Local structural studies on Co doped ZnS nanowires by synchrotron X-ray atomic pair distribution function and micro-Raman shift

Abstract

Micro-flowers consisting nanowires of pure and cobalt-doped ZnS samples were synthesized via a hydrothermal method. The atomic structures of the obtained nanowires (NWs) were studied by X-ray atomic pair distribution function (PDF) analysis and total synchrotron X-ray scattering. The PDF method was used to describe the wurtzite structure of the as-synthesized samples and the results suggested that a cation–cation (Zn²⁺–Zn²⁺) distance of 3.8343–3.7733 Å with a coordination number ∼4 for the wurtzite structure. The diameters of the samples were evaluated using the PDF data and were in good agreement with the TEM results. Raman spectra of the Co-doped ZnS NWs exhibited a first-order phonon mode at 349.34, 347.08, and 344.70 cm⁻¹, corresponding to the A₁/E₁ longitudinal-optical-phonon vibration mode, in addition to the strong surface optical (SO) phonon mode observed at 330 and 333 cm⁻¹. In the Raman spectra, the peak position of the longitudinal-optical-phonon plasmon coupled mode was shifted to higher frequencies with increasing the cobalt concentration. It was observed that the energy band gap kept on decreasing with increases in the cobalt content. XPS analysis revealed that the chemical state and S vacancies could be controlled by varying the amount of cobalt content in the NWs. The FTIR spectra demonstrated that the sharp peaks were mainly due to all manner of bending vibrations within the NWs observed in the fingerprint region (400–1500 cm⁻¹). The peaks observed at 1038.5, 1163; 1027, 1150; 1011, 1150; 1019.4, 1133 cm⁻¹; and 1011, 1158 cm⁻¹ were due to the formation of a microstructure for the pure and Co-doped ZnS samples, respectively.