Sensors & Transducers Magazine, Vol.39, Issue 1, January 2004, pp.106-111
Sensors & Transducers
ISSN 1726- 5479
© 2004 by IFSA
http://www.sensorsportal.com
Lanthanide Doping Bis[octakis(octyloxy)phthalocyaninato] Complexes Based Langmuir-Blodgett Films for NO2 Gas Sensors Application
Yadong Jiang1, Dan Xie2
1School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu, 610054 P.R.China
2Institute of Microelectronics, Tsinghua University, Beijing 100084, P. R. China
Phone: ++86-28-83202616, e-mail:
Received: 16 December 2003 /Accepted: 14 January 2004 /Published: 18 January 2004
Abstract: A new series of sandwich-like lanthanide doping bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] complexes Ln[Pc*]2 (Pc*=Pc(OC8H17)8, Ln=Sm, Pr, Er) were used as NO2 gas-sensing materials is described in the article. The gas-sensing films of Ln[Pc*]2 were prepared by Langmuir-Blodgett (LB) technique and the NO2 gas-sensing properties of Ln[Pc*]2 LB films were studied. The sensitive properties of Ln[Pc*]2 LB films to NO2 gas was monitored by the change of conductivity during gas exposure. Therein, Sm[Pc*]2 has the best sensitivity and responsivity to NO2 gas. The detecting range is from 0~100ppm, and the response and recovery time of 11-layer Sm[Pc*]2 LB film to 20ppm NO2 at room temperature is 16 s and 80 s, respectively. The thinner the film, the faster the response and recovery become. Recovery time in air is longer than that in pure N2.
Keywords: Gas sensor, NO2 gas, Bis[phthalocyaninato] complexes, Langmuir-Blodgett film
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1. Introduction
It has long been known that the conductivities of phthalocyanines(Pcs) and metal-substituted phthalocyanines(M-Pc) are excellent gas-sensing materials owing to their thermal and chemical stability, especially sensitive to the presence of certain electrophilic gases such as nitrogen dioxide (NO2). Therefore, such materials have a wide appliaction in gas sensors, detectors [1,2]. There are lots of reports about gas sensors based on Pcs or M-Pc by Langmuir-Blodgett (LB) technique [1-4].LB technique is a promising means to develop highly-ordered conducting organic thin films. Because such ultrathin films have high ratios of surface area to bulk volume, the use of organic gas-sensitive substances and LB deposition technique have a great potential for improving the performance of gas sensors. By good molecular packing of gas-sensing groups aligned near the surface, an efficient and quick response can be expected [1-4]. However, much research focus on mono-phthalocyanine, and there has been no systematical research on the gas-sensitivity of bis-phthalocyaninato. References 5 and 6 reported the synthesis and electrochemical properties of substituted bis[phthalocyaninato] lanthanide double-deckers, which show great potential for application in molecular electronics, gas sensors, electrochromic and molecular magnetic devices [5,6]. In this paper, we report the preparartion of lanthanide doping bis[phthalocyaninato] complexes Ln[Pc*]2(Pc*=Pc(OC8H17)8; Ln=Sm, Pr, Er) films by LB technique and the sensing properties to NO2 gas of these LB films were studied.
2. Experiments
The lanthanide doping bis[phthalocyaninato] complexes Ln[Pc*]2 were synthesized by the method described in reference [6]. The molecular structure of Ln[Pc*]2 is shown in Fig. 1. Ln[Pc*]2 has a sandwich-type structure with rare earth metal at the center of two phthalocyanine ligands facing each other in a staggered arrangement [7].
Fig. 1: Molecular structure of Ln[Pc*]2 (Pc*=Pc(OC8H17)8; R= OC8H17). / Fig. 2: Schematic presentation of the interdigitated electrodes gas sensor with LB film deposited.(A: sensitive film; B: electrodes; C: substrate; D: down-lead)Ln[Pc*]2 LB films were deposited with WM-1 LB instrument made in Southeast University of Nanjing, China. Different spreading solution were prepared by dissolving Sm[Pc*]2, Pr[Pc*]2 and Er[Pc*]2 in chloroform respectively. The concentrations of the three kinds of solution were 1.2~1.3mmol/ml. In order to produce stable Langmuir films, Ln[Pc*]2 was mixed with stearic acid (SA) in different molar ratios of 1:3. The subphase solution was 10-4M Cd2+ subphase (pH5.7). The monolayer was then compressed at a speed of 3mm/min and the surface pressure was monitored by Wilhelmy balance. The mixture of Ln[Pc*]2 and auxiliary solvents were deposited respectively on silicon substrate to form uniform LB flms. Planar interdigitated gold electrodes consisting of 50 pairs of fingers were lithographically patterned onto the surface of a silicon substrate, therein, the electrode width and gap were both 20mm.
The gas-sensing properties were studied by placing the samples in a chamber through which gas could be passed. NO2 gas was diluted with nitrogen (N2) using National Standards Research Center MF-2 model gas blender and these gases had purity levels of 99.99%. The electrical conductivity of LB films was measured using a Changzhou Tonghui TH2682 high resistance meter [8].
3. Gas-sensing Properties of Ln[Pc*]2 LB films to NO2 Gas
In order to study the gas-sensing property, LB films were exposed to NO2 gas repeatedly. Each cycle of exposure to NO2 and recovery in air or pure N2 was recorded constantly from 0 to 500 s. Fig.3 shows the relative change of resistance of 11-layer Ln[Pc*]2 LB film to different concentrations of NO2 gas. It is found that the resistance decreases with the increase of NO2 concentration. The relation between the relative change of resistance and the gas concentration is close to linearity. To the same concentration of NO2 gas, the change of resistance of Sm[Pc*]2 LB film was the most. It indicates Sm[Pc*]2 LB film is more sensitive to NO2 gas than Pr[Pc*]2 and Er[Pc*]2.
Fig. 3: The plot of the change of relative resistance of 11-layer Ln[Pc*]2 LB films vs.the concentrations of NO2 gas
Fig. 4 shows the response-recovery properties to 100ppm NO2 gas of Er[Pc*]2 LB films with different numbers of layers at room temperature. The conductivity increases sharply with time at first when LB film contacts with NO2 gas, which may be due to the surface adsorption effect, and then increases slowly, which may be due to the bulk diffusion effect [10]. The interaction process between LB film and the adsorbed gas is a dynamical process. When the LB film is exposed to NO2 gas, the adsorption and desorption processes will simultaneously occur. Having attained dynamic equilibrium, the number of the adsorbed gas molecules will be equal to the number of the desorbed gas molecules. Then the conductivity attains a stable value. The recovery also shows a rapid decrease followed by a the surface effect and bulk effect. Because NO2 gas is an oxidizing gas, the gas sensing mechanism is realized through the charge transfer interaction in which the gas molecule to be sensed acts as a planar p-electron acceptor forming a redox couple, and the positive charge produced is delocalized over the two phthalocyanine macrocycles causing the increase of the conductivity [11, 12].
From Fig. 4, it is also found that the change of conductivity decreased on the second exposure cycle, and both the response time and recovery time increased. Initial physisorption of NO2 molecules followed by chemisorption and/or diffusion into bulk will lead to the formation of acceptor states. These will be located on the surface and will extend into the bulk if inward diffusion occurs, forming NO2- molecules. Removal of the NO2 ambient and exposure to nitrogen leads to the slow thermal desorption of adsorbed NO2 from the film surface, decreasing the acceptor concentration and thus the conductivity. Additionally, desorption from different types of ad-sites will be at different rates and strongly bounded NO2 molecules may effectively be retained permanently at room temperature [13,14]. Re-exposure to NO2 gas will repopulate the weaker binding sites leading to a rather smaller effect than on the first cycle.
Fig. 4: Response-recovery properties of Er[Pc*]2 LB films with various number of film layers to 100ppm NO2 gas at different temperatures
For Sm[Pc*]2 and Pr[Pc*]2 LB films, the similar variations were obtained. Table 1 gives the response time and the recovery time of 11-layer Ln[Pc*]2 LB films to different concentrations of NO2 gas. The response time and recovery time shortened when being exposed to higher concentration of NO2 gas. To the same NO2 concentration, Sm[Pc*]2 LB films has the faster response and recovery than Pr[Pc*]2 and Er[Pc*]2. The response time of 11-layer Sm[Pc*]2 LB film to 20ppm NO2 is 16 s, the recovery time in air and pure N2 is about 100 s and 80 s at room temperature, respectively.
Table 1. The response time and recovery time of 11-layer Ln[Pc*]2 LB film to different concentrations of NO2 gas at room temperature
Concentration of NO2 (ppm) / Ln[Pc*]2 / 20 / 40 / 60 / 80 / 100Response time (s) / Er[Pc*]2
Pr[Pc*]2
Sm[Pc*]2 / 80
27
16 / 60
25
12 / 45
20
10 / 30
18
8 / 20
15
5
Recovery time in air (s) / Er[Pc*]2
Pr[Pc*]2
Sm[Pc*]2 / 180
120
100 / 120
90
70 / 80
60
50 / 50
45
35 / 30
25
18
Recovery time in N2 (s) / Er[Pc*]2
Pr[Pc*]2
Sm[Pc*]2 / 135
100
80 / 100
60
50 / 60
40
30 / 35
25
18 / 20
15
10
Compared with the three curves of Fig. 4, we can see the response and recovery velocity becomes slow with the increase of the number of LB film layers. The response time of 11-layer and 31-layer Er[Pc*]2 LB film to 100ppm NO2 gas at room temperature is 20 s and 40 s, respectively. Thus it can be seen that the thinner the film, the faster the speed of adsorption/bulk diffusion and desorption becomes which also can be seen from Fig. 5. To the same NO2 concentration, thickness of LB film will affect the response property: the thinner the LB film, the faster the response becomes.
Fig. 5: The plot of response time of Ln[Pc*]2 LB film to 20ppm NO2 vs.the number of LB film layers(25℃).
Table.1 shows the recovery properties have the relationship with the condition of recovery. The recovery speed in air is slower than that in pure N2. It may be due to the adsorption of O2. When being exposed in vacuum (0.02MPa), the recovery time of the three LB films was almost less than 10 s. Therefore, the recovery time of Er[Pc*]2 LB film in air before the second gas exposure cycle is about 30 s. While the recovery time in pure N2 is about 20 s.
5. Conclusions
Lanthanide doping bis[phthalocyaninato] complexes Ln[Pc*]2 (Ln=Sm, Pr, Er) were used as NO2 gas sensing materials, and the films of Ln[Pc*]2 were prepared by LB technique. Ln[Pc*]2 LB films are sensitive to NO2 gas, therein, Sm[Pc*]2 has the best sensitivity and responsivity. The response time of 11-layer Sm[Pc*]2 LB film to 20ppm NO2 is 16 s, the recovery time in air and pure N2 is about 100 s and 80 s at room temperature, respectively. Therefore, lanthanide doping bis[phthalocyaninato] complexes are promising organic materials to develop gas sensors with further improved properties. The gas-sensing property of LB films to NO2 gas can be affected by the thickness of LB film. The thinner the film, the faster the response and recovery become. Recovery time in air is longer than that in pure N2.
References
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