Solar-driven H2 evolution over CuNb2O6: Effect of two polymorphs (monoclinic and orthorhombic) on optical property and photocatalytic activity

Abstract

We synthesized two different polymorphs (monoclinic and orthorhombic phases) of CuNb2O6 as a d0–d9 photocatalyst by a solid-state reaction route and performed photocatalytic H2 evolution from water under AM 1.5G solar light irradiation. The monoclinic phase (P21/c) was transformed to the orthorhombic phase (Pcna) at ambient temperature (>700 °C), and XRD data revealed that the direct environment of octahedral sites was occupied by Cu2+ cations. The difference in the local crystal structure of the CuO6 octahedral in the two polymorphs of CuNb2O6 was closely related to the optical absorption property. For monoclinic CuNb2O6, the optical band gap transition was a metal-to-metal charge transfer between Cu2+ and Nb5+, and d-d absorption of Cu2+ was a forbidden transition by the Laporte selection rule due to its symmetric CuO6 octahedral. For orthorhombic CuNb2O6, d-d absorption of Cu2+ was partially allowed because of its distorted CuO6 octahedral. We experimentally revealed that d-d absorption of Cu2+ contributed a significant effect on the photocatalytic activity of both polymorphs of CuNb2O6; the photocatalytic activity of the monoclinic phase was insensitive to d-d transition in Cu2+ ions, while the photocatalytic activity of the orthorhombic phase was strongly influenced by d-d transition in Cu2+ ions because an electron configuration as d9 readily induces structural distortion by Jahn-Teller effects in an octahedral crystal field. The distortion was avoided by trapping photo-generated electrons, leading to a decrease in photocatalytic activity. We discuss the photocatalytic activities of both polymorphs of CuNb2O6 in conjunction with structural, optical and electrochemical properties. We also show that a deposition of platinum nanoparticles on CuNb2O6 further improved the photocatalytic activity.