A recent article in Nature Communications examined the role of van der Waals interactions between hydrophilic ZnO surfaces and hydrophobic aliphatic alkyl chains in mitigating catalyst deactivation during aliphatic aldehyde sensing.
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Background
Surface functionalization of nanostructured metal oxides with catalysts is a promising and versatile method across various chemical research fields. It is extensively employed in catalysis-based electrical sensing for several volatile organic compounds (VOCs) on nanostructured metal oxide surfaces.
Among various surface functionalization methods for metal oxides, organic molecular modification is one of the simplest methods, exploiting intermolecular interactions and affinity between surface-decorated molecules and target analytes. However, this type of modification diverges from traditional catalysis-based mechanisms, as it reduces catalytic density on metal oxide surfaces, a phenomenon known as catalyst deactivation.
Catalyst deactivation is a common issue in catalysis-based electrical sensing of VOCs via catalytic processes on metal oxides, which can significantly impair catalytic performance. Current methods to counteract surface deactivation, such as ultraviolet radiation or high-temperature annealing, are harsh and unsuitable for catalyst surfaces modified with organic materials.
Methods
Single-crystalline ZnO nanowires were grown hydrothermally on a ZnO seed layer/SiO2/p-Si substrate. The resulting ZnO nanowire array was annealed for one hour at 600 °C to prevent surface electrical insulation. Structural characterization was performed using field emission-scanning electron microscopy (FE-SEM).
Solutions of 0.05 mM octadecylphosphonic acid (ODPA), methylphosphonic acid (MPA), or hexadecyl phosphonic acid (D-HDPA) were prepared using toluene (ODPA and D-HDPA) or methanol (MPA) solvent to form a uniform self-assembled monolayer (SAM) over ZnO sample.
The sensing device was fabricated using the hydrothermally grown ZnO nanowires dispersed in acetone and dropped onto a 100-nm SiO2/n+-type Si substrate with Ti-Pt pad electrodes. Target aldehydes were tested in a chamber of a temperature-controlled probe station with purified air as the carrier gas (N2:O2 = 4:1) at different temperatures (150, 200, 250, 300, and 350 °C). The desorbed gas at each temperature was then analyzed using gas chromatography-mass spectrometry (GC-MS).
Molecular adsorption on ZnO nanowires was achieved by exposing the substrate to saturated nonanal vapor (a biomarker in human breath) for 10 minutes at 50 °C. These nonanal-adsorbed ZnO nanowires were then used for further measurements.
Fourier-transform infrared (FTIR) and p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS) spectra of surface molecules on the ZnO nanowires were obtained at room temperature using an FTIR spectrometer. The ZnO nanowire devices were also electrically characterized with a semiconductor analyzer. Finally, density functional theory (DFT) calculations were conducted to investigate the sensing mechanism.
Results and Discussion
The resistance of both bare and ODPA-modified ZnO nanowire sensors decreased consistently upon exposure to nonanal vapor, confirming the ability of the ZnO nanowires to detect nonanal molecules electrically. However, the recovery process varied significantly between the two types of sensors when air was reintroduced into the test chamber.
The resistance of the bare sensor did not fully recover with airflow, resulting in a permanent change in resistance. In contrast, the resistance of the ODPA-modified sensors returned completely to its initial value. Additionally, ODPA-modified sensors demonstrated a faster recovery, even at high concentrations of nonanal vapor.
ODPA modifications did not significantly influence the electric transport properties of ZnO nanowires. Thus, differences in sensor resistance under airflow were attributed to structural variations in the ZnO nanowire array. Additionally, improvements in sensor performance due to ODPA modification occurred independently of nanowire structural variations.
FTIR measurements of nonanal on ZnO nanowire samples showed that ODPA modifications suppressed catalyst deactivation by weakening the adsorption states of carboxylates on ZnO surfaces, aligning with experimental sensing results.
DFT-based desorption experiments with MPA-modified ZnO nanowires further highlighted the role of van der Waals interactions between alkyl chains and the ZnO surface in reducing catalyst deactivation. The presence of metal elements with multiple electrons strongly influenced these interactions, which could decrease if light-element molecules, such as water, interpose between the alkyl chains and the metal oxide surface.
Conclusion
The researchers demonstrated that weak van der Waals interactions between hydrophobic aliphatic alkyl chains and hydrophilic ZnO surfaces help inhibit catalyst deactivation during the electrical sensing of aldehydes on ZnO sensor surfaces. These interactions have generally been undervalued in catalytic molecular sensing.
ODPA modification considerably reduced the recovery time of the ZnO nanowire sensor when exposed to nonanal vapor without compromising sensitivity in electrical measurements. The desorption temperature of the carboxylates on ODPA-modified ZnO (<150 °C) was significantly less than that on bare ZnO nanowires (>300 °C), exhibiting mitigated catalyst poisoning.
Thus, engineering strong interactions between alkyl chains and metal oxide nanostructures through organic or inorganic modifications offers a promising approach for developing chemical sensors and heterogeneous catalysts with improved durability and selectivity.
Journal Reference
Nakamura, K., et al. (2024). Van der Waals interactions between nonpolar alkyl chains and polar oxide surfaces prevent catalyst deactivation in aldehyde gas sensing. Nature Communications. DOI: 10.1038/s41467-024-53577-8, https://www.nature.com/articles/s41467-024-53577-8
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