Transformations of Cold-Compressed Multiwalled Boron Nitride Nanotubes Probed by Infrared Spectroscopy

Multiwalled boron nitride nanotubes (BNNTs) were compressed at room temperature in diamond anvil cells up to 35 GPa, followed by decompression. For the first time, in situ infrared absorption spectroscopy was used to monitor BNNT structural transformations. These BNNTs were found to undergo pressure-induced transformations from a hexagonal to a more closely packed wurtzite structure at 11 GPa, which is similar to that exhibited by bulk hexagonal BN (h-BN). However, when BNNTs are compressed, they exhibit quantitative differences compared with bulk h-BN in terms of transformation completeness and reversibility. Our findings provide unambiguous evidence that sp(3) bondings that form in significantly different yields originated from different morphologies of the starting BN materials. The unique transformation mechanism proposed for BNNTs provides new information for developing BNNTs as potential advanced materials with more desirable properties than those of carbon nanotubes.