Synthesis of Ag2O Nanocrystals with Systematic Shape Evolution from Cubic to Hexapod Structures and Their Surface Properties



We report the development of a facile method for the synthesis of Ag2O crystals with systematic shape evolution from cubic to edge- and corner-truncated cubic, rhombicuboctahedral, edge- and corner-truncated octahedral, octahedral, and hexapod structures by mixing AgNO3, NH4NO3, and NaOH at molar ratios of 1:2:11.8. A sufficient volume of NaOH solution was first added to a mixture of AgNO3 and NH4NO3 solution to promote the formation of Ag(NH3)2+ complex ions and the growth of Ag2O nanocrystals with good morphological control. The crystals are mostly submicrometer-sized. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy characterization has been performed to determine the crystalline surface facets. A band gap value of approximately 1.45 eV has been found for the octahedral Ag2O crystals. By changing the molar ratios of AgNO3/NH4NO3/NaOH to 1:2:41.8, corner-depressed rhombicuboctahedra and elongated hexapods were obtained as a result of enhanced crystal growth along the [100] directions. Smaller nanocubes with average sizes of approximately 200 and 300 nm and octapods can also be prepared by adjusting the reagent molar ratios and their added volumes. Both the octahedra and hexapods with largely silver atom-terminated {111} surface facets responded repulsively and moved to the surface of the solution when dispersing in a solution of positively charged methylene blue, but can be suspended in a negatively charged methyl orange solution. The cubes and octapods, bounded by the {100} faces, were insensitive to the molecular charges in solution. The dramatic facet-dependent surface properties of Ag2O crystals have been demonstrated.