Titania Inverse-Opal Photonic Crystals Incorporating Gold Nanoparticles in Void Spaces for Photoabsorption Enhancement
Authors: Ohtani Bunsho , Raja-Mogan Tharishinny , Takashima Mai , Kowalska Ewa 
Published: December 2020

Designing of three-dimensional (3D) materials such as inverse-opal (IO) photonic crystals (PCs) has been identified as an effective pathway to enhance light harvesting to longer electromagnetic absorption regions such as visible and infrared [1]. IO PCs exhibit photonic band gap (PBG) and this band structure predicts the translation of photons with reduced velocity, namely as slow photons at certain crystallographic directions. The photonic effect from titania (TiO2)-IO structure was reported to enhance light-material interactions thus allowing better absorption of light at the wavelength at which the materials absorb poorly [2]. On the other hand, utilization of gold-nanoparticles (Au-NPs) on TiO2 could aid in charge separation and play its role as an amplifier for visible-light absorption due to its effects originating from localized surface plasmon resonance (LSPR) [3]. In this study, TiO2-IO PC with incorporated gold nanoparticles (Au-NPs) per void space was developed by implementing five important steps to achieve a good quality of TiO2-IO structure. Both photonic effects due to the slow photons in TiO2-IO structure and LSPR effects contributed by Au-NPs were expected to give visible-light absorption by the photocatalyst material.
The experimental procedure consists of five steps as shown in Scheme 1 starting from (i) synthesis of Au-NPs, (ii) silica (SiO2) coating on Au-NP to form Au/SiO2 core-shells, (iii) self-assembly of Au/SiO2 to form high-quality opal structure, (iv) infiltration of TiO2 precursor via forced impregnation method and (v) selective etching of SiO2 to obtain the final TiO2-IO structure. Photodegradation of acetic acid was carried out in the presence of the photocatalyst material by using blue LED light (450 nm) with optical power of 0.3 W. SiO2 containing Au-NP (Au/SiO2) was used as the opal template for the 3D macroporous TiO2-IO containing Au-NP per void space (TiO2(Au)-IO). Figures 1a and 1b are images observed for Au/SiO2 opal structure exhibiting face-centered cubic (FCC) arrangement. As is observed, the closely packed opal structure could be obtained as the minimum variation among each particle was less than 5 %. However, extra precautions should be taken to avoid cracking on the opal structure to sustain the quality of the final desired material. Figures 1c and 1d are images of TiO2(Au)-IO which was successfully obtained after TiO2 infiltration via forced impregnation method followed by the removal of SiO2 to form the nanovoids. The strategy that has been implemented to enhance the light harvesting process was by designing the TiO2-IO structure with different diameter of nanovoids, which can be controlled by altering the SiO2 shell thickness in the second step of the synthesis process.
Photocatalytic activities of those samples were evaluated by measuring the amount of carbon-dioxide (CO2) generated from acetic acid by TiO2(Au)-IO with different diameter of nanovoid diameters (Au-NP size: 44 nm) under LED light irradiation (4 h). Enhanced CO2 evolution was detected with TiO2(Au)-IO with nanovoid diameter of 215 nm suggesting the successful matching of photonic and LSPR effects in TiO2(Au)-IO.
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[2] V. Jovic, H. Idriss and G.I.N. Waterhouse. Chem. Phys. 479 (2016) 109–121.
[3] S.M. Yoo, S.B. Rawal, J.E. Lee, J. Kim, H. Ryu, D. Park and W.E. Lee. Appl. Catal. A Gen. 499 (2015) 47–54.

Figure 1