Designing High Performance Carbon/ZnSn(OH)₆-Based Humidity Sensors

Abstract

In this work, pure phase and carbon/ZnSn(OH)₆ samples were synthesized by a hydrothermal method. The composite sample’s structure, morphology, and functional groups were investigated by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Subsequently, ZnSn(OH)₆ samples were modified with different carbon contents, and their humidity-sensing properties were investigated. The introduction of carbon increased the specific surface area of pure ZnSn(OH)₆ samples, thus significantly improving the sensors’ humidity sensing response. The C10-ZnSn(OH)₆ sensor exhibited a high response, up to three orders of magnitude, a humidity hysteresisof 13.5%, a fast response time of 3.2 s, and a recovery time of 24.4 s. The humidity sensor’s possible humidity sensing mechanism was also analyzed using the AC complex impedance puissance method with a simulated equivalent circuit. These results revealed that ZnSn(OH)₆ can effectively detect ambient humidity and that the introduction of carbon significantly improves its humidity-sensing performance. The study provides an effective strategy for understanding and designing ZnSn(OH)₆-based humidity sensors.

1. Introduction

Sensors have attracted great interest in many fields due to their unique, excellent performance and broad application prospects. Humidity sensors are widely used in respiratory monitoring, industrial corrosion, agriculture, environmental analysis, and other fields [1,2,3,4]. As the core of humidity sensors, the research has been focused on develo** sensitive substrates, including metal oxides, polymers, natural nanoclay, cellulose, metal hydroxides, and some two-dimensional materials (black phosphorus, black arsenic), etc. [5,6,7,8,9,10,11].

ZnSn(OH)₆ is a metal hydroxide in which metal atoms are octahedrally coordinated with oxygen atoms to form a Sn(OH)₆ polyhedra and a Zn(OH)₆polyhedra. These polyhedra share an O-angle connection to create a structural framework, forming a face-centered cubic crystal structure (FCC). ZnSn(OH)₆ is non-toxic, easy to prepare, andlow-cost; therefore, it is widely applied in photocatalysis [12], gas sensors [13], flame retardant, smoke suppression [14], negative materials for ion batteries [15], inorganic fillers [16], and many other fields. Humidity detection is closely linked to the material’s microstructure, crystal defects, specific surface area, active sites, and surface hydrophilic groups [17,18]. Our previous research reported that the main lattice of ZnSn(OH)₆ contains abundant hydroxyl groups essential for the adsorption and desorption of water molecules [9]. The sensing mechanism of humidity sensors involves the water molecules’ adsorption by hydrophilic functional groups on their surface, causing an increase in ionic conductivity and a change in resistive properties (resistance or capacitance). Thus, a ZnSn(OH)₆ crystal would respond highly to humidity signals [19]. Additionally, its wider band gap (4.35 eV) [20] allows for higher stability in signal detection, broadening its application prospects in microelectronics.

The materials’ sensing performance is closely related to the adsorption and desorption of chemical signals, as well as the catalytic activity on the sensing material surface. Modification with precious metals, such as Pt, Ag, or Au, onto sensitive substrate is still a meaningful way to improve the chemical activity [21,22,23]. However, using precious metal modification increases the preparation cost and causes metals to aggregate during loading, having adverse effects on sensing performance improvement. Therefore, develo** other substitutes for precious metals has been a focus. Carbon-based materials, such as carbon nanotubes, graphene, and other carbon-based materials, have received significant attention, and exhibit unique physical structures and excellent electrical properties [8,24]. Carbon-based materials have been the most promising alternative to noble metal modification [25]. In this research, we employed a simple and effective wet chemical method for synthesizing a carbon/ZnSn(OH)₆ octahedron. The fabricated carbon/ZnSn(OH)₆ humidity sensor demonstrates excellent humidity detection performance. The possible humidity sensing mechanism of sensors is analyzed using the AC complex impedance spectrum with simulated equivalent circuits. The study results provide evidence to explore and design ZnSn(OH)₆-based humidity-sensitive materials.

2. Experimental Details

2.1. Materials

Zinc acetate (Zn(AC)₂·2H₂O), tin tetrachloride pentahydrate (SnCl₄·5H₂O), sodium hydroxide (NaOH), dextran (C6nH10nO5n), ethanol (C₂H₅OH), lithium chloride (LiCl), magnesium chloride (MgCl₂), potassium carbonate (K₂CO₃), magnesium nitrate (Mg(NO₃)₂), copper chloride (CuCl₂), sodium chloride (NaCl), potassium chloride (KCl), and potassium nitrate (KNO₃) were purchased from Sinopharm Chemical Reagent Co., Ltd., Bei**g, China. All of the reagents used in this experiment were of analytical grade.

2.2. Synthesis of Carbon/ZnSn(OH)₆

Firstly, 0.7 g SnCl₄·5H₂O and 0.44 g Zn(AC)₂·2H₂O were dissolved in 5 mL deionized water under continuous stirring. After 10 min, 8 mL NaOH solution (3 M) was slowly added dropwise to the above solution, and then 10 mL 40% aqueous ethanol solution was added. Subsequently, 5 mL C6nH10nO5n solution was dropped into the above mixture and stirred for 30 min. The mixed solution was transferred to a 50mL high-pressure reactor and heated to 160 °C for 6 h. After the reaction, the residuals were cooled naturally, and washed with ethanol and deionized water several times. Finally, the precipitate was dried under vacuum at 60 °C for 6 h. The final product obtained was ground to fine powder. The mass ratios of C6nH10nO5n to ZnSn(OH)₆ were 0 wt%, 5 wt%, 10 wt%, and 15 wt%, and labelled as ZnSn(OH)₆, C5-ZnSn(OH)₆, C10-ZnSn(OH)₆, and C15-ZnSn(OH)₆, respectively (Figure 1).

2.3. Materials Characterization

The samples’ crystal structure and composition were determined by X-ray diffractometer (XRD, D8 advance, Bruker, Karlsruhe, Germany) with a Cu Kα radiation (λ = 1.5418 nm) in the 2θ range of 10–80°. The micro-morphology of samples was observed by a field emission scanning electron microscope (SEM, Simgma 300, ZEISS, Oberkochen, Germany) at an accelerating voltage of 5 kV. The internal nanostructures and elemental distribution of samples were characterized by transmission electron microscopy (TEM, TecnaiG2 F20, FEI, Hillsborough, OR, USA) and energy spectrometry (EDS, JEOL JEM-3010, Tokyo, Japan) at an operating voltage of 200 kV.

The elemental compositions of materials were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250** images of C10-ZnSn(OH)₆ samples. ZnSn(OH)₆ morphology is observed as an octahedron shape with a dimension around 1 μm (Figure 3a–c). SEM images of ZnSn(OH)₆ after carbon inducing is shown in Figure 3d–g. Each face of the octahedron is approximately a regular triangle. The morphology of the C10-ZnSn(OH)₆ sample is similar to that of pure ZnSn(OH)₆, and the octahedron-like morphological structure remains almost unchanged. Moreover, the C10-ZnSn(OH)₆ crystal grows, reducing the aggregation of grains. Obvious cracks were observed on the surface of the C10-ZnSn(OH)₆; the relatively rough surface of it would result in a large surface area.

Zn, Sn, O, and C elements in C10-ZnSn(OH)₆ are detected (Figure 3h–l is the map** image of all elements), and the elements are evenly distributed; in particular, Figure 3l suggests that carbon existed on the C10-ZnSn(OH)₆ surface. Additionally, the weight percentages of carbon in C5-ZnSn(OH)₆, C10-ZnSn(OH)₆, and C15-ZnSn(OH)₆ samples were tested and found to be 5.99, 10.09, and 14.18, respectively, using an energy dispersive spectrometer, which is basically consistent with the theoretical results.

Figure 4 displays the TEM and HRTEM images of the C10-ZnSn(OH)₆ samples, further confirming their octahedral morphology. The average side length of the octahedron is about 0.36 μm (Figure 4a). Additionally, one lattice plane of 0.26 nm is observed in HRTEM images, corresponding to (221) planes of the C10-ZnSn(OH)₆, indicating an excellent crystallinity of the octahedron (Figure 4b,c). This analysis is consistent with the SEM and XRD results, further confirming the successful preparation of ZnSn(OH)₆.

The XPS analysis of the C10-ZnSn(OH)₆ sample is shown in Figure 5. The whole spectra illustrate that Zn, Sn, O, and C exist in this sample. In the Zn 2p spectrum, the peaks at 1044.71 eV and 1021.75 eV binding energies correspond to 2p_1/2 and 3p_3/2 for Zn²⁺, respectively (Figure 5a) [26]. Similarly, in the Sn 3d spectrum, the binding energy peaks at 495.26 eV and 486.97 eV originated from 3d_3/2 and 3d_5/2 for Sn⁴⁺, respectively (Figure 5b) [27]. Additionally, three peaks are obtained by fitting the O1s peaks, where the peaks at 530.39 eV, 531.58 eV, and 533 eV correspond to lattice oxygen, defective oxygen, and surface adsorbed oxygen in C10-ZnSn(OH)₆, respectively (Figure 5c) [28,29,30]. In the spectrum of C 1s, the characteristic peaks at 284.80 eV, 286.59 eV, and 288.03 eV are from C-C, C-O-C, and O-C=O bonds, respectively (Figure 5d) [31]. The XPS results further demonstrate the presence of carbon in C10-ZnSn(OH)₆.

The ZnSn(OH)₆ crystal structure model is established through the crystallographic dataof its PDF card. The crystal structure of ZnSn(OH)₆ illustrates that its surface contains abundant hydrophilic hydroxyl groups (Figure 6a). These hydroxyl groups would contribute to form H bonds with water molecules in humidity detection, facilitate ion mobility, and manifest macroscopically as a decrease in the resistance of sensors. Therefore, the content of hydroxyl groups in humidity-sensing materials is closely related to its sensitivity.

The microstructure and surface defects of materials play an essential role in sensing performance. Therefore, the samples were analyzed by FT-IR spectroscopy to determine further the functional groups on the surface, as shown in Figure 6b. The 3228.59 cm⁻¹ and 3113.23 cm⁻¹ spectra exhibit two broad peaks related to OH stretching vibration and H-bonded OH stretch, respectively [32]. The absorption peak near 2298.28 cm⁻¹ may result from asymmetric stretching vibrations of atmospheric CO₂ or CO₂ adsorbed on the sample surface [33]. A strong absorption peak of Sn-OH bending vibrations is also observed at 1175.52 cm⁻¹ [34]. The absorption peaks at 777.74 cm⁻¹ and 540.99 cm⁻¹ are attributed to the hydrogen bonding between water molecules and the Sn-OH stretching [35]. The FT-IR spectrum of the C10-ZnSn(OH)₆ and the pure sample are basically consistent. The hydroxyl group is hydrophilic with a high affinity for water molecules. It provides mobile protons in different humidity environments, which increases electrical properties and ultimately affects the proton transport mechanism.

Nitrogen adsorption and desorption tests are conducted to further investigate the influence of carbon modification on ZnSn(OH)₆ (Figure 7). The specific surface areas of pure ZnSn(OH)₆ and C10-ZnSn(OH)₆ are 2.14 m²/g and 4.24 m²/g, respectively. The C10-ZnSn(OH)₆ exhibits a larger specific surface area. The higher specific surface area may provide more active sites on the material surface, which are beneficial to reactions and promote further response. Thus, it is inferred that the humidity-sensitive response of C10-ZnSn(OH)₆ may be enhanced.

3.2. Humidity-Sensitive Performance

Figure 8a shows the impedance variation curves with ambient humidity for pure phase ZnSn(OH)₆ and carbon/ZnSn(OH)₆ sensors at 100 Hz and 1 V. The impedance response of the pure ZnSn(OH)₆ sensor is two orders of magnitude in the humidity range of 11–95% RH. The impedance of the C10-ZnSn(OH)₆ sensor decreases from 30,910 kΩ to 17.3 kΩ, realizing three orders of magnitude. The sensor response (S_R) can be defined as S_R = (Ra − Rg)/Rg × 100, where Ra represents the impedance at 11% RH and Rg represents the impedance at 95% RH [36]. Therefore, the response of ZnSn(OH)₆, C5-ZnSn(OH)₆, C10-ZnSn(OH)₆, and C15-ZnSn(OH)₆ sensors are calculated to be 30,595%, 9594%, 176,027%, and 9396%, respectively. Meanwhile, the C10-ZnSn(OH)₆ sensor behaves with a good linearity. C5-ZnSn(OH)₆ and C15-ZnSn(OH)₆ sensors exhibit a relatively low response and poor linearity. Therefore, C10-ZnSn(OH)₆ was an optimal choice for subsequent testing.

Figure 8b shows the relationship between impedance and relative humidity for the C10-ZnSn(OH)₆ sensor at different frequencies (40 Hz, 100 Hz, 1 kHz, 10 kHz, and 100 kHz). The operating frequency significantly affects the humidity-sensitive performance. As the operating frequency increases, the humidity-sensitive response tends to level off, indicating that the polarization process in the water is smaller than the electric field change [37]. Overall, the humidity sensor exhibits an unstable humidity response at 40 Hz and a poor response at higher frequencies. Therefore, 100 H_Z was selected as the optimum operating frequency for subsequent measurement.

Figure 9a presents a humidity hysteresis graph of the C10-ZnSn(OH)₆ sensor. The sensor was placed in 11%, 33%, 43%, 54%, 64%, 75%, 85%, and 95% RH humidity ambient, and the impedance was recorded. Then, the sensor was placed in high to low humidity environments, in order, to obtain the hysteresis properties of the sensor. The calculation method for hysteresis is as follows: γH = ±ΔH_max/2F_FS, where ΔH_max represents the maximum difference in impedance at the same relative humidity, and F_FS represents the full scale output [38]. The impedance is recorded when stable, showing a hysteresis of about 13.5%. Hydrophilic hydroxyl groups can form hydrogen bonds with water molecules. During the adsorption process, the formation of hydrogen bonds helps to enhance the conductivity of the material. During the desorption process, as water molecules dissociate, hydrogen bonds gradually disappear, leading to a decrease in the material’s conductivity and an increase in impedance. This leads to the inconsistent response speed of the C10-ZHS humidity sensor during adsorption and desorption processes when relative humidity environment changes, resulting in the moisture hysteresis phenomenon.

The response/recovery time of the sensor is shown in Figure 9b. The response time is 90% of the total impedance change time of the sensor from 11% RH to 95% RH (as the red color shows). Similarly, the recovery time is required to reach a 90% change in total impedance during the desorption process from 95% RH to 11% RH (as the blue color shows). The results reveal that the C10-ZnSn(OH)₆ sensor’s response time is 3.2 s, and the recovery time is 24.4 s. To explore the repeatability of the sensor, the response and recovery characteristics are tested after consecutive cycles (Figure 9c). The C10-ZnSn(OH)₆ sensor shows high stability and repeatability over several successive cycles. The average response time of the device is 4.2 s, and the recovery time is 27.8 s. Long-term stability is a crucial humidity-sensitive characteristic that reflects sensors’ practical application performance. To investigate the sensors’ stability, the impedance fluctuation with the humidity of the C10-ZnSn(OH)₆ sensor was explored for 30 days, with measurement every 5 days at intervals from 11% to 95% RH (Figure 9d). It was found that the C10-ZnSn(OH)₆ sensor was stable at low relative humidity, while there was some fluctuation at high relative humidity, showing relatively reliable stability overall.

In

Table 1. A comparison of the response/recovery time of humidity sensors across different studies.

Material	Measurement Range	Tresponse	Trecovery	Ref.
SnO2-TiO2	11–95% RH	19.1 s	181 s	[5]
c-IPN polymer	30–90% RH	38 s	70 s	[6]
Cu2ZnSnS4	11–97% RH	105 s	36 s	[39]
Sn-doped NiO	10–90% RH	43 s	38 s	[40]
PANIHCl/ZnO	10–90% RH	13 s	171 s	[41]
C10-ZnSn(OH)6	11–95% RH	3.2 s	24.4 s	This work

, the response/recovery time of the C10-ZnSn(OH)₆ humidity sensor fabricated in this work was compared with recent research. It can be seen that the C10-ZnSn(OH)₆ humidity sensor exhibits excellent response/recovery characteristics.

3.3. Mechanistic Analysis of the Sensors

To further investigate the humidity sensing mechanism, the complex impedance spectrum (CIS) of the sensor was obtained at 11%, 33%, 43%, 54%, 64%, 75%, 85%, and 95% RH, and equivalent circuits were fitted to analyze its sensing characteristics. The C10-ZnSn(OH)₆ sensor impedance variation was measured at different humidity ambient from 11% RH to 95% RH over the operating frequency range of 40 Hz to 100 kHz (Figure 10). Figure 10a,b shows that at a relatively low humidity of 11% RH and 33% RH, the CIS curve is an arc with a large curvature. Under low humidity conditions, water molecules in the environment first exist on the sample surface, and then are ionized under a electrostatic force (H₂O = H⁺ + OH⁻). As the number of hydrophilic hydroxyl increases, a hydroxyl group is formed. When hydroxyl groups contact water molecules, the negative charge of the oxygen in the hydroxyl group interacts with the positive charge of the hydrogen in the water molecule, forming hydrogen bonds. The formation of hydrogen bonds enhances the interaction force between hydroxyl groups and water molecules, thereby promoting their binding with water molecules and enhancing the adsorption of sensitive materials towards water molecules [42]. As shown in Figure 10c,d, the adsorbed water molecules gradually increase with the increase in humidity (43–54%RH). The proton (H⁺) combines with the water molecules adsorbed on it through the hydroxyl group to form H₃O⁺ [9], prone to leakage conductive flow, thus giving the CIS curve as a semicircular shape. The primary carrier in this stage is H₃O⁺. Figure 10e–h depicts that as the relative humidity further increases (64%, 75%, 85%, and 95% RH), the CIS curve consists of a semicircle at high frequencies and a trailing line at low frequencies. At this point, the accumulation of water molecules formed a physical adsorption layer, and a liquid water layer grew up on the surface of the sensing sample [43]. On the liquid water layer, the H₃O⁺ releases a proton to an adjacent water molecule, releasing a proton to its following water molecule, realizing the free movement of protons in the liquid water layer. This is the Grotthuss chain reaction (H₂O + H₃O⁺→ H₃O⁺ + H₂O) [44], occurring at low frequencies. Therefore, a straight line is observed at the end of the CIS curve. Thus, in high-humidity environments, both H₃O⁺ and protons act as the leading carriers for conduction.

The equivalent circuit diagram of CIS for C10-ZnSn(OH)₆ is presented in Figure 10i,j. At 11–54% RH, the semi-circular curve of CIS represents a sensing mechanism by dielectric correlation theory [9]. An equivalent circuit is fitted as a resistor (R_f) connected in parallel with a constant phase element (CPE_f), as shown in Figure 10i. The equivalent circuit is composed of Rf and CPE_f in parallel and then in series with the Warburg impedance (Z_w) under a high humidity environment (64–95% RH), as shown in Figure 10j. Therefore, the C10-ZnSn(OH)₆ sensor shows different humidity sensing mechanisms at low and high relative humidity. In addition, according to

Table 2. Errors in CIS curve equivalent circuit fitting for the C10-ZnSn(OH)₆ sensor.

Relative Humidity	11% RH	33% RH	43% RH	54% RH	64% RH	75% RH	85% RH	95% RH
Overall error rate	1.17%	1.29%	2.09%	1.83%	0.91%	0.38%	0.25%	0.87%

, it can be concluded that the maximum fitting error of the equivalent circuit is less than 2.1%, demonstrating a good fitting effect.

For the C10-ZnSn(OH)₆ sample, the introduction of carbon mainly affects the surface state of ZnSn(OH)₆; the surface of carbon modified samples appears rougher with a larger specific surface area, which is more conducive to the active sites on the surface to fully contact water molecules and enhance their adsorption capacity for water molecules. When the C10-ZnSn(OH)₆ sensoris working, H⁺ and H₃O⁺ are still the main carriers for conductivity. The sensing schematic diagram of the C10-ZnSn(OH)₆ humidity sensor is shown in Figure 11. In low relative humidity environments, the sensing layer of C10-ZnSn(OH)₆ mainly adsorbs water molecules through chemical adsorption. As the relative humidity increases, it gradually transforms into the physical adsorption of water molecules. In a high relative humidity environment, the Grotthuss chain reaction is formed, and protons skip to conduct.

4. Conclusions

The synthesis and characterization of pure ZnSn(OH)₆ and carbon/ZnSn(OH)₆ are discussed in this research. ZnSn(OH)₆ humidity sensors modified with different carbon contents were fabricated, and their sensing performance was explored. By introducing carbon, the roughness of the surface of ZnSn(OH)₆ grains was increased, resulting in a doubling of their specific surface area and an increase in active sites on the surface of C10-ZnSn(OH)₆, which is beneficial for the adsorption of water molecules. The C10-ZnSn(OH)₆ sensor exhibits a high impedance response close to three orders of magnitude, a humidity hysteresis of 13.5%, a fast response time (3.2 s), and a recovery time of 24.4 s. FT-IR and crystal structure analyses reveal that the C10-ZnSn(OH)₆ sample surface is rich in hydroxyl groups, which contribute significantly to the adsorption of water molecules. The humidity sensor’s possible sensing mechanism was also analyzed using the AC complex impedance spectrum with a simulated equivalent circuit. In conjunction with ionic conductivity theory, protons and hydrated hydrogen ions act as the main conducting carriers, with proton ions playing an essential role in low- and high-humidity environments. These results demonstrate the superior performance of ZnSn(OH)₆ for effective humidity detection and that carbon modification significantly improves the humidity performance of pure ZnSn(OH)₆.