A novel high-purity V₂C MXene two-dimensional carbide, was successfully synthesized by etching V₂AlC with sodium fluoride and hydrochloric acid at 90°C for 72 h. From the analysis of X-ray diffraction, energy dispersive spectra, and X-ray photoelectron spectroscopy, the purity of as-synthesized V₂C MXene was >90 wt% with a few impurities of Na₅Al₃F₁₄ and V₂AlC. The V₂C MXene made by this method was much purer than those made by HF etching at room temperature. The as-prepared V₂C MXene showed excellent electrochemical properties as anode of lithium-ion batteries. The capacity can be 260 mAh g⁻¹ if discharged under 370 mA g⁻¹. The capacity was increased with charge cycles at high charge rate (500 mA g⁻¹). It was suggested that V₂C with high purity can be promising anode material with excellent performance.

Double-walled carbon nanotubes are coaxial nanostructures composed of exactly two single-walled carbon nanotubes, one nested in another. This unique structure offers advantages and opportunities for extending our knowledge and application of the carbon nanomaterials family. This review seeks to comprehensively discuss the synthesis, purification and characterization methods of this novel class of carbon nanomaterials. An emphasis is placed on the double wall physics that contributes to these structures’ complex inter-wall coupling of electronic and optical properties. The debate over the inner-tube photoluminescence provides an interesting illustration of the rich photophysics and challenges associated with the myriad combinations of the inner and outerwall chiralities. Outerwall selective covalent chemistry will be discussed as a potential solution to the unattractive tradeoff between solubility and functionality that has limited some applications of single-walled carbon nanotubes. Finally, we will review the many different uses of double-walled carbon nanotubes and provide an overview of several promising research directions in this new and emerging field.