School of Chemistry - Theses

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Author: Tan, Jeannie Ziang Yie × Start date: 2000 × Type: PhD thesis × Subject: TiO2 × Subject: photocatalysis ×

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    Fabrication of PVDF–TiO2 electrospun membranes incorporating with carbon nitride for solar fuel production and organic pollutant photodecomposition
    Tan, Jeannie Ziang Yie ( 2016)
    Semiconductor–mediated photocatalysis for the decomposition of pollutants and production of industrially important species, i.e., methane by photoreduction of CO2 (g) is an emerging technology. However, problems, including low quantum efficiency, visible light inactivity and the difficulty to deploy and recover the photocatalyst, have to be mitigated. In order to enhance the photocatalytic activity of titanium dioxide, the sensitization of TiO2 with visible light active carbon nitrides (CNx) was proposed. Nonetheless, as an important step in the fabrication of a photocatalytic device, the integration of photocatalytic nanoparticles into a solid matrix, such as an electrospun fibrous membrane, forms a research objective in this thesis. A low temperature synthesis route to fabricate TiO2 nanoparticles with different crystal phase compositions was developed. The Ti–precursor concentration (9–45 mM) and the presence of Cl– during hydrothermal treatment influenced the TiO2 crystal phase composition. Overall, anatase–rich TiO2 samples showed higher photocatalytic decomposition activity than rutile–rich samples. However, all samples and a commercial TiO2 reference produced only trace amounts of methane during CO2 photoreduction. A polyvinylidene fluoride (PVDF)–TiO2 nanocomposite was fabricated by electrospinning followed by a low temperature hydrothermal treatment to induce the in situ growth of TiO2 nanoparticles on the electrospun PVDF nanofibres. The crystal phase composition of TiO2 was tuned by manipulating the concentration of the Ti–precursor (0.030–0.125 M) and acidity (pH <0–6.5) in the hydrothermal solution. The surface accessibility, crystal phase composition and the presence of Ti3+ within the nanocomposite significantly influenced the photocatalytic activity for CO2 reduction and organic pollutant decomposition. The maximum production of methane was 19.8 µmol per gram of photocatalyst per hour (quantum efficiency for the photomethanation reaction, Q. E.CH4 : 0.44 %) under UV irradiation. The visible light absorption of the PVDF–TiO2 nanocomposite was enhanced by the addition of CNx. A facile, low temperature wet–chemical synthesis was developed for CNx. The synthesized CNx possessed C=O functional groups that resulted in a negatively charged surface across pH 3–9, and led to an enhanced adsorption capacity and organic pollutant photodegradation under visible light irradiation. CNx also showed a relatively high capacity for heavy metal ion adsorption. Unfortunately, the CNx particles were too large for successful incorporation into the PVDF–TiO2 nanofibres. As an alternative, graphitic–CNx quantum dots (g–CNQDs) were synthesized by microwave heating, and were introduced into the PVDF–TiO2 nanofibres during electrospinning. The g–CNQDs were evenly distributed along the nanofibres, and significantly extended the photoresponse of the nanocomposite into the visible range. Methane production from CO2 photoreduction increased with the amount of g–CNQDs incorporated into the nanocomposite, with a maximum production of 39.8 µmol of methane per gram of photocatalyst per hour (Q.E.CH4 = 0.58%) under simulated sunlight irradiation.