Principle Scientists: Ben Murray
B.J. Murray, Q. Li, J.T. Newberg, E.J. Menke, J.C. Hemminger, and R.M. Penner*, Shape- and Size-Selective Electrochemical Synthesis of Dispersed Silver(I) Oxide Colloids. Nano Letters 5 (2005) 2319.
Silver (I) oxide is a direct-gap seminconductor with a bandgap of 1.3 eV, give or take 0.1 eV. This material has no real applications that we are aware of, but it is important to us because we believe it contaminates the surfaces of electrodeposited silver nanowires, and that it exerts a rather large influence on the properties of these nanowires, especially when we employ these nanowires in chemical sensors [more]. While studying the electrochemistry of silver nanowires, we made an interesting discovery completely by accident. We found that within a narrow pH range, a silver electrode can produce a dispersed Ag2O sol - visible as a brown plume descending from the anodically-poised electrode. You can see this in the movie below.
When the nano- and microparticles within this sol were examined using an SEM, we found that the predominent particle shape depended on the applied potential: At the threshold of the anodic wave responsible for sol formation, the particles were cube-shaped with holes in the center of each face (these are called "hopper" particles); the application of a more positive potential produced a transition to dendritic, "flower"-shaped particles. Typical SEM images are shown below.
This is a somewhat bizarre observation, but it is easily understood - at least at a qualitative level - with reference to classical colloid growth theory, such as that presented in the text by Sugawara (Sunagawa, I., Crystals - Growth, Morphology and Perfection. ed.; Cambridge University Press: 2005). To make a long story short, the formation of Ag2O particles in this experiment involves two discrete stpes: Step 1: The anodic dissolution of silver metal to form solvated silver ions, and, Step 2: the diffusion and reaction of these ions with OH- to form Ag2O. The rate of Step 1 controls the relative super saturation (RSS) of Ag2O and controls the particle shape. Higher RSS, caused by a faster Ag+ production rate, induces more rapid nanocrystal growth which demands that growth occur at the corners of the cubic Ag2O unit cell, along the [111] crystallographic direction. This is shown schematically below.
