P-n junction based Ag2O@Ag@Coated functionalized carbon nanotubes and their efficient visible-light photocatalytic reduction performances

Abstract

Abstract A simple deposition method for synthesizing different loadings of Ag2O@Ag (0.4%–8.5%) coated oxygen functionalized carbon nanotubes (CNT); using a triblock copolymer template, is adopted followed by annealing at different temperatures (250 °C and 400 °C). These synthesized materials are characterized by XRD, TEM-SAED, N2 sorptiometry, XPS and photoluminescence spectroscopy. The catalysts electrode surfaces are electrochemically characterized by cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy. Remarkably, the photocatalyst 4%Ag2O@Ag/CNT annealed at 400 °C (7 nm) exhibits the highest reduction rate constant of 4-nitrophenol (4-NP) under dark (0.0112 s−1) and visible light illumination (0.028 s−1), followed by 8.5%Ag2O@Ag/CNT (dark, 0.008 s−1) annealed at 250 °C. The generation of Ag2O p-type on the annealed CNT surface establishes the formation of p–n junction; confirmed by the Mott-Schottky analysis, mediated by Ag nanoparticles of surface plasmon resonance bands at 460 and 555 nm. The 8.5%Ag2O@Ag/CNT250 electrode shows a specific capacitance of 225 F g−1 exceeding that of 4%Ag2O@Ag/CNT400 (186 F g−1) at a current density of 1.0 A g−1, achieving lower internal resistance at the former electrode. This proposes that the charge storage and its transfer rate are not the only factor affecting the reduction of 4-NP. This emphasizes the importance of the reactants facile adsorption as well as the facile creation of the reductive reactive species. The kinetics of the reduction reaction as well as the reaction mechanism are well elucidated and discussed.