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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Titania-doped chromium oxide (CTO) can serve as an active gas-sensing material. It has been successful in commercial gas sensors due to its good gas-sensing performance and stability in humid environment; especially with respect of volatile organic compounds (VOCs). CTO fibers with reticular structure were prepared by electrospinning technology and the sensing behavior upon exposure to ethanol was characterized in this report. The gas sensors made of CTO fibers show a good response to ethanol and stability for a long term at operating temperature of 400°C. The experimental results have been analyzed and simulated using the response equations. The humidity effects on the sensor performance were also evaluated. The results indicate that the CTO fibers could be used in practical applications.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

The development of reliable and selective solid-state gas sensors is crucial for many industrial applications. Titanium-substituted chromium oxides (CTO) are p-type semiconducting oxides that have been studied as thick-film sensors to detect various types of gases and organic vapors.[1] It has a number of benefits contributing to the practical applications, such as the base line stability, selectivity, and negligible humidity interference.[2, 3] The CTO film sensors were reported for monitoring ammonia with an operating temperature of 500°C.[4] The CTO microspheres with various compositions of TiO2 had been studied and the sensor with a composition of Cr1.95Ti0.05O3 showed maximum response to ethanol vapor.[5] Wollenstein et al. reported that the resistance of CTO thin film was nearly not influenced by relative humidity (RH) when RH was above 10%.[6] The polycrystalline Cr1.8Ti0.2O3 thick film synthesized by sol–gel was exposed to various concentrations of alcohols.[7] The results indicated that the material had a high sensitivity even with a very low concentrations of alcohol. The Cr2−xTixOz films fabricated via an electrostatic spray-assisted vapor deposition (ESAVD) were also tested toward ammonia and ethanol over the temperature range of 200–500°C.[8] Compared with the traditional thin- and thick-film sensors, reticular structures based on metal oxide fibers have several advantages such as very large surface-to-volume ratio comparable to the extension of surface charge region, superior stability owing to the high crystallinity.[9] However, there have only been rare reports on the CTO fibers explored so far for ethanol detection. Ethanol is one of the most important vapors in breathalyzers, wine making, medical processes, and food industries. Because gas sensors are applied in the air, the interference from humidity must be avoided first.[10] Therefore, the preparation and characterizations of CTO fibers synthesized by electrospinning technology are reported in this work. The gas-sensing properties of the CTO fibers sensors are examined. In addition, the effects of humidity on the sensing properties of these fibers are presented and discussed.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Fabrication of Fibers

In a typical electrospinning process, 0.7 g Cr(NO3)3·9H2O and 12 μL titanium isopropoxide (Ti(OiPr)4; Sigma-Aldrich, St. Louis, MO) were dissolved in a mixed solvent contained 4.6 mL of DMF (dimethylformamide) and 5.4 mL of ethanol under vigorous stirring for 30 min. Thereafter, 0.8 g PVP (polyvinyl pyrrolidone, Aldrich, Mw = 1,300,000) was added into above solution with stirring for 6 h at room temperature until a homogeneous solution was obtained. Then the mixture was loaded into a glass syringe and connected to high-voltage power supply. A voltage of 16 kV was applied between cathode (a flat aluminum foil) and anode (a syringe nozzle) with a distance of 15 cm. To remove PVP completely, the as-spun fibers were calcined at temperature from 700 to 1000°C in air for 4 h. A slow heating rate (1°C/min) was selected to ensure the removal of organic phase without destroying the fibrillar structure and to avoid disintegration of the oxide fibers.

Materials Characterization

X-ray diffraction (XRD) analysis was characterized by using a Shimadzu XRD-6000 (Shimadzu, Kyoto, Japan) in 2θ region of 20–80° with CuKα (0.15406 nm) radiation. Scanning electron microscopy (SEM) was performed using a Hitachi S-4800I microscope (Hitachi, Tokyo, Japan). The composition was determined by electron probe microanalysis (EPMA) using a Shimadzu EPMA-1600.

Sensor Measurement

The as-synthesized samples were mixed with deionized water in a weight ratio of 100:25 to form a paste. Then the paste was coated on alumina tubes, on which two platinum wires had been previously installed at each end. After the alumina tubes were heated at 600°C for 2 h, a small Ni–Cr alloy wire was placed through the tube as a heater, which provided an operating temperature for the sensor. To improve their stability and repeatability, the gas sensors were aged at a heating temperature 300°C for 48 h in air.

Gas-sensing properties of the as-fabricated sensors were measured using a static test system. The resistance of the sensors was recorded by a data acquisition card, which was connected to a computer to save the real-time data, and the whole system was governed by a computer automatically. Target gas was injected into a test chamber (50 L in volume) by a syringe through a rubber plug. For a required concentration, the volume of the target gas (V) can be calculated as follows:

  • display math(1)

Where C is the target gas concentration (ppm), and v% is the volume fraction of bottled gas.

The sensing response of a sensor was defined as Ra/Rg, where Ra, Rg represented the resistances of a gas sensor in air and in a target gas, respectively. The time taken by the sensor to achieve 90% of the total resistance change was defined as the response time in the case of adsorption or the recovery time in the case of desorption.[11]

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Materials Characterization

The XRD patterns of as-synthesized CTO fibers were calcined at various temperatures between 700°C and 1000°C to investigate the evolution of the crystalline phases as shown in Fig. 1. Sharp peaks in Fig. 1 reflect the formation of a well-ordered crystal at 1000°C.The patterns of the CTO were identical to that of Cr2O3 (JCPDS File no.38–1479), and no peaks corresponding to TiO2 or CTO are observed, which may due to the small mass percent of TiO2 in CTO fibers.[12] Formation and solid solution behavior of TiO2 in Cr2O3 are expected when both Cr3+ and Ti4+ ions have the same ionic radius (0.061 nm).[7] With the help of Scherrer equation, as calculated by the relatively large diffraction peaks assigning to (012), (104), (110), and (116) planes, the average crystallite size of the CTO samples are about 27, 32, 38, and 42 nm, respectively.

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Figure 1. XRD patterns for as-synthesized CTO fibers.

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Features of the reticular structure of fibers were also examined by SEM. Figure 2 shows SEM micrographs of a typical structure of the CTO fibers: (i) as-synthesized composite fibers and (ii) the calcined fibers, the inset is the CTO fibers with high amplification. It can be observed that the distributions of fibers are fairly random with no distinct alignment. The surface of the hybrid fibers is smooth and the diameters are around 500 nm. After the calcination process, the diameters of fibers shrink significantly to ~200 nm with the removal of organic phase, and the surface of the fibers becomes coarse, which consisted of nanocrystallines.

image

Figure 2. SEM micrographs of (a) as-synthesized composite fibers and (b) the calcined fibers of CTO, the inset is the CTO fibers with high amplification.

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The EPMA study of CTO showed that the content of the as-prepared CTO is Cr1.93Ti0.07O2.7. Table 1 lists the compositions of the as-prepared fibers. We can see from Table 1 that the as-prepared fibers generate an abundance of oxygen vacancies in these samples, which is responsible for the active sites to reversibly adsorb oxygen.[13] As the gas-sensing catalytic activity is related to the surface adsorbed oxygen,[14] it can be inferred that the catalytic activity is controlled by the nonstoichiometric character of the gas-sensing material.[15] Hence, the nonstoichiometric “CTO” could enhance and improve the activity of catalytic reaction on its surface.

Table 1. Composition of As-synthesized CTO Fibers
ElementsWt%Mol%
Cr68.15940.811
O29.70157.798
Ti2.141.391

Gas-Sensing Properties

Here, the investigation focuses on the ethanol-sensing properties of the as-synthesized CTO fibers. The behavior of the gas-sensing properties are determined by the interactions of oxygen ions and gas molecules around the surface oxygen vacant sites of the sensing material.[16] These reactions' processes can be expressed as follows:

  • display math(2)
  • display math(3)
  • display math(4)

where R- represents the carbonic long chain. The gas response is believed to be due to surface catalytic reaction of ethanol with the O forming a surface hydroxyl species and an unstable alkoxyl radical as an intermediate product, which releases the surface-trapped electrons, then oxidizes to water and carbon dioxide at the working temperature of 400°C.[17]

The responses of the CTO fibers tested in 1–1000 ppm of ethanol are presented in Fig. 3. The operating temperature of the sensor was 400°C. In our measurement, the response time and recovery time are about 110 and 200 s to 50 ppm ethanol, respectively.

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Figure 3. Response of the CTO fibers versus ethanol concentration, the inset is the response and recovery characteristics curves in the range of 1–25 ppm.

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The experimental data could be fitted by the sensor response law developed by Gurlo et al.[18] The correlation between the sensor response (S) and gas concentration (C) is approximated by an equation in a form of = 1 + aCbin this report, where a and b are variables, that can be fitted to = (0.2144 ± 0.00307) and = (0.6186 ± 0.0022), respectively.

The sensor noise can be calculated using the residual between experimental data and the corresponding curve-fitting value using the root-mean-square deviation (rmsd).[19]

  • display math(5)

Where yi is the measured data point, y is the corresponding value calculated from the curve-fitting equation, and N is the number of data points used in the curve fitting. The sensor noise is 0.0315 for the ethanol sensor in Fig. 3. According to the IUPAC definition,[20] when the signal-to-noise ratio equals 3, the signal is considered to be a true signal.

  • display math(6)

Using the above equations, the minimum response is calculated to be 1.095 and the ethanol detection limit is about 0.268 ppm in Fig. 3.

The cross-response (selectivity) of CTO sensors to formaldehyde, ethanol, ammonia, and isopropanol were examined as shown in Fig. 4. The results indicate that the CTO sensors are more sensitive to ethanol and isopropanol than to formaldehyde and ammonia. The sensors show almost the same response to ethanol and isopropanol, and the main reason relating to this phenomenon may be that ethanol and isopropanol have the same functional group (-OH) involved in the redox reaction. The different activity energy for the interaction between oxygen ions on the surface of CTO fibers with diverse functional groups of gas molecules is predictable. As proposed in reference,[21] to obtain high selectivity of gas-sensing materials among a variety of gas vapors, a combination of the sensor process and catalyst design approach should be developed. Further investigations are needed in this respect to achieve the selective sensing properties of the CTO fibers.

image

Figure 4. Cross-responses of CTO fibers sensors to formaldehyde, ethanol, ammonia, and isopropanol.

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In addition, it was known that the sensing performance of gas-sensing materials was considerably influenced by the humidity of the environment. Figure 5 shows the change in the base resistance and the response of gas sensor-based CTO fibers to ethanol at 15% RH and 50% RH, respectively. In general, surface resistance of metal oxide-sensing materials is influenced by the presence of moisture as the preadsorbed oxygen on the surface can be displaced by competitive water physisorption.[22] For the p-type CTO fibers, an increase in RH led to an increase in resistance. The relative variation in base resistance was found to be 4.75% and the sensor relative response decreased by a factor of 13.38% and 7.45% to 50 and 100 ppm ethanol when the humidity varied from 15% RH to 50% RH, respectively. However, even after reduction, the relative response of the sensor was high enough for a good sensor categorization, and the response and recovery characteristics remained unchanged.

image

Figure 5. Resistance of the CTO fibers to different concentrations of ethanol in humid air of 15% RH and 50% RH at 400°C.

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Figure 6 gives the response transient curves of two CTO sensors to 100 ppm ethanol of three cycles in 20% RH. We can see that two sensors have good consistency and reproducibility in three response–recovery cycles.

image

Figure 6. Response transient curves of two CTO sensors to 100 ppm ethanol of three cycles.

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The sensor stability test was performed every 5 days for 30 days as shown in Fig. 7. There are almost no changes in the response values, which suggests that these sensors could be operated to continuously monitor ethanol. The better stability of the fiber mats sensors is explained in reticular framework of reduced propensity of fibers to sinter at elevated temperature.[23]

image

Figure 7. Stability of CTO sensor in different ethanol concentrations.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

The CTO fibers with reticular structure for gas-sensing applications had been prepared by electrospinning technology. The process showed that CTO fibers become coarse after calcined at 1000°C. The electrical response of CTO fibers showed well-defined sensing properties over a wide range of concentration to ethanol. Detection limit of 0.268 ppm to ethanol vapor for the sensor fibers were calculated. The baseline resistance and the cross-sensitivity of these CTO sensors were not significantly influenced by water vapor. The sensors of CTO fibers exhibit good sensitivity to both ethanol and isopropanol, according to the catalytic reaction on the surface of CTO fibers, both molecules have similar activity energy involved due to the same functional group (-OH) reacting with oxygen ions. In conclusion, the features of good stability and negligible water influences compared with sensors with film structures make the potential application for CTO fibers sensor in practical detection.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

The authors thank The National Natural Science Foundation of China (61176068, 61001054, 61131004) and the 973 Projects (2011CB302105) for financial support.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Section
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References