The fabrication process for silicon sub-oxide nanotubes (SiO 2 NTs) is illustrated schematically in Fig. 1. An amorphous layer of SiO 2 is deposited onto commercial AAO templates via vapor phase deposition through thermal degradation of PDMS in air under vacuum. Our improved synthesis procedure eliminates the need to sand excess SiO 2 deposits off of the fragile AAO templates after deposition. By placing the AAO templates downstream from, rather than covering, the PDMS blocks and performing the deposition process under vacuum, we are better able to control SiO 2 deposition throughout the AAO template, preventing unwanted thick deposits of SiO 2 . SiO 2 conformally coats all exposed surfaces of the AAO including the top and bottom of the template, creating a connected tubular network of SiO 2 . The AAO is subsequently removed via a heated phosphoric acid bath to leave SiO 2 NTs. After rinsing several times in DI water to remove phosphoric acid, the tubes are sonicated to separate the bundles of SiO 2 NTs into individual tubes. The connected SiO 2 NT network obtained after AAO removal is not mechanically sound, therefore the tubes must be sonicated apart so that they may be handled facilely in powder form. A 20 nm coating of SiO 2 on a 13 mm diameter AAO with a thickness of 50 μm produces a volumetric density of SiO2 of 0.515 gcm−3 and an areal density of 2.57 mgcm−2.

Figure 1 Schematic representation of the fabrication process for SiO 2 NTs. Full size image

SEM images in Fig. 2 reveal the tubular morphology of the SiO 2 NTs as well as their high aspect ratio. Bundles of SiO 2 NTs occur due to deposition of SiO 2 on the tops and bottoms of the AAO templates, but brief sonication serves to easily liberate the tubes. The SEM image in Fig. 2(a) reveals the excellent uniformity of the SiO 2 coating across all dimensions of the AAO templates and the interconnected nature of the SiO 2 NTs after removal of the AAO template. These small bundles occur after a brief period of sonication and further sonication serves to fully separate all of the tubes, as seen in Fig. 2(b). The tubes have a very high aspect ratio of 250:1 at a length of 50 μm and an average diameter of 200 nm. Lengthy periods of sonication fully separate and shorten the SiO 2 NTs, revealing their tubular morphology as seen in Fig. 2(c). The SEM image in Fig. 2(d) reveals the branched morphology of the SiO 2 NTs, which serves to further increase the surface area of the tubes and is purely a result of anodization in the presence of aluminum imperfections43.

Figure 2 SEM image of (a) partially separated SiO 2 NTs showing NT bundles and dispersed NTs, (b) fully separated dispersed SiO 2 NTs, (c) separated SiO 2 NTs showing tubular morphology and (d) image showing branched morphology. Scale bars are 25 μm, 10 μm, 2 μm and 1 μm for (a), (b), (c) and (d), respectively. (e) TEM image of dispersed SiO 2 NTs and inset showing tube diameter and wall thickness. Scale bar is 400 nm and 50 nm for the inset. (f) XRD analysis of SiO 2 NTs. Full size image

TEM images reveal the wall thickness is 20 nm and highly uniform throughout the length of the tubes as in Fig. 2(e). The branched nature of the NTs is confirmed via TEM and no evidence suggests porosity exists in the walls. TEM confirms the SiO 2 NTs have an average diameter of 200 nm, which is expected given the commercial AAO template specifications. Based on the highly random fracture patterns generated via sonication, we conclude that the tubes are composed of amorphous SiO 2 . XRD analysis confirms that the SiO 2 NTs are amorphous as seen in Fig. 2(f). Regardless of initial crystallinity, SiO 2 NTs will undoubtedly be amorphous in subsequent cycles44,45.

Scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) are further performed to confirm the composition of the as-prepared nanotube samples. The STEM-EDS sample was simply prepared via transferring vacuum-dried SiO 2 NTs onto a copper TEM grid. As shown in Figure 3a, SiO 2 NTs are randomly oriented and a selected area EDS mapping was performed on the region within the yellow box. EDS microanalysis on the selected region shows the SiO 2 NTs consists of primarily Si and O (Figure 3b). EDS element mapping micrographs of Si and O suggest a very uniform distribution of these two elements. Traceable amount of C, Al, P (wt% < 1%) were observed due to carbon contaminates, unetched AAO and unremoved H 3 PO 4 etchant, respectively. An EDS quantitative analysis on the selected region was performed to characterize the weight and atomic percentages of elements and to confirm the existence of SiO 2 , as in Fig. 3 e–f. We believe the contaminants and the copper grid both contribute to the oxygen peak due to the existence of respective oxides from each.

Figure 3 (a) SEM image of SiO 2 NTs. (b) EDS spectra of SiO 2 NTs on selected region (yellow rectangle) of image a. (c–d) show the EDS microanalysis of element Si and O for this selected region. Scale bar: 5 μm. (e–f) EDS quantitative analysis of selected region. Full size image

The Langmuir and BET surface areas were measured to be 45.17 and 26.64 m2g−1, respectively, for the as-prepared SiO 2 NTs. The obtained surface area and pore distribution suggest the as-prepared SiO 2 NTs have limited surface area and porosity as in Fig. 4. This surface area is more than double the surface area of as-received AAO templates which is expected given the tubular morphology46. This limited surface area is beneficial in reducing the amount of SEI layer formation in the first few cycles by limiting active material contact with the electrolyte47. However, the rate capability suffers as a result of the limited surface area due to a higher reliance on bulk diffusion of Li into SiO 2 .

Figure 4 (a) Type IV N 2 adsorption and desorption isotherms for SiO 2 NTs. (b) Pore size distribution of SiO 2 NTs. Full size image

The electrochemical performance of SiO 2 NTs was characterized by fabricating 2032 coin cells with SiO 2 anodes and Li metal counter electrodes. Cyclic voltammetry (CV) was performed in the 0–3.0 V range with a scan rate of 0.1 mVs−1, shown in Fig. 4(a). The CV plot is shown to 1.75 V to emphasize the noteworthy reactions taking place at lower voltages. Decomposition of the electrolyte and formation of the SEI layer occurs at the broad peak of 0.43 V as in Fig. 5(a). A much broader, less discernable peak occurs at 1.40 V which can be attributed to a reaction between electrolyte and electrode and the beginning of SEI formation48. Both of these peaks become undiscernible in the 2nd cycle suggesting SEI formation takes place mostly during the first cycle and that these initial reactions are irreversible. During the initial charge cycle a noticeable peak occurs at 0.33 V, which can be attributed to dealloying. In subsequent cycles this peak becomes very pronounced and shifts downward to 0.25 V. The sharpening and growth of this dealloying peak implies a rate enhancement in the kinetic process of delithiation of SiO 2 NTs. The kinetic enhancement may be due to the formation of an embedded nano-Si phase as it has been reported that one of the oxidation peaks of Si is 0.25 V during Li extraction from Li x Si49. By the 10th cycle there is an emergence of an anodic peak located at 0.22 V while the peak at 0.01 V has decreased. It is known in the literature that the 0.01 V and 0.22 V peaks are associated with the lithiation of Si48,50. The CV curves are in good agreement with the charge-discharge profiles in Fig. 5(c) and Fig. 5(d).

Figure 5 (a) CV of SiO 2 NTs using a scan rate of 0.1 mVs−1. (b) Charge-discharge capacities versus cycle number using a C rate of 100 mAg−1. (c) Galvanostatic voltage profiles for SiO 2 NTs at a C/2 rate at selected cycles. (d) Galvanostatic voltage profiles for SiO 2 NTs at selected C rates. Full size image