A high long-term stability is crucial for room-temperature gas-sensitive metal oxide semiconductors (MOSs) to find practical applications. A series of Pd-SnO2 mixtures with 2, 5, and 10 wt% Pd separately were prepared from SnO2 and Pd powders. Through pressing and sintering, Pd-SnO2 composite nanoceramics have been successfully prepared from the mixtures, which show responses of 50, 100, and 60 to 0.04% CO-20% O2-N2 at room temperature for samples of 2, 5, and 10 wt% Pd, respectively. The room-temperature CO-sensing characteristics were degraded obviously after dozens of days’ aging for all samples. For samples of 5 wt% Pd, the response to CO was decreased by a factor of 4 after 20 days of aging. Fortunately, some rather mild heat treatments will quite effectively reactivate those aged samples. Heat treatment at 150 °C for 15 min in air tripled the response to CO for a 20 days-aged sample of 5 wt% Pd. It is proposed that the deposition of impurity gases in air on Pd in Pd-SnO2 composite nanoceramics has resulted in the observed aging, while their desorption from Pd through mild heat treatments leads to the reactivation. More studies on aging and reactivation of room-temperature gas sensitive MOSs should be conducted to achieve high long-term stability for room-temperature MOS gas sensors.

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Here, we used a simple hydrothermal route to controllably synthesize a connection-enriched two-dimensional WO3 network comprising nanorods and a transfer-favored two-dimensional WO3 network comprising nanowires. The network compositions were studied by X-ray powder diffraction. Their structures were observed by transmission electron microscope and scanning electron microscope imaging. We used ethanol vapor to test the gas-sensing performances of the two kinds of networks. The gas sensitivity test results showed that the connection-enriched two-dimensional WO3 network comprising nanorods and a transfer-favored two-dimensional WO3 network comprising nanowires obtained a gas response of 12.5 and 27.5, to 200 ppm ethanol at 300 ℃. We attribute these differences to greater density of electron transmission channels in the former network.

The slight but cumulative influence of impurity gases in air poses a great threat to the long-term stability of room-temperature gas sensors. Room-temperature hydrogen-sensitive Pt–SnO2 composite nanoceramics of 5 wt% Pt were prepared through pressing and sintering. The response of a sample was over 10,000 after being exposed to 500 ppm H2S–20% O2–N2 at room temperature, and the room-temperature hydrogen sensing capacity was seriously degraded even for samples that had aged dozens of days since H2S exposure. Mild heat treatments such as 160 °C for 10 min were found able to fully activate those H2S-exposed samples. As the peak of S 2p electron was clearly detected in H2S-exposed samples, it was proposed that for room-temperature hydrogen-sensitive Pt–SnO2 composite nanoceramics, H2S exposure induced degradation results from the poisoning of Pt by H2S deposited on it, which can be removed through a mild heat treatment. Periodic mild heat treatment should be a convenient and effective measure for room-temperature metal oxide gas sensors to achieve long-term stability through preventing the accumulation of impurity gases in air deposited on them.

Pt-WO3 composite nanoceramic produced by pressed and sintered with 5 wt% Pt and WO3 nanoparticles has a response multiple of about 80 for 800 ppm H2-20% O2-N2 and a response multiple of about 10 for 100 ppm H2-20% O2-N2 at room temperature of 40 relative humidity(RH), showing an impressive sensing capacity for low concentration hydrogen. However, Pt-WO3 composite nanoceramic will show the aging phenomenon mainly with the rapid decline of recovery rate in the open air, which limits its application in practical equipment. Fortunately, Pt-WO3 composite nanoceramic can be reactivated by heat treatment at 200°C for 1 h, thus restoring its pre-aging hydrogen sensitivity capacity. This may be because heat treatment refixed some weak-bonds Pt particles to the original site.