Jin Niu‡
,
Su Zhang‡,
Yue Niu,
Ranran Song,
Huaihe Song*,
Xiaohong Chen,
Jisheng Zhou and
Song Hong
State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing, 100029, P. R. China. E-mail: songhh@mail.buct.edu.cn; Fax: +86-10-6443-4916; Tel: +86-10-6443-4916
First published on 9th June 2014
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.
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).
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.
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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