Eco-friendly mass production of MoS2 flakes in pure water for performance enhancement of triboelectric nanogenerator

Abstract

Among 2D materials, MoS2 has a conspicuous negative charging tendency owing to its large work function. This study investigates the modified work function of MoS2 caused by direct exfoliation and dispersion in pure water, which is an efficient mass production method from an environmental and economic perspective. We prepared a vacuum-filtered film using water dispersion of MoS2 flakes. An X-ray photoelectron spectroscopy analysis demonstrated that either or both hydroxyl and water molecules were adsorbed onto the MoS2, and a partial transformation from MoS2 to MoO3 occurred during the exfoliation process in pure water. The Raman spectroscopy signals also indicate the formation of MoO3. Kelvin probe force microscopy revealed that the work function of MoS2, exfoliated in pure water, was enhanced (5.14 eV) compared to that of the as-received MoS2 powder (4.56 eV). The filtered film was annealed at 650 °C under H2/H2S gas to remove functional groups and transform the Mo oxide to MoS2 by sulfurization. Accordingly, the work function of the annealed film returned to approximately that of the as-received powder (4.64 eV). To explore the potential application of MoS2 dispersion in water as triboelectric nanogenerators, we measured the output voltages generated from the filtered and annealed films when in contact with polyethylene terephthalate. The results showed that a higher voltage was generated from the filtered film (6.58 V) than from the annealed film (3.48 V). Furthermore, when using the MoS2 with enhanced work function in an electron-acceptor layer in polyimide, the triboelectric nanogenerator (TENG) with the PI/MoS2:PI/PI structure exhibited approximately 3.7 times greater power density than that with the MoS2 having non-enhanced work function. We envisage that the MoS2, which is produced in an eco-friendly in pure water at low cost, can be used in TENG applications without considerable technical difficulty because of the successful modification of its electron affinity.