Chromium-Decorated Double-Walled Carbon Nanotubes for Preferential Room-Temperature Xylene Sensing

Room-temperature discrimination of closely related aromatic volatile organic compounds remains challenging for chemiresistive sensors. Here, chromium-modified double-walled carbon nanotubes (Cr-DWNTs) were prepared using an endohedral filling process and were investigated for preferential xylene detection. X-ray diffraction showed substantial transformation of the CrCl₂ precursor to rhombohedral Cr₂O₃ identified as the dominant crystalline chromium-containing phase. Transmission electron microscopy and elemental analysis supported association of the chromium-containing phase with the DWNT structure. Raman spectroscopy showed retention of the characteristic D, G, and 2D bands with only a small change in the ID/IG ratio, indicating preservation of the graphitic framework after chromium modification. Hall-effect measurements showed positive Hall coefficients for both pristine DWNTs and Cr-DWNTs, confirming hole-dominated transport, while Cr-DWNTs exhibited a lower apparent Hall carrier concentration together with substantially higher carrier mobility and conductivity. Cr-DWNTs exhibited a pronounced preferential response toward xylene over 0.1-10 ppm. The xylene response increased from approximately 0.96% at 0.1 ppm to 5.93% at 10 ppm and was enhanced by approximately 1.9-2.6 times relative to pristine DWNTs. Within 0.1-1 ppm, the low-concentration xylene sensitivity was 2.36% ppm^-1 (R2=0.958), with a calculated limit of detection of approximately 37 ppb. The enhanced response is attributed to the combination of the intrinsic affinity of graphitic carbon toward methyl-substituted aromatics and interfacial electronic transduction within the DWNT network. These results demonstrate that chromium modification provides an effective strategy for amplifying room-temperature xylene selectivity in CNT-based chemiresistive sensors.