Tuning the surface electronic structure of WS2 with Zn- and Cu-phthalocyanine for improved hydrogen evolution reaction: Experimental and DFT investigation

WS2 nanosheets have been successfully demonstrated to be promising materials for several important catalytic reactions including Hydrogen Evolution Reaction (HER) and Oxygen Reduction Reactions (ORR) either stand-alone or by functionalizing and as composites. Tungsten disulfide (WS2) is known to exist in two phases: 2H semiconducting and 1 T conducting. The 2H phase is commonly obtained by simple exfoliation of bulk WS2. Due to the semiconducting nature of the 2H phase, catalytic activity towards HER is very feeble compared to its 1 T counterpart. In this work, we have enhanced the activity of 2H WS2 nanosheets by interfacial modification through metal phthalocyanine (Pc) molecule (Cu-Pc and Zn-Pc) functionalization. We observe that the HER activity shows significant enhancement with the functionalization, with Zn-Pc showing better performance than the Cu-Pc. This functionalization creates an electronically coupled inorganic-organic heterostructure which shows improved response to HER. The WS2 functionalized nanosheets were studied in-depth with X-ray diffraction (XRD), X-ray photo electron spectroscopy (XPS) and High-Resolution Transmission Electron Micro-scopy (HRTEM) analyses. Electrochemical studies like linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) demonstrates that the heterostructures deliver enhanced charge transfer kinetics. The Zn-Pc functionalized nanosheets display an overpotential of 233 mV for attaining a current density of 10 mA. cm-2 and the overpotential decreased to 165 mV after cycling due to the surface reconstruction, when the stability was assessed with chronoamperometry. Density Functional Theory (DFT) studies reveal that the charge transfer due to the adsorption of Zn-Pc onto WS2 nanosheets is comparatively better than that of Cu-Pc. This work recounts that by simple metal phthalocyanine functionalization, charge transfer characteristics of WS2 can be markedly improved, thereby signifying the importance of interface modification in two-dimensional (2D) heterostructures.