Dissertation committee:
Λυκοδήμος Βλάσης, Επίκουρος Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ
Γλένης Σπυρίδων, Επίκουρος Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ
Στεφάνου Νικόλαος, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ
Γαρδέλης Σπυρίδων, Αναπληρωτής Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ
Μαυρόπουλος Φοίβος, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ
Τσακμακίδης Κοσμάς, Επίκουρος Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ
Κόντος Αθανάσιος, Αναπληρωτής Καθηγητής, Σ.Ε.Μ.Φ.Ε, ΕΜΠ
Summary:
Graphene oxide (GO) attracts particular scientific interest because its reduction enables large-scale production of graphene but also because of its unique electronic properties, which make it suitable for integration into a variety of applications, such as photovoltaic devices and advanced photocatalytic materials. GO is composed of graphene sheets with randomly distributed surface oxygen groups, which cause the presence of mixed sp2/sp3 hybridization and a highly heterogeneous electronic structure comprising conductive and insulating regions sp2 and sp3, respectively. Photonic crystal-assisted TiO2 photocatalysis has been attracting significant attention as an advanced photon management approach that combines light harvesting with the macro/mesoporous structured materials properties permitting enhanced mass transport and high adsorption. In this work, surface modification of TiO2 photonic crystals by graphene oxide (GO) nanocolloids was explored as an integrated approach to further improve their performance by combing the advantages of better light harvesting, surface area and transport with the enhanced adsorption capability and charge separation that GO can induce.
In the beginning, the electronic and magnetic properties of graphene oxide before and after its chemical reduction by sodium borohydride (NaBH4) were investigated. Raman spectroscopy, IR spectroscopy and XRD measurements revealed a drastic removal of the functional oxygen groups and a significant growth of the sp2 graphitic domains after the reduction. Static magnetization and EPR spectroscopy indicated the presence of strong paramagnetism in GO along with weak antiferromagnetic interactions at low temperatures, mainly due to the existence of high spin (S=2) magnetic moments, attributed to spatially “isolated” magnetic clusters, stemming from exchange coupled localized spins. The chemical reduction resulted in the decrease of the paramagnetism along with the increase of the diamagnetism and the appearance of a weak Pauli contribution, reflecting the removal of the defects spins and the concomitant recovery of the sp2 areas. In the case of reduced graphene oxide (rGO), EPR measurements, also, revealed the occurrence of two distinct spin systems, a major one possibly originating from of localized defect states strongly coupled with itinerant spins within the sp2 domains and a minor one due to edge/vacancy defects spins weakly coupled with the conduction electrons, indicative of structural inhomogeneity of rGO.
Subsequently, photonic band gap engineered TiO2 inverse opal films were fabricated by the convective evaporation-induced co-assembly of polystyrene colloidal spheres with the titania precursor, leading to well-ordered nanocrystalline photonic films with controlled structural and optical properties, which were surface functionalized by GO nanocolloids (nanoGO). The loading of GO nanosheets was determined by the films’ macropore size, with minimal effects on their long range periodicity and photonic properties according to scanning and transmission electron microscopy, specular and diffuse UV-Vis reflectance, Raman spectroscopy and N2 porosimetry measurements. The photocatalytic performance of the films was evaluated on the aqueous phase degradation of the pollutant methylene blue (MB). Under UV-vis irradiation, the nanoGO deposition led to a marked improvement of the photocatalytic efficiency by enhancing the MB adsorption on the films and reducing the electron-hole recombination of TiO2. The increase of the dye adsorption was corroborated by means of Raman spectroscopy and was ascribed to the electrostatic interactions between the oxygen groups of GO and the pollutant. The stronger charge separation was a result of electron transport from titania to GO and was verified using EPR and photoluminescence spectroscopy. The functionalization of the films, also, led to a significant improvement of the photocatalytic activity under visible light, as the higher MB adsorption enhanced the self-sensitized dye degradation mechanism on titania’s surface. Under both UV-Vis and Vis light, slow photon amplification was identified, when the low energy edge of the inverse opal stop band (in water) approached the MB electronic absorption. Furthermore, the presence of the slow photons was confirmed by the selective response of the Raman signal of MB on the photonic films.
Finally, thermal reduction of GO-titania photonic films at different temperatures (200 and 500 oC) was explored as a means to further enhance the interfacial electron transfer and, thus, improve their photocatalytic performance. It was evidenced that post-reduction did not affect the highly ordered macroporous structure of the photonic films, leaving the photonic stop bands positions intact. Raman, EDX and XPS spectroscopies disclosed, however, that upon thermal trreatment, not only were the sp2 domains partially recovered, but also the amount of nanoGO on the modified films was moderated. Although the losses of oxygen functional groups and the GO nanosheets led to a decrease in the dye adsorption, aqueous phase photodegradation of the MB dye under UV-vis and visible light showed that thermal reduction of the GO-TiO2 photonic films at 200oC, in synergy with slow photon amplification, improved the MB photocatalytic degradation rate, indicative of enhanced charge separation due to the lower work function and higher conductivity of the rGO nanosheets. The intensification of interfacial charge transfer was further supported by both PL spectroscopy and photocatalytic experiments under UV-vis light using salicylic acid as emerging water pollutant. In that case, the SA photocatalytic degradation was drastically increased on the post-treated rGO-TiO2 inverse opals despite the absence of photonic effects.
Keywords:
graphene oxide (GO), reduced graphene oxide (rGO), titanium dioxide (TiO2), photonic crystals, slow photons, photocatalysis
