A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

Abstract

A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3−x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm−2 up to >0.5 mA cm−2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm−2) at an applied bias of −0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.

Metastability and Photoelectrochemical Properties of Cu2SnO3 and Cu2–xLixTiO3: Two Cu(I)-Based Oxides with Delafossite Structures

“…It has been suggested that the bare surfaces of Cu­(I) oxide films are poorly catalytic and results in significant charge recombination and/or photocorrosion of Cu­(I) to Cu(0) . However, several studies have demonstrated that moderate oxidation of Cu­(I) oxide films produces nano-islands of “CuO” at the particles’ surfaces, which can greatly improve the measured photocurrents. − The proposed mechanism of the enhancement is due to a favorable type-II band offset between the Cu­(I) oxide film and CuO. As shown in Figure , a type-II band offset would also develop between Cu 2 SnO 3 /Cu 2– x Li x TiO 3 and CuO.…”

Perspective—Multinary Oxide Semiconductors for Solar Fuels Generation: Closing the Performance Gap between Theory and Practice

“…An additional beneficial feature of these compounds is that they are highly defect tolerant, with defect energies that typically fall within the valence band, and thus reduces the potential number of recombination centers that can drastically reduce photocurrents. 24,21 Bismuth and copper vanadates.-Among the oxides considered here, it is safe to conclude that BiVO 4 has received the most attention. 26 Theoretically, BiVO 4 can reach a STH conversion efficiency of ∼9%; 27 however, until now, a maximum STH conversion efficiency of only ∼1.1% has been achieved.…”

“…Copper(I) tantalates.-These compounds are p-type semiconductors owing to the facile oxidation of Cu(I) to Cu(II), and are thus employed as photocathodes. 15,16,21 15,21 As a result, the valence band maximum (VBM) is at higher energy than the Cu 3d 10 configuration, 22 which facilitates hole doping; Cu vacancies are Further details of these compounds may be found in Table I. The shaded bars show the measured photocurrents culled from the literature (see also Table I).…”

“…24 Similar results were found for Cu 3 Ta 7 O 19 , where the presence of CuO nanoislands led to an increase of the photocathodic current from ∼0.1 mA cm −2 to ∼0.9 mA cm −2 . 21 Niobium doping has been studied for Cu 5 Ta 11 O 30 16 with the results shown in Table I. In general, the values quoted for the photocurrents for these compounds above and in Table I, must be regarded as only representative of what have been achieved to date.…”

Metastability and Photoelectrochemical Properties of Cu2SnO3 and Cu2-xLixTiO3: Two New Cu(I)-Based Oxides with Delafossite Structures

“…54 However, several studies have demonstrated that moderate oxidation of Cu(I)-oxide films produces nano-islands of 'CuO' at the particles' surfaces which can greatly improve the measured photocurrents. [55][56][57] The proposed mechanism of the enhancement is due to a favorable type-II band offset between the Cu(I)-oxide film and CuO. As shown in Figure 9 Polycrystalline films of II were oxidized by heating in air at 400°C for 30 min, following previously reported procedures, 55 and the resulting photocurrents of these films are plotted in S8.…”