Hollow or Yolk–Shell-Type Photocatalyst

Abstract

The emerging hollow and yolk–shell-type photocatalyst composites show attractive photocatalytic performance by multifunction assembly, accelerating the separation of photocatalytic induced hole–electron and enhancing the light absorption efficiency. Meanwhile, more active sites can be formed due to the exposed inner surface of the shell. This chapter focuses on providing a state-of-the-art introduction about the synthesis routes and photocatalytic application of this kind of composites.

The photocatalytic efficiency of bulk semiconductor particles generally suffers from poor diffusion efficiency of photo-generated carriers from the inside to the surface and low absorption efficiency to the incident light. The introduction of cavity inside the semiconductor particle has been proven effective to form accessible active sites throughout the bulk particle and enhance the light absorption efficiency through multiple scattering effects. Moreover, through reasonably spatial arrangement of different components in a single particle, the hole–electron separation degree can be enhanced, and functions such as plasmon resonance absorption and magnetic separation can be extra assembled. In this chapter, we mainly discuss the synthesis and characteristics-tuning of hollow-, yolk–shell-, hierarchical-, and tubular-type photocatalyst and the structural effect on the photocatalytic performance.

9.1 Synthesis

The generation of void inside the semiconductor can be achieved mainly through non-template (Kirkendall, Ostwald ripening) and hard-template (silica, carbon, polymer colloids) routes [1,2,3]. Both methods can form cavitory semiconductors with versatile structures and highly tunable dimensions concerning core, shell, and void.

9.1.1 Non-template Route

Ostwald ripening is the most commonly used strategy for non-template synthesis of cavitory semiconductor [2, 4, 5]. A typical example is the formation of hollow anatase TiO₂ nanospheres using TiF₄ as the precursor in aqueous system and under hydrothermal conditions (Fig. 9.1) [2]. Inner nanospace and highly organized crystallites in the shell structure and surface regions can be created with a wide range of controlling parameters. The interior void enlarges with the prolonging ripening process.

Fig. 9.1

(a) Schematic illustration (cross-sectional views) of the ripening process and two types (i & ii) of hollow structures. Evolution (TEM images) of TiO₂ nanospheres synthesized with 30 mL of TiF₄ (1.33 mM) at 180 °C with different reaction times: (b) 2 h (scale bar = 200 nm), (c) 20 h (scale bar = 200 nm), and (d) 50 h (scale bar = 500 nm) (Reprinted with the permission from ref. [2]. Copyright 2004 American Chemical Society)

Bian et al. prepared mesoporous titania spheres with tunable chamber structure, where TiOSO₄ as the titania precursor was solvothermally reacted in glycerol, alcohol, and ethyl ether [6]. Judicious choice of the alcohol molecules (e.g., methanol, ethanol, and propanol) and reaction time affords the synthesis of spheres with adjustable morphology, size, and interior structure that is tunable from solid, sphere-in-sphere, to hollow. The formation of such spheres may involve aggregation of titania building clusters into spheres and their subsequent reaction, dissolution, and redeposition process. Specially, during the solvothermal condition, etherifying reactions between alcohol and glycerol produce water continuously. Titania building clusters may be generated through alcoholysis reaction or hydrolysis–condensation reactions of TiOSO₄. Solid spheres first formed from the clusters aggregation, which contain a large number of hydrolyzable ligands due to slow reaction kinetics. Water continuously produced from the etherifying reactions further drives the hydrolysis–condensation reaction forward, leading to the dissolution and rearrangement of the surface building clusters (Fig. 9.2).

Fig. 9.2

SEM and TEM (insets) images of the titania spheres synthesized for (a) 1/24, (b) 0.5, (c) 1, (d) 2, (e) 7, and (f) 14 days, showing transiting interior structure from dense, to sphere-in-sphere, to hollow and surface morphology from smooth to prickly (Reprinted with the permission from ref. [6]. Copyright 2007 American Chemical Society)

Hierarchical macro-/mesoporous titania can be prepared without the addition of templates or auxiliary additives at room temperature by the simple dropwise addition of tetrabutyl titanate (TBOT) to pure water and then calcined at various temperatures (Fig. 9.3) [7]. Firstly, the contact between the TBOT droplets and distilled water immediately produces a thin, dense semipermeable titania membrane at the droplet interface. Then, the distilled water diffuses through the spherical outer membrane, and the reaction proceeding inwards is approximately perpendicular to the external surface of the particles. The water/alcohol channels within the TBOT droplets undergo spontaneous radial patterning caused by the hydrodynamic flow of the solvent. The calcination temperature has a strong effect on the structures and photocatalytic activity of the prepared titania. At 300 °C, the calcined sample shows the highest photocatalytic activity. The calcination temperature higher than 500 °C caused the destruction of the hierarchical macro-/mesoporous structure, thus leading to the decrease of the photocatalytic activity.

Fig. 9.3

Schematic procedure for the formation mechanism of the hierarchical macro-/mesoporous titania (Reproduced from ref. [7] by permission of John Wiley & Sons Ltd)

9.1.2 Hard-Template Route

Compared with template-free method, hard-template route allows higher tunability over the pore volume, layer numbers, and composition. SiO₂, carbon, and polymer are the commonly used hard templates, which can be removed through acid–base etching, calcination, or dissolution. Yin et al. produced hollow TiO₂ using SiO₂ as the hard template [8], where amorphous TiO₂ was first coated around SiO₂ and then crystallized into anatase phase. SiO₂ was finally etched through NaOH to form interior void (Fig. 9.4).

Fig. 9.4

Top: schematic illustration of the water-assisted crystallization strategy for converting an amorphous TiO₂ layer to mesoporous crystalline shells. Bottom, typical TEM images of the samples at each preparation step: (a) SiO₂@TiO₂ core–shell structures prepared by sol–gel coating; (b) SiO₂@TiO₂ core–shell structures after water-assisted crystallization; and (c) mesoporous TiO₂ hollow nanostructures after removing SiO₂ cores. (d) XRD patterns of samples (a) and (c), showing the transition from the amorphous to the anatase phase after water refluxing (Reproduced from ref. [8] by permission of John Wiley & Sons Ltd)

Hierarchical macro-/mesoporous silica materials co-incorporated with Cr and Ti (MM–Si–Cr–Ti) were directly synthesized by adopting close-packed array of polystyrene (PS) microsphere as hard template for macropore and triblock copolymer Pluronic P123 as a soft template through a simple soaking-calcination way [9], where the Si/Ti ratio was fixed at 200 and Si/Cr ratio varied from 200 to 10. Ti specie is highly dispersed in the porous matrix, and Cr specie mainly exists as tetra-coordinated CrO₃ when Si/Cr ≤ 50 and transforms to a mixture of CrO₃ and crystallized hexa-coordinated Cr₂O₃ when the Si/Cr ratio is higher than 50 (Fig. 9.5).

Fig. 9.5

SEM (a, b) and TEM (c, d) images of MM–Si–Cr–Ti prepared with molar ratio of Si–Cr–Ti = 200/10/1 (Reprinted from ref. [9], Copyright 2015, with permission from Elsevier)

Hierarchically ordered macro-/mesoporous TiO₂ films (denoted as H–TiO₂) with high thermal stability and crystallinity were synthesized using a confined evaporation-induced self-assembly (EISA) method [10]. P123 is used as a soft template to create the mesopores, and 3D periodic colloidal crystal PS are used as a hard template to create macropores (Scheme 9.1). The surfactant sulfuric acid carbonization method is applied to retain the ordered mesostructure during the high-temperature crystallization (550, 650 °C), while the PS spheres can generate amorphous carbon by heating treatment under an inert environment, which can support the inverse opal macrostructure from collapse. The hierarchically porous TiO₂ films present high surface areas of up to 240 m² g⁻¹ and huge pore volume of ca. 1.2 cm³ g⁻¹. The photoelectrocatalytic water splitting performance of the hierarchical porous TiO₂ films is excellent and is much higher than that of pristine mesoporous TiO₂ films. The photoconversion efficiency is up to 5.23% for the N-doped hierarchically porous TiO₂ films.

Scheme 9.1

Scheme of the synthesis process of the hierarchically ordered macro-/mesoporous TiO₂ films. The Ti precursor solution containing triblock copolymer Pluronic P123 and H₂SO₄ was impregnated into the interspace of the PS colloidal crystal films. After an EISA and aging process, the as-formed polymer–H–TiO₂ composite films were calcined under N₂ atmosphere at a high temperature (400, 550, 650 °C), and the in situ carbon that came from the carbonization of PS spheres and Pluronic P123 was generated both in macropores and mesopores to form the carbon–H–TiO₂ composite films. After the calcination in air to remove the carbon, the hierarchically ordered macro-/mesoporous TiO₂ films (H–TiO₂) with high crystallinity could be obtained (Reproduced from ref. [10] by permission of John Wiley & Sons Ltd)

9.2 Spatial Arrangement of Different Functions

The fabrication of yolk–shell structure allows the spatial arrangement of different functions. The Ostwald ripening process depicted in Fig. 9.1 can be used to encapsulate the metal core into the interior cavity. For example, Au/TiO₂ was fabricated through the following steps [11]: (1) formation of metallic cores, (2) aggregation of TiO₂ nanocrystallites around a single metallic core, (3) evacuation of central TiO₂ crystallites by Ostwald ripening, and (4) manipulation of the metal core to a desired size via controlling the growth of the inside metal. For the size manipulation, HAuCl₄ was first introduced into the vacant space of the nanoreactor, and the HAuCl₄-soaked Au/TiO₂ nanoreactors were then placed in a solution of cetyltrimethylammonium bromide (CTAB) and ascorbic acid. It is believed that a slow interdiffusion between the HAuCl₄ inside the nanoreactor and the CTAB/ascorbic acid solution outside the nanoreactor would ensure a slow growth of Au cores while preventing TiO₂ shells from direct metal deposition. The size of Au core can be tuned in the range of 150–250 nm (Figs. 9.6 and 9.7).

Fig. 9.6

(a)–(d) TEM images of Au/TiO₂ core–shell nanoreactors after enlargement of Au cores. All bar scales: 200 nm (Reprinted with the permission from ref. [11]. Copyright 2007 American Chemical Society)

Fig. 9.7

(a) Schematic illustration of the “hydrothermal etching assisted crystallization” strategy for the formation of the Fe₃O₄@titanate double-shelled yolk–shell microspheres. TEM images of a single sphere synthesized at 150 C with hydrothermal treatment time: (b) 0 min, (c) 20 min, (d) 2 h, (e) 12 h, and (f) 24 h (Reprinted with the permission from ref. [12]. Copyright 2011 American Chemical Society)

Li et al. reported a facile “hydrothermal etching assisted crystallization” route to synthesize Fe₃O₄@titanate yolk–shell microspheres with ultrathin nanosheets-assembled double-shell structure [12]. The as-prepared microspheres possess a uniform size, tailored shell structure (Fig. 9.8), good structural stability, versatile ion-exchange capability, high surface area, and large magnetization and exhibit a remarkable photocatalytic performance.

Fig. 9.8

The PTIM-MS structure and the mechanism for photocatalytic oxidation. Pt and MnOx are spatially separated by the TiO₂–In₂O₃ heterogeneous double-layered shell. (a) The reaction process. A represents an electron acceptor; here, NaIO₃ was used. (b) Simplified band structure of the catalyst. The CB positions of In₂O₃ and TiO₂ are 0.63 and 0.40 eV vs NHE, and the VB positions of In₂O₃ and TiO₂ were calculated to be 2.17 and 2.80 eV vs NHE according to the corresponding bandgaps (Reproduced from ref. [14] by permission of John Wiley & Sons Ltd)

9.3 Acceleration of Photo-Carrier Separation

Submicrometer-sized yolk–shell-structured Au@TiO₂ was synthesized by controllably hydrolyzing TiF₄ in Au nanoparticle solution under hydrothermal condition [13]. Both the size and the shell thickness could be easily adjusted by changing the amount of TiF4 in reaction mixture. Notably, when these Au@TiO2 hollow submicrospheres were used as the working electrodes for DSSCs, an obvious improvement in conversion efficiency is achieved compared to those solar cells based on TiO2 hollow spheres or TiO2 nanoparticles. DSSCs fabricated with Au–P25 composite materials and Au/TiO₂ core–shell nanowires can achieve an efficiency of 3.3% and 4.53%, respectively. As comparison, Au@TiO₂ hollow submicrospheres achieve a remarkably higher efficiency of 8.13%. Such a large enhancement should be ascribed to the fact that the sealed Au particles in the TiO₂ shells can effectively suppress both back electron transfer and energy transfer from dye to Au particles (Scheme 9.2).

Scheme 9.2

Illustrations of (a) formation process of Au@TiO₂ hollow submicrospheres and (b) the charge separation process in the DSSCs with the photoanode of Au@TiO₂ hollow submicrospheres (Reproduced from ref. [13] by permission of John Wiley & Sons Ltd)

Pt@TiO₂@In₂O₃@MnO_x mesoporous hollow spheres (PTIM-MSs) was designed and synthesized as shown in Fig. 9.8, which combine the advantages of spatially separated cocatalysts (Pt and MnO_x) and thin heterojunctions (TiO₂@In₂O₃ shell) to simultaneously reduce bulk and surface recombination [14]. Spatially separated cocatalysts drive electrons and holes near the surface to flow in opposite directions, reducing their recombination. Thin heterojunctions can effectively separate charges in the bulk phase and enable their transfer to the surface–subsurface region where they can be easily trapped by cocatalysts for surface reactions. Furthermore, In₂O₃ serves as a sensitizer to enhance light absorption. In combination with other advantages, such as a large surface area, long light-scattering path, and surface reaction kinetics promoted by cocatalysts, the PTIM-MS system is an excellent photocatalyst of both water oxidation and selective benzyl alcohol oxidation.

A new strategy for design and synthesis of Co₉S₈ hollow cubes decorated by CdS QDs was developed by Zhang et al. [15]. The hybrid Z-scheme system is obtained by a simple hydrothermal method containing dimethyl sulfoxide (DMSO) as a solvent and hollow Co(OH)₂ cubes as a template. DMSO solvent served as a sulfur source to react with Cd²⁺, producing CdS QDs, and was also used as the anion exchange reagent to react with easily prepared Co(OH)₂ cubes. The hollow CdS–Co₉S₈ cubes exhibit efficient solar light harvesting, as well as impressively enhanced hydrogen evolution reaction (HER) activity and stability under solar light irradiation compared to that of the pure Co₉S₈ and CdS catalysts. An efficient Z-scheme building block and the multiple reflections of solar light within the cavity of hollow cubes are responsible for substantially enhanced HER activity and stability (Fig. 9.9).

Fig. 9.9

The preparative process for CdS–Co₉S₈ and corresponding TEM images; scale bar = 200 nm (Reproduced from ref. [15] by permission of John Wiley & Sons Ltd)

MnO_x@CdS/CoP hollow spheres with spatially separated surfaces were fabricated by loading the dual cocatalysts (inside MnO_x and outside CoP) for enhanced photocatalytic H₂ evolution activities [16]. CdS shells (hollow spheres) can be realized by using SiO₂ spheres as sacrificial templates, where the MnO_x and CoP NPs are selectively anchored on the inner and outer surface of CdS shells, respectively. That is, the photo-generated electrons and holes can directionally migrate to the locations of dual cocatalysts for reduction and oxidation reaction, respectively. As expected, in this case, the recommendation of electrons and holes has been significantly restrained, and the MnO_x@CdS/CoP catalyst shows a reduction surface and an enhanced photocatalytic activity for H₂ evolution (Fig. 9.10).

Fig. 9.10

Comparison of photocatalytic activities of the titania spheres with solid, sphere-in-sphere, and hollow structure. Inset shows a schematic illustration of multireflections within the sphere-in-sphere structure (Reprinted with the permission from ref. [6]. Copyright 2007 American Chemical Society)

9.4 Multiple Light Scattering

The hollow or yolk–shell structure allows the multiple reflections of incident light within the interior cavity, thus enhancing the photocatalytic activity due to the improved light absorption efficiency (Fig. 9.11) [6]. The destroying of sphere-in-sphere structure by grinding the spheres causes a dramatic decrease of photocatalytic activity that is similar to that of the solid spheres. As schematically illustrated in the inset, a sphere-in-sphere structure with an appropriate inner sphere diameter allows more efficient use of the light source and therefore offers an improved catalytic activity. The hollow and yolk–shell structures with smaller inner spheres show lower activity due to the reduced light reflection efficiency.

Fig. 9.11

(Left) SEM and (b) HRTEM images of SnO₂ MHSs. (c) SEM and (d) HRTEM images of TiO₂–SnO₂ MHSs. (Right) I–V characteristics of DSSCs with the photoelectrode films of TiO₂–SnO₂ MHSs, TiO₂–nano-SnO₂, SnO₂ MHSs, nano-SnO₂, and TiO₂ (P25) nanoparticles. The inset illustrates the multiple reflecting and scattering of light in the multilayered hollow spheres (Reproduced from ref. [16] by permission of John Wiley & Sons Ltd)

Qian et al. designed TiO₂-coated multilayered SnO₂ hollow microspheres (MHS) for dye-sensitized solar cells [16]. Multilayered spherical SnO₂–C composite was first formed through a condensation polymerization and carbonization of sucrose accompanied by hydrolysis of SnCl₄ in the hydrothermal reaction. Then, hollow SnO₂ were obtained by removal of carbon via calcination. Compared to TiO₂, SnO₂ has higher electron mobility and larger bandgap. However, SnO₂-based DSSCs generally have lower conversion efficiencies than TiO₂, which are attributed to a faster interfacial electron recombination and lower trapping density. On the other hand, SnO₂ has a lower isoelectric point (pH = 4–5) than anatase TiO2 (pH = 6–7), which leads to less adsorption of the dye with acidic carboxyl groups. To solve these problems, coating a thin layer of an isolating oxide, such as TiO₂, ZnO, Al2O3, or MgO, has been adopted to improve the conversion efficiency of SnO₂ photoelectrodes. TiO₂–SnO₂ multilayered hollow microspheres (SnO₂ MHSs) were synthesized by two steps (Fig. 9.8). First, SnO₂ MHSs were prepared by a chemically induced self-assembly reaction of aqueous sucrose–SnCl₄ solution under hydrothermal condition. The second step was to coat TiO₂ nanocrystallites onto the SnO₂ MHSs by impregnating in TiCl4 and then hydrolyzing TiCl₄ to form a surface layer of TiO₂.

The distinct photovoltaic behavior of the TiO₂–SnO₂ MHSs is its large short-circuit current (JSC, 14.6 mA cm⁻²) compared with TiO₂–nano-SnO₂ (J_SC, 10.3 mA cm⁻²) and TiO₂ (J_SC, 11.1 mA cm⁻²) nanoparticles. This enhanced photocurrent could be attributed to better dye adsorption, due to increased active surface area, or better light-harvesting efficiency, due to the hollow spherical structure. Since the amount of dye adsorption on the TiO₂–SnO₂ MHSs is very similar to those of TiO₂–nano-SnO₂ and TiO₂ nanoparticles, the larger JSC value for the TiO₂–SnO₂ MHSs is most likely given rise by an enhanced light harvesting due to multiple light reflecting and scattering in between the hierarchical spherical shells of the TiO₂–SnO₂ MHSs, rather than an increased surface area. The JSC value for the SnO₂ MHSs is 50% higher than that of nano-SnO₂, suggesting that the enhanced JSC value for the TiO₂–SnO₂ MHSs compared with the TiO₂–nano-SnO₂ is due to the hierarchical spherical structure rather than the TiCl₄ treatment. The inset in Fig. 9.11 (right) illustrates the reflecting and scattering of light in a TiO₂–SnO₂ MHS. It is believed that the shells with the different sizes in a microsphere cannot only multireflect but also scatter the incident light of different wavelengths in the range of visible light. The improved photocurrent for the TiO₂–SnO₂MHSs is mainly contributed to its multilayered hollow spherical structure, which provides an effective way to enhance light-harvesting efficiency.

Photonic-crystal-based optical coupling offers a unique way of light–matter interaction to increase light harvesting, especially around the absorption edge of a semiconductor. A photonic crystal is a periodic dielectric structure that can forbid the propagation of light in a certain crystal direction within a certain spectrum regime, called a photonic stop-band. The light in a photonic crystal undergoes strong coherent multiple scattering and travels with very low group velocity near the photonic stop-band edges, referred to as slow light. Such a slow-light effect can considerably increase the effective optical path length, therefore leading to a delay and storage of light in photonic materials. 3D photonic crystal design was utilized to enhance incident photon-to-electron conversion efficiency (IPCE) of WO3 photoanodes. Large-area and high-quality WO3 photonic crystal photoanodes with inverse opal structure were prepared (Fig. 9.12). The photonic stop-bands of these WO3 photoanodes were tuned experimentally by variation of the pore size of inverse opal structures. It was found that when the red edge of the photonic stop band of WO3 inverse opals overlapped with the WO3 electronic absorption edge at Eg = 2.6–2.8 eV, a maximum of 100% increase in photocurrent intensity was observed under visible light irradiation (λ > 400 nm) in comparison with a disordered porous WO3 photoanode (Fig. 9.13). When the red edge of the stop band was tuned well within the electronic absorption range of WO3, noticeable but less amplitude of enhancement in the photocurrent intensity was observed. It was further shown that the spectral region with a selective IPCE enhancement of the WO3 inverse opals exhibited a blueshift in wavelength under off-normal incidence of light, in agreement with the calculated stop band edge locations. The enhancement could be attributed to a longer photon– matter interaction length as a result of the slow-light effect at the photonic stop-band edge, thus leading to a remarkable improvement in the light-harvesting efficiency. The present method can provide a potential and promising approach to effectively utilize solar energy in visible light-responsive photoanodes [17].

Fig. 9.12

SEM images of WO₃ inverse opals: (a) WO₃-200, (b) WO₃-260, and (c) WO₃-360. (d) Photograph of the inverse opal WO₃ photoanodes under white light illumination, WO₃-200, WO₃-260, and WO₃-360, from left to right. (e–g) SEM images of cross-sectional view of each inverse opal. The thickness is estimated to be about 2.5 μm (17 layers) for WO₃-200 (e), 2.6 μm (14 layers) for WO₃-260 (f), and 2.7 μm (11 layers) for WO₃-360 (g) (Reprinted with the permission from ref. [17]. Copyright 2011 American Chemical Society)

Fig. 9.13

Photocurrent potential curves of different WO₃ photoanodes measured under (a) UV–visible light irradiation (λ > 300 nm) and (b) visible light irradiation (λ > 400 nm) (Reprinted with the permission from ref. [17]. Copyright 2011 American Chemical Society)

A range of TiO₂ inverse opals with tunable macroporous size were synthesized using different sized PS arrays as hard templates [18]. After a simple heating treatment in vacuum, Ti³⁺-doped TiO₂ inverse opals were obtained. The optical responses of TiO₂- and Ti³⁺-doped TiO₂ inverse opals could be enhanced by choosing PS arrays with appropriate size as hard templates due to the slow light effect of inverse opal structure, thus coupling the physical and chemical enhancement for the light absorption. The photocatalytic efficiency was evaluated by the photodegradation of AO7, and it is proved that the cooperation of slow light effect and Ti³⁺ doping is an effective way to improve visible light-driven photocatalytic performance of TiO₂ photocatalyst (Scheme 9.3).

Scheme 9.3

Scheme of slow light effect (physical enhancement) and vacuum activation (chemical enhancement) (Reprinted from ref. [18], Copyright 2014, with permission from Elsevier)

To achieve the coupling between the slow photon effect and plasmon resonance absorption, three-dimensional ordered assembly of TiO₂ hollow nanospheres deposited with Au nanoparticles (Au/TiO₂–3DHNSs) was designed (Fig. 9.14) [19]. A photonic stop band centered at 500 nm was observed from the reflection spectrum (Fig. 9.15). As such, the slow photon effect occurring at the edges of the stop band is expected to appear in the range of 550–620 nm, which matches well with the surface plasmon resonance (SPR) absorption of Au nanoparticles. The wavelength matching of SPR absorption and photonic band edge would be expected to increase the SPR intensity since an increase of the effective path length of light in the photonic band edge regions would result in a significant enhancement of the interaction between photons and Au NPs, which was verified by the decreased SPR absorption from crushed microarray structures (Fig. 9.14).

Fig. 9.14

(a) TEM; (b) STEM; (c, d) HRTEM images of Au/TiO₂–3DHNSs (Reproduced from ref. [19] by permission of John Wiley & Sons Ltd)

Fig. 9.15

(a) UV–vis diffuse reflectance spectra for Au/TiO₂–3DHNSs and the TiO₂–3DHNSs reference sample. The shaded region in (A) shows the presence of photonic stop band. (b) UV–vis absorption spectra of Au/TiO₂–3DHNSs before and after being crushed. Inset in (B) shows a digital photo of Au/TiO₂–3DHNSs (Reproduced from ref. [19] by permission of John Wiley & Sons Ltd)

The photocatalytic activity of Au/TiO₂–3DHNSs for the decomposition of isopropanol to CO₂ under visible light illumination (λ = 420 nm) was investigated using Au/TiO₂–P25, crushed Au/TiO₂–3DHNSs, and Au/TiO₂–HNSs without the 3D ordered structure as the reference samples. Among all of samples, CO₂ produced from Au/TiO₂–3DHNSs is much higher than those from other samples. Therefore, the 3D ordered assembly of HNSs with periodic voids exhibits the slow photon effect which enhances the visible light absorption and, as a consequence, increases the photocatalytic activity of Au/TiO₂–3DHNSs. Even under the UV light irradiation, Au/TiO₂–3DHNSs still show the highest activity, which can be mainly attributed to the multiple light scattering resulting from the unique 3DHNS structure.