Release of Hydrogen During Transformation from Porous Silicon to Silicon Oxide at Normal Temperature

international journal of hydrogen energy 36 (2011) 4513e4517

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Release of hydrogen during transformation from porous silicon to silicon oxide at normal temperature

Changyong Zhan a,b,*, Paul K. Chu b,**, Ding Ren a, Yunchang Xin b, Kaifu Huo b, Yu Zou a, N.K. Huang a a Key Laboratory of Radiation and Technology of Education Ministry of China, Institute of Nuclear Science and Technology, Sichuan University, Chengdu, 610064, China b Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China article info abstract

Article history: The use of porous silicon as an energy carrier is investigated. NaOH and solid Mg alloy are Received 12 October 2010 used to introduce OH in water to react with the porous silicon and the porous silicon Accepted 1 January 2011 treated with Mg alloy in water is converted to transparent silicon oxide hydride. The Available online 5 February 2011 amount and release rate of hydrogen from the reaction between porous silicon and water are determined and the efﬁciency is also studied. The total amount of released hydrogen Keywords: does not vary much with the pH value but the release rate is sensitive to the pH value. The Hydrogen release average amount of hydrogen produced form porous silicon can reach 63.2 mmol per gram

Porous silicon of porous silicon. A moderate rate of about 1.77 mol of H2 per mol of porous silicon can be Magnesium obtained per day with the aid of the Mg alloy. This technique is potential useful in Silicon oxide supplying hydrogen to fuel cells at normal temperature. Electrochemical etching Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1. Introduction Porous silicon which can be readily formed in an HF solution and organic solvent has promising applications in that The energetic behavior of nanocrystalline porous silicon is of hydrogen can adhere chemically [8]. nano-porous silicon with practical interest because porous silicon can be used as an a porosity of 90% and pore size of 5 nm can absorb energy carrier [1e3]. An energy carrier which can release 34 mmol g 1 and the maximum amount of molecular energy gradually, continuously, and controllably is necessary hydrogen per gram of the initial porous silicon nanostructure for miniaturized fuel cells. Pichonat and Gauthier-Manuel is determined to be about 100 mmol in theory [5]. Chemical, reported the realization of miniaturized fuel cells by using heating and photoactivating treatments to nano-porous a porous silicon membrane and silane grafted on the pore silicon for producing hydrogen have been studied [5,7,9,10]. walls and showed that the supply of hydrogen was one of the An ammonia solution has been used in the chemical treat- limitations of the fuel cells [4]. Porous silicon has many SieH ment, but hydrogen desorption is not efﬁcient due to the bonds which can react with OH to produce H2 suggesting that incomplete reaction. The amount of hydrogen released from the materials can serve as a hydrogen reservoir [5,6] and it also the chemical treatment of porous silicon and the possible has a possible application in a photoelectrochemical system release rate have not been explored systematically by exper- for water splitting [7]. iments. Mg should be a good material in the chemical

* Corresponding author. Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China. Tel.: þ86 28 85412230; fax: þ86 28 85410252. ** Corresponding author. Tel.: þ852 34427724. E-mail addresses: [email protected] (C. Zhan), [email protected] (P.K. Chu). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.003 4514 international journal of hydrogen energy 36 (2011) 4513e4517

promising applications in portable fuel cells at normal Table 1 e Amounts of hydrogen produced from porous silicon (PS) and reaction time under different conditions. temperature. In this work, we study the reaction between porous silicon Reaction Reactants Reaction Hydrogen and OH under different conditions such as pH values. Our pH time amount (mL) experiments disclose that porous silicon is converted to 12 PS, water, 1.5 h 5.5 transparent silicon oxide in water while releasing hydrogen in NaOH the presence of an OH source. NaOH solution and Mg alloy 11 PS, water, 8 h 5.1 are used as the sources of OH and the amount and release NaOH 7e8 PS, water, 1 day 5 rate of hydrogen from the porous silicon are determined. The Mg alloy mechanism is also proposed and discussed.

2. Experimental details Table 2 e Amounts of hydrogen produced and the reaction time for the porous silicon etched at 150 mA. Porous silicon was prepared from 0.01e0.15 U-cm p-type (100) Reaction pH Reaction time Hydrogen amount silicon wafer by electrochemical etching in an HF and ethanol 7e8 1 day 9.7 mixture using a two-electrode conﬁguration with graphite as 7e8 1 day 10 the counter electrode. The volume ratio of HF solution (48%) 7e8 1 day 8.8 and ethanol was 1:1 during etching progress. The etching current was controlled by a direct power source with voltage and current rating of 72 V/1.2 A. Before etching, the silicon treatment of porous silicon rendering hydrogen desorption was rinsed in acetone, alcohol, and distilled water for 10 min from a controllable hydrogen reservoir by controlling the and then immerged in 5% HF for 10 min to clean the surface quantity of reacting Mg. Porous silicon consumes OH oxide. The etched area was about 2 cm2 and etching time was releasing hydrogen and Mg reacts with water to form H2 and 10 min. Afterwards, the as-prepared porous silicon was dried Mg(OH)2. A self-sustained reaction thus occurs in water and with argon and stored in acetone to reduce oxidization. this system consisting of porous silicon and Mg can be used as Two methods were used to accomplish hydrogen release an energy carrier to supply hydrogen. This method has from the porous silicon. The porous silicon was put in water at

Fig. 1 e Cross-sectional images of porous silicon etched at: (a) 50 mA, (b) 100 mA, and (c) 150 mA; (d) surface and (e) cross- sectional images of transparent silicon oxide ﬁlm formed from porous silicon. international journal of hydrogen energy 36 (2011) 4513e4517 4515

values. The reaction rate diminishes as the pH value decreases. The amount of released hydrogen changes slightly from 5.5 mL to 5 mL at pH ¼ 12, implying that the bulk silicon is only corroded slightly by OH . Table 2 shows the stability of hydrogen produced at pH of 7e8 in the presence of a magnesium alloy from the porous silicon samples etched at the current of 150 mA. The weight of the porous silicon is 0.006 0.0004 g. The maximum amount of hydrogen amount can reach 10 mL. The average amount is 9.23 mL from the reaction which corresponds to 63.2 mmol per gram of initial

porous silicon, that is, about 1.77 mol of H2 per mol of porous silicon per day on the average. Of course, we did not count the hydrogen produced from the reaction of Mg and water in the ﬁnal data. For reference, the theoretical value of hydrogen released from silicon reacting with water is 71.4 mmol g 1 [5]. Our experiments show that the process is stable and controllable. These data mean that the porous silicon as a hydrogen supplier is stable and easily controlled.

3.2. SEM and XPS analyses

Fig. 1(a)e(c) depicts the cross-sectional images of porous silicon prepared at different etching currents. A porous silicon layer 14e35 mm thick can be obtained by adjusting the etching current from 50e150 mA. The porous silicon thickness increases with higher etching current and the porous layer has a loose structure compared to the bulk. The porous silicon is composed of nanopores with sizes of several to more than ten nanometers and silicon nanocrystals with size of several nanometers [11]. The pores increase the surface area thereby making porous silicon chemically more reactive than its bulk e Fig. 2 Fitted Si2p XPS spectra: (a) freshly prepared porous counterpart. silicon and bulk silicon and (b) reacted sample. The SEM images of the surface and cross section of the produced surface layer after separating from the sample reacting in the presence of Mg are shown in Fig. 1(d) and (e). The different pH values and in water with an Mg alloy (AZ31D) materials formed from the porous silicon are transparent silicon þ ¼ 2þ providing OH . The equation of reaction is: 2H2O Mg Mg oxide. Fig. 1(e) indicates it has a very different structure þ þ 2OH H2 which provides alkalescency in water. Different pH compared to the initial porous structure as shown inFig. 1(a)e(c). values of 12, 11, 10, and 7e8 adjusted by NaOH were used to However, if the porous silicon is put in NaOH solutions with determine the release rate and amount of released hydrogen. The pH ¼ 12 and 11, transparent silicon oxide isnot formed due to the displacement method was used to determine the amount of formation of sodium silicate. The results suggest that the pH hydrogen released from the reactions. The reacted samples were value is an important parameter to control the process. blow dried by air and the reacted layers formed from porous Fig. 2(a) displays the ﬁtted Si2p XPS results acquired from the silicon were removed by a 5% HF aqueous solution and an elec- freshly prepared porous silicon and bulk silicon (for compar- tronic precision balance was used for quantiﬁcation. ison). Using the Si2p bonding energy of elemental silicon at

Scanning electron microscopy (SEM, JEOLJSM-6335F) was 99.2 eV as the reference, the Si2p peak of the porous silicon is utilized to characterize the structures and Fourier transform ﬁtted by three peaks. The peak at 99.2 eV is due to elemental infrared spectroscopy (FTIR, PerkinElmer Spectrum 100) was silicon, the one at 98.8 eV arises from SieH, and the one at used to determine the chemical bonds in the freshly prepared 100.5 eV correspond to surface SieOH. The chemical shift and reacted samples. X-ray photoelectron spectroscopy (XPS, towards low energy indicates that porous silicon has a stronger Physical Electronics PHI 5802) was conducted to provide reducibility than silicon due to the existence of SieH dangling chemical information. bond which make porous silicon react with slightly alkalescent solution [12]. Fig. 2(b) shows the XPS results of the transparent silicon oxide ﬁlm formed from porous silicon. This XPS 3. Results and discussion measurement is performed after sputtering off approximately

50 nm from the surface. The two Si2p peaks at 99.5 and 102.9

3.1. Hydrogen amount correspond to SieSi and SiOx (x < 2), respectively [13]. The peak at 105.1 eV may be due to the remaining SieF bond. Mg is not Table 1 shows the amounts of hydrogen released from the detected by XPS meaning that the Mg concentration in the chemical treatment of porous silicon (PS) at different pH product is quite small and serves as a catalyst in the reactions. 4516 international journal of hydrogen energy 36 (2011) 4513e4517

Fig. 3 e FTIR spectra of porous silicon etched at: (a) 10 mA, (b) 50 mA, (c) 100 mA, and (d) 200 mA.

e ¼ It has been reported that porous silicon with H dangling correspond to Si Hx (x 1, 2, 3) bending, Si-H2 wagging, and e bonds is not stable in water. The mainly reactions are shown Si H2 scissor modes, respectively [14,15]. The FTIR spectrum in the following [5,6,9]: acquired from the sample etched at a current of 10 mA shows that little SieH bonds are found. As the etching current increases, the absorption intensity of SieH bond increases e e þ ¼ e e þ Si H H2O Si OH H2 (1) obviously. The CeH bond appears from the sample etched at the current of 200 mA because SieH bond reacts with alcohol e e e þ ¼ e e e e þ Si Si OH H2O Si O Si OH H2 (2) e to form Si OCH2CH3. The FTIR spectrum of the reacted product in Fig. 4 shows an obvious pattern of silicon oxide These two actions account for porous silicon as energy hydride. The sharp peaks at 510.5 cm 1, 1271 cm 1, and carrier for hydrogen fuel cells. In our experiments, it is found 1 e e e e e e e ¼ 2254 cm are due to Si O Si bending as well as Si O Si and that Si Si (OH)y ( y 1, 2, 3) is not stable in water and e Si OH stretching modes, respectively [12,16]. The H2O degenerates to SieOeSieOH ﬁnally. Additionally, oxidization of porous silicon is limited by SieSi bonds in air or oxygen [12], but it is not the case in water provided with OH . The surface SieO bonds without H prevent reaction (2) from continuing because the porous silicon reacts hardly with water albeit at a pH of 11. This means that the reaction (2) is based on reaction (1). Hence, the SieH bond is important to the release of hydrogen from porous silicon. XPS indicates that the amount of hydrogen is limited by the SieO and SieF bonds formed during electrochemical etching and the formation of silicon sub-oxide during the reactions.

3.3. FTIR analysis

Fig. 3(a)e(d) shows the FTIR spectra in the range of 400e4000 cm 1 using etching currents of 10 mA, 50 mA, 100 mA, and 200 mA. The peaks around 2100 cm 1 are attrib- uted to SieHx (x ¼ 1, 2, 3) stretching modes and those around 1030 and 1110 cm 1 stem from the SieOeSi stretching mode Fig. 4 e FTIR spectrum of the transparent product produced [5,12,13]. The peaks around 600e673, 806, and 914.5 cm 1 from porous silicon. international journal of hydrogen energy 36 (2011) 4513e4517 4517

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