Urchin-like Na-doped zinc oxide nanoneedles for low-concentration and exclusive VOC detections

In the early-stage diagnosis of lung cancer, the low-concentration (< 5 ppm) volatile organic compounds (VOCs) are extensively identified to be the biomarkers for breath analysis. Herein, the urchin-like sodium (Na)-doped zinc oxide (ZnO) nanoneedles were synthesized through a hydrothermal strategy with the addition of different contents of citric acid. The Na-doped ZnO gas sensor with a 3 : 1 molar ratio of Na⁺ and citric acid showed outstanding sensing properties with an optimal selectivity to various VOCs (formaldehyde (HCOH), isopropanol, acetone, and ammonia) based on working temperature regulation. Specifically, significantly enhanced sensitivity (21.3@5 ppm) compared with pristine ZnO (~7-fold), low limit of detection (LOD) (298 ppb), robust humidity resistance, and long-term stability of formaldehyde sensing performances were obtained, which can be attributed to the formation of a higher concentration of oxygen vacancies (20.98%) and the active electron transitions. Furthermore, the improved sensing mechanism was demonstrated by the exquisite band structure and introduction of the additional acceptor level, which resulted in the narrowed bandgap of ZnO.