Research on gas-sensing properties of lead sulfide-based sensor for detection of NO2 and NH3 at room temperature

Sensors and Actuators B 140 (2009) 116–121

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

Research on gas-sensing properties of lead sulfide-based sensor for detection of NO2 and NH3 at room temperature Tiexiang Fu ∗ School of Chemistry and Bioengineering, Changsha University of Science & Technology, Chiling Road, Changsha 410077, Hunan, PR China

a r t i c l e

i n f o

Article history: Received 20 November 2008 Received in revised form 28 March 2009 Accepted 31 March 2009 Available online 10 April 2009 Keywords: Nitrogen dioxide Ammonia Gas-sensing properties PbS Sensor

a b s t r a c t In the present paper, the gas-sensing characteristics of novel gas sensors based on PbS were investigated. The sensors exhibited high responses to NO2 and NH3 at room temperature. No response to other gases was observed. The response magnitude of sensors based on PbS prepared with Na2 S at 50 ◦ C was 266.4 to NO2 at operating voltage 5 V. The response and recovery time was about 54 and 43 s, respectively. The response magnitude of PbS prepared with Na2 S at 80 ◦ C was about 301 to NH3 at operating voltage 5 V. The response and recovery time was about 46 and 67 s, respectively. The influence of the working temperature and operating voltage on the sensor response was also studied. The sensing mechanism of the sensor was discussed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Nitrogen dioxide (NO2 ) and ammonia (NH3 ) are foul-smelling and harmful gases. Nitrogen dioxide is created by the high temperature combustion of coal, chemical production, natural gas or oil in power plants and also by the combustion of gasoline in internal combustion engines. One of the consequences of NO2 being released into the atmosphere is the formation of photochemical smog. In addition, NO2 is a cause of acid rain and is involved in the depletion of ozone in the stratosphere. Ammonia is the most abundant alkaline component in the atmosphere, and it has been focused air quality regulatory attention on the livestock and poultry industries [1,2]. Other typical sources of ammonia include fertilizers, soils and production of chemicals, etc. The importance of the detection of these gases is evident. There are needs for nitrogen dioxide and ammonia sensors are used in many situations including leakdetection in air-conditioning systems and environmental sensing of trace amounts ambient NH3 or NO2 in air, and the automatic control of the chemical engineering production process involving ammonia or nitrogen dioxide [3]. Generally, because these gases are toxic, it is necessary to be able to sense low levels (∼ppm) of NO2 or NH3 , but the ability to sense high levels (∼%) of NO2 and NH3 gases should also be required in certain areas such as the automatic control of the chemical engineering production process. Sensors based on metal oxide [4–14], porous silicon [15], SiC [3] and carbon nanotubes [16–18] have been widely used for the

detection of NO2 or NH3 . But the majority of the sensors showed a high response and good selectivity to NH3 or NO2 only at high temperatures. In recent years, considerable research has been done to investigate the NO2 - or NH3 -sensing properties of metal complex [19–21]. A highly selective NH3 sensor based on potassium trisoxalateferrate(III) complex [22] and a NO2 gas sensor based on complex [Cr(bipyO2 )Cl2 ]Cl [23] have also been reported by our group. Lead sulfide (PbS) is normally used as optical and semiconductor materials [24]. The practical application of PbS in a gas sensor has only been studied by Y. Shimizu and his group. They carried out systematically research determine the gas-sensing properties of the Pb1−x Cdx S (x = 0.1, 0.2)-based and the metal-mono sulfidebased (NiS, CdS, SnS and PbS) solid electrolyte sensor elements. The sensor elements gave good SO2 sensitivity at 300–400 ◦ C [25,26]. But there is no report about the NO2 - and NH3 -sensing properties of PbS till date. In present paper, the gas-sensing characteristics of PbS were investigated for the need to develop sensitive, reliable and low cost sensors for toxic gases. Novel gas sensors were fabricated based on a single component of PbS layer and their responses to toxic gases were studied. The influences of preparation conditions of PbS on the sensor response and selectivity were also investigated. 2. Experimental 2.1. Preparation of lead sulfide

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Na2 S used for precipitating agent. Lead acetate (10 g) was dissolved in 100 mL of water, and 80 mL of a Na2 S solution (0.38 mol/L)

T. Fu / Sensors and Actuators B 140 (2009) 116–121

117

Fig. 1. Schematic structure of the sensor.

was slowly dropped into this lead acetate solution (0.3 mol/L) and then stirred at a temperature of 50 ◦ C. The resulting precipitate was washed with distilled water several times and filtered. Finally, PbS particles were dried 2 h at 150 ◦ C. Another kind of PbS particle was prepared at reaction temperature of 80 ◦ C similar to the method and procedure described above. H2 S used for precipitating agent. 0.03 mol H2 S was slowly injected into 100 mL of lead acetate solution (0.3 mol/L) and vigorously stirred at temperature 50 ◦ C. The resulting precipitate was washed with distilled water several times and filtered. Finally, PbS particles were dried 2 h at 150 ◦ C. Another kind of PbS particle was prepared similar to the method and procedure described above, only with a reaction temperature of 80 ◦ C. 2.2. Sensor fabrication PbS particles were mechanically milled in a PVA (polyvinyl alcohol) medium using an agate pestle and mortar for 4 h to form a paste. The paste was coated on an aluminum oxide tube on which a pair of interdigited electrodes made of graphite film was previously coated and two platinum leads had been installed on different graphite films. The element was dried at 105 ◦ C for 4 h. The thickness of the PbS layer as a gas-sensing layer was about 5 ␮m. A platinum heating wire having a 0.05 mm diameter was inserted to form a side-heated gas sensor (see Fig. 1). Finally, the sensing element was soldered on a pedestal with a vent hole, and then was cased in a plastic vessel with a reticulate vent. Four kinds of PbS were used to make the sensors under the same process conditions. 2.3. Measurement of the sensing characteristics Because the sensor’s interelectrode gap influences the electrical resistance and response to gases, it is very important to take this fact into account and make comparisons only for devices having identical interelectrode gaps. Therefore, sensors having an interelectrode gap of 0.1 mm only were studied. The graphic measuring principle is shown in Fig. 2. Measurements of gas-sensing properties were carried out using a system with an airtight chamber of 10 L. To ensure the environment in the airtight chamber was nearly at atmospheric pressure, a dilatation film was connected to the airtight chamber. The dilatation film could be distended when the chamber was overly filled with sample gases. The sample gases could also circulate by using a miniature fan installed in the airtight chamber. The sensor element was exposed to sample gases of variable concentrations, containing a known amount of air. During the sensing measurement, the sample gas concentrations were altered by injecting sample gases into the airtight chamber using a sample injector. A heating voltage (Vh ) was supplied to the coil for heating the sensor and the working temperature of the sensor element was

Fig. 2. Electric circuit for gas-sensing measurement.

changed from room temperature to 85 ◦ C by controlling the heater voltage. Operating voltages (VO ) of 2 V, 5 V, 10 V and 20 V dc were applied across the circuit, and the voltage outputs (VS ) across the sampling resistor were recorded (range: from 0.1 to 2000 mV). The electrical resistance of a sensor was measured in sample gases and also in air. The transformational relation between the resistance (R) of a sensor and the output voltage of the sampling resistor in circuit is given by the following formula: R = (VO − VS )RS /VS where RS is the resistance values of the sampling resistor. The electrical resistance of a sensor was measured in sample gases and also in air. The sensor response was defined as RS = Ra /Rg , where Ra is the baseline resistance of the sensor in dry air and Rg is the resistance value in the presence of a gas to be measured. To examine reproducibility of the sensor response, the sensor was exposed alternately to 0.9 and 1.2% NO2 (or 3.1 and 6.5% NH3 ) gas and to dry air at room temperature for a repeated five cycles alternately. The dry air and the NO2 (or NH3 ) gas were maintained under constant conditions during the measurements. A calibration curve, the relation between sensor response and NO2 (or NH3 ) concentration, was obtained by changing the NO2 (or NH3 ) gas concentration from 0.004 to 9.23% at room temperature. The response or recovery time is the time for the voltage change to reach 90% of the total change from V(out)a to V(out)g or vice versa. The response and recovery time of sensors depended on the thickness of the gas-sensing layer. The thinner the layer, the shorter the response or recovery time was, in general. However, the responses to NO2 and NH3 decreased when the thickness of the gas-sensing layer was too thin. The response and recovery time of the sensor in the present study were all measured at the optimizing thickness of gas-sensing layer, about 5 ␮m. Two sensors were heated for a week continuously, and then the stability of the sensors was examined by comparing the data of sensing properties at room temperature before and after the heating.

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3. Results and discussions 3.1. Gas-sensing properties 3.1.1. Sensor response and selectivity Fourteen kinds of sample gases such as ammonia (NH3 ), water vapor (H2 O), hydrogen sulfide (H2 S), sulfur dioxide (SO2 ), nitrogen dioxide (NO2 ), formaldehyde (HCHO), methanol (CH3 OH), ethanol (C2 H5 OH), acetone (CH3 COCH3 ), benzene (C6 H6 ), toluene (C7 H8 ), ethyl ether (C4 H10 O), tetrahydrofuran (C4 H8 O) and chloroform (CHCl3 ) were tested. The concentrations of these gases were changed from 0 to 9.23% (the ratio of sample gas volume to air volume). The sensor responses to sample gases were measured at nine working temperatures (25, 30, 35, 40, 45, 55, 65, 75, 85 ◦ C) and four operating voltages (2, 5, 10 and 20 V), and the measurements were replicated five times for each operating voltage and each temperature. The results obtained from the sensor response study are shown in Fig. 3(a) and (b). Only representative data at room temperature and 5 V operating voltage are shown in Fig. 3(a) and (b), respectively. Fig. 3 shows that sensors had a very high response to NH3 and NO2 gases at room temperature. No response to H2 S, SO2 , water vapor (H2 O) and organic solvent vapors was observed. However, the sensor responses to NH3 and NO2 depend on the PbS preparation condition. The response magnitude of the sensors based on PbS prepared with Na2 S at 50 ◦ C was about 266.4 to NO2 and 21.2 to NH3 (see Fig. 3(a)), respectively. The response magnitude of PbS prepared with Na2 S at 80 ◦ C was about 105 to NO2 and 296 to NH3 (see Fig. 3(b)), respectively. The experimental results of antiinterference at room temperature and 5 V operating voltage showed that, as long as the concentration of NO2 and NH3 is more than 0.6%, other gases will not interfere with the determination of NH3 and NO2 . The behavior of the sensors is also very similar at other operating voltages and other working temperatures. However, NH3 will not interfere with the determination of NO2 only with the sensors based on PbS prepared with Na2 S at 50 ◦ C. 3.1.2. Sensor response to NO2 To investigate the influences of preparation condition of PbS on the sensor response, two cheap reagents, hydrogen sulfide and sodium sulfide were used as precipitation agents in order to reduce sensor costs and two temperatures, 50 and 80 ◦ C, were chosen to examine the effects of temperature. The temperature selection considers two points: First, control the temperature below the boiling point to prevent the solution was evaporated excessively resulting in concentration changes. Second, is separated evenly between the room temperature and the boiling point. Sensors fabricated from PbS prepared under four conditions (Na2 S at 50 ◦ C, Na2 S at 80 ◦ C, H2 S at 50 ◦ C and H2 S at 80 ◦ C) were studied at four operating voltages (2, 5, 10 and 20 V). The highest values of response to NO2 at room temperature are shown in Table 1. All sensors are sensitive to NO2 . The most responsive sensor to NO2 was that based on PbS prepared with Na2 S at 50 ◦ C. This sensor had the highest response value at 5 V operating voltages. The response and recovery time of sensors at optimal thickness of gas-sensing layer (5 ␮m) are about Table 1 Sensor’s highest value of response to NO2 in a room temperature (each highest response value appears at a different concentration). Preparation of PbS with

◦

Na2 S at 50 C Na2 S at 80 ◦ C H2 S at 50 ◦ C H2 S at 80 ◦ C

Operating voltage (V) 2

5

10

20

96.1 37.6 51.2 72.9

266.4 105.0 84.9 232.7

82.7 64.2 17.7 49.2

32.5 50.4 13.7 18.8

Fig. 3. Typical responses to different test gases of two sensors based on PbS at room temperature and 5 V operating voltage. (a) PbS were prepared with Na2 S at 50 ◦ C, and (b) using Na2 S at 80 ◦ C.

54 and 43 s, respectively. The average relative deviation was about 2.8%. The second most responsive sensor to NO2 was the sensor based on PbS prepared with H2 S at 80 ◦ C. The response and recovery time of the sensor was about 135 and 98 s, respectively. The average relative deviation was about 7.3%. The influences of precipitation agents and temperature of preparation PbS on the sensor response were not clearly explained. One of possible reasons is that sulfur ion concentration, chemical reaction rate and temperature influence the surface area and the activation absorption center (position and quantity) of precipitates. The sensor response to NO2 depends on the gas concentration. The sensor response curve to NO2 at the optimal operating voltage, based on PbS prepared with Na2 S at 50 ◦ C, is shown in Fig. 4.

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Fig. 4. The correlation between NO2 gas concentration and responses of the sensors based on PbS was prepared with Na2 S at 50 ◦ C at the optimal operating voltage and in a room temperature.

The variations of the sensor response may be divided into three stages. Before 0.25% NO2 , the response increased slightly with an increase in NO2 concentration. When the NO2 concentration was between 0.25and 1.55%, the response increased rapidly from 6.5 to 266. When the NO2 concentration exceeded 1.55%, there is a slight decrease with increasing NO2 concentration. Other sensor’s response curves also indicate similar responses to NO2 gas, respectively. The operating voltage had an influence on sensor’s response to NO2 gas at room temperature. All the sensor responses to NO2 increased and then decreased with an increase in operating voltage at room temperature. The optimum operating voltage of sensor was 5 V. 3.1.3. Sensor response to NH3 Table 2 shows the highest values of sensor’s response to NH3 at room temperature. From Table 2, we can see that the most responsive sensor to NH3 was that based on PbS prepared with Na2 S at 80 ◦ C. This sensor had the highest response value at 5 V operating voltages. The response and recovery time was about 46 and 67 s, respectively. The average relative deviation was about 3.6%. The sensor based on PbS prepared with H2 S at 50 ◦ C had a very high response to NH3 , too. The response and recovery time was about 78 and 94 s, respectively. The average relative deviation was about 3.4%. Comparison with the sensor response to NO2 , the temperature and the precipitating agents of preparation PbS have an opposite influence to the sensor response to NH3 . The possible reasons are Table 2 Sensor’s highest value of response to NH3 in a room temperature (each highest response value appears at a different concentration). Preparation of PbS with

◦

Na2 S at 50 C Na2 S at 80 ◦ C H2 S at 50 ◦ C H2 S at 80 ◦ C

Operating voltage (V) 2

5

10

20

11.8 12.9 12 11.2

21.2 296.0 225.1 27.2

14.3 69.4 43.6 18

17.1 31.9 17.5 15.2

119

Fig. 5. The correlation between NH3 gas concentration and responses of the sensors based on PbS was prepared with Na2 S at 80 ◦ C at the optimal operating voltage and in a room temperature.

that NH3 and NO2 on the surface of PbS absorption ways and means of electron transfer are different. The variations in response of the sensor based on PbS prepared with Na2 S at 80 ◦ C with the concentration of NH3 at the optimal operating voltage are also shown in Fig. 5. The response curve to NH3 at 5 V operating voltage may be divided into two stages. Before 7.08% NH3 , the response increased rapidly from about 1 to 296. When the concentration exceeded 7.08% NH3 , it slightly increased with an increase in the NH3 concentration. The operating voltage also had an influence on sensors response to NH3 gas at room temperature. The sensor responses to NH3 all increased and then decreased as the operating voltage increasing. The sensor responses to NH3 at 2 V operating voltage were lower than those at 20 V. The optimum operating voltage of the sensor was also 5 V. 3.2. Relationship between gas response and temperature The temperature dependences of the sensor response at 5 V operating voltage are presented in Fig. 6. Fig. 6(a) shows that sensor based on PbS prepared with Na2 S at 50 ◦ C in responses of NO2 slightly decreased as the temperature increased from room temperature to 47 ◦ C. After that, the responses decreased sharply. When the temperature exceeded 75 ◦ C, the sensor did not respond to NO2 . Fig. 6(b) shows that responses to NH3 of the sensor based on PbS prepared with Na2 S at 80 ◦ C slightly decreased as the temperature increased from room temperature to 42 ◦ C. After that, the responses decreased sharply. When the temperature exceeded 60 ◦ C, the sensor did not respond to NH3 . Similar results were also obtained from the studies on the temperature dependence of responses of the sensor based on PbS prepared under other conditions to NO2 and NH3 . Two sensors were aged a week by being heated continuously and keeping their temperature at 120 ◦ C by an electric heater of platinum wire to test the stability of the sensor. Then the sensor response measurements of NO2 and NH3 gases were replicated

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Fig. 6. The responses as a function of temperature for two sensors based on PbS at operating voltage 5 V. PbS was prepared under different conditions: (a) with Na2 S at 50 ◦ C, sensor responses to NO2 ; (b) with Na2 S at 80 ◦ C, sensor responses to NH3 .

at five concentrations. The measurement results showed popularly consistent with what was done above. The average relative deviations were in the range of about ±2.7 to NO2 and ±3.8% to NH3 , respectively. The sensors’ electrical resistance in air changed very poorly. The stability of the gas-sensing layers’ resistance in air ensures a stable level for the gas sensors’ applications. 3.3. Discussion about the gas-sensing mechanism It is known that each NO2 molecule has one lone electron pair and ␲-bonding electrons and each NH3 molecule has two lone electron pairs, which can be donated to other species. Therefore, NO2 and NH3 are donors. When the sensor is exposed to NO2 or NH3 gas, the gas molecules are adsorbed on the surface of the gas-sensing

layer and the weak coordinate bonds can be formed between NO2 or NH3 and lead(II) in PbS molecules. Electrons are transferred from NO2 or NH3 to the gas-sensing layer and decreasing sensors electrical resistance. The responses should increase with an increase in NO2 or NH3 concentration until reaching the absorption saturation. The comparison between the response curve to NH3 and theoretical presumption shows a good agreement with each other. The saturation response point corresponds to the saturation point of gases adsorbed. However, the response curve to NO2 is not the same. The response increased significantly with an increase in concentration until the NO2 gas concentration exceeded a certain value, as it can be seen in Fig. 4. The relation curves between the response and gases concentration show a saturation point and an inflexion. The response saturation point corresponds to the saturation point of gases adsorbed, but the inflexion phenomenon shows that the sensing layer’s gas adsorption may be divided into two stages. First, as a result of Van der Waals attraction, when the distance between NO2 molecules and the surface of the gas-sensing layer is shortened, the potential energy is decreased to the minimum of a nadir. At this stage, it is a physical adsorption and few electrons transfer to the layers to make the electrical resistance decrease. Second, when the distance is shortened further, there is a deeper potential well which has a more steady adsorption, a weak coordination adsorption. The weak coordinate bonds can be formed with transferring electrons partly from NO2 to the gas-sensing layers and thus decreases the electrical resistance of the layer at this state. There is a threshold value of concentration between the first and second stage. Only when the gas concentration exceeds the threshold value will the second adsorption appear. The responses to NO2 increase sharply with an increase in concentration. Since the operating voltage is necessary for driving electrons in gas-sensing layers, the operating voltage is one of the main factors that influences the sensor response to NO2 and NH3 gases. However, when the operating voltage is too high, the sensor responses to NO2 and NH3 gases will decrease. The reason for this behavior is obvious. An increase in operating voltage results in an increase in electron concentrations in the layer and at grain boundaries. Since there is electrostatic repulsion between electronic excess negative charges in the layer and gaseous NO2 or NH3 molecules, the amount of NO2 and NH3 adsorption quantities in the layers decreases. As a consequence, a decrease in response to NO2 or NH3 of the gassensing layer is observed. It is understandable that temperature affected the sensor response. Increasing temperature can speed up gas molecules’ motion to weaken the weak coordination adsorption. As a result, the sensor responses decreased because of a decrease in the amount of molecules adsorbed and electrons transferred from NO2 or NH3 to the gas-sensing layer. 4. Conclusions In summary, The response and selectivity of four sensor types based on PbS prepared under four conditions were tested at different temperatures and operating voltages. These sensors showed high response to NO2 and NH3 at room temperature. No response to H2 S, SO2 , water vapor (H2 O) and organic solvent vapors was obtained. The best response to NO2 was the sensor based on PbS prepared with Na2 S at 50 ◦ C. The response value reached 266 to 1.55% of NO2 at 5 V operating voltage. The best response to NH3 was the sensor based on PbS prepared with Na2 S at 80 ◦ C. The response value reached 301 to 8.08% of NH3 at 5 V operating voltage. The response and recovery time was generally satisfactory. Such sensors have the advantages of being simple to fabricate and having cheaper prices.

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Biography Tiexiang Fu is currently a professor at the School of Chemistry and Bioengineering, Changsha University of Science & Technology. His current research interests are orientation complexes, gas sensors and electronic nose systems.