High-Energy Faceted SnO2-Coated TiO2 Nanobelt Heterostructure for

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High-Energy Faceted SnO2‑Coated TiO2 Nanobelt Heterostructure for Near-Ambient Temperature-Responsive Ethanol Sensor Guohui Chen,† Shaozheng Ji,† Haidong Li,† Xueliang Kang,† Sujie Chang,† Yana Wang,† Guangwei Yu,† Jianren Lu,§ Jerome Claverie,∥ Yuanhua Sang,*,† and Hong Liu*,†,‡ †

State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Science, Beijing 100864, China § School of Information Science and Engineering, Shandong University, Jinan, Shandong 250100, China ∥ NanoQAM Research Center, Department of Chemistry, University of Quebec at Montreal, 2101 rue Jeanne-Mance, CP 8888, Montreal, Quebec H3C3P8, Canada ‡

S Supporting Information *

ABSTRACT: A SnO2 gas sensor was prepared by a two-step oxidation process whereby a Sn(II) precursor was partially oxidized by a hydrothermal process and the resulting Sn3O4 nanoplates were thermally oxidized to yield SnO2 nanoplates. The SnO2 sensor was selective and responsive toward ethanol at a temperature as low as 43 °C. This low sensing temperature stems from the rapid charge transport within SnO2 and from the presence of high-energy (001) facets available for oxygen chemisorption. SnO2/TiO2 nanobelt heterostructures were fabricated by a similar two-step process in which TiO2 nanobelts acted as support for the epitaxial growth of intermediate Sn3O4. At temperatures ranging from 43 to 276 °C, the response of these branched nanobelts is more than double the response of SnO2 for ethanol detection. Our observations demonstrate the potential of low-cost SnO2-based sensors with controlled morphology and reactive facets for detecting gases around room temperature. KEYWORDS: high energy facet, SnO2 nanoplates, room temperature sensor, ethanol, TiO2 nanobelts, heterostructure hierarchical 2D SnO2 nanosheets with high index (113̅) and (102)̅ facets15 were reported to exhibit better gas-sensing performance than the conventional SnO2 nanocrystals, which are usually covered by (110), (101), and (100) facets with low surface energy.16 Importantly, in a typical crystal growth process, low-energy facets are predominant because these are the most stable ones. Tto obtain a sensing material working at a low temperature, one needs to create a nanomaterial with a large number of high-energy facets. Nanostructured SnO2 can be synthesized by wet-chemical methods or by sol−gel methods. Furthermore, SnO2 nanoparticles can also be obtained by oxidizing tin oxide of lower degree of oxidation. Previous studies have shown that calcination of tin monoxide (SnO) in air at elevated temperature results in the formation of SnO2 with controlled morphology.17−19 This technique allows for the preparation of 2D layered structures.20−22 The mixed oxide Sn3O4 is known to be an intermediate during the oxidative transformation of SnO into SnO2.18 As revealed by theoretical theoretical calculations, Sn3O4 adopts a (101)-layered rutile-type structure,22 with onethird of the Sn atoms in divalent tetrahedral coordination sites,

1. INTRODUCTION Gas sensors are becoming increasingly ubiquitous in our society. They are typically constituted of a semiconductor sensing material encased in an electronic transducing device. Sensors based on metal oxide semiconductors have undergone extensive development, because they usually offer acceptable sensing performance in terms of sensitivity, selectivity, stability, response time, and cost.1−3 However, they also need to be operated at high temperatures, which creates severe safety, stability, and miniaturization issues. Thus, an essential trend in sensor technology resides in the discovery of materials that are responsive at low operating temperature, and most ideally at room temperature.4,5 Sensors based on SnO2 exhibit high sensitivity and selectivity at working temperatures above 200 °C.6−8 SnO2 nanocrystals with specific crystal facets have been synthesized in order to lower the working temperature and to increase the sensing activity. These facets9−11 influence the transport rate of the electrons and the adsorption enthalpy of the analytes.12 Active facets can generally be tuned by decreasing the scale of 0D SnO2 nanoparticles; however, it is at the cost of a conductivity decrease for only a limited enhancement in sensitivity.13 Hierarchical architectures of SnO2 exhibiting highly reactive surfaces have attracted intensive research. For example, 1D tetragonal SnO2 nanorods with high energy (221) facets14 and © XXXX American Chemical Society

Received: September 13, 2015 Accepted: October 20, 2015

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DOI: 10.1021/acsami.5b08630 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Then, the obtained product was isolated from the solution by centrifugation, which was washed thoroughly with deionized water for several times, and dried at 70 °C for 10 h. Finally, anatase TiO2 nanobelts with coarsened surface were acquired by thermal annealing at 600 °C for 2 h. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns were taken on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα (λ = 0.15406 nm) radiation. Scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDS) were performed on a HITACHI S-4800 field-emission scanning electron microscope. Transmission electron microscopy (TEM), high-resolution transmission electron microscopic (HRTEM), and selected area electron diffraction (SAED) were acquired from a JEOL JEM 2100 microscope. X-ray photoelectron spectroscopy (XPS) was recorded with an ESCALAB 250 system. Fourier-transform infrared (FTIR) spectra were collected on a Nicolet Avatar 370 infrared spectrometer in the range of 400- 3800 cm−1 using pressed KBr discs. The Agilent 4294A precision impedance analyzer and KSL-1100X furnace were adopted to measure the change of resistance upon exposing the sensor device to different temperatures. Mott−Schottky (M−S) plots were obtained with a Gamry electrochemical station (reference 3000, USA). 2.4. Gas Sensing Analyses. A WS-3A gas sensing system (Zhengzhou Winson Electronics Technology Co. Ltd.) was adopted to measure the gas sensing performance. The synthesized samples were mixed with water under constant grinding. Then the slurry was spin-coated onto ceramic tubes with two interdigitated Au electrodes on its top surface, to form a thin sensitive film (about 3 μm). After drying at room temperature for 24h, the tubes were soldered to the test chamber by platinum wires connected to the Au electrodes and a resistive heating wire was inserted into the tube to control the working temperature. The gas sensors were further heat-treated at 200 °C for 24h on an ambient atmosphere to improve the contact between the sample and electrodes. Additional experimental details are provided in the Supporting Information. The gas concentration was controlled by injecting a certain volume of ethanol, methanol or acetone, and purging with air to recover the sensor resistance. The relative humidity was around 50% and the room temperature was about 25 °C. The gas response is calculated by the ratio Ra/Rg, where Ra and Rg are the measured resistances of air and tested gases, respectively. The resistances were measured by monitoring the terminal voltage of the load resistor at a test circuit voltage of 5 V. Figure S1 illustrates the working principle of the electric circuit system.

two-thirds in tetravalent octahedral sites, and oxygen vacancies in (101) planes.23−25 The predicted crystalline structure of heterovalent Sn3O4 has been experimentally confirmed and its properties as photocatalyst and gas sensor have also been reported.26,27 As shown in this work, Sn3O4 could also serve as sacrificial template to prepare SnO2 nanostructures with unique crystalline structures and physicochemical properties. It is generally difficult for a gas sensor based on a single oxide semiconductor to satisfy all requirements on sensitivity, selectivity, stability, and working temperature. Sensors based on two or more components have been explored to improve gas sensing performance.28,29 Among the various oxides, 1D TiO2, such as TiO2 nanobelts, nanorods, and nanotubes, are promising candidates for the design and fabrication of hierarchical nanostructures.30 TiO2 nanobelts possess a large surface area, and provide sufficient space for the nucleation and growth of a second generation of nanoparticles on its surface. Thus, a series of 1D TiO2-based sensors such as Fe2O3/TiO2, V2O5/TiO2, ZnO/TiO2 have been reported to exhibit enhanced sensitivity.31−33 Herein, a SnO2/TiO2 nanobelt heterostructure was synthesized by the in situ oxidation of Sn3O4 nanoplates supported on the surface of TiO2 nanobelts. As the resulting SnO2 nanoplates exhibit a large number of high energy facets, the SnO2/TiO2 nanobelt heterostructures is sensitive and selective to ethanol, even at temperatures close to ambient temperature. This remarkable property can be attributed to the combination of several factors: the presence of a heterojunction, the large sensing surface SnO2 nanoplates with high energy facets, and the rapid electron transport in TiO2 nanobelts.34 We have therefore demonstrated that it is feasible to design a gas sensor that functions at near-ambient temperature using a facile and economical synthetic approach.

2. EXPERIMENTAL SECTION 2.1. Preparation of SnO2 Nanoplates. All chemicals, including tin(II) chloride dihydrate (SnCl2·2H2O), sodium citrate dihydrate (Na3C6H5O7·2H2O), Titania P25 (TiO2: ca. 80% anatase and 20% rutile), sodium hydroxide (NaOH), hydrochloric acid (HCl), and sulfuric acid (H2SO4), were purchased from Sinopharm and used without any further treatment. In a typical experiment, Sn3O4 nanosheets were synthesized by hydrothermal treatment of the transparent precursor solution,35 which was prepared by dissolving 5.0 mmol of SnCl2·2H2O, 12.5 mmol of Na3C6H5O7·2H2O, and 2.5 mmol of NaOH in 40 mL of deionized water and stirred for 30 min. The solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. After cooling to room temperature, the obtained yellow powder was washed with deionized water and ethanol, then dried at 80 °C for 12 h. To oxidize Sn3O4 to SnO2, calcination in air was formed by ramping the temperature at a rate of 10 °C/min up to 550 °C and by maintaining this temperature for 2 h. 2.2. Construction of SnO2/TiO2 Heterostructure. Hierarchical SnO2/TiO2 nanobelts were fabricated by similar process with addition of certain amount TiO2 nanobelts (10.0, 20.0, and 30 mmol) into the precursor solution. The nanobelts were presynthesized according to a previous reported alkali-hydrothermal method with minor modifications.26 Briefly, 0.8 g of P25 and 80 mL of 10 M NaOH aqueous solution were mixed and stirred for 1 h before transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 72 h. After cooling down to room temperature, the Na2Ti3O7 product was washed with deionized water to remove the excess NaOH, followed by immersing in 100 mL of 0.1 M HCl for 24 h to produce H2Ti3O7 nanobelts. To coarsen the obtained H2Ti3O7 nanobelts, we mixed 40 mL of 0.02 M H2SO4 aqueous solution with the H2Ti3O7 in a 50 mL of Teflon-lined stainless steel autoclave and heated at 100 °C for 12 h.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Property of SnO2. To prepare SnO2 with high energy facets, Sn3O4 obtained by hydrothermal process was selected as precursor material. The thermal behavior of Sn3O4 was investigated by Thermogravimetric Analysis (TGA) in air (Figure 1A). Two distinct weight loss steps were observed: the first weight loss of 0.89% before 200 °C is due to the evaporation of absorbed water, and the second step (0.55%) occurring at around 200−350 °C was attributed to the elimination of structural water (hydroxyl groups). Fourier-transform infrared (FTIR) spectra of Figure 1B confirmed this dehydration process. The peaks at 3421, 1606, and 1365 cm−1, corresponding to −OH stretching vibrations, almost disappeared after calcination at 350 °C for 2 h. The characteristic peaks at 588, 528, and 482 cm−1 assigned to Sn− O stretching vibrations36 did not significantly change. Above 350 °C, the TGA curve indicated a weight increase and the color of the sample changed from yellow (Figure 1A-a) to black (Figure 1A-b), as expected for the oxidation of Sn3O4. The oxidation degree of Sn in the calcined Sn3O4 sample was determined by X-ray photoelectron spectroscopy (XPS) (Figure 1C). The Sn 3d spectrum shows the characteristic splittings of Sn 3d5/2 and Sn 3d3/2 at binding energies ranging from 484 to 488 eV and from 493 to 497 eV, respectively. The B

DOI: 10.1021/acsami.5b08630 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (A) Typical SEM image of the Sn3O4 nanoplates; (B) SEM image of the SnO2 obtained after calcination of Sn3O4 at 550 °C for 2 h (inset: corresponding TEM image) (C) HRTEM image of SnO2 (inset: TEM image of the particle, showing in a red square the HRTEM zone); (D) SAED patterns of SnO2 (inset: corresponding crystal lattice, Sn atoms in gray, O atoms in red).

Figure 1. (A) TGA of Sn3O4 (insets Sn3O4 (a) before and (b) after calcination at 550 °C for 2 h); (B) FTIR spectra of Sn3O4 before (i) and after calcination at (ii) 200 °C and (iii) 350 °C for 2 h; (C) XPS spectra (Sn 3d lines) of the samples (a) before and (b) after calcination at 550 °C for 2h. D) XRD patterns of the samples (a) before and (b) after calcination at 550 °C for 2 h, the standard data for Sn3O4 (JCPDS No. 16−0737) and SnO2 (JCPDS No. 41−1445) are presented for the sake of comparison.

prepared SnO2 nanoplates are single crystals with a (001) exposed facet. The inset in Figure 2D illustrates the schematic atom distribution according to the diffraction pattern. As calculated by Slater et al.,39 the (001) facets of SnO2 have a higher surface energy (2.363 J m−2) than common low-index facets such as (110) (1.401 J m−2), (101) (1.554 J m−2), and (100) (1.648 J m−2). We infer the presence of high energy facets in SnO2 could be beneficial to gas sensing. Accordingly, we measured the related gas sensing performance of Sn3O4 and SnO2 prepared above. Figure 3A compares the ethanol response at different working temperatures, ranging from 43 to 276 °C. Here, the gas response is defined as the ratio of the resistance in air (Ra) to the resistance in the presence of the gaseous analyte (Rg). It can be seen that Sn3O4 and SnO2 share

deconvolutions of the Sn 3d5/2 and Sn 3d3/2 peaks revealed the presence of two major peaks at 485.9 and 494.4 eV (Sn(II) species) each with a shoulder at 486.7 and 495.3 eV (Sn(IV) species), respectively.35 The quantitative analysis of the XPS data indicates that the atomic ratio of Sn(II)/Sn(IV) is about 2:1, which is consistent with the stoichiometric ratio of Sn3O4. After calcination, the Sn 3d5/2 (485−488.5 eV) and Sn 3d3/2 (493.5−496.5 eV) binding energies match with the core levels of Sn4+ at 486.7 and 495.3 eV, respectively. The signals related to Sn2+ related have disappeared, indicating that the conversion of Sn2+ to Sn4+ is complete during the thermal treatment. This transformation was also monitored by X-ray diffraction (XRD). Diffraction peaks of the synthesized Sn3O4 (Figure 1D-a) correspond to the standard triclinic Sn3O4 phase (JCPDS card No. 16−0737).37 After calcination at 550 °C for 2 h, the peaks match those of the tetragonal SnO2 phase (JCPDS card No. 41−1445), with no extra peak observed, which is indicative of the high purity of the product. The above characterizations confirm that triclinic Sn3O4 has been transformed to the tetragonal SnO2. The samples were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in order to characterize their morphology and microstructure. The Sn3O4 aggregates (Figure 2A), consists of nanoplates with size ranging from 100 to 200 nm and thickness about 10 nm. The subsequent calcination leads to a decrease in size and an increase in thickness. As revealed in Figure 2B, the irregular SnO2 nanoplates are 20 to 200 nm long and around 20 nm thick. The noticeable decrease of the nanoplate surface area possibly arises from the strain induced by the phase transformation.38 Figure 2C displays a high resolution transmission electron microscopy (HRTEM) image of a typical SnO2 nanoplate. The perpendicularly crossed lattice fringes correspond to the (110) and (11̅0) plane of SnO2. The corresponding selected area electron diffraction (SAED) pattern in Figure 2D can be indexed to the [001] zone axis of single-crystal tetragonal SnO2, which implies that the as-

Figure 3. (A) Variation in the response (Ra/Rg) to ethanol vapor for (a) Sn3O4 and (b) SnO2 obtained by calcining Sn3O4 at 550 °C for 2h; (B) response of SnO2 sensor to (a) ethanol, (b) methanol, and (c) acetone gases at a concentration of 100 ppm at different working temperatures (inset: same curves for a Sn3O4 sensor); (C) dynamical response−recovery curves toward ethanol for (a) Sn3O4 and (b) SnO2 sensors at a working temperature of 43 °C; (D) electrical resistance vs temperature for (a) Sn3O4 and (b) SnO2 sensors. C

DOI: 10.1021/acsami.5b08630 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the same optimum working temperature at about 240 °C, and exhibit similar gas response at temperatures above 200 °C. For both Sn3O4 and SnO2 sensors, the response of ethanol is much larger than those of acetone and methanol, which suggests that the sensors show selectivity for ethanol at high temperature. At temperatures below 100 °C, the response of the Sn3O4 sensor to the three gases is very low, with only a marginally higher response for ethanol. By stark contrast, for the SnO2 sensor, the responses to methanol and acetone are very low, but the response to ethanol remains very high (higher than 7), and commensurate with the response observed at higher temperature. At 43 °C, the response to ethanol is 5−6 times higher than for methanol and acetone, which indicates a very high selectivity to ethanol. To the best of our knowledge, this remarkable response to ethanol and selectivity at 43 °C has never been reported before for a SnO2-based sensor. Thus, it is a proof of concept that SnO2 can be used as selective sensor at near room temperature. Figure 3C depicts the dynamic gas responses of Sn3O4 and SnO2 sensors at a working temperature of 43 °C. The response of Sn3O4 is negligible while that of SnO2 reaches 7.65, although it takes almost 40 min for the SnO2 sensors to fully convert the ethanol concentration to an electrical signal. The increase over time of the response in Figure 3C-b corresponds to a decrease in the sensor resistance upon exposure to the reducing gas, corresponding to the typical behavior of a n-type semiconductor. In air, oxygen molecules are believed to adsorb at the surface of SnO2, which could easily trap free electrons to form chemisorbed oxygen species, such as O−.40 When the ethanol vapor is injected into the sensor, the gas molecules tend to react with the ionized oxygen species as shown in the following reversible reaction C2H5OH + O− ⇄ C2H5OH − O + e−

Figure 4. SEM images with different magnifications of (A, B) TiO2 nanobelts with coarsened surface, Sn3O4/TiO2 nanobelt heterostructure with Sn/Ti molar ratio of (C, D) Sn/Ti = 2/1; (E, F) Sn/ Ti = 4/1; (G, H) Sn/Ti = 6:1.

of the Sn3O4 nanoplates could be tuned by varying the amount of TiO2 nanobelts in the precursor solution. Figure 4C, D show the resulting SEM images of Sn3O4/TiO2 nanobelt heterostructures with molar ratio Sn/Ti = 2/1. The Sn3O4 nanolates are dispersed perpendicularly on the TiO2 nanobelts. Similar hierarchical TiO2 nanobelts with obvious higher density are observed by increasing the Sn/Ti to 4/1 (Figure 4E, F) and 6/ 1(Figure 4G, H), and excess aggregated Sn3O4 nanoplates are also detected in Figure 4H. XRD patterns of Sn3O4/TiO2 nanobelts heterostructures, pure TiO2 and Sn3O4 are shown in Figure 5A. The TiO2 nanobelts and Sn3O4 nanoplates can be indexed as anatase TiO2 (JCPDS card, no. 21−1272) and triclinic-phase Sn3O4 (JCPDS card, no. 16−0737). For Sn3O4/TiO2 heterostructures (Figure 5A-b−d), all the diffraction peaks can be indexed to either Sn3O4 or to TiO2, and the corresponding peaks of Sn3O4

(1)

The oxidation reaction increases the electron density and thus decreases the resistance of the sensor. After purging with air, the oxidized ethanol desorbs from the surface of SnO2 and the resistance recovers its initial level. The high surface energy and electrical conductivity of the sensing material are supposed to play a key role in the formation of the surface-absorbed ionized oxygen species, which possibly explains the low temperature at which SnO2 functions. The exposed high energy (001) surface leads to high chemical activity, such as high density of dangling bonds, which is beneficial for the electron injection process and for the adsorption of oxygenated species. The resistance of the gas sensing devices based on Sn3O4 and SnO2 were recorded at various temperatures (Figure 3D and Figure S2). As the temperature decreases, the resistance of Sn3O4-based sensor increases sharply, whereas the resistance of the SnO2 sensor increases less drastically. This is indicative of a better electrical transport in SnO2 and thus a higher concentration of O− at the surface of the SnO2 nanoplates.41 As for the selectivity toward ethanol, it is possibly linked to the relatively stronger interaction between ethanol and the surface-absorbed oxygen species. 3.2. Preparation and Property of Heterostructures. To further improve the sensor property of SnO2, we assembled the above Sn3O4 nanoplates on TiO2 nanobelts to yield Sn3O4/ TiO2 nanobelt heterostructures. Surface coarsened TiO2 nanobelts (Figure 4A, B) with length over ten micrometers, width of 100−200 nm and thickness about 20−40 nm acted as backbone for the nucleation and hydrothermal growth of Sn3O4.26 Through a thermal process, the Sn3O4/TiO2 was oxidized in a SnO2/TiO2 nanobelt heterostructure. The density

Figure 5. (A) XRD patterns and (B) variation of the response (Ra/Rg) to ethanol vapor at a concentration of 100 ppm of (a) Sn3O4, Sn3O4/ TiO2 heterostructures with Sn/Ti molar ratio of (b) Sn/Ti = 2/1; (c) Sn/Ti = 4/1; (d) Sn/Ti = 6:1, and (e) TiO2; (C) gas sensing response of the Sn3O4/TiO2 heterostructure with molar ratio Sn/Ti = 4/1 versus concentrations of ethanol vapor at different working temperatures. (D) Response of the Sn3O4/TiO2 heterostructure (Sn/Ti = 4/ 1) sensor to various test gases (concentration of 300 ppm) at various working temperatures. D

DOI: 10.1021/acsami.5b08630 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces become stronger with the increase of Sn/Ti molar ratio, thus confirming the successful growth and attachment of Sn3O4 nanoplates. Energy dispersive X-ray spectroscopy (EDS) analysis (Figure S3) also confirms the presence of element Sn, Ti and O in Sn3O4/TiO2 heterostructure (Sn/Ti = 4/1). The gas sensing performance of the Sn3O4/TiO2 heterostructures toward ethanol (100 ppm) was investigated. The TiO2 sensor (Figure 5B-e) exhibits a low response even at working temperatures above 250 °C, whereas the Sn3O4 (Figure 5B-a) and Sn3O4/TiO2 heterostructures (Figure 5Bb−d) sensors show much higher responses from 140 to 295 °C, with a maximum at about 250 °C. The Sn 3 O 4 /TiO 2 heterostructure with molar ratio Sn/Ti = 4/1 (Figure 5B-c) has the most prominent response enhancement (about 13), thus demonstrating that Sn3O4 assembled on TiO2 nanobelts with proper Sn/Ti ratio can improve the sensing response. The enhancement can be explained by the presence of a heterostructure between Sn3O4 and TiO2 nanobelts which is beneficial to the interband electron transfer. Furthermore, the gas sensing performance of the optimal Sn3O4/TiO2 heterostructure (Sn/Ti = 4/1) toward ethanol at different concentrations and at different working temperatures was also conducted. The result of those experiments is summarized in Figure 5C, suggesting a sensing performance which is concentration and temperature dependence sensing performance. The Sn3O4/TiO2 heterostructure (Figure 5D, Sn/Ti = 4/ 1) has also a much higher response toward ethanol than methanol or acetone. On the basis of the results above, we selected the Sn3O4/ TiO2 nanobelt heterostructure with molar ratio Sn/Ti = 4/1 to prepare the SnO2/TiO2 nanobelt heterostructure by calcination at 550 °C for 2h. The XRD pattern (Figure 6A-c) reveals that the dominant peaks of the SnO2/TiO2 heterostructure are attributed to the tetragonal SnO2 (Figure 6A-b). In the zoom-in spectrum, the two prominent diffraction peaks of anatase TiO2 (Figure 6A-a) are marked at 2θ = 25.28 and 48.05°. The weak intensity of TiO2 may arise from the formation of tiny SnO2

nanoplates that cover the TiO2 nanobelts. As observed from the corresponding SEM images in Figure 6B, C, the oxidized SnO2 nanoplates with length ranging from 20 to 200 nm are scattered perpendicularly to the TiO2 nanobelts. Such geometry restricts the X-rays from accessing the inner part of the TiO2 nanobelts and consequently weakens the intensity of the TiO2 diffraction pattern. XPS spectra of the SnO2/TiO2 nanobelts reveals the presence of Sn and Ti atoms in the Sn4+ and Ti4+ state respectively (Figure S4). In addition, the clear lattice interface shown in the HRTEM image (Figure 6D) proves the formation of a heterostructure between SnO2 and TiO2. The growth direction of the TiO2 nanobelt is (101) with 0.35 nm interplanar distance. In the SnO2 part, the (110) plane has almost the same interplanar distance so that each (110) SnO2 plane extends a (101) TiO2 (101) plane. The atomic model in Figure 6D illustrates the heterojunction between SnO2 and TiO2. Due to the near-perfect match between the (110) SnO2 and (101) TiO2 planes, the heterostructure favors electron transport and sensing performance. Gas-sensing performance of the SnO2/TiO2 nanobelt heterostructure was investigated at operating temperatures ranging from 43 to 276 °C. As for the SnO2 sensor, the SnO2/ TiO2 sensor shows selectivity to ethanol, with an optimal operating temperature of about 250 °C, at which the response reaches 25.7 (Figure 7A). By raising the temperature to 276 °C, the response decreases because of the adsorption/desorption kinetics of ethanol.27 We also find that the response of the SnO2/TiO2 sensor toward ethanol vapor only slightly decreased by reducing the working temperature, indicating its high sensitivity at low temperature. It is worth mentioning that the overall sensitivity of the heterostructure is twice higher than the one of a SnO2 sensor under the same test conditions, which implies that adding TiO2 nanobelts can expedite electron transport and enhance chemisorbed oxygen activity. The TiO2 nanobelts also act as backbone for the epitaxial growth of Sn3O4 and subsequent SnO2 nanoplates. Thus, TiO2 prevents the aggregation of SnO2, leading to more interaction sites for analytic gases. Also, the heterojunction between the two n-type semiconductors of SnO2 and TiO2 is thought to be responsible for the enhancement of the interband electron transfer, which boosts the carrier concentration (ND) as verified by Mott− Schottky (MS) method (Figure S5).42 The dynamic responses of the SnO2/TiO2 heterostructure when exposed to 100 ppm ethanol at different temperatures are shown in Figure 7B. The response of the SnO 2 /TiO 2 undergoes a drastic rise upon the injection of ethanol and a decline to its initial value after exposing the sensor to air for some time. The response time is shortened at higher temperatures because of the active dynamics of ethanol at high temperature. Figure 7C shows the typical sensing profile of the SnO2/TiO2 sensor operating at 43 °C upon exposure to ethanol at different concentrations, and the inset plot denotes the corresponding response value. It is seen that the sensor has a wide response range for ethanol vapor ranging from 10 to 500 ppm, and shows concentration-dependent activity as the response decreased with decreasing ethanol concentration. Impressively, the sensor exhibits a sensitivity of about 11.2 in response to trace amount of 10 ppm ethanol at the temperature of 43 °C, although it takes more than 40 min to fully respond and about 5 min to recover its initial resistance. On the basis of the above observations, we can conclude that the SnO2/TiO2 nanobelt heterostructures are remarkable sensors for detecting ethanol at temperatures approaching room temperature.

Figure 6. (A) XRD patterns of (a) TiO2, (b) SnO2, (c) SnO2/TiO2 obtained by calcination of Sn3O4/TiO2 (Sn/Ti = 4/1) at 550 °C for 2h (top is an zoom-in spectrum of SnO2/TiO2 diffractogram with 2θ ranging from 20° to 50°); (B, C) SEM images of SnO2/TiO2 with different magnifications; (D) HRTEM image of SnO2/TiO2 with clear interface and atomic illustration (Sn atoms in gray, Ti atoms in blue, O atoms in red) of the heterojunction. E

DOI: 10.1021/acsami.5b08630 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. (A) Response (Ra/Rg) of the SnO2/TiO2 nanobelt sensor to ethanol, methanol and acetone vapor at a concentration of 100 ppm with respect to working temperature; dynamical response−recovery curve of the SnO2/TiO2 nanobelt sensor (B) to 100 ppm ethanol at different working temperatures and (C) at a working temperature of 43 °C for various ethanol concentrations; the inset figure shows the corresponding response as a function of the ethanol concentration.

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4. CONCLUSION We propose a facile method to prepare SnO2 by thermal oxidation of sacrificial template Sn3O4 nanoplates. The phase conversion from Sn3O4 to SnO2 was achieved at 550 °C, yielding to the formation of SnO2 nanoplates with high energy (001) exposed facets. The SnO2 sensors are sensitive toward ethanol over a wide range of working temperatures, starting from the low temperature of 43 °C. Furthermore, the synthetic process was extended to the fabrication of SnO2/TiO2 nanobelt heterostructures whereby TiO2 nanobelts act as support for the nucleation and growth of SnO2 precursors. The heterostructure formed between SnO2 and TiO2 facilitates the interfacial electron transfer and hence increases the charge carrier concentration, resulting in an enhancement of gas response in terms of sensitivity and selectivity toward ethanol. Our results demonstrate that it is feasible to fabricate efficient SnO2based gas sensors that are efficient at low temperature, which may have promising applications for safe gas detection, especially in flammable or explosive environments.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are thankful for funding from the National Natural Science Foundation of China (Grant 51402172, 51372142, 50802055), Innovation Research Group (IRG: 51321091), The Fundamental Research Funds of Shandong University (2015JC017, 2014QY003).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08630. Sketch and schematic electrical circuit of the gas sensor; instruments adopted to measure the change of resistance upon exposing the sensor device to different temperatures; EDS of the Sn3O4/TiO2 heterostructure; XPS of the SnO2/TiO2 heterostructure; Mott−Schottky plots of the SnO2 and SnO2/TiO2 electrodes measured in 1 M NaOH solution with a frequency of 100 kHz (PDF) F

DOI: 10.1021/acsami.5b08630 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.5b08630 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX