Synthesis of nitrogen-doped carbon dot/ tin disulfide nanosheet composite electro-catalysts for dye-sensitized solar cells.

Nitrogen-doped carbon dots (N-CDs) and vertically-grown tin disulfide (SnS2) nanosheets are synthesized via hydrothermal method and chemical vapor deposition (CVD) technique, respectively. The SnS2 nanosheets are directly fabricated on flexible carbon cloth (CC), and then their basal planes are decorated with N-CDs. The as-prepared composite electrodes are used as the counter electrode for the application in dye-sensitized solar cells (DSSCs). The characterizations of N-CDs and SnS2 nanosheets are studied by high resolution transmission electron microscopy (HR-TEM), scanning electron microscopic (SEM), energy dispersive X-ray spectrometer (EDS), Raman spectrometer and X-ray photoelectron spectroscopy (XPS) ect. Moreover, the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and photocurrent-density voltage (J-V) are utilized to understand the electro-catalytic performance of N-CDs/SnS2/CC composite counter electrode. The N-CDs/SnS2/CC composite electrode shows higher cathodic reduction current density and lower charge transfer resistance in CV and EIS measurements, respectively, as compared to those of the electrodes with N-CDs or SnS2 alone. Meanwhile, the DSSC using N-CDs/SnS2/CC exhibits cell efficiency (η) of 7.68%, which is higher than those of cells having SnS2/CC (η=7.54%) and N-CDs/CC (η=5.66%) counter electrodes, repectively; it also reaches 94% cell efficiency of the cell using Pt/CC counter electrode (η=8.15%). The design concept of the modification of the basal planes by defect-rich carbon dots (i.e., N-CDs) and highly-exposed edge sites (i.e., vertically-grown SnS2 nanosheets) makes promising route to enhance the performance of two-dimensional electro-catalysts.