A convenient way of manufacturing silicon nanotubes on a silicon substrate
Graphical abstract
Introduction
Nano silicon (Si) material attracted extensive interests for their potential and practical applications on account of its rich resources, convenient chemical modifications [1], [2] and different forms of Si nanoparticles (NPs) [3], [4], [5], [6], [7]. For instance, silicon quantum dots have the promises to be used in labeling biomolecules [5]. Silicon nanowires (SiNWs) arrays on a silicon substrate were applied in depositing metallic NPs to improve the sensitivity of some analytic techniques [6], [7]. Many routes of preparing SiNWs arrays [6], [8], [9], [10], [11] on a silicon substrate were described in previous reports. In these reports, nano Ag [10], [12], Au [13], [14] Co [15] or Cu [16] was firstly deposited on a wafer. Then the wafer was put into aqueous solutions of hydrofluoric acids (HF) and hydrogen peroxides (H2O2) for a predetermined time to prepare SiNWs on a silicon substrate.
Compared with SiNWs, the forming of silicon nanotubes (SiNTs) are relatively difficult. According to the previous reports the processes of preparing SiNTs were rather complicate. Chemical vapor deposition (CVD) in nano channels of an aluminium oxide substrate was a candidate to prepare SiNTs [17], [18]. Silane, hydrogen and argon gasses were mixed at a chamber under a defined pressure and temperature for a predetermined time. The decomposition of the silane gases resulted in the deposition of silicon atom on the walls of shaped nano channels until the accomplishment of SiNTs. Molecular beam epitaxy [19] (MBE) was another way of synthesizing SiNTs. SiNTs grew on surface of nano channels of a porous aluminium oxide template under high vacuum and a high temperature. The silicon atoms were supplied by an electron-beam evaporator with a growth rate of 0.07 Å s−1. Patolsky [20], [21] reported an innovative method of preparing SiNTs with well-controlled inner diameter and wall thickness using single-crystalline germanium-core nanowires as a sacrificial base for the epitaxial growth of SiNTs. The germanium nanowires template, obtained by an ultrahigh-vacuum CVD system, could be dissolved ultimately by wet-chemical etching. Such nanostructures of silicon were utilized in making photocathodes for water reduction [22], [23], [24]. Porous Si nanowires, as anode materials, had prospective applications in manufacturing next generation lithium ion battery [25].
Herein, we report a facile route of preparing SiNTs on a silicon substrate without requirements of highly purified reagents and harsh conditions of high vacuum and high temperature. Also there is no need for the expensive equipments. To do so, a porous silicon (PSi) slice was first obtained via electroless chemical etching aided by the deposited Ag NPs through galvanic displacement reaction. Then the SiNTs were formed on the silicon substrate by putting the PSi slice in a solution of cobalt ions (Co2+) and ammonium fluorides (NH4F) at a defined temperature for a defined time.
Section snippets
Materials
Silver nitrate (AgNO3, ≥99.8%), H2O2 (30% in water), HF (40%), Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, ≥99.0%) and NH4F (98%) were purchased from Aladdin (Shanghai) Reagent Company and were used as received without any further purification. Doubly distilled water was used throughout the work. Double-side polished silicon slice (p-type, boron doped, electrical resistivity: 8–10 Ω cm) with a size of 10 mm × 5 mm was chosen to prepare the PSi and the SiNTs.
SiNTs preparation on a silicon substrate and characterization techniques
A Si(111) slice was cleaned with
Preparation of the PSi slice
When a cleaned silicon slice was immersed in aqueous solution of HF and AgNO3, surface Si-Hx (x = 1, 2 or 3) species [27] are produced instantly. The silver ions are reduced by the set Si-Hx species producing Ag NPs-decorated silicon slice [28] according to the reactions of 2≡SiH + 12F− → 2[SiF6]2− + H2 + 2e and 2Ag+ + 2e → 2Ag (herein, the monohydride of SiH species are chosen as an example). The surface Ag NPs are shown in the insert of Fig. 1(a). Surface Si elements are consumed giving [SiF6]
Conclusion
We have prepared SiNTs facilely on a silicon substrate without the needing of harsh synthesis conditions. Such a novel approach provides a convenience way of manufacturing SiNTs. The small NPs of Ag which deposited for preparing PSi slice play a vital role for the formation of SiNTs. It is these Ag NPs that bring about the micro pores on a PSi slice, which facilitate the entering of Co2+ and the successive etching reactions. The inner and outer silicon elements are consumed simultaneously until
Acknowledgements
This work was supported by Natural Science Foundation of Anhui Province (1508085MB31).
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