Microporous and Mesoporous Materials 174 (2013) 10–13

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Microporous and Mesoporous Materials

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Short Communication

Self-sealing SiO2 pores on silicon formed by oxidation of microporous silicon

Changyong Zhan a, Yu Zou a, Jianchun Wu a, Ding Ren a, Bo Liu a, Kaifu Huo b, Ningkang Huang a,Paul K. Chu b,⇑a Key Laboratory of Radiation Physics and Technology of Education Ministry of China, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, Chinab Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

a r t i c l e i n f o

Article history:Received 13 December 2012Received in revised form 18 January 2013Accepted 21 February 2013Available online 4 March 2013

Keywords:Silicon poresMicroporous siliconSilicon dioxide poresIon beam sputteringMagnesium

1387-1811/$ - see front matter � 2013 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.micromeso.2013.02.030

⇑ Corresponding author. Tel.: +852 34427724.E-mail address: paul.chu@cityu.edu.hk (P.K. Chu).

a b s t r a c t

Self-sealing silicon pores with size of 140–160 nm are formed by electrochemical etching. The micropo-rous silicon (MPS) covers the openings and side walls of straight silicon pores (SSPs). The self-sealing SiO2

pores with diameters of 195–205 nm are fabricated from the self-sealing SSPs in an aqueous alkalinemedium containing magnesium via oxidation of MPS. The thicknesses of the surface layers coveringthe SSPs and SiO2 pores are 200 nm and 140 nm, respectively, and the depth of the pores is more than60 lm. The SiO2 making up the sub-micron pores has a nanoporous structure which allows water andhydrogen molecules to pass through in an aqueous solution.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

SiO2 pores have potential applications in separation, functional-ization, guiding of highly charged ions, and so on [1–6]. It can beused in organic–inorganic nanocomposite materials by graftingwith organic groups [6,7] and to separate colloids, particles, andmicroorganisms in aqueous solutions [2,6,8]. Owing to the diversefunctionalities, a wide range of SiO2 pore size and hydrophilicsurface is needed. Nanoporous and mesoporous SiO2 has beenfabricated by direct synthesis in surfactants [3,5,7,9] and thecross-section of the SiO2 pores is limited to about 10 nm or lessby the size of the surfactant aggregates [10]. Sub-micrometerSiO2 pores with size between 100 nm and 1 lm have attractedattention in recent years due to the demand of big pores in separa-tion and functionalization. Sub-micrometer SiO2 pores can be pre-pared by colloidal crystallization [10], supercritical fluids [5], andthermal oxidation of silicon pores [4]. Macroporous silica with amultilamellar structure is synthesized by mixed-surfactant-basedsynthesis [8], but it is still a challenge to fabricate hierarchically or-dered macro–mesostructures efficiently [6].

Microporous silicon (MPS) can be oxidized by OH� in water toform silicon oxide. For instance, a transparent silicon dioxide filmis formed when microporous silicon is placed in an aqueous alka-line medium containing Mg [11]. Electrochemical etching has beenshown to be an effective method to produce macro- or micro-pores

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in silicon. Microporous silicon can be formed on straight macro-pore walls on p-type silicon [12–14] and microporous siliconcovering the top opening of straight macropores in n-type silicon[13–15]. This offers a new method to prepare sub-micrometerSiO2 pores by oxidizing porous silicon produced by electrochemicaletching. Crystalline silicon can be used as the substrate to improvethe hardness of the SiO2 pores and the Mg + H2O system yields abetter hydrophilic surface with Si-OH bonds [11].

In this work, microporous silicon covering both the pore walland opening is prepared by electrochemical etching in an HF solu-tion. Sub-micrometer SiO2 pores are formed by oxidizing themicroporous silicon on crystalline silicon and the SiO2 shows ananoporous structure. The pores are characterized and the mecha-nism is discussed.

2. Experimental details

The straight silicon pores (SSP) with MPS covering the top open-ing and pore wall were prepared on 1–60 X-cm n-type (100) sili-con by electrochemical etching in an HF and ethanol mixtureusing a 2-electrode configuration with graphite as the counterelectrode under room light. The volume ratio of HF (48%) to etha-nol and etching current were 1:1.2 and 120 mA, respectively, witha limiting voltage of 50 V to avoid higher voltage puncturing of thesurface nanostructure. The etched area was about 3 cm2 and etch-ing time was 10 min. The etched sample was put in water contain-ing the Mg alloy (AZ31) which provided OH� ata pH value of 7–8.The reaction under alkaline conditions is:

C. Zhan et al. / Microporous and Mesoporous Materials 174 (2013) 10–13 11

2H2OþMg!Mg2þ þ 2OH� þH2 reaction: ð1Þ

The prepared MPS had many Si-H dangling bonds [11,16] andMPS reacted with OH� to form SiO2:

� Si-HþH2O ¼ �Si-OHþH2 reaction and ð2Þ� Si-Si� OHþH2O ¼ �Si-O-Si-OHþH2 reaction: ð3Þ

Mg was not detected by X-ray photoelectron spectroscopy (XPS)meaning that the OH� produced in reaction (1) served as a catalystin reactions (2 and 3). The MPS was oxidized to SiO2 to form theself-sealing SiO2 pores. Oxidization of MPS in Mg + H2O proceededslowly for several hours. The SiO2 was removed by 5% HF and final-ly the silicon macropores were obtained from the SSP.

To characterize the inner structure of the etched samples, ionbeam sputtering was used to remove the surface covering thepores. The current and voltage of argon ions were 14 mA and1.9 kV, respectively. The sputtering time was 20 min and the sput-tering rate was about 30 nm/min. The difference in the sputteringrates of silicon, MPS, and SiO2 helped to distinguish the structuresand no chemical reaction occurred during ion sputtering.

A power supply (ITECT IT6123) was used to electrochemicallyetch n-type silicon. Ion beam sputtering was performed in a plas-ma immersion ion implantation system offering multiple functionssuch as ion implantation and film deposition. Scanning electronmicroscopy (SEM, JEOL JSM-6335F) and field-emission scanningelectron microscopy (FE-SEM, JEOL JSM 7001F) were utilized tocharacterize the structures of the samples. Au was evaporated onthe samples prior to electron microscopy examination. X-ray pho-toelectron spectroscopy (XPS) was performed to determine thechemical shifts.

3. Results and discussion

Fig. 1 depicts the SEM image of the sample produced using40 mA/cm2 and volume ratio of HF to ethanol of 1–1.2. The etchedsurface is uniform and disordered pores are formed using the opti-mized etching parameters. The pore opening is not clear due to thesurface cover. The pore density is 0.65–0.85 lm�2 which is higherthan those reported previously [17–19]. Fig. 2(a) and (b) depict thecross-section of the SSPs and SiO2 pores. Fig. 2(a) shows that thebreakdown of the space charged region and current burst playkey roles in the surface and deep etching, respectively [17–19].Fig. 2(b) shows that the SiO2 pores are obtained by oxidizing the

Fig. 1. SEM image of the sample produced using 40 mA/cm2 and a volume ratio ofHF to ethanol of 1–1.2.

Fig. 2. Cross-section images: (a) SSPs and (b) SiO2 pore. (c) Si2p XPS spectra ofsurfaces of SSP and SiO2 pore samples.

MPS covering SSP in (Mg + H2O), indicating that water can passthrough the surface oxide layer to oxidize the MPS at a depth.Fig. 2(c) displays the Si2p XPS spectra of the surfaces of the freshlyprepared SSP and SiO2 pores. The Si2p peaks of the SSP and SiO2

pore samples are at 99.2 eV and 103.2 eV, respectively, confirmingthe formation of SiO2. The O/Si atom ratio is 2.13 which implies theexistence of extra O due to bonded H2O. Complete conversion ofMPS into hydrated dioxide in the alkaline solution can be inferredfrom infrared spectra [11] and results in the literature [16]. Asshown in Fig. 2(a), the surface and side wall of the SSPs are coveredby MPS and it is different from the published structures [12–14].The SSPs underneath the surface MPS has a pore tip where currentburst begins during electrochemical etching. As shown in Fig. 2(b),the SiO2 pores have a similar self-sealing structure. However, thethicknesses of the surface layers on SSP and SiO2 pores are200 nm and 140 nm, respectively, and the SiO2 pore diameter isbigger. This confirms a shrinkage of 30% when the sample isexposed to air compared to surface MPS due to the porosity of

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MPS and dehydration of SiO2(H2O)x. Even so, the SiO2 making upthe pores shows a porous structure as shown in Fig. 2(b).

Fig. 3(a) and (b) show the cross-section of the SSP and SiO2 poreconfirming the existence of MPS and SiO2, respectively. Althoughthe SiO2 pore is covered by a surface SiO2 film, water can penetratethe surface SiO2 film to oxidize the inner MPS. This means that thesurface SiO2 film inherits the porous structure of PS in the aqueoussolution. Oxidization of silicon proceeds gradually along the micro-or meso-pores in SiO2(H2O)x. The micro- or meso-pore size inwater cannot be determined accurately because of shrinkage inair. The depth of the SSP and SiO2 pore can be more than 60 lmand depends on the duration of electrochemical etching. With re-gard to the self-sealing SSP, the depth reaches a maximum of about60 lm, because a longer time destroys the surface and inner MPS.The diameters of the SSP and SiO2 pores are in the ranges of 140–160 nm and 195–205 nm, respectively. The uncertainty in themeasured data is caused by nonuniform etching of Si. The thick-nesses of the MPS and SiO2 in the deep pores are 120–160 nmand 100–160 nm, respectively and the aspect ratios are up to 400for SSP and 300 for the SiO2 pores.

To examine the pore side shape, an ion beam is used to sputterthe surface. Fig. 4(a) and (b) show the SEM images of the SSPs andSiO2 pores and the inset shows a single pore after about 600 nmhas been sputtered off. The structure around the pore is different.Owing to shrinkage of SiO2(H2O)x during conversion from MPS,the SiO2 pore evolves into a nearly circular shape. The size of sput-tered openings is 50–75 nm and 105–120 nm for the SSPs and SiO2

pores, respectively. The self-sealing submicrometer SiO2 pores can

Fig. 3. Cross-section images (in depth): (a) SSP and (b) SiO2 pore.

Fig. 4. SEM images: (a) SSPs and (b) SiO2 pores after sputtering off about 600 nmwith the inset showing a single pore.

be transformed to ring holes and through-holes with no chemicalreaction by using ion beam sputtering.

To confirm that NPS in the deep SSP is oxidized, a solution con-taining HF and ethanol is used to remove the surface and innerSiO2. MPS does not dissolved in the HF solution. Fig. 5 (a) and (b)show the surface and cross-sections of the Si square macroporesderived from the SiO2 pores in Si, respectively. The size of the Simacropores is between 320 and 500 nm, confirming nonuniformelectrochemical etching of the side walls of the pores. Fig. 5(b) cor-roborates that the depth of the pores is more than 60 lm. Manybranches are found on the macropore walls, but The SSP and sub-micrometer SiO2 pores show no such branches.

The presence of Mg yields a moderate pH value (7–8) for theoxidation of MPS. NaOH is also used to oxidize the microporous sil-icon but does not show the same effectiveness. Oxidation of MPS isa slow process as shown in the aforementioned reactions (2) and(3). Reaction (2) works only on surface Si–H bonds and dehydra-tion between two neighboring Si–Si–OHs forms the Si–Si–O–Si–Sibonds. Si–O–Si bonds are stronger than Si–Si bonds and the Si-Sibonds are destroyed enabling oxidization. The rapid reaction be-tween NaOH and MPS increases the dehydration time enabling dis-solution of silicon atoms in NaOH. A low concentration of NaOH atpH of 7–8 renders control of the reaction rate difficultly.

The submicrometer SiO2 pores prepared by oxidation of micro-porous silicon in silicon can be controlled by electrochemical etch-ing parameters. This method yields straight SiO2 pores with a

Fig. 5. (a) Surface and (b) Cross-section images of Si macropores derived from SiO2

pores.

C. Zhan et al. / Microporous and Mesoporous Materials 174 (2013) 10–13 13

multilamellar structure. Future studies will focus on the oxidationof MPS and etching of SSP as they impact potential applications toseparation, functionalization, and guiding of highly-charged ions.

4. Conclusion

Self-sealing SSPs with size of 140–160 nm are formed byelectrochemical etching. The MPS covers the openings and sidewalls of the SSPs and self-sealing SiO2 pores with diameters of195–205 nm are fabricated from the self-sealing SPSs in an aque-ous alkaline medium containing Mg. The depth of the pores is morethan 60 lm. Oxidization of MPS to SiO2(H2O)x on crystalline siliconis a new method to prepare nanostructured SiO2 pores and formsub-micrometer and nanosize structures at the same time in anaqueous solution.

Acknowledgments

The work was financially sponsored by Hong Kong ResearchGrants Council (RGC) General Research Funds (GRF) Nos. CityU112510 and 112211 and National Natural Science Foundation ofChina Nos. 11005076, 11075112 and 11205107.

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