Electrochemistry Communications 28 (2013) 95–99

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Electrochemistry Communications

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Good sensitivity and high stability of humidity sensor using micro-arc oxidationalumina film

Xinghua Guo a, Keqin Du a,⁎, Hao Ge b, Quanzhong Guo a, Yong Wang a, Fuhui Wang a

a State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Science, Shenyang, Chinab Liaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang, China

⁎ Corresponding author. Tel.: +86 24 23895348; fax:E-mail address: kqdu@imr.ac.cn (K. Du).

1388-2481/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.elecom.2012.11.036

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 November 2012Received in revised form 29 November 2012Accepted 29 November 2012Available online 8 December 2012

Keywords:Humidity sensorSensitivityStabilityMicro-arc oxidationAl2O3

Humidity sensor based on thinmicro-arc oxidation alumina (MOA) filmwas fabricated by a transient self-feedbackcontrol (TSFC) technique for the first time. The MOA film consisted of a polycrystalline nature of α and γ-Al2O3

phases,while high surface/volume ratio led to remarkable adsorption of theMOAfilm.Moreover, the humidity per-formance of the MOA sensor indicated good sensitivity of 8045.55 at 98% RH. Fast response and recovery times ofthe MOA sensor were 90 s and 30 s, respectively, at 20 °C and 1000 Hz. Exposing the MOA sensor to differenthumidities for longer durations indicated an excellent stability due to theα-Al2O3 phase in theMOAfilm. Themech-anism in the sensor, to humidity, was explained by using equivalent circuits of (RC) and (C(RZ)). In the low RHrange, the process was mainly dominated by the migration of protons through the adsorbed water moleculelayer on the surface of theMOA film; while, in the high RH range, the process was due to the faster diffusion of pro-tons in the single or multiple layers that were formed on the materials surface.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Humidity is the most frequently used parameter that influencesvarious chemical, physical and biological processes. Therefore, hu-midity must be monitored and controlled. From the viewpoints of me-chanical strength, temperature capability, reliability and (low) cost, thinfilmsmade fromporous anodic alumina (PAA) appear to bemore suitablecandidates for commercial humidity sensors, which have received muchattention relating to material technology, sensor fabrication and perfor-mance characterization in recent years [1–5]. Unfortunately, the sensorshows significant degradation of sensitivity and drift in the capacitancecharacteristics after long durations at high humidity due to the formationof a new boehmite phase (γ-Al2O3 H2O) [6]. Thismeans that it is very im-portant for alumina humidity sensors to be stable against water.

Recently, we employed a specific densification treatment of transientself-feedback control (TSFC) to design a new sensor based on a thinMOAfilm. According to our previous studies [7,8], theMOA filmwhen used asthe humidity sensing material has the following advantages: firstly, theceramic MOA film when under a high electric field has greater hardnessand better mechanical strength. Secondly, the MOA film was designedflexibly to form a larger specific surface through a complex voltagewave-form, consisting of a matrix square voltage (MSV) waveform and ahigh-frequency square voltage (HFSV) waveform. Thirdly, a more stablemicrostructure containing ceramicα-Al2O3was obtained by dynamicallyaltering the frequency of the HFSV waveform. Therefore, we propose ahumidity sensor, based on the above fabrication process, that uses the

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thin MOA film to obtain better sensitivity, a faster response and greaterstability. In addition, a conventional PAA sensor was fabricated to act asa baseline against which the MOA sensor's humidity sensing propertiescould be compared. Lastly, the sensing mechanism was derived fromthe analysis of Nyquist diagrams.

2. Experimental

Aluminum sheets (thickness: 0.5 mm; purity: >99.999%) wereobtained from Joinworld Co., Ltd. (Xinjiang, China). The PAA filmwas prepared by using a two-step anodization technique followinga previously published method [9]. The first step anodization and sec-ond step anodization were performed in a 1:1 sulfuric/oxalic acidmixture at 26 V and 5 °C for 3 h each. After the first step anodization,the formed alumina was etched off in an aqueous solution containing1.8% CrO3 (wt.%) and 6% H3PO4 (wt.%) at 60 °C for 2 h. After the sec-ond anodization, the remaining aluminum substrate was removed bysoaking in a CuCl2-based solution (100 ml of HCl (38%)+100 ml ofH2O+3.4 g of CuCl2·H2O) at room temperature for about 10 min.The perforated porous alumina template was prepared by removingthe bottom part (barrier layer) of the template in 5 wt.% H3PO4 at35 °C.

The MOA film was processed by using a pulse power supply(Duercoat IV), which was able to deliver the monopolar polar pulsecarrier wave described (M-TSFC) in Ref. [7,8]. As shown in Fig. 1(a)and (b), the TSFC mode exported a complex square voltage waveformthrough loading the HFSV waveform on the MSV waveform. Theanode was an Al sheet while the cathode was a stainless steel plate.The MOA process time was 40 min, and the temperature of the

a b

dc

fe

Fig. 1. (a) (b)Oscillograms of TSFC mode; (c) (d) Morphologies and (e) (f) phases of PAA film and MOA films.

96 X. Guo et al. / Electrochemistry Communications 28 (2013) 95–99

electrolyte was maintained below 35 °C during the entire processusing a Durachill cooling system from Polyscience Co. The electro-lytes, NaOH (3–5 g/L), Na2SiO3 (2–5 g/L) and organic addition agent(1 g/L), were dissolved in deionized water at room temperature(25 °C). The thin PAA and MOA films were transformed into humiditysensors by sputtering a 40–80 nm layer of platinum on both sides.

This served as the working electrode in the subsequent metal electro-deposition process.

Surface morphologies of the two films were characterized by scan-ning electron microscopy (SEM, Hitachi S4880). The X-ray diffraction(XRD) data of thin films were collected using a Philips X'Pert Pro withCu-Kα radiation and 2° grazing angle. Brunauer–Emmett–Teller (BET)

0 20 40 60 80 100

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PAA 25 1000Hz 1000Hz

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Time (Days)

Fig. 2. Humidity performances of PAA and MOA sensors: (a) sensitivity; (b) (c) response and recovery transient; (d) (e) stability exposed in water vapor for a longer duration.

97X. Guo et al. / Electrochemistry Communications 28 (2013) 95–99

surface area measurements were performed on an Autosorb 1 in N-2-adsorption mode. Humidity sensing performances of the sensorswere tested as a function of relative humidity (RH) with a TH2617LCR analyzer (Changzhou, China) and an applied AC voltage of 1 V

and 1 kHz in a home-built testing chamber. The humidity mechanismwas studied throughNyquist diagramsperformed on a PAR 273 electro-chemical station (frequency range: 105 Hz to 10 Hz). The chamber wasfilled with saturated solutions of LiCl, MgCl2, Mg(NO3)2, NaCl, and

98 X. Guo et al. / Electrochemistry Communications 28 (2013) 95–99

K2SO4, to calibrate the RH to 11%, 33%, 55%, 75% and 98%, respectively, atroom temperature. The sensors were stored in a dry environmentwhennot being used, and then suspended above each solution, withouttouching it, in the sealed chamber for each experiment.

3. Results and discussion

Fig. 1(c)–(f) shows SEM images and XRD patterns of PAA andMOAfilms. It can be seen from Fig. 1(c) and (e) that the nanopores of thePAA film, consisting of γ-Al2O3 phase, were uniform and highly or-dered with an average diameter of about 45 nm. In contrast, the mor-phology of the MOA film (Fig. 1d) showed that a large number ofanomalous pores (0.5–2 μm in diameter) and ceramic particles weresuperposed together in the film, which formed a complex and inter-laced network structure. It was indicated by the XRD pattern (Fig. 1f)that the MOA film had a polycrystalline nature from the α and γ-Al2O3

films. The BET surface areas of the PAA and MOA films were about 13.4and 17.2 mm2/g, respectively. These results can be used to concludethat theMOAfilmwould be a goodmaterial for forming humidity sensorsdue to its polycrystalline phase and high surface/volume ratio.

The RHdependent sensitivity values, defined as the ratio of the capac-itancemeasured for the actual vapor concentration against the respectivevalue for 11% RH, are compared in Fig. 2(a) [10]. The sensitivity of theMOA sensorwas considerably enhanced in comparison to the sensor fab-ricated from the PAA sheet. The sensitivity of the MOA sensor at 98% RHwas about 8045.55, thus being 292.458 times higher than that of the PAAsensor (27.51), and it demonstrated good sensitivity. Fig. 2(b) and (c)show the response and recovery behavior of the PAA and MOA sensorsat 25 °C and 1000 Hz. It can be seen that the MOA sensor showed a fastresponse (90 s) and recovery (30 s) to water vapor, whereas the PAAsensor exhibited slow response (140 s) and recovery (75 s) times. Thestability of the PAA andMOA sensors to different humidities for differenttimes are shown in Fig. 2(d) and (e). The capacitance of the PAA sensorgreatly increased at different humidities (Fig. 2d). The reason for this is

a

c

0 500 1000 1500 2000 2500 30000

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Zre(kΩ)

Zim

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)

11%

33%

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98%

fit lines

×40×25

×12×5

×1

Fig. 3. Nyquist diagrams of (a) PAA and (b) MOA sensors (The multiplied numbers are the

that moisture seepage in the cell structure reacts with the amorphousor γ-Al2O3 film to form a new boehmite phase (γ-Al2O3 H2O) when ex-posed for longer durations or at high humidities, which leads to a declinein porosity near the surface and degradation of the sensor's characteris-tics [6]. In contrast, at the five testing humidities and durations therewas very little change exhibited in the capacitance of the MOA film(Fig. 2e). This result indicated that the MOA film has a strong structureresulting in good stability to water when exposed for a longer duration,due to the α-Al2O3 phase in the MOA film.

The sensing mechanisms of the above-mentioned humidity sen-sors were investigated through the analysis of the Nyquist diagramsat different RH. It can be seen from Fig. 3(a) that the Nyquist diagramsof PAA sensor for 11% RH looked like an inclined semicircle. As RH in-creased (55% RH) a low frequency line connectedwith the high frequen-cy semicircle, which seemed to be the beginning of another semicircle.The higher the RH, the longer the line and the smaller the first semicirclebecame.When RHwas 98%, the high frequency semicircle appeared as aminimum value which connected to an extremely long line. For theNyquist diagrams of the MOA sensor (Fig. 3b) the appearance of thelow frequency line advanced at 33% RH. When the RH was 55%, thecurve looked like a line. The lines became shorter as RH increased.

The semicircle represents a kind of polarization mechanism [11–13]and can be fitted to an equivalent parallel circuit (RC) [13] in Fig. 3(c).The parallel Rf and Cf are the resistance and capacitance of the PAA andMOA films. At low RH of PAA (11%–33%RH) and MOA (11%RH) sensors,these semicircular Nyquist diagrams indicate that the migration of theproton through the adsorbed water molecule layer, on the surface ofthe porous PAA and MOA films, is the main process. Under this circum-stance, no continuous aquatic layer is formed owing to insufficient ad-sorption of water vapor [14]. With increasing adsorption, linear tailsoccurred at lower frequencies (PAA 55%–75%RH and MOA 33%RH) thatwere accompanied by a significant decrease in the semicircular part ofthe diagram. The linear tail represents a typical Warburg impedancecaused by diffusion of reactants at the electrode/electrolyte interface

b

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) ×10

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magnification of the Nyquist plots.); Equivalent circuits of (c) (RC) and (d) (C(RZ)).

99X. Guo et al. / Electrochemistry Communications 28 (2013) 95–99

[15]. The element ofWarburg impedance Zi was added in serieswith Rf tothe equivalent circuit (C(RZ)) (Fig. 3d) [13],which represents the faradaicimpedance (including the Warburg impedance) at the electrode/film in-terface. A 45° line (PAA 98%RH and MOA 55%–98%RH) formed on theNyquist diagrams for all frequencies when adsorption was increased fur-ther. Here, the electrolytic conductionmainly arises from the faster diffu-sion of protons in the single or multiple layers that are formed on thematerial's surface [12,13].

The transition threshold of the two sensing mechanisms for theMOA sensor is about 33% RH at room temperature; in contrast tothe transition point of 55% RH for the PAA sensor (Fig. 3). A straightline at the full frequency range for the PAA sensor only occurred at98% RH, in contrast a straight line at the full frequency range was ob-served at 55% RH for the MOA sensor. In combination with the de-creasing value of the transition threshold, this behavior indicatesthat the MOA film enhances water adsorption on the sensor surfacebecause of its high surface/volume ratio.

4. Conclusion

(1) TheMOA filmwith a large specific surface consisted of a polycrys-talline naturewithα andγ-Al2O3 phases. The high surface/volumeratio probably led to remarkable adsorption of the MOA film.

(2) The sensor based on the MOA film for humidity performs withgood sensitivity (8045.55 at 98% RH), fast response (90 s) and re-covery (30 s) to water vapor; and, has excellent stability to waterwhen exposed for a longer duration due to the α-Al2O3 phase inthe MOA film.

(3) In the low RH range, the process was mainly dominated by themigration of protons through the adsorbed water molecule

layer on the surface of the MOA film; while, in the high RHrange, the process was due to the faster diffusion of protons inthe single or multiple layers that were formed on the materialssurface.

Acknowledgment

Thisworkwas supported by a project of theNational Natural ScienceFoundation (50971126).

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