Electrochemical and Photoelectrochemical Properties of Nano-Islands of Zinc and Niobium Oxides Deposited on Aluminum Thin Film by RF Magnetron Reactive Sputtering

Abstract

Zinc oxide (ZnO) and niobium oxide (NbO x ) with a nano-island structure were deposited by a sputtering method on Al-coated glass substrates. Cells with a (ZnO or NbO x )/Al/glass|KNO 3 aq.|Al/ glass structure were assembled, and electrochemical and photoelectrochemical properties were evaluated. The ZnO and NbO x electrodes had higher electrode potentials than the counter Al/glass electrode, and electron flows from the counter electrode to the ZnO and NbO x electrodes through the external circuit were commonly confirmed. In the ZnO-based cell, only faint photocurrent generation was seen, where Zn and Al elution from the ZnO electrode was found. In the NbO xbased cell, however, stable generation of electricity was successfully achieved, and electrode corrosion was not recognized even in microscopic observations. A photoelectrochemical conversion model was proposed based on potential-pH diagrams. In the case of nano-island structures formed at shorter NbO x deposition time, it was concluded that the photoelectrochemical reactions, which were proceeded in the immediate vicinity of the boundary among nano-islands, substrate, and electrolyte solution, were predominant for the photoelectrochemical conversion, and in the case of film structures with longer deposition time, the predominant reactions took place at the film surface.

Structural analyses and reverse Monte Carlo modeling of niobium oxide amorphous film prepared by sputtering method

“…NbO x specimens with the thickness of 100 nm with a continuous structure were deposited on various substrates by an RF magnetron reactive sputtering method, where a metallic Nb was sputtered under a 0.39 Pa of an Ar:O 2 = 1:1 mixture with an RF power of 200 W (more detailed conditions are given in else-where 24) ). An oxygen-free graphite sheet (PERMA-FOIL μ ) was used as a substrate for the measurements of Rutherford back scattering (RBS) and X-ray fluorescence (XRF) to determine the composition.…”

“…24) In the PECs with a NbO x /Al/ glass«KNO 3 aq.«Al/glass structure, faint but stable electricity generation has been successfully observed during light irradiation. The maximum power density was obtained, when the NbO x deposits were not in continuous film structure but in a discrete nano-island structure.…”

“…It was concluded that the PEC reactions proceeded in the immediate vicinity of the boundary among the nano-islands of niobium oxide, substrate metal, and electrolyte solution. However, actual mechanisms of power generation have not been solved completely, because NbO x deposits are amorphous 24) and hence their atomic structures have been unknown. If the atomic structures of NbO x deposits are clarified, their electronic structures will be also elucidated, and it is consequently expected that the power generation mechanism is revealed.…”

Investigation of electronic structure of amorphous niobium oxide based on the density functional theory calculation of crystalline niobium pentoxide polymorphs

“…Among rare metal elements, niobium is comparatively abundant, and hence materials derived from niobium compounds are supposed to be advantageous in regards to cost. 11) In the authors' research group, a photoelectrochemical cell (PEC) with NbO x /Al/glass«KNO 3 aq.«Al/ glass has been developed, 12) where small but stable power generation was observed. Niobium oxide, NbO x was deposited by an RF-magnetron sputtering method, and larger output was obtained when the NbO x deposits were not in film, but in a nano-island structure.…”

“…The detailed preparing condition was given in Ref. 12. Optical absorption spectra have been measured for the films deposited on a quartz glass substrate with different sputtering time.…”