Morphology and photoluminescence of self-assembled CaWO4:Sm3+ microspheres: effect of pH and surfactant concentration

Materials

All chemicals of analytical grades were used as received without further purification. Calcium nitrate tetrahydrate [Ca(NO ₃ ) ₂ ·4H ₂ O, Himedia, 99% pure], sodium tungstate dihydrate (Na ₂ WO ₄ ·2H ₂ O, Sigma-Aldrich, 99.995% pure), samarium nitrate hexahydrate [Sm(NO ₃ ) ₃ ·6H ₂ O, Sigma-Aldrich, 99.9% pure], sodium dodecyl sulphate (C ₁₂ H ₂₃ SO ₄ Na, Sigma-Aldrich, 98.5% pure) were used for the synthesis. Double-distilled water was used as the solvent and reaction medium.

Synthesis of CaWO4:Sm3+ microspheres

In a typical synthesis, 25 mL of 0.02 M SDS solution was mixed with 25 mL of solution containing 0.19 M Ca(NO ₃ ) ₂ ·4H ₂ O and 0.01 M Sm(NO ₃ ) ₃ ·6H ₂ O such that the Ca:Sm atomic ratio is 95:5. The mixture was stirred for homogenous complexation of the metal ions with the SDS molecules. To the complex mixture, 25 mL of 0.2 M Na ₂ WO ₄ ·2H ₂ O solution was added with constant stirring for an hour to achieve thorough mixing and complete precipitation. The whole mixture was then transferred to a Teflon-lined autoclave and kept in the pre-heated oven at 150 °C for 3 h. The autoclave was then removed and cooled to room temperature. The products were collected by centrifugation, washed with water and acetone. Finally, the products were dried at 40 °C for further analysis. The pH of the reaction was adjusted by addition of NaOH (1 mol L ⁻¹ ) or HCl (1 mol L ⁻¹ ). For the samples prepared at different SDS concentrations, i.e. 50 and 100 mM, the pH is adjusted to 9. The samples prepared at different pH and 25 mM SDS concentration are labelled as 3H25, 5H25, 7H25, 9H25 and 11H25 corresponding to pH 3, 5.2, 7, 9 and 10.8. The samples prepared at 50 and 100 mM SDS (pH 9) are labelled as 9H50 and 9H100.

Characterization of materials

The crystal structure of the prepared samples was characterized by PANalytical’s X-ray diffractometer (X′-PertPRO) with CuKα (1.5406 Å) radiation with Ni filter. The powdered sample was placed in the sample container and it was mounted at the sample holder. The X-ray tube was operated at 20 mA and 40 kV. The X-ray diffraction (XRD) pattern was recorded in 2 θ degree ranging from 10° to 60° at step size of 0.05° s ⁻¹ . Unit cell parameters were calculated using UnitCell Program ©Tim Holland and Simon Redfern. The average size of the crystallite was calculated by the Scherrer’s formula [ ¹⁷ ]:

Scanning electron microscope (SEM FEI Quanta 250) operating at accelerating voltage of 20 kV was used to record the morphology of the synthesized samples and transmission electron microscope (TEM JEOL JEM-2100, Japan available at SAIF, NEHU) operating at 200 kV was used for studying the shape and size of the materials. For SEM, samples were sputter coated and dispersed uniformly on a carbon grid. For TEM, powder samples were dispersed in methanol under ultrasonication for an hour. A drop of the dispersed particles was put over the carbon-coated copper grid and evaporated to dryness at room temperature. It was mounted inside the sample chamber. The photoluminescence spectra were recorded using Perkin Elmer LS55 spectrophotometer.

XRD study

Figure  ¹ shows the XRD patterns of CaWO ₄ :Sm ³⁺ crystals obtained at different pH and varied surfactant concentration in the range of 10°–60° in 2 θ . Eventually, the CaWO ₄ :Sm ³⁺ crystals have scheelite type tetragonal structure in accordance with the standard diffraction data for bulk CaWO ₄ (JCPDS No. 77-2233). The results indicate that the CaWO ₄ :Sm ³⁺ samples are crystalline and free from impurity phases. The absence of impurity phases indicates the successful doping of Sm ³⁺ ions in the CaWO ₄ lattice. Further, the change in pH or the varied surfactant concentration has no adverse effect on the crystal phases. The cell parameters were calculated to be a  =  b  = 5.252 Å, c  = 11.371 Å and the unit cell volume, V  = 313.68 Å ³ . The crystallite size calculated using the Scherer’s formula corresponding to the diffraction peak of plane (112) was found to be 15–20 nm. Comparative studies of the intensities of the (112) diffraction peaks with the change of pH and surfactant concentration have been shown in Fig.  ² a, b. In Fig.  ² a, it was observed that the intensity of the (112) diffraction peak was decreased with the increase in pH of the reaction medium. This indicated that the crystallinity of the samples was lowered with the increase in pH of the medium. In Fig.  ² b, it was observed that the sample prepared at an increasing amount of SDS has lower diffraction intensity. Thus, the crystallinity of the samples was lowered with the increasing SDS concentration. Meanwhile, the sample prepared at higher 100 mM SDS concentration showed a slight shift of (112) diffraction peak towards larger 2 θ angle.

Fig. 1

Fig. 2

SEM and TEM study

The SEM images of the CaWO ₄ :Sm ³⁺ samples prepared at different pH are shown in Fig.  ³ a–f. All the samples have spherical morphology with size less than or equal to 2 µm. The size of the microspheres seemed to become slightly larger with the increasing pH of the reaction medium. However, the monodispersity of the samples prepared at pH 5 and 7 is quite high compared to other samples. Below pH 4, the CMC is decreased [ ¹⁸ ]. As a result of which, the number of SDS per micelle is decreased and the number of Ca ²⁺ as counter-ions in the Stern layer attaching to the polar head of SDS will be decreased. When the WO ₄ ²⁻ ions approach the Ca ions, only fewer units of CaWO ₄ molecules will be formed and merged to obtain CaWO ₄ :Sm ³⁺ nano-/micro-crystals. Above pH 4, the CMC of SDS does not change at all. This is why the CaWO ₄ microspheres prepared at pH 3 has smaller size compared to that at pH 5 (absence of HCl and NaOH). At pH 7 and higher, the increasing number of OH ⁻ ions from NaOH will influence the growth of microspheres. The OH ⁻ ions will get attached to the surface of the CaWO ₄ :Sm ³⁺ . This explains the growth of slightly larger microspheres at high pH. In Fig.  ⁴ a–f, the SEM images of the samples prepared at different SDS concentrations with pH being 9 are shown. It is quite obvious that particles tend to grow in size randomly as well as the monodispersity is destroyed with increasing surfactant concentration. Larger particles and aggregates of smaller particles were also formed. The change in shape of the CaWO ₄ :Sm ³⁺ particles have been attributed to the change of micellar structure of SDS at different concentrations arising from the intermicellar interactions. Larger micelles could also be formed due to the stronger hydrophobic interactions. That is why the CaWO ₄ :Sm ³⁺ particles prepared at higher SDS concentrations are larger and aggregated particles are obtained.

Fig. 3

Fig. 4

The TEM analysis gives further insight into the morphological construction of the microspheres. Figure  ⁵ a–f shows the TEM micrographs of samples prepared at pH 9 and 11 (at 25 mM SDS) and at 100 mM (pH 9). The TEM images have shown that 9H25 and 11H25 do not contain particles of uniform dimensions in the nanometer regime. They rather have intertwined or interconnected framework of nanodimensional particles. However, the particles of 11H25 assume a more reasonable shape and size although they exist in aggregates. Roughly, the particles have size in the range of 15–30 nm. Thus, it is quite obvious that the microspheres so formed are aggregates or self-assembled from nanodimensional units and mesoporous in nature. While the sample 9H100 consists of rather granular nanoparticles with size ranging from 10 to 30 nm. Hence, it can be concluded that higher pH and higher SDS concentration are not favourable for formation of assembled mesoporous microspheres.

Fig. 5

Photoluminescence study

A typical photoluminescence excitation spectrum of CaWO ₄ :Sm ³⁺ microspheres is shown in Fig.  ⁶ . In WO ₄ ²⁻ groups, ligand to metal charge transfer (LMCT) occurs by excitation of electrons from the O 2p states to W 5d states to yield the excitation band at 240 nm [ ¹⁹ ]. The low intense excitation peaks in the region of 300–450 nm corresponds to the f–f transition of Sm ³⁺ . The exhibition of the LMCT of WO ₄ ²⁻ group in the excitation spectrum when monitoring emission at 645 nm (Sm ³⁺ emission) indicates an energy transfer from WO ₄ ²⁻ groups to the activator ion (Sm ³⁺ ) [ ²⁰ ]. The photoluminescence properties of the self-assembled microspheres were studied under UV excitation at 240 nm as shown in Fig.  ⁷ . It is obvious that the activator Sm ³⁺ ions exhibit their characteristic emission in the material as a result of energy transfer from WO ₄ ²⁻ host to them. The four emission bands at 565, 600, 645 and 707 nm correspond to ⁴ G _5/2  →  ⁶ H _5/2 , ⁶ H _7/2 , ⁶ H _9/2 and ⁶ H _11/2 transitions in Sm ³⁺ ions [ ²¹ ]. According to Judd–Ofelt theory, the electric dipole transitions occur between energy levels differing in ΔJ by more than one unit (as much as six units in some cases) and the emission centre should not be situated at a centre of inversion [ ²² , ²³ ]. The predominance of the electric dipole transition ⁴ G _5/2  →  ⁶ H _9/2 (Δ J  = 2) (which is not commonly observed in other hosts) is due to removal of inversion centre in SmO ₈ clusters.

Fig. 6

In Fig.  ⁷ a, the dependence of luminescence intensity with pH of the reaction medium is shown. It is observed that the intensity has an inverse relationship with pH. This can be attributed to the decrease of luminescent quencher –OH and increasing crystallinity as the pH decreases. It is observed from the XRD pattern in Fig.  ² that the crystallinity of the constituent nanodimensional particles increases with decreasing pH of the reaction medium. Hence, the increasing crystallinity could be a reason for luminescence enhancement [ ²⁴ ]. During the recombination process in insulators, thinly populated non-stoichiometric defects often occur at the surface of oxide materials as the electrons and holes tend to diffuse through the material towards the surface [ ²⁵ ]. In CaWO ₄ materials prepared by hydrothermal methods, defect-related radiative and non-radiative relaxation channels co-exist in the crystals. The competition between these relaxation channels affect luminescence [ ²⁶ ]. If the excited emission centre is in the vicinity of the non-radiative channel (quenching centre), the luminescence efficiency is weakened [ ⁵ ]. Such quenching centres are highly populated in samples of low crystallinity. This further added an explanation to increased emission intensity of the hydrothermally prepared samples with increasing crystallinity.

Fig. 7

In Fig.  ⁷ b, the emission spectra of CaWO ₄ :Sm ³⁺ samples prepared at different SDS concentrations are shown. The emission intensity is found to decrease with the increasing amount of SDS in the preparation. Several reasons for this observation could be decrease in crystallinity, organic quenching and non-uniform self-assembly of CaWO ₄ :Sm ³⁺ particles. The crystallinity is decreased with the increasing amount of SDS in the reaction medium. Also, adsorption of organic chain onto the particles assembly could serve as a source of energy sink where it is used up in the vibrational relaxation. Further, the uniform self-assembly with quite high monodispersity apparently lowers the surface to volume ratio and hence the surface energy of the nanocrystals. This could reduce the non-radiative channels in the luminescence [ ²⁷ ]. But in this case, monodispersity is destroyed and self-assembly turns out to be non-uniform with increasing SDS concentration. Hence, these factors are accountable for decreasing emission intensities. Altogether, the mode of self-assembly and morphology could induce variation in packing densities and the amount of light scattered as a result of which emission intensities vary with pH and surfactant concentration.