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Preparation of Ultra-Sensitive Humidity-Sensing Films by Aerosol Deposition Jun-Ge Liang, Cong Wang, Zhao Yao, Ming-Qing Liu, Hong-Ki Kim, Jong-Min Oh, and Nam-Young Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14082 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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ACS Applied Materials & Interfaces

Preparation of Ultra-Sensitive Humidity-Sensing Films by Aerosol Deposition Jun-Ge Liang1, Cong Wang2 *, Zhao Yao1, Ming-Qing Liu1, Hong-Ki Kim3, Jong-Min Oh3,*, Nam-Young Kim1,*

J.-G. Liang, Z. Yao, M.-Q. Liu, N.-Y. Kim RFIC Center, Kwangwoon University, Seoul 139-701, Republic of Korea Email: [email protected] (Nam-Young Kim) C. Wang School of Information and Engineering, Harbin Institute of Technology, Harbin, China Email: [email protected] (Cong Wang) H.-K. Kim, J.-M. Oh Department of Electronic Materials Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea Email: [email protected] (Jong-Min Oh)

Keywords: aerosol deposition, humidity sensor, post-annealing, ultra-high sensitivity, superior detection balance, mechanistic modeling. 1 ACS Paragon Plus Environment

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Abstract: Aerosol deposition (AD) is a novel ceramic film preparation technique exhibiting the advantages of room-temperature operation and highly efficient film growth. Despite these advantages, AD has not been used for preparing humidity-sensing films. Herein, roomtemperature AD was utilized to deposit BaTiO3 films on a glass substrate with a Pt interdigital capacitor, and their humidity sensing performances were evaluated in detail, with further optimization performed by post-annealing at temperatures of 100, 200, ..., 600 °C. Sensor responses (i.e., capacitance variations) were measured in a humidity chamber for relative humidities (RHs) of 20–90%, with the best sensitivity (461.02) and a balanced performance at both low and high RHs observed for the chip annealed at 500 °C. Besides, its response and recovery time were extremely fast respectively at 3 and 6 s and kept a stable recording with the maximum error rate of 0.1% over a 120-h aging test. Compared to other BaTiO3-based humidity sensors, the above chip required less thermal energy for its preparation but featured a more than twofold higher sensitivity and a superior detection balance at RHs of 20–90%. Cross-sectional transmission electron microscopy imaging revealed that the prepared film featured a transitional variable-density structure, with moisture absorption and desorption being promoted by a specific capillary structure. Finally, a bilayer physical model was developed to explain the mechanism of enhanced humidity sensitivity by the prepared BaTiO3 film.

1. Introduction Industrial processing, environmental control, agricultural production, and other processes require accurate and reliable humidity monitoring, consequently utilizing humidity sensors. These sensors respond to ambient humidity by generating electrical signals and exhibit good linearity, high sensitivity, low hysteresis, fast response, and long-term operational stability.1–7

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Humidity-sensing films are the core component of humidity sensors, exhibiting wellreproducible and regular variations of optical power absorption, conductivity, and permittivity in response to ambient humidity changes.8,9 Specifically, the variation of optical absorption directly affects the received power at the output port, the material conductivity change reflects on the resistance, whereas permittivity variation determines the measured capacitance.1,2,10 The above parameters are affected by factors such as the internal porous structure and hydrophilic surface morphology of the hygroscopic material.2 However, moisture-sensitive films directly contact the ambient environment, often experiencing extreme temperatures and high humidity and being exposed to chemical contaminants and complex gas mixtures. Therefore, in addition to good humidity sensing performance, prospective sensing materials should exhibit high mechanical strength and chemical resistance, with ceramic films therefore being superior to organic polymer ones.3 The susceptibility of ceramic surfaces to hydration is more pronounced than that of other humidity-sensing materials such as polymers, which facilitates the water dissociation and protonation.4 Porous ceramics or nanocrystalline materials comprising ABO3-perovskite-type BaTiO3 have been extensively studied as humidity sensing materials5 due to the well-known sensitivity of A-site alkaline earth elements (e.g., Ba) to humidity.6 Moreover, the incorporation of Ti ions is known to reduce the electrical conductivity and dielectric loss of ferrites, which is very important for better humidity sensing.11 The realization of broadcoverage humidity sensors requires the sensing materials to exhibit well-balanced sensitivity at both low and high relative humidities (RHs). However, ceramic materials generally exhibit poor sensitivity at low RH, mainly due to their non-uniform inner pore distribution and singular ionic sensing (Grotthuss) mechanism.1 The above problem can be mitigated by utilizing both ionic and electronic charge carriers12 and/or adjusting the internal pore distribution to afford a capillary state.13



A large number of ceramic humidity-sensing films have been prepared, e.g., Tripathy14 fabricated a uniformly porous humidity-sensing material (CaMgFe1.33Ti3O12) by stepwise solid-state sintering, whereas He et al. synthesized nanofibrous BaTiO3 using a combination of electrospinning and calcination.15 Moreover, other conventional film preparation techniques such as sol-gel,16 stearic acid gel,17 and wet chemical methods18 have also been tested. However, these solution-phase methods require high-temperature annealing/sintering to enhance film recrystallization and therefore exhibit certain shortcomings hindering their industrial mass application, namely (1) significant thermal cost, (2) complex and timeconsuming operation, and (3) limited shelf life of required raw material solutions. The above problems may be circumvented by utilizing aerosol deposition (AD), a technique featuring the direct deposition of ceramic particles at velocities of 100–600 m/s and room temperature (RT).19 Although AD has already been utilized in the fabrication of hydrophilic materials,20 optical devices,20,21 high-K capacitors,23–25 and implanted dental brackets,26 as well as some sensing applications, such as gas sensors27, piezoelectric devices,19 its applicability to the synthesis of functional humidity-sensing materials has not been tested yet. Limitations of AD include weak particle-to-particle bonds, surface macroscopic defects, and high inner porosity, which can cause leakage currents and unstable permittivity properties and impair the electrical performance. However, these limitations turn out to be advantages in humidity-sensing applications wherein mesoporous inner structure and high-roughness surface can enhance the moisture absorption and desorption. Besides, since impact consolidation effects enable direct ceramic particle–substrate binding, high-temperature and solution-phase processing can be excluded, which improves the energy efficiency of the deposition process and allows ceramic films to be fabricated on flexible substrates. Moreover, the long shelf life of starting materials coupled with the advantages of cost effectiveness, operation simplicity, and efficient film growth (1–5 µm/min) make AD an ideal option for


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industrial mass production, with the desired surface morphology and internal microstructure obtained by controlling the particle size of starting materials, gas flow rate, and scan time. This pilot study aimed to investigate the feasibility of preparing humidity-sensing films by AD using BaTiO3 powder as the raw material. To further improve their sensing performance, the deposited films were post-annealed at different temperatures, and their grain growth state, crystal lattice, internal microstructure, and surface morphology were characterized in detail to model surface morphology variation at different treatment temperatures. Additionally, the physical models were constructed to explain the enhanced sensitivity of the prepared sensors based on their cross-sectional structure. Thus, our investigation provides first-time evidence of the potential of AD for preparing ultra-sensitive humidity-sensing films.

Figure 1. Schematic representation of (a) synthesis of humidity-sensing films by AD, (b) post-annealing of as-deposited films, and (c) humidity sensing experiments.

2. Experimental Section 2.1. Sensor Fabrication by AD and Post-Annealing 5 ACS Paragon Plus Environment


AD is a room-temperature process based on the shock-loading solidification (SLS) of accelerated particles on the substrate (Figure 1a). First, a carrier gas is passed through loose BaTiO3 powder contained in a vibrating chamber, thereby producing a fluidized bed. Driven by a pressure difference, the aerosolized particles are transported from the aerosol chamber through a nozzle to the evacuated deposition chamber. Then the aerosol is accelerated by the narrow-sized nozzle to velocity around 100 to 600 m/s, forming an aerosol jet at its outlet. Herein, helium was used to accelerate the BaTiO3 aerosol to achieve higher gas flow speeds and particle impact velocity. The chamber gas flow rate was maintained at 7–8 L/min by an internal pressure of 1–7 Torr. BaTiO3 particles were deposited on the glass substrate at a scanning speed of 1–2 µm/min, with other process parameters (e.g., size of nozzle orifice, nozzle-to-substrate distance, and operation time, Table S1) chosen to achieve a final deposition thickness of ~1 µm. The accelerated BaTiO3 particles generally experienced consecutive impact and fragmentation on the substrate, which caused crystal lattice distortion, internal stress accumulation, and grain size reduction. Post-annealing is known to be an effective way of relieving crystal structure distortion, promoting crystallization and producing the desired capillary microstructure.17,28,29,30 Therefore, as-deposited films were subjected to 2-h postannealing in ambient atmosphere at 100, 200, ..., 600 °C (100PA, 200PA, ..., 600PA samples), with the furnace temperature increased and decreased at a rate of 5 °C/min (Figure 1b).

2.2. Crystallinity, Microstructure, and Surface Morphology Characterization The crystal structure of the prepared films was characterized by X-ray diffraction (XRD) (ATX-G, Rigaku Co., Japan) using Cu Kα1 radiation (λ = 1.54060 Å) and a voltage/current of 40.0 keV/30.0 mA. Scanning was performed for 2θ = 20–70° at a rate of 2°/min. The average crystallite size was estimated using the Scherrer equation:


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= ,

(1)

where D is the crystallite size, k a constant equal to 0.9, β is the full width at half maximum, and θ is the Bragg angle. Atomic force microscopy (AFM) (XE150, PSIA, USA) imaging in non-contact mode was employed to characterize the surface morphology of samples, with the obtained data analyzed by XEI software (Park Systems Corp.) as AFM top-view, 3Dmorphology-view, and 2D Fourier filter transform power spectra. Moreover, cross-sectional microstructure was also characterized by scanning electron microscopy (SEM) (S-4800, Hitachi, UK) imaging. The distributions of inner voids and grains were evaluated based on the results of transmission electron microscopy (TEM) (JEM-2100F, JEOL, USA) imaging.

2.3. Humidity Sensing An inductance-capacitance-resistance (LCR) meter (HIOKI IM 3536) was utilized to measure capacitance variation (Figure 1c). The frequency was set to 100 Hz, which is known to be optimal for evaluating the material capacitance.31 The applied voltage and temperature were fixed at 1 V and 23 °C, respectively. Sensors were tested in a humidity chamber at RHs of 20–90%. Sensitivity (s), a representative parameter characterizing the humidity sensor performance, was calculated as ∆

= ∆ .

(2)

The statistical evaluation of sensitivity was separately performed at both low (20–60%) and high (60–90%) RH to inspect the corresponding humidity detection capabilities and determine if the developed approach improves the generally poor performance of ceramic humidity sensors at low RH. Based on the above statistics, the ratio of L-sensitivity to H-sensitivity was calculated to express the balance of sensing capability at low and high RH (L/H balance), with higher values indicating better performance. The response and recovery time were 7 ACS Paragon Plus Environment


measured under a sudden humidity change from/to the ambient humidity in the measurement room (30%) to/from 90% RH. The aging test was implemented at different level RHs (20%, 50%, 90%) for 120 h with continuous capacitance recording. Additionally, a humidity sensing model was developed to study the sensing mechanism of the specific AD-prepared film.

3. Results and Discussion 3.1. Crystallinity of BaTiO3 Films During AD film growth, BaTiO3 particles underwent two-step fragmentation, with the first step ascribed to the effect of SLS (i.e., the impact to the substrate or the precursor BaTiO3 layer),22 whereas the second one was attributed to the hammering effect of the subsequently deposited high-speed particles.32 The above double fragmentation not only resulted in severely reduced grain size, but also induced crystal lattice distortion and residual stress accumulation, thus significantly impacting crystal structure properties.33 Figure 2a shows the XRD pattern of as-deposited BaTiO3 film, revealing its cubic perovskite structure. The expansion of diffractions peaks at 2θ ≈ 45.37° (corresponding to reflections from the (200) plane, Figure 2b) revealed the occurrence of a cubic-to-tetragonal phase change at high annealing temperatures. The crystallite size of the as-deposited film was calculated as 17.4 nm, which, together with a 0.69° peak shift in comparison to bulk BaTiO3, revealed the presence of crystal lattice distortions and residual stress.


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Figure 2. (a) XRD patterns, (b) expansion of the area corresponding to reflections from the (200) plane, and (c) AFM images (2 mm × 2 mm) of as-deposited BaTiO3 film and that annealed at 200, 400, and 600 °C (corresponding samples denoted as RT, 200PA, 400PA, and 600PA). The dash-dotted line in (a) and (b) represents the position of the (200) peak of bulk BaTiO3.

The height of BaTiO3 peaks increased along with the eased peak shift and crystallite size increased with increasing post-annealing temperature in the range of 200–400 °C (200PA400PA), which was ascribed to the increase of dielectric permittivity in response to increased ferroelectricity and domain wall contributions.34 Since the capacitance variation of the fabricated humidity sensors at different RHs was caused by changes of the combined permittivity of the BaTiO3 film and condensed water, the above permittivity increase was viewed as a favorable factor. The peaks of 600°C-treated (600PA) films well matched those of the standard BaTiO3 tetragonal perovskite phase, indicating that high-temperature 9 ACS Paragon Plus Environment


annealing induced a gradual restoration of the crystal structure. The accompanying 2 µm × 2

µm AFM images supported the occurrence of grain growth on the surface, with the absence of clear grains in AFM images of as-deposited films explained by grain agglomeration caused by the impact of high-speed particles (Figure 2c). In contrast, the 200PA film started showing clear grains on its surface, with larger and clearer grains identified in 400PA and 600PA films. Additionally, increased amounts of surface meso-cracks were observed inside grains at increased annealing temperatures, which was thought to favor the adsorption of water vapor. In addition to 2-h constant-temperature annealing, heating up and cooling down at 5 °C/min also exhibited a certain influence, resulting in the total thermal effect being higher than expected.

3.2. Cross-Sectional Microstructure of BaTiO3 Films As side-view SEM images of as-deposited films (Figure 3a) revealed their densely laminated uniform structure featuring tiny grains and visible crevices, with the increased grain size and number of crevices after post-annealing at 200 °C indicating obvious agglomeration. A temperature increase from 400 to 600 °C resulted in further grain growth and agglomeration, reducing the amount of crevices. During AD, kinetic energy was converted into thermal energy, giving rise to SLS and the hammering effect as well as inducing a further transformation into the energy of grain/substrate bonding. Due to the more pronounced hammering effect experienced by the first-deposited (bottom) film layer, it was more densely laminated than the top layer, wherein the cross-sectional density is distributed in a transitional state (Figure 3b). Moreover, a high porosity was present in the top layer, progressively decreasing in the direction toward the bottom layer. The TEM cross-sectional graphs of 500PA chips in repetitive experiments and 200PA, 400PA 600PA chips all showed this transitional-density structure, which was verified 10 ACS Paragon Plus Environment

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as a consistent structural characteristic of AD-prepared film (Figure S1). As discussed above, double fragmentation resulted in residual stress accumulation and crystal structure distortion of the bottom layer, severely affecting the overall dielectric capacity. Moreover, the relative less hammering effects experienced by the top layer resulted in loose grain-to-grain bonding and high porosity, which, together with the presence of surface defects, could result in leakage current.32 Besides, the presence of large-sized pores was thought to adversely affect sensing at low RH, since the detectable humidity limit is known to decrease with decreasing pore radius.1

(a)



(b)

Figure 3. (a) Cross-sectional SEM images of as-deposited and post-annealed chips. (b) TEM image of the 500 PA film showing the different densities of bottom, central, and top layers.

The dielectric and humidity absorption properties of the as-prepared film were improved by post annealing, which induced a solid-phase transformation from a non-uniform state to a uniform one.17 The required heating time and temperature were determined by the initial film structure wherein an ununiformed inner structure needs a long distance to complete this reaction, with excessive annealing possibly leading to deteriorated humidity sensing performance. Considering the bottom layer, thermal treatment promoted crystal growth and relieved inner stress/crystal structure distortion, thus resulting in crystal property optimization. Simultaneously, the above treatment induced grain expansion in the top layer, thus enhancing grain-to-grain bonding, which, in turn, resulted in decreased pore size to afford a capillary structure to extend the minimum RH detection limit.

3.3. Surface Morphology Adjustment AFM imaging of films annealed between RT and 300 °C (Figure 4a) revealed that thermal treatment resulted in gradual repair of gross surface defects, with the root mean square (RMS) 12 ACS Paragon Plus Environment

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roughness decreasing from 80.5 to 49.8 nm. However, annealing at 400 °C led to surface morphology deterioration and increased RMS roughness to 60.2 nm, whereas annealing at 500 °C resulted in optimized flatness (RMS roughness = 45.4 nm). Finally, deteriorated surface flatness was observed at 600 °C (RMS roughness = 57.5 nm). Due to the consistent roughness change at temperatures between RT and 300 °C, thermal treatment in this range was defined as the initial adjustment stage, with other cases (400, 500, and 600 °C) classified as the indefinite variation stage due to the observed irregular pattern change, and this irregular morphology change was again verified as a consistent rule by the repetitive experiments (Figure S2). Power spectral density (PSD) is one of the parameters used to represent surface roughness, having an advantage over other parameters such as RMS roughness due to containing information on how each frequency component contributes to the total roughness of the surface. High-frequency power (H) implies the high amount of matter-to-matter transition, whereas large low-frequency power (L) implies large overall altitude variations. In this case, the value of H indicated the amount of small surface grains and crevices, which are known to be important for humidity sensing.17 Hence, we introduced the H/L ratio as a new parameter, with its high values implying a surface with a large amount of grains and crevices and a few of large agglomerates and surface defects, thus corresponding to properties favoring efficient humidity sensing.



(a)

(b)


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Figure 4. (a) AFM and Fourier filter transform characterization of deposited films annealed between RT and 600 °C. (b) Statistics of surface parameters (RMS, H, L, and H/L) and the corresponding variation patterns.

The surface patterns obtained at different annealing temperatures were further classified into five different states (Figure 4b). Thus, upon going from RT to 200 °C, H increased from 0.75 to 1.00, whereas L decreased from 1.0 to 0.16, leading to an increase of H/L from 0.05 to 0.43, which corresponded to states 1, 2, and 3, respectively. As the temperature was increased further, surface morphologies corresponding to states 4 and 5 were observed. However, the change of PSD parameters also showed non-consistent rule here, which could also be regarded as an indefinite variation. Notably, the 500PA film featured the highest H/L value due to exhibiting a high H and the lowest L (state 5). However, both H and L values were high in the case of 300PA, 400PA, and 600PA films (state 4). The high hardness of the glass substrate caused severe fragmentation of impacted BaTiO3 forming the initial film layer,35 with non-uniform grain size and distribution leading to heterogeneous density and surface defects. The subsequently deposited high-speed BaTiO3 particles exhibited a hammering effect on the previously deposited BaTiO3 thin layer, causing its further densification and simultaneously exerting an etching effect on the not fully fragmented region,36 with the etching/densification balance determined by both the precursor density distribution and the kinetic energy of high-speed BaTiO3 particles. Thus, the hammering effect induced further densification of highly dense areas, whereas not fully fragmented areas usually featuring crater defects were etched and densified, which explains the high RMS roughness (80.5 nm) of as-deposited films.



The post-annealing–induced surface morphology variation was caused by both internal solidstate reactions and the abovementioned growth of surface grains. During the initial adjustment stage (RT to 300 °C), the main effect of annealing corresponded to the expansion of individual grains and the shrinkage of crevices inside the film. The above expansion repaired severe defects,37 increasing the amount of clear surface grains and meso-cracks, i.e., resulting in better flatness and higher H/L values. However, after the above repair, the further increase of annealing temperature shifted the main process from grain expansion to mutual extrusion. The continuous extrusion of inner grains caused by the non-uniform film structure increased surface roughness (400PA), with further temperature/heating time increases causing further grain expansion and defect filling (500PA) or deterioration (600PA), which was regarded as an indefinite variation, since the heating-induced grain expansion was based on the nonuniformity of the film structure.

3.4. Humidity Sensing Properties Figure 5a shows the variation of sensor capacitance for RH = 20–90%, with the corresponding sensitivity statistics summarized in Figure 5b. At 20% RH, all sensors exhibited capacitances of ~30 pF, with increased capacitances with distinct variation patterns observed at higher RH. Sensors annealed below 200 °C failed to detect humidity variations at low RH levels (20–60%), whereas their capacitance increased at high RH (60–90%), with larger capacitance variation observed for higher post-annealing temperatures. The sensor sensitivity in the high RH region dominated the overall sensitivity (20–90% RH), and poor L/H balance was seen. Notably, the 300PA sensor showed both improved low-RH detection ability (L-sensitivity = 1.40) and an enhanced H-sensitivity (21.13), thus featuring a better sensitivity balance (L/H = 0.07). The capacitance of the 400PA chip varied from pico- to nanofarads, in contrast to that of the 300PA one. Moreover, the former sensor also showed an optimized L-sensitivity of 5.94, 16 ACS Paragon Plus Environment

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which, however, was not as significant as the improved H-sensitivity of 391.7. The best result was observed for the 500PA chip, corresponding to a sensitivity of 461.02 and a balance of 0.55. The 600PA chip featured a decreased sensitivity of 106.77, which was also lower than that of 400PA (171.04). However, 600PA sensors achieved better L-sensitivity (14.59) and balance (0.06) compared to the respective parameters of 400PA chips (5.94 and 0.02). The response and recovery time were extremely fast for the 500PA sensor at 3 and 6 s, respectively (Figure 5c). And this sensor also achieved a stable capacitance value with a maximum error rate of 0.1% over 120 h of continuous testing under different RH levels (Figure 5d).

(a)



(b)

(c)


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(d)

Figure 5. Humidity measurements: (a) variation of capacitance with RH, (b) sensitivities at low RH (L-sensitivity) and high RH (H-sensitivity), as well as overall sensitivity and its balance at low and high RHs (L/H-balance), (c) response and recovery test, and (d) 120-h room temperature aging test in different ambient humidity of 500PA sensor.

3.5. Humidity Sensing Mechanism Figure 6a shows the bilayer structure of deposited BaTiO3 films, with the bottom layer being denser and featuring smaller grains and minimized porosity, whereas the top layer exhibited a sparse structure and contained larger grains. The automatically-generated porous structure by AD played a critical role in the interactions and storage of condensed water and the high open-pore ratio on top layer also enhanced the moisture absorption.

3.5.1 Working Mechanism of Humidity-Caused Conduction All condensed water expressed the protons tunneling effect which contributes to the increased dielectric permittivity and conductivity of the sensitive layer. BaTiO3 films prepared by AD could sense RH variation due to exhibiting both protonic and electronic conduction, the combined variation of which influenced the dielectric permittivity of the sensing layer and thus induced regular capacitance variation.

Protonic conduction: As shown in Figure 6b, Ba2+ and Ti4+ could bind the OH– of condensed water, whereas O2– could bind H+ to form the first aquatic layer, which, taken together, corresponded to the chemical absorption of water. Subsequently, physical water absorption resulted in the formation of the second aquatic layer. The protons inside the above two layers were not mobile, i.e., unavailable for charge transport. However, according to the Grotthuss mechanism, continuous moisture condensation results in unhindered proton transfer between water molecules via hydrogen bonding.1 At room temperature (23 oC), water 19 ACS Paragon Plus Environment


molecules ionize to H+ (or H3O+) and OH− ions (H2O→H++OH− or 2H2O→OH−) when they were adsorbed on BaTiO3 grains surface. H+ (or H3O+) ions play an important role in conduction when RH is getting higher. Therefore, proton conductance is generally expected to occur at higher RH, explaining why the measured capacitance variation exhibited an exponential pattern.

Electronic conduction: Conversely, electron conduction dominated at low RH, which was designated as the donate effect.13 Thus, adsorption of moisture on the surface of BaTiO3 grains lowered their potential barrier and active energy, releasing the accumulated electrons and making them available for conductance. The surface O2– ions, which will be replaced by H2O molecules to induce the release of electrons, also contributes to the donate effect.

3.5.2 Effects of Structural Properties on Moisture Absorption In addition to promoting electron participation, another method of improving sensing performance at low RH features the adjustment of the internal structure of the sensing film to a capillary state comprising smaller pores. In current case, high-temperature post-annealing promoted grain growth, resulting in intergranular crevice shrinkage and enhancing humidity sensitivity and low-RH sensing. According to previous studies, the sensitivity of humidity sensors is co-determined by constituent material properties, internal microstructure, and surface hydrophilicity.1,2 Herein, sensitivity was positively correlated with annealing temperature (from RT to 500 °C), with thermal treatment inducing regular changes of the internal microstructure but causing an indefinite surface morphology variation above 300 °C. The enhancement of humidity sensing ability was attributed to the synergistic effects of surface morphology and internal structural adjustment induced by thermal treatment. The best-performing 500PA sensor, which is in the indefinite surface variation range and might not being suitable for the repetitive industrial 20 ACS Paragon Plus Environment

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production. However, the desired surface roughness (high H/L) can possibly be realized by post-etching of the above sensor.23,38

(a)

(b)

Figure 6. (a) Structural characteristics of the AD-fabricated humidity sensing film and (b) corresponding humidity sensing mechanism.

3.5.3 Best Humidity-Sensing Performance of 500PA Chip The best sensitivity and detection balance observed for 500PA were ascribed to its enhanced BaTiO3 dielectric properties, good surface hydrophilicity enabling high H/L, and ideal capillary porous structure, in which these properties could be optimized through moderate thermal treatment (500 °C for 2 h), whereas excessive thermal effects (600 °C) caused the deterioration of surface morphology and hindered moisture absorption due to pore restriction (Figure 4 and S1). 21 ACS Paragon Plus Environment


BaTiO3 crystal properties promotion: During AD process, BaTiO3 particles underwent twostep fragmentation, with the first ascribed to the effect of shock-loading solidification, whereas the second was attributed to the hammering effect of subsequently deposited highspeed particles. The double fragmentation resulted in residual stress and crystal structure distortion, severely affecting the BaTiO3 grain’s crystal properties. The high-temperature treatment could relieve the residual stress and crystal distortion. However, increasing temperature after completing the solid-state reaction, although the grains growth would continue, the thermal effects on BaTiO3 precursor was not obvious anymore.

Surface hydrophilicity enhancement: A large amount of surface nano-scale grains and crevices were favorable to improve the hydrophilicity. The 500 °C thermal treatment further promoted the surface grain expansion and defects filling, resulting in better flatness and higher H/L. However, 600 °C treatment with excessive thermal effect caused a deterioration on surface hydrophilicity.

Inner microstructure adjustment: The 500 °C treatment induced a thermal solid-phase reaction to transform the inner structure from a non-uniform to a uniform state, which improved the dielectric properties and enhanced humidity absorption. The time length and temperature of the thermal treatment were determined by the initial film structural state wherein an ununiformed structure was generally in need of a long distance to complete this reaction, with excessive annealing possibly leading to deteriorated humidity sensing performance. Comparing the results of this series experiments, 500 °C treatment for 2 h was identified as the superior option.

3.6. Modeling of Humidity Sensing


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In view of the specific cross-sectional microstructure, the increased cross-sectional density was expected to inhibit the penetration of water vapor into deeper-lying film layers. Under these conditions, moisture condensation should mainly occur in the top layer, implying that the relative permittivity of the film should be depth-dependent and change with RH. Besides influencing moisture absorption/desorption, the cross-sectional transitional density should also show different humidity sensitivities with the uniformed structure. The transitional density structure could be viewed as being approximately equivalent to the layered structure; based on this and the TEM image of the 500PA film were built two models with different laminated structures (Figure 7). Thus, the above models utilized the same thickness and void volume/distribution. However, the bilayer model featured layers with different void fractional volumes (VFVs), with the bottom layer having a more compact grain contact than the top layer.

Case 1: Monolayer hygroscopic film The VFV of the overall film was calculated as 32.84%, with the fractional volumes content of water at different RHs calculated as shown below.40

γ = , = RH%/100.

(3)

where is the maximum fractional volume at room temperature (32.84% in this case),

represents the temperature dependence of the adsorption coefficient, and denotes the temperature dependence of the dielectric constant of water !"

!"

defined as40

= 78%1 − 4.6 × 10+, - − 298 + 8.8 × 10+1 - − 298! 2,

(4)

where T is the absolute temperature. Since the measurements were performed at room temperature, γ and

!"

γ =32.84% × ".45!,

could be calculated as !"

= 78.54.

(5) 23

ACS Paragon Plus Environment


The mixed dielectric constant

Page 24 of 37

of the BaTiO3-based hygroscopic film could be expressed

as41

= % 6

where

8 9

!7 −

:;7

8 9

:;7 < +

8 9

:;7 2

5

,

(6)

is the dielectric constant of the AD-prepared film. Since the humidity

measurements revealed that capacitance did not significantly change at very low RH, the value measured =>? at 20% RH (=>? = 28.8 pF) was used to approximate capacitance

under dry conditions. As shown in Figure 7a, =>? featured contributions from the

interdigital capacitor (IDC)-generated film capacitance =@AB , substrate capacitance =CD , and the measurement line loss =B of 18.0 pF:

=>? = =CD + =@AB + =B .

(7)

=CD is the substrate capacitance generated by the IDC in the absence of the sensing film (i.e., corresponding to the value obtained when the above film is replaced by an infinitely thick air layer). Hence, this parameter was determined by the dielectric constant of ambient vapor and substrate parameters such as dielectric constant (

CD

= 4.6), dielectric loss (0.006),

thickness (1 µm), and IDC geometric structure. Simulation using the Advanced Design System 2016.01 (ADS; Figure S3) afforded =CD = 2.85 pF at a frequency of 100 Hz,

allowing =@AB to be calculated as 7.95 pF. Based on the equivalent circuit, the relationship

between =CD , =@AB , and the corresponding dielectric constants was expressed as EFGHI EJKL

=

MNOP QJKL

,

(8)

allowing the IDC measured dielectric constant ε:;7 of the 500PA film to be calculated as 12.83, with the mixed dielectric constant

of moisture-impregnated BaTiO3 at different RH

expressed as 24 ACS Paragon Plus Environment

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= 0.6345 ".45! + 2.345 ， = RH%/100.

(9)

The interior and exterior unit capacitances were calculated as42 = =

UV@AB W, X ,

(10)

W, X is the cell constant for internal and where L is the length of electrode fingers, and V@AB

external IDC units determined by the metallization ratio W and the height-to-width ratio X. In turn, the above variables were expressed as Y

,

!\

,

W=

YZ[

X=

YZ[

(11)

(12)

Based on the above model, sensitivity was calculated as 2.2.

(a)

(b)



(c)

Page 26 of 37

(d)

Figure 7. Models of (a) monolayer and (b) bilayer hygroscopic films; (c) Hashin-Shtrikman bounds of AD-prepared BaTiO3 as a function of BaTiO3 content and (d) dependence of sensitivity on the FV2/FV1 ratio determined using Neumann and Dirichlet boundary conditions. Case 2: Bilayer hygroscopic film In this model, the film was approximated as a bilayer structure with 0.5-µm-thick bottom and top layers. Since the AD-fabricated film featured a transitional-density cross-section structure, the relative permittivities of each layer ( ] ,

!)

were expected be different and could be

estimated based on the fractional volume of BaTiO3 using the Hashin-Shtrikman bounds.43 The above permittivities, depending on the connected grain fraction (%) and the top and bottom limits, were calculated using Equation 11 and 12 and plotted in Figure 7c. ;^

=

:

:;7 _:;7

=

+

:;7 _:;7

Àa _Àa

+

−

bNOP bcGd QNOP +QcGd e

QNOP bcGd ZQcGd bNOP Zf+]QNOP

Àa _Àa

−

bNOP bcGd QNOP +QcGd e

QNOP bcGd ZQcGd bNOP Zf+]QcGd

(13)

(14)

Therefore, the relative permittivities of each layer of water-impregnated BaTiO3 at 90% and 20% RH could be calculated using Equation 7. The permittivity reduction observed upon going from one layer to the next was assumed to act as an electric field barrier, with these 26 ACS Paragon Plus Environment

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layers cascading in a parallel-type configuration and Neumann boundary conditions assumed at their interfaces. For the case of increasing permittivity, when the electric filed is more strongly guided away from the electrode plane, the layers were assumed to be coupled in series, and Dirichlet boundary conditions were applied at the layer interface.44,45 Hence, the capacitance of the bilayer film at 20% and 90% RH could be calculated as Parrallel: = = U% Series:

]

pq

]

= %s r

]

QI8

]+ ! V@AB W, X]

−

]

t

]

v w,a8 QIe uFGHI

+

+ ]

! V@AB W, X! 2. ]

v w,ae QIe uFGHI

2.

(15)

(16)

Denoting the VFV of the bottom layer as FV1 and that of the top layer as FV2, the change of sensitivity for FV2/FV1 = 1–14 was simulated (Figure 7d). As a result, the sensitivity increased with increasing FV2/FV1 in the range of 1–8.4, implying that larger FV2/FV1 ratios enhanced sensitivity based on the fixed thickness and overall VFV. However, as FV2/FV1 was increased above 8.4, the simulated sensitivity decreased, which was ascribed to the inability of the bottom layer with a small FV1 to well sense the RH variation via mixed permittivity changes. For FV2/FV1 = 1, corresponding to the monolayer structure, the sensitivity was calculated as 0.80, which was close to the previously calculated sensitivity of 2.2 based on the measured relative permittivity. The above result verified the suitability of the Hashin-Shtrikman bounds theory for estimating the relative permittivity of BaTiO3 films. Besides, two-dimensional image processing using Matlab R2015b allowed the 500PA TEM image to be utilized for calculating bottom and top layer VFVs as 6.17 and 58.40%, respectively (Figure 7b), with the FV2/FV1 ratio thus equaling 2.84. The measured sensitivity of 461.02 was rather close to the simulated value of 572.96, which shows that the developed model could well reproduce experimental results. According to the above model, at a fixed hygroscopic film thickness of 1 µm and VFV constant, the higher amplitude of cross-sectional density variation should make the 27 ACS Paragon Plus Environment


Page 28 of 37

homogeneous structured film more sensitive to humidity in a certain range. The specific transitional-density structure resulting from the AD-induced hammering effects could realize the above physical model, which was verified as a superior method to prepare the high sensitivity humidity sensitive film. Although the above model validated the advantage of transitional structure for improving sensitivity and predicted the influence of inner density distribution variation, it still exhibited some limitations that affected its prediction accuracy. First, although the selected TEM image was representative, it still could not accurately reflect the pore distribution in the whole film. Second, the density-transitional type film was modeled by a bilayer structure, which could affect modeling accuracy. Third, during humidity sensing, capillary condensation of water vapor would occur only in the mesoporous size,46 where an extremely large or small FV ratio would lead to poor sensitivity to humidity; this probability was not considered in the present model. Considering these limitations, the predictions provided by the developed model should be treated with caution.

Table 1. Performances of the current and previously reported humidity sensors. Work

Material Preparation technique

Ref. 15a)

BaTiO3 sol-gel

Auxiliary Annealing Sensitivity L/H methods temperature and balance time

2 h at 800 °C Electrospinning Sol-gel, 15 kV, 12-h drying

214.21

0.05

Ref. 13a)

BaTiO3 powder

Screen printing

1 V at 0.5 h at 600– 140.00 100% 650 °C to RH for powder; 0.5 h at 24 h 500 °C

0.17

Ref. 16a)

BaTiO3

Spin coating

Not required

0.04

sol-gel

2-h drying at 148.33 200 °C, 1-h sintering at 600 °C


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Ref. 18b)

BaTiO3 powder

Cold isostatic Cold 8-h sintering at 154 pressing and isostatic 1250 °C, 1 h at pressing 600 °C sintering at 150 MPa

No data

Ref. 18b)

BaTiO3 powder

Cold isostatic Cold 8-h sintering at 11 pressing and isostatic 1250 °C, 1 h at pressing 600 °C sintering at 150 MPa

No data

500PA

BaTiO3 powder

AD

Not 500 °C for 2 h required

461.02

0.55

a)

IDC-based sensors. b) Metal-insulator-metal capacitor–based sensors. The capacitance variation of all sensors was measured at a frequency of 100 Hz.

4. Conclusion This study presents the first step toward the fabrication of novel humidity-sensing films by AD, allowing the preparation of sensors exhibiting superior sensitivity and L/H balance compared to those of other BaTiO3–based sensors, with the second best sensitivity achieved by He et al., who prepared a nanofibrous film at a high voltage and subjected it to 2-h drying and 2-h heating at 800 °C (Table 1). Notably, other works required at least two-step annealing at temperatures above 500 °C. In current case, post-annealing enhanced grain growth and film recrystallization, thus changing the internal and surface microstructure. Films annealed under certain conditions featured the desired internal capillary porous structure and a high-H/L surface morphology, which are important factors influencing their humidity sensing performance. The above comparison verified that the AD-prepared hygroscopic film could achieve superior performance, with the AD method itself featuring the additional advantages of room-temperature operation, highly efficient film growth, operation simplicity, and the long shelf life of the utilized raw materials, thus being suitable for mass production. Besides, although the non-annealed AD-prepared film failed to detect humidity at low RH, it still allowed the fabrication of ceramic humidity-sensing films on certain thermally unstable 29 ACS Paragon Plus Environment


substrates such as flexible materials or 3D electronic integrated systems, worked on high RH detection.

ASSOCIATED CONTENT

Supporting Information Supporting Information is available free of charge on the ACS Publications website. Simulation of an IDC on a glass substrate in air by Advanced Design System (Figure S1). AD conditions (Table S1).

AUTHOR INFORMATION

First author J.-G. Liang

Corresponding author C. Wang Email: [email protected] J.-M. Oh Email: [email protected] N.-Y. Kim Email: [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was supported by the This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; Ministry of 30 ACS Paragon Plus Environment

Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Science, ICT & Future Planning) (No.2011-0030079 and No. 2017R1C1B5017013) and a grant supported from the Korean government (MEST) No. 2015R1D1A1A09057081. This work was also supported by a Research Grant of Kwangwoon University in 2017. ABBREVIATIONS AD Aerosol deposition; PA post-annealing; RH relative humidity; XRD X-ray diffraction; AFM Atomic force microscopy; TEM transmission electron microscopy; PSD Power spectral density;

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(37) Yao, Z.; Wang, C.; Li, Y.; Kim, H.-K.; Kim, N.-Y. Effects of Starting Powder and Thermal Treatment on the Aerosol Deposited BaTiO3 Thin Films Toward Less Leakage Currents. Nanoscale Res. Lett. 2014, 9, NO. 435. (38) Kim, D.; Park, J.; Lee, J.; Lee, M.; Kim, H.; Oh, J.; Seong, T.; Kim, D.; James, S.; van Hest, M.; Chandra, S.; Yoon, S. Tuning Hydrophobicity with Honeycomb Surface Structure and Hydrophilicity with CF4 Plasma Etching for Aerosol - Deposited Titania Films. J. Am. Ceram. Soc. 2012, 95, 3995-3961. (39) Shibata, H.; Ito, M.; Asakursa, M.; Watanabe, K. A Digital Hygrometer using a Polyimide Film Relative Humidity Sensor. IEEE Trans. Instrum. Meas. 1996, 45, 564569. (40) Hasted, J. B. Aqueous Dielectrics, Chapman and Hall: London, 1973; pp. 37–38. (41) Schubert, P. J.; Nevin, J. H. A Polyimide-Based Capacitive Humidity Sensor. IEEE Trans. Electron Devices 1985, 32, 1220-1223. (42) Blume, S. O. P.; Ben-Mrad, R.; Sullivan, P. E. Modelling the Capacitance of Multi-Layer Conductor-Facing Interdigitated Electrode Structures. Sens. Actuators, B 2015, 213, 423433. (43) Torquato, S. Random Heterogeneous Materials: Microstructure and Macroscopic Properties, Springer: New York, 2005; pp. 552–592. (44) Igreja, R.; Dias, C. Analytical Evaluation of the Interdigital Electrodes Capacitance for a Multi-Layered Structure. J. Sens. Actuators, A 2004, 112, 291-301. (45) Igreja, R.; Dias, C. Extension to the Analytical Model of the Interdigital Electrodes Capacitance for a Multi-Layered Structure. J. Sens. Actuators, A 2011, 172, 392-399. 36 ACS Paragon Plus Environment

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BRIEFS AD serves as another option to prepare ultra-sensitive ceramic hygroscopic film. SYNOPSIS This study presented the first research on aerosol-deposition (AD) based ultra-sensitive humidity-film preparation. AD technique deposits BaTiO3 film with specific transitional variable-density structure, which enhanced the capacity of moisture absorption and desorption. Featured with the room-temperature fabrication, cost effectiveness, operation simplicity, and efficient film growth, AD was proved as another option suitable for mass production of high performance functional humidity-sensing film.

ToC figure


Preparation of Ultrasensitive Humidity-Sensing ... - ACS Publications

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