To selectively detect trace carbon monoxide (CO typically lower than 100 ppb) at low temperature is still a great challenge for chemiresistive gas sensors. In this work, we introduce In 2 O 3 nanocubes with exposed {001} facets to couple with MOFderived porous Co 3 O 4 nanosheets (NSs) forming p−n heterojunction for trace CO detection. Benefiting from the vacancy storing effect of the sub-valent band on In 2 O 3 nanocube {001} facets and the p−n heterojunction effect, the electron−hole recombination is effectively inhibited, and the carrier lifetime is prolonged. As a result, the prepared CoIn0.1-based gas sensor shows a good selectivity and ultralow detection limitation of 25 ppb toward CO at relatively low temperature of 175 °C. In addition, the CoIn0.1-based CO sensor also exhibits high response (272.89%) and ultrafast response/ recovery time (2 s/13 s) to 100 ppm CO, which can be ascribed to the fast gas diffusion in the porous Co 3 O 4 NSs and the highindex facet effect of In 2 O 3 nanocubes. This work provides a method to design and synthesize trace CO gas sensors via modulating the band structure of p−n heterojunctions constructed by two-dimensional metal oxide NSs and oxide-exposed high-index facets. KEYWORDS: CO gas sensors, ppb-level, Co 3 O 4 nanosheets, In 2 O 3 nanocubes, heterostructure

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Boron nitride nanosheets (BNNSs) have attracted more and more attention in the field of thermal management due to their excellent thermal and electrical properties. However, the low dispersity and low production yield are still a great baffle for its application. Herein, high-performance hydroxyl and amino bifunctionalized BNNSs (HA-BNNSs) were successfully prepared by the citric acid-assisted ball milling one-step method. Based on the additional friction provided by citric acid during ball milling, the aspect ratio of HA-BNNSs was up to 499 and the production yield was as high as 89.7%. Simultaneously, arising from the high functionalization rate, the HA-BNNSs show excellent dispersity in a variety of polar solvents, particularly in 50% aqueous ethanol solution. The dispersibility of HA-BNNSs was much improved to 39.8 mg/mL. Based on the good dispersibility of HA-BNNSs and strong interface interactions between HA-BNNSs and the matrix, HA-BNNSs have been introduced to aramid nanofibers (ANFs) to fabricate layered HA-BNNSs/ANF composite films for thermal management applications. In comparison with the pure BNNSs/ ANF, the HA-BNNSS/ANF composite exhibits improved mechanical strength (85.6−175.6 MPa) and better thermal conductivity (8−39.5 W/m•K) at a relatively low filler content (≤50%). In summary, benefiting from the bifunctional group, improved dispersibility, and high aspect ratio, the HA-BNNSs have great application potential in the fields of composite reinforcement, drug delivery, and photocatalysis.