Carbon nitride nanotube-based photocatalysts for solar to chemical energy conversion

Solar-to-chemical energy conversion via photocatalysis has attracted tremendous attention as a potential solution to the worldwide energy shortage and environmental issues. In this thesis, a range of graphitic carbon nitride nanotube (C3N4 NT)-based photocatalysts were developed and fabricated via the facile methods. The work started with visible-light active bare C3N4 NTs for photocatalytic hydrogen (H2) generation, even in the absence of any co-catalyst. Upon good dispersion of the non-noble co-catalysts, Ag-Cu nanoparticles (NPs) on the bare C3N4 NTs, it exhibited twice the H2 evolution rate of the bare C3N4 NTs and about 1.5 times higher than that of the Pt/C3N4 NTs. The improved activity is attributed to its unique tubular nanostructure, strong metal-support interaction, and efficient photo-induced electron-hole separation compared to their bare and monometallic counterparts, evidenced by complementary characterisation techniques. It reveals that the H2 production rates correlate well with the oxidation potentials of the sacrificial reagents used. Triethylamine (TEA) outperforms other sacrificial reagents including triethanolamine (TEOA) and methanol (MeOH). Mechanistic studies on the role of various sacrificial reagents in photocatalytic H2 generation demonstrate that irreversible photodegradation of TEA into diethylamine and acetaldehyde via monoelectronic oxidation contributes to the improved hydrogen yield. Similarly, TEOA is oxidised to diethanolamine and glycolaldehyde, while MeOH is unable to quickly capture the photo-induced holes and remains intact due to the high oxidation potential. Tunable heterojunction architectures of cobalt oxide (CoOx) nanoparticles were confined on well-arrayed C3N4 NTs by using a facile one-pot method but under different annealing atmospheres. A Type II heterojunction of cobalt monoxide nanoparticles (CoO NPs)/C3N4 NTs was obtained after annealing under vacuum, and fine CoO NPs less than 8 nm in size were homogeneously anchored on the surface of C3N4 NTs. A Type I heterojunction of tricobalt tetraoxide (Co3O4)/C3N4 NTs were formed under air condition, and Co3O4 NPs in the size range of 10 to 50 nm were aggregated on the surface. The photocatalytic activities of these two heterojunctions were evaluated with H2 production from water. The strategically developed CoO/C3N4 NTs with 7 wt. % CoO shows the highest H2 yield under visible light irradiation and the best stability among the photocatalysts studied in this work. Comprehensive characterisation results reveal that the superior catalytic performance of CoO/C3N4 NTs may be attributed to the uniformly distributed smaller nanoparticles on the well-arrayed nanotubes, the longer lifetime of excited electrons, the faster charge transfer and the stronger electronic interaction between the heterojunctions. The Kelvin probe force microscopy results verify that the CoO/C3N4 NT and Co3O4/C3N4 NT nanocomposites form a Type II and Type I heterojunction, respectively, and charge transfer pathways and reaction mechanisms are therefore established. To further extend the light absorption of C3N4 NTs toward near-infrared (NIR) light, upconversion nanoparticles (UCNPs), NaYF4:Yb,Tm,Gd (NYFG) and NaYF4:Yb,Tm (NYF) were decorated on C3N4 NTs separately by a facile technique to construct heterojunction structures. It is found that, with a loading content of 15 wt. %, NYFG/C3N4 NTs exhibited the highest H2 generation with an apparent quantum efficiency (AQE) of 0.80 ‰, about 1.4 times higher than that of NYF/C3N4 NTs under 980 nm laser irradiation. This enhanced photocatalytic activity is attributed to the synergistic effect, stronger interaction, higher emission intensity, and faster charge transfer between the two nanocomposites. The energy transfer between NYFG NPs and C3N4 NTs was investigated by the steady-state and dynamic fluorescence spectroscopy. The emitted photons were absorbed by C3N4 NTs via a fluorescence resonance energy transfer process, leading to a high photocatalytic activities. This work highlights the potential of developing near-infrared (NIR) responsive catalysts for energy and environmental applications. The last part of the thesis works on the photocatalytic fixation of N2 to NH3 under NIR light irradiation over UCNPs decorated C3N4 NTs with nitrogen vacancies (NV-C3N4 NTs). NYF/NV-C3N4 NTs with a mass ratio of 15 % exhibited a higher ammonia synthesis rate of 0.80 mmol L-1 gcat-1 (0.99 % for AQE), which is higher than the other catalysts reported so far under NIR light irradiation. This catalyst also provided about three times higher activity than the bare C3N4 NTs under UV-filtered solar light. Characterisation results reveal that the abundant NVs play an important role in increasing the active sites, light absorption and energy migration. NYF NPs endow the nanostructure with NIR light response. Moreover, the mechanism of the energy transfer pathway was investigated. This work paves the way towards the development of NIR responsive heterogeneous photocatalysts for solar to chemical energy conversion.