Formation of a Ni(OH)2/NiOOH active redox couple on nickel nanowires for formaldehyde detection in alkaline media

Highly ordered, Ni(OH)(2)-Ni-nanowire-based receptor elements were electrochemically fabricated and tested for formaldehyde (HCHO) detection by monitoring their oxidation ability in alkaline media. In order to normalize the electrochemical output currents, the Ni nanowires' electrochemically active surface area was assessed using an oxalate-based method after the template was released. The electrochemical transformation of the Ni-nanowire surfaces to a Ni(OH)(2)/NiOOH redox couple was performed in 0.5-mol L-1 KOH using cyclic voltammetry at 200 mV s(-1). The transformation was monitored for two cases: without KOH modification and with KOH-modified Ni nanowires. It was shown that the non-modified Ni nanowires possess a poor electrochemical response to HCHO oxidation, mainly due to the formation of a NiO surface layer. On the other hand, the modified Ni nanowires donated an electron to the HCHO oxidation reaction, resulting in high output-current densities, attributed to the thin Ni(OH)(2)/NiOOH layer, its amorphous state (TEM/SAED) and its small work function, due to electron doping from under the layered Ni. The modified Ni-nanowire-based electrodes had high sensitivity, reproducibility, selectivity and a low detection limit (0.8 mu mol L-1). The developed HCHO Ni-nanowirebased electrodes' characteristics surpass other Ni-based nanostructured electrodes and have limits of detection comparable to those achieved with noble metals. (C) 2019 The Authors. Published by Elsevier Ltd.