Theoretical and experimental study of the interaction of small molecules with metal oxide surfaces for sensing applications

Stringent gas monitoring of harmful gases associated with industrial activities has received considerable attention lately with the research and development of novel nanostructured semiconducting gas sensors. Zinc oxide (ZnO) nanostructures have shown electrical properties highly suitable for gas sensing, and can be manipulated with specific properties for desired applications; however, the adsorption of different molecules on the non-polar ZnO surfaces remain poorly understood. <i>Ab-initio</i> density functional theory (DFT) and some experimental techniques were used to study the electronic and structural properties of different gases on ZnO surfaces for gas sensing applications.<br><br> The two low index non-polar ZnO surfaces, namely the (1010) and (2110) surfaces were studied as these are commonly found on different ZnO nanostructures. Nitrous oxide (N₂O) and ethanol (CH₃CH₂OH) were adsorbed on each surface and the effect of surface facet, gas coverage, and the presence of surface defects (oxygen vacancies) was investigated. It was found that nitrous oxide physisorbed on both stoichiometric surfaces at 1 ML coverage, with evidence of weak chemisorption on the (2110) surface. Multiple stable structures were found with N₂O adsorbing via the O or the terminal N atom to a surface Zn atom. The calculated charge transfer for each surface indicated that N₂O behaves as a charge acceptor, withdrawing charge from the surface after adsorption, in line with other theoretical and experimental results.<br><br> Ethanol also adsorbed in multiple stable orientations on the stoichiometric and defect surfaces at ¼, ½ and 1 ML coverages, with the binding being stronger than for adsorbed N₂O. Ethanol can form two types of adsorbate-substrate interactions; one being between the ethanol O atom and a surface Zn atom; and the other being a hydrogen bond between the ethanol hydroxyl H atom and a surface O atom. The formation of a hydrogen bond stabilises the adsorbate-substrate interaction, and can be identified by a red shift in the adsorbate OH stretching mode. Due to the reduced number of surface oxygen atoms on the defect surfaces, the binding was weaker as either a hydrogen bond could not form or it was not as strong as on the stoichiometric surfaces. In contrast to nitrous oxide, ethanol was calculated to donate charge to the surface, behaving as a reducing gas.<br><br> Experimentally, several characterisation techniques: Raman, Fourier Transform Infrared (FTIR) Spectroscopy and X-Ray Diffraction (XRD) were used to detect the presence of hydrogen gas (H₂) adsorbed on nanopowdered ZnO surfaces having a high proportion of the low index (1010) surface. We propose that the interaction of hydrogen gas on the ZnO surfaces can lead to the formation of surface hydroxyl groups, as was indicated by the FTIR spectroscopy. The presence of hydrogen was not observed for highly crystalline ZnO nanopowders with larger grain sized, while for smaller grain sized powders, clear peak shifts confirm the presence of hydrogen. <br><br>Our theoretical and experimental findings have shed new light on the design and synthesis of potential nanostructured gas sensors and their interaction with different gases.<br>