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

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. Among them, lanthanide oxysulfide Ln2O2S (Ln = lanthanide) are indirect band gap semiconductors, with wide gaps, except for Ce2O2S. (Gd,Ce)2O2S anisotropic nanoparticles with a hexagonal structure were obtained over the whole composition range and exhibit colors varying from white to brown with increasing Ce concentration. Band gap 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, because of the limited thickness of the lamellar nanoparticles, the band gap of the nanoparticles is direct as validated by density functional theory on slabs. The fine control of the band gap over a wide range, solely triggered by the cation ratio, is rarely described in the literature and is highly promising for further development of this class of compounds. We propose a multiregime mechanism to rationalize the band gap 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.