Oxygen-vacancies-rich Ti_nO_2n-1 quantum dots embedded Ti₃CNT_x/CNTs foam by HCl induced self-assembly and spontaneous oxidation towards strong and multiband microwave absorption

Ti₃CNT_x/Ti_nO_2n-1 foam and Ti₃CNT_x/CNTs/Ti_nO_2n-1 foam were prepared by hydrochloric acid (HCl) induced self-assembly and freeze-drying, during which oxygen-vacancies-rich Ti_nO_2n-1 quantum dots in situ grown on Ti₃CNT_x nanoflakes as a result of their slight and spontaneous oxidation. The well-developed pore structure, abundant interfaces, unique dangling bonds, defects and terminated groups on highly conductive Ti₃CNT_x nanoflakes, as well as the oxygen vacancies in Ti_nO_2n-1 quantum dots render the foams with superior impedance matching, intensive multiple reflection and dielectric loss. By immersing the foams into molten paraffin (absorber content: 4.2 wt%), excellent microwave absorption (MA) properties were achieved: an effective absorption bandwidth (EAB) of 5.2 GHz (1.55 mm) and a reflection loss (RL) of −81.3 dB (1.85 mm) for Ti₃CNT_x/Ti_nO_2n-1foam, and an EAB of 6.4 GHz and a RL of −54.3 dB (1.85 mm) for Ti₃CNT_x/CNTs/Ti_nO_2n-1 foam (15 wt% CNTs). Moderate addition of CNTs enhanced the conduction loss and polarization loss, leading to a larger EAB. When adjusting the thickness (1.2–5 mm), the EAB of Ti₃CNT_x/Ti_nO_2n-1 foam (14.7 GHz) and Ti₃CNT_x/CNTs/Ti_nO_2n-1 foam (15.3 GHz) both covered the C, X and Ku band. This work offers a simple and facile method for constructing Ti₃CNT_x-based foams as excellent MA materials.