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Hydrogen (H 2) is the cleanest energy but also dangerous, thus the real-time detection of H 2 leakage is crucially important. Herein, we develop ultra-large micro-sized Sn 3 O 4 nanosheet hierarchies via a facile hydrothermal method using polyvinyl pyrrolidone as a regulating agent. Structural analyses reveal the as-prepared Sn 3 O 4 nanosheets are single-crystalline with {010}-facet exposure and Sn2+-deficiency. When examined as gas sensing materials, the {010}-faceted Sn2+-deficient Sn 3 O 4 nanosheets demonstrate highly efficient H 2 sensing with excellent selectivity/response (two times higher than those for NO 2 , SO 2 , H 2 S, and H 2 O) and stability upon long-term testing (over 37 days). The sensor displays fast response /recovery times of 9.4/24 s to 10 ppm H 2 at a low working temperature of 150 °C, and even well responds to 50 ppb H 2 with a theoretical limit of detection (LOD) of 0.54 ppb. The excellent H 2 sensing performance of the Sn 3 O 4 nanosheets has been verified by the first-principle calculation based on density functional theory (DFT), suggesting that the Sn2+-deficient {010} surface of Sn 3 O 4 provides abundant adsorption sites for hydrogen molecules. Moreover, we demonstrate the practical application of Sn 3 O 4 sensor embedded into a miniaturized deployable module, which realizes the real-time on-line H 2 monitoring with excellent repeatability and selectivity for long-term operation. [Display omitted] • We predicted that Sn 3 O 4 NSs with Sn2+ defect were sensitive to H 2 by DFT. • The Sn 3 O 4 NSs have been successfully synthesized by hydrothermal method. • H 2 -sensitive properties were tested comprehensively to verify the DFT calculation. • We fabricated a miniaturized deployable module to monitor the hydrogen leak. [ABSTRACT FROM AUTHOR]