The National Institute for Occupational Safety and Health (NIOSH) has established exposure limits for sulfur-based volatile components, particularly hydrogen sulfide (H2S), at 20 ppm for an 8-h exposure and 50 ppm for durations under 10 min. Detecting such toxic gases at low levels necessitates innovative sensor fabrication. This study introduces a unique sensor design, involving the direct thermophoretic deposition of SnO2 aerosol streams on one side and densely compacted SnO2 thick films via ultrasonic spray pyrolysis (UPS) on the other side, acting as a heater. Analyzing flame-made SnO2 particles using Brunauer–Emmett–Teller (BET), XRD, and transmission electron microscopy (TEM) techniques revealed highly crystalline particles approximately 8 nm in size. Methyl mercaptan (CH3SH) and H2S were employed as analyte gases, ranging from 20 ppb to 25 ppm and from 20 ppb to 50 ppm, respectively. The results indicate that the flame-made SnO2 exhibits significant potential for developing gas sensors that are highly sensitive to CH3SH and H2S gases across a broad concentration range. The sensor demonstrates a linear increase in response at lower concentrations, saturating at concentrations exceeding 20 ppm. Consequently, highly sensitive gas sensors capable of detecting very low levels can be manufactured, suitable for machine learning applications in environmental monitoring, healthcare, and industrial safety.

Ultra-sensitive H2S and CH3SH Sensors Based on SnO2 Porous Structures Utilizing Combination of Flame and Ultrasonic Spray Pyrolysis Methods

We demonstrated highly stable and reversible H2 sensors using a dual electron-conduction channel comprising Pd-coated SnO2 nanorods (NRs) and a parallel conduction channel containing Cr/native oxide/p-type Si interface. The dual-channel sensor exhibited highly reversible resistance changes during H2 in-and-out cycles at operating temperatures of 25 °C− 100 °C, while typical single-channel sensors with SnO2 NRs fabricated on SiO2 (300 nm)/Si substrates showed irreversible response because of insufficient recovery from Pd-Hx to Pd-Ox. In particular, the sensor showed significantly high repeatability, long-term stability, and selectivity for H2 sensing at 80 °C. Additionally, the time taken to detect H2 above 1% was less than 2 s and the response to H2 above 500 ppm was unaffected by high humidity. These results indicate that the dual-channel Pd-coated SnO2 NR sensor is suitable for applications such as hydrogen leak detectors that require high speed and precision. Moreover, it was found that the sensor could detect H2 below 0.5 ppm even at low temperatures. This work provides a pathway for improving gas-sensing performance by interface engineering, enabling various practical hydrogen sensor applications. 

Highly Stable and Reversible Hydrogen Sensors Using Pd-coated SnO2 Nanorods and an Electrode–Substrate Interface as a Parallel Conduction Channel

• Surprising room-temperature CO sensing capacity is first revealed for Pt-SnO2 composite ceramics. • Room-temperature CO sensing capacity degrades rather quickly for most Pt-SnO2 ceramics. • Pt-SnO 2 ceramics of 10 wt% Pt show an unusually stable room-temperature CO sensing capacity. • Room-temperature CO sensing capacity of aged Pt-SnO 2 ceramics is fully restored through a heat-treatment at 200 °C for 10 min. Pt-SnO 2 composite nanoceramics with 1, 2, 5, and 10 wt% Pt separately were prepared through pressing and sintering. All samples showed strong and fast response to CO at room temperature. The responses of samples of 1, 2, 5 wt% Pt decreased very quickly with time, while that of samples of 10 wt% Pt was much more stable. Brief heat-treatments had been tested and the CO-sensing capability of a 384 h aged sample of 10 wt% Pt was fully restored to that as-sintered through a heat-treatment of 200 °C for 10 min. It is proposed that the deposition of impurity gases in air results in the observed aging and brief heat treatments are able to remove them. Two closely related strategies, namely a suitable Pt content and a periodic heat-treatment, are suggested for room temperature CO-sensitive Pt-SnO 2 composite nanoceramics to ensure a high long-term stability and a reasonable cost simultaneously. 

Achieving a high long-term stability for room temperature CO-sensitive Pt-SnO2 composite nanoceramics through two strategies

Composite ceramics of metal oxides and noble metals have received much attention for sensing reducing gases at room temperature. Presently, composite ceramics of SnO2 and noble metals have been prepared and investigated for sensing oxidizing NO2 at room temperature. While dramatic increases in resistance were observed for both 1 wt% Pt–SnO2 and 5 wt% Au–SnO2 composite nanoceramics after being exposed to NO2 at room temperature, the largest increase in resistance was observed for 1 wt% Pt–5 wt% –Au–SnO2 composite nanoceramics among the three composites. The response to 0.5 ppm NO2-–20% O2–N2 was as high as 875 at room temperature, with a response time of 2566 s and a recovery time of 450 s in the air of 50% relative humidity (RH). Further investigation revealed that water molecules in the air are essential for recovering the resistance of Pt–Au–SnO2 composite nanoceramics. A room temperature NO2-sensing mechanism has been established, in which NO2 molecules are catalyzed by Pt–Au to be chemisorbed on SnO2 at room temperature, and desorbed from SnO2 by the attraction of water molecules in the air. These results suggest that composite ceramics of metal oxides and noble metals should be promising for room temperature sensing, not only reducing gases, but also oxidizing gases.

/pdf/no2-sensing-capability-of-pt-au-sno2-composite-nanoceramics-2pbn4erv.pdf

NO2 Sensing Capability of Pt–Au–SnO2 Composite Nanoceramics at Room Temperature

Soluble noble metal salts are widely used for loading noble metals as nano-catalysts in many applications. In this paper, Pt-SnO2 composite nanoceramics were prepared from SnO2 nanoparticles and H2PtCl6 using two Pt-loading methods separately: For the solution reduction method, the H2PtCl6 solution was added to a suspension of SnO2 and zinc powder to form Pt on SnO2 nanoparticles; and for the impregnation method, Pt was formed from H2PtCl6 in the course of sintering. Although a series of samples prepared using both Pt loading methods showed a strong response to H2 at room temperature, the ones prepared using the solution reduction method exhibited much better room temperature hydrogen sensing characteristics. For two samples of 0.5 wt% Pt and sintered at 825°C, the response value for the sample prepared using the solution reduction method was 5300 to 1% H2-20% O2-N2, which was much larger than that of 150 for the sample prepared using the impregnation method. Samples prepared using the two Pt loading methods have similar microstructures as characterized by XRD, FESEM, EDS, TEM, and HRTEM. However, the residual chlorine content in those using the impregnation method was found to be higher than that using the solution reduction method through analysis. It is proposed that the striking difference in the room temperature hydrogen sensing characteristics among samples prepared using these two different Pt loading methods separately resulted from their different chlorine removal process. This study demonstrates the importance of a proper method in loading noble metals from their soluble salts as nano-catalysts in many applications. 

Room-Temperature Hydrogen Sensitive Pt-SnO2 Composite Nanoceramics: A Comparison Study on Two Different Pt Loading Methods

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.

/pdf/aging-and-heat-treatment-of-room-temperature-hydrogen-1qb17qtc.pdf

Aging and Heat-treatment of Room-temperature Hydrogen Sensitive Pt-WO3 Composite Nanoceramics

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.

/pdf/aging-behavior-and-heat-treatment-for-room-temperature-co-kmvht6in.pdf

Aging Behavior and Heat Treatment for Room-Temperature CO-Sensitive Pd-SnO2 Composite Nanoceramics

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.

/pdf/influences-of-impurity-gases-in-air-on-room-temperature-84hp63nn.pdf

Influences of Impurity Gases in Air on Room-Temperature Hydrogen-Sensitive Pt–SnO2 Composite Nanoceramics: A Case Study of H2S

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