Vertically Aligned MoS2 Quantum Dots/Nanoflakes Heterostructure: Facile Deposition with Excellent Performance toward Hydrogen Evolution Reaction

A facile, low-cost, and straightforward procedure was developed for vertical deposition of MoS2 quantum dots (QDs)/nanoflakes (NFs) on fluorine-doped tin oxide (FTO) substrate without any binder. Bulk MoS2 powder was exfoliated in a bath sonicator followed by tip sonicator inside a water-ethanol (0.5/0.5 volume ratio) solution. The obtained MoS2 QDs/NFs are deposited on FTO substrate via a simple electrophoretic technique which resulted in vertical deposition of MoS2 nanoflakes. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) confirmed the formation of MoS2 nanoflakes with thickness of similar to 20 nm and lateral dimension of similar to 400 nm; transmission electron microscopy (TEM) and AFM analysis revealed that many single- or two-layer MoS2 quantum dots exist with lateral size of similar to 5 nm on average on the basal plains of MoS2 nanoflakes. Electrochemical measurements indicated that the vertical MoS2 QDs/NFs/FTO electrode has a Tafel slope of 74 mV/decade and charge transport resistance (R-ct) of 16 Omega which is similar to 1.9 and 2.7 times smaller than the Tafel slope and R-ct of the nonvertical MoS2 NFs/FTO electrode, respectively. This unique morphology exhibited excellent stability for the electrocatalytic hydrogen evolution reaction (HER) after a repeating linear sweep voltammetry (LSV) test for 1000 cycles.