Uniform graphitic carbon nitride films : thermal vapor condensations and photoelectrochemical applications
均勻類石墨結構氮化碳薄膜 : 熱氣固化合成及光電化學應用
Student thesis: Doctoral Thesis
Photoelectrochemical (PEC) water splitting for H2 production by semiconductors has received a great deal of attention, since H2 can be used to ease the pressure of the energy crisis and alleviate the associated environmental problems. The semiconductor as photoelectrode should have a band structure in which the conduction band edge and valence band edge straddle the electrochemical potentials for H2 and O2 evolutions, sufficient overpotential for efficient charge carrier transfer to the electrolyte, narrow band gap to absorb the light in the visible range and efficient charge transport properties. However, most of the semiconductors are not qualified for the application. Graphitic carbon nitride (g-CN) has been considered as a good photocatalyst for solar fuel conversion and pollutant degradation due to its moderate band gap of 2.7 eV, good chemical stability under light irradiation, low cost, and nontoxity. Unfortunately, applications of g-CN in PEC fuel production have been hindered by the significant challenge of deposition of uniform g-CN films on solid substrates. In this thesis work, we developed a thermal vapor condensation method to deposit uniform g-CN films at atmosphere. We found that melamine was the ideal precursor for depositing g-CN films. We could tune the surface morphology and film thickness of the g-CN film effectively by changing the substrate, precursor mass and processing temperature, respectively. Based on the time-dependent density functional based tight-binding (TD-DFTB) calculations, XRD pattern and UV/Vis spectra, we confirmed that the structure unit of the g-CN film was tri-s-triazine. The energy levels above the lowest unoccupied molecular orbital (LUMO) are all π* states, while the energy levels below the highest occupied molecular orbital (HOMO) were a mixture of lone pair (LP), π and σ states. The absorption peaks at 307, 366 and 386 nm were due to the π-π*, π-π* and LP-π* transitions. The active energy states including π*, π, and lone-pair states facilitated their efficient (6% quantum yield in the solid state) photoluminescence. We attributed the efficient luminescence to the transitions from the π* states to σ, π and LP states, respectively. Furthermore, we systematically investigated photoelectrochemical properties of the g-CN films. We found that the photocurrent density of the g-CN films was enhanced with increasing the processing temperature, which was due to the intimate contact between the film and substrate, enhanced light absorption, decreased charge transport and charge transfer resistance. The g-CN film grown at 600 °C exhibited the best photocurrent density, which was as high as 0.12 mA cm-2, the highest to date for g-CN based photoanode, at the bias of 1.55 V vs reversible hydrogen electrode with Na2S as sacrificial reagent. Further increase in the growth temperature to 650 °C leaded to the decomposition of the film. Sacrificial reagent is needed to facilitate the charge transfer from the energy levels where the charge carriers cannot directly transfer to the E(OH-/O2). The success in this study demonstrates the potential for the g-CN films to be applied in multiple electronic and optoelectronic devices.
- Condensation, Photoelectrochemistry, Thin films, Graphite