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Compositing TiO2 with metal sulfide to construct heterojunction is an effective approach to improve the carriers’ separation efficiency and photocatalytic activity. However, TiO2 is an N-type semiconductor and most metal sulfide is also N-type semiconductor. It will form n–n heterojunction between TiO2 and metal sulfide. The photoinduced electrons in conduction band of metal sulfide hardly flow into the conduction band of TiO2, which will weaken the improvement of carriers’ separation efficiency. In this work, anatase TiO2 nanosheets with coexposed {101} and {001} facets were composited with porous ZnS to construct a novel n–p–n dual heterojunction. This TiO2/ZnS composite displays 108% improvement of photocatalytic activity compared to that of pristine TiO2 nanosheets. As comparison, P25 with mainly exposed {101} facets/porous ZnS with an n–n single heterojunction only show 2% enhancement of photocatalytic activity than that of P25. The n–p–n dual heterojunction displays an obvious advantage than common TiO2/ZnS n–n heterojunction. In the n–p–n dual heterojunction, first, photoinduced electrons at CB of {001} facets will flow into the CB of {101} facets, while photoinduced holes at VB of {101} facets will flow into the VB of {001} facets; second, photoinduced electrons at CB of ZnS will flow into the CB of {001} facets, while photoinduced holes at VB of {001} facets will flow into the VB of ZnS. In this way, it realizes carriers’ separation in the n–p–n dual heterojunction. This work improves a new strategy to employ crystal facets of photocatalysts to construct n–p–n dual heterojunction with metal sulfide for enhancing the photocatalytic activity.

Trimethylamine (TMA) sensors based on metal oxide semiconductors (MOS) have drawn great attention for realtime seafood quality evaluation. However, poor selectivity and baseline drift limit the practical applications of MOS TMA sensors. Engineering core@shell heterojunction structures with accumulation and depletion layers formed at the interface is regarded as an appealing way for enhanced gas sensing performances. Herein, we design porous hollow Co 3 O 4 @ZnO cages via a facile ZIF-67@ZIF-8-derived approach for TMA sensors. These sensors demonstrate great TMA resistive sensing performance (linear response at moderate TMA concentrations (<33 ppm)), and a high sensitivity of ∼41 is observed when exposed to 33 ppm TMA, with a response/recovery time of only 3/2 s. This superior performance benefits from the Co 3 O 4 @ZnO porous hollow structure with maximum heterojunctions and high surface area. Furthermore, great capacitive TMA sensing with linear sensitivity over the full testing concentration range (0.33−66 ppm) and better baseline stability were investigated. A possible capacitive sensing mechanism of TMA polarization was proposed. For practical usage, a portable sensing prototype based on the Co 3 O 4 @ZnO sensor was fabricated, and its satisfactory sensing behavior further confirms the potential for real-time TMA detection.

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