Encapsulation of Au nanoparticles with well-crystallized anatase TiO₂ mesoporous hollow spheres for increased thermal stability

Abstract

Uniform Au_encap/TiO₂ hollow microspheres, in which sub-10 nm Au nanoparticles are coated with a mesoporous anatase TiO₂ shell, are prepared by a protected-calcinating process. The method involves the preparation of SiO₂@Au@TiO₂ colloidal composites, sequential deposition of carbon and then SiO₂ layers through solvothermal and sol–gel processes, crystallization of TiO₂ by calcination and finally etching of the inner and outer silica layers to produce the hollow structure. The protected crystallization process suppresses the excessive growth of TiO₂ and eventually produces mesoporous anatase shells with high surface area. Additionally, by simply controlling the crystallization temperature, it can be convenient to tune the porosity and crystallinity of the TiO₂ shell, meanwhile maintaining the small size of Au nanoparticles. When used as the catalyst for the reduction of 4-nitrophenol, the synthesized Au_encap/TiO₂ catalysts exhibit significantly enhanced catalytic performance. Moreover, the obtained Au_encap/TiO₂ hollow spheres show a superior thermal stability, as it resists sintering during the additional calcination at 500 °C, whereas the sample prepared by deposited Au nanoparticles on commercial P25(Au/P25) was found to sinter severely.

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1. Introduction

The advantages of nanosized noble metal particles in heterogeneous catalysis have been widely recognized in the past few years.^1,2 At the nanoscale, the particles exhibit fascinating physical, chemical and biological properties that are often different to the bulk counterparts, which bestows them with high potentials for catalysis, sensor, biomedicine and so on.^3–5 As one of the most investigated nanoparticle systems, gold nanoparticles (Au NPs) have been successfully applied in various catalytic reactions including low-temperature CO oxidation,⁶ photodegradation of organic pollutants,⁷ Fischer–Tropsch reaction⁸ and so forth.⁹ Recent advances in colloid chemistry have enabled us to prepare Au NPs with tunable size, shape and composition.^10,11

However, challenges still exist. A bottleneck problem for Au nanocatalysts is their lack of stability at realistic technical conditions.⁶ It is well known that Au NPs with small sizes are usually tend to diffuse, coalesce and sinter under the harsh conditions, and that leads to loss of the unique properties registered in the original nanoparticles.¹² To solve the problem of migration and aggregation, many strategies have been carried out. Among them, encapsulating the nanoparticles in a porous oxide shell have been proposed as one of the effective means to improve the thermal stability of the metal nanocatalysts.^13–15 Using this strategy, many catalysts with core–shell or yolk–shell structures have been prepared, in which the individual gold particle is isolated by the oxide shell (MO_x). The shell restrict the migration of the neighboring gold particles, and therefore the particle can be prevented from sintering. Meanwhile, construction pore in the oxide shell may allow the chemical species to reach the buried catalytic particles and therefore the reaction activity is improved.^16,17 Nevertheless, in this architecture, usually named nanoreactors,¹⁸ only one metal particle exists in each composite particle. The little gold contents in the catalysts may affect their catalytic performance. Actually, many obtained MO_x coated Au nanocatalysts shows lower activity in comparison with the supported Au/MO_x catalysts with the same catalysts dosage.^6,19,20

To increase the amount of Au NPs in catalytic particles, people have developed a new construction that combines the supported metal nanoparticles and surface-encapsulation methods.^14,21,22 In their systems, the Au loading is increased largely and the catalytic activity is enhanced to some extent. However, the inorganic coating used here is mostly limited to SiO₂, for it's easy to synthesize and high-temperature stability.^13,23 It is generally believed that the SiO₂ is an inert support, which has weak metal–support interaction.²⁴ In many catalytic reactions, SiO₂ does not contribute to the catalytic activity, and thus the gold supported on SiO₂ catalysts usually possess poor activity.^25,26 As a result, the metal oxides are of increasing interest matrix for its synergistic effect in the promotion of catalytic reaction. However, the metal oxides are mainly used as the support to disperse the metal particles still now. In this architecture, the degree of integration between Au NPs and metal oxides is relatively low, which deceases the contact area of the two components. It is well known that the surface encapsulation can significantly increase the interaction of the active metal and the oxide shell. Thus, it is highly desirable to encapsulate the supported Au NPs with metal oxide.

Generally speaking, two methods, “surfactant-templating” and “surface-protected etching”, have been used to synthesis the permeable SiO₂ shells.^27,28 However, few reports about the preparation of the activated metal oxide porous shell, especially for the crystalline TiO₂. One big challenge is that classical calcination schema to crystallization the amorphous TiO₂ often leads to significant decrease in the surface area of the TiO₂ shells.²⁹ To solve this problem, Yin and us have recently proposed many methods based on “surface-protected calcination” and obtained anatase TiO₂ with well-developed crystallinity and favorable mesoporous structure.^30,31 These greatly inspire us to develop novel porous anatase TiO₂ shell encapsulated Au nanoparticles.

Herein, we report such a gold nanocatalyst composed of porous anatase TiO₂ hollow shell with Au NPs embedded in the inner surfaces (Au_encap/TiO₂, Fig. 1). The structure is different from those of reported Au@TiO₂ catalysts for abundant sub-10 nm gold particles in one catalytic particle in our catalyst system.²⁰ The synthetic procedure involves several sequential steps as follows: (1) synthesis of SiO₂@Au@TiO₂ colloidal composites through sol–gel processes, (2) sequential coating of the colloidal composites with carbon and SiO₂ layer, (3) calcination to crystallization of the TiO₂ shell, (4) remove the overall silica by etching with NaOH solution. Thanks to the effective metal–TiO₂ interaction, the obtained catalysts show superior thermal stability, as the encapsulated Au NPs can resist sintering up to 500 °C in air. Impressively, when these Au_encap/TiO₂ microspheres are used as the catalyst for reduction of 4-nitrophenol (4-NP), they exhibit considerably enhanced catalytic activity and reusability.

Fig. 1 Schematic illustration of our silica-protected calcination procedure developed for the encapsulation of supported Au nanoparticles with mesoporous anatase TiO₂ hollow spheres.

2. Experimental section

Synthesis of SiO₂/Au/TiO₂

The SiO₂/Au/TiO₂ were prepared by modified the strategy described by Yin.³² Briefly, spherical silica microspheres, about 200 nm in diameter, were first synthesis by mixing tetraethylorthosilicate (TEOS, 1.72 mL), concentrated ammonia solution (28 wt%, 1.24 mL), ethanol (46 mL) and deionized water (8.6 mL) at room temperature for 6 h, and then functionalized with amino groups by refluxing a mixture of isopropanol (50 mL) and 3-aminopropyl-triethoxysilane (APTES, 1 mL) at 80 °C for 12 h. In a separate reaction, the Au nanoparticles were synthesized by mixing a solution of HAuCl₄ (356 μL) with deionized water (75 mL) and sodium citrate solution (40 mg mL⁻¹, 0.55 mL), quickly injecting a sodium borohydride solution (0.1 mol L⁻¹, 4.5 mL) under vigorous stirring. The resulting citrate-stabilized Au nanoparticles, 5 nm in diameter, were then adsorbed on the SiO₂–NH₂ colloids. The TiO₂ layer was deposited by mixing the SiO₂–Au colloidal spheres with hydroxypropyl cellulose (HPC, 0.2 g), ethanol (50 mL) and deioned water (0.2 mL). After stirring for 30 min, a mixed solution containing titanium tert-butoxide (TBOT, 2 mL) and 10 mL anhydrous ethanol was slowing injected, and then followed by refluxing at 85 °C for 100 min. The particles were isolated by centrifugal separation, washed with ethanol. To obtain a thicker TiO₂ shell, the TiO₂ coating process was repeated.

Sequential coating with carbon and SiO₂

The carbon layer was prepared by a solvothermal reaction. In a typical experiment, the SiO₂/Au/TiO₂ spheres were mixed with a 20 mL PVP solution (M_w ∼ 50 000, 0.5 g), then centrifuging the mixture to remove unbound polymer and dispersing the solid into a mixture of ethanol, deioned water (5 mL) and glucose (1 g). The mixture was then sealed into a Teflon-lined autoclave. After reaction at 180 °C for 12 h, the carbon coated particles were centrifuged, washed and dispersed in a PVP solution for additional surface modified. Then the suspension was mixed with ethanol (36 mL), water (0.86 mL), aqueous ammonia (0.64 mL) and TEOS (0.86 mL), followed by stirring for 6 h, the SiO₂/Au/TiO₂/C/SiO₂ composite particles were collected by centrifugation and washing.

Calcination and etching

The calcination and etching process was according to the method described in ref. 30. Briefly, the calcination was operated in a muffle furnace at desired temperature for 4 h. The remove of the silica layer was by the chemical etching using NaOH as the etching agent, and the product was labeled as “Au_encap/TiO₂(C)”.

Characterization

Transmission electron microscopy (TEM) experiments were conducted on a JEM-1230 microscope operated at 100 kV. The samples for the TEM measurements were suspended in ethanol and supported onto a Cu grid. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance Diffractometer (Germany) with Cu Kα radiation (λ = 1.5406 Å). The nitrogen adsorption and desorption isotherms were measured at −196 °C on an ASAP 2020 (Micromeritics USA). Prior to measurements, the sample was degassed at 120 °C for 10 h. The specific surface area was determined from the linear part of the BET equation (P/P₀ = 0.05–0.25). The pore size distribution was derived from the desorption branch of the N₂ isotherm using the Barrett–Joyner–Halenda (BJH) method. UV-Vis absorption spectra analysis was performed on a Shimadzu UV 3600 spectrometer.

Catalytic reduction of p-nitrophenol (4-NP)

To study catalytic properties of the prepared Au/TiO₂ hollow nanoparticles, reduction of 4-NP by NaBH₄ was chosen as a model reaction. In a typical experiment, aqueous solutions of 4-NP (0.2 mL, 0.005 M) and NaBH₄ (2 mL, 0.2 M) were added to a quartz cuvette. After adding 0.5 mL of aqueous dispersion of the catalyst particles (0.5 mg mL⁻¹), the yellow solution was gradually faded as the reaction proceeded. The reaction temperature was maintained at 25 °C and UV-Vis spectra of the solution were recorded every 2 min to monitor the progress of the reduction reaction.

3. Result and discussion

The work is to synthesize a catalytic system with enhanced thermal stability and high Au loading in which several sub-10 nm Au nanoparticles are encapsulated in each well-crystallized mesoporous TiO₂ hollow shell. To obtain the nanocatalyst, multi-steps were carried out, of which the first step was to prepare SiO₂/Au/TiO₂ composite spheres by sequential deposition of Au and TiO₂ on SiO₂ bead. Fig. 2a and b showed the typical TEM images obtained after Au and TiO₂ deposition, respectively. Tens of Au NPs were seen on each SiO₂ microspheres surface. All the Au NPs possessed approximately the same size with about 4.6 nm in size. The Au/SiO₂ composite spheres were then coated with a layer of amorphous TiO₂ by hydrolyzing TBOT in the presence of HPC. HPC served as a surfactant that modified the surface characteristic of SiO₂ and facilitated the deposition of TiO₂.³³As revealed by TEM images, the deposited TiO₂ layer was uniform with about 12 nm thick. No major changes in the size of Au NPs (average size: 5.57 nm) were observed after being coated with TiO₂. It was usually suggested that the strong chemical affinity between Au and primary amines of APTES can protected Au NPs from sintering in the SiO₂ coating process at room temperature.^21,22 Similarly, in our coating process, the strong electrostatic interaction was still robust to prevent the Au NPs from growing even in a relatively high temperature (85 °C).

Fig. 2 Representative TEM images of (a) Au/SiO₂, (b) TiO₂/Au/SiO₂ and (c) Au/TiO₂-P25.

In the second step, the TiO₂/Au/SiO₂ particles were subjected to a solvothermal process, where the glucose was carbonized to form carbon coated particles. In our previous work, to avoid the dissolution of SiO₂ core in the carbonization process, pure ethanol was used as the solvent instead of conventional water, and the thickness of carbon layer can be conveniently controlled by tuning the concentration of glucose.³⁰ In this process, the carbonized of previously dissolved glucose in ethanol may offer the water for the dissolution of residual glucose and promote the further carbonization of glucose. In the work, to accelerate the carbonization rate, we added a small amount of water in the solvothermal process to dissolve more glucose. The result showed the time of carbonization can be reduced to 12 h, and meantime SiO₂ core was well reserved (date not show.)

The carbon coated particles were followed by an encapsulation process with the outer SiO₂ layer as described by Yin.^29,31 In order to crystallize the TiO₂ shell, the composite microspheres were subjected to calcination treatment at different temperature. After removal of the SiO₂ species in NaOH solution, the crystal structure of the Au_encap/TiO₂ samples were characterized by XRD measurements. Fig. 3 illustrated the XRD patterns of the final samples at different calcination temperature. The Au_encap/TiO₂ (C0, 500) sample (direct calcination at 500 °C without the C and SiO₂ coating) exhibited a significant improvement in crystallinity of anatase TiO₂ even at a lower calcination temperature. The introduction of C and SiO₂ layer may offer the space for TiO₂ to grow into anatase TiO₂, meanwhile inhibit the over-growth of TiO₂, as evidenced by the broader XRD peaks corresponding to anatase TiO₂. With the calcination temperature increased, the peaks were enhanced correspondingly, demonstrating that the grain size of anatase TiO₂ nanocrystals could be well-controlled by tuning the calcination temperature. The XRD pattern also told us that the growth of Au NPs were suppressed for all the TiO₂ coated samples, as the XRD peaks corresponding to Au were indistinct. The sandwich structures with the Au NPs embedded inside the TiO₂ shell protected the former from moving together and coagulating in the calcination process,³² even for the Au_encap/TiO₂ (C0, 500) sample, in which the TiO₂ shell was extensive crystallization and grain growth.

Fig. 3 XRD patterns of the Au_encap/TiO₂ samples. (a) Au_encap/TiO₂ (C0, 500), (b) Au_encap/TiO₂ (C, 500), (c) Au_encap/TiO₂ (C, 600) and (d) Au_encap/TiO₂ (C, 650).

Characterization of TEM also confirmed the minor changes of Au NPs in the preparation process of Au_encap/TiO₂ catalyst. As shown in Fig. 4, it can be clear that the Au_encap/TiO₂ samples remained virtually unchanged upon calcinations and etching, demonstrating that the TiO₂ shell was quite effective at preserving the size and distribution of the Au NPs even in the growth of TiO₂ grains, which should be attributed to the strong interaction between Au and TiO₂ by the encapsulation. To better identify Au and TiO₂ particles, the magnified TEM image was given in the inset of Fig. 4a. The crystal lattices with d-space equivalent to 0.35 nm and 0.23 nm were corresponded to (101) planes of anatase TiO₂ and (110) planes of Au NPs, respectively. Notably, Au NPs of the Au_encap/TiO₂ (C0, 500) was slightly bigger with respect to the carbon and silica coated samples, accompanied with the severe structure collapse. It was known that significant increase in the grain size of the anatase nanocrystals was inevitable without the protection of the outer carbon and silica layer, which may reduce the interaction between Au and TiO₂ particles and result in a growth of Au NPs. Another unfavorable effect was that the significant increase in the grain size of the TiO₂ shell may block the outside molecules to reach the buried active Au NPs (see later.)

Fig. 4 TEM images of the prepared Au_encap/TiO₂ catalysts. (a) Directly calcined TiO₂/Au/SiO₂ at 500 °C and the magnified TEM image (inset). (b) Calcined the SiO₂/TiO₂/Au/SiO₂ particles at 500 °C. (c) and (d) Calcined the SiO₂/TiO₂/Au/SiO₂ particles at 650 °C. All the samples were calcined in air for 4 h and etched by NaOH in the final step.

N₂ physisorption measurements were further used to investigate the structure properties of the Au_encap/TiO₂ samples. The N₂ adsorption/desorption isotherms together with the corresponding pore size distribution of the samples were shown in Fig. 5. As can be observed, both the Au_encap/TiO₂ (C, 500) and Au_encap/TiO₂ (C, 650) samples exhibited type IV curves according to the IUPAC nomenclature with two hysteresis loops in the relative pressure range of 0.4–1.0, revealing bimodal pore-size distributions in the mesoporous and macroporous region.³⁴ The first hysteresis loop at a low relative pressure (0.4–0.8), corresponding to the filling of the mesopores formed by the stacking of small TiO₂ grains. The second hysteresis loop at a high relative pressure (0.8–1.0), corresponding to the filling of the macropores produced by remove of inner silica core. The surface area are calculated to be as high as about 341.7 m² g⁻¹ and 297.8 m² g⁻¹ for the Au_encap/TiO₂ (C, 500) and Au_encap/TiO₂ (C, 650) samples, respectively. The slightly decrease of the special surface area should be account for the controlled grain growth of TiO₂ by coating with carbon and silica. The detailed pore size distribution calculated based on the BJH method also revealed the growth of TiO₂ grains, as evidenced by an increase in the pore diameter after increased the calcination temperature form 500 °C to 650 °C. By contrast, the capillary condensation steps of the Au_encap/TiO₂ (C0, 500) sample was shifted to a higher relative pressure (P/P₀ = 0.6–1.0), which indicated a larger average pore size. Indeed, the pore size distribution curve confirmed that the Au_encap/TiO₂ (C0, 500) sample possessed mesopores of about 20 nm. The surface area was decreased to 153.2 m² g⁻¹ correspondingly. As mentioned before, without the protection of carbon and silica shells, the grain size of the anatase nanocrystals were significant increased, and thus lead to severe structure breakage of TiO₂ shell and significant decrease in surface area.

Fig. 5 N₂ adsorption–desorption isotherms and corresponding pore size distribution curves (inset). (a) Au_encap/TiO₂ (C0, 500), (b) Au_encap/TiO₂ (C, 500), (c) Au_encap/TiO₂ (C, 650).

A better identifying the environment around Au NPs was obtained through the UV-visible absorption characterization of the Au_encap/TiO₂ samples (Fig. 6). As shown in Fig. 6a, the absorption band at 523 nm was arised from the excitation of surface plasmon mode of the Au NPs in the SiO₂ surface. By contrast, when the original Au/SiO₂ particles were covered with the TiO₂ shell, the surface plasmon peak became too weak to be observed (Fig. 6b), possibly because the dense TiO₂ shell confined the transfer of free electrons to the surroundings, which was suggested to give rise to the plasmon resonance.³⁵ However, after converting the TiO₂ shell to a porous one, the characteristic absorption band of Au NPs returned (Fig. 6c and d), which suggested that Au NPs became exposed again after calcination treatment. Additionally, the maximum absorption peak was strengthened for the Au_encap/TiO₂ (C, 650) sample with respect to the Au_encap/TiO₂ (C0, 500), further demonstrating that Au NPs of the Au_encap/TiO₂ samples prepared by the protected calcination were more convenient to contact with the environment.

Fig. 6 UV-vis absorption spectra of (a) the original Au/SiO₂ particles, (b) TiO₂/Au/SiO₂ particles, (c) Au_encap/TiO₂ (C0, 500) particles and (d) Au_encap/TiO₂ (C, 650) particles. All the particles were dispersed in ethanol solution.

It was also noteworthy that the peak plasmon absorption of the Au_encap/TiO₂ samples obviously red shifted as compared with the as-prepared Au/SiO₂ sample, and the Au_encap/TiO₂ (C0, 500) sample exhibited a maximum shift. Generally, two reasons, encapsulation and particle aggregation, were accounted for the red shift.³⁶ In this work, the shift in plasmon from 523 nm for the original Au/SiO₂ particles to 547 nm for the Au_encap/TiO₂ (C, 650) was principally upon encapsulation, due to the increase in the refractive index of the composite spheres upon the incorporated Au NPs by crystalline TiO₂ shells. In contrast, the further red-shift of the Au_encap/TiO₂ (C0, 500) sample was mainly caused by the increase in the Au particle dimension. As mentioned before, the Au NPs of the Au_encap/TiO₂ (C0, 500) sample was slightly bigger than Au_encap/TiO₂ (C, 650), and thus leading to a red shift of about 10 nm.

The reduction of 4-NP by sodium borohydride was chosen as a model reaction^21,27,37 for studying the catalytic performances of the Au/TiO₂ catalysts. Since only a small amount of catalyst particles were used, light scattering due to the Au and TiO₂ was negligible in comparison to characteristic absorption by 4-NP at 400 nm. No change in the absorption peak at 400 nm was determined after standing for 2 h, suggesting that the reduction reaction did not proceed in the absence of Au catalyst. After addition of a trace amount of catalysts into the solution, the reaction proceeded, as evidenced by the decrease in the absorption at 400 nm and the gradual increase in the absorption at 295 nm. Considering the concentration of NaBH₄ largely exceeds that of 4-NP, the reaction rate can be assumed to be independent of NaBH₄ concentration. So a pseudo-first-order rate kinetics with respect to 4-NP can be applied to evaluate the catalytic rate.^38,39 Fig. 7a showed the linear relationships between ln(C_t/C₀) and reaction time in the reaction catalyzed by different Au/TiO₂ samples, and the plots well matched the first-order reaction kinetics. According to the linear relationship, we calculated the reaction rate constant k from the slope of the straight lines. The rate constant k of the as-prepared TiO₂/Au/SiO₂ particles (without calcination treatment) was 0.18 min⁻¹. While for the catalysts after the protecting calcination at 500 °C, the value of k increased to 0.41 min⁻¹, revealing a good activity of Au catalysts after calcination process. Increased the calcination temperature, k further increased to 0.65 min⁻¹ and 1.03 min⁻¹ for 600 °C and 650 °C, respectively, clearly demonstrating that the efficiency of catalysts was improved with the increase of the calcination temperature.

Fig. 7 (a) Relationship of ln(C_t/C₀) and reaction time for the reduction of 4-NP catalyzed by different samples. (b) Conversion of 4-NP in 4 successive cycles of reduction with the Au_encap/TiO₂ (C, 650) and Au_encap/TiO₂ (C0, 500).

As discussed above, the calcination treatment can help to increase the catalytic activity, which might be ascribed to the following reasons. First, mesopores in TiO₂ shells formed by the calcination treatment accelerated the mass transfer rate of the 4-NP. It is known that the pore of the pristine sol–gel derived TiO₂ shell (in TiO₂/Au/SiO₂ catalyst) is restricted to micropore, which would make it difficult for 4-NP to reach the buried Au NPs and result in a relatively lower reactive activity. The calcination process can successfully transform the TiO₂ shell into the mesoporous one, and thus better mass diffusion is achieved for the catalysis. Second, the encapsulation of Au NPs with TiO₂ layer is quite effective at preventing aggregation of the Au NPs even in the growth of TiO₂ grains. The preserving of the size of the Au NPs may retain the number density of active sites for the reduction reaction. Third, the higher crystallinity of TiO₂ may promote the electron transfer between Au and TiO₂, which may enhance the metal–support interaction. Thus, the calcination treatment has a significantly influence on the catalytic activity and higher catalytic activity can be obtained as for the Au_encap/TiO₂ (C, 650) catalyst. As a comparison, the amorphous TiO₂ in TiO₂/Au/SiO₂ catalyst not only prevents the diffusion of 4-NP to the buried Au surface, but also possesses the weaker metal–support interaction, thus the as-prepared TiO₂/Au/SiO₂ exhibited the lowest activity for the reduction of 4-NP.

To investigate the reusability, the catalysts were separated from solution by centrifugation, rinsed with deionized water and reused for the next cycle of catalysis. As shown in Fig. 7b, only a slight decrease in the catalytic activity was observed for Au_encap/TiO₂ (C, 650) after 4 successive cycles, demonstrating the remarkable stability of Au_encap/TiO₂ (C, 650). It should be pointed out that though the initial activity of the Au_encap/TiO₂ (C0, 500) was higher than Au_encap/TiO₂ (C, 500), the conversion dropped severely after 4 cycles. Such decrease in activity can be explained by the serious loss of catalyst during centrifugation and washing. The Au_encap/TiO₂ (C0, 500) is composed by the small particles fragments, which are difficult to settle down from the solution with respect to the intact Au_encap/TiO₂ particles and unavoidably leading to a serious loss of catalyst. Apparently, the unbroken anatase TiO₂ shell synthesized by protected-calcination is sufficient for stabilizing Au NPs by preventing their aggregation and erosion.

Finally, to investigate the thermal stability of the catalyst, the Au_encap/TiO₂ particles were further calcined at 500 °C for 4 h. For comparison, a supported catalyst was prepared by mixing the commercial P25 TiO₂ with as-synthesized Au colloids and evaporating the aqueous solution at 70 °C (Au/TiO₂-P25). The size of Au NPs in the as-prepared Au/TiO₂-P25 catalyst was about 6.3 nm (Fig. 2c), which was comparable to that of TiO₂/Au/SiO₂ particles (5.57 nm). After calcined, Au NPs already started to coalesce and sintered to 43 nm on the Au/TiO₂-P25 sample (Fig. 8a) On the other hand, exposure of the TiO₂ protected samples to similar treatments resulted in virtually no changes in the Au diameter after calcination. Over these catalysts, the integrity of the TiO₂ shells were varied. Re-calcination of the Au_encap/TiO₂ (C, 500) samples led to a change in the texture of the hollow structure with respect to the Au_encap/TiO₂ (C, 650) samples, apparently because the re-growth of TiO₂ nanocrystals. The anatase TiO₂ was well developed after the initial calcination at 650 °C. As a result, the TiO₂ crystal growth in Au_encap/TiO₂ (C, 650) catalyst were limited. However, the growth of TiO₂ in Au_encap/TiO₂ (C, 500) sample were inhibited in the initial calcination at 500 °C by the SiO₂ shell, thus the further TiO₂ crystallization was inevitable after removed the SiO₂ layer. But it should be underlined that only a small fraction of the hollow structure was affected, most of the TiO₂ shells maintained their structure after re-calcination. Thus, the results clearly indicated that our catalysts exhibited excellent sinter resistance and superior thermal stability.

Fig. 8 TEM images of the samples after further calcination at 500 °C. (a) Au/TiO₂-P25, (b) Au_encap/TiO₂ (C, 500), (c) Au_encap/TiO₂ (C, 650).

4. Conclusions

We have demonstrated a successful synthesis of Au_encap/TiO₂ catalyst constructed by the sub-10 nm Au NPs imbedded TiO₂ nanocrystals via a protected calcination process. The calcination treatment allows the crystallization of TiO₂ in a controllable manner, and leads to a well-developed crystallinity, favorable porous structure and small size of the Au NPs. Moreover, the porosity and crystallinity can be well controlled by the change of calcination temperature. The Au_encap/TiO₂ catalyst shows high activity for the reduction of 4-NP attributed to the small gold size, convenient mass pass and higher crystallinity of TiO₂. The anatase TiO₂ shell also prevents the Au NPs from sintering after an additional calcination at 500 °C. The combination of anatase TiO₂ and Au NPs in our catalyst may extend the application of the particles to photocatalysis.

Acknowledgements

The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant no. 21376051, 21306023, 21106017, and 51077013), Natural Science Foundation of Jiangsu (Grant no. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant no. BA2011086), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20100092120047), Key Program for the Scientific Research Guiding Found of Basic Scientific Research Operation Expenditure of Southeast University (Grant no. 3207043101) and Instrumental Analysis Fund of Southeast University.

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