Remarkably enhanced photoluminescence of hexagonal GdPO4·nH2O:Eu with decreasing size

The hexagonal rhabdophane-type GdPO(4) hydrate (GdPO(4).nH(2)O) was synthesized via a simple hydrothermal process. The size and morphology of the products can be tunable by adjusting the pH of reaction systems through the addition of aqueous NaOH. The nanorods with a width of 50-100 nm and a length of about 1 microm were obtained in the absence of NaOH (pH = 2), while a significant reduction of size (width: approximately 10 nm, length: approximately 50 nm) was observed for the product synthesized in the presence of NaOH (pH = 10). Surprisingly, the small-sized product exhibits a remarkably enhanced photoluminescence quantum yield and long excited state lifetime in comparison with those of the large-sized product. This abnormal luminescence phenomenon is discussed and explained. The EDS and XPS measurements revealed the presence of Na(+) in the small-sized samples. These Na(+) cations were probably bonded to the surface O(2-) dangling bonds, which thus reduces the number of surface defects that usually serve as the nonradiative energy transfer center channels. A considerable reduction of surface defect centers results in the increase of the emission efficiency and excited state lifetime in a small-sized sample. Obviously, the controlled synthesis of rare-earth-doped nanoparticles with a small size, but with relatively strong luminescence, is significant for their applications in the areas of technologies including optoelectronics, sensing and bioimaging.