Hydrothermal Synthesis of Hierarchical SnO2 Nanostructures for Improved Formaldehyde Gas Sensing

Enhanced sensitivity of Au@Bi2WO6 flower-like materials to formaldehyde

Bi2WO6 flower-like materials (FMs) were prepared by a hydrothermal method, followed by an in-situ reduction method to prepare Au@Bi2WO6 FMs. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy were employed to characterize the samples. It was discovered that the calculated OV content of Au@Bi2WO6 FMs is 25.16% whereas that of Bi2WO6 FMs is 20.81%, offering appropriate active sites for the absorption of gases and thus enhancing outstanding sensing property. Moreover, the detection of volatile and hazardous substances such as formaldehyde, methanol, acetone, benzene, toluene, and xylene was carried out to assess the efficacy of the Au@Bi2WO6 FMs sensors. The optimal operating temperatures for the Bi2WO6 FMs and Au@Bi2WO6 FMs sensors were 290 and 260 °C, respectively. Compared with Au@Bi2WO6 FMs sensor and Bi2WO6 FMs one, the best response of the front was 250 (900)-100 (800) ppm formaldehyde whereas that of the latter was 90 (230). Therefore, Au@ Bi2WO6 FMs have good response and selectivity, which are promising candidates for formaldehyde detection.

Highly Dispersed Pt-Incorporated Mesoporous Fe2O3 for Low-Level Sensing of Formaldehyde Gas

Highly dispersed Pt-incorporated mesoporous Fe₂O₃ (Pt/m-Fe₂O₃) of 4 μm size is prepared through a simple hydrothermal reaction and thermal decomposition procedures. Furthermore, the formaldehyde gas-sensing properties of Pt/m-Fe₂O₃ are investigated. Compared with our previous mesoporous Fe₂O₃-based gas sensors, a gas sensor based on 0.2% Pt/m-Fe₂O₃ shows improved gas response by over 90% in detecting low-level formaldehyde gas at 50 ppb concentration, an enhanced selectivity of formaldehyde gas, and a lower degradation of sensing performance in high-humidity environments. Additionally, the gas sensor exhibits similar properties as the previous sensor, such as operating temperature (275 °C) and long-term stability. The enhancement in formaldehyde gas-sensing performance is attributed to the attractive catalytic chemical sensitization of highly dispersed Pt nanoparticles in the mesoporous Fe₂O₃ microcube architecture.

UV-Enhanced Formaldehyde Sensor Using Hollow In2O3@TiO2 Double-Layer Nanospheres at Room Temperature

Hollow In₂O₃@TiO₂ double-layer nanospheres were prepared via a facile water bath method using the sacrifice template of carbon nanospheres. It is shown that the size of the In₂O₃/TiO₂ nanocomposites is 150-250 nm, the thickness of the In₂O₃ shell is about 10 nm, and the thickness of the TiO₂ shell is about 15 nm. The sensing performances of the synthesized In₂O₃/TiO₂ nanocomposites-based chemiresistive-type sensor to formaldehyde (HCHO) gas under UV light activation at room temperature have been studied. Compared to the pure In₂O₃- and pure TiO₂-based sensors, the In₂O₃/TiO₂ nanocomposite sensor exhibits much better sensing performances to formaldehyde. The response of the In₂O₃/TiO₂ nanocomposite-based sensor to 1 ppm formaldehyde is about 3.8, and the response time and recovery time are 28 and 50 s, respectively. The detectable formaldehyde concentration can reach as low as 0.06 ppm. The role of the formed In₂O₃/TiO₂ heterojunctions and the involved chemical reactions activated by UV light have been investigated by AC impedance spectroscopy and the in situ diffuse reflectance Fourier transform infrared spectroscopy. The improvement of the sensing properties of In₂O₃/TiO₂ nanocomposites could be attributed to the nanoheterojunctions between the two components and the "combined photocatalytic effects" of UV-light-emitting diode irradiation. Density functional theory calculations demonstrated that introducing heterojunctions could improve the adsorption energy and charge transfer between formaldehyde and sensing materials.

Synthesis of ZIF-8 Coating on ZnO Nanorods for Enhanced Gas-Sensing Performance

Firstly, ZnO nanorods were prepared by a relatively simple method, and then self-sacrificed by a water bath heating method to generate a commonly used porous ZIF-8 and firmly attached to the ZnO surface. The successful synthesis of synthetic composites was demonstrated with various detection methods. The gas-sensing results show that the ZIF-8-coated ZnO with a core-shell structure exhibits better response than the raw ZnO because of the increased specific surface area and active sites.

Green Synthesis of Metal Oxides Semiconductors for Gas Sensing Applications

During recent decades, metal oxide semiconductors (MOS) have sparked more attention in various applications and industries due to their excellent sensing characteristics, thermal stability, abundance, and ease of synthesis. They are reliable and accurate for measuring and monitoring environmentally important toxic gases, such as NO₂, NO, N₂O, H₂S, CO, NH₃, CH₄, SO₂, and CO₂. Compared to other sensing technologies, MOS sensors are lightweight, relatively inexpensive, robust, and have high material sensitivity with fast response times. Green nanotechnology is a developing branch of nanotechnology and aims to decrease the negative effects of the production and application of nanomaterials. For this purpose, organic solvents and chemical reagents are not used to prepare metal nanoparticles. On the contrary, the synthesis of metal or metal oxide nanoparticles is done by microorganisms, either from plant extracts or fungi, yeast, algae, and bacteria. Thus, this review aims at illustrating the possible green synthesis of different metal oxides such as ZnO, TiO₂, CeO₂, SnO₂, In₂O₃, CuO, NiO, WO_3, and Fe₃O₄, as well as metallic nanoparticles doping.