Development of hybrid metal oxide nanostructures for photocatalytic CO2 conversion and acetone gas sensing

Abstract

Nanomaterials have been investigated in various fields, such as energy catalysts, medical systems, and gas sensors, due to their novel properties differentiated from bulk materials from large surface areas and highly active surface energy. In addition, these properties can be precisely controlled by the size, composition, surface facet, and shape of nanomaterials. In recent years, the design of complex nanostructures is drawing much attention to emphasize the desired properties for application fields. Significantly, the hybrid nanostructures add the properties of distinct materials and show additional synergy effects between them. For instance, charge depletion regions are formed at the p-n junction of two different semiconductors in gas sensing. The enhancement of charge separation with directionality occurs through the Z-scheme mechanism in photocatalysts. In this study, we demonstrate the design strategies of hybrid nanostructures and their applications for photocatalytic CO$_2$ reduction reaction (CO$_2$RR) and gas sensors. In chapter 2, we report ZnO-CuO core-hollow nanocubes for highly sensitive acetone gas sensors. The design principle was that the p-type CuO hollow nanocubes were grown on the n-type ZnO cores to form well-defined p-n heterojunctions. Another advantage was the generation of noble-metal free chemiresistors. The average gas response was measured to be 11.14 at 1 ppm of acetone gas, which was a remarkable sensitivity among noble metal-free sensors for volatile organic compounds (VOCs). The unique and well-defined morphology of ZnO-CuO with ultrasmall grain size, high surface area, and p-n heterojunction contribute to this outstanding gas sensing performance. A large amount of gas analytes adsorbed on the core-shell-like structure makes the charge flow through inter-particle p-p junctions, and the p-n junctions in individual particles enhance the sensitivity. Besides, the small grain sizes of ZnO and CuO domains maximize the charge depletion region. Our strategy of precise design and synthesis would enable low concentration detections with high selectivity for versatile gas species. In chapter 3, we represent g-C$_3$N$_4$-Cu$_2$O and TiO$_2$-Cu$_2$O-Au hybrid nanostructure for photocatalytic CO$_2$ reduction. The Cu$_2$O nanocubes were deposited on graphitic carbon nitride (g-C$_3$N$_4$) nanosheet for photocatalytic CO$_2$ conversion in aqueous media. The production rate was 53 μmol/h (5.3 mmol/gcat∙h) with a selectivity of 98% for CH$_4$. The g-C$_3$N$_4$ thin layer effectively transferred photo-excited electrons to Cu$_2$O, and the defect-less Cu$_2$O nanocubes could selectively convert CO$_2$ into CH$_4$. Also, the synergistic hybrid effect of Cu$_2$O and g-C$_3$N$_4$ could facilitate charge separation through the Z-scheme mechanism. This study proposes a design strategy via a suitable combination of semiconductors with well-defined nanostructures for photocatalytic CO$_2$RR. Then, TiO$_2$-Cu$_2$O-Au hollows were prepared for enhancing the visible-light activity of photocatalytic CO$_2$RR. This catalyst exhibited an enhanced photocatalytic activity under the visible region and better stability than those of the TiO$_2$-Cu$_2$O catalyst. The detailed mechanism needs to be discussed further, but the main contribution of the Au domains enhanced the absorption of the visible light and promoted charge separation, resulting in excellent CO$_2$RR performances.