Hollow ZnSnO3 Cubes with Controllable Shells Enabling Highly

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Hollow ZnSnO3 Cubes with Controllable Shells Enabling Highly Efficient Chemical Sensing Detection of Formaldehyde Vapors Tingting Zhou, Tong Zhang, Rui Zhang, Zheng Lou, Jianan Deng, and Lili Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03112 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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ACS Applied Materials & Interfaces

Hollow ZnSnO3 Cubes with Controllable Shells Enabling

Highly

Efficient

Chemical

Sensing

Detection of Formaldehyde Vapors Tingting Zhou,1 Tong Zhang,1,3 Rui Zhang,1 Zheng Lou,2 Jianan Deng,1 Lili Wang1* 1. State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China 2. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China 3. State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 100083, China E-mail address: [email protected] *Corresponding author: E-mail address: [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

Structural hierarchy, inherently hollow nanostructured materials preferentially possessing high surface area demand attention due to their alluring sensing performances. However, the activity of hollow and structural hierarchy nanomaterials generally remains suboptimal due to their boring space structure and large lateral size, which greatly hampers and limits the availability of inner space active sites. Here, hollow ZnSnO3 cubes with a controllable interior structure were successfully prepared through a simple and low-cost co-precipitation approach followed with subsequent calcination process. The solid-, single-shelled-, double-shelled- and multi-shelled ZnSnO3 hollow cubes could be selectively tailored by repeated addition of alkaline solution. The multi-shelled architecture displayed outstanding sensing properties for formaldehyde vapors due to large specific surface area, less agglomerations, abundant interfaces, thin shells and high proportion porous structure, which act synergistically to facilitate charge transfer and promote target gas adsorption.

KEYWORDS Hollow nanostructured, Controllable Shells ZnSnO3, High Active Site, High Response, Formaldehyde Sensing


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1.


INTRODUCTION

The novel design and synthesis of high-performance gas sensing materials is of great significance to meet the increasing demand for detection of toxic and harmful gases. Hollow micro-/nanostructured materials with controllable morphologies (hollow structure, core-shell structure, core-in hollow shell structure and multi-shelled hollow structure) have stimulated great interest because of attractive properties such as low density, great permeation and less agglomerations.1-3 Among different candidates, the multilevel hollow architecture is currently, without any doubt, the most intensively studied material for various fields, especially chemical sensor.4-10 A prominent advantage is that the sensing materials with multi-shelled hollow structure show advantages over solid and single-shelled hollow materials, as the distinct structure not only maintains the hollow structural feature but also provides larger surface area, which is beneficial to gas diffusion and mass transportation in active layers.11-14 As examples, hollow microspheres with multi-shelled structure have been reported to be highly active sensing layer that shows excellent sensing performance, compared to single-shelled hollow structures.15 Although considerable progress on hollow materials has been reported to exhibit high sensing performance, however, few of research specially focused on the relationship between gas sensing performances and the shell. In our work, we make efforts to give a systemic comparison of sensing abilities of hollow structured materials with tunable shell numbers of 1-3 shells. Moreover, the multi-shelled hollow materials prepared are always spherical in geometrical structure or a copy of template structure in literature. Therefore, new strategies are needed for the preparation of materials with controllable shells and designed shape to achieve novel sensing structure model. Here, we presented well-defined ZnSnO3 cubes with controllable shelled numbers. It is worth noting that the method does not involve capping agents or additional templates. The number


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of shell can be easily controlled by adding alkaline solution repeatedly. Consequently, the ZnSnO3 multi-shelled cubes based sensor exhibit better formaldehyde sensing performances including the high response and the fast response process, which is relation to the thin shell thickness, less aggregations, large surface-to-volume proportion and good permeation. This work further confirms the potential of multilevel nanostructure for chemical sensor applications. 2.

EXPERIMENTAL SECTION

2.1. Materials. Stannic chloride (SnCl4·5H2O), sodium citrate (C6H5Na3O7·2H2O), zinc chloride (ZnCl2), sodium hydroxide (NaOH), ethanol (C2H5OH) were of analytical grade and purchased from Shanghai Chemical Corp. Deionized (DI) water was used for all experiments. 2.2. Synthesis Process. Synthesis of ZnSnO3 solid cubes: ZnSnO3 solid cubes were synthesized on the basis of the reported method.16,17 In a typical synthesis, 5mL, 0.2 M SnCl4 ethanol solution was combined with 10 mL of mixture containing 0.1 M sodium citrate solution and 0.1 M ZnCl2 solution. After stirring for 30 min, 25 mL, 0.41 M NaOH aqueous solution was slowly dropped into the above solution under continuously stirring for 30 min. After that, the white ZnSn(OH)6 solid cube precipitate was centrifuged and washed 4-5 times with ethanol and deionized water. Finally, the precursor was annealed at 150 °C, 300 °C and 450 °C in nitrogen environment to obtain ZnSnO3 solid cubes. Synthesis of ZnSnO3 single-shelled cubes: ZnSnO3 single-shelled cubes were synthesized on the basis of the reported method.16,17 5mL, 0.2 M SnCl4 ethanol solution was combined with 10 mL of mixture containing 0.1 M ZnSn(OH)6 solid cubes solution, 0.1 M sodium citrate solution and 0.1 M ZnCl2 solution, and then, added 25 mL, 0.41 M NaOH aqueous solution. After stirring for 30 min, 15 mL, 2 M of NaOH aqueous solution was dropped into the


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suspension to convert ZnSn(OH)6 solid cubes into the ZnSn(OH)6 single-shelled cubes. Finally, the precursor was annealed with the same heating rate in nitrogen environment to obtain ZnSnO3 single-shelled cubes. Synthesis of ZnSnO3 double-shelled cubes and ZnSnO3 multi-shelled cubes: ZnSnO3 materials with double and multi shells were synthesized via similar method with single-shelled cubes and double-shelled ZnSn(OH)6 as the seed materials, respectively. 2.3. Characterization. The XRD spectrum of all the samples was measured by a X-ray diffraction with Cu Kα radiation (XRD, Rigaku D/Max-2550 diffractometer, λ = 1.5403 Å). The microstructures of samples were observed on a field emission scanning electron microscope (FESEM, JEOL JSM-7500F) and transmission electron microscopy (TEM, JEOL JEM-2100F). Energy dispersive X-ray spectrometry (EDS) attached to the FESEM was used to investigated the composition. Chemical binding analysis was conducted by an X-ray photoelectron spectrograph (XPS, ESCALAB MKK II), operated using Mg as the exciting source. JWBK132F analyzer was utilized to analyze the porosity and specific surface area of the ZnSnO3 materials. A CGS-8 series Intelligent Test Meter (China, ELITE TECH) was employed to determine the gas sensing performance. 3.

RESULTS AND DISCUSSIONS 3.1. Material Synthesis and Structural Characterization.

The strategy of preparing the ZnSnO3 hollow structures with controllable shells is highly repeatable and revealed in Figure 1. (I) Synthesis of ZnSn(OH)6 solid cubes. The samples were obtained by the fast co-precipitation reaction of Zn2+, Sn4+ and OH-, when sodium citrate solution is present at room temperature (eq 1).18 Depending on the intrinsic cubic crystal structure, spontaneous formation of cube-like precursors was realized. (II) Synthesis of


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ZnSn(OH)6 single-shelled cubes. Step II involves deposition of ZnSn(OH)6 layers onto the surface of the pre-grown ZnSn(OH)6 solid cubes. It has been reported that the external ZnSn(OH)6 cube is more stable.17 Benefiting from the amphoteric nature, [Zn(OH)4]2- and [Sn(OH)6]2- could be generated and dissolved with the addition of excess OH- (eq 2). (III) Synthesis of ZnSn(OH)6 double-shelled cubes. Similarly, ZnSn(OH)6 double-shelled cubes could be easily prepared with alkaline etching by using single-shelled ZnSn(OH)6 cubes as both templates and precursors. (IV) Synthesis of ZnSn(OH)6 multi-shelled cubes. On the basis of the above preparation, ZnSn(OH)6 multi-shelled cubes were constructed with continuous separation of the adjacent inner/outer shells when ZnSn(OH)6 double-shelled cubes served as seed materials. (V) Finally, the conversation from as-obtained different ZnSn(OH)6 cubes to ZnSnO3 cubes due to thermal-induced dehydration via the annealing treatment in N2 environment (eq 3).19 Zn2++Sn4++6OH-→ZnSn(OH)6

(1)

ZnSn(OH)6+4OH-→Zn(OH)42-+Sn(OH)62-

(2)

ZnSn(OH)6→ZnSnO3+3H2O

(3)

Figure 2a-h shows the typical morphologies of ZnSnO3 cubes with solid/ single/double/multishelled structures characterized by SEM and TEM observations. The FESEM images of ZnSnO3 solid cubes, which have cubic structure and smooth surfaces in Figure 2a. After hollowing process, the materials inherit the geometric shape and obtain a rough cavity, which seem like a box as shown in Figure 2b. By repeated etching and thermal annealing process, the cubes with two shells and even more shells are present, which can be identified from some broken cubes in Figure 2c-d. The bigger cubes incubate the smaller counterparts two or more times producing the architecture of box in box. The multi-shelled hollow structures can provide not only numerous interstitial spaces between shells but also a lot of spaces in cavities. The


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TEM images in Figure 2e-h further reveal the inner structures of the different ZnSnO3 samples. The shells and void spaces can be distinguished by the contrast of the bright regions and dark edges. Thus, the shell number can be simply controllable from 1 to 3 through the continuous addition of alkaline solution. Size uniformity was recorded in Figure 2i-l. The diameters of most ZnSnO3 solid cubes are between 400 and 600 nm (Figure 2i). Because of the previous deposition of ZnSn(OH)6 layers on the surface of solid ZnSn(OH)6 cubes, the average size of particles of ZnSnO3 single-shelled cubes is a little bigger than that of ZnSnO3 solid cubes (Figure 2j). The size of most ZnSnO3 double-shelled cubes increases obviously and are varied from 700 to 900 nm (Figure 2k). For ZnSnO3 multi-shelled cubes, most particles possess a size in the range from 900 to 1100 nm (Figure 2l). As a result, this method can directly affect the particle size. To identify the phase composition of the samples, X-ray diffraction was characterized. The XRD diffraction peaks match well with the perovskite structural ZnSn(OH)6 JCPDS card no. 73-2384 (Figure S1), and no impure phases are observed. After calcination in N2, all of the products exhibit amorphous phases (Figure 2m). Energy dispersive spectroscopy (EDS) analysis reveals the existence of Zn, Sn and O elements and the molar ratio of Zn to Sn is about 1:1 (Figure S2). To investigate the Brunauer-Emmett-Teller (BET) specific surface area and the porosity of the ZnSnO3 materials, nitrogen adsorption-desorption isotherms and pore size distribution analysis were performed. The N2-BET values of the solid/single/double /multishelled cubes were calculated to be 37, 70, 86 and 98 m2 g−1, respectively (Figure 2n). It is obvious that ZnSnO3 cubes with multi-shelled structure possess a larger BET surface area, which can support sufficient adsorption sites and is beneficial for enhancing the gas sensing performance. The peaks of pore size distribution of the solid cubes, single-shelled cubes, double-shelled cubes and multi-shelled cubes centered at 2.19, 2.3, 2.88 and 2.58 nm,


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respectively (Figure 2o). The mesoporous structures enable gas to easily access or leave from the surface regions of ZnSnO3 multi-shelled cube, facilitating a fast mass transfer.20 Based on these results, ZnSnO3 multi-shelled cubes can be selected for gas sensor fabrication because of their uniform morphology properties, larger surface area, and mesoporous structure (Figure 3a). The TEM image in Figure 3b suggests that the synthesized multi-shelled ZnSnO3 retains cubic structure and further shows potent evidence that the ZnSnO3 cubes are made up of an exterior shell and two interior shells. A representative morphology is provided in Figure 3c, also revealing ZnSnO3 box-in-box structure with three shells. EDX elemental mappings are recorded in Figure 3d-g. Zn, Sn and O elements existed along the shape of the multi-shelled cubic structure with homogeneous dispersion. It has been reported that the thickness of shells is one of the critical parameters to influence the gas sensing performances of oxide semiconductors with hollow architecture.21 Thus, the shell thickness of different ZnSnO3 products is investigated. Figure S3a shows a typical dense, solid ZnSnO3 cube with the diameter of about 550 nm. After the first alkaline etching, the ZnSnO3 cube obtained an interior cavity and a porous shell of about 110 nm in thickness (Figure S3b). Figure S3c suggests that the shell thickness of most single-shelled ZnSnO3 cubes (S1) is mainly 100-120 nm. After the second alkaline etching, the inner shell thickness (D1) and outer shell thickness (D2) of a double-shelled cube, which are 90 nm and 70 nm in size, respectively, were clearly observed (Figure S3d). The inner shell thicknesses of most ZnSnO3 double-shelled cubes are varied from 70 to 90 nm (Figure S3e). The outer shell thicknesses of most ZnSnO3 double-shelled cubes are in the range from 50 to 70 nm (Figure S3f). From the closer observation of the dark periphery of the cube, the multi-shelled ZnSnO3 cube has a spacious internal hollow space with a diameter of about 500 nm. The outer shell, secondary shell and the inner shell are with thickness of about 30 nm, 50 nm and 40 nm, respectively, as


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shown in Figure 3h. After the third alkaline etching, all of the shells of multi-shelled cubes became thinner. The inner shell thickness (M1), secondary shell thickness (M2) and the outer shell thickness (M3) are mainly 20-50 nm, 40-60 nm and 20-40 nm, respectively (Figure 3i-k). Thus, the shell thickness of ZnSnO3 hollow cubes can be tuned effectively with this synthetic method. The elements of chemical states and surface chemical composition were investigated by X-ray photoelectron spectroscopy (XPS). In Figure 3l, the spectrum shows the presence of Zn, Sn and O elements. The peaks located at 1021.6 and 1044.7 eV (Figure 3m) can be ascribed into Zn 2p3/2 and Zn 2p1/2 of Zn2+.22 Two peaks nearby 494.8 and 486.3 eV (Figure 3n) are associated with Sn 3d3/2 and Sn 3d5/2 of Sn4+, respectively.23 Figure 3o presents the binding energy of 530.2 eV for O 1s, which can be assigned to the metal-oxygen bonds.24 Thus, ZnSnO3 multi-shelled cubes have been successfully synthesized through a step-by-step etching method. 3.2. Fabrication and Measurement of Gas Sensor. Figure 4a shows the schematic of the sensor structures based on ZnSnO3 samples with solid cubes, single-shelled cubes, double-shelled cubes and multi-shelled cubes. A paste was formed by mixing the as-obtained samples with deionized water. The paste used as the sensitive body with the thickness of about 10 µm was coated on a ceramic tube installed platinum wires and two gold electrodes. A heat wire (Ni-Cr alloy) served as a resistor was placed in the ceramic tube. The theoretic diagram of the test circuit was displayed in Figure 4b. Figure 4c displays the gas sensor experimental setup, which were performed on a CGS-8 series Intelligent Test Meter (China, ELITE TECH). The sensor response, which is calculated based on the resistance ratio (Ra/Rg, where Ra is initial resistance and Rg is the measured resistance in formaldehyde sample), was recorded with injections of different formaldehyde gas concentrations in a home-made


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gas chamber. The response/recovery behaviors are defined as the time that the resistance change to the 90% of total resistance after injecting target gas (adsorption) and dry air (desorption) into the gas chamber, respectively.6 To investigate the structural stability of the ZnSnO3 cubes during repeated formaldehyde sensing measurement for 30th cycles, we give the images of different ZnSnO3 samples before and after sensing tests. Figure 4d displays well-defined cube-shaped morphology with uniform distribution, which implies that the ZnSnO3 samples are very stable in ambient atmosphere. After measurement, a few of hollow cubes are crack and disintegrate in view (Figure 4e). Most of the ZnSnO3 cubes in the sensor were found to basically retain their original morphology, suggesting a good stability of the amorphous ZnSnO3 materials. The current-voltage (I-V) performances of all four samples were measured and the results are displayed in Figure 4f. The curve for ZnSnO3 multi-shelled cubes is lower than that for the solid cubes, single-shelled cubes and double-shelled cubes, which relates to the decreasing shell thickness and high surface area. When the shell is thin enough, the entire primary particles become active during gas sensing reaction.21,25 In addition, the gas sensing performance is mainly depended on the outermost layer. The outer shell layers, which play a more significant role in sensing process, became thinner and thinner. A semipermeable and thin outermost layer could provide more effective electron-depleted surface region from the inner/outer shells because of high gas accessibility.26 Moreover, the conductivity is very sensitive to the negatively charged oxygen species absorbed the surface of ZnSnO3. Because the ZnSnO3 multi-shelled cubes have larger surface areas, the amount of oxygen that can absorb and cover on the surface of ZnSnO3 is maximized. That is, more oxygen molecules can trap electrons from the conduction band of ZnSnO3 to form oxygen ions, resulting the increase of resistance. Thus, the tendency


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of the curves for ZnSnO3 multi-shelled cubes is less steep compared with the other ZnSnO3 products. The gas sensing performances to 100 ppm of methylbenzene, ammonia, acetone and formaldehyde were measured at 190-240 °C (Figure 5a-d). Apparently, all of the ZnSnO3 samples showed high responses to formaldehyde and relatively low responses to methylbenzene, ammonia and acetone. Therefore, the polar plots of the gas responses suggest that the selective detection for formaldehyde is possible by using ZnSnO3 sensors. When compared to the other ZnSnO3-based nanostructures, the multi-shelled cubes obtained a higher response to formaldehyde and the maximum response was obtained at 220 °C. The responses to 100 ppm formaldehyde were 10.7, 15.4, 24.1 and 37.2 for solid cubes, singleshelled cubes, double-shelled cubes and multi-shelled cubes at 220 °C, respectively. Among them, the ZnSnO3 multi-shelled cubes-based sensor revealed about 3.5, 2.4 and 1.5 times enhancement in response, respectively, which are believed to be responsible for the larger specific surface areas provided by multilevel hollow-structure. This result shows a direct relationship between the gas sensing properties and the shell number, and demonstrates that the gas sensing performance can be effectively improved by controlling internal structures of hollow materials. Figure 5e-h shows the dynamic sensing transients of the ZnSnO3 materials. When the sensor was exposed to reducing gases, the distinct decrease in resistance revealed gas sensing characteristics of n-type metal oxide semiconductors.27 Moreover, the original resistances in air increased with the shape evolution from the solid structures to the complex multi-shelled hollow cubes, which could be attributed to the form of thin shell structures, high surface area and less agglomerations. Figure 6a shows the transient response/recovery sensing curves of the ZnSnO3 multishelled cube sensor toward formaldehyde with concentrations ranging from 10 to 200 ppm at


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220 °C. It can be seen that the multi-shelled cube sensor provided a stable baseline in the dry air condition. After formaldehyde was injected, the sensor got a positive response. When the sensor was exposed in air again, it presented recovery characteristic. With the corresponding formaldehyde concentration increasing, the response of multi-shelled cube sensor increased. The response of the sensor based on the ZnSnO3 multi-shelled cubes to 10, 20, 50, 100 and 200 ppm were 12.8, 17.5, 24.9, 37.2 and 44.6, respectively. The response/recovery time is also a critical parameter for the application of gas sensors.28 Figure 6b shows the testing results of ZnSnO3 multi-shelled cubes based sensor. The response time and the recovery time were about 1 s and 59 s when the formaldehyde concentration was 100 ppm. The response of multi-shelled cube sensor toward formaldehyde with concentrations ranging from 10 to 2000 ppm at the optimum operating temperature of 220 °C is shown in Figure 7a. It can be observed that the sensor exhibited a rapid increase of response with a linear trend and increased by 24.4 under low formaldehyde concentration (10-100 ppm) (Figure 7b). However, when the formaldehyde concentration was above 500 ppm, the responses of multi-shelled cube sensor showed a slower growth trend and increased by 5.2 in the concentration range of 1500 ppm (500-2000 ppm). This phenomenon can be ascribed to the surface active occupation of formaldehyde gas or the lack of adsorbed oxygen species.29,30 The long-term stability of a gas sensor is necessary for the practical applications. The results in Figure 7c showed that the multi-shelled cube sensor tended to remain relatively stable during a longterm stability measurement of 30 days. The sensor showed a decrease of about 5%, implying a good reliability. The dynamic 5-cycle response curve to 100 ppm formaldehyde was displayed in (Figure S4). In the whole process, the sensor exhibited the excellent response and recovery properties, which indicated that the sensor obtained good repeatability for formaldehyde detection.


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In our work, the related formaldehyde gas sensing performances were summarized in the Table 1. The ZnSnO3 cubes with multi-shelled hollow structure possessed a high response, short response time and fast recovery process compared with ZnSnO3 with fewer shells. The response/recovery time decreased with the shell number increasing, which can be imparted to the ZnSnO3 multi-shelled cubes through the formation of thin, permeable shell structure (Figure S3). In order to further consider the application value of the ZnSnO3 multi-shelled cubes-based sensor, a comparison about the formaldehyde sensing ability of different materials was listed in Table 2. The response (37.2) of ZnSnO3 multi-shelled cubes to formaldehyde is higher than that of other materials and the response time (1s) is the fastest in these literatures.19,31-35 3.3. Gas Sensing Mechanism of the ZnSnO3 Multi-Shelled Cubes. The sensing mechanism that is widely accepted is the electrical conductivity taken place in the surface of the materials, which involves serial processes: adsorption-oxidation-desorption. When ZnSnO3 multi-shelled cubes are in the air, oxygen molecules can absorb on the surface of ZnSnO3 shell and translate into more reactive oxygen ions (O−)36,37 by trapping electrons. Compared with solid-, single-shelled- and double-shelled ZnSnO3 cubes, the thinner, porous structure enable multiple oxygen molecules to diffuse entire sensing surface (Figure 7d). Benefiting from the appealing structure, the electron depletion layer can form on the both outer shell and interior shells, leading to a high resistance value.38 In formaldehyde gas, the electrons are released back into the sensing material, which results in a dramatic decrease of resistance and also a rapid increase of response. From the N2 adsorption-desorption isotherm results, it can be clearly verified that multi-shelled cubes obtained a large surface area. It provides more surface active sites and is helpful for multilevel shells to adsorb more target gas.26 Moreover, the complex structure remains hollow interior and stable shells without


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structural collapse, which could prevent them from aggregating6,12,23 and also improve the stability of gas sensing performances. When ZnSnO3 multi-shelled hollow cubes break away from the formaldehyde gas, the resistances return to the original state in the air. Thus, the improving gas performances for ZnSnO3 multi-shelled hollow cubes can be attributed to (1) thinner shell layer (2) porous, gas-accessible, multi-shelled structures and (3) high surface area with less agglomeration. 4. CONCLUSIONS In summary, we demonstrate a simple, low-cost, easy-controlled method to prepare welldefined hollow ZnSnO3 cubes with variable shell number of 1-3 shells by a co-precipitation and alkaline etching route. The increase of shell number, which ensures the large surface area and more active sites, reflects directly the formaldehyde sensing performance of different ZnSnO3 materials. The formaldehyde responses of ZnSnO3 multi-shelled cubes are about 3.5/2.4/1.5fold higher than that of solid-, single-shelled-, and double-shelled ZnSnO3 cubes, respectively. Resulting sensors based on ZnSnO3 multi-shelled cubes also possess fast response time (1 s) and good reproducibility (30 days), which are suitable for effective and continuous detection of formaldehyde. The outstanding performances all point to ZnSnO3 multi-shelled cube providing a good platform to monitoring formaldehyde. It is anticipated that the unique sensing properties of ZnSnO3 multi-shelled cubes make it of the potential materials in gas sensor field.

ASSOCIATED CONTENT SUPPORTING INFORMATION Supporting Information. The XRD patterns, EDS spectrum and detailed sensing performance of sensing materials are listed. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Postdoctoral Science Foundation of China (No. 2015M571361, 2016M601131 and 2016T90251) and the Natural Science Foundation Committee (NSFC, Grant No. 51502110 and 61504136).


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FIGURE CAPTIONS Figure 1. Illustration of the formation of different ZnSnO3 cubes. Figure 2. FESEM images, TEM images and size distribution (diameter) of prepared ZnSnO3 cubes: (a, e and i) solid cubes (b, f and j) single-shelled cubes (c, g and k) double-shelled cubes and (d, h and l) multi-shelled cubes. The XRD patterns (m), nitrogen adsorptiondesorption isotherm (n) and pore size distribution (o) of different ZnSnO3 cubes. Figure 3. (a) Schematic diagrams showing the structure of the ZnSnO3 multi-shelled cube gas sensor. (b) Low-magnification TEM images of the precursors. (c) TEM image of an individual ZnSnO3 multi-shelled cube. (d-g) EDX mapping images of an individual ZnSnO3 multi-shelled cube. (h) TEM image of the shell thickness of the ZnSnO3 multi-shelled cube. Shell thickness distribution of (i) M1, (j) M2 and (k) M3. XPS spectrum of (l) survey spectrum, (m) Zn 2p spectrum, (n) Sn 3d spectrum and (o) O 1s spectrum for the ZnSnO3 multi-shelled cubes. Figure 4. Schematic diagrams of (a) the ZnSnO3-based sensor, (b) the electrical circuit for measuring the gas sensor and (c) gas sensing test equipment. SEM images of (d) ZnSnO3 samples before gas sensing test and (e) ZnSnO3 samples after gas sensing test (f) I–V curves of gas sensing devices based on ZnSnO3 samples at room temperature. Figure 5. Polar graphs and gas responses (Ra/Rg) of the (a) solid cube, (b) single-shelled cube, (c) double-shelled cube and (d) multi-shelled cube sensors to various gases at 190-240 °C, respectively. Response transients (220 °C) of (e) solid cube, (f) single-shelled cube, (g) double-shelled cube and (h) multi-shelled cube sensors to 100 ppm formaldehyde. Figure 6. (a) Response/recovery graphs of the ZnSnO3 multi-shelled cubes based gas sensor to formaldehyde with the concentration from10 ppm to 200 ppm at 220 °C. (b) Response/


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recovery time graphs of the ZnSnO3 multi-shelled cubes based gas sensor to 100 ppm formaldehyde at 220 °C. Figure 7. (a) The relationship of response vs the formaldehyde concentration for ZnSnO3 multi-shelled cube sensor. (b) Response of the ZnSnO3 multi-shelled cube sensor to 10-100 ppm formaldehyde and to 500-2000 ppm formaldehyde. (c) Long-term stability of the gas sensor based on ZnSnO3 multi-shelled cubes with 100 ppm formaldehyde. (d) Schematic of sensing mechanism of ZnSnO3 multi-shelled cube samples. Error bars show the standard deviation for n = 3 measurements. TABLE Table 1. Comparison of formaldehyde-sensing performances of gas sensors based on various ZnSnO3 structures. Table 2. Comparison of formaldehyde sensing ability of different gas sensors.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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Figure 7.


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Table 1

ZnSnO3 material

BET/m2g-1

R0/MΩ

Res./Ra/Rg

Tr1/s

Tr2/s

Solid cubes

37

63

10.7

6

80

Single-shelled cubes

70

93

15.4

4

73

Double-shelled cubes

86

115

24.1

2

69

Multi-shelled cubes

98

136

37.2

1

59

♦R0: resistance in air; ♦Res.: response ♦Tr1: response time; ♦Tr2: recovery time


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Table 2

Materials

Con./[ppm]

ZnSnO3 porous cubes

Res. /[Ra/Rg]

Tem/ [ºC]

Tres/[s]

Ref.

100

36.8

300

3

[19]

ZnSnO3 nanocages

50

~5

210

25

[31]

Pd-SnO2 nanofibers

100

~18.5

160

2

[32]

20

8.2

240

1

[33]

2000

9.9

200

120

[34]

100

24.6

160

3

[35]

ZnSnO3 multi-shelled cubes 100

37.2

220

1

In2O3 porous polyhedra Pt-NiO In/Pd-SnO2 microsphere

[This work]

♦Con.: Concentration; ♦Res.: response ♦Tem: Temperature; ♦Tres: Response time; ♦Ref.: Reference


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TOC Graphic


33

Hollow ZnSnO3 Cubes with Controllable Shells Enabling Highly

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