Transformation of sluggish higher valent molybdenum into electrocatalytically active amorphous carbon doped MoO2/MoO3-x nanostructures using phyllanthus reticulatus fruit extract as natural reducing agent in supercritical fluid processing

Abstract

Electrochemical hydrogen evolution by molybdenum dioxide and molybdenum sub-oxides has gathered much attention owing to their excellent stability and high catalytic activity. However, the reduction of higher valent molybdenum oxides to lower valent molybdenum oxides using conventional chemical process is a challenging issue. Here, supercritical fluid (SCF) and hydrothermal assisted direct synthesis of carbon doped lower valent molybdenum oxides i.e., MoO2/MoO3-x using ammonium heptamolybdate tetrahydrate and phyllanthus reticulatus fruit extract is demonstrated. The phyllanthus reticulatus fruit extract was used as natural reducing agent and it facilitated the reduction, nucleation and surface capping of lower valent molybdenum oxides. XRD and XPS analysis confirmed the phase pure monoclinic MoO2 nanostructures formation with mildly oxidized molybdenum suboxides (MoO3-x) thin layer on the surface. The partial conversion of phyllanthus reticulatus fruit extract to carbon nanostructures that were doped in MoO2/MoO3-x nanostructures was further confirmed using Raman spectroscopic analysis. The carbon doped MoO2/MoO3-x nanostructures prepared by SCF and hydrothermal method exhibited superior electrochemical hydrogen evolution reaction with the low overpotential of 160 and 207 mV at a current density of 10 mA cm−2, respectively.