NH 3 sensing property and mechanisms of quartz surface acoustic wave sensors deposited with SiO 2 , TiO 2 , and SiO 2 -TiO 2 composite films

Pristine SiO2, TiO2 and composite SiO2-TiO2 films of 200 nm thick were coated on surface of quartz acoustic wave (SAW) sensors with sol-gel and spin coating technique. Their performance and mechanisms for sensing NH3 were systematically investigated. Sensors made with the TiO2 and SiO2-TiO2 films showed positive frequency shifts, whereas SiO2 film exhibits a negative frequency shift to NH3 gas. it is believed that the negative frequency shift was mainly caused by the increase of NH3 mass loading on the sensitive film while the positive frequency shift was associated to the condensation of the hydroxyl groups (-OH) on the film making the film stiffer and lighter, when exposed to NH3 gas. It demonstrated that humidity played a significant factor on the sensing performance. Comparative studies exhibited that the sensor based on the composite SiO2-TiO2 film had a much better sensitivity to NH3 at a low concentration level (1 ppm) with a response of 2 KHz, and also showed fast response and recovery, excellent selectivity, stability and reproducibility

NH3-Sensing Mechanism Using Surface Acoustic Wave Sensor with AlO(OH) Film

In this study, AlO(OH) (boehmite) film was deposited onto a surface acoustic wave (SAW) resonator using a combined sol-gel and spin-coating technology, and prepared and used as a sensitive layer for a high-performance ammonia sensor. The prepared AlO(OH) film has a mesoporous structure and a good affinity to NH3 (ammonia gas) molecules, and thus can selectively adsorb and react with NH3. When exposed to ammonia gases, the SAW sensor shows an initial positive response of the frequency shift, and then a slight decrease of the frequency responses. The sensing mechanism of the NH3 sensor is based on the competition between mass-loading and elastic-loading effects. The sensor operated at room temperature shows a positive response of 1540 Hz to 10 ppm NH3, with excellent sensitivity, selectivity and stability

Real-time monitoring of airborne molecular contamination on antireflection silica coatings using surface acoustic wave technology

Real time monitoring of contamination on antireflection (AR) silica coatings in high peak power laser systems (HPLs) is critically needed in order to avoid reductions of transmission and laser damage to optical surfaces. Herein we proposed to apply a surface acoustic wave (SAW) sensor to real-time monitor trace amounts of airborne molecular contaminants (AMCs) adsorbed on the AR silica coatings. The silica coating is found to be susceptible to AMCs because of its mesoporous structure, huge surface area and polar nature. The adsorbed AMCs caused the increased mass on the silica coating of the SAW sensor, which resulted in a significant increase of its frequency shift. The fabricated sensor showed a high sensitivity of ∼-490 mm2 ng−1Hz and an excellent linearity vs. the areal density of adsorbed AMCs since the frequency shift of the sensor is linearly related to the change of mass of the silica coating

Heterostructured NiO/ZnO nanorod arrays with significantly enhanced H2S sensing performance

H2S gas sensors were fabricated using p-n heterojunctions of NiO/ZnO, in which the ZnO nanorod arrays were wrapped with nickel oxide (NiO) nanosheets via a hydrothermal synthesis method. When the H2S gas molecules were adsorbed and then oxidized on the surfaces ZnO, the free electrons will be released. The increase of electron concentration on the ZnO boosts the transport speeds of electrons on both sides of the NiO/ZnO p-n junction, which significantly improved the sensing performance and selectivity for H2S detection, if compared to those using the pure ZnO nanorod arrays. The response toward 20 ppm of H2S was 21.3 at 160 oC for the heterostructured NiO/ZnO sensor, while the limit of detection was 0.1 ppm. Additionally, it was found that when the sensor was exposed to H2S at an operating temperature below 160 oC, the resistance of sensor was significantly decreased, indicating its n-type semiconductor nature, whereas when the operating temperature was above 160 oC, the resistance was significantly increased, indicating its p-type semiconductor nature. Finally, the sensing mechanism of the NiO/ZnO heterostructured H2S gas sensor was discussed in detail

High humidity enhanced surface acoustic wave (SAW) H2S sensors based on sol–gel CuO films

Surface acoustic wave (SAW) sensors based on sol–gel processed porous and nanoparticulate CuO films were explored for H2S detection operated at room temperature. The SAW sensor showed a negative frequency shift when exposed to H2S gas, due to the mass loading effect on the sensitive CuO film. It has a high sensitivity of H2S, a detection limit of 500 ppb and a good selectivity to H2S gas compared with H2, C2H5OH, acetone, NH3, CO, NO, NO2 gases. Relative humidity (RH) was identified to have a critical influence on the sensing performance of the sensor. The responses became much faster and stronger with the increase of RH. The significant enhancement in the sensing performance is attributed to that the chemical reaction between H2S and CuO is hindered and the physical adsorption of CuO to H2S is enhanced

Bi-Metal Phosphide NiCoP: An Enhanced Catalyst for the Reduction of 4-Nitrophenol

Porous phosphide NixCoyP composite nanomaterials are successfully synthesized at different Ni/Co ratios (=0, 0.5, 1, and 2) to reduce 4-nitrophenol. The X-ray diffraction and X-ray photoelectron spectroscopy results demonstrate that the products are CoP, NiCoP/CoP, NiCoP, and NiCoP/Ni2P when the Ni/Co ratio is 0, 0.5, 1, and 2, respectively. The products exhibit different catalytic performance for reduction of 4-nitrophenol at room temperature. Among them, the pure NiCoP delivers a better catalytic efficiency with k app = 677.4 × 10 − 2   min − 1 and k = 338.7   ( Lg − 1 min − 1 ) , due to the synergy between Ni and Co atoms. The sequence of catalytic efficiency of different samples is CoP < NiCoP/CoP < NiCoP/Ni2P < NiCoP

Cellulose nano-crystals as a sensitive and selective layer for high performance surface acoustic wave HCl gas sensors

We report that cellulose nano-crystals (CNCs) can be used as a sensitive and selective layer in surface acoustic wave (SAW) sensors for in-situ HCl gas detection. CNCs were prepared through directly hydrolysis of cotton fiber and were spin-coated on quartz SAW resonators to form the sensitive layer. The CNCs have been identified to have abundant hydroxyl groups on their surfaces, which can act as the perfect adsorption sites for H2O, which can further act as the active sites for HCl gas adsorption. The absorption of HCl on the CNCs layer, thus leads to an increase of its mass, causing negative responses of the SAW sensors. Ambient humidity and thickness of CNCs layer are found to have significant influences on the responses of the SAW sensor. With an 80-nm-thick CNCs layer, the sensor shows a response of −2 kHz to 1 ppm HCl at 25 °C and relative humidity of 50% with an excellent selectivity and recovery characteristics

A fast response & recovery H2S gas sensor based on α-Fe2O3 nanoparticles with ppb level detection limit

H2S gas sensor based on α-Fe2O3 nanoparticles was fabricated by post-thermal annealing of Fe3O4 precursor which was synthesized using a facile hydrothermal route. The characteristic techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were adopted to characterize the chemical composition and microstructure of the obtained samples. Gas-sensing performance of the sensor was investigated at different operation temperatures from 100 °C to 400 °C. Results showed that the sensor exhibited the best sensitivity, reproducibility and long-term stability for detecting H2S gas at an operating temperature of 300 °C. The detection limit towards H2S gas was 0.05 ppm, and the response time and recovery time was 30 s and 5 s, respectively. In addition, sensing mechanism of the sensor towards H2S was discussed