Photocatalytic Behavior of Strontium Aluminates Co-Doped with Europium and Dysprosium Synthesized by Hydrothermal Reaction in Degradation of Methylene Blue

Strontium aluminates co-doped with europium and dysprosium were prepared by a hydrothermal reaction through a sintering process at lower temperatures. The physicochemical properties of the strontium aluminates co-doped with europium and dysprosium were characterized and compared with those of strontium aluminates prepared by a sol–gel method. The photocatalytic properties of the strontium aluminates co-doped with europium and dysprosium were evaluated through the photocatalytic decomposition of methylene blue dye. The strontium aluminates co-doped with europium and dysprosium prepared by the hydrothermal reaction exhibited good phosphorescence and photocatalytic activities that were similar to those prepared by the sol–gel method. The photocatalytic activity of these catalysts for methylene blue degradation was higher than that of the titanium dioxide (TiO2) photocatalyst.

The sol-gel process has attracted considerable interest in obtaining novel chemical compositions and relatively lower reaction temperatures, resulting in homogeneous products [9]. The process enables the synthesis of phosphors with a small size. The inorganic salt-based sol-gel approach has attracted greater interest than the alkoxide-based sol-gel process in the preparation of strontium aluminate luminescent materials [10,11] because inorganic salts are usually non-toxic and cheaper than alkoxides.
Many methods for preparing SAOs have been reported, such as high temperature solid-state reactions [12,13], sol-gel methods [14][15][16], co-precipitation methods [17], and hydrothermal reaction methods [18,19]. To prepare SAOs by sol-gel method, the mixed reactant sol process should be calcined at temperatures higher than 1000 • C as lower temperatures will not lead to SAOs with good crystallinity [20]. The calcination temperature should be lowered to accomplish low-cost preparation of SAOs.
This paper reports the preparation of SAOs by a hydrothermal reaction through a sintering process at lower temperatures. The physical properties and phosphorescence of SAOs were characterized and compared with those of SAOs prepared by the sol-gel method. The photocatalytic decomposition of methylene blue (MB) dye using SAO was performed to estimate its photocatalytic activities for the photocatalytic degradation of methylene blue dye. Figure 1 presents X-ray diffraction (XRD) patterns of the SAOs obtained by a hydrothermal reaction and sol-gel method. Single-phase SAOs were obtained from the two methods. The positions and intensities of the main peaks of the two SAOs corresponded entirely to the standard card (No 34-0379). This suggests that the products were the SrAl 2 O 4 phase. The XRD patterns of SAOs prepared by hydrothermal reaction showed many reflections. Small quantities of Sr 3 Al 2 O 6 and SrAl 4 O 7 were observed on the particle surface. Figure 2 presents scanning electron microscopy (SEM) images of the SAOs synthesized by the hydrothermal reaction and sol-gel methods. The SAOs were polycrystalline, and the particles were sintered into irregular shapes due to the high calcination temperature. The crystallinity of SAOs prepared by the sol-gel method was superior to that prepared by the hydrothermal reaction. In particular, SEM images of the SAOs prepared by a hydrothermal reaction showed smaller crystals on the surface of the particles. This suggests that the smaller crystals on the particle surface in the SEM images are the Sr 3 Al 2 O 6 and SrAl 4 O 7 phases. good crystallinity [20]. The calcination temperature should be lowered to accomplish low-cost preparation of SAOs. This paper reports the preparation of SAOs by a hydrothermal reaction through a sintering process at lower temperatures. The physical properties and phosphorescence of SAOs were characterized and compared with those of SAOs prepared by the sol-gel method. The photocatalytic decomposition of methylene blue (MB) dye using SAO was performed to estimate its photocatalytic activities for the photocatalytic degradation of methylene blue dye. Figure 1 presents X-ray diffraction (XRD) patterns of the SAOs obtained by a hydrothermal reaction and sol-gel method. Single-phase SAOs were obtained from the two methods. The positions and intensities of the main peaks of the two SAOs corresponded entirely to the standard card (No 34-0379). This suggests that the products were the SrAl2O4 phase. The XRD patterns of SAOs prepared by hydrothermal reaction showed many reflections. Small quantities of Sr3Al2O6 and SrAl4O7 were observed on the particle surface. Figure 2 presents scanning electron microscopy (SEM) images of the SAOs synthesized by the hydrothermal reaction and sol-gel methods. The SAOs were polycrystalline, and the particles were sintered into irregular shapes due to the high calcination temperature. The crystallinity of SAOs prepared by the sol-gel method was superior to that prepared by the hydrothermal reaction. In particular, SEM images of the SAOs prepared by a hydrothermal reaction showed smaller crystals on the surface of the particles. This suggests that the smaller crystals on the particle surface in the SEM images are the Sr3Al2O6 and SrAl4O7 phases.   Figure 3 presents the energy-dispersive X-ray (EDX) spectra of the SAOs prepared by the hydrothermal reaction and sol-gel methods. Peaks for Sr and Al were observed. The intensities of the Sr and Al peaks in SAOs prepared by a hydrothermal reaction were similar to those prepared by the sol-gel method. Figure 4 presents Fourier transform infrared (FTIR) spectra of the SAOs. SrAl2O4 belongs to a distorted stuffed tridymite structure. Tridymite is a member of the nepheline family of structures consisting of a corner-sharing tetrahedral framework that distorts to form large cation-occupying cavities. In SrAl2O4, the framework is built up by AlO4 tetrahedra and the structural channels are occupied by Sr 2+ ions [21]. The XO4 molecule will have four degenerate normal modes of vibrations: Symmetric stretching (γs), symmetric bending (δs), antisymmetric stretching (γas), and antisymmetric bending (δd) [22].   Figure 3 presents the energy-dispersive X-ray (EDX) spectra of the SAOs prepared by the hydrothermal reaction and sol-gel methods. Peaks for Sr and Al were observed. The intensities of the Sr and Al peaks in SAOs prepared by a hydrothermal reaction were similar to those prepared by the sol-gel method. Figure 4 presents Fourier transform infrared (FTIR) spectra of the SAOs. SrAl 2 O 4 belongs to a distorted stuffed tridymite structure. Tridymite is a member of the nepheline family of structures consisting of a corner-sharing tetrahedral framework that distorts to form large cation-occupying cavities. In SrAl 2 O 4 , the framework is built up by AlO 4 tetrahedra and the structural channels are occupied by Sr 2+ ions [21]. The XO 4 molecule will have four degenerate normal modes of vibrations: Symmetric stretching (γ s ), symmetric bending (δ s ), antisymmetric stretching (γ as ), and antisymmetric bending (δ d ) [22].  Figure 3 presents the energy-dispersive X-ray (EDX) spectra of the SAOs prepared by the hydrothermal reaction and sol-gel methods. Peaks for Sr and Al were observed. The intensities of the Sr and Al peaks in SAOs prepared by a hydrothermal reaction were similar to those prepared by the sol-gel method. Figure 4 presents Fourier transform infrared (FTIR) spectra of the SAOs. SrAl2O4 belongs to a distorted stuffed tridymite structure. Tridymite is a member of the nepheline family of structures consisting of a corner-sharing tetrahedral framework that distorts to form large cation-occupying cavities. In SrAl2O4, the framework is built up by AlO4 tetrahedra and the structural channels are occupied by Sr 2+ ions [21]. The XO4 molecule will have four degenerate normal modes of vibrations: Symmetric stretching (γs), symmetric bending (δs), antisymmetric stretching (γas), and antisymmetric bending (δd) [22].              Figure 6 presents the emission spectra of the SAOs prepared by the hydrothermal reaction and sol-gel methods. The luminescence properties of the SAO particles were measured in the solid state at room temperature. Regardless of the preparation methods, the SAO particles exhibited similar emission peaks centered at approximately 615 nm under an excitation of 266 nm. The intensities of the peaks for the SAOs prepared by both synthesis methods were similar, which was associated with the typical 4f 6 5d 1 →4f 7 transition of the Eu 2+ ion in SrAl 2 O 4 . This strongly affected the nature of the Eu 2+ surroundings, where the shielding function of the electrons in the inner shell split the mixed states of 4f and 5d by the crystal field [23]. The special emissions of Dy 3+ and Eu 3+ were not observed in the spectra. The Eu 3+ ions in the precursor were reduced to Eu 2+ in a weak reducing atmosphere. The Eu 2+ ions in the precursor were reduced to Eu + . The Dy 3+ ions were oxidized to Dy 4+ during excitation [24]. Simultaneously, thermal vibrations of the surrounding ions and local vibrations in the lattice structure resulted in luminescence spectra with broad bands [25]. The SAOs prepared by the hydrothermal reaction through lower temperature calcination exhibited a similar emission intensity to the SAOs prepared by the sol-gel method through a higher temperature calcination.  Figure 6 presents the emission spectra of the SAOs prepared by the hydrothermal reaction and sol-gel methods. The luminescence properties of the SAO particles were measured in the solid state at room temperature. Regardless of the preparation methods, the SAO particles exhibited similar emission peaks centered at approximately 615 nm under an excitation of 266 nm. The intensities of the peaks for the SAOs prepared by both synthesis methods were similar, which was associated with the typical 4f 6 5d 1 →4f 7 transition of the Eu 2+ ion in SrAl2O4. This strongly affected the nature of the Eu 2+ surroundings, where the shielding function of the electrons in the inner shell split the mixed states of 4f and 5d by the crystal field [23]. The special emissions of Dy 3+ and Eu 3+ were not observed in the spectra. The Eu 3+ ions in the precursor were reduced to Eu 2+ in a weak reducing atmosphere. The Eu 2+ ions in the precursor were reduced to Eu + . The Dy 3+ ions were oxidized to Dy 4+ during excitation [24]. Simultaneously, thermal vibrations of the surrounding ions and local vibrations in the lattice structure resulted in luminescence spectra with broad bands [25]. The SAOs prepared by the hydrothermal reaction through lower temperature calcination exhibited a similar emission intensity to the SAOs prepared by the sol-gel method through a higher temperature calcination.     Figure 8A presents the UV-vis spectra of a pure MB solution, MB solution containing SAO prepared by a hydrothermal reaction, and MB solution containing SAO prepared by the sol-gel method. The absorbance increased with the injection of SAOs prepared by different methods in the MB solution. Figure 8B presents the absorbance of the MB solution containing SAO prepared by hydrothermal reaction and the sol-gel method as a function of the irradiation time of UV light at 300 nm. The absorbance decreased with the injection of SAO into the MB solution despite the short irradiation time. This indicates that the injection of SAOs into the MB solution leads to rapid degradation of the MB dye, and the degradation rate was higher in the MB solution containing SAO prepared by the sol-gel method.  Figure 8A presents the UV-vis spectra of a pure MB solution, MB solution containing SAO prepared by a hydrothermal reaction, and MB solution containing SAO prepared by the sol-gel method. The absorbance increased with the injection of SAOs prepared by different methods in the MB solution. Figure 8B presents the absorbance of the MB solution containing SAO prepared by hydrothermal reaction and the sol-gel method as a function of the irradiation time of UV light at 300 nm. The absorbance decreased with the injection of SAO into the MB solution despite the short irradiation time. This indicates that the injection of SAOs into the MB solution leads to rapid degradation of the MB dye, and the degradation rate was higher in the MB solution containing SAO prepared by the sol-gel method.  Figure 8A presents the UV-vis spectra of a pure MB solution, MB solution containing SAO prepared by a hydrothermal reaction, and MB solution containing SAO prepared by the sol-gel method. The absorbance increased with the injection of SAOs prepared by different methods in the MB solution. Figure 8B presents the absorbance of the MB solution containing SAO prepared by hydrothermal reaction and the sol-gel method as a function of the irradiation time of UV light at 300 nm. The absorbance decreased with the injection of SAO into the MB solution despite the short irradiation time. This indicates that the injection of SAOs into the MB solution leads to rapid degradation of the MB dye, and the degradation rate was higher in the MB solution containing SAO prepared by the sol-gel method.  Figure 9a presents the changes in MB concentration with different initial MB concentrations on a TiO 2 photocatalyst. The MB concentrations decreased due to the photocatalytic decomposition of MB. The rate of MB degradation was faster at lower initial concentrations of MB than at higher initial concentrations because the photoefficiency increases with decreasing dye concentration. In addition, a large amount of dye might be adsorbed on the TiO 2 surface, which can prevent the dye molecules from coming in contact with the free radicals and electron holes. Figure 9b shows the photocatalytic degradation of MB on TiO 2 and the SAO photocatalysts at the same initial concentration of MB. The degradation of MB was faster with the SAO photocatalysts than that with the TiO 2 photocatalyst. This suggests that SAOs have higher photocatalytic activity than the TiO 2 photocatalyst. The higher photocatalytic activity of SAO was attributed to its higher photosensitivity, which was defined in the DRS results. The SAOs showed a lower bandgap than TiO 2 . A lower bandgap of SAOs led to enhanced photocatalytic activity than TiO 2 . containing SAO prepared by sol-gel method; (B) variation of absorbance of the MB solution containing (a) SAO prepared by hydrothermal reaction, and (b) SAO prepared by sol-gel method as a function of irradiation time of UV light at 300 nm. Figure 9a presents the changes in MB concentration with different initial MB concentrations on a TiO2 photocatalyst. The MB concentrations decreased due to the photocatalytic decomposition of MB. The rate of MB degradation was faster at lower initial concentrations of MB than at higher initial concentrations because the photoefficiency increases with decreasing dye concentration. In addition, a large amount of dye might be adsorbed on the TiO2 surface, which can prevent the dye molecules from coming in contact with the free radicals and electron holes. Figure 9b shows the photocatalytic degradation of MB on TiO2 and the SAO photocatalysts at the same initial concentration of MB. The degradation of MB was faster with the SAO photocatalysts than that with the TiO2 photocatalyst. This suggests that SAOs have higher photocatalytic activity than the TiO2 photocatalyst. The higher photocatalytic activity of SAO was attributed to its higher photosensitivity, which was defined in the DRS results. The SAOs showed a lower bandgap than TiO2. A lower bandgap of SAOs led to enhanced photocatalytic activity than TiO2.

Preparation of SAOs
The SAOs were prepared by both a sol-gel method and hydrothermal reaction. In both methods, aluminum nitrate nonahydrate (Al(NO 3 ) 3 ·9H 2 O, 99%; Duksan, Ansan-City, Korea), strontium nitrate The chelating reagent solution was prepared using the appropriate amount of aqueous citric acid solution and added dropwise to the above solution. A boric acid solution was then added to the chelating reagent solution. The mixture was concentrated at 80 • C with stirring until it changed to a high viscosity translucent gel. The mixture was then calcined in an electric muffle furnace at 1100 • C for 3 h in a weak reducing atmosphere using a hydrogen-containing gas mixture (Ar:H 2 = 95:5).
SAOs were also prepared by a hydrothermal reaction. The mixture of SAO precursors and the chelating agent solution were poured in an autoclave. The autoclave containing the reactant was heated to 130 • C with stirring using a magnetic stirrer. The hydrothermal reaction conditions were maintained for 5 h. After the hydrothermal reaction, the product was dried at 110 • C for 12 h. The product was then calcined at 550 • C for 4 h in a reducing atmosphere. Commercially available TiO 2 (Degussa, P25, Krefeld, Germany) with a particle size and specific area of ≈30 nm and ≈50 m 2 /g, respectively, was also used for photocatalytic decomposition.

Photocatalytic Decomposition of Methylene Blue
The MB solution (100 mL) was mixed with the photocatalysts as a reactant mixture. The photocatalyst loading was adjusted to 5 mL because of the significant difference in the densities between TiO 2 and SAOs. The reactant mixture was stirred in the dark for 1 h to reach adsorption equilibrium. The reactant mixture was irradiated with UV light with stirring. Samples were taken at regular intervals. They were the centrifuged and the photocatalysts were separated. The concentrations of the samples were analyzed by UV-visible spectrophotometry (Shimadzu UV-2450, Tokyo, Japan). The concentrations of MB were determined from the calibrated absorbance at 665 nm using a spectrophotometer.
The photocatalytic degradation of MB was carried out using a glass reactor-installed UV lamp system. The reactor was kept in the dark to prevent the dispersion of UV light during the photoreaction. The photoreaction temperature was maintained at 25 • C. The UV array consisted of two 10 W UV-A lamps. The UV emission wavelength and light strength was 365 nm and 30 Lx, respectively.

Characterization of the SAOs
The phase of the SAO particles was determined by XRD (Rigaku Model D/max-II B, Texas, USA). XRD was conducted at 40 kV and 30 mA with a scan speed of 5 • /min, scanning angle from 10 • to 60 • , and a step of 0.02 • using Cu Kα radiation. The crystal size and morphology of the SAOs were investigated by SEM (Hitachi S-4700, Tokyo, Japan). Their chemical components were analyzed by EDX (NORAN Z-MAX 300 series, Tokyo, Japan). The N 2 isotherms of the SAOs were investigated using a volumetric adsorption apparatus (Mirae SI, Porosity-X, Gwangju, Korea) at liquid N 2 temperature. The sample was pretreated at 150 • C for 1 h before exposure to nitrogen gas. The surface areas of the samples were calculated using the BET equation [27]. Transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan) was performed using a LaB 6 filament and operated at 200 kV.
The composition of the phosphors was analyzed using an EDX micro analyzer (JEOL JSM-840A, Tokyo, Japan) mounted on the microscope.
The photoluminescence was evaluated by photoluminescence spectroscopy (PL, Spectrograph 500i, Acton Research Co., Acton, MA, USA) with a 0.2 nm resolution at room temperature. The samples were excited at 226 nm using a He-Cd laser. The UV-vis diffuse reflectance spectra were measured using a UV-vis spectrometer (Shimadzu UV-2450, Tokyo, Japan) in the region, 200-700 nm, with BaSO 4 as the reflectance standard. The optical bandgap (E gap ) was calculated using the Kubelka-Munk method for indirect electronic transitions.

Conclusions
Strontium aluminates co-doped with europium and dysprosium was prepared by a hydrothermal reaction through a sintering process at lower temperatures. The physicochemical properties of the SAOs characterized by SEM-EDX, photoluminescence, and UV-visible DRS were similar to those of the SAOs prepared by the sol-gel method. Although SAOs had been calcined at lower temperatures, their characteristics matched the standard. The photocatalytic activity for the photodecomposition of MB dye was higher than that of the TiO 2 photocatalyst. The SAOs exhibited higher photocatalytic activity than the TiO 2 photocatalyst. The higher photocatalytic activity of SAO was attributed to its higher photosensitivity.