UV light driven high-performance room temperature surface acoustic wave NH3 gas sensor using sulfur-doped g-C3N4 quantum dots

ZnO@Ti3C2Tx MXene Hybrid Composite-Based Schottky-Barrier-Coated SAW Sensor for Effective Detection of Sub-ppb-Level NH3 at Room Temperature under UV Illumination

“…The initial and last terms in eq were pertained to the mass loading and electroacoustic/electrical conductivity effects, leading to the negative Δ f , respectively, while the middle term indicates the elastic loading effect, resulting in positive Δ f . , However, our experimental results displayed a magnificent negative Δ f under NH 3 gas, which can be ascribed to either mass loading or electrical conductivity loading effects, and thus, the elastic loading was terminated from the sensing mechanism. ,, To further explore the causes for high negative Δ f , a thin conductive film (Au ≈ 30 nm) was sputtered on the SAW sensing area prior to the addition of a ZnO@MXene layer (Figure a), because the Au thin-film-coated sensor can only detect the nonelectrical signal changes and eliminates the acoustoelectric interactions due to the shielding effect of conductive film. , According to Figure a, the Au-film-coated SAW sensor exhibited negligible Δ f to NH 3 as compared to uncoated sensor, signifying that the influence of the mass loading effect on Δ f was extremely week, and therefore, it was conferred that the negative Δ f in our results was triggered from the acoustoelectric interaction. , Furthermore, the resistive type ZnO@MXene sensor (Figure b) shows that the resistance of the sensor was reduced (conductivity increased) in the presence of NH 3 (20 ppm) with a high response of ∼39.16% as compared to the coexistence gases (Figure c), indicating the potential sensing capability of the hybrid heterostructure. The increase in sensing layer conductivity of the SAW sensor under NH 3 gas via exchanging the electron transfer leads to the attenuations in acoustic wave frequency, resulting in high negative Δ f . , …”

“…3,4,14 To further explore the causes for high negative Δf, a thin conductive film (Au ≈ 30 nm) was sputtered on the SAW sensing area prior to the addition of a ZnO@MXene layer (Figure 4a), because the Au thin-filmcoated sensor can only detect the nonelectrical signal changes and eliminates the acoustoelectric interactions due to the shielding effect of conductive film. 3,14 According to Figure 4a, the Au-film-coated SAW sensor exhibited negligible Δf to NH 3 as compared to uncoated sensor, signifying that the influence of the mass loading effect on Δf was extremely week, and therefore, it was conferred that the negative Δf in our results was triggered from the acoustoelectric interaction. 3,4 Furthermore, the resistive type ZnO@MXene sensor (Figure 4b) shows that the resistance of the sensor was reduced (conductivity increased) in the presence of NH 3 (20 ppm) with a high response of ∼39.16% as compared to the coexistence gases (Figure 4c), indicating the potential sensing capability of the hybrid heterostructure.…”

“…Despite its numerous advantages, ammonia (NH 3 ) has been extensively utilized in chemical products, fertilizer manufacturing, food processing, and agricultural fields; however, it causes excessive NH 3 emissions into the atmosphere, posing a serious threat to human health and ecological sustainability. − Even a few ppm of NH 3 causes detrimental health hazards including respiratory irritation as well as eye and skin burns. ,− Therefore, to ensure human health and safety, a high-performance gas sensor with remarkable sensitivity and selectivity is crucial, particularly for detecting low-ppm levels of NH 3 at room temperature (RT). − Recently, surface acoustic wave (SAW) sensors, employing diverse sensing layers (heterostructures, metal oxide semiconductors, two-dimensional materials), have emerged as an ideal solution for the selective and sensitive RT NH 3 detection, due to their cost-effectiveness, accuracy, low-power consumption, portability, rapid response, and wireless operations, which establish them as superior alternatives to the conventional sensor devices. ,,,− …”

“…Further, the mechanism behind the outstanding NH 3 sensing performances boosted by the synergistic integration of the ZnO@MXene hybrid heterostructure-based SAW sensor was determined using the following standard equation (eq ). ,, Δ f f 0 = − C m false( f o false) Δ ρ s + 4 f o C e h false( normalΔ G ′ false) − K 2 2 × Δ true[ 1 1 + ( v o 2 C s 2 / σ s 2 ) true] …”

“…As a result, the Schottky barrier height was lowered, and the overall conductivity was enhanced significantly (eq ), which further leads to the outstanding sensing performances by strongly inhibiting the acoustic wave propagation (Figure g). ,,− Furthermore, the photogenerated holes could aid to enhance the neutralization of O 2 – ions, leading to the fast desorption of adsorbed NH 3 molecules, resulting in quick recovery time. ,,,, Moreover, according to the Grotthuss chain reaction at higher RH levels, water molecules dissociate into hydroxyl (OH – ) ions that maintain strong bonding with hydroxy groups of the ZnO@MXene hybrid nanocomposite and hydronium (H 3 O + ) ions, which can then act as a charge carrier to attract a greater number of NH 3 molecules. , The adsorbed NH 3 molecules would further react with the preadsorbed H 2 O molecules and potentially solvates to form NH 4 + and OH – radicals (eq ). ,, Further, the NH 4 + conducts an oxidation reaction with adsorbed O 2 – (eq ) and releases a massive amount of electrons to the CB of the ZnO@MXene heterostructure leading to the high conductivity, consequently resulting in a significant negative Δ f at higher RH values. ,, h ν ↔ h h v + + normale h v − normalO 2 ( gas ) + normale h v − ↔ normalO 2 − ( h ν ) normalO 2 ( gas ) …”