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Rapid sensitive sensing platform based on yolk-shell hybrid hollow sphere for detection of ethanol
Sensors and Actuators B 256 (2018) 479–487
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Research Paper
Rapid sensitive sensing platform based on yolk-shell hybrid hollow sphere for detection of ethanol Rui Zhang, Tong Zhang, Tingting Zhou, Lili Wang ∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 20 June 2017 Received in revised form 19 September 2017 Accepted 13 October 2017 Available online 5 November 2017 Keywords: Hollow nanostructure Double shells Hybrid materials Gas sensor High performance
a b s t r a c t The sensing layer composited from the inorganic particles, organic materials, polymers could be regarded as an important and functional component used in electronic devices, such as chemical sensor. Furthermore, it is very important for these materials to own hierarchical and porous structures leading to a large effective specific surface area. Among these, hybrid materials with yolk double-shelled architecture can further realize the required surface chemical reactions. However, it is rarely reported in gas sensors. Herein, a novel SnO2 -TiO2 hollow nanostructure consisting of double shells was successfully designed and synthesized. Due to the special multi-shelled structure and abundant hetero-interface, the sensing layer based on SnO2 -TiO2 realizes a rapid response rate within 1.7 s to ethanol gas and alluring reproducibility (15 days), which are superior over those for compact structure with similar sensors. The results indicated that the SnO2 -TiO2 yolk double-shelled microsphere have the huge potential for designing high performance practical ethanol devices in environmental monitoring and drunk driving. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, a great challenge to human beings was posed based on environmental problems, such as environment humidity, dust, toxic and hazardous gases. So, the rise of chemical sensors (CSs) is very important for monitoring the environmental change in real time [1–3]. For this purpose, high-performance chemical sensors are needed to be applied in various fields [4]. However, traditional gas sensing layers such as SnO2 , ZnO, and ␣-Fe2 O3 , suffered from several obstacles, for instance, poor sensitive selectivity, low catalytic activity, high cost, ultra-high working temperature and low electron conductivity [5–9]. To meet the ever-increasing demand for high performance sensing devices, it is imperative to design and fabricate efficient sensing materials that could provide a high sensing activity and effective utilization rate of surface area. As we know, the gas reaction and sensing performance are highly relied on the exposed surface area [10,11]. That is to say, a high specific surface area could achieve more effective active sites, leading to a higher sensitivity [12]. Hence, assembling tunable shell on the valuable core not only reduces the consumption of precious materials but also increases the dispersibility, functionality and stability [13]. Compared with the micro-sized particles, 0D structured nanoparticles possess the shorter diffusion length and
∗ Corresponding author. E-mail address: lili [email protected] (L. Wang). https://doi.org/10.1016/j.snb.2017.10.064 0925-4005/© 2017 Elsevier B.V. All rights reserved.
larger gas-sensing layer contact area. However, the low catalytic activity of single-component usually leads to poor sensing activity. Furthermore, hybrid structured nanomaterials usually exhibit higher reactive activity [14–17]. Inspired by the concept, we design a new type of SnO2 -TiO2 hybrid materials with yolk double-shelled spheres that were prepared by using resorcinol-formaldehyde (RF) without any other additional surfactants. Systematic studies indicated that RF can not only passivate the surface of metal oxide semiconductor (MOS) particles to avoid aggregating but also act as precursor to form RF resin, which reveals dual functions. In this study, ethanol sensing behavior was significantly improved through the design of SnO2 TiO2 yolk double-shelled heterostructure. The aim of this study is to explore morphological effects as well as ethanol sensing performance of SnO2 -TiO2 nanocomposites with an especial emphasis on improving sensing response and ultrafast response.
2. Experiment 2.1. Chemical materials Tin (II) sulfate (SnSO4 , 99%), titanium (IV) sulfate (TiSO4 , 96%), resorcinol (C6 H6 O2 , 99.5%) and formaldehyde solution (HCHO, 37–40%) were the initial chemicals without any further purification after received. Moreover, deionized water was also used in this experiment.
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2.2. Chemical synthesis Preparation of SnO2 -TiO2 yolk double-shelled microsphere: SnO2 TiO2 yolk double-shelled microsphere was prepared by a facile and simple template-assisted hydrothermal method with a slight modification [18]. Typically, 1.6 g of Ti(SO4 )2 , 0.86 g of SnSO4 and 0.64 g C6 H6 O2 were added into the mixed solution of 12 ml of deionized water and 1.6 ml of formaldehyde under continuous stirring. Then, the mixture was transferred into a 20 ml Teflon-lined stainless steel autoclave and heated at 85 ◦ C for uninterrupted 48 h. The obtained product was collected from the cooling autoclave and washed for 4 times using deionized water. After dried for overnight at 60 ◦ C, the powder was transferred to the muffle furnace and heating at 500 ◦ C for 3 h at a rate of 5 ◦ C/min. Preparation of SnO2 -TiO2 nanoparticles: Solid SnO2 -TiO2 nanoparticles (NPs) were prepared by a facile hydrothermal method. Typically, 1.6 g of Ti(SO4 )2 and 0.86 g of SnSO4 were added into the solution of 12 ml of deionized water and 1.6 ml of formaldehyde under continuous stirring. Then, the mixture was transferred into a 20 ml Teflon-lined stainless steel autoclave and heated at 85 ◦ C for uninterrupted 48 h. The obtained product was collected from the cooling autoclave and washed for 4 times using deionized water. After dried for overnight at 60 ◦ C, the powder was transferred to the muffle furnace and heating at 500 ◦ C for 3 h at a rate of 5 ◦ C/min. 2.3. Characterization X-ray diffraction (XRD) was used to identify the crystal structures of the as-synthesized materials using Cu K␣ radiation ( = 0.154 nm) on a Rigaku TTRIII X-ray diffractometer (40 kV). Field-emission scanning electron microscope (FESEM) and highresolution transmission electron microscope (HRTEM) were used to observe the micro-morphology on a JEOL JSM-7500F microscope and Tecnai G2 20S-Twin microscope, respectively. The composition of the as-synthesized products was tested using a PREVAC X-ray photoelectron spectroscopy (XPS) system. And Brunauer-EmmettTeller (BET) was carried on a JW-BK132F analyzer. 2.4. Fabrication and measurement of the gas sensor Indirect-heating structure sensor was used in this work to obtain sensing properties (Fig. S1). The typical fabrication process was described as follows: the as-synthesized composite powder was transferred to an agate mortar and mixed with deionized water to form a paste coated onto the surface of a tube, which is made from alumina. A pair of Au electrodes was printed on the each side of the tube and four platinum wires were installed at each end. A Ni-Cr alloy wire was inserted crossing the alumina tube and used to control the working temperature. Then, the tube was connected to a bakelite base. Next, the gas sensor was aged for 24 h in air in order to ensure the stability and repeatability. The response to target gases is defined as: S = Ra /Rg , where Rg is the electrical resistance in the target gas and Ra is the resistance in the atmosphere. And the response/recovery time was defined as the time cost of changes of 90% resistance. What’s more, in this experiment, the sensor was tested under the around 33% RH environment. 3. Results and discussion 3.1. Structural and morphological characteristics The smart designed of SnO2 -TiO2 composite has been illustrated in Scheme 1. The yolk double-shelled morphology results from the following processes: (1) formation of Sn2+ -Ti2+ @RF composite spheres by polymerization of the resorcinol and formaldehyde
along with the decomposition of tin and titanium salts; (2) combustion of polymeric component, decomposition or oxidation of the outer part of Sn2+ -Ti2+ @RF sphere into SnO2 -TiO2 outermost shell and then subsequent contraction of the inner Sn2+ -Ti2+ @RF sphere; (3) repeated procedure (2). For obtaining hierarchical structure, the Sn2+ -Ti2+ @RF colloidal spheres were obtained mainly through the process of polymerization of resorcinol in a mixture of formaldehyde and deionized water. Firstly, emulsion droplets were formed by the hydrogen bonding of resorcinol, formaldehyde and water firstly. Then Sn2+ -Ti2+ @RF polymer colloidal spheres generally grow into the bigger size through ageing polymerization. However, the Sn2+ and Ti2+ ions are considered to continuously interact with anionic SO4 2− in the absence of RF, resulting in solid and compact particles because of aggregation. It seems that resorcinol acted two major roles in this synthesis system: (1) It served as a surfactant active agent, which could stabilize the as-prepared Sn2+ -Ti2+ core-particles, as no additional surfactants introduced; (2) It could form the resorcinol-formaldehyde (RF) resin shell through reacting with formaldehyde around the core-particles in the meanwhile. Therefore, the gas sensor based on SnO2 -TiO2 yolk double-shelled microsphere is expected to display high response and rapid response speed in comparison with SnO2 -TiO2 nanoparticles because of the sufficient content volume and porous surface. In short, the porous yolk double-shelled structure is promising for gas detection in the efficient industrial applications. After the hydrothermal process, the final product was obtained. The as-fabricated structure of Sn-Ti precusor was observed with a diameter of about 3 m, as shown in Fig. 1(a). Fig. 1(b) presents the XRD patterns of the Sn-Ti precusor and SnO2 -TiO2 yolk doubleshelled microsphere. XRD results reveal that the crystal structure of hierarchical product was coincident with of tetragonal SnO2 (JCPDS No. 41-1445) and rutile TiO2 (JCPDS No. 21-1272) after calcination. However, in the pattern of Sn-Ti precusor, only the (101) and (200) peaks of TiO2 NPs could be observed, which was probably caused by the low intensity of other peaks of SnO2 NPs. The thermal decomposition of the as-synthesized precursors was investigated using Thermogravimetric Analysis (TGA) (Fig. 1(c)). The heat-treatment project during TGA analysis was described as follows: the as-synthesized precursor was heated to 800 ◦ C at a heating rate of 10 ◦ C/min. The small weight loss (∼6%) till ∼270 ◦ C could be explained by the removal of residual solvent. Heating in air flow to 800 ◦ C results in a weight loss of about 66.5% because of the removal of adsorbed water and polymer, which indicated that the loading amount of tin and titanium oxide was about 33.5% in the initial as-synthesized composite. Moreover, about 63% weight loss appeared at 470 ◦ C, and that is very close to the weight loss of that at 800 ◦ C. Thus, the calcination temperature was fixed at 500 ◦ C, and it could be confirmed that the impurity and organic matter removed completely. The porous surface of the structure of yolk double-shelled microsphere has been observed after calcination which is beneficial for the gas sensing reaction. The SEM and enlarged SEM images (Fig. 1(d and e)) reveal that the microsphere consists of lots of irregularly nano-sized particles, which leads to a larger number of channels. The inner shell could be observed clearly from the broken microsphere in Fig. 1(f and g). And large space appeared between the inner shell and outer shell, which results in a high surface area of 71.9 m2 g−1 . In addition, the most pore size of SnO2 -TiO2 yolk double-shelled microsphere is 3.7 nm (Fig. 1(h and i)). TEM and HRTEM images with the analysis of elements by EDS mapping were recorded (Fig. 2(a–d, f–i) and S2). More interestingly, the obtained microsphere presents a yolk double-shelled structure (Fig. 2(a and b)). In addition, the structure of the exterior shell was also investigated by HRTEM image (Fig. 2(d)). As can be seen, the as-synthesized hierarchical microsphere with porous shell owns large amount of cavities. The shell was rough and composed of
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Scheme 1. Schematic illustration of the one-step synthesis of (a) SnO2 -TiO2 composite nanoparticles and (b) SnO2 -TiO2 yolk double-shelled microsphere.
Fig. 1. (a) SEM image of Sn-Ti precursor before calcination; (b) XRD patterns of uniform Sn-Ti composites, the column show XRD standard card of SnO2 (JCPDS 41-1445) and TiO2 (JCPDS 21-1272); (c) Schematic illustrations of formation of SnO2 -TiO2 yolk double-shelled microsphere. The black curve represents the combustion process; (d–g) SEM and enlarged SEM images of SnO2 -TiO2 yolk double-shelled microsphere; (h) N2 adsorption-desorption isotherms and (i) pore size distribution for SnO2 -TiO2 yolk double-shelled microsphere.
small sized nanoparticles with the diameter of about 13 nm. Two lattice planes of 0.35 nm and 0.33 nm were also observed in the HRTEM, which were in accordance with (101) of TiO2 and (110) of SnO2 , respectively. The pattern of selected area electron diffraction (SAED) presents a polycrystalline structure of SnO2 -TiO2 yolk double-shelled microsphere (Fig. 2(e)). Moreover, the obtained product prepared under the similar condition without formaldehyde was agglomerate composite nanoparticles (Fig. S3). Fig. 2(h and i) shows the EDS mapping of the hierarchical structure. We can see that element of Sn was mainly distributed in the center, with the element of Ti distributing in the outer shell pre-
vailingly in the meanwhile. The result showed that the hierarchical structure was a mixture of SnO2 and TiO2 , and only the yolk and the inner shell were mainly made of SnO2 and the outer shell was mainly made of TiO2 . The probable reason for the special distribution of SnO2 and TiO2 in the inner and outer microsphere is proposed as follows: the radius of Sn2+ (0.069 nm) is larger than that of Ti2+ (0.061 nm) [19]. So, the diffusion coefficient of Ti2+ is greater than that of Sn2+ . Thus, the hierarchical structure of the tinrich in the core and titanium-rich in the outer shell could be formed [20].
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Fig. 2. (a–c) Typical TEM, (d) HRTEM images and (e) the related SAED pattern of SnO2 -TiO2 yolk double-shelled microspheres; (f) STEM image and (g–i) the corresponding EDS mapping showing the dispersion of Sn, Ti and O elements in the SnO2 -TiO2 yolk double-shelled microsphere; (j–l) XPS spectrum of SnO2 -TiO2 yolk double-shelled microsphere: (j) Sn 3d spectrum, (k) Ti 2p spectrum, (l) O 1s spectrum.
The full-scale XPS spectrum was shown in Fig. S4, which illustrates the composition and element valence states of assynthesized sample. The obtained binding energies in the XPS analysis were adjusted for specimen charging through referencing the C 1s line at 284.5 eV. The core level states of tin located at ∼495.2 and ∼486.8 eV are attributed to the spin orbital splitting of Sn4+ 3d3/2 and Sn4+ 3d5/2 , respectively, which reveals the existence of tin oxide [21]. Additionally, the difference value between Sn4+ 3d3/2 and Sn4+ 3d5/2 (8.4 eV) well matches the standard spectrum of X-ray Photoelectron Spectroscopy of Sn in the Handbook [22]. XPS spectra of Ti 2p was shown in Fig. 2(k), main peaks centered at 464.6 eV and 458.9 eV were contributed to Ti 2p3/2 and Ti 2p1/2 [23]. Fig. 2(l) shows that the profile of O 1s is asymmetric, which confirms the presence of adsorbed oxygen physically [19].
In the high-resolution O 1s spectra, two fitted peaks at 530.3 eV and 531.9 eV were observed, which are the characteristic peaks of surface lattice oxygen (OL ) and surface chemisorbed oxygen (OC ) [24]. The obtained results indicate the good coupling between TiO2 and SnO2 , which is agreed to the XRD pattern in Fig. 1(b). 3.2. Ethanol sensing properties It is known that selectivity is quite an important functional character for a gas sensor, since that the poor selectivity could limit the application and cause mistaken alarm [25]. Selectivity of SnO2 TiO2 yolk double-shelled microsphere-based sensor to 200 ppm of different tested gases at discrepant operating temperature was shown in Fig. 3(a). The sensor showed good selectivity to ethanol
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Fig. 3. (a) Nine-axe spiderweb graph for evaluating the working temperature and selectivity of SnO2 -TiO2 yolk double-shelled microsphere-based sensor; (b) Responses of SnO2 -TiO2 composite-based sensors towards 200 ppm of carbon monoxide, styrene, acetone, ethanol, methanol, trimethylamine, ammonia, methane and toluene; (c) Dynamic sensing curves and (d) Enlarged resistance change of response/recovery process based on SnO2 -TiO2 yolk double-shelled microsphere towards 10–500 ppm ethanol.
at the optimum of 300 ◦ C compared with other working temperatures. The highest response to 200 ppm ethanol is about 9.4, which is higher than those of other gases. To investigate the influence of morphology on the sensor signal, simultaneous responses to various gases measurements of the resistance to ethanol were car-
ried on SnO2 -TiO2 composite nanoparticles (Fig. 3(b)). It could be clearly seen that SnO2 -TiO2 composites show the similar selectivity to the target gases. However, compared with that of SnO2 -TiO2 nanoparticles, the sensor based on SnO2 -TiO2 yolk double-shelled microsphere shows a higher response to each test gas. Moreover,
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Fig. 4. Relationship of response vs concentration of (a) 10–2000 ppm and (b) 10–200 ppm ethanol for SnO2 -TiO2 yolk double-shelled microsphere-based sensor; (c) Five cycles resistance variation of the sensor based on SnO2 -TiO2 yolk double-shelled microspheres towards 50 ppm ethanol; (d) Long-term stability (15 days) of the gas sensor based on SnO2 -TiO2 yolk double-shelled microsphere towards 200 ppm ethanol. Error bars show the standard deviation for n = 3 measurements.
the response to ethanol has a significant enhancement by building of yolk double-shelled structure. Dynamic changes in the resistance caused by step changes in ethanol concentration from 10 to 500 ppm were measured at the working temperature of 300 ◦ C (Fig. 3(c)), which indicated that the response increases with the concentration of ethanol increases (Fig. S5(a)). The resistance of SnO2 -TiO2 yolk double-shelled microsphere-based sensor decreases when ethanol is admitted, which is a typical character of conductivity for n-type SnO2 and TiO2 semiconductors in reaction with a reducing gas [26]. The sensing transients to 10–500 ppm ethanol at 300 ◦ C showed relatively stable response and recovery characteristics. In addition, the enlarged response and recovery resistance transient to 10–500 ppm ethanol at 300 ◦ C was shown in Fig. 3(d). With ethanol concentration changing (from 10 to 500 ppm), the response and recovery times varies in the range of 1–7 s and 4–14 s, respectively, which was concluded in Fig. S5(b). The response/recovery time of the sensor based on various materials reported in the literature are listed in Table 1, which shows the superiority of SnO2 -TiO2 yolk double-shelled microsphere [27–32]. Such rapid response and recovery speed could be attributed to the special yolk double-shelled architecture with porous surface, the structure could offer more contact surface, sufficient pores and good permeability for adsorbed oxygen and target gas molecules, which leads to more ethanol molecules adsorption and in favor of gas diffusion. Fig. 4(a) shows the responses of SnO2 -TiO2 yolk double-shelled microsphere-based sensor to different concentrations of ethanol in the range from 10 to 2000 ppm at 300 ◦ C. It is clear that the sensor presents an excellent response to ethanol when exposed
Table 1 Comparison of response/recovery time of various sensing materials-based sensors to ethanol. Materials
Tem. (◦ C)
Con. (ppm)
Tres (s)
Trecov (s)
Ref.
SnO2 porous films SnO2 hollow spheres SnO2 film TiO2 nanobelt TiO2 layer TiO2 sphere SnO2 -TiO2 sphere SnO2 -TiO2 sphere
300 300 300 400 350 350 300 300
100 100 100 100 100 300 100 200
36 4 7.1 21 240 128 3 1.7
– 10 36.9 45 – 348 8.4 13.6
[27] [28] [29] [30] [31] [32] this work this work
Tem.: Temperature, Con.: Concentration, Tres : Response time, Trecov : Recovery time, Ref.: Reference.
to 10, 20, 50, 100, 200, 500, 1000 and 2000 ppm ethanol with the values of 1.6, 2.4, 3.3, 4.9, 9.4, 13.4, 20 and 24, respectively. Moreover, Fig. 4(b) shows the responses of sensor at relatively low ethanol concentrations. When exposed to 10 ppm of ethanol, the response is about 1.6, indicating gas response could be obtained in detecting low concentration of ethanol using SnO2 -TiO2 yolk double-shelled microsphere as potential gas-sensing material. For further exploring the gas sensing properties of SnO2 -TiO2 yolk double-shelled microsphere-based sensor, five characteristic cycle curves of response and recovery to 50 ppm ethanol were presented in Fig. 4(c). Obviously, no major changes appeared in the response/recovery times and resistance values, demonstrating the good repeatability of the sensor. Humidity may have impact on the sensing performance of the gas sensor. Thus, the long-term stability of the gas sensor was tested in the 33% RH air (Fig. 4(d)). Uninter-
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Fig. 5. (a) Schematic of possible gas sensing mechanisms; (b) HRTEM image of SnO2 -TiO2 yolk double-shelled microsphere; (c) Band diagram of the interface between SnO2 and TiO2 grains.
rupted testing of 15 days showed that the sensing performance of the device was stable, which demonstrated the long-term stability of the gas sensor based on SnO2 -TiO2 yolk double-shelled microsphere. Moreover, the relationship of relative humidity and ethanol sensing response was further investigated. Fig. S5(c) showed the relationship between response towards 200 ppm ethanol and relative humidity. It indicated that only slight variation in response appeared when RH < 40%. But, in high relative humidity of around 90%, the response changed to half value approximately. The above results indicated that the humidity has a negligible impact on the gas sensing response under low relative humidity environment. 3.3. Gas sensing mechanism For explaining the gas sensing improvement of SnO2 -TiO2 yolk double-shelled microsphere, two factors should be taken into account: (1) morphology effect and (2) electronic properties. It is well-known that surface structure and the specific surface area have fatal impact on gas sensing properties, and the sensor based on a hollow or hierarchical structure is preponderant compared with solid and bulk materials [33]. Firstly, the yolk double-shelled structure offers a more efficient means to harvest gas efficiency (Fig. 5(a)). Such structure enables multiple diffusion
of oxygen and target gases between the outer spherical shell and the interior shell. Secondly, another valid influence factor that rules the sensitivity of the sensor is the porosity of the composite material. In this work, not only the outer surface but also the inner shell surface shows rough, which were composed of nanoparticles. As a consequence, porous SnO2 -TiO2 yolk double-shelled microsphere, which has more active sites with better permeability, is good for rapid diffusion and transmission of gas molecules across not only the inner surface but also the outer surface of the sensor material, resulting in a fast response and recovery. The result shows that the porous double-shelled structure becomes the fatal influence factor for enhancing the gas sensing properties, including sensitivity and response/recovery time. Beyond that, the fundamental sensing mechanism of metal oxide semiconductor (MOS) has been believed to involve gas adsorption, charge transfer and desorption [34]. Firstly, the free electrons from the conductance band of SnO2 and TiO2 nanoparticles are transferred to oxygen molecules, leading to the formation of O− when the sensor exposed to the air at the working temperature of 300 ◦ C [35]. Then, an electron depletion layer formed on the surface of nanoparticles, resulting in high resistance. When the ethanol molecule is introduced, it is adsorbed on the oxygen sites
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preferably and released electrons to the sensing material, increasing conductivity of the sensor (Fig. S6). In addition, hetero building between SnO2 and TiO2 gives rise to the electron transport from TiO2 to SnO2 through band bending for the propose of Fermi energy balance, leading to creating a potential barrier between the hetero-interface (Fig. 5(b and c)). The potential barrier hinder the electron transportation in the inner structure, thus the surface electrons offer an obvious chance for oxygen species to be adsorbed, thus response of the sensor was increased significantly [19]. As is well known, reactions on the surface of the sensing materials between adsorbed oxygen species and target gas molecules affect the sensing properties seriously. And the additional of TiO2 to SnO2 could increase the basic properties of the composite because SnO2 has both basic and acidic characteristics but TiO2 has basic property naturally. Thus, CO2 and H2 O were the final product converted by ethanol molecules from the dehydrogenation process. The reaction was described in the following: C2 H5 OH → CH3 CHO + H2 CH3 CHO + 5O− → 2CO2 + 2H2 O + 5e− 4. Conclusion In summary, yolk double-shelled microsphere with heterostruc¨ ture has been successfully prepared via a simple classical Stober method combined with hydrothermal method, which is facile and easily scalable. The as-synthesized material was in micrometerscale and formed of small SnO2 /TiO2 nanocrystals with relatively rough surface. The yolk double-shelled microsphere-based sensor showed enhanced gas sensing performance compared with that of composite nanoparticles, including fast response/recovery speed (1.7 s/13.6 s), high response (9.4), and good reproducibility (15 days). The enhanced sensing performance may be due to the special multi-shelled hierarchical structure and fabrication of hetero structure. That is to say, the built-in electric field caused the variability of carrier (electron) transmission mode in the depletion layers. Moreover, this research set an example of the possibility to exhibit micro-device with a special multi-shelled structure and enhance the sensing properties of ethanol sensing sensor by structure modification, which enables new means to apply other hetero-nanostructures for gas sensors. Acknowledgements This work was supported by the Postdoctoral Science Foundation of China (No. 2015M571361), the Natural Science Foundation Committee (NSFC, Grant No. 51502110 and 61673191) and Central Universities Basic Scientific Research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.snb.2017.10.064. References [1] A. mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: a review, Ceram. Int. 42 (2016) 15119–15141. [2] T. Wagner, S. Haffer, C. Weinberger, D. Klaus, M. Tiemann, Mesoporous materials as gas sensors, Chem. Soc. Rev. 42 (2013) 4036–4053. [3] Z.F. Dai, H. Dai, Y. Zhou, D.L. Liu, G.T. Duan, W.P. Cai, Y. Li, Monodispersed Nb2 O5 microspheres: facile synthesis, air/water interfacial self-assembly, Nb2 O5 -based composite films, and their selective NO2 sensing, Adv. Mater. Interfaces 2 (2015), 1500167-(7).
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Biographies
Rui Zhang received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2017. As a Ph.D. student, her research interest is gas sensors based on metal oxide semiconductor composite materials.
Tong Zhang completed her MS degree in semiconductor materials in 1992 and her Ph.D. in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronics Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors, and humidity sensors.
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Tingting Zhou received her BE degree from the College of Electronics Science and Engineering, Jilin University, China in 2015. As a Ph.D. student, her research interest is gas sensors based on p-type metal oxide semiconductor materials.
Lili Wang is an Associate Professor in the College of Electronic Science and Engineering at Jilin University. She received his Ph.D. degree from Jilin University in 2014. Her current research focuses on functional nano/biomaterials and sensors, including semiconductor, graphene, biosensors, pressure sensors and chemical sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Research Paper
Rapid sensitive sensing platform based on yolk-shell hybrid hollow sphere for detection of ethanol Rui Zhang, Tong Zhang, Tingting Zhou, Lili Wang ∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 20 June 2017 Received in revised form 19 September 2017 Accepted 13 October 2017 Available online 5 November 2017 Keywords: Hollow nanostructure Double shells Hybrid materials Gas sensor High performance
a b s t r a c t The sensing layer composited from the inorganic particles, organic materials, polymers could be regarded as an important and functional component used in electronic devices, such as chemical sensor. Furthermore, it is very important for these materials to own hierarchical and porous structures leading to a large effective specific surface area. Among these, hybrid materials with yolk double-shelled architecture can further realize the required surface chemical reactions. However, it is rarely reported in gas sensors. Herein, a novel SnO2 -TiO2 hollow nanostructure consisting of double shells was successfully designed and synthesized. Due to the special multi-shelled structure and abundant hetero-interface, the sensing layer based on SnO2 -TiO2 realizes a rapid response rate within 1.7 s to ethanol gas and alluring reproducibility (15 days), which are superior over those for compact structure with similar sensors. The results indicated that the SnO2 -TiO2 yolk double-shelled microsphere have the huge potential for designing high performance practical ethanol devices in environmental monitoring and drunk driving. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, a great challenge to human beings was posed based on environmental problems, such as environment humidity, dust, toxic and hazardous gases. So, the rise of chemical sensors (CSs) is very important for monitoring the environmental change in real time [1–3]. For this purpose, high-performance chemical sensors are needed to be applied in various fields [4]. However, traditional gas sensing layers such as SnO2 , ZnO, and ␣-Fe2 O3 , suffered from several obstacles, for instance, poor sensitive selectivity, low catalytic activity, high cost, ultra-high working temperature and low electron conductivity [5–9]. To meet the ever-increasing demand for high performance sensing devices, it is imperative to design and fabricate efficient sensing materials that could provide a high sensing activity and effective utilization rate of surface area. As we know, the gas reaction and sensing performance are highly relied on the exposed surface area [10,11]. That is to say, a high specific surface area could achieve more effective active sites, leading to a higher sensitivity [12]. Hence, assembling tunable shell on the valuable core not only reduces the consumption of precious materials but also increases the dispersibility, functionality and stability [13]. Compared with the micro-sized particles, 0D structured nanoparticles possess the shorter diffusion length and
∗ Corresponding author. E-mail address: lili [email protected] (L. Wang). https://doi.org/10.1016/j.snb.2017.10.064 0925-4005/© 2017 Elsevier B.V. All rights reserved.
larger gas-sensing layer contact area. However, the low catalytic activity of single-component usually leads to poor sensing activity. Furthermore, hybrid structured nanomaterials usually exhibit higher reactive activity [14–17]. Inspired by the concept, we design a new type of SnO2 -TiO2 hybrid materials with yolk double-shelled spheres that were prepared by using resorcinol-formaldehyde (RF) without any other additional surfactants. Systematic studies indicated that RF can not only passivate the surface of metal oxide semiconductor (MOS) particles to avoid aggregating but also act as precursor to form RF resin, which reveals dual functions. In this study, ethanol sensing behavior was significantly improved through the design of SnO2 TiO2 yolk double-shelled heterostructure. The aim of this study is to explore morphological effects as well as ethanol sensing performance of SnO2 -TiO2 nanocomposites with an especial emphasis on improving sensing response and ultrafast response.
2. Experiment 2.1. Chemical materials Tin (II) sulfate (SnSO4 , 99%), titanium (IV) sulfate (TiSO4 , 96%), resorcinol (C6 H6 O2 , 99.5%) and formaldehyde solution (HCHO, 37–40%) were the initial chemicals without any further purification after received. Moreover, deionized water was also used in this experiment.
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2.2. Chemical synthesis Preparation of SnO2 -TiO2 yolk double-shelled microsphere: SnO2 TiO2 yolk double-shelled microsphere was prepared by a facile and simple template-assisted hydrothermal method with a slight modification [18]. Typically, 1.6 g of Ti(SO4 )2 , 0.86 g of SnSO4 and 0.64 g C6 H6 O2 were added into the mixed solution of 12 ml of deionized water and 1.6 ml of formaldehyde under continuous stirring. Then, the mixture was transferred into a 20 ml Teflon-lined stainless steel autoclave and heated at 85 ◦ C for uninterrupted 48 h. The obtained product was collected from the cooling autoclave and washed for 4 times using deionized water. After dried for overnight at 60 ◦ C, the powder was transferred to the muffle furnace and heating at 500 ◦ C for 3 h at a rate of 5 ◦ C/min. Preparation of SnO2 -TiO2 nanoparticles: Solid SnO2 -TiO2 nanoparticles (NPs) were prepared by a facile hydrothermal method. Typically, 1.6 g of Ti(SO4 )2 and 0.86 g of SnSO4 were added into the solution of 12 ml of deionized water and 1.6 ml of formaldehyde under continuous stirring. Then, the mixture was transferred into a 20 ml Teflon-lined stainless steel autoclave and heated at 85 ◦ C for uninterrupted 48 h. The obtained product was collected from the cooling autoclave and washed for 4 times using deionized water. After dried for overnight at 60 ◦ C, the powder was transferred to the muffle furnace and heating at 500 ◦ C for 3 h at a rate of 5 ◦ C/min. 2.3. Characterization X-ray diffraction (XRD) was used to identify the crystal structures of the as-synthesized materials using Cu K␣ radiation ( = 0.154 nm) on a Rigaku TTRIII X-ray diffractometer (40 kV). Field-emission scanning electron microscope (FESEM) and highresolution transmission electron microscope (HRTEM) were used to observe the micro-morphology on a JEOL JSM-7500F microscope and Tecnai G2 20S-Twin microscope, respectively. The composition of the as-synthesized products was tested using a PREVAC X-ray photoelectron spectroscopy (XPS) system. And Brunauer-EmmettTeller (BET) was carried on a JW-BK132F analyzer. 2.4. Fabrication and measurement of the gas sensor Indirect-heating structure sensor was used in this work to obtain sensing properties (Fig. S1). The typical fabrication process was described as follows: the as-synthesized composite powder was transferred to an agate mortar and mixed with deionized water to form a paste coated onto the surface of a tube, which is made from alumina. A pair of Au electrodes was printed on the each side of the tube and four platinum wires were installed at each end. A Ni-Cr alloy wire was inserted crossing the alumina tube and used to control the working temperature. Then, the tube was connected to a bakelite base. Next, the gas sensor was aged for 24 h in air in order to ensure the stability and repeatability. The response to target gases is defined as: S = Ra /Rg , where Rg is the electrical resistance in the target gas and Ra is the resistance in the atmosphere. And the response/recovery time was defined as the time cost of changes of 90% resistance. What’s more, in this experiment, the sensor was tested under the around 33% RH environment. 3. Results and discussion 3.1. Structural and morphological characteristics The smart designed of SnO2 -TiO2 composite has been illustrated in Scheme 1. The yolk double-shelled morphology results from the following processes: (1) formation of Sn2+ -Ti2+ @RF composite spheres by polymerization of the resorcinol and formaldehyde
along with the decomposition of tin and titanium salts; (2) combustion of polymeric component, decomposition or oxidation of the outer part of Sn2+ -Ti2+ @RF sphere into SnO2 -TiO2 outermost shell and then subsequent contraction of the inner Sn2+ -Ti2+ @RF sphere; (3) repeated procedure (2). For obtaining hierarchical structure, the Sn2+ -Ti2+ @RF colloidal spheres were obtained mainly through the process of polymerization of resorcinol in a mixture of formaldehyde and deionized water. Firstly, emulsion droplets were formed by the hydrogen bonding of resorcinol, formaldehyde and water firstly. Then Sn2+ -Ti2+ @RF polymer colloidal spheres generally grow into the bigger size through ageing polymerization. However, the Sn2+ and Ti2+ ions are considered to continuously interact with anionic SO4 2− in the absence of RF, resulting in solid and compact particles because of aggregation. It seems that resorcinol acted two major roles in this synthesis system: (1) It served as a surfactant active agent, which could stabilize the as-prepared Sn2+ -Ti2+ core-particles, as no additional surfactants introduced; (2) It could form the resorcinol-formaldehyde (RF) resin shell through reacting with formaldehyde around the core-particles in the meanwhile. Therefore, the gas sensor based on SnO2 -TiO2 yolk double-shelled microsphere is expected to display high response and rapid response speed in comparison with SnO2 -TiO2 nanoparticles because of the sufficient content volume and porous surface. In short, the porous yolk double-shelled structure is promising for gas detection in the efficient industrial applications. After the hydrothermal process, the final product was obtained. The as-fabricated structure of Sn-Ti precusor was observed with a diameter of about 3 m, as shown in Fig. 1(a). Fig. 1(b) presents the XRD patterns of the Sn-Ti precusor and SnO2 -TiO2 yolk doubleshelled microsphere. XRD results reveal that the crystal structure of hierarchical product was coincident with of tetragonal SnO2 (JCPDS No. 41-1445) and rutile TiO2 (JCPDS No. 21-1272) after calcination. However, in the pattern of Sn-Ti precusor, only the (101) and (200) peaks of TiO2 NPs could be observed, which was probably caused by the low intensity of other peaks of SnO2 NPs. The thermal decomposition of the as-synthesized precursors was investigated using Thermogravimetric Analysis (TGA) (Fig. 1(c)). The heat-treatment project during TGA analysis was described as follows: the as-synthesized precursor was heated to 800 ◦ C at a heating rate of 10 ◦ C/min. The small weight loss (∼6%) till ∼270 ◦ C could be explained by the removal of residual solvent. Heating in air flow to 800 ◦ C results in a weight loss of about 66.5% because of the removal of adsorbed water and polymer, which indicated that the loading amount of tin and titanium oxide was about 33.5% in the initial as-synthesized composite. Moreover, about 63% weight loss appeared at 470 ◦ C, and that is very close to the weight loss of that at 800 ◦ C. Thus, the calcination temperature was fixed at 500 ◦ C, and it could be confirmed that the impurity and organic matter removed completely. The porous surface of the structure of yolk double-shelled microsphere has been observed after calcination which is beneficial for the gas sensing reaction. The SEM and enlarged SEM images (Fig. 1(d and e)) reveal that the microsphere consists of lots of irregularly nano-sized particles, which leads to a larger number of channels. The inner shell could be observed clearly from the broken microsphere in Fig. 1(f and g). And large space appeared between the inner shell and outer shell, which results in a high surface area of 71.9 m2 g−1 . In addition, the most pore size of SnO2 -TiO2 yolk double-shelled microsphere is 3.7 nm (Fig. 1(h and i)). TEM and HRTEM images with the analysis of elements by EDS mapping were recorded (Fig. 2(a–d, f–i) and S2). More interestingly, the obtained microsphere presents a yolk double-shelled structure (Fig. 2(a and b)). In addition, the structure of the exterior shell was also investigated by HRTEM image (Fig. 2(d)). As can be seen, the as-synthesized hierarchical microsphere with porous shell owns large amount of cavities. The shell was rough and composed of
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Scheme 1. Schematic illustration of the one-step synthesis of (a) SnO2 -TiO2 composite nanoparticles and (b) SnO2 -TiO2 yolk double-shelled microsphere.
Fig. 1. (a) SEM image of Sn-Ti precursor before calcination; (b) XRD patterns of uniform Sn-Ti composites, the column show XRD standard card of SnO2 (JCPDS 41-1445) and TiO2 (JCPDS 21-1272); (c) Schematic illustrations of formation of SnO2 -TiO2 yolk double-shelled microsphere. The black curve represents the combustion process; (d–g) SEM and enlarged SEM images of SnO2 -TiO2 yolk double-shelled microsphere; (h) N2 adsorption-desorption isotherms and (i) pore size distribution for SnO2 -TiO2 yolk double-shelled microsphere.
small sized nanoparticles with the diameter of about 13 nm. Two lattice planes of 0.35 nm and 0.33 nm were also observed in the HRTEM, which were in accordance with (101) of TiO2 and (110) of SnO2 , respectively. The pattern of selected area electron diffraction (SAED) presents a polycrystalline structure of SnO2 -TiO2 yolk double-shelled microsphere (Fig. 2(e)). Moreover, the obtained product prepared under the similar condition without formaldehyde was agglomerate composite nanoparticles (Fig. S3). Fig. 2(h and i) shows the EDS mapping of the hierarchical structure. We can see that element of Sn was mainly distributed in the center, with the element of Ti distributing in the outer shell pre-
vailingly in the meanwhile. The result showed that the hierarchical structure was a mixture of SnO2 and TiO2 , and only the yolk and the inner shell were mainly made of SnO2 and the outer shell was mainly made of TiO2 . The probable reason for the special distribution of SnO2 and TiO2 in the inner and outer microsphere is proposed as follows: the radius of Sn2+ (0.069 nm) is larger than that of Ti2+ (0.061 nm) [19]. So, the diffusion coefficient of Ti2+ is greater than that of Sn2+ . Thus, the hierarchical structure of the tinrich in the core and titanium-rich in the outer shell could be formed [20].
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Fig. 2. (a–c) Typical TEM, (d) HRTEM images and (e) the related SAED pattern of SnO2 -TiO2 yolk double-shelled microspheres; (f) STEM image and (g–i) the corresponding EDS mapping showing the dispersion of Sn, Ti and O elements in the SnO2 -TiO2 yolk double-shelled microsphere; (j–l) XPS spectrum of SnO2 -TiO2 yolk double-shelled microsphere: (j) Sn 3d spectrum, (k) Ti 2p spectrum, (l) O 1s spectrum.
The full-scale XPS spectrum was shown in Fig. S4, which illustrates the composition and element valence states of assynthesized sample. The obtained binding energies in the XPS analysis were adjusted for specimen charging through referencing the C 1s line at 284.5 eV. The core level states of tin located at ∼495.2 and ∼486.8 eV are attributed to the spin orbital splitting of Sn4+ 3d3/2 and Sn4+ 3d5/2 , respectively, which reveals the existence of tin oxide [21]. Additionally, the difference value between Sn4+ 3d3/2 and Sn4+ 3d5/2 (8.4 eV) well matches the standard spectrum of X-ray Photoelectron Spectroscopy of Sn in the Handbook [22]. XPS spectra of Ti 2p was shown in Fig. 2(k), main peaks centered at 464.6 eV and 458.9 eV were contributed to Ti 2p3/2 and Ti 2p1/2 [23]. Fig. 2(l) shows that the profile of O 1s is asymmetric, which confirms the presence of adsorbed oxygen physically [19].
In the high-resolution O 1s spectra, two fitted peaks at 530.3 eV and 531.9 eV were observed, which are the characteristic peaks of surface lattice oxygen (OL ) and surface chemisorbed oxygen (OC ) [24]. The obtained results indicate the good coupling between TiO2 and SnO2 , which is agreed to the XRD pattern in Fig. 1(b). 3.2. Ethanol sensing properties It is known that selectivity is quite an important functional character for a gas sensor, since that the poor selectivity could limit the application and cause mistaken alarm [25]. Selectivity of SnO2 TiO2 yolk double-shelled microsphere-based sensor to 200 ppm of different tested gases at discrepant operating temperature was shown in Fig. 3(a). The sensor showed good selectivity to ethanol
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Fig. 3. (a) Nine-axe spiderweb graph for evaluating the working temperature and selectivity of SnO2 -TiO2 yolk double-shelled microsphere-based sensor; (b) Responses of SnO2 -TiO2 composite-based sensors towards 200 ppm of carbon monoxide, styrene, acetone, ethanol, methanol, trimethylamine, ammonia, methane and toluene; (c) Dynamic sensing curves and (d) Enlarged resistance change of response/recovery process based on SnO2 -TiO2 yolk double-shelled microsphere towards 10–500 ppm ethanol.
at the optimum of 300 ◦ C compared with other working temperatures. The highest response to 200 ppm ethanol is about 9.4, which is higher than those of other gases. To investigate the influence of morphology on the sensor signal, simultaneous responses to various gases measurements of the resistance to ethanol were car-
ried on SnO2 -TiO2 composite nanoparticles (Fig. 3(b)). It could be clearly seen that SnO2 -TiO2 composites show the similar selectivity to the target gases. However, compared with that of SnO2 -TiO2 nanoparticles, the sensor based on SnO2 -TiO2 yolk double-shelled microsphere shows a higher response to each test gas. Moreover,
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Fig. 4. Relationship of response vs concentration of (a) 10–2000 ppm and (b) 10–200 ppm ethanol for SnO2 -TiO2 yolk double-shelled microsphere-based sensor; (c) Five cycles resistance variation of the sensor based on SnO2 -TiO2 yolk double-shelled microspheres towards 50 ppm ethanol; (d) Long-term stability (15 days) of the gas sensor based on SnO2 -TiO2 yolk double-shelled microsphere towards 200 ppm ethanol. Error bars show the standard deviation for n = 3 measurements.
the response to ethanol has a significant enhancement by building of yolk double-shelled structure. Dynamic changes in the resistance caused by step changes in ethanol concentration from 10 to 500 ppm were measured at the working temperature of 300 ◦ C (Fig. 3(c)), which indicated that the response increases with the concentration of ethanol increases (Fig. S5(a)). The resistance of SnO2 -TiO2 yolk double-shelled microsphere-based sensor decreases when ethanol is admitted, which is a typical character of conductivity for n-type SnO2 and TiO2 semiconductors in reaction with a reducing gas [26]. The sensing transients to 10–500 ppm ethanol at 300 ◦ C showed relatively stable response and recovery characteristics. In addition, the enlarged response and recovery resistance transient to 10–500 ppm ethanol at 300 ◦ C was shown in Fig. 3(d). With ethanol concentration changing (from 10 to 500 ppm), the response and recovery times varies in the range of 1–7 s and 4–14 s, respectively, which was concluded in Fig. S5(b). The response/recovery time of the sensor based on various materials reported in the literature are listed in Table 1, which shows the superiority of SnO2 -TiO2 yolk double-shelled microsphere [27–32]. Such rapid response and recovery speed could be attributed to the special yolk double-shelled architecture with porous surface, the structure could offer more contact surface, sufficient pores and good permeability for adsorbed oxygen and target gas molecules, which leads to more ethanol molecules adsorption and in favor of gas diffusion. Fig. 4(a) shows the responses of SnO2 -TiO2 yolk double-shelled microsphere-based sensor to different concentrations of ethanol in the range from 10 to 2000 ppm at 300 ◦ C. It is clear that the sensor presents an excellent response to ethanol when exposed
Table 1 Comparison of response/recovery time of various sensing materials-based sensors to ethanol. Materials
Tem. (◦ C)
Con. (ppm)
Tres (s)
Trecov (s)
Ref.
SnO2 porous films SnO2 hollow spheres SnO2 film TiO2 nanobelt TiO2 layer TiO2 sphere SnO2 -TiO2 sphere SnO2 -TiO2 sphere
300 300 300 400 350 350 300 300
100 100 100 100 100 300 100 200
36 4 7.1 21 240 128 3 1.7
– 10 36.9 45 – 348 8.4 13.6
[27] [28] [29] [30] [31] [32] this work this work
Tem.: Temperature, Con.: Concentration, Tres : Response time, Trecov : Recovery time, Ref.: Reference.
to 10, 20, 50, 100, 200, 500, 1000 and 2000 ppm ethanol with the values of 1.6, 2.4, 3.3, 4.9, 9.4, 13.4, 20 and 24, respectively. Moreover, Fig. 4(b) shows the responses of sensor at relatively low ethanol concentrations. When exposed to 10 ppm of ethanol, the response is about 1.6, indicating gas response could be obtained in detecting low concentration of ethanol using SnO2 -TiO2 yolk double-shelled microsphere as potential gas-sensing material. For further exploring the gas sensing properties of SnO2 -TiO2 yolk double-shelled microsphere-based sensor, five characteristic cycle curves of response and recovery to 50 ppm ethanol were presented in Fig. 4(c). Obviously, no major changes appeared in the response/recovery times and resistance values, demonstrating the good repeatability of the sensor. Humidity may have impact on the sensing performance of the gas sensor. Thus, the long-term stability of the gas sensor was tested in the 33% RH air (Fig. 4(d)). Uninter-
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Fig. 5. (a) Schematic of possible gas sensing mechanisms; (b) HRTEM image of SnO2 -TiO2 yolk double-shelled microsphere; (c) Band diagram of the interface between SnO2 and TiO2 grains.
rupted testing of 15 days showed that the sensing performance of the device was stable, which demonstrated the long-term stability of the gas sensor based on SnO2 -TiO2 yolk double-shelled microsphere. Moreover, the relationship of relative humidity and ethanol sensing response was further investigated. Fig. S5(c) showed the relationship between response towards 200 ppm ethanol and relative humidity. It indicated that only slight variation in response appeared when RH < 40%. But, in high relative humidity of around 90%, the response changed to half value approximately. The above results indicated that the humidity has a negligible impact on the gas sensing response under low relative humidity environment. 3.3. Gas sensing mechanism For explaining the gas sensing improvement of SnO2 -TiO2 yolk double-shelled microsphere, two factors should be taken into account: (1) morphology effect and (2) electronic properties. It is well-known that surface structure and the specific surface area have fatal impact on gas sensing properties, and the sensor based on a hollow or hierarchical structure is preponderant compared with solid and bulk materials [33]. Firstly, the yolk double-shelled structure offers a more efficient means to harvest gas efficiency (Fig. 5(a)). Such structure enables multiple diffusion
of oxygen and target gases between the outer spherical shell and the interior shell. Secondly, another valid influence factor that rules the sensitivity of the sensor is the porosity of the composite material. In this work, not only the outer surface but also the inner shell surface shows rough, which were composed of nanoparticles. As a consequence, porous SnO2 -TiO2 yolk double-shelled microsphere, which has more active sites with better permeability, is good for rapid diffusion and transmission of gas molecules across not only the inner surface but also the outer surface of the sensor material, resulting in a fast response and recovery. The result shows that the porous double-shelled structure becomes the fatal influence factor for enhancing the gas sensing properties, including sensitivity and response/recovery time. Beyond that, the fundamental sensing mechanism of metal oxide semiconductor (MOS) has been believed to involve gas adsorption, charge transfer and desorption [34]. Firstly, the free electrons from the conductance band of SnO2 and TiO2 nanoparticles are transferred to oxygen molecules, leading to the formation of O− when the sensor exposed to the air at the working temperature of 300 ◦ C [35]. Then, an electron depletion layer formed on the surface of nanoparticles, resulting in high resistance. When the ethanol molecule is introduced, it is adsorbed on the oxygen sites
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preferably and released electrons to the sensing material, increasing conductivity of the sensor (Fig. S6). In addition, hetero building between SnO2 and TiO2 gives rise to the electron transport from TiO2 to SnO2 through band bending for the propose of Fermi energy balance, leading to creating a potential barrier between the hetero-interface (Fig. 5(b and c)). The potential barrier hinder the electron transportation in the inner structure, thus the surface electrons offer an obvious chance for oxygen species to be adsorbed, thus response of the sensor was increased significantly [19]. As is well known, reactions on the surface of the sensing materials between adsorbed oxygen species and target gas molecules affect the sensing properties seriously. And the additional of TiO2 to SnO2 could increase the basic properties of the composite because SnO2 has both basic and acidic characteristics but TiO2 has basic property naturally. Thus, CO2 and H2 O were the final product converted by ethanol molecules from the dehydrogenation process. The reaction was described in the following: C2 H5 OH → CH3 CHO + H2 CH3 CHO + 5O− → 2CO2 + 2H2 O + 5e− 4. Conclusion In summary, yolk double-shelled microsphere with heterostruc¨ ture has been successfully prepared via a simple classical Stober method combined with hydrothermal method, which is facile and easily scalable. The as-synthesized material was in micrometerscale and formed of small SnO2 /TiO2 nanocrystals with relatively rough surface. The yolk double-shelled microsphere-based sensor showed enhanced gas sensing performance compared with that of composite nanoparticles, including fast response/recovery speed (1.7 s/13.6 s), high response (9.4), and good reproducibility (15 days). The enhanced sensing performance may be due to the special multi-shelled hierarchical structure and fabrication of hetero structure. That is to say, the built-in electric field caused the variability of carrier (electron) transmission mode in the depletion layers. Moreover, this research set an example of the possibility to exhibit micro-device with a special multi-shelled structure and enhance the sensing properties of ethanol sensing sensor by structure modification, which enables new means to apply other hetero-nanostructures for gas sensors. Acknowledgements This work was supported by the Postdoctoral Science Foundation of China (No. 2015M571361), the Natural Science Foundation Committee (NSFC, Grant No. 51502110 and 61673191) and Central Universities Basic Scientific Research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.snb.2017.10.064. References [1] A. mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: a review, Ceram. Int. 42 (2016) 15119–15141. [2] T. Wagner, S. Haffer, C. Weinberger, D. Klaus, M. Tiemann, Mesoporous materials as gas sensors, Chem. Soc. Rev. 42 (2013) 4036–4053. [3] Z.F. Dai, H. Dai, Y. Zhou, D.L. Liu, G.T. Duan, W.P. Cai, Y. Li, Monodispersed Nb2 O5 microspheres: facile synthesis, air/water interfacial self-assembly, Nb2 O5 -based composite films, and their selective NO2 sensing, Adv. Mater. Interfaces 2 (2015), 1500167-(7).
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Biographies
Rui Zhang received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2017. As a Ph.D. student, her research interest is gas sensors based on metal oxide semiconductor composite materials.
Tong Zhang completed her MS degree in semiconductor materials in 1992 and her Ph.D. in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronics Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors, and humidity sensors.
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Tingting Zhou received her BE degree from the College of Electronics Science and Engineering, Jilin University, China in 2015. As a Ph.D. student, her research interest is gas sensors based on p-type metal oxide semiconductor materials.
Lili Wang is an Associate Professor in the College of Electronic Science and Engineering at Jilin University. She received his Ph.D. degree from Jilin University in 2014. Her current research focuses on functional nano/biomaterials and sensors, including semiconductor, graphene, biosensors, pressure sensors and chemical sensors.