Effect of redox reactions on the thermoluminescence characteristics of Cu-doped NaLi2PO4 phosphors

Abstract

A Cu-doped NaLi₂PO₄ phosphor material was successfully synthesized through the high-temperature solid state diffusion method. It was mainly doped with Cu₂Cl₂ and CuCl₂ salts for impurities in the form of Cu⁺ and Cu²⁺, respectively. Formation of the material in the single phase of the phosphor material was confirmed by powder XRD. Morphological and compositional characterization was done using XPS, SEM and EDS techniques. The materials were annealed in reducing, (10% H₂ in Ar) and CO/CO₂ (by burning charcoal in a closed system), as well as in oxidizing (air) atmospheres at different temperatures. ESR and PL studies were conducted for studying redox reactions due to annealing and its effect on TL characteristics. It is known that the impurity Cu could exist in Cu²⁺, Cu⁺ and Cu⁰ forms. The material was doped with two different salts (Cu₂Cl₂ and CuCl₂) as sources of the impurities in two different forms i.e., Cu⁺ and Cu²⁺, however, it was found that it gets incorporated in both the forms inside the material. Also, annealing in different atmospheres not only changed their ionic states but also affected the sensitivity of these phosphors. It was observed that at ∼10 Gy, NaLi₂PO₄:Cu(ii) is around 3.3 times, 3.0 times and almost equally sensitive than commercially available TLD-900 phosphor on annealing in air, 10% H₂ in Ar and CO/CO₂ at 400, 400 and 800 °C, respectively. However, NaLi₂PO₄:Cu(i) becomes 1.8 times sensitive after annealing in CO/CO₂ at 800 °C as compared to TLD-900. With high sensitivity, both the materials NaLi₂PO₄:Cu(ii) and NaLi₂PO₄:Cu(i) are good candidates for radiation dosimetry with a wide dose response (mGy-5.0 kGy).

Fig. 1. PXRD patterns of the NaLi₂PO₄ phosphors: (a) PXRD pattern of the NaLi₂PO₄:Cu(ii) phosphor material, (b) PXRD pattern of the NaLi₂PO₄:Cu(i) phosphor material, (c) crystal structure. Theoretically fitted by Rietveld refinement patterns along with JCPDS data (file # 80-2110) are shown for comparison. The lattice parameters used for the fittings are also shown in the inset.

Fig. 2. A typical EDS spectrum of NaLi₂PO₄:Cu(ii) (0.1 mol%) material. The SEM image and the elemental composition are also shown in the inset.

Fig. 3. (a) Optimization of concentration of the impurity in the host matrix NaLi₂PO₄. TL glow curve of the material doped with different impurity (Cu²⁺) concentrations (0.05–2.0 mol%) and exposed at 10 Gy of γ-rays from Co⁶⁰ source. The materials were annealed at 400 °C in air. Inset shows TL intensity of NaLi₂PO₄:Cu(ii) vs. Cu²⁺ impurity concentration, (b) TL glow curve of the material doped with different impurity (Cu⁺) concentrations (0.05–1.0 mol%) and exposed at 10 Gy of gamma-rays from Co⁶⁰ source. Inset shows TL intensity of NaLi₂PO₄:Cu(i) vs. Cu⁺ impurity concentration.

Fig. 4. (a) Optimization of the annealing temperature effect on NaLi₂PO₄:Cu(ii). TL glow curve of the material with doped Cu²⁺ impurity (0.1 mol%) concentration and exposed at 10 Gy of gamma-rays from the Co⁶⁰ source. The plot of TL intensity (peak height) vs. annealing temperature is also shown in the inset, (b) optimization of the annealing temperature effect on NaLi₂PO₄:Cu(i). TL glow curve of the material doped with Cu⁺ impurity (0.1 mol%) concentration and exposed at 10 Gy of gamma-rays from the Co⁶⁰ source. The plot of TL intensity (peak height) vs. annealing temperature is also shown inside.

Fig. 5. (a) NaLi₂PO₄:Cu(ii) phosphor material annealed in various atmosphere at different temperatures 400, 600 and 800 °C for 1.0 h, (b) NaLi₂PO₄:Cu(i) phosphor material annealed in various atmosphere at different temperatures 400, 600 and 800 °C for 1.0 h.

Fig. 6. Change in colour of the powder samples after annealing: (a) NaLi₂PO₄:Cu(ii) annealed in air atmosphere at various temperatures, (b) NaLi₂PO₄:Cu(ii) annealed in 10% H₂ in Ar atmosphere at various temperatures, (c) NaLi₂PO₄:Cu(ii) annealed in CO/CO₂ atmosphere at various temperatures, (d) NaLi₂PO₄:Cu(i) annealed in air atmosphere at various temperatures, (e) NaLi₂Po₄:Cu(i) annealed in 10% H₂ in Ar atmosphere at various temperatures, (f) NaLi₂PO₄:Cu(i) annealed in CO/CO₂ atmosphere at various temperatures. All the images were taken in room light with high-resolution Canon DSLR camera EOS 1500D.

Fig. 7. (a) ESR spectra of the microcrystalline NaLi₂PO₄:Cu(ii) phosphor materials annealed in air, 10% H₂ in Ar and CO/CO₂ atmospheres, (b) ESR spectra of the microcrystalline NaLi₂PO₄:Cu(i) phosphor materials annealed in air, 10% H₂ in Ar and CO/CO₂ atmospheres. The ESR spectrum for the DPPH (g = 2.0037) has also been given for calibration.

Fig. 8. (a) XPS survey of NaLi₂PO₄:Cu(ii) annealed in various atmospheres for Cu ions, (b) XPS deconvoluted spectrum of NaLi₂PO₄:Cu(ii) as prepared sample for Cu 2p_3/2 and Cu 2p_1/2, (c) XPS spectrum of NaLi₂PO₄:Cu(ii) annealed in 10% H₂ in Ar atmosphere at 400 °C for Cu 2p_3/2 and Cu 2p_1/2, (d) XPS spectrum of NaLi₂PO₄:Cu(ii) annealed in CO/CO₂ atmosphere at 800 °C for Cu 2p_3/2 and Cu 2p_1/2.

Fig. 9. (a) XPS survey of NaLi₂PO₄:Cu(i) annealed in various atmospheres for Cu ions, (b) XPS deconvoluted spectrum of NaLi₂PO₄:Cu(i) as prepared sample for Cu 2p_3/2 and Cu 2p_1/2, (c) XPS spectrum of NaLi₂PO₄:Cu(i) annealed in 10% H₂ in Ar atmosphere at 400 °C for Cu 2p_3/2 and Cu 2p_1/2, (d) XPS spectrum of NaLi₂PO₄:Cu(i) annealed in CO/CO₂ atmosphere at 800 °C for Cu 2p_3/2 and Cu 2p_1/2.

Fig. 10. (a) XPS deconvoluted spectrum of NaLi₂PO₄:Cu(i) as prepared sample for O 1s and its vacancies, (b) XPS spectrum of NaLi₂PO₄:Cu(i) annealed in 10% H₂ in Ar atmosphere at 400 °C for O 1s and its vacancies, (c) XPS spectrum of NaLi₂PO₄:Cu(i) annealed in CO/CO₂ atmosphere at 800 °C for O 1s and its vacancies.

Fig. 11. (a) XPS deconvoluted spectrum of NaLi₂PO₄:Cu(ii) as prepared sample for O 1s and its vacancies, (b) XPS spectrum of NaLi₂PO₄:Cu(ii) annealed in 10% H₂ in Ar atmosphere at 400 °C for O 1s and its vacancies, (c) XPS spectrum of NaLi₂PO₄:Cu(ii) annealed in CO/CO₂ atmosphere at 800 °C for O 1s and its vacancies.

Fig. 12. (a) PL emission and excitation spectra of NaLi₂PO₄:Cu(i) annealed in air atmosphere at 400, 600 and 800 °C, (b) PL emission and excitation spectra of NaLi₂PO₄:Cu(i) annealed in CO/CO₂ atmosphere at 400, 600 and 800 °C, (c) PL emission and excitation spectra of NaLi₂PO₄:Cu(i) annealed in 10% H₂ in Ar at 400, 600 and 800 °C, (d) PL emission and excitation spectra of NaLi₂PO₄:Cu(ii) annealed in 10% H₂ in Ar at 400 and 600 °C, (e) PL emission and excitation spectra of NaLi₂PO₄:Cu(ii) annealed in CO/CO₂ atmosphere at 600 and 800 °C, (f) PL emission and excitation spectra of NaLi₂PO₄:Cu(ii) annealed in 10% H₂ in Ar at 400, 600 and 800 °C.

Fig. 13. (a) Comparison of TL sensitivity of the NaLi₂PO₄:Cu(ii) materials annealed in air, in 10% H₂ in Ar and in CO/CO₂ atmospheres annealed at 400, 400 and 800 °C, respectively, with that of CaSO₄:Dy (TLD-900) phosphor, (b) similar comparison of NaLi₂PO₄:Cu(i) phosphors annealed under same conditions.

Fig. 14. (a) Dose response (curve a) of NaLi₂Po₄:Cu(ii) (0.1 mol%) material annealed at 400 °C in air in the dose range of 1.0 to 10 Gy. The dose response (curve b) of CaSo₄:Dy (TLD-900) is also given for comparison. Dose response (curve c) of NaLi₂Po₄:Cu(i) (0.1 mol%) material annealed at 400 °C in air in the dose range of 1.0 to 10 Gy. Dose response (curve d and e) of individual peaks (peak a and peak b) are also are also given for clarity, (b) dose response (curve a) of NaLi₂PO₄:Cu(ii) (0.1 mol%) material annealed at 400 °C in air in the dose range of 10.0 Gy to 10 kGy. Dose response (curve b) of NaLi₂PO₄:Cu(i) (0.1 mol%) material annealed at 400 °C in air in the dose range of 10.0 Gy to 10 kGy. Dose response (curve c and d) of individual peaks (peak a and peak b) are also are also given for clarity.

Fig. 15. (a) TL fading of NaLi₂PO₄:Cu(ii), TL fading was taken immediately after the irradiation, i.e., day 1 to 60 days, (b) TL fading NaLi₂PO₄:Cu(i) from day 1 to 60 days.