Improved photo-luminescence behaviour of Eu³⁺ activated CaMoO₄ nanoparticles via Zn²⁺ incorporation

Abstract

Zn²⁺ (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:2Eu³⁺ nanophosphors have been synthesized via the polyol method using ethylene glycol (EG) as both capping agent and reaction medium at 150 °C. From XRD analysis, all 900 °C annealed Zn co-doped CaMoO₄:Eu³⁺ nanophosphors have a tetragonal scheelite phase. Some extra phase evolution has been observed for the as-prepared Zn doped samples. The intensity and crystallinity of XRD patterns increase as heat treatment increases to 900 °C. The valence states of the involved compositions (Zn co-doped CaMoO₄:Eu) were investigated by X-ray photoelectron spectroscopy (XPS) and it was found that Ca, Mo, Eu and Zn are in their +2, +6, +3 and +2 oxidation states, respectively. TG-DSC studies of the as-prepared samples corroborate their thermal stability. A TEM (Transmission electron microscopy) study reveals that the particles have spherical morphology. Photoluminescence studies have been carried out under ∼266, and 395 nm excitation wavelengths. Zn co-doping in the CaMoO₄:Eu matrix produces a high distortion and modifies the crystal field around the Eu³⁺ ion and improves the PL intensity. CIE co-ordinates of the 900 °C annealed 10 at% Zn co-doped CaMoO₄:Eu sample under 266 nm excitation is x = 0.64 and y = 0.35, which are close to the standard of NTSC (x = 0.67 and y = 0.33). These investigations reveal that Zn co-doped CaMoO₄:Eu³⁺ nano-materials can be used as potential red emitting phosphors, an area which is a bottleneck in the development of low cost LEDs.

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1. Introduction

White light-emitting diodes (w-LEDs) are promising and are supposed to be the next generation in illumination due to their high energy efficiencies, long lifetimes, good reliability and environmental friendliness compared to conventional incandescent lamps. It is well known that w-LEDs are produced by mixing red, green and blue (RGB) LEDs, combing a blue LED with a yellow phosphor of (Y,Gd)₃(Al,Ga)₅O₁₂:Ce³⁺, or using near-ultraviolet (UV) LED-stimulated RGB phosphors. The major challenge in developing near-UV LEDs is to explore highly efficient tri-color phosphors that possess an excitation wavelength that matches well with the emission spectrum of the near-UV LED chips.^1–3 The current red phosphors based on Eu³⁺ activated oxides, sulphides and nitrides have disadvantages such as low reliability, high toxicity and luminous efficacy, compared with blue and green phosphors. Therefore, it is necessary and urgent to explore novel red-emitting phosphors that can be efficiently excited in the near UV range. The scheelite structured molybdates have a vast number of industrial applications and are used in scintillators, solid state lasers, fluorescent lamps and photocatalysis. One of the most fascinating aspects of these materials is their ability to generate white light from a single phasic phosphor material.^4–6 Calcium molybdate, CaMoO₄ is a self activated blue-green host with a phonon energy ∼815 cm⁻¹.⁷ Photoluminescence properties can be tuned by doping with different rare earth ions. In the CaMoO₄ lattice, Ca atoms have 8 co-ordinations and make [CaO₈] polyhedra while Mo atoms have 4-co-ordinations building [MoO₄] polyhedra.^7,8 MoO₄²⁻ units in the CaMoO₄ lattice have broad and intense absorption bands. This occurs due to charge transfer from oxygen to metal in the near-UV region. A blue-green light emission is observed due to a transition into MoO₄²⁻ units in the lattice host. Eu³⁺-doped CaMoO₄ phosphors can be efficiently excited in the near-UV region, spanning from 250 to 400 nm.⁹ Yan et al. has reported a 3 times improvement in red emission intensity in CaMoO₄:Eu via bismuth co-doping.¹⁰ Recently, enhancements in luminescent intensity in CaMoO₄:Eu and in Y₂WO₆:Ln (Ln = Sm, Eu and Dy) through Gd³⁺ co-doping have been reported.^11,12 A single-component white-light phosphor is normally produced by co-doping a sensitizer and an activator into the same host matrix. A few prominent investigations have been performed to improve the luminescent properties of the CaMoO₄ phosphor by co-doping other metal ions. However, the low efficacy of these nanomaterials has limited their general application. Consequently, it is of great interest to significantly enhance the emission intensity of phosphors in order to enhance their potential applications.

The energy transfer mechanism from a sensitizer to an activator such as in Eu²⁺/Mn²⁺, Ce³⁺/Mn²⁺, and Ce³⁺/Eu²⁺ couples has been investigated in many inorganic hosts, and an effective resonance-type multi polar interaction has been verified in NaSr₄(BO₃)₃:Ce³⁺/Mn²⁺, NaBa₄(BO₃)₃:Ce³⁺/Mn²⁺, Sr₃Sc(PO₄)₃:Eu²⁺/Mn²⁺, Sr₃B₂O₆:Ce³⁺/Eu²⁺, and so on. Moreover, some single-phase phosphors, such as Ca₁₀K(PO₄)₇:Eu²⁺/Mn²⁺, Ca₉Y(PO₄)₇:Eu²⁺/Mn²⁺, Ca₉Y(PO₄)₇:Ce³⁺/Eu²⁺, and Ca₉Y(PO₄)₇:Ce³⁺/Mn²⁺, are used to improve the emission intensity for n-UV LED (light emitting diode) applications.^11,13,14 Also an improvement in photoluminescence intensity has been reported with co-doping of charge compensators such as Li⁺, K⁺, Na⁺ and Bi³⁺ in phosphors or with a SiO₂ coating over phosphor particles. In this direction, non-radiative rates are reduced by either charge compensation, improved crystallinity or the extent of the decrease in surface dangling bonds/OH bonds from the inner core phosphor by using a shell.¹⁵ Su and co-workers also reported enhanced photoluminescence in a CaWO₄ based red phosphor via Eu and Na co-doping.¹⁶

There are some reports of Zn doped nano-phosphors and their improvement in photoluminescence intensity in the literature^17–21 but no such study on Eu³⁺–Zn²⁺ co-doped CaMoO₄ materials has been found to the best of our knowledge.

Many reports have been published on synthesis routes as well as luminescent properties of molybdates doped with different lanthanide ions. Synthesis routes such as solid state reaction, auto-combustion, sol–gel, and solvo-thermal have been reported for CaMoO₄ and/or Ln³⁺ doped CaMoO₄ in the literature, with an emphasis of controlling the crystal size, morphology, and the composition, which are crucial for a high quantum efficiency.²² Among these polyol synthesis has been effectively been used to prepare the metal-nanoparticles. In this methodology, ethylene glycol (EG) is used as a capping agent with a low synthesis temperature (∼150 °C). The use of poly alcohols, like ethylene glycol, as reducing agents for obtaining metallic nanoparticles has some advantages, as the by-products obtained during this process are ketones or carboxylic acids, they can be easily removed from the reaction mixture.²³

Herein we have prepared a series of CaMoO₄ based red phosphors including CaMoO₄:Eu³⁺ (Eu³⁺ = 2 at%) and Ca_1−x−yMoO₄ : xEu : yZn (x = 2, y = 2, 5, 7 and 10 at%). The detailed photoluminescence properties of Eu³⁺ doped CaMoO₄ with different concentrations of Zn²⁺ ions that emit light in the visible range were studied for ASP and 900 °C annealed samples. The detailed structural and downshifting phenomenon and involved decay kinetics have been studied for Zn co-doping in the Eu³⁺ activated CaMoO₄ phosphor matrix.

2. Synthesis

Zn²⁺ (Zn²⁺ = 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu (here the concentration of Eu³⁺ is taken as 2 at% optimal) was prepared using ethylene glycol (EG) as capping agent with a reaction medium at 150 °C. Zinc oxide (ZnO, AR grade), calcium carbonate (CaCO₃, AR grade), europium oxide (Eu₂O₃, 99.99%, Sigma Aldrich) and ammonium molybdate ((NH₄)₂Mo₇·4H₂O, AR grade) were used as sources of Ca²⁺, Eu³⁺ and MoO₄²⁻. In a typical synthesis procedure of a 1 g sample of 5 at% Zn²⁺ co-doped CaMoO₄:Eu nanoparticles, 0.3312 g of CaCO₃ and 0.0189 g of Eu₂O₃ and 0.0146 g of ZnO were dissolved together in concentrated nitric acid (HNO₃). The mixture was heated at 80 °C to remove the excess acid this process of removal of excess acid was repeated five times after addition of de-ionized water (5 ml). To this solution, 0.6351 g of (NH₄)₂Mo₇·2H₂O was added followed by 50 ml of EG. The pH of the solution was adjusted to 8–9 using urea. The resulting solution was then stirred for 1 hour. This solution was then transferred to a two neck round bottom flask and was heated up to 150 °C for 3 hours under refluxing conditions in a condenser until precipitation was complete. The white precipitate so obtained was collected by centrifugation and washed 5 times in methanol to remove excess of EG and finally it was washed with acetone and dried at 90 °C for 2 hours in ambient conditions to yield the final white product. Finally, the as prepared samples were divided in 2 parts. One part of the sample was annealed at 900 °C in an ambient atmosphere at a heating rate of 2 °C min⁻¹ for 4 hours in an alumina crucible and the other part was left untreated.

2.1 Characterization techniques

Phase confirmation of the synthesized samples was examined by Rigaku-Miniflex-II X-ray diffractometer with Ni filtered Cu K_α radiation (λ = 1.5405 Å) at 30 kV and 15 mA. All patterns were recorded over the range 10 ≤ 2θ ≤ 70° with a step size of 0.02°. The chemical composition and valence state of the elements were analysed by X-ray photoelectron spectroscopy (XPS) using a monochromatic AlKα (hν = 1486.6 eV) X-ray source and a hemispherical analyzer (SPECS, HSA3500). The recorded spectra were charge-corrected to the C 1s ∼284.6 eV as the reference. The size and morphology of the samples were inspected using a Transmission Electron Microscope (TEM) JEOL JSM 100CX operating at an accelerating voltage of 200 kV. The samples for TEM were prepared by depositing a drop of a colloidal ethanol solution of the powder sample onto a carbon-coated copper grid. Simultaneous DSC/TGA spectra were recorded using NETZSCH STA 449 F1 Jupiter. DSC and TG analyses were carried out using 10 mg of the sample at a heating rate of 10 °C min⁻¹ from 35 to 800 °C, in nitrogen atmosphere under a flow of 60 cm³ min⁻¹. The Raman spectra of the as prepared and annealed samples were measured with Renishaw micro-Raman spectrometer attached with 633 nm laser as an excitation source. Photoluminescence excitation (PLE), emission (PL) and lifetime measurements were performed using a Fluorolog-3 spectrofluorometer (Model: FL3-11, Horiba Jobin Yvon). The 266 nm excitation wavelength of a Nd:YAG laser and CCD (charged coupled device) detector (Ocean Optics, QE 65000) were also used for emission measurement.

3. Results and discussion

3.1 Structural studies

3.1.1 XRD study. XRD patterns of Zn²⁺ (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu ASP, and annealed at 900 °C samples are shown in Fig. 1(a) and (b). It is evident from the figure that even the as-prepared (ASP) sample shows highly crystalline behaviour with tetragonal structure. However some extra peaks (marked as #) that had smaller intensities were observed in the ASP samples with Zn co-doping at 2θ = 24.05, 26.26 and 28.57°. Peak intensity of these peaks increases with Zn doping. These peaks do not match with JCPDS card no. 29-0351. The evolution of these peaks may be assigned to MoO_n·mH₂O (m and n are whole integers), other Eu–Mo–O related compounds present on the sample. Similar results have been reported for CaMoO₄ and SrWO₄ doped systems with Tb³⁺ and Eu³⁺ different activator ions.^22,23

Fig. 1 XRD patterns of Zn²⁺ (0, 2, 5, 7 and 10 at%) doped CaMoO₄:Eu³⁺ nanoparticles for (a) ASP and (b) 900 °C annealed samples. Symbols marked as (#) show the extra phase evolution.

All diffraction peaks for 900 °C annealed samples match well with JCPDS card no. 29-0351 (a = 5.226 Å, c = 11.43 Å and V = 312.17 ų). The lattice parameters for ASP 2 at% Zn²⁺-doped CaMoO₄ samples are a = 5.230 Å, c = 11.462 Å, V = 313.56 ų and for 900 °C annealed samples they are a = 5.231 Å, c = 11.46 Å, V = 313.57 ų. Overall, cell volume decreases on annealing the samples as compared to ASP samples with Zn²⁺ co-doping in CaMoO₄:Eu host matrix. The unit-cell constants and the calculated average crystallite sizes of the ASP and 900 °C annealed samples of CaMoO₄ with different concentrations of Zn²⁺ ions are summarized in Table S1 (ESI†).

The diffraction pattern intensities for 5, 7 and 10 at% Zn²⁺-doped CaMoO₄:Eu were found to be slightly less than those for 2 at% doped samples, which may be due to defects created at higher concentrations of Zn²⁺-doping in CaMoO₄:Eu (Fig. 1(a)). Samples annealed at ∼900 °C show a slightly higher degree of crystalline behaviour than the ASP samples, which is shown in Fig. 1(b).

The average crystallite sizes were estimated with the Scherrer formula,

where, D is the average crystallite size, λ is the wavelength of the X-rays (1.5405 Å), and θ and β are the diffraction angle and full-width at half maximum (FWHM) of the peak in the XRD pattern, respectively. The strongest four peaks (1 0 1) at 2θ = 18.71, (1 1 2) at 2θ = 28.71, (2 0 0) at 2θ = 34.48 and (204) at 2θ = 47.10 were used to calculate the average crystallite size (D) of the powders. It can be seen that the lattice parameters and the cell volume decrease as the Zn²⁺ concentration increases from 2 to 10 at%. This is due to substitution of Ca²⁺ (CN = 8, 1.12 Å) ions⁷ by Zn²⁺ (CN = 6, 0.74 Å) ²⁴ and Eu³⁺ (CN = 8, 1.06 Å) ions.⁷

Rietveld analyses was performed for the Zn (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu annealed at 900 °C samples using the FullProf software.²⁵ Typical Rietveld fitting for undoped and 2 at% Zn co-doped CaMoO₄:Eu are shown in Fig. 2(a) and (b). The Wyckoff positions of atoms based on space group I4₁/_a (88) and Z = 4 (number of CaMoO₄ formula units per unit cell) in CaMoO₄ unit cell are:^7,8 Ca: (4b: 0, 0.25, 0.0625), Mo: (4a: 0, 0.025, 0.125) and O: (16f: x, y, z) with angles (α = β = γ = 90°).

Fig. 2 Rietveld XRD patterns demonstrating observed and calculated difference and corresponding Bragg positions of (a) un-doped and (b) 2 at% Zn co-doped CaMoO₄:Eu.

The Pseudo-Voigt function was used to model the peak profiles and six coefficient polynomial was used to describe the background.

3.1.2 TEM study. Fig. 3 illustrates the TEM micrograph of 10 at% Zn²⁺ doped CaMoO₄:Eu nanoparticles annealed at 900 °C. Most of these nanoparticles have irregular shapes and a few have a spherical shape. It is well known that the uniformity of the size and shape is controlled by nucleation.²³ In polyol synthesis, reaction was started after adding ethylene glycol.

ASP samples show high agglomeration (shown in Fig. S1 (ESI†)) and Zn inclusion improves the grain growth of the particle shape evolution. During the heating process, nucleation and crystal growth occurred. This results in irregular shapes and agglomerated particles.²⁶ The particle sizes estimated from TEM are ∼25–52 nm, which is in agreement with the calculated sizes from XRD studies. The typical particle size (shown in Fig. 3 itself) has been estimated to be ∼25 nm.

Fig. 3 TEM micrograph of 10 at% Zn co-doped CaMoO₄: Eu sample annealed at 900 °C.

3.1.3 DSC/TGA study. Fig. 4 shows the simultaneous DSC/TGA curves along with DTG curves of ASP 2 at% Zn²⁺ co-doped CaMoO₄:Eu³⁺ sample. The sample was measured in the temperature range of 35 to 800 °C with a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. In the TG curve, a weight loss of ∼11% between 80 to 250 °C, 1.35% in the range 250–600 °C and no appreciable loss was observed beyond 600 °C. A marginal weight loss is observed in the vicinity of around 650 (TG/DTG curve) which may be due to an artefact. Similar observations have been reported in the literature.^27,28 The mass loss until 300 °C is attributed to complete dehydration of the powders while the mass loss until 600 °C is due to the evaporation of organic constituents like EG and methanol.

Fig. 4 Simultaneous DTG/TGA and DSC isotherms of 2 at% ASP Zn²⁺ co-doped CaMoO₄:Eu³⁺.

To study the heat flow as a function of temperature in the inert gas (N₂) atmosphere associated with transitions in the 2 at% Zn²⁺ co-doped CaMoO₄:Eu³⁺ ASP sample DSC was recorded (Fig. 4). The curve shows only endothermic peaks. The peak around 110 °C represents the mass loss due to evaporation of water and methanol. These results show that the prepared nano-phosphors are thermally stable and can be used in lighting and display devices.

3.1.4 Raman study. Fig. 5 shows the Raman spectra for Zn²⁺ (0 and 10 at%) co-doped CaMoO₄ samples annealed at 900 °C under 633 nm laser excitation. The Scheelite structure shows 26 modes of vibrations (3Ag, 5Au, 5Bg, 3Bu, 5Eg, 5Eu) in which 13 modes are Raman active (3Ag, 5Bg, 5 Eg) and (4Au, 4Eu) are IR active. The three B_u vibrations are silent modes whereas one A_u and one E_u mode are acoustic vibrations.^7,29 8 Raman-active modes were observed in the Raman spectrum and the other (3B_g and 2E_g) vibration modes were not detectable, this may be due to their low intensities. However, due to strong interaction between the O–Ca–O and O–Mo–O bonds in the clusters, the Raman spectra exhibited intense and sharp bands.^30,31 Strong intense bands at 875, 843, and 791 cm⁻¹ are attributed to the A_g, B_g and E_g modes, respectively. Bands at 202 and 389 cm⁻¹ can be attributed to the B_g and A_g modes. The band at 320 cm⁻¹ is the superposition of A_g (ν₂) and E_g (ν₂) modes of vibrations. The three E_g internal modes are observed at 791, 139 and 107 cm⁻¹. The four Raman bands below 267 cm⁻¹ are due to external modes (lattice modes) whereas the five bands below 920 cm⁻¹ correspond to internal/optical modes (within MoO₄²⁻).³²

Fig. 5 Room temperature Raman spectra of Zn free and 10 at% Zn co-doped CaMoO₄:Eu samples annealed at 900 °C.

All Raman-active modes of CaMoO₄ crystals reported in this work are in good agreement with those reported in the literature.^33,34 In fact, the shifts observed on these bands can be attributed to the degree of interaction between the O–Mo–O bonds and distortions on the [MoO₄] clusters induced by the structural order/disorder in the lattice.

3.1.5 XPS study. The compositional and chemical state of the Zn (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu samples, has been examined by X-ray photoelectron spectroscopy (XPS). XPS spectra of Ca, Mo, O and Zn for Zn (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu are shown in Fig. 6. The XPS survey spectrum for 5 at% Zn doped CaMoO₄:Eu sample at 900 °C comprising core BE levels of Ca, Mo, O, Eu/Zn is shown in Fig. S2 (ESI†) was obtained in the range of 0–1100 eV. Fig. 6(a) shows the XPS spectra of Ca (2p) for 0, 2, 5, 7 and 10 at% Zn²⁺ co-doped CaMoO₄:Eu ASP samples. For the Zn²⁺ free CaMoO₄:Eu sample, the peaks corresponding to Ca (2p) have a core BE of ∼346.64 (2p_3/2) and 350.19 eV (2p_1/2) with the corresponding full width at half maximum (FWHM) values being ∼1.7 and 2.0 eV. Peaks were convoluted using Gaussian function. Typical fitting of Ca (2p) spectra has been shown in Fig. S3 (ESI†). On increasing Zn²⁺ (0, 2, 5, 7 and 10 at%) co-doping concentration, there are slight changes in BE values to higher eV. Moreover, the integrated intensity ratio of (2p_3/2) to (2p_1/2) (I_Ca) is found to be 1.71, 1.79, 169, 1.51 and 1.41 for 0, 2, 5 7 and 10 at% Zn²⁺ co-doping, respectively. These results confirm the +2 oxidation state of Ca. Fig. 6(b) shows the peaks at ∼232.39 and 235.53 eV, which correspond to the core BE of Mo (3d_5/2) and Mo (3d_3/2), respectively for the 900 °C Zn²⁺ free CaMoO₄:Eu sample. Integrated intensity ratios of (3d_5/2) to (3d_3/2) (I_Mo) were found to be 1.31, 1.37, 1.41, 1.49 and 1.39 for 0, 2, 5, 7 and 10 at% Zn²⁺ co-doping samples, respectively (Fig. 6(b)). There is no significant change in BE on Zn²⁺ co-doping. Also, (I_Mo) was found to be 1.39, 1.32, 1.38, 1.43 and 1.39 for 0, 2, 5, 7 and 10 at% Zn²⁺ co-doping samples, respectively. The lack of any significant change in the 3d_3/2–3d_5/2 BE in the Mo spectral region suggests that Mo ions remain in their Mo⁶⁺ state.⁷ Similar behaviour was reported in the literature for Ca–Bi–Mo oxide and for the molybdenum phosphate glass.³⁵ Typical fitting of a Mo (3d) spectrum has been shown in Fig. S4 (ESI†).

Fig. 6 XPS spectra of (a) Ca (2p), (b) Mo (3d), (c) O (1s) and (d) Zn (2p).

A peak at ∼141.1 eV, which corresponds to Eu³⁺ (4d_3/2) and an absence of a peak at ∼127.1 eV corresponding to Eu²⁺ (4d_5/2) is observed. This confirms the high probability of Eu³⁺ in the sample (shown in Fig. S5 (ESI†)). It is also confirmed from the photoluminescence study (discussed later). Typical XPS spectra of Eu³⁺ showing core binding energy and intensity with Zn (0, 2 and 10 at%) co-doped CaMoO₄:Eu annealed at 900 °C samples are shown in Fig. S5 (ESI†). Intensity of these Eu³⁺ peaks are very small for annealed samples and improves with increasing Zn²⁺ concentration.

In addition, O 1s spectral regions have been used to obtain information regarding the presence of oxygen vacancies in the sample (Fig. 6(c)). Peaks were de-convoluted using Gaussian function. Two peaks are well fitted at BE ∼529.7 (P₁) and 531.43 eV (P₂) having FWHM ∼1.7 and 1.79 eV, respectively. Typical peak fitting of O 1s spectra for the 10 at% Zn doped at 900 °C annealed sample has been shown in Fig. S6 (ESI†). On increasing Zn²⁺ co-doping the peak position changes slightly by ±0.1–0.2 eV (Fig. 6(c)). Overall peaks show an asymmetric nature in the higher BE side. This may be due to defects and the oxygen vacancies that were created by Zn²⁺ doping. There are few reports which indicate that the high energy side of the O(1s) peak arises due to hydroxyl groups –OH or other radicals on the sample surface such as CO or CO₂.³⁶ However, the asymmetric behaviour of high energy peaks (∼530.5 eV) in an O 1s spectrum is the signature of the presence of an oxygen ion vacancy in the lattice.³⁷ Vacancies decrease on annealing of the sample surface.⁷ The Core BE peaks of 10 at% Zn doped CaMoO₄:Eu were observed at ∼1024.56 and ∼1048.4 eV and correspond to Zn2p_3/2 and Zn2p_1/2 (Fig. 6(d)).³⁸

4. Optical studies

4.1 Excitation study

Excitation spectra of ASP Zn co-doped (0, 2, 5, 7, and 10 at%) CaMoO₄:Eu nano-phosphors at an emission wavelength of 615 nm are shown in Fig. 7. A broad band from 230 to 320 nm is observed which arises due to a combination of the ligand to metal charge transfer O²⁻ → Mo⁶⁺ and charge transfer band (CTB) from the completely filled 2p orbitals of O²⁻ to the partially filled f–f orbitals of the Eu³⁺ ions (O²⁻ → Eu³⁺),^9,22,32 and the intra f–f transitions of Eu³⁺ around 360 (⁷F₀ → ⁵D₄), 376 nm (⁷F₀ → ⁵G₃), (382 (⁷F₀ → ⁵G₄), 395 (⁷F₀ → ⁵L₆), 415 (⁷F₀ → ⁵D₃), 464 (⁷F₀ → ⁵D₂) and 532 nm (⁷F₀ → ⁵D₁) are observed. The wavelength corresponding to the peak of Eu/Mo–O CTB band around 230–320 nm decreases from 280 to 266 nm and the corresponding FWHM decreases from 51 to 37 nm with an increase in Zn²⁺ ion concentration up to 2 at%. This blue shift in the Mo/Eu–O CT band with Zn²⁺ ion concentration can be related to some Mo/Eu based compound formation which may result in phase segregation, (also observed from ASP XRD data of Zn doped samples). The excitation spectra of 900 °C annealed Zn co-doped (0, 2, and 10 at%) CaMoO₄:Eu³⁺ nano-phosphors at an emission wavelength of 615 nm has been given in Fig. S7 (ESI†).

Fig. 7 Excitation spectra of ASP, Zn (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu samples monitoring emission at a wavelength of 615 nm.

The intensity of the bands increases up to 10 at% Zn²⁺ co-doping and the position of the Eu/Mo–O charge-transfer band (CTB) is shifted to a higher wavelength by ∼2–5 nm due to annealing of the samples at 900 °C compared with the ASP samples; similar observations have been reported for the Mo–O CTB in CaWO₄ and W–O CTB in CaMoO4 host.^29,32

When an electron is transferred from oxygen to Mo, an electronic transition takes place which gives rise to the Mo–O charge transfer band (CTB). In ASP samples, there are a relatively large number of dangling bonds over the particle surface and the lattice is less ordered as compared to that of annealed samples. There is also a higher degree of ionic character between Mo and O for the as-prepared samples as compared to that for the 900 °C annealed samples, and this results in lower energy absorption for annealed samples. On annealing the degree of co-valent character increases. Consequently, the position of the Mo–O charge-transfer band is shifted to a higher wavelength by ∼2–5 nm upon annealing the samples as compared to the as-prepared samples.²⁸

4.2 Emission study

The Eu³⁺ ion has generally been selected as the activator ion to investigate the luminescence properties of rare earth tungstate/molybdate materials as it shows emissions in the visible region. Since the ground electronic state configuration of the Eu³⁺ ion has ⁷F₀ non-degenerate and non-overlapping ^2S+1L_J multiplets. Therefore the Eu³⁺ ion can be used as a structural probe for investigating the local environment in a host matrix.³⁹ It is well documented that the symmetry of the crystal sites of doped Eu³⁺ ions will determine the relative intensity of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions. If the ⁵D₀ → ⁷F₁ magnetic dipole transition is dominant in the spectrum, this indicates that europium is located in a site with inversion symmetry. If the ⁵D₀ →⁷F₂ electric dipole transition is dominant this means that Eu³⁺ is located in a site without inversion symmetry.⁴⁰

Fig. 8(a) shows the PL emission spectra of ASP Zn (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu³⁺ under 266 nm excitation. All the samples show strong ⁵D₀ →⁷F₂ (615 nm), ⁵D₀ → ⁷F₁ (590 nm), ⁵D₀ → ⁷F₃ (654 nm) and ⁵D₀ → ⁷F₄ (705 nm) emission lines upon 266 nm excitation. It is observed that the electric dipole transition at 615 nm is dominant over the magnetic dipole transition at 590 nm. It is well documented that the electric dipole transition is hypersensitive to its environment and the parity allowed transition originating from ⁵D₀ → ⁷F₁ is insensitive to the crystal field environment. It is suggested that most of the Eu³⁺ enters into the lattice site centres without inversion symmetry.^11,23,40 Emission intensity increases up to 2 at% Zn²⁺ doped CaMoO₄:Eu may be due to substitutional and crystal field effects. After 2 at% Zn doping, intensity decreases. Zn²⁺ co-doping may create vacancies that act as a sensitizer, mixing the charge-transfer states. Zn²⁺ addition increased the PL intensity by increasing the radiative transition probability. However, an increase in the Zn²⁺ concentration over a certain limit generates a significant amount of oxygen ion vacancies in the lattice. Consequently, the crystal lattice collapses, and the luminescence intensity decreases.⁴²

Fig. 8 Emission spectra of Zn²⁺ (0, 2, 5, 7 and 10 at%) co-doped CaMoO₄:Eu for (a) ASP and (b) 900 °C annealed samples at a 266 nm excitation wavelength.

Similar behaviour has been reported for an Li⁺ doped Y₂O₃:Eu system.⁴³ The PL band positions with different luminescence intensities have been observed for 900 °C annealed samples as compared to ASP Zn doped samples (shown in Fig. 8(b)). Photoluminescence intensity increases ∼3 times for 10 at% Zn²⁺ co-doped CaMoO₄:Eu as compared to Zn free sample annealed at 900 °C samples. Emission intensity increases up to 10 at% Zn²⁺ doping. On annealing the sample crystallinity increases, non-radiative decay rate decreases and thus radiative rate is improved. This means that co-doping of Zn²⁺ improves luminescence. Improvement of luminescence is due to the substitution of Ca²⁺ sites by Zn²⁺ ions. Zn²⁺ doping may change the crystal field and asymmetricity around the Eu³⁺ ion which leads electric dipole transitions. Also the energy absorbed by the Zn²⁺ fully or partly transfers into Eu³⁺, raising the activation energy of Eu³⁺ and increasing the transition processes, leading to an improvement of the emitting intensity and red color purity. It is likely that the dipole moment of the transitions increases upon co-doping of Zn²⁺ ions. In addition, improved crystallinity as well as removal of organic moieties (such as PEG) gives rise to enhanced luminescence after heat treatment at 900 °C. Also PL study of Zn doped CaMoO₄:Eu for the ASP and annealed at 900 °C samples under 395 nm excitation has been carried out and it showed a similar pattern intensity as under 266 nm excitation (shown in Fig. S8 and S9 (ESI†)).

The ratio of the integrated area of electric dipole transition (⁵D₀ → ⁷F₂) to magnetic dipole transition (⁵D₀ → ⁷F₁) has been probed to study the structural distortion around the Eu³⁺ ion, and is known as the asymmetric ratio (A₂₁).^11,23 In our case the asymmetric ratio is represented as,

where subscript ‘2’ and ‘1’ refer to transitions of ⁵D₀ → ⁷F_J, j = 2 and 1, I₂ and I₁ are the intensities of electric and magnetic dipole transitions, respectively calculated after background correction subjected to the peaks under Gaussian fit and λ is the wavelength respectively. The value of A₂₁ for the ASP Zn (0, 2, 5, 7 and 10 at%) co-doped samples is ∼7.4, 9.8, 8.5, 8.1 and 7.4. The value of A₂₁ for the Zn (0, 2, 5, 7 and 10 at%) co-doped samples annealed at 900 °C is 7.8, 8.7, 9.2 and 10.6 at 266 nm excitation. Also A₂₁ value for ASP and 900 °C samples are in the range ∼6.1 to 9.3 under 395 nm excitation. Higher values of A₂₁ clearly demonstrate that Eu³⁺ occupies the site without inversion symmetry. Overall, the A₂₁ values increase on annealing because of a decrease of the non-radiative rates. The increase in A₂₁ values demonstrates the high distortion present in the host lattice which appears to be highly red emitting.^11,23,40,41

4.3 Decay analysis

The decay curves of the level ⁵D₀ (615 nm) of Eu³⁺ have been measured and are shown in Fig. 9. The excitation wavelength is fixed at 395 nm. The decay curves for Eu³⁺ emissions can be well fitted by using bi-exponential curve fitting which is expressed as:Where I₁ and I₂ are the intensities at different time intervals, τ₁ and τ₂ are their corresponding lifetimes. The bi-exponential fitting of 10 at% Zn²⁺ doped as-prepared and 900 °C annealed CaMoO₄:Eu phosphors under 395 nm excitation are shown in Fig. 9(a) and (b). The decay profile has been recorded monitoring the emission at 615 nm at 395 nm excitation (direct excitation of Eu³⁺). The decay profile shows its bi-exponential behaviour. It corroborates that the availability of Eu³⁺ ion on the surface (τ₁) and core (τ₂) of the particles. Further, the average decay life times can be calculated as:

Fig. 9 Decay curve of 10 at% Zn co-doped CaMoO₄:Eu (a) ASP and (b) 900 °C annealed sample under 395 nm excitation.

The life time values obtained using bi-exponential decay equation for ASP and 900 °C annealed 10 at% Zn²⁺ co-doped CaMoO₄:Eu are ∼0.58 and 0.76 ms, respectively under 395 nm excitation.

These values are in good agreement with the reported values for other Eu³⁺ doped compounds.^9,11 As for Mo–O CTB (∼266 nm) excitation, luminescence decay follows a non-exponential equation because the initial stage energy transfer from Mo–O or Zn²⁺ to the excited states of Eu³⁺ occurs and then the decays from the ⁵D₀ level. The details of energy transfer rate and mechanism have been reported in literature.^11,22 In the case of samples annealed at 900 °C, the lifetime values are higher than those of the as-prepared samples. This may be due to a reduction in non-radiative rates (R₀) as compared to the radiative rate. Non-radiative rate R₀ is expressed as:

Where α and β are constants. ΔE is the difference in energy between excited and ground states of the activator (Eu³⁺) ions, ν_max are the highest available vibrational modes of the surroundings of the rare earth ion. In the case of the Eu³⁺ ion, ΔE is ∼10 000–15 000 cm⁻¹ and the value is comparable with the third overtone stretching vibrations of the –OH functional group (∼3500 cm⁻¹). This functional group arises from the water molecules absorbed or associated during the synthesis of the nanomaterials. EG and aqueous media are used and are the source of the water. R₀ values become large when ΔE ∼ 2hν_max. In the case of the ASP samples, a significant extent of non-radiative transfer of energy from the excited states of the Eu³⁺ ions to the different vibrational modes of –OH species occurs which leads to a reduction in the Eu³⁺ emission. Similar reports have been documented for Eu³⁺/Mn²⁺ doped CaF₂ and Fe₃O₄ hybrid structures.⁴⁴ Recently the radiative and non-radiative decay rates of CdSe nanorods were found to be modified in Au/CdSe tetrapod structures and the non-radiative rate changes from 1.91 × 10⁷ s⁻¹ to 9.33 × 10⁹ s⁻¹ were observed for CdSe nanorod to Au/CdSe tetrapod structures.⁴⁵

The radiative decay rate constants are defined as:¹¹

(k_r) = 1/τ_av (6)

Calculated values of radiative rate constants for 10 at% Zn²⁺ co-doped CaMoO₄:Eu³⁺ for ASP and 900 °C annealed sample are 1.724 × 10³ and 1.315 × 10³ s⁻¹, respectively.

4.4 CIE studies

Fig. 10 shows the Commission Internationale de l’Eclairage (CIE) chromaticity diagram for Zn²⁺ (0 and 10 at%) co-doped CaMoO₄:Eu³⁺ annealed at 900 °C phosphors excited at 266 nm. CIE coordinates vary for ASP and 900 °C annealed samples at 266 and 395 nm excitation. Typical CIE coordinates for 10 at% ASP Zn²⁺ co-doped sample is (0.58, 0.34) at 266 nm excitation. CIE color coordinates for Zn free and 10 at% Zn²⁺ co-doped CaMoO₄:Eu annealed samples at 266 nm excitation are (x1 = 0.58, 0.36) and (x2 = 0.64, 0.35), which lie well within the red region. It is worthwhile to observe that under 266 and 395 nm (see Table S2 (ESI†), the color coordinates are located in the shallow red for ASP and lie in red region, for 900 °C annealed samples, respectively. Red emission color is due to the characteristic emission of Eu³⁺ ion. Detailed CIE coordinates have been calculated for Zn co-doped samples for ASP and samples annealed at 900 °C under 266 and 395 nm excitation, given in Table S2 (see ESI†).

Fig. 10 CIE diagram of Zn²⁺ free and 10 at% Zn²⁺ co-doped CaMoO₄:Eu annealed at 900 °C.

5. Conclusion

Highly nano-crystalline nanoparticles of Zn co-doped CaMoO₄:Eu have been prepared using a polyol synthesis. Tetragonal scheelite single phase has been confirmed through XRD study. Characteristics valence states for Ca, Mo, Zn and Eu have been probed through an XPS study and they were in their formal +2, +6, +2 and +3 oxidation states. TEM study confirms the spherical morphology of the Zn doped samples. Characteristic Raman modes of vibrations have been observed for the CaMoO₄ host. Enhanced photoluminescence for 900 °C annealed samples has been observed as compared to ASP via Zn-doping, it may be due to the reduction of –OH ions and organic moieties at higher temperature. Also Zn²⁺ co-doping in CaMoO₄:Eu matrix produces high asymmetricity. The asymmetric ratio is ∼7.4 to 10.6 and ∼6.1 to 9.3 under 266 and 395 nm excitation wavelengths, which reflects that it is a high red emitter. Zn²⁺ co-doping favours the crystallinity and changes the crystal field around the Eu³⁺ ion, resulting in a significant enhancement in PL intensity. CIE coordinates for ASP, it is lie in the shallow red region while for annealed samples, it is lie in red region under 266 and 395 nm excitation. Studies corroborate the potentiality of these samples as a promising red phosphor for w-LED applications at much lower cost.

Acknowledgements

One of the authors BPS is thankful for financial assistantship to Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing the Senior Research Fellowship. Also author Maheshwary acknowledges the Central Research Fellowship provided by University Grants Commission (UGC), India. BPS gratefully acknowledges Dr R. S. Ningthoujam, Chemistry Division, BARC, India for his support and encouragements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06692a

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