The electrochemical polymerization of polyaniline (PANI) was studied using correlative measurements of electrochemistry and UV-vis spectroscopy, i.e., spectroelectrochemistry. The ...electropolymerization of PANI was performed in an acidic medium (1 M HCl) containing 0.1 M aniline with cyclic voltammetry (CV) in a potential window from −0.3 to 1 V and a 50 mV s−1 scan rate. At the same time, UV-vis absorbance spectra in the wavelength range from 200 to 900 nm were measured for every 10 mV change in the CV. The CV results show the oxidation of the monomer at a high positive potential (0.9 V vs Ag), the continuous growth of the PANI film and the transformation between the three best-known forms of PANI redox in the potential range between −0.3 V and 1 V. In parallel, the spectroscopic study confirmed the formation of PANI oxidation. The spectroscopic results showed the formation of the final conductive PANI product (emeraldine salt) due to the absorbance of the formed charge carriers (polarons, bipolarons) during the polymerization. The correlative electrochemical/spectroscopy study gave an additional dimension to the PANI polymerization mechanism, where not only was the oxidation the lead type of reaction, but the reduction was also found to play an important role.
Highly ordered, Ni(OH)2Ni-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 μmol L−1). The developed HCHO Ni-nanowire-based electrodes’ characteristics surpass other Ni-based nanostructured electrodes and have limits of detection comparable to those achieved with noble metals.
The effect of Ni/Cu-coating residuals on the magnetic properties and microstructures of samarium–cobalt (SmCo5) magnets was studied. SmCo5 magnets with 0.0, 0.5, 1.0, 2.0, 3.0 and 4.0 wt.% of added ...Ni/Cu (85 wt.% Ni/15 wt.% Cu) were prepared using a conventional sintering route. The magnetic properties of the magnets were found to be consistent up to 2 wt.% Ni/Cu. Any further increase in the Ni/Cu content resulted in a significant reduction in the magnetic properties, to lower than values that would be commercially acceptable. SEM/EDS studies showed that two major phases, i.e., the SmCo5 matrix phase and Sm2O3 were present in all the sintered SmCo5 magnets. The presence of Sm2Co7 as a minor phase fraction was detected in the sintered SmCo5 magnets containing up to 2 wt.% Ni/Cu. A 2 wt.% Ni/Cu addition to magnets resulted in the presence of two new phases with compositions close to SmCo and Sm2Co17 in addition to SmCo5 and Sm2O3 as major phases in the SEM-observed microstructure. These newly formed phases are present in small fractions and are presumably homogenously distributed at the grain boundaries of the magnets. As they are known to act as nucleation sites for reverse magnetic domains, they effectively reduce the intrinsic grain boundary magnetic strength, leading to a drop in the coercivity. We concluded that the sintered SmCo5 magnets could be recycled with up to 2 wt.% Ni/Cu as a residual from the coating under our sintering and heat treatment conditions.
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•Electrodeposited Ni films were modified by potential cycling in KOH.•KOH modification increases the amount of structurally disordered Ni(OH)2/NiOOH.•The amount of structurally ...disordered Ni(OH)2/NiOOH influences the catalytic properties.•The KOH-modified Ni films provide better catalytic activity towards HCHO oxidation.
The main challenge with electrocatalysis is finding low-cost electrocatalysts that can work efficiently to oxidize the HCHO. Here, we propose a mechanism for the voltammetric formation of a highly active, structurally disordered β-Ni(OH)2/β-NiOOH redox pair on the surface of electrodeposited Ni thin films to achieve an extraordinary catalytic performance with respect to HCHO oxidation in alkaline media. We report electrochemical, XRD and FT-IR measurements on as-deposited and voltammetrically treated (i.e., KOH-modified) Ni thin films, and calculations based on the electrical charge to investigate the changes in the surface composition, crystal structure and related HCHO oxidation activity. We found that the KOH-modification process plays a crucial role in the formation of surface highly active, disordered β-Ni(OH)2/β-NiOOH. The KOH-modified Ni film with the largest amount of the structurally disordered β-Ni(OH)2/β-NiOOH resulted in improved catalytic performance, i.e., an onset overpotential reduced by 400 mV and a catalytic rate increased by 69 mV dec−1. The presented technique has a wide range of applications and provides advances with a novel design idea and a new synthesis strategy for the preparation of highly active, structurally disordered Ni(OH)2/NiOOH redox systems on the surface of Ni thin films and other Ni-based nanostructured electrocatalysts for HCHO oxidation.
In this study, we introduce a novel approach using correlative analysis techniques to unravel detailed insights into the environmental influences on crystal growth. Tabular and bipyramidal wulfenite ...samples from the Mežica mine in north-eastern Slovenia were analysed to combine the morphological aspects of crystal growth with the atomic-resolution reconstruction of the positions of lead (Pb) and molybdenum (Mo) atoms in the parent crystal lattice. These combined data also allow us to present the formation mechanism that enables the development of bipyramidal or tabular morphologies in wulfenite. The bipyramidal and tabular crystals are chemically pure wulfenite (PbMoO
), as confirmed by various advanced diffraction and spectroscopy techniques. However, each habit includes multiple inclusions, mostly consisting of carbonates, Pb-Fe oxides, Pb oxides and, more rarely, Pb vanadate (descloizite). The differences in the morphologies can be attributed to compositional changes during precipitation from a meteoric solution and thus, we propose a growth mechanism consisting of three different phases of growth. This innovative approach emphasises the importance of understanding the origin of crystal habits, as can help to decipher how external influences can affect the crystal structure and its surface, leading to the dissolution of preferred surfaces and the selective release of Pb and Mo.
The electrochemical oxidation of gold nanoparticles (NPs) has been examined in H2SO4 and acidified NaCl by combining multiple techniques including scanning electrochemical cell microscopy (SECCM) and ...in-situ electrochemical transmission electron microscopy (EC-TEM). Our findings provide novel insights into the intricate oxidation dynamics of Au NPs, which are determined by the interplay between passivation and dissolution. SECCM chronoamperometric measurements in the Cl− containing electrolyte reveal distinct current peak events during the dissolution process. We attribute these events to delayed one-by-one NP dissolution following the rapid breakdown of the passive layer formed during the initial stages of anodic polarization, and exposure of the metallic gold core to Cl− ions. Statistical analysis of these peak events further uncovers relationships between the applied potential and the distribution of the peak descriptors over time. The analysis of the charge consumed during each event indicates that NPs undergo partial dissolution interrupted by rapid re-passivation. As the potential increases, the peak current rises and the duration of the peak events decreases due to intensified dissolution and faster re-passivation. The onset time of the peak events displays a highly stochastic nature. In-situ EC-TEM measurements support these findings by confirming that Au NPs dissolve one-by-one at different time intervals and stages, revealing a core-shell structure during the dissolution process. The shell, which is more resistant, leads to a delayed, particle-by-particle dissolution once it is broken down. Quantification of the EC-TEM video allows recreating an equivalent current-time transient which presents peak events similar to these measured by SECCM. These findings contribute to our understanding of the complex and stochastic behavior of nanoparticle dissolution, which cannot be fully explained by traditional (macro) electrochemistry alone. The combination of SECCM and EC-TEM offers high potential to understand complex nanomaterial degradation pathways, essential for the design of durable electrocatalysts for electrochemical conversion and storage applications.
The nucleation and dissolution of gold nanoparticles (NPs) on a glassy carbon substrate have been studied with a local electrochemical approach based on Scanning Electrochemical Cell Microscopy ...(SECCM). By performing hundreds of locally-resolved voltammograms, a distribution of responses for nucleation was found when scanning multiple regions of the substrate, addressing experimentally the statistical nature of the nucleation and growth process at the micro-scale. Moreover, the confinement of the electrochemical cell to the SECCM meniscus enables us to resolve a diversity of events during the electrochemical dissolution of previously electrodeposited NPs, characterized as current spikes during the voltammetric scans. These current events do not manifest in the macro-scale due to the averaging of the electrochemical response over the large electrode area and reveal previously untapped information from the electrochemical nucleation, growth, and dissolution of supported metal nanoparticles. This approach opens up new opportunities for the rational design (electrodeposition) of functional nanostructured materials and the evaluation of their durability under electrochemical polarization (resistance to electrodissolution). The ability to study these considering the heterogeneous nature of the supports and the differences within nanomaterial ensembles is essential for applications in electrochemical conversion and storage.
Simple, low-cost methods for sensing volatile organic compounds that leave no trace and do not have a detrimental effect on the environment are able to protect communities from the impacts of ...contaminants in water supplies. This paper reports the development of a portable, autonomous, Internet of Things (IoT) electrochemical sensor for detecting formaldehyde in tap water. The sensor is assembled from electronics, i.e., a custom-designed sensor platform and developed HCHO detection system based on Ni(OH)
-Ni nanowires (NWs) and synthetic-paper-based, screen-printed electrodes (pSPEs). The sensor platform, consisting of the IoT technology, a Wi-Fi communication system, and a miniaturized potentiostat can be easily connected to the Ni(OH)
-Ni NWs and pSPEs via a three-terminal electrode. The custom-made sensor, which has a detection capability of 0.8 µM/24 ppb, was tested for an amperometric determination of the HCHO in deionized (DI) and tap-water-based alkaline electrolytes. This promising concept of an electrochemical IoT sensor that is easy to operate, rapid, and affordable (it is considerably cheaper than any lab-grade potentiostat) could lead to the straightforward detection of HCHO in tap water.
Ammonia (NH3) present in biological fluids is biomarker for several disease states. In this work the well-known interaction between polyaniline (PANI) and NH3 was used for the fabrication of an ...amperometric sensory platform to detect aqueous NH3 at neutral i.e. biological pH. Understanding PANI's electrochemical synthesis and redox behaviour in acidic and neutral media was used to determine the NH3 detection mechanism. The latter is based on the PANI deprotonation reaction, NH4+ oxidation and follow-up PANI reduction and oxidation. Chronoamperometry was applied as an NH3-detection method in a 50 µL background electrolyte in which a 1 µL sample was injected, triggering instantaneous changes to the current. The sensory platform's detection limit, based on pure electrochemically synthesised PANI (PANIel) (24.64 µM), was reduced 17 times (1.44 µM) with the addition of 20 nm Au nanoparticles. This Au-decorated PANIel sensory platform showed excellent reversibility, reusability and the possibility for continuously cycled NH3 measurements, with a recovery rate of 90–99.5% for artificial saliva samples of different pHs. This demonstrates its suitability for more complex samples. The developed sensory platform presents potential from the industry point of view, as a base for an NH3 batch injection analysis and for preventive home medical self-care.
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