Synthesis of Sm3+: Ligd(WO4)2 Phosphors Using Ammonium Paratungstate

White light LED (WLED) have been widely used in various lighting and display products in the market. Conventionally, WLED functions by applying InGaN blue LED chips with Ce3+:YAG yellow phosphor. However, there is a shortage of color purity owing to the absence of red component in its light emission.

Samarium (Sm3+) was infrequently investigated as red phosphors. Interestingly, Sm3+ observed to have a strong reddish-orange. Hence, Sm3+ doped oxide materials worth to be used for red/reddish-orange phosphors for the warm WLED application with low color temperature and high CR.

Thus, a sol-gel method was adopted to prepare the Sm3+ activated LiGd(WO4)2 phosphors as a material to be used in the WLED.

The synthesis process is as below:

LiNO3 (99.99%), Gd2O3 (99.99%), Sm2O3 (99.99%), and ammonium paratungstate (APT) were applied as starting materials.

An appropriate amount of metal nitrates, LiNO3 and APT were taken and dissolved separately in deionized water. Gd2O3 and Sm2O3 were converted into Gd(NO3)3 and Sm(NO3)3 solution by dissolving in dil. HNO3 and the citric acid was added as a chelator to form metallic nitrates. The Li, Gd, Sm nitrates were finally mixed together and the pH of the solution was adjusted to 5.5 by adding ammonia. Ethylene glycol was added to the above mixture under constant stirring and heating around 80 °C to promote the citrate polymerization. The final mixture was dried to form a semi-transparent polymer gel. The polymer gel was pre-fired at 250 °C followed by heating at 800 °C under the air atmosphere. The phase purity, particle morphology, and vibrational properties were investigated using XRD, SEM, and Raman analysis.

In conclusion, Single-phase Sm3+ activated LGW phosphors were prepared using Pechini sol-gel method. On near UV (405 nm) excitation, the Sm3+ activated LGW samples exhibit strong reddish-orange emission and it shows effective energy transfer from the LGW host matrix to the Sm3+ activator. The SxLGW shows better performance at an optimal doping concentration of x = 0.01 with a critical distance of the Sm3+ activator in the LGW host to be 3.0 nm. The Dexter theoretical calculation suggested that the electric dipole-dipole interaction predominant for the energy transfer between Sm3+ ions. Chromaticity coordinate of the Sm3+ (x = 0.01) activated LGW phosphors was found to be (0.568, 0.425) that corresponds to reddish-orange and the CP factor was estimated as 82.4%.

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