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[Takayuki Nakane](https://orcid.org/0000-0003-0282-169X), [Takashi Naka](https://orcid.org/0000-0002-0645-6952), Minako Nakayama, [Tetsuo Uchikoshi](https://orcid.org/0000-0003-3847-4781)

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[Humidity Sensitivity of Chemically Synthesized ZnAl2O4/Al](https://mdr.nims.go.jp/datasets/20ba280b-d600-48ef-834a-62e9ca2fb098)

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Humidity Sensitivity of Chemically Synthesized ZnAl2O4/AlCitation: Nakane, T.; Naka, T.;Nakayama, M.; Uchikoshi, T.Humidity Sensitivity of ChemicallySynthesized ZnAl2O4/Al. Sensors2022, 22, 6194. https://doi.org/10.3390/s22166194Academic Editor: Eduard LlobetReceived: 30 June 2022Accepted: 9 August 2022Published: 18 August 2022Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2022 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).sensorsArticleHumidity Sensitivity of Chemically Synthesized ZnAl2O4/AlTakayuki Nakane *, Takashi Naka, Minako Nakayama and Tetsuo UchikoshiFine Particles Engineering Group, Research Center for Functional Materials, National Institute for MaterialsScience, 1-2-1 Sengen, Tsukuba 305-0047, Ibaraki, Japan* Correspondence: nakane.Takayuki@nims.go.jp; Tel.: +81-29-859-2325Abstract: Humidity sensitivity is evaluated for chemically synthesized ZnAl2O4/Al devices. We suc-ceeded in synthesizing the ZnAl2O4/Al device by applying chemical techniques only. Hydrothermaltreatment for the anodized aluminum (AlOx/Al) gives us the device of the ZnAl2O4/Al structure.All fabrication processes were conducted under 400 ◦C. The key was focusing on ZnAl2O4 as thesensing material instead of MgAl2O4, which is generally investigated as the humidity sensor. Theevaluation of this ZnAl2O4/Al device clarified its effectiveness as a sensor. Both electrical capacitance,Cp, and the resistivity, Rp, measured by an LCR meter, obviously responded to the humidity withgood sensitivity and appreciable repeatability. Our synthesis technique is possible in principle toimprove on the process for the device with a complex structure providing a large surface area. Thesecharacteristics are believed to expand the application study of spinel aluminate devices as the sensor.Keywords: gas sensor; humidity; ZnAl2O4; hydrothermal synthesis; anodization; oxide; devicedesign; environmental monitoring; semiconductor1. IntroductionHumidity is an important parameter. For instance, the semiconductor industry, foodpackaging industry, medical industry, and textile industry utilize humidity sensors formonitoring/controlling it during the production/storage process. For this usage, poly-meric humidity sensors [1–3] have advantages for cost, but they have issues with stability,especially under conditions at high temperatures and/or at high humidity. The stabilityunder high-humidity conditions is not only an issue for polymeric sensors but also forceramics-based sensors. For example, humidity sensors based on aluminum oxide exhibitan excellent response [4–6], but high-humidity conditions have the possibility to transformaluminum oxide into boehmite (γ-AlO(OH)). Moreover, high flexibility for the shape de-sign becomes important for all sensing devices corresponding to the advanced technology,the sophisticated industry, and the enhancement of the functionality. Standing on thisbackground, novel ceramics sensors are still required, and numerous effective materialsare also reported [7–14]. MgAl2O4 is one of the prospective materials for these discus-sions [15–17]. A wide range response (2–98 %RH) is the attractive point of this material asa humidity sensor [18]. Then, this material is chemically quite stable under atmospheres ofhigh temperatures and high humidity. In addition, the sustainability of the resource supplyand low material cost is also one of the expectable points of MgAl2O4. The most importantissue for a MgAl2O4 humidity sensor is thought to be the effective technique of reducingthe fabrication cost.The humidity sensing device of MgAl2O4 is generally fabricated as a thick film bya technique based on solid-state reactions [15,17,19–21]. It requires a high heating tem-perature of over 1000 ◦C. This kind of technique is reliable for study in the laboratory,but establishing the industrial fabrication technique seems to be difficult, especially whendiscussing the fabricating cost and the flexibility for the shape design of the product. On theother hand, thin or thick film fabrication does not require such a high temperature; however,they tend to require well-sintered powder [16,22]. These approaches also have issues withSensors 2022, 22, 6194. https://doi.org/10.3390/s22166194 https://www.mdpi.com/journal/sensorshttps://doi.org/10.3390/s22166194https://doi.org/10.3390/s22166194https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/sensorshttps://www.mdpi.comhttps://orcid.org/0000-0003-3847-4781https://doi.org/10.3390/s22166194https://www.mdpi.com/journal/sensorshttps://www.mdpi.com/article/10.3390/s22166194?type=check_update&version=1Sensors 2022, 22, 6194 2 of 13the flexibility of the shape design of the product. Standing on these considerations, thechemical synthesis route is desirable in order to develop the fabrication technique meetingthe advancing requirements. Here, we inspired the chemical synthesis of MgAl2O4 fromthe AlOx/Al device. The chemical preparation technique of the AlOx/Al device is alreadyestablished in this case as the anodization technique of aluminum, thus we should consideronly the chemical reaction of MgAl2O4 from AlOx. Aluminum compounds such as spineloxide generally require high-temperature heating conditions to form, so hydrothermalsynthesis is thought to be suitable as the chemical synthesis technique for our concept.Note that hydrothermal synthesis is generally conducted at a temperature under 200 ◦C,but it seems to be too low to react the aluminum oxide with metal ions such as Mg2+. Inthis case, the application of high temperature over 200 ◦C is one approach.On the other hand, the verification of the target material is also meaningful. MgAl2O4is known as typical spinel oxide with the compositional formula expressed as AB2O4,where A and B indicate the di- and trivalent metal cations, respectively. The cations existingat each site (i.e., A2+ and B3+) form the characteristic tetrahedral and octahedral structureswith the coordination anion (i.e., O2−), respectively. The various attractive properties foundin spinel oxides mainly originate in the electronic state of these two metal cations. Thehumidity sensitivity of MgAl2O4 is considered to originate in the connections betweenchemisorbed/physisorbed water and cations, even though the mechanism is not clearedfrom the first principal approach yet. Both MgO [23] and γ-Al2O3 with spinel structure [5]show humidity sensitivity, and this property is reported in the device of MgFe2O4 [15,24]and ZnAl2O4 [25,26]. Thus, we cannot judge easily the most important element of humiditysensitivity in MgAl2O4, but the previous report comments on the better sensitivity ofMgAl2O4 than MgFe2O4 [15]. Consequently, we focused on ZnAl2O4 instead of MgAl2O4,because we experimentally felt that the fabrication of ZnAl2O4 was easier than that ofMgAl2O4. In addition, ZnAl2O4 is a more famous material as a catalyst [27,28] thanMgAl2O4, so ZnAl2O4 has expectable surface activity. There are few studies reportingthe humidity sensitivity of ZnAl2O4, unfortunately, so identifying this property itself isthought to still be an important trial for this spinel oxide. However, the easiness of thechemical synthesis of ZnAl2O4 can become the breakthrough in the investigations of thehumidity sensitivity of spinel oxide.From this background, this work tried to fabricate the ZnAl2O4/Al device usingonly the water chemical technique, i.e., the anodization of aluminum and hydrothermaltreatment for the AlOx/Al precursor. All of the fabrication processes were conducted under400 ◦C as one of the characteristic points in contrast with numerous previous works. Then,our ZnAl2O4/Al device showed good sensitivity and appreciable repeatability.2. Materials and Methods2.1. Preparation of ZnAl2O4/Al DeviceFigure 1 summarizes the flow of the sample preparation process with schematicillustrations of the cross-sectional image of the sample at each step and real picturesof them. The first step of the sample preparation is the anodization of an aluminumplate. The aluminum plate (99.999%, 10 × 50 × 0.5 mm) was cleaned in acetone (99.5%,FUJIFILM Wako Pure Chemical Corp., Tokyo, Japan) using the ultrasonic bath for 15 minand electropolished at 4 ◦C in the solution with 40 ml of perchloric acid (60%, KantoChemical Co., Inc., Tokyo, Japan) and 160 ml of Ethanol (99.5%, FUJIFILM Wako PureChemical Corp., Tokyo, Japan). The electropolish was performed with the constant currentmode of 1 A for 4 min. The electropolished Al plate was washed with distilled water anddried. Then, it was anodized in the solution of 0.5 M sulfuric acid with the constant currentmode of 0.01 A for 15 min at 22 ◦C. The solution was diluted with sulfuric acid (96.0%,Kanto Chemical Co., Inc., Tokyo, Japan) by distilled water. After washing with the distilledwater and the drying processes, we could obtain the AlOx/Al precursor plate. Note thatwe express the amorphous aluminum oxide as AlOx in this manuscript for convenience.Sensors 2022, 22, 6194 3 of 13Sensors 2022, 22, x FOR PEER REVIEW 3 of 13   Chemical Co., Inc., Tokyo, Japan) by distilled water. After washing with the distilled wa-ter and the drying processes, we could obtain the AlOx/Al precursor plate. Note that we express the amorphous aluminum oxide as AlOx in this manuscript for convenience.    Figure 1. The flow of the sample preparation process with schematic illustrations of the cross-sec-tional image of the sample at each step and real pictures of them. The upper picture shows the scale (left side), electropolished Al plate (second from the left), anodized electropolished plate (AlOx/Al: second from the right) and hydrothermal synthesized sample (ZnAl2O4/Al: right side). Bottom pic-ture shows the device after measurement. Hydrothermal (HT) synthesis was conducted for the AlOx/Al precursor plate as the next step. The precursor plate was inserted into a tube-type vessel (inner size 12 Φ× 96, Hastelloy C22, TSC-0011: Taiatsu Techno Corp., Tokyo, Japan) with 5 ml of Zn solution, and it was closed tightly. This Zn solution was created from ZnNO3·6H2O (99%: FUJIFILM Wako Pure Chemical Corp., Tokyo, Japan) and distilled water. The concentration was 0.5 M. The vessel was set in the furnace heated at 400 °C for 1 h, and it was quenched to the water bath. The inner pressure was estimated as 38 MPa, so this hydrothermal synthesis was run under the supercritical region. The obtained ZnAl2O4/Al sample was finally washed with distilled water/ethanol and dried again.  The obtained ZnAl2O4/Al sample was masked and set in the quick coater (SC-701: Sanyu Electron Co., Ltd., Tokyo, Japan). Then, Au sputtering was conducted with the con-dition of 5 mA for 5 minutes (the estimated thickness of Au was 50 nm). This process formed an Au electrode on ZnAl2O4 with the size of 1 × 10 mm square (the sample dis-cussing the humidity sensitivity at each atmosphere) or 7 mm Φ (the sample discussing the repeatability of the sensitivity). On the other hand, the sample’s edge was mechani-cally polished in order to remove the oxide layer for contact with the bottom Al layer. This ZnAl2O4/Al device was glued onto the same size of PTFE plate (thickness of 1 mm) expecting mechanical support and electrical insulation. Finally, Cu-wires were con-nected to the top electrode and bottom Al layer at room temperature by using silver paste in an area of 1 mm Φ. Then, it was dried in air for one night at least.   Cleaning and Electropolishing    Al → AlAnodization Al → AlOx/AlHydrothermal Synthesis AlOx/Al → ZnAl2O4/AlAu SputteringDevice makingIn the solution(Electropolished)Out of the solution (Connected to power supply with clip)In the solution(Anodized)Out of the solutionZnAl2O4 layerOxidized surface by HTAlOx layerAu Electrode Masked areaElectropolished Anodized HTPTFE plateSilver pasteLCR meterMechanically polishing area after HT processAl plateAl core layerFigure 1. The flow of the sample preparation process with schematic illustrations of the cross-sectional image of the sample at each step and real pictures of them. The upper picture showsthe scale (left side), electropolished Al plate (second from the left), anodized electropolished plate(AlOx/Al: second from the right) and hydrothermal synthesized sample (ZnAl2O4/Al: right side).Bottom picture shows the device after measurement.Hydrothermal (HT) synthesis was conducted for the AlOx/Al precursor plate as thenext step. The precursor plate was inserted into a tube-type vessel (inner size 12 Φ× 96,Hastelloy C22, TSC-0011: Taiatsu Techno Corp., Tokyo, Japan) with 5 ml of Zn solution,and it was closed tightly. This Zn solution was created from ZnNO3·6H2O (99%: FUJIFILMWako Pure Chemical Corp., Tokyo, Japan) and distilled water. The concentration was 0.5 M.The vessel was set in the furnace heated at 400 ◦C for 1 h, and it was quenched to the waterbath. The inner pressure was estimated as 38 MPa, so this hydrothermal synthesis was rununder the supercritical region. The obtained ZnAl2O4/Al sample was finally washed withdistilled water/ethanol and dried again.The obtained ZnAl2O4/Al sample was masked and set in the quick coater (SC-701:Sanyu Electron Co., Ltd., Tokyo, Japan). Then, Au sputtering was conducted with thecondition of 5 mA for 5 minutes (the estimated thickness of Au was 50 nm). This processformed an Au electrode on ZnAl2O4 with the size of 1 × 10 mm square (the samplediscussing the humidity sensitivity at each atmosphere) or 7 mm Φ (the sample discussingthe repeatability of the sensitivity). On the other hand, the sample’s edge was mechanicallypolished in order to remove the oxide layer for contact with the bottom Al layer.This ZnAl2O4/Al device was glued onto the same size of PTFE plate (thickness of1 mm) expecting mechanical support and electrical insulation. Finally, Cu-wires wereconnected to the top electrode and bottom Al layer at room temperature by using silverpaste in an area of 1 mm Φ. Then, it was dried in air for one night at least.2.2. Characterization and EvaluationsPrepared devices were characterized by a grazing incidence X-ray diffractometer (GI-XRD, Smart-Lab, Rigaku Corp., Tokyo, Japan), scanning electron microscopy (SEM, SU-8000:Hitachi High-Tech Corp., Tokyo, Japan), and energy-dispersive X-ray spectroscopy (SEM-EDS, JSM-6500F: JEOL Ltd., Tokyo, Japan) for the phase contents, the surface morphology,Sensors 2022, 22, 6194 4 of 13and the chemical homogeneity, respectively. The XRD pattern was also used for calculatingthe lattice constant and average grain size of the spinel layer. A large difference in the SEMimages of the two different apparatus was the probe currents; they were 10 µA (SU-8000)and 104 µA (JSM-6500F), respectively. The EDS analysis requires a large probe current,although it buries the sensitive surface information.The humidity sensitivity was evaluated by an LCR meter (IM3533: Hioki E.E. Corp.,Tokyo, Japan). The schematic illustration of the measuring configuration is drawn inFigure 2. The prepared device was set in the measurement chamber (inner volume < 0.2 L)and vacuumed by a rotary vacuum pump. Here, Cu wires from the top and bottomelectrodes were connected to the LCR meter of the outside in order to measure the electricalcapacitance, Cp, and the resistivity, Rp, of the ZnAl2O4 layer. The amplitude voltage ofthe applied AC field was 1.0 V, and the frequency was controlled in the range between10 Hz and 200 kHz. The monitoring pressure gauge of the chamber was a Bourdon tubetype. After the equilibrium state in the chamber, an AC measurement was performed inorder to obtain the calibration data as that of 0 %RH. Then, humidity-controlled gas in thegas box was introduced to the chamber. The gas box was composed of acrylic resin (innervolume 8 L), and a reference humidity meter was put inside the box (this humidity meter isa simple, commercial one. Thus, the absolute value of %RH is not thought to be sufficientlycalibrated, but it was believed to be relatively reliable). After the confirmation of the gaugeindicating ambient pressure, LCR measurement was performed again in order to obtainthe data of each humidity. In this paper, we define SC and SR as the equations as follows.SC = (Cp (x) − Cp (0))/Cp (0) (1)SR = (Rp (x) − Rp (0))/Rp (0) (2)Sensors 2022, 22, x FOR PEER REVIEW 4 of 13   2.2. Characterization and Evaluations Prepared devices were characterized by a grazing incidence X-ray diffractometer (GI-XRD, Smart-Lab, Rigaku Corp., Tokyo, Japan), scanning electron microscopy (SEM, SU-8000: Hitachi High-Tech Corp., Tokyo, Japan), and energy-dispersive X-ray spectroscopy (SEM-EDS, JSM-6500F: JEOL Ltd., Tokyo, Japan) for the phase contents, the surface mor-phology, and the chemical homogeneity, respectively. The XRD pattern was also used for calculating the lattice constant and average grain size of the spinel layer. A large difference in the SEM images of the two different apparatus was the probe currents; they were 10 µA (SU-8000) and 104 µA (JSM-6500F), respectively. The EDS analysis requires a large probe current, although it buries the sensitive surface information. The humidity sensitivity was evaluated by an LCR meter (IM3533: Hioki E.E. Corp., Tokyo, Japan). The schematic illustration of the measuring configuration is drawn in Fig-ure 2. The prepared device was set in the measurement chamber (inner volume < 0.2 L) and vacuumed by a rotary vacuum pump. Here, Cu wires from the top and bottom elec-trodes were connected to the LCR meter of the outside in order to measure the electrical capacitance, Cp, and the resistivity, Rp, of the ZnAl2O4 layer. The amplitude voltage of the applied AC field was 1.0 V, and the frequency was controlled in the range between 10 Hz and 200 kHz. The monitoring pressure gauge of the chamber was a Bourdon tube type. After the equilibrium state in the chamber, an AC measurement was performed in order to obtain the calibration data as that of 0 %RH. Then, humidity-controlled gas in the gas box was introduced to the chamber. The gas box was composed of acrylic resin (inner volume 8 L), and a reference humidity meter was put inside the box (this humidity meter is a simple, commercial one. Thus, the absolute value of %RH is not thought to be suffi-ciently calibrated, but it was believed to be relatively reliable). After the confirmation of the gauge indicating ambient pressure, LCR measurement was performed again in order to obtain the data of each humidity. In this paper, we define SC and SR as the equations as follows. SC = (Cp (x) – Cp (0))/Cp (0) (1) SR = (Rp (x) – Rp (0))/Rp (0) (2) Here, Cp (x) and Rp (x) mean the parallel capacitance and parallel resistance at the humidity of x %RH, respectively. Then, the Cp and Rp values in vacuumed conditions be-fore measurement were used as the calibration data of Cp (0) and Rp (0).  Figure 2. Schematic illustration of the measurement configuration for evaluating the humidity sen-sitivity of the sample device. Gas BoxMeasurement ChamberAir leakVacuum pumpPressure gaugeBall valveLCR meterAir leakFigure 2. Schematic illustration of the measurement configuration for evaluating the humiditysensitivity of the sample device.Here, Cp (x) and Rp (x) mean the parallel capacitance and parallel resistance at thehumidity of x %RH, respectively. Then, the Cp and Rp values in vacuumed conditionsbefore measurement were used as the calibration data of Cp (0) and Rp (0).The humidity in the gas box was controlled by using the saturated salt solution method.The carrier gas was ambient air. We waited until the reference sensor put in the gas boxshowed a stable %RH value. It was longer than 6 h at least. Table 1 summarizes theinorganic salts and %RH values used in this work. Here, ZnNO3·6H2O was exploited tobuild up 43 and 53 %RH. The equilibrium %RH should be a repeatable value depending onthe chemical composition of the salt. However, we could obtain different stable %RH valuesSensors 2022, 22, 6194 5 of 13by controlling the ratio of ZnNO3·6H2O and water. It means the quasi-equilibrium states,but the %RH value does not fluctuate after a long waiting time of over 12 h. Therefore, weused these gases with different humidity.Table 1. List of salt used for controlling the %RH value in the gas box.Salt %RHMgCl2 16CH3COOK 23ZnNO3·6H2O 43ZnNO3·6H2O 52NaHCO3 78Na2SO4 843. Results and Discussions3.1. Characterization of ZnAl2O4/Al DeviceThe picture of the obtained samples at each preparation step is shown in Figure 1. Itdemonstrates that the ZnAl2O4/Al sample (arrowed as HT) after the hydrothermal processof supercritical water for 1 h still maintains the metallic luster of aluminum substratesimilarly to the samples after electropolishing and/or anodization, even though the surfaceslightly becomes to be cloudy.Figure 3 shows the XRD pattern of hydrothermally synthesized samples. The promi-nent peaks were all assigned as ZnAl2O4 (04-007-6610 of ICDD database) or Al (00-004-0787of it), so that the sample is considered to consist of the ZnAl2O4 layer on the Al plate,i.e., the ZnAl2O4/Al structure as intended. The lattice constants and crystalline size ofZnAl2O4 were estimated as 8.120 Å and 5.8 nm, respectively.Sensors 2022, 22, x FOR PEER REVIEW 5 of 13   The humidity in the gas box was controlled by using the saturated salt solution method. The carrier gas was ambient air. We waited until the reference sensor put in the gas box showed a stable %RH value. It was longer than 6 h at least. Table 1 summarizes the inorganic salts and %RH values used in this work. Here, ZnNO3·6H2O was exploited to build up 43 and 53 %RH. The equilibrium %RH should be a repeatable value depending on the chemical composition of the salt. However, we could obtain different stable %RH values by controlling the ratio of ZnNO3·6H2O and water. It means the quasi-equilibrium states, but the %RH value does not fluctuate after a long waiting time of over 12 h. There-fore, we used these gases with different humidity.  Table 1. List of salt used for controlling the %RH value in the gas box. Salt %RH MgCl2 16 CH3COOK 23 ZnNO3·6H2O 43 ZnNO3·6H2O 52 NaHCO3 78 Na2SO4 84 3. Results and Discussions 3.1. Characterization of ZnAl2O4/Al Device The picture of the obtained samples at each preparation step is shown in Figure 1. It demonstrates that the ZnAl2O4/Al sample (arrowed as HT) after the hydrothermal process of supercritical water for 1 h still maintains the metallic luster of aluminum substrate sim-ilarly to the samples after electropolishing and/or anodization, even though the surface slightly becomes to be cloudy.  Figure 3. shows the XRD pattern of hydrothermally synthesized samples. The prom-inent peaks were all assigned as ZnAl2O4 (04-007-6610 of ICDD database) or Al (00-004-0787 of it), so that the sample is considered to consist of the ZnAl2O4 layer on the Al plate, i.e., the ZnAl2O4/Al structure as intended. The lattice constants and crystalline size of Z nAl2O4 were estimated as 8.120 Å  and 5.8 nm, respectively.  Figure 3. XRD patterns of the ZnAl2O4/Al sample. The peaks of ZnAl2O4 were all indexed in this figure. On the other hand, the peak positions of Al were only indicated by a red dash line. The 400 and 440 peaks were overlapped with the small Al peaks. Figure 4 shows the surface SEM images of the AlOx layer of the anodized AlOx/Al precursor plate and of the ZnAl2O4 layer after hydrothermal treatment. The homogeneous surface of the AlOx/Al precursor plate seems to consist of quite small grains. This mor-phology was speculated to become the base of the microstructure of the ZnAl2O4 layer after the hydrothermal synthesis process. However, average grain sizes seem to become 10 20 30 40 50 60 70Intensity (a.u.)2θ (degrees)Al220311331 422 511 440400Figure 3. XRD patterns of the ZnAl2O4/Al sample. The peaks of ZnAl2O4 were all indexed in thisfigure. On the other hand, the peak positions of Al were only indicated by a red dash line. The 400and 440 peaks were overlapped with the small Al peaks.Figure 4 shows the surface SEM images of the AlOx layer of the anodized AlOx/Alprecursor plate and of the ZnAl2O4 layer after hydrothermal treatment. The homogeneoussurface of the AlOx/Al precursor plate seems to consist of quite small grains. This mor-phology was speculated to become the base of the microstructure of the ZnAl2O4 layerafter the hydrothermal synthesis process. However, average grain sizes seem to becomesmall with hydrothermal treatment. The image of high magnification for the ZnAl2O4/Alsample shows the small grains with a size of less than 10 nm. This is consistent with thecalculated results from XRD patterns. It implies the possibility that hydrothermal treatmentpromotes the sample to enhance the porous morphology.Sensors 2022, 22, 6194 6 of 13Sensors 2022, 22, x FOR PEER REVIEW 6 of 13   small with hydrothermal treatment. The image of high magnification for the ZnAl2O4/Al sample shows the small grains with a size of less than 10 nm. This is consistent with the calculated results from XRD patterns. It implies the possibility that hydrothermal treat-ment promotes the sample to enhance the porous morphology. On the other hand, the images of low magnification indicate the small grains distrib-uted homogeneously on the surface of the sample. This homogeneity was additionally verified from the viewpoint of the element mapping for Zn, Al, and O. Figure 5 shows the EDS mapping images of the ZnAl2O4/Al sample. Some grains and different textures are slightly observed in the SEM images, although it was basically homogeneous. EDS map-ping images could not detect this small roughness due to the inhomogeneity (see the im-ages for Area 1). The relatively homogeneous distribution of each element was the same for the images of high magnification (see the images for Area 2).  Figure 4. SEM images of the surface of the anodized AlOx layer (left side) and hydrothermally synthesized ZnAl2O4 layer (right side).  Figure 5. Mapping images observed by SEM-EDS for ZnAl2O4/Al sample. Area 1 shown on the top was 3000 times scale, and Area 2 (bottom) was 13,000 times, respectively. 10µm100 nmZnAl2O4/Al × 4000ZnAl2O4/Al × 300k10µm100 nmAlOx/Al × 4000AlOx/Al × 300k10umAREA1 1 - Al 1 - Zn 1 - OAREA2 2 - Al 2 - Zn 2 - O300 nmFigure 4. SEM images of the surface of the anodized AlOx layer (left side) and hydrothermallysynthesized ZnAl2O4 layer (right side).On the other hand, the images of low magnification indicate the small grains dis-tributed homogeneously on the surface of the sample. This homogeneity was additionallyverified from the viewpoint of the element mapping for Zn, Al, and O. Figure 5 showsthe EDS mapping images of the ZnAl2O4/Al sample. Some grains and different texturesare slightly observed in the SEM images, although it was basically homogeneous. EDSmapping images could not detect this small roughness due to the inhomogeneity (see theimages for Area 1). The relatively homogeneous distribution of each element was the samefor the images of high magnification (see the images for Area 2).Sensors 2022, 22, x FOR PEER REVIEW 6 of 13   small with hydrothermal treatment. The image of high magnification for the ZnAl2O4/Al sample shows the small grains with a size of less than 10 nm. This is consistent with the calculated results from XRD patterns. It implies the possibility that hydrothermal treat-ment promotes the sample to enhance the porous morphology. On the other hand, the images of low magnification indicate the small grains distrib-uted homogeneously on the surface of the sample. This homogeneity was additionally verified from the viewpoint of the element mapping for Zn, Al, and O. Figure 5 shows the EDS mapping images of the ZnAl2O4/Al sample. Some grains and different textures are slightly observed in the SEM images, although it was basically homogeneous. EDS map-ping images could not detect this small roughness due to the inhomogeneity (see the im-ages for Area 1). The relatively homogeneous distribution of each element was the same for the images of high magnification (see the images for Area 2).  Figure 4. SEM images of the surface of the anodized AlOx layer (left side) and hydrothermally synthesized ZnAl2O4 layer (right side).  Figure 5. Mapping images observed by SEM-EDS for ZnAl2O4/Al sample. Area 1 shown on the top was 3000 times scale, and Area 2 (bottom) was 13,000 times, respectively. 10µm100 nmZnAl2O4/Al × 4000ZnAl2O4/Al × 300k10µm100 nmAlOx/Al × 4000AlOx/Al × 300k10umAREA1 1 - Al 1 - Zn 1 - OAREA2 2 - Al 2 - Zn 2 - O300 nmFigure 5. Mapping images observed by SEM-EDS for ZnAl2O4/Al sample. Area 1 shown on the topwas 3000 times scale, and Area 2 (bottom) was 13,000 times, respectively.3.2. Humidity Sensitivity of ZnAl2O4/Al DevicesThe above section shows the success in the chemical preparation of ZnAl2O4/Al sam-ples. In this section, we try to evaluate the humidity sensitivity of chemically synthesizedZnAl2O4/Al devices.Sensors 2022, 22, 6194 7 of 13Figure 6A plots the Sc values of the ZnAl2O4/Al devices against the %RH values.The Sc value calculated by Equation (1) in Figure 6A changed corresponding to the %RHcondition at the measurement atmosphere. The relationship looks linear (see blue dashedline); however, the Sc value at 0 %RH is defined as 0. Therefore, this relation is speculatedto have two different slopes (see red dashed line in the figure) rather than a simple linearline (blue dashed line). This clear relationship was shown for the data measured at 10 kHzof the applied AC field. It should not be an individual relation only for the data measuredat 10 kHz if the Sc values of our ZnAl2O4/Al devices truly worked as the humidity sensor.Figure 6B supports this consideration. It plots the relationship between the Sc and %RHvalues obtained at the different frequencies. Each relationship seems to keep the correlationthat the increase in %RH enhances the value of Sc at each frequency. However, the accuracyis not thought to be constant. The sensitive response seems to be prominent correspondingto increases in the frequency; on the other hand, decreases in the values of Sc result indegrading the sensitive response. It is important for discussing the reliability of the absoluteSc value in the case of a low %RH value especially. The inserted figure in Figure 6B plotsthe Sc values at 52 %RH against the measurement frequency. The low-frequency data showsome scattering, but it settles at the high-frequency region over 2kHz. The maximum Scseems to be obtained at around 20 kHz. This result is not considered to be a universal trend,since the capacitance measurement using AC voltage for the insulator strongly dependson the measurement configuration. However, this result is considered to indicate at leastthat high-frequency measurements give us reliable values for discussing the humiditysensitivity using the capacitance data.Sensors 2022, 22, x FOR PEER REVIEW 7 of 13   3.2. Humidity Sensitivity of ZnAl2O4/Al Devices The above section shows the success in the chemical preparation of ZnAl2O4/Al sam-ples. In this section, we try to evaluate the humidity sensitivity of chemically synthesized ZnAl2O4/Al devices. Figure 6A plots the Sc values of the ZnAl2O4/Al devices against the %RH values. The Sc value calculated by Equation (1) in Figure 6A changed corresponding to the %RH con-dition at the measurement atmosphere. The relationship looks linear (see blue dashed line); however, the Sc value at 0 %RH is defined as 0. Therefore, this relation is speculated to have two different slopes (see red dashed line in the figure) rather than a simple linear line (blue dashed line). This clear relationship was shown for the data measured at 10 kHz of the applied AC field. It should not be an individual relation only for the data measured at 10 kHz if the Sc values of our ZnAl2O4/Al devices truly worked as the humidity sensor. Figure 6B supports this consideration. It plots the relationship between the Sc and %RH values obtained at the different frequencies. Each relationship seems to keep the correla-tion that the increase in %RH enhances the value of Sc at each frequency. However, the accuracy is not thought to be constant. The sensitive response seems to be prominent cor-responding to increases in the frequency; on the other hand, decreases in the values of Sc result in degrading the sensitive response. It is important for discussing the reliability of the absolute Sc value in the case of a low %RH value especially. The inserted figure in Figure 6B plots the Sc values at 52 %RH against the measurement frequency. The low-frequency data show some scattering, but it settles at the high-frequency region over 2kHz. The maximum Sc seems to be obtained at around 20 kHz. This result is not consid-ered to be a universal trend, since the capacitance measurement using AC voltage for the insulator strongly depends on the measurement configuration. However, this result is considered to indicate at least that high-frequency measurements give us reliable values for discussing the humidity sensitivity using the capacitance data.  Figure 6. (A) Dependence of the Sc on the %RH in the atmosphere of ZnAl2O4/Al device. (B) De-pendence of relationship between the Sc and %RH values on the applied AC frequency to ZnAl2O4/Al device. The inserted figure plots the Sc values at %RH = 52 against the applied AC fre-quency. On the other hand, we found an additional interesting result in the evaluating pro-cess using an LCR meter. It is shown in Figure 7 plotting the SR values calculated by Equa-tion (2) against the %RH values. In this case, SR looks to change linearly corresponding to the %RH at the measurement atmosphere. The inset figure is the similar plot inserted in 0 20 40 60 80 1000.01.02.03.04.05.06.0S C%RH10 kHz(A)10kHzS C1kHz100kHz100Hz101 102 103 104 1050.00.51.01.52.02.53.0Frequency (Hz)%RH = 520 20 40 60 80 1000.01.02.03.04.05.06.0(B)S C%RH10HzFigure 6. (A) Dependence of the Sc on the %RH in the atmosphere of ZnAl2O4/Al device. (B) Depen-dence of relationship between the Sc and %RH values on the applied AC frequency to ZnAl2O4/Aldevice. The inserted figure plots the Sc values at %RH = 52 against the applied AC frequency.On the other hand, we found an additional interesting result in the evaluating processusing an LCR meter. It is shown in Figure 7 plotting the SR values calculated by Equation (2)against the %RH values. In this case, SR looks to change linearly corresponding to the %RHat the measurement atmosphere. The inset figure is the similar plot inserted in Figure 6B.In this case, a higher frequency measurement over 10 kHz seems to be better for obtainingreliable data.Sensors 2022, 22, 6194 8 of 13Sensors 2022, 22, x FOR PEER REVIEW 8 of 13   Figure 6B. In this case, a higher frequency measurement over 10 kHz seems to be better for obtaining reliable data.  Figure 7. Dependence of the SR on the %RH in the atmosphere of the ZnAl2O4/Al device. The in-serted figure plots the SR values at %RH = 52 against the applied AC frequency. Finally, the repeatability of the humidity sensitivity is validated for the ZnAl2O4/Al device. This experiment was conducted without a gas box since the capacity could not reproduce the same pressure for the inside of the chamber. Consequently, this property was checked by repeating the vacuuming (for setting the %RH  0) and inserting ambient air (=59 %RH on the experiment day). The total experimental time was not so long, hence we regard the humidity as constant. Figure 8 shows the time dependence of the capaci-tance values, i.e., Cp (0)–Cp (59) of the ZnAl2O4/Al device. Experiments were started from the vacuumed state and the leak valve was opened (gas in) by closing the valve connected to the rotary pump. Air was introduced to the measurement chamber quickly, and it was re-vacuumed again (gas out) after confirmation of the inside pressure indicating atmos-pheric value. This experiment was repeated three times, and the data were plotted with different colors. Figure 8A plots the data against the absolute time, and Figure 8B plots it against the relative time of each cycle. Figure 8 visibly shows the history. The response speeds seem to depend on the leaking/vacuuming rate rather than the ability of the device; however, this figure seems to demonstrate the good repeatability of the ZnAl2O4/Al. Here, dashed, short dashed, and dot lines indicate the times where the pressure gauge indicates -0.02 MPa, -0.06 MPa, and -0.08 MPa, respectively. The position of these lines also seems to replicate. The small difference is considered to be due to the low accuracy. Probably, the reproducibility of this timing would become higher if we could measure the data with the appropriate configuration recording the inner pressure accurately. The repeatability of our device was also identified for the data of Rp. Figure 9 shows the results plotted in the same format as Figure 8. In this case, the data variation is wider at the higher-pressure region around the ambient atmosphere, and the start position of the gas in/gas out is clearer than that of Figure 8. 100 ×SR%RH10 kHz101102103104105−3.0−2.5−2.0−1.5−1.0−0.50.00.5%RH = 52Frequency100 ×SRFigure 7. Dependence of the SR on the %RH in the atmosphere of the ZnAl2O4/Al device. Theinserted figure plots the SR values at %RH = 52 against the applied AC frequency.Finally, the repeatability of the humidity sensitivity is validated for the ZnAl2O4/Aldevice. This experiment was conducted without a gas box since the capacity could notreproduce the same pressure for the inside of the chamber. Consequently, this propertywas checked by repeating the vacuuming (for setting the %RH ≈ 0) and inserting ambientair (=59 %RH on the experiment day). The total experimental time was not so long, hencewe regard the humidity as constant. Figure 8 shows the time dependence of the capacitancevalues, i.e., Cp (0)–Cp (59) of the ZnAl2O4/Al device. Experiments were started from thevacuumed state and the leak valve was opened (gas in) by closing the valve connectedto the rotary pump. Air was introduced to the measurement chamber quickly, and itwas re-vacuumed again (gas out) after confirmation of the inside pressure indicatingatmospheric value. This experiment was repeated three times, and the data were plottedwith different colors. Figure 8A plots the data against the absolute time, and Figure 8B plotsit against the relative time of each cycle. Figure 8 visibly shows the history. The responsespeeds seem to depend on the leaking/vacuuming rate rather than the ability of the device;however, this figure seems to demonstrate the good repeatability of the ZnAl2O4/Al. Here,dashed, short dashed, and dot lines indicate the times where the pressure gauge indicates−0.02 MPa, −0.06 MPa, and −0.08 MPa, respectively. The position of these lines also seemsto replicate. The small difference is considered to be due to the low accuracy. Probably, thereproducibility of this timing would become higher if we could measure the data with theappropriate configuration recording the inner pressure accurately. The repeatability of ourdevice was also identified for the data of Rp. Figure 9 shows the results plotted in the sameformat as Figure 8. In this case, the data variation is wider at the higher-pressure regionaround the ambient atmosphere, and the start position of the gas in/gas out is clearer thanthat of Figure 8.Sensors 2022, 22, 6194 9 of 13Sensors 2022, 22, x FOR PEER REVIEW 9 of 13    Figure 8. Repeatability of the Cp of ZnAl2O4/Al device against changing pressure between the vacu-umed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Cp is plotted against absolute time. (B) Cp is plotted against the relative time of each cycle.  Figure 9. Repeatability of the Rp of ZnAl2O4/Al device against changing pressure between the vacu-umed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Rp is plotted against absolute time. (B) Rp is plotted against the relative time of each cycle 4. Discussion 4.1. Chemical Synthesis of ZnAl2O4/Al Device This work succeeded in the chemical synthesis of the ZnAl2O4/Al device by applying hydrothermal treatment and the anodization technique. All fabrication processes were conducted under 400 °C, and it was possible in principle to improve on the process for the device with a complex structure providing a large surface area. These characteristics are believed to expand the study of the application of spinel aluminate devices as sensors. The key to this chemical synthesis is changing the target to ZnAl2O4 from the widely in-vestigated MgAl2O4. At least, our background experiments could not synthesize a MgAl2O4/Al device by hydrothermal treatment at 400 °C using the solution formed from Cp(nF/cm2)Time (s)-0.02 MPa-0.06 MPa-0.08 MPa%RH = 5910 kHz1st2ed3rd(A)(B)Gas inGas inGas inGas out Gas outGas out%RH = 5910 kHz(A)Gas outGas in-0.02 MPa-0.06 MPa-0.08 MPa1st2ed3rd(B)Time (s)Rp()Gas in Gas inGas out Gas outFigure 8. Repeatability of the Cp of ZnAl2O4/Al device against changing pressure between thevacuumed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Cp is plotted against absolutetime. (B) Cp is plotted against the relative time of each cycle.Sensors 2022, 22, x FOR PEER REVIEW 9 of 13    Figure 8. Repeatability of the Cp of ZnAl2O4/Al device against changing pressure between the vacu-umed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Cp is plotted against absolute time. (B) Cp is plotted against the relative time of each cycle.  Figure 9. Repeatability of the Rp of ZnAl2O4/Al device against changing pressure between the vacu-umed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Rp is plotted against absolute time. (B) Rp is plotted against the relative time of each cycle 4. Discussion 4.1. Chemical Synthesis of ZnAl2O4/Al Device This work succeeded in the chemical synthesis of the ZnAl2O4/Al device by applying hydrothermal treatment and the anodization technique. All fabrication processes were conducted under 400 °C, and it was possible in principle to improve on the process for the device with a complex structure providing a large surface area. These characteristics are believed to expand the study of the application of spinel aluminate devices as sensors. The key to this chemical synthesis is changing the target to ZnAl2O4 from the widely in-vestigated MgAl2O4. At least, our background experiments could not synthesize a MgAl2O4/Al device by hydrothermal treatment at 400 °C using the solution formed from Cp(nF/cm2)Time (s)-0.02 MPa-0.06 MPa-0.08 MPa%RH = 5910 kHz1st2ed3rd(A)(B)Gas inGas inGas inGas out Gas outGas out%RH = 5910 kHz(A)Gas outGas in-0.02 MPa-0.06 MPa-0.08 MPa1st2ed3rd(B)Time (s)Rp()Gas in Gas inGas out Gas outFigure 9. Repeatability of the Rp of ZnAl2O4/Al device against changing pressure between thevacuumed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Rp is plotted against absolutetime. (B) Rp is plotted against the relative time of each cycle.4. Discussion4.1. Chemical Synthesis of ZnAl2O4/Al DeviceThis work succeeded in the chemical synthesis of the ZnAl2O4/Al device by applyinghydrothermal treatment and the anodization technique. All fabrication processes wereconducted under 400 ◦C, and it was possible in principle to improve on the process forthe device with a complex structure providing a large surface area. These characteristicsare believed to expand the study of the application of spinel aluminate devices as sensors.The key to this chemical synthesis is changing the target to ZnAl2O4 from the widelySensors 2022, 22, 6194 10 of 13investigated MgAl2O4. At least, our background experiments could not synthesize aMgAl2O4/Al device by hydrothermal treatment at 400 ◦C using the solution formedfrom Mg nitrate. The XRD patterns show the Al2O3 structure. Probably, the synthesistemperature was not enough for synthesizing MgAl2O4. This trend sometimes occursfor fabricating ZnAl2O4 and MgAl2O4 by the same method. In addition, the nitrate saltused for preparing the solution of hydrothermal synthesis is also key to the success of thechemical synthesis. The reason has not been cleared yet. However, the solution of othersalts drastically oxidizes and involves the device structure of AlOx/Al. Therefore, thecounter anion of ZnNO3·6H2O is considered to work for preventing the metal aluminumfrom radical oxidation.On the other hand, this work prepared an AlOx/Al precursor plate created from thesolution with 0.5 M of sulfuric acid. This condition is quite simple as a technique foranodizing the Al plate. For example, anodization using boric acid yields a barrier-typeAlOx layer on Al; in contrast with that, sulfuric acid yields a porous-type AlOx layer [29,30].Our experimental data shown in Figure 4 demonstrate the trend, even though it is not agenerally known honeycomb structure. However, this is considered to be an importantpoint in the future for investigating the humidity sensitivity of our ZnAl2O4/Al device. Thisproperty strongly depends on the morphology of the surface state of the spinel aluminatelayer [17,19,25]. Then, Figure 4 implies that this microstructure of the final product devicedepends on the anodization process rather than the hydrothermal process. It means thatwe can chemically control the morphology of the device more intricately.The chemical synthesis has some advantages for the industrial application of ceramicsmaterials; however, it also has disadvantages for fabricating the oxide, with high crys-tallinity exhibiting excellent performance. Chemical synthesis often degrades performancecompared with that of the oxide formed by solid-state reaction. In fact, our sample issynthesized at a low temperature under 400 ◦C. This temperature is low enough to inducesome disorders in the product oxide. The lattice constant of the ZnAl2O4 of our deviceseems to be longer than that of the sample sintered by a solid-state reaction at 1300 ◦C(≈ 8.092 Å) [31]. The reason has not been cleared yet, but cation deficiency, site exchange,impurity substitution, etc. are considered as possibilities. These considerations imply thatthere is further potential to improve the performance of our ZnAl2O4/Al device. Thiswork, as the first trial for the chemical synthesis of a ZnAl2O4/Al device, has not optimizedthe fabricating condition of the device. Further investigation is required for optimizingit and for clarifying the correlation between the strict structural characteristics and theperformance of the humidity sensor.4.2. Humidity Sensitivity of ZnAl2O4/Al DeviceExperimental results for the ZnAl2O4/Al device show the relationship between the Scand %RH values. It appears to have good sensitivity and appreciable repeatability as thehumidity sensor. This point should be discussed in this section.At first, the relationship between Sc and %RH seems to have two different slopes (seered dashed line in Figure 6A). A previous study performed for MgAl2O4 devices reportsthe existence of three slopes [17]. It is a commonly discussed trend, and the difference in thedominant response is concluded as the reason for this change in the slope. Our result is alsoconsidered to be consistent with these reports. For a lower humidity region, the small slopeindicates the dominance of the chemisorption water; contrary to that, the physisorptionof water is dominant for the middle humidity region. Here, we can expect an additionalchange in the slope for the region over 70 %RH, because the condensation of water willoccur in the device macrospore. However, our experiments frequently vacuumed thechamber and the porosity seemed to be low. Therefore, the influence of water condensationwas considered to be small. This is the reason for the two different slopes. On the otherhand, this speculation should be common with the trend of SR, although it appears tobe a linear relationship. This is speculated to be due to the absolutely small value of theresistance owing to the large area of the electrode and thin thickness of the ZnAl2O4 layer.Sensors 2022, 22, 6194 11 of 13Next, the humidity sensitivity of ZnAl2O4 should be compared with the general trend.This paper is thought to be the first report on a ZnAl2O4/Al device synthesized by a low-temperature chemical technique only. Although, previous studies report the effectivenessof ZnAl2O4 fabricated by solid-state reactions [25,26]. They report that the data originatingin the capacitance value were enhanced corresponding to the increase in the %RH value.On the other hand, the data originating in the resistance value become small correspondingto the increase in the %RH value. Our experimental results shown in Figures 6 and 7 areconsistent with these trends. Therefore, we believe that Figures 6 and 7 certainly indicatethe response of the ZnAl2O4/Al device to its atmospheric humidity.The high responsibilities shown in Figures 8 and 9 can impress onto us the effectivenessof our devices as vacuum indicators. The above discussion concluded their effectivenessas humidity sensors, so this response is also considered to originate in their sensitivity tochanges in atmospheric humidity. However, it is also interesting if the ZnAl2O4/Al deviceworked as a vacuum sensor. Figures 8 and 9 show the high sensitivity of our device in a low-pressure region rather than a high-pressure one. This is considered to be the next issue forthis device. In addition, ZnAl2O4 is well-known as a catalyst material [27,28]. Consequently,we speculated that a ZnAl2O4/Al sample is effective not only as a humidity sensor but alsoas a device for other gas detection. These points are also the next issues of our device.Finally, both capacitance and resistance were good indicators for sensing the humidityresponse. We experimentally felt that the stability of the absolute measured value was betterfor resistance rather than capacitance. In addition, Figures 8 and 9 show the high sensitivityof our device in a low-pressure region rather than a high-pressure one. This is importantsince a low %RH is the focus of the humidity sensor for the next generation. Standing onthis viewpoint, resistivity is considered to be a good indicator. However, the inset figure inFigure 7 indicates the importance of the measurement at the high-frequency region. If wecan use the device effectively at a frequency of around 60 Hz, it is expected to acceleratethe application of the ZnAl2O4/Al device. This viewpoint supports the utilization of thecapacitance data as the indicator.5. ConclusionsThis work succeeded in the chemical synthesizing of a ZnAl2O4/Al device by applyinghydrothermal treatment and the anodization technique. The electrical capacitance, Cp, andthe resistivity, Rp, measured by an LCR meter, obviously respond to changes in humidity.This clear response was obtained from high-frequency measurements.Humidity sensitivity is basically discussed for the device of MgAl2O4, and the de-vices are generally fabricated by solid-state reactions applying heat treatment at higherthan 1000 ◦C. On the other hand, the sensing material of our device was ZnAl2O4, andthe preparation process was all exhibited chemically at under 400 ◦C. However, phasecharacterization indicates the device structure of ZnAl2O4/Al (see Figure 3). Then, theexperimental data evaluated for this device were consistent with previous few reports aboutthe humidity sensitivity of ZnAl2O4 [25,26]. Therefore, we concluded that our ZnAl2O4/Aldevice worked as a humidity sensor and that the ceramics sensor of spinel aluminate couldbe chemically synthesized by applying the anodization and hydrothermal technique.Our ZnAl2O4/Al device demonstrates good sensitivity and appreciable repeatabilityas a humidity sensor, although the preparation procedure was more convenient than othertechniques standing on solid-state reactions or CVD or PVD. This technique is possiblein principle to improve on the process for the device with a complex structure providinga large surface area. Therefore, our investigation is believed to expand the applicationstudy for spinel aluminate devices as the sensor. This work is the first trial to verify theeffectiveness, thus there are some issues to investigate more carefully. For example, deepverification for the performance (reproducibility, hysteresis, response speed, life, etc.) isthought to be an issue. In addition, the sample quality, morphological analysis, devicestructure, and measurement configurations should be optimized appropriately. We believeSensors 2022, 22, 6194 12 of 13that our ZnAl2O4/Al device is worth investigating for these issues. Therefore, furtherinvestigation is required.Author Contributions: Conceptualization, T.N. (Takayuki Nakane); methodology, T.N. (TakayukiNakane) and T.N. (Takashi Naka); Resources, T.N. (Takayuki Nakane) and T.N. (Takashi Naka);software, T.N. (Takayuki Nakane); validation, T.N. (Takayuki Nakane) and T.N. (Takashi Naka);investigation, T.N. (Takayuki Nakane) and M.N.; writing—original draft preparation, T.N. (TakayukiNakane); writing—review and editing, T.N. (Takayuki Nakane), T.N. (Takashi Naka), and T.U.;supervision, T.N. (Takayuki Nakane); project administration, T.N. (Takayuki Nakane); fundingacquisition, T.N. (Takayuki Nakane) and T.U. All authors have read and agreed to the publishedversion of the manuscript.Funding: This research received no external funding.Acknowledgments: In this section, you can acknowledge any support given which is not covered bythe author’s contribution or funding sections. This may include administrative and technical support,or donations in kind (e.g., materials used for experiments). The authors would like to acknowledge T.Hiroto of the Materials Analysis Station in the National Institute for Materials Science for his technicalsupport about GI-XRD. We also acknowledge H. Segawa and Y. Saito of the Electroceramics Group inthe National Institute for Materials Science for their kind support in acquiring the anodization technique.Conflicts of Interest: The authors declare no conflict of interest.References1. Fei, T.; Jiang, K.; Liu, S.; Zhang, T. Humidity sensors based on Li-loaded nanoporous polymers. Sens. Actuators B Chem. 2014, 190,523–528. [CrossRef]2. 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[CrossRef] [PubMed]http://doi.org/10.1590/1980-5373-mr-2017-0729http://doi.org/10.1039/C5CE00252Dhttp://doi.org/10.1016/S0955-2219(99)00153-3http://doi.org/10.1039/C6RA01255Hhttp://doi.org/10.1007/s00339-021-04662-yhttp://doi.org/10.1016/0924-0136(96)85112-0http://doi.org/10.1016/j.ceramint.2013.02.077http://doi.org/10.1007/s10853-021-06709-0http://doi.org/10.1016/j.apcata.2009.09.035http://doi.org/10.1080/21870764.2018.1439692http://doi.org/10.1021/cr60259a005http://doi.org/10.1016/j.tsf.2011.01.188http://doi.org/10.1039/C4DT01599Ahttp://www.ncbi.nlm.nih.gov/pubmed/25407768 Introduction  Materials and Methods  Preparation of ZnAl2O4/Al Device  Characterization and Evaluations  Results and Discussions  Characterization of ZnAl2O4/Al Device  Humidity Sensitivity of ZnAl2O4/Al Devices  Discussion  Chemical Synthesis of ZnAl2O4/Al Device  Humidity Sensitivity of ZnAl2O4/Al Device  Conclusions  References