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Deep eutectic solvent route synthesis of zinc and copper vanadate n-type semiconductors – mapping oxygen vacancies and their effect on photovoltage

Abstract

Ternary metal oxides M2V2O7-δ (M = Zn and Cu) were synthesized by dissolving binary metal oxide precursors in an environmentally benign deep eutectic solvent (DES), which is a eutectic mixture of a hydrogen bond donor and acceptor, followed by annealing in open crucible. The unique reaction environment provided by the evolved ammonia allows for stabilization of oxygen vacancies and reduced oxidation states of metal ions within an oxide matrix without a need for any post-treatment with flammable reducing agents. According to the comprehensive characterization, including X-ray photoelectron spectroscopy (XPS), neutron powder diffraction, and UV-Vis spectroscopy, the oxygen deficiency is accompanied by reduced oxidation states of metal centers (Cu+ or V+4), resulting in oxides with mixed-valent metal oxidation states. The amount of oxygen vacancies can be tuned by changing the annealing temperature providing control over band gaps of the ternary metal oxides and mid-gap states from the reduced metal centers. All synthesized vanadates are n-type semiconductors as stems from the negative photovoltages obtained by surface photovoltage spectroscopy (SPS). A decay of the photovoltage with increasing annealing temperatures is attributed to electron trapping and electron/hole recombination at V+4 and Cu+ mid-gap states. This work shows for the first time the impact of oxygen vacancies on the electronic structure of DES synthesized oxides for the solar energy conversion applications.

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Publication details

The article was received on 25 Jan 2019, accepted on 08 Apr 2019 and first published on 08 Apr 2019


Article type: Paper
DOI: 10.1039/C9TA00957D
Citation: J. Mater. Chem. A, 2019, Accepted Manuscript
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    Deep eutectic solvent route synthesis of zinc and copper vanadate n-type semiconductors – mapping oxygen vacancies and their effect on photovoltage

    S. Hong, R. M. Welch, F. Osterloh and J. V. Zaikina, J. Mater. Chem. A, 2019, Accepted Manuscript , DOI: 10.1039/C9TA00957D

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