Introduction

In recent years, bulk and nano materials in several areas of pure and applied sciences have captivated a great interest amongst the researchers. Because of their fascinating and tremendous properties with great potential in many applications such as solid state lasers, lamp industry, colour displays, etc (Senthil et al, 2001; Tamrakar et al, 2013“a”; Tamrakar et al, 2013“b”; Tamrakar et al, 2014“a”; Tamrakar et al, 2014“b”) these bulk and nano materials having great interests of research.

Experimental Section

2.1. Materials and Synthesis

Samarium oxide (Sm₂O₃) and samarium nitrate (Sm(NO₃)₃.6H₂O) were used as the rare earth sources of K₃Gd(PO₄)₂:Sm³⁺ in the following synthesis methods. Urea (NH₂CONH₂) and citric acid monohydrate (C₆H₈O₇.H₂O) were used as fuels in combustion and citrate gel combustion methods. The raw materials in carbonate form were used in solid state method whereas in case of combustion and citrate gel method it could be used in nitrate form.

2.1.1. Synthesis of undoped K₃Gd(PO₄)₂ and Sm³⁺ doped K₃Gd(PO₄)₂ by solid state method

The raw materials potassium carbonate (K₂CO₃), gadolinium oxide (Gd₂O₃), ammonium dihydrogen orthophosphate (NH₄H₂PO₄) and samarium oxide (Sm₂O₃) of high purity were mixed and grounded together with the smallest possible amount of ethanol in an agate mortar for an hour to obtain a homogeneous mixture. The stoichometric amount of starting materials was weighed according to the balanced chemical reactions for the undoped K₃Gd(PO₄)₂ (equation (1)) and Sm³⁺ doped K₃Gd(PO₄)₂ (equation (2)) and are given as follows:

The mixed powder was transferred to the alumina crucible and placed into the muffle furnace at 850 °C for 7 hours. The prepared phosphors were cooled to room temperature and grounded to obtain fine powder.

2.1.2. Synthesis of undoped K₃Gd(PO₄)₂ and Sm³⁺ doped K₃Gd(PO₄)₂ by combustion method with urea (NH₂CONH₂)

Urea (NH₂CONH₂) as a fuel used for the preparation of the undoped K₃Gd(PO₄)₂ and Sm³⁺ doped K₃Gd(PO₄)₂ by combustion method. The reagents used were potassium nitrate (KNO₃), gadolinium oxide (Gd₂O₃), ammonium dihydrogen orthophosphate (NH₄H₂ PO₄), urea (NH₂CONH₂) and samarium nitrate (Sm(NO₃)₃.6H₂O) of high purity. The materials were weighed according to the balanced chemical reactions (equations (3 & 4)) given below:

The stoichometric ratio of reagents was kept at unity, so that the heat liberated during combustion be maximized for complete combustion. The weighed reagents were dissolved in a small amount of distilled water and thoroughly mixed in an agate mortar to obtain a paste. The obtained paste was transferred to the alumina crucible and inserted into the pre-heated muffle furnace sustained at 600. The combustion process occurs with the evolution of the large amount of gasses. The whole reaction takes 3-5 minutes to complete. The final white foamy product was cooled to room temperature and ground to obtain fine powder. The fine powder then further annealed at 850 for 3 h to get complete crystallanity.

2.1.3. Synthesis of undoped K₃Gd(PO₄)₂ and Sm³⁺ doped K₃Gd(PO₄)₂ by citrate-gel combustion method with citric acid (C₆H₈O₇.H₂O)

Potassium nitrate (KNO₃), gadolinium oxide (Gd₂O₃), ammonium dihydrogen orthophosphate (NH₄H₂ PO₄), samarium nitrate (Sm (NO₃)₃.6H₂O), citric acid monohydrate (C₆H₈O₇.H₂O) of high purity was used as the starting materials for the preparation of the undoped K₃Gd(PO₄)₂ and Sm³⁺ doped K₃Gd(PO₄)₂. The citrate gel was prepared according to the following chemical reactions:

The reactants in stoichometric amounts were weighed according to equations (5 & 6) and dissolved together in 10 ml distilled water. The mixed solution was heated to 85°C on a hot plate with continuous stirring for 2 hours to obtain a viscous gel. The obtained gel was placed into the pre-heated muffle furnace maintained at 600. After several minutes, the gel boiled followed by evolution of huge amounts of gases. Eventually, spontaneous ignition occurred and the gel underwent combustion. The whole process ended within a few minutes. The final products were cooled to room temperature and grounded to obtain fine powder. The fine powder, then further annealed at 850 for 3 h to ensure that all the unwanted impurities were evaporated and only the higher temperature pure monoclinic phase of K₃Gd(PO₄)₂ remains as the final product.

2.2. Characterization

The X-ray powder diffraction patterns of the synthesized powders were recorded by using Philips X’pert MPD system with Cu Kα radiation (1.5406 Å) operated at 40 kV and 30 mA. The 2θ was varied in the range of 10^° ≤ 2θ ≤ 60° with step size of 0.01° (2θ) and count time of 18s /step. The lattice parameters were calculated using unit cell program [ …]. The diffuse reflectance spectra were recorded using a Shimadzu UV-VIS-2600 double beam spectrophotometer coupled with an ISR (integrating sphere assembly). The photoluminescence (PL) (excitation and emission spectra) and the lifetime measurements of the synthesized phosphors were recorded using a Cary-Eclipse Spectrofluorometer equipped with a 150W Xenon lamp as an excitation source with slit width 5 nm and 2.5 nm for excitation and emission monochromator. All the measurements were performed at room temperature.

3. Results and discussion

3.1. Phase formation

The powder XRD patterns of the Sm³⁺ doped K₃Gd(PO₄)₂ samples synthesized by solid state, combustion and citrate gel combustion methods were measured. The patterns are shown in figure 1 and compared with that of the JCPDS card 049-1085 (K3Gd(PO4)2). The XRD patterns

of Sm3+ doped K3Gd(PO4)2 phosphor indicates a pure phase of the standard K3Gd(PO4)2 and all the peaks were in good agreement which belongs to the monoclinic phase with space group P2_1/m [20]. Also the XRD shows that the formed materials are in crystalline and homogeneous forms.

The particle size of the prepared samples have been calculated from the full width half maximum (FWHM) of the intense peaks using Debye Scherer formula.

Here, D is particle size, k is the Scherrer’s constant (0.89), β is FWHM (full width half maximum), λ = 1.54 A° is the wavelength of X-ray source (Cu (Kα) radiation), θ is Bragg angle of the X-ray diffraction peak.

The average particle size of the Sm³⁺ doped K₃Gd(PO₄)₂ phosphor prepared by solid state method was found approximately 39 nm, while those prepared by combustion and citrate gel methods were found to be 23 nm and 27 nm. The decrease in the particle size is due to increase in the FWHM of the XRD peaks for the phosphor synthesized by combustion method and citrate gel combustion method. The variation of particle size with FWHM is also tabulated in table {}.

References