Rare earth metal doped molybdenum sulfide microflowers reinforced with graphene for the photocatalytic annihilation of ciprofloxacin drug

The photocatalytic activity of molybdenum disulfide (MoS₂) is often hindered by its limited visible light absorption and substantial charge carrier recombination. In this study, holmium (Ho³⁺)-doped MoS₂ (Ho@MoS₂) has been developed through a facile hydrothermal method. The Ho@MoS₂ was then incorporated into graphene derivatives, i.e., reduced graphene oxide (rGO), via ultrasonication. The physicochemical properties of the synthesized materials were characterized using X-rays diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, thermal gravimetric analysis, electrochemical impedance analysis, ultraviolet-visible spectroscopy, current-voltage, and Mott-Schottky measurements. X-rays diffraction analysis confirmed the formation of the hexagonal phase for both MoS₂ and Ho@MoS₂, while SEM images revealed micro-flower structures encased within graphene sheets. Ultraviolet-visible spectroscopy showed enhanced light absorption and a reduced band gap for Ho@MoS₂ (1.86 eV) compared to pure MoS₂ (1.99 eV). Mott-Schottky and electrochemical impedance analyses indicated improved charge carrier generation and lower internal resistance following Ho³⁺ doping and graphene integration. When applied for the photocatalytic treatment of a representative pharmaceutical drug, Ciprofloxacin (CIP), Ho@MoS₂-rGO demonstrated a significantly enhanced degradation efficiency with 96.1 % CIP removal within 105 min, which is approximately 2.4 and 1.5 times higher than that of Ho@MoS₂ (63.2 %) and MoS₂ (39.8 %), respectively. This enhanced photocatalytic activity is attributed to the electron-capturing ability of Ho³⁺ ions, the high conductivity of Ho@MoS₂-rGO, favorable band potential (−1.58 V), and reduced series resistance (177 Ω). Additionally, the semiconductor‑carbon (S[sbnd]C) heterojunction within Ho@MoS₂-rGO further promotes efficient charge separation and mobility of reactive oxygen species (ROS). The obtained outstanding photocatalytic performance of Ho@MoS₂-rGO offers its potential use for wastewater treatment.