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Si nanowire directly grown on a liquid metal substrate—towards wafer scale transferable nanowire arrays with improved visible-light sterilization
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 145601 (http://iopscience.iop.org/0957-4484/25/14/145601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.117.10.200 This content was downloaded on 26/06/2014 at 10:57
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 145601 (6pp)
doi:10.1088/0957-4484/25/14/145601
Si nanowire directly grown on a liquid metal substrate—towards wafer scale transferable nanowire arrays with improved visible-light sterilization Hui Wang1 , Jian-Tao Wang1 , Xue-Mei Ou1 , Chun-Sing Lee2 and Xiao-Hong Zhang1,3 1
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China 2 Center of Super-Diamond and Advanced Film (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, People’s Republic of China 3 Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China E-mail: [email protected] Received 25 October 2013, revised 21 January 2014 Accepted for publication 30 January 2014 Published 12 March 2014
Abstract
Integrating vertically aligned nanowires (NWs) on a functional substrate is important for the application of NWs in wafer scale assemblies and functional devices. However, vertically aligned NWs via the current epitaxial growth route can only be prepared on crystalline wafers. A convenient method is thus presented to overcome NW substrate limitations. Liquid metal is proposed to serve as a substrate for the initial growth of vertically aligned NWs. NWs could then be harvested from the growth substrate and integrated with functional substrates. Fabricated vertically aligned silicon NWs (SiNWs) were grown on molten Sn and then integrated into a flexible transparent poly(dimethylsiloxane) film to obtain a SiNW/functional substrate device. The device showed enhanced visible-light absorption ability and refreshable visible-light bactericidal activities with a bacterial reduction rate of close to 100%, indicating that growth with molten metal as a substrate could be a promising approach for extending the function and application of NWs. Keywords: Si nanowire, sterilization, bionic, PDMS, vapor–liquid–solid growth S Online supplementary data available from stacks.iop.org/Nano/25/145601/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
these SiNWs, is of interest because of the demonstrated possibility of the wafer scale integration of SiNWs into functional devices, as well as novel properties such as enhanced light absorption [1, 2]. This configuration has boosted extensive 1D SiNW device research in various fields, from solar energy and photocatalysis to cell biology [3–8]. In most of these
One-dimensional (1D) Si nanowires (NWs) have unique physical and chemical properties that make them useful in a broad range of applications. Their vertically ordered configuration, including free-standing SiNWs and the substrate that holds 0957-4484/14/145601+06$33.00
1
c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 145601
H Wang et al
Figure 1. (a) Water hyacinth growing on a water surface. (b) SiO2 formed a floating layer on the liquid Sn surface. (c) Sn substrate
vaporized and formed Sn liquid micro-droplets on the SiO2 floating layer. (d) Sn droplets catalyzed the VLS growth of Si/SiO2 NW. (e) SEM image of the as-produced NW array.
applications, vertically ordered SiNWs were grown on crystalline wafer by epitaxial growth through the vapor–liquid– solid (VLS) process [9–17]. Since its first presentation in 1964, the VLS growth method had been widely used and studied for high-quality 1D nanostructure synthesis [18, 19]. Nevertheless, apart from the rigid single crystalline wafer, SiNWs require diversified functional substrates to maximize their biocompatibility and visible-light absorption capacity, thus making SiNWs useful in various applications (such as flexible bionic devices). For instance, functional polymer substrates aid the development of SiNWs as potential artificial photosynthetic assemblies in splitting water to release H2 and O2 [15]. Unfortunately, in the traditional VLS growth technique, the options of functional substrates for SiNW application are few because the growth process depends on the crystalline substrate to provide a required crystal surface for the orderly growth of SiNWs [16]. Despite the intense efforts that have been made to develop new VLS growth methods with functional substrates, these methods still depend on crystalline substrate and epitaxial growth [14, 17]. The most common method of transferring SiNWs to a functional substrate involves the removal of VLS-grown SiNW arrays from crystal Si wafer with mechanical force and integrating them into other materials [7]. However, both large-scale functionalization and efficient transformation are difficult because SiNWs are epitaxially grown with the Si wafer as a whole. This rigid substrate may also affect the growth features of SiNWs. Therefore, a new synthesis strategy that meets the requirements of wafer scale transferable SiNW array growth and functionalization for novel applications is highly desirable. In this paper, a new strategy of transferring and functionalizing vertically ordered SiNW arrays for visible-light sterilization is presented to address the issue of extending the function and application of SiNWs to flexible bionic devices. This method involved using liquid metal as a growth substrate to both assist the vertical growth of SiNW arrays by the VLS process and to integrate them conveniently into a flexible polymer substrate. This approach was inspired by the growth of water hyacinths on river surfaces (figure 1(a)), i.e., water hyacinths freely float on the water surface and move with the wind in a vertical, orderly manner. Similarly, SiNWs were grown in this study using Sn as the liquid surface because
of its low melting point and high catalytic activity [20]. Vertically ordered Si/SiO2 nanowire arrays were easily grown on the surface of liquid Sn by the VLS process (figures 1(b)– (d)). They were functionalized by controlled embedding in transparent poly(dimethylsiloxane) (PDMS) and conveniently removed from the metal substrate. The SiNW/PDMS assembly demonstrated high flexibility and good optical absorption capacity. The assembly also revealed the visible-light bactericidal activity of elemental Si for the first time, with a bacterial reduction rate of almost 100%. The liquid metal substrate showed three significant advantages over its traditional solid counterpart for the growth of wafer scale transferable NW arrays. First, the liquid surface had horizontal mobility that could weaken the swaying of growing NWs induced by the fluctuation of carrier gas flow. Consequently, the NWs grew longer and thinner, and remained in an upright position compared with traditional solid substrates; thus, these NWs had large aspect ratios and a unique performance. Second, the liquid substrate offered more uniform temperature and material distribution that enabled uniform growth over a large area. Third, the liquid metal underwent substantial shrinkage upon solidification. This phenomenon led to loose contact between the SiNWs and the cooled solid metal, thereby allowing convenient harvesting of the NWs. It is worth noting, in addition to transfer of SiNW arrays to flexible substrates for bionic devices, this approach may also be used to extend the electronic applications of SiNW arrays after transferring them to functional substrates for improved electrical contact. 2. Experimental methods
Growth of vertically ordered Si/SiO2 nanowire arrays. Si/SiO2 core/shell NWs were prepared by thermally evaporating SiO powder in a tube furnace. The temperature in the tube was measured using a movable thermocouple. An alumina boat loaded with 1.00 g of SiO powder (Aldrich, 325 meshes, 99.9%) was placed at the middle of a 130 cm-long alumina tube. A slab of Sn (10 cm × 2 cm × 0.5 cm) loaded onto another Al2 O3 boat was placed 16.5 cm downstream of the SiO source. The length of the furnace from the source to the growth area can importantly affect the growth. After pumping down the system to a pressure of 5 × 10−2 mbar, nitrogen 2
Nanotechnology 25 (2014) 145601
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was continuously fed into the tube to maintain a pressure to 350 mbar. The system was then heated to 1350 ◦ C (temperature at the SiO source) at a rate of 50 ◦ C min−1 , and maintained at this temperature for growth for about 80 min. After cooling naturally to room temperature, a yellowish-brown product of approximately 2 cm × 8 cm was obtained on the Sn surface. While the thermal couple recorded a temperature change from 600 to 1200 ◦ C over this growth zone, the actual temperature range is expected to be smaller due to the high thermal conductivity of the liquid metal. As grown product was first examined with a Philips XL 30 FEG SEM. As-prepared and HF acid treated samples were then dispersed in ethanol. Samples for transmission electron microscopy (TEM, Philips, CM20, operated at 200 kV) were prepared by dropping the dispersions onto a carbon-coated copper TEM sample grid. Optical absorption of the products was measured on a dual beam, ultraviolet, visible light and near infrared spectrophotometer (Varian Cary-5000) with an integrating sphere.
The SEM image revealed that the product was composed of vertically ordered nanowires. Individual cables on the top layer were further studied by transmission electron microscopy (TEM) and SEM (figure S1 available at stacks.iop.org/Nano/2 5/145601/mmedia), which showed that each cable comprised a single crystalline SiNW core surrounded by amorphous SiO2 . This SiO2 shell protected the crystalline SiNWs within. Two ends of the cables were studied by TEM and a spherical particle was found at the bottom of each SiNW, indicating that individual SiNW growth followed a VLS process with Sn as the catalyst [20]. The sample was then investigated by x-ray diffraction. The resulting spectrum demonstrated strong peaks of silicon with a cubic crystalline structure and weak peaks of Sn with a body-centered crystalline structure (figure S2 available at stacks.iop.org/Nano/25/145601/mmedia). Characterization of the morphology and chemical composition of the as-prepared products indicated that vertically ordered Si/SiO2 nanowires were successfully grown on molten Sn. The liquid metal substrate might be a key factor in the product’s vertically ordered growth. During growth, the Transfer of nanowires to PDMS substrates. The PDMS base liquid Sn substrate adsorbed SiO vapor, and solidified SiO and curing agent (Sylgard 184, Dow Corning) were mixed at was disproportionated into SiO2 and Si. Si dissolved into a 10:1 w/w ratio and then diluted with methylene chloride the metal substrate, whereas SiO2 spread out on the liquid −1 (0.25 g ml ) before dropping onto the Si/SiO2 core/shell metal surface and formed a floating SiO2 layer because of 2 NW array (about 0.1 ml casting solution for 1 cm subits poor solubility in liquid Sn (figure 1(b)). When the SiO2 strate). The methylene chloride was slowly evaporated and layer completely covered the surface of the liquid substrate, no the monomer was polymerized at ambient conditions for 24 h. further adsorption of SiO vapor occurred. As a low-meltingThe SiNW/PDMS assembly was then torn off from the metal substrate and etched for 5 h in a premixed solution of 30 ml point metal, the Sn substrate vaporized and then deposited onto of DI water, 2.5 ml of 18 mol l−1 H2 SO4 , 10 ml of 44% aq. the SiO2 layer, forming Sn liquid micro-droplets (figure 1(c)). HF acid, and 2.5 ml of ethanol. The SiNW/PDMS assembly These droplets catalyzed the Si/SiO2 nanowire VLS growth on was rinsed with copious amounts of DI water, and sterilized in the liquid substrate (figure 1(d)) [20]. It is worth noting that the liquid metal substrate might also be a key factor in the product’s 75% alcohol before storage in physiological saline solution. easy harvesting. When the liquid substrate solidified at the end Bactericidal activity under visible light. An E. coli solu- of growth, it was easily separated from the product because of tion (4 × 109 CFU ml−1 , 5 ml) was separated from LB the difference between the thermal expansion coefficients of liquid medium by centrifugation and washed thrice with the liquid substrate and the solid nanowires. 5 ml sterile PBS solution. The solution was then diluted to In addition to their convenient growth and easy harvest4 × 108 CFU ml−1 with sterile PBS solution. The resultant ing, the vertically ordered Si/SiO2 nanowires demonstrated solution was used as the working bacterial dilution. 3 ml of advantages of easy integration/transfer into other functional the working bacterial dilution was poured into three glass cells substrates (figures 2(a) and (b)). As an example, flexible (1 cm × 1 cm × 5 cm), which were exposed to a 100 mW cm−2 transparent PDMS was used as a functional substrate. It can xenon lamp with a 400 nm cut-off filter in the presence be prepared by casting dimethylsiloxane solution onto the of SiNW/PDMS (1 cm2 ), blank PDMS film (1 cm2 ), and as-prepared product [7, 8]. The casting route can be extended to free-standing hydrogen-terminated SiNWs (the same amount many other polymers, but PDMS is often preferred because it is of SiNWs as in other samples). After a specified time, 2.5 µl inexpensive, flexible, nontoxic to cells, impermeable to water, solution was taken out and diluted with sterile PBS solution. and permeable to O2 and CO2 [21–25]. After curing the film Afterwards, 200 µl of the dilution was incubated on an LB agar under ambient conditions, the whole product was transferred plate for 16 h at 37 ◦ C. The bacterial colonies were counted, to a hydrofluoric acid (HF) solution (10%, v/v) to expose the and the percentile reduction was calculated in CFU ml−1 . ends of the SiNWs from the PDMS film. The exposed length of the SiNWs can be controlled by both the etching time in the acid solution and the growth time during free-floating growth 3. Results and discussion (figures 2(c)–(e)). SEM images of the Si/SiO2 nanowires The free-floating growth of vertically ordered Si/SiO2 core/shell embedded in PDMS (hereafter referred to as SiNW/PDMS) NWs was performed in a tube furnace using SiO as an indicated that vertically ordered SiNWs with length ranging evaporation source and a slab of Sn (10 cm × 2 cm × 0.5 cm) from several tens to hundreds of micrometers were integrated as the growth substrate. After removal from the Sn substrate, into the PDMS membrane. The uniformly distributed SiNWs the product was analyzed by scanning electron microscopy penetrate through the PDMS film (figure S3 available at sta cks.iop.org/Nano/25/145601/mmedia); they have an average (SEM). 3
Nanotechnology 25 (2014) 145601
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Figure 2. (a), (b) Schematic illustrations of harvesting the NW arrays from the growth substrate (solid Sn) and transferring them to PDMS. (c) SEM images of the SiNW/PDMS grown for 80 min and pickled for 5 h. (d) Grown for 120 min and pickled for 8 h. (e) Top view of image (c). (f) Visible absorption spectra of samples.
presence of SiNW/PDMS with a 100 mW cm−2 xenon lamp with a 400 nm cut-off filter to provide visible-light irradiation (figure 3(a), top), the E. coli bacterial colonies that formed on the agar plates significantly decreased. No colony-forming units were found after illumination, indicating a bacterial reduction rate of almost 100% (figure 3(a), bottom). The bactericidal activity was also easily refreshed by treating the used film with aq. HF (5%, v/v) for several minutes. Three features of the bacterial reduction kinetics for SiNW/PDMS, H-SiNWs, and blank PDMS film were observed (figure 3(b)). First, the SiNW/PDMS assembly had higher bactericidal efficiency than H-SiNWs. About 80% of E. coli can be killed with the SiNW/PDMS assembly within the first 2 min of illumination. By contrast, only 2.3% of the E. coli can be killed with H-SiNWs. After 7 min, the bacterial reduction rate of the SiNW/PDMS assembly was close to 100%. By contrast, after 1 h of illumination, a threshold ratio of about 45.7% was found for H-SiNWs. Third, blank PDMS film also initially caused weak bacterial colony reduction that increased after 40 min. The variation in bacterial colony reduction may be due to the adsorption and growth of E. coli on blank PDMS film. The above bacterial reduction kinetics revealed that, after integration into the PDMS film, vertically ordered SiNWs developed efficient and refreshable visible-light bactericidal activity. This new function can be attributed to the specific morphology of the vertically ordered SiNWs and the functional PDMS substrate. First, the vertically ordered structure enhanced the photo-absorption ability of SiNWs [9], producing more hydroxyl radicals or singlet oxygen on the surface of the SiNWs and improving their bactericidal kinetics [26, 27] (figure S4 available at stacks.iop.org/Nano/25/145601/mme dia). Second, the hydrophobic PDMS substrate, which could bring the E. coli on its surface, could increase the opportunities
diameter of about 164 nm (from 124 to 240 nm). SiNWs’ density in the film is about 0.51 mg cm−2 . The SiNW/PDMS assembly is highly flexible, can firmly adhere onto human skin, and can be peeled off by hand. Under laboratory conditions, the size of the resulting product was only limited by the surface area of the Sn substrate. The integration of SiNWs into the flexible PDMS improved their convenient application and endowed the assembly with interesting properties, such as visible-light absorption. Accordingly, the visible absorption spectra of the SiNW/PDMS assembly, as-prepared Si/SiO2 nanowires on Sn, and HFtreated SiNWs (H-SiNWs) randomly dispersed in water with uniform size and the same mass of Si were compared. The results showed that the visible-light absorption of the SiNW/PDMS assembly was 1.28 times higher than that of the as-prepared Si/SiO2 nanowire array, and 3.64 times higher than that of H-SiNWs (based on spectrum integration) (figure 2(f)). Moreover, the blank PDMS film showed very low light adsorption. These results indicated that SiNW/PDMS had the highest visible-light adsorption among the three samples. The enhancement in light adsorption may be due to the specific morphology of the SiNW/PDMS assembly. Si is a highly refractive index material; the calculated absorption and emission efficiencies of SiNWs with a diameter of 200 nm under a wavelength of 350 nm incident light could be 901% and 449%, respectively, indicating that SiNWs have strong resonant field enhancement of incident light [26]. In addition, light can also be scattered around in the assembly, leading to optical trapping [9, 14]. The visible-light bactericidal activity of the SiNW/PDMS assembly against Escherichia coli (E. coli) was subsequently examined. The SiNW/PDMS assembly showed enhanced bactericidal activity compared with H-SiNWs. After 10 min of illumination of E. coli in sterile PBS solution and in the 4
Nanotechnology 25 (2014) 145601
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Figure 3. Visible-light bactericidal activity of SiNW/PDMS. (a) E. Coli growth in sterile PBS solution with SiNW/PDMS before (top) and after (bottom) 10 min of illumination with a 100 mW cm−2 xenon lamp with 400 nm UV cut-off filter. (b) Bacterial reduction kinetics in sterile PBS solution for SiNW/PDMS, H-SiNWs, and blank PDMS film.
for the contact between E. coli and SiNWs, which thus can reach an enhanced bactericidal efficiency as high as 100% [28]. In addition, it is worth noting that the chemical characteristics of the H-terminated SiNW surface can be modified by various approaches, thus it is then expected that further improved bactericidal efficiency might be achieved by modifying its surface to tailor surface stability, electrical properties et al [29, 30].
[2] Oh I, Kye J and Hwang S 2012 Nano Lett. 12 298 [3] Zhao J, Li J, Ying P, Zhang W, Meng L and Li C 2013 Chem. Commun. 49 4477 [4] Liu Z, Wang H, Ou X M, Lee C S and Zhang X H 2012 Chem. Commun. 48 2815 [5] Yang T, Wang H, Ou X M, Lee C S and Zhang X H 2012 Adv. Mater. 46 6199 [6] Zhang C, Chen P, Liu J, Zhang Y, Shen W, Xu H and Tang Y 2008 Chem. Commun. 28 3290 [7] Plass K E, Filler M A, Spurgeon J M, Kayes B M, Maldonado S, Brunschwig B S, Atwater H A and Lewis N S 2009 Adv. Mater. 21 325 [8] Spurgeon J M, Boettcher S W, Kelzenberg M D, Brunschwig B S, Atwater H A and Lewis N S 2010 Adv. Mater. 22 3277 [9] Kelzenberg M D, Boettcher S W, Petykiewicz J A, Turner-Evans D B, Putnam M C, Warren E L, Spurgeon J M, Briggs R M, Lewis N S and Atwater H A 2010 Nature Mater. 9 239 [10] Boettcher S W et al 2011 J. Am. Chem. Soc. 133 1216 [11] Tsakalakos L, Balch J, Fronheiser J, Korevaar B A, Sulima O and Rand J 2007 Appl. Phys. Lett. 91 233117 [12] Goodey A P, Eichfeld S M, Lew K K, Redwing J M and Mallouk T E 2007 J. Am. Chem. Soc. 129 12344 [13] Wei W, Bao X Y, Soci C, Ding Y, Wang Z L and Wang D 2009 Nano Lett. 9 2926 [14] Alper J P, Gutes A, Carraro C and Maboudian R 2013 Nanoscale 5 4114 [15] Spurgeon J M, Walter M G, Zhou J, Kohl P A and Lewis N S 2011 Energy Environ. Sci. 4 1772 [16] Kayes B M, Filler M A, Putnam M C, Kelzenberg M D, Lewis N S and Atwater H A 2007 Appl. Phys. Lett. 91 103110 [17] Nasi L, Calestani D, Fabbri F, Ferro P, Besagni T, Fedeli P, Licci F and Mosca R 2012 Nanoscale 5 1060 [18] Wagner R S and Ellis W C 1964 Appl. Phys. Lett. 4 89 [19] Wacaser B A, Dick K A, Johansson J, Borgstr¨om M T, Deppert K and Samuelson L 2009 Adv. Mater. 21 153 [20] Wang H, Zhang X H, Meng X M, Zhou S M, Wu S K, Shi W S and Lee S T 2005 Angew. Chem. Int. Edn 44 6934 [21] Mukhopadhyay R 2007 Anal. Chem. 9 3248 [22] Vaidya A A and Norton M L 2004 Langmuir 20 11100
4. Conclusion
In conclusion, this work reported a strategy of growing wafer scale transferable SiNW arrays for easy integration into functional substrates for new applications. A reusable low-meltingpoint metal was applied as a growth substrate that provided a flexible surface on which the NWs vertically grew and from which they could be easily harvested. This finding revealed the advantages of a liquid metal substrate in the growth and functionalization of vertically ordered SiNWs. New properties of the SiNW array, such as visible-light bactericidal activity with a bacterial reduction rate of almost 100%, demonstrated its potential application as a disposable visible-light sterilization material for drinking water sterilization, food packaging, and photodynamic therapy. Acknowledgments
This work was partially supported by the National Basic Research Program of China (973 program) (Grant No. 2012CB932400, 2010CB934500), the National Natural Science Foundation of China (Grant No. 51172246, 51128301, 51373188), and the Major Research Plan of the National Natural Science Foundation of China (Grant No. 91233110, 91333208). References [1] Boettcheer S W, Spurgeon J M, Putnam M C, Warren E L, Turner-Evans D B, Kelzenberg M D, Maiolo J R, Atwater H A and Lewis N S 2010 Science 327 185 5
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[27] Shao M W, Cheng L, Zhang X H, Ma D D D and Lee S T 2009 J. Am. Chem. Soc. 131 17738 [28] Difilippo E L and Eganhouse R P 2010 Environ. Sci. Technol. 44 6917 [29] Paska Y and Haick H 2012 ACS Appl. Mater. Interfaces 4 2604 [30] Assad O, Puniredd S R, Stelzner T, Christiansen S and Haick H 2008 J. Am. Chem. Soc. 130 17670
[23] Simmons A, Padsalgikar A D, Ferris L M and Poole-Warren L A 2008 Biomaterials 29 2987 [24] Leclerc E, Sakai Y and Fujii T 2004 Biotechnol. Prog. 20 750 [25] Tanaka Y, Morishima K, Shimizu T, Kikuchi A, Yamato M, Okano T and Kitamori T 2006 Lab Chip 6 230 [26] Br¨onstrup G, Jahr N, Leiterer C, Cs´aki A, Fritzsche W and Christiansen S 2010 ACS Nano 12 7113
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Si nanowire directly grown on a liquid metal substrate—towards wafer scale transferable nanowire arrays with improved visible-light sterilization
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 145601 (http://iopscience.iop.org/0957-4484/25/14/145601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.117.10.200 This content was downloaded on 26/06/2014 at 10:57
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 145601 (6pp)
doi:10.1088/0957-4484/25/14/145601
Si nanowire directly grown on a liquid metal substrate—towards wafer scale transferable nanowire arrays with improved visible-light sterilization Hui Wang1 , Jian-Tao Wang1 , Xue-Mei Ou1 , Chun-Sing Lee2 and Xiao-Hong Zhang1,3 1
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China 2 Center of Super-Diamond and Advanced Film (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, People’s Republic of China 3 Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China E-mail: [email protected] Received 25 October 2013, revised 21 January 2014 Accepted for publication 30 January 2014 Published 12 March 2014
Abstract
Integrating vertically aligned nanowires (NWs) on a functional substrate is important for the application of NWs in wafer scale assemblies and functional devices. However, vertically aligned NWs via the current epitaxial growth route can only be prepared on crystalline wafers. A convenient method is thus presented to overcome NW substrate limitations. Liquid metal is proposed to serve as a substrate for the initial growth of vertically aligned NWs. NWs could then be harvested from the growth substrate and integrated with functional substrates. Fabricated vertically aligned silicon NWs (SiNWs) were grown on molten Sn and then integrated into a flexible transparent poly(dimethylsiloxane) film to obtain a SiNW/functional substrate device. The device showed enhanced visible-light absorption ability and refreshable visible-light bactericidal activities with a bacterial reduction rate of close to 100%, indicating that growth with molten metal as a substrate could be a promising approach for extending the function and application of NWs. Keywords: Si nanowire, sterilization, bionic, PDMS, vapor–liquid–solid growth S Online supplementary data available from stacks.iop.org/Nano/25/145601/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
these SiNWs, is of interest because of the demonstrated possibility of the wafer scale integration of SiNWs into functional devices, as well as novel properties such as enhanced light absorption [1, 2]. This configuration has boosted extensive 1D SiNW device research in various fields, from solar energy and photocatalysis to cell biology [3–8]. In most of these
One-dimensional (1D) Si nanowires (NWs) have unique physical and chemical properties that make them useful in a broad range of applications. Their vertically ordered configuration, including free-standing SiNWs and the substrate that holds 0957-4484/14/145601+06$33.00
1
c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 145601
H Wang et al
Figure 1. (a) Water hyacinth growing on a water surface. (b) SiO2 formed a floating layer on the liquid Sn surface. (c) Sn substrate
vaporized and formed Sn liquid micro-droplets on the SiO2 floating layer. (d) Sn droplets catalyzed the VLS growth of Si/SiO2 NW. (e) SEM image of the as-produced NW array.
applications, vertically ordered SiNWs were grown on crystalline wafer by epitaxial growth through the vapor–liquid– solid (VLS) process [9–17]. Since its first presentation in 1964, the VLS growth method had been widely used and studied for high-quality 1D nanostructure synthesis [18, 19]. Nevertheless, apart from the rigid single crystalline wafer, SiNWs require diversified functional substrates to maximize their biocompatibility and visible-light absorption capacity, thus making SiNWs useful in various applications (such as flexible bionic devices). For instance, functional polymer substrates aid the development of SiNWs as potential artificial photosynthetic assemblies in splitting water to release H2 and O2 [15]. Unfortunately, in the traditional VLS growth technique, the options of functional substrates for SiNW application are few because the growth process depends on the crystalline substrate to provide a required crystal surface for the orderly growth of SiNWs [16]. Despite the intense efforts that have been made to develop new VLS growth methods with functional substrates, these methods still depend on crystalline substrate and epitaxial growth [14, 17]. The most common method of transferring SiNWs to a functional substrate involves the removal of VLS-grown SiNW arrays from crystal Si wafer with mechanical force and integrating them into other materials [7]. However, both large-scale functionalization and efficient transformation are difficult because SiNWs are epitaxially grown with the Si wafer as a whole. This rigid substrate may also affect the growth features of SiNWs. Therefore, a new synthesis strategy that meets the requirements of wafer scale transferable SiNW array growth and functionalization for novel applications is highly desirable. In this paper, a new strategy of transferring and functionalizing vertically ordered SiNW arrays for visible-light sterilization is presented to address the issue of extending the function and application of SiNWs to flexible bionic devices. This method involved using liquid metal as a growth substrate to both assist the vertical growth of SiNW arrays by the VLS process and to integrate them conveniently into a flexible polymer substrate. This approach was inspired by the growth of water hyacinths on river surfaces (figure 1(a)), i.e., water hyacinths freely float on the water surface and move with the wind in a vertical, orderly manner. Similarly, SiNWs were grown in this study using Sn as the liquid surface because
of its low melting point and high catalytic activity [20]. Vertically ordered Si/SiO2 nanowire arrays were easily grown on the surface of liquid Sn by the VLS process (figures 1(b)– (d)). They were functionalized by controlled embedding in transparent poly(dimethylsiloxane) (PDMS) and conveniently removed from the metal substrate. The SiNW/PDMS assembly demonstrated high flexibility and good optical absorption capacity. The assembly also revealed the visible-light bactericidal activity of elemental Si for the first time, with a bacterial reduction rate of almost 100%. The liquid metal substrate showed three significant advantages over its traditional solid counterpart for the growth of wafer scale transferable NW arrays. First, the liquid surface had horizontal mobility that could weaken the swaying of growing NWs induced by the fluctuation of carrier gas flow. Consequently, the NWs grew longer and thinner, and remained in an upright position compared with traditional solid substrates; thus, these NWs had large aspect ratios and a unique performance. Second, the liquid substrate offered more uniform temperature and material distribution that enabled uniform growth over a large area. Third, the liquid metal underwent substantial shrinkage upon solidification. This phenomenon led to loose contact between the SiNWs and the cooled solid metal, thereby allowing convenient harvesting of the NWs. It is worth noting, in addition to transfer of SiNW arrays to flexible substrates for bionic devices, this approach may also be used to extend the electronic applications of SiNW arrays after transferring them to functional substrates for improved electrical contact. 2. Experimental methods
Growth of vertically ordered Si/SiO2 nanowire arrays. Si/SiO2 core/shell NWs were prepared by thermally evaporating SiO powder in a tube furnace. The temperature in the tube was measured using a movable thermocouple. An alumina boat loaded with 1.00 g of SiO powder (Aldrich, 325 meshes, 99.9%) was placed at the middle of a 130 cm-long alumina tube. A slab of Sn (10 cm × 2 cm × 0.5 cm) loaded onto another Al2 O3 boat was placed 16.5 cm downstream of the SiO source. The length of the furnace from the source to the growth area can importantly affect the growth. After pumping down the system to a pressure of 5 × 10−2 mbar, nitrogen 2
Nanotechnology 25 (2014) 145601
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was continuously fed into the tube to maintain a pressure to 350 mbar. The system was then heated to 1350 ◦ C (temperature at the SiO source) at a rate of 50 ◦ C min−1 , and maintained at this temperature for growth for about 80 min. After cooling naturally to room temperature, a yellowish-brown product of approximately 2 cm × 8 cm was obtained on the Sn surface. While the thermal couple recorded a temperature change from 600 to 1200 ◦ C over this growth zone, the actual temperature range is expected to be smaller due to the high thermal conductivity of the liquid metal. As grown product was first examined with a Philips XL 30 FEG SEM. As-prepared and HF acid treated samples were then dispersed in ethanol. Samples for transmission electron microscopy (TEM, Philips, CM20, operated at 200 kV) were prepared by dropping the dispersions onto a carbon-coated copper TEM sample grid. Optical absorption of the products was measured on a dual beam, ultraviolet, visible light and near infrared spectrophotometer (Varian Cary-5000) with an integrating sphere.
The SEM image revealed that the product was composed of vertically ordered nanowires. Individual cables on the top layer were further studied by transmission electron microscopy (TEM) and SEM (figure S1 available at stacks.iop.org/Nano/2 5/145601/mmedia), which showed that each cable comprised a single crystalline SiNW core surrounded by amorphous SiO2 . This SiO2 shell protected the crystalline SiNWs within. Two ends of the cables were studied by TEM and a spherical particle was found at the bottom of each SiNW, indicating that individual SiNW growth followed a VLS process with Sn as the catalyst [20]. The sample was then investigated by x-ray diffraction. The resulting spectrum demonstrated strong peaks of silicon with a cubic crystalline structure and weak peaks of Sn with a body-centered crystalline structure (figure S2 available at stacks.iop.org/Nano/25/145601/mmedia). Characterization of the morphology and chemical composition of the as-prepared products indicated that vertically ordered Si/SiO2 nanowires were successfully grown on molten Sn. The liquid metal substrate might be a key factor in the product’s vertically ordered growth. During growth, the Transfer of nanowires to PDMS substrates. The PDMS base liquid Sn substrate adsorbed SiO vapor, and solidified SiO and curing agent (Sylgard 184, Dow Corning) were mixed at was disproportionated into SiO2 and Si. Si dissolved into a 10:1 w/w ratio and then diluted with methylene chloride the metal substrate, whereas SiO2 spread out on the liquid −1 (0.25 g ml ) before dropping onto the Si/SiO2 core/shell metal surface and formed a floating SiO2 layer because of 2 NW array (about 0.1 ml casting solution for 1 cm subits poor solubility in liquid Sn (figure 1(b)). When the SiO2 strate). The methylene chloride was slowly evaporated and layer completely covered the surface of the liquid substrate, no the monomer was polymerized at ambient conditions for 24 h. further adsorption of SiO vapor occurred. As a low-meltingThe SiNW/PDMS assembly was then torn off from the metal substrate and etched for 5 h in a premixed solution of 30 ml point metal, the Sn substrate vaporized and then deposited onto of DI water, 2.5 ml of 18 mol l−1 H2 SO4 , 10 ml of 44% aq. the SiO2 layer, forming Sn liquid micro-droplets (figure 1(c)). HF acid, and 2.5 ml of ethanol. The SiNW/PDMS assembly These droplets catalyzed the Si/SiO2 nanowire VLS growth on was rinsed with copious amounts of DI water, and sterilized in the liquid substrate (figure 1(d)) [20]. It is worth noting that the liquid metal substrate might also be a key factor in the product’s 75% alcohol before storage in physiological saline solution. easy harvesting. When the liquid substrate solidified at the end Bactericidal activity under visible light. An E. coli solu- of growth, it was easily separated from the product because of tion (4 × 109 CFU ml−1 , 5 ml) was separated from LB the difference between the thermal expansion coefficients of liquid medium by centrifugation and washed thrice with the liquid substrate and the solid nanowires. 5 ml sterile PBS solution. The solution was then diluted to In addition to their convenient growth and easy harvest4 × 108 CFU ml−1 with sterile PBS solution. The resultant ing, the vertically ordered Si/SiO2 nanowires demonstrated solution was used as the working bacterial dilution. 3 ml of advantages of easy integration/transfer into other functional the working bacterial dilution was poured into three glass cells substrates (figures 2(a) and (b)). As an example, flexible (1 cm × 1 cm × 5 cm), which were exposed to a 100 mW cm−2 transparent PDMS was used as a functional substrate. It can xenon lamp with a 400 nm cut-off filter in the presence be prepared by casting dimethylsiloxane solution onto the of SiNW/PDMS (1 cm2 ), blank PDMS film (1 cm2 ), and as-prepared product [7, 8]. The casting route can be extended to free-standing hydrogen-terminated SiNWs (the same amount many other polymers, but PDMS is often preferred because it is of SiNWs as in other samples). After a specified time, 2.5 µl inexpensive, flexible, nontoxic to cells, impermeable to water, solution was taken out and diluted with sterile PBS solution. and permeable to O2 and CO2 [21–25]. After curing the film Afterwards, 200 µl of the dilution was incubated on an LB agar under ambient conditions, the whole product was transferred plate for 16 h at 37 ◦ C. The bacterial colonies were counted, to a hydrofluoric acid (HF) solution (10%, v/v) to expose the and the percentile reduction was calculated in CFU ml−1 . ends of the SiNWs from the PDMS film. The exposed length of the SiNWs can be controlled by both the etching time in the acid solution and the growth time during free-floating growth 3. Results and discussion (figures 2(c)–(e)). SEM images of the Si/SiO2 nanowires The free-floating growth of vertically ordered Si/SiO2 core/shell embedded in PDMS (hereafter referred to as SiNW/PDMS) NWs was performed in a tube furnace using SiO as an indicated that vertically ordered SiNWs with length ranging evaporation source and a slab of Sn (10 cm × 2 cm × 0.5 cm) from several tens to hundreds of micrometers were integrated as the growth substrate. After removal from the Sn substrate, into the PDMS membrane. The uniformly distributed SiNWs the product was analyzed by scanning electron microscopy penetrate through the PDMS film (figure S3 available at sta cks.iop.org/Nano/25/145601/mmedia); they have an average (SEM). 3
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Figure 2. (a), (b) Schematic illustrations of harvesting the NW arrays from the growth substrate (solid Sn) and transferring them to PDMS. (c) SEM images of the SiNW/PDMS grown for 80 min and pickled for 5 h. (d) Grown for 120 min and pickled for 8 h. (e) Top view of image (c). (f) Visible absorption spectra of samples.
presence of SiNW/PDMS with a 100 mW cm−2 xenon lamp with a 400 nm cut-off filter to provide visible-light irradiation (figure 3(a), top), the E. coli bacterial colonies that formed on the agar plates significantly decreased. No colony-forming units were found after illumination, indicating a bacterial reduction rate of almost 100% (figure 3(a), bottom). The bactericidal activity was also easily refreshed by treating the used film with aq. HF (5%, v/v) for several minutes. Three features of the bacterial reduction kinetics for SiNW/PDMS, H-SiNWs, and blank PDMS film were observed (figure 3(b)). First, the SiNW/PDMS assembly had higher bactericidal efficiency than H-SiNWs. About 80% of E. coli can be killed with the SiNW/PDMS assembly within the first 2 min of illumination. By contrast, only 2.3% of the E. coli can be killed with H-SiNWs. After 7 min, the bacterial reduction rate of the SiNW/PDMS assembly was close to 100%. By contrast, after 1 h of illumination, a threshold ratio of about 45.7% was found for H-SiNWs. Third, blank PDMS film also initially caused weak bacterial colony reduction that increased after 40 min. The variation in bacterial colony reduction may be due to the adsorption and growth of E. coli on blank PDMS film. The above bacterial reduction kinetics revealed that, after integration into the PDMS film, vertically ordered SiNWs developed efficient and refreshable visible-light bactericidal activity. This new function can be attributed to the specific morphology of the vertically ordered SiNWs and the functional PDMS substrate. First, the vertically ordered structure enhanced the photo-absorption ability of SiNWs [9], producing more hydroxyl radicals or singlet oxygen on the surface of the SiNWs and improving their bactericidal kinetics [26, 27] (figure S4 available at stacks.iop.org/Nano/25/145601/mme dia). Second, the hydrophobic PDMS substrate, which could bring the E. coli on its surface, could increase the opportunities
diameter of about 164 nm (from 124 to 240 nm). SiNWs’ density in the film is about 0.51 mg cm−2 . The SiNW/PDMS assembly is highly flexible, can firmly adhere onto human skin, and can be peeled off by hand. Under laboratory conditions, the size of the resulting product was only limited by the surface area of the Sn substrate. The integration of SiNWs into the flexible PDMS improved their convenient application and endowed the assembly with interesting properties, such as visible-light absorption. Accordingly, the visible absorption spectra of the SiNW/PDMS assembly, as-prepared Si/SiO2 nanowires on Sn, and HFtreated SiNWs (H-SiNWs) randomly dispersed in water with uniform size and the same mass of Si were compared. The results showed that the visible-light absorption of the SiNW/PDMS assembly was 1.28 times higher than that of the as-prepared Si/SiO2 nanowire array, and 3.64 times higher than that of H-SiNWs (based on spectrum integration) (figure 2(f)). Moreover, the blank PDMS film showed very low light adsorption. These results indicated that SiNW/PDMS had the highest visible-light adsorption among the three samples. The enhancement in light adsorption may be due to the specific morphology of the SiNW/PDMS assembly. Si is a highly refractive index material; the calculated absorption and emission efficiencies of SiNWs with a diameter of 200 nm under a wavelength of 350 nm incident light could be 901% and 449%, respectively, indicating that SiNWs have strong resonant field enhancement of incident light [26]. In addition, light can also be scattered around in the assembly, leading to optical trapping [9, 14]. The visible-light bactericidal activity of the SiNW/PDMS assembly against Escherichia coli (E. coli) was subsequently examined. The SiNW/PDMS assembly showed enhanced bactericidal activity compared with H-SiNWs. After 10 min of illumination of E. coli in sterile PBS solution and in the 4
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Figure 3. Visible-light bactericidal activity of SiNW/PDMS. (a) E. Coli growth in sterile PBS solution with SiNW/PDMS before (top) and after (bottom) 10 min of illumination with a 100 mW cm−2 xenon lamp with 400 nm UV cut-off filter. (b) Bacterial reduction kinetics in sterile PBS solution for SiNW/PDMS, H-SiNWs, and blank PDMS film.
for the contact between E. coli and SiNWs, which thus can reach an enhanced bactericidal efficiency as high as 100% [28]. In addition, it is worth noting that the chemical characteristics of the H-terminated SiNW surface can be modified by various approaches, thus it is then expected that further improved bactericidal efficiency might be achieved by modifying its surface to tailor surface stability, electrical properties et al [29, 30].
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4. Conclusion
In conclusion, this work reported a strategy of growing wafer scale transferable SiNW arrays for easy integration into functional substrates for new applications. A reusable low-meltingpoint metal was applied as a growth substrate that provided a flexible surface on which the NWs vertically grew and from which they could be easily harvested. This finding revealed the advantages of a liquid metal substrate in the growth and functionalization of vertically ordered SiNWs. New properties of the SiNW array, such as visible-light bactericidal activity with a bacterial reduction rate of almost 100%, demonstrated its potential application as a disposable visible-light sterilization material for drinking water sterilization, food packaging, and photodynamic therapy. Acknowledgments
This work was partially supported by the National Basic Research Program of China (973 program) (Grant No. 2012CB932400, 2010CB934500), the National Natural Science Foundation of China (Grant No. 51172246, 51128301, 51373188), and the Major Research Plan of the National Natural Science Foundation of China (Grant No. 91233110, 91333208). References [1] Boettcheer S W, Spurgeon J M, Putnam M C, Warren E L, Turner-Evans D B, Kelzenberg M D, Maiolo J R, Atwater H A and Lewis N S 2010 Science 327 185 5
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[27] Shao M W, Cheng L, Zhang X H, Ma D D D and Lee S T 2009 J. Am. Chem. Soc. 131 17738 [28] Difilippo E L and Eganhouse R P 2010 Environ. Sci. Technol. 44 6917 [29] Paska Y and Haick H 2012 ACS Appl. Mater. Interfaces 4 2604 [30] Assad O, Puniredd S R, Stelzner T, Christiansen S and Haick H 2008 J. Am. Chem. Soc. 130 17670
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