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Fabrication and properties of anodic alumina humidity sensor with through-hole structure LING ZhiYuan†, CHEN ShuoShuo, WANG JinChi & LI Yi Department of Electronic Materials Science and Engineering, Institute of Materials, South China University of Technology, Guangzhou 510640, China
Through-hole structural humidity sensor was fabricated by radio-frequency magnetron sputtering deposition of gold electrodes on two sides of anodic aluminum oxide (AAO) membranes which were prepared by two-step anodization procedure at 0―5℃ and 40 V in 0.5 mol/L oxalic acid electrolyte. The investigation on the impedance at various humid conditions showed a linear relationship between impedance and relative humidity over the range of 12%―97% RH. Other excellent properties such as rapid response and good reproducibility were also obtained. humidity sensor, anodic aluminum oxide, nanopore electrode
1 Experimental The through-hole structure of the humidity sensor based www.scichina.com | csb.scichina.com | www.springerlink.com
on AAO is shown in Figure 1. The through-hole AAO membrane is between the top and bottom electrodes which are nanopores arrays. The holes on the metal electrodes are in accordance with those on the AAO membrane, and the structure with through-hole from the bottom to the top electrode is formed. 1.1 Fabrication of AAO membrane High-purity aluminum sheet (99.99%) was used as the start materials. Prior to anodizing, the aluminum sheet was annealed at 600℃ for 1 h in vacuum (5×10−4 Pa), and degreased in acetone and ethanol for 10 min of ultrasonic cleaning, separately. Next, the sheet was electropolished at 25℃ and 21 V for 5 min in a 1:4 by volume mixture of perchloric acid and ethanol. Subsequently, anodization was carried out at 0―5℃ and 40 V in a 0.5 mol/L oxalic acid solution by the graphite as cathode and aluminum sheet as anode. After the first anodization for 12 h, the alumina was removed completely at room temperature in a 5% phosphoric acid. Received March 1, 2007; accepted May 12, 2007 doi: 10.1007/s11434-008-0008-z † Corresponding author (email:
[email protected]) Supported by the research fund of Key Lab of Specially Functional Materials, Ministry of Education, China
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Anodic alumina has attracted much attention as a promising material for humidity sensor since the influence of water vapour on electrical properties was first reported by Ansbacher and Jason[1] in 1950. According to the sensing － mechanism mentioned elsewhere[2 4], the main reasons to investigate AAO template for the humidity sensor are as follows. First, the existence of large quantity of negatively charged impurity ions[5] makes it easier to adsorb water molecules than other oxides. Secondly, the self-assembled nanopores are uniform, highly ordered and with controllable diameters[6,7], so that not only the surface area is increased by orders of magnitude, but also the stable sensors can be obtained by mass production. Regarding to － the sensor’s structure, the former researchers[8 12] would keep the barrier layer on the anodic alumina, and the holes would exist only in one end of the AAO template. Few reports have concerned about humidity sensors with through-hole structure, and no porous metal electrodes have been adopted. In this paper, we tried to combine through-hole structural AAO with porous metal electrodes to fabricate a novel humidity sensor.
tron sputtering (JPG560 ultra-high Vacuumcy Multifunction Sputtering Instrument) with the gold target (99.99%). The base pressure, deposition pressure, the Ar flux and the deposition time were 1×10−5 Pa, 1.2 Pa, 3.5 sccm, and 5 min, respectively. After deposition, the sample was annealed for 1 h at 300℃.
2 Results and discussion
Figure 1 Through-hole structural humidity sensor based on AAO.
Then, the second anodization was conducted for 24 h at the same condition as above. The remaining aluminum was removed in a saturated CuCl2 solution. Finally, the barrier layer was dissolved and the pores were widened by chemical etching in a 5% phosphoric acid solution for 25 min. 1.2 Preparation of electrodes Au electrodes on both sides of the anodic alumina membrane were deposited by radio-frequency magne-
SEM graphs of the fabricated AAO membranes are shown in Figure 2. The thickness of the membranes and the diameter of the pore are 100 μm and 85 nm, respectively. The aspect ratio of the pore is about 1180, the distance between two pores is about 104 nm, and the density of the pores is about 1010/cm2. Hexagonal nanopores are aligned orderly, which are normal to the surface of the membranes and parallel to each other. Figure 3 shows the SEM graphs of gold electrodes deposited on the AAO membranes by RF magnetron sputtering system. The gold electrodes replicate the surface pattern of the AAO membranes, and form an ordered nanopores structure with diameter of 60 nm and
Figure 2 SEM (LEO1530VP) graphs of the AAO membranes. (a) Top view; (b) cross sectional view.
Figure 3 Gold electrodes deposited on the AAO membranes by RF magnetron sputtering. (a) Top view; (b) cross sectional view.
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LING ZhiYuan et al. Chinese Science Bulletin | January 2008 | vol. 53 | no. 2 | 183-187
tributing large amount of anions ( C 2 O 24 − and OH − )[5];
when magnetron sputtering, Au+ ions were electrostatically attracted on the surface of the AAO membranes by the negative charge; when vacuum evaporating, the Au particles which have no charges can be deposited in the nanopores. The equivalent circuit of the sensor shown in Figure 1 can be illustrated by Figure 4. According to this circuit, the relationship between impedance and angular frequency is R1 R2 Z= . (1) R1 + R2 + jω R1 R2 (C1 + C2 )
By logarithmic transforms of both sides of eq. (2), a linear relationship can be obtained as log Z ≈ − log ω + log
1 . C1 + C2
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thickness of 190 nm. Figure 3(b) indicates that the deposited gold appears only on the surface of the AAO membranes, not into holes. This result disagrees with the result given by ref. [13] in which the authors replicated the metallic copper nanopore array membrane by vacuum evaporation, but similar to the result of ref. [14] on the fabrication of large area ZnO nanopore array membranes by magnetron sputtering. We assume that the mechanism was as follows. The surface of anodic alumina was negative charged by dis-
(3)
Relative humid conditions of 12%, 33%, 53%, 75% and 97% were obtained by saturated aqueous solutions of LiCl, MgCl2, Mg(NO3)2, NaCl and K2SO4 at room-temperature. Electrical properties were measured by HP 4194A impedance/phase-gain analyzer. Figure 5 shows the impedance spectrum of the through-hole structural humidity sensor at various humid conditions.
Figure 4 Equivalent circuit of the through-holes AAO humidity sensor. R1 and C1 are the resistance and capacitance of the AAO membrane, respectively; R2 and C2 are resistance and capacitance of the water molecules absorbed on the AAO membranes.
If R0 =
R1 R2 , the real part of the impedance Z can R1 + R2
be described as Z = When
the
R0 1 + [ω R0 (C1 + C2 )]
2
angular
[ω R0 (C1 + C2 )]
2
frequency
>> 1,
Z ≈
1 . ω (C1 + C2 )
.
increases
(2) till
The sample was kept in each humid condition for 30 min before testing. Linear relationship of log|Z| and logω is indicated in Figure 5. The angular frequency ω0 increases with the increase of the relative humidity (ω0 is defined as the angular frequency where the linear relationship of curves is just obtained in Figure 5). While ω≥ω0, all the curves of |Z|-ω at different relative humilities merge to a straight line. By linear fitting the correspondence of |Z|-ω at RH=12%, an equation log Z ≈ − log ω + 10 is obtained. The results suggest that the variation of |Z| is decided by R2 but independent of C2 for C1>>C2, and that water molecules are chemisorption and physisorption on the inner surface of the AAO membrane while capillary condensation water does not exist as mentioned in ref. [2] (the permittivity of water is 80, whereas AAO membrane less than 5[15,16]). While ω < ω0, there is a nonlinear relationship between |Z| and ω. This is similar to the results mentioned in refs. [8－12, 17]. Nonlinear error (ef) of |Z| and RH at different ω were calculated by eq. (4) (Figure 6), and an extreme point was found in the spectrum. ef reaches the
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Figure 5 Relationship between |Z| and ω at different humid conditions.
minimum value of 9.2% at ω≈62.8 kHz. ef =
ΔZ
max
Z 97% − Z 12%
,
(4)
where Δ|Z|max is the maximum deviation between the fitting curve and the experimental data; |Z|97% and |Z|12% are the real parts of the impedance at 97%RH and 12%RH, respectively.
Figure 7 Reproducibility of |Z| with RH at ω ≈ 62.8 kHz.
Figure 6 Nonlinear error of |Z| with RH at various frequencies.
We changed relative humid conditions from 12%RH to 97%RH step by step (5 min for one step) and then backwards, and |Z| of the sensor was measured at the frequency of 62.8 kHz. Good reproducibility was obtained (Figure 7). The response time τres, defined as the time needed to reach 90% of the final signal for a given relative humidity, and recovery time τrec, defined as the time taken for the signal to come to within 10% of the initial valued, were determined by alternately exposing the sensors to 12%RH and 75%RH. As shown in Figure 8, the desirable reproducibility of |Z| is obtained, and the τres and τrec are shorter than those reported in refs. [8―12]. In the present paper, τres≈25 s and τrec≈5.8 s are obtained at ω≈62.8 kHz, and the result of τres much longer than τrec has not been reported elsewhere. Obviously, through-hole structural humidity sensor based on AAO membrane has excellent characteristics in linearity, reproducibility and response/recovery, ― compared with the published results[8 12]. To analyze the mechanism of present sensors, it is assumed that the most important influences are from two factors: one is the diameter of nanopores on AAO membranes (85 nm) 1
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Figure 8 Response and recovery characteristics of the sensor at different frequencies of 31.4, 62.8 and 125.6 kHz.
together with that of metal electrodes (60 nm), which avoids the appearance of capillary condensed water; the other one is the through-hole structure which provides an effective channel for the absorption and desorption of water molecules. Because no capillary condensed water is accumulated in the pores, physisorption water molecules connected by hydrogen bond are the main factor affecting the impedance. Therefore, when the relative humidity varies over the range of 12%―97%, the water molecules entering the holes can be absorbed or desorbed fast and uniformly from the chemisorption layer of the water molecules in the inner surface on the AAO membrane[17], and consequently, the properties of the sensor, such as linearity, reproducibility and response/recovery, are improved. ronmental sensing. J Nanosci Nanotechn, 2003, 3(4): 277―293[DOI] 3
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