Improving the Gas Detection Abilities of ZIF-8 Coated ZnO through Titanium Metal Doping
Authors: Nuftolla Aidarbek , Soltabayev Baktiyar , Mentbayeva Almagul 
Published: December 2024

Worldwide atmospheric pollution is one of the biggest and deadliest issues as the effect of prolonged exposure to gases such as nitrogen oxides (NOx), carbon dioxide (CO2), carbon monoxide (CO) and volatile organic compounds (VOC) cause severe adverse effects towards environmental and human well-being [1]. Significant research efforts were made for the development of gas sensors that can precisely detect the amount of toxic gases present in air. Metal oxide semiconductors (MOS) are being intensively developed due to their inherent properties like high sensitivity to toxic gases, ability to be incorporated into small, miniaturized devices capable of high mobility and ease of usage [2]. ZnO is at the forefront of MOS based gas sensors, becoming increasingly popular over the years for several suitable properties like wide band gas energy, high binding energy and high electron mobility [3]. Regardless, further improvement through the addition of doping elements and the creation of a heterostructure is necessary. This study investigates the effects of titanium (Ti) element doping of ZnO and the encapsulation strategy by zeolitic imidazolate framework-8 (ZIF-8). ZIFs are a part of metal organic frameworks (MOFs) that have attracted considerable attention due to their supremely porous structure that is quite suitable for gas sensing purposes [4]. It was reported that the encapsulation of ZnO core by ZIF-8 increases the selectivity via the molecular sieving mechanism common for MOFs, meaning that the gas particles smaller than the aperture sizes of the ZIF-8 shell may pass through to the ZnO core, while the same isn’t viable for bigger particles. This study focuses on the combination of both doping by Ti element and the encapsulation by ZIF-8 shell for the synergistic effect on base ZnO material for gas sensing applications that has not been reported yet.
Initially, zinc nitrate hexahydrate was chosen as zinc source, while titanium isopropoxide was added for the doping purpose. Both the doping and encapsulation synthesis processes took place in a controlled temperature and pressure environment in a Teflon-lined stainless-steel autoclave, further aiding the formation of crystalline nanoparticles. Figure 1 shows the XRD, FTIR, and TEM characterization results for the synthesized products. The difference in FTIR spectra between the ZnO@ZIF-8 and doped/undoped ZnO samples is the presence of peaks characteristic to ZIF-8 such as 1146, 1311, 992, 759, 692 cm-1. Peak 425 cm-1 corresponds to Zn-N stretching vibration band and indicates the presence of nitrogen characteristic to methylimidazole ligands present in ZIF-8. The peaks in range 400-500 cm-1 are jointly attributable to Ti-O and Zn-O vibrational stretching modes, while weak peaks from 1100 to 1600 are due to OH bending vibrational mode that is likely to be present due to the precursor nature of the doping element.
The structural modification of ZnO is evident from the XRD graph in Fig. 1a. The peaks (100), (002), (101), (102), (110), (103), (112) correspond to hexagonal ZnO structure. Ti element has a smaller ionic radius that Zn and therefore the substitution to Zn site leads to a decrease in the size of the crystals, from 39.68 to 33.61 nm, and the increase in stress and strain, 6.356 * 10-6 Å-2 to 8.862 * 10-6 Å-2 and 2.93 * 10-3 to 3.44*10-3, respectively, as compared to before. Moreover, the shift in the angle of diffraction greatly hints at the optimized structure. Peaks (110), (200), (211), (220), (310), (222) are attributable to ZIF-8 that constitute the shell and is seen from the TEM image presented in Fig. 1d. The reported results are expected to alter the base sensitivity and selectivity of ZnO sensing material towards NO2.
Gas sensing performance and further characterization of prepared Ti-doped ZnO@ZIF-8 sensing material will be presented at the conference.

References

[1] Q. Zhou, W. Chen, L. Xu, R. Kumar, Y. Gui, and Z. Zhao, “Highly sensitive carbon monoxide ( CO ) gas sensors based on Ni and Zn doped SnO 2 nanomaterials,” Ceram. Int., vol. 44, no. 4, pp. 4392–4399, 2018.
[2] B. Soltabayev et al., “Quick NO Gas Sensing by Ti-Doped Flower-Rod-like ZnO Structures Synthesized by the SILAR Method,” ACS Appl. Mater. Interfaces, vol. 14, no. 36, pp. 41555–41570, 2022.
[3] J. Xu, J. Han, Y. Zhang, and B. Xie, “Studies on alcohol sensing mechanism of ZnO based gas sensors,” vol. 132, pp. 334–339, 2008.
[4] B. Yu, F. Wang, W. Dong, J. Hou, P. Lu, and J. Gong, “Self-template synthesis of core – shell ZnO @ ZIF-8 nanospheres and the photocatalysis under UV irradiation,” vol. 156, pp. 50–53, 2015.

Figure 1