Quantitative Structure–Activity Relationship of Nanowire Adsorption to SO₂ Revealed by In Situ TEM Technique

Abstract

A quantitative structure−activity relationship (QSAR) is revealed based on the real-time sulfurization processes of ZnO nanowires observed via gas-cell in situ transmission electron microscopy (in situ TEM). According to the in situ TEM observations, the ZnO nanowires with a diameter of 100 nm (ZnO-100 nm) gradually transform into a core−shell nanostructure under SO 2 atmosphere, and the shell formation kinetics are quantitatively determined. However, only sparse nanoparticles can be observed on the surface of the ZnO-500 nm sample, which implies a weak solid− gas interaction between SO 2 and ZnO-500 nm. The QSAR model is verified with heat of adsorption (−ΔH°) and aberration-corrected TEM characterization. With the guidance of the QSAR model, the following adsorbing/sensing applications of ZnO nanomaterials are explored: (i) breakthrough experiment demonstrates the application potential of the ZnO-100 nm sample for SO 2 capture/storage; (ii) the ZnO-500 nm sample features good reversibility (RSD = 1.5%, n = 3) for SO 2 sensing, and the detection limit reaches 70 ppb.

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“…In another study, Wang et al [ 36 ] demonstrated a quantitative analysis of kinetics of solid–gas reactions between ZnO nanowires and SO 2 gas molecules using in situ TEM technique ( Figure 2 c) and recorded the real-time morphological evolution of the ZnO nanowires during SO 2 gas exposure. When the experiment was carried out at room temperature, no obvious morphology change was observed, and the morphology change only occurred when the temperature was increased to 70 °C [ 36 ].…”

“…In another study, Wang et al [ 36 ] demonstrated a quantitative analysis of kinetics of solid–gas reactions between ZnO nanowires and SO 2 gas molecules using in situ TEM technique ( Figure 2 c) and recorded the real-time morphological evolution of the ZnO nanowires during SO 2 gas exposure. When the experiment was carried out at room temperature, no obvious morphology change was observed, and the morphology change only occurred when the temperature was increased to 70 °C [ 36 ]. As shown in Figure 2 d–f, a homogenous shell gradually formed on the outer layer of ZnO-100 nm nanowire due to the diffusion of SO 2 molecules through the ZnO surface where the shell thickness increased from 0 to 16 nm by increasing the exposure time from 0 to 258 s. This is attributed to the irreversible reaction between SO 2 gas molecules and ZnO surface where atoms on the nanowires surface were gradually consumed by SO 2 to form a zinc sulfite hydrate (ZnSO 3 .2.5H 2 O) shell [ 36 ].…”

“…When the experiment was carried out at room temperature, no obvious morphology change was observed, and the morphology change only occurred when the temperature was increased to 70 °C [ 36 ]. As shown in Figure 2 d–f, a homogenous shell gradually formed on the outer layer of ZnO-100 nm nanowire due to the diffusion of SO 2 molecules through the ZnO surface where the shell thickness increased from 0 to 16 nm by increasing the exposure time from 0 to 258 s. This is attributed to the irreversible reaction between SO 2 gas molecules and ZnO surface where atoms on the nanowires surface were gradually consumed by SO 2 to form a zinc sulfite hydrate (ZnSO 3 .2.5H 2 O) shell [ 36 ]. Further increase in the exposure time up to 516 s led to the formation of a dense shell layer (21 nm) around the ZnO surface ( Figure 2 f), resulting in the slower reaction rate between SO 2 gas molecules and atoms on the ZnO surface.…”

“…However, the sensing material never recovered to the baseline ( Figure 2 g, blue line). In fact, the high reactivity of the ZnO-100 nm sample towards SO 2 gas is due to the poisoning effect of SO 2 on ZnO surface, and consequently, irreversible solid-gas reaction between thin nanowires and gas molecules [ 36 ]. In contrast, a reversible sensing response to SO 2 was observed for the ZnO-500 nm nanowires ( Figure 2 g, pink line), indicating the important role nanostructures play in solid–gas reaction kinetics where ZnO-500 nm is a suitable sample as a reversable SO 2 sensing material while ( Figure 2 h) ZnO-100 nm could be used in SO 2 gas capturing applications.…”

“…The experimentally observed LOD towards detecting SO 2 gas molecules was lower than 2 ppm for ZnO-500 nm nanowires. A LOD of 70 ppb was reported using the empirical method and a signal-to-noise ratio of 3 [ 36 ].…”