Morphology and photoluminescence of self-assembled CaWO4:Sm3+ microspheres: effect of pH and surfactant concentration
Self-assembled CaWO4:Sm3+ microspheres were prepared via surfactant (sodium dodecyl sulfate) mediated hydrothermal method. The effect of pH and the concentration of surfactant on the morphology and photoluminescence of the synthesized phosphors have been studied. Samples were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence spectroscopy. The microspheres were found to have an average size of 1–2 µm. They were mesoporous in nature and constituted of nanocrystals of about 10–30 nm dimension. The TEM images revealed the interlinking framework of the nano-sized constituents which consequently lead to the formation of mesoporous microspheres. The lowering of pH causes a slight reduction in the size of microspheres which could have been attributed to loss of OH from the nanoparticle surface and subsequent retardation in the adsorption of growing molecular CaWO4:Sm3+ units. Also, as pH increases, crystallinity decreases. The increase in amount of SDS reduced the crystallinity of the materials, destroyed the monodispersity of microspheres and lowered the luminescence output. It was found that lower pH and higher monodispersity of microspheres are quite favourable for high luminescence output.
KeywordsCaWO4 microspheres Hydrothermal synthesis Lanthanides Luminescence
The synthesis and fabrication of nano- or microstructures with uniformly well-defined size and morphology is highly important. The physicochemical properties of nanomaterials depend on their size and shape [1, 2, 3]. The frequency of active centres on the exposed facets and the surface energy of nanostructures could induce many novel nanoscale effects and provides technological potentials. For instance, under UV excitation, ellipsoid β-Ga2O3:Dy exhibits higher emission intensity and quantum yield compared to spindles or microspheres . The luminescence intensity of 1-μm hexagonal NaYF4:Yb,Er is weaker than that of 80-nm cubic spherical samples . The weak intensity of luminescence has been attributed to the presence of non-radiative traps. Spherically fine phosphor microparticles also show high luminescence yield as a result of efficiently high packing densities and low light scattering .
Alkaline earth tungstates (AWO4; A = Ca, Sr, Ba) has been extensively studied as luminescent hosts, scintillation detectors, optoelectronics, lasers and catalysis [7, 8, 9, 10]. In photoluminescence, tungstate has been gaining interest due to its physical and chemical stability, low phonon threshold energy, wide band gap semiconducting nature and intrinsic blue emission. The WO4 group can generate blue emission (≈420 nm) on absorption of ultraviolet (UV) radiation mostly in the short wavelength region. This intrinsic blue emission arises from charge transfer transitions between O 2p → W 5d states within the WO4 unit. The blue emission can be harnessed for alteration of luminescent properties by doping with rare earth (RE) ions since most of the RE ions can be absorbed in the blue region.
CaWO4 phosphors are synthesized by several methods such as co-precipitation, solid state reaction, hydro/solvothermal (conventional and microwave), electrospinning, spray pyrolysis, sol–gel route, and pulsed-laser deposition. Of these methods, hydrothermal method is proven to be a promising one for the synthesis of low- and high-order novel architectural designs of nanomaterials through a well-controlled manipulation of appropriate reaction parameters such as temperature, time of hydrothermal ageing, pH of reaction, surfactants or other templates [11, 12, 13]. It has an added advantage of relatively low-temperature growth and size-controlled synthesis. Certain reports have been made on the importance of surfactants and polymers such as sodium dioctyl sulfosuccinate (AOT), polyethylene glycol (PEG), polypeptides in the self-assembly and morphology control of nano- or micro-dimensional materials [14, 15, 16, 17]. Nanotubes and hollow microspheres with high monodispersity can be synthesized using surfactant as soft template via microwave-assisted or conventional hydrothermal method.
In this work, 5 at.% Sm3+-doped CaWO4 (hereafter CaWO4:Sm3+) microspheres were synthesized by conventional hydrothermal method in the presence of sodium dodecyl sulphate (SDS). The effect of variation of pH of the reaction medium and surfactant concentration on the morphology and photoluminescence of the phosphor materials has been studied. The mode of assembly and the size variation of the microspheres have been found to influence the luminescence properties.
All chemicals of analytical grades were used as received without further purification. Calcium nitrate tetrahydrate [Ca(NO3)2·4H2O, Himedia, 99% pure], sodium tungstate dihydrate (Na2WO4·2H2O, Sigma-Aldrich, 99.995% pure), samarium nitrate hexahydrate [Sm(NO3)3·6H2O, Sigma-Aldrich, 99.9% pure], sodium dodecyl sulphate (C12H23SO4Na, 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(NO3)2·4H2O and 0.01 M Sm(NO3)3·6H2O 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 Na2WO4·2H2O 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
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.
Results and discussion
SEM and TEM study
In Fig. 7b, the emission spectra of CaWO4:Sm3+ 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 CaWO4:Sm3+ 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 . 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.
In summary, this work presents the successful synthesis of CaWO4:Sm3+ microspheres via conventional hydrothermal method using SDS as soft template. Highly monodipersed CaWO4:Sm3+ microspheres were obtained at different pH and 25 mM SDS concentration. The pH range of 5–7 is found to be quite favourable for highly monodipersed microsphere formation. However, higher concentration of SDS decreased the monodispersity of microsphere and crystallinity. The microspheres are found to be mesoporous in nature with the constituting nanodimensional units having the size of 10–30 nm as revealed from the micrographs of SEM and TEM. Since the CMC of SDS does not vary much with increase of pH, all samples prepared at different pH have almost the same size. While the samples prepared at different concentration of SDS have change in the shape. This could be attributed to the change of micellar structure of SDS in the solution. The lowering of crystallinity of the samples has also been determined by the mechanism of self-assembly in the micellar structure of SDS. All the samples showed pink-red emission upon excitation at 240 nm. The emission intensities of the CaWO4:Sm3+ microspheres decrease with higher pH and higher SDS concentration. The monodispersity index can be a factor in luminescence yield as observed from the emission intensity of CaWO4:Sm3+ microspheres prepared at different SDS concentration. Beside these, the mode of self-assembly and morphology can be an indispensable factor controlling luminescence output. Hence, this work represents a simple method to design the synthesis of self-assembled microspheres.
Goutam S. N. is grateful to University Grants Commission (UGC, New Delhi) for providing Senior Research Fellowship. The authors acknowledged Physics Department, Manipur University for XRD and SEM facility; SAIF, NEHU, Shillong for TEM facility.
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