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Study on the fabrication techniques of nanocomposites based on g-C3N4 and TiO2 and their photocatalytic activity

초록/요약

Nanosized TiO2 for photocatalytic water splitting to produce H2 for the future hydrogen economy has attracted numerous attentions because of its low cost and nontoxicity. To solve its barriers such as the fast recombination of photoexcited electron-hole pairs and the weak response to visible light of TiO2, many attempts have been investigated to make it work under the visible light irradiation. There are many modification techniques that can be used to promote the limitation of TiO2, such as coupling with another visible light responsive material. From this point of view, in this study, g-C3N4/TiO2 composites were obtained by several different fabrication methods. TiO2 nanotubes were synthesized by an alkaline hydrothermal process and g-C3N4 was obtained from melamine as a precursor. The as-prepared catalysts were thoroughly characterized by SEM, TEM, XRD, UV-vis DRS and XPS. The photocatalytic performances of the catalysts were evaluated from the hydrogen evolution in aqueous solution with methanol under a halogen lamp operating at 22 V. We found that the photoactivity of the g-C3N4/TiO2 composites was largely enhanced compared to that of TiO2 nanotubes. The enhanced photoactivity is generally attributed to the cooperative role of g-C3N4 and TiO2 in charge separation as well as enhanced absorption by g-C3N4. In addition, the magnitude of the enhancement is found to be strongly dependent on the detailed preparation methods. The origin of the above results is expected to be due to different interfacial bonding structures between TiO2 nanotubes and g-C3N4. To further enhance the photoactivity by the control of surface defects and surface modification, the modified g-C3N4 nanosheets was obtained via the acid treatment and ultrasonication for exfoliation. Then, the modified g-C3N4/TiO2 nanotubes composites have been successfully fabricated by mixing the as-prepared TiO2 nanotubes and the modified g-C3N4 nanosheets. The photoactivity of the modified g-C3N4/TiO2 nanotubes composites was significantly enhanced compared to that of bulk g-C3N4/TNT composites. The enhanced photoactivity is generally attributed to the modified 2D nature of the g-C3N4 nanosheets. Also, the stronger interfacial bonding with TiO2 can promote the charge transfer between g-C3N4 and TiO2 for an efficient charge separation. Thus, it can be suggested that controlling the interfacial bonding structure in the composite structure can be an efficient way of enhancing photoactivity of the photocatalysts based on the composite structure. The introduction of nitrogen into TiO2 can reduce the bandgap for a visible activity in photocatalysis as well as carbon can enhance visible light absorption and photoactivity. Nevertheless, N- and C-doped TiO2 performances largely depends on the different bonding structure and atoms distribution on the surface and in the bulk. Based on this point of view, surface nitridation for the anatase TiO2 films have been examined in the pure N2, N2-H2 RF afterglows and pure N2, N2-H2, N2-CH4 microwave afterglows. Optical emission spectroscopy (OES) was applied to observe various excited species in the afterglows and XPS was employed to characterize the N species doped into TiO2 films. The results reveal that the control of gas composition in the N2 RF and microwave plasmas can be used to tune the surface nitridation performances for oxide films.

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목차

Chapter 1 Study on the photocatalytic water splitting for H2 evolution over different methods for composing g-C3N4 and TiO2 1
1.1 Introduction 1
1.2 Experimental details 7
1.2.1 Preparation of TiO2 nanotubes (TNT) 7
1.2.2 Preparation of g-C3N4 (gCN) 7
1.2.3 Preparation of g-C3N4/TiO2 (gCN/TiO2) composites 8
1.2.4 Characterization techniques and methods 10
1.2.5 Photocatalytic hydrogen evolution 11
1.3 Results and discussion 12
1.3.1 Analysis of TNT 12
1.3.2 Analysis of gCN 17
1.3.3 Analysis of gCN-on-TNT, TiO2-on-gCN and pm-gCN-TNT composites 18
1.3.4 Measurement of photocatalytic activity for H2 evolution 26
1.4 Conclusion 33
Chapter 2 Study on the photocatalytic water splitting for H2 evolution over modified g-C3N4/TiO2 nanotubes composites 34
2.1 Introduction 34
2.2 Experimental details 37
2.2.1 Preparation of the modified g-C3N4 nanosheets 37
2.2.2 Preparation of the modified g-C3N4/TNT composites 38
2.2.3 Characterization techniques and methods 39
2.2.4 Photocatalytic hydrogen evolution 39
2.3 Results and discussion 40
2.3.1 Analysis of the modified g-C3N4/TNT composite 40
2.3.2 Measurement of photocatalytic activity for H2 evolution 45
2.4 Conclusion 50
Chapter 3 Study on the surface nitridation for TiO2 films in the N2 based RF and microwave afterglows 51
3.1 Introduction 51
3.2 Experimental details 54
3.2.1 Preparation of the samples for N2 based RF afterglows 54
3.2.2 Preparation of the samples for N2 based microwave afterglows 54
3.2.3 Experimental set-up for RF plasmas 54
3.2.4 Experimental set-up for microwave plasmas 56
3.2.5 Sample treatments in RF afterglows 57
3.2.6 Sample treatments in microwave afterglows 57
3.2.7 Characterization techniques and methods 57
3.3 Results and discussion 58
3.3.1 Analysis of TiO2 films surface nitridation in the N2 and N2-H2 RF afterglows 58
3.3.2 Analysis of TiO2 films surface nitridation in the N2, N2-H2 and N2-CH4 microwave afterglows 65
3.4 Conclusion 75
References 76

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