Ammonium tetrafluoroborate, a cheap and commonly used chemical, was successfully utilized in this study to synthesize BN-coated MgF_2 nanowires and collapsed BN nanotubes, although previous investigations have indicated that the compound cannot be used as a chemical vapour deposition (CVD) reaction precursor of amorphous or crystalline BN. Our study reveals that when MgCl_2 is used as a promoter a crystalline BN phase can be obtained on a large scale. The MgCl_2 also controls the product morphology, resulting in collapsed BN-coated MgF_2 nanowires at the first stage of CVD. The detailed morphology of the composite nanowires also depends on the reaction temperature. Increase in temperature stimulates the nanowire formation. The MgF_2 inclusions can be fully removed via a simple high-temperature evaporation procedure, forming collapsed BN nanotubes of high purity and yield.
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