A novel cataluminescence gas sensor based on MgO thin film

A novel cataluminescence gas sensor based on MgO thin film

Sensors and Actuators B 148 (2010) 292–297 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevie...

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Sensors and Actuators B 148 (2010) 292–297

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

A novel cataluminescence gas sensor based on MgO thin film Ying Tao, Xiaoan Cao ∗ , Yan Peng, Yonghui Liu Environmental Science and Engineering Institute, Guangzhou University, No. 230 Wai-Huan West Road, 510006 Guangzhou, PR China

a r t i c l e

i n f o

Article history: Received 17 February 2010 Received in revised form 21 April 2010 Accepted 28 April 2010 Available online 5 May 2010 Keywords: 2-Methoxyethanol 2-Ethoxyethanol MgO film Chemiluminescence Gas sensor

a b s t r a c t Using MgO film as sensing material, a cataluminescence sensor was proposed by the determination of ethylene glycol ethers (2-ethoxyethanol and 2-methoxyethanol). This ethylene glycol ethers sensor showed high sensitivity and specificity. With detection limits of 1.0 ppm and 1.4 ppm, the linear ranges of cataluminescence intensity versus ethylene glycol ethers concentrations were 2.0–2000 ppm for 2ethoxyethanol and 2.0–1500 ppm for 2-methoxyethanol, respectively. The response time was less than 5 s. Foreign substances passed through the surface of MgO film without response, such as ammonia, benzene, ethyl acetate, acetaldehyde, vinyl acetate, methanol, acetone, ethanol, acetic acid, formaldehyde, and isopropyl ether. The sensor could determine 2-ethoxyethanol and 2-methoxyethanol whether they existed alone or together in air samples. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Ethylene glycol ethers are frequently used as solvents, detergents, and emulsifiers alone or as components in numerous industrial and domestic products such as paints, varnishes, inks, cosmetics, and cleaning agents industrial. There exist extensive literatures reporting reproductive, testicular, embryotoxic, teratogenic and hematologic toxicity of ethylene glycol ethers in animal experiments [1–3]. In the 1990s ethylene glycol ethers were reported be responsible for a higher risk level of miscarriage amongst women workers in semiconductor factories in USA [4,5]. 2-Methoxyethanol (EM, CAS No. 109-86-4) and 2ethoxyethanol (EE, CAS No. 110-80-5) are two typical members of industrial solvents collectively known as ethylene glycol ethers. They are mainly used as solvents, colorant, and stabilizer in dyes industry. The standard permitted concentrations of EM and EE vapors in ambient air are less than 8.8 ppm and 8.9 ppm ruled by the Occupational Exposure Limit for hazardous agents in the workplace (GBZ 2-2002, China). Short-chained ethylene glycol ethers are phased out due to their health hazards, EE has been forbidden in Germany. However, EM and EE are still widely used in most of other countries [6,7]. The ethylene glycol ethers may readily enter the body by inhalation as well as dermal uptake. A rapid detecting technique of EM or EE is very important in modern industry.

∗ Corresponding author. Tel.: +86 20 39366937; fax: +86 20 39366946. E-mail address: [email protected] (X. Cao). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.04.043

Primary methods determined EM and EE are based on biological monitoring [8], gas chromatograph equipped with flame ionization detector (GC-FID) [9,10], gas chromatography–mass spectrometry (GC/MS) [11], and UV/vis spectrometer [12]. They possessed high performance and sensitivity, however, with difficult operation and no real-time detection. Gas sensors have advantages of on-site and real-time detection of hazard vapors. There is no specific sensor for detection of EM and EE. So, ethylene glycol ethers sensor was developed to detect gaseous quickly in workplace. In 1976, Breysse et al. observed that a weak catalytic luminescence phenomenon occurred during catalytic oxidation of CO on ThO2 surface [13]. This chemiluminescence mode was defined as cataluminescence (CTL). In the 1990s, Nakagawa et al. reported CTL sensors using ␥-Al2 O3 as catalyst material for determination of ethanol and acetone [14–18]. Zhang et al. developed the CTL sensors, a porous alumina film with 0.5 mm thick was used for detecting saccharides [19]. Many nano-materials such as TiO2 , ZrO2 , BaCO3 , SrCO3 , ZnO, ␥-Al2 O3 , La2 O3 , ␥-Al2 O3 + Nd2 O3 , V2 Ti4 O13 , and Y2 O3 were applied to detect organic vapors of ethanol, acetaldehyde, pinacolyl alcohol, propane, butane, acetone, ethylene dichloride, benzaldehyde, ethyl ether, and acetic acid with high sensitivity and specificity [20–33]. Cao reported that the vinyl acetate sensor based on MgO nanoparticles showed good cataluminescence characters [34]. New development of CTL array sensors can distinguish several different gases [35]. CTL gas sensors have advantages of on-site and real-time detection, small size, and no need of lamp-house. Nevertheless, it is still a challenge for scientists to obtain better sensitive and specific CTL sensors to some toxic gases since their low level of concentrations in the air.

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Fig. 1. Schematic diagram of the CTL detection system.

In chemistry sensors, film catalysts were used to improve stability, sensitivity and specificity. ZnO and SnO2 thin film sensors were reported to enhance the sensitivity of CO and NO2 [36,37]. MgO is a widely used catalysis, its thin film has also been considered as a suitable material for technological applications. It can be used as sensor elements for humidity sensor [38] and CH3 OH adsorption [39]. However, there are few thin films used in the cataluminescence sensors. Most of nano-materials in cataluminescence sensors are in powder state and might easily come off the substrates. Since thin films could attach firmly and uniformly to the substrate, MgO thin film was synthesized and characterized in the present study. A new CTL-based gas sensor using MgO thin film for the detection of ethylene glycol ethers vapors was developed. MgO thin film exhibited the highest sensitivity of EE and EM and showed good analytical characters. To the best of our knowledge, EE and EM sensors have been reported in the present work firstly. 2. Materials and methods 2.1. Apparatus The schematic diagram of the detection system was shown in Fig. 1. The catalyst was coated as a layer on the ceramic heating tubes which were coated with nano-size gold. The 5 mm in diameter ceramic heating tube was put into a 12 mm inner-diameter quartz tube. The temperature of the catalysts could be adjusted by controlling the voltage of the heating tube. The air from the pump was mixed with detecting vapor and flowed through the quartz tube, a catalytic reaction occurred on the surface of catalysts. The CTL intensity was measured by a BPCL Ultra Weak Chemiluminescence Analyzer (Biophysics Institute of Chinese Academy of Science, PR China). 2.2. Synthesis and characterization of thin film materials All regents were of analytical grade. Indium chloride (InCl3 ·4H2 O) was obtained from Reagent No. 1 Factory of Shanghai Chemical Reagent Co., Ltd. Magnesium chloride (MgCl2 ·6H2 O), sodium citrate (Na3 C6 H5 O7 ·2H2 O), polyethylene glycol (PEG6000), zirconium nitrate (Zr(NO3 )4 ·3H2 O), and oxalic acid were purchased from Tianjin Damao Chemical regent Factory. In2 O3 , MgO and ZrO2 were prepared by sol–gel method. (1) In2 O3 precursor was synthesized as follows: 0.65 g InCl3 ·4H2 O and 1.0 g PEG-2000 were dissolved in 30 mL distilled water with vigorous stirring for 30 min. Then 10% ammonia solution was added until a white precipitate of In(OH)3 was formed, further addition of ammonia solution resulted in dissolving of the precipitate until

Fig. 2. TFE-SEM photo of MgO thin film.

pH value reached 5. Then stirred for another 1 h. (2) MgO precursor was synthesized as follows: 2.5 g MgCl2 ·6H2 O and 2.5 g PEG-6000 were dissolved in 50 mL distilled water with vigorous stirring for 20 min, NaOH solution was added into the mixture until pH value reached 12. Keep on stirring for 1 h. (3) ZrO2 precursor was synthesized as the above method, the molar ratio of zirconium nitrate to oxalic acid was 4.5:1. Fabrication of the thin films: Ceramic heating tubes which were coated with gold were used as the solid substrate for film growth. The substrates were washed with hydrochloric acid, ethanol, and distilled water in sequence and dried in oven at last. Thin film systems were prepared by using the dip coating method from each colloidal and sol–gel solution. MgO thin films were synthesized in the following way: The ceramic substrates were immersed into the MgO precursor solution for five times, dried at 80 ◦ C for 1 h and decomposed to MgO when calcined at 450 ◦ C for 1 h. Other films such as In2 O3 and ZrO2 were synthesized following the same method. MgO film produced in the present work was characterized for its surface morphology and chemical composition. The microstructure of the film was investigated by Quanta 400 thermal field emission environment scanning electron microscope (TFE-SEM). Specimens were observed at accelerating voltages <20 kV. Fig. 2 showed the surface morphology of the film. The thickness of the film was measured by dektak 150 surface profiler (Veeco Instruments Inc.). Subsequently, chemical compositions of the annealed films were determined using the X-ray photoelectron spectroscopy (XPS, model: ESCALAB 250; manufacturer: Thermo Fisher Scientific). XPS analysis was using a Mono Al K␣ (1486.6 eV), X-ray source operating at 150 W. The vacuum of the analysis chamber was better than 2 × 10−9 mbar. All spectra were acquired at a fixed analyzer energy mode. Pass energy of 150 eV was applied to wide scans,

Table 1 Chemical composition of the MgO thin film surface. Name

Peak BE (eV)

FWHM (eV)

Atomic percent (%)

C 1s O 1s Mg 1s Cl 2p F 1s Na 1s

284.75 532.52 1305.89 199.71 685.95 1073.97

1.83 2.75 2.92 2.59 2.12 3.98

31.53 44.84 18.36 1.61 0.99 2.66

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Table 2 Comparison of signal-to-noise ratio of six air pollutions on some materials. Materials

In2 O3 ZrO2 MgO

Signal-to-noise ratio (S/N)

Selective conditions

EM

EE

Ethanol

Formaldehyde

Acetone

Benzene

0.73 1.69 64.52

0.63 Not detectable 104.91

0.7 1.19 0.43

0.43 Not detectable 0.55

1.24 1.08 1.06

Not detectable 0.59 Not detectable

296 ◦ C, 425 m 279 ◦ C, 425 m 279 ◦ C, 425 m

while 20 eV was used for narrow region scans. The binding energies were calibrated with reference to C 1s at 284.8 eV for adventitious hydrocarbon contamination. The analysis area was 500 ␮m with a typical analysis depth of 5–10 nm.

3. Results and discussion 3.1. Structure of the thin film material The surface morphology of MgO thin film was shown in Fig. 2. It could be seen in Fig. 2 that MgO film has a plate like structure. The thickness of the films depends on the dig time. And the thicknesses of MgO and In2 O3 films are 0.23 ␮m and 0.20 ␮m, respectively. XPS analysis was used to detect the element ingredients of MgO film. Table 1 summarized the atomic weight ratios of elements and surface chemical compositions of MgO film, which were calculated from the experimental data. A single peak of Mg 1s was obtained at 1305 eV, which indicated that the material was MgO. Air pollutions caused high atomic weight ratios of elements C and O.

Fig. 4. Wavelength dependence of CTL intensity of EE and EM vapors. (1) EE: temperature: 279 ◦ C, flow rate: 300 mL/min, and concentration: 500 ppm; (2) EM: temperature: 279 ◦ C, flow rate: 210 mL/min, and concentration: 500 ppm.

3.3. CTL response profile of ethylene glycol ethers on MgO film 3.2. Estimation of the thin film materials for the sensing mode To select the appropriate sensing materials for ethylene glycol ethers sensor, In2 O3 , ZrO2 , and MgO thin films were examined. The CTL responses on the surface of these films were detected when all the organic vapors were 500 ppm. All detection conditions in Table 2 were optimal conditions of using each sensing material based on the experiments’ results. The data showed that MgO film was the best material to obtain the highest signal-to-noise ratio. It could be attributed to a combination of materials’ differences in (i) diffusivity, (ii) adsorption behavior, and (iii) intrinsic reaction rate (restrictions on the size of the transition state) [40,41]. Therefore, MgO film was selected for the subsequent study in the present work. The work on MgO film showed that film at thickness of 0.2 ␮m was stable, good reproducibility, and will not come off the substrate. So, all the thin films were prepared about this thickness.

The CTL response profiles of EE and EM on MgO film were investigated by injecting each vapor into the carrier gas with certain flow rates. Fig. 3 showed the CTL response profiles of samples containing EE or EM at 279 ◦ C with a bandpass filter of 425 nm. Curves 1 and 2 denoted the results for EE and EM with the same concentration of 500 ppm. The CTL response profiles were similar to each other. The peak of each curve appeared at about 5 s after sample injection and the half decay time of each curve was about 45 s, which indicated that the EE and EM sensing were fast processes.

3.4. Optimization of wavelengths In order to investigate the effect of wavelength on the CTL intensity, six bandpass filters (400, 425, 440, 460, 490, and 535 nm) were used to select the optimal wavelength. The noise signal produced from incandescent radiation of the ceramic heater substrate increased towards longer wavelength, so signal-to-noise ratios were used to show the CTL intensity. From Fig. 4, the optimal wavelength for CTL was 425 nm to detect EE and EM. This wavelength was used for the quantitative analysis because of the lower incandescent radiation.

3.5. Optimization of working temperature

Fig. 3. CTL response profiles of EE and EM. (1) EE: temperature: 279 ◦ C, flow rate: 300 mL/min, wavelength: 425 nm, and concentration: 500 ppm; (2) EM: temperature: 279 ◦ C, flow rate: 210 mL/min, wavelength: 425 nm, and concentration: 500 ppm.

The effect of working temperature on CTL was shown in Fig. 5. As shown in Fig. 5, the incandescent radiation noise of substrate increased markedly above 320 ◦ C, therefore 279 ◦ C was selected as the optimum detection temperature owing to the maximum signal-to-noise ratio. Moreover, the noise produced from incandescent radiation was still at a very low level at this temperature. Therefore, 279 ◦ C was used for the subsequent study. The optimum detection temperature of EM was the same as EE’s.

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Fig. 5. Temperature dependence of CTL intensity of EE and EM vapors. (1) EE: wavelength: 425 nm, flow rate: 300 mL/min, and concentration: 500 ppm; (2) EM: wavelength: 425 nm, flow rate: 210 mL/min, and concentration: 500 ppm.

Fig. 7. CTL responses of different compounds on the sensor. Temperature: 279 ◦ C; wavelength: 425 nm; flow rate: 250 mL/min; and concentration: 500 ppm.

Fig. 6. Effect of flow rate of carrier air on CTL intensity of EE and EM vapors. Temperature: 279 ◦ C; wavelength: 425 nm and concentration: 500 ppm.

linear regression equations of EE and EM were described by I = 7.289C + 2.504 (r = 0.9981) and I = 5.088C − 47.578 (r = 0.9976), respectively. Where I was the relative CTL intensity, C was the concentration of EE or EM (ppm) and r was the regression coefficient. The detection limit was 1.0 ppm for EE and 1.4 for EM. Since the regression equations of EE and EM were a little different, approximate total amounts of ethylene glycol ethers could be obtained if they coexisted in one sample (see sample 1 in Table 3). According to the Occupational Exposure Limit for hazardous agents in the workplace (GBZ 2-2002, China), the maximum allowable concentrations of EE and EM vapor in air are less than 8.9 ppm and 8.8 ppm. The detection limits of the EE and EM vapors are below the standard permitted concentrations, therefore, the sensor can be used for air quality monitoring of ethylene glycol ethers in workplace.

3.6. Optimization of flow rate of carrier gas 3.8. Specificity of MgO thin film gas sensor The flow rate dependence on CTL intensity was investigated ranged from 50 to 395 mL/min. As illustrated in Fig. 6 (EE), the CTL intensity increased gradually with the increase of flow rate from 50 to 300 mL/min. When the flow rate was above 300 mL/min, the CTL intensity deceased slightly. Fig. 6 (EM) showed the similar trend, however, 210 mL/min was the inflexion. The results showed that the catalytic oxidation was under a diffusion controlled condition when the flow rate was below 300 mL/min or 210 mL/min. The reason for this case might be that the reaction time between EE, EM and MgO film would not be sufficient when flow rates of carrier gas increased to above 300 mL/min or 210 mL/min. So, 300 mL/min and 210 mL/min were finally chosen for detection.

Eleven kinds of vapors, which might coexist with EE and EM in air, were detected respectively under the optimal conditions. As shown in Fig. 7, vapors of ammonia, benzene, and ethyl acetate had no interference with EE or EM. While, acetaldehyde, vinyl acetate, methanol, acetone, ethanol, acetic acid, formaldehyde, and isopropyl ether caused interferences at levels around 0.77%, 2.01%, 0.87%, 0.66%, 0.40%, 0.61%, 0.48%, and 1.93% compared with the response of EE. Therefore, the sensor was feasibility for the determinations of EE and EM in air owing to significantly specificity to ethylene glycol ethers. 3.9. Lifetime of the sensor

3.7. Analytical characteristics Under the optimal conditions described above, the calibration curves of CTL intensity versus EE or EM concentration were linear in the range of 2.0–2000 ppm or 2.0–1500 ppm. The

The lifetime of the sensor was tested at the optimal conditions. The CTL intensities were measured once per 5 h by continuously introducing 500 ppm of EE for 100 h in the carrier gas through the sensor. No significant decrease of CTL intensity was observed dur-

Table 3 Ethylene glycol ethers analysis in artificial samples. Sample no. 1

Composition EE EM

Prepared values (ppm) 1000 250

Measured values (ppm, n = 6) 1370 ± 87

2

EE Ethanol

1000 200

1080 ± 62

Recovery (%) 110 108

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ing the 100 h detection. The relative standard deviations were 2.6% (n = 26) for 500 ppm EE, 2.4% (n = 26) for 500 ppm EM. The results indicated the durability of the MgO film based sensor. 3.10. Determination of ethylene glycol ethers in the synthetic samples In order to test the feasibility of the sensor, two artificial samples containing known concentrations of EE and EM, EE and ethanol were analyzed under the optimal conditions. Since EE and EM showed a little different sensitivity, approximate total amounts of EE and EM could be obtained if they coexisted in one sample. As shown in Table 3, the recovery of sample 1 was obtained from the ratio of measured value to aggregate prepared value of EE and EM, while the recovery of sample 2 was obtained from the ratio of measured value to prepared value of EE. Satisfactory recoveries were obtained. As a result of this experiment, EE and EM could be detected separately or when they coexist. 3.11. Mechanism The mechanisms about the oxidation of EE and EM on nanocatalysts have not been studied yet. Lv et al. reported that ethyl ether would oxidized to acetaldehyde and then to acetic acid, during which main luminous intermediates CH3 CO* were generated and emitted light [42]. Therefore, possible mechanisms of the cataluminescence of EM and EE are as the following: For EM :

MgO

CH3 OCH2 CH2 OH + O2 −→CH3 OCH2 CHO∗

CH3 OCH2 CHO∗ → CH3 CHO + h For EE :

MgO

CH3 CH2 OCH2 CH2 OH + O2 −→CH3 CH2 OCH2 CHO∗

CH3 CH2 OCH2 CHO∗ → CH3 CHO + h When EM or EE vapors passed through the surface of the MgO thin film, they were catalytically oxidized by O2 in the air. The electronically excited methoxy acetaldehyde (CH3 OCH2 CHO*) or ethoxy acetaldehyde (CH3 CH2 OCH2 CHO*) could be produced and absorbed on the MgO film during the reaction and generated photoemission when they returned to their ground states. 4. Conclusions The EE and EM vapors sensor based on MgO thin film had been investigated for the first time in this paper. The results showed that the sensor possessed rapid response, high sensitivity, satisfactory durability, and excellent specificity to EE and EM among some possible coexistence substances in air. The sensor can detect EE and EM whether the gas exist alone or together. It shows the prospect for EE and EM determination in industry and environment monitoring. This paper is also valuable for the future research to develop cataluminescence sensors based on thin film catalysts. Acknowledgements The authors gratefully thank for the financial support by the National Natural Science Foundation of China (20677013), Natural Science Foundation of Guangdong Province, China (8151009101000130). References [1] B.D. Hardin, Reproductive toxicity of the glycol ethers, Toxicology 27 (1983) 91–102.

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Biographies Ying Tao completed her undergraduate studies in the Chemical Technology Institute, Wuhan University of Science and Technology, China. And is currently a postgraduate student at the Environmental Science and Engineering Institute, Guangzhou University, China. Her major is Environmental Chemistry and focus on cataluminescence and the application of the gas sensor. Xiaoan Cao received her BS degree in 1982 from Department of Chemistry, Jiangxi University, China. She is currently working in the Institute of Environmental Science and Engineering, Guangzhou University. She has been a professor of Guangzhou University since 2002. Her interest is in the investigation of cataluminescence and the application to the gas sensor. Yan Peng received her MS degree in 2001 from East China Geological Institute (now, East China Institute of Technology) China. Now she is a PhD candidate of China University of Geosciences and majors in environmental engineering. Her research interest is focused mainly on analytical of volatile organic compounds in indoor air. Yonghui Liu received her PhD degree in 2004 from Dalian University of Science & Technology, China. Now she works in Institute of Environmental Science & Engineering at Guangzhou University, China. Her major research interests focus mainly on VOCs detection and degradation.