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. 2020 Jul 21;5(30):18818–18825. doi: 10.1021/acsomega.0c01946

Humidity-Sensing Properties of a BiOCl-Coated Quartz Crystal Microbalance

Qiao Chen †, Ning-bo Feng ‡,*, Xian-he Huang †,*, Yao Yao †, Ying-rong Jin ‡, Wei Pan †, Dong Liu †
PMCID: PMC7408249  PMID: 32775883

Abstract

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The performance of a bismuth oxychloride (BiOCl)-based quartz crystal microbalance (QCM) humidity sensor was studied using an oscillating circuit method. The BiOCl powder was prepared by a hydrolysis method. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were used to characterize the BiOCl sample. Its humidity-sensing property was analyzed by combining it with a QCM at room temperature (25 °C). Experimental results indicated that the BiOCl-based QCM sensor showed good humidity characteristics from 11.3 to 97.3%, such as good logarithmic frequency response to humidity levels (R2 = 0.994), fast response time (5.2 s)/recovery time (4.5 s), good reversibility, stability, repeatability, and low humidity hysteresis. In addition, the response to human nose breaths showed excellent practicability. Finally, the humidity sensing mechanism of the BiOCl-based QCM humidity sensor was discussed in detail. This work demonstrates that BiOCl is a promising candidate material for humidity detection.

1. Introduction

Accurate detection of humidity levels plays an important role in aerospace, residential environments, agricultural production, food processing, pharmaceutical synthesis, and other fields.1 Thus, a large number of researchers are committed to developing high-performance sensors (e.g., accurate results, high sensitivity, excellent reliability, rapid response/recovery time, good repeatability, low cost, etc.).2−4 So far, depending on different working principles, humidity sensors are classified into resistive humidity sensors,5−9 capacitive sensors,10,11 optical fiber humidity sensors,12,13 and bulk acoustic wave (BAW) humidity sensors.14,15 Among them, quartz crystal microbalance (QCM), a typical BAW device, is becoming a research hotspot with advantages of high sensitivity, low cost, easy operation, and digital output.16−19

The ambient humidity can change the adsorption–desorption equilibrium of water molecules on the surface of humidity-sensitive materials, thereby changing the inherent parameters of the resonator and the frequency shift of a QCM. According to previous work, eq 1 can be used to calculate the frequency shift for a non-rigid thin-film QCM sensor.20

1. 1

where Δf and Δm are frequency shift and additional mass attached onto the surface of the QCM. A and f0 are the effective area and the operating frequency of the QCM, respectively. μq and ρq are the shear modulus and density of the quartz, respectively. ρL and μL are the density and viscosity of the non-rigid sensing layers, respectively. It is found that the frequency shift from the eq 1 is composed of two parts. The first half on the right side is the frequency shift caused by the mass of the adsorbed water molecules (namely, mass effect), and the second half is caused by the viscosity change of the sensing material (namely, viscosity effect).

In general, a QCM transducer without a sensing material shows negligible response to changes in ambient humidity. Thus, the sensing materials are a key component in determining the performance of QCM humidity sensors. Till now, several kinds of sensing materials have been used for humidity detection, mainly including carbonic materials,21,22 polymers,23,24 bacterial cellulose membranes,25,26 oxides,27,28 phosphorus,29,30 hydroxides,31 and metal–organic framework materials.32 In addition, various nanocomposites have also been synthesized to provide larger specific surface area and more active sites to improve the performance.33−44 Walia et al. demonstrated the recoverable humidity sensing capabilities of black phosphorus, with detection levels down to 10% RH with a broad dynamic range of 10–90% RH (30 °C).45 A novel copper metal–organic framework has also been demonstrated by Zhou et al. to have good linearity humidity sensing properties at room temperature (about 28.7 Hz/% RH with a good linearity coefficient of 0.993).46 Jalili et al. showed a versatile humidity sensor made from pure graphene oxide which exhibits high sensitivity (66.5 Hz/% RH) and the lowest limit of detection ever reported (0.006% RH at 27 °C).47 However, existing humidity sensing materials suffer from low sensitivity, long response/recovery time, or high cost.31 Metal oxides are promising candidates for humidity sensors because of their fast response/recovery time, low cost, high sensitivity, easy production, and good portability. Gao et al. coated colloidal SnO2 nanowires with large specific surface area on the QCM to develop a humidity sensor, showing high sensitivity (23.17 Hz/% RH) and fast response (10 s)/recovery time (3 s) in the range of 11–97%.28 Zhang et al. fabricated a humidity sensor by coating MOF-derived hollow ball-like TiO2 on the QCM, which exhibits excellent humidity sensing characteristics in a wide RH range (0–97% RH), such as high sensitivity (33.8 Hz/% RH) and fast dynamic response/recovery times (5 s/2 s).48 These works show that metal oxides still have great potential for development in high-performance humidity sensors.

Bismuth oxychloride (BiOCl) is an important ternary semiconducting compound with a band gap of 3.2–3.5 eV. It has been widely used in various fields, such as gas sensing, dye, pharmaceutical, photocatalysis, photoluminescence, optoelectronic devices, solar cells, and others because of its unique and excellent optical, catalytic, electrical, magnetic, and luminescent properties.49−51 However, to the best of our knowledge, there is no literature on the humidity-sensing performance of BiOCl. In this paper, BiOCl was prepared by a hydrolysis method and used as a humidity sensing material coated on a QCM transducer, and then, we systematically studied the humidity properties of the BiOCl-based QCM sensor, including sensitivity, humidity hysteresis, dynamic response, response time/recover time, stability, repeatability, and cross selectivity. Finally, the response to human nose breaths was tested, and the humidity sensing mechanism was discussed in detail.

2. Results and Discussion

2.1. Structural Characterization of the BiOCl Sample

The scanning electron microscopy (SEM) picture of the synthesized BiOCl sample is shown in Figure 1a. It can be seen that BiOCl shows loose porous layered structures comprised of uniform wafers with a diameter of about 500 nm. This loose porous structure makes BiOCl powder to have a large specific surface area and provides more adsorption sites for water molecules. Therefore, the BiOCl can become a good humidity-sensing material. The X-ray diffraction (XRD) spectrum of the BiOCl sample is given in Figure 1b. All diffraction peaks could be well indexed to BiOCl (JCPDS card no. 06-0249), which shown the well crystallization of the prepared BiOCl powder. Figure 1c shows the Fourier transform infrared spectroscopy (FTIR) spectra of BiOCl powder. The peak at about 528 cm–1 is assigned to the symmetrical stretching vibration of the O–Bi bond, and the absorption peak at 825 cm–1 corresponds to the Cl– ion. The peak at 1384 cm–1 corresponds to the nitrate anion. The peak at 3440 and 3132 cm–1 are attributed to the stretching vibration and deformation vibration of the −OH group at wet environment, respectively.52,53

Figure 1.

Figure 1

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(a) SEM image, (b) XRD powder patterns, and (c) FTIR spectra of the BiOCl sample.

2.2. Humidity Sensing Properties and Application of the Sensor

The blank test was performed by exposing the uncoated QCM sensor to a relative humidity that increased from 11.3 to 97.3%, and it showed a negligible response (less than 5 Hz) to the changes in the relative humidity. The equivalent motional resistance values before and after the materials deposition are about 13.3 and 24.2 Ω, respectively. Thus, according to the literature,20 the Q factors are about 42,000 and 23,000. The frequency shifts before and after attaching BiOCl film are 9,984,702 and 9,980,698 Hz, respectively. The density of BiOCl is 7.73 g·cm–3.54 According to the Sauerbrey equation,55 the amount and thickness of the BiOCl film on the surface of the QCM electrode are about 17.7 μg/cm2 and 23 nm, respectively. The frequency shift of the BiOCl-based QCM sensor in air was less than 8 Hz in 2 h, indicating stability of the sensor and testing environment.

The dynamic response curve of the BiOCl-based QCM sensor (Hz) versus time (s) is plotted in Figure 2a. The sensor can respond well to the relative humidity, and the frequency was decreased (625 Hz) dramatically with the relative humidity increase from 11.3 to 97.3%. Moreover, the adsorbent and desorbent velocities were fast and symmetric, suggesting the reversibility of the process of adsorption and desorption of water molecules.

Figure 2.

Figure 2

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(a) Dynamic response curve of the BiOCl-based QCM sensor. (b) Logarithmic fitting curve of log|Δf| vs RH. (c) Humidity hysteresis of the BiOCl-based QCM humidity sensor. (d) Response and recovery times of the BiOCl-based sensor.

As shown in Figure 2b, the BiOCl-based QCM humidity sensor was exhibiting good logarithmic increase (log|Δf|) toward RH with a high regression coefficient (R2 = 0.994) from 32.8 to 97.3%, instead of the linear relationship between Δf and the relative humidity. This result is similar to most of previous QCM-based humidity sensors.29,48,56 Brunauer–Emmett–Teller (BET) adsorption theory is usually used to explain this phenomenon. The adsorption of water molecules to the surface of the BiOCl film is multi-molecular layer adsorption. According to the BET adsorption theory, the amount of water vapor adsorption on the QCM is approximately logarithmic with different RH levels. As a result, the frequency shift and the relative humidity is in a logarithmic relationship. The exponential frequency shift of the BiOCl-based QCM humidity sensor shown in Figure 3c is similar to the type III adsorption isotherm.57

Figure 3.

Figure 3

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(a) BVD equivalent electrical circuit for a QCM. (b) Relative shifts in crystal’s motional resistance versus RH.

Humidity hysteresis is often used to evaluate the reliability of a humidity sensor. The humidity hysteresis characteristics of the BiOCl-based QCM sensor shown in Figure 2c were obtained by exposing the sensor to various humidity levels from low RH to high RH and from high RH to low RH, respectively. As shown in Figure 2c, the sensor showed a low hysteresis (2% at 54% RH) during the cyclic humidity operation, indicating good reliability of the obtained sensor. The small humidity hysteresis observed is due to water molecules being trapped in the deep layer of the BiOCl film without complete desorption.

The response time and recovery time are often used to reflect the response characteristics of a humidity sensor. The sensor was first suspended in the 11.3% environment. After the frequency is completely stable, the sensor was switched to 84.3% environment immediately. The real-time frequency variation is shown in Figure 2d. The results suggest that the BiOCl-based QCM humidity sensor has extremely fast-response speed with a response/recovery time of 5.2 s/4.5 s. We have compared the performance of our sensor with some humidity sensors reported in previous publications (please see Table 1). It can be seen from this table that the response/recovery time is faster than that of most of reported QCM humidity sensors.

Table 1. Performance Comparison of QCM Humidity Sensors in Other Literatures.

material range sensitivity response/recovery time references
MWCNTs/GO 10–80% RH 9.8 Hz/% RH 4 s/3 s (58)
CdS 17–85% RH 11.8 Hz/% RH 36 s (59)
GO 6.4–97% RH 22.1 Hz/% RH 45 s/24 s (22)
UFR/nano-silica 11–83% RH 10.3 Hz/% RH 12 s/25 s (60)
ZnO 11–97% RH 1 Hz/% RH 36 s/12 s (61)
ZnS 22–97% RH 10 Hz/% RH 42 s/259 s (62)
PANI 20–80% RH 3 Hz/% RH 5 s/40 s (63)
anodized alumina 27–59% RH 2 Hz/% RH 5 s/5 s (64)
SnO2–SiO2 11–96% RH 9.4 Hz/% RH 14 s/16 s (65)
BiOCl 11–97% RH 7.3 Hz/% RH 5.2 s/4.5 s this work
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The Butterworth-Van Dyke (BVD) equivalent electrical circuit model shown in Figure 3a is used to describe the electro-acoustic model of a QCM sensor.66,67 In this model, R1, C1, L1, and C0 are the motional resistance, motional capacitance, motional inductance, and static capacitance, respectively. The quality-factor (Q-factor) is defined as the ratio of the energy stored (Estore) to the energy dissipated (Edissipated) during an oscillation period and used to quantificationally reflect the stability of QCM.68

2.2. 2

According to the proportional relationship between R1 and the energy dissipation Edissipated, a larger R1 will result in poor stability of a QCM. Thus, the change in R1 of the BiOCl-based QCM sensor versus RH was also measured to study the stability of the sensor. Figure 3b plots the motional resistance change as a function of RH. It is observed that all the R1 values of the BiOCl-based QCM humidity sensor value was within 50 Ω in the humidity range of 11.3 to 97.3% RH, which is smaller than that of most previous QCM-based humidity sensors.22,25,29 This result indicates the BiOCl-based QCM humidity sensor shows good stability in a large humidity range.

The repeatability is also an important parameter to evaluate the performance of a humidity sensor. The dynamic repeatability of the BiOCl-based QCM humidity sensor was studied by moving the sensors between the 11.3% RH environment [provided by a saturated lithium chloride (LiCl) solution] and the 84.3% RH environment (provided by a saturated KCl solution) for three cycles. The output frequency of the BiOCl-based QCM humidity sensor was recorded in real time in Figure 4a throughout the entire process. In addition, the repeatability of frequency response and motional resistance change as a function of RH is shown in Figure 4b,c, respectively. It can be seen from the figure that the frequency response and motional resistance change of the sensor have good repeatability. Here, the ratio of standard deviation to the mean value of multiple measurements was used to descript the repeatability of sensor. The calculated repeatability of the BiOCl-based QCM (including) is less than 2.4% RH, indicating that the frequency response and motional resistance change of the sensor have good repeatability.

Figure 4.

Figure 4

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(a) Dynamic repeatability of the sensor between the 11.3% RH and 84.3% RH environment for three cycles. The repeatability of (b) frequency response and (c) motional resistance change as a function of RH.

The long-term stability investigation of the BiOCl-based QCM humidity sensor was performed by measuring the sensor frequency shift after 3 months of storage and use. We compared the frequency shifts of the humidity sensor under different relative humidities after 3 months with day 1, and the result is shown in Figure 5a. The frequency shifts of the sensor after 3 months at a humidity point are in good agreement with that of day 1. The results indicate the performance of the BiOCl-based QCM humidity sensor degrades insignificantly over time.

Figure 5.

Figure 5

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(a) Long-term stability of the sensor; (b) selectivity of the BiOCl-based QCM sensor at room temperature.

Cross-sensitivity of is also important to evaluate the performance of a QCM humidity sensor. Therefore, we tested the response of acetone, methane, ethanol, ammonia, and NO2 gas at a concentration of 500 ppm to this BiOCl-based QCM sensor. The result is shown in Figure 5b. It can be seen that the humidity sensor showed good selectivity to water molecules against acetone, methane, ethanol, ammonia, and NO2 at high RH. But at the same time, we should also consider the influence of the cross-sensitivity of BiOCl film at low RH because the humidity-sensing response of BiOCl is relatively low.

Exhaled gases and inhaled gases have different levels of humidity when breathing. We placed the BiOCl-based sensor 1.5 cm from the nose, and tested the response of human nose breaths. Experimental result is shown in Figure 6. The resonant frequency of the BiOCl-based QCM sensor decreased during exhalation and increased during breathing. The sensor can immediately come back to the baseline when the nose breath is off, which shows good repeatability and excellent practicability. This result indicates that the BiOCl-based QCM humidity sensor has potential for biomedical measurement, contactless switching, and so forth.

Figure 6.

Figure 6

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Sensing performance of the sensor under nose breath.

2.3. Discussion on the Humidity Sensing Mechanism

BiOCl is also a typical oxide, so its humidity sensing mechanism is similar to that of other oxide materials. The BiOCl can establish an adsorption–desorption equilibrium with the help of van der Waals force and hydrogen bonding in a certain humidity environment, as shown in Figure 7.69 At first, water molecules are chemisorbed on active sites of BiOCl powder, and then, water molecules are physisorbed onto this hydroxyl layer by attaching with two neighboring hydroxyl groups through hydrogen double bonds. The chemisorbed hydroxyls and the first physisorbed layer are immobile. With the humidity continuing to increase, an extra layer (the second physisorbed layer) on top of the first physically adsorbed layer forms. New adsorption–desorption equilibrium will be established as the humidity is changed. Accordingly, the mass of the QCM surface will also be changed, eventually causing a frequency shift. Specifically, water molecules desorb from the surface of the sensing material into an environment in a relative humidity lower than the ambient humidity. Decreased mass causes the increase in the QCM frequency. Conversely, water molecules absorb from the surface of the sensing material from the environment in a relative humidity high than the environmental humidity. Increased mass leads to the decrease in the QCM frequency.

Figure 7.

Figure 7

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Schematic diagram of the molecular structure diagram of BiOCl and the water adsorption model of the BiOCl-based QCM humidity sensor.

3. Conclusions

In this paper, we systemically studied the humidity sensing properties of BiOCl powder by combining with QCM transducer. BiOCl powder was obtained and characterized by SEM, XRD, and FTIR. The humidity sensing experiment was performed at room temperature (25 °C). The experimental results demonstrated that the BiOCl-based QCM sensor showed excellent humidity characteristics from 11.3 to 97.3%, such as good logarithmic frequency response to humidity levels (R2 = 0.994), fast response time (5.2 s)/recovery time (4.5 s), good reversibility, stability, repeatability humidity hysteresis, and cross selectivity. Moreover, the response of this sensor to human nose breaths was tested, and it showed good repeatability and excellent practicability. Finally, the humidity sensing mechanism was discussed in detail. Our work demonstrates that BiOCl is a potential candidate material for humidity detection.

4. Materials and Methods

4.1. Materials

Bismuth nitrate (Bi(NO3)3·5H2O), hydrochloric acid (HCl, 37 wt %), absolute ethyl alcohol, ammonium hydroxide (NH3·H2O, 2 mol/L), deionized (DI) water, LiCl, Magnesium chloride (MgCl2), magnesium nitrate (Mg(NO3)2), sodium chloride (NaCl), potassium chloride (KCl), and potassium sulfate (K2SO4) were purchased from Shanghai Aladdin Biochemical Polytron Technologies Inc. All the solvents and chemicals were of analytical grade and used without further purification.

4.2. Preparation and Characterization of BiOCl Powder

One gram of Bi(NO3)3·5H2O was dissolved in 30 mL of HCl at room temperature, and then, the solution was diluted to 60 mL with DI water. The pH value of the solution was adjusted to 7 with ammonia hydroxide. The milky white precipitate was obtained. The main reactions were as follows

4.2. 3
4.2. 4
4.2. 5
4.2. 6

Then, the solution was filtered with a vacuum filter. The resultant precipitate was washed by absolute ethyl alcohol three times. Finally, obtained BiOCl powder was dried at 120 °C for 24 h. A scanning electron microscope (SEM: Nova Nano-SEM450, Czech) and an X-ray diffractometer (XRD: Bruker D8 ADVANCE) were used to determine the morphology and crystal structure of the BiOCl nanostructures, respectively. FTIR of BiOCl powder was recorded on a 5DX FTIR (5DX, Nicolet Co., USA) spectrometer using KBr powder-pressed pellets.

4.3. Preparation of BiOCl-Based QCM Sensor and Humidity-Sensing Apparatus

The QCM was purchased from Wuhan Hitrusty Electronics Co. Ltd. China. The fundamental frequency of the AT-cut, “plano–plano” QCM is 10 MHz. In this work, the circular silver electrodes were formed by thermal evaporation process, and the thicknesses are 1000 Å. At the same time, a 15 Å thick Cr layer was sequentially deposited to enhance the adhesion between silver electrode and quartz plate. The diameters of the quartz wafer and the silver electrode are 8 and 5 mm, respectively. The humidity-sensing experiment setup is shown in Figure 8. A QCM sensor, a commercial phase-locked loop oscillator (PLO-10i, Maxtek Inc., Santa Fe Springs, CA, USA), a frequency counter (53131A, Agilent Technologies, Santa Clara, CA, USA), a digital multimeter (34401A, Agilent), and several saturated salt solutions were used. The PLO-10i is connected to the QCM sensor and provides frequency signal. The frequency counter is used for recording the resonant frequency. Saturated LiCl, MgCl2, Mg(NO3)2, NaCl, KCl, and K2SO4 solutions are used to yielded approximately 11.3, 32.8, 54.3, 75.3, 84.3, and 97.3% RH levels at 25 °C, respectively.

Figure 8.

Figure 8

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Schematic diagram of the humidity sensing experimental setup.

Fifty milligrams of BiOCl powder were dissolved in 50 mL of pure water and sonicated for 2 h to form a homogeneous dispersion solution. Before depositing the BiOCl film, the QCM was rinsed with DI water and ethanol, and then dried in air for 2 h. The resonant frequency of QCM in air was measured and recorded as f1. Then, 4 μL of the BiOCl solution was deposited to the electrode on one side by a drop-casting method, and the QCM coated with the BiOCl film was dried at room temperature for 4 h. The performance of the BiOCl-based QCM sensor was investigated by exposing the sensor to various relative humidities.

Acknowledgments

We would like to thank the funding form National Natural Science Foundation of China [project no. 61871098].

The authors declare no competing financial interest.

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