Chemical synthesis and characterization of Cu doped ZnS nano-powder

Abstract

In the past few years, II–VI compounds semiconductor nanocrystal have received much interest because their optical properties are different from the corresponding bulk crystals [1, 2]. Among II–VI compounds, ZnS is one of the most promising host materials for the production of commercial thin film phosphors for electroluminescent (EL) device application [3–7]. Since its absorption and emission lie in the ultraviolet regime, the studies of its small size particles did not evoke much interest until the 1980s. In 1994, Bhargava and Gallagher first synthesized ZnS semiconductor microcrystallites doped with Mn2+ ions; they found that the photoluminescence of Mn2+doped in ZnS nanoparticles had much higher quantum efficiency than bulk crystals [8]. Hereafter, the metal doped ZnS nanoparticles have received increasing interest. Recently, many methods have been developed in preparation of Cu doped ZnS nanopowder [9, 10]. In this paper, we report a new chemical synthesis method for preparation of Cu doped ZnS nanopowder as shown in Fig. 1. This process concluded two steps: First, selection to synthesize different lengths of the zinc methacrylate polymer by adjusting the molar ratio of mercaptoacetic acid to monomer of the methacrylic acid; second, the polymer reaction with sodium sulfide and cupric sulfate, giving the Cu doped ZnS nanopowder. During formation of the nanoparticles, the nanoparticles were capped by the methacrylic polymer, which passivated the surface atoms, eliminated the energy levels inside of gap, and increased the luminescence intensity. The size of nanoparticles can be well controlled from 1.8 nm to 3 nm by adjusting the molar ratio of mercaptoacetic acid to monomer of methacrylic acid. The production of Cu doped ZnS nanopowder was also characterized by TEM, XRD and PL. Methacrylic acid (99%) monomers were purified by distillation under reduced pressure prior to use. The cupric sulfate (99%), zinc acetate dihydrate (99%), sodium sulfide (99%), potassium persulfate (99.5%) and mercaptoacetic acid (HSCH2COOH) (85%) were used without further purification. All of the chemicals were purchased from Beijing chemical reagent corporation. To synthesize Cu doped ZnS nanopowder, 40 × 10−6 m3 of water, 0.03 mol of zinc acetate, and 5.5 × 10−6 m3 (6.49 × 10−4 molar) of methacrylic acid were added to a three-neck flask equipped with condenser, then heated and stirred. When the temperature increased about 60 ◦C, 40 × 10−6 m3 of 0.005 M potassium persulfate aqueous solution and 10−5 m3 of 0.032 M (3.2 × 10−6 molar) mercaptoacetic acid aqueous solution were added, respectively. The temperature was kept at 80 ◦C for 1 h. 20 × 10−6 m3 of 0.008 M CuSO4 aqueous solution was added into reactor vessel and stirred for 5 min. 40 × 10−6 m3 of 0.8 M sodium sulfide aqueous solution was dropped into the reactor at a constant rate. When the Na2S aqueous solution was complete, the reaction was continued for an additional 30 min at 80 ◦C. The powder was well separated from the solution by centrifuging, rinsed with methanol and dried in vacuum. The Cu doped ZnS nano-powder was obtained. The PL spectra of Cu doped ZnS nanopowder were recorded with a Hitachi F-4500 fluorescence spectrophotometer. The spectra were obtained by exciting the sample with 368 nm wavelength at room temperature. X-ray powder diffraction was performed on the Cu doped ZnS powder on a Rigaku RU-200B rotaflex diffractometer using Cu Kα λ = 0.15406 nm. TEM