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Abstract

The present dissertation deals with the electrochemical synthesis of Ni-based electrodes, i.e. receptor elements that enable fast, completely repeatable and feasible testing of formaldehyde (HCHO) by monitoring their oxidation ability in alkaline media. The main objectives of this work were to electrochemically synthesise 2-D films and 1-D nanowires based on metallic Ni and catalytically active Ni(OH)2/NiOOH surface species.
It was shown that electro-catalytic oxidation of HCHO is highly dependent on the surface composition of Ni-based electrodes, the formation and crystallinity of which can be controlled by electrodeposition conditions (pH, substrate, etc.). A detailed electrochemical study supported with X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) analysis showed that Ni-Ni(OH)2/NiOOH electrodes can be prepared via direct electrodeposition or indirect processing which includes the electrodeposition of Ni/NiO and subsequent KOH-transformation of catalytically inactive NiO to active Ni(OH)2/NiOOH.
Direct synthesis is only possible by using strongly acidic conditions in an electrochemical deposition. Directly deposited Ni(OH)2/NiOOH-Ni film, i.e. “fine-grained” Ni film, was prepared from NiSO4 electrolyte (pH = 2.5) by applying a constant potential of -1.0. The pH of 2.5 was adjusted by adding H2SO4. Fine-grained Ni film was electrodeposited on Au substrate that exhibits preferential crystallographic orientation in (1 1 1) direction. XRD confirmed the presence of electrodeposited Ni (Ni2+ + 2e- ->Ni0) with the same texture in (1 1 1). Also, XRD and FT-IR confirmed the presence of crystalline hexagonal α-Ni(OH)2 which chemically precipitates on the surface of metallic Ni due to the massive production of OH- in water splitting reaction (𝐻2𝑂+2 𝑒− → 𝐻2+2 𝑂𝐻−).
The indirect preparation of Ni-based receptor elements was introduced in the case of “coarse-grained” Ni film and Ni nanowires. The “coarse-grained” Ni film was electrodeposited from NiSO4 solution (pH = 5.5) by applying a constant potential of -1.0. Also, “coarse-grained” Ni film was electrodeposited on Au (1 1 1). The XRD and FT-IR confirmed the presence of metallic Ni (with the preferential (1 1 1)-orientation), and NiO2 which is mainly present due to the rapid passivation of metallic Ni upon exposure to air or aqueous solutions. The XRD analysis revealed the absence of Ni(OH)2 as the mechanism of electrodeposition was different due to the increase of electrolyte pH. The Ni nanowires were electrodeposited from NiSO4-H3BO3 electrolyte into Al2O3 template that was covered by isotropic Au at the bottom. The XRD revealed the presence of isotropic Ni and NiO after the removal of the template.
After electrodeposition of “coarse-grained” Ni film and Ni nanowires, the surface transformation, i.e. KOH-modification (from inactive NiOx to catalytically active Ni(OH)2/NiOOH) was introduced. A comprehensive study of surface transformation was performed using cyclic voltammetry under controlled experimental conditions that include: the potential range (up to 1.0 V or 0.6 V), scan rate (10, 200 or 400 mV s-1) and a number of cycles (1–50). Each surface transformation of KOH-modified electrodes was supported by testing the catalytic ability towards HCHO oxidation and by means of analysis (XRD, FT-IR or TEM). It has been shown that inactive NiOx is completely transformed to active Ni(OH)2/NiOOH by using the following parameters in KOH-modification process: potential range up to +0.6 V, the scan rate of 200 mV s-1, and 50 cycles. XRD confirmed the presence of crystalline hexagonal α-Ni(OH)2 that has grown on a preferentially (1 1 1)-oriented fcc Ni in coarse-grained Ni film substrate, while the presence of amorphous Ni(OH)2 on the surface of the isotropic Ni nanowires substrate was proven by TEM and FT-IR.
It was shown that the KOH-modified Ni-based electrodes are able to electro-catalyze an HCHO oxidation, due to the formation of an active Ni(OH)2/NiOOH surface layer. Based on a comparison of the output current density (current / electrochemically active surface, determined by the ‘oxalate method’) and the electrochemical detection potential, it was shown that KOH-modified Ni nanowires exhibit better catalytic properties for HCHO detection in comparison to KOH-modified fine- and coarse-grained Ni films. The better catalytic activity was attributed to the amorphous nature (TEM, FT-IR) of thin Ni(OH)2/NiOOH surface layer and its low work function, due to electron doping from under layered Ni. On the other hand, the HCHO electrochemical oxidation of KOH-modified Ni films resulted in lower output-current densities and higher onset overpotential attributed to the α-Ni(OH)2/γ-NiOOH surface layer.
The catalytic properties of KOH-modified Ni-based electrodes were supported by a study of sensory properties by determination of the following parameters: limit of detection, linear range, sensitivity, and selectivity. It is worth mentioning that the developed HCHO detection platform that is based on KOH-modified Ni nanowires surpasses other Ni-based nanostructured electrodes and has limits of detection (8 μmol L-1) comparable to those achieved with noble metals (5–10 μmol L-1). Thus, it shows great potential for the successful implementation in future HCHO sensing applications.