Band Gap Engineering from Cation Balance: The Case of Lanthanide Oxysulfide Nanoparticles

Abstract

Among the inorganic compounds, many oxides and sulfides are known to be semiconductors. At the crossroads of these two families, oxysulfide MxOySz compounds were much less investigated because they are scarce in nature and complex to synthesize. Amongst them, lanthanide oxysulfide Ln2O2S (Ln = lanthanide) are indirect bandgap semiconductors, with wide gaps, except Ce2O2S. (Gd,Ce)2O2S anisotropic nanoparticles with hexagonal structure were obtained over the whole composition range and exhibit colors varying from white to brown with increasing Ce concentration. Bandgap engineering is thus possible, from 4.7 eV for Gd2O2S, to 2.1 eV for Gd0.6Ce1.4O2S, while the structure is preserved with a slight lattice expansion. Surprisingly, due to the limited thickness of the lamellar nanoparticles, the bandgap of the nanoparticles is direct as validated by density functional theory on slabs. The fine control of the bandgap over a wide range, solely triggered by the cation ratio, is rarely described in the literature and highly promising for further development of this class of compounds. We propose a multi-regime mechanism to rationalize the bandgap engineering over the whole composition range. This should inspire the design of other bimetallic nanoscaled compounds, in particular in the field of visible light photocatalysis.