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

Abstract

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.

Keywords
Preparation; Nanomaterials; Ethanol; Gas sensor

1. Introduction

The leak of flammable, toxic and explosive gases in industrial processes will cause serious environmental and safety problems¹. 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 applications⁴. 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 circuitry⁶.

With the fast development of nanotechnology, many nanosized semiconductors, including SnO₂, ZnO, ZnS, WO₃, TiO₂ etc., have been fabricated and employed as gas sensors⁷. 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 sizes⁹. Hence, much attentions have been paid on synthesis of a variety of PbS nanostructures, such as nanoparticles¹⁰, quantum dots¹¹, nanorods¹², nanobelts¹³ nanowires¹⁴, cubes¹⁵, dendrites¹⁶, star-like¹⁷ and hierarchical structures¹⁸ 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 devices¹⁹. 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 detection²⁰. Shimizu et al. fabricated the metal-mono sulfide based (NiS, CdS, SnS and PbS) solid electrolyte sensors, and found their good SO₂ 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 H₂²³. Fu obtained PbS sensors with high responses to NO₂ and NH₃²⁴.

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.

2. Experimental

2.1. Preparation of PbS nanomaterials

3.2 mmol of Pb(Ac)₂·3H₂O (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 ºC 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 ºC for 10 h.

2.2. Characterizations

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 10º to 80º 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).

2.3. 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 ºC) 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,

$\mathit{Sensitivity}=\frac{{\mathit{R}}_{\mathit{a}}}{{\mathit{R}}_{\mathit{g}}}$

where R_a and R_g are the electrical resistance of the sensor in air and in test gas, respectively.

3. Results and discussion

3.1. Morphology and Structure of PbS nanomaterials

The phase composition and structure of the synthesized product are investigated by XRD. Figure 1 demonstrates the representative XRD pattern of the as-prepared product, which show the diffraction peaks at 2θ values 26.09º, 30.19º, 43.17º, 51.09º, 53.52º, 62.64º, 68.98º, 71.04º and 79.27º. All the diffraction peaks are assigned to (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes of a face-centered-cubic structured PbS (JCPDS No. 05-592), and no other peaks are observed, suggesting pure PbS is obtained²⁸. 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 PbS⁹.

Figure1
XRD patterns of PbS sample.

Figure 2
(a) SEM, (b) TEM, and (c) HRTEM images of PbS.

3.2. 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 a maximum of 3.04 at 400 ºC, and then decreases. The phenomenon can be explained by the fact that it is hard for ethanol molecules to react with adsorbed oxygen species (O^-, O₂^-, O^2-) at lower operating temperature due to insufficient thermal energy, while higher operating temperature makes the adsorbed oxygen species (O^-, O₂^-, O^2-) reduce and thus limits gas response towards ethanol molecules²⁹. Therefore, the optimal operating temperature is 400 0C for the PbS sensor.

Figure 3
Sensitivity of the PbS sensor at different operating temperatures

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 ºC. 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 the response-recovery curves of the PbS sensor towards ethanol with different concentrations at operating temperature of 400 ºC. Generally, the times to reach 90 % variation in resistance upon exposure to ethanol and air are defined as the 90 % response time (t_response) and the 90 % recovery time (t_recovery), respectively. As observed, the PbS sensor exhibits fast response and recovery to 20, 50, 100, 200, and 300 ppm ethanol. The response times are all less than 1 s and the recovery time upon exposure to 20, 50, 100, 200, and 300 ppm ethanol are 6 s, 7 s, 9 s, 10 s and 11 s, respectively. The fast response and recovery speed suggests that the diffusion of ethanol and its oxidation by oxygen species (O^-, O₂^-, O^2-) are very rapid in the star-shaped structure.

Figure 4
Sensitivity of PbS sensor to different ethanol concentrations.

Figure 5
Response-recovery curves of the PbS sensors to different ethanol concentrations.

3.3. 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^-, O₂^-, O^2-) on the surface of the PbS sensor³⁰. 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 ºC³¹. 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^-, O₂^-, 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^-, O₂^-, 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)).

(1) $O_2\left(\mathrm{gas}\right)\to O_2\left(\mathrm{ads}\right)$

(2) $O_2\left(\mathrm{ads}\right)+e^{-}\to O_2{}^{-}\left(\mathrm{ads}\right)$

(3) $O_2{}^{-}\left(\mathrm{ads}\right)+e^{-}\to 2O^{-}\left(\mathrm{ads}\right)$

(4) $O^{-}\left(\mathrm{ads}\right)+e^{-}\to O^{2-}\left(\mathrm{ads}\right)$

(5) $C_2{H}_5\mathrm{OH}\left(\mathrm{gas}\right)\to C_2{H}_5\mathrm{OH}\left(\mathrm{ads}\right)$

(6) $\begin{array}{c}{C}_2{H}_5\mathrm{OH}\left(\mathrm{ads}\right)+{\mathrm{6O}}^{-}\left(\mathrm{ads}\right)\to \hfill \\ 2{\mathrm{CO}}_2\left(\mathrm{gas}\right)+3H_{2}O\left(\mathrm{gas}\right)+6e^{-}\hfill \end{array}$

(7) $\begin{array}{c}{C}_2{H}_5\mathrm{OH}\left(\mathrm{ads}\right)+6O^{2-}\left(\mathrm{ads}\right)\to \hfill \\ 2{\mathrm{CO}}_2\left(\mathrm{gas}\right)+3H_{2}O\left(\mathrm{gas}\right)+12e^{-}\hfill \end{array}$

4. 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 ºC. 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^-, O₂^-, O^2-) on the surface of the PbS sensor.

5. Acknowledgement

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 Scholars, Ministry of Human Resources and Social Security of China, and the National Natural Science Foundation of China (21276035, 21476034).

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Publication Dates

• Publication in this collection
  03 Oct 2016
• Date of issue
  Nov-Dec 2016

History

• Received
  12 Nov 2015
• Reviewed
  11 Aug 2016
• Accepted
  04 Sept 2016