Unraveling the molecular and growth mechanism of colloidal black In2O3−x

Abstract

Black metal oxides with varying concentrations of O-vacancies display enhanced optical and catalytic properties. However, direct solution syntheses of this class of materials have been limited despite being highly advantageous given the different synthetic handles that can be leveraged towards control of the targeted material. Herein, we present an alternate colloidal synthesis of black In₂O_3−x nanoparticles from the simple reaction between In(acac)₃ and oleyl alcohol. Growth studies by PXRD, TEM, and STEM-EDS coupled to mechanistic insights from ¹H, ¹³C NMR revealed the particles form via two paths, one of which involves In⁰. We also show that variations in the synthesis atmosphere, ligand environment, and indium precursor can inhibit formation of the black In₂O_3−x. The optical spectrum for the black nanoparticles displayed a significant redshift when compared to pristine In₂O₃, consistent with the presence of O-vacancies. Raman spectra and surface analysis also supported the presence of surface oxygen vacancies in the as-synthesized black In₂O_3−x.