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Rice-straw-like structure of silicon nanowire arrays for a hydrogen gas sensor
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 475502 (http://iopscience.iop.org/0957-4484/24/47/475502) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.193.117.53 This content was downloaded on 03/06/2014 at 19:11
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 475502 (8pp)
doi:10.1088/0957-4484/24/47/475502
Rice-straw-like structure of silicon nanowire arrays for a hydrogen gas sensor Bohr-Ran Huang, Ying-Kan Yang and Hsien-Lung Cheng Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, Republic of China E-mail: [email protected]
Received 27 June 2013, in final form 13 September 2013 Published 31 October 2013 Online at stacks.iop.org/Nano/24/475502 Abstract A rice-straw-like silicon nanowire (SiNW) array was developed for hydrogen gas sensing applications. The straight-aligned SiNW array sensor was first fabricated by the metal-assisted electroless etching (MAEE) technique. Rice-straw-like SiNW arrays were formed using a repeated MAEE technique. Hydrogen sensing characteristics were measured for gas concentrations from 20 to 1000 ppm at room temperature. The rice-straw-like SiNW-array-based hydrogen gas sensor performed with low noise and a high response (232.5%) for 1000 ppm hydrogen gas. It was found that the rice-straw-like SiNW-array hydrogen gas sensor had a much better response (approximately 2.5 times) than the straight-aligned SiNW-array sensor. The rice-straw-like SiNW-array structure effectively increased the surface area and the concentration of silicon oxide, which provided additional binding sites for gas molecules. Thus, the rice-straw-like SiNW-array-based hydrogen gas sensor possessed good sensing properties and has the potential for mass production of sensing devices. (Some figures may appear in colour only in the online journal)
1. Introduction
small dimensions, as well as their potential application in sensor devices. Wang et al [7] developed ZnO nanowires for hydrogen gas sensing. Kumar et al [8] reported Pd-decorated single-walled carbon nanotubes (SWNTs) for hydrogen sensing. Peng et al [9] first demonstrated porous silicon nanowires (SiNWs) for NOx sensing at room temperature. This paper focused primarily on SiNW-based gas sensors, due to their high response, quick response and recovery time, and low operating temperature [10–12]. SiNWs are a promising material for gas sensing mainly due to their substantially large surface to volume ratio and strong adsorption for gases [11]. Additionally, they have an advantage of being able to control the surface morphology through the variation of the formation parameters. Furthermore, SiNWs are easy to integrate with other Si-based nano-devices [10, 13]. Recently, SiNWs have been synthesized using two basic approaches, the bottom-up and top-down approaches. The bottom-up approach involves chemical vapor deposition [14], laser ablation [15], and physical vapor deposition [16].
Since it is odorless, colorless, has a wide flammable range (4.65–75%), and is explosive over a wide range of concentrations (15–59%) at standard atmospheric temperature, hydrogen is an extremely dangerous gas. However, it is also essential in many research and industrial applications, such as fuel cells, automobile engines, industrial processing, chemical production, and cryogenic cooling. Hydrogen gas cannot be detected by the human senses, and other means are therefore required to detect its presence and quantify its concentration. Therefore, rapid and accurate hydrogen gas concentration measurements are essential to alert people to the formation of potentially explosive mixtures with air, and to help prevent the risk of an explosion [1, 2]. Potential hydrogen sensors have been investigated using several materials, including SnO2 [3], In2 O3 [4], TiO2 [5], and WO3 [6]. However, one-dimensional nanomaterials have recently attracted much attention, due to their importance in understanding fundamental properties at 0957-4484/13/475502+08$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 475502
B-R Huang et al
Figure 1. Schematic illustration of the major processes involved in the fabrication of: (a) straight-aligned SiNW-array gas sensors; and (b) rice-straw-like SiNW-array gas sensors.
The top-down approach allowed the preparation of SiNWs via dimensional reduction of bulk Si by lithography and etching. Electron beam lithography, reactive ion etching [17], and the recently developed metal-assisted electroless etching (MAEE) technique for silicon have been widely used to fabricate SiNWs [9, 18]. The MAEE technique was the most cost-effective method to synthesize SiNWs, as this approach is suitable for large areas, up to wafer scale. This paper describes the MAEE and repeated MAEE techniques for fabricating straight-aligned and rice-straw-like SiNW arrays, respectively, for hydrogen gas sensing. The rice-straw-like SiNW arrays provide excellent electrical performance. The morphology of the structures was investigated and the effects of these factors on hydrogen sensing properties are discussed.
2.2. Fabrication of rice-straw-like SiNW arrays The rice-straw-like SiNW arrays were prepared using the repeated MAEE technique. After the cleaning process, the silicon wafer was first immersed in a mixture of 5 M HF and 25 mM AgNO3 etching solution for 1 h at room temperature. The sample was then immediately immersed in HNO3 aqueous solution for 120 s, before being rinsed with DI water and blown dry in air. After drying in air, the sample was immersed in the etching solution a second time for 1.5 h; this sample was designated as RS1. Another silicon wafer was first immersed in a mixture of 5 M HF and 35 mM AgNO3 etching solution for 1 h at room temperature. Then, the sample was immediately immersed in HNO3 aqueous solution for 120 s, rinsed with DI water, and blown dry in air. After drying in air, the sample was immersed in a mixture of 5 M HF and 15 mM AgNO3 etching solution for 1.5 h; this sample was designated as RS2. Afterward, all samples were immediately removed from the silver film by leaving them in HNO3 aqueous solution for 120 s. Finally, the samples were rinsed with DI water and blown dry in air.
2. Experimental details 2.1. Fabrication of straight-aligned SiNW arrays Straight-aligned SiNW arrays were synthesized using the MAEE technique [17]. First, a p-type (1–10 cm, B-doped, 520 µm) Cz silicon (100) wafer was ultrasonically cleaned in acetone, isopropyl alcohol, and deionized (DI) water, for 30 min in each cleaning solution. Then the cleaned silicon wafer was immersed in a mixture of 5 M hydrofluoric acid (HF) aqueous and 25 mM silver nitrate (AgNO3 ) solution, for periods of 2, 2.5 and 3 h at room temperature; these samples were designated as SA1, SA2, and SA3, respectively. In this step, Ag+ ions dissolve in the solution which was randomly deposited on the sample surface by galvanic displacement. The sample surface in contact with the Ag+ ions was locally oxidized, becoming SiO2 , and then the oxide layer was etched by hydrofluoric acid [19, 20]. Following the MAEE process, the silicon wafer was wrapped in a thick silver film consisting of high-density tree-like dendritic structures [20]. To remove the capped silver, the as-prepared samples were dipped into a 30-wt% HNO3 aqueous solution for 120 s. Finally, the samples were rinsed with DI water and blown dry in air.
2.3. Preparation and measurement of the sensors Interdigitated platinum (Pt) electrodes, with 18 fingers and a gap interval of 0.1 mm, were deposited on the straight-aligned SiNW arrays and rice-straw-like SiNW arrays using DC sputtering to form chemiresistor gas sensors. A schematic diagram of the fabrication procedures for straight-aligned SiNW-array-based hydrogen sensors and rice-straw-like SiNW-array-based hydrogen sensors is presented in figure 1. For gas detection studies, each hydrogen sensor was characterized in a stainless steel chamber equipped with electrical feedthroughs, through which hydrogen gas of 99.99% purity diluted with dry air flowed under the control of a mass flow controller. The two electrodes of each sensor were connected to a computer controlled Keithley 237 source meter at a fixed voltage of 5 V, and the real-time electrical resistance response was then recorded with various hydrogen concentrations at room temperature. 2
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Figure 2. FESEM images of the surface and tilted cross-section morphology for straight-aligned SiNW arrays with (a) 2, (b) 2.5, and (c) 3 h of MAEE time. (d) Length and top surface density of straight-aligned SiNW arrays with different MAEE time. Table 1. Extracted length and density of straight-aligned and rice-straw-like SiNW arrays with various etching conditions. Straight-aligned SiNW-array gas sensor
Rice-straw-like SiNW-array gas sensor
Sample
SA1
SA2
SA3
RS1
RS2
Condition (concentration of AgNO3 , time) Length of SiNW arrays (µm) Density of SiNW arrays (%)
25 mM, 2h 21.9 35.2
25 mM, 2.5 h 28.0 30.7
25 mM, 3h 38.3 21.7
25 mM, 1 h + 25 mM, 1.5 h 29.5 35.1
35 mM, 1 h + 15 mM, 1.5 h 28.4 50.3
diameters ranging from 20 to 300 nm and lengths proportional to the electroless etching time (approximately 21.9, 28.0, and 38.3 µm for SA1, SA2, and SA3, respectively). Furthermore, it was also observed that the density of the SiNW arrays is around 35.2, 30.7, and 21.7% for SA1, SA2, and SA3, respectively. Figure 2(d) shows the length and density of straight-aligned SiNW arrays with different MAEE duration. It is observed that the density of SiNWs decreases with the MAEE time, which is consistent with other reports [21]. The extracted length and density of straight-aligned SiNW arrays with various etching conditions are summarized in table 1. The hydrogen sensors are characterized by recording their resistance change when they are exposed to environments with hydrogen molecules of different concentrations (20–1000 ppm) at room temperature. Representative hydrogen response curves versus time, with varying hydrogen gas concentrations for straight-aligned SiNW-array gas sensors, are shown in figure 3(a). Here, the sensor response is defined as [(Rhydrogen − Rair )/Rair ] × 100, where Rhydrogen is the resistance of the hydrogen sensor when exposed to ambient hydrogen, and Rair is the resistance in a dry air environment. Surface resistance changes are mainly due to changes in the free electron concentration, due to charge exchange
2.4. Material characterization The morphologies of the straight-aligned SiNW arrays and rice-straw-like SiNW arrays were examined by field emission scanning electron microscopy (FESEM) (JEOL JSM-6700F, operated at 15 kV). The density of SiNW arrays was obtained by performing image analysis of the FESEM photo using the ImageJ image-processing program developed by the US National Institute of Health. The chemical binding of SiNW arrays was investigated by Fourier-transform infrared (FTIR) spectrometry (BomemDA8.3), using an unetched silicon wafer to obtain the background spectrum. The crystal structure of the SiNW arrays was analyzed using a transmission electron microscope (TEM, Philips Tecnai F20 G2 FEI-TEM).
3. Results and discussion FESEM images in figures 2(a)–(c) show the different morphologies of straight-aligned SiNW arrays with different MAEE time. They are observed as free-standing SiNWs, arrayed perpendicularly to the silicon surface. Dense and well-oriented SiNW arrays were formed with nanowire 3
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Figure 3. (a) Hydrogen response versus time with hydrogen concentration ranging from 20 to 1000 ppm for the straight-aligned SiNW-array gas sensors. (b) Hydrogen response, (c) response time, and (d) recovery time versus different hydrogen concentration levels for the straight-aligned SiNW-array gas sensors.
between adsorbed species from the gas and the semiconductor surface [10]. Moreover, Campanella et al [22] indicated that trapped electrons were found at the interface of the silicon walls and hydrogen molecules, and are responsible for the change in resistance. Typical curves obtained from these sensors show that their resistance quickly increases as these sensors are exposed to hydrogen gas. After the introduction of hydrogen gas, the resistance increases with time and saturates at a value that depends on the gas concentration. In general, sensors exhibit a higher response when they are exposed to hydrogen with higher concentrations. That is, when these sensors are surrounded by dry air, their resistance drops and essentially returns to its original value. However, the straight-aligned SiNW-array gas sensors can even detect, with observable signals, hydrogen concentrations down to levels of 20 ppm. Figure 3(a) shows the time dependence of the response for the straight-aligned SiNW-array hydrogen gas sensors with different electroless etching times, at various hydrogen concentrations from 20 to 1000 ppm. The response shown in figure 3(b) is the summarized response for environments with different hydrogen concentrations. The response observed is 56.1, 94.9, and 82.8% for SA1, SA2, and SA3, respectively, for a 1000 ppm hydrogen gas concentration. It indicates that the SA2 sample exhibits the highest response since it possesses the larger surface area. Kanungo et al [11] indicate that an increase to a larger surface area produces a higher gas response since there is an increase in the interaction of hydrogen with the sensing surface. In addition, Oh et al [21] reported that the surface area varies proportionally with the density and length of SiNW arrays. Hence, the surface area could be given in proportion
to the length of the SiNW arrays × the density of SiNW arrays, denoted by A. A was 7.7, 8.6, and 8.3 for SA1, SA2, and SA3, respectively. SA2 possessed the larger surface area, yielding the highest response of the three samples. Figures 3(c) and (d) show response times and recovery times versus different hydrogen concentration levels, respectively, for the straight-aligned SiNW-array hydrogen gas sensor. Response time is defined as the time required to reach 90% of the maximum change. Recovery time is defined as the time required to reach 10% of the minimum change [23]. The response times of SA1, SA2, and SA3 are around 256, 276, and 296 s, respectively, for a hydrogen concentration of 20 ppm, and decrease to about 89, 20, and 143 s for a concentration of 1000 ppm, respectively. It should be noted that SA2 has the fastest response time at higher hydrogen concentrations. SA2 also has the fastest recovery time over the full concentration range, and recovery times vary from 152 s for a 20 ppm hydrogen concentration to 209 s for a 1000 ppm hydrogen concentration. As mentioned earlier, since SA2 provided the largest effective surface area to adsorb and desorb hydrogen gas, it has the shortest response and recovery times. Rice-straw-like SiNW arrays are shown in figures 4(a) and (b). Unlike the straight-aligned SiNW arrays, the top of the rice-straw-like SiNW arrays comprises thin bunches of rice-straw-like structures. During the repeated MAEE etching process, silver particles were easily formed on the tips and upper side walls of the nanowires. Thus, the tip and upper side wall of nanowires are further etched. Moreover, the bottom of nanowires are covered by silver particles at this stage, and elongated nanowires are formed. Afterward, the tips of the 4
Nanotechnology 24 (2013) 475502
B-R Huang et al
Figure 4. FESEM images of the surface and tilted cross-section morphology for rice-straw-like SiNW arrays of (a) RS1 and (b) RS2. (c) The proposed etching process of rice-straw-like SiNW arrays and the proposed current flow during the hydrogen sensing measurement.
Figure 5. FTIR spectra of straight-aligned SiNW arrays and rice-straw-like SiNW arrays.
Figure 6. TEM image showing a magnified view for the rice-straw-like SiNW arrays of RS2.
nanowires that are off-axis to gravitation will bend more, and eventually the torque leads to bending and agglomeration of the nanowires after the silver is removed. This phenomenon is attributed to the strong van der Waals attraction of the nanowires [24]. Figure 4(c) shows a schematic representation of the repeated etching process for rice-straw-like SiNW arrays. The length of rice-straw-like SiNW arrays for RS1 and RS2 is 29.5 and 28.4 µm, yielding densities of 35.1 and 50.3%, respectively. Figure 5 shows FTIR spectra of the straight-aligned SiNW-array gas sensor (SA1) and the rice-straw-like SiNWarray gas sensors (RS1 and RS2) in the range from 500 to 4000 cm−1 . The broad peak in the range 3000–3700 cm−1 is due to the O–H stretching vibration. Symmetric and asymmetric C–H stretching vibrations located in the range
between 2800 and 3000 cm−1 were observed. The peak at around 1736 cm−1 is a C–O stretching vibration. The asymmetric Si–O–Si stretching vibrations are distributed in the range 1000–1300 cm−1 . The bending and wagging vibration modes in Si–H are located at 880 cm−1 . It was found that RS1 and RS2 have an obvious peak at around 814 cm−1 , corresponding to the O–Si–O bending mode [25, 26]. As shown in figure 6, the TEM image of RS2 shows a sharp contrast between the bright region of the amorphous phase of the silicon oxides in the sidewall and the periodic structure of single-crystal silicon in the center of the SiNW. 5
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Figure 7. Comparison of (a) and (b) hydrogen response, (c) response times, and (d) recovery times versus different hydrogen concentration levels for different rice-straw-like SiNW arrays.
SiNW-array gas sensors. Ali et al [10] indicated that a higher concentration of silicon oxide creates a greater response to hydrogen gas. Fourthly, the repeated electroless etching process for rice-straw-like SiNW-array gas sensors may produce a type of incomplete covalent bond. This type of bond provides a type of gas sensing in nanowire systems [27]. As shown in figures 7(c) and (d), for rice-straw-like SiNW-array gas sensors, the response times for RS1 and RS2 are about 152 and 199 s, respectively, at hydrogen concentrations of 20 ppm, and decrease to about 41 and 30 s, respectively, for concentrations of 1000 ppm. The recovery times are about 142 and 192 s for RS1 and RS2, respectively, at hydrogen concentrations of 20 ppm, increasing to around 162 and 249 s, respectively, for concentrations of 1000 ppm. In order to test the selectivity for hydrogen of the rice-straw-like SiNW-array gas sensors, the gas response to O2 , CO2 and N2 for RS2 with a gas concentration of 1000 ppm have also been investigated and summarized in figure 8. It is observed that the gas response for O2 , CO2 and N2 is much lower in comparison with its gas response to hydrogen. It is shown that the rice-straw-like SiNW arrays possess high selectivity to detect hydrogen at room temperature. In this study, a response of about 232.5% is achieved for rice-straw-like SiNW-array gas sensors. A comparison between the sensing performances of the sensor and literature reports [28–35] is summarized in table 2. The response is comparable to those featured in previous reports with different sensing materials. Furthermore, the response is superior to that of porous silicon gas sensors [34], or silicon nanostructures modified with noble metal gas sensors [11, 35]. Moreover, the repeated electroless etching process of rice-straw-like SiNW arrays could be investigated further
Figure 7(a) shows the time dependence of the response for the straight-aligned SiNW-array hydrogen gas sensors with different electroless etching times, at various hydrogen concentrations from 20 to 1000 ppm. The response shown in figure 7(b) is the summarized response of the sensor in environments with different hydrogen concentrations. The responses for SA1 and SA2 are 191.1 and 232.5, respectively, indicating that SA2 has a better response than SA1. Furthermore, the rice-straw-like SiNW-array gas sensors display a significantly better response than the straight-aligned SiNW-array gas sensors, which is due to the increase of surface area. As mentioned earlier, A is proportional to the surface area. However, A for RS1 and RS2 is 10.4 and 14.3, respectively. It can be observed that the rice-straw-like SiNW-array gas sensors exhibit a higher surface area than the straight-aligned SiNW-array gas sensors. Moreover, RS2 exhibits a higher surface area than RS1. Hence, the higher response for rice-straw-like SiNW-array gas sensors could be due to the following suggestion. Firstly, as shown in figure 4(c), for the straight-aligned SiNW arrays, the current flows along from the top of the nanowires down to the substrate between the Pt electrodes. However, the rice-straw-like SiNW arrays provide more current paths in addition to the path along the substrate between the Pt electrodes (dotted line). The primary current path (solid line) flows along the top surface of the SiNWs since more active conducting sites exist due to the rice-straw-like SiNW nanostructure. This leads to more active sensing sites in the measurement. Secondly, the rice-straw-like SiNW-array gas sensors possess a larger surface area and therefore provide a higher contact area with the hydrogen gas. Thirdly, there is a higher concentration of Si–O–Si bonding in rice-straw-like 6
Nanotechnology 24 (2013) 475502
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Table 2. Comparison of recently developed nanostructure-based hydrogen gas sensors at room temperature. Sensing materials
Response (%)
Response time (s)
Recovery time (s)
Reference
ZnO nanorod Cd-doped ZnO nanowires ZnO/In2 O3 core–shell nanorod Al- and V-doped TiO2 nanostructures Pd-doped reduced graphene oxide CNT/Ni composite film Porous silicon Pd-modified porous silicon Pd-coated silicon nanowires Rice-straw-like structure silicon nanowires
294 (1000 ppm) 274 (100 ppm) 20.5 (500 ppm) 0.6 (1000 ppm) 26 (1000 ppm) 14.1 (200 ppm) 0.4 84 (10 000 ppm) 180 (1000 ppm) 232.5 (1000 ppm)
60 14 — — — 422 160 8 — 31
277 11 — — — 130 69 207 — 249
[28] [29] [30] [31] [32] [33] [34] [11] [35] Present work
References [1] H¨ubert T, Brett L B, Black G and Banach U 2011 Hydrogen sensors—a review Sensors Actuators B 157 329–52 [2] Zeng X Q, Latimer M L, Xiao Z L, Panuganti S, Welp U, Kwok W K and Xu T 2011 Hydrogen gas sensing with networks of ultrasmall palladium nanowires formed on filtration membranes Nano Lett. 11 262–8 [3] Shen Y, Yamazaki T, Liu Z, Meng D, Kikuta T, Nakatani N, Saito M and Mori M 2009 Microstructure and H2 gas sensing properties of undoped and Pd-doped SnO2 nanowires Sensors Actuators B 135 524–9 [4] Xu X et al 2013 Template-free synthesis of novel In2 O3 nanostructures and their application to gas sensors Sensors Actuators B 185 118–23 [5] Srivastava S, Kumar S, Singh V N, Sigh M and Vijay Y K 2011 Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing Int. J. Hydrog. Energy 36 6343–55 [6] Calavia R, Mozalev A, Vazquez R, Gracia I, Can´e C, Ionescu R and Llobet E 2010 Fabrication of WO3 nanodot-based microsensors highly sensitive to hydrogen Sensors Actuators B 149 352–61 [7] Wang H T, Kang B S, Ren F, Tien L C, Sadik P W, Norton D P, Pearton S J and Lin J 2005 Hydrogen-selective sensing at room temperature with ZnO nanorods Appl. Phys. Lett. 86 243503 [8] Kumar M K, Reddy A L M and Ramaprabhu S 2008 Exfoliated single-walled carbon nanotube-based hydrogen sensor Sensors Actuators B 130 653–60 [9] Peng K Q, Wang X and Lee S T 2009 Gas sensing properties of single crystalline porous silicon nanowires Appl. Phys. Lett. 95 243112 [10] Ali N K, Hashim M R and Aziz A A 2008 Effects of surface passivation in porous silicon as H2 gas sensor Solid-State Electron. 52 1071–4 [11] Kanungo J, Saha H and Basu S 2010 Effect of porosity on the performance of surface modified porous silicon hydrogen sensors Sensors Actuators B 147 145–51 [12] Chen Z H, Jie J S, Luo L B, Wang H, Lee C S and Lee S T 2007 Applications of silicon nanowires functionalized with palladium nanoparticles in hydrogen sensors Nanotechnology 18 345502 [13] Barillaro G, Nannini A and Pieri F 2003 APSFET: a new, porous silicon-based gas sensing device Sensors Actuators B 93 263–70 [14] Gunawan O and Guha S 2009 Characteristics of vapor–liquid–solid grown silicon nanowire solar cells Sol. Energy Mater. Sol. Cells 93 1388–93 [15] Morales A M and Lieber C M 1998 A laser ablation method for the synthesis of crystalline semiconductor nanowires Science 279 208–11
Figure 8. Gas sensor response based on rice-straw-like SiNW arrays for O2 , CO2 , N2 , and H2 . The inset shows the response versus gas type with a concentration of 1000 ppm.
with different concentrations for optimal conditions that may effectively promote the response of the SiNW-array-based gas sensors.
4. Conclusion A rice-straw-like SiNW-array hydrogen sensor operating at room temperature was fabricated by a repeated electroless etching technique. In this paper, the rice-straw-like SiNWarray hydrogen sensor displayed a high response (232.5%), a fast response time (31 s), and an appropriate recovery time (249 s), for a hydrogen concentration of 1000 ppm. The rice-straw-like SiNW arrays significantly improved the sensor response, due to the increased surface area and the increased concentration of silicon oxide. Therefore, this repeated MAEE technique offers a simple and cost-effective way to mass produce SiNW-array-based gas sensors.
Acknowledgments This work was partially supported by the National Science Council of Taiwan under grant No. 100-2221-E-011-052MY3 and by the National Taiwan University of Science and Technology. 7
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[16] Schubert L, Werner P, Zakharov N D, Gerth G, Kolb F M, Long L, G¨osele U and Tan T Y 2004 Silicon nanowhiskers grown on h111i Si substrates by molecular-beam epitaxy Appl. Phys. Lett. 84 4968–70 [17] Hsu C M, Connor S T, Tang M X and Cui Y 2008 Wafer-scale silicon nanopillars and nanocones by Langmuir–Blodgett assembly and etching Appl. Phys. Lett. 93 133109 [18] In H J, Field C R and Pehrsson P E 2011 Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection Nanotechnology 22 355501 [19] Huang Z, Geyer N, Werner P, Boor J de and G¨osele U 2011 Metal-assisted chemical etching of silicon: a review Adv. Mater. 23 285–308 [20] Qiu T and Chu P K 2008 Self-selective electroless plating: an approach for fabrication of functional 1D nanomaterials Mater. Sci. Eng. R 61 59–77 [21] Oh J Y, Jang H J, Cho W J and Islam M S 2012 Highly sensitive electrolyte-insulator—semiconductor pH sensors enabled by silicon nanowires with Al2 O3 /SiO2 sensing membrane Sensors Actuators B 171/172 238–43 [22] Campanella L 1990 Principles of chemical sensors J. Electroanal. Chem. Interfacial Electrochem. 299 73–4 [23] Huang B R and Lin T C 2011 A novel technique for fabrication of horizontally-aligned CNTs nanostructure for hydrogen gas sensing Int. J. Hydrog. Energy 36 15919–26 [24] Qiu T, Wu X L, Shen J C, Ha P C T and Chu P K 2006 Surface-enhanced Raman characteristics of Ag cap aggregates on silicon nanowire arrays Nanotechnology 17 5769–72 [25] Lin L, Sun X, Tao R, Feng J and Zhang Z 2011 The synthesis and photoluminescence properties of selenium-treated porous silicon nanowire arrays Nanotechnology 22 075203
[26] Lin J C, Huang B R and Yang Y K 2013 IGZO nanoparticle-modified silicon nanowires as extended-gate field-effect transistor pH sensors Sensors Actuators B 184 27–32 [27] Joshi R and Kumar A 2011 Room temperature gas detection using silicon nanowires Mater. Today 14 52 [28] Hassan J J, Mahdi M A, Chin C W, Hassan H A and Hassan Z 2013 Room temperature hydrogen gas sensor based on ZnO nanorod arrays grown on a SiO2 /Si substrate via a microwave-assisted chemical solution method J. Alloys Compounds 546 107–11 [29] Lupana O, Chow L, Pauport´e Th, Ono L K, Cuenya B R and Chai G 2012 Highly sensitive and selective hydrogen single-nanowire nanosensor Sensors Actuators B 173 772–80 [30] Huang B R and Lin J C 2012 Core–shell structure of zinc oxide/indium oxide nanorod based hydrogen sensors Sensors Actuators B 174 389–93 [31] Li Z, Ding D and Ning C 2013 p-type hydrogen sensing with Al- and V-doped TiO2 nanostructures Nanoscale Res. Lett. 8 25 [32] Pandey P A, Wilson N R and Covington J A 2013 Pd-doped reduced graphene oxide sensing films for H2 detection Sensors Actuators B 183 478–87 [33] Lin T C and Huang B R 2013 Temperature effect on hydrogen response for cracked carbon nanotube/nickel (CNT/Ni) composite film with horizontally aligned carbon nanotubes Sensors Actuators B 185 548–52 [34] Naderi N, Hashim M and Amran T S T 2012 Enhanced physical properties of porous silicon for improved hydrogen gas sensing Superlatt. Microstruct. 51 626–34 [35] Noh J S, Lim H, Kim B S, Lee E, Cho H H and Lee W 2011 High-performance vertical hydrogen sensors using Pd-coated rough Si nanowire J. Mater. Chem. 21 15935–9
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Rice-straw-like structure of silicon nanowire arrays for a hydrogen gas sensor
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 475502 (http://iopscience.iop.org/0957-4484/24/47/475502) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.193.117.53 This content was downloaded on 03/06/2014 at 19:11
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 475502 (8pp)
doi:10.1088/0957-4484/24/47/475502
Rice-straw-like structure of silicon nanowire arrays for a hydrogen gas sensor Bohr-Ran Huang, Ying-Kan Yang and Hsien-Lung Cheng Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, Republic of China E-mail: [email protected]
Received 27 June 2013, in final form 13 September 2013 Published 31 October 2013 Online at stacks.iop.org/Nano/24/475502 Abstract A rice-straw-like silicon nanowire (SiNW) array was developed for hydrogen gas sensing applications. The straight-aligned SiNW array sensor was first fabricated by the metal-assisted electroless etching (MAEE) technique. Rice-straw-like SiNW arrays were formed using a repeated MAEE technique. Hydrogen sensing characteristics were measured for gas concentrations from 20 to 1000 ppm at room temperature. The rice-straw-like SiNW-array-based hydrogen gas sensor performed with low noise and a high response (232.5%) for 1000 ppm hydrogen gas. It was found that the rice-straw-like SiNW-array hydrogen gas sensor had a much better response (approximately 2.5 times) than the straight-aligned SiNW-array sensor. The rice-straw-like SiNW-array structure effectively increased the surface area and the concentration of silicon oxide, which provided additional binding sites for gas molecules. Thus, the rice-straw-like SiNW-array-based hydrogen gas sensor possessed good sensing properties and has the potential for mass production of sensing devices. (Some figures may appear in colour only in the online journal)
1. Introduction
small dimensions, as well as their potential application in sensor devices. Wang et al [7] developed ZnO nanowires for hydrogen gas sensing. Kumar et al [8] reported Pd-decorated single-walled carbon nanotubes (SWNTs) for hydrogen sensing. Peng et al [9] first demonstrated porous silicon nanowires (SiNWs) for NOx sensing at room temperature. This paper focused primarily on SiNW-based gas sensors, due to their high response, quick response and recovery time, and low operating temperature [10–12]. SiNWs are a promising material for gas sensing mainly due to their substantially large surface to volume ratio and strong adsorption for gases [11]. Additionally, they have an advantage of being able to control the surface morphology through the variation of the formation parameters. Furthermore, SiNWs are easy to integrate with other Si-based nano-devices [10, 13]. Recently, SiNWs have been synthesized using two basic approaches, the bottom-up and top-down approaches. The bottom-up approach involves chemical vapor deposition [14], laser ablation [15], and physical vapor deposition [16].
Since it is odorless, colorless, has a wide flammable range (4.65–75%), and is explosive over a wide range of concentrations (15–59%) at standard atmospheric temperature, hydrogen is an extremely dangerous gas. However, it is also essential in many research and industrial applications, such as fuel cells, automobile engines, industrial processing, chemical production, and cryogenic cooling. Hydrogen gas cannot be detected by the human senses, and other means are therefore required to detect its presence and quantify its concentration. Therefore, rapid and accurate hydrogen gas concentration measurements are essential to alert people to the formation of potentially explosive mixtures with air, and to help prevent the risk of an explosion [1, 2]. Potential hydrogen sensors have been investigated using several materials, including SnO2 [3], In2 O3 [4], TiO2 [5], and WO3 [6]. However, one-dimensional nanomaterials have recently attracted much attention, due to their importance in understanding fundamental properties at 0957-4484/13/475502+08$33.00
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Figure 1. Schematic illustration of the major processes involved in the fabrication of: (a) straight-aligned SiNW-array gas sensors; and (b) rice-straw-like SiNW-array gas sensors.
The top-down approach allowed the preparation of SiNWs via dimensional reduction of bulk Si by lithography and etching. Electron beam lithography, reactive ion etching [17], and the recently developed metal-assisted electroless etching (MAEE) technique for silicon have been widely used to fabricate SiNWs [9, 18]. The MAEE technique was the most cost-effective method to synthesize SiNWs, as this approach is suitable for large areas, up to wafer scale. This paper describes the MAEE and repeated MAEE techniques for fabricating straight-aligned and rice-straw-like SiNW arrays, respectively, for hydrogen gas sensing. The rice-straw-like SiNW arrays provide excellent electrical performance. The morphology of the structures was investigated and the effects of these factors on hydrogen sensing properties are discussed.
2.2. Fabrication of rice-straw-like SiNW arrays The rice-straw-like SiNW arrays were prepared using the repeated MAEE technique. After the cleaning process, the silicon wafer was first immersed in a mixture of 5 M HF and 25 mM AgNO3 etching solution for 1 h at room temperature. The sample was then immediately immersed in HNO3 aqueous solution for 120 s, before being rinsed with DI water and blown dry in air. After drying in air, the sample was immersed in the etching solution a second time for 1.5 h; this sample was designated as RS1. Another silicon wafer was first immersed in a mixture of 5 M HF and 35 mM AgNO3 etching solution for 1 h at room temperature. Then, the sample was immediately immersed in HNO3 aqueous solution for 120 s, rinsed with DI water, and blown dry in air. After drying in air, the sample was immersed in a mixture of 5 M HF and 15 mM AgNO3 etching solution for 1.5 h; this sample was designated as RS2. Afterward, all samples were immediately removed from the silver film by leaving them in HNO3 aqueous solution for 120 s. Finally, the samples were rinsed with DI water and blown dry in air.
2. Experimental details 2.1. Fabrication of straight-aligned SiNW arrays Straight-aligned SiNW arrays were synthesized using the MAEE technique [17]. First, a p-type (1–10 cm, B-doped, 520 µm) Cz silicon (100) wafer was ultrasonically cleaned in acetone, isopropyl alcohol, and deionized (DI) water, for 30 min in each cleaning solution. Then the cleaned silicon wafer was immersed in a mixture of 5 M hydrofluoric acid (HF) aqueous and 25 mM silver nitrate (AgNO3 ) solution, for periods of 2, 2.5 and 3 h at room temperature; these samples were designated as SA1, SA2, and SA3, respectively. In this step, Ag+ ions dissolve in the solution which was randomly deposited on the sample surface by galvanic displacement. The sample surface in contact with the Ag+ ions was locally oxidized, becoming SiO2 , and then the oxide layer was etched by hydrofluoric acid [19, 20]. Following the MAEE process, the silicon wafer was wrapped in a thick silver film consisting of high-density tree-like dendritic structures [20]. To remove the capped silver, the as-prepared samples were dipped into a 30-wt% HNO3 aqueous solution for 120 s. Finally, the samples were rinsed with DI water and blown dry in air.
2.3. Preparation and measurement of the sensors Interdigitated platinum (Pt) electrodes, with 18 fingers and a gap interval of 0.1 mm, were deposited on the straight-aligned SiNW arrays and rice-straw-like SiNW arrays using DC sputtering to form chemiresistor gas sensors. A schematic diagram of the fabrication procedures for straight-aligned SiNW-array-based hydrogen sensors and rice-straw-like SiNW-array-based hydrogen sensors is presented in figure 1. For gas detection studies, each hydrogen sensor was characterized in a stainless steel chamber equipped with electrical feedthroughs, through which hydrogen gas of 99.99% purity diluted with dry air flowed under the control of a mass flow controller. The two electrodes of each sensor were connected to a computer controlled Keithley 237 source meter at a fixed voltage of 5 V, and the real-time electrical resistance response was then recorded with various hydrogen concentrations at room temperature. 2
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Figure 2. FESEM images of the surface and tilted cross-section morphology for straight-aligned SiNW arrays with (a) 2, (b) 2.5, and (c) 3 h of MAEE time. (d) Length and top surface density of straight-aligned SiNW arrays with different MAEE time. Table 1. Extracted length and density of straight-aligned and rice-straw-like SiNW arrays with various etching conditions. Straight-aligned SiNW-array gas sensor
Rice-straw-like SiNW-array gas sensor
Sample
SA1
SA2
SA3
RS1
RS2
Condition (concentration of AgNO3 , time) Length of SiNW arrays (µm) Density of SiNW arrays (%)
25 mM, 2h 21.9 35.2
25 mM, 2.5 h 28.0 30.7
25 mM, 3h 38.3 21.7
25 mM, 1 h + 25 mM, 1.5 h 29.5 35.1
35 mM, 1 h + 15 mM, 1.5 h 28.4 50.3
diameters ranging from 20 to 300 nm and lengths proportional to the electroless etching time (approximately 21.9, 28.0, and 38.3 µm for SA1, SA2, and SA3, respectively). Furthermore, it was also observed that the density of the SiNW arrays is around 35.2, 30.7, and 21.7% for SA1, SA2, and SA3, respectively. Figure 2(d) shows the length and density of straight-aligned SiNW arrays with different MAEE duration. It is observed that the density of SiNWs decreases with the MAEE time, which is consistent with other reports [21]. The extracted length and density of straight-aligned SiNW arrays with various etching conditions are summarized in table 1. The hydrogen sensors are characterized by recording their resistance change when they are exposed to environments with hydrogen molecules of different concentrations (20–1000 ppm) at room temperature. Representative hydrogen response curves versus time, with varying hydrogen gas concentrations for straight-aligned SiNW-array gas sensors, are shown in figure 3(a). Here, the sensor response is defined as [(Rhydrogen − Rair )/Rair ] × 100, where Rhydrogen is the resistance of the hydrogen sensor when exposed to ambient hydrogen, and Rair is the resistance in a dry air environment. Surface resistance changes are mainly due to changes in the free electron concentration, due to charge exchange
2.4. Material characterization The morphologies of the straight-aligned SiNW arrays and rice-straw-like SiNW arrays were examined by field emission scanning electron microscopy (FESEM) (JEOL JSM-6700F, operated at 15 kV). The density of SiNW arrays was obtained by performing image analysis of the FESEM photo using the ImageJ image-processing program developed by the US National Institute of Health. The chemical binding of SiNW arrays was investigated by Fourier-transform infrared (FTIR) spectrometry (BomemDA8.3), using an unetched silicon wafer to obtain the background spectrum. The crystal structure of the SiNW arrays was analyzed using a transmission electron microscope (TEM, Philips Tecnai F20 G2 FEI-TEM).
3. Results and discussion FESEM images in figures 2(a)–(c) show the different morphologies of straight-aligned SiNW arrays with different MAEE time. They are observed as free-standing SiNWs, arrayed perpendicularly to the silicon surface. Dense and well-oriented SiNW arrays were formed with nanowire 3
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Figure 3. (a) Hydrogen response versus time with hydrogen concentration ranging from 20 to 1000 ppm for the straight-aligned SiNW-array gas sensors. (b) Hydrogen response, (c) response time, and (d) recovery time versus different hydrogen concentration levels for the straight-aligned SiNW-array gas sensors.
between adsorbed species from the gas and the semiconductor surface [10]. Moreover, Campanella et al [22] indicated that trapped electrons were found at the interface of the silicon walls and hydrogen molecules, and are responsible for the change in resistance. Typical curves obtained from these sensors show that their resistance quickly increases as these sensors are exposed to hydrogen gas. After the introduction of hydrogen gas, the resistance increases with time and saturates at a value that depends on the gas concentration. In general, sensors exhibit a higher response when they are exposed to hydrogen with higher concentrations. That is, when these sensors are surrounded by dry air, their resistance drops and essentially returns to its original value. However, the straight-aligned SiNW-array gas sensors can even detect, with observable signals, hydrogen concentrations down to levels of 20 ppm. Figure 3(a) shows the time dependence of the response for the straight-aligned SiNW-array hydrogen gas sensors with different electroless etching times, at various hydrogen concentrations from 20 to 1000 ppm. The response shown in figure 3(b) is the summarized response for environments with different hydrogen concentrations. The response observed is 56.1, 94.9, and 82.8% for SA1, SA2, and SA3, respectively, for a 1000 ppm hydrogen gas concentration. It indicates that the SA2 sample exhibits the highest response since it possesses the larger surface area. Kanungo et al [11] indicate that an increase to a larger surface area produces a higher gas response since there is an increase in the interaction of hydrogen with the sensing surface. In addition, Oh et al [21] reported that the surface area varies proportionally with the density and length of SiNW arrays. Hence, the surface area could be given in proportion
to the length of the SiNW arrays × the density of SiNW arrays, denoted by A. A was 7.7, 8.6, and 8.3 for SA1, SA2, and SA3, respectively. SA2 possessed the larger surface area, yielding the highest response of the three samples. Figures 3(c) and (d) show response times and recovery times versus different hydrogen concentration levels, respectively, for the straight-aligned SiNW-array hydrogen gas sensor. Response time is defined as the time required to reach 90% of the maximum change. Recovery time is defined as the time required to reach 10% of the minimum change [23]. The response times of SA1, SA2, and SA3 are around 256, 276, and 296 s, respectively, for a hydrogen concentration of 20 ppm, and decrease to about 89, 20, and 143 s for a concentration of 1000 ppm, respectively. It should be noted that SA2 has the fastest response time at higher hydrogen concentrations. SA2 also has the fastest recovery time over the full concentration range, and recovery times vary from 152 s for a 20 ppm hydrogen concentration to 209 s for a 1000 ppm hydrogen concentration. As mentioned earlier, since SA2 provided the largest effective surface area to adsorb and desorb hydrogen gas, it has the shortest response and recovery times. Rice-straw-like SiNW arrays are shown in figures 4(a) and (b). Unlike the straight-aligned SiNW arrays, the top of the rice-straw-like SiNW arrays comprises thin bunches of rice-straw-like structures. During the repeated MAEE etching process, silver particles were easily formed on the tips and upper side walls of the nanowires. Thus, the tip and upper side wall of nanowires are further etched. Moreover, the bottom of nanowires are covered by silver particles at this stage, and elongated nanowires are formed. Afterward, the tips of the 4
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Figure 4. FESEM images of the surface and tilted cross-section morphology for rice-straw-like SiNW arrays of (a) RS1 and (b) RS2. (c) The proposed etching process of rice-straw-like SiNW arrays and the proposed current flow during the hydrogen sensing measurement.
Figure 5. FTIR spectra of straight-aligned SiNW arrays and rice-straw-like SiNW arrays.
Figure 6. TEM image showing a magnified view for the rice-straw-like SiNW arrays of RS2.
nanowires that are off-axis to gravitation will bend more, and eventually the torque leads to bending and agglomeration of the nanowires after the silver is removed. This phenomenon is attributed to the strong van der Waals attraction of the nanowires [24]. Figure 4(c) shows a schematic representation of the repeated etching process for rice-straw-like SiNW arrays. The length of rice-straw-like SiNW arrays for RS1 and RS2 is 29.5 and 28.4 µm, yielding densities of 35.1 and 50.3%, respectively. Figure 5 shows FTIR spectra of the straight-aligned SiNW-array gas sensor (SA1) and the rice-straw-like SiNWarray gas sensors (RS1 and RS2) in the range from 500 to 4000 cm−1 . The broad peak in the range 3000–3700 cm−1 is due to the O–H stretching vibration. Symmetric and asymmetric C–H stretching vibrations located in the range
between 2800 and 3000 cm−1 were observed. The peak at around 1736 cm−1 is a C–O stretching vibration. The asymmetric Si–O–Si stretching vibrations are distributed in the range 1000–1300 cm−1 . The bending and wagging vibration modes in Si–H are located at 880 cm−1 . It was found that RS1 and RS2 have an obvious peak at around 814 cm−1 , corresponding to the O–Si–O bending mode [25, 26]. As shown in figure 6, the TEM image of RS2 shows a sharp contrast between the bright region of the amorphous phase of the silicon oxides in the sidewall and the periodic structure of single-crystal silicon in the center of the SiNW. 5
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Figure 7. Comparison of (a) and (b) hydrogen response, (c) response times, and (d) recovery times versus different hydrogen concentration levels for different rice-straw-like SiNW arrays.
SiNW-array gas sensors. Ali et al [10] indicated that a higher concentration of silicon oxide creates a greater response to hydrogen gas. Fourthly, the repeated electroless etching process for rice-straw-like SiNW-array gas sensors may produce a type of incomplete covalent bond. This type of bond provides a type of gas sensing in nanowire systems [27]. As shown in figures 7(c) and (d), for rice-straw-like SiNW-array gas sensors, the response times for RS1 and RS2 are about 152 and 199 s, respectively, at hydrogen concentrations of 20 ppm, and decrease to about 41 and 30 s, respectively, for concentrations of 1000 ppm. The recovery times are about 142 and 192 s for RS1 and RS2, respectively, at hydrogen concentrations of 20 ppm, increasing to around 162 and 249 s, respectively, for concentrations of 1000 ppm. In order to test the selectivity for hydrogen of the rice-straw-like SiNW-array gas sensors, the gas response to O2 , CO2 and N2 for RS2 with a gas concentration of 1000 ppm have also been investigated and summarized in figure 8. It is observed that the gas response for O2 , CO2 and N2 is much lower in comparison with its gas response to hydrogen. It is shown that the rice-straw-like SiNW arrays possess high selectivity to detect hydrogen at room temperature. In this study, a response of about 232.5% is achieved for rice-straw-like SiNW-array gas sensors. A comparison between the sensing performances of the sensor and literature reports [28–35] is summarized in table 2. The response is comparable to those featured in previous reports with different sensing materials. Furthermore, the response is superior to that of porous silicon gas sensors [34], or silicon nanostructures modified with noble metal gas sensors [11, 35]. Moreover, the repeated electroless etching process of rice-straw-like SiNW arrays could be investigated further
Figure 7(a) shows the time dependence of the response for the straight-aligned SiNW-array hydrogen gas sensors with different electroless etching times, at various hydrogen concentrations from 20 to 1000 ppm. The response shown in figure 7(b) is the summarized response of the sensor in environments with different hydrogen concentrations. The responses for SA1 and SA2 are 191.1 and 232.5, respectively, indicating that SA2 has a better response than SA1. Furthermore, the rice-straw-like SiNW-array gas sensors display a significantly better response than the straight-aligned SiNW-array gas sensors, which is due to the increase of surface area. As mentioned earlier, A is proportional to the surface area. However, A for RS1 and RS2 is 10.4 and 14.3, respectively. It can be observed that the rice-straw-like SiNW-array gas sensors exhibit a higher surface area than the straight-aligned SiNW-array gas sensors. Moreover, RS2 exhibits a higher surface area than RS1. Hence, the higher response for rice-straw-like SiNW-array gas sensors could be due to the following suggestion. Firstly, as shown in figure 4(c), for the straight-aligned SiNW arrays, the current flows along from the top of the nanowires down to the substrate between the Pt electrodes. However, the rice-straw-like SiNW arrays provide more current paths in addition to the path along the substrate between the Pt electrodes (dotted line). The primary current path (solid line) flows along the top surface of the SiNWs since more active conducting sites exist due to the rice-straw-like SiNW nanostructure. This leads to more active sensing sites in the measurement. Secondly, the rice-straw-like SiNW-array gas sensors possess a larger surface area and therefore provide a higher contact area with the hydrogen gas. Thirdly, there is a higher concentration of Si–O–Si bonding in rice-straw-like 6
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Table 2. Comparison of recently developed nanostructure-based hydrogen gas sensors at room temperature. Sensing materials
Response (%)
Response time (s)
Recovery time (s)
Reference
ZnO nanorod Cd-doped ZnO nanowires ZnO/In2 O3 core–shell nanorod Al- and V-doped TiO2 nanostructures Pd-doped reduced graphene oxide CNT/Ni composite film Porous silicon Pd-modified porous silicon Pd-coated silicon nanowires Rice-straw-like structure silicon nanowires
294 (1000 ppm) 274 (100 ppm) 20.5 (500 ppm) 0.6 (1000 ppm) 26 (1000 ppm) 14.1 (200 ppm) 0.4 84 (10 000 ppm) 180 (1000 ppm) 232.5 (1000 ppm)
60 14 — — — 422 160 8 — 31
277 11 — — — 130 69 207 — 249
[28] [29] [30] [31] [32] [33] [34] [11] [35] Present work
References [1] H¨ubert T, Brett L B, Black G and Banach U 2011 Hydrogen sensors—a review Sensors Actuators B 157 329–52 [2] Zeng X Q, Latimer M L, Xiao Z L, Panuganti S, Welp U, Kwok W K and Xu T 2011 Hydrogen gas sensing with networks of ultrasmall palladium nanowires formed on filtration membranes Nano Lett. 11 262–8 [3] Shen Y, Yamazaki T, Liu Z, Meng D, Kikuta T, Nakatani N, Saito M and Mori M 2009 Microstructure and H2 gas sensing properties of undoped and Pd-doped SnO2 nanowires Sensors Actuators B 135 524–9 [4] Xu X et al 2013 Template-free synthesis of novel In2 O3 nanostructures and their application to gas sensors Sensors Actuators B 185 118–23 [5] Srivastava S, Kumar S, Singh V N, Sigh M and Vijay Y K 2011 Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing Int. J. Hydrog. Energy 36 6343–55 [6] Calavia R, Mozalev A, Vazquez R, Gracia I, Can´e C, Ionescu R and Llobet E 2010 Fabrication of WO3 nanodot-based microsensors highly sensitive to hydrogen Sensors Actuators B 149 352–61 [7] Wang H T, Kang B S, Ren F, Tien L C, Sadik P W, Norton D P, Pearton S J and Lin J 2005 Hydrogen-selective sensing at room temperature with ZnO nanorods Appl. Phys. Lett. 86 243503 [8] Kumar M K, Reddy A L M and Ramaprabhu S 2008 Exfoliated single-walled carbon nanotube-based hydrogen sensor Sensors Actuators B 130 653–60 [9] Peng K Q, Wang X and Lee S T 2009 Gas sensing properties of single crystalline porous silicon nanowires Appl. Phys. Lett. 95 243112 [10] Ali N K, Hashim M R and Aziz A A 2008 Effects of surface passivation in porous silicon as H2 gas sensor Solid-State Electron. 52 1071–4 [11] Kanungo J, Saha H and Basu S 2010 Effect of porosity on the performance of surface modified porous silicon hydrogen sensors Sensors Actuators B 147 145–51 [12] Chen Z H, Jie J S, Luo L B, Wang H, Lee C S and Lee S T 2007 Applications of silicon nanowires functionalized with palladium nanoparticles in hydrogen sensors Nanotechnology 18 345502 [13] Barillaro G, Nannini A and Pieri F 2003 APSFET: a new, porous silicon-based gas sensing device Sensors Actuators B 93 263–70 [14] Gunawan O and Guha S 2009 Characteristics of vapor–liquid–solid grown silicon nanowire solar cells Sol. Energy Mater. Sol. Cells 93 1388–93 [15] Morales A M and Lieber C M 1998 A laser ablation method for the synthesis of crystalline semiconductor nanowires Science 279 208–11
Figure 8. Gas sensor response based on rice-straw-like SiNW arrays for O2 , CO2 , N2 , and H2 . The inset shows the response versus gas type with a concentration of 1000 ppm.
with different concentrations for optimal conditions that may effectively promote the response of the SiNW-array-based gas sensors.
4. Conclusion A rice-straw-like SiNW-array hydrogen sensor operating at room temperature was fabricated by a repeated electroless etching technique. In this paper, the rice-straw-like SiNWarray hydrogen sensor displayed a high response (232.5%), a fast response time (31 s), and an appropriate recovery time (249 s), for a hydrogen concentration of 1000 ppm. The rice-straw-like SiNW arrays significantly improved the sensor response, due to the increased surface area and the increased concentration of silicon oxide. Therefore, this repeated MAEE technique offers a simple and cost-effective way to mass produce SiNW-array-based gas sensors.
Acknowledgments This work was partially supported by the National Science Council of Taiwan under grant No. 100-2221-E-011-052MY3 and by the National Taiwan University of Science and Technology. 7
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[16] Schubert L, Werner P, Zakharov N D, Gerth G, Kolb F M, Long L, G¨osele U and Tan T Y 2004 Silicon nanowhiskers grown on h111i Si substrates by molecular-beam epitaxy Appl. Phys. Lett. 84 4968–70 [17] Hsu C M, Connor S T, Tang M X and Cui Y 2008 Wafer-scale silicon nanopillars and nanocones by Langmuir–Blodgett assembly and etching Appl. Phys. Lett. 93 133109 [18] In H J, Field C R and Pehrsson P E 2011 Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection Nanotechnology 22 355501 [19] Huang Z, Geyer N, Werner P, Boor J de and G¨osele U 2011 Metal-assisted chemical etching of silicon: a review Adv. Mater. 23 285–308 [20] Qiu T and Chu P K 2008 Self-selective electroless plating: an approach for fabrication of functional 1D nanomaterials Mater. Sci. Eng. R 61 59–77 [21] Oh J Y, Jang H J, Cho W J and Islam M S 2012 Highly sensitive electrolyte-insulator—semiconductor pH sensors enabled by silicon nanowires with Al2 O3 /SiO2 sensing membrane Sensors Actuators B 171/172 238–43 [22] Campanella L 1990 Principles of chemical sensors J. Electroanal. Chem. Interfacial Electrochem. 299 73–4 [23] Huang B R and Lin T C 2011 A novel technique for fabrication of horizontally-aligned CNTs nanostructure for hydrogen gas sensing Int. J. Hydrog. Energy 36 15919–26 [24] Qiu T, Wu X L, Shen J C, Ha P C T and Chu P K 2006 Surface-enhanced Raman characteristics of Ag cap aggregates on silicon nanowire arrays Nanotechnology 17 5769–72 [25] Lin L, Sun X, Tao R, Feng J and Zhang Z 2011 The synthesis and photoluminescence properties of selenium-treated porous silicon nanowire arrays Nanotechnology 22 075203
[26] Lin J C, Huang B R and Yang Y K 2013 IGZO nanoparticle-modified silicon nanowires as extended-gate field-effect transistor pH sensors Sensors Actuators B 184 27–32 [27] Joshi R and Kumar A 2011 Room temperature gas detection using silicon nanowires Mater. Today 14 52 [28] Hassan J J, Mahdi M A, Chin C W, Hassan H A and Hassan Z 2013 Room temperature hydrogen gas sensor based on ZnO nanorod arrays grown on a SiO2 /Si substrate via a microwave-assisted chemical solution method J. Alloys Compounds 546 107–11 [29] Lupana O, Chow L, Pauport´e Th, Ono L K, Cuenya B R and Chai G 2012 Highly sensitive and selective hydrogen single-nanowire nanosensor Sensors Actuators B 173 772–80 [30] Huang B R and Lin J C 2012 Core–shell structure of zinc oxide/indium oxide nanorod based hydrogen sensors Sensors Actuators B 174 389–93 [31] Li Z, Ding D and Ning C 2013 p-type hydrogen sensing with Al- and V-doped TiO2 nanostructures Nanoscale Res. Lett. 8 25 [32] Pandey P A, Wilson N R and Covington J A 2013 Pd-doped reduced graphene oxide sensing films for H2 detection Sensors Actuators B 183 478–87 [33] Lin T C and Huang B R 2013 Temperature effect on hydrogen response for cracked carbon nanotube/nickel (CNT/Ni) composite film with horizontally aligned carbon nanotubes Sensors Actuators B 185 548–52 [34] Naderi N, Hashim M and Amran T S T 2012 Enhanced physical properties of porous silicon for improved hydrogen gas sensing Superlatt. Microstruct. 51 626–34 [35] Noh J S, Lim H, Kim B S, Lee E, Cho H H and Lee W 2011 High-performance vertical hydrogen sensors using Pd-coated rough Si nanowire J. Mater. Chem. 21 15935–9
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