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NaNbO3 nanocomposite with stable humidity-sensing properties at room temperature
Sensors & Actuators: B. Chemical 283 (2019) 643–650
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
An urchin-like SnO2/NaNbO3 nanocomposite with stable humidity-sensing properties at room temperature
T
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Jiuyang Zhang, Yuhua Zhen , Haoyue Xue, Xiaoxin Gao, Wenxin Wang, Yingda Li, Tasawar Hayat, Njud S. Alharbi College of Material Science and Engineering, China University of Petroleum, QingDao 266580 China
A R T I C LE I N FO
A B S T R A C T
Keywords: Urchin-like structure Hydrothermal method SnO2 NaNbO3 Humidity
An urchin-like nano-composite of SnO2/NaNbO3 was fabricated by the hydrothermal method and the influence of the Sn/Nb ratio in the samples on their humidity-sensing properties was investigated. The phases and morphologies of these composites and their elemental distributions were analyzed by X-ray diffraction scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The sensor based on the optimum Sn/Nb ratio of 1:0.4 exhibited remarkable humidity-sensing properties, including a good response (S = 4823.8) that was 8.72 times higher than that of the pure SnO2 sensor (S = 553.01). The composites showed rapid response and recovery times (3/9 s), as well as good stability, linearity, and excellent selectivity. The fabrication and humiditysensing mechanisms were systematically analyzed using analog computations and Nyquist diagrams, respectively. Compared with pure NaNbO3 and SnO2 sensors, our easily prepared SnO2/NaNbO3 sensor demonstrates good sensing properties and holds great promise for use in humidity-sensing applications.
1. Introduction Humidity sensors play an important role in monitoring and adjusting the environmental humidity in various scientific fields such as biology, chemistry, and physics [1]. The use of these sensors has gradually expanded to more applications and research fields over time [2,3]. However, owing to the complex environments of laboratories and factories, a variety of gases and vapors may be present at room temperature. Therefore, the demand for stable and sensitive humidity sensors, with good response/recovery times and selectivity, as well as other properties, is increasing [4]. Hitherto, myriad materials such as polymers, metal oxides, and carbon nanostructures have been innovatively employed to fabricate humidity sensors [5,6]. Among these, SnO2, an important n-type semiconductor with a wide band gap (Eg = 3.6 eV at 300 K), has attracted much attention owing to its simple structure, high sensitivity, favorable selectivity, and lower cost as compared with ZnO, In2O3, and other metal oxide semiconductors [7,8]. Moreover, SnO2 exhibits conductivity at or below room temperature [9]. Composites of SnO2 with TiO2, ZnO, and Fe, have been previously reported, and all displayed good sensing properties under
different conditions [10–13]. Hence, SnO2 doped or composited with other substances has been widely explored in the research and development of gas or humidity sensors [14–17]. Sodium niobate (NaNbO3), which has a perovskite structure, is used to produce energy storage devices and piezoelectric regulators owing to its anti-ferroelectric-toferroelectric phase transition [18–20]. In previous studies, humidity was found to influence the electrical properties of NaNbO3 system piezoceramics [21,22], suggesting that a relationship between relative humidity and NaNbO3 behavior possibly exists. Moreover, because of its high chemical stability, NaNbO3 is also used in sensors, and has excellent sensing properties, especially toward humidity. Hence, it has attracted considerable attention for its potential as a stable sensor material [23–25]. Nanoline NaNbO3 has been fabricated by complex methods, and owing to its unique electrical properties, shows large impedance in sensing tests. This inspired us to investigate its potential for enhancing sensor performance [26,27]. Materials with urchin-like structures have been produced previously by the hydrothermal method [28,29]. In such structures, a needle or slice of one material is inlayed or inserted in the ball-like skeleton of another material, producing an increased specific surface area.
Abbreviations: BFGS, Broyden–Fletcher–Goldfarb–Shanno method; DFT, density functional theory; EDS, energy-dispersive spectroscopy; GGA, generalized gradient approximation; IDE, interdigitated electrode; OTFG, on-the-fly-generated; PBE, Perdew–Burke–Ernzerhof functional; RH, relative humidity; RT, room temperature; SCF, self-consistent field; XRD, X-ray powder diffraction ⁎ Corresponding author. E-mail address: [email protected] (Y. Zhen). https://doi.org/10.1016/j.snb.2018.12.035 Received 19 July 2018; Received in revised form 3 December 2018; Accepted 8 December 2018 Available online 10 December 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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Although the composition of the urchin-like structure is non-homogeneous, its structure facilitate can contact with the tested gas as well as electron transport. Thus, the material with the urchin-like structures can display good properties in gas sensing. In this paper, we report the novel urchin-like structures of SnO2/ NaNbO3 composite-based humidity sensors prepared through a simple hydrothermal method [7,27,30,31]. We composited NaNbO3 with SnO2 in different ratios and explored the resulting physical phases [23,32]. The reaction activity of Nb toward OHˉ is higher than that of Sn, as demonstrated by experiments and theoretically verified by analog computations. A heterojunction structure was formed between the NaNbO3 and SnO2, thereby remarkably enhancing the electrical properties [31,33]. In addition, the fabrication and sensing mechanisms of the composite are discussed based on analog computations and Nyquist diagrams, respectively. The results and related analyses shed light on the impact of humidity on SnO2/NaNbO3, and project its potential for high-humidity sensing and other selective sensing applications.
Fig. 1. Structure of humidity sensor.
sensor (containing the IDE) with a digital electrical bridge instrument, Mydream Electronic, ModelLCR-TH2828. The response/recovery time (T) and response (S) are both normalized evaluation figures for the performance of the humidity sensors. T is the time taken by the sensor to attain 90% of its final value. In this paper, for the humidity sensing tests, S is defined as S = |ZRH11|/|ZRHy|, where ZRH11 and ZRHy are the impedance of the sensor at 11% and y% (y = 95) RH levels, respectively [36,37]. The humidity surrounding of gas sensing testing experiment is 11% RH all the time, so S is defined as S = |Zair|/|Zgas| [14].
2. Experimental procedures 2.1. Materials and sample fabrication In this work, we synthesized NaNbO3/SnO2 nanocomposites with different ratios of constituents by the hydrothermal method. All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). They were of analytical grade and used without further purification. To identify the optimal alkali content, SnCl4·5H2O (7.5 mmol) was added to absolute ethanol (40 mL) and NaOH (60 mmol) was dissolved in deionized water (40 mL). Then, varying amounts of Nb2O5 (x mol%; x = 0.2, 0.4, 0.6, 0.8, and 1) were then dissolved in the abovementioned ethanol solution. Each alcohol solution was added dropwise to the alkali solution under magnetic stirring for 20 min until evenly mixed. Then, the solution was sonicated at 50 °C for 30 min and transferred to a 180 mL Teflon-lined stainless steel autoclave, where it was heated at 180 °C for 6 h. After cooling to room temperature (RT) naturally, the precipitate was transferred into a centrifuge tube and centrifugally washed several times with deionized water and absolute ethanol. Finally, it was dried at 60 °C for 6 h and calcined at 450 °C for 3 h in a muffle furnace.
3. Results and discussion 3.1. Crystal structure of SnO2/NaNbO3 SnO2/NaNbO3 nanopowders with different Sn/Nb ratios prepared by the hydrothermal method were characterized by XRD, and the profiles are shown in Fig. 2. The diffraction peaks of SnO2 are identical to the standard values in the JCPDS reference card (PDF#41-1445), while the peaks due to NaNbO3 in the as-prepared SnO2/NaNbO3 composites are in good agreement with those in JCPDS card PDF#191221 [4,38]. All the samples exhibit relatively strong diffraction peaks, suggesting that the hydrothermally prepared crystals have good crystallinity. With increasing Sn4+/Nb5+ molar ratios, the peaks undergo a series of changes. When the Sn/Nb ratio is 1:0.4, two substantial sets of peaks coexist. As the content of Nb increases to 0.6, the SnO2 peaks are replaced by Na3NbO4 (PDF#22-1391), which is an intermediate product that can continue to react with OHˉ to generate NaNbO3 [39,40]. With increasing Nb content (1:0.8 ratio and higher), the SnO2 peaks disappear completely while Nb2O5 (PDF#19-0862) peaks appear [41].
2.2. Material characterization To investigate the purity of the samples and to analyze their crystal structures, X-ray powder diffraction (XRD) experiments were conducted on a Dandong DX-2700 X-ray diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). The microscopic morphologies and the distribution of elements in the samples were recorded by field-emission scanning electron microscopy (FESEM, Hitachi S-4800) with energy-dispersive spectroscopy (EDS). 2.3. Humidity sensing and sensor properties The sample powder was dispersed in absolute ethanol in a weight ratio of 20:1 to form slurry, and then drop-coated onto a pretreated interdigitated electrode (IDE), which had been ultrasonically cleaned using acetone, deionized water, and ethanol several times (Fig. 1). One end of a copper wire was polished and adhered to the IDE by lead-free soldering, and a solder ball was welded on the other end. The IDE was then dried at 120 °C for 1 h. Different humidity atmospheres were produced by dissolving LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, KI, NaCl, KCl, and K2SO4 in an airtight vessel at 45 °C, respectively, and then cooling to RT to form saturated solutions to obtain approximately 11, 22, 33, 43, 54, 69, 75, 84, and 95% relative humidity (denoted as RH) levels [34,35]. The various impedance responses of the sensors in the different humidity environments were assessed by connecting the
Fig. 2. XRD patterns of the nanocomposites prepared with different ratios of Sn and Nb: (a) 1:1, (b) 1:0.8, (c) 1:0.6, and (d) 1:0.4. 644
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Fig. 3. (a, b, and c) SEM images and (d) EDS map of the SnO2/NaNbO3 composite with Sn/Nb = 1:0.4.
shown in Fig. 3(d). In the chosen area, an evident cluster of needles inlaid on the surface of the internal cubic structure can be observed. During hydrothermal synthesis, the crystal nuclei are distributed nonuniformly before crystal growth occurs. When characterized by EDS, the penetrability of the electron beam enables it to pierce the needles inlaid on the surface; thus, the elements and their distribution were obtained for both parts of the urchin-like structure. In the selected area, Sn is distributed densely in the needlelike structures, whereas Nb is distributed densely on the cubic structures and is absent in the needlelike structures. Na and O exist equally in both parts. Combined with the result of Fig. 2 (XRD), it is obvious that the main products in the composite with Sn/Nb = 1:0.4 are SnO2 and NaNbO3. Therefore, needlelike SnO2 is generated and inlaid on the cubic NaNbO3 to form the urchin-like structure. The thorny structure dramatically improves the specific surfaces area and increases the contact with water molecules, enabling remarkable humidity-sensing performance. Fig. 4. Impedance changes as a function of humidity at different frequencies for the SnO2/NaNbO3 composite with Sn/Nb = 1: 0.4.
3.2. Choice of testing frequency The electrical bridge instrument was used to test the Sn/Nb = 1:0.4 sample at different frequencies to obtain a plot of impedance as a function of humidity (Fig. 4). The AC frequency can impact the electrical properties of a sensor. Because the net polarization of the water molecules changes as the AC frequency decreases, dipole generation is more difficult, and the vibrational relaxation of the water molecules will be limited. Thus, the sensors show humidity insensitivity. Therefore, it was necessary to first determine an appropriate frequency for humidity-sensing testing. Fig. 4 shows that the impedance decreases with an increase in RH from 11% to 95% for all the chosen frequencies, among which the most varied impedance changed by five orders of magnitude from 10 MΩ to 1 kΩ. At low humidity levels, the impedance decreases more gently, and the impedance shows a linear change at high frequencies. This indicates that the sensitivity at high RH is lower than at low RH. Among the various measurement frequencies, the relationships differ. With increasing frequency, the impedance value decreases overall, but the change is more significant at low frequencies. One of the criteria for selecting a sensor’s testing frequency is a linear relationship between impedance and humidity. Better linearity results
The diffraction peaks of NaNbO3, as the product of the reaction between Nb2O5 and NaOH, are present throughout the phase transformation. However, the peaks due to SnO2 decrease, and those due to Na3NbO4 (another intermediate, produced from NaNbO3) and the original reactant Nb2O5 evolve as the Nb2O5 doping content increases. In a fixed reaction system with a certain amount of NaOH, these changes illustrate that Nb5+ competes with Sn4+ to combine with OHˉ. With increasing Nb5+ content, the Nb5+ preferentially reacts with OHˉ, and thus, the quantity of OHˉ reacting with Sn4+ decreases. Thus, the diffraction peaks due to SnO2 disappear gradually. This was also verified computationally, by comparing the adsorption energies of the hydroxyl ion on the respective Nb and Sn surfaces. Morphological examination of the ratio of 1:0.4 nanocomposites reveals an urchin-like structure; SEM images are presented in Fig. 3(a–c). The inner skeleton of the structure comprises a stack of cubic NaNbO3, with needlelike SnO2 inlaid on its surface. The length of the cube is about 500 nm, whereas the needles are 6.5 μm in length and 150 nm in width in all orientations. The elemental distributions are
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impedance recovers to its initial value after responding to the gases, especially in dry gas atmospheres. 3.4. Computational details for formation mechanism We next considered the mechanism of composite formation. To verify that Nb2O5 preferentially reacts with OHˉ versus SnCl4·5H2O, the OHˉ adsorption behavior on both surfaces was studied using density functional theory (DFT). The Nb2O5 (100) and SnCl4·5H2O (100) surfaces were chosen to examine the adsorption of OHˉ. As shown in Fig. 8(a) and (b), the Nb2O5 (100) surface was modeled by 183 atoms, and the SnCl4·5H2O (100) surface by 232 atoms. In both models, the atoms between the red and black dashed lines were relaxed and regarded as surface atoms, whereas the atoms below the black dashed line were fixed and considered as bulk-phase atoms. The surfaces were separated by a 15 Å vacuum region to ensure that there was no significant interaction between the slabs. Fig. 8(c) and (d) show three high-symmetry adsorption sites (top site (T site), bridge site (B site), and hollow site (H site)) on the Nb2O5 (100) and SnCl4·5H2O (100) surfaces, respectively, where the adsorption of OHˉ atoms was allowed. All calculations were performed using DFT within the CASTEP (Cambridge Sequential Total Energy Package) code [42]. The exchange and correlation potentials were constructed by the generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE) functional. To indicate the interactions between the valence electrons and ionic core, on-the-fly-generated (OTFG) ultrasoft pseudopotentials were employed. Before property calculations and geometry relaxation, the convergence criteria for structure optimization and energy calculations were set as follows: the convergence value of the average total energy of atoms was 1 × 10−5 eV/atom, the maximum force that one atom was subjected to 0.03 eV/Å, the maximum stress was 0.05 GPa, the maximum displacement of an atom was 0.001 Å, and the self-consistent field (SCF) convergence threshold was 1 × 10-6 eV/atom. All atoms in the considered systems were relaxed using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method, which was treated as the minimization algorithm. A 3 × 3 × 1 Monkhorst-Pack k-point grid was specified in the calculation. The cut-off energy of the plane-wave basis set was set as 490 eV. The adsorption energy Eads can reflect the combining capacity between OHˉ and the surface. The following formula was used to compute the adsorption energy [43]:
Fig. 5. Response and recovery curves in different humidity environments.
in more-stable measurements, and better measurement precision improves practical applicability. Therefore, by linearizing these data and calculating the goodness of fit, we determined that at 1000 Hz, the impedance and RH exhibit the best linearity. Consequently, 1000 Hz was chosen as the optimal frequency to test the humidity-sensing properties of the samples. 3.3. Humidity-sensing properties of SnO2/NaNbO3 The optimal frequency was used to test the humidity-sensing properties of the SnO2/NaNbO3 samples. Each sample was placed in an environment of 11% RH for 1 min, transferred to a 22% RH environment for 1 min, and then back to the 11% RH environment. The above process was repeated until the humidity reached 95%. The data in Fig. 5 show the relationship between the impedance of the samples and time from 11% to 95% RH. SnO2 does not exhibit sensing stability at any humidity level. Although NaNbO3 is more stable than SnO2, its response and recovery in all RHs are unsatisfactory. Both pure substances are similar in terms of humidity sensitivity, in that the impedance does not recover to the initial value after responding to humidity. This phenomenon shows that both single substances sense humidity by chemisorbing water molecules. Compared with the single materials, the SnO2/NaNbO3 composites exhibit significantly superior properties. When Sn/Nb = 1:0.4, obvious response behavior is evident from 33% RH, a relatively lowhumidity environment, and the sensor is faster (T = 3/9 s) compared to the pure SnO2 material (6/21 s), as calculated from Fig. 6. Furthermore, NaNbO3 doping not only affects the response and recovery, but also improves the sensor’s response, with the Sn/Nb = 1:0.4 composite (S = 4823.8) exhibiting a value 8.72 times larger than that of SnO2 (S = 553.01). Moreover, not only the T and S values, but also the sensing range and stability improve. The difference between pure SnO2 and NaNbO3 lies in whether or not the impedance can recover to its initial value. However, as the doping amount continues to increase, the stability and repeatability of the sensors are reduced, and the impedance does not recover to its initial value. Hence, the optimal composition of Sn/Nb = 1:0.4 was used in the following tests. The response sensitivities of the different SnO2/NaNbO3 sensors to humidity are shown in Fig. 7(a). All of the composites except for 1:0.4 SnO2/NaNbO3 display response sensitivities on the order of 101. Based on the XRD analysis (Fig. 2), this may be attributed to incomplete formation of the SnO2/NaNbO3 composite during hydrothermal synthesis. The humidity sensor with Sn/Nb = 1:0.4 was exposed to different atmospheres to measure its selectivity toward various gases; Fig. 7(b) shows these responses. Compared with other gases, the sensor exhibits excellent response and recovery properties when exposed to humidity. To better quantify this behavior, the response of the sensor toward various gases was calculated (Fig. 7(c)). The response to humidity (103) was much higher than that for other gases (∼10°), which indicates that the composite possesses significant gas selectivity. In addition, the response and recovery curves in Fig. 7(b) show that the
Eads = EOH+sur – (EOH + Esur)
(1)
where EOH + sur is the energy of OHˉ interacting with the Nb2O5 (100) or SnCl4·5H2O (100) surface, Esur is the total energy of the clean surface, and EOH is the energy of OHˉ. To investigate the bonding mode for OHˉ adsorption on the surfaces, the electron density difference, Δρ(r), was calculated using the following formula [44]: Δρ(r) = ρtotal(r) – ρM(r) – ρsurf(r)
(2)
where ρtotal(r), ρM(r), and ρsurf(r) are the electron densities of the adsorption system, isolated ion (OHˉ), and substrate surfaces, respectively. 3.4.1. DFT calculations In these calculations, we considered all the possible adsorption sites for OHˉ on the Nb2O5 (100) and SnCl4·5H2O (100) surfaces. However, only the T site was a stable adsorption site for OHˉ on either surface. Table 1 lists the adsorption energies, bond distances, and angles between the OHˉ and surfaces after adsorption. Clearly, OHˉ adsorption on the Nb2O5 (100) or SnCl4·5H2O (100) surface is favorable in the top sites (Table 1) because the signs of the adsorption energies are negative. The adsorption energy on the Nb2O5 (100) surface is −6.35 eV, whereas that on the SnCl4·5H2O (100) surface is −2.24 eV. The high 646
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Fig. 6. Response and recovery curves for different ratios of SnO2/NaNbO3 composites as well as the pure samples in 11% RH and 95% RH.
Fig. 7. Responses of different samples (a) and the composite with Sn/Nb = 1:0.4 to several gases (c), as well as the selectivity of the latter (b). 647
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Fig. 8. Models and adsorption sites of Nb2O5 (100) and SnCl4·5H2O (100) surfaces: (a) side view of the Nb2O5 (100) surface; (b) side view of the SnCl4·5H2O (100) surface; (c) top view of the Nb2O5 (100) surface; and (d) top view of the SnCl4·5H2O (100) surface.
3.5. Humidity-sensing mechanism
Table 1 Adsorption data for OHˉ on the Nb2O5 (100) and SnCl4·5H2O (100) surfaces. Surface
Stable adsorption site
Adsorption energy (eV)
Bond length (Å)
Nb2O5 (100) SnCl4·5H2O (100)
Top Top
−6.35 −2.24
1.896 2.019
To better understand the sensing mechanism, complex impedance plots of the 1:0.4 SnO2/NaNbO3 sensor from 320 Hz to 10 kHz in different RH environments were analyzed (Fig. 10). The electrical properties of a humidity sensor are related to the water adsorption and desorption processes that take place on the active sites of the sensing material, and are significantly related to the surface area exposed to the environment. There are two kinds of absorbed water molecules, i.e., chemisorbed and physisorbed. When water molecules reach the surface of a material, they are chemisorbed first, and the chemisorbed layers exhibit no relationship with the RH. With the introduction of humidity, the chemisorbed layer is covered by physisorbed layers, which are easily affected by humidity. Moreover, considering the almost fully recovered sensing performance of the sensors mentioned above, physisorption is the dominant part of the interaction process between the water molecules and the material surface. This is the reason why the chemisorbed water molecules on the surface cannot be completely removed at room temperature [2]. As humidity is measured at room temperature, the mechanism by which absorbed water molecules react with the Lewis acid and base sites on the SnO2 surface to release electrons is not operative. At room temperature, water physisorption
adsorption energy indicates that OHˉ can easily react with the Nb2O5 (100) surface. In addition, the bond length of OeNb is 1.896 Å, whereas that of OeSn is 2.019 Å; a shorter bond length indicates that the Nb2O5 (100) surface has stronger adsorptivity. Fig. 9 shows the differences in the electron densities of the OHˉ ions adsorbed on the top sites for each surface. Clearly, electrons are transferred from the substrate to the adsorbate when adsorption occurs. This indicates that the adsorption of OHˉ on both surfaces is via chemisorption. In conclusion, the calculated results verify that the chemical reactivity of Nb2O5 toward OHˉ is higher than that of SnCl4·5H2O, which explains the series of changes observed in the X-ray diffraction patterns.
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Fig. 9. Electron-density difference views around OHˉ adsorbed on (a) Nb2O5 (100) and (b) SnCl4·5H2O (100) surfaces. Blue and yellow regions indicate increasing and decreasing electron density, respectively. The value of the isosurface is ± 0.03 e/Å3 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
values are extraordinarily high. (2) As the RH increases from 33% to 69% RH, the additional water molecules lead to larger numbers of hydroxyl ions and protons, and charge transport is secured by protonhopping between the chemisorbed hydroxyl groups. Simultaneously, water molecules become polarized to form the polarized charges involved in conduction. Hence, the straight line gradually becomes curved and, eventually, semicircular. During this period, the electrochemical reaction is controlled by charge transfer, while diffusion can be neglected. (3) When the RH further increases to 75%, physisorbed layers are formed on the chemisorbed layers. At this stage, H2O molecules are autodissociated into H3O+ and OHˉ, and H3O+ tends to be the major charge carrier. In the physisorbed layers, H3O+ releases a proton to the neighboring water molecules, which accept it while releasing another proton and so forth; consequently, the proton can move freely in the layers. This exchange is known as the Grotthuss chain mechanism: H2O + H3O+ = H3-O+ + H2O [1,33,34]. Consequently, the front part of the curve appearing at high frequency is a semicircle, which indicates that it is controlled by kinetics, and the latter part of the curve appearing at low frequency is a 45° line, which is controlled by mass transfer. Thus, this process is controlled by both charge transfer and diffusion. Moreover, at higher RH, all the gaps are filled with water molecules and the physisorbed water layers show liquid-like behavior. Based on the model of the ion transport mechanism reported by Casalbore-Miceli et al. [47], ions can dissolve in the absorbed water, and the dielectric constant, which is a function of the absorbed water, renders them free from the interaction of the opposite charges. Therefore, more ions could transfer freely in the entire nanostructure, resulting in a sharp decrease in impedance, which is expressed in the impedance spectra where a part of the semicircle fades away and the straight line becomes obvious. During the humidity sensing process, SnO2, as an n-type semiconductor, the absorbed oxygen molecules on the SnO2 surface can be ionized to O2−, O−, or O2− species by obtaining electrons from the SnO2 needles because the number of electrons is larger than the number of holes [8,30]. However, this ionization process cannot occur on the surface of NaNbO3, which is a p-type semiconductor. The molecules of reducing gas react with negative oxygen ions and release electrons back to SnO2 when the sensor is exposed in methane or other release gases environments, so the impedance declines. Because NaNbO3 is more sensitive to humidity than the chosen release gas, the sensor expresses good selectivity.
Fig. 10. Nyquist plots of a sensor containing the SnO2/NaNbO3 composite with Sn/Nb = 1:0.4 at different RHs. −
competes with pre-adsorbed oxygen species (e.g., O2−, O−, O2 ) for the absorption sites on the nano-SnO2 surface. As the number of water molecules increases, the surface of the nano-SnO2 becomes wetted and a chemisorbed water layer is formed [11,37,45]. For NaNbO3, the hydrophilic Na–O bonds are sensitive to water molecules through electrostatic interactions and are likely to react with the chemisorbed water that covers the surface of the cubic NaNbO3. Furthermore, the impedance decreases and the absorption sites for water increase due to the presence of Na+, which favorably impacts the formation of chemisorbed water layers [22,25]. Using Fig. 10, a detailed interaction process between the sensing material and water under varying humidity conditions can be described as follows: (1) at low RH (11% and 22% RH), only a few molecules are absorbed in the gaps between the NaNbO3 and SnO2, primarily forming chemisorbed water layers. Then, the absorbed water molecules ionize into H3O+ and OHˉ ions. OHˉ absorbs on the surfaces of the Nb and Sn atoms to form a hydroxyl group layer, as suggested by the abovementioned DFT model calculations [46]. This layer benefits the subsequent absorption of water molecules that combine with the hydroxyl groups to further form a physically absorbed water layer. Since ion transport is difficult in the chemisorbed water layers, the intrinsic impedance spectra present an almost straight line and the impedance
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Rep. 3 (2013) 2714. [33] J.-H. Lee, et al., Extraordinary improvement of gas-sensing performances in SnO2 nanofibers due to creation of local p–n heterojunctions by loading reduced graphene oxide nanosheets, ACS Appl. Mat. Interfaces. 7 (2015) 3101–31034. [34] D. Zhang, J. Tong, B. Xia, Humidity-sensing properties of chemically reduced graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano selfassembly, Sens. Actuators, B (2014) 66–72. [35] L. Li, et al., Structure and humidity sensing properties of SnO2 zigzag belts, Cryst. Res. Technol. 45 (2010) 539–544. [36] D. Zhang, et al., Facile fabrication of MoS2-modified SnO2 hybrid nanocomposite for ultrasensitive humidity sensing, ACS Appl. Mat. Interfaces 8 (2016) 14142–14149. [37] Q. Kuang, et al., High-sensitivity humidity sensor based on a single SnO2 nanowire, J. Am. Chem. Soc. 129 (2007) 6070–6071. [38] S. Lin, et al., Raman scattering investigations of the low-temperature phase transition of NaNbO3, J. Raman Spectrosc. 37 (2006) 1442–1446. [39] S. Yamazoe, et al., Needle-like NaNbO3 synthesis via Nb6O198−cluster using Na3NbO4 precursor by dissolution–precipitation method, Chem. Lett. 42 (2013) 380–382. [40] A. Dias, V. Ciminelli, Fundamental thermodynamic studies as predictive tools on the behavior of ceramic materials in different hydrothermal environments, J. Solut. Chem. 38 (2009) 843–856. [41] R.R. Pereira, et al., Nanostructured rare earth doped Nb2O5: structural, optical properties and their correlation with photonic applications, J. Lumin. 170 (2016) 707–717. [42] M.C. Payne, et al., Iterative minimization techniques for ab initio total-energy calculations: molecular dynamics and conjugate gradients, Rev. Mod. Phys. 64 (1992) 1045. [43] K. Kośmider, R. Kucharczyk, L. Jurczyszyn, CO adsorption on the Ni2Pb/Ni (1 1 1) surface alloy: ADFT study, Appl. Surf. Sci. 267 (2013) 4–7. [44] V. Orazi, et al., DFT study of methanol adsorption on PtCo (111), Appl. Surf. 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The better humidity performance of the NaNbO3-doped humidity sensors (especially with 0.4 mol% NaNbO3), including the response and recovery times as well as responses, compared to that of the pure SnO2, may be attributed to their urchin-like structures in which NaNbO3 nano-cubes stack to form a porous skeleton and with needle SnO2 inlays [33]. The urchin-like structure not only enables permeation of the water molecules in the skeleton to form a water absorption layer and fill the gaps between the inner adjacent NaNbO3 cubes, but facilitates the contact of SnO2 with NaNbO3 to form p-n junctions. A heterojunction is proposed at the interface of the p-type semiconductor NaNbO3 and ntype semiconductor SnO2; upon the adsorption of water molecules, electrons would pass more easily to the p-n junction and further contribute to electron transfer from SnO2 to NaNbO3. This process leads to a sharp decrease in impedance and the enhanced sensing response as well as responses under different humidity conditions. 4. Conclusions We used the hydrothermal method to fabricate SnO2/NaNbO3 nanocomposites, characterized their structure, determined their humiditysensing properties, and explained their fabrication and sensing mechanisms using analog computational methods and complex impedance plots, respectively, while considering their heterojunction behavior. The Sn/Nb = 1:0.4 nanocomposite possessed an urchin-like structure with a large specific surface area that facilitated the adsorption of water molecules, resulting in good sensing properties, including a high response (S = 4823.8), rapid response and recovery times (3/9 s), good stability, linearity, and remarkable selectivity, which were superior to those of the pure SnO2 sensor. The improved humidity-sensing properties indicate the potential application of these NaNbO3-based sensors in different fields. Moreover, the results reveal the influence of humidity on the electrical properties of a NaNbO3-doped ceramic system. Acknowledgements This work was partially supported by Natural Science Foundation of China (Grant No. 51772168). References [1] B.M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc. 74 (1991) 697–708. [2] Z. Chen, C. Lu, Humidity sensors: a review of materials and mechanisms, Sens. lett. 3 (2005) 274–295. [3] Z. Ren, et al., Hierarchically nanostructured materials for sustainable environmental applications, Front. Chem. 1 (2013) 18. [4] M. Wu, et al., Hydrothermal synthesis of SnO2 nanocorals, nanofragments and nanograss and their formaldehyde gas-sensing properties, Mat. Sci. Semicond. Proc. 16 (2013) 1495–1501. [5] Z. Li, et al., Highly sensitive and stable humidity nanosensors based on LiCl doped TiO2 electrospun nanofibers, J. Am. Chem. Soc. 130 (2008) 5036–5037. [6] D. Zhang, et al., Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film, Sens. Actuators, B. 203 (2014) 263–270. [7] Q. Lin, Y. Li, M. Yang, Tin oxide/graphene composite fabricated via a hydrothermal method for gas sensors working at room temperature, Sen. Actuators, B. 173 (2012) 139–147. [8] P. Sun, et al., Dispersive SnO2 nanosheets: hydrothermal synthesis and gas-sensing properties, Sens. Actuators, B. 156 (2011) 779–783. [9] M. Zhang, et al., Hydrothermally synthesized SnO2-graphene composites for H2 sensing at low operating temperature, Mater. Sci. Eng. B. 209 (2016) 37–44. [10] G. Chen, et al., High-energy faceted SnO2-coated TiO2 nanobelt heterostructure for near-ambient temperature-responsive ethanol sensor, ACS Appl. Mat. Interfaces. 7 (2015) 24950–24956. [11] N.M. Sin, et al., Fabrication of nanocubic ZnO/SnO2 film-based humidity sensor with high sensitivity by ultrasonic-assisted solution growth method at different Zn: Sn precursor ratios, Appl. Nanosci. ANPACY Q. 4 (2014) 829–838. [12] Y. Zhen, et al., Ultrafast breathing humidity sensing properties of low-dimensional Fe-doped SnO2 flower-like spheres, RSC Adv. 6 (2016) 27008–27015. [13] X. Wang, H. Fan, P. Ren, Self-assemble flower-like SnO2/Ag heterostructures: correlation among composition, structure and photocatalytic activity, Colloids Surf. A. 419 (2013) 140–146.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
An urchin-like SnO2/NaNbO3 nanocomposite with stable humidity-sensing properties at room temperature
T
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Jiuyang Zhang, Yuhua Zhen , Haoyue Xue, Xiaoxin Gao, Wenxin Wang, Yingda Li, Tasawar Hayat, Njud S. Alharbi College of Material Science and Engineering, China University of Petroleum, QingDao 266580 China
A R T I C LE I N FO
A B S T R A C T
Keywords: Urchin-like structure Hydrothermal method SnO2 NaNbO3 Humidity
An urchin-like nano-composite of SnO2/NaNbO3 was fabricated by the hydrothermal method and the influence of the Sn/Nb ratio in the samples on their humidity-sensing properties was investigated. The phases and morphologies of these composites and their elemental distributions were analyzed by X-ray diffraction scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The sensor based on the optimum Sn/Nb ratio of 1:0.4 exhibited remarkable humidity-sensing properties, including a good response (S = 4823.8) that was 8.72 times higher than that of the pure SnO2 sensor (S = 553.01). The composites showed rapid response and recovery times (3/9 s), as well as good stability, linearity, and excellent selectivity. The fabrication and humiditysensing mechanisms were systematically analyzed using analog computations and Nyquist diagrams, respectively. Compared with pure NaNbO3 and SnO2 sensors, our easily prepared SnO2/NaNbO3 sensor demonstrates good sensing properties and holds great promise for use in humidity-sensing applications.
1. Introduction Humidity sensors play an important role in monitoring and adjusting the environmental humidity in various scientific fields such as biology, chemistry, and physics [1]. The use of these sensors has gradually expanded to more applications and research fields over time [2,3]. However, owing to the complex environments of laboratories and factories, a variety of gases and vapors may be present at room temperature. Therefore, the demand for stable and sensitive humidity sensors, with good response/recovery times and selectivity, as well as other properties, is increasing [4]. Hitherto, myriad materials such as polymers, metal oxides, and carbon nanostructures have been innovatively employed to fabricate humidity sensors [5,6]. Among these, SnO2, an important n-type semiconductor with a wide band gap (Eg = 3.6 eV at 300 K), has attracted much attention owing to its simple structure, high sensitivity, favorable selectivity, and lower cost as compared with ZnO, In2O3, and other metal oxide semiconductors [7,8]. Moreover, SnO2 exhibits conductivity at or below room temperature [9]. Composites of SnO2 with TiO2, ZnO, and Fe, have been previously reported, and all displayed good sensing properties under
different conditions [10–13]. Hence, SnO2 doped or composited with other substances has been widely explored in the research and development of gas or humidity sensors [14–17]. Sodium niobate (NaNbO3), which has a perovskite structure, is used to produce energy storage devices and piezoelectric regulators owing to its anti-ferroelectric-toferroelectric phase transition [18–20]. In previous studies, humidity was found to influence the electrical properties of NaNbO3 system piezoceramics [21,22], suggesting that a relationship between relative humidity and NaNbO3 behavior possibly exists. Moreover, because of its high chemical stability, NaNbO3 is also used in sensors, and has excellent sensing properties, especially toward humidity. Hence, it has attracted considerable attention for its potential as a stable sensor material [23–25]. Nanoline NaNbO3 has been fabricated by complex methods, and owing to its unique electrical properties, shows large impedance in sensing tests. This inspired us to investigate its potential for enhancing sensor performance [26,27]. Materials with urchin-like structures have been produced previously by the hydrothermal method [28,29]. In such structures, a needle or slice of one material is inlayed or inserted in the ball-like skeleton of another material, producing an increased specific surface area.
Abbreviations: BFGS, Broyden–Fletcher–Goldfarb–Shanno method; DFT, density functional theory; EDS, energy-dispersive spectroscopy; GGA, generalized gradient approximation; IDE, interdigitated electrode; OTFG, on-the-fly-generated; PBE, Perdew–Burke–Ernzerhof functional; RH, relative humidity; RT, room temperature; SCF, self-consistent field; XRD, X-ray powder diffraction ⁎ Corresponding author. E-mail address: [email protected] (Y. Zhen). https://doi.org/10.1016/j.snb.2018.12.035 Received 19 July 2018; Received in revised form 3 December 2018; Accepted 8 December 2018 Available online 10 December 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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Although the composition of the urchin-like structure is non-homogeneous, its structure facilitate can contact with the tested gas as well as electron transport. Thus, the material with the urchin-like structures can display good properties in gas sensing. In this paper, we report the novel urchin-like structures of SnO2/ NaNbO3 composite-based humidity sensors prepared through a simple hydrothermal method [7,27,30,31]. We composited NaNbO3 with SnO2 in different ratios and explored the resulting physical phases [23,32]. The reaction activity of Nb toward OHˉ is higher than that of Sn, as demonstrated by experiments and theoretically verified by analog computations. A heterojunction structure was formed between the NaNbO3 and SnO2, thereby remarkably enhancing the electrical properties [31,33]. In addition, the fabrication and sensing mechanisms of the composite are discussed based on analog computations and Nyquist diagrams, respectively. The results and related analyses shed light on the impact of humidity on SnO2/NaNbO3, and project its potential for high-humidity sensing and other selective sensing applications.
Fig. 1. Structure of humidity sensor.
sensor (containing the IDE) with a digital electrical bridge instrument, Mydream Electronic, ModelLCR-TH2828. The response/recovery time (T) and response (S) are both normalized evaluation figures for the performance of the humidity sensors. T is the time taken by the sensor to attain 90% of its final value. In this paper, for the humidity sensing tests, S is defined as S = |ZRH11|/|ZRHy|, where ZRH11 and ZRHy are the impedance of the sensor at 11% and y% (y = 95) RH levels, respectively [36,37]. The humidity surrounding of gas sensing testing experiment is 11% RH all the time, so S is defined as S = |Zair|/|Zgas| [14].
2. Experimental procedures 2.1. Materials and sample fabrication In this work, we synthesized NaNbO3/SnO2 nanocomposites with different ratios of constituents by the hydrothermal method. All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). They were of analytical grade and used without further purification. To identify the optimal alkali content, SnCl4·5H2O (7.5 mmol) was added to absolute ethanol (40 mL) and NaOH (60 mmol) was dissolved in deionized water (40 mL). Then, varying amounts of Nb2O5 (x mol%; x = 0.2, 0.4, 0.6, 0.8, and 1) were then dissolved in the abovementioned ethanol solution. Each alcohol solution was added dropwise to the alkali solution under magnetic stirring for 20 min until evenly mixed. Then, the solution was sonicated at 50 °C for 30 min and transferred to a 180 mL Teflon-lined stainless steel autoclave, where it was heated at 180 °C for 6 h. After cooling to room temperature (RT) naturally, the precipitate was transferred into a centrifuge tube and centrifugally washed several times with deionized water and absolute ethanol. Finally, it was dried at 60 °C for 6 h and calcined at 450 °C for 3 h in a muffle furnace.
3. Results and discussion 3.1. Crystal structure of SnO2/NaNbO3 SnO2/NaNbO3 nanopowders with different Sn/Nb ratios prepared by the hydrothermal method were characterized by XRD, and the profiles are shown in Fig. 2. The diffraction peaks of SnO2 are identical to the standard values in the JCPDS reference card (PDF#41-1445), while the peaks due to NaNbO3 in the as-prepared SnO2/NaNbO3 composites are in good agreement with those in JCPDS card PDF#191221 [4,38]. All the samples exhibit relatively strong diffraction peaks, suggesting that the hydrothermally prepared crystals have good crystallinity. With increasing Sn4+/Nb5+ molar ratios, the peaks undergo a series of changes. When the Sn/Nb ratio is 1:0.4, two substantial sets of peaks coexist. As the content of Nb increases to 0.6, the SnO2 peaks are replaced by Na3NbO4 (PDF#22-1391), which is an intermediate product that can continue to react with OHˉ to generate NaNbO3 [39,40]. With increasing Nb content (1:0.8 ratio and higher), the SnO2 peaks disappear completely while Nb2O5 (PDF#19-0862) peaks appear [41].
2.2. Material characterization To investigate the purity of the samples and to analyze their crystal structures, X-ray powder diffraction (XRD) experiments were conducted on a Dandong DX-2700 X-ray diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). The microscopic morphologies and the distribution of elements in the samples were recorded by field-emission scanning electron microscopy (FESEM, Hitachi S-4800) with energy-dispersive spectroscopy (EDS). 2.3. Humidity sensing and sensor properties The sample powder was dispersed in absolute ethanol in a weight ratio of 20:1 to form slurry, and then drop-coated onto a pretreated interdigitated electrode (IDE), which had been ultrasonically cleaned using acetone, deionized water, and ethanol several times (Fig. 1). One end of a copper wire was polished and adhered to the IDE by lead-free soldering, and a solder ball was welded on the other end. The IDE was then dried at 120 °C for 1 h. Different humidity atmospheres were produced by dissolving LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, KI, NaCl, KCl, and K2SO4 in an airtight vessel at 45 °C, respectively, and then cooling to RT to form saturated solutions to obtain approximately 11, 22, 33, 43, 54, 69, 75, 84, and 95% relative humidity (denoted as RH) levels [34,35]. The various impedance responses of the sensors in the different humidity environments were assessed by connecting the
Fig. 2. XRD patterns of the nanocomposites prepared with different ratios of Sn and Nb: (a) 1:1, (b) 1:0.8, (c) 1:0.6, and (d) 1:0.4. 644
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Fig. 3. (a, b, and c) SEM images and (d) EDS map of the SnO2/NaNbO3 composite with Sn/Nb = 1:0.4.
shown in Fig. 3(d). In the chosen area, an evident cluster of needles inlaid on the surface of the internal cubic structure can be observed. During hydrothermal synthesis, the crystal nuclei are distributed nonuniformly before crystal growth occurs. When characterized by EDS, the penetrability of the electron beam enables it to pierce the needles inlaid on the surface; thus, the elements and their distribution were obtained for both parts of the urchin-like structure. In the selected area, Sn is distributed densely in the needlelike structures, whereas Nb is distributed densely on the cubic structures and is absent in the needlelike structures. Na and O exist equally in both parts. Combined with the result of Fig. 2 (XRD), it is obvious that the main products in the composite with Sn/Nb = 1:0.4 are SnO2 and NaNbO3. Therefore, needlelike SnO2 is generated and inlaid on the cubic NaNbO3 to form the urchin-like structure. The thorny structure dramatically improves the specific surfaces area and increases the contact with water molecules, enabling remarkable humidity-sensing performance. Fig. 4. Impedance changes as a function of humidity at different frequencies for the SnO2/NaNbO3 composite with Sn/Nb = 1: 0.4.
3.2. Choice of testing frequency The electrical bridge instrument was used to test the Sn/Nb = 1:0.4 sample at different frequencies to obtain a plot of impedance as a function of humidity (Fig. 4). The AC frequency can impact the electrical properties of a sensor. Because the net polarization of the water molecules changes as the AC frequency decreases, dipole generation is more difficult, and the vibrational relaxation of the water molecules will be limited. Thus, the sensors show humidity insensitivity. Therefore, it was necessary to first determine an appropriate frequency for humidity-sensing testing. Fig. 4 shows that the impedance decreases with an increase in RH from 11% to 95% for all the chosen frequencies, among which the most varied impedance changed by five orders of magnitude from 10 MΩ to 1 kΩ. At low humidity levels, the impedance decreases more gently, and the impedance shows a linear change at high frequencies. This indicates that the sensitivity at high RH is lower than at low RH. Among the various measurement frequencies, the relationships differ. With increasing frequency, the impedance value decreases overall, but the change is more significant at low frequencies. One of the criteria for selecting a sensor’s testing frequency is a linear relationship between impedance and humidity. Better linearity results
The diffraction peaks of NaNbO3, as the product of the reaction between Nb2O5 and NaOH, are present throughout the phase transformation. However, the peaks due to SnO2 decrease, and those due to Na3NbO4 (another intermediate, produced from NaNbO3) and the original reactant Nb2O5 evolve as the Nb2O5 doping content increases. In a fixed reaction system with a certain amount of NaOH, these changes illustrate that Nb5+ competes with Sn4+ to combine with OHˉ. With increasing Nb5+ content, the Nb5+ preferentially reacts with OHˉ, and thus, the quantity of OHˉ reacting with Sn4+ decreases. Thus, the diffraction peaks due to SnO2 disappear gradually. This was also verified computationally, by comparing the adsorption energies of the hydroxyl ion on the respective Nb and Sn surfaces. Morphological examination of the ratio of 1:0.4 nanocomposites reveals an urchin-like structure; SEM images are presented in Fig. 3(a–c). The inner skeleton of the structure comprises a stack of cubic NaNbO3, with needlelike SnO2 inlaid on its surface. The length of the cube is about 500 nm, whereas the needles are 6.5 μm in length and 150 nm in width in all orientations. The elemental distributions are
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impedance recovers to its initial value after responding to the gases, especially in dry gas atmospheres. 3.4. Computational details for formation mechanism We next considered the mechanism of composite formation. To verify that Nb2O5 preferentially reacts with OHˉ versus SnCl4·5H2O, the OHˉ adsorption behavior on both surfaces was studied using density functional theory (DFT). The Nb2O5 (100) and SnCl4·5H2O (100) surfaces were chosen to examine the adsorption of OHˉ. As shown in Fig. 8(a) and (b), the Nb2O5 (100) surface was modeled by 183 atoms, and the SnCl4·5H2O (100) surface by 232 atoms. In both models, the atoms between the red and black dashed lines were relaxed and regarded as surface atoms, whereas the atoms below the black dashed line were fixed and considered as bulk-phase atoms. The surfaces were separated by a 15 Å vacuum region to ensure that there was no significant interaction between the slabs. Fig. 8(c) and (d) show three high-symmetry adsorption sites (top site (T site), bridge site (B site), and hollow site (H site)) on the Nb2O5 (100) and SnCl4·5H2O (100) surfaces, respectively, where the adsorption of OHˉ atoms was allowed. All calculations were performed using DFT within the CASTEP (Cambridge Sequential Total Energy Package) code [42]. The exchange and correlation potentials were constructed by the generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE) functional. To indicate the interactions between the valence electrons and ionic core, on-the-fly-generated (OTFG) ultrasoft pseudopotentials were employed. Before property calculations and geometry relaxation, the convergence criteria for structure optimization and energy calculations were set as follows: the convergence value of the average total energy of atoms was 1 × 10−5 eV/atom, the maximum force that one atom was subjected to 0.03 eV/Å, the maximum stress was 0.05 GPa, the maximum displacement of an atom was 0.001 Å, and the self-consistent field (SCF) convergence threshold was 1 × 10-6 eV/atom. All atoms in the considered systems were relaxed using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method, which was treated as the minimization algorithm. A 3 × 3 × 1 Monkhorst-Pack k-point grid was specified in the calculation. The cut-off energy of the plane-wave basis set was set as 490 eV. The adsorption energy Eads can reflect the combining capacity between OHˉ and the surface. The following formula was used to compute the adsorption energy [43]:
Fig. 5. Response and recovery curves in different humidity environments.
in more-stable measurements, and better measurement precision improves practical applicability. Therefore, by linearizing these data and calculating the goodness of fit, we determined that at 1000 Hz, the impedance and RH exhibit the best linearity. Consequently, 1000 Hz was chosen as the optimal frequency to test the humidity-sensing properties of the samples. 3.3. Humidity-sensing properties of SnO2/NaNbO3 The optimal frequency was used to test the humidity-sensing properties of the SnO2/NaNbO3 samples. Each sample was placed in an environment of 11% RH for 1 min, transferred to a 22% RH environment for 1 min, and then back to the 11% RH environment. The above process was repeated until the humidity reached 95%. The data in Fig. 5 show the relationship between the impedance of the samples and time from 11% to 95% RH. SnO2 does not exhibit sensing stability at any humidity level. Although NaNbO3 is more stable than SnO2, its response and recovery in all RHs are unsatisfactory. Both pure substances are similar in terms of humidity sensitivity, in that the impedance does not recover to the initial value after responding to humidity. This phenomenon shows that both single substances sense humidity by chemisorbing water molecules. Compared with the single materials, the SnO2/NaNbO3 composites exhibit significantly superior properties. When Sn/Nb = 1:0.4, obvious response behavior is evident from 33% RH, a relatively lowhumidity environment, and the sensor is faster (T = 3/9 s) compared to the pure SnO2 material (6/21 s), as calculated from Fig. 6. Furthermore, NaNbO3 doping not only affects the response and recovery, but also improves the sensor’s response, with the Sn/Nb = 1:0.4 composite (S = 4823.8) exhibiting a value 8.72 times larger than that of SnO2 (S = 553.01). Moreover, not only the T and S values, but also the sensing range and stability improve. The difference between pure SnO2 and NaNbO3 lies in whether or not the impedance can recover to its initial value. However, as the doping amount continues to increase, the stability and repeatability of the sensors are reduced, and the impedance does not recover to its initial value. Hence, the optimal composition of Sn/Nb = 1:0.4 was used in the following tests. The response sensitivities of the different SnO2/NaNbO3 sensors to humidity are shown in Fig. 7(a). All of the composites except for 1:0.4 SnO2/NaNbO3 display response sensitivities on the order of 101. Based on the XRD analysis (Fig. 2), this may be attributed to incomplete formation of the SnO2/NaNbO3 composite during hydrothermal synthesis. The humidity sensor with Sn/Nb = 1:0.4 was exposed to different atmospheres to measure its selectivity toward various gases; Fig. 7(b) shows these responses. Compared with other gases, the sensor exhibits excellent response and recovery properties when exposed to humidity. To better quantify this behavior, the response of the sensor toward various gases was calculated (Fig. 7(c)). The response to humidity (103) was much higher than that for other gases (∼10°), which indicates that the composite possesses significant gas selectivity. In addition, the response and recovery curves in Fig. 7(b) show that the
Eads = EOH+sur – (EOH + Esur)
(1)
where EOH + sur is the energy of OHˉ interacting with the Nb2O5 (100) or SnCl4·5H2O (100) surface, Esur is the total energy of the clean surface, and EOH is the energy of OHˉ. To investigate the bonding mode for OHˉ adsorption on the surfaces, the electron density difference, Δρ(r), was calculated using the following formula [44]: Δρ(r) = ρtotal(r) – ρM(r) – ρsurf(r)
(2)
where ρtotal(r), ρM(r), and ρsurf(r) are the electron densities of the adsorption system, isolated ion (OHˉ), and substrate surfaces, respectively. 3.4.1. DFT calculations In these calculations, we considered all the possible adsorption sites for OHˉ on the Nb2O5 (100) and SnCl4·5H2O (100) surfaces. However, only the T site was a stable adsorption site for OHˉ on either surface. Table 1 lists the adsorption energies, bond distances, and angles between the OHˉ and surfaces after adsorption. Clearly, OHˉ adsorption on the Nb2O5 (100) or SnCl4·5H2O (100) surface is favorable in the top sites (Table 1) because the signs of the adsorption energies are negative. The adsorption energy on the Nb2O5 (100) surface is −6.35 eV, whereas that on the SnCl4·5H2O (100) surface is −2.24 eV. The high 646
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Fig. 6. Response and recovery curves for different ratios of SnO2/NaNbO3 composites as well as the pure samples in 11% RH and 95% RH.
Fig. 7. Responses of different samples (a) and the composite with Sn/Nb = 1:0.4 to several gases (c), as well as the selectivity of the latter (b). 647
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Fig. 8. Models and adsorption sites of Nb2O5 (100) and SnCl4·5H2O (100) surfaces: (a) side view of the Nb2O5 (100) surface; (b) side view of the SnCl4·5H2O (100) surface; (c) top view of the Nb2O5 (100) surface; and (d) top view of the SnCl4·5H2O (100) surface.
3.5. Humidity-sensing mechanism
Table 1 Adsorption data for OHˉ on the Nb2O5 (100) and SnCl4·5H2O (100) surfaces. Surface
Stable adsorption site
Adsorption energy (eV)
Bond length (Å)
Nb2O5 (100) SnCl4·5H2O (100)
Top Top
−6.35 −2.24
1.896 2.019
To better understand the sensing mechanism, complex impedance plots of the 1:0.4 SnO2/NaNbO3 sensor from 320 Hz to 10 kHz in different RH environments were analyzed (Fig. 10). The electrical properties of a humidity sensor are related to the water adsorption and desorption processes that take place on the active sites of the sensing material, and are significantly related to the surface area exposed to the environment. There are two kinds of absorbed water molecules, i.e., chemisorbed and physisorbed. When water molecules reach the surface of a material, they are chemisorbed first, and the chemisorbed layers exhibit no relationship with the RH. With the introduction of humidity, the chemisorbed layer is covered by physisorbed layers, which are easily affected by humidity. Moreover, considering the almost fully recovered sensing performance of the sensors mentioned above, physisorption is the dominant part of the interaction process between the water molecules and the material surface. This is the reason why the chemisorbed water molecules on the surface cannot be completely removed at room temperature [2]. As humidity is measured at room temperature, the mechanism by which absorbed water molecules react with the Lewis acid and base sites on the SnO2 surface to release electrons is not operative. At room temperature, water physisorption
adsorption energy indicates that OHˉ can easily react with the Nb2O5 (100) surface. In addition, the bond length of OeNb is 1.896 Å, whereas that of OeSn is 2.019 Å; a shorter bond length indicates that the Nb2O5 (100) surface has stronger adsorptivity. Fig. 9 shows the differences in the electron densities of the OHˉ ions adsorbed on the top sites for each surface. Clearly, electrons are transferred from the substrate to the adsorbate when adsorption occurs. This indicates that the adsorption of OHˉ on both surfaces is via chemisorption. In conclusion, the calculated results verify that the chemical reactivity of Nb2O5 toward OHˉ is higher than that of SnCl4·5H2O, which explains the series of changes observed in the X-ray diffraction patterns.
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Fig. 9. Electron-density difference views around OHˉ adsorbed on (a) Nb2O5 (100) and (b) SnCl4·5H2O (100) surfaces. Blue and yellow regions indicate increasing and decreasing electron density, respectively. The value of the isosurface is ± 0.03 e/Å3 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
values are extraordinarily high. (2) As the RH increases from 33% to 69% RH, the additional water molecules lead to larger numbers of hydroxyl ions and protons, and charge transport is secured by protonhopping between the chemisorbed hydroxyl groups. Simultaneously, water molecules become polarized to form the polarized charges involved in conduction. Hence, the straight line gradually becomes curved and, eventually, semicircular. During this period, the electrochemical reaction is controlled by charge transfer, while diffusion can be neglected. (3) When the RH further increases to 75%, physisorbed layers are formed on the chemisorbed layers. At this stage, H2O molecules are autodissociated into H3O+ and OHˉ, and H3O+ tends to be the major charge carrier. In the physisorbed layers, H3O+ releases a proton to the neighboring water molecules, which accept it while releasing another proton and so forth; consequently, the proton can move freely in the layers. This exchange is known as the Grotthuss chain mechanism: H2O + H3O+ = H3-O+ + H2O [1,33,34]. Consequently, the front part of the curve appearing at high frequency is a semicircle, which indicates that it is controlled by kinetics, and the latter part of the curve appearing at low frequency is a 45° line, which is controlled by mass transfer. Thus, this process is controlled by both charge transfer and diffusion. Moreover, at higher RH, all the gaps are filled with water molecules and the physisorbed water layers show liquid-like behavior. Based on the model of the ion transport mechanism reported by Casalbore-Miceli et al. [47], ions can dissolve in the absorbed water, and the dielectric constant, which is a function of the absorbed water, renders them free from the interaction of the opposite charges. Therefore, more ions could transfer freely in the entire nanostructure, resulting in a sharp decrease in impedance, which is expressed in the impedance spectra where a part of the semicircle fades away and the straight line becomes obvious. During the humidity sensing process, SnO2, as an n-type semiconductor, the absorbed oxygen molecules on the SnO2 surface can be ionized to O2−, O−, or O2− species by obtaining electrons from the SnO2 needles because the number of electrons is larger than the number of holes [8,30]. However, this ionization process cannot occur on the surface of NaNbO3, which is a p-type semiconductor. The molecules of reducing gas react with negative oxygen ions and release electrons back to SnO2 when the sensor is exposed in methane or other release gases environments, so the impedance declines. Because NaNbO3 is more sensitive to humidity than the chosen release gas, the sensor expresses good selectivity.
Fig. 10. Nyquist plots of a sensor containing the SnO2/NaNbO3 composite with Sn/Nb = 1:0.4 at different RHs. −
competes with pre-adsorbed oxygen species (e.g., O2−, O−, O2 ) for the absorption sites on the nano-SnO2 surface. As the number of water molecules increases, the surface of the nano-SnO2 becomes wetted and a chemisorbed water layer is formed [11,37,45]. For NaNbO3, the hydrophilic Na–O bonds are sensitive to water molecules through electrostatic interactions and are likely to react with the chemisorbed water that covers the surface of the cubic NaNbO3. Furthermore, the impedance decreases and the absorption sites for water increase due to the presence of Na+, which favorably impacts the formation of chemisorbed water layers [22,25]. Using Fig. 10, a detailed interaction process between the sensing material and water under varying humidity conditions can be described as follows: (1) at low RH (11% and 22% RH), only a few molecules are absorbed in the gaps between the NaNbO3 and SnO2, primarily forming chemisorbed water layers. Then, the absorbed water molecules ionize into H3O+ and OHˉ ions. OHˉ absorbs on the surfaces of the Nb and Sn atoms to form a hydroxyl group layer, as suggested by the abovementioned DFT model calculations [46]. This layer benefits the subsequent absorption of water molecules that combine with the hydroxyl groups to further form a physically absorbed water layer. Since ion transport is difficult in the chemisorbed water layers, the intrinsic impedance spectra present an almost straight line and the impedance
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The better humidity performance of the NaNbO3-doped humidity sensors (especially with 0.4 mol% NaNbO3), including the response and recovery times as well as responses, compared to that of the pure SnO2, may be attributed to their urchin-like structures in which NaNbO3 nano-cubes stack to form a porous skeleton and with needle SnO2 inlays [33]. The urchin-like structure not only enables permeation of the water molecules in the skeleton to form a water absorption layer and fill the gaps between the inner adjacent NaNbO3 cubes, but facilitates the contact of SnO2 with NaNbO3 to form p-n junctions. A heterojunction is proposed at the interface of the p-type semiconductor NaNbO3 and ntype semiconductor SnO2; upon the adsorption of water molecules, electrons would pass more easily to the p-n junction and further contribute to electron transfer from SnO2 to NaNbO3. This process leads to a sharp decrease in impedance and the enhanced sensing response as well as responses under different humidity conditions. 4. Conclusions We used the hydrothermal method to fabricate SnO2/NaNbO3 nanocomposites, characterized their structure, determined their humiditysensing properties, and explained their fabrication and sensing mechanisms using analog computational methods and complex impedance plots, respectively, while considering their heterojunction behavior. The Sn/Nb = 1:0.4 nanocomposite possessed an urchin-like structure with a large specific surface area that facilitated the adsorption of water molecules, resulting in good sensing properties, including a high response (S = 4823.8), rapid response and recovery times (3/9 s), good stability, linearity, and remarkable selectivity, which were superior to those of the pure SnO2 sensor. The improved humidity-sensing properties indicate the potential application of these NaNbO3-based sensors in different fields. Moreover, the results reveal the influence of humidity on the electrical properties of a NaNbO3-doped ceramic system. Acknowledgements This work was partially supported by Natural Science Foundation of China (Grant No. 51772168). References [1] B.M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc. 74 (1991) 697–708. [2] Z. Chen, C. Lu, Humidity sensors: a review of materials and mechanisms, Sens. lett. 3 (2005) 274–295. [3] Z. Ren, et al., Hierarchically nanostructured materials for sustainable environmental applications, Front. Chem. 1 (2013) 18. [4] M. Wu, et al., Hydrothermal synthesis of SnO2 nanocorals, nanofragments and nanograss and their formaldehyde gas-sensing properties, Mat. Sci. Semicond. Proc. 16 (2013) 1495–1501. [5] Z. Li, et al., Highly sensitive and stable humidity nanosensors based on LiCl doped TiO2 electrospun nanofibers, J. Am. Chem. Soc. 130 (2008) 5036–5037. [6] D. Zhang, et al., Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film, Sens. Actuators, B. 203 (2014) 263–270. [7] Q. Lin, Y. Li, M. Yang, Tin oxide/graphene composite fabricated via a hydrothermal method for gas sensors working at room temperature, Sen. Actuators, B. 173 (2012) 139–147. [8] P. Sun, et al., Dispersive SnO2 nanosheets: hydrothermal synthesis and gas-sensing properties, Sens. Actuators, B. 156 (2011) 779–783. [9] M. Zhang, et al., Hydrothermally synthesized SnO2-graphene composites for H2 sensing at low operating temperature, Mater. Sci. Eng. B. 209 (2016) 37–44. [10] G. Chen, et al., High-energy faceted SnO2-coated TiO2 nanobelt heterostructure for near-ambient temperature-responsive ethanol sensor, ACS Appl. Mat. Interfaces. 7 (2015) 24950–24956. [11] N.M. Sin, et al., Fabrication of nanocubic ZnO/SnO2 film-based humidity sensor with high sensitivity by ultrasonic-assisted solution growth method at different Zn: Sn precursor ratios, Appl. Nanosci. ANPACY Q. 4 (2014) 829–838. [12] Y. Zhen, et al., Ultrafast breathing humidity sensing properties of low-dimensional Fe-doped SnO2 flower-like spheres, RSC Adv. 6 (2016) 27008–27015. [13] X. Wang, H. Fan, P. Ren, Self-assemble flower-like SnO2/Ag heterostructures: correlation among composition, structure and photocatalytic activity, Colloids Surf. A. 419 (2013) 140–146.
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