Maximizing the photo catalytic and photo response properties of multimodal plasmonic Ag/WO3−x heterostructure nanorods by variation of the Ag size

Abstract

High quality nearly monodisperse colloidal WO_3−x nanorods with an aspect ratio ∼18 were synthesized using the thermal decomposition technique. The effects of a capping agent and an activating agent on the nanorod aspect ratio have been studied. Excess carrier concentration due to large oxygen vacancy and smaller width of the nanorods compared to the Bohr exciton radius gives rise to an increase of the band gap. Shape anisotropy in nanorods results in two plasmonic absorbance bands at about 890 nm and 5900 nm corresponding to short axis and long axis plasmon modes. The short axis mode reveals an excellent plasmonic sensitivity of ∼345 nm per refractive index. A plasmonic photocatalysis process based on WO_3−x nanorods has been developed to synthesize Ag/WO_3−x heterostructures consisting of multiple Ag dots with ∼2 nm size, randomly decorated on the surface of the WO_3−x nanorods. Long time irradiation leads to an increase in the size (5 nm) of Ag nanocrystals concomitant with decrease in the number of Ag nanocrystals attached per WO_3−x nanorod. Plasmonic photocatalysis followed by thermal annealing produces only one Ag nanocrystal of size ∼10 nm on each WO_3−x nanorod. Red shifting and broadening of plasmon bands of Ag nanocrystals and WO_3−x nanorods confirm the formation of heterostructures between the metal and semiconductor. Detailed transmission electron micrograph analysis indicates the epitaxial growth of Ag nanocrystals onto WO_3−x nanorods. A high photocurrent gain of about 4000 is observed for Ag (10 nm)/WO_3−x heterostructures. The photodegradation rate for Rhodamine-B and methylene blue is maximum for Ag (10 nm)/WO_3−x heterostructures due to efficient electron transfer from WO_3−x nanorods to Ag nanocrystals. Metal plasmon–semiconductor exciton coupling, prominent plasmon absorbance of metal nanoparticles, and formation of an epitaxial interface are found to be the important factors to achieve the maximum photocatalytic activity and fabrication of a high speed photodetector device by employing the heterostructures.