Nucleation and growth mechanisms of palladium, nanoflower-shaped, and its performance as electrocatalyst in the reduction of Cr(VI)

Abstract

Palladium electrocrystallization on Ti film is studied through theoretical models of nucleation and growth. At the applied potential + 0.1 V, palladium electrodeposition followed a mechanism through these steps: adsorption, 2D nucleation with ad-atoms incorporation-controlled growth, and 3D nucleation with diffusion-controlled growth. At − 0.3 V, the metallic nucleus and its growth were accompanied by proton reduction, followed by hydrogen adsorption on palladium nucleation sites; thus nucleation turned more progressive. When Pd° was electrodeposited on glassy carbon (GC) electrode at + 0.05 V and + 0.1 V, this led to Volmer–Weber mechanism, with a 3D instantaneous nucleation and diffusion-controlled growth. The structural and morphological characterizations of both substrates determined a Pd° fcc lattice and nanoflowers of Pd° between 100 and 125 nm. Pd/Ti systems are applied in electroanalytic determinations, performing the nanoflowers as highly sensitive electrocatalysts in the chromium(VI) determination in acidic media. Pd/Ti showed a reduction peak at + 0.58 V Ag/AgC in deoxygenated 0.1 mol L−1 H2SO4. Unlike Ti film, the palladium nanostructures contributed to reduction of Cr(VI). Dependence of current peak versus Cr(VI) concentration with Pd/Ti electrocatalysts was linear from 1.9 10−5–10−4 mol L−1, even when the electrode was tested in a real water sample. For comparison, a glassy carbon (GC) electrode and Pd/GC electrode synthesized at + 0.05 V were assayed with Cr(VI). Unlike Ti film, GC contributed to reduction of Cr(VI). Scan rate studies confirmed that Cr(VI) reduction with Pd/Ti is a diffusion-controlled process. Differential pulse voltammetry (DPV) allowed Pd/Ti electrode to obtain a detection limit (LOD) of 5.8 ppb Cr(VI) and a quantitation limit (LOQ) of 19 ppb Cr(VI) in 0.1 mol L−1 H2SO4.