Spatially separated bimetallic cocatalysts on hollow-structured TiO₂ for photocatalytic hydrogen generation

Abstract

Efficient charge separation and light harvesting of photocatalysts (e.g., TiO₂) are the key issues to be considered in the design of solar-energy conversion systems. In particular, the charge separation of noble metal-decorated TiO₂ materials could be greatly improved via decreasing the size of noble metal particles (NPs). Furthermore, designing specific morphologies such as hollow structures can improve light harvesting ability. Herein, a hybrid hollow TiO₂ with spatially separated bimetals (Pd@TiO₂@Au) was prepared, which demonstrated enhanced charge separation. By choosing zeolite as the sacrificial substrate, ultrasmall Pd NPs and Au NPs were decorated in the inner and outer shells of hollow TiO₂, respectively. The separated bimetals could pull the photoexcited electrons away from the surface of TiO₂ for more efficient charge separation. The as-prepared Pd@TiO₂@Au catalyst exhibited a superior photocatalytic H₂ evolution rate up to 272.3 μmol h⁻¹, which was higher than most of the TiO₂-based photocatalysts.

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Introduction

Nowadays, the problem of energy shortage and environmental pollution is becoming more and more serious in the world.^1–5 Hydrogen (H₂) as a kind of clean fuel has drawn intensive attention for its superior energy density and environmental friendliness.^1–3 Converting solar energy into H₂ by photocatalytic water splitting is one of the most efficient ways for H₂ production. A large number of semiconductor photocatalysts including titanium dioxide (TiO₂), CdS, Ta₃N₅, ZnO and g-C₃N₄ have been widely used for photocatalytic H₂ generation.^6,7 Among them, TiO₂ has been considered as one of the most promising photocatalysts owing to its low price, high safety and superior chemical stability.^8–11 However, the fast recombination of the photogenerated electron–hole pairs strongly hinders the photocatalytic hydrogen production. Recently, noble metal nanoparticles (NPs) (e.g., Au, Pt, and Ag NPs) were decorated on photocatalysts to enhance the catalytic properties.^12–15 In particular, the bimetal decorated TiO₂ (such as Pd–Au, Pt–Cu, and Au–Pt) can exhibit further improved photocatalytic efficiency compared to the monometal decorated ones.^16–20 Among them, most of the bimetals were randomly decorated outside the TiO₂ with the inner interface not being used. Thus, the spatially separated bimetals on the inner and outer surfaces of TiO₂ are expected to be able to enhance the photocatalytic properties by fully utilizing the photo-induced charges on the interface of excited TiO₂.

On the other hand, recent investigations showed that the size of noble metal NPs has a great influence on the photocatalytic properties of the nanohybrids.^6,21 It has been demonstrated that the smaller noble metal NPs can induce a lower Fermi level, which can cause improved charge transfer and an elevated separation rate of electron–hole pairs.²¹ Unfortunately, it is hard to obtain ultrasmall noble metal clusters since they are prone to aggregation. Nowadays, zeolites have been widely used for the confinement synthesis of ultrasmall metal NPs and even sub-nanometric metallic clusters, in which the nanospace of zeolites can restrict the growth of the metals.^22–24 Also, the ordered microporous structures of zeolites can eliminate the aggregation of metal species and thus improve the catalytic properties and stability.²⁵

Furthermore, it has been demonstrated that the nanostructures of the hybrids of noble metal–TiO₂ play an important role in improving the photocatalytic properties.^11,26–30 Notably, the nano hybrids with a yolk–shelled hollow morphology can induce higher photocatalytic properties.³⁰ On the one hand, the hollow structure can enhance light utilization efficiency by realizing repeated light reflection and refraction; on the other hand, the hollow structure can increase the surface-to-volume ratio, which can provide more active reaction sites and enhance mass transfer.

Herein, we designed a hollow-structured TiO₂ with spatially separated bimetallic NPs decorated in the core and outer shells, respectively. In detail, the hollow TiO₂ sphere was obtained by choosing silicalite-1 (MFI) zeolite with embedded ultrasmall Pd NPs as a supporting substrate followed by etching of the zeolite. The TiO₂ exhibited a hollow structure and the ultrasmall Pd NPs were decorated inside the hollow TiO₂ sphere. Then, Au NPs working as the cocatalyst were decorated on the outer surfaces of the TiO₂ shell. The obtained sandwich-like hollow-structured Pd@TiO₂@Au with spatially separated cocatalysts can significantly enhance the photocatalytic properties of TiO₂ by combining the surface plasmon resonance (SPR) effect of Au NPs and the smaller size effect of Pd NPs. Specifically, Pd@TiO₂@Au exhibited a superior photocatalytic H₂ evolution rate up to 272.3 μmol h⁻¹. Such performance is far superior to most of the TiO₂-based photocatalysts for water-splitting.

Experimental

Materials

Chloroauric acid tetrahydrate (HAuCl₄·4H₂O, A.R., Shanghai Chemical Factory), tetraethylorthosilicate (TEOS) (Sinopharm Chemical Reagent Co., Ltd), tetrabutyl orthotitanate (TBOT) (98%, Macklin), tetrapropylammonium hydroxide (TPAOH) (25 wt%, Tianjin Guangfu Fine Chemical Research Institute), ethanol (99%, Tianjin Guangfu Fine Chemical Research Institute), palladium chloride (PdCl₂, Pd 59%, Aladdin), ethylenediamine (NH₂CH₂CH₂NH₂, Tianjin Fuchen Chemical Reagents Factory), ammonia (NH₃·H₂O, Tianjin Yongsheng Fine Chemical Co., Ltd), sodium hydroxide (NaOH, A.R., Tianjin Yongsheng Fine Chemical Co., Ltd), and deionized water (resistance >18 MΩ cm⁻¹).

Preparation of silicalite-1 (S1) zeolite

S1 was prepared by using the hydrothermal method at 170 °C for 4 days, the molar composition of which is 1.0SiO₂ : 0.4TPAOH : 35H₂O. Specifically, 13 g of TPAOH was mixed with deionized water followed by stirring for 10 min, then 8.32 g of TEOS was added to the above solution. After being stirred for 6 h, the mixture was transferred to a Teflon-lined stainless-steel autoclave and heated at 170 °C for 4 days under static conditions. After washing with deionized water and ethanol three times, the product was then dried at 60 °C. Finally, conventional S1 was obtained by calcinating the above product at 550 °C in air for 8 h.

Synthesis of Pd@silicalite-1 (Pd@S1)

Pd@S1 was synthesized using the same method for the preparation of S1 except for the addition of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ after the hydrolysis of TEOS.²⁷ After calcination, the product was subsequently reduced with H₂ at 400 °C for 2 h. In this way, Pd@S1 was obtained.

Preparation of hollow Pd@TiO₂, hollow TiO₂ and TiO₂ NPs

Typically, 150 mg of the above-synthesized zeolite or Pd@S1 was dispersed in 200 mL of absolute ethanol. After adding 0.9 mL of ammonia solution (28 wt%) into the system followed by ultrasonication for half an hour, TBOT (2.0 mL) was added into the solution drop by drop. Then, the mixture was stirred at 45 °C for 24 hours. After washing with deionized water and ethanol several times and calcination at 450 °C for 2 h, Pd@S1@TiO₂ NPs were obtained. In order to get hollow structured Pd@TiO₂, 50 mg of Pd@S1@TiO₂ NPs was treated with 30 mL of NaOH solution (3 M) under stirring for 12 h. After centrifugation and washing with water and ethanol 3 times, hollow Pd@TiO₂ was obtained. When it comes to the synthesis of hollow TiO₂ NPs, S1 was used to replace Pd@S1. The conventional TiO₂ NPs were prepared using the same method for preparing hollow TiO₂ except without adding zeolite as the template.

Preparation of sandwich-like Pd@TiO₂@Au, hollow TiO₂@Au, Pd@TiO₂@Pt and hollow TiO₂@Pt

The sandwich-like Pd@TiO₂@Au was synthesized by photo-deposition of the Au NPs on the surfaces of Pd@TiO₂. Specifically, the starting gel of sandwich-like Pd@TiO₂@Au was obtained by dispersing Pd@TiO₂ into HAuCl₄ (0.06%) solution and adjusting the pH to 8 (adjusted by 0.1 M NaOH). After irradiation at 200–1100 nm for 20 minutes, Au³⁺ was converted into Au NPs. In this way, the sandwich-like Pd@TiO₂@Au was obtained. The hollow TiO₂@Au was obtained by replacing Pd@TiO₂ with hollow TiO₂. The sandwich-like Pd@TiO₂@Pt was synthesized using the same method except for replacing HAuCl₄ by H₂PtCl₆. The hollow TiO₂@Pt was prepared by replacing Pd@TiO₂ with hollow TiO₂.

Synthesis of Pd-im/silicalite-1 (Pd-im/S1) and Pd-im@TiO₂

For comparison, Pd-im/S1 was obtained using the impregnation method.²⁷ Typically, 0.23 mL of (NH₄)₂PdCl₄ solution (0.28 M) was added into 1 g of S1. After stirring for an hour, the mixture was dried overnight at 80 °C. Finally, Pd-im/S1 was obtained by reducing the dried mixture with hydrogen. Then, Pd-im@TiO₂ was prepared using the same method for preparing Pd@TiO₂ except for replacing Pd@S1 by Pd-im/S1.

Synthesis of Pd–Au-im@TiO₂ and Pd–Pt-im@TiO₂

Pd–Au-im@TiO₂ and Pd–Pt-im@TiO₂ were prepared using the impregnation approach. The metal loadings in Pd–Au-im@TiO₂ and Pd–Pt-im@TiO₂ were controlled to be comparable with those of sandwich-like Pd@TiO₂@Au and Pd@TiO₂@Pt accordingly. First, Pd@S1 was impregnated with HAuCl₄ and H₂PtCl₆ solutions, respectively. Then, Pd–Au-im@S1 and Pd–Pt-im@S1 were obtained by reducing the above two mixtures with hydrogen at 400 °C for 2 h. Finally, Pd–Au-im@TiO₂ and Pd–Pt-im@TiO₂ were obtained by growing TiO₂ on the outside surface of Pd–Au-im@S1 and Pd–Pt-im@S1, respectively, followed by etching the zeolite with NaOH solution. The methods for the growth of TiO₂ and the etching details are the same as those for the preparation of sandwich-like Pd@TiO₂@Au.

Material characterization

Powder X-ray diffraction was performed on a Rigaku Smart Lab X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) in the 2θ range from 4° to 80°. Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectrometry images were obtained using a JSM-7800F (Japan) electron microscope. The transmission electron microscopy (TEM) images and EDX spectrometry images were obtained using a Tecnai F20 electron microscope. The metal loading was determined using inductively coupled plasma (ICP) analyses on a PerkinElmer Optima 3300 DV ICP instrument. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250 spectrometer. UV-vis absorption spectra were obtained using a SHIMADZU UV-2550 spectrophotometer (200–800 nm). The photoluminescence (PL) spectral measurements were conducted on a SHIMADZU RF-5301pc spectrofluorophotometer. The PL lifetime decay curves were plotted using a HORIBA Scientific FluoroMax-4 spectrofluorometer.

Photocatalytic measurements

Typically, 10 mg of the photocatalyst was dispersed in 20 mL of an aqueous solution containing 4 mL of methanol, then the solution was transferred into a quartz vessel. After being vacuumed for 20 min to remove the dissolved air, the vessel was irradiated by a 300 W Xenon lamp under simulated solar light (200–1100 nm) with a light intensity of 250 mW cm⁻². The gas products generated from photocatalytic water splitting were analyzed periodically using an Agilent 7890A gas chromatograph (GC) with a thermal conductivity detector (TCD).

Photocurrent measurements

The electrochemical experiments were performed on a CHI 660E electrochemistry work station in Na₂SO₄ (0.5 M) solution. Platinum wire and the AgCl/Ag electrode (saturated with KCl) were used as the counter electrode and the reference electrode, respectively. The working electrodes were made by loading the samples on indium-tin oxide (ITO). In detail, 10 mg of the photocatalysts was dispersed in 200 μL of deionized water, and then the obtained slurry was dropped onto ITO to form a square with the area of 1 cm² followed by drying at 50 °C for 30 min. Following dropping 20 μL of Nafion on the surface of the photocatalysts, the photoelectrodes of hollow TiO₂, hollow TiO₂@Pt, Pd@TiO₂ and Pd@TiO₂@Au were obtained. Light illumination was provided by a 300 W Xe lamp with the light intensity of 300 mW cm⁻².

Results and discussion

The synthesis process of the sandwich-like hollow-structured Pd@TiO₂@Au photocatalyst is shown in Fig. 1. First, ultrasmall Pd NPs were obtained by using an MFI type zeolite (silicalite-1) as the host matrix and [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ as the metal precursor, respectively, using the method developed by our group.³¹ Specifically, the precursor of Pd NPs was introduced to the hydrothermal crystallization process of silicalite-1 (S-1). After calcination and followed by reduction with H₂, the ultrasmall Pd NPs confined in silicalite 1 (Pd@S1) were obtained. Subsequently, Pd@S1 was covered with a layer of amorphous TiO₂ after mixing Pd@S1 in tetrabutyl titanate (TBOT) and ammonia solution at 45 °C for 24 h. Following calcination at 450 °C for 2 h, Pd@S1@TiO₂ with anatase TiO₂ was obtained. Afterwards, Pd@S1@TiO₂ was transformed into hollow-structured Pd@TiO₂ after being treated with the NaOH solution. In this way, the sandwich-like hollow-structured Pd@TiO₂@Au was achieved by photo-reducing HAuCl₄ on the outer surface of Pd@TiO₂. For comparison, hollow TiO₂ and conventional TiO₂ NPs were also prepared by using pure S1 as the sacrificial substrate and without adding zeolites, respectively. In addition, Pd-im@TiO₂ with larger Pd NPs was achieved by growing TiO₂ on the outside surface of Pd-im/S1 (obtained using the impregnation method). In order to further elucidate the effect of spatially separated cocatalysts on the sandwich-like hollow structured Pd@TiO₂@Au photocatalyst, Pd–Au-im@TiO₂ with both the cocatalysts (Pd and Au NPs) decorated on the inner surface of hollow TiO₂ was prepared.

Fig. 1 Schematic illustration for the preparation of the sandwich-like Pd@TiO₂@Au photocatalyst.

As shown in the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Fig. 2 and Fig. S1, S2, ESI†), S1, Pd@S1, Pd@TiO₂ and the sandwich-like Pd@TiO₂@Au all display the shape of a hexagonal prism with the size of about 200–300 nm. According to the TEM images of Pd@S1 obtained by the in situ process, the ultrasmall Pd clusters (<2 nm) are uniformly located inside the zeolite crystals (Fig. 2a and b). In comparison, the Pd-im/S1 sample prepared by the impregnation method possesses a larger size of Pd NPs (∼10 nm), as shown in Fig. 2c. The TEM image of Pd@S1@TiO₂ in Fig. 2d shows that a layer of TiO₂ with the thickness of about 35 nm covers the outer surface of Pd@S1. As shown in Fig. 2e and f, both Pd@TiO₂ and Pd@TiO₂@Au exhibit a hollow structure. Due to the small size, the inner Pd NPs are invisible. The Au NPs in Pd@TiO₂@Au decorated on the outer surface of hollow Pd@TiO₂ are about 10 nm in size. In contrast, the Pd NPs in the hollow Pd-im@TiO₂ (Fig. 2g) are more obvious for their larger size. In addition, the hexagonal S1@TiO₂ (Fig. 2h) and hollow TiO₂ (Fig. 2i) and conventional TiO₂ NPs (Fig. S3, ESI†) were also obtained.

Fig. 2 TEM image of Pd@S1 in lower (a) and higher magnification (b); (c) TEM image of Pd-im/S1; (d) TEM image of Pd@S1@TiO₂; (e) TEM image of Pd@TiO₂; (f) TEM image of Pd@TiO₂@Au; (g) TEM image of Pd-im@TiO₂; (h) TEM image of S1@TiO₂; (i) TEM image of hollow TiO₂.

The SEM and TEM elemental mappings of Pd@TiO₂ and Pd@TiO₂@Au are shown in Fig. 3a–i and Fig. S4 (ESI†), revealing the existence of Pd, Ti, O and Au elements. The Pd, Ti and O elements are distributed uniformly in hollow-structured Pd@TiO₂ and Pd@TiO₂@Au. Furthermore, it appears that the Au NPs are deposited on the outside surface of hollow TiO₂ in sandwich-like Pd@TiO₂@Au (Fig. S4, ESI†). According to the inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis, the Au and Pd loading in the sandwich-like hollow-structured Pd@TiO₂@Au is 4.89 and 0.62 wt%, respectively.

Fig. 3 Elemental mapping images of Pd@TiO₂ (a)–(d) and sandwich-like Pd@TiO₂@Au (e)–(i): (a and e) the corresponding overlapped elemental mapping images of Pd, Ti, and O and Pd, Ti, O and Au respectively, and the insets in (a) and (e) are the corresponding SEM images; (b and f) Pd mapping images; (c and g) Ti mapping images; (d and h) O mapping images; and (i) Au mapping image.

The as-prepared sandwich-like hollow-structured Pd@TiO₂@Au photocatalyst was further characterized using X-ray diffraction (XRD). The as-synthesized S1 and Pd@S1 exhibit the typical diffraction peaks of the MFI zeolite structure (Fig. S5, ESI†).²² For Pd@S1@TiO₂, the diffraction peaks belonging to anatase-TiO₂ can be obviously seen (Fig. S6, ESI†).²⁹ According to Fig. S6, S7 (ESI†) and Fig. 4a, all the TiO₂-containing samples (TiO₂ NP_S, hollow TiO₂, Pd@TiO₂, hollow TiO₂@Au and Pd@TiO₂@Au) show typical diffraction peaks of anatase-TiO₂. Besides, the peaks of face-centered-cubic Au can be seen in the XRD profiles of hollow TiO₂@Au and Pd@TiO₂@Au.²⁹

Fig. 4 (a) XRD profiles and UV-vis spectra (b) of hollow TiO₂, Pd@TiO₂, hollow TiO₂@Au and sandwich-like Pd@TiO₂@Au; (c)–(f) XPS spectra of sandwich-like Pd@TiO₂@Au.

The UV-vis spectra in Fig. 4b indicate that the existence of both Pd NPs and Au NPs can enhance the light absorption of hollow TiO₂. Furthermore, Pd@TiO₂@Au with spatially separated Pd NPs and Au NPs exhibits much higher light absorption efficiency than the hollow TiO₂ and monometal cocatalyst decorated ones (Pd@TiO₂ and hollow TiO₂@Au). Besides, TiO₂@Au and Pd@TiO₂@Au with decorated Au NPs show much higher light absorption ability in the visible light range, which may be caused by the elevated light absorption due to the SPR effect and the light scattering of Au NPs.²⁷

The X-ray photoelectron spectroscopy (XPS) analyses of sandwich-like Pd@TiO₂@Au are shown in Fig. 4c–f. There are two peaks belonging to Ti 2p that appear at around 458.8 and 464.7 eV, indicating the Ti⁴⁺ valence state in octahedral coordination with oxygen.³² The signal of O 1s exhibits a peak at 530.2 eV corresponding to the Ti–O bonds in the TiO₂ lattice, and the peak located at the shoulder (532.4 eV) is related to the oxygen in the surface hydroxyl groups.³³ The peak at 335.1 attributed to Pd 3d_5/2 is observed in Pd@TiO₂@Au, which is related to the zero-valent Pd NPs.³¹ The Au 4f spectrum displays two peaks at 83.3 and 87.0 eV with the splitting of 3.7 eV, indicating the metallic nature of Au.³⁴

The photocatalytic H₂ evolution activities of sandwich-like Pd@TiO₂@Au together with hollow TiO₂, Pd@TiO₂, and hollow TiO₂@Au were investigated under simulated solar light. In the photocatalytic reaction system, methanol (CH₃OH) was added as the sacrificial agent to react with the holes. As shown in Fig. 5a–b and Fig. S8 (ESI†), all of the hollow-structured photocatalysts exhibit enhanced hydrogen generation compared to the TiO₂ NPs. The H₂ evolution rate of hollow TiO₂ (14.3 μmol h⁻¹) is about 2 times faster than that of the TiO₂ NPs (6.5 μmol h⁻¹ in Fig. S8, ESI†). This phenomenon can be attributed to the elevated light harvesting induced by the hollow structure. Notably, Pd@TiO₂@Au with spatially separated bimetallic cocatalysts shows greatly improved photocatalytic properties. The H₂ generation rate of Pd@TiO₂ and TiO₂@Au reaches up to 210.8 and 119.5 μmol h⁻¹, respectively. Significantly, Pd@TiO₂@Au gives a H₂ generation rate up to 272.3 μmol h⁻¹. For comparison, we also prepared sandwich-like Pd@TiO₂@Pt by using Pt to replace Au to further test whether different separated bimetallic cocatalysts on TiO₂ could still work (Fig. 5c). The morphologies of Pd@TiO₂@Pt, hollow TiO₂@Pt and Pt-im/Pd@S1 are shown in Fig. S9 and S10 (ESI†). As expected, Pd@TiO₂@Pt with similar types of spatially separated cocatalysts (Pd NPs and Pt NPs) shows an enhanced hydrogen evolution rate (648.6 μmol h⁻¹) compared to TiO₂ NPs and hollow TiO₂. In addition, Pd@TiO₂@Pt gives a higher elevated hydrogen evolution rate than Pd@TiO₂ (210.8 μmol h⁻¹) and TiO₂@Pt (464.0 μmol h⁻¹). To further demonstrate the effects of the spatially separated cocatalysts on TiO₂, Pd–Au-im@TiO₂ and Pd–Pt-im@TiO₂ with both bi-noble metal NPs decorated within the hollow cavity were also prepared to compare with Pd@TiO₂@Au and Pd@TiO₂@Pt correspondingly (Fig. 5d). The H₂ generation rates of Pd–Au-im@TiO₂ and Pd–Pt-im@TiO₂ are 159.3 and 264.2 μmol h⁻¹, respectively, which are much lower than that of Pd@TiO₂@Au and Pd@TiO₂@Pt decorated with the spatially separated bimetallic cocatalyst. Therefore, we can conclude that spatially separated bi-noble metals play an important role in enhancing the photocatalytic efficiency.

Fig. 5 (a) H₂ generation over the sandwich-like Pd@TiO₂@Au catalyst, hollow TiO₂, Pd@TiO₂ and hollow TiO₂@Au; (b) H₂ evolution rates based on the corresponding catalysts in Fig. 5a; (c) H₂ generation over the Pd@TiO₂@Pt catalyst and the corresponding controls; (d) H₂ generation over Pd–Au-im@TiO₂ and Pd–Pt-im@TiO₂ for comparison with Pd@TiO₂@Au and Pd@TiO₂@Pt.

On the other hand, the size effect of Pd NPs plays an important role in the photocatalytic efficiency of Pd@TiO₂. For comparison, Pd-im@TiO₂ with larger sized Pd NPs was also obtained by replacing Pd@S1 with Pd-im/S1 to work as the sacrificial substrate. As shown in Fig. S11 (ESI†), Pd-im@TiO₂ shows a lower hydrogen evolution rate (163.5 μmol h⁻¹) than Pd@TiO₂ (210.8 μmol h⁻¹). Therefore, the smaller sized Pd NPs synthesized by in situ crystallization of the zeolite can further improve the catalytic properties of TiO₂. It is worth mentioning that the hydrogen generation rate of sandwich-like Pd@TiO₂@Au (272.3 μmol h⁻¹) with the quantum efficiency of 1.64% (Fig. S12, ESI†) is superior to most of the TiO₂-based photocatalysts for photocatalytic hydrogen generation under similar conditions (Table S1, ESI†).^34–42

Fig. 6a shows the photocurrent generation performance of hollow TiO₂, Pd@TiO₂, hollow TiO₂@Au and sandwich-like Pd@TiO₂@Au. It can be clearly seen that the existence of Pd NPs and Au NPs can improve the photocurrent density. Specifically, the sandwich-like Pd@TiO₂@Au with spatially separated cocatalysts exhibits the highest photocurrent density, demonstrating the efficient photoinduced charge separation properties. To further investigate the charge separation properties of the as-prepared photocatalysts, the photoluminescence (PL) emission spectrum was recorded. As shown in Fig. 6b, all the samples exhibit characteristic peaks at around 400 nm. In detail, all the TiO₂ hybrids with decorated cocatalysts (Pd NPs or Au NPs) show weaker PL emission compared to hollow TiO₂, which may be attributed to the lower Fermi level of Pd NPs and Au NPs.²¹ Furthermore, the PL emission of Pd@TiO₂ is lower than that of hollow TiO₂@Au, suggesting that Pd@TiO₂ possesses greater charge transfer ability than hollow TiO₂@Au. Notably, the sandwich-like Pd@TiO₂@Au exhibits the weakest PL emission among all of the samples, further confirming the superior charge separation performance. To better understand the behaviour of the photo-excited charge carriers, the time-resolved fluorescence decay spectra of the as-prepared photocatalysts were also studied (Fig. 6c). According to the bar chart of PL decay time shown in Fig. 6d, the sandwich-like Pd@TiO₂@Au displays a longer decay time than the other samples, further indicating the superior charge separation of the photocatalyst with spatially separated bimetals.

Fig. 6 Photocurrent generation performance (a), PL spectra (b), PL lifetime decay curves (c) and the bar chart of PL decay time (d) of hollow TiO₂, Pd@TiO₂, hollow TiO₂@Au and sandwich-like Pd@TiO₂@Au.

The proposed mechanism of photocatalytic hydrogen generation over the sandwich-like Pd@TiO₂@Au catalyst is shown in Fig. 7. Upon irradiation by incident light, holes are generated on the valence band (VB) of TiO₂ and photo-induced electrons are produced on the conduction band (CB). Subsequently, the photo-induced electrons can be quickly transferred from TiO₂ to the cocatalysts (Pd NPs and Au NPs).^43,44 In addition, the SPR effect induced by the Au NPs on the outside surface of TiO₂ can stimulate an enhanced electronic field, which can not only improve the generation rate of electron–hole pairs, but also facilitate the charge transfer.⁴⁵ Meanwhile, charge separation can be further improved by the ultrasmall size effect of Pd NPs located on the inner surface of TiO₂.⁴⁶ In this way, more efficient charge transfer and quantum efficiency can be obtained. Moreover, the photo-generated holes on the VB of TiO₂ can be used to oxidize the scavenger (CH₃OH) to produce H⁺ ions. Afterward, the H⁺ ions can combine with the electrons on the CB of TiO₂ or cocatalysts to produce H₂. Specifically, the spatially separated bimetallic cocatalysts can further enhance the photocatalytic properties of sandwich-like Pd@TiO₂@Au due to the enhanced charge separation, as shown in Fig. 6.

Fig. 7 Proposed mechanism of photocatalytic hydrogen generation over the Pd@TiO₂@Au catalyst.

Conclusions

A hollow-structured TiO₂ decorated with spatially separated bimetallic cocatalysts (Pd@TiO₂@Au) was obtained by using a zeolite as the sacrificial substrate. Pd@TiO₂@Au exhibits superior photocatalytic hydrogen generation ability based on the factors below: first, the hollow structure of TiO₂ can not only enhance light utilizing ability by repeated light reflection and refraction, but also provide more active reaction sites and facilitate mass transfer through the enhanced specific surface area. Second, the spatially separated bimetallic cocatalysts (Pd and Au) can further increase the photocatalytic charge transfer and separation by fully utilizing the photo-induced charges on both the inner and outer surfaces of the excited TiO₂. Thirdly, the SPR effect induced by the Au NPs on the outside surface of TiO₂ can stimulate an enhanced electronic field, which can not only improve the generation rate of electron–hole pairs, but also facilitate the charge transfer. Fourth, the ultra-small Pd NPs synthesized by the in situ crystallization process of zeolite can further elevate the charge separation efficiency. The facile approach demonstrated here holds great promise for the design of highly efficient photocatalysts for the application of environmental purification and solar-to-hydrogen energy conversion.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the National Key Research and Development Program of China (Grant 2016YFB0701100), the National Natural Science Foundation of China (Grant 21835002 and 21621001), the 111 Project (B17020), the Post Doctoral Innovative Talent Support Program (BX20180122) and the China Postdoctoral Science Foundation (2019M651195) for supporting this work.

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Footnote

† Electronic supplementary information (ESI) available: Additional characterization results including SEM images, TEM images, EDS mapping images, XRD patterns, and time courses of photocatalytic H₂ evolution. See DOI: 10.1039/d0qm00042f

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