Towards Si@SiO₂ core–shell, yolk–shell, and SiO₂ hollow structures from Si nanoparticles through a self-templated etching–deposition process

Abstract

Si@SiO₂ core–shell, yolk–shell, and SiO₂ hollow structures can be obtained when Si nanoparticles are simply treated with ammonia–water–ethanol solution at room temperature. Their formation mechanism is attributed to the self-templated etching–deposition process.

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Silica wet chemistry is an attractive topic, which has received great attention in recent years.^1,2 With its non-toxicity and highly biocompatible properties, silica shows great potential in biochemistry especially for drug/gene delivery³ and cell imaging.⁴ Silicon is a traditional, but the most important, semiconductor material and has already been applied in many fields such as chips, memory devices, electronics and photovoltaics.^5–7 Silicon is also considered as a promising candidate for the anode of lithium ion batteries because of its ultrahigh theoretical capacity.⁸ In recent years, silicon or silica nanomaterials with various morphologies such as coating,⁹ yolk–shell,^10,11 and hollow structures^12–15 have been developed. For example, Yin and coworkers reported that when an amorphous SiO₂ colloid was treated by NaBH₄ in an aqueous solution, it would undergo a spontaneous dissolution–regrowth process in which these solid particles were transformed to yolk–shell and further hollow SiO₂ structures spontaneously.^1,16 Moreover, yolk–shell¹⁷ or hollow SiO₂ particles¹⁸ were obtained from selectively etching SiO₂ solid particles using poly(vinylpyrrolidone) as protector and NaOH solution as etchant. Cui et al. designed a novel Si@void@carbon yolk–shell nanocomposite in which Si could expand freely without breaking the outer carbon shell during the lithiation–delithiation process. This nanocomposite performed with ultrahigh specific capacity and excellent durability as an anode for lithium ion batteries.^19,20

Herein, we report that when simply treated with ammonia–water–ethanol solution, solid Si nanoparticles could transform to Si@SiO₂ core–shell, yolk–shell and further SiO₂ hollow structures. Although various routes have been developed for the preparation of SiO₂ or Si core–shell, yolk–shell and hollow nanomaterials, to the best of our knowledge, few works have been focused on the fabrication of these structures from pure Si nanoparticles by a solution based etching method. The formation is attributed to the etching of the Si core and self-templated deposition of SiO₂ shell simultaneously induced by the concentration gradient of water in nano-regions. This facile method can advance the design of Si-based materials for potential applications such as anode materials for lithium-ion batteries, catalyst, drug delivery and nanoreactors.^8,10,21,22

The ammonia–water–ethanol solution is composed of 2 ml concentrated ammonia (14.5 M), 120 ml ethanol and a certain volume of deionized water. The morphologies of the obtained materials could be simply modulated by changing the water volume or the reaction time. For easy understanding, we only name the water volume instead of the whole solution component in this paper (e.g., the solution composed of 2 ml concentrated ammonia, 120 ml ethanol and 20 ml water is named as 20 ml H₂O solution). The morphologies of the precursor, commercially available Si nanoparticles (Shanghai ST-Nano Science & Technology Co., Ltd; diameter, 20–160 nm, average diameter is 70 nm, see the ESI†) are given in Fig. 1a1 and a2. Most of these particles tend to merge together to form a chain-like structure. The Si nanoparticles exhibit high crystallinity and from the zoomed image (Fig. 1a2) of the marked area in Fig. 1a1, a barely amorphous coating is observed on the surface of this particle. However, treatment with 20 ml H₂O solution even for a short time of 10 min can cause the formation of Si@SiO₂ core–shell structure. A loose amorphous shell with the thickness of ca. 10 nm (Fig. 1b1) is coated on the surface of the highly crystal Si core (Fig. 1b2). With the elongation of the treating time, the Si core is continuously etched and Si@SiO₂ yolk–shell (Fig. 1c1) or SiO₂ hollow structures (Fig. 1d1) are further obtained. The yolk–shell nanoparticles show a highly crystalline but rougher core and a denser amorphous shell (Fig. 1c2) compared with that of the Si@SiO₂ core–shell structure. Fig. 1c3 and 1d2 give the elemental distribution investigation carried out by elemental mapping and line-scan profile, which further prove the formation of Si@SiO₂ yolk–shell and SiO₂ hollow structures.

Fig. 1 (a1, a2) Pure Si nanoparticles; samples after reacted with 20 ml H₂O solution for (b1, b2) 10 min, (c1, c2, c3) 1 h, and (d1, d2) 24 h.

Si@SiO₂ core–shell structures can also be achieved when Si nanoparticles were treated by ammonia–ethanol solution with various water concentrations (Fig. 2a1, a2, b and c1–c3). However, these SiO₂ shells show much difference not only for the thickness but also the structural integrity and permeability. Briefly, the etching–deposition process is accelerated accompanied by more vigorous gas release with increasing water concentration in ammonia–ethanol solution, which results in the formation of much rougher SiO₂ shells. From the HRTEM images, a homogeneous SiO₂ coating is produced when Si nanoparticles are treated by 10 ml H₂O solution for 24 h (Fig. 2a1, a2), and the integrated Si core–SiO₂ shell structure remains unchanged in the 10 ml H₂O solution even after the extended reaction time of 96 h (Fig. 2b). This is due to the dense SiO₂ shell formed under such low reaction rate showing poor permeability, which inhibits the subsequent etching of the inner Si cores. On the other hand, a discontinuous SiO₂ shell emerges when Si nanoparticles are treated by 30 ml H₂O solution for only 10 min (Fig. 2c1–c3). Along with the continuous etching and spontaneous deposition thereafter, these Si core–SiO₂ shell nanoparticles reconstruct to hollow SiO₂ nanoparticles after 1 h reaction (Fig. 2d).

Fig. 2 Samples after reaction with 10 ml H₂O solution for (a1 and a2) 24 h, and (b) 96 h; 30 ml H₂O solution for (c1–c3) 10 min and (d) 1 h; retreatment of the samples using 20 ml H₂O solution for 24 h, (e) sample obtained from 10 ml H₂O solution treatment for 24 h and (f) sample obtained from 20 ml H₂O solution treatment for 1 h.

To probe the permeability of these SiO₂ shells, re-treatment by 20 ml H₂O solution for 24 h was carried out on the Si@SiO₂ core–shell structure obtained from 10 ml H₂O solution (Fig. 2a1 and a2) and the Si@SiO₂ yolk–shell structure from 20 ml H₂O solution (Fig. 1c1–c3). From the results shown in Fig. 2e and f, the core–shell structure still remains unchanged, while in the yolk–shell structure, the Si cores are totally removed after retreatment. Pore structural parameters of these two materials (core–shell structure obtained from 10 ml H₂O solution and the Si@SiO₂ yolk–shell structure from 20 ml H₂O solution) show that the latter has larger specific surface area (SSA) and total pore volume (TPV) than the former (Table S1†). Hollow structures obtained from 10 ml and 20 ml H₂O solution have also been measured. After treated with 20 ml H₂O, the obtained hollow silica has larger SSA and TPV but a smaller micropore surface area (Table S1†). This indicates that the properties of the SiO₂ shell, mainly its porosity and permeability, are also much dependent on the H₂O concentration. Since H₂O can promote the etching of Si, deposition of SiO₂ and release of H₂ gas,^23–25 slow reaction rate under low H₂O concentration results in the deposition of compact SiO₂ shell which can prevent the further etching of Si cores, whereas under large H₂O concentration, loose and permeable SiO₂ shell is generated. Further experiments have been carried out to see the effects of ammonia concentration on the structure transformation. The results show that a high concentration of ammonia can accelerate the structure transformation from yolk–shell to hollow, which can be seen elsewhere in the ESI.†

Si + 2NH₃·H₂O + H₂O → (NH₄)₂SiO₃ + 2H₂ (1)

(NH₄)₂SiO₃ + H₂O → 2NH₃·H₂O + SiO₂ (2)

According to the discussion above, the possible formation mechanism is put forward and the scheme is shown in Fig. 3. It is commonly known that Si can be etched by alkaline aqueous solutions such as NaOH or KOH aqueous solution.^23–25 When ammonia–water–ethanol solution is used as a weak etching medium, NH₃·H₂O and H₂O is firstly reacted with Si (Reaction 1 in Fig. 3) as shown in eqn (1). It can be clearly observed that (1) the etching rate is much dependent on the H₂O concentration; (2) Si is etched with the release of H₂, and faster H₂ release can cause much rougher SiO₂ shells;²⁵ and (3) the rapid hydrolysis of (NH₄)₂SiO₃ leads to the deposition of SiO₂ coatings (eqn (2)). From this point of view, it seems that only Si@SiO₂ core–shells can be formed during the etching of Si and subsequent deposition of SiO₂. However, the permeability of the SiO₂ shell is much notable in the formation procedure. Under low H₂O concentration, the slow reaction rates of Si etching, SiO₂ deposition and H₂ release lead to the formation of dense SiO₂ shells with poor permeability, which could prevent the Si core from further etching, resulting in the formation of only Si@SiO₂ core–shell structure. On the other hand, when Si nanoparticles are treated by solutions with certain high H₂O concentrations, loose SiO₂ shell with high permeability can primarily be formed possibly owing to the fast release of H₂. In this case, NH₃·H₂O and H₂O is allowed to diffuse inside of the SiO₂ shell, thus causing the continuous etching of the Si core. Due to the consumption of water during etching of the inside Si core, hydrolysis of (NH₄)₂SiO₃ (Reaction 2) is inferred to be inhibited inside the SiO₂ shell. However, when the (NH₄)₂SiO₃ diffuses out of the permeable SiO₂ coatings, there is enough water for (NH₄)₂SiO₃ to hydrolyze, leading to the formation of the outer SiO₂ shells. Consequently, yolk–shell structure and further SiO₂ hollow structure are fabricated by self-templated deposition. In this procedure, etching proceeds from the beginning of the reaction while SiO₂ deposition appears to have a delayed onset, which is very similar to the work reported by Yin et al.¹

Fig. 3 Schematic illustration of the formation of Si@SiO₂ core–shell, yolk–shell and SiO₂ hollow structures. The blue part represents SiO₂, and the brown part represents Si.

In summary, we report the spontaneous transformation of solid Si nanoparticles to Si@SiO₂ core–shell, yolk–shell, and SiO₂ hollow structures when treated with ammonia–water–ethanol solution. The structure of these particles and permeability of the SiO₂ shell can be modulated by the H₂O concentration and reaction time. The transformation mechanism is attributed to the etching of the Si core and self-template deposition of the SiO₂ shell.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51202009 and 51272019), New Teachers' Fund for Doctor Stations, Ministry of Education of China (20120010120004), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, additional data on Si nanoparticles, Si@SiO₂ core–shell, yolk–shell and SiO₂ hollow nanoparticles. See DOI: 10.1039/c4ra02236j

‡ These authors contributed equally to this work.

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