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Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina
Journal Pre-proofs Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina C.K. Chung, O.K. Khor, E.H. Kuo, C.A. Ku PII: DOI: Reference:
S0167-577X(19)31553-8 https://doi.org/10.1016/j.matlet.2019.126921 MLBLUE 126921
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
5 October 2019 25 October 2019 30 October 2019
Please cite this article as: C.K. Chung, O.K. Khor, E.H. Kuo, C.A. Ku, Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina, Materials Letters (2019), doi: https:// doi.org/10.1016/j.matlet.2019.126921
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina C.K. Chung*, O.K. Khor, E.H. Kuo and C.A. Ku Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan *E-mail: [email protected], Tel: 886-6-2757575 ext. 62111, Fax: 886-6-2352973 Abstract Total effective surface area is an important factor for environmentally sensing performance. The porous anodic aluminum oxide (AAO) film with a high density of nanopores leads to a tremendous surface area for absorbing water molecules. But such an AAO humidity sensor formed in oxalic acid exhibits a low response of capacitance, especially under the low relative humidity (RH). Here, we demonstrate total effective surface area principle to greatly enhance the performance of AAO capacitive humidity sensor using small anodizing potential in oxalic acid. For pore-dependent surface area, the AAO pore wall would directly affect the absorbance of water molecules and the response of capacitive sensor. Decreasing the anodizing potential reduces both of the pore diameter and interpore distance proportionally but increases the surface area inversely. Therefore, the AAO sensor formed at small 20 V can greatly increase the amount of water molecules absorbed on the wall for enhancing 2~3 times response under low-to-high RH compared to those at 40 and 50 V. The good stability and reliable response/recovery time are also obtained for the AAO sensor synthesized at 20 V in oxalic acid. Keywords: Porous materials; Ceramics; Sensors; Thick films; Alumina; Surfaces
1. Introduction Total effective surface area is an important factor for environmentally sensing performance. In daily life, the capacitive humidity sensors are important and useful for many 1
industrial, food quality and environmental monitoring based on the capacitive, resistive, masssensitive or electric magnetic features [1]. The anodic aluminum oxide (AAO) has high density of nanopores and huge surface area for the humidity detection in a capacitive sensor [2]. Several studies have investigated the performance of the porous alumina humidity sensor and the surface absorption mechanism for the sensitivity characteristics. Regarding sensing mechanism, Khanna et al. [3] reported that water molecules initially chemisorbed on the metal oxide. Then the following water molecules physically bound with the hydroxyl groups by hydrogen bonds to form the first physisorbed layer. Therefore the capacitance increases slowly with relative humidity (RH) rising up to 40%–50% and then has a sharp increase after 50% RH [4]-[6]. This type of sensor suffers from insufficient response or sensitivity over a wide humidity range and could not satisfy the demands of their applications at low humidity. Some studies showed changing the pore size by pore widening for promoting performance [1,7] but the sensitive capacitance just had a little bit improved. This method also didn’t increase the pore density of AAO sensor. Therefore it wonders whether there is some method to enhance the response and linearity of capacitive AAO sensors under low-to-high humidity conditions. Here, we present a method to enhance the capacitive humidity sensors of AAO by employing various anodizing potential in oxalic acid. The pore density increases with decreasing anodizing potential for the reduced interpore distance. Thus, the enhanced surface area at a small potential of 20 V can promote the number of water molecules absorbed as much on the pore wall that highly contributes to dielectric constant and capacitance for high capacitance response. It also exhibits good stability and reliable response/recovery time. 2. Experimental procedures An electropolished pure aluminum foil (99.99%, Alfa Aesar, USA) of 0.25 mm thick with an exposed area of 1.76 cm2 was used for 2-step anodization in 0.3 M oxalic acid at 25 oC via the three-electrode electrochemical cell and the potentiostat [8]. The anodizing potentials of
2
20~50 V were controlled for AAO synthesis with the close thickness about 16 µm at different time. A 10 nm Pt thin film was deposited on AAO as one electrode of the capacitive sensor [10]. The morphology and thickness of AAO films were examined by Ultra-High-Resolution Field-Emission-Scanning-Electron Microscope (UHR-FESEM, AURIGA Zeiss, Germany). The pore size was estimated using the gray-scale imaging technique with commercial software (ImageJ). The sensor measurement system was reported in our previous works [10] with a controllable humidity level (15% ~ 80%). The capacitance of AAO humidity sensor was measured via a LCR meter (HIOKI 3522-50, Japan) at 1 kHz and 1.0 Vpp. The sensors were placed at the steady humidity level for 5 min before recording the capacitance values. The measurement of sensor was repeatedly 3 times to obtain the average value of each humidity. Regarding the response/recovery behavior, the AAO humidity sensor was measured at RH 15% for 2 min and then rapidly increasing to RH 55% for 2 min and repeated it for 3 times. 3. Results and discussion
Fig. 1 SEM micrographs and cross sections of the AAO films formed by 2-step anodization at
3
(a) 20 V for 3 h, (b) 40 V for 1 h and (c) 50 V for 40 min to keep the close thickness about 16 μm. Figs. 1(a)-(c) show the SEM plane-views and cross-sections of the AAO films formed at 20 V for 3 h, 40 V for 1 h and 50 V for 40 min, respectively. The analyzed AAO mean pore diameter and standard deviation were 31.8±3.6 nm, 53.1±3.4 nm and 71.0±3.8 nm for various anodizing voltages of 20, 40 and 50 V, respectively, as listed in Table 1. Also, the AAO interpore distance was 54 nm for 20 V, 111 nm for 40 V and 139 nm for 50V. It reveals that both the pore size and interpore distance increase with increasing anodizing voltage. The close thicknesses of three samples are 16.0 μm, 15.9 μm and 16.2 μm for the 20 V, 40 V and 50 V, respectively, as listed in Table 1.
Fig. 2 (a) The relationship between the capacitance and relative humidity of the AAO humidity sensor at 20 V, 40 V and 50 V. (b) The capacitance response value corresponding to relative humidity of AAO sensor. Fig. 2 (a) shows the relationship between the capacitance and relative humidity of AAO 4
humidity sensors at various anodizing potential. In the case of 40 V (red line), the responsive capacitance of sensor is 0.65 nF at the lowest RH of 15%. Increasing RH to 45%, the capacitance just slightly increases to 4.06 nF. As the RH is over than 45%, the capacitance markedly increases. For example, the sensor at RH of 80% exhibits a capacitance of about 52.3 nF. The non-linear response of the present sensor at 40V is consistent with the capacitive AAO humidity sensor in [4]. Also, the capacitance of AAO sensor formed at 50 V exhibits the similar trend with the sensor at 40 V although the sensor at 50 V has a larger pore size. In contrast, the AAO sensor at 20 V exhibits an excellent performance in the low-to-high RH range. For example, the capacitance of the 20 V sensor is 12.9 nF at RH 45%, which is about 3 times higher than the sensors of 40 V and 50 V. When the RH increase to 80%, the sensor capacitance of 117.3 nF also improved 2~3 times higher than the others. That is, the sensor at low 20 V can greatly increase the capacitance in the low-to-high RH. The capacitance response (Rc=C/C15, where C15 is the sensor capacitance at minimum 15% RH and ∆C is the absolute value of capacitance change induced by the increase of RH relative to 15% RH) is also shown in Fig. 2(b). The capacitance response grows faster during middle range, especially between RH of 45% and 70%. At RH of 80%, the AAO sensor fabricated at 40V has a little better response because of its low initial value (0.65 nF), compared with the one at 20V, which is 1.52 nF. This causes the sensor response to be amplified in response calculation. It is noted that the 50 V sensor is little higher than that 40 V, but all of them has a similar trend at the whole RH. Therefore, it reveals that only the anodizing potential or the pore diameter parameter does not fully explain the evolution of capacitance behavior. To elucidate the relationship between the anodizing potential and capacitance, the interpore distance and the surface area have been investigated. According to the report of Ebihara et al. [11], the interpore distance (Dint) of AAO with oxalic acid is linearly proportional with the anodizing potential (U) and can be expressed as Dint = -1.7 + 2.81 U, U ≥ 20 V
5
Here, the interpore distance of AAO at 20 V, 40 V and 50 V was 54 nm, 111 nm and 139 nm, respectively (Figs. 1(a)-(c)). The pore density which is defined as the total number of pores (n) occupying the surface area of 1 cm2 is expressed by the equation 𝑛 =
2 × 1014 3𝐷𝑖𝑛𝑡2
[12].
Because the Dint directly increases with the anodizing potential, the pore density will decrease with increasing anodizing potential. That is, the pore density is 3.81010, 9.4109 and 6.0109 unit/cm2 for the anodizing potential at 20, 40 and 50 V, respectively, as listed in Table 1. The total surface area of AAO for the absorbed water molecules can be expressed by the equation: A= n × 𝜋𝑑 × h, where the d is the diameter of AAO pore and h the thickness of AAO. The calculated surface area of AAO sensors formed in 1 cm2 at 20, 40 and 50 V was 6.1102, 2.5102, 2.0102 cm2, respectively, as listed in Table 1. It reveals that the surface area has a slight difference between the 40 V and 50 V AAOs. But the surface area of AAO at 20 V is 2~3 times larger than those at 40 V and 50 V. The major contribution of AAO capacitive humidity sensor is from water molecules with an inherent dielectric constant of about 80, which is much higher than that of either air of 1 and the empty porous AAO film of 9.3 ~ 11.5. When the RH increases, the water molecules start absorb on the surface of pore wall and contribute to the dielectric constant. Therefore, the total surface area of the AAO pore wall can directly affect the capacitive sensor. Decreasing the anodizing potential reduces both the pore diameter and interpore distance proportionally, but increases the surface area inversely. The larger total surface area of AAO enhances more water molecules absorbed on the pore wall as shown in Fig 3(a). It leads to the enhancement of water molecules contribution to the dielectric constant and capacitance, no matter what at low or high RH. On the contrary, the high anodizing potential results in the larger pore diameter and pore distance that reduces total surface area as well as the number of water molecules bonding with the pore wall due to water molecules can rest at the pore center, as shown in Fig 3(b). Thus it is the reason why the sensor
6
at a small anodizing potential of 20 V exhibits an excellent performance and linear-like response under low-to-high RH.
Fig. 3 Schematic diagrams of water molecules absorbed on the AAO pore wall in the: (a) small and (b) large pore size.
Figure 4 The response time (Trs(10-90)) and recovery time (Trc(10-90)) of the AAO sensor at 20 V in 0.3 M oxalic acid. The response time (Trs(10-90)) and recovery time (Trc(10-90)) corresponding to the AAO sensor at 20 V in oxalic acid is shown in Fig 4. The response time is defined as the total time to reached 90% of the stable capacitance for a given RH (55%) and the recovery time defined as the time
7
needed to come to within 10% of the initial capacitance under RH(15%). It is noted that the second peak is little higher than the others. Since the graph is the relationship between the capacitance value and relative humidity measured by the commercial sensor that cannot display the slight humidity change, it leads to the difficulty on controlling chamber humidity. On the other hand, our AAO sensor is precise enough to detect the small change, so that is why the second peak is little higher. The reliable and stability of the AAO sensor was determined by 3 times alternately exposing to the chamber with capacitance measured at 1 kHz. It reveals that the response of the AAO sensor formed at 20 V is a stable and reliable one. And the average of the response time and recovery time are 45 s and 36 s, respectively. 4. Conclusions The enhancement of AAO capacitive humidity sensors formed at various anodizing potential is examined. The AAO sensor at a small anodizing potential exhibits a linear-like response under low-to-high RH condition because the AAO has a larger effective surface area. Table 1 lists the detailed pore parameters of AAO formed at 20~50 V. Decreasing the anodizing potential reduces both the pore diameter and interpore distance for the enhanced total surface area for absorbing more water molecules on the walls. Therefore, the AAO humidity sensor at 20 V achieves a greatly linear-like response in capacitance and it improves 2~3 times higher capacitance than the others at 40 and 50 V. In addition, the reliable response and recovery time are measured to be 45 s and 36 s, respectively. Acknowledgement This work is partially sponsored by the Ministry of Science and Technology (MOST) under contract No MOST 106-2221-E-006-101-MY3. References [1] P. Bindra, A. Hazra, J Mater Sci: Mater Electron 29 (2018) 6129-6148. [2] C.K. Chung, O.K. Khor, C.J. Syu and S.W. Chen, Sens. Actuator B-Chem., 210 (2015)
8
69-74. [3] V.K. Khanna, R.K. Nahar, Appl. Surf. Sci., 28 (1987) 247-264. [4] R.K. Nahar, Sens. Actuator B-Chem., 63 (2000) 49-54. [5] V.K. Khanna, R.K. Nahar, Journal of Physics D-Applied Physics, 19 (1986) L141-L145. [6] Y. Kim, B. Jung, H. Lee, H. Kim, K. Lee, H. Park, Sensors and Actuators B: Chemical, 141 (2009) 441-446. [7] M.A. Kashi, A. Ramazani, H. Abbasian, A. Khayyatian, Sens. Actuator A-Phys., 174 (2012) 69-74. [8] C.K. Chung, W.T. Chang, M.W. Liao, H.C. Chang, Materials Letters 88 (2012) 104-107. [9] C.K. Chung, T.Y. Liu, W.T. Chang, Microsystem Technologies 16 (2010) 1451-1456, 2010. [10] S.W. Chen, O.K. Khor, M.W. Liao, C.K. Chung, Sens. Actuator B-Chem., 199 (2014) 384-388. [11] H.T. K. Ebihara, M. Nagayama, J. Met. Finish. Soc. Japan, 33 (1982) 156–164. [12] G.D. Sulka, W.J. Stepniowski, Electrochimica Acta, 54 (2009) 3683-3691. Table caption Table 1 The anodizing voltage and analyzed data of AAO structure. Anodizing potential (V)
20
40
50
Interpore distance (nm)
54
111
139
Mean pore diameter (nm)
31.8
53.1
71.0
Thickness (μm)
16.0
15.9
16.2
Pore density (unit/cm2)
3.81010
9.4109
6.0109
Surface area (cm2)
6.1102
2.5102
2.0102
9
Highlights Enhancing total effective surface area can enhance alumina humidity sensing. Reducing anodizing potential for the increased surface area benefits capacitance sensing. The alumina humidity sensor formed at 20 V enhances 2~3 times capacitance than those at 40-50 V. An enhanced linear-like relationship between the capacitance and humidity occurs. The theoretic surface area enhancement mechanism is presented.
10
S0167-577X(19)31553-8 https://doi.org/10.1016/j.matlet.2019.126921 MLBLUE 126921
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
5 October 2019 25 October 2019 30 October 2019
Please cite this article as: C.K. Chung, O.K. Khor, E.H. Kuo, C.A. Ku, Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina, Materials Letters (2019), doi: https:// doi.org/10.1016/j.matlet.2019.126921
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina C.K. Chung*, O.K. Khor, E.H. Kuo and C.A. Ku Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan *E-mail: [email protected], Tel: 886-6-2757575 ext. 62111, Fax: 886-6-2352973 Abstract Total effective surface area is an important factor for environmentally sensing performance. The porous anodic aluminum oxide (AAO) film with a high density of nanopores leads to a tremendous surface area for absorbing water molecules. But such an AAO humidity sensor formed in oxalic acid exhibits a low response of capacitance, especially under the low relative humidity (RH). Here, we demonstrate total effective surface area principle to greatly enhance the performance of AAO capacitive humidity sensor using small anodizing potential in oxalic acid. For pore-dependent surface area, the AAO pore wall would directly affect the absorbance of water molecules and the response of capacitive sensor. Decreasing the anodizing potential reduces both of the pore diameter and interpore distance proportionally but increases the surface area inversely. Therefore, the AAO sensor formed at small 20 V can greatly increase the amount of water molecules absorbed on the wall for enhancing 2~3 times response under low-to-high RH compared to those at 40 and 50 V. The good stability and reliable response/recovery time are also obtained for the AAO sensor synthesized at 20 V in oxalic acid. Keywords: Porous materials; Ceramics; Sensors; Thick films; Alumina; Surfaces
1. Introduction Total effective surface area is an important factor for environmentally sensing performance. In daily life, the capacitive humidity sensors are important and useful for many 1
industrial, food quality and environmental monitoring based on the capacitive, resistive, masssensitive or electric magnetic features [1]. The anodic aluminum oxide (AAO) has high density of nanopores and huge surface area for the humidity detection in a capacitive sensor [2]. Several studies have investigated the performance of the porous alumina humidity sensor and the surface absorption mechanism for the sensitivity characteristics. Regarding sensing mechanism, Khanna et al. [3] reported that water molecules initially chemisorbed on the metal oxide. Then the following water molecules physically bound with the hydroxyl groups by hydrogen bonds to form the first physisorbed layer. Therefore the capacitance increases slowly with relative humidity (RH) rising up to 40%–50% and then has a sharp increase after 50% RH [4]-[6]. This type of sensor suffers from insufficient response or sensitivity over a wide humidity range and could not satisfy the demands of their applications at low humidity. Some studies showed changing the pore size by pore widening for promoting performance [1,7] but the sensitive capacitance just had a little bit improved. This method also didn’t increase the pore density of AAO sensor. Therefore it wonders whether there is some method to enhance the response and linearity of capacitive AAO sensors under low-to-high humidity conditions. Here, we present a method to enhance the capacitive humidity sensors of AAO by employing various anodizing potential in oxalic acid. The pore density increases with decreasing anodizing potential for the reduced interpore distance. Thus, the enhanced surface area at a small potential of 20 V can promote the number of water molecules absorbed as much on the pore wall that highly contributes to dielectric constant and capacitance for high capacitance response. It also exhibits good stability and reliable response/recovery time. 2. Experimental procedures An electropolished pure aluminum foil (99.99%, Alfa Aesar, USA) of 0.25 mm thick with an exposed area of 1.76 cm2 was used for 2-step anodization in 0.3 M oxalic acid at 25 oC via the three-electrode electrochemical cell and the potentiostat [8]. The anodizing potentials of
2
20~50 V were controlled for AAO synthesis with the close thickness about 16 µm at different time. A 10 nm Pt thin film was deposited on AAO as one electrode of the capacitive sensor [10]. The morphology and thickness of AAO films were examined by Ultra-High-Resolution Field-Emission-Scanning-Electron Microscope (UHR-FESEM, AURIGA Zeiss, Germany). The pore size was estimated using the gray-scale imaging technique with commercial software (ImageJ). The sensor measurement system was reported in our previous works [10] with a controllable humidity level (15% ~ 80%). The capacitance of AAO humidity sensor was measured via a LCR meter (HIOKI 3522-50, Japan) at 1 kHz and 1.0 Vpp. The sensors were placed at the steady humidity level for 5 min before recording the capacitance values. The measurement of sensor was repeatedly 3 times to obtain the average value of each humidity. Regarding the response/recovery behavior, the AAO humidity sensor was measured at RH 15% for 2 min and then rapidly increasing to RH 55% for 2 min and repeated it for 3 times. 3. Results and discussion
Fig. 1 SEM micrographs and cross sections of the AAO films formed by 2-step anodization at
3
(a) 20 V for 3 h, (b) 40 V for 1 h and (c) 50 V for 40 min to keep the close thickness about 16 μm. Figs. 1(a)-(c) show the SEM plane-views and cross-sections of the AAO films formed at 20 V for 3 h, 40 V for 1 h and 50 V for 40 min, respectively. The analyzed AAO mean pore diameter and standard deviation were 31.8±3.6 nm, 53.1±3.4 nm and 71.0±3.8 nm for various anodizing voltages of 20, 40 and 50 V, respectively, as listed in Table 1. Also, the AAO interpore distance was 54 nm for 20 V, 111 nm for 40 V and 139 nm for 50V. It reveals that both the pore size and interpore distance increase with increasing anodizing voltage. The close thicknesses of three samples are 16.0 μm, 15.9 μm and 16.2 μm for the 20 V, 40 V and 50 V, respectively, as listed in Table 1.
Fig. 2 (a) The relationship between the capacitance and relative humidity of the AAO humidity sensor at 20 V, 40 V and 50 V. (b) The capacitance response value corresponding to relative humidity of AAO sensor. Fig. 2 (a) shows the relationship between the capacitance and relative humidity of AAO 4
humidity sensors at various anodizing potential. In the case of 40 V (red line), the responsive capacitance of sensor is 0.65 nF at the lowest RH of 15%. Increasing RH to 45%, the capacitance just slightly increases to 4.06 nF. As the RH is over than 45%, the capacitance markedly increases. For example, the sensor at RH of 80% exhibits a capacitance of about 52.3 nF. The non-linear response of the present sensor at 40V is consistent with the capacitive AAO humidity sensor in [4]. Also, the capacitance of AAO sensor formed at 50 V exhibits the similar trend with the sensor at 40 V although the sensor at 50 V has a larger pore size. In contrast, the AAO sensor at 20 V exhibits an excellent performance in the low-to-high RH range. For example, the capacitance of the 20 V sensor is 12.9 nF at RH 45%, which is about 3 times higher than the sensors of 40 V and 50 V. When the RH increase to 80%, the sensor capacitance of 117.3 nF also improved 2~3 times higher than the others. That is, the sensor at low 20 V can greatly increase the capacitance in the low-to-high RH. The capacitance response (Rc=C/C15, where C15 is the sensor capacitance at minimum 15% RH and ∆C is the absolute value of capacitance change induced by the increase of RH relative to 15% RH) is also shown in Fig. 2(b). The capacitance response grows faster during middle range, especially between RH of 45% and 70%. At RH of 80%, the AAO sensor fabricated at 40V has a little better response because of its low initial value (0.65 nF), compared with the one at 20V, which is 1.52 nF. This causes the sensor response to be amplified in response calculation. It is noted that the 50 V sensor is little higher than that 40 V, but all of them has a similar trend at the whole RH. Therefore, it reveals that only the anodizing potential or the pore diameter parameter does not fully explain the evolution of capacitance behavior. To elucidate the relationship between the anodizing potential and capacitance, the interpore distance and the surface area have been investigated. According to the report of Ebihara et al. [11], the interpore distance (Dint) of AAO with oxalic acid is linearly proportional with the anodizing potential (U) and can be expressed as Dint = -1.7 + 2.81 U, U ≥ 20 V
5
Here, the interpore distance of AAO at 20 V, 40 V and 50 V was 54 nm, 111 nm and 139 nm, respectively (Figs. 1(a)-(c)). The pore density which is defined as the total number of pores (n) occupying the surface area of 1 cm2 is expressed by the equation 𝑛 =
2 × 1014 3𝐷𝑖𝑛𝑡2
[12].
Because the Dint directly increases with the anodizing potential, the pore density will decrease with increasing anodizing potential. That is, the pore density is 3.81010, 9.4109 and 6.0109 unit/cm2 for the anodizing potential at 20, 40 and 50 V, respectively, as listed in Table 1. The total surface area of AAO for the absorbed water molecules can be expressed by the equation: A= n × 𝜋𝑑 × h, where the d is the diameter of AAO pore and h the thickness of AAO. The calculated surface area of AAO sensors formed in 1 cm2 at 20, 40 and 50 V was 6.1102, 2.5102, 2.0102 cm2, respectively, as listed in Table 1. It reveals that the surface area has a slight difference between the 40 V and 50 V AAOs. But the surface area of AAO at 20 V is 2~3 times larger than those at 40 V and 50 V. The major contribution of AAO capacitive humidity sensor is from water molecules with an inherent dielectric constant of about 80, which is much higher than that of either air of 1 and the empty porous AAO film of 9.3 ~ 11.5. When the RH increases, the water molecules start absorb on the surface of pore wall and contribute to the dielectric constant. Therefore, the total surface area of the AAO pore wall can directly affect the capacitive sensor. Decreasing the anodizing potential reduces both the pore diameter and interpore distance proportionally, but increases the surface area inversely. The larger total surface area of AAO enhances more water molecules absorbed on the pore wall as shown in Fig 3(a). It leads to the enhancement of water molecules contribution to the dielectric constant and capacitance, no matter what at low or high RH. On the contrary, the high anodizing potential results in the larger pore diameter and pore distance that reduces total surface area as well as the number of water molecules bonding with the pore wall due to water molecules can rest at the pore center, as shown in Fig 3(b). Thus it is the reason why the sensor
6
at a small anodizing potential of 20 V exhibits an excellent performance and linear-like response under low-to-high RH.
Fig. 3 Schematic diagrams of water molecules absorbed on the AAO pore wall in the: (a) small and (b) large pore size.
Figure 4 The response time (Trs(10-90)) and recovery time (Trc(10-90)) of the AAO sensor at 20 V in 0.3 M oxalic acid. The response time (Trs(10-90)) and recovery time (Trc(10-90)) corresponding to the AAO sensor at 20 V in oxalic acid is shown in Fig 4. The response time is defined as the total time to reached 90% of the stable capacitance for a given RH (55%) and the recovery time defined as the time
7
needed to come to within 10% of the initial capacitance under RH(15%). It is noted that the second peak is little higher than the others. Since the graph is the relationship between the capacitance value and relative humidity measured by the commercial sensor that cannot display the slight humidity change, it leads to the difficulty on controlling chamber humidity. On the other hand, our AAO sensor is precise enough to detect the small change, so that is why the second peak is little higher. The reliable and stability of the AAO sensor was determined by 3 times alternately exposing to the chamber with capacitance measured at 1 kHz. It reveals that the response of the AAO sensor formed at 20 V is a stable and reliable one. And the average of the response time and recovery time are 45 s and 36 s, respectively. 4. Conclusions The enhancement of AAO capacitive humidity sensors formed at various anodizing potential is examined. The AAO sensor at a small anodizing potential exhibits a linear-like response under low-to-high RH condition because the AAO has a larger effective surface area. Table 1 lists the detailed pore parameters of AAO formed at 20~50 V. Decreasing the anodizing potential reduces both the pore diameter and interpore distance for the enhanced total surface area for absorbing more water molecules on the walls. Therefore, the AAO humidity sensor at 20 V achieves a greatly linear-like response in capacitance and it improves 2~3 times higher capacitance than the others at 40 and 50 V. In addition, the reliable response and recovery time are measured to be 45 s and 36 s, respectively. Acknowledgement This work is partially sponsored by the Ministry of Science and Technology (MOST) under contract No MOST 106-2221-E-006-101-MY3. References [1] P. Bindra, A. Hazra, J Mater Sci: Mater Electron 29 (2018) 6129-6148. [2] C.K. Chung, O.K. Khor, C.J. Syu and S.W. Chen, Sens. Actuator B-Chem., 210 (2015)
8
69-74. [3] V.K. Khanna, R.K. Nahar, Appl. Surf. Sci., 28 (1987) 247-264. [4] R.K. Nahar, Sens. Actuator B-Chem., 63 (2000) 49-54. [5] V.K. Khanna, R.K. Nahar, Journal of Physics D-Applied Physics, 19 (1986) L141-L145. [6] Y. Kim, B. Jung, H. Lee, H. Kim, K. Lee, H. Park, Sensors and Actuators B: Chemical, 141 (2009) 441-446. [7] M.A. Kashi, A. Ramazani, H. Abbasian, A. Khayyatian, Sens. Actuator A-Phys., 174 (2012) 69-74. [8] C.K. Chung, W.T. Chang, M.W. Liao, H.C. Chang, Materials Letters 88 (2012) 104-107. [9] C.K. Chung, T.Y. Liu, W.T. Chang, Microsystem Technologies 16 (2010) 1451-1456, 2010. [10] S.W. Chen, O.K. Khor, M.W. Liao, C.K. Chung, Sens. Actuator B-Chem., 199 (2014) 384-388. [11] H.T. K. Ebihara, M. Nagayama, J. Met. Finish. Soc. Japan, 33 (1982) 156–164. [12] G.D. Sulka, W.J. Stepniowski, Electrochimica Acta, 54 (2009) 3683-3691. Table caption Table 1 The anodizing voltage and analyzed data of AAO structure. Anodizing potential (V)
20
40
50
Interpore distance (nm)
54
111
139
Mean pore diameter (nm)
31.8
53.1
71.0
Thickness (μm)
16.0
15.9
16.2
Pore density (unit/cm2)
3.81010
9.4109
6.0109
Surface area (cm2)
6.1102
2.5102
2.0102
9
Highlights Enhancing total effective surface area can enhance alumina humidity sensing. Reducing anodizing potential for the increased surface area benefits capacitance sensing. The alumina humidity sensor formed at 20 V enhances 2~3 times capacitance than those at 40-50 V. An enhanced linear-like relationship between the capacitance and humidity occurs. The theoretic surface area enhancement mechanism is presented.
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