For the first time, Pd nanoparticle-functionalized, rGO nanosheet-modified 3D rod-like CuO (Pd–CuO/rGO) with a nanowire hierarchical structure was successfully synthesized using a facile hydrothermal strategy. Interestingly, the developed Pd–CuO/rGO sensor exhibited more prominent features compared with the CuO, Pd–CuO and CuO/rGO sensors towards trace NO₂ gas at room temperature (RT, 23 °C). In particular, the Pd–CuO/rGO sensor demonstrated an ultra-high response of 64.2 towards 100 ppm NO₂ gas at RT, which was approximately 3.4 times that of the pristine CuO sensor, 2.7 times that of the Pd–CuO sensor and 2 times that of the CuO/rGO sensor, respectively. Simultaneously, a response/recovery time of 10.7/9.2 s towards 10 ppm NO₂ gas at RT was indicated for the Pd–CuO/rGO sensor, and the extremely low limit of detection was down to 50 ppb, ensuringreal-time detection at a ppb-level NO₂ gas concentration. Furthermore, the fabricated sensors had reliable consistency, excellent repeatability and long-term stability. In addition to the contribution from the porous rGO-modified CuO providing abundant active places for gas adsorption and diffusion, Pd and partially oxidized Pd NPs that chemically and electronically sensitized the CuO supporter greatly improved the sensitivity and selectivity of the fabricated Pd–CuO/rGO sensor. Moreover, abundant surface oxygen vacancies were also to some extent beneficial for the sensing performance. Our scientific and systematic work has great potential for practical applications in extremely harsh environments.

Hydrogen gas sensing is critical for energy storage, industrial safety, and environmental monitoring. However, traditional sensors still face challenges in selectivity, sensitivity, and stability. This work introduces an innovative N-polar GaN/AlGaN high-electron-mobility transistor (HEMT) with a 10 nm Pd catalytic layer as a hydrogen sensor. The device achieves ppm-level H₂ detection with rapid recovery and reusability, which is comparable to or even exceeds the performance of conventional Ga-polar HEMTs. The N-polar structure enhances sensitivity through its unique polarization-induced 2DEG and intrinsic back barrier, while the Pd layer catalyzes H₂ dissociation, forming a dipole layer that can modulate the Schottky barrier height. Experimental results demonstrate superior performance at both room temperature and elevated temperatures. Specifically, at 200 °C, the sensor exhibits a response of 102% toward 200 ppm H₂, with response/recovery times of 150 s/17 s. This represents a 96% enhancement in sensitivity and a reduction of 180 s/14 s in response/recovery times compared to room-temperature conditions (23 °C). These findings highlight the potential of N-polar HEMTs for high-performance hydrogen sensing applications.