Polymerization-Induced Aggregation Approach toward Uniform Pd Nanoparticle-Decorated Mesoporous SiO₂/WO₃ Microspheres for Hydrogen Sensing

Abstract

Hydrogen as an important clean energy source with a high energy density has attracted extensive attention in fuel cell vehicles and industrial production. However, considering its flammable and explosive property, gas sensors are desperately desired to efficiently monitor H2 concentration in practical applications. Herein, a facile polymerization-induced aggregation strategy was proposed to synthesize uniform Si-doped mesoporous WO3 (Si-mWO3) microspheres with tunable sizes. The polymerization of the melamine–formaldehyde resin prepolymer (MF prepolymer) in the presence of silicotungstic acid hydrate (abbreviated as H4SiW) leads to uniform MF/H4SiW hybrid microspheres, which can be converted into Si-mWO3 microspheres through a simple thermal decomposition treatment process. In addition, benefiting from the pore confinement effect, monodispersed Pd-decorated Si-mWO3 microspheres (Pd/Si-mWO3) were subsequently synthesized and applied as sensitive materials for the sensing and detection of hydrogen. Owing to the oxygen spillover effect of Pd nanoparticles, Pd/Si-mWO3 enables adsorption of more oxygen anions than pure mWO3. These Pd nanoparticles dispersed on the surface of Si-mWO3 accelerated the dissociation of hydrogen and promoted charge transfer between Pd nanoparticles and WO3 crystal particles, which enhanced the sensing sensitivity toward H2. As a result, the gas sensor based on Pd/Si-mWO3 microspheres exhibited excellent selectivity and sensitivity (R air/R gas = 33.5) to 50 ppm H2 at a relatively low operating temperature (210 °C), which was 30 times higher than that of the pure Si-mWO3 sensor. To develop intelligent sensors, a portable sensor module based on Pd/Si-mWO3 in combination with wireless Bluetooth connection was designed, which achieved real-time monitoring of H2 concentration, opening up the possibility for use as intelligent H2 sensors.

Maillard Reaction Inspired Microexplosion toward Fast Synthesis of Two-Dimensional Mesoporous Tin Oxides for Efficient Chemiresistive Gas Sensing

“…The UV–vis diffuse reflectance spectroscopy (DRS) characterization (Figure S12a) shows that the Pd/mSnO 2 - n sample (e.g., Pd/mSnO 2 -0.5) presents a stronger visible light absorption than that of mSnO 2 . Calculation based on the Tauc/Davis–Mott model (Figure S12b) indicates that the band gap of Pd/mSnO 2 -0.5 (3.35 eV) is slightly lower than that for mSnO 2 (3.47 eV), implying that the surface modification of Pd NPs can efficiently influence the electronic structure of SnO 2 . After Pd modification, the decreased band gap results in an easier electron excitation process, thus leading to greater electron transfer efficiency.…”

“…In the Pd 3d spectrum (Figure S13c), the splitting of the peak reveals that the dominating Pd species are zero-valence states, and the peaks at 335.5 and 340.8 eV can be assigned to Pd 0 3d 5/2 and Pd 0 3d 3/2 , respectively. Meanwhile, the oxidized Pd species also exist, where the peaks at 337.6 and 342.9 eV correspond to Pd 2+ 3d 5/2 and Pd 2+ 3d 3/2 , respectively …”

“…First, the two-dimensional mesoporous nanostructures provide a large number of gas–solid interfaces with easily accessible active sites, thus facilitating gas diffusion. Second, the spillover effect of the exposed Pd NPs on the surface of SnO 2 can induce the formation of extra adsorbed oxygen species, which is beneficial for accelerating the surface oxidation reaction, thus increasing the sensitivity of the sensor . Third, the electron sensitization of Pd can induce the formation of a Schottky barrier, resulting in an increase of baseline resistance and sensing response .…”

“…Meanwhile, the oxidized Pd species also exist, where the peaks at 337.6 and 342.9 eV correspond to Pd 2+ 3d 5/2 and Pd 2+ 3d 3/2 , respectively. 49 Gas Sensing Performance of Pd/mSnO 2 . Acetone (CH 3 COCH 3 ) is identified as a biomarker molecule related to diabetes, and the acetone concentration in the exhaled breath of diabetics is over 1.8 ppm, much higher than healthy people.…”

“…Second, the spillover effect of the exposed Pd NPs on the surface of SnO 2 can induce the formation of extra adsorbed oxygen species, which is beneficial for accelerating the surface oxidation reaction, thus increasing the sensitivity of the sensor. 49 Third, the electron sensitization of Pd can induce the formation of a Schottky barrier, resulting in an increase of baseline resistance and sensing response. 47 Moreover, the p−n heterojunctions between PdO species and SnO 2 alter the band structure (Figure S17) and increase the EDL thickness of SnO 2 , thus accelerating the electron transfer during the sensing process and remarkably enhancing the sensitivity and response speed.…”