- Email: [email protected]
SiNWs electrode
Sensors and Actuators B 142 (2009) 298–303
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
An amperometric ethanol sensor based on a Pd–Ni/SiNWs electrode Bairui Tao a,b , Jian Zhang a,∗ , Shichao Hui a , Lijuan Wan a a Laboratory of Poling Materials and Devices of MOE and State Key Laboratory of Transducer Technology, East China Normal University, 500 Dong Chuan Road, Shanghai, 200241, China b College of Communications and Electronic Engineering, Qiqihar University, 42 Wenhua Street, Qiqihar, Heilongjiang, 161006, China
a r t i c l e
i n f o
Article history: Received 28 May 2009 Received in revised form 3 August 2009 Accepted 4 August 2009 Available online 12 August 2009 Keywords: Ethanol sensor Nickel/Silicon nanowires Palladium Electroless plating Electrocatalysis
a b s t r a c t A new amperometric ethanol sensor has been developed. The sensor uses the silicon nanowires covered with co-deposited palladium–nickel (Pd–Ni/SiNWs) as the working electrode. The detection of ethanol concentration is based on the response currents resulted from the electro-catalytic oxidation of ethanol. The performance of the sensor was characterized by cyclic voltammetry and fixed potential amperometry techniques. In 1 M KOH solution containing different ethanol concentrations, the sensor shows a good sensitivity of 7.48 mA mM−1 cm−2 and the corresponding detection limit (signal-to-noise ratio = 3) of 6 M for cyclic voltammetry. Meanwhile, it also displays a sensitivity of 0.76 mA mM−1 cm−2 and the corresponding detection limit of 10 M for fixed potential amperometry. The results demonstrate that the Pd–Ni/SiNWs electrodes are potential as the electrochemical integrated sensors for ethanol detection. © 2009 Published by Elsevier B.V.
1. Introduction The detection of ethanol concentration is important for medicine, brewing, beverage, traffic safety and etc. For example, the ethanol level assays are necessary for the control of drunken driving to avoid traffic accidents. Although liquid chromatograph and mass spectroscopy can determine the concentrations of ethanol with high precision, they are expensive and not suitable for portable use [1,2]. So lots of ethanol sensors have been explored which can detect the ethanol quickly and cost-effectively [3,4]. For the most commonly-used semiconductor-based ethanol sensors, their applications are limited due to some drawbacks such as high working temperature (≥300 ◦ C), complicated fabrication, high cost, and so on [5]. In principle, the electrochemical ethanol sensors, based on detection of the response currents resulted from the ethanol oxidation, have the highest sensitivity and accuracy among all of the sensors [6]. The three-electrode or two-electrode configurations are employed in these sensors. Therefore, the main jobs are focused on the development of new electrodes which can electrooxidize the ethanol easily and effectively. Several sensors had been reported to detect the ethanol such as using the alcohol oxidase enzyme modified electrode [7,8], the cobalt–nickel oxide electrode [9], the nickel foil electrode [10], the sputtered Ni/Pt/Ti on an Al2 O3 substrate as
∗ Corresponding author. Tel.: +86 21 54345203; fax: +86 21 54345119. E-mail address: [email protected] (J. Zhang). 0925-4005/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.snb.2009.08.004
the working electrode [11], etc. Furthermore, it is a significant challenge to establish electrodes without enzyme or platinum for the low cost and efficient ethanol sensors. Recently, the development of the ordered porous frameworks or nanostructures with a large surface area, permanent porosity and high thermal stability such as nanowires, nanotubes, nanoparticles and polymer nanocomposites open up a window for establish new electrodes [12–15]. In this paper, a robust electrochemical ethanol sensor based on a Pd–Ni/SiNWs electrode is presented. The SiNWs were first fabricated by chemical etching as backbone, and the Pd–Ni/SiNWs composite structure was then obtained by electroless co-plating nickel and trace palladium onto the surface of SiNWs. The Pd–Ni/SiNWs composite electrode exhibits high catalytic activity for the oxidation of ethanol. And ethanol sensing performances are studied by cyclic voltammetry (CV) and fixed potential amperometry technology. 2. Experimental 2.1. Raw materials 1 0 0-Orientated single-polished N-type heavily Sb-doped silicon-wafers with resistivity of 0.006–0.025 cm were used as the substrates for SiNWs growth. AgNO3 , NiSO4 ·6H2 O, (NH4 )2 SO4 , PdCl2 , NH4 F, KOH, Triton X-100 wetting agent, hydrofluoric acid, sodium dodecyl sulfate, sodium succinate, sodium citrate, ammonia and ethanol were all analytical reagents and used without further purification.
B. Tao et al. / Sensors and Actuators B 142 (2009) 298–303
Fig. 1. Schematic diagram of the ethanol sensing detection system.
2.2. Fabrication of Pd–Ni/SiNWs electrode 2.2.1. Formation of SiNWs The SiNWs were first fabricated by wet chemical etching with the etchant of AgNO3 (25 mM): HF (15%) = 1:1 (volume ratio) and the formation of SiNWs in detailed can be found elsewhere [16]. Next, all the chips were rinsed in diluted nitric acid (∼30% in volume) and deionized water to remove the residuals on their surface. 2.2.2. Electroless co-plating Pd–Ni onto SiNWs After cleaning carefully, the SiNWs chips were dipped into the Triton X-100 solution (1%) to activate for 30 s. Next, The Pd–Ni/SiNWs nanocomposites were prepared by coating Ni and trace Pd simultaneously onto the SiNWs with electroless co-plating. The recipe for Ni electroless plating bath can be found elsewhere [17]. The electroless co-plating Pd–Ni was conducted by adding PdCl2 100 mg L−1 into the Ni electroless plating bath. After plating, these chips were annealed at 400 ◦ C using a rapid thermal annealing system in Ar atmosphere for 300 s. Finally, the Pd–Ni/SiNWs chips as-received were used for material characterization and working electrode of the ethanol sensor development. 2.3. Electrode characterization and system for ethanol sensing detection The morphology and structure of the prepared Pd–Ni/SiNWs nanocomposite electrode were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The working electrode was constructed by gluing a copper leading wire onto the Pd–Ni/SiNWs chips with the electrode area of 0.5 × 0.5 cm2 . The electrochemical properties of the electrode were performed using an electrochemical workstation (LK3200A, Lanlike, China) with a platinum foil counter electrode and a saturated Ag/AgCl reference electrode in 1 M KOH alkaline solution. Fig. 1 shows the schematic diagram of the ethanol sensing detection system, where, S is the solution of sample, C the carrier fluid of 1 M KOH solution, R the deionized water, P the peristaltic pump, V the 6-channel valve, D the tri-electrode cell. During testing process, the solution of sample with various concentration ethanol was injected through S channel by the peristaltic pump P. 3. Results and discussion 3.1. Characterizations of the Pd–Ni/SiNWs electrode The SEM morphology images of the as-prepared Pd–Ni/SiNWs formed on the Si substrate are shown in Fig. 2(a)–(c), respectively. Fig. 2(a) is the top view of the Pd–Ni/SiNWs. It can be seen that the length of the SiNWs formed the bundle-like structure are uniform, ∼50 m. Fig. 2(b) is the partial enlarged view. It can be observed that the diameter of nanowires range from 60 to 300 nm. Fig. 2(c) is the cross sectional view. It is found that an additional layer has been plated onto the SiNWs’ surface.
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The additional layer was further examined by TEM. And the TEM picture is shown in Fig. 3(a). It is found that the thickness of the additional layer is about 10 nm. And the additional layer covered the silicon nanowires uniformly. The EDS spectrum of Pd–Ni/SiNWs nanocomposites is shown in Fig. 3(b). It is verified that the layer plated onto the SiNWs is Ni and trace Pd. In addition, according to the EDS spectrums of multiple measurements in different parts of Pd–Ni/SiNWs, only the characteristic peaks of Ni and Pd are found. No characteristic peaks of Si are found. It is implied that Ni coating formed is a dense layer and some Ni or Pd agglomerate particles are formed on the Ni film. Here, Cr is the contamination due to the sample holder. 3.2. The ethanol electrooxidation on the Pd–Ni/SiNWs electrode To verify the fabricated Pd–Ni/SiNWs electrode which has the capability to sense the ethanol, the electrochemical behavior of the electrode in 1 M KOH solution with or without ethanol of 10 mM was first examined in a three-electrode electrochemical cell using cyclic voltammetry (CV), with a potential window from −0.95 to 0.6 V at the scan rate of 10 mV s−1 . The CV curves are shown in Fig. 4, in which the solid curve, A, corresponds to the scanning results without ethanol addition and the dash curve, B, corresponds to the case when the 10 mM ethanol was added. Comparing with the solid curve A, two oxidation current peaks are observed clearly in curve B at the potential of ∼−0.28 and 0.52 V, which is attributed to the electrocatalytic of Pd and Ni, respectively [18,19]. It is concluded that the Pd–Ni/SiNWs electrode as-received shows the electro-oxidation capability to ethanol, which can be used for ethanol detection. 3.3. Ethanol sensing with cyclic voltammetry technique In the ethanol sensing procedure, the optimum sweeping potential range from −0.6 to 0.1 V is selected for avoiding the interferences such as the redox reaction of Ni, OH− and oxygen dissolved in solution etc. [18–20]. In this research, the oxidation peak currents are used as the criterion for the ethanol detection. In this study, the scanning rate is fixed at 50 mV s−1 unless otherwise specified. 3.3.1. Stability of Pd–Ni/SiNWs electrode The stability of electrode was first conducted in the 1 M KOH solution without ethanol addition. The results are shown in Fig. 5(a). It indicates that after 100 cycle CVs, the maximum variation of the current is less than 1% at the same potential. Fig. 5(b) shows the 100 cycle CVs of the prepared electrode in 1 M KOH solution with 1 M ethanol addition. During the forward sweeping, the oxidation peak at potential of −0.01 V is mainly attributed to the ethanol electrocatalytic oxidation reaction on the Pd sites. The mechanism of ethanol oxidation on Pd in alkaline solution has been reported according to the in situ IR measurement [21]. The intermediates of ethanol oxidation are CH3 COO− , small amounts of CH3 CHO, Pd2 (CO)ads and etc. The ideal end-product is CO2 with H2 O [20–22]. During the reverse sweeping, the reduction of Pd oxides to Pd and production of active sites take place, the re-oxidation of ethanol or carbonaceous species formed in the anodic process occurs on clean Pd surface and backward peak at −0.36 V appears. These have important meaning to keep the stability of Pd–Ni/SiNWs electrode. It can be also observed that the peak current decrease almost linearly with the CV times increasing. After 100 times cycling, the loss of peak current is less than 9.5% at the potential of ∼0 V, which correspond to the peak current variation of ∼1% per cycle. The loss mechanism is probably attributed to the consumption of ethanol in oxidation process. It
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Fig. 2. Micro-morphology of Pd–Ni/SiNWs: (a) the top view, (b) the partial enlarged view and (c) the cross sectional view.
is concluded that the Pd–Ni/SiNWs electrode has the superior stability.
due to the transition of the electrode process from a mass-transport controlled to a surface-reaction controlled process [23].
3.3.2. Sensitivity and linearity The CVs results of the prepared electrode with different ethanol concentrations in 1 M KOH solution are shown in Fig. 6, where the curve a–e correspond to the ethanol concentration of 3.4, 6.8, 10.2, 13.6 and 17.1 mM, respectively. It is indicated that the oxidation peak currents increase from 5.04 to 31.09 mA and the oxidation peak potential shift from −0.40 to −0.23 V when ethanol concentration increase from 3.4 mM to 17.1 M, which is mainly caused by the electrode polarization. During the reverse sweeping, the potential of Pd+2 reduction peak and the carbonaceous species oxidation peak is shift right when the ethanol concentrate increase. Accordingly, the relationship between the abstracted oxidation peak current values and ethanol concentration levels can be employed to detect ethanol. The results are depicted in Fig. 7 when the ethanol concentrations increase from 0 to 34.2 mM every 3.4 mM. The linear fitting results are also shown in the inset of this figure, where, the equation of linear fitting equation is expressed as I (mA) = −0.71 + 1.87C (mM) with R2 = 0.998. The sensitivity of the prepared Pd–Ni/SiNWs electrode is ∼7.48 mA mM−1 cm−2 and the concentration of the detection limit is 6 M for the signal-to-noise ratio of 3 [11]. When the ethanol concentration is larger than 20.4 mM, a sharp decrease of response current can be observed, which is probably
3.4. Ethanol sensing with fixed potential amperometry The response of Pd–Ni/SiNWs electrode to ethanol is also examined using fixed potential amperometry at −0.25 V. The initial volume of each sample is 100 mL in the tri-electrode cell. The experiment is performed on solution containing without ethanol in 1 M KOH. After the background current was stable, the ethanol solution of 99.7% was injected by the peristaltic pump with about 20 L every 30 s (∼3.4 mM every 30 s) drop by drop. The current–time responses of the Pd–Ni/SiNWs electrode to ethanol are shown in Fig. 8. The steady state values of the response current are plotted in the inset of Fig. 8 corresponding to the ethanol concentration ranging from 0 to 20.4 mM. The linearly fitting equation is I (mA) = −0.09 + 0.19C (mM) with R2 = 0.997. The sensitivity of the prepared Pd–Ni/SiNWs electrode to the ethanol concentration is ∼0.76 mA mM−1 cm−2 with the detection limit concentration of ∼10 M for the signal-to-noise ratio of 3. 3.5. Discussion A comparison of some basic parameters between this sensor prototype and the other developed ethanol sensor systems is described in Table 1.
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Fig. 3. TEM and EDS of Pd–Ni/SiNWs: (a) the TEM image, (b) the EDS spectrum.
The study has demonstrated that the Pd–Ni/SiNWs electrode exhibit high sensitivity, excellent stability to ethanol. It could be attributed to three aspects: Firstly, the electrode of Pd–Ni/SiNWs with high volume of surface ratio and small curvature radii of nanowires not only increases the reaction area but also promote electron-transfer reactions, which can lower the activation energy and catalyze the electrooxidation ethanol easily [24]. This can promote the sensitivity. Secondly, the backbone of SiNWs itself could support the active materials (Pd–Ni) well. Especially, the electrode annealed at 400 ◦ C can form the NiSi layer between Ni and SiNWs interface, which can resist to OH− etching [25,26]. This can lead to the improved stability.
Fig. 4. Cyclic voltammograms in 1 M KOH with and without 10 mM ethanol.
Fig. 5. Cyclic voltammograms of 100 times cycling: (a) in 1 M KOH solution; (b) in 1 M KOH with 1 M ethanol.
Thirdly, it is believed that the nanoscale effects of Pd–Ni layer about 10 nm between Ni and SiNWs interface could play a very importance role during the electro-catalytic oxidation steps of the ethanol in alkaline solution. The detail mechanism need further study.
Fig. 6. Cyclic voltammograms in 1 M KOH solution containing different ethanol concentrations.
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Table 1 Comparison of some basic parameters for ethanol sensors. Working electrode Pd–Ni/SiNWs Pd–Ni/SiNWs Alcohol oxidase Co-Ni oxide RuO2 -modified Ni Ni/Pt/Ti
Detection technique Cyclic voltammetry Fixed potential amperometry Fixed potential amperometry Voltammetry Fixed potential amperometry Fixed potential amperometry
Sensitivity −1
−2
∼7.48 mA mM cm ∼0.76 mA mM−1 cm−2 ∼859 nA mM−1 cm−2 ∼375 nA mM−1 4.92 A ppm−1 cm−2 3.08 A mM−1 cm−2
Detection limit
Reference
6 M 10 M 29.7 M 32 M – –
This work This work [8] [9] [10] [11]
Acknowledgement This work is supported by the National Natural Science Foundation of China (Grant No. 60672002) and by Innovation Program of Shanghai Municipal Education Commission (Grant No. 09ZZ46). We are grateful for the financial support. References
Fig. 7. The relationship between the abstracted oxidation peak current values of CVs and ethanol concentration.
Fig. 8. The current–time responses of the Pd–Ni/SiNWs electrode.
4. Conclusion In conclusion, we have fabricated the Pd–Ni/SiNWs nanocomposite electrode with the combination of wet chemical etching of Si and electroless plating of Pd–Ni nucleus to determine the ethanol concentration. It exhibits high sensitivity, excellent stability. The detection limit is 6 and 10 M with cyclic voltammetry and fixed potential amperometry at −0.25 V, respectively. The sensor with simple, low cost, easy integration, highly sensitive and excellent stability is attractive for the fabrication of efficient integrated amperometric ethanol sensors.
[1] S.M. Rao, Spectrographic technique for determining refractive indexes, Opt. Eng. 36 (1997) 162–166. [2] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species, Science 293 (2001) 1289. [3] A.O.S.S. Rangel, I.V. Toth, Determination of alcohol in wines by flow-injection spectrophotometry using gas-diffusion and an immobilized enzyme reactor, Am. J. Enol. Vitic. 50 (1999) 259–263. [4] R. Phathaitep, G. Atcharawan, C. Torranin, M. Pongsri, M. Nikorn, C. Supab, Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction, Ceram. Int. 35 (2009) 649–652. [5] G. Neri, A. Bonavita, G. Micali, N. Donato, F.A. Deorsola, P. Mossino, I. Amato, B. De Benedetti, Ethanol sensors based on Pt-doped tin oxide nanopowders synthesised by gel-combustion, Sens. Actuators B 117 (2006) 196–204. [6] Y.C. Weng, T.C. Chou, Ethanol sensors by using RuO2 -modified Ni electrode, Sens. Actuators B 85 (2002) 246–255. [7] A.M. Azevedo, D.M.F. Prazeres, J.M.S. Cabral, L.P. Fonseca, Ethanol biosensors based on alcohol oxidase, Biosens. Bioelectron. 21 (2005) 235–247. [8] M.M. Barsan, C.M.A. Brett, An alcohol oxidase biosensor using PNR redox mediator at carbon film electrodes, Talanta 74 (2008) 1505–1510. [9] E.T. Hayes, B.K. Bellingham, H.B. Mark Jr., A. Galal, An amperometric aqueous ethanol sensor based on the electrocatalytic oxidation at a cobalt-nickel oxide electrode, Electrochim. Acta 41 (1996) 337–344. [10] Y.C. Weng, T.C. Chou, Ethanol sensor by using by using RuO2 -modified Ni electrode, Sens. Actuators B 85 (2002) 246–255. [11] Y.C. Weng, J.F. Rick, T.C. Chou, A sputtered thin film of nanostructured Ni/Pt/Ti on Al2 O3 substrate for ethanol sensing, Biosens. Bioelectron. 20 (2004) 41–51. [12] T.K. Maji, R. Matsuda, S. Kitagawa, A flexible interpenetrating coordination framework with a bimodal porous functionality, Nat. Mater. 6 (2007) 142–148. [13] C.-D. Wu, A. Hu, L. Zhang, W. Lin, A homochiral porous metal-organic framework for highly enantioselective heterogeneous asymmetric catalysis, J. Am. Chem. Soc. 127 (2005) 8940–8941. [14] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Reticular synthesis and the design of new materials, Nature 423 (2003) 705–714. [15] N. Elfström, A.E. Karlström, J. Linnros, Silicon nanoribbons for electrical detection of biomolecules, Nano Lett. 3 (2008) 945–949. [16] L.J. Wan, W.L. Gong, K.W. Jiang, H.L. Li, B.R. Tao, J. Zhang, Preparation and surface modification of silicon nanowires under normal conditions, Appl. Surf. Sci. 254 (2008) 4899–4907. [17] B.R. Tao, J. Zhang, F.J. Miao, H.L. Li, L.J. Wan, Y.T. Wang, Capacitive humidity sensors based on Ni-SiNWs nanocomposites, Sens. Actuators B 136 (2008) 144–150. [18] Y.L. Lo, B.J. Hwang, In situ Raman studies on cathodically deposited nickel hydroxide films and electroless Ni-P electrodes in 1 M KOH, Langmuir 14 (1998) 944–950. [19] Y.Y. liao, T.C. Chou, An amperometric alcohol by using electrolyses nickel as working electrode, Electroanalysis 12 (2000) 55–59. [20] Z.X. Liang, T.S. Zhao, J.B. Xu, L.D. Zhu, Mechanism study of the ethanol oxidation reaction on palladium in alkaline media, Electrochim. Acta 54 (2009) 2203–2208. [21] K. Nishimura, K. Kunimatsu, Enyo, Electrocatalysis on Pd + Au alloy electrodes: Part III. IR spectroscopic studies on the surface species derived from CO and CH3 OH in NaOH solution, J. Electroanal. Chem. 260 (1989) 167. [22] R.N. Singh, A. Singh, Anindita, Electrocatalytic activity of binary and ternary composite films of Pd, MWCNT, and Ni for ethanol electro-oxidation in alkaline solutions, Carbon 47 (2009) 271–278. [23] F. Schweppe, M. Martin, E. Frornm, Model on hydride formation describing surface control, diffusion control and transition regions, J. Alloys Compd. 261 (1997) 254–258.
B. Tao et al. / Sensors and Actuators B 142 (2009) 298–303 [24] J.O. Howell, R.M. Wightman, Ultrafast voltammetry and voltammetry in highly resistive solutions with microvoltammetric electrodes, Anal. Chem. 56 (1984) 524–529. [25] C.J. Tsai, P.L. Chung, K.H. Yu, Stress evolution of Ni/Pd/Si reaction system under isochronal annealing, Thin Solid Films 365 (2000) 72–76. [26] M. Bhaskaran, S. Sriram, L.W. Sim, Nickel silicide thin films as masking and structural layers for silicon bulk micro-machining by potassium hydroxide wet etching, J. Micromech. Microeng. 18 (2008), 095002 (3pp).
Biographies Bairui Tao received the MS degree in Electrical Engineering from East China Normal University, China in 2007. Since September 2007 he has been working towards his PhD degree at the Department of Electronic Science and Technology, School of Information Science and Technology, East China Normal University. His current research interests include semiconductor materials, IC design, devices and nanosensors. Jian Zhang received his master degree from Changchun Institute of Optics and Fine Mechanics, and PhD degree from Shanghai Institute of Metallurgy, Chinese Academy of Sciences (CAS), China, in 1994 and 1997, respectively. From 1997 to 1998, he
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worked as the Assistant Professor in State Key Laboratory of Transducer, CAS, China. From 1998 to 2000, he worked as a Research Fellow in Nanyang Technological University, Singapore. From August 2000 to December 2003, he worked as a Research Fellow in Institute of Materials Research and Engineering, Singapore. From January 2004, he worked as a professor in Department of Electrical Engineering, East China Normal University. His current research interests include micro sensors and arrays, microfabrication, gas sensors and biosensors. Shichao Hui received the BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, fuel cells, supercapacitors and nanosensors. Lijuan Wan received the MS degree in Electrical Engineering from East China Normal University, China in 2007. Since September 2007 she has been working towards her PhD degree at Department of Electronic Science and Technology, School of Information Science and Technology, East China Normal University. Her current research interests include nanomaterials, microfabrication and sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
An amperometric ethanol sensor based on a Pd–Ni/SiNWs electrode Bairui Tao a,b , Jian Zhang a,∗ , Shichao Hui a , Lijuan Wan a a Laboratory of Poling Materials and Devices of MOE and State Key Laboratory of Transducer Technology, East China Normal University, 500 Dong Chuan Road, Shanghai, 200241, China b College of Communications and Electronic Engineering, Qiqihar University, 42 Wenhua Street, Qiqihar, Heilongjiang, 161006, China
a r t i c l e
i n f o
Article history: Received 28 May 2009 Received in revised form 3 August 2009 Accepted 4 August 2009 Available online 12 August 2009 Keywords: Ethanol sensor Nickel/Silicon nanowires Palladium Electroless plating Electrocatalysis
a b s t r a c t A new amperometric ethanol sensor has been developed. The sensor uses the silicon nanowires covered with co-deposited palladium–nickel (Pd–Ni/SiNWs) as the working electrode. The detection of ethanol concentration is based on the response currents resulted from the electro-catalytic oxidation of ethanol. The performance of the sensor was characterized by cyclic voltammetry and fixed potential amperometry techniques. In 1 M KOH solution containing different ethanol concentrations, the sensor shows a good sensitivity of 7.48 mA mM−1 cm−2 and the corresponding detection limit (signal-to-noise ratio = 3) of 6 M for cyclic voltammetry. Meanwhile, it also displays a sensitivity of 0.76 mA mM−1 cm−2 and the corresponding detection limit of 10 M for fixed potential amperometry. The results demonstrate that the Pd–Ni/SiNWs electrodes are potential as the electrochemical integrated sensors for ethanol detection. © 2009 Published by Elsevier B.V.
1. Introduction The detection of ethanol concentration is important for medicine, brewing, beverage, traffic safety and etc. For example, the ethanol level assays are necessary for the control of drunken driving to avoid traffic accidents. Although liquid chromatograph and mass spectroscopy can determine the concentrations of ethanol with high precision, they are expensive and not suitable for portable use [1,2]. So lots of ethanol sensors have been explored which can detect the ethanol quickly and cost-effectively [3,4]. For the most commonly-used semiconductor-based ethanol sensors, their applications are limited due to some drawbacks such as high working temperature (≥300 ◦ C), complicated fabrication, high cost, and so on [5]. In principle, the electrochemical ethanol sensors, based on detection of the response currents resulted from the ethanol oxidation, have the highest sensitivity and accuracy among all of the sensors [6]. The three-electrode or two-electrode configurations are employed in these sensors. Therefore, the main jobs are focused on the development of new electrodes which can electrooxidize the ethanol easily and effectively. Several sensors had been reported to detect the ethanol such as using the alcohol oxidase enzyme modified electrode [7,8], the cobalt–nickel oxide electrode [9], the nickel foil electrode [10], the sputtered Ni/Pt/Ti on an Al2 O3 substrate as
∗ Corresponding author. Tel.: +86 21 54345203; fax: +86 21 54345119. E-mail address: [email protected] (J. Zhang). 0925-4005/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.snb.2009.08.004
the working electrode [11], etc. Furthermore, it is a significant challenge to establish electrodes without enzyme or platinum for the low cost and efficient ethanol sensors. Recently, the development of the ordered porous frameworks or nanostructures with a large surface area, permanent porosity and high thermal stability such as nanowires, nanotubes, nanoparticles and polymer nanocomposites open up a window for establish new electrodes [12–15]. In this paper, a robust electrochemical ethanol sensor based on a Pd–Ni/SiNWs electrode is presented. The SiNWs were first fabricated by chemical etching as backbone, and the Pd–Ni/SiNWs composite structure was then obtained by electroless co-plating nickel and trace palladium onto the surface of SiNWs. The Pd–Ni/SiNWs composite electrode exhibits high catalytic activity for the oxidation of ethanol. And ethanol sensing performances are studied by cyclic voltammetry (CV) and fixed potential amperometry technology. 2. Experimental 2.1. Raw materials 1 0 0-Orientated single-polished N-type heavily Sb-doped silicon-wafers with resistivity of 0.006–0.025 cm were used as the substrates for SiNWs growth. AgNO3 , NiSO4 ·6H2 O, (NH4 )2 SO4 , PdCl2 , NH4 F, KOH, Triton X-100 wetting agent, hydrofluoric acid, sodium dodecyl sulfate, sodium succinate, sodium citrate, ammonia and ethanol were all analytical reagents and used without further purification.
B. Tao et al. / Sensors and Actuators B 142 (2009) 298–303
Fig. 1. Schematic diagram of the ethanol sensing detection system.
2.2. Fabrication of Pd–Ni/SiNWs electrode 2.2.1. Formation of SiNWs The SiNWs were first fabricated by wet chemical etching with the etchant of AgNO3 (25 mM): HF (15%) = 1:1 (volume ratio) and the formation of SiNWs in detailed can be found elsewhere [16]. Next, all the chips were rinsed in diluted nitric acid (∼30% in volume) and deionized water to remove the residuals on their surface. 2.2.2. Electroless co-plating Pd–Ni onto SiNWs After cleaning carefully, the SiNWs chips were dipped into the Triton X-100 solution (1%) to activate for 30 s. Next, The Pd–Ni/SiNWs nanocomposites were prepared by coating Ni and trace Pd simultaneously onto the SiNWs with electroless co-plating. The recipe for Ni electroless plating bath can be found elsewhere [17]. The electroless co-plating Pd–Ni was conducted by adding PdCl2 100 mg L−1 into the Ni electroless plating bath. After plating, these chips were annealed at 400 ◦ C using a rapid thermal annealing system in Ar atmosphere for 300 s. Finally, the Pd–Ni/SiNWs chips as-received were used for material characterization and working electrode of the ethanol sensor development. 2.3. Electrode characterization and system for ethanol sensing detection The morphology and structure of the prepared Pd–Ni/SiNWs nanocomposite electrode were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The working electrode was constructed by gluing a copper leading wire onto the Pd–Ni/SiNWs chips with the electrode area of 0.5 × 0.5 cm2 . The electrochemical properties of the electrode were performed using an electrochemical workstation (LK3200A, Lanlike, China) with a platinum foil counter electrode and a saturated Ag/AgCl reference electrode in 1 M KOH alkaline solution. Fig. 1 shows the schematic diagram of the ethanol sensing detection system, where, S is the solution of sample, C the carrier fluid of 1 M KOH solution, R the deionized water, P the peristaltic pump, V the 6-channel valve, D the tri-electrode cell. During testing process, the solution of sample with various concentration ethanol was injected through S channel by the peristaltic pump P. 3. Results and discussion 3.1. Characterizations of the Pd–Ni/SiNWs electrode The SEM morphology images of the as-prepared Pd–Ni/SiNWs formed on the Si substrate are shown in Fig. 2(a)–(c), respectively. Fig. 2(a) is the top view of the Pd–Ni/SiNWs. It can be seen that the length of the SiNWs formed the bundle-like structure are uniform, ∼50 m. Fig. 2(b) is the partial enlarged view. It can be observed that the diameter of nanowires range from 60 to 300 nm. Fig. 2(c) is the cross sectional view. It is found that an additional layer has been plated onto the SiNWs’ surface.
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The additional layer was further examined by TEM. And the TEM picture is shown in Fig. 3(a). It is found that the thickness of the additional layer is about 10 nm. And the additional layer covered the silicon nanowires uniformly. The EDS spectrum of Pd–Ni/SiNWs nanocomposites is shown in Fig. 3(b). It is verified that the layer plated onto the SiNWs is Ni and trace Pd. In addition, according to the EDS spectrums of multiple measurements in different parts of Pd–Ni/SiNWs, only the characteristic peaks of Ni and Pd are found. No characteristic peaks of Si are found. It is implied that Ni coating formed is a dense layer and some Ni or Pd agglomerate particles are formed on the Ni film. Here, Cr is the contamination due to the sample holder. 3.2. The ethanol electrooxidation on the Pd–Ni/SiNWs electrode To verify the fabricated Pd–Ni/SiNWs electrode which has the capability to sense the ethanol, the electrochemical behavior of the electrode in 1 M KOH solution with or without ethanol of 10 mM was first examined in a three-electrode electrochemical cell using cyclic voltammetry (CV), with a potential window from −0.95 to 0.6 V at the scan rate of 10 mV s−1 . The CV curves are shown in Fig. 4, in which the solid curve, A, corresponds to the scanning results without ethanol addition and the dash curve, B, corresponds to the case when the 10 mM ethanol was added. Comparing with the solid curve A, two oxidation current peaks are observed clearly in curve B at the potential of ∼−0.28 and 0.52 V, which is attributed to the electrocatalytic of Pd and Ni, respectively [18,19]. It is concluded that the Pd–Ni/SiNWs electrode as-received shows the electro-oxidation capability to ethanol, which can be used for ethanol detection. 3.3. Ethanol sensing with cyclic voltammetry technique In the ethanol sensing procedure, the optimum sweeping potential range from −0.6 to 0.1 V is selected for avoiding the interferences such as the redox reaction of Ni, OH− and oxygen dissolved in solution etc. [18–20]. In this research, the oxidation peak currents are used as the criterion for the ethanol detection. In this study, the scanning rate is fixed at 50 mV s−1 unless otherwise specified. 3.3.1. Stability of Pd–Ni/SiNWs electrode The stability of electrode was first conducted in the 1 M KOH solution without ethanol addition. The results are shown in Fig. 5(a). It indicates that after 100 cycle CVs, the maximum variation of the current is less than 1% at the same potential. Fig. 5(b) shows the 100 cycle CVs of the prepared electrode in 1 M KOH solution with 1 M ethanol addition. During the forward sweeping, the oxidation peak at potential of −0.01 V is mainly attributed to the ethanol electrocatalytic oxidation reaction on the Pd sites. The mechanism of ethanol oxidation on Pd in alkaline solution has been reported according to the in situ IR measurement [21]. The intermediates of ethanol oxidation are CH3 COO− , small amounts of CH3 CHO, Pd2 (CO)ads and etc. The ideal end-product is CO2 with H2 O [20–22]. During the reverse sweeping, the reduction of Pd oxides to Pd and production of active sites take place, the re-oxidation of ethanol or carbonaceous species formed in the anodic process occurs on clean Pd surface and backward peak at −0.36 V appears. These have important meaning to keep the stability of Pd–Ni/SiNWs electrode. It can be also observed that the peak current decrease almost linearly with the CV times increasing. After 100 times cycling, the loss of peak current is less than 9.5% at the potential of ∼0 V, which correspond to the peak current variation of ∼1% per cycle. The loss mechanism is probably attributed to the consumption of ethanol in oxidation process. It
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Fig. 2. Micro-morphology of Pd–Ni/SiNWs: (a) the top view, (b) the partial enlarged view and (c) the cross sectional view.
is concluded that the Pd–Ni/SiNWs electrode has the superior stability.
due to the transition of the electrode process from a mass-transport controlled to a surface-reaction controlled process [23].
3.3.2. Sensitivity and linearity The CVs results of the prepared electrode with different ethanol concentrations in 1 M KOH solution are shown in Fig. 6, where the curve a–e correspond to the ethanol concentration of 3.4, 6.8, 10.2, 13.6 and 17.1 mM, respectively. It is indicated that the oxidation peak currents increase from 5.04 to 31.09 mA and the oxidation peak potential shift from −0.40 to −0.23 V when ethanol concentration increase from 3.4 mM to 17.1 M, which is mainly caused by the electrode polarization. During the reverse sweeping, the potential of Pd+2 reduction peak and the carbonaceous species oxidation peak is shift right when the ethanol concentrate increase. Accordingly, the relationship between the abstracted oxidation peak current values and ethanol concentration levels can be employed to detect ethanol. The results are depicted in Fig. 7 when the ethanol concentrations increase from 0 to 34.2 mM every 3.4 mM. The linear fitting results are also shown in the inset of this figure, where, the equation of linear fitting equation is expressed as I (mA) = −0.71 + 1.87C (mM) with R2 = 0.998. The sensitivity of the prepared Pd–Ni/SiNWs electrode is ∼7.48 mA mM−1 cm−2 and the concentration of the detection limit is 6 M for the signal-to-noise ratio of 3 [11]. When the ethanol concentration is larger than 20.4 mM, a sharp decrease of response current can be observed, which is probably
3.4. Ethanol sensing with fixed potential amperometry The response of Pd–Ni/SiNWs electrode to ethanol is also examined using fixed potential amperometry at −0.25 V. The initial volume of each sample is 100 mL in the tri-electrode cell. The experiment is performed on solution containing without ethanol in 1 M KOH. After the background current was stable, the ethanol solution of 99.7% was injected by the peristaltic pump with about 20 L every 30 s (∼3.4 mM every 30 s) drop by drop. The current–time responses of the Pd–Ni/SiNWs electrode to ethanol are shown in Fig. 8. The steady state values of the response current are plotted in the inset of Fig. 8 corresponding to the ethanol concentration ranging from 0 to 20.4 mM. The linearly fitting equation is I (mA) = −0.09 + 0.19C (mM) with R2 = 0.997. The sensitivity of the prepared Pd–Ni/SiNWs electrode to the ethanol concentration is ∼0.76 mA mM−1 cm−2 with the detection limit concentration of ∼10 M for the signal-to-noise ratio of 3. 3.5. Discussion A comparison of some basic parameters between this sensor prototype and the other developed ethanol sensor systems is described in Table 1.
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Fig. 3. TEM and EDS of Pd–Ni/SiNWs: (a) the TEM image, (b) the EDS spectrum.
The study has demonstrated that the Pd–Ni/SiNWs electrode exhibit high sensitivity, excellent stability to ethanol. It could be attributed to three aspects: Firstly, the electrode of Pd–Ni/SiNWs with high volume of surface ratio and small curvature radii of nanowires not only increases the reaction area but also promote electron-transfer reactions, which can lower the activation energy and catalyze the electrooxidation ethanol easily [24]. This can promote the sensitivity. Secondly, the backbone of SiNWs itself could support the active materials (Pd–Ni) well. Especially, the electrode annealed at 400 ◦ C can form the NiSi layer between Ni and SiNWs interface, which can resist to OH− etching [25,26]. This can lead to the improved stability.
Fig. 4. Cyclic voltammograms in 1 M KOH with and without 10 mM ethanol.
Fig. 5. Cyclic voltammograms of 100 times cycling: (a) in 1 M KOH solution; (b) in 1 M KOH with 1 M ethanol.
Thirdly, it is believed that the nanoscale effects of Pd–Ni layer about 10 nm between Ni and SiNWs interface could play a very importance role during the electro-catalytic oxidation steps of the ethanol in alkaline solution. The detail mechanism need further study.
Fig. 6. Cyclic voltammograms in 1 M KOH solution containing different ethanol concentrations.
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Table 1 Comparison of some basic parameters for ethanol sensors. Working electrode Pd–Ni/SiNWs Pd–Ni/SiNWs Alcohol oxidase Co-Ni oxide RuO2 -modified Ni Ni/Pt/Ti
Detection technique Cyclic voltammetry Fixed potential amperometry Fixed potential amperometry Voltammetry Fixed potential amperometry Fixed potential amperometry
Sensitivity −1
−2
∼7.48 mA mM cm ∼0.76 mA mM−1 cm−2 ∼859 nA mM−1 cm−2 ∼375 nA mM−1 4.92 A ppm−1 cm−2 3.08 A mM−1 cm−2
Detection limit
Reference
6 M 10 M 29.7 M 32 M – –
This work This work [8] [9] [10] [11]
Acknowledgement This work is supported by the National Natural Science Foundation of China (Grant No. 60672002) and by Innovation Program of Shanghai Municipal Education Commission (Grant No. 09ZZ46). We are grateful for the financial support. References
Fig. 7. The relationship between the abstracted oxidation peak current values of CVs and ethanol concentration.
Fig. 8. The current–time responses of the Pd–Ni/SiNWs electrode.
4. Conclusion In conclusion, we have fabricated the Pd–Ni/SiNWs nanocomposite electrode with the combination of wet chemical etching of Si and electroless plating of Pd–Ni nucleus to determine the ethanol concentration. It exhibits high sensitivity, excellent stability. The detection limit is 6 and 10 M with cyclic voltammetry and fixed potential amperometry at −0.25 V, respectively. The sensor with simple, low cost, easy integration, highly sensitive and excellent stability is attractive for the fabrication of efficient integrated amperometric ethanol sensors.
[1] S.M. Rao, Spectrographic technique for determining refractive indexes, Opt. Eng. 36 (1997) 162–166. [2] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species, Science 293 (2001) 1289. [3] A.O.S.S. Rangel, I.V. Toth, Determination of alcohol in wines by flow-injection spectrophotometry using gas-diffusion and an immobilized enzyme reactor, Am. J. Enol. Vitic. 50 (1999) 259–263. [4] R. Phathaitep, G. Atcharawan, C. Torranin, M. Pongsri, M. Nikorn, C. Supab, Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction, Ceram. Int. 35 (2009) 649–652. [5] G. Neri, A. Bonavita, G. Micali, N. Donato, F.A. Deorsola, P. Mossino, I. Amato, B. De Benedetti, Ethanol sensors based on Pt-doped tin oxide nanopowders synthesised by gel-combustion, Sens. Actuators B 117 (2006) 196–204. [6] Y.C. Weng, T.C. Chou, Ethanol sensors by using RuO2 -modified Ni electrode, Sens. Actuators B 85 (2002) 246–255. [7] A.M. Azevedo, D.M.F. Prazeres, J.M.S. Cabral, L.P. Fonseca, Ethanol biosensors based on alcohol oxidase, Biosens. Bioelectron. 21 (2005) 235–247. [8] M.M. Barsan, C.M.A. Brett, An alcohol oxidase biosensor using PNR redox mediator at carbon film electrodes, Talanta 74 (2008) 1505–1510. [9] E.T. Hayes, B.K. Bellingham, H.B. Mark Jr., A. Galal, An amperometric aqueous ethanol sensor based on the electrocatalytic oxidation at a cobalt-nickel oxide electrode, Electrochim. Acta 41 (1996) 337–344. [10] Y.C. Weng, T.C. Chou, Ethanol sensor by using by using RuO2 -modified Ni electrode, Sens. Actuators B 85 (2002) 246–255. [11] Y.C. Weng, J.F. Rick, T.C. Chou, A sputtered thin film of nanostructured Ni/Pt/Ti on Al2 O3 substrate for ethanol sensing, Biosens. Bioelectron. 20 (2004) 41–51. [12] T.K. Maji, R. Matsuda, S. Kitagawa, A flexible interpenetrating coordination framework with a bimodal porous functionality, Nat. Mater. 6 (2007) 142–148. [13] C.-D. Wu, A. Hu, L. Zhang, W. Lin, A homochiral porous metal-organic framework for highly enantioselective heterogeneous asymmetric catalysis, J. Am. Chem. Soc. 127 (2005) 8940–8941. [14] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Reticular synthesis and the design of new materials, Nature 423 (2003) 705–714. [15] N. Elfström, A.E. Karlström, J. Linnros, Silicon nanoribbons for electrical detection of biomolecules, Nano Lett. 3 (2008) 945–949. [16] L.J. Wan, W.L. Gong, K.W. Jiang, H.L. Li, B.R. Tao, J. Zhang, Preparation and surface modification of silicon nanowires under normal conditions, Appl. Surf. Sci. 254 (2008) 4899–4907. [17] B.R. Tao, J. Zhang, F.J. Miao, H.L. Li, L.J. Wan, Y.T. Wang, Capacitive humidity sensors based on Ni-SiNWs nanocomposites, Sens. Actuators B 136 (2008) 144–150. [18] Y.L. Lo, B.J. Hwang, In situ Raman studies on cathodically deposited nickel hydroxide films and electroless Ni-P electrodes in 1 M KOH, Langmuir 14 (1998) 944–950. [19] Y.Y. liao, T.C. Chou, An amperometric alcohol by using electrolyses nickel as working electrode, Electroanalysis 12 (2000) 55–59. [20] Z.X. Liang, T.S. Zhao, J.B. Xu, L.D. Zhu, Mechanism study of the ethanol oxidation reaction on palladium in alkaline media, Electrochim. Acta 54 (2009) 2203–2208. [21] K. Nishimura, K. Kunimatsu, Enyo, Electrocatalysis on Pd + Au alloy electrodes: Part III. IR spectroscopic studies on the surface species derived from CO and CH3 OH in NaOH solution, J. Electroanal. Chem. 260 (1989) 167. [22] R.N. Singh, A. Singh, Anindita, Electrocatalytic activity of binary and ternary composite films of Pd, MWCNT, and Ni for ethanol electro-oxidation in alkaline solutions, Carbon 47 (2009) 271–278. [23] F. Schweppe, M. Martin, E. Frornm, Model on hydride formation describing surface control, diffusion control and transition regions, J. Alloys Compd. 261 (1997) 254–258.
B. Tao et al. / Sensors and Actuators B 142 (2009) 298–303 [24] J.O. Howell, R.M. Wightman, Ultrafast voltammetry and voltammetry in highly resistive solutions with microvoltammetric electrodes, Anal. Chem. 56 (1984) 524–529. [25] C.J. Tsai, P.L. Chung, K.H. Yu, Stress evolution of Ni/Pd/Si reaction system under isochronal annealing, Thin Solid Films 365 (2000) 72–76. [26] M. Bhaskaran, S. Sriram, L.W. Sim, Nickel silicide thin films as masking and structural layers for silicon bulk micro-machining by potassium hydroxide wet etching, J. Micromech. Microeng. 18 (2008), 095002 (3pp).
Biographies Bairui Tao received the MS degree in Electrical Engineering from East China Normal University, China in 2007. Since September 2007 he has been working towards his PhD degree at the Department of Electronic Science and Technology, School of Information Science and Technology, East China Normal University. His current research interests include semiconductor materials, IC design, devices and nanosensors. Jian Zhang received his master degree from Changchun Institute of Optics and Fine Mechanics, and PhD degree from Shanghai Institute of Metallurgy, Chinese Academy of Sciences (CAS), China, in 1994 and 1997, respectively. From 1997 to 1998, he
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worked as the Assistant Professor in State Key Laboratory of Transducer, CAS, China. From 1998 to 2000, he worked as a Research Fellow in Nanyang Technological University, Singapore. From August 2000 to December 2003, he worked as a Research Fellow in Institute of Materials Research and Engineering, Singapore. From January 2004, he worked as a professor in Department of Electrical Engineering, East China Normal University. His current research interests include micro sensors and arrays, microfabrication, gas sensors and biosensors. Shichao Hui received the BS degree in Microelectronics from East China Normal University, China in 2008. Since September 2008, she has been working towards her MS degree in Electrical Engineering from East China Normal University. Her current research interests include semiconductor materials, fuel cells, supercapacitors and nanosensors. Lijuan Wan received the MS degree in Electrical Engineering from East China Normal University, China in 2007. Since September 2007 she has been working towards her PhD degree at Department of Electronic Science and Technology, School of Information Science and Technology, East China Normal University. Her current research interests include nanomaterials, microfabrication and sensors.