Formation mechanism and optical properties of CdMoO₄ and CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho and Er) microspheres synthesized via a facile sonochemical route

Abstract

In the present work, large-scale uniform CdMoO₄ and CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho, Er) microspheres have been successfully synthesized via a facile sonochemical route. XRD, FE-SEM, EDS, the Brunauer–Emmett–Teller (BET) surface area, and photoluminescence (PL) spectra were used to characterize the samples. The results show that the CdMoO₄:Ln³⁺ can be directly indexed to the tetragonal CdMoO₄ phase with high purity. The influence of the reaction time and reactants on the size and shapes of the CdMoO₄ microspheres has been studied, and the results revealed that the ultrasonication time and reactants play a crucial role in determining the final morphologies of the samples. Additionally, the PL properties of the CdMoO₄ and CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho and Er) microspheres were investigated in detail. It can be seen that the CdMoO₄:Pr³⁺, Sm³⁺, Eu³⁺, Dy³⁺, Ho³⁺ and Er³⁺ samples are located in the red, yellow, red, white, yellow and green regions, respectively. Simultaneously, this novel and efficient pathway could open new opportunities for further investigating the properties of molybdate materials.

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1. Introduction

Metal molybdates (AMoO₄, A = Ca, Sr, Ba, Pb and Cd) have novel properties and great potential applications in various fields such as PL, catalysis, scintillator materials, magnetic materials, and microwave applications.^1–3 Among these materials, cadmium molybdate is an important material owing to its excellent physical and chemical properties, electronic structure and relatively low band gap energy as compared to the above scheelite structures. Cadmium molybdate is one of the metallic molybdate compounds with a scheelite structure and has a body-center orthorhombic unit cell with C^4h₆ space group. Each site of Cd is surrounded by eight oxygen and Mo is surrounded by four equivalents oxygen.⁴ This unique combination of physical and chemical properties of CdMoO₄ in term of molecular and electronic versatility, reactivity and stability suggests that a CdMoO₄ may be a promising substrate materials.⁵ Therefore, it is of great importance to study the optical properties of this material for potential applications.

Fabrication of multi-dimensional (MD) hierarchical inorganic materials with highly specific size and morphology is a key aspect of modern materials science in many fields such as, advanced materials, catalysis, medicine, photoelectric material, ceramics, pigments, etc.^6–8 So far, some approaches have been demonstrated to be most effective for the preparation of multi-dimensional structures, such as sol–gel method, solvothermal route, microwave and microemulsion methods.^9–12 However, these methods tend to be rather complicated with obvious drawbacks of the final products and limit the practical applications. Large-scale use will require the development of simple low-cost approaches to the synthesis of inorganic functional materials. Therefore, it is still highly desirable to develop a facile, simple and mild method for the preparation of MD structure materials.

High intensity ultrasound has found many important applications in organic synthesis, materials and organometallic chemistry, and industrial manufacturing processes.¹³ The chemical and physical effects of ultrasound therefore arise not from a direct interaction between chemical species and sound waves, but rather from the physical phenomenon of acoustic cavitation: the formation, growth, and implosive collapse of bubbles. Sonochemistry originates from the extreme transient conditions induced by ultrasound, which produces unique hot spots that can achieve temperatures above 5000 K, pressures exceeding 1000 atmospheres, and heating and cooling rates in excess of 10¹⁰ K s⁻¹.^14–16 These conditions are distinct from other conventional synthetic techniques such as photochemistry, wet chemistry, hydrothermal synthesis. To the best of our knowledge, the sonochemical approach for CdMoO₄ preparation has not yet been reported.

Herein, we employed a facile sonochemical route without using any catalysts or templates to prepare uniform and monodisperse CdMoO₄ and CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho and Er) microspheres. In addition, the possible formation mechanism of the microspheres is proposed. The energy transfer process from host to rare earth ions were investigated systematically. Particularly, CdMoO₄:Ln³⁺ phosphors have controlled morphology and potential applications as an UV-convertible phosphors for LEDs. This work provides some insight into the design of other well-defined molybdate functional materials by sonochemistry synthesis.

2. Experimental section

2.1 Synthesis

CdCl₂·2.5H₂O (A. R.), Cd(CH₃COO)₂·2H₂O (A. R.), (NH₄)₆Mo₇O₂₄·4H₂O (A. R.), Na₂MoO₄·2H₂O (A. R.), Pr₆O₁₁ (99.99%), Ln₂O₃ (99.99%) (Ln = Sm, Eu, Dy, Ho Er) and absolute ethanol were purchased from Beijing Fine Chemicals Co. (China). Aqueous solutions of Ln(NO₃)₃ were obtained by dissolving rare earth oxides in dilute HNO₃ solution under heating with agitation. All the reagents were used as-received without further purification.

In a typical procedure, 5 mmol (1.1417 g) of CdCl₂·2.5H₂O was added into 25 mL of deionized water to form a solution (labeled as A). Meantime, the same amount of Na₂MoO₄·2H₂O (1.2097 g) was added into 25 mL of deionized water to form a colorless solution (labeled as B). Then solution B was added dropwise to solution A under vigorous magnetic stirring, and a white precipitate was yielded immediately, and the suspension was labeled as C. After above process, the resulting precursor C was ultrasound irradiated for 30 min by an ordinary ultrasound cleaning bath. At the end of the sonication, a temperature of about 50 °C was reached under ambient air without cooling. Subsequently, the products were separated from the solution by centrifugation, washed several times with deionized water and absolute ethanol to remove any ionic residual, and then dried in vacuum at 80 °C for 12 h. At last, the final CdMoO₄ samples were obtained and collected for characterizations. CdMoO₄:Ln³⁺ samples were prepared in a similar procedure except adding a stoichiometric amount (5 mol%) of Ln(NO₃)₃ together with CdCl₂·2.5H₂O were added as the initial materials, as described above. The specification of the ultrasound cleaning bath (Fig. S1†) used are as follows: Manufacturer: KQ-300, Kunshan ultrasonic instrument co., LTD, China; frequency: 40 kHz; Rated output power: 300 W.¹⁵

2.2 Characterization

The samples were examined by XRD measurements performed on a Rigaku D/max-II B X-ray diffractometer with monochromatic Cu Kα radiation (k = 0.15406 nm). The XRD data were collected by using a scanning mode in the 2θ ranging from 10° to 80° with a scanning step size of 0.02° and a scanning rate of 4.0° min⁻¹. SEM images and EDS of the products were obtained on field emission scanning electron microscope (Hitachi S-4800, Japan), employing an accelerating voltage of 5 kV or 15 kV. The FT-IR spectroscopy (Iraffinity-1, Shimadzu, Japan) was used to confirm the surface chemical structure of the products in the wave number range of 400–4000 cm⁻¹ by KBr disk method. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of the powders was analyzed by nitrogen adsorption on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.). Before nitrogen adsorption measurement, the sample was degassed in vacuum at 180 °C for 2 h. The PL excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W Xe lamp as the excitation source. The quantum efficiency (QE) was measured using the integrating sphere on the time resolved and steady state fluorescence spectrometers (FLS920, Edinburgh Instruments), and a Xe900 lamp was used as an excitation source. In addition to the BET surface area, all of the measurements were performed at room temperature.

3. Results and discussion

3.1 The basis of characterizations of the as-prepared CdMoO₄ samples

X-ray diffraction analysis was used to examine the crystal structure of the products. The XRD pattern of the as-prepared CdMoO₄ product was shown in Fig. 1a. All of the observed diffraction peaks can be perfectly indexed to those of the tetragonal phase of CdMoO₄ with cell constants of a = 5.155 Å and c = 11.194 Å (JCPDS no. 07-0209).¹⁷ No additional peaks of other phases have been found, indicating the formation of a pure single phase can be easily obtained by this method. And the sharp strong diffraction peaks indicate the good crystallinity of the as-synthesized products. The EDS spectrum shows strong peaks of Cd, Mo and O (Fig. 1b). No other impurity peaks can be detected, which confirms the pure phase of CdMoO₄ and can effectively support the XRD result of the samples. Quantitative analysis shows that the atom ratio of Cd : Mo : O is about 1 : 1 : 4, giving a stoichiometric composition of CdMoO₄. The analysis of EDS shows that the CdMoO₄ has been prepared successfully by sonochemistry method.

Fig. 1 (a) The XRD patterns (b) EDS spectrum of the as-synthesized CdMoO₄ microspheres. (c) Nitrogen adsorption–desorption isotherm loop (d) pore-size distribution calculated from the adsorption branch by the BJH model.

The nitrogen adsorption–desorption isotherm and pore-size distribution curve were investigated to evaluate the surface area and pore size distribution of the CdMoO₄ microspheres. As seen in Fig. 1c, the N₂ adsorption–desorption isotherm can be identified as type IV in the IUPAC (International Union of Pure and Applied Chemistry) classification with a distinct hysteresis loop. The hysteresis loop in the low relative (P/P₀) range of 0.5–1 may be ascribed to the presence of mesopores as well as macropores on the surface of the microspheres. The pore volumes and BET specific surface area are about 0.013 cm³ g⁻¹ and 1.34 m² g⁻¹ of the CdMoO₄ microspheres, respectively. Combined with the Barrett–Joyner–Halenda (BJH) pore size distribution (Fig. 1d), the CdMoO₄ microspheres exhibit hierarchical porosity: mesopores (2–50 nm) together with macropores (>50 nm).³⁰

FT-IR spectroscopy was performed on the as-prepared CdMoO₄ sample, as illustrated in Fig. S2.† The bands at 435, 771, 1651 and 2360 cm⁻¹ are observed. The absorption band at 435 cm⁻¹ has been attributed to the bending vibration of Mo–O. A strong absorption band at 771 cm⁻¹ is related to O–Mo–O stretches of the MoO₄ tetrahedron.¹⁸ This implies that the crystalline CdMoO₄ phase has formed in the as-prepared particles. The absorption band centered at 2360 cm⁻¹ is assigned to the CO₂ on the surface of the sample. The absorption band located at 1651 cm⁻¹ can be ascribed to the O–H bending vibration of water adsorbed on the surface of the samples.¹⁹ Because the samples were prepared in aqueous solution, the surface of particles can be covered inevitably with the absorbed water molecules. The FT-IR result provides additional evidence for the formation of the CdMoO₄ product after the sonochemical process, which is in good accordance with the XRD and EDS results.

It should be emphasized that doping a small amount of Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho, Er) in the CdMoO₄ host does not change the phase, crystallization, and morphology of the CdMoO₄ products in our present study, which agrees well with the previous literatures.^34,44 Therefore, here we only choose pure CdMoO₄ host as an example to illustrate the morphology of the samples. The morphology of the obtained CdMoO₄ sample was examined by FE-SEM. Fig. 2a shows FE-SEM image of the sample obtained under ultrasonic reaction for 30 min. The sample is composed of microspheres with a diameter of 2–3 μm. A high-magnification SEM image (Fig. 2b) reveals that microspheres is composed of tens of irregular nanoparticles (NPs). The device fabricated using spherical shaped particles should enhance the screen brightness and improve the resolution due to lower scattering of the emitted light and higher packing densities over irregular shaped particles. So our product may have great potential applications in the LEDs field.^1,34

Fig. 2 (a) Low- and (b) high-magnification SEM images of the CdMoO₄ samples.

It was found that samples sizes, morphologies could be greatly influenced via the types of the reactants. We found that the Mo and Cd sources are critical to the formation of the microspheres structures. The XRD patterns of the as-prepared CdMoO₄ products obtained by means of different types of reactants were shown in Fig. S3.† All of the observed diffraction peaks can be perfectly indexed to those of the tetragonal phase of CdMoO₄. Replacing Na₂MoO₄·2H₂O with (NH₄)₆Mo₇O₂₄·4H₂O as the Mo source while keeping the other reaction conditions the same results in the formation of rugby-like microstructures, as shown in Fig. 3a–d. The rugby-like structures have thickness of 1 μm and diameters of 2 μm (Fig. 3c). Fig. 3d reveals that rugby-like microstructures is composed of myriad nanoparticles with size of circa 60–100 nm. Replacing CdCl₂·2.5H₂O with Cd(CH₃COO)₂·2H₂O as the Cd source while keeping the other reaction conditions the same results in the formation of small size microspheres, as shown in Fig. 3e. The size of the microspheres dropped to 1 μm (Fig. 3f). Comparing the two types of Mo sources, using Na₂MoO₄·2H₂O as the precursor might have the advantage of relatively fast release of MoO₄²⁻ ions so that the reaction kinetics favors the generation of CdMoO₄ microspheres. Comparing the two types of Cd sources, The microspheres obtained from CdCl₂·2.5H₂O (Fig. 2) have larger particle size than those obtained from Cd(CH₃COO)₂·2H₂O. The CH₃COO⁻ ligand in Cd(CH₃COO)₂·2H₂O, can act as a surfactant and provide steric hindrance. It is obvious that using Cd(CH₃COO)₂·2H₂O as Cd source results in microspheres structures formation with small size.²⁰ Through the transformation of reactants (Mo and Cd sources) can be found that the control of microspheres morphology are greatly influenced by Mo source. And the control of microspheres size are greatly influenced by Cd source.

Fig. 3 SEM images of the CdMoO₄ obtained by using (NH₄)₆Mo₇O₂₄·4H₂O as the Mo source (a–d), Cd(CH₃COO)₂·2H₂O as the Cd source (e and f).

In order to monitor the effect of the concentration of reactants on the size and morphology, we change the concentration of reactants from [Cd²⁺] = [MoO₄²⁻] = 0.02 M to [Cd²⁺] = [MoO₄²⁻] = 0.14 M. As shown in Fig. 4, it can be seen that the as-obtained products are mainly composed of large scale microspheres, implying that a high yield can be realized in the adopted reaction conditions. We found that with the change of concentration of reactants, the product can maintain the spherical morphology. By careful observation, the concentration of reactants have a great influence on the uniformity and monodispersity of CdMoO₄ microspheres. Too high or too low of the concentration of reactants are not conducive to fine morphology. Thus, choosing an optimal condition ([Cd²⁺] = [MoO₄²⁻] = 0.10 M) is of substantial importance to synthesize CdMoO₄ with an ideally spherical shape, narrow size distribution, and showing low-agglomeration.

Fig. 4 The SEM images of CdMoO₄ crystals prepared at different concentration of the reagents [Cd²⁺] = [MoO₄²⁻] = 0.02 M (a), 0.06 M (b), 0.10 M (c), and 0.14 M (d), respectively.

3.2 Formation mechanism research based on time-dependent experiments

To investigate the details of the sonochemical conversion from precursor to final products, we have carried out a series of experiments by employing different sonication times without changing other preparation conditions. Fig. 5a–f represent the SEM images of the precursor intermediates obtained at different reaction periods of 0 (a), 5 (b), 10 (c), 15 (d), 20 (e) and 30 min (f), respectively. When the reaction was performed in the absence ultrasonic irradiation, the product is only composed of amorphous nano-/micron-blocks without a discernable morphology (Fig. 5a). After ultrasonic reaction for 5 min, the nano-/micron-blocks become NPs, and some microspheres architectures could occasionally be observed, but there are still some NPs left as shown in Fig. 5b. As the reaction went on, more and more well-defined microspheres were generated though some NPs still existed in the product (Fig. 5c–e). As illustrated in Fig. 5f, a longer reaction time of 30 min gave rise to an increase in the number of microspheres, and microspheres became the predominant product. Therefore, the experiments have proved that ultrasonic irradiation played an important role in the formation of CdMoO₄ microspheres. The XRD patterns of the products were collected at different reaction stages are provided in the Fig. S4.† It can be seen that all the diffraction peaks can be indexed into the pure tetragonal phase and no trace of additional peaks from other phases can be detected, implying that the samples are of high purity.

Fig. 5 The SEM images of the products collected at different ultrasound reaction time (a) 0, (b) 5, (c) 10, (d) 15, (e) 20 min and (f) 30 min, respectively.

On the basis of the above experimental process, the possible formation mechanism of such CdMoO₄ microspheres structures could be carefully elucidated as follows. We believe that the sonochemical formation of 3D CdMoO₄ microspheres structure by means of “dissolution & recrystallization – growth – self-assembled”. A possible reaction scheme of the formation process for CdMoO₄ can be summarized as follows:²¹

CdCl₂·2.5H₂O → Cd²⁺ + 2Cl⁻ + 2.5H₂O (1)

Na₂MoO₄·2H₂O → 2Na⁺ + MoO₄²⁻ + 2H₂O (2)

Cd²⁺ + MoO₄²⁻ → CdMoO₄↓ (3)

At first, the reactants CdCl₂·2.5H₂O and Na₂MoO₄·2H₂O were dissolved in the aqueous solution (eqn (1) and (2)). When a Na₂MoO₄ solution is added to CdCl₂·2.5H₂O solution, free Cd²⁺ ions can react with the vicinal MoO₄²⁻ to form numerous tiny CdMoO₄ crystalline nuclei, which led to the formation of CdMoO₄ nano-/micron-blocks (eqn (3)).¹³ The small K_sp (solubility product constant) of CdMoO₄ drives the whole reaction, which can be seen as internal driving force for reaction.²² When the solution is irradiated by high intensity of ultrasound, acoustic cavitation in reaction system occurs. Bubbles collapse in liquid and produce enormous amount of heat energy. Meanwhile, ultrasound irradiation will generate strong tiny jets in water and facilitate the nanoparticles to distribute uniformly.²³ Therefore the CdMoO₄ nano-/micron-blocks would crack into small nanoparticles and further disperses in the presence of the strong shock wave of the ultrasound through a dissolution & recrystallization process, which led to the growth of CdMoO₄ nanoparticles.²⁴ Sonication was helpful to obtain relatively small and uniform particles. The same observation have been reported in the previous literature.^25,26 The small nanoparticles are aggregated into nanoparticles via a process known as growth involving coarsening.²⁷ And these nanoparticles further assembled into microspheres structure to reduce the surface area and energy.²⁸ A plausible formation mechanism of self-assembled CdMoO₄ microspheres is illustrated in Fig. 6.

Fig. 6 Schematic illustration of the possible formation mechanism for the CdMoO₄ samples.

3.3 Optical properties of CdMoO₄ host and CdMoO₄:Ln³⁺ phosphors

Molybdate compounds have been shown to be an efficient host lattice for Ln³⁺, bringing about all kinds of PL properties. As we known, MoO₄²⁻ group has excellent absorption in the near UV region. Hence, the energy transfer process from MoO₄²⁻ to Ln³⁺ can easily occur, which can greatly enhance the PL efficiency of Ln³⁺ doped materials.²⁹ To our knowledge, the PL properties of CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho, Er) which obtained through sonochemical approach have been rarely investigated yet.

Fig. 7a shows the UV-vis absorbance spectrum of CdMoO₄ sample. The sample has a broad band range from 200 to 385 nm, which is due to the O–Mo charge transfer transitions (CTT) of MoO₄²⁻ groups. The steep shape of the spectrum indicated that the absorption was not due to the transition from the impurity level but the band-gap transition.^16,30 The combination between absorbance and PL measurements allows to discover the energy levels in the materials and the optical band gap value. The optical band gap energy (E_gap) was calculated by the method proposed by Wood and Tauc. According to ref. 31, the optical band gap is associated with absorbance and photon energy by the following equation (eqn):

hνa ∝ (hν − E_gap)ⁿ

where a is the absorbance, h is the Planck constant, v is the frequency, E_gap is the optical band gap, and n is a constant associated with the different types of electronic transitions (For n = 1/2, 2, 3/2 and 3, the transitions are the direct allowed, indirect allowed, direct forbidden, and indirect forbidden, respectively.). According to Lacomba-Perales et al. the molybdates with scheelite-type tetragonal structure present a direct allowed electronic transition.³² So, in our study, n = 1/2 was chosen as the criterion in eqn. The plot of (aE_photon)² versus E_photon is shown in Fig. 7b. The E_gap value of CdMoO₄ was evaluated extrapolating the linear portion of the curve or tail. UV-vis measurements on the sample showed a typical value of 3.38 eV.

Fig. 7 (a) The UV-vis diffuse reflectance spectrum. (b) The plot of (aE_photon)² ∼ E of the CdMoO₄ samples.

PL properties of these CdMoO₄ microspheres are being studied. Fig. 8 shows the PL spectra of the CdMoO₄ samples obtained under different reactants (a) CdCl₂·2.5H₂O + Na₂MoO₄·2H₂O (b) Cd(CH₃COO)₂·2H₂O + Na₂MoO₄·2H₂O (c) CdCl₂·2.5H₂O + (NH₄)₆Mo₇O₂₄·4H₂O, respectively.

Fig. 8 Room-temperature PL spectra of the CdMoO₄ samples obtained under different reactants. (a) CdCl₂·2.5H₂O + Na₂MoO₄·2H₂O (b) Cd(CH₃COO)₂·2H₂O + Na₂MoO₄·2H₂O (c) CdCl₂·2.5H₂O + (NH₄)₆Mo₇O₂₄·4H₂O.

With excitation at 330 nm, the spectra show that all the three samples exhibited broad emission band peaks at 470 nm, which is mainly attributed to the charge transfer transition (CTT) between the O 2p orbitals and the Mo 4d orbitals within the MoO₄²⁻ complex.¹ It is commonly accepted that the PL properties of inorganic materials are strongly dependent on their sizes, morphologies, and crystallinity. From Fig. 8, it can be clearly seen that the emission spectra of the products synthesized at different reactants are similar in shape, but different in the intensity to some extent, indicating that the luminescent properties are closely correlated with the morphologies and sizes of the materials. Under identical measurement conditions, the microspheres (black line) have the highest relative emission intensity, while the rugby-shaped structures exhibit the lowest intensity. It is thus inferred that the morphology of the 3D microspheres (large size) is more beneficial than the morphology of rugby-shaped and microspheres (small size) to the emission. The possible reasons for the different of emission intensities can be deduced as follows. On the one hand, the microspheres (large size) has an average diameter of 2–3 μm, which is much bigger than the rest of microstructures. The enhanced luminescence performance due to microspheres (large size) has less surface defect than the other samples. On the other hand, the electronic structures could be modified by means of the changes of the sizes and shapes of samples, which influences the excitation of the carriers from the valence band to the conduction band and the numbers of photons released from the surfaces of samples.^6,15 Compared to the previous reported PL spectrum of CdMoO₄ nano-octahedra with the edge length of about 25 nm, our sample has a red shift of approximately 30 nm. It is obvious that the CdMoO₄ microspheres possess higher specific surface areas than that of CdMoO₄ nano-octahedra due to their complex morphology, which might lead to the red shift of the luminescence peak.¹⁷ All of the aforementioned observations support the concept of morphology-dependent properties of CdMoO₄ samples and emphasize the importance of morphology and size control.

As an example of latent application, various lanthanide ions (Ln³⁺: Ln = Pr, Sm, Eu, Dy, Ho and Er) have been doped into the microspheres and the corresponding PL properties were investigated in detail. As demonstrated by XRD patterns of the as-synthesized samples doped with different types of Ln³⁺ shown in Fig. S5.† One can see that the diffraction peaks of CdMoO₄:Ln³⁺ samples agree well with tetragonal scheelite-type CdMoO₄, indicating that the Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho, Er) ions have been effectively doped into the CdMoO₄ host lattice.

Fig. 9 presents the excitation and emission spectra of the CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho and Er) samples. The excitation and emission spectra of the CdMoO₄:Pr³⁺ phosphor at room temperature are given in Fig. 9a and b. In the excitation spectra monitored at 606 nm, a broad band with a maximum at 322 nm can be observed, which can be assigned to the CTB from the oxygen ligands to the central molybdates atom within the MoO₄²⁻ groups. As shown in Fig. 9a, it can be seen that some sharp lines are in the 400–500 nm long wavelength range, including 453 nm (³H₄ → ³P₂), 477 nm (³H₄ → ³P₁), and 490 nm (³H₄ → ³P₀), respectively.³³ Upon excitation into the MoO₄²⁻ group at 322 nm, the characteristic emission spectra of the Pr³⁺ ions at 534 nm (³P₀ → ³H₄), 561 nm (³P₀ → ³H₅), 597 and 606 nm (¹D₂ → ³H₄), 623 nm (³P₀ → ³H₆) and 654 nm (³P₀ → ³F₂) can be detected (Fig. 9b).

Fig. 9 Excitation and emission spectra of CdMoO₄:Pr³⁺ (a and b) CdMoO₄:Sm³⁺ (c and d) CdMoO₄:Eu³⁺ (e and f) CdMoO₄:Dy³⁺ (g and h) samples.

Sm³⁺ ions with 4f⁵ configuration has complicated energy levels and various possible transitions of f–f levels. The transitions between these f–f levels are highly selective and of sharp line spectra.³⁴ The excitation and emission spectra of CdMoO₄:Sm³⁺ are depicted in Fig. 9c and d. The intense and broad absorption band centered at 339 nm is assigned to the combination of the CTB of O²⁻ → Mo⁶⁺ and Sm³⁺ in MoO₄²⁻ group. All the sharp peaks of the excitation spectra are due to the excitation from ground-level ⁶H_5/2 to higher energy levels of Sm³⁺ ion. As shown in Fig. 9c, it can be seen that some sharp lines are in the 390–500 nm wavelength range, including 408 nm (⁶H_5/2 → ⁴K_11/2), 423 nm (⁶H_5/2 → ⁶P_5/2 + ⁴M_19/2), 478 nm (⁶H_5/2 → ⁴I_11/2 + ⁴M_15/2), respectively. The strongest excitation sharp line is corresponding to ⁶H_5/2–⁴K_11/2 transition of Sm³⁺ located at 408 nm. The emission spectrum of CdMoO₄:Sm³⁺ consists of three sharp peaks in the visible region near 563, 596 and 606, 645 nm, which are assigned to the intra-4f-shell transitions from the excited level ⁴G_5/2 to ground levels ⁶H_5/2, ⁶H_7/2, ⁶H_9/2, respectively (Fig. 9d). The strongest emission peak located at 563 nm is originated from ⁴G_5/2 → ⁶H_5/2 typical transition of Sm³⁺.³⁵

Fig. 9e and f illustrate the excitation (λ_em = 615 nm) and emission (λ_ex = 330 nm) spectra of the CdMoO₄:Eu³⁺ phosphor. From Fig. 9e, the intense and broad absorption band centered at 330 nm is assigned to the combination of the CTB transitions of O²⁻ → Mo⁶⁺ and O²⁻ → Eu³⁺ groups.^4,36 As shown in Fig. 9e, it can be seen that some sharp lines are in the 370–500 nm long wavelength range, including 382 (⁷F₀ → ⁵L₇), 395 (⁷F₀ → ⁵L₆), 417 (⁷F₀ → ⁵D₃), 467 nm (⁷F₀ → ⁵D₂), respectively. The broad excitation band is ascribed to the intra-configurational 4f–4f transitions of Eu³⁺ in the host lattices. Two of the strongest absorptions are at 395 nm (⁷F₀ → ⁵L₆) and 467 nm (⁷F₀ → ⁵D₂), respectively. As shown in Fig. 9f, the main emission at 615 nm, originating from the electric dipole transitions and hypersensitive to the environment of Eu³⁺, other f–f transitions of Eu³⁺ ion, such as 591 and 650 nm, relatively weak, which are associated with the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₃, respectively.^36,37 When excited by 330, 395 and 467 nm, the emissions of the phosphor are shown in Fig. S6,† respectively. The emission intensities decrease with increasing the excitation wavelength. It should be noticed that the red emission intensity excited at 330 nm is far greater than that of excited at near-UV radiation (395 nm) and blue region 467 nm, suggesting that the energy transfer from MoO₄²⁻ to Eu³⁺ is very efficient. When the excitation wavelength is 330 nm, our sample is twice the intensity of the commercial phosphor Y₂O₃:Eu³⁺ (Grirem Advanced Materials Co., Ltd.). Compared with Y₂O₃:Eu³⁺ phosphor, CdMoO₄:Eu³⁺ is much more stable, efficient and suitable, therefore, CdMoO₄:Eu³⁺ may have potential application as a red phosphor in the LEDs field (Fig. S7†). The quantum efficiency of the phosphor is measured by the integrated sphere method and can be calculated according to the equation Q = (E_c − (1 − A))E_b/L_aA, where E_c is the integrated luminescence of the powder caused by direct excitation, E_b is the integrated luminescence of the powder caused by indirect illumination from the sphere and the term L_a is the integrated excitation profile from an empty integrated sphere (blank control). A is the powder absorbance.³⁸ The quantum efficiency for the CdMoO₄:0.05Eu³⁺ phosphor with 330 nm excitation were calculated by integrating emission counts from the 550 to 650 nm wavelength range. The values is found to be about 43.94%. The values is higher than that of the commercial red-emitting Y₂O₃:Eu³⁺ (12.2%) and Y₂O₂S:Eu³⁺ (35.0%), respectively.^39,40 Thus, the CdMoO₄:0.05Eu³⁺ phosphor demonstrates a high quantum efficiency and potential application.

Fig. 9g and h shows the excitation and emission spectra of CdMoO₄:Dy³⁺ phosphor. The broadband centered at 330 nm is attributed to a CTB, which occurs by electron delocalization from O²⁻ → Mo⁶⁺ and O²⁻ → Dy³⁺ in MoO₄²⁻. There are some sharp lines between 340 and 500 nm, which correspond to f–f transitions within Dy³⁺. The excitation peaks locate at 353, 368, 391, 429, 455 and 477 nm, which are attributed to the transitions from the ⁶H_15/2 ground state to ⁶P_7/2, ⁶P_5/2, ⁴F_7/2 + ⁴I_13/2, ⁴G_11/2, ⁴I_15/2, and ⁴F_2/9 respectively.⁴¹ The emission spectrum is composed of two groups of narrow bands and the emission peaks are located at 478, 487 and 575 nm (Fig. 9h). The emission corresponds to the ⁴F_9/2 → ⁶H_15/2 (478 and 487 nm) and ⁴F_9/2 → ⁶H_13/2 (575 nm) transitions of Dy³⁺. In CdMoO₄ host, if Dy³⁺ occupies an inversion symmetry site in the crystal lattice, the magnetic dipole transition is the dominant transition. while the electric dipole transition will be the dominant transition if Dy³⁺ occupies an unsymmetrical site in the crystal lattice.⁴² The ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ belongs to electric dipole transitions and ⁴F_9/2 → ⁶H_15/2 transition belongs to magnetic dipole transitions. It can be seen from Fig. 9h that the strong yellow emission line at 575 nm (⁴F_9/2 → ⁶H_13/2) is the dominant band, which indicates that Dy³⁺ ions do not occupy the inversion symmetry site.

Fig. 10a exhibits the emission spectrum of CdMoO₄:Ho³⁺ sample. The emission spectrum is composed of two groups of bands and the emission peaks are located at 543, 644 and 660 nm. The emission corresponds to the ⁵F₄ → ⁵I₈ (543 nm) and ⁵F₅ → ⁵I₈ (644 and 660 nm) transitions of Ho³⁺, respectively.⁴³ Fig. S8a† shows the excitation of CdMoO₄:Ho³⁺ phosphor. The broadband centered at 320 nm is attributed to a CTB, which occurs by electron delocalization from O²⁻ to Mo⁶⁺ and Ho³⁺ in MoO₄²⁻. There are some sharp lines between 350 and 500 nm, which correspond to f–f transitions within Ho³⁺. The excitation peaks locate at 361, 402, 418, 451 and 481 nm, which are attributed to the transitions from the ⁵I₈ ground state to ⁵G₂, ⁵G₄, ⁵G₅, ⁵F₁ and ⁵F₂ respectively.⁴⁴ Fig. 10b exhibits the emission spectrum of CdMoO₄:Er³⁺ sample. The emission spectrum is composed of three groups of bands and the emission peaks are located at 525, 531, 545, 554, 656 and 668 nm. The emission corresponds to the ²H_11/2 → ⁴I_15/2 (525 and 531), ⁴S_3/2 → ⁴I_15/2 (545 and 554) and ⁴F_9/2 → ⁴I_15/2 (656 and 668) transitions of Er³⁺, respectively.⁴⁵ Fig. S8b† shows the excitation of CdMoO₄:Er³⁺ phosphor. The broadband centered at 318 nm is attributed to a CTB, which occurs by electron delocalization from O²⁻ to Mo⁶⁺ and Er³⁺ in MoO₄²⁻. There are some sharp lines between 350 and 500 nm, which correspond to f–f transitions within Er³⁺. The excitation peaks locate at 365, 378, 407 and 488 nm, which are attributed to the transitions from the ⁴I_15/2 ground state to ⁴G_7/2, ⁴G_11/2, ²H_9/2, and ⁴F_7/2, respectively.⁴⁶

Fig. 10 Emission spectra of CdMoO₄:Ho³⁺ (a) CdMoO₄:Er³⁺ (b) samples.

According to the excitation and emission spectra of CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho, Er), the broad band originating from the molybdate hosts is predominant of all excitation spectra monitored at the prominent characteristic emissions of Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho, Er), indicating that the doped Ln³⁺ ions can be excited through host absorption and the corresponding energy transfer process is highly efficient. This suggests the tetragonal CdMoO₄ is ideal host material for the luminescence of Ln³⁺ cations. To help explain the mechanism of multi-colour emission and the paths of energy transfer during PL, a schematic diagram is shown in Fig. 11.⁴⁷ In the first place, the electrons of the host are first excited from the ¹A₁ ground state to the ¹T₂ excited state by absorbing ultraviolet radiation. Subsequently, when Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho and Er) were incorporated into the host of CdMoO₄, the energy transfer from MoO₄²⁻ to Ln³⁺. In the end, the energy can non-radiatively relaxes from high energy levels to the lower excited energy levels by multi-phonon relaxation. And the multi-colour emissions occur through representative transitions of Ln³⁺.^48,49 Based on the detailed PL analysis above, a conclusion can be drawn that CdMoO₄ is efficient host to sensitize Ln³⁺. Meanwhile, sonochemical route is more convenient and could save time and energy for the synthesis of luminescent materials.

Fig. 11 Schematic diagram of MoO₄²⁻ and Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho and Er) energy levels and excitation, emission, and energy transfer in CdMoO₄:Ln³⁺ phosphors.

Chromaticity coordinates are one of the vital factors for evaluating performance of the prepared phosphors. We investigated the PL color of the as-obtained samples by using the 1931 CIE (Commission Internationale de L'Eclairage) system. Fig. 12 presents the corresponding CIE coordinates positions, which show the different emission colors for the as-synthesized CdMoO₄: (A) Pr³⁺, (B) Sm³⁺, (C) Eu³⁺, (D) Dy³⁺, (E) Ho³⁺ and (F) Er³⁺ respectively. It can be seen that the CdMoO₄: (A) Pr³⁺, (B) Sm³⁺, (C) Eu³⁺, (D) Dy³⁺, (E) Ho³⁺ and (F) Er³⁺ samples are located in the red, yellow, red, white, yellow and green regions, respectively. A detailed conclusion for the excitation and emission peaks, CIE coordinates, colors and correlated color temperature (CCT) are listed in Table 1.

Fig. 12 CIE chromaticity diagram showing the emission colors for CdMoO₄: (A) Pr³⁺, (B) Sm³⁺, (C) Eu³⁺, (D) Dy³⁺, (E) Ho³⁺ and (F) Er³⁺ phosphors.

Table 1 Summary of the PL properties of the CdMoO₄:Ln³⁺ (Ln = Pr, Sm, Eu, Dy, Ho and Er) microsphere samples

Ln^3+	Excitation peaks (nm)/transition	Emission peaks (nm)/transition	CIE coordinates (x, y)	Colors	CCT (K)
Pr	322/CTB, 453/(^3H4 → ^3P2), 477/(^3H4 → ^3P1), 490/(^3H4 → ^3P0)	534/(^3P0 → ^3H4), 561/(^3P0 → ^3H5), 597, 606/(^1D2 → ^3H4), 623/(^3P0 → ^3H6), 654/(^3P0 → ^3F2)	x = 0.624 y = 0.376	Red	1887
Sm	339/CTB, 408/(^6H5/2 → ^4K11/2), 423/(^6H5/2 → ^6P5/2 + ^4M19/2), 478/(^6H5/2 → ^4I11/2 + ^4M15/2)	563/(^4G5/2 → ^6H5/2), 596, 606/(^4G5/2 → ^6H7/2), 645/(^4G5/2 → H9/2)	x = 0.554 y = 0.443	Yellow	1994
Eu	330/CTB, 382/(^7F0 → ^5L7), 395/(^7F0 → ^5L6), 417/(^7F0 → ^5D3), 467/(^7F0 → ^5D2)	591/(^5D0 → ^7F1), 615/(^5D0 → ^7F2), 650/(^5D0 → ^7F3)	x = 0.666 y = 0.334	Red	3339
Dy	330/CTB, 353/(^6H15/2 → ^6P7/2), 368/(^6H15/2 → ^6P5/2), 391/(^6H15/2 → ^4F7/2 + ^4I13/2), 429/(^6H15/2 → ^4G11/2), 455/(^6H15/2 → ^4I15/2), 477/(^6H15/2 → ^4F2/9)	478, 487/(^4F9/2 → ^6H15/2), 575/(^4F9/2 → ^6H13/2)	x = 0.351 y = 0.360	White	4808
Ho	320/CTB, 361/(^5I8 → ^5G2), 402/(^5I8 → ^5G4), 418/(^5I8 → ^5G5), 451/(^5I8 → ^5F1), 481/(^5I8 → ^5F2)	543/(^5F4 → ^5I8), 644, 660/(^5F5 → ^5I8)	x = 0.432 y = 0.561	Yellow	3933
Er	318/CTB, 365/(^4I15/2 → ^4G7/2), 378/(^4I15/2 → ^4G11/2), 407/(^4I15/2 → ^2H9/2), 488/(^4I15/2 → ^4F7/2)	525, 531/(^2H11/2 → ^4I15/2), 545, 554/(^4S3/2 → ^4I15/2), 656, 668/(^4F9/2 → ^4I15/2)	x = 0.275 y = 0.705	Green	6311

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4. Conclusions

In summary, the present work provides an efficient sonochemistry process for the fabrication of novel 3D architectures of CdMoO₄ and CdMoO₄:Ln³⁺ microspheres. The investigation of synthesis parameters reveals that the ultrasonic time in the reaction system should be responsible for the formation of the CdMoO₄ microspheres. The reasonable formation mechanism of the microspheres was proposed on the basis of time-dependent experiments. PL spectra of CdMoO₄ show that the optical properties could be modulated through different morphologies and sizes. The as-prepared CdMoO₄:Ln³⁺ phosphors show abundant and excellent luminescent properties after incorporating Ln³⁺ cations under ultraviolet light excitation, which have potential applications in advanced flat panel displays and light emitting diodes. Notably, the results show that it is possible to design and synthesize microspheres by sonochemistry method. And the work can also be extended to other well-defined molybdate functional materials with controllable morphologies in the absence of any surfactants and templates.

Acknowledgements

This present work was financially supported by Mineral and Ore resources Comprehensive Utilization of Advanced Technology Popularization and Practical Research (no. 12120113088300); and the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources (no.: OSR-5); the authors thank Bingnan Wang and Shasha Yi for their cooperation and kind help.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1. Schematic illustration for reaction system and equipment. Fig. S2. FT-IR spectrum of CdMoO₄ samples. Fig. S3. XRD patterns of CdMoO₄ obtained by using different reactants. (A = CdCl₂·2.5H₂O; B = Na₂MoO₄·2H₂O; C = (NH₄)₆Mo₇O₂₄·4H₂O; D = (CH₃COO)₂Cd·2H₂O). Fig. S4. XRD patterns of samples at different reaction time (a) 0, (b) 5, (c) 10, (d) 15, (e) 20 min and (f) 30 min, respectively. Fig. S5. XRD patterns of CdMoO₄: (a) Pr³⁺, (b) Sm³⁺, (c) Eu³⁺, (d) Dy³⁺, (e) Ho³⁺ and (f) Er³⁺ phosphors. Fig. S6. Emission spectra of CdMoO₄:Eu³⁺ sample under 330, 395 and 467 nm wavelength excitation. Fig. S7. Emission spectra of CdMoO₄:Eu³⁺ compared with commercial red phosphor Y₂O₃:Eu³⁺. Fig. S8. Excitation spectra of CdMoO₄:Ho³⁺ (a) CdMoO₄:Er³⁺ (b) samples. See DOI: 10.1039/c4ra05038j

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