The response magnitude of sensors based on PbS prepared with Na2S at 50◦C was 266.4 to NO2at operating voltage 5 V.. The response and recovery time of sensors at optimal thickness of gas
Trang 1Tiexiang 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 xxx
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 NO2and NH3at room temperature No response to other gases was observed The response magnitude of sensors based on PbS prepared with Na2S at 50◦C was 266.4
to NO2at operating voltage 5 V The response and recovery time was about 54 and 43 s, respectively The response magnitude of PbS prepared with Na2S at 80◦C was about 301 to NH3at 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
temper-ature combustion of coal, chemical production, natural gas or oil
in power plants and also by the combustion of gasoline in
inter-nal combustion engines One of the consequences of NO2 being
released into the atmosphere is the formation of photochemical
smog In addition, NO2is a cause of acid rain and is involved in the
depletion of ozone in the stratosphere Ammonia is the most
abun-dant alkaline component in the atmosphere, and it has been focused
air quality regulatory attention on the livestock and poultry
indus-tries[1,2] Other typical sources of ammonia include fertilizers,
soils and production of chemicals, etc The importance of the
detec-tion of these gases is evident There are needs for nitrogen dioxide
and ammonia sensors are used in many situations including
leak-detection in air-conditioning systems and environmental sensing of
trace amounts ambient NH3or NO2in 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 NO2or NH3, but
the ability to sense high levels (∼%) of NO2and NH3gases 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
∗ Tel.: +86 7315726803.
E-mail address:ftxjy1@163.com
detection of NO2 or NH3 But the majority of the sensors showed
a high response and good selectivity to NH3or NO2only at high temperatures In recent years, considerable research has been done
to investigate the NO2- or NH3-sensing properties of metal com-plex [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 semiconduc-tor 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−xCdx S (x = 0.1, 0.2)-based and the metal-mono
sulfide-based (NiS, CdS, SnS and PbS) solid electrolyte sensor elements The sensor elements gave good SO2sensitivity 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 Na2S used for precipitating agent Lead acetate (10 g) was
dis-solved in 100 mL of water, and 80 mL of a Na2S solution (0.38 mol/L) 0925-4005/$ – see front matter © 2009 Elsevier B.V All rights reserved.
doi: 10.1016/j.snb.2009.03.075
Trang 2Fig 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
H 2 S used for precipitating agent 0.03 mol H2S 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
alco-hol) 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
previ-ously 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 5m A platinum
heating wire having a 0.05 mm diameter was inserted to form a
side-heated gas sensor (seeFig 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
inter-electrode gap of 0.1 mm only were studied The graphic measuring
principle is shown inFig 2 Measurements of gas-sensing
prop-erties 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
vari-able 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 RSis the resistance values of the sampling resistor The elec-trical resistance of a sensor was measured in sample gases and also
in air The sensor response was defined as RS = Ra/Rg, where Rais the
baseline resistance of the sensor in dry air and Rgis 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)ato V(out)gor vice versa The response and recovery time of sensors depended on the thick-ness of the gas-sensing layer The thinner the layer, the shorter the response or recovery time was, in general However, the responses
to NO2and NH3decreased 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 5m
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 heat-ing
Trang 3and four operating voltages (2, 5, 10 and 20 V), and the
measure-ments were replicated five times for each operating voltage and
each temperature
The results obtained from the sensor response study are shown
inFig 3(a) and (b) Only representative data at room temperature
and 5 V operating voltage are shown inFig 3(a) and (b),
respec-tively.Fig 3shows that sensors had a very high response to NH3
and NO2gases at room temperature No response to H2S, SO2, water
vapor (H2O) and organic solvent vapors was observed However, the
sensor responses to NH3and NO2depend on the PbS preparation
condition The response magnitude of the sensors based on PbS
prepared with Na2S at 50◦C was about 266.4 to NO2 and 21.2 to
NH3(seeFig 3(a)), respectively The response magnitude of PbS
prepared with Na2S at 80◦C was about 105 to NO2 and 296 to
NH3(seeFig 3(b)), respectively The experimental results of
anti-interference at room temperature and 5 V operating voltage showed
that, as long as the concentration of NO2and NH3is more than 0.6%,
other gases will not interfere with the determination of NH3and
NO2 The behavior of the sensors is also very similar at other
oper-ating voltages and other working temperatures However, NH3will
not interfere with the determination of NO2only with the sensors
based on PbS prepared with Na2S at 50◦C
3.1.2 Sensor response to NO 2
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
con-siders 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 (Na2S at 50◦C, Na2S at 80◦C,
H2S at 50◦C and H2S at 80◦C) were studied at four operating
volt-ages (2, 5, 10 and 20 V) The highest values of response to NO2at
room temperature are shown inTable 1 All sensors are sensitive
to NO2 The most responsive sensor to NO2was that based on PbS
prepared with Na2S 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 (5m) are about
Table 1
Sensor’s highest value of response to NO 2 in a room temperature (each highest
response value appears at a different concentration).
Preparation of PbS with Operating voltage (V)
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 Na 2 S at 50 ◦ C, and (b) using Na 2 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 H2S 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 influ-ence the surface area and the activation absorption center (position and quantity) of precipitates
The sensor response to NO2depends on the gas concentration The sensor response curve to NO2 at the optimal operating volt-age, based on PbS prepared with Na2S at 50◦C, is shown inFig 4
Trang 4Fig 4 The correlation between NO2 gas concentration and responses of the sensors
based on PbS was prepared with Na 2 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 NO2concentration When the NO2concentration 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 NO2concentration Other sensor’s
response curves also indicate similar responses to NO2gas,
respec-tively
The operating voltage had an influence on sensor’s response
to NO2gas 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 NH 3
Table 2shows the highest values of sensor’s response to NH3at
room temperature FromTable 2, we can see that the most
respon-sive sensor to NH3was that based on PbS prepared with Na2S 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 H2S 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 NH 3 in a room temperature (each highest
response value appears at a different concentration).
Preparation of PbS with Operating voltage (V)
Fig 5 The correlation between NH3 gas concentration and responses of the sensors based on PbS was prepared with Na 2 S at 80 ◦ C at the optimal operating voltage and
in a room temperature.
that NH3and NO2on 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 Na2S at 80◦C with the concentration of NH3 at the optimal operating voltage are also shown inFig 5 The response curve to
NH3at 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 NH3concentration
The operating voltage also had an influence on sensors response
to NH3gas at room temperature The sensor responses to NH3all increased and then decreased as the operating voltage increasing The sensor responses to NH3at 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 inFig 6.Fig 6(a) shows that sen-sor based on PbS prepared with Na2S at 50◦C in responses of NO2 slightly decreased as the temperature increased from room tem-perature 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 NH3of the sensor based on PbS prepared with Na2S 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 sen-sor 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 NO2and
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
Trang 5Fig 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 Na 2 S
at 50◦C, sensor responses to NO 2 ; (b) with Na 2 S at 80◦C, sensor responses to NH 3
at five concentrations The measurement results showed
popu-larly 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 NO2molecule has one lone electron pair
and-bonding electrons and each NH3molecule has two lone
elec-tron pairs, which can be donated to other species Therefore, NO2
and NH3are donors When the sensor is exposed to NO2or NH3gas,
the gas molecules are adsorbed on the surface of the gas-sensing
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 dis-tance between NO2molecules and the surface of the gas-sensing layer is shortened, the potential energy is decreased to the min-imum of a nadir At this stage, it is a physical adsorption and few electrons transfer to the layers to make the electrical resis-tance decrease Second, when the disresis-tance is shortened further, there is a deeper potential well which has a more steady adsorp-tion, a weak coordination adsorption The weak coordinate bonds can be formed with transferring electrons partly from NO2to 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 concen-tration exceeds the threshold value will the second adsorption appear The responses to NO2increase 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 NO2and NH3gases However, when the operating voltage is too high, the sensor responses to
NO2 and NH3gases 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 NO2or NH3molecules, the amount
of NO2and NH3adsorption quantities in the layers decreases As
a consequence, a decrease in response to NO2or NH3of the gas-sensing 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 NO2or 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 dif-ferent temperatures and operating voltages These sensors showed high response to NO2and NH3at room temperature No response
to H2S, SO2, water vapor (H2O) and organic solvent vapors was obtained The best response to NO2was the sensor based on PbS prepared with Na2S at 50◦C The response value reached 266 to 1.55% of NO2at 5 V operating voltage The best response to NH3was the sensor based on PbS prepared with Na2S 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 sen-sors 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
Bioengineer-ing, Changsha University of Science & Technology His current research interests are orientation complexes, gas sensors and electronic nose systems.