수정진동자와 H2Pc LB 박막을 이용한 가스 센서의 NO2 감지 특성

권현정, 이영진, 장용근, 김종득

한국과학기술원 화학공학과

The Application of Octa-Substituted H2Pc

Langmuir-Blodgett Films for NO2 Measurement

H. J. Kwon, Y.-J. Lee, Y. K.Chang, and J.-D. Kim

Dept.of Chem. Eng., KAIST

Introduction

As nitrogen dioxide(NO2) has been a serious air polluting agent of late, a gas sensor for its monitoring has become on growing demand.  In this study, piezoelectric quartz crystal microbalance(QCM) device with phthalocyanine film was adopted for detecting NO2 level in air owing to its high sensitivity and low cost.  For an added NO2 mass uniformly deposited over the surface of QCM, the shift of resonant frequency occurs proportionally to the mass. Langmuir-Blodgett(LB)  technique was employed in order to produce extremely thin sensing films onto substrates with a high degree of control over their thickness and molecular architecture.

In our preceding paper, CuPc(OR)8 (R=2-ethylhexyl) LB film was investigated and found to be a reusable and reliable sensing element for NO2 in pure nitrogen gas based system[1].

In this study, H2Pc(OR)8 (R=2-ethylhexyl)(Abbreviated to H2Pc) LB film, which was postulated to have higher sensitivity, was examined as a sensing film. The monolayer behavior of H2Pc molecules on liquid surface was observed on UV spectroscopy and thickness of the film transferred onto silicon wafer was measured.    NO2 concentration was quantified by H2Pc-QCM sensor system under the condition that sensor cell was maintained at 35℃ constantly and carrier gas was dry air.  The effect of humidity was also examined.

Materials and Methods

Sensing material used in this study was octa-substituted phthalocyanines whose structure and synthetic scheme was shown in other paper[2].  Eight substituted side chains, 2-ethylhexyloxy groups, endow phthalocyanines with greater solubility in organic solvents and discotic liquid crystalline properties[3]. Phthalocyanines were deposited onto the surface of 9 MHz QCM using LB technique.

KSV 5000 system (KSV chemicals, Finland) which has moving-barrier type LB trough was used for the film deposition.  During film deposition process, the surface pressure was maintained at 30 mN/m for H2Pc at room temperature.

The apparatus for the sensing experiment was shown in our preceding paper[4].  NO2 sample gas (1.35% N2 based) was prepared by cascade dilution and injected into the sensor cell with mixed carrier gas flow which continuously purged sensor cell at 2 l/min flow.  A humidified air was made by using a bubbler.  The temperature in the sensor cell was controlled to remain at 35℃ constantly.

AV spectroscopy(MCPD, Otsuka electronics, Japan) was used for scanning absorption spectrum on water surface.  In order to determine the thickness and the refractive index of sensing film, ellipsometry(Rudolph Research/Auto(II)) with specific wave length of 633 nm was used.   Thin film X-ray diffraction(D/MAX-RC, Rigaku) with CuK□-ray(□=1.54) scanned the film to measure the lattice spacing of it.

Results and Discussion

The surface pressure - area isotherm of octa-substituted phthalocyanines at room temperature have been already reported[1].  Repetitive dipping and withdrawing of the substrate through the air/water interface constructed Y-type multilayered structure of H2Pc[5].

Absorption spectrum of H2Pc monolayer on water surface was measured in the visible and the ultraviolet region with an increase of surface pressure(Fig. 1).    Characteristic Q band in the region of 650-700 nm was not shifted under 60 mN/m  but over.  Since partial aggregation  results in blue-shifted Q bands[6],  H2Pc  monolayer is considered to be compressed uniformly without aggregation until the surface pressure reaches 60 mN/m.

After this monolayer was transferred onto the surface of silicon wafer, film thickness and refractive index were measured by ellipsometry(Table 1).  One layer thickness was calculated to be 67.9, 70.2 and 63.3Å respectively.  And the thin film X-ray diffraction was used to estimate the lattice spacing of the 20-layer and 40 layer H2Pc films. From the Bragg's equation n□=2dsin□, the lattice spacing was calculated to be 22.29Å and 21.8Å respectively.  When the mass difference of QCM attributed to transferred H2Pc film was measured to be 11000Hz while CuPc being 5700Hz.  Computer-simulated molecular model offers that one molecular size is about 27Å.  Allowing for these results, H2Pc is expected to be transferred nearly with double layers each time onto substrate when  the surface pressure was 30 mN/m at room temperature.

Table   The thickness and refractive index of H2Pc LB film onto silicon wafer were measured by ellipsometry

No. of strokes	Thickness (Å)	Refractive index
4	271	2.033
6	421	1.914
10	580	1.821

The H2Pc sensing film was exposed to 150 ppm of NO2 flow for 1 hour before subsequent runs.  This pretreatment process make the sensing film reusable by  saturating irreversible sensing sites[1]. And then sensing experiments was carried out under the condition that carrier gas was dry air and the temperature of the sensor cell remained at 35℃ constantly. Resonant frequency change for 12 minutes and intial slope  at various NO2 concentrations were shown in Fig.2 and Fig.3.  Initial slope was defined as initial frequency change for 2 minutes, which means the speed of detection.  The frequency change and initial slope were observed to have nearly linear relationship with NO2 concentration and the minimum resolution reached to 1 ppm.   Both frequency change in a given time and initial slope could be used for quantification of NO2 concentration.

The effect of humidity in carrier gas was examined. At exposure to 10 ppm of NO2 flow, frequency change for 20 minutes was 150 Hz in 43% humidity while being  23Hz in dry air(Fig.4). The interference of humidity was so severe, provoking a significant frequency change, that its entrapment was indispensible to obtain reliable frequency change.

Conclusion

A gas sensor system using QCM and H2Pc LB film has been developed for the purpose of detecting NO2 level at the air.  H2Pc molecules were compressed on water surface without aggregation and well- transferred onto substrate at the dipping surface pressure of 30 mN/m.  It showed a significant frequency change having  nearly linear relationship with NO2 concentration in air based system, and the resolution reached 1 ppm.

Fig. 2 Frequency change for 12 minutes vs. NO2 concentration

Fig.   UV absorption spectra of H2Pc monolayer on the water surface while surface pressure was increasing.  Characteristic Q band in the region of 650 - 700 nm blue-shifted over 60 mN/m of surface pressure.

Fig. 4  Frequency change of 20-layer H2Pc sensing film in dry air and in 43% of humidity at exposure to 10 ppm of NO2

Fig.3 Initial slopes of 20 -layer H2Pc LB film in dry air at exposure to various concentrations of NO2

References

[1] Y.-J. Lee, S.-A.Choi, H.-J. Kwon, Y. -M.Koo, Y. K. Chang and J.-D. Kim, "A study of NO2 sensing quartz crystal microbalance sensor using metallophthalocyanine LB films", Theories and applications of Chem. Eng. 3, No.1, 933, 1997

[2] S.-R. Kim, S.-A.Choi, J.-D. Kim, and K.H. Choi, S.K.Park, Y.H.chang, "The characteristics of metallophthalocyanines mono-. multilyer and application to the gas sensor for NO2", Synthetic Metals 71, 2293, 1995

[3] S.-A. Choi, Y.-J. Lee, H. J. Kwon, Y. K. Chang, and J.-D. Kim , "Alternatively Stacked Langmuir-Blodgett Film of Phospholipid and ZnO as an Olfactory Sensing Membrane", Mol. Cryst. Liq. Cryst., 295, 149-152, 1997

[4] S.-R. Kim, J.-D. Kim, and K.H.choi, Y.H.chang, "NO2-sensing properties of octa(2-ethylhexyloxy) metallophthalocyanine LB films using quartz crystal microbalance", Sensors and Actuators B , 1996

[5] K.H.Choi, "Discotic liquid crystalline octa-(2-ethylhexyloxy) and octa (1,4,7,10 -tetrazaundecyl) metallophthalocyanines; syntheses, characterization and applications", Ph.D. thesis, 1996