer takes the exchange interaction; otherwise, it takes the electric multipole interaction. The distance R c can be calculated using the crystal structure data method through the following equation [39]:

$$ R_{\text{c}} = 2\left[ {\frac{3V}{{4\pi X_{\text{C}} N}}} \right]^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}}} $$
(2)

where N is the number of molecules in the unit cell, V the cell volume and X c the total concentration of Tb3+ and Eu3+ ions. For NaScMo2O8 host (N = 2,V = 370.3 Å3), with increasing Eu3+ concentration (x = 0.002, 0.025, 0.05 and 0.1) in the NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ sample, the R c was calculated to be 18.9, 16.8, 15.2 and 13.3 Å at total concentration (X c) of 0.052, 0.075, 0.10 and 0.15, respectively. All these R c values are far greater than 4 Å, which indicates that the energy transfer mechanism of Tb3+ → Eu3+ is governed by electric multipolar interaction in NaScMo2O8 host [40].

The energy transfer mechanism for multipolar interactions can be further discussed by the following equation [41]:

$$ \eta_{\text{S0}} /\eta_{\text{S}}\, \infty\, C^{{n}/{3}} $$
(3)

Many studies have reported that the value of η so/η s can be approximately estimated by the luminescence intensity ratio (I S0/I S) of Tb3+ as follows:

$$ I_{\text{S0}} /I_{\text{S}}\, \infty\, C^{{n}/{3}} $$
(4)

where C is the total concentration of Tb3+ and Eu3+ and n = 6, 8 and 10 are dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively. I S and I S0 are the emission intensity of Tb3+ in the presence and absence of Eu3+. The relationships between I S0/I S and \( {\text{C}_{(\text{Tb}^{3+}+\text{Eu}^{3+})}}^{n/3} \) are presented in Fig. 6b. When n = 6, the linear behavior fitting value R2 is 0.986 which is better than others (n = 8, R 2 = 0.977 and n = 10, R 2 = 0.94). The fitting results illustrate that the energy transfer mechanism between Tb3+ and Eu3+ in NaScMo2O8 host is dominated by dipole–dipole electric multipolar interaction.

Figure 7 shows detailed schematic for the ET processes of MoO4 2− → Tb3+ → Eu3+ in NaScMo2O8 host. Firstly, upon UV irradiation, the energy is absorbed by MoO4 2−. The electrons in the ground state (1A1) of MoO4 2− shift to its 1B (1T2) level [42] and then a very small part of them return to the lowest excited 1B (1T1) level to emit the characteristic emission of MoO4 2− (Fig. S3 in Supporting Information); meanwhile, other excited electrons transferred energy to Tb3+ and Eu3+. On the one hand, the Tb3+ ion shows its characteristic emissions: The energy on Tb3+ higher levels relaxes to the lowest excited energy level 5D4 via multi-phonon relaxation [40] and then return to the ground state, engendering the emissions of Tb3+ (5D4 → 7F6, 5, 4, 3). On the other hand, the energy absorbed by Tb3+ be transferred to higher excited energy level of Eu3+ (5D1) via dipole–dipole interaction. Finally, the energy on 5D1 level relaxes to 5D0 level, giving out red emissions based on 5D0 → 7F0, 1, 2, 3 transitions of Eu3+.

Figure 7
figure 7

Schematic for the ET processes of MoO4 2− → Tb3+ → Eu3+ in NaScMo2O8 host

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Upconversion luminescence

In Fig. 8, under 980 nm excitation, the emission of NaScMo2O8:RE3+ (RE = Yb/Er, Yb/Ho) showed green (Yb3+/Er3+: 4S3/2, 2H11/2 → 4I15/2; Yb3+/Ho3+: 5S2 → 5I8) luminescence [43]. To gain further insight into the UC emission properties of NaScMo2O8:RE3+ (RE = Yb/Er, Yb/Ho), the concentration of Er3+ (or Ho3+) was fixed at 0.01 and the concentration of the Yb3+ was changed from 0 to 0.2 (or from 0 to 0.25). From Fig. 8a, it can be found that the UC emission spectra of NaSc(0.99-x)Mo2O8:xYb3+, 0.01Er3+ samples do not show an obvious difference in shape and emission bands except for the emission intensity. All of them exhibit three emission bands centered at 536, 556 and 662 nm, which can be ascribed to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. Upon variation of the Yb3+ concentration from 0 to 0.2, the intensity of UC emission spectra first increases and then decreases. When Yb3+ concentration is increased from 0 to 0.1, more Yb3+ become available to furnish and transfer energy to the Er3+, resulting in the higher emission intensity. Once exceeded its limit (x = 0.1), the interatomic distance between Yb3+ and Er3+ became short with further increase in the Yb3+ concentration, which remarkably enhances the probability of energy migration to the quenching center caused by resonance transfer and then results in the decrease in UC emission intensity. This phenomenon is similar to that of NaSc(0.99-x)Mo2O8:xYb3+, 0.01Ho3+, which is given in Fig. 8b. The strong emission band centered at 541 nm, and the relatively weak emission band at 649 nm can be attributed to the 5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+ ions, respectively. When x = 0.2, the sample shows the strongest upconversion luminescence (Fig. 8b).

Figure 8
figure 8

UC emission spectra of a the NaSc(0.99-x)Mo2O8: xYb3+, 0.01Er3+ (x = 0–0.2) and b the NaSc(0.99-x)Mo2O8:xYb3+, 0.01Ho3+ (x = 0–0.25) under 980 nm excitation, respectively

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Conclusion

In summary, NaScMo2O8:RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho) phosphors were synthesized by a simple surfactant-free hydrothermal route combined with subsequent calcination at 800 °C. Various ways are enumerated in this paper to prove the ET of MoO4 2− → Tb3+ → Eu3+. The NaSc(0.95−x)Mo2O8:0.05Tb3+, xEu3+ exhibit strong multicolor emissions from green to red due to the effective ET from Tb3+ to Eu3+ by dipole–dipole interaction. Upon 980 nm excitation, Yb3+/Er3+- and Yb3+/Ho3+-doped NaScMo2O8 both exhibit strong green emission. Besides, the Yb3+ concentration doped in the NaSc(0.99-x)Mo2O8:xYb3+, 0.01Er3+ and NaSc(0.99-x)Mo2O8:xYb3+, 0.01Ho3+ phosphors have been optimized at x = 0.1 and 0.2, respectively. Owing to their excellent DC/UC luminescence properties, the NaScMo2O8:RE3+ (RE = Tb, Eu, Tb/Eu, Yb/Er, Yb/Ho) phosphors have great potential application in the fields of color displays and light-emitting devices.