Photoelectrochemical Properties of (K, Na)NbO3 (KNN) Thin Film

Abstract

Photoelectrochemical (PEC) and photocatalytic (PC) processes driven by solar energy, e.g., PEC and PC water splitting, have been paid increasing attention because the processes are promising candidates to address environmental issues and energy shortages. For decades, considerable efforts have been made to find a suitable semiconductor material for the processes. Recently, sodium niobate (NaNbO3) and potassium niobate (KNbO3), which have typical ABO3 perovskite structures, attract much interst in the field of photocatalysis because NaNbO3 and KNbO3 show excellent PC activities for H2 generation (1-4). The band gap of KNbO3 is 3.1 eV, which is slightly narrower than that of NaNbO3 (3.4 eV), and thus KNbO3 shows a higher PC activity than NaNbO3 due to the narrower band gap and higher mobile charge carriers (3). The Nb-related perovskite oxides have other interesting properties in terms of ferroelectric, piezoelectric, dielectric, and optical properties. Moreover, they exhibit photovoltaic effects due to spontaneous electric polarisation (i.e., due to their ferroelectric property). In these regards, potassium sodium niobate, (K, Na)NbO3 (KNN), has also been studied extensively. However, nothing has been studied on its PEC and PC behaviour, probably because the bulk band gap of KNN is relatively wide (4.28 eV) (5). Thus, this work aims to demonstrate the PEC behaviour of KNN thin films. Figure 1(a) shows cyclic voltammograms (CVs) for a KNN thin film in 0.1 M (= mol/L) NaOH solution measured with and without ultraviolet-visible light irradiation. The potential (E) was scanned between -0.86 and 0.64 V vs. SHE at a rate of 0.1 Vs-1. The film was deposited onto a Pt/Ti/SiO2/Si substrate by using a radio frequency magnetron sputtering method. The KNN film exhibited a characteristic of n-type semiconductors: the oxidative photocurrent, which is likely to be due to the oxygen evolution reaction (4OH－ → O2 + 2H2O + 4e－), started to flow at -0.5 V and increased as E increased. Figure 1b shows photo images of the KNN film before and after cycling the potential 100 times under the light irradiation. The film was hardly affected by the potential cycles, i.e., it was stable during the PEC reaction. We can thus say that the KNN is an active material for the PEC water splitting, which will be discussed in the presentation. FIGURE CAPTION Figure 1. (a) Photoelectrochemical behavior of a KNN thin film (the surface area: 130 mm2) measured in a 0.1 M NaOH solution. A 100W mercury lamp was employed to irradiate the KNN film electrode. (b) Photo images of the KNN film taken (left) before and (right) after cycling the potential 100 times. The potential was scanned between -0.66 and 0.64 V. Figure 1