ar ticle

Copyright © 2012 by American Scientific Publishers All rights reserved. Printed in the United States of America

Journal of Nanoengineering and Nanomanufacturing Vol. 1, pp. 1–9, 2012 (www.aspbs.com/jnan)

Specific H2S Gas Sensor Based on Metal Nanoparticles, Sulfur and Nitrogen/Single-Walled Carbon Nanotube-Modified Field Effect Transistor Shirin Nasresfahani1, 3 , Mohammad Mahdi Doroodmand2, 3, ∗ , Mohammad Hossein Sheikhi1, 3 , and Ahmad Reza Ghasemi1, 3 1

School of Electrical and Computer engineering, University of Shiraz, Shiraz, Iran Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran 3 Nanotechnology Research Center, Shiraz University, Shiraz, Iran 2

ABSTRACT

1. INTRODUCTION Hydrogen sulfide (H2 S) is a colorless, bad smelling and poisonous gas. It is generated in sewage, coal and natural gas processing and petroleum industries. Owning to the highly toxicity of H2 S,1 this gas produces severe effects on the nervous system at low concentration. Also at higher concentrations, H2 S causes life threatening.1 Therefore, quantitative detection of H2 S gas ranging from a few parts per billion (ppb) to the hundred parts per million (ppm) is of great important for both oil and natural gas industries and human safety. Exciting detection of H2 S rely mainly on electrochemical2–4 and metal oxide semiconductor devices.5–11 While there are commercially a number of sensing devices, drawbacks of existing H2 S sensors including high operating temperatures, low selectivity, cross sensitivity ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: Accepted:

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problems and high cost, making them inappropriate for environmental monitoring.10–12 Over the last decade, one-dimensional nanostructures such as carbon nanotubes (CNTs) have been attracted substantial attention because of their unique structural, electronic, optical, thermal and mechanical properties.12–15 Chemical sensors based on CNTs offer high sensitivity and low power consumption.16–19 This is due to the fact that, CNTs have nano-sized morphology and high aspect ratio, which make them suitable for high sensitive and rapid gas adsorption of gaseous species. Interaction between carbon nanostructures and any gas molecules causes significant change in the electrical properties of CNTs. One of the most interesting transduction systems, employed during the last years, is the “Metal-OxideSemiconductor Field Effect Transistor” (MOSFET). Since then, many theoretical and experimental studies have described the performance of the FETs. FETs are adopted to provide physical/chemical changes into an electrical signal. They have been used as detection system in potentiometric sensors.20 However in accordance with the literature review, few studies have been focused on CNT-based field

2157-9326/2012/1/001/009

doi:10.1166/jnan.2012.1020

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A novel hydrogen sulfide (H2 S) sensor is fabricated using commercial metal oxide field effect transistor (MOSFET), individually modified with Fe or Ag-doped single-walled carbon nanotubes (SWCNTs). In this study, integrated circuit (IC: BS250) was selected as selective probe for H2 S detection. For this purpose, plastic cover on the surface of drain was drilled to bare the drain surface, followed by modification with nitrogen and sulfur-doped SWCNTs by chemical vapor deposition (CVD) process. The CVD-synthesized SWCNTs were then electrochemically modified with Ag or Fe nanoparticles. Accordance to the figures of merit, fabricated sensor was linear from 150 to 920 parts per billion (ppb). Detection limit was also 85 ppb. Relative standard deviation (RSD) for five replicate analyses was 3.26%. Based on 90% of maximum response (t90 , response time was ∼52 s. Calibration sensitivity was measured to 0.30 mV/ppb. No interference was observed, when introducing at least 500 folds of interferences such as vapors of H2 O, HCl, HClO4 , HNO3 , HIO4 , gaseous species like O2 , H2 , CO, CO2 , NO2 , hydrocarbons such as C2 H2 , CH4 and also volatile orgasmic compounds (VOCs) to 400 ppb of H2 S solution. Reliability of the sensor was also evaluated via determination of the amounts of H2 S in different industrial samples. KEYWORDS: H2 S Gas Sensor, Field Effect Transistor, Carbon Nanotubes, Metal Nanoparticles.

Specific H2 S Gas Sensor Based on Metal Nanoparticles, Sulfur and Nitrogen/SWCNT-Modified Field Effect Transistor

effect transistor-based (FET) to detect H2 S.21 Compared to other kinds of gaseous sensors, CNT-based FETs are usually reproducible and low cost. However, CNTs have some unique characteristics, but gas sensors based on pristine CNTs are suffer from some shortcoming including low sensitivity to analyte due to low adsorption,22 low selectivity,16 and long recovery time.23 In this study, a novel, specific and sensitive H2 S FET sensor was fabricated via modification of the drain of a commercial FET device with Fe and Ag nanoparticles-doped SWCNTs as a gas-partitioning membrane.

2. EXPERIMENTAL DETAILS

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2.1. Instrumentation In this study, H2 S with 99.9% purity percentage was purchased from “Technical Gas Services” company, Germany. Gases including argon and nitrogen with 99.0% purity were also from “Parsballon” company, Iran. Chemical vapor deposition (CVD) process was used for synthesis of any forms of carbon nanostructures including activated carbon, single-walled carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs), carbon nanofibers (CNFs) and fullerene (C60 , C70 , etc.). For this purpose, acetylene (Parsballon” company, Iran) was used as the source of carbon. Ferrocene, thiophene and pyridine (Fluka) were also utilized as the sources of Fe nanoparticles, sulfur and nitrogen, respectively. Mass flow controller (MFC) was a digital metal steel instrument (model: UFC-1661, SN: All43024300), USA. Also, high vacuum pump was related to the 2-stage Edwards E2M2 company, Crawley Sussex, England. Spectroscopic techniques such as Raman spectrometry (Thermo Nicolet Almega dispersive, Raman Spectrometer) and patterned X-ray diffraction (XRD, D8 Advance, Brüker AXS) were employed for characterization of the carbon nanostructures. Transmisstion and scanning electron micrographs were attained using TEM (CN-10, Philips, 100 KV) and SEM (XL-30 FEG, Philips, 20 KV) instruments. 2.2. Fabrication of Sensor 2.2.1. Gas-Sensitive MOSFET Preparation The integrated circuit (IC: BS250) was used as a FET sensor. To modify the FET with nitrogen/sulfur-doped Fe/SWCNTs or Ag/SWCNTs, the plastic cover of the FET was carefully removed to reach the yellow layer of the drain using a small drill (diameter: 0.5 mm). This step was the most sensitive process in the preparation of the drain of the FET for modification with Fe or Ag-doped SWCNTs. Therefore, care should be made for regular removing of the plastic cover. For more confidence about the correctness of the performance of the FET during drilling and removing of the plastic cover of the FET, it was electronically 2

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tested via following the cutoff, active and saturated modes of the FET using a designed electronic circuit. Also, the silicide supported on the drain surface was characterized using elemental analysis. The current–voltage (I–V) curve of the MOSFET after baring the drain was investigated as shown in Figure 1(A). 2.2.2. Thin Film-Functionalized SWCNTs For modification of the FET, an optimum amount (0.02 g) of nitrogen/sulfur-doped SWCNTs was directly coated on the surface of the drain of the FET sensor using CVD process. For this purpose, the nitrogen and sulfurdoped SWCNT bundles were synthesized at temperature ∼1300 C in an inert atmosphere of argon, using acetylene as the source of SWCNTs, ferrocene as the source of iron nanoparticles and pyridine as the source of nitrogen, followed by activation and purification of SWCNT bundles via purging O2 to the production line or using ultraviolet (UV) radiation, and reduction of nitrogen and sulfur atoms doped on SWCNTs using H2 as reducing agents. The synthesized nitrogen/sulfur-doped SWCNTs were then directly coated on silicide support of the drain surface of the FET device. Figure 1(B) shows the TEM image of the CVD-synthesized SWCNT bundles. Inset of Figure 1(B) shows the photographic image of drain-bared FET device. The SWCNT bundlers are also characterized by Raman spectroscopy (Fig. 1(C)) and patterned XRD spectrometry (Fig. 1(D)). 2.2.3. Electrodeposition of Metal/Metal oxide Nanoparticles To improve the linearity and selectivity of the sensor to H2 S, modification of two types of metal/metal oxide nanoparticles to functionalized SWCNTs was attempted by electrodeposition method. The schematic of electronic circuit used to electrodeposit nanoparticles is schematically shown in Figure 1(SI). When putting the FET sensor into aqueous solution; the pins of the FET sensor should be covered with heat shrink. Ag and Fe nanoparticles were then deposited using 10 × 10−6 M and 10 × 10−4 M of AgNO3 and (NH4 2 Fe(SO4 2 solutions, respectively. This process involves the formation of either a progressive nucleation or instantaneous deposition.24 In the progressive deposition, a certain DC voltage is applied to the sensing probe in a certain time interval for electrochemical deposition of electroactive species. In instantaneous deposition, at first, more negative DC potential is applied to the FET device, followed by applying less negative potential, while controlling the deposition time. The average diameter and size distribution of the electrodeposited species strongly depend on the applied potential and the deposition time of electrodeposition process. In the case of instantaneous nucleation, all the nuclei form instantaneously on the probe substrate, and subsequently J. Nanoeng. Nanomanuf., 1, 1–9, 2012

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(B)

(A)

(C) (D)

grow during the electrodeposition time. In contrast, the number of formed nuclei is a function of time of electrodeposition in the progressive nucleation. These nuclei gradually grow and overlap, and therefore, the progressive nucleation process exhibits zones of reduced nucleation rate around the growing stable nuclei.24 Figure 2 shows the SEM images of electrodeposition of silver and iron nanoparticles on SWCNT-modified FET sensor using single and double pulses at different voltages. According to the SEM images (Fig. 2), the average size of electrodeposited FE and Ag nanoparticles is estimated to ∼80 nm.

amplification of the change in electrical current generated according to the adsorption of H2 S on drain surface membrane. Therefore, the potentiometric response of the FET was changed according to the adsorption of H2 S onto the nitrogen/sulfur-doped Ag/SWCNTs. The response of the sensor was then amplified. Figure 2(SI) shows the electronic schematic designed for measuring the response of the H2 S FET sensor. The data related to the adsorption process were processed in PC using a program written in Visual Basic. 2.4. Procedure

2.3. Apparatus for H2 S Detection The setup of the instrumentation system used to detect H2 S gas is shown in Figure 3. It consists of two glass tubing (i.d. 2.0 cm and length 5.0 cm) connected to each other through flange. Each glass tube has a side arm (i.d. 3.0 mm and length 1.0 cm). One of the side arm was connected to the vacuum pump through a valve and the other side arm was connected to the H2 S cylinder thorough as mass flow controller to control and detect the concentration of the standard solution of H2 S in N2 (as diluent). To detect and measure H2 S, the FET device was positioned inside the chamber through third side-arm tubing. To measure the responses of the H2 S sensing FET during the adsorption of H2 S on nitrogen/sulfur-doped Fe/ or Ag/SWCNTs on the drain surface, the FET was set in the active mode for J. Nanoeng. Nanomanuf., 1, 1–9, 2012

Prior the H2 S sensor testing, the FET was put inside a 0.1 M of NaOH solution for ∼6 min, to eliminate the memory effect of the H2 S FET sensor from any formerly adsorbed H2 S. Afterward, the FET device was heated at temperature to 60 C for ∼5 min in air atmosphere, to oxidize metal oxide nanoparticles. For H2 S detection, the chamber was vacuumed using vacuum pump for ∼30 s to decrease the pressure of the chamber to ∼0.5 torr. Then, flow controller was set to mix selected ratio of H2 S and N2 for generation of standard H2 S solution and directly injection into the test chamber, in which the H2 S FET device was positions. After about each 30-time applications of the H2 S FET sensor in sensing high concentrations of H2 S gaseous sample, in order to eliminate the residual H2 S, the FET sensor 3

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Fig. 1. Characterization of H2 S FET sensor including (A) I–V curve of the MOSFET after baring the drain surface, (B) TEM image of SWCNTs deposited on the FET sensor using CVD process, inset: the photographic image of the drain-bared FET device, (C) Raman spectrum of CVD-synthesized SWCNTs, and (D) XRD pattern of drain-modified SWCNTs.


(A)

(B)

800 nm

800 nm (C)

(D)

800 nm

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800 nm

Fig. 2. SEM images of electrodeposition of metal nanoparticles on SWCNT-modified FET sensor including (A) Ag nanoparticles using single pulse (VApplied = −080 V, pulse time duration: 600 s) in 10 × 10−6 M AgNO3 solution, (B) Ag nanoparticles using single pulse (VApplied = −040 V, pulse time duration: 180 s) in 10 × 10−6 M AgNO3 solution, (C) Ag nanoparticles using double pulses (VApplied1 = −080 V, VApplied2 = −020 V, pulse time duration: 720 s) in 10 × 10−6 M AgNO3 solution and (D) Fe nanoparticles using double pulses (VApplied1 = −080 V, VApplied2 = −020 V, pulse time duration: 450 s) in 10 × 10−4 (NH4 2 Fe(SO4 2 solution.

was again put inside the NaOH solution (0.1 M) for ∼5 min to desorb the adsorbed H2 S. Then, the FET device was heated at temperature to 60 C for ∼5 min in air atmosphere, to regenerate any previously reacted sulfurous anions. 2.5. Real Sample Analysis To validate the proposed analysis approach, a recovery test was performed. Known amounts of H2 S were spiked to a gaseous sample of air and the recovery percentages were calculated.

Fig. 3.

4

Setup of system used to detect H2 S gas.

3. RESULT AND DISCUSSION 3.1. Optimization of the Operating Parameters Several parameters were optimized to reach the highest sensitivity and reproducibility for H2 S detection and determination. The parameters having influence on the sensitivity and reproducibility of the sensor for H2 S detection include the type of FET device, the appropriate voltages applied to the FET, the type of CNT membrane deposited on the FET, the effect of iron or silver nanoparticles doped on CNTs and the effect of nitrogen and sulfur atoms positioned in the CNT matrix. In this study, since it was aimed to fabricate H2 S gas sensor using commercial FET devices, different kinds of FET such as BSS92, BSS89 and BS250 were tested. Among them it was observed that, the plastic cover of some types of FETs such as BSS92 was so hard that it was impossible to securely remove the plastic cover of the FETs. Also among other tested FETs such as BSS89 and BS250, maximum surface area was observed for the drain surface. However, in the ion-selective FETs (ISFETs) in which the modified gate is considered as detecting probe,20 but, when using commercial FET devices as probes, due to the more surface area of the drain as well as because of simple possibility for regular and reproducible removing of the plastic of the drain surface using a drill, therefore, the drain was bared for modification with carbon nanomaterials. The results exhibited that, BS250 had the highest drain J. Nanoeng. Nanomanuf., 1, 1–9, 2012

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second one that is biased in the saturated region, acts as pseudo-reference electrode,27 used to evaluate the potentiometric response of the sensor to the H2 S gas. The proton sensitivity of FET sensor before and after modification with carbon nanomaterials was evaluated using different buffer solutions of acetic acid and sodium hydroxide ranging from pH 4.5 to 7, as reported in Table I. According to the results, the drain-bared FET sensor may be burnt at pH lower than 4.0. Also, low sensitive response is observed at pH higher than 7.5. Compared to the drainbared FET sensor, pseudo-nernstian responses were evaluated for the FET sensor when modified with carbon nanostructures. The results reveal that, the recommended procedure is easily capable to bare the drain surface of FET device via removing of the plastic cover of the FET device without damaging the FET. To have specific, more sensitive and more reproducible H2 S FET sensor, the adsorption of H2 S on the SWCNT membrane should be promoted. For this purpose, doping of metal nanoparticles such as iron or silver nanoparticles on SWCNT matrix is necessary. This is due to the synergetic effect of carbon nanostructures and metal nanoparticles for the adsorption and reversible reaction of H2 S molecules. The similar behavior of metal nanoparticles such as copper oxide and silver has been reported on CNTs and metal oxide supports.28 29 Also, selective, stronger and reproducible interaction is existed between H2 S and SWCNT matrix, when CNT support is modified with “Lewis” based such as nitrogen or sulfur atoms. This is due to the formation of “Zwither” ion during the adsorption of H2 S on CNT support. Therefore, the sensitivity of the H2 S FET sensor is the result of contribution of SWCNTs, nitrogen and sulfur atoms and nanoparticles such as iron or silver. In this study, however, partially, the same morphologies are evaluated according to the SEM images (Fig. 2) according to the proposed procedure, but, the results (Table II) reveal that, presence of Fe nanoparticles in the SWCNT matrix, significantly reduces the sensitivity of the FET sensor. To interpret the negative effect of iron nanoparticles, the morphologies of Fe and Ag nanoparticles are studied in detail. To have H2 S FET sensor with maximum sensitivity, oxidation of metal nanoparticles is also necessary. Therefore, metal oxide nanoparticles act as active sites for H2 S adsorption and reaction. In this study, oxygen was used for oxidation of metal nanoparticles. Based on the recommended procedure on the FET sensor, the results reveals

Table I. Response of FET sensor to pH in aqueous solution. Drain-modified FET Sensor Bare drain Drain-modified SWCNTs ∗

pH range

Sensitivity mV/decade

Correlation Coefficient (R)

Response Time (t90 ) (sec.)

RSD (%)

5–6.5 4.5–7

−90.0 −77.5

0.982 0.991

10.0 14.0

5.34 3.12

V is the output voltage (Vout of the electronic circuit.


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surface area (∼4 mm2 , and also maximum amplification gain compared to other FET devices. Therefore, BS250 was selected as the transducer for H2 S sensing probe. When putting the FET sensor into aqueous solution, the pins of the FET sensor should be covered with insulating materials such as heat shrink in order to limit the response of the sensor only to the interaction between electrolyte and modified drain surface. Also, the short circuit caused by the direct connection of the pins of FET sensor may lead to burn FET. To modify the FET sensor with CNTs, different forms of synthesized carbon nanomaterials including SWCNTs, multi-walled CNTs (MWCNTs), carbon nanofibers (CNFs) and fullerene (C60 , C70 , etc.), were tested to reach the maximum reproducible response to the H2 S detection. In the fabricated H2 S sensor, maximum adsorption was observed for H2 S on the semiconducting SWCNT bundles compared to that of other forms of carbon nanomaterials such as fullerene, MWCNTs and CNFs. Also, maximum sensitivity was evaluated for SWCNT bundle with (6, 5) coordination, approved based on the radial breathing mode (RBM) of the Raman spectrum (280 cm−1 )25 and the rotation angle of each unit cell of SWCNT (27 ), according to the high resolution scanning tunneling microscopy (STM).25 Therefore, the FET sensor, modified with the semiconducting SWCNT bundles with (6, 5) coordination, had more active surface sites for the adsorption of H2 S molecules. Also, the FET sensor had lower background resistance (5.3%) and more improved linear dynamic range, which made the FET sensor suitable for the quantitative analysis of H2 S. Before studying the sensitivity as well as the selectivity of the SWCNT-based MOSFET device to the H2 S gas, the reliability of the proposed procedure for preparation of the drain of the MOSFET device was studied in detail. Since the drain of the MOSFET device is made of silicide, therefore, the presence of hydroxyl groups in silica semiconductor is significantly sensitive to the hydrogen ion (H+ . Thus, this intrinsic property of silica semiconductor can be considered as appropriate probe for evaluation of the reliability of the proposed procedure during the preparation of the MOSFET devise for H2 S sensing application. Figure 1(SI) depicts the measuring electric circuit. In this electronic circuit, the MOSFET was biased in triode (active) region. Two operational amplifiers (op-amps) are also used to measure the electrical responses of the MOSFET sensor to H2 S.26 The first op-amp converts the change in source-drain current to voltage. Whereas, the


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Table II. Comparison between the effects of different forms of nitrogen/sulfur-doped carbon nanostructure on the response of the H2 S FET sensor introduced with 450 ppb H2 S.1 Drain-modified carbon substrate

Sensitivity24 (%)

Liner range (ppm)

Saturated limit (ppb)

T390 (sec.)

RSD4 (%)

100 30 20 10 10 140 05

250-800 300-750 350-700 350-600 350-550 300-750 400-500

1200 1000 850 850 600 1200 600

78 86 95 95 150 83 128

3.49 3.81 4.19 4.72 4.87 3.81 5.18

SWCNTs MWCNTs CNFs Fullerene (C60 Activated carbon Ag/SWCNTs Fe/SWCNTs

Data are the average of three independent analyses. 2 Calculated according to Eq. (1). 3 Response time. 4 Response, when FET sensor was introduced to 450 ppb H2 S gas solution.

that, oxidation of silver and iron nanoparticles leads to form Ag2 O and FeOOH nanoparticles, respectively. As formation of FeOOH nanoparticles simply causes the protonation of sulfur and nitrogen atoms, the active sites are significantly limited. Therefore, no significant response was observed when doping Fe nanoparticles (Table II). This phenomenon is considered as an appropriate evidence for suggestion of the proposed mechanism of the interaction between H2 S and FET sensor. The effects of silver oxide nanoparticles, doped on the SWCNT matrix is shown based on the trace of the response of the H2 S FET sensor in Figure 4. Also, figures of merit for fabricated FET sensor, modified with silver oxide, sulfur and nitrogen-doped SWCNTs are shown in Table III. The amplification gain of the FET is strongly dependent to the temperature. In this study, the effect of temperature was studied via circulation of water around the cell containing H2 S FET sensor. The results (Fig. 5) showed that, at temperatures above ∼42 C, a decrease in sensitivity was observed due to the noisy background signals produced by the compassion of the FET to the temperature. This also leads to the less capability of H2 S molecules for adsorption of the nitrogen and sulfur-doped SWCNTs. However, in the fabricated H2 S FET sensor, the highest sensitivity was observed at ∼38 C, but, to have more Ag2O/Sulfur/NitrogenSWCNTs 0.3 0.25

Response (V)

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1

Ag/Sulfur/NitrogenSWCNTs

0.2

Ag/Sulfur/NitrogenMWCNTs

0.15 0.1 0.05 0

2

4

6

8

10

12

14

16

18

20

22

Time (min)

Fig. 4. Trace showing the effects of the modification of carbon nanostructures with silver/silver oxide nanoparticles, and nitrogen/sulfur atoms, when exposing the H2 S FET sensor to 500 ppb of H2 S standard solution.

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precise H2 S FET sensor with maximum sensitivity, ambient temperature was selected to evaluate the potentiometric responses of the H2 S FET sensor.

3.2. Analytical Figures of Merit In this experiment, the effects of voltages of source-gate and gate-drain were optimized via sweeping the direct current (DC) voltages applied to the FET sensor. In the fabricated H2 S FET sensor, maximum reproducibility was observed at the middle regions of the active dynamic mode of the H2 S FET sensor. The optimum DC voltage applied to the source-gate (VSG and source-drain (VSD voltages were 3.0 V and 0.30 V, respectively. The FET sensor was sensitive for H2 S detection yielding linear dynamic range from 150 ppb up to 920 ppb. The trace of H2 S standard solution is illustrated in Figure 6. Good quality of the FET sensor was observed for replicate analyses of different concentrations of H2 S. The calibration curve of the fabricated H2 S FET sensor is also shown in the inset of Figure 6. However, the CNT-modified FET sensor has large active surface area and plenty of active sites at which large amounts of H2 S molecules can easily adsorbed and reacted. But to have reproducible response, it is recommended to remove any previously interacted H2 S with carbon nanomaterials. To reach this purpose, it is proposed to investigate the salting out effects of basic solutions such as NaOH. In this study, since the H2 S FET sensor is treated in basic solution in open circuit, therefore, high concentration of basic solution does not damage the FET device. Consequently, a NaOH solution (0.1 M) was used for rapid elimination of any formerly adsorbed H2 S from the FET sensor. The results reveals that, exposing the FET sensor into the NaOH solution (0.1 M) can remove more than 99% of the memory effect of the FET sensor, caused by any previously H2 S species. The detection limit was defined as the concentration of hydrogen sulfide giving a steady state signal equal to the blank signal plus triple values of the standard deviation of the blank. In this study, the limit of detection was found as J. Nanoeng. Nanomanuf., 1, 1–9, 2012

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Table III. Figures of merit for fabricated H2 S FET sensor modified with Ag2 O/nitrogen/Sulfur-doped SWCNTs. Equation1

Linear range (ppb)

3

Sensitivity2 (%)

Detection limit (ppb)

Saturated limit (ppb)

T390 (sec.)

RSD (%)

0.991

21.0

85

1500

52

3.26

∗

V = −00003 C − 00983

150–920 1

Correlation coefficient (R)

V is the output voltage (Vout of the electronic circuit; C is the concentration of H2 S in ppb. 2 Response, when FET sensor was introduced to 450 ppb H2 S gas solution. Response time.

85 ppb for H2 S species. Also, the relative standard deviation (RSD) for five replicate analyses was evaluated to 3.26%. The performance of the H2 S FET sensor is evaluated by sensitivity and response time.30 In this study, the response time was defined as the time interval for 90% of maximum response (t90 of the H2 S FET sensor. This was evaluated as ∼52 s. Sensitivity is also defined by the fraction of the greatest difference in the output response of the H2 S FET sensor for a constant concentration of H2 S gaseous solution defined by the Eq. (1):31 (1)

where V0 is the response of the H2 S FET sensor, when introduced to N2 as diluent gas, and Vout shows maximum response of the sensor, when exposing the FET sensor to a H2 S gas solution. This was evaluated to 21.0% when introduced 450 ppb of H2 S solution. The interference affects of at least 500 folds of some foreign species such as, acidic vapors such as HCl, HNO3 , H3 PO4 , and also the effects of volatile organic compounds (VOCs) such as benzene, toluene, ethyl benzene, etc. as well as gaseous including water vapor, acetylene, methane, oxygen, carbon dioxide, carbon monoxide, nitrogen dioxide and helium in nitrogen were investigated. No significant interfering effect was observed for each foreign gas. The selectivity coefficients are reported in Table IV according to Eq. (2) defined as: KH2 S/Int =

CH2 S SInt /SH2 S CInt

(2)

Response (V)

0.2 0 0

10

20

30

40

50

Temperature (ºC) Fig. 5. Effect of temperature on the response of fabricated the FET sensor, when introduction of 400 ppb of H2 S to the FET sensor. J. Nanoeng. Nanomanuf., 1, 1–9, 2012

900 ppb

3.3. Proposed Mechanism of H2 S Sensing FET Sensor During the last decades, different mechanisms are suggested for the operation of the gaseous sensors. Among Selectivity coefficients of different foreign gaseous species.

Foreign gaseous

0.4

900 ppb

In this equation, CH2 S and Cint are the concentrations of H2 S and interfering gaseous species, respectively, whereas SH2 S and Sint are the calibration sensitivity of each H2 S and foreign species, respectively.32 The small quantities of selectivity coefficients reveal the specificity of the proposed procedure for H2 S recognition and determination. To validate the proposed analysis methodology, a recovery test was achieved. For this purpose, known amounts of H2 S were spiked to a gaseous sample of air and the recovery percentages were determined. The results are shown in Table V.

Table IV.

0.6

400 ppb

Fig. 6. Trace revealing the reproducibility of fabricated H2 S FET sensor for several analyses of 400 ppb and 900 ppb H2 S standard solutions. Calibration curve of Ag/SWCNT-doped H2 S FET sensor ranging from 150–920 (Inset).

1 0.8

400 ppb

HNO3 , HClO4 , HCl, HIO4 , CO H2 O H2 CH4 C2 H2 VOCs NO2 CO2 O2

Tolerance ratio of interference/H2 S1

Selectivity coefficient of H2 S2

2000

0.08

500 1000 1000 1000 1000 500 2000 2000

0.26 0.12 0.16 0.14 0.23 0.12 No response No response

1 The concentration of H2 S is 450 ppb. 2 The values of selectivity coefficients were calculated using Eq. (2).

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Sensitivity = Vout − V0 ∗ 100

400 ppb


Table V. Recovery of spiked H2 S in real samples. Real sample

H2 S in sample (ppb)

H2 S added (ppb)

H2 S found (ppb)

Recoverya (%)

Real sample

45

120 150

163 197

98.33 101.33

a

Recovery percentage = (Conc. of the spiked H2 S-Found value for the background of H2 S FET sensor)/(Found value of the H2 S spiked)∗ 100.

Electrochemical Reduction

H2S Adsorption

H 2S Adsorption

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Ag2O Regeneration

H2S Desorption

Scheme 1. Proposed mechanism for selective nanoparticles/SWCNT-modified FET sensor to H2 S.

response

Ag

these proposal mechanisms, change in the “Schottky” barrier and the “Charge Transfer” process are considered as the most probable mechanisms.33 34 According to the evidences, charge transfer mechanism is proposed for the fabricated H2 S FET sensor. The evidences related to the mechanism of this sensor are as follows: Among different types of carbon nanostructures such as SWCNTs, MWCNTs, CNFs and fullerenes, SWCNT membrane was considered as the most appropriate supports for H2 S adsorption. This phenomenon reveals the importance of basal plane of the SWCNT support compared to the edge plane of MWCNTs.35 36 As, it is expected, presence of functional groups such as hydroxyl (–OH) groups in the edge plane of MWCNT matrix, partially prevent the dissociation of H2 S into HS− and S2− . This is due to the higher acidic strength of carboxylic

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acid (pKa1 = 476), compared to the H2 S (pKa1 = 697), thereby, leading to the low sensitivity of modified MWCNTs with H2 S. This reveals that, to have significant and sensitive response of the FET sensor to H2 S, dissociation of H2 S is important. The dissociation of H2 S is also promoted using “Lewis” bases such as nitrogen and sulfur atoms. In this study, nitrogen and sulfur atoms were positioned in the SWCNT matrix during the synthesis of CNTs by CVD process, using pyridine and thiophene as the source of nitrogen and sulfur, respectively. This leads to the formation of n-type semiconducting SWCNTs.37 According to the results, reaction of H+ with sulfur and nitrogen atoms strongly promotes the sensitivity of the fabricated sensor during the dissociation of H2 S and transfer of charge on the SWCNT membrane. This reveals the important effects of “charge transfer” mechanism in the fabricated H2 S sensor. In spite of the presence of sulfur and nitrogen atoms in the SWCNT matrix, but, high capacity of the SWCNT bundles for adsorption of O2 (4.6%), results to the conversion of n-type SWCNT into p-type SWCNTs. In this study, this behavior was evaluated via current–voltage (I–V) characteristics of semiconducting SWCNTs in carbon nanotube field effect transistors (CNFET) structure (Fig. 1(A)). The same behavior has also been reported for the effect of oxygen in the conversion of the semiconductivity of CNTs.38–40 The p-type semiconducting SWCNTs can easily interact with electron-denoting molecules such as H2 S, leading to provide charge transfer. This phenomenon also points to the “charge transfer” mechanism for the fabricated H2 S sensor. To have H2 S FET sensor with maximum sensitivity, metal nanoparticles should be oxidized. This is due to the lower work function of metal oxides, compared to metal species.41 Therefore, metal oxide nanoparticles, act as active sites for H2 S adsorption and reaction. Also, oxidation process leads to form Ag2 O and FeOOH nanoparticles, respectively. Reduction of the sensitivity of FeOOH/SWCNT-based FET sensor is due to the protonation of sulfur and nitrogen atoms by FeOOH.41 This also reveals the importance of nitrogen and sulfur atoms as another active site in the performance of FET sensor based on “charge transfer” mechanism. Reversible reaction of metal oxide nanoparticles such as silver oxide (Ag2 O) with H2 S, leads to the formation of

Table VI. Comparisons between the fabricated H2 S sensor and other sensors. Mechanism of gas detection Detection limit Interference Operating temperature Sensitivity Response time (T90 Recovery time Linear dynamic range

8

Fabricated H2 S sensor

Optical H2 S sensor [42]

Resistance H2 S sensor [43]

Potentiometric H2 S sensor [2]

85 ppb No interference Room temperature 21% 52 s ∼ 60s in NaOH solution 150–920 ppb

3 ppm — 320 C — 15 s 120 s 8–2000 ppm

Several ppm Ethanol, Acetone 160 C — 68 s 30 s 50–300 ppm

Several ppm — 300 C 74 mV/decade 12 s 30 s 5–50 ppm


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Ag2 S, causing significantly better interaction of H2 S with SWCNT matrix. The reverse reaction is also occurred, when exposing the FET sensor to oxygen. The influence of various probable equilibria on the adsorption of H2 S on SWCNTs is shown in Scheme 1. All the mentioned evidences reveal the importance of charge transfer mechanism for the fabricated H2 S sensor.

4. CONCLUSIONS

Acknowledgment: The authors thank the support of this work by Shiraz University Research Council.

References and Notes 1. P. A. Patnaik, comprehensive Guide to the Hazardous Properties of Chemical Substances, Wiley, New York (1999). 2. X. Liang, Y. He, F. Liu, B. Wang, T. Zhong, B. Quan, and G. Lu, Sens. Actuators, B 125, 544 (2007). 3. Y. Wang, H. Yan, and E. Wang, Sens. Actuators, B 87, 115 (2002). 4. N. Miura, Y. Yan, G. Lu, and N. Yamazoe, Stockholm, Sweden 95, 845 (1995). 5. W. H. Tao and C. H. Tsai, Sens. Actuators, B 81, 237 (2002). 6. Y. Wang, F. Kong, B. Zhu, S. Wang. S. Wu, and W. Huang, Mater. Sci. Eng., B 140, 98 (2007). 7. A. Khanna, R. Kumar, and S. S. Bhatti, Appl. Phys. lett. 82, 4388 (2003). 8. C. Jin, T. Yamazaki, K. Ito, T. Kikuta, and N. Nakatani, Vacuum 80, 723 (2006). 9. G. H. Jain, L. A. Patil, M. S. Wagh, D. R. Patil, S. A. Patil, and D. P. Amalnerkar, Sens. Actuators, B 117, 159 (2006). 10. J. Tamaki, T. Maekawa, N. Miura, and N. Yamazoe, IEEE 150 (1991). 11. H. Liu, S. P. Gonga, Y. X. Hua, J. Q. Liu, and D. X. Zhou, Sens. Actuators, B 140, 190 (2009). 12. R. Saito, G. Dresselhaus, and M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London (1998). 13. M. S. Dresselhaus, G. Dresselhaus, and P. Avoris, Carbon Nanotubes: Synthesis, Structures, Properties and Applications, Springer Verlag, (2001).


9

ARTICLE

This article presents a simple, accurate, specific and low cost H2 S gas sensor. The fabricated H2 S sensor is based on a modified MOSFET transistor in which source-drain current is influenced during the H2 S gas adsorption. In this study, it was founded that, metal nanoparticles such as silver can markedly change the SWCNTs for H2 S sensing properties. Table VI shows the comparisons between the fabricated FET sensor and the previous reported H2 S CNT sensor. The fabricated H2 S FET sensor shows outstanding advantages such as specificity, high sensitivity, simplicity, and the ease of fabrication. In comparison with the commercial gaseous sensors, which are usually expensive, the present method for H2 S detection is beneficial.

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