Formaldehyde is a hazardous indoor volatile organic pollutant, thus selective and precise detection at a low concentration level is crucial. In this paper, NiO/NiFe₂O₄ nanocomposites were prepared through a simple two-step hydrothermal route and their gas sensing performances were investigated. SEM and TEM results showed that the composite products were NiO nanotetrahedrons decorated with numerous separated NiFe₂O₄ nanoparticles on the outer layer surface. XPS characterization confirmed that the as formed NiFe₂O₄ exhibited a p-type conductivity due to the hole transfer between Ni³⁺ and Ni²⁺ and therefore p-p isotype heterojunctions formed in the composite products. The element ratio of Fe to Ni was optimized to gain higher gas-sensing performance through adjusting the feeding amount of Fe³⁺ in the synthesis process. As a consequence, the gas sensor based on the sample NiFe-0.008 showed the highest response of 33.3 to 200 ppm formaldehyde vapor at 240 °C and a low detection limit of 200 ppb. The composite sensor also showed fast response/recovery time and long term stability. The enhanced gas sensing properties to formaldehyde can be attributed to the unique tetrahedral morphology and p-p heterojunction structure, which can provide more active sites and significantly enlarge resistance variation. This work will provide a new perspective of p-p heterojunction for the practical application in formaldehyde gas sensor.

甲醛是室内有害的挥发性有机污染物，因此在低浓度下进行选择性和精确检测至关重要。本文通过简单的两步水热法制备了NiO / NiFe ₂ O ₄纳米复合材料，并研究了其气敏性能。SEM和TEM结果表明，复合产品为NiO纳米四面体，其外层表面装饰有大量分离的NiFe ₂ O ₄纳米颗粒。XPS表征证实，由于Ni ³⁺和Ni ²⁺之间的空穴转移，形成的NiFe ₂ O ₄表现出p型导电性因此在复合产物中形成了pp同型异质结。通过调整Fe ³⁺的进料量，优化了Fe与Ni的元素比，以获得更高的气敏性能。在合成过程中。结果，基于样品NiFe-0.008的气体传感器在240°C时表现出33.3至200 ppm甲醛蒸气的最高响应和200 ppb的低检测限。复合传感器还显示出快速的响应/恢复时间和长期稳定性。增强的对甲醛的气敏特性可以归因于独特的四面体形态和pp异质结结构，可以提供更多的活性位点并显着增大电阻变化。这项工作将为pp异质结在甲醛气体传感器中的实际应用提供新的视角。