Facultad de Física

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Browsing Facultad de Física by Subject "13 Climate action"

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    Study on the properties of CO2 adsorption and CO detection by materials based on modified graphene oxide and copper oxide composites
    (2025) Roble Albeal, Martín Cristián; Díaz, Donovan; Pontificia Universidad Católica de Chile. Instituto de Física
    This doctoral thesis focused in two related topics, carbon dioxide (CO2) adsorption and carbon monoxide (CO) detection. Both research areas shared a common material, graphene oxide (GO). For CO2 adsorption, chemical modification of GO, which produced modified GO (mGO), was done using a variety of chemical compounds. For CO detection, composite materials based on hydrothermally-produced copper oxide (CuO) and GO were studied. GO was synthesized via Tour’s method and modified using ammonia (mGO-N1), hydrazine (mGO-N2), sodium sulfide (mGO-S1), sodium sulfate (mGO-S2), and pure water (mGO-H), using a condensation reflux system and under the same reaction temperature and time. Modifications aimed to introduce heteroatoms (N, S) and alter the oxygen functional groups inGO. The obtained materials were characterized by a variety of techniques, and their CO2 adsorption capacity measured using a quartz crystal microbalance (QCMB) setup. As determined from UV-Visible absorption and X-ray Photoelectron spectroscopies, the modified GO presented shifts in their UV-Vis absorption peak (π→π* transition going from 228 nm in GO to 277 nm in mGO-N2), and nitrogen doping (4.5 at% for mGO-N1), while S-doping was ineffective (only 0.4% for mGO-S2). Raman analysis suggested that vacancy-type defects could dominate over other vacancy types, but the significance of this result was limited by the low crystallinity of the materials. Scanning electron Microscopy (SEM) revealed significant differences in the conformation of the mGO films. From QCMB measurements, it was found that mGO-N1 achieved the highest CO₂ adsorption capacity (~1.2 wt% at 45 torr), attributed mostly to N-dopants acting as Lewis base sites. On the contrary, mGO-H exhibited the lowest capacity due to its lack of adsorption sites (N-doping, oxygen functional groups) and visible diffusion channels. Furthermore, it was observed that film thickness in GO influenced kinetics, with medium thicknessoptimizing CO2 adsorption capacity in the adsorption timeframes studied. Selectivity tests confirmed mGO-N1’s superior CO₂/CO discrimination, over GO and mGO-H. For CO detection, CuO microplates with estimated lengths and thicknesses of around 2-3mm and 35 nm, respectively, were hydrothermally synthesized with/without addition of GO. Additionally, composites were also prepared by physically mixing CuO with GO, followed by thermal reduction at 150°C and 250°C. The obtained materials were deposited on gold/platinum interdigitated electrodes, and placed inside a testing chamber where their response towards CO was studied under flow-regime.SEM confirmed CuO’s planar morphology, while XRD identified tenorite as the single phase in CuO. For CuO-rGO composites, SEM identified the presence of both phases in composites with high content of GO, but identification of this phase was limited for the composites with low GO content. Similarly, CuO-rGO composite analyzed by XRD lacked a diffraction peak attributable to GO/rGO, due to high dispersion of the GO sheets and low mass content. However, Raman spectroscopy and XPS confirmed the effective formation of a CuO-rGO composite. In particular, XPS analysis revealed a higher reduction at 250°C (90% unoxidized carbon vs. 60% at150°C), as expected. On CO detection, it was found that all the tested materials (CuO, hydrothermal and physically-mixed composites, and rGO) exhibited p-type response towards CO. However, pure CuO sensors outperformed CuO-rGO composites in the studied temperature range, showing a comparable or higher response towards 150 ppm of CO. The optimal response of CuO sensor was found at ~165°C (S = 1.38 for 150 ppm CO, where S = RCO / Rair is the ration between the sensor resistances under CO exposure and pure air, respectively). Physically mixed CuO-GO outperformed thermally reduced CuO-rGO composites, but their unstable response difficult reallife applications. Instability refers to significant dispersion in the response at fixed temperatures of 45 and 55ºC, of 20 and 10%, respectively. Post-thermal reduction (150°C) enhanced stability but reduced sensitivity. It was found that rGO sensor showed negligible recovery at lowtemperatures, requiring thermal "reset" steps. When compared with the literature, the observed superior performance of CuO over CuOrGO composites seems to be against the expected outcomes. It is hypothesized that the planar morphology of the CuO microstructures studied play a role in the response decrease of the composites. Interestingly, the high-yet-unstable responses of unreduced CuO-GO composites suggest that GO/rGO could play a role beyond electrical modulation through heterojunction formation between p-p semiconductors, owing to the insulating nature of GO.