One-pot microwave synthesis of core–shell C@Bi ₂ S ₃ nanoflake-like structure for humidity-dependent photodetection applications

The microwave-assisted hydrothermal synthesis was developed to synthesize C@Bi₂S₃ core-shell structures. The poly(ethylene glycol) (PEG) served as a solution viscosity modifier and carbon source in this method. The SEM (Scanning Electron Microscopy) analysis confirmed formation of Bi₂S₃ nanoflakes with thickness in the range of 50 to 130 nm. TEM (Transmission Electron Microscopy) examination revealed on the flake surface a presence of an amorphous carbon layer with a typical thicknesses ranging from 1 to 10 nm. The thin carbon film did not affect the optical properties of bismuth sulfide, resulting in a direct energy bandgap of 1.33(3) eV. A flexible visible light photodetector was fabricated via deposition of core-shell C@Bi₂S₃ nanoflakes onto interdigitated electrodes made of a highly conductive PDMS/CNT composite. The nonlinear current-voltage characteristics of device were observed suggesting the mixed conduction mechanism arising from Ohmic transport and space charge limited current. The enhancement of dark current and photocurrent with the increase of relative humidity (RH) from 40% to 75% was explained by taking into account the Grotthuss mechanism. The remarkable responsivity of 11.5 mA W⁻¹ and the specific detectivity of 1.56 × 10¹⁰ Jones were achieved under light illumination with wavelength of 465 nm and intensity of 19.2 μW cm⁻² at highest RH. The determined values of specific detectivity and responsivity were found to be competitive or even better than those reported for other photodetectors constructed from Bi₂S₃. The obtained results clearly indicate that the proposed C@Bi₂S₃-based device can be used for accurate monitoring of relative humidity and sensitive detection of visible light.