Titania photocatalysts have been intensively examined for both mechanism study and possible commercial applications for more than 30 years. Although various reports have already been published on titania, including comprehensive review papers, the morphology-governed activity, especially for novel nanostructures, has not been reviewed recently. Therefore, this paper presents novel, attractive, and prospective titania photocatalysts, including zero-, one-, two-, and three-dimensional titania structures. The 1D, 2D, and 3D titania structures have been mainly designed for possible applications, e.g., (i) continuous use without the necessity of particulate titania separation, (ii) efficient light harvesting (e.g., inverse opals), (iii) enhanced activity (fast charge carriers' separation, e.g., 1D nanoplates and 2D nanotubes). It should be pointed out that these structures might be also useful for mechanism investigation, e.g., (i) 3D titania aerogels with gold either incorporated inside the 3D network or supported in the porosity, and (ii) titania mesocrystals with gold deposited either on basal or lateral surfaces, for the clarification of plasmonic photocatalysis. Moreover, 0D nanostructures of special composition and morphology, e.g., magnetic(core)-titania(shell), mixed-phase titania (anatase/rutile/brookite), and faceted titania NPs have been presented, due to their exceptional properties, including easy separation in the magnetic field, high activity, and mechanism clarification, respectively. Although anatase has been usually thought as the most active phase of titania, the co-existence of other crystalline phases accelerates the photocatalytic activity significantly, and thus mixed-phase titania (e.g., famous P25) exhibits high photocatalytic activity for both oxidation and reduction reactions. It is believed that this review might be useful for the architecture design of novel nanomaterials for broad and diverse applications, including environmental purification, energy conversion, synthesis and preparation of "intelligent" surfaces with self-cleaning, antifogging, and antiseptic properties.

Noble metal (NM)-modified wide-bandgap semiconductors with activity under visible light (Vis) irradiation, due to localized surface plasmon resonance (LSPR), known as plasmonic photocatalysts, have been intensively studied over the last few years. Despite the novelty of the topic, a large number of reports have already been published, discussing the optimal properties, synthesis methods and mechanism clarification. It has been proposed that both efficient light harvesting and charge carriers’ migration are detrimental for high and stable activity under Vis irradiation. Accordingly, photonic crystals (PCs) with photonic bandgap (PBG) and slow photon effects seem to be highly attractive for efficient use of incident photons. Therefore, the study on PCs-based plasmonic photocatalysts has been conducted, mainly on titania inverse opal (IO) modified with nanoparticles (NPs) of NM. Although, the research is quite new and only several reports have been published, it might be concluded that the matching between LSPR and PBG (especially at red edge) by tuning of NMNPs size and IO-void diameter, respectively, is the most crucial for the photocatalytic activity.

Here, we report a novel structured material, titania inverse-opal photonic crystal with or without a single gold nanoparticle in each void, to provide a photoabsorption design strategy as enhanced photoreaction rates, only when wavelengths of photoirradiation, photoabsorption (by gold nanoparticles or titania), and photonic-bandgap edge are trebly matched.

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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. References [1] X. Zheng, D. Li, X. Li, L. Yu, P. Wang, X. Zhang, J. Fang, Yu. Shao and Y. Zheng. Phys. Chem. Chem. Phys. 16 (2014) 15299–15306. [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

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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. [1] X. Zheng, D. Li, X. Li, L. Yu, P. Wang, X. Zhang, J. Fang, Yu. Shao and Y. Zheng. Phys. Chem. Chem. Phys. 16 (2014) 15299–15306. [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

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. [1] X. Zheng, D. Li, X. Li, L. Yu, P. Wang, X. Zhang, J. Fang, Yu. Shao and Y. Zheng. Phys. Chem. Chem. Phys. 16 (2014) 15299–15306. [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