Photothermal effect in solid-state MWCNT: Possible signatures of thermal anisotropy

The photothermal effect of multi-walled carbon nanotubes (MWCNTs) in solid-state was investigated by employing a sensitive heat detection technique, i.e., photo-Differential Scanning Calorimetry using an incoherent, broadband, continuous wave UV-visible light source of the range 250–450 nm. MWCNTs showed considerable photothermal activity, with the highest value recorded being ∼116.1 W/g arising from ∼ 0.2 mg of the sample under an illumination intensity of ∼ 706.8 mW/cm2. Moreover, a time-dependent two-step response in the heat output was obtained as soon as the irradiation source was turned ON or OFF. In view of the huge anisotropy in thermal conductivity as was reported in the case of MWCNTs, we attribute their origin to the quick detection of heat generated in the outer layer and a second gradual, delayed increase in heat output combining the contributions from the inner tubes as well, which gets conveyed to the exterior layer after a certain time-interval owing to the weak van der Waals interaction b...