Boric acid nanotubes, nanotips, nanorods, microtubes, and microtips

Abstract

Thin films containing boron, carbon, and oxygen were synthesized by plasma-enhanced chemical vapor deposition (PE-CVD). When the thin films were exposed to humid atmosphere, nano- or micro-scale boric acid structures were observed to grow spontaneously. Depending on the relative humidity, temperature, and exposure time to the humid atmosphere, these boric acid structures were either nanotubes, nanotips, nanorods, microtubes, or microtips. X-ray photoelectron spectroscopy (XPS) studies on the thin film surface suggested that the thin films contained B2O3, BCO2, and C-C clusters. The relative amounts of boron, carbon, and oxygen in the thin films were about 41.3, 14, and 43.9% respectively. Secondary ion mass spectrometry (SIMS) results indicated that the elemental distribution of boron, carbon, and oxygen in the thin films was rather homogeneous. X-ray diffraction (XRD) was used to study the time evolution of these boric acid structures at 23 degreesC and 45 +/- 3% relative humidity. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the structures showed that the boric acid tubes and tips had a hollow structure, whereas the boric acid rods were solid. The boric acid structures dehydrated into similarly shaped amorphous structures in a vacuum. In-situ X-ray diffraction studies in a vacuum showed that at 23 degreesC and 45 +/- 3% relative humidity (at one atmosphere pressure), the dissociation rate law of the boric acid structures at 250 and 500 mTorr was [-ln(1 -alpha)](1/2) = kt; whereas at 1000 mTorr, the dissociation rate law was [-ln(1 - alpha)] = kt, where a, k, and t were the fraction of decomposed reactant, the reaction rate constant, and the time, respectively.