Next Article in Journal
Research on the Microstructure and Properties of a Flux-Cored Wire Gas-Shielded Welded Joint of A710 Low-Alloy High-Strength Steel
Previous Article in Journal
kHz, 10s TW, Femtosecond Source Based on Yb:YAG Thin Disk Laser Pumped OPCPA of Low Quantum Defect
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Crystals
Volume 13
Issue 3
10.3390/cryst13030482
crystals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

TiO2-SnS2 Nanoheterostructures for High-Performance Humidity Sensor

by
Wencheng Yu
1,
Duo Chen
1,2,*,
Jianfei Li
1,* and
Zhenzhen Zhang
1,2,*
1
International School for Optoelectronic Engineering, Qilu University of Technology (Shandong Academy of Science), Jinan 250353, China
2
Laser Institute, Shandong Academy of Sciences, Jinan 250100, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(3), 482; https://doi.org/10.3390/cryst13030482
Submission received: 19 February 2023 / Revised: 4 March 2023 / Accepted: 7 March 2023 / Published: 11 March 2023
Browse Figures
Review Reports Versions Notes

Abstract

:
The larger surface-to-volume ratio of the hierarchical nanostructure means it has attracted considerable interest as a prototype gas sensor. Both TiO2 and SnS2 can be used as sensitive materials for humidity sensing with excellent performance. However, TiO2-SnS2 nanocomposites are rarely used in humidity detection. Therefore, in this work, a new humidity sensor was prepared by a simple one-step synthesis process based on nano-heterostructures, and the humidity sensing performance of the device was systematically characterized by much faster response/recovery behavior, better linearity and greater sensitivity compared to pure TiO2 or SnS2 nanofibers. The enhanced sensitivity of the nanoheterostructure should be attributed to its special hierarchical structure and TiO2-SnS2 heterojunction, which ultimately leads to a significant change in resistance upon water molecule exposure. In consideration of its non-complicated, cost-effective fabrication process and environmental friendliness, the TiO2-SnS2 nanoheterostructure is a hopeful candidate for humidity sensor applications.

1. Introduction

In recent years, semiconductor composites have become one of the hottest topics all over the world. When two different semiconductors are combined together, some novel properties may appear. TiO2-MoS2 [1] and TiO2-Sn3O4 [2] show an enhanced photocatalytic activity. TeO2-SnO2 [3] and CuO-ZnO [4] have improved gas-sensing properties. Similarly, TiO2-SnO2 [5] and TiO2-ZnO [6] display excellent ultraviolet responsivity. Metal oxides are stable in structure; easy to synthesize; have good application prospects in photoelectric measurement, gas detection, etc.; and are a class of cost-effective materials. Among metal oxide semiconductors, titanium dioxide (TiO2) is one of the most widely used wide-band-gap oxide semiconductors. It has excellent physical and chemical properties and low prices and has been widely used in practical life. Since Fujisima and Honda published their work on the catalytic water splitting of TiO2 under ultraviolet light irradiation in the 1970s [7], the application of TiO2 has been rapidly extended to the fields of optoelectronics [8], photocatalysis [9], photo/electrochromic [10] and gas detection [11]. Moreover, TiO2 has good chemical stability and controllable morphology and is a good matrix material [12]. As an n-type semiconductor material, SnS2 belongs to layered metal sulfide with a hexagonal CdI2 crystal structure, and it is a novel two-dimensional material [13]. SnS2 has a wide energy band gap (about 2.18 eV [13]) and strong anisotropic optical properties. Therefore, SnS2 is often used in gas-sensitive materials [14], photoelectric equipment [15], optical materials [16] and other fields. TiO2 and SnS2 have been studied as active materials in humidity sensors. A humidity sensor based on sol–gel-prepared TiO2 film has been reported by Giampiero Montesperelli et al., which shows high humidity sensitivity at the minimum relative humidity (RH) values (4–10% RH) at 40 ℃ [17]. Lakshmi Deepika Bharatula et al. demonstrated a SnS2 nanoflake micro-nano sensor device that can work within the scope of 11–97% RH at room temperature [18]. TiO2-SnS2 nanocomposites are generally used as photocatalytic materials. For example, Marin Kovacic et al. used TiO2-SnS2 nanocomposites as solar-active photocatalytic materials for water treatment [19]. Then, in the field of humidity sensors, carbon-based nanomaterials are commonly used as sensing materials. Due to the large specific surface area of carbon nanotubes, they have good adsorption characteristics for water molecules and are good moisture-sensitive materials for humidity sensors, and they also meet the development trend of the integration and miniaturization of humidity sensors. Hai M. Duong et al. used a high-temperature CVD furnace to produce continuous macroscopic fibers and films made from CNT superfibers and used them in the field of humidity sensing [20]. In addition, Hai Minh Duong et al. found that since the properties of post-treated CNT fibers are comparable to many commercial high-strength fibers, such as carbon fiber T300, Dyneema and Twaron, they can be utilized as reinforcements for advanced composites. Nanotube-based composites made from unstructured CNT powder have been extensively applied as structural materials for a wide range of applications, such as automotive and aerospace applications [21]. Therefore, carbon-based nanomaterials also have a wide range of promising applications.
In the investigations to date, many TiO2 and SnS2-related composites have been used in humidity sensors. Dongzhi Zhang et al. fabricated a humidity sensor based on a WS2/SnO2 nanocomposite with improved sensitivity and rapid response compared to pure WS2 and pure SnO2, and it also performs quite well in detecting human respiration [22]. Dongzhi Zhang et al. prepared SnS2/Zn2SnO4 hybrid spherical films as sensitive materials for humidity sensors utilizing a layer-by-layer self-assembly technique. They found that the SnS2/Zn2SnO4 hybrid thin-film sensors made significant progress in humidity sensors compared to single-SnS2 and single-Zn2SnO4 nanomaterials, achieving accurate measurements of human breath, sweat, urine and water droplets [23]. Yun Wang et al. successfully fabricated tubular TiO2-SnO2 fibers (FIT-TSF) using a general crystal-phase-induced formation strategy. The prepared FIT-TSF exhibited excellent sensing performance with third-order impedance variation, an ultra-fast response time of 5 s, a recovery time of 8 s and good reproducibility [24]. Irene Cappelli et al. analyzed the performance of different humidity sensors based on TiO2 nanoparticles and correlated them with different chemical/physical phenomena, and they found that when relative humidity is greater than 70%, the presence of condensate changes the electrical properties of the sensor, resulting in a smaller equivalent resistance value and a larger equivalent capacitance value of the sensor. The sensor has the advantages of a relatively fast response, a large measurement range and good stability [25]. However, there are few studies using TiO2-SnS2 nanocomposites for humidity detection.
In this paper, we synthesized high-quality TiO2 nanoribbons by the hydrothermal method and acid treatment, and we dispersed SnS2 nanoparticles on TiO2 nanoribbons to form TiO2-SnS2 nanoheterostructures. The morphology and structure of bare TiO2 nanoribbons and SnS2-TiO2 nanoheterostructures were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectrum and X-ray diffraction. The optical properties of the bare TiO2 and TiO2-SnS2 nanoheterostructures were characterized by reflection spectroscopy. The humidity sensors were prepared using bare TiO2 nanoribbons, SnS2 nanoparticles and TiO2-SnS2 nanoheterostructures as active materials, and their humidity detection performance at room temperature was investigated. Finally, through comparative experiments, we found that the resistance changes of the detector based on TiO2-SnS2 are linear with the relative humidity, while the resistance change of the two detectors based on pure TiO2 and pure SnS2 is not linear in the process of humidity change. In addition, the resistance of the two detectors based on pure TiO2 and pure SnS2 can reach 1010 ohms under low relative humidity, which is difficult to measure accurately. The resistance of TiO2-SnS2 is only in the order of kiloohms, which can be easily detected with an ordinary multimeter. This is coupled with the fact that the synthesis process of TiO2-SnS2 nanoheterostructures in this work is simpler and less costly than that of general metal oxide composites. Therefore, the humidity sensor based on a TiO2-SnS2 nanostructure is more suitable for daily applications.

2. Materials and Methods

Analytically pure Titania P25 (TiO2: ca. 80% anatase (CAS. 13463-67-7) and 20% rutile (CAS. 1317-80-2)), sodium hydroxide (NaOH (CAS. 1310-73-2)), hydrochloric acid (HCl (CAS. 7647-01-0)), sulfuric acid (H2SO4 (CAS. 7664-93-9)), tin(Ⅳ) chloride (SnCl4·5H2O (CAS. 10026-06-9)) and thioacetamide (CAS. 62-55-5) were used without further purification. A homogeneous solution was made by mixing 0.8 g of P25 TiO2 with 80 mL of aqueous 10 M NaOH. The mixture was then shifted to a 100 mL Teflon (CAS. 9002-84-0) stainless steel autoclave and heated at 180 °C for 72 h. Na2Ti3O7 nanoparticles were gained after thoroughly washing the obtained powder with deionized water. H2Ti3O7 nanospheres were produced by immersing 0.47 g of Na2Ti3O7 nanospheres into 58.8 mL of 0.1 M hydrochloric acid for 24 h. Finally, H2Ti3O7 nanospheres (0.285 g) were etched in 14.25 mL of 0.02 M H2SO4 aqueous solution at 100 °C for 2 h to obtain rough nanospheres. The products were separated from the solution by centrifugation, washed thoroughly with deionized water in turn, and then annealed at 600 °C for 2 h to obtain TiO2 nanospheres.
TiO2-SnS2 nanoheterostructures were prepared by a simple hydrothermal co-precipitation method. In a typical process, 2.5 mmol SnCl4·5H2O and 25 mmol thioacetamide were dissolved in 18 mL of deionized water to make a transparent solution, to which a certain amount of pre-synthesized TiO2 nanobelts (molar ratio of Sn/Ti = 1/1) was added. Then, the solution was injected into a 20 mL Teflon-lined stainless-steel autoclave and maintained at 160 °C for 12 h. The obtained TiO2-SnS2 nanoheterostructures were washed with deionized water and dried at 70 °C. Bare SnS2 was also prepared using a similar method.
The synthesized powders were dispersed in deionized water. Then, several drops of the obtained suspension were directly dropped onto a precleaned alumina (CAS. 1344-28-1) substrate, followed by thermal annealing at 100 °C for 30 min. Finally, interlaced gold electrodes (width: 100 μm; pitch: 200 μm) were deposited on the sample surface by thermal evaporation for humidity detection measurements.
The crystal structure of the samples was examined by X-ray diffraction (XRD, XD-3, PG Instruments Ltd., Beijing, China) and Raman (Bruker, Ltd., Billerica, MA, USA). The surface morphology of the samples was characterized by using field emission scanning electron microscopy (SEM; Hitachi S-4800, Hitachi, Ltd., Chiyoda, Tokyo, Japan). Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were collected on a JEOL JEM 2100F electron microscope (JEM 2100F, JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 200 kV. We used a saturated salt solution humidity generator to measure the humidity sensitivity of the sample. LiCl- (CAS. 7447-41-8), MgCl2·6H2O- (CAS. 7791-18-6), NaBr- (CAS. 7647-15-6), NaCl- (CAS. 20510-56-9), KCl- (CAS. 7447-40-7) and KNO3- (CAS. 14797-55-8) saturated salt solutions, whose corresponding humidity at room temperature was 11.30%, 32.78%, 57.57%, 75.29%, 84.34% and 93.58%, respectively, were chosen as humidity generators. Taking the relative humidity of the lithium-chloride-saturated salt solution as the background humidity, the response of TiO2, SnS2 and TiO2-SnS2 was measured under different moderate conditions.

3. Results and Discussion

As illustrated in Figure 1, the crystal structure of the synthetic TiO2 nanobelts and TiO2-SnS2 nanoheterostructures was investigated by XRD. Figure 1a depicts the XRD pattern of TiO2 nanobelts, with all diffraction peaks matching those of anatase TiO2 (JCPDS card, no. 21-1272). For the TiO2-SnS2 (Figure 1b), besides the diffraction peaks from anatase TiO2, all other peaks can be indexed to hexagonal-structured SnS2 (JCPDS card, no. 23-0677). Figure 2 shows the Raman spectrums of TiO2, SnS2 and TiO2-SnS2. For the TiO2, as shown in Figure 2a, three distinct peaks, centered at 237, 252 and 294 cm−1, can be observed. Figure 2b shows the Raman spectral lines of the SnS2 that has one distinct peak, centered at 310 cm−1. The red curve in Figure 2 shows that the TiO2-SnS2 has three peaks centered at 235, 257 and 285 cm−1, which roughly correspond to the three distinct peaks of TiO2, and has one peak centered at 309 cm−1, corresponding to the peak of SnS2. No unambiguous signal from others is observed. These results confirm the successful deposition of SnS2 on TiO2 nanobelts, which is further proved by SEM and TEM in the following part.
Figure 3 shows the SEM images of the TiO2 nanobelts and the TiO2-SnS2 nanoheterostructures. The surface of the TiO2 nanobelts is relatively smooth, and the nanobelts are straight, as shown in Figure 3a. A higher-magnification SEM image (Figure 3b) shows that there are many randomly distributed dark depressions on the nanobelts as a result of the acid etching of the nanobelts. The morphology of the TiO2-SnS2 is shown in Figure 3c. The surface of the TiO2 nanobelts is rough due to the deposition of SnS2 nanoparticles. For better observation, a single-TiO2 nanobelt decorated with SnS2 is checked under high magnification, as shown in Figure 3d. It can be observed that the SnS2 nanoparticles are scattered on the TiO2 nanoribbons in a random shape.
The morphology of the TiO2 nanobelts is characterized by TEM, as shown in Figure 4a. The nanobelts are uniform and straight, with widths of 60–180 nm. A higher-magnification TEM image (Figure 4b) shows that there are many bright and dark regions randomly distributed on the nanobelts, which indicates that the nanobelt surface becomes rough due to acid etching. However, the single-crystal structure of TiO2 is reserved even after acid etching, as demonstrated by HRTEM and the electron diffraction pattern in Figure 4c. The morphology of the TiO2-SnS2 nanostructure is shown in Figure 4d. SnS2 nanoparticles are deposited on the surface of the TiO2 nanobelt evenly. In order to facilitate better observation, a SnS2-decorated single-TiO2 nanobelt is checked at high magnification, as shown in Figure 4e. It can be seen that the size of SnS2 nanoparticles is small and relatively homogeneous. As exhibited by HRTEM analysis of the TiO2-SnS2 nanoheterostructure in Figure 4f, SnS2 nanoparticles with a size of approximately 10 nm are distributed on the TiO2 nanobelt uniformly and densely.
Figure 5a shows the optical reflection spectra of bare TiO2 and TiO2-SnS2 nanoheterostructures. A sharp decrease in the reflectivity can be observed at approximately 560 nm for TiO2-SnS2 and 370 nm for TiO2, which can be attributed to the interband absorption of TiO2-SnS2 [26,27] and anatase TiO2 [28]. The TiO2-SnS2 nanostrcuture exhibits lower reflection than the bare TiO2 in visible light, which comes from the increased light absorption by SnS2 nanoparticles [29]. Figure 5b shows the plots of (F(R)E)2 as a function of photon energy E for TiO2-SnS2 and TiO2 samples (F(R) is the Kubelka–Munk function; F(R) = (1−R)2/2R, where R is the reflectance). The band gap can be determined by the linear extrapolation of (F(R)E)2 to 0. The deduced band gap is about 3.36 eV for TiO2 and 2.31 eV for TiO2-SnS2. Because of its strong absorption in visible light, the TiO2-SnS2 nanostructure may be used as the active layer in a visible-light photodetector.
A saturated salt solution humidity generator, also known as a fixed-point humidity generator, has lots of advantages, such as simple equipment, cost-effectiveness, stable humidity value, easy recovery after damage, good reproducibility and so on. In this article, the humidity sensitivity of samples is measured using the saturated salt solution humidity generator. LiCl-, MgCl2·6H2O-, NaBr-, NaCl-, KCl- and KNO3-saturated salt solutions, whose corresponding humidity at room temperature is 11.30%, 32.78%, 57.57%, 75.29%, 84.34% and 93.58%, respectively, are chosen as humidity generators. The relative humidity of LiCl-saturated salt solution is taken as the background humidity, and the response curves of TiO2, SnS2 and TiO2-SnS2 at different humidity levels are shown in Figure 6. As shown in Figure 6a, the resistance of the SnS2-TiO2-based sensor is much smaller than that of the bare TiO2 or SnS2-based sensor, and the change in resistance from RH 11% to RH 93% reaches three orders of magnitude. However, in low relative humidity, the TiO2 nanobelt device is in a high-resistance state (Figure 6b), and the resistance value is too high for an ordinary instrument. The response at different RH of the SnS2 device is shown in Figure 6c. The resistance of this device is basically unchanged at lower RH, and the resistance change reaches three orders of magnitude from low RH to high RH. Figure 6d–f show the resistance of TiO2-SnS2, TiO2 and SnS2 sensors at different RH from 32.78% to 93.58%. The resistance change of the TiO2-SnS2 device is close to a linear relation, while the performance of the other two devices is unsatisfactory. Furthermore, the resistance of TiO2 or SnS2 devices reaches 1010 ohm, which is hard to be detected by ordinary instruments. In comparison, the resistance of TiO2-SnS2 is only in the order of kiloohm, which can be easily detected by a multimeter. In other words, the device based on TiO2-SnS2 nanostructures is more practical for daily-life applications.
Pure TiO2 and SnS2 have different responses to humidity, and the composite of the two materials TiO2-SnS2 shows different response mechanisms. First of all, we describe the sensing mechanisms of monolithic components (i.e., pure TiO2 and SnS2). The moisture sensing of TiO2 is mainly caused by oxygen vacancies [30], and compounds containing alkali ions usually show significant hydrophilic properties due to their surface alkalinity [31]. When H2O combines with variations, the conductivity is promoted. For SnS2, the sensing mechanism here is dominant by proton conduction [32]. If abundant water molecules were adsorbed at the SnS2 surface, proton conduction would be formed. As Figure 7a shows, when SnS2 (band gap Eg = 2.18 eV [13]) loads on TiO2 (Eg = 3.2 eV [33]), a potential barrier develops at the TiO2-SnS2 heterojunction. The equivalent resistance of the whole system is the series resistance of the Rt, Rs and Rh. Rt means the resistance of TiO2, Rs means the resistance of SnS2, and Rh means the resistance of the heterojunction. When the sensor is exposed to a low relative humidity environment, water molecules are adsorbed on the TiO2 surface. It reduces the Rt. Therefore, equivalent resistance reduces. As the relative humidity increases, water molecules are further adsorbed due to the electrostatic effect of the OH- groups, and a physical adsorption water layer is formed [34]. Protons transfer from water molecules to SnS2, and the potential barrier height reduces further, as shown in Figure 7b. The layer facilitates the transfer of H2O or H3O+ [35,36]. The quick transfer of ions in the aqueous layer significantly decreases the impedance, which gives rise to the high sensitivity of the sensor.
Table 1 shows a comparison of the performance of some humidity sensors using various materials as sensitive materials. The main comparisons are made in terms of the fabrication method, measuring range, and response time. From the aspect of the fabrication method, this work uses a one-step hydrothermal method to prepare the samples, and the production process is relatively simple. From the aspect of measuring range, other parts of the material are similar to the samples prepared in this work and all have a large measurement range. From the aspect of response time, there exist other sensitive materials with more rapid response times, but the response time of the samples in this work is also faster. Dongzhi Zhang et al. used a simple one-step hydrothermal route for the preparation of microelectrode polyimide substrates to synthesize SnO2 nanoparticles and SnO2/RGO hybrids using hydrothermal methods and used them as sensitive materials to fabricate humidity sensors [37]. The sensor has the advantages of high sensitivity and fast measurement speed, but processing the material into a sensor is relatively tedious. Ravindra Kumar Jha and Prasanta Kumar Guha synthesized WS2 nanosheets in a binary mixture of acetone and acetone by ultrasound and used them as a sensing material for a humidity sensor [38]. The sensor has a good linear relationship with the response of humidity, and the repeatability and stability are also good, but the disadvantage of this sensor is that the measurement range is relatively small. Ming-Zhi Yang et al. fabricated an integrated humidity microsensor using a commercial 0.18 μm complementary metal oxide semiconductor (CMOS) process [39]. The advantage of this sensing is the synthesis of a miniature humidity sensor that is very convenient, but one of the biggest limitations of this sensor is that the measurement range is too small. Yinghua Tan et al. prepared hollow MoS2 micro@nano-sphere composites by a one-step hydrothermal method and used this material as a sensitive material to prepare humidity sensors [40]. They found that the sensor has high sensitivity and good stability through implementation, but the sensor has a relatively small measurement range compared to our humidity sensor. Hui Yang et al. prepared capacitive humidity sensors by sequentially coating aqueous suspensions of zinc oxide (ZnO) nanopowders and polyvinylpyrrolidone-reduced graphene oxide (PVP-RGO) nanocomposites dropwise on cross-finger electrodes [41]. The sensor showed a significant improvement in sensitivity and linearity compared with PVP RGO/ZnO, PVP-RGO and ZnO for the ZnO/PVP-RGO sensor. However, the preparation process of this sensor material is more tedious than the preparation process of the sensitive material used in this paper. Hengchang Bi et al. fabricated a miniature capacitive humidity sensor using a graphene oxide thin film as a humidity sensing material [42]. Compared with the conventional capacitive humidity sensor, this sensor has a high sensitivity at 15–95% relative humidity, which is more than 10 times more sensitive than the best of the conventional sensors. However, the fabrication process of this sensor is relatively tedious. Comparing all aspects, we can find that TiO2-SnS2 nanoheterostructures are indeed an excellent candidate to be used as sensitive materials for humidity sensors.
From the comparison of experimental results, the resistance of the TiO2-SnS2-based sensor is much smaller than that of the bare TiO2 or SnS2-based sensor, and the change in resistance from RH 11% to RH 93% reaches three orders of magnitude, from 0.05 MΩ to 100 MΩ. However, at low relative humidity, the TiO2 nanoribbon devices are in a high resistance state, which is too high for ordinary instruments. The resistance of the SnS2 nanoparticle devices is essentially unchanged at lower relative humidity, and the change in resistance from low to high relative humidity reaches three orders of magnitude. Resistance of the TiO2-SnS2 device varies linearly with humidity, while the other two devices do not perform as well. In addition, the resistance of TiO2 or SnS2 devices reaches 1010 ohms, which is difficult to detect with ordinary instruments. In contrast, the resistance of TiO2-SnS2 is only kiloohms, which can be easily detected with a multimeter. In addition, the roughness of the TiO2 surface also affects the performance of the humidity sensor, which can be expressed as the larger the specific surface area of TiO2, the better the effect of the humidity sensor, and conversely, the smaller the specific surface area of TiO2, the worse the effect of the humidity sensor [43]. In terms of the humidity sensing performance of sensitive materials, the humidity sensors based on TiO2-SnS2 nanostructures are more suitable for everyday applications.

4. Conclusions

In summary, high-quality rough TiO2 nanobelts were synthesized, and SnS2 nanoparticles were loaded on the TiO2 nanobelts by a hydrothermal method. The humidity detectors were fabricated using the powders of TiO2-SnS2, TiO2 and SnS2. Our research results show that the measurement range of TiO2-SnS2 nanoheterostructures in humidity detection is 11–93%, the response time is 60 s, and the linearity between resistance and humidity is good. In addition, the preparation of TiO2-SnS2 nanoheterostructures only uses a one-step hydrothermal method, the preparation process is very simple and convenient, and the cost of preparation is relatively low. These advantages will further increase the possibility of TiO2-SnS2 nanoheterostructures being excellent candidates for humidity detectors.
However, there are still some limitations to the TiO2-SnS2 we have synthesized. The structure of the TiO2-SnS2 nanoheterostructure we synthesized now is not very regular, and the performance of the humidity sensor needs to be further improved. In the next research, we will enhance the structure and number of heterojunctions, further modulate the structural properties of the interface to improve the humidity response of the sensor, and gradually explore the specificity of the sensor for some other gases to increase the practicality of the sensor.

Author Contributions

Conceptualization, D.C. and W.Y.; methodology, D.C.; software, W.Y.; validation, D.C.; formal analysis, W.Y.; investigation, J.L.; resources, D.C.; data curation, W.Y.; writing-original draft preparation, D.C.; writing-review and editing, W.Y.; visualization, J.L.; supervision, Z.Z.; project administration, Z.Z.; funding acquisition, D.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Shandong Provincial Natural Science Foundation (ZR2021QF133), Basic research projects of science, education and industry integration pilot projects of Qilu University of Technology (2022PX037).

Data Availability Statement

The data that support the findings of this study are available within this article.

Acknowledgments

The authors acknowledge the experimental support of the International School for Optoelectronic Engineering at Qilu University of Technology (Shandong Academy of Science).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Jia, P.Y.; Guo, R.T.; Pan, W.G.; Huang, C.Y.; Tang, J.Y.; Liu, X.Y.; Qin, H.; Xu, Q.Y. The MoS2/TiO2 heterojunction composites with enhanced activity for CO2 photocatalytic reduction under visible light irradiation. Colloids Surf. A Physicochem. Eng. Asp. 2019, 570, 306–316. [Google Scholar] [CrossRef]
  2. Hu, J.; Tu, J.; Li, X.; Wang, Z.; Li, Y.; Li, Q.; Wang, F. Enhanced UV-Visible Light Photocatalytic Activity by Constructing Appropriate Heterostructures between Mesopore TiO2 Nanospheres and Sn3O4 Nanoparticles. Nanomaterials 2017, 7, 336. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  3. Choi, M.S.; Bang, J.H.; Mirzaei, A.; Na, H.G.; Jin, C.; Oum, W.; Kim, S.S.; Kim, H.W. Exploration of the use of p-TeO2-branch/n-SnO2 core nanowires nanocomposites for gas sensing. Appl. Surf. Sci. 2019, 484, 1102–1110. [Google Scholar] [CrossRef]
  4. Cai, L.; Li, H.; Zhang, H.; Fan, W.; Wang, J.; Wang, Y.; Wang, X.; Tang, Y.; Song, Y. High performance gas sensor based on ZnO/CuO heterostructures. In Proceedings of 2019 International Conference on Optoelectronics and Measurement ICOM; Springer: Singapore, 2021; Volume 726, pp. 191–198. [Google Scholar]
  5. Chen, D.; Wei, L.; Meng, L.; Chen, Y.; Tian, Y.; Yan, S.; Mei, L.; Jiao, J. High-Performance Self-P formance Self-Powered UV Detected UV Detector Based or Based on SnO2-TiO2 Nanomace Arrays. Nanoscale Res. Lett. 2018, 13, 92. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  6. Shao, D.; Sun, H.; Xin, G.; Lian, J.; Sawyer, S. High quality ZnO–TiO2 core–shell nanowires for efficient ultraviolet sensing. Appl. Surf. Sci. 2014, 314, 872–876. [Google Scholar] [CrossRef]
  7. Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38. [Google Scholar] [CrossRef] [PubMed]
  8. Essalhi, Z.; Hartiti, B.; Lfakir, A.; Mari, B.; Thevenin, P. Optoelectronics properties of TiO2: Cu thin films obtained by sol gel method. Opt. Quantum Electron. 2017, 49, 301. [Google Scholar] [CrossRef]
  9. Nakata, K.; Fujishima, A. TiO2 photocatalysis: Design and applications. J. Photochem. Photobiol. C Photochem. Rev. 2012, 13, 169–189. [Google Scholar] [CrossRef]
  10. Şilik, E.; Pat, S.; Özen, S.; Mohammadigharehbagh, R.; Yudar, H.H.; Musaoğlu, C.; Korkmaz, Ş. Electrochromic properties of TiO2 thin films grown by thermionic vacuum arc method. Thin Solid Film. 2017, 640, 27–32. [Google Scholar] [CrossRef]
  11. Chen, D.; Yu, W.; Wei, L.; Ni, J.; Li, H.; Chen, Y.; Tian, Y.; Yan, S.; Mei, L.; Jiao, J. High sensitive room temperature NO2 gas sensor based on the avalanche breakdown induced by Schottky junction in TiO2-Sn3O4 nanoheterojunctions. J. Alloys Compd. 2022, 912, 165079. [Google Scholar] [CrossRef]
  12. Hamidi, F.; Aslani, F. TiO2-based Photocatalytic Cementitious Composites: Materials, Properties, Influential Parameters, and Assessment Techniques. Nanomaterials 2019, 9, 1444. [Google Scholar] [CrossRef] [Green Version]
  13. Gaur, R.; Jeevanandam, P. Synthesis of SnS2 Nanoparticles and Their Application as Photocatalysts for the Reduction of Cr(VI). J. Nanosci. Nanotechnol. 2018, 18, 165–177. [Google Scholar] [CrossRef] [PubMed]
  14. Yang, Z.; Su, C.; Wang, S.; Han, Y.; Chen, X.; Xu, S.; Zhou, Z.; Hu, N.; Su, Y.; Zeng, M. Highly sensitive NO2 gas sensors based on hexagonal SnS2 nanoplates operating at room temperature. Nanotechnology 2019, 31, 075501. [Google Scholar] [CrossRef]
  15. Han, X.; Xing, J.; Xu, H.; Huang, Y.; Li, D.; Lu, J.; Li, P.; Wu, Y. Remarkable improved photoelectric performance of SnS2 field-effect transistor with Au plasmonic nanostructures. Nanotechnology 2020, 31, 215201. [Google Scholar]
  16. Liu, M.; Wu, H.; Liu, X.; Wang, Y.; Lei, M.; Liu, W.; Guo, W.; Wei, Z. Optical properties and applications of SnS2 SAs with different thickness. Opto-Electron. Adv. 2021, 4, 200029. [Google Scholar] [CrossRef]
  17. Montesperelli, G.; Pumo, A.; Traversa, E.; Gusmano, E.; Bearzotti, A.; Montenero, A.; Gnappi, G. Sol—Gel processed TiO2-based thin films as innovative humidity sensors. Sens. Actuators B Chem. 1995, 25, 705–709. [Google Scholar] [CrossRef]
  18. Bharatula, L.D.; Erande, M.B.; Mulla, I.S.; Rout, C.S.; Late, D.J. SnS2 nanoflakes for efficient humidity and alcohol sensing at room temperature. RSC Adv. 2016, 6, 105421–105427. [Google Scholar] [CrossRef]
  19. Kovacic, M.; Kusic, H.; Fanetti, M.; Stangar, U.L.; Valant, M.; Dionysiou, D.D.; Bozic, A.L. TiO2-SnS2 nanocomposites: Solar-active photocatalytic materials for water treatment. Environ. Sci. Pollut. Res. 2017, 24, 19965–19979. [Google Scholar] [CrossRef] [PubMed]
  20. Duong, H.M.; Tran, T.Q.; Kopp, R.; Myint, S.M.; Peng, L. Direct Spinning of Horizontally Aligned Carbon Nanotube Fibers and Films from the Floating Catalyst Method. In Nanotube Superfiber Materials, 2nd ed.; William Andrew Publishing: Norwich, NY, USA, 2019; pp. 3–29. [Google Scholar]
  21. Duong, H.M.; Myint, S.M.; Tran, T.Q.; Le, D.K. Post-spinning treatments to carbon nanotube fibers. In Carbon Nanotube Fibers and Yarns; Elsevier: Amsterdam, The Netherlands, 2020; pp. 103–134. [Google Scholar]
  22. Zhang, D.; Cao, Y.; Li, P.; Wu, J.; Zong, X. Humidity-sensing performance of layer-by-layer self-assembled tungsten disulfide/tin dioxide nanocomposite. Sens. Actuators B Chem. 2018, 265, 529–538. [Google Scholar] [CrossRef]
  23. Zhang, D.; Zong, X.; Wu, Z.; Zhang, Y. Hierarchical Self-Assembled SnS2 Nanoflower/Zn2SnO4 Hollow Sphere Nanohybrid for Humidity Sensing Applications. ACS Appl. Mater. Interfaces 2018, 10, 32631–32639. [Google Scholar] [CrossRef] [PubMed]
  24. Wang, Y.; Zhao, X.; Feng, Z.; Lu, G.; Sun, T.; Xu, Y. Crystalline-phase-induced formation of fibre-in-tube TiO2–SnO2 fibres for a humidity sensor. CrystEngComm 2017, 19, 5528–5531. [Google Scholar] [CrossRef]
  25. Cappelli, I.; Fort, A.; Lo Grasso, A.; Panzardi, E.; Mugnaini, M.; Vignoli, V. RH Sensing by Means of TiO2 Nanoparticles: A Comparison among Different Sensing Techniques Based on Modeling and Chemical/Physical Interpretation. Chemosensors 2020, 8, 89. [Google Scholar] [CrossRef]
  26. Shanmugaratnam, S.; Selvaratnam, B.; Baride, A.; Koodali, R.; Ravirajan, P.; Velauthapillai, D.; Shivatharsiny, Y. SnS2/TiO2 Nanocomposites for Hydrogen Production and Photodegradation under Extended Solar Irradiation. Catalysts 2021, 11, 589. [Google Scholar] [CrossRef]
  27. Xia, F.F.; Yang, F.L.; Hu, J.; Zheng, C.Z.; Yi, H.B.; Sun, J.H. Enhanced visible light absorption performance of SnS2 and SnSe2 via surface charge transfer doping. RSC Adv. 2018, 8, 40464–40470. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  28. Nagaveni, K.; Hegde, M.S.; Ravishankar, N.; Subbanna, G.N.; Madras, G. Synthesis and Structure of Nanocrystalline TiO2 with Lower Band Gap Showing High Photocatalytic Activity. Langmuir 2004, 20, 2900–2907. [Google Scholar] [CrossRef] [PubMed]
  29. Gao, J.; Sun, X.; Zheng, L.; He, G.; Wang, Y.; Li, Y.; Liu, Y.; Deng, J.; Liu, M.; Hu, J. 2D Z-scheme TiO2/SnS2 Heterojunctions with Enhanced Visible-light Photocatalytic Performance for Refractory Contaminants and Mechanistic Insight. New J. Chem. 2021, 45, 16131–16142. [Google Scholar] [CrossRef]
  30. Li, T.Y.; Si, R.J.; Sun, J.; Wang, S.T.; Wang, J.; Ahmed, R.; Zhu, G.B.; Wang, C.C. Giant and controllable humidity sensitivity achieved in (Na plus Nb) co-doped rutile TiO2. Sens. Actuators B Chem. 2019, 293, 151–158. [Google Scholar] [CrossRef]
  31. Boukoussa, B.; Hakiki, A.; Nunes-Beltrao, A.P.; Hamacha, R.; Azzouz, A. Assessment of the intrinsic interactions of nanocomposite polyaniline/SBA-15 with carbon dioxide: Correlation between the hydrophilic character and surface basicity. J. CO2 Util. 2018, 26, 171–178. [Google Scholar] [CrossRef]
  32. HHassan, H.U.; Mun, J.; Kang, B.S.; Song, J.Y.; Kim, T.; Kang, S.-W. Sensor based on chemical vapour deposition-grown molybdenum disulphide for gas sensing application. RSC Adv. 2016, 6, 75839–75843. [Google Scholar] [CrossRef]
  33. Zhang, H.; Gao, Y.; Zhu, G.; Li, B.; Gou, J.; Cheng, X. Synthesis of PbS/TiO2 nano-tubes photoelectrode and its enhanced visible light driven photocatalytic performance and mechanism for purification of 4-chlorobenzoic acid. Sep. Purif. Technol. 2019, 227, 115697. [Google Scholar] [CrossRef]
  34. Anderson, J.H.; Parks, G.A. Electrical conductivity of silica gel in the presence of adsorbed water. J. Phys. Chem. 1968, 72, 3662–3668. [Google Scholar] [CrossRef]
  35. Agmon, N. The Grotthuss mechanism. Chem. Phys. Lett. 1995, 244, 456–462. [Google Scholar] [CrossRef]
  36. Casalbore-Miceli, G.; Yang, M.J.; Camaioni, N.; Mari, C.M.; Li, Y.; Sun, H.; Ling, M. Investigations on the ion transport mechanism in conducting polymer films. Solid State Ion. 2000, 131, 311–321. [Google Scholar] [CrossRef]
  37. Zhang, D.; Chang, H.; Li, P.; Liu, R.; Xue, Q. Fabrication and characterization of an ultrasensitive humidity sensor based on metal oxide/graphene hybrid nanocomposite. Sens. Actuators B Chem. 2016, 225, 233–240. [Google Scholar] [CrossRef]
  38. Jha, R.K.; Guha, P.K. Liquid exfoliated pristine WS2 nanosheets for ultrasensitive and highly stable chemiresistive humidity sensors. Nanotechnology 2016, 27, 475503. [Google Scholar] [CrossRef]
  39. Yang, M.Z.; Dai, C.L.; Wu, C.C. Sol-Gel Zinc Oxide Humidity Sensors Integrated with a Ring Oscillator Circuit On-a-Chip. Sensors 2014, 14, 20360–20371. [Google Scholar] [CrossRef] [Green Version]
  40. Tan, Y.; Yu, K.; Yang, T.; Zhang, Q.; Cong, W.; Yin, H.; Zhang, Z.; Chen, Y.; Zhu, Z. The combinations of hollow MoS2 micro@nano-spheres: One-step synthesis, excellent photocatalytic and humidity sensing properties. J. Mater. Chem. C 2014, 2, 5422–5430. [Google Scholar] [CrossRef]
  41. Yang, H.; Ye, Q.; Zeng, R.; Zhang, J.; Yue, L.; Xu, M.; Qiu, Z.-J.; Wu, D. Stable and Fast-Response Capacitive Humidity Sensors Based on a ZnO Nanopowder/PVP-RGO Multilayer. Sensors 2017, 17, 2415. [Google Scholar] [CrossRef] [Green Version]
  42. Bi, H.; Yin, K.; Xie, X.; Ji, J.; Wan, S.; Sun, L.; Terrones, M.; Dresselhaus, M.S. Ultrahigh humidity sensitivity of graphene oxide. Sci. Rep. 2013, 3, 2714. [Google Scholar] [CrossRef] [Green Version]
  43. Zhao, X.; Chen, X.; Ding, X.; Yu, X.; Chen, X.; Liu, F. Humidity Sensing Properties and Negative Differential Resistance Effects of TiO2 Nanowires. IEEE Sens. J. 2021, 21, 18477–18482. [Google Scholar] [CrossRef]
Crystals 13 00482 g001 550
Figure 1. XRD patterns of (a) the TiO2 belts and (b) the TiO2-SnS2 nanostructure.
Figure 1. XRD patterns of (a) the TiO2 belts and (b) the TiO2-SnS2 nanostructure.
Crystals 13 00482 g001
Crystals 13 00482 g002 550
Figure 2. Raman spectrums of (a) the TiO2, (b) the SnS2 and the TiO2-SnS2.
Figure 2. Raman spectrums of (a) the TiO2, (b) the SnS2 and the TiO2-SnS2.
Crystals 13 00482 g002
Crystals 13 00482 g003 550
Figure 3. SEM images of (a,b) the TiO2 nanobelts and (c,d) the TiO2-SnS2.
Figure 3. SEM images of (a,b) the TiO2 nanobelts and (c,d) the TiO2-SnS2.
Crystals 13 00482 g003
Crystals 13 00482 g004 550
Figure 4. TEM images (a,b) and HRTEM image (c) of the TiO2 nanobelts. TEM images (d,e) and HRTEM image (f) of TiO2-SnS2. Insets: (b) TEM image of single-TiO2 nanobelt, (c) the electron diffraction pattern image of TiO2 and (e) TEM image of single-SnS2-TiO2 nanoheterostructure.
Figure 4. TEM images (a,b) and HRTEM image (c) of the TiO2 nanobelts. TEM images (d,e) and HRTEM image (f) of TiO2-SnS2. Insets: (b) TEM image of single-TiO2 nanobelt, (c) the electron diffraction pattern image of TiO2 and (e) TEM image of single-SnS2-TiO2 nanoheterostructure.
Crystals 13 00482 g004
Crystals 13 00482 g005 550
Figure 5. (a) The reflection spectra of TiO2-SnS2 and TiO2 nanobelts. (b) Plots of (F(R)E)2 versus photon energy E.
Figure 5. (a) The reflection spectra of TiO2-SnS2 and TiO2 nanobelts. (b) Plots of (F(R)E)2 versus photon energy E.
Crystals 13 00482 g005
Crystals 13 00482 g006 550
Figure 6. The humid responsivity (a) and (d) at different RH of TiO2-SnS2, the humid responsivity (b) and (e) at different RH of TiO2, and the humid responsivity (c) and (f) at different RH of SnS2.
Figure 6. The humid responsivity (a) and (d) at different RH of TiO2-SnS2, the humid responsivity (b) and (e) at different RH of TiO2, and the humid responsivity (c) and (f) at different RH of SnS2.
Crystals 13 00482 g006
Crystals 13 00482 g007 550
Figure 7. The energy band variations of the TiO2-SnS2 heterojunction after adsorbing water molecules at (a) without water and (b) with water.
Figure 7. The energy band variations of the TiO2-SnS2 heterojunction after adsorbing water molecules at (a) without water and (b) with water.
Crystals 13 00482 g007
Table
Table 1. Comparison of the main features of previously reported humidity sensors.
Table 1. Comparison of the main features of previously reported humidity sensors.
Active MaterialsFabrication MethodMeasuring RangeResponseReference
Graphene/SnO2Hydrothermal11–97%RH560.85[37]
WS2Liquid exfoliation40–80%RH37.5[38]
ZnOSol–gel method40–90%RH-[39]
MoS2Hydrothermal17.2–89.5%RH67.34[40]
ZnO/PVP/RGODrop-casting15–95%RH-[41]
Graphene oxideSolution dripping15–95%RH378[42]
TiO2-SnS2Hydrothermal11–93%RH60This work
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Yu, W.; Chen, D.; Li, J.; Zhang, Z. TiO2-SnS2 Nanoheterostructures for High-Performance Humidity Sensor. Crystals 2023, 13, 482. https://doi.org/10.3390/cryst13030482

AMA Style

Yu W, Chen D, Li J, Zhang Z. TiO2-SnS2 Nanoheterostructures for High-Performance Humidity Sensor. Crystals. 2023; 13(3):482. https://doi.org/10.3390/cryst13030482

Chicago/Turabian Style

Yu, Wencheng, Duo Chen, Jianfei Li, and Zhenzhen Zhang. 2023. "TiO2-SnS2 Nanoheterostructures for High-Performance Humidity Sensor" Crystals 13, no. 3: 482. https://doi.org/10.3390/cryst13030482

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop