We investigated the static charge generation by contact electrification between Au and polydimethylsiloxane (PDMS) and the redox reaction by the static charge in the aqueous phase, to reveal the mechanism of contact electrification and redox reaction which may be applied to mechanical-to-chemical energy harvesting. First, the static charge distribution on the equipotential Au was probed through Kelvin probe force microscopy (KPFM) in air after the contact with patterned PDMS. Positive charges are localized on the contact areas indicating the ion migration while the polarity becomes negative after water contact. Second, the redox reaction by the charged Au was electrochemically monitored using open circuit potential (OCP), stripping voltammetry, and copper underpotential deposition (UPD). All electrochemical experiments consistently resulted in the reduction of the reactant by the charged Au within the highly dielectric water media. We concluded that the reduction is not driven by the discharge of static charge on Au but by reducing radicals.

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Electrical field (e-field) enhancement in gold nanohelices (AuNHs) with approximately 130 nm helical diameter (80 nm wire diameter) and various helical pitches (80−170 nm) is investigated with finite-difference time-domain (FDTD) simulation. The dimensions of the AuNHs are empirically determined from electron microscopic images of AuNHs that were synthesized via surfactant-assisted seedmediated growth. In contrast to a Au cylinder with 80 nm diameter, the e-field is effectively enhanced in the AuNH by a longitudinally incident electromagnetic wave (λ of approximately 600 nm) to the AuNH axis. In particular, the dipole distribution of the AuNHs is distinguished by transverse and longitudinal incidence: higher volume and more uniform hot spots exist in AuNHs with longitudinal incidence. A maximum surface-enhanced Raman scattering (SERS) enhancement of 10 6 is obtained in AuNHs with a 100 nm pitch. SERS enhancement values that are obtained from the simulations are compared with experimentally measured SERS enhancement of 4-mercaptobenzoic acid that is loaded on the AuNHs. In addition, the power law dependence of the helical gap on SERS enhancement is relatively stronger compared to the nanogap in gold dimer nanostructures. Hence, AuNHs are satisfactory SERS templates with resonance tuning ability because of their unique structural characteristic (helical gap).

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Grain particle size-controlled Cu 2 O structures were fabricated by oxidizing copper microfilms by soaking them in a 4 M NaOH solution. A microfilm of Cu 2 O was obtained from one-step heating (80 °C for 1 h) of a multilayered film of Cu/Au/Ti (1000/50/10 nm) on a glass substrate followed by naturally cooling under ambient conditions. On the other hand, molding by a micro contact stamp during fabrication of the Cu 2 O structure was carried out to obtain Cu 2 O structures with a relatively smaller particle size than that of the Cu 2 O microfilm, which was obtained by confinement of the Cu 2 O crystal growth within the molded space of the micro contact stamp. The Cu 2 O phases in the microfilm and the molded Cu 2 O samples were confirmed with X-ray diffraction, micro-Raman spectroscopy, and diffuse reflectance spectroscopy. The light absorption capabilities of the Cu 2 O samples were characterized by finite-difference time-domain simulations; the wavelength-scale particle size resulted in absorbance enhancement near the Cu 2 O band gap, and the mirror effect of the Au film underneath the Cu 2 O samples strengthened the absorbance over the entire monitored wavelength region. Moreover, photoelectrochemical characterization proved that the photocurrent of the wavelength-scale Cu 2 O particles (obtained with the molding procedure) increased by approximately 2−5 times in the cathodic process, and the onset potential decreased by approximately 0.3 eV compared to that of the Cu 2 O microfilm. The enhanced light absorption and relatively large surface area, due to the wavelength-scale particle size of Cu 2 O in the molded sample, would result in an enhancement of the photocatalytic activity of Cu 2 O materials.