A Single-Source Solid-Precursor Method for Making Eco-Friendly Doped Semiconductor Nanoparticles Emitting Multi-Color Luminescence

Abstract

A novel synthesis method is presented for the preparation of eco-friendly, doped semiconductor nanocrystals encapsulated within oxide-shells, both formed sequentially from a single-source solid-precursor. Highly luminescent ZnS nanoparticles, in situ doped with Cu+–Al3+ pairs and encapsulated with ZnO shells are prepared by the thermal decomposition of a solid-precursor compound, zinc sulfato-thiourea-oxyhydroxide, showing layered crystal structure. The precursor compound is prepared by an aqueous wet-chemical reaction involving necessary chemical reagents required for the precipitation, doping and inorganic surface capping of the nanoparticles. The elemental analysis (C, H, N, S, O, Zn), quantitative estimation of different chemical groups (SO2−4 and NH−4) and infrared studies suggested that the precursor compound is formed by the intercalation of thiourea, and/or its derivatives thiocarbamate (CSNH−2), dithiocarbamate (CS2NH−2), etc., and ammonia into the gallery space of zinc-sulfato-oxyhydroxide corbel where the ZnII ions are both in the octahedral as well as tetrahedral coordination in the ratio 3 : 2 and the dopant ions are incorporated within octahedral voids. The powder X-ray diffraction of precursor compound shows high intensity basal reflection corresponding to the large lattice-plane spacing of d = 11.23 Å and the Rietveld analysis suggested orthorhombic structure with a = 9.71 Å, b = 12.48 Å, c = 26.43 Å, and β = 90°. Transmission electron microscopy studies show the presence of micrometer sized acicular monocrystallites with prismatic platy morphology. Controlled thermolysis of the solid-precursor at 70–110 °C leads to the collapse of layered structure due to the hydrolysis of interlayer thiourea molecules or its derivatives and the S2− ions liberated thereby reacts with the tetrahedral ZnII atoms leading to the precipitation of ZnS nanoparticles at the gallery space. During this process, the dopant ions situated at octahedral voids gets incorporated into the nano-ZnS lattice and results in bright photoluminescence. On further heat treatment above 1100 °C, the corbel zinc-oxyhydroxide sheets undergo dehydroxylation to form ZnO which eventually encapsulates the ZnS nanoparticles at the gallery leading to significant enhancement in the luminescence quantum efficiency, up to ∼22%. The emission color of thus formed nano-ZnS/micro-ZnO composites could be tuned over wide spectral ranges from 480 to 618 nm and the spectral changes are attributed to a number of factors including lattice defects, Cu+–Al3+ dopant-pairs and iso-electronic oxygen in nano-ZnS and oxygen-vacancy or -interstitial centers in non-stoichiometric ZnO.