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

Zahirul Sohag,Shaun O'Donnell,Paul A Maggard,Lindsay Fuoco

doi:10.3390/molecules26226830

Zahirul Sohag, Shaun O'Donnell + Show 2 more

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https://doi.org/10.3390/molecules26226830

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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.

Highlights

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
Many mixed-metal oxide semiconductors containing a Cu(I) cation have emerged as promising p-type semiconducting photoelectrodes [1,2]
Cu(I) cation within a metal-oxide semiconductor drives the formation of a higher-energy valence band edge, stemming from the 3d10 orbitals

Summary

Introduction

Cu(I) cation within a metal-oxide semiconductor drives the formation of a higher-energy valence band edge, stemming from the 3d10 orbitals. This results in a shortened energetic distance to the conduction band of an early transition-metal or main group oxide, as found in several previous examples, such as CuRhO2 [3], Cu2 WO4 [4], CuNbO3 [5], Cu3 VO4 [6], and others. While only a few promising p-type semiconducting oxides have previously been reported, e.g., Cu2 O [7,8] and CaFe2 O4 [9,10], many new members of this emerging class of Cu(I)-containing oxides have formed p-type semiconductors. These new ternary Cu(I)-containing oxides represent an intriguing and quickly growing class of p-type semiconductors

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