Mesoporous g-C ₃ N ₄ , TiO ₂ and g-C ₃ N ₄ /TiO ₂ photonic films with a chiral nematic structure: slow photonic effect inducing improved H ₂ generation
Masa Johar , Cong Wang, Mohamed-Nawfal Ghazzal Institut de Chimie Physique, Université Paris Saclay, France
Abstract: Energy crises have occurred in the last few decades. Photocatalysis is a promising green and sustainable process for converting solar light energy into chemical energy. Various semiconductors, including graphitic carbon nitride (g-C 3 N 4 ) and titanium dioxide (TiO 2 ), have attracted attention as promising photocatalysts due to their unique properties. However, a common challenge among all photocatalysts is their limited photocatalytic activity, which can be attributed to high recombination of photogenerated charges and a low absorption factor. Extensive research has been dedicated to exploring new chemical and physical strategies to enhance the light harvesting capabilities of photocatalysts. Photonic structures, engineered to manipulate the propagation of light and utilizing phenomena such as the slow photon effect, are employed to enhance light absorption capabilities. This work proposes the fabrication of three different bioinspired photonic crystal systems: SiO 2 / g-C 3 N 4 , SiO 2 / TiO 2 , and SiO 2 / g-C 3 N 4 / TiO 2 films with a chiral-nematic structure. This fabrication method combines soft chemistry (sol-gel chemistry) with bio-template nanomaterials, specifically cellulose nanocrystals (CNCs), see figure 1. The resulting films exhibit iridescent colors and Bragg peak reflection, which can be controlled by adjusting the sol formulation. Characterization techniques such as UV-vis spectroscopy, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS) were used to analyze the films. The photocatalytic activity was evaluated for H 2 generation under Mercury lamp illumination. The results demonstrate that the structure can be controlled by the adjusting the sol formulation, thereby influencing the optical properties and the position of the Bragg peak reflection. It was observed that adjusting the Bragg position directly affects the photocatalytic performance of the photocatalyst, exploiting the slow photon effect.
Figure 1. Synthetic route for the Au-SiO 2 / g-C 3 N 4 / TiO 2 with chiral nematic structure derived from cellulose nanocrystals as a biotemplate and TMOS, urea and TAA as precursors. References 1. Wang, C. et al. A sol gel biotemplating route with cellulose nanocrystals to design a photocatalyst for improving hydrogen generation. J. Mater. Chem. A 8, 10779 10786 (2020). 2. Gesesse, G. D. et al. Enhanced Photogenerated Charge Carriers and Photocatalytic Activity of Biotemplated Mesoporous TiO 2 Films with a Chiral Nematic Structure. Chem. Mater. 31, 4851 4863 (2019). 3. Lin, W. et al. Bioinspired Mesoporous Chiral Nematic Graphitic Carbon Nitride Photocatalysts modulated by Polarized Light. ChemSusChem 11, 114 119 (201 8). 4. Chen, J. I., von Freymann, G., Choi, S. Y., Kitaev, V., & Ozin, G. A. (2006). Amplified photochemistry with slow photons. Advanced Materials , 18 (14), 1915-1919.
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