English

• 1. INTRODUCTION

  The detection and control of humidity are very important in industry, agriculture, and many other fields^[1-3]. Traditional humidity sensors (e.g., mechanical hygrometer) cannot provide a comprehensive application with performance needs for high sensitivity, fast response and small volume^[1]. Many researchers have committed to develop high-performance humidity sensors based on various detection techniques such as capacitive-type^[4], resistive-type^[5], field effect transistor (FET)-type^[6], etc. Among them, the capacitive-type humidity sensors show very promising performance^[7], in which water molecules induce capacitance change of the sensing materials. People have developed various functional materials as sensing layers, including carbon materials (e.g., graphene^[8] and carbon nanotubes^[9]), ceramics (e.g., alumina^[10], titanium dioxide^{[11, 12]} and silicon oxide^[2]), semiconductor metal oxides (e.g., tin oxide^{[13, 14]} and indium oxide^{[15, 16]}), and polymers (e.g., polymer electrolytes^{[17, 18]} and conductive polymers^[19]).

  Tin dioxide (SnO₂), an n-type wide bandgap semiconductor (3.4~4.0 eV), has many unique properties such as low electrical resistivity, excellent chemical and physical stability^[20], making it widely used in humidity sensors^{[13, 14]}. Feng et al.^[21] prepared a humidity sensor based on three-dimensional (3D) hierarchical SnO₂ as a sensing layer coated on the interdigital electrodes (IDEs) made of golden. Li et al.^[22] have reported a high temperature humidity sensor based on WO₃/SnO₂ composite hollow nanospheres as sensing materials. Yang et al.^[23] have studied controllable assembly of SnO₂ nano-cubes onto TiO₂ electrospun nanofibers toward humidity sensing applications. Although capacitive-type humidity sensors with SnO₂ as sensing material show very promising properties, sensitivity and reliability of the sensors still need to be improved^[24]. Researchers have made a lot of efforts to improve the sensing performance of SnO₂ for humidity.

  In recent years, graphene oxide (GO) has been widely used as humidity sensing materials due to the existence of various functional groups such as hydroxyl, epoxy and carboxyl groups on its surface^[25-27]. The functional groups can be chemically modified for humidity sensing, which has made GO a promising material for high-performance humidity sensors^[27]. When compounded with SnO₂, GO can provide a large specific surface area for effective dispersing of SnO₂ particles which in turn prevent GO from agglomeration^[28]. As a result, the GO/SnO₂ composite sensing layer is able to improve sensor sensitivity to some extent^[29]. Usually, precious metal electrodes are deposited on a substrate, and coated with sensing materials to form humidity sensors_.^{[30, 31]} However, such method is complex and expensive, and it is necessary to develop a more convenient and cost-efficient way to fabricate high-performance humidity sensors. Reduced graphene oxide (rGO) can be used as sensing electrode materials in capacitive-type humidity sensors^{[8, 32, 2]}. Here we introduce a capacitive-type humidity sensor composed of laser-scribed graphene (LSG) as a sensing electrode and GO/SnO₂ as a sensing layer. The laser-scribed graphene (LSG) is a rGO interdigital pattern electrode resulted from the reducing of GO within a GO/SnO₂ composite layer by laser scribing method, and the un-scribed GO/SnO₂ composite layer perform duties as sensing materials. The sensor fabrication is a one-step process which is facile and cost-efficient. Such sensor exhibits good sensing performance with high sensitivity, quick response time, and good stability.

  2. EXPERIMENTAL

  2.1 Syntheses of GO and GO/SnO₂ composite layers

  Graphite and SnO₂ (≥99%) were purchased from Aladdin Co. Ltd. All chemicals used in this work were used without further treatments. GO was synthesized by an oxidative treatment of natural graphite using a modified Hummer's method^[33]. The obtained GO solid (400 mg) was added to DI water (100 mL), and then dispersed ultrasonically for 30 minutes. Five parts of GO dispersions (20 mL each) were placed in different vials and 40, 80, 120, and 160 mg of SnO₂ were added to prepare GO/SnO₂ suspensions with GO: SnO₂ mass ratios of 1:0.5, 1:1, 1:1.5 and 1:2, respectively. The GO/SnO₂ suspensions were named as GO-S_0.5, GO-S₁, GO-S_1.5 and GO-S₂, respectively, while GO-S₀ stands for a pure GO dispersion (4 mg/mL). A polyimide film was adhered to the DVD disc and divided into 5 equal parts. Then, 1 mL of each GO/SnO₂ suspension was evenly coated onto each part of the polyimide film and dried at 30 ℃ overnight to form GO/SnO₂ layers.

  2.2 Fabrication of humidity sensors

  The fabrication process of LSG/GO/SnO₂ sensors (named as LSG-GS) is shown in Fig. 1a~1e. A standard LightScribe DVD drive was used to scribe the GO/SnO₂ composite layer on polyimide substrate for 6 times based on a designed interdigital electrodes (IDEs) pattern (electrode width was 0.33 mm, gap size was 0.3 mm, and the area of each single electrode was 4.8mm × 0.33mm). After laser scribing, the polyimide film was peeled off from the DVD disc, and the scribed IDEs areas were cut off. Conductive silver paint was applied on both sides of IDEs and copper foil was attached to the edge of the two electrodes. Finally, LSG-GS humidity sensors were encapsulated with silicone rubber. Fig. 2 is the Real pictures of IDEs and LSG-GS sensor. The sensors were named as LSG-GS₀, LSG-GS_0.5, LSG-GS₁, LSG-GS_1.5 and LSG-GS₂, corresponding to the GO-S₀, GO-S_0.5, GO-S₁, GO-S_1.5 and GO-S₂ suspensions, respectively.

  Figure 1

  

  Figure 1.  Schematic illustration of the fabrication process for the LSG-GS sensors

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  Figure 2

  

  Figure 2.  Real pictures of IDEs (a) and LSG-GS sensor (b)

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  2.3 Characterizations of the materials

  The surface morphologies of GO layer, SnO₂ particles, GO/SnO₂ layer and LSG/SnO₂ layer were characterized by scanning electron microscopy (SEM, Hitachi su-8010). The structures of GO layer, SnO₂ particles, GO/SnO₂ layer and LSG/SnO₂ layer were analyzed by X-ray diffraction (XRD, Bruker D8 advance).

  2.4 Evaluation of humidity sensors

  In order to characterize the performance of humidity sensors, each LSG-GS sensor was exposed to different RH levels (11~97% RH) at 25 ℃. Saturated solutions of lithium chloride, potassium acetate, magnesium chloride, potassium carbonate, magnesium sulfate, copper chloride, sodium chloride, potassium chloride, and potassium sulfate with different concentrations were put into different sealed containers to obtain 11%, 23%, 33%, 42%, 54%, 61%, 75%, 84% and 97% RH, respectively^[34]. The humidity sensors were placed in various containers with different RH environments, and a LCR instrument (Agilent 4284A) was used to monitor the capacitances response at various measurement frequencies. Impedance spectra were measured by electrochemical workstation (1 Hz to 1 MHz, Shanghai Chenhua Instrument Co. Ltd.). For a stability test, the sensor was monitored for 41 days at different RH levels (23%, 61%, 84% and 97% RH, respectively).

  3. RESULTS AND DISCUSSION

  3.1 SEM and XRD characterization

  Fig. 3a~3e show the SEM images of GO layer, SnO₂ particles, GO/SnO₂ layer, LSG/SnO₂ and un-scribed GO/SnO₂ layers, and LSG/SnO₂ layer, respectively. Fig. 3a exhibits a wrinkled morphology of GO layer. Fig. 3b displays spheroidal morphology of SnO₂ particles, and 3c reveals the dispersing of SnO₂ particles within GO sheet. Fig. 3d presents the SEM images of GO/SnO₂ surface (the un-scribed part) in direct contrast to the LSG/SnO₂ region (the scribed part). Compared to the original GO/SnO₂ surface, laser reduction results in characteristic exfoliation and a large expansion of the LSG layers^[35], which increases the LSG sensing electrode area. Fig. 3e is a larger version of the folded part of Fig. 3d.

  Figure 3

  

  Figure 3.  SEM images of (a) GO layer, (b) SnO₂ particles, (c) GO/SnO₂ layer, (d) LSG/SnO₂ and un-scribed GO/SnO₂ layers, and (e) LSG/SnO₂ layer

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  Fig. 4 illustrates the XRD pattern of GO layer, SnO₂ particles, GO/SnO₂ composite sensing layer, and LSG/SnO₂ layer. A typical XRD pattern of GO displays its characteristic peak at 10.98°, as shown in Fig. 4a. The characteristic peaks of SnO₂ are given in Fig. 4b. Fig. 4c demonstrates that GO/SnO₂ composite sensing layer is a mixture of GO and SnO₂ and there is no extra phase. As shown in Fig. 4d, the characteristic peak at 16.9° represents LSG, indicating that GO is reduced to LSG during the DVD laser scribing, while the peaks of SnO₂ remain unchanged.

  Figure 4

  

  Figure 4.  XRD pattern of GO layer, SnO₂ particles, GO/SnO₂ composite sensing layer and LSG/SnO₂ layer

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  3.2 Humidity sensing performance

  Table 1 shows the capacitances of LSG-GS₀, LSG-GS_0.5, LSG-GS₁, LSG-GS_1.5 and LSG-GS₂ sensors under 97% RH at various measurement frequencies (at 25 ℃). The results indicate that the response capacitance increases with the GO/SnO₂ ratio in the sensing layer. The sensor with a mass ratio of 1:1 (i.e., the LSG-GS₁ sensor) gives the maximum response capacitance. As the ratio further increases, the response capacitance decreases. In addition, as the measurement frequency increases, the response capacitance of the sensor decreases. Therefore, the LSG-GS₁ sensor was used for further studies and 50 Hz was chosen as the measurement frequency in the further experiments. Fig. 5 indicates the relationship between capacitance and RH (ranging from 11% to 97%) at various measurement frequencies (i.e., 50, 100, 500, and 1 kHz, respectively) for the LSG-GS₁ sensor. The response capacitance of the LSG-GS₁ sensor increases exponentially with the increase of RH. Within the RH range measured, the capacitance at 50 Hz is the highest compared to those at other frequencies because the absorbed water molecules help to enhance the polarization effect and increase the dielectric constant, resulting in the increase of the capacitance^[36].

  Table 1

  

  Table 1.  Capacitance of LSG-GS₀, LSG-GS_0.5, LSG-GS₁, LSG-GS_1.5 and LSG-GS₂ Sensors under 97% RH at Various Measurement Frequencies

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  Figure 5

  

  Figure 5.  Capacitance of LSG-GS₁ sensor at a RH range between 11% and 97% RH at different frequencies (50, 100, 500 and 1 kHz, respectively)

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  The sensor sensitivity is defined as: S = ΔC/ΔRH*100% (pF/%RH), where ΔC is a change of the response capacitance resulted from a change of relative humidity ΔRH. Fig. 6a compares the sensitivity of the LSG-GS₀ sensor with the LSG-GS₁ sensor. When the RH increases from 11% to 97%, capacitance of the LSG-GS₁ sensor changes from 12 to 121700 pF, giving a sensitivity of 1414.98 pF/%RH based on the above formula, while the LSG-GS₀ sensor only gives a sensitivity of 419.52 pF/%RH. The LSG-GS₁ sensor is 3.37 times more sensitive than the LSG-GS₀ sensor. It is possible that the addition of SnO₂ in the sensing materials provides more oxygen functional groups to form hydrogen bonds with water molecules^[23] and therefore improves the sensing sensitivity. In addition, SnO₂ particles are interspersed between GO layers to prevent accumulation of GO, which results in increasing specific surface area of the sensing layer, promoting absorption of water molecules, and ultimately improving sensing sensitivity. Fig. 6b plots the adsorption and desorption characteristic of LSG-GS₁ humidity sensors, measured by increasing RH from 11% to 97% and then restoring the RH to 11%. It is worth noting that the sensor has a highly reversible sensitivity. The sensing curves for both adsorption and desorption processes almost coincide, indicating a very small hysteresis.

  Figure 6

  

  Figure 6.  (a) Sensitivity comparison between LSG-GS₁ sensor and LSG-GS₀ sensors; (b) Hysteresis characteristics of the LSG-GS₁ humidity sensor

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  Response-recovery behavior is one of the important characteristics to evaluate the performance of humidity sensors. We define the time for the sensor to reach a 90% change of the total capacitance as the response time during adsorption process or the recovery time during desorption process. Fig. 7 shows the response-recovery characteristics of the LSG-GS₁ sensor measured at 50 Hz. The adsorption time is 20 s when the RH is increased from 23% to 87%, and desorption time is 18 s when the RH is decreased from 87% to 23%. Kuang et al.^[20] reported a high-sensitivity humidity sensor based on a single SnO₂ nanowire, in which the response and recovery time was 120~170 s and 20~60 s, respectively. Lin et al.^[38] prepared a graphene/TiO₂ sensor, which exhibited the response and recovery time of approximately 128 and 68 s. The LSG-GS₁ sensor from this study has very good response-recovery properties when compared with some results reported by others^{[20, 37-39]}.

  Figure 7

  

  Figure 7.  Response-recovery characteristics of the LSG-GS₁ sensor at 50 Hz

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  Stability is also an important factor to judge the performance of sensors. Fig. 8 shows the capacitance change of the LSG-GS₁ sensor exposed to 23%, 61%, 84% and 97% RH, respectively for 41 days. The response of the sensor stays almost the same during the measurement time under each testing RH environment, demonstrating a highly stable sensor.

  Figure 8

  

  Figure 8.  Humidity sensing stability of the LSG-GS₁ sensor monitored for 41 days at 50 Hz

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  3.3 Humidity sensing mechanism

  Electrochemical impedance spectroscopy (EIS) is used to study the humidity sensing mechanism of the LSG-GS₁ sensors. Fig. 9 shows complex impedance spectra of the LSG-GS₁ humidity sensor at different RH levels (i.e., changing from 23% to 97%) tested at frequency ranging from 1 Hz to 1 MHz. Z' and Z'' are the real and imaginary parts of EIS, and some of them are magnified on the same plane to make convenient comparison. At a low RH level under 42%, the complex impedance spectrum appears to be a semicircle. In this case, the adsorbed water molecules are discontinuous on the surface of GO/SnO₂ sensing layer, which results in poor ion conduction. The conductivity of the sensor mainly depends on the intrinsic electrons of the GO/SnO₂ composite, which demonstrates very high impedances^[40]. The semicircle shrinks with RH increasing. At a high RH level above 42%, more water molecules are adsorbed onto the sensing surface to form continuous paths, which enhances the ion conduction and the complex impedance diagram appears to be a semicircle with a straight line. The semicircle is contributed by the intrinsic electrons of GO/SnO₂ composite and the line is due to the ion conduction within water paths on the GO/SnO₂ sensing layer^[41]. As the RH increases, the semicircle shrinks more, which reflects the adsorbed water molecules become more dominant and the contribution of the intrinsic electrons from GO/SnO₂ composite become smaller^[42].

  Figure 9

  

  Figure 9.  Complex impedance spectra of LSG-GS₁ humidity sensor at different relative humidity

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  4. CONCLUSION

  In this work, GO/SnO₂ humidity sensors are fabricated by one-step laser scribing process which is facile and cost-efficient. The sensing performance of the sensors is investigated by exposing the sensor to different RH levels. The test results show that the LSG-GS₁ humidity sensor has both fast response-recovery behavior, high sensitivity, and long-term stability. The LSG-GS₁ humidity sensor exhibits a sensitivity value of 1414.98 pF/% RH, which is more than 3.37 times of the LSG-GS₀ sensor. Considering the facile and cost-efficient technology of laser scribing and the excellent performance of the LSG-GS₁ humidity sensor, the sensor is expected to have potential for practical applications in humidity detection.

  
  
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• 

  Figure 1  Schematic illustration of the fabrication process for the LSG-GS sensors

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  Figure 2  Real pictures of IDEs (a) and LSG-GS sensor (b)

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  Figure 3  SEM images of (a) GO layer, (b) SnO₂ particles, (c) GO/SnO₂ layer, (d) LSG/SnO₂ and un-scribed GO/SnO₂ layers, and (e) LSG/SnO₂ layer

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  Figure 4  XRD pattern of GO layer, SnO₂ particles, GO/SnO₂ composite sensing layer and LSG/SnO₂ layer

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  Figure 5  Capacitance of LSG-GS₁ sensor at a RH range between 11% and 97% RH at different frequencies (50, 100, 500 and 1 kHz, respectively)

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  Figure 6  (a) Sensitivity comparison between LSG-GS₁ sensor and LSG-GS₀ sensors; (b) Hysteresis characteristics of the LSG-GS₁ humidity sensor

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  Figure 7  Response-recovery characteristics of the LSG-GS₁ sensor at 50 Hz

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  Figure 8  Humidity sensing stability of the LSG-GS₁ sensor monitored for 41 days at 50 Hz

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  Figure 9  Complex impedance spectra of LSG-GS₁ humidity sensor at different relative humidity

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  Table 1.  Capacitance of LSG-GS₀, LSG-GS_0.5, LSG-GS₁, LSG-GS_1.5 and LSG-GS₂ Sensors under 97% RH at Various Measurement Frequencies

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