Core–shell α–Fe₂O₃@SnO₂/Au hybrid structures and their enhanced gas sensing properties

Abstract

Hybrid nanostructures with controlled morphology and tunable compositions are of vital significance to both fundamental studies and practical applications. A novel core–shell heterostructure of α-Fe₂O₃@SnO₂/Au ternary hybrid nanospindles has been successfully fabricated, which were composed of α-Fe₂O₃ nanospindles as the cores and Au-decorated SnO₂ coatings as the shells. The obtained core-shell structures have been characterized by means of XRD, SEM, TEM and XPS. By virtue of their special architectures and multiple compositions, the α-Fe₂O₃@SnO₂/Au hybrid structures demonstrated enhanced gas sensing performances such as higher sensitivity and faster response-recovery, in comparison to pristine α-Fe₂O₃ nanospindles. This work proposes an example for the design of complex nanostructures with multiple functional compositions for high performance gas sensors.

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Introduction

The fabrication of hybrid nanostructures is of vital significance to both fundamental studies and practical applications, as the hybrid materials integrate the different physical and chemical properties of two or more compositions.^1–5 The obtained hybrid nanomaterials usually exhibit superior device performance or functional properties over their single components due to the synergistic interaction or the combination of the merits of individual components.^2,4,6,7 To date, versatile composite nanostructures have been reported by many groups, such as ZnO@SnO₂ core–shell tetrapods,² SnO₂-on-α-Fe₂O₃,^3,4,8α-Fe₂O₃-on-SnO₂⁶ and ZnO-on-SnO₂⁷ hierarchical architectures CuO@SnO₂,⁹α-Fe₂O₃@ZnO¹⁰ and α-Fe₂O₃@SnO₂¹¹ core–shell nanorods, α-Fe₂O₃@SnO₂ core-shell nanorattles,^12–14MnO₂@SnO₂ nanowires,¹⁵ and ZnO/SnO₂ nanosheets.¹⁶ More importantly, these hybrid nanostructures have demonstrated better performances in comparison to their single counterparts for various applications including optics,^2,7 gas sensors,^8–12lithium-ion batteries,^6,12 photocatalysis,^4,16 and supercapacitors.¹⁵ Besides metal oxides, noble metal nanoparticles such as Au, Ag, Pt and Pd are also frequently combined with metal oxides to form metal-oxide hybrid nanomaterials for enhanced applications such as gas sensors,^17–21 photocatalysis,^22–24 low-temperature CO oxidation^25–30 and electrocatalysts³¹ by virtue of the unique electronic and catalytic properties of noble metals^25,32 and the synergic metal-support interaction.^29,30 To further expand the functionalities of composite nanomaterials, it is highly attractive to functionalize the hetero-nanostructures based on metal oxides with noble metals to generate more complex nanostructures with multiple compositions. To our knowledge, such hybrid structures as well as their advanced applications have been rarely reported.

In this work, we report a novel hybrid structure of ternary α-Fe₂O₃@SnO₂/Au core–shell nanorattles, which are constructed by α-Fe₂O₃ nanospindles as the inner cores and Au-functionalized SnO₂ coatings as the outer shells. The obtained α-Fe₂O₃@SnO₂/Au core–shell structures are expected to provide enhanced device performance for gas sensors due to their complex architecture and multiple compositions.

Experimental

Materials

Chemicals such as FeCl₃·6H₂O, Na₂SnO₃·3H₂O, NaH₂PO₄, urea, ethanol, lysine, and NaBH₄ were of analytical grade and purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). HAuCl₄·4H₂O was obtained from Yingdaxigui Chemical Reagent Company (Tianjin, China). Distilled water was used throughout the experiments.

Synthesis of α-Fe₂O₃@SnO₂ core–shell nanospindles

α-Fe₂O₃@SnO₂ core–shell nanospindles were prepared following the literature method with some modifications.¹⁴ First, α-Fe₂O₃ nanospindles were synthesized according to the ref. 33. Typically, 80 mL of aqueous solution containing 2.0 × 10⁻² M FeCl₃ and 4.5 × 10⁻⁴ M NaH₂PO₄ was placed in a 100 mL Teflon-lined autoclave and kept at 100 °C for 72 h. The red precipitate was centrifuged and washed several times with distilled water and absolute ethanol, and then dried at 80 °C. Subsequently, 0.053 g of α-Fe₂O₃ nanospindles was dispersed into a mixture of 15 mL ethanol and 25 mL distilled water by ultrasonication for 15 min. Then, 1.2 g of urea and 0.125 g of Na₂SnO₃·3H₂O were completely dissolved in the above dispersion under stirring. The dispersion was transferred into a 50 mL Teflon-lined autoclave and kept at 160 °C for 24 h. The products was centrifuged and washed several times with distilled water and absolute ethanol, and then dried at 80 °C.

Synthesis of Au-functionalized α-Fe₂O₃@SnO₂ core–shell nanospindles

The deposition of Au nanoparticles onto α-Fe₂O₃@SnO₂ is performed according to our previous work.^20,21,34 0.0514 g of α-Fe₂O₃@SnO₂ core–shell nanospindles was dispersed into 30 mL distilled water by ultrasonication for 15 min, followed by the addition of 1.5 mL of 0.01 M HAuCl₄ and 2 mL of 0.01 M lysine solution. The dispersion was stirred for 30 min and then 3 mL of 0.1 M fresh NaBH₄ (excess) solution was added to reduce HAuCl₄ to Au nanoparticles. After further stirring for 40 min, the precipitate was collected by centrifugation and washed several times with distilled water and absolute ethanol, and dried at 80 °C. The product was calcined at 300 °C in air for 1 h to remove lysine.

Characterizations and gas sensing tests

The crystalline structure of the product was characterized by powder X-ray diffraction (XRD, Rigaku D/max-2500, Cu Kα, λ = 1.5418 Å). The morphology was observed by scanning electron microscopy (SEM, Shimadzu SS-550, 15 kV) and transmission electron microscopy (TEM, Philips FEI Tecnai 20ST, 200 kV). The chemical composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD spectrometer, Al Kα X-ray monochromator).

Gas sensing tests were performed on a commercial HW-30A Gas Sensing Measurement System (HanWei Electronics Co., Ltd., Henan, China). The gas sensor was fabricated by coating an aqueous slurry of the materials onto an alumina tube with a diameter of 1 mm and length of 4 mm, positioned with a pair of Au electrodes and four Pt wires on both ends of the tube. A Ni–Cr alloy coil through the tube was employed as a heater to control the operating temperature by tuning the heating voltage. The working voltage for gas sensing test is 5 V. The analyte gas such as ethanol was introduced into the testing chamber on HW-30A by a microsyringe, using air as the diluting and reference gas. The sensor sensitivity is defined as the ratio of R_a/R_g, where R_a and R_g are the electrical resistances of the sensor in air and in test gas, respectively. The response time (τ_res) or recovery time (τ_rec) is defined as the time for the sensor to reach 90% of its maximum response. Details of the sensor fabrication, photograph, and gas sensing test principle can be seen in our previous work.^34,35

Results and discussion

The α-Fe₂O₃@SnO₂/Au core–shell structures were created by a three-step procedure, as shown in Fig. 1. Hydrothermally derived α-Fe₂O₃ nanospindles were used as the template to grow the SnO₂ shells by a second hydrothermal method. An in situreduction route was used to deposit Au nanoparticles onto the binary α-Fe₂O₃@SnO₂ core–shell nanospindles. Fig. 2a and b display the SEM images of the pristine α-Fe₂O₃ and α-Fe₂O₃@SnO₂ core–shell nanospindles. By comparison, it can be seen that the pristine α-Fe₂O₃ nanospindles have two sharp tips, while the two tips of α-Fe₂O₃@SnO₂ nanospindles become relatively round, which is caused by the outer SnO₂ coatings. In addition the α-Fe₂O₃@SnO₂ nanospindles have a little larger diameter compared with the pristine α-Fe₂O₃ nanospindles, as indicated by the white arrows, which apparently results from the SnO₂ coatings on the α-Fe₂O₃ nanospindles. The crystal phase and composition of the samples were analyzed by XRD. Fig. 3 compares their XRD patterns. The pattern of α-Fe₂O₃ agrees well with the hematite phase (JCPDS No. 33-0644). Besides the diffraction peaks of hematite, α-Fe₂O₃@SnO₂ also contains the peaks of SnO₂ corresponding to the tetragonal rutile structure (JCPDS No.41-1445). As for α-Fe₂O₃@SnO₂/Au structures, three additional peaks are detected, corresponding to Au (111), (200), and (220) planes. The average crystallite size of SnO₂ is calculated to be 5.1 nm by the Scherer equation based on the (110) peak.

Fig. 1 (a) Schematic procedure for the synthesis of α-Fe₂O₃@SnO₂/Au structures.

Fig. 2 SEM images of (a) α-Fe₂O₃ and (b) α-Fe₂O₃@SnO₂ nanospindles.

Fig. 3 XRD patterns of α-Fe₂O₃, α-Fe₂O₃@SnO₂ and α-Fe₂O₃@SnO₂/Au nanospindles.

Detailed structural information of the α-Fe₂O₃@SnO₂/Au structures was observed by TEM. Fig. 4a shows the TEM image of pristine α-Fe₂O₃ nanospindles, which have a relatively smooth surface compared with the α-Fe₂O₃@SnO₂/Au composite nanospindles shown in Fig. 4b. From Fig. 4b, one can see that each particle exhibits a typical core–shell structure. The α-Fe₂O₃ nanospindles are well encapsulated in the center by the SnO₂ shells with a thickness of 16–18 nm and the tips of these core–shell nanospindles are not as spiculate as that of pristine α-Fe₂O₃. Interestingly, the diameter and length of the α-Fe₂O₃ nanospindle cores are slightly decreased compared with those in Fig. 4a. This phenomenon may be caused by the partial dissolution of α-Fe₂O₃ in the hydrothermal growth process of SnO₂ shells. The dispersion of Au nanoparticles on the α-Fe₂O₃@SnO₂/Au hybrid nanospindles could be observed by the TEM images in Fig. 4c and d. It is seen that Au nanoparticles are highly dispersed on the surface of α-Fe₂O₃@SnO₂ nanospindles, suggested by their dark contrast against the support. The inset in Fig. 4d displays the size distribution histogram for the Au nanoparticles in Fig. 4d, indicating a particle size range of 2–11 nm with a mean size of 5.9 nm. TEM observation indicates that Au nanoparticle-decorated α-Fe₂O₃@SnO₂ core–shell nanospindles were successfully synthesized.

Fig. 4 TEM images of (a) α–Fe₂O₃ and (b–d) α–Fe₂O₃@SnO₂/Au nanospindles. Inset of (d) is the size distribution histogram of Au nanoparticles.

The surface composition of α-Fe₂O₃@SnO₂/Au core–shell nanospindles were further characterized by XPS analysis. The high resolution XPS spectra of Fe, Sn and Au are shown in Fig. 5a. In the spectrum of Fe 2p in Fig. 5a, the two peaks at 710.7 and 724.1 eV are assigned to Fe 2p_3/2 and Fe 2p_1/2. The interference peak at 715.9 eV is from Sn 3p. Fig. 5b displays two peaks with a binding energy of 486.3 and 494.7 eV corresponding to Sn 3d_5/2 and Sn 3d_3/2, respectively. The spectrum of Au 4f in Fig. 5c presents significant signals for Au 4f_7/2 at 83.3 eV and Au 4f_5/2 at 87.1 eV. Compared with the binding energy of bulk Au at 83.8 eV, the Au nanoparticles decorated herein displays a negative shift of ca. 0.5 eV. This negative shift of binding energy has been reported in previous publications, and was attributed to the electronic interaction between noble metal and metal oxide supports.^30,36–38 Note that this kind of metal–support interaction has been taken as an important factor in the mechanism study of catalysts.^36,37,39 The mass content of Au in the α-Fe₂O₃@SnO₂/Au core–shell hybrids was 4.80% detected by XPS analysis.

Fig. 5 XPS spectra of (a) Fe 2p, (b) Sn 3d and (c) Au 4f.

As important n-type metal oxides, both α-Fe₂O₃ and SnO₂ have been widely utilized in applications such as catalysis and gas sensors. A combination of the two materials to form a hetero structure could provide more enhanced performances.^4,6,8,12 Due to its exceptional catalytic activity, Au nanoparticles have been loaded onto various semiconductors as a promoter to improve catalyst and gas sensor properties. Herein, the α-Fe₂O₃@SnO₂/Au core–shell nanospindles are expected to have excellent gas sensing performances. Therefore, gas sensing tests were conducted to examine the sensor performance of this novel hybrid structure.

In order to find the optimum operating temperature of the sensor, the sensitivities of α-Fe₂O₃@SnO₂/Au core–shell nanospindles to 50 ppm ethanol at different operating temperature are collected in Fig. 5. One can see that the sensor has the highest sensitivity at 320 °C, therefore, all the following sensor tests were performed at this temperature. Fig. 6a displays the dynamic response-recovery curves of pristine α-Fe₂O₃, α-Fe₂O₃@SnO₂ and α-Fe₂O₃@SnO₂/Au sensors to ethanol with gas concentrations of 2, 10, 50, 100, 200, 500 and 800 ppm at 320 °C. It shows that the response of the three sensors increased with the increasing gas concentration. However, the increase amplitudes in the response of α-Fe₂O₃@SnO₂/Au with gas concentration are more prominent than the other two sensors. To each gas concentration, the α-Fe₂O₃@SnO₂/Au sensor shows the highest response. Furthermore, by comparison, it is seen that the response-recovery speed of α-Fe₂O₃@SnO₂ is faster than that of pristine α-Fe₂O₃, while that of α-Fe₂O₃@SnO₂/Au was further increased. Take the response-recovery curves of the three sensors to 100 ppm ethanol for example, as shown in Fig. 6b, the response time (τ_res = 5 s) and recovery time (τ_rec = 5 s) of α-Fe₂O₃@SnO₂/Au are much shorter than that of α-Fe₂O₃@SnO₂ (τ_res = 9 s and τ_rec = 9 s) and pristine α-Fe₂O₃ (τ_res = 15 s and τ_rec = 12 s). The relationship between sensitivities and ethanol concentrations of the three sensors are compared in Fig. 7a. The sensitivities of α-Fe₂O₃@SnO₂/Au to each ethanol concentration are much higher than those of α-Fe₂O₃@ SnO₂ and pristine α-Fe₂O₃. In order to further exploit the sensing properties of α-Fe₂O₃@SnO₂/Au, other gases such as methanol, acetone, chloroform, and H₂S were also tested to examine the selectivity of the sensor. All the analyte gases are of the same concentration (100 ppm). From Fig. 7b, it can be seen that the α-Fe₂O₃@SnO₂/Au hybrid structure shows the highest sensitivity to ethanol (3.11), higher than that of acetone (2.23) and chloroform (2.14) and much higher than that of methanol (1.75) and H₂S (1.64), implying that the α-Fe₂O₃@SnO₂/Au nanospindles are more sensitive to ethanol gas and could be a very promising candidate for gas sensor materials.

Fig. 6 (a) Dynamic response-recovery curves to different ethanol concentrations, and (b) response-recovery curves to 100 ppm ethanol of α-Fe₂O₃, α-Fe₂O₃@SnO₂ and α-Fe₂O₃@SnO₂/Au nanospindles. (F denotes α-Fe₂O₃, FS is α-Fe₂O₃@SnO₂ and FSA is α-Fe₂O₃@SnO₂/Au.)

Fig. 7 (a) Sensor sensitivity as a function of ethanol concentration of α-Fe₂O₃, α-Fe₂O₃@SnO₂ and α-Fe₂O₃@SnO₂/Au nanospindles, and (b) sensor sensitivity of α-Fe₂O₃@SnO₂/Au to different gases. (F denotes α-Fe₂O₃, FS is α-Fe₂O₃@SnO₂ and FSA is α-Fe₂O₃@SnO₂/Au.)

From the sensing results discussed above, we can see that the sensing performance of the α-Fe₂O₃@SnO₂/Au structures have been significantly improved by coating SnO₂ shells on α-Fe₂O₃ nanospindles and further functionalization of Au nanoparticles. Consequently, the enhanced sensing performance should be ascribed to at least the following factors, including the unique core–shell structure, synergic effect between α-Fe₂O₃ and SnO₂ components, catalytic activity of Au nanoparticles and the metal-support interaction. However, deep efforts are needed to reveal a clear sensing mechanism for the complex nanostructures.

Conclusions

In conclusion, α-Fe₂O₃@SnO₂/Au core–shell hybrid nanospindles were successfully fabricated via two steps of hydrothermal procedures and subsequent decoration of Au nanoparticles. Gas sensing studies demonstrate that the ternary hybrid structures exhibited significantly enhanced sensing performance in terms of higher sensitivity and faster response-recovery speed in comparison to pristine α-Fe₂O₃ nanospindles. This work has shed some new light on the synthesis of complex nanostructures combined with multiple functional compositions, which are expected to be promising for many other applications such as photocatalysis.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 20871071), and the Applied Basic Research Programs of Science and Technology Commission Foundation of Tianjin (Nos. 09JCYBJC03600 and 10JCYBJC03900).

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