Synthesis of 3D composite materials based on ultrathin LDH nanowalls grown in situ on graphene surface and fast-response NO2 gas sensing performance at room temperature†

To enhance the gas sensing response performance of LDH materials, this study employed a hydrothermal synthesis method using sodium citrate as an inducer and urea as a precipitant. Graphene with excellent conductivity was used as a substrate. By controlling the solution's alkalinity, sheet-like NiFe-LDHs were successfully induced and assembled on the ultra-thin graphene surface. SEM and AFM characterizations confirmed that the flower-ball morphology of the LDHs, formed by the aggregation of nanosheets, created ultra-thin nanosheets of 6–8 nm that fully covered both sides of the 3–4 nm GO, rendering the material highly porous and well ordered (specific surface area of 111.39 m² g⁻¹). At ambient temperature (RH = 26%), the sample NF/rGO2 with 0.12 g of sodium citrate exhibited extremely high sensitivity and rapid response to 100 ppm NO₂, with a response value and response/recovery time of 22.30 and 2.8/46 s, respectively. Moreover, the sensor demonstrated high selectivity and remarkable long-term stability for up to 100 days. The superior gas sensing performance can be attributed to the unique morphology of the composite material: the inhibited growth of LDHs on the graphene surface exposed numerous basic sites between layers, enhancing NO₂ adsorption capability. Additionally, the staggered and orderly arrangement of ultra-thin LDHs significantly improved the electron transport rate. Therefore, the response/recovery time of the gas sensing material was considerably shortened, enhancing the gas sensing performance of the material. This study provides a novel approach for the preparation and synthesis of high-sensitivity and high-performance NO₂ sensors at room temperature.