Synthesis of Star-Shaped Lead Sulfide (PbS) Nanomaterials and theirs Gas-Sensing Properties

Star-shaped PbS nanomaterials are synthesized by a hydrothermal method. Morphology and structure of the PbS nanomaterials are analyzed by SEM, HRTEM and XRD. Gas-sensing properties of the as-prepared PbS sensor are also systematically investigated. The results show star-shaped PbS nanostructure consists of four symmetric arms in the same plane and demonstrate good crystallinity. With the increase of ethanol concentration, the sensitivity of the PbS sensor significantly increases and demonstrates an almost linear relationship at the optimal operating temperature of 400 oC. Moreover, the fast response-recovery towards ethanol is also observed, which indicates its great potential on ethanol detection.


Introduction
The leak of flammable, toxic and explosive gases in industrial processes will cause serious environmental and safety problems 1 .Many traditional detection methods, such as gas chromatography, mass spectrometers, and optical spectroscopy etc., have been used for the accurate quantification of these gases.However, the use of these techniques often requires expensive and bulky instruments, skilled operators and tedious sample preparation processes.Therefore, the design of miniaturized smart sensors for fast and real-time detection has become an important and urgent strategy in the modern industrial production 2,3 .
Up to date, several types of gas sensors, including semiconductor gas sensors, liquid electrolyte gas sensors, solid electrolyte gas sensors and catalytic combustion gas sensors etc., have been developed for commercial applications 4 .A number of gas-sensing materials, such as semiconductor, intrinsically conducting polymer, conducting polymer composite etc. have been adopted in these sensors based on various transduction units, for example, chemiresistive, surface acoustic wave, quartz crystal microbalance, optical transducers and metal-oxide-semiconductor field-effect transistor (MOSFET) 5 .Among them, chemiresistive gas sensors based on semiconductor have been subjected to extensive research and development due to their advantageous features such as low cost, high sensitivity, fast response/ recovery time, and simplicity in device structure and circuitry 6 .
With the fast development of nanotechnology, many nanosized semiconductors, including SnO 2 , ZnO, ZnS, WO 3 , TiO 2 etc., have been fabricated and employed as gas sensors 7 .Lead sulfide (PbS) is an important IV-VI semiconductor with a narrow band gap energy (0.41eV) and large excition Bohr radius (18 nm), And it has been widely studied in many fields, such as solar cells, IR photodetectors and display devices etc. 8 .As previous report, nanosized semiconducting materials usually exhibit a wide range of electrical and optical properties that depend sensitively on both morphologies and sizes 9 .Hence, much attentions have been paid on synthesis of a variety of PbS nanostructures, such as nanoparticles 10 , quantum dots 11 , nanorods 12 , nanobelts 13 nanowires 14 , cubes 15 , dendrites 16 , star-like 17 and hierarchical structures 18 etc.Although we always stress that the morphology and size of nanomaterials significantly affect their properties, the internal relationship is not yet clear and still needs to be investigated for the further development of high-performance electrical and optical devices 19 .Recently, PbS nanomaterials have been successfully applied as promising candidates for fabricating gas sensors by a few research groups.Markov and Maskaeva reported their works on PbS films used for nitrogen oxides detection 20 .Shimizu et al. fabricated the metal-mono sulfide based (NiS, CdS, SnS and PbS) solid electrolyte sensors, and found their good SO 2 sensitivity 21,22 .Kaci et al. synthesized PbS thin films by modified polyol process on amorphous a-SiC and p-Si(100) substrates, which exhibited highly sensitive with fast response to H 2

23
. Fu obtained PbS sensors with high responses to NO 2 and NH 3 24 .Ethanol detection are great important in many fields including control of fermentation processes, safety testing of food packaging, and monitoring drunken driving etc. 25 .However, to the best of our knowledge, there is not any study about the detection of ethanol gas using PbS gas sensors.In this work, we synthesized star-shaped PbS nanomaterials using a simple hydrothermal method, investigated their morphologies and structure characteristics, and further assess their gas sensing properties and potential in ethanol detection.
a College of Environmental Science and Engineering, Dalian Maritime University, 1 Linghai Road 116026, Dalian, China

Preparation of PbS nanomaterials
3.2 mmol of Pb(Ac) 2 •3H 2 O (CAS number: 6080-56-4 ) and 80 mmol thiourea (CAS number: 62-56-6) were added to 80 mL of deionized water under stirring.The mixture was sealed in a Teflon-lined stainless steel autoclave of 100 mL capacity and heat at 170 °C for 12 h, and then cooled to room temperature.The resulting precipitates were collected by centrifugation and washed three times by deionized water and ethanol (CAS number: 64-17-5) to remove possible impurities, and subsequently dried at 60 o C for 10 h.

Fabrication and measurement of PbS gas sensor
PbS nanomaterials were mixed with several drops of ethanol to form a slurry, and then the slurry was brush-coated onto the surfaces of an alumina tube with two Au electrodes and four Pt wires.A Ni-Cr heating wire was inserted into the alumina tube and used as a heater.The alumina tube was then welded onto a pedestal with six probes to obtain the final sensor unit.Gas sensing tests were performed on a WS-30A static gas-sensing system (HanWei Electronics Co., Ltd., Henan, China) using ambient air as the dilute and reference gas, which is operated at the controllable ambient temperature (25 ± 1 o C) and relative humidity ( ~ 30 ± 5 % RH).A typical testing procedure is presented as follows: Firstly, the PbS sensor is put into the test chamber (18 L in volume).After the resistance of the PbS sensor is stable, the calculated amount of tested solution is manually injected into the test chamber by a microsyringe, and then, the tested solution is evaporated by a quick evaporator and mixed with air immediately by two installed fans, which constitutes the measurement atmosphere.Afterwards, the test chamber is opened and the PbS sensor begins to recover in air 26,27 .
The sensor sensitivity is defined as follows,

Morphology and Structure of PbS nanomaterials
The phase composition and structure of the synthesized product are investigated by XRD. Figure 1 222), ( 400), ( 331), (420), and (422) planes of a facecentered-cubic structured PbS (JCPDS No. 05-592), and no other peaks are observed, suggesting pure PbS is obtained 28 .The sharp and strong intensity of XRD peaks suggest that the samples have high crystallinity.The morphology of the PbS nanomaterials are observed by SEM and TEM images.As shown in Figure 2a.The PbS nanomaterials show the morphology of star-shaped nanostructures.All the products observed exhibit similarities in shape and morphology but slightly varied in size.TEM image reveals that individual star-shaped PbS nanostructure consists of four symmetric arms in the same plane extend radially from the center (Figure 2b).From the HRTEM image of the PbS sample (Figure 2c), it can be seen that the fringe spacing of pure PbS is about 0.295 nm, which corresponding to the interplanar spacing of (200) planes of the face-centered-cubic structured PbS 9 .

=
where R a and R g are the electrical resistance of the sensor in air and in test gas, respectively.

Gas sensing properties of PbS nanomaterials
Operating temperature plays an important role in determining the response of the sensor to ethanol. Figure 3 present the sensitivity of the PbS sensor towards 100 ppm ethanol at various operating temperature.It is obvious that the PbS sensor demonstrate the increasing sensitivity with the increase of the operating temperature and reaches  -, O 2-) at lower operating temperature due to insufficient thermal energy, while higher operating temperature makes the adsorbed oxygen species (O -, O 2 -, O 2-) reduce and thus limits gas response towards ethanol molecules 29 .Therefore, the optimal operating temperature is 400 0C for the PbS sensor.
Figure 4 displays a variation between the sensitivity of the PbS sensor and ethanol concentration from 10 ppm to 300 ppm at operating temperature of 400 o C. As observed, the detection limit of the PbS sensor toward ethanol is 10 ppm.With the increase of ethanol concentration, the sensitivity of the PbS sensor significantly increases and demonstrates an almost linear relationship between them.The response and recovery times are also an important parameter to assess the gas-sensing property of the PbS sensor.Figure 5 provides

Gas-sensing mechanism of PbS sensor
The gas-sensing mechanism of the PbS sensor towards ethanol can be interpreted by the interaction between ethanol molecules and the absorbed oxygen species (O -, O 2 -, O 2-) on the surface of the PbS sensor 30 .Generally, the type of absorbed oxygen species is determined by the operating temperature, and O 2− becomes the main form of oxygen species at the operating temperature of 400 °C31 .When the PbS sensor contacts with the air, oxygen molecules in the air will adsorb on the surfaces of the PbS sensor and trap electrons from the conduction band of the PbS sensor, forming oxygen species (O -, O 2 -, O 2-) (Eqs.(1)-( 4)), which results in the formation of the electron depletion layer on the surface of the PbS sensor and makes the resistance of the PbS sensor increase.As ethanol is introduced into the system, ethanol molecules will react with the oxygen species (O -, O 2 -, O 2-) adsorbed on the surface of the PbS sensor, thus the captured electrons are released back to conduction band of the PbS sensor, which leads to a decreased resistance (Eqs.(5)-( 7)).
Scholars, Ministry of Human Resources and Social Security of China, and the National Natural Science Foundation of China (21276035, 21476034).

Conclusions
In summary, we have synthesized star-shaped PbS nanomaterials using a simple hydrothermal method.The sensitivities of the as-prepared PbS sensor towards 100 ppm ethanol increase with operating temperature and then reach a maximum, The PbS sensor reveals fast response and recovery time for the detection of ethanol concentration in the range of 10-300 ppm at the optimal operating temperature of 400 o C. The gas-sensing mechanism of the PbS sensor towards ethanol can be interpreted by the interaction between ethanol molecules and the absorbed oxygen species (O -, O 2 -, O 2-) on the surface of the PbS sensor.

Acknowledgement
X-ray diffraction (XRD) patterns of PbS nanomaterials were recorded using a D/Max-2400 diffractometer (Cu Kα radiation, λ =1.54055Å) in a range of diffraction angle 2θ from 10 o to 80 o to analyze the diffraction peaks of PbS.The morphologies of PbS nanomaterials were observed by a scanning electron microscope (SEM) (Philips XL30 FEG) and a high-resolution transmission electron microscopy (HRTEM) (FEI/Philips Techal 12 BioTWIN).

Figure 3 :
Figure 3: Sensitivity of the PbS sensor at different operating temperatures

Figure 4 :
Figure 4: Sensitivity of PbS sensor to different ethanol concentrations.

Figure 5 :
Figure 5: Response-recovery curves of the PbS sensors to different ethanol concentrations.
This work was supported by the Scientific Research Project of Education Department of Liaoning Province (L2013203), The Natural Science Foundation of Liaoning Province (2014025014), the Fundamental Research Funds for the Central Universities (3132016327), the Scientific Public Research Foundation of Liaoning Province (2013003007), the Science and Technology Foundation for Overseas Chinese