(Invited) Metal Nanoparticles Modified Carbon Film Electrodes for Chemical Sensing
Authors: Niwa Osamu , Shiba Shunsuke , Otani Kazuya , Takahashi Shota , Kamata Tomoyuki , Kato Dai 
Published: May 2020

Electrocatalytic performance of metal nanoparticles (NPs) has been studied to apply for electrochemical devices such as fuel cells and electrochemical sensors. We have developed metal NPs embedded carbon film electrodes by co-sputtering of metal and carbon [1]. Various kinds of metals including Pt, Pd, Au, Ni, Cu and their alloys can be fabricated in the carbon film by using unbalanced magnetron (UBM) sputtering and applied for detecting hydrogen peroxide, glucose [1], heavy metals [2,3] and sugar markers [4, 5]. The electrocatalytic activity of metal NPs can be modulated by changing electronegativity of neighbor materials. Here, we proposed two electrode structures to modulate the electrocatalytic activity of metal NPs by forming metal NPs on hybrid carbon film electrodes, The electrodes were then applied applied for detecting small organic molecules such as alcohols and sugars.
The first electrode structure is Ni(OH)2 NPs electrodeposited on the surface of nitrogen terminated carbon film electrodes. The carbon film was formed by UBM sputtering and the surface was treated by N2 plasma. After treatment, the NiNPs were electrodeposited by applying negative potential(Fig.1 left figure of Electrode Fabrication). The size of NiNPs became smaller by decreasing potential from -1000 to -1300 mV. By taking account of this property, we adjusted the potential to obtain similar NiNPs size on both N2 plasma treated and untreated carbon surfaces. After deposition, the potentials of both electrodes were cycled between 0 and 0.70 V (vs Ag/AgCl) to form surface Ni(OH)2. The redox reaction peaks of Ni(OH)2 oxidation and NiOOH reduction shifted positive and negative directions, respectively at Ni(OH)2 modified untreated carbon film(Fig.1 centor figure of Electrode Fabrication). with increasing potential scan rate. In contrast , peak separation increase at Ni(OH)2 modified N2 plasma treated carbon film is greatly decreased suggesting fast redox reaction. We applied both electrodes for electrocatalytic oxidation of glucose in alkaline media(Fig.1 left of Applications). The oxidation current of glucose at Ni(OH)2 modified N2 plasma treated carbon film electrode shows much larger current than that at Ni(OH)2 modified N2 untreated carbon film electrode. The current of former electrode continued to slightly increase when the glucose concentration is around 7 mM. In contrast, the current of latter electrode was saturated below 3 mM.
The other structure we proposed was fabricated by electroplating the Ni onto PdNPs or AuNPs embedded carbon film electrodes. By using the overpotential difference between PdNPs and carbon surface, we successfully electrodeposited NiNPs selectively only on the embedded PdNPs, resulting Ni-Pd heterodimer structure on the electrode surface(Fig.1 right of Electrode Fabrication). The average size of deposited NiNPs onto PdNPs is smaller than that deposited on the pure carbon film electrode because the embedded PdNPs worked as growth core of NiNPs. We cycled the potential in strong alkaline solution to form Ni(OH)2 on the surface of NiNPs. The oxidation peak potential of Ni(OH)2 /Pd heterodimers, which corresponds to surface NiOOH formation reaction, slightly increases with increasing the scan rates from 1 mV/s to 100 mV/s. In contrast, the oxidation peak potential of Ni(OH)2 NP modified pure carbon film(Fig.1 center figure of Electrode Fabrication) shifted more positively (about 0.2 V) with increasing the scan rate, which suggests that the oxidation and reduction cycle of Ni(OH)2 /Pd heterodimer is much faster than that of Ni(OH)2 NP. Both electrodes were applied to electrochemically oxidize methanol. When the concentration of methanol is below 1M, the sensitivity of Ni(OH)2 NP modified carbon film electrode is relatively higher than that at Ni(OH)2 /Pd heterodimer modified carbon film electrode. However, the curent at Ni(OH)2 /Pd heterodimer modified electrodes was superior to that at Ni(OH)2 NP modified electrode when the methanol concentration was higher than 1 M (Fig.1 right figure of Applications). This could be due to the fast regeneration of catalytic sites (NiOOH) resulting in higher turnover rates for methanol oxidation reaction. The above behavior of Ni(OH)2 /Pd heterodimer modified carbon film electrode could be more suitable for energy device such as fuel cell electrode and Ni(OH)2 modified carbon film electrode is more suitable to detect alcohol sensing electrode.
In conclusion, the electrocatalytic activity of NiNPs can be largely modulated by changing substrates, suggesting that the design of electrode surfaces are very important to realize high performance electrodes for chemical sensors.

References

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