Heteroatom-doped photoreduced graphene oxide photocatalysts for removal of volatile organic compounds / Tai Xin Hong

Tai , Xin Hong (2022) Heteroatom-doped photoreduced graphene oxide photocatalysts for removal of volatile organic compounds / Tai Xin Hong. PhD thesis, Universiti Malaya.

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Abstract

Indoor air quality (IAQ) has become a great concern as people today spend most of their time indoors, particularly during the Covid-19 pandemic era. Indoor air pollutants such as volatile organic compounds (VOCs) significantly deteriorate air quality and endanger human health. Photocatalytic oxidation (PCO) is a promising method for air remediation because of its ability to operate under ambient conditions to degrade and mineralise VOCs into harmless compounds such as carbon dioxide (CO2). Recently, the use of metal-free photocatalyst has emerged as a cost-effective, sustainable, lightweight, and earth abundance approach for various photocatalytic applications. Graphene oxide (GO) is a promising metal-free carbon-based photocatalyst that showed photocatalytic activities for dye degradation, water splitting, and CO2 reduction. Nevertheless, the application of GO and heteroatom (e.g, boron (B), nitrogen (N), and fluorine (F)) doped GO in the PCO of air pollutants have not been explored yet. There are two common strategies to enhance the photoactivity and properties of GO, namely reduction and heteroatom-doping. Therefore, an effective, green, and scalable method to simultaneously reduce and dope GO was developed. In this study, GO was transformed into photoreduced graphene oxide (PRGO) and doped with heteroatom via a facile photoirradiation technique. The photoactivity of the PRGO was 2.4 times better than GO due to the narrower band gap and slower charge carrier recombination rate. Besides that, GO was simultaneously reduced and doped via photoirradiation with B, N, and F heteroatoms as BPRGO, NPRGO, and FPRGO, respectively. The VOCs photodegradation efficiencies of the photocatalysts followed the pseudo-first-order kinetic (k) as according to this sequence: NPRGO (100%, k = 0.38 h-1) > FPRGO (80.4%, k = 0.26 h-1) > BPRGO (67.7%, k = 0.19 h-1) > PRGO (27.0%, k = 0.06 h-1) > GO (13.7%, k = 0.03 h-1). Additionally, VOCs were successfully mineralised into CO2 by the heteroatom-doped PRGO photocatalysts with mineralisation efficiency up to 100% for NPRGO-0.5. The improved photocatalytic activities of the heteroatoms-doped PRGO were attributed to their increment of charge carrier densities after doping, which resulted in slower charge carrier recombination rates. Among the heteroatom-doped PRGO, the NPRGO had the slowest charge carrier recombination rate because of its n-type conductivity. As an n-type semiconductor, the N dopants formed a shallow donor level near the conduction band of NPRGO. During photocatalysis, if a photoexcited electron falls from the conduction band of NPRGO, the shallow dopant level could trap the electron and reemit it back to the conduction band easily, therefore preventing the electron from recombining with a hole at the valence band. Moreover, the NPRGO could be reused for five PCO cycles without any significant loss in photoactivity. Through mechanism studies, it was determined that the photogenerated hole was the most significant reactive species in the PCO processes. This study provides new insights into a scalable photoirradiation method for producing effective metal-free PRGO-based photocatalysts for air purification.

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