Hydrothermal synthesis of tin-doped indium oxide nanoparticles using pamoic acid as an organic additive and their photoluminescence properties

We report a facile hydrothermal method using the disodium salt of pamoic acid (Na₂PA) as an organic additive, In(NO₃)₃·xH₂O as an indium precursor, and SnCl₂·2H₂O as a tin precursor for preparation of ITO nanoparticles (NPs) at low temperature (200 °C). After drying at 110 °C, the as hydrothermally synthesized materials were found to be monodisperse spherical NPs with diameters in the range of ca. 30–40 nm as an intermediate NP product, as confirmed by field emission scanning electron microscopy, transmission electron microscopy and XRD analysis. The XRD analysis confirmed the presence of InOOH in the intermediate NPs. For comparison, the formation of the intermediate NPs under the same reaction conditions was also examined using two analogs of PA instead of Na₂PA, i.e., 3-hydroxy-2-naphthalene carboxylic acid (3H2NA) or 2-naphthol (2NP). These additives yielded spherical NPs but with different sizes and different homogeneities compared to the NPs formed using Na₂PA. In terms of size and homogeneity of the intermediate NPs, the additives followed the order Na₂PA > 2NP > 3H2NA. However, in photoluminescence (PL) studies, the intermediate prepared using 3H2NA showed the highest intensity followed by the intermediates formed using Na₂PA and 2NP. After calcination at 420 °C, only the NPs obtained with Na₂PA were converted to ITONPs; the other NPs remained in the InOOH form. These results correlated with the corresponding TGA analysis. Interestingly, the ITONPs prepared using Na₂PA did not change their morphology during calcination at 420 °C. The morphology of the ITONPs prepared using Na₂PA was found to be comparable in terms of homogeneity and shape to that of a commercially available ITO nanopowder. However, the ITONPs prepared using Na₂PA showed superior PL intensities compared to the commercial ITONPs.