A simple, rapid, one-step approach for preparation of Ag@TiO₂ nanospheres with multiple cores as effective catalyst

Abstract

Novel Ag@TiO₂ nanostructures with multiple Ag nanoparticles as cores and a crystalline TiO₂ as the outer shell have been successfully achieved via a facile and one-step solvothermal route. The synthetic approach is simple, rapid, and environmentally friendly, in which no any sacrificial template, toxic reagent or surfactant is emploited. Moreover, the as-prepared Ag@TiO₂ products show an uniform and spherical morphology as well as a large specific surface area (225.9 m² g⁻¹). The time-dependent experiments reveal that the formation of Ag@TiO₂ nanospheres includes a nucleation, aggregation and self-assembly process. Apart form this, the rattle-type Ag@TiO₂ nanoparticles can be also obtained by only tuning the amount of tetrabutyl titanate (TBOT) added in the precursor. When employed as catalyst for reduction of 4-nitrophenol (4-NP), the Ag@TiO₂ nanospheres prepared exhibit a superior catalytic activity and a good cycle stability, benefiting from their unique multiple-cored nanostructure and the effective synergistic effect between Ag nanoparticles and TiO₂ shell. The present method also provides a great possibility for preparation of other metal@TiO₂ nanocomposites and their promising applications in catalysis, electrochemistry, and purification, and so on.

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1. Introduction

In recent years, noble metal nanoparticles used as catalysts have attracted significant attention due to their excellent performances for reduction of aromatic compounds.^1–4 For example, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) can be carried out in the presence of some noble metal nanoparticles. However, the agglomeration of metal nanoparticles is inevitable in practical application, which will seriously deteriorate their performances. Therefore, to address the issues, it's necessary to control the crystal size and dispersibility of the particles on the support materials to effectively avoid agglomerating, thus improving their activities. Currently, many materials such as SiO₂,^5–7 TiO₂,^8–10 Fe₃O₄,^11–13 and carbon materials^14,15 possessing high surface area and good chemical stability have been employed as the ideal matrix for constructing the metal@support composites. In the available materials,† TiO₂ is regarded as one of the most extensively studied semiconductor materials, which has been widely employed as an efficient support because of its tailorable physicochemical properties, good structural stability, excellent catalytic activity and cost-effectiveness.^16–18

Recently, a lot of synthetic routes have been developed to prepare the TiO₂-supported metal composite catalysts.^19–22 In most cases, the composites are fabricated by blending or depositing noble metal nanoparticles on the TiO₂ surface.^23–25 Despite this kind of metal–TiO₂ composites can also get good catalytic performance at first, the cycling stability of these catalysts is relatively poor owing to the aggregation and falling off of the active nanoparticles during long-term reaction process. Particularly, the construction of core–shell structured metal–TiO₂ composites has become a promising strategy for protecting the metal particles from aggregation and dropping, thus keeping a good cycling stability for catalysts.^26,27 To prepare the metal@TiO₂ core–shell materials, a two-step method is usually used including the production of metal nanoparticles as core and subsequent coating of TiO₂ shell. For instance, Tang et al.²⁸ reported a hydrothermal route for synthesis of hollow Au@TiO₂ microspheres by controlling the hydrolysis of TiF₄ in Au nanoparticle solution prepared in advance. However, the tedious process enhances the consumption of the raw material and energy, and goes against the large scale production. Furthermore, TiF₄ is often employed as titanium source, which has also a negative impact on environment. Notably, a one-step approach towards Au@TiO₂ core–shell composite was developed by Zhao et al.²⁹ In two cases, most of the materials have only a single metal core with large crystal size. In contrast, multiple nanoparticles as the core are more advantageous than single one as catalyst because that could maximize the synergistic effect between active species and support material to a certain extent.^30,31 Nevertheless, the formation of the nanostructure with multiple cores commonly needs a multistep synthetic process, such as the template-deposition and surface-protected etching method. As far as we know, few work reports the fabrication of metal–TiO₂ composites with multiple nanoparticles as the core only through a simple and one-step method.

Herein, we present a facile, easy, and one-step solvothermal protocol for preparing the Ag@TiO₂ nanospheres with multiple Ag nanocores and the crystalline TiO₂ as the protective shell by only using TBOT, AgNO₃ and ethanol as the starting materials. Based on the time-dependent experiments, we explore the possible formation process for growth of Ag@TiO₂ nanostructure. Besides that, the special rattle-type Ag@TiO₂ nanoparticles can be also achieved by simply tuning the amount of tetrabutyl titanate (TBOT) in the precursor. The reduction reaction of 4-nitrophenol (4-NP) indicates that the Ag@TiO₂ nanospheres demonstrate a superior catalytic activity and a good cycling stability on account of their unique structure, well-defined morphology, and the enhanced synergistic effect between Ag nanoparticles and TiO₂ shell.

2. Experimental section

2.1 Materials

Tetrabutyl titanate (TBOT), ethanol, silver nitrate (AgNO₃), sodium borohydride (NaBH₄), 4-nitrophenol (4-NP), the reagents mentioned were all purchased from Sinopharm Chemical Reagent Co., Ltd. All chemical reagents were used as provided without further purification. Distilled water was used in throughout the experiments.

2.2 Synthesis of Ag@TiO₂ nanospheres

In a typical procedure, 30 ml of ethanol, 0.12 ml of TBOT, and 0.12 ml of AgNO₃ solution (0.1 M) were homogeneously mixed together and stirred at room temperature for 10 min to form a clear solution. Then, the solution was transferred into a Teflon-lined stainless steel autoclave and treated at 180 °C for 6 h. The resulting precipitates were filtered, washed thoroughly with water and ethanol several times, and dried at 60 °C overnight to achieve the Ag@TiO₂ products. To further explore the formation process of the Ag@TiO₂, a series of products were also prepared by varying the precursor compositions and reaction time.

2.3 Characterizations

Scanning electron microscopy (SEM, JEOL JSM-6700F), transmission electron microscopy (TEM, JEOL 200CX), high resolution transmission electron microscopy (HRTEM, JSM-2010F) were used to observe the structural features of the samples. Elemental qualitative analysis was conducted by the energy-dispersive X-ray spectroscopy (EDX, OXFORD INCA). X-ray powder diffraction (XRD) patterns were obtained on the Japan Rigakul D/max-2550 instrument using CuKα radiation (λ = 0.154 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250Xi (Thermofisher, England) using monochromatic AlKα X-ray source operating. Raman spectrum was measured with Renishaw inVia Raman microscope using 514 nm lasers as a light source. N₂ adsorption–desorption isotherms were tested on a QUADRASORB SI surface area & pore size analyzer at 77 K. The Brunauer–Emmett–Teller (BET) specific surface area was calculated from the desorption data. UV-Vis spectra were collected on a HITACHI U-3010 UV-Vis spectrophotometer.

2.4 Catalytic reduction of 4-nitrophenol

The catalytic performances of Ag@TiO₂ products were evaluated through the reduction reaction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH₄) as a model. The whole process was carried out in aqueous media at room temperature. Typically, NaBH₄ solution (0.5 ml, 0.6 M) was firstly added to a 4-NP solution (100 μl, 0.005 M). Thereafter, the Ag@TiO₂ nanospheres solution (1 ml, 1 wt%) was added to initiate the catalytic reduction. Immediately, the solution was quickly subjected to UV-Vis measure in the range of 250–550 nm. In addition, the reusability of the catalyst was also investigated under the same condition.

3. Results and discussion

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are used to observe the structural features of the products. Fig. 1a shows a typical SEM image of the as-prepared Ag@TiO₂ products. Clearly, the product shows a uniform and spherical morphology, and a relatively coarse surface assembled with abundant TiO₂ nanoparticles. Meanwhile, the obtained Ag@TiO₂ nanospheres show a relatively narrow particle size distribution (inset in Fig. 1a) and the average diameter is around 250 nm. Fig. 1b and c present the TEM images of the Ag@TiO₂ nanospheres. The spheres possess a well-defined morphology and good dispersibility, in keeping with the SEM observation. Apart from some small Ag nanoparticles, large one is also occasionally seen, which may come from the growth of small Ag nanoparticles in the composite (Fig. 1c). HRTEM image in Fig. 1d clearly reveals that the small Ag nanoparticles with an average size of 10 nm are homogeneously dispersed in the TiO₂ nanospheres. As marked by the red circles, the unique nanostructure is totally different with the traditional single-core structure, which may make for the full contact and synergistic effect between Ag nanoparticles and TiO₂ outer shell. The elemental composition and distribution of Ag@TiO₂ are studied by STEM analysis. As depicted in Fig. 1e, Ti, O and Ag elements can be well detected and uniformly dispersed throughout the Ag@TiO₂ nanospheres, confirming that the Ag nanoparticles are introduced into the TiO₂ nanospheres. The EDX pattern from Fig. 1f shows that no significant Ag species are recorded on the outer surface, indicating that almost all of the Ag nanoparticles are encapsulated in the TiO₂ chamber, which is consistent with the HRTEM image (Fig. 1d).

Fig. 1 (a) SEM image (inset is the corresponding particle size distribution), (b and c) TEM images, (d) HRTEM image, (e) STEM image and the corresponding elemental mapping, and (f) EDX pattern of the typical Ag@TiO₂ nanospheres.

Fig. 2a shows the XRD pattern of the Ag@TiO₂ product. In the pattern, the strong diffraction peaks at 2θ = 25.4° and 48° are ascribed to the (101) and (200) crystal plane, respectively. Those are usually identified as the characteristic peaks of the anatase crystal phase of TiO₂ (JCPDS no. 21-1272), suggesting a high crystallinity of the product. Besides the diffraction peaks of well-crystallized TiO₂, the sharp peaks at around 2θ = 44.3, 64.4 and 77.4° are assigned to the (200), (220) and (311) facets of face-centered cubic (fcc) Ag (JCPDS no. 87-0720).³² More information on the elemental composition and chemical state of the Ag@TiO₂ sample is provided by XPS technique (Fig. 2b). Both Ti and O elements are observed in the composite with sharp photoelectron peaks appearing at binding energy of 458 eV (Ti 2p) and 531 eV (O 1s), respectively. Additionally, corresponding to the high-resolution XPS spectrum of Ag (inset in Fig. 2b), the Ag@TiO₂ sample shows two distinct peaks at 367 eV and 373 eV, which are ascribed to the binding energy of Ag 3d_5/2 and Ag 3d_3/2, respectively. All the results indicate that the Ag nanoparticles have been successfully encapsulated into the TiO₂ nanospheres.³³

Fig. 2 (a) XRD pattern, and (b) XPS spectrum (inset is the high-resolution XPS spectrum of Ag 3d) of the typical Ag@TiO₂ nanospheres.

To further confirm the microstructure of Ag@TiO₂, Raman measurement is collected at room temperature. As shown in Fig. S1a,† the peaks at 147, 394, and 640 cm⁻¹ are ascribed to E_g, B_1g, and E_g modes, respectively, which belong to the characteristics of anatase TiO₂.^34,35 UV-Vis spectrum indicates that the Ag@TiO₂ has a weak absorption peak in the range of 400–550 nm (Fig. S1b†), which is mainly ascribed to the surface plasmon resonance (SPR) response of Ag cores.³⁶ The porous property of Ag@TiO₂ is further confirmed by the N₂ adsorption–desorption isotherm. In Fig. S2,† the Ag@TiO₂ sample displays a type-IV isotherm with a well-defined hysteresis loop and a narrow pore size distribution at ∼3.4 nm, revealing its mesoporous characteristics. Those are mainly derived from the aggregation of primary nanocrystals, in accordance with the TEM images (Fig. 1c and d). Specifically, the Ag@TiO₂ product shows a high BET surface area of 225.9 m² g⁻¹. Such a large surface area and relatively narrow pore size distribution are in favor of the full contact between catalyst and substrate molecules, as well as the diffusion of substrate molecules.

As we know, the nucleation and growth of the crystals are closely related to the concentration of hydrolyzed species. Some studies have reported that the amount of Ti source has a significant influence on the structure and morphology of the products.^8,37 To this end, the dosage effect of TBOT is systematically investigated by TEM images. As presented in Fig. 3a, when the amount of TBOT solution is reduced by half, the quasi-spherical Ag@TiO₂ nanoparticles are formed and a small amount of aggregation appears. Further decreasing TBOT amount to 0.04 ml (Fig. 3b), the product becomes more disperse. Interestingly, we find that the formed nanoparticles have a distinct hollow structure according to the contrast of light and shade in TEM image, and the small Ag nanoparticles are homogeneously encapsulated into the hollow TiO₂ nanoparticles. The results indicate that the rattle-type Ag@TiO₂ nanoparticles can be easily prepared by simply adjusting the TBOT amount in the precursor. When 0.02 ml of TBOT is employed, this structure is still kept well (Fig. 3c). What's more, the spherical morphology becomes more uniform and the average diameter is about 180 nm. Nonetheless, when we continue to decrease the dosage of TBOT to 0.01 ml, only some irregular and broken TiO₂ nanospheres are produced (Fig. 3d). According to the above observations, we tentatively deduce the possible reason for generation of rattle-type structure. On one hand, with decreasing the TBOT amounts, the formed H₂O amounts from the redox and dehydration of alcohol in the reaction system relatively increase. On the other hand, the Ostwald ripening process easily occurs in the presence of abundant water. That may be the reason for the formation of the rattle-type structure.^38,39 In addition, it is worth mentioning that the Ag nanoparticles inside TiO₂ spheres can grow larger because the hollow structure can provide the space and reduce the obstacle, which may create the chance for the aggregation of Ag nanoparticles to some extent.

Fig. 3 TEM images of the Ag@TiO₂ nanostructures synthesized with different TBOT amounts: (a) 0.06 ml, (b) 0.04 ml, (c) 0.02 ml, and (d) 0.01 ml.

The growth process of Ag@TiO₂ nanostructure is further explored by the time-dependent experiments. Fig. 4 shows the TEM images of structural evolution for those products obtained at different reaction time. At the initial stage, only some irregular Ag nanoparticles and titania clusters are generated when the reaction time is only 20 min (Fig. 4a). Increasing the reaction time to 30 min (Fig. 4b), some solid nanoparticles with ill-defined morphology begin to appear due to the aggregation of these titania building clusters. Further prolonging the reaction time to 1 h (Fig. 4c), the titania building clusters disappear and some spherical nanoparticles are seen. Upon increasing the reaction time to 6 h, the Ag@TiO₂ nanoparticles with well-defined morphology and better dispersibility are obtained at last (Fig. 4d). Fig. 4e briefly depicts the formation process of the Ag@TiO₂ nanostructure. At the initial stage of the reaction, Ag⁺ ions are reduced to Ag nanoparticles by the hydroxyl groups of ethanol during the high-temperature solvothermal process. Then, the generated H₂O by the reduction reaction and dehydration of ethanol further promotes the hydrolysis of the titanium species. Subsequently, the Ag nanoparticles are quickly coated by the produced titania building clusters. More significantly, these TiO₂ clusters can also prevent the Ag nanoparticles from aggregation. As the hydrothermal reaction proceeds, the TiO₂ building clusters gradually aggregate into nanospheres in order to reduce the surface energy of the whole system. In the process, the TiO₂ nanocrystals begin to grow, resulting in the formation of porous outer shell. At last, the Ag@TiO₂ nanospheres with uniform morphology and unique nanostructure are achieved.⁴²

Fig. 4 TEM images of the Ag@TiO₂ nanospheres synthesized at different reaction time: (a) 20 min, (b) 30 min, (c) 1 h, and (d) 6 h; (e) schematic illustration for the growth process of the Ag@TiO₂ nanospheres.

To investigate the catalytic performance of Ag@TiO₂ nanospheres, the reduction reaction of 4-nitrophenol (4-NP) by NaBH₄ in aqueous phase is applied as a typical example.⁴¹ The previous studies have proved that NaBH₄ cannot reduce 4-NP without any catalyst,^6,40 even though keeping the mixed solution of 4-NP and NaBH₄ for a long time. Yet, once adding the NaBH₄ solution, the absorption maximum shifts from 316 nm to 400 nm, meaning that 4-NP has been completely deprotonated at the presence of NaBH₄ and 4-nitrophenolate ion forms (Fig. 5a). The intensity of the absorption peak at 400 nm gradually decreases as the reaction continues (Fig. 5b), while a small shoulder at 300 nm gradually increases owing to the formation of 4-aminophenol (4-AP). Remarkably, the full conversion of 4-NP to 4-AP can be performed within 4 min, showing an outstanding catalytic activity of the Ag@TiO₂ nanospheres. Besides that, the catalytic performance of pure Ag nanoparticles is also tested. As shown in Fig. S3,† the catalytic efficiency of pure Ag nanoparticles is slightly higher than that of Ag@TiO₂ nanostructures in the first cycle. That may be due to that the Ag nanoparticles are totally exposed in the reaction solution at the beginning. However, the cycle stability of the Ag nanoparticles is poor due to the particles aggregation and lack of protection in subsequent cycles.

Fig. 5 (a) UV-Vis spectra of 4-nitrophenol before and after adding NaBH₄ solution, (b) time-dependent UV-Vis spectra for the reduction of 4-NP with the Ag@TiO₂ as catalyst, (c) cycling stability, and (d) TEM image of Ag@TiO₂ nanospheres after 5 cycles.

The results also demonstrate that although Ag nanoparticles are entirely covered by TiO₂ shell in the composite, the permeable TiO₂ porous shell can still enable 4-NP molecules to access the Ag nanoparticles, thus facilitating the subsequent catalytic reaction. Not only that, the presence of the porous TiO₂ shells can keep the good dispersivity of the Ag nanoparticles in the reaction, suggesting a good cycling stability. Furthermore, the unique nanostructure with multiple cores can not only provide more active sites and accelerate the transfer of the substrate molecules, but also maximize the synergistic interaction between Ag nanoparticles and TiO₂ shell, which contribute to the superior catalytic performance. Beyond that, as can be seen from the digital photos (inset in Fig. 5b), the color of the resultant liquid turns from light yellow to absolutely transparent colorless, revealing a full reaction process has been finished. Nevertheless, in terms of catalysts, the cycling stability is another important index. As shown in Fig. 5c, the Ag@TiO₂ nanospheres exhibit a robust cycle stability, the degradation rate of 4-NP is still kept above 96% even after 5 cycles. Especially, the morphology and structure of the Ag@TiO₂ nanospheres are still well remained after the 5th cycle (Fig. 5d). The result further proves that the as-prepared Ag@TiO₂ nanospheres have a good structural stability benefiting from the active Ag nanoparticles protected by the porous TiO₂ shell, which are suitable for more applications including catalysis, nanoreactor, and so on.

4. Conclusions

In conclusion, we have demonstrated a facile, scalable, and one-step hydrothermal route for synthesis of the Ag@TiO₂ nanospheres with a high specific surface area and uniform morphology. Compared with the traditional metal@TiO₂ core–shell nanostructure, the multiple Ag nanoparticles as core are homogeneously dispersed in the resulting Ag@TiO₂ nanospheres, which provides more active sites and greatly enhances the synergistic effect between Ag nanoparticles and TiO₂ shell. The reduction of 4-NP indicates that the Ag@TiO₂ nanospheres show an excellent performance including a superior catalytic activity and good cycling stability. This work may also open a new window for preparing other metal@TiO₂ nanostructure with multiple nanoparticles as core. Besides that, such a hybrid nanostructure can also be used in photocatalysis, electrochemistry, and purification, etc.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (11275121, 21471096, 21371116), and Program for Innovative Research Team in University (IRT13078).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21173a

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