Chemiresistive Sensor Based on Zinc Oxide Nanoflakes for CO2

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Chemiresistive Sensor Based on Zinc Oxide Nanoflakes for CO2 Detection Srinivasulu Kanaparthi, and Shiv Govind Singh ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01763 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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Chemiresistive Sensor Based on Zinc Oxide Nanoflakes for CO2 Detection Srinivasulu Kanaparthi and Shiv Govind Singh* Department of Electrical Engineering, Indian Institute of Technology Hyderabad, Kandi, 502285, Telangana, India. *Corresponding author: Email: [email protected]

Abstract

Ultra-fast detection of dynamic variations in carbon dioxide (CO2) gas concentration with good sensitivity is crucial in many applications that range from respiration monitoring to fire detection. However, the chemical inertness of the CO2 sensor makes its detection with high sensitivity quite difficult and only a few materials were reported which can sense CO2 effectively. Nevertheless, the sensors based on these materials exhibited very low sensitivity with large response times and thus they are not suitable for many practical applications. Here, we report a highly sensitive, reversible and ultra-fast detection of carbon dioxide gas in air using a resistive gas sensor based on ZnO nanoflakes. Excellent sensitivity (0.1125 ppm-1 for 600 ppm) with ultra-fast response (< 20 seconds) is observed upon exposure of the sensor to 200-1025 ppm CO2 at 250 0C. The sensing mechanism of the device is explained by the oxygen vacancy model. Further, the effect of temperature and cross-sensitivity of the sensor to other gases were experimentally investigated. Being highly sensitive and faster, this CO2 sensor can be utilized in numerous applications where high response and recovery times along with good sensitivity are extremely important.

Keywords CO2 gas sensor, ZnO gas sensor, ZnO nanoflakes, environmental monitoring, pollution monitoring

Introduction Carbon dioxide (CO2) sensors play a significant role in diverse applications such as air quality monitoring, greenhouse gas monitoring, fire detection, intelligent food packaging, medical


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diagnosis and consumer electronics [1-8]. Conventional techniques to measure CO2 are optical methods that include infrared spectroscopy and fluorescence and gas chromatography [9-13]. Despite their excellent sensitivity and selectivity in detecting various gases including CO2, they are bulky and expensive which limit their use to certain specific applications. Thus, the development of inexpensive and miniaturized CO2 sensors has been gained significant interest given the chemical inertness of CO2 gas. Metal oxide gas sensors operating at lower temperatures cannot detect the gases with sufficient sensitivity required for practical applications [14-17]. Alternatively, the graphene and other 2D materials based gas sensors exhibit high response at room temperature [18-21]. However, they suffer from the irreversible response and large response or recovery times. Moreover, the gas sensors working at ambient temperature are highly sensitive to temperature variations and relative humidity. The effect of temperature and humidity can be nullified by using post signal processing techniques if the response time is low. Unfortunately, the response time of most of the room temperature gas sensors is several minutes to hours. During this time, the temperature and humidity may vary multiple times and it affects the final output of the sensor. Thus the sensor working at ambient temperature with high response and recovery times is not reliable. Therefore, the use of metal oxide semiconductors for gas sensing is preferable in many aspects despite their relatively high power consumption owing to their fast response. There has been a fairly large number of reports on CO2 sensing based on metal oxides and their composites which have high operating temperature [22-30]. However, the sensitivity of these sensors is comparatively too low and respond slowly. Therefore, it is required to implement a sensor that can detect CO2 in the air with excellent sensitivity and yet with good response and recovery times. In this work, we synthesized ZnO nanoflakes (NF) at low temperature and utilized them in sensing of CO2 gas. The NF were synthesized by a simple precipitation method at low temperature using zinc precursor and sodium hydroxide. As prepared ZnO exhibited very high sensitivity as well as ultra-fast response to CO2 gas. The high sensitivity is attributed to the large surface to volume ration of thin nanoflakes. The high sensitivity combined with the ultra-fast response of the sensor makes it a great alternative to the traditional bulky and expensive instruments and it can be utilized in a range of applications despite its relatively high power consumption compared to room



temperature sensors. Moreover, this sensor is a perfect choice where high-temperature gas sensing is required.

2. Experimental Methods 2.1 Materials Zinc acetate dihydrate (Zn (CH3COO)2. 2H2O)), silver (AgNO3) paste, isopropanol, and sodium hydroxide (NaOH) were procured from Sigma Aldrich, USA. Ethanol (C2H5OH) was obtained from Hychem Laboratories, India and DI water was prepared from Millipore DI water purification system. 2.2 Synthesis of ZnO nanoflakes ZnO nanoflakes were synthesized by a simple precipitation method. First, 658 mg of Zn (CH3COO)2. 2H2O was dissolved in 30 mL DI water. Next, 600 mg NaOH was added to the above precursor solution and stirred for 40 min at 50 0C. Then, the solution was washed with ethanol and DI water several times using Eppendorf centrifuge at 5000 rpm for 5 minutes. Finally, the solution was transferred to a Petri dish followed by dried it overnight at 60 0C and the white precipitate was collected with a tiny spatula. 2.4 Materials characterization The structural properties of ZnO nanostructures were characterized with powder X-Ray diffraction (XRD, PAN analytic X’pert pro) with X-Ray wavelength of CuKα1, 𝜆 = 1.5406 Å in the 2θ range of 200 to 800. Zeiss Field Emission Scanning Electron Microscope (FESEM) was used to investigate the surface morphology of the ZnO dropcasted on polyimide substrate. EHT = 10 kV and working distance of 8.5 mm were used during the characterization. Surface area analysis was carried out using BET surface Analyzer (MicroMeritics ASAP2020) at liquid nitrogen temperature (77 K) 2.5 Device fabrication and sensing The sensor is fabricated on polyimide substrate (Figure 1(a)) by screen printing ZnO dispersed in isopropanol using aluminum stencil mask. First, the ZnO solution was dropped with a micropipette


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and squeegeed with a mini squeegee to get relatively uniform deposition. Finally, the mask was removed and the electrodes were made using silver paste such that the dimensions of the device are 2 mm x 3 mm (width x length). To investigate the performance of the device, it was placed in a gas chamber that provided with heater and mass flow controllers (MFC). Electrical readings were recorded using a Keithley 2450 source meter. The device was allowed to stabilize under intended temperature conditions prior to the exposure of CO2 and other interference analytes. The desired concentration of CO2 was achieved by diluting 10000 ppm CO2 with synthetic air (80% N2 and 20% O2). Similarly, the response of the sensor to other gases like Ammonia (NH3) and Hydrogen Sulphide (H2S) was observed by flowing the corresponding diluted gases instead of CO2.

3. RESULTS AND DISCUSSION

To get high sensitivity in gas sensing, materials with a high surface to volume ratio are required. Figure 1 (b) and 1 (c) show the surface morphology of ZnO film deposited on polyimide substrate at different magnifications. It can be observed that ZnO has two-dimensional flakes or sheets morphology. As the thickness of flakes is very less, the surface to volume ratio of the synthesized material is very high and thus it is possible to realize highly sensitive sensors with this material. To determine the crystal structure of ZnO nanoflakes, XRD analysis is carried out. Figure 2 shows the polycrystalline nature of ZnO nanostructures. The peaks in the spectra (JCPDS No. 36-1451) reveal the hexagonal wurtzite structure of ZnO. The crystallite size (D) (40.7 nm), lattice constants a (3.2502 Å) and c (5.1939 Å) of ZnO nanoflakes were calculated from the XRD data (Supporting Information). The surface area of the material was determined to be 32 m2/g with BET surface analyzer.



Figure 1. (a) Schematic of ZnO chemiresistive gas sensor; and (b), (c) Morphology of ZnO nanoflakes

Figure 2. XRD spectra of ZnO NF


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Figure 3. (a) Dynamic response of the sensor to CO2; (b) Response of the sensor (in %) as a function of the concentration of CO2; (c) Repeatability of the sensor; (d) Response and Recovery times of the sensor as a function of the concentration of the gas

To measure the response of the sensor, a DC voltage of 5 V was applied and the current through the sensor was recorded continuously as it was exposed to target gas repeatedly at different temperatures. We present more details of the response of the sensor upon repeated exposure and removal of CO2 gas in Figure 3a. The response (normalized conductance) of 5 cycles were recorded corresponding to the exposure and removal of CO2 concentrations of 1025 ppm, 800 ppm, 600 ppm, 400 ppm, and 200 ppm, respectively. During the experiment, the conductance recovered to its original value completely upon removal of CO2 exposure. The response followed exponential behavior (Figure 3b) with a concentration of CO2 and it is fitted with the nonlinear equation 𝑦 = 𝑦0 + 𝐴 ∗ |𝑥 − 𝑥𝑐 |𝑃

(1)

where y0 (112.49), A (4.1e-4), xc (94.93) and P (1.799) are constants and x is the gas concentration. It is in agreement with the standard behavior of semiconducting gas sensors given by y=APk where P is the gas partial pressure which is directly proportional to concentration of the gas. The sensor sensitivity (S) is given by the first order derivative of the response curve as represented below:



𝑑𝑦

𝑆(%) =

𝑑𝑥

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|𝑥−𝑥 | 𝑃

= 𝐴𝑃 (𝑥−𝑥𝑐)

(2)

𝑐

The sensitivity was estimated to be 0.1125 at 600 ppm CO2 and this high sensitivity is attributed to large surface area of the material. To know the repeatability of the sensor, it was exposed to the CO2 of the same concentrations for 4 cycles (Figure 3c). The response of the sensor is repeatable with a small baseline drift. This baseline drift can be corrected with appropriate algorithms. Further, the response and recovery times of the sensor was determined to be 9-17 sec and 9-17 sec respectively in the CO2 concentration range of 400 ppm to 1025 ppm (Figure 3d). The response time (tr)of the reversible gas sensor is 1

𝐾

𝑘

1+𝑃𝑔 𝐾

𝑡𝑟 = ( ) ∗ (

)

(3)

Where k is forward rate constant, K is reverse rate constant and Pg is the partial pressure of the gas which is proportional to the concentration of the gas. For low concentrations of target gas, it can be

written as 𝑡𝑟 𝑙𝑖𝑚𝑃

𝑔 →0

= 𝑙𝑖𝑚𝑃𝑔 → 0

1 𝑘

𝐾 ) 1+𝑃𝑔 𝐾

( )∗ (

=

𝐾 𝑘

(constant)

(4)

Thus the response time of the sensor is constant irrespective of the concentration of the gas when the concentration of the gas is low. In this case we obtained an average response time of 13 sec with small fluctuations of +/- 4 sec. It is consistent with the results reported in the literature [31]. The lower limit of CO2 in air is around 380-400 ppm and we investigated the response of the sensor to CO2 even at 200 ppm. Thus, it is not necessary to find limit of detection of CO2. Moreover, it is the best report when we consider the combined parameters sensitivity, response and recovery times and the operating temperature (relatively low compared to other sensors whose operating temperature is ~ 400 0C) together. The CO2 gas sensing mechanism can be explained by oxygen vacancy principle. This principle is based on oxygen vacancies present on the metal oxide surface which determines the chemiresisting behavior of the sensor. ZnO is rich in oxygen deficiency and these vacancies act as electron donors and thus it behaves as an n-type semiconductor. The surface resistivity and


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subsequently the gas sensing behavior was controlled through alternate reduction-reoxidation of ZnO by oxygen gas [32-34]. It involves the following steps: (1) The oxygen reacts with positively oxygen vacancies to form oxygen ions at its lattice site. 𝑉𝑂•• + 2𝑒´ (𝐶𝐵) +

1 2

𝑂2 (𝑔𝑎𝑠) ↔ 𝑂𝑂⨯

(5)

This reaction reduces the conductivity of the sensor as the ZnO loses electrons in presence of oxygen environment. (2) The CO2 gas reacts with 𝑂𝑂⨯ , to form metastable Carbon tetroxide (CO3) [34]. 𝐶𝑂2(𝑎𝑑𝑠) + 𝑂𝑂⨯ ↔ 𝐶𝑂3 (𝑔𝑎𝑠) + 𝑉𝑂⨯

(6)

CO3 has a short lifetime and thus it further dissociates into CO2 and O2. (3) The neutral oxygen vacancy ionizes to give the electrons back to the conduction band 𝑉𝑂⨯ ↔ 𝑉𝑂• + 𝑒´

(7)

𝑉𝑂• ↔ 𝑉𝑂•• + 𝑒´

(8)

Thus, the adsorption of the CO2 gas on ZnO increase the free carrier concentration and the conductivity increases upon exposure to the CO2 gas. The drift current density through the sensor is 𝐽 = 𝑛𝑞𝜇𝐸, where n is the free electron density, q is the electron charge, μ is the mobility of the electron and E is the electric field. Thus, the current is directly proportional to the charge carrier density, provided the variations in scattering potential is negligible. Therefore, the current through the sensor increases with increase in the concentration of CO2. In other words, the current density can be written as 𝐽 =

𝐸𝐿 𝑅𝐴

, where A is area of cross section and L is the length of the sensing film

and R is the resistance of the film. Therefore, the resistance decreases with adsorption of CO2 gas.



Figure 4. (a) The response of the sensor as a function gas concentration at three different temperatures; (b),( c) corresponding heat mapping and contour plot; (d) cross-sensitivity of the sensor to other gases such as NH3 and H2S at 250 0C

To investigate the effect of temperature on the performance of the sensor, we further repeated the experiments at higher temperatures 300 0C and 350 0C. We observed that the response decreases at 300 0C and further increases to 350 0C as shown in Figure 4a and Figure S1, Supporting Information (SI). This behavior can be attributed to temperature dependent adsorption-desorption, reaction processes and carrier concentration [35]. All these parameters affect the number of electrons participated in the surface reaction and thus it is difficult to predict the behavior of the sensor with temperature. From 2D heat map in Figure 4b, it can be noted that the response of the sensor is nonlinear with temperature and the relation between temperature and response is unique for different concentrations and thus the sensor produces non-redundant response for the same gas concentration at different temperatures. This can be very useful in electronic nose applications because the temperature can be used as one of the independent parameters as it greatly reduces the number of sensors required to discriminate the different gases and quantify them. Moreover, it can be observed that there is no notable improvement in the response of the sensor even if we increase the temperature sensor by 100 0C. Therefore, the operating temperature of 250 0C is good in terms of both response and recovery times and power consumption if we want to use it at a single temperature.


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To investigate the effect of the relative humidity (RH) on sensor response, it was exposed to 1000 ppm CO2 gas at different RH levels (20 to 80). The response increases from 20% RH to 50% RH and then it decreases as shown in Figure 4(c). It can be explained with two different mechanisms [36]. (1) The first one is the dissociation of H2O and the reaction between neutral H atom and the lattice oxygen + − 𝐻2 𝑂𝑔𝑎𝑠 + 𝑍𝑛𝑍𝑛 + 𝑂𝑜 → (𝑍𝑛𝑍𝑛 − 𝑂𝐻 − ) + (𝑂𝐻)+ 𝑂+ 𝑒

(9)

+ where 𝑍𝑛𝑍𝑛 − 𝑂𝐻 − is isolated hydroxyl group and OH+ is rooted hydroxyl group. Due to low

electron affinity of rooted hydroxyl group, it ionizes and donates electron to the conduction band. (2) The second one is binding of zinc with hydroxyl groups resulted from reaction of hydrogen with lattice oxygen and subsequent ionization of electrons to produce oxygen vacancies (𝑉𝑂++ ). + 𝐻2 𝑂𝑔𝑎𝑠 + 2𝑍𝑛𝑍𝑛 + 𝑂𝑜 → 2(𝑍𝑛𝑍𝑛 − 𝑂𝐻 − ) + 𝑉𝑂++ + 2𝑒 −

(10)

From the above two mechanisms, it can be concluded that the adsorption of water increases the free carrier concentration and oxygen vacancies which in turn accelerates the adsorption of oxygen species on the zinc oxide [37,38]. The total concentration of adsorption sites [St] can be written as [St] = [St0] + k0 pH2O

(11)

where [St0] is the number of adsorption sites with no humidity and pH2O is the partial pressure of water vapor which is directly proportional to the relative humidity. Thus the increase in RH results in increase in adsorption sites and variation in coverage rate of oxygen species and hydroxyl groups. At low RH levels, the oxygen species and hydroxyl groups covered on the film are relatively less and CO2 find less number of oxygen species to react with and thus the response is relatively low. The oxygen species covered on the film increases with increase in RH and subsequently CO2 react with more oxygen species and subsequently response of the sensor increases. However, beyond certain humidity level (50% RH), the large coverage of hydroxyl groups on the film limit the coverage of oxygen species which leads to decrease in the response. One of the most important requirements of the gas sensing system is the cross sensitivity of the sensor to other interference gases. It is well known that ZnO is cross-sensitive to other gases like ammonia (NH3) and hydrogen sulfide (H2S). However, the cross-sensitivity to more gases is a necessary criterion to design the gas sensors in an array to detect and quantify the multiple gases



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simultaneously. To know the performance of the sensor towards other gases it was exposed to different gases at different concentrations at 250 0C. We used lower concentration for other gases as their concentration is very low compared to CO2 in the environment. It was observed that the response of the sensor towards 1000 ppm is sufficiently higher than that of others as shown in Figure 4(d). Therefore, this sensor can be used for sensing CO2 selectively in indoor and outdoor environment. Table 1. Comparison of ZnO NF sensor with other sensors reported in the literature (The values are either reported or calculated from the results reported)

Material

Temperature (0C)

Concentration of the gas (ppm)

Sensitivity (% ppm-1)

Response time

Recovery time

Reference

Poly(Ionic liquid) Nanoparticle CaO-In2O3

Room Temperature

150-2400

~ 0.004

~ 40 min

[39]

230

300-5000

~ 0.03

Room Temperature

50-5000

~ 0.00714

> 1000 sec >10 min

[22]

PEIfunctionalized PANI film LaOCl – SnO2 Reduced Graphene Oxide PEI/starch functionalized carbon nanotubes 50% La loaded ZnO

35 min (300 ppm) >1000 sec >10 min

400 Room Temperature

250-4000 0-1500

0.0333 0.0118

3-20 sec 4 min

4-19 sec 4 min

[23] [41]

Room Temperature

500-100000

0.000101

~ 15 sec (100 % CO2)

~ 15 sec (100 % CO2)

[42]

400

500-2500

~0.0035

380 250 250

500-2000 2000-50000 500-10000

< 0.02 ~0.015 ~0.0038

38 sec (5000 ppm) 15 min 300 sec 10 min

[24]

La1-x Srx FeO3 CdO Ag-BaTiO3CuO ZnO:Ca ZnO thin film

90 sec (5000 ppm) 11 min 200 sec 15 min

450 300

0-10000 2000-10000

~0.008 ~0.0005

300 250

0-10000 200-1025

0.001 0.1125

~10 sec 20-50 sec ~ 10 sec 9-17 sec

[28] [29]

ZnO:Al:Ca ZnO nanoflakes

~10 sec 10-20 sec ~10 sec 9-17 sec


[40]

[25] [26] [27]

[30] This work

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The CO2 sensing performance of ZnO nanoflakes was compared with that of emerging nanomaterials such as Graphene, CNT, Polyaniline and other 1-D metal oxide nanostructures at similar concentration levels. Higher sensitivity and ultra-fast detection are very important at this concentration from the human safety and health perspective. This comparison cites research reports that have utilized both undoped and doped materials. As depicted in Table 1, the reported sensor outperforms other sensors in the literature as it has high sensitivity as well as short response and recovery times. These results indicate that the reported CO2 sensors based on ZnO nanoflakes that were prepared at low temperature can be effective to achieve superior sensing performance compared to their counterparts.

Conclusion In summary, we have developed an ultra-fast responsive, highly sensitive and reversible chemiresistive CO2 gas sensor based on ZnO nanoflakes synthesized by simple and facile wet chemical method at low temperature. The sensor exhibited an exponential response with a sensitivity of 0.1125 to 600 ppm and sub-20 sec response and recovery times in the range of 4001025 ppm CO2 concentration. The high sensitivity of the sensor can be attributed to the large surface to volume ratio of the thin two dimensional nanoflakes. The response and recovery times of the sensors are very low so that they can be used in many practical applications that include safety, health and environment monitoring. Further, we discussed the effect of temperature on the response of the sensor and observed that the variation of the sensor response is nonlinear with temperature and hence the temperature can be used as one of the independent parameters to differentiate different gases and quantify them with less number of sensors. This can greatly reduce the hardware complexity. Finally, this study reports a sensor which has better sensitivity as well as better response and recovery times compared to other sensors reported in the literature.

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.xxxxx. XRD calculations, response of the sensor at different temperatures, selectivity and parameters that affect the performance of the sensor (PDF).



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Chemiresistive Sensor Based on Zinc Oxide Nanoflakes for CO2

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