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Mesoporous assembled SnO2 nanospheres: Controlled synthesis, structural analysis and ethanol sensing investigation
Sensors and Actuators B 181 (2013) 629–636
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Mesoporous assembled SnO2 nanospheres: Controlled synthesis, structural analysis and ethanol sensing investigation Yun Kuang, Guobing Chen, Xiaodong Lei, Liang Luo ∗ , Xiaoming Sun ∗ State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 11 November 2012 Received in revised form 16 February 2013 Accepted 18 February 2013 Available online 28 February 2013 Keywords: Tin dioxide Nanosphere Assembly Mesoporous Gas sensing
a b s t r a c t A series of mesoporous SnO2 nanospheres were prepared in ethanol/water mixed solvents with poreforming surfactant (CTAB) involved. Surprisingly, the nanospheres assembled from SnO2 nanocones exhibited higher sensitivity to ethanol than hollow and amorphous-core counterparts, and more importantly, such submicrometer-sized spheres all showed linear responses to external ethanol concentration, similar to those extremely small spheres. Careful structure characterizations revealed their inner structure and crystallization behavior differences. It was finally concluded that the growth model difference significantly affected the inner structures (or assembly fashions) of these nanospheres, and consequently determined their sensing performances. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nanomaterials are natural choices for gas sensing applications because sensing is a process involves adsorption/desorption on surface while nanomaterials have significantly enlarged surface [1]. This advantage has been even strengthened as the depletion depth was proved to be several nanometers [2]. Understanding or exploring the influence of nanostructure features (e.g. particle size, specific surface area and crystallinity) to sensing properties would be of fundamental importance for constructing better sensors [3,4]. Tin dioxide (SnO2 ), as an n-type semiconductor with outstanding gas sensing property [1–8], has been commonly commercialized to monitor flammable gases. Various fabrication methods have been developed, including chemical vapor deposition [9], hard template methods (e.g. anodic aluminum oxide (AAO) membranes [10], carbon spheres [11]) and soft templates (e.g. cetyltrimethylammonium bromide (CTAB) [5,12], sodium dodecylbenzenesulfonate (SDBS)) [13]. Various morphologies of SnO2 nanocrystals have also been prepared by above methods, including nanowires [14,15], nanorods [16], nanoribbons/nanobelts [2,17], urchin-like spheres [18], dendrites [19], hollow structures [20] and mesoporous structures [21–23]. Among them, spherical particles are believed to be one of the ideal candidates because they are easy
∗ Corresponding authors. Tel.: +86 10 64448751. E-mail addresses: [email protected] (L. Luo), [email protected] (X. Sun). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.02.073
to assemble into condensed films and thus facilitate electron transport, which can be used for better gas sensor construction [24–26]. We and other groups have demonstrated that hollow spheres [20,27] were better choice for gas senor application than solid-core counterparts because the macroporous structure induced better response and shorter recovery time [28]. Very recently, Wang’s group demonstrated that the shell thickness of hollow spheres significantly affected their sensing performance [29]. Great efforts have been evidenced that both the structures [14–23] and the preparation methods [5,9–13] of nanomaterials can influence the sensing performance. However, since difference on structures usually stems from different synthetic methods, it is sometimes not conclusive for the structural effects [30]. Therefore, creating a series of different comparable model structures by the same method but with slightly altered parameters would be helpful to understand the relationship between structure and sensing performance, and thus provide information for further improvement of gas sensing performance. In this work, three types of mesoporous assembled SnO2 nanospheres (nanocones-assembled spheres, hollow spheres and solid spheres) were prepared by solvothermal method in ethanol/water mixed solvents with CTAB as a structuredirecting agent [5]. Besides CTAB, volume ratio of ethanol to water also showed a significant influence on the inner structure of SnO2 nanospheres. Interestingly, the nanocones-assembled SnO2 nanospheres showed a superior gas response performance than hollow and amorphous-core counterparts. Further research revealed that the difference on inner structures [31] of nanospheres played a dominant role in determining sensing performance; this
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structure control process provided new strategies to enhance the performance of assembled nanostructures. 2. Experimental 2.1. Materials All reagents were purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. 2.2. Synthesis of SnO2 nanospheres In a typical procedure, 5 mL of ethanolamine, 1 mmol of sodium stannate trihydrate (Na2 SnO3 ·3H2 O), and 2 mmol of cetyltrimethylammonium bromide (CTAB) were added to 22.5 mL of an ethanol–water mixture with different r (volume ratio of ethanol to water) values, to achieve a slightly white translucent or clear solution (depending on the value of r), with gentle stirring by glass rod for several minutes. Afterward, the mixture was transferred into a 40 mL Teflon-lined stainless steel autoclave, sealed tightly and maintained at 180 ◦ C for 24 h. After the autoclave cooled down to room temperature, the white product was obtained by centrifugation and washed with deionized water and absolute ethanol before drying at 80 ◦ C overnight. 2.3. Characterization X-ray diffraction (XRD) patterns were collected on a Shimadzu XRD-6000 diffractometer with Cu K␣ radiation (40 kV, 30 mA, ˚ recorded with 2 ranging from 20◦ to 80◦ . The size = 1.5418 A), and morphology of as-synthesized samples were characterized by field emission scanning electron microscopy (FE-SEM) on a Zeiss SUPRA 55 SEM operating at 10 or 20 kV, and by transmission electron microscopy (TEM) with a Hitachi H-800TEM operating at 200 kV. More structural details were revealed by high-resolution transmission electron microscope (HRTEM) on a JEOL-3010 TEM at an acceleration voltage of 300 kV. The specific surface area (SSA) was examined using multipoint Brunauer–Emmett–Teller (BET) method on an AUTOSORB-1 nitrogen adsorption apparatus. 2.4. Alcohol-sensing tests Alcohol-sensing tests were performed in a WS-30A measuring system (Zhengzhou Winsen Electronics Technology, China). The test chamber is an isolated environment for target gases. The testing system contains one quick-evaporating heater and two electric fans to obtain a uniformly distributed and accurate gas environment. In a typical test, an appropriate quantity of ethylene glycol was added to the as-prepared SnO2 , and the mixture was gently ground in an agate mortar to form a gel. The mixture was then coated onto the surface of ceramic tubes to obtain thin films. A resistor wire coil was inserted in the tube as a heater, which was able to provide working temperature (300 ◦ C) of the sensors. Absolute ethanol was injected directly onto a metal-plate heater in the test chamber and evaporated by heating. 0.51 L, 1.02 L, 5.1 L and 10.2 L alcohol were injected to create 10 ppm, 20 ppm, 50 ppm, 100 ppm and 200 ppm environment, respectively. Gas sensitivity of the sensor was defined as the ratio of the resistance in air (˝a ) to that in ethanol gas (˝g ), i.e. S = ˝a /˝g . 3. Results and discussion SnO2 nanospheres were fabricated following previous reported method with some modifications [5]. A typical fabrication process involved solvothermal treatment using CTAB as a
Fig. 1. XRD patterns of SnO2 samples prepared with r = (a) 1:3.5; (b) 2:1; (c) 3:1; (d) standard data of SnO2 (JCPDS 41-1445).
structure-directing agent and an ethanol–water mixture as solvent with different volume ratios (r): r = 1:3.5, 2:1 and 3:1. Fig. 1 shows standard JCPDS data and XRD patterns of the asprepared samples. All the diffraction peaks were indexed to rutile structure of SnO2 (space group: P42/mnm, JCPDS 41-1445); a little right-shift of the peaks were attributed to the existance of sodium. However, the relative intensity of the peaks deviated from the standard data. (1 0 1) peaks and (0 0 2) peaks (marked with asterisk in Fig. 1) were strengthened while (1 1 0) peaks were weakened; calculations based on Scherrer Formula [8] indicated that the average crystal size calculated from [1 1 0] (Fig. 1a: 13.23 nm, b: 10.40 nm, c: 10.00 nm) were all smaller than [1 0 1] (Fig. 1a: 19.60 nm, b: 14.91 nm, c: 14.96 nm) for all the three types of nanospheres, implying oriented growth of SnO2 nanocrystals. All the three products showed spherical morphologies under SEM (Fig. 2a, d, and g). However, broken individual spheres revealed the inner structure difference. The SnO2 nanospheres shown in Fig. 2a (r = 1:3.5) were assembled by nanocones with sharp ends located at the centers (Fig. 2b) and grew radially from the centers. The cone-like morphology was clearer after ultrasonication for 1 h when the assembled structure collapsed (Fig. 2c). Fig. 2d and f shows the SnO2 nanospheres prepared with ethanol–water r = 2:1, which, according to a broken sphere, was hollow structure, ∼46 nm in wall thickness (Fig. 2e). Fig. 2g shows a SEM image of the product prepared with r = 3:1, and the product was also spherical but with rough surface (Fig. 2i). Unlike the product prepared with r = 2:1, these nanospheres appeared solid-cores (Fig. 2h). To get further insight into the structure of SnO2 nanospheres, TEM and high-resolution TEM (HRTEM) were employed. Fig. 3a and b gives the low-magnification and high-magnification TEM images of the nanocones, respectively. The samples were obtained by ultrasonicating the assembled spheres for more than 1 h. Fig. 3c shows a HRTEM image of the selected area in Fig. 3b, the clear crystal lattices indicated good crystallinity of the cones. The 0.262 nm lattice spacing perpendicular to the growth direction of cones was consistent with that of (1 0 1) crystal plane lattice, indicating the preferential growth direction was [1 0 1] [32]. As shown in Fig. 3d and e, the bright fields in the centers of spheres confirmed that the nanospheres prepared with r = 2:1 were hollow; while those prepared with r = 3:1 were solid (Fig. 3g and h). This confirmed the results from SEM observation. Besides, HRTEM images (Fig. 3f and i) showed the lattice spacing of 0.33 nm (hollow spheres) and 0.32 nm (solid spheres), which coincided with (1 1 0) lattice fringes, indicating that the surface section also had good crystallinity, and grew along [1 1 0] direction.
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Fig. 2. SEM images of as-synthesized SnO2 samples. (a)–(c) SEM images of SnO2 nanocones assembled spheres (AS), r = 1:3.5; (d)–(f) SEM images of SnO2 hollow spheres (HS), r = 2:1; (g)–(i) SEM images of SnO2 solid spheres (SS), r = 3:1.
We confirmed the role of CTAB in the three systems by calculating the supersaturation degrees. As a cationic surfactant, the first critical micelle concentration (CMC) of CTAB in water and ethanol is 9 × 10−4 M and 0.24 M, respectively. In all the three systems, the concentration of CTAB was higher than the first CMC and thus CTAB molecules tended to self-assemble into microemulsions. The emulsions worked as shape-controlling (nucleation stage) and pore-forming (growth stage) agents in consequence of mesoporous SnO2 nanospheres’ formation. However, control experiment without CTAB yielded irregular SnO2 nanoparticles, evidencing the importance of CTAB. To understand why the volume ratio of ethanol to water played a dominant role in tailoring the inner structures of the nanospheres, we performed a series of experiments to explore the formation mechanism. Centrifugation was first used to collect the nuclei formed before solvothermal step for the nucleation process investigation. When r = 1:3.5, only ∼8 wt% was obtained (using original Na2 SnO3 ·3H2 O as 100%). After 500 ◦ C calcination, the weight lowered to 2% (Table S1), implying high CTA+ ions content in the precipitate. Following in situ precipitation and growth on these nuclei accompanied with pore-forming process lead to well crystallized radial nanoconesassembled structures. However, under r = 2:1 condition, ∼55 wt% solid was obtained after centrifugation. And as r increased to 3:1, the nuclei weight ratio increased to ∼77%. Moreover, the solid
weight did not lose much after 500 ◦ C calcinations (<3%, Table S1), indicating the low content of CTA+ ions. It meant far less growth units were left in the solution system after nucleation step under these two conditions (r = 2:1 and r = 3:1). Subsequently, the solvent ratio also affected the inner structure of the spheres by influencing the solubility of CTA+ /SnO3 2− ionpair and the crystallization diffusion distance. We monitored the formation process of the three types of assembled nanospheres by SEM/TEM at different reaction intervals (1 h, 12 h and 24 h, Fig. 4). When r = 1:3.5, ethanol volume was relatively low and water was rich, so most of the CTA+ /SnO3 2− ion-pairs were dissolved and the supersaturation degree of SnO3 2− was low, which resulted in few nuclei; after 1 h growth process, the spheres grew to near 700 nm in diameter. Meanwhile, because of the high solubility of SnO3 2− , the crystallization played a dominant role in the ripening process. Combined with the pore-forming effect achieved by CTAB microemulsions, mesoporous nanocone-assembled spheres were obtained. However, when r = 2:1, increased solvent molar ratio caused poor solubility of SnO3 2− , resulting in high supersaturation degree, and thus more nuclei formed at the beginning. After growing for 1 h, the sphere size reached about 200 nm in diameter. Meanwhile, increased solvent molar ratio balanced the crystallization speed and the dissolution–precipitation speed, thus formed a hollow shell in 24 h, as expressed by modified Kirkendall effect [33]. When the solvent ratio was increased to 3:1, higher r value brought
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Fig. 3. TEM and high-resolution TEM (HRTEM) images of the SnO2 samples. (a, d, and g) Low-magnification TEM images of SnO2 nanocones (after ultrasonication), SnO2 hollow spheres and SnO2 solid spheres; (b, e, and h) HRTEM images of SnO2 nanocones (after ultrasonication), SnO2 hollow spheres and SnO2 solid spheres; (c, f, and i) HRTEM images of selected area of (b, e, and h).
about extremely poor solubility of SnO3 2− . Under this condition, in situ crystallization and conversion resulted in SnO2 nanospheres with crystallized shells and relative big amorphous cores. Pure ethanol and water were also used to investigate the effect of the solvent ratio. Pure water produced nanocones while pure ethanol only produced irregular SnO2 particles (Fig. S1). On the basis of above results, the solvent ratio r value would affect both nucleation and ripening process, and finally lead to different assembled nanostructures with different stability. The smallest cores and thickest crystallized shells made nanocones-assembled nanospheres (AS) thermally stable; even 48 h of solvothermal treatment could not change the structure (Fig. 5a), while core-lacking hollow spheres (HS) and solid spheres (SS) with largest amorphous cores and thinnest crystallized shells collapsed after long-time (48 h) solvothermal treatment resulting in nanoparticles (Fig. 5b and c), which implied that growth model difference significantly affected the inner structures (or assembly fashions) and thus influenced the thermostability. The as-prepared three types of SnO2 nanospheres had similar size, similar appearance, and very similar synthesis methods, thus providing ideal models for investigation on inner structure for gas-sensing properties. All the sensors were made by coating the gel of SnO2 nanospheres onto ceramic tubes and then calcined at 300 ◦ C for 2 h. Fig. 6 shows the response curves of the
three types of sensors upon exposure to different concentrations (10–200 ppm) of ethanol at working temperature of 300 ◦ C. It was clearly seen that AS showed higher sensitivity (defined as ˝a /˝g ) than HS and SS (Fig. 6a and b). For instance, S = 120.0 was observed on AS, higher than HS (S = 71.3) and SS (S = 48.1) under exposure to 200 ppm ethanol vapor, in spite that the recovery time for AS was longer than the other two, which implied a deeper adsorption of ethanol molecules, i.e. thicker crystallized shell. The higher adsorption depth usually needs longer time to get balanced/saturated to environmental ethanol vapor [34]. This was evidenced by the “sloped heads” of response peaks for AS samples at low ethanol vapor concentration (e.g. 10 ppm), as marked with arrows in Fig. 6a. For HS and SS samples, the response curves always showed “flat” heads on the contrary, implying a shallower adsorption depth. The sensitivity of sensors showed an approximately linear relationship to environmental ethanol vapor pressure (Fig. 6c). It can be expressed as S = A[C]N
(I)
where S is the sensitivity of the sensor, A is a constant and C is the gas concentration. N = 1 or 1/2, which depends on the size of SnO2 nanoparticles: generally, N = 1 when the size of the SnO2 nanoparticle is comparable with the space–charge thickness (6 nm); N = 1/2 when the size of the SnO2 nanoparticle increases to more than
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Fig. 4. Schematic illustration of the formation mechanism of three types of SnO2 nanospheres.
20 nm [5]. The linear dependence of the sensitivity on the ethanol concentration suggested that the SnO2 nanospheres sensors were working similar to extremely small nanoparticles, though the assembled particles were hundreds of nanometers in size. To understand why the big sized nanospheres worked like small nanoparticles, why the adsorption depth and recovery behavior differed, Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption measurements was performed. The result (Fig. 7a) showed that the specific surface area of nanoconeassembled spheres was 25.61 m2 /g, a bit larger than hollow spheres (21.85 m2 /g) and solid spheres (19.25 m2 /g). These curves were consistent with characteristic processes between adsorption and desorption from the mesoporous materials according to the IUPAC nomenclature [5]. The corresponding pore size distribution analysis (Fig. 7b) indicated that the pore size of the three assembled
nanospheres all centered at 3.76 nm, which exactly matched the length of two CTA+ cations [35], implying that the pores should form and locate between the nanorods when they assembled/stacked, as revealed in Fig. 7c and d. To further confirm the secondary structure, the nanocones-assembled spheres were treated with ultrasonication and characterized with HRTEM, as shown in Fig. S3: the primary particles consisted of nanorods with size of 10–13 nm. Therefore, CTAB molecules should not only act as soft templates for directing spherical nuclei but also help assembling these nanorods into regular cone-like structures by selective adsorption and hydrophobic interactions. These long tail organic molecules converted to pores at following washing and calcination process. When the assembled spheres were working as gas sensors, these pores worked as tunnels for ethanol molecules to pass through. The highest surface area of AS implied enrichment of such
Fig. 5. SEM images of three SnO2 samples synthesized at 180 ◦ C for 48 h: (a) r = 1:3.5; (b) r = 2:1; (c) r = 3:1.
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Fig. 6. (a) Response curves of nanocone assembled spheres (AS), hollow spheres (HS) and solid spheres (SS) on cycling between increasing concentration (10–200 ppm) of ethanol and ambient air. (b) Partly magnified response curves at a concentration of 100 ppm to exhibit the recovery time as marked with asterisks. (c) The corresponding gas-sensing curves versus ethanol concentration.
tunnels, possibly by elongating the tunneling distance from center to surface. For HS and SS, their inner surface or solid core did not contribute so much to the surface area, but lose some surface area by shortening the tunnel distance. Such proposal was in accord with the longest balance and recovery time of AS samples in the three types of nanospheres. Microtomb technology was applied to further confirm the structure of the three samples (Fig. 8). One could see from the half sphere section that the nanocones of the assembled spheres were highly crystallized (Fig. 8a); hollow sphere also possessed high crystalline assembled rods in shells, but much shorter (Fig. 8b); while solid sphere had a big poorly crystallized core, with only quite short crystallized rods on the outer surface (Figs. 8c and S2b). Moreover, mesoporous structure of HS microtomb section can be
seen directly from the magnified TEM image (Fig. S2a). The pores between nanorods were nearly 3 nm, which was consistent with the BET results. Based on the above results, we thus proposed a possible electron flow mechanism to illustrate the sensitivity difference, as shown in Fig. 9. It is known that sensitivity is defined as: S=
˝a ˝g
(II)
where ˝a and ˝g represent the resistance of the samples exposed to air and targeted gas, respectively. However, the resistance of the three kinds of spheres before exposure to the targeted gas were far larger than after exposure and were almost equal to the load
Fig. 7. (a) Nitrogen adsorption/desorption isotherms obtained at 77 K and (b) pore size distribution of the as-synthesized SnO2 samples (c and d) high magnification SEM image of AS surface, the scale bar is 200 nm.
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Fig. 8. Cross-sectional TEM images of (a) nanocones assembled spheres, (b) hollow spheres and (c) solid spheres.
resistor resistance ˝0 , thus, the sensitivity S could be calculated by: S=
˝0 ˝g
(III)
∂˝ =− ∂t
Resistance is defined as: L ˝= A
(IV)
where L is the length of the conductor, A is the cross-section area of the conductor and is the electrical resistivity. As the electrical resistivity of amorphous SnO2 is much higher than crystallized ones, electrons tend to flow through crystallized zones (Fig. 9a–c). Therefore, an equivalent hollow spherical resistor model was established in Fig. 9d to calculate the resistance of the three types of spheres based on Eq. (IV). Section integral was used when calculating the resistance as shown below:
R−t
˝1 =
dx (R2 − x2 ) − [(R − t)2 − x2 ]
0 R−a
˝2 = R−t
dx = 2R (R2 − x2 )
˝ = 2(˝1 + ˝2 ) =
1 t
−
Partial derivative revealed the negative correlation between the resistance ˝ and the thickness of the crystallized layer, which means, the thicker the crystallized layer, the lower the resistance of the sphere.
ln
=
(R − t) (2Rt − t 2 )
2R − a t + ln 2R − t a
(V)
2R − a t 1 1 ln + + ln 2R − t R 2R − t a
(VI)
(VII)
where ˝ represents the whole resistance of the spheres which composed of two sections of resistance, ˝1 and ˝2 . R represents the radius of the spheres, t represents the thickness of crystallized outer layer and a represents the contact depth of the spheres.
1 t
−
1 2R − t
2 <0
(VIII)
Therefore, when exposed to ethanol, nanocone-assembled sphere had the lowest resistance ˝g while hollow sphere had a middle one and solid sphere had the largest one. That was why the AS had the largest sensitivity while SS had the lowest and HS got the middle according to Eq. (III). As well known, well crystallized SnO2 nanoparticles have much better transport performance than amorphous ones [7]. Hence, the sensor performance could be considered as a reflection of the crystallization, assembly and tunnel difference of assembled nanospheres. All the three samples had crystallized and assembled shells (the AS could be considered as an extremely small core with a very thick shell), which were assembled from the primary nanorods through selective adsorption and hydrophobic interactions of CTAB. Combustion of those CTA+ ions at calcination step made several pores with size of 3.76 nm, which worked as tunnels for gas absorption/desorption. As the sensors were made and exposed to ethanol vapor, the ethanol molecules moved through the tunnels into the mesoporous shells and induced electric signal response [36]. The tunnels also made the primary nanorods exposed completely to targeted vapors, thus the whole 10 nm-sized nanorods were included in the depletion space charge layer and therefore the samples exhibited approximately linear response according to Eq. (I). Since only the crystallized shells gave response, the sensitivity of nanocones-assembled nanospheres was even higher than thinner-shell hollow spheres. Solid core spheres had only very thin crystallized shell and thus performed worst. For sure, the saturating and recovery time for AS would be longer due to longer diffusion distance of trapped ethanol molecules within 3.76 nm tunnels. The solid spheres have the shortest recovery time due to their thinnest shells. 4. Conclusions
Fig. 9. Possible mechanism of electron flow in (a) nanocone assembled sphere, AS. (b) Hollow sphere, HS. (c) Solid sphere, SS. (d) Equivalent circuit of the spheres.
A series of mesoporous assembled SnO2 nanospheres, including nanocones-assembled spheres, hollow spheres and solid spheres, were obtained by cetyltrimethylammonium bromide (CTAB) directed solvothermal method. The submicrometer-sized spheres exhibited linear response to ethanol concentration, which was previously observed only on nanoparticles less than 10 nm. Systematic analysis on inner structure and sensor performance revealed that the crystallization behavior, assembly fashion and pore structures all determined the sensing performance. Regular aligned primary nanorods and consequent radically ordered pore
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structure were believed the source of the uncommon performance. Considering that the spherical morphology is helpful for dense assembly and submicrometer size of hierarchical structure is beneficial to long term stability, such hierarchical structure should be inspirative to further optimizing the nanostructures for constructing better gas sensors. Supporting information It contains weighing data, EDS spectra, and supplementary SEM, HRTEM images. Acknowledgements This work was supported by NSFC, the Foundation for Authors of National Excellent Doctoral Dissertations of P.R. China, the Program for New Century Excellent Talents in Universities, the 973 Program (2011CBA00503 and 2011CB932403), and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.02.073. References [1] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sensors and Actuators B 140 (2009) 319–336. [2] M. Law, H. Kind, B. Messer, F. Kim, P.D. Yang, Photochemical sensing of NO2 with SnO2 nanoribbon nanosensors at room temperature, Angewandte Chemie International Edition 41 (2002) 2405–2408. [3] G. Korotcenkov, Gas response control through structural and chemical modification of metal oxide films: state of the art and approaches, Sensors and Actuators B 107 (2005) 209–232. [4] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Oxide materials for development of integrated gas sensors—a comprehensive review, Critical Reviews in Solid State and Materials Sciences 29 (2004) 111–188. [5] G.C. Xi, Y.T. He, Q. Zhang, H.Q. Xiao, X. Wang, C. Wang, Synthesis of crystalline microporous SnO2 via a surfactant-assisted microwave heating method: a general and rapid method for the synthesis of metal oxide nanostructures, Journal of Physical Chemistry C 112 (2008) 11645–11649. [6] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Letters 5 (2005) 667–673. [7] X.G. Han, M.S. Jin, S.F. Xie, Q. Kuang, Z.Y. Jiang, Y.Q. Jiang, Z.X. Xie, L.S. Zheng, Synthesis of tin dioxide octahedral nanoparticles with exposed high-energy {2 2 1} facets and enhanced gas-sensing properties, Angewandte Chemie International Edition 48 (2009) 9180–9183. [8] J. Xu, D. Wang, L. Qin, W. Yu, Q. Pan, SnO2 nanorods and hollow spheres: controlled synthesis and gas sensing properties, Sensors and Actuators B 137 (2009) 490–495. [9] C. Martinez, B. Hockey, C. Montgomery, S. Semancik, Porous tin oxide nanostructured microspheres for sensor applications, Langmuir 21 (2005) 7937–7944. [10] G. Wang, J. Park, M. Park, X. Gou, Synthesis and high gas sensitivity of tin oxide nanotubes, Sensors and Actuators B 131 (2008) 313–317. [11] W. Caihong, X. Chu, M. Wu, Highly sensitive gas sensors based on hollow SnO2 spheres prepared by carbon sphere template method, Sensors and Actuators B 120 (2007) 508–513. [12] C. Aprile, L. Teruel, M. Alvaro, H. Garcia, Structured mesoporous tin oxide with electrical conductivity. Application in electroluminescence, Journal of the American Chemical Society 131 (2009) 1342–1343. [13] Q. Zhao, Y. Xie, T. Dong, Z. Zhang, Oxidation–crystallization process of colloids: an effective approach for the morphology controllable synthesis of SnO2 hollow spheres and rod bundles, Journal of Physical Chemistry C 111 (2007) 11598–11603. [14] A. Leonardy, W. Hung, D. Tsai, C. Chou, Y. Huang, Structural features of SnO2 nanowires and Raman spectroscopy analysis, Crystal Growth and Design 9 (2009) 3958–3963. [15] E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Quasione dimensional metal oxide semiconductors: preparation, characterization and application as chemical sensors, Progress in Materials Science 54 (2009) 1–67. [16] J.Q. Sun, J.S. Wang, X.C. Wu, G.S. Zhang, J.Y. Wei, S.Q. Zhang, H. Li, D.R. Chen, Novel method for high-yield synthesis of rutile SnO2 nanorods by oriented aggregation, Crystal Growth and Design 6 (2006) 1584–1587.
[17] E. Comini, G. Faglia, G. Sberveglieri, Z.W. Pan, Z.L. Wang, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Applied Physics Letters 81 (2002) 1869–1871. [18] G. Cheng, K. Wu, P. Zhao, Y. Cheng, X. He, K. Huang, Controlled growth of oxygen-deficient tin oxide nanostructures via a solvothermal approach in mixed solvents and their optical properties, Nanotechnology 18 (2007) 355604. [19] D.F. Zhang, L.D. Sun, G. Xu, C.H. Yan, Size-controllable one-dimensinal SnO2 nanocrystals: synthesis, growth mechanism, and gas sensing property, Physical Chemistry Chemical Physics 8 (2006) 4874–4880. [20] X. Sun, J. Liu, Y. Li, Use of carbonaceous polysaccharide microspheres as templates for fabricating metal oxide hollow spheres, Chemistry – A European Journal 12 (2006) 2039–2047. [21] Y. Wang, C. Ma, X. Sun, H. Li, Synthesis of mesoporous structured material based on tin oxide, Microporous and Mesoporous Materials 49 (2001) 171–178. [22] M. Tiemann, Porous metal oxides as gas sensors, Chemistry – A European Journal 13 (2007) 8376–8388. [23] T. Hyodo, N. Nishida, Y. Shimizu, M. Egashira, Preparation and gas-sensing properties of thermally stable mesoporous SnO2 , Sensors and Actuators B 83 (2002) 209–215. [24] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Nearly monodisperse Cu2 O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chemistry of Materials 18 (2006) 867–871. [25] Y. Xia, B. Gates, Y. Yin, Y. Lu, Monodispersed colloidal spheres: old materials with new applications, Advanced Materials 12 (2000) 693–713. [26] J. Ge, Y. Hu, Y. Yin, Highly tunable superparamagnetic colloidal photonic crystals, Angewandte Chemie International Edition 119 (2007) 7572–7575. [27] H.C. Zeng, Synthesis and self-assembly of complex hollow materials, Journal of Materials Chemistry 21 (2011) 7511–7526. [28] Q. Zhao, Y. Gao, X. Bai, C. Wu, Y. Xie, Facile synthesis of SnO2 hollow nanospheres and applications in gas sensors and electrocatalysts, European Journal of Inorganic Chemistry 2006 (2006) 1643–1648. [29] X. Lai, J. Li, B.A. Korgel, Z. Dong, Z. Li, F. Su, J. Du, D. Wang, General synthesis and gas-sensing properties of multiple-shell metal oxide hollow microspheres, Angewandte Chemie International Edition 50 (2011) 2738–2741. [30] Z.R. Dai, Z.W. Pan, Z.L. Wang, Novel nanostructures of functional oxides synthesized by thermal evaporation, Advanced Functional Materials 13 (2003) 9–24. [31] X. Xu, J. Zhuang, X. Wang, SnO2 quantum dots and quantum wires: controllable synthesis, self-assembled 2D architectures, and gas-sensing properties, Journal of the American Chemical Society 130 (2008) 12527–12535. [32] G. Cheng, J. Wang, X. Liu, K. Huang, Self-assembly synthesis of single-crystalline tin oxide nanostructures by a poly (acrylic acid)-assisted solvothermal process, Journal of Physical Chemistry B 110 (2006) 16208–16211. [33] B. Liu, H.C. Zeng, Fabrication of ZnO “dandelions” via a modified kirkendall process, Journal of the American Chemical Society 126 (2004) 16744–16746. [34] J. Zhang, S. Wang, Y. Wang, M. Xu, H. Xia, S. Zhang, W. Huang, X. Guo, S. Wu, ZnO hollow spheres: preparation, characterization, and gas sensing properties, Sensors and Actuators B 139 (2009) 411–417. [35] F. Zhao, Y.-K. Du, J.-K. Xu, S.-F. Liu, Determination of surfactant molecular volume by atomic force microscopy, Colloid Journal 68 (2006) 784–787. [36] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, Journal of Electroceramics 7 (2001) 143–167.
Biographies Yun Kuang received his B.S. from Beijing University of Chemical Technology in 2011 and now he is pursuing his PhD under the supervision of Prof. Xiaoming Sun at State Key Laboratory of Chemical Resource Engineering in Beijing University of Chemical Technology. Guobing Chen received his B.S. and M.Sci. from Beijing University of Chemical Technology in 2008 and 2011, he is now working as a research scientist in Azure Wind Environmental Protection Technology Co., Ltd. His main research interests focuses on gas sensing materials for environmental monitoring devices. Xiaodong Lei received his received his M.Sci. and Ph.D. from Beijing University of Chemical Technology in 2003 and 2006. After that, he joined the faculty of Beijing University of Chemical Technology. His current research interests focuses on Layered inorganic functional materials, water treatment and recycling of waste resources. Liang Luo received his Ph.D. in Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, China, in 2010. He is now working at Department of Inorganic Chemistry, Faculty of Science, Beijing University of Chemical Technology. His current research interests cover design, preparation of inorganic nanomaterials and their applications as gas sensors and optical devices. Xiaoming Sun received his B.S. and Ph.D. in Department of Chemistry, Tsinghua University at 2000 and 2005, respectively. He finished his postdoctoral work at Stanford University with Prof. Hongjie Dai and joined the faculty of Beijing University of Chemical Technology at 2008. His main research interests focus on controlled synthesis, separation, assembly, and consequent property investigations (e.g. Energy-storage, Gas-Sensing, Catalysis) on inorganic nanomaterials.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Mesoporous assembled SnO2 nanospheres: Controlled synthesis, structural analysis and ethanol sensing investigation Yun Kuang, Guobing Chen, Xiaodong Lei, Liang Luo ∗ , Xiaoming Sun ∗ State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 11 November 2012 Received in revised form 16 February 2013 Accepted 18 February 2013 Available online 28 February 2013 Keywords: Tin dioxide Nanosphere Assembly Mesoporous Gas sensing
a b s t r a c t A series of mesoporous SnO2 nanospheres were prepared in ethanol/water mixed solvents with poreforming surfactant (CTAB) involved. Surprisingly, the nanospheres assembled from SnO2 nanocones exhibited higher sensitivity to ethanol than hollow and amorphous-core counterparts, and more importantly, such submicrometer-sized spheres all showed linear responses to external ethanol concentration, similar to those extremely small spheres. Careful structure characterizations revealed their inner structure and crystallization behavior differences. It was finally concluded that the growth model difference significantly affected the inner structures (or assembly fashions) of these nanospheres, and consequently determined their sensing performances. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nanomaterials are natural choices for gas sensing applications because sensing is a process involves adsorption/desorption on surface while nanomaterials have significantly enlarged surface [1]. This advantage has been even strengthened as the depletion depth was proved to be several nanometers [2]. Understanding or exploring the influence of nanostructure features (e.g. particle size, specific surface area and crystallinity) to sensing properties would be of fundamental importance for constructing better sensors [3,4]. Tin dioxide (SnO2 ), as an n-type semiconductor with outstanding gas sensing property [1–8], has been commonly commercialized to monitor flammable gases. Various fabrication methods have been developed, including chemical vapor deposition [9], hard template methods (e.g. anodic aluminum oxide (AAO) membranes [10], carbon spheres [11]) and soft templates (e.g. cetyltrimethylammonium bromide (CTAB) [5,12], sodium dodecylbenzenesulfonate (SDBS)) [13]. Various morphologies of SnO2 nanocrystals have also been prepared by above methods, including nanowires [14,15], nanorods [16], nanoribbons/nanobelts [2,17], urchin-like spheres [18], dendrites [19], hollow structures [20] and mesoporous structures [21–23]. Among them, spherical particles are believed to be one of the ideal candidates because they are easy
∗ Corresponding authors. Tel.: +86 10 64448751. E-mail addresses: [email protected] (L. Luo), [email protected] (X. Sun). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.02.073
to assemble into condensed films and thus facilitate electron transport, which can be used for better gas sensor construction [24–26]. We and other groups have demonstrated that hollow spheres [20,27] were better choice for gas senor application than solid-core counterparts because the macroporous structure induced better response and shorter recovery time [28]. Very recently, Wang’s group demonstrated that the shell thickness of hollow spheres significantly affected their sensing performance [29]. Great efforts have been evidenced that both the structures [14–23] and the preparation methods [5,9–13] of nanomaterials can influence the sensing performance. However, since difference on structures usually stems from different synthetic methods, it is sometimes not conclusive for the structural effects [30]. Therefore, creating a series of different comparable model structures by the same method but with slightly altered parameters would be helpful to understand the relationship between structure and sensing performance, and thus provide information for further improvement of gas sensing performance. In this work, three types of mesoporous assembled SnO2 nanospheres (nanocones-assembled spheres, hollow spheres and solid spheres) were prepared by solvothermal method in ethanol/water mixed solvents with CTAB as a structuredirecting agent [5]. Besides CTAB, volume ratio of ethanol to water also showed a significant influence on the inner structure of SnO2 nanospheres. Interestingly, the nanocones-assembled SnO2 nanospheres showed a superior gas response performance than hollow and amorphous-core counterparts. Further research revealed that the difference on inner structures [31] of nanospheres played a dominant role in determining sensing performance; this
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structure control process provided new strategies to enhance the performance of assembled nanostructures. 2. Experimental 2.1. Materials All reagents were purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. 2.2. Synthesis of SnO2 nanospheres In a typical procedure, 5 mL of ethanolamine, 1 mmol of sodium stannate trihydrate (Na2 SnO3 ·3H2 O), and 2 mmol of cetyltrimethylammonium bromide (CTAB) were added to 22.5 mL of an ethanol–water mixture with different r (volume ratio of ethanol to water) values, to achieve a slightly white translucent or clear solution (depending on the value of r), with gentle stirring by glass rod for several minutes. Afterward, the mixture was transferred into a 40 mL Teflon-lined stainless steel autoclave, sealed tightly and maintained at 180 ◦ C for 24 h. After the autoclave cooled down to room temperature, the white product was obtained by centrifugation and washed with deionized water and absolute ethanol before drying at 80 ◦ C overnight. 2.3. Characterization X-ray diffraction (XRD) patterns were collected on a Shimadzu XRD-6000 diffractometer with Cu K␣ radiation (40 kV, 30 mA, ˚ recorded with 2 ranging from 20◦ to 80◦ . The size = 1.5418 A), and morphology of as-synthesized samples were characterized by field emission scanning electron microscopy (FE-SEM) on a Zeiss SUPRA 55 SEM operating at 10 or 20 kV, and by transmission electron microscopy (TEM) with a Hitachi H-800TEM operating at 200 kV. More structural details were revealed by high-resolution transmission electron microscope (HRTEM) on a JEOL-3010 TEM at an acceleration voltage of 300 kV. The specific surface area (SSA) was examined using multipoint Brunauer–Emmett–Teller (BET) method on an AUTOSORB-1 nitrogen adsorption apparatus. 2.4. Alcohol-sensing tests Alcohol-sensing tests were performed in a WS-30A measuring system (Zhengzhou Winsen Electronics Technology, China). The test chamber is an isolated environment for target gases. The testing system contains one quick-evaporating heater and two electric fans to obtain a uniformly distributed and accurate gas environment. In a typical test, an appropriate quantity of ethylene glycol was added to the as-prepared SnO2 , and the mixture was gently ground in an agate mortar to form a gel. The mixture was then coated onto the surface of ceramic tubes to obtain thin films. A resistor wire coil was inserted in the tube as a heater, which was able to provide working temperature (300 ◦ C) of the sensors. Absolute ethanol was injected directly onto a metal-plate heater in the test chamber and evaporated by heating. 0.51 L, 1.02 L, 5.1 L and 10.2 L alcohol were injected to create 10 ppm, 20 ppm, 50 ppm, 100 ppm and 200 ppm environment, respectively. Gas sensitivity of the sensor was defined as the ratio of the resistance in air (˝a ) to that in ethanol gas (˝g ), i.e. S = ˝a /˝g . 3. Results and discussion SnO2 nanospheres were fabricated following previous reported method with some modifications [5]. A typical fabrication process involved solvothermal treatment using CTAB as a
Fig. 1. XRD patterns of SnO2 samples prepared with r = (a) 1:3.5; (b) 2:1; (c) 3:1; (d) standard data of SnO2 (JCPDS 41-1445).
structure-directing agent and an ethanol–water mixture as solvent with different volume ratios (r): r = 1:3.5, 2:1 and 3:1. Fig. 1 shows standard JCPDS data and XRD patterns of the asprepared samples. All the diffraction peaks were indexed to rutile structure of SnO2 (space group: P42/mnm, JCPDS 41-1445); a little right-shift of the peaks were attributed to the existance of sodium. However, the relative intensity of the peaks deviated from the standard data. (1 0 1) peaks and (0 0 2) peaks (marked with asterisk in Fig. 1) were strengthened while (1 1 0) peaks were weakened; calculations based on Scherrer Formula [8] indicated that the average crystal size calculated from [1 1 0] (Fig. 1a: 13.23 nm, b: 10.40 nm, c: 10.00 nm) were all smaller than [1 0 1] (Fig. 1a: 19.60 nm, b: 14.91 nm, c: 14.96 nm) for all the three types of nanospheres, implying oriented growth of SnO2 nanocrystals. All the three products showed spherical morphologies under SEM (Fig. 2a, d, and g). However, broken individual spheres revealed the inner structure difference. The SnO2 nanospheres shown in Fig. 2a (r = 1:3.5) were assembled by nanocones with sharp ends located at the centers (Fig. 2b) and grew radially from the centers. The cone-like morphology was clearer after ultrasonication for 1 h when the assembled structure collapsed (Fig. 2c). Fig. 2d and f shows the SnO2 nanospheres prepared with ethanol–water r = 2:1, which, according to a broken sphere, was hollow structure, ∼46 nm in wall thickness (Fig. 2e). Fig. 2g shows a SEM image of the product prepared with r = 3:1, and the product was also spherical but with rough surface (Fig. 2i). Unlike the product prepared with r = 2:1, these nanospheres appeared solid-cores (Fig. 2h). To get further insight into the structure of SnO2 nanospheres, TEM and high-resolution TEM (HRTEM) were employed. Fig. 3a and b gives the low-magnification and high-magnification TEM images of the nanocones, respectively. The samples were obtained by ultrasonicating the assembled spheres for more than 1 h. Fig. 3c shows a HRTEM image of the selected area in Fig. 3b, the clear crystal lattices indicated good crystallinity of the cones. The 0.262 nm lattice spacing perpendicular to the growth direction of cones was consistent with that of (1 0 1) crystal plane lattice, indicating the preferential growth direction was [1 0 1] [32]. As shown in Fig. 3d and e, the bright fields in the centers of spheres confirmed that the nanospheres prepared with r = 2:1 were hollow; while those prepared with r = 3:1 were solid (Fig. 3g and h). This confirmed the results from SEM observation. Besides, HRTEM images (Fig. 3f and i) showed the lattice spacing of 0.33 nm (hollow spheres) and 0.32 nm (solid spheres), which coincided with (1 1 0) lattice fringes, indicating that the surface section also had good crystallinity, and grew along [1 1 0] direction.
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Fig. 2. SEM images of as-synthesized SnO2 samples. (a)–(c) SEM images of SnO2 nanocones assembled spheres (AS), r = 1:3.5; (d)–(f) SEM images of SnO2 hollow spheres (HS), r = 2:1; (g)–(i) SEM images of SnO2 solid spheres (SS), r = 3:1.
We confirmed the role of CTAB in the three systems by calculating the supersaturation degrees. As a cationic surfactant, the first critical micelle concentration (CMC) of CTAB in water and ethanol is 9 × 10−4 M and 0.24 M, respectively. In all the three systems, the concentration of CTAB was higher than the first CMC and thus CTAB molecules tended to self-assemble into microemulsions. The emulsions worked as shape-controlling (nucleation stage) and pore-forming (growth stage) agents in consequence of mesoporous SnO2 nanospheres’ formation. However, control experiment without CTAB yielded irregular SnO2 nanoparticles, evidencing the importance of CTAB. To understand why the volume ratio of ethanol to water played a dominant role in tailoring the inner structures of the nanospheres, we performed a series of experiments to explore the formation mechanism. Centrifugation was first used to collect the nuclei formed before solvothermal step for the nucleation process investigation. When r = 1:3.5, only ∼8 wt% was obtained (using original Na2 SnO3 ·3H2 O as 100%). After 500 ◦ C calcination, the weight lowered to 2% (Table S1), implying high CTA+ ions content in the precipitate. Following in situ precipitation and growth on these nuclei accompanied with pore-forming process lead to well crystallized radial nanoconesassembled structures. However, under r = 2:1 condition, ∼55 wt% solid was obtained after centrifugation. And as r increased to 3:1, the nuclei weight ratio increased to ∼77%. Moreover, the solid
weight did not lose much after 500 ◦ C calcinations (<3%, Table S1), indicating the low content of CTA+ ions. It meant far less growth units were left in the solution system after nucleation step under these two conditions (r = 2:1 and r = 3:1). Subsequently, the solvent ratio also affected the inner structure of the spheres by influencing the solubility of CTA+ /SnO3 2− ionpair and the crystallization diffusion distance. We monitored the formation process of the three types of assembled nanospheres by SEM/TEM at different reaction intervals (1 h, 12 h and 24 h, Fig. 4). When r = 1:3.5, ethanol volume was relatively low and water was rich, so most of the CTA+ /SnO3 2− ion-pairs were dissolved and the supersaturation degree of SnO3 2− was low, which resulted in few nuclei; after 1 h growth process, the spheres grew to near 700 nm in diameter. Meanwhile, because of the high solubility of SnO3 2− , the crystallization played a dominant role in the ripening process. Combined with the pore-forming effect achieved by CTAB microemulsions, mesoporous nanocone-assembled spheres were obtained. However, when r = 2:1, increased solvent molar ratio caused poor solubility of SnO3 2− , resulting in high supersaturation degree, and thus more nuclei formed at the beginning. After growing for 1 h, the sphere size reached about 200 nm in diameter. Meanwhile, increased solvent molar ratio balanced the crystallization speed and the dissolution–precipitation speed, thus formed a hollow shell in 24 h, as expressed by modified Kirkendall effect [33]. When the solvent ratio was increased to 3:1, higher r value brought
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Fig. 3. TEM and high-resolution TEM (HRTEM) images of the SnO2 samples. (a, d, and g) Low-magnification TEM images of SnO2 nanocones (after ultrasonication), SnO2 hollow spheres and SnO2 solid spheres; (b, e, and h) HRTEM images of SnO2 nanocones (after ultrasonication), SnO2 hollow spheres and SnO2 solid spheres; (c, f, and i) HRTEM images of selected area of (b, e, and h).
about extremely poor solubility of SnO3 2− . Under this condition, in situ crystallization and conversion resulted in SnO2 nanospheres with crystallized shells and relative big amorphous cores. Pure ethanol and water were also used to investigate the effect of the solvent ratio. Pure water produced nanocones while pure ethanol only produced irregular SnO2 particles (Fig. S1). On the basis of above results, the solvent ratio r value would affect both nucleation and ripening process, and finally lead to different assembled nanostructures with different stability. The smallest cores and thickest crystallized shells made nanocones-assembled nanospheres (AS) thermally stable; even 48 h of solvothermal treatment could not change the structure (Fig. 5a), while core-lacking hollow spheres (HS) and solid spheres (SS) with largest amorphous cores and thinnest crystallized shells collapsed after long-time (48 h) solvothermal treatment resulting in nanoparticles (Fig. 5b and c), which implied that growth model difference significantly affected the inner structures (or assembly fashions) and thus influenced the thermostability. The as-prepared three types of SnO2 nanospheres had similar size, similar appearance, and very similar synthesis methods, thus providing ideal models for investigation on inner structure for gas-sensing properties. All the sensors were made by coating the gel of SnO2 nanospheres onto ceramic tubes and then calcined at 300 ◦ C for 2 h. Fig. 6 shows the response curves of the
three types of sensors upon exposure to different concentrations (10–200 ppm) of ethanol at working temperature of 300 ◦ C. It was clearly seen that AS showed higher sensitivity (defined as ˝a /˝g ) than HS and SS (Fig. 6a and b). For instance, S = 120.0 was observed on AS, higher than HS (S = 71.3) and SS (S = 48.1) under exposure to 200 ppm ethanol vapor, in spite that the recovery time for AS was longer than the other two, which implied a deeper adsorption of ethanol molecules, i.e. thicker crystallized shell. The higher adsorption depth usually needs longer time to get balanced/saturated to environmental ethanol vapor [34]. This was evidenced by the “sloped heads” of response peaks for AS samples at low ethanol vapor concentration (e.g. 10 ppm), as marked with arrows in Fig. 6a. For HS and SS samples, the response curves always showed “flat” heads on the contrary, implying a shallower adsorption depth. The sensitivity of sensors showed an approximately linear relationship to environmental ethanol vapor pressure (Fig. 6c). It can be expressed as S = A[C]N
(I)
where S is the sensitivity of the sensor, A is a constant and C is the gas concentration. N = 1 or 1/2, which depends on the size of SnO2 nanoparticles: generally, N = 1 when the size of the SnO2 nanoparticle is comparable with the space–charge thickness (6 nm); N = 1/2 when the size of the SnO2 nanoparticle increases to more than
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Fig. 4. Schematic illustration of the formation mechanism of three types of SnO2 nanospheres.
20 nm [5]. The linear dependence of the sensitivity on the ethanol concentration suggested that the SnO2 nanospheres sensors were working similar to extremely small nanoparticles, though the assembled particles were hundreds of nanometers in size. To understand why the big sized nanospheres worked like small nanoparticles, why the adsorption depth and recovery behavior differed, Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption measurements was performed. The result (Fig. 7a) showed that the specific surface area of nanoconeassembled spheres was 25.61 m2 /g, a bit larger than hollow spheres (21.85 m2 /g) and solid spheres (19.25 m2 /g). These curves were consistent with characteristic processes between adsorption and desorption from the mesoporous materials according to the IUPAC nomenclature [5]. The corresponding pore size distribution analysis (Fig. 7b) indicated that the pore size of the three assembled
nanospheres all centered at 3.76 nm, which exactly matched the length of two CTA+ cations [35], implying that the pores should form and locate between the nanorods when they assembled/stacked, as revealed in Fig. 7c and d. To further confirm the secondary structure, the nanocones-assembled spheres were treated with ultrasonication and characterized with HRTEM, as shown in Fig. S3: the primary particles consisted of nanorods with size of 10–13 nm. Therefore, CTAB molecules should not only act as soft templates for directing spherical nuclei but also help assembling these nanorods into regular cone-like structures by selective adsorption and hydrophobic interactions. These long tail organic molecules converted to pores at following washing and calcination process. When the assembled spheres were working as gas sensors, these pores worked as tunnels for ethanol molecules to pass through. The highest surface area of AS implied enrichment of such
Fig. 5. SEM images of three SnO2 samples synthesized at 180 ◦ C for 48 h: (a) r = 1:3.5; (b) r = 2:1; (c) r = 3:1.
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Fig. 6. (a) Response curves of nanocone assembled spheres (AS), hollow spheres (HS) and solid spheres (SS) on cycling between increasing concentration (10–200 ppm) of ethanol and ambient air. (b) Partly magnified response curves at a concentration of 100 ppm to exhibit the recovery time as marked with asterisks. (c) The corresponding gas-sensing curves versus ethanol concentration.
tunnels, possibly by elongating the tunneling distance from center to surface. For HS and SS, their inner surface or solid core did not contribute so much to the surface area, but lose some surface area by shortening the tunnel distance. Such proposal was in accord with the longest balance and recovery time of AS samples in the three types of nanospheres. Microtomb technology was applied to further confirm the structure of the three samples (Fig. 8). One could see from the half sphere section that the nanocones of the assembled spheres were highly crystallized (Fig. 8a); hollow sphere also possessed high crystalline assembled rods in shells, but much shorter (Fig. 8b); while solid sphere had a big poorly crystallized core, with only quite short crystallized rods on the outer surface (Figs. 8c and S2b). Moreover, mesoporous structure of HS microtomb section can be
seen directly from the magnified TEM image (Fig. S2a). The pores between nanorods were nearly 3 nm, which was consistent with the BET results. Based on the above results, we thus proposed a possible electron flow mechanism to illustrate the sensitivity difference, as shown in Fig. 9. It is known that sensitivity is defined as: S=
˝a ˝g
(II)
where ˝a and ˝g represent the resistance of the samples exposed to air and targeted gas, respectively. However, the resistance of the three kinds of spheres before exposure to the targeted gas were far larger than after exposure and were almost equal to the load
Fig. 7. (a) Nitrogen adsorption/desorption isotherms obtained at 77 K and (b) pore size distribution of the as-synthesized SnO2 samples (c and d) high magnification SEM image of AS surface, the scale bar is 200 nm.
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Fig. 8. Cross-sectional TEM images of (a) nanocones assembled spheres, (b) hollow spheres and (c) solid spheres.
resistor resistance ˝0 , thus, the sensitivity S could be calculated by: S=
˝0 ˝g
(III)
∂˝ =− ∂t
Resistance is defined as: L ˝= A
(IV)
where L is the length of the conductor, A is the cross-section area of the conductor and is the electrical resistivity. As the electrical resistivity of amorphous SnO2 is much higher than crystallized ones, electrons tend to flow through crystallized zones (Fig. 9a–c). Therefore, an equivalent hollow spherical resistor model was established in Fig. 9d to calculate the resistance of the three types of spheres based on Eq. (IV). Section integral was used when calculating the resistance as shown below:
R−t
˝1 =
dx (R2 − x2 ) − [(R − t)2 − x2 ]
0 R−a
˝2 = R−t
dx = 2R (R2 − x2 )
˝ = 2(˝1 + ˝2 ) =
1 t
−
Partial derivative revealed the negative correlation between the resistance ˝ and the thickness of the crystallized layer, which means, the thicker the crystallized layer, the lower the resistance of the sphere.
ln
=
(R − t) (2Rt − t 2 )
2R − a t + ln 2R − t a
(V)
2R − a t 1 1 ln + + ln 2R − t R 2R − t a
(VI)
(VII)
where ˝ represents the whole resistance of the spheres which composed of two sections of resistance, ˝1 and ˝2 . R represents the radius of the spheres, t represents the thickness of crystallized outer layer and a represents the contact depth of the spheres.
1 t
−
1 2R − t
2 <0
(VIII)
Therefore, when exposed to ethanol, nanocone-assembled sphere had the lowest resistance ˝g while hollow sphere had a middle one and solid sphere had the largest one. That was why the AS had the largest sensitivity while SS had the lowest and HS got the middle according to Eq. (III). As well known, well crystallized SnO2 nanoparticles have much better transport performance than amorphous ones [7]. Hence, the sensor performance could be considered as a reflection of the crystallization, assembly and tunnel difference of assembled nanospheres. All the three samples had crystallized and assembled shells (the AS could be considered as an extremely small core with a very thick shell), which were assembled from the primary nanorods through selective adsorption and hydrophobic interactions of CTAB. Combustion of those CTA+ ions at calcination step made several pores with size of 3.76 nm, which worked as tunnels for gas absorption/desorption. As the sensors were made and exposed to ethanol vapor, the ethanol molecules moved through the tunnels into the mesoporous shells and induced electric signal response [36]. The tunnels also made the primary nanorods exposed completely to targeted vapors, thus the whole 10 nm-sized nanorods were included in the depletion space charge layer and therefore the samples exhibited approximately linear response according to Eq. (I). Since only the crystallized shells gave response, the sensitivity of nanocones-assembled nanospheres was even higher than thinner-shell hollow spheres. Solid core spheres had only very thin crystallized shell and thus performed worst. For sure, the saturating and recovery time for AS would be longer due to longer diffusion distance of trapped ethanol molecules within 3.76 nm tunnels. The solid spheres have the shortest recovery time due to their thinnest shells. 4. Conclusions
Fig. 9. Possible mechanism of electron flow in (a) nanocone assembled sphere, AS. (b) Hollow sphere, HS. (c) Solid sphere, SS. (d) Equivalent circuit of the spheres.
A series of mesoporous assembled SnO2 nanospheres, including nanocones-assembled spheres, hollow spheres and solid spheres, were obtained by cetyltrimethylammonium bromide (CTAB) directed solvothermal method. The submicrometer-sized spheres exhibited linear response to ethanol concentration, which was previously observed only on nanoparticles less than 10 nm. Systematic analysis on inner structure and sensor performance revealed that the crystallization behavior, assembly fashion and pore structures all determined the sensing performance. Regular aligned primary nanorods and consequent radically ordered pore
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structure were believed the source of the uncommon performance. Considering that the spherical morphology is helpful for dense assembly and submicrometer size of hierarchical structure is beneficial to long term stability, such hierarchical structure should be inspirative to further optimizing the nanostructures for constructing better gas sensors. Supporting information It contains weighing data, EDS spectra, and supplementary SEM, HRTEM images. Acknowledgements This work was supported by NSFC, the Foundation for Authors of National Excellent Doctoral Dissertations of P.R. China, the Program for New Century Excellent Talents in Universities, the 973 Program (2011CBA00503 and 2011CB932403), and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.02.073. References [1] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sensors and Actuators B 140 (2009) 319–336. [2] M. Law, H. Kind, B. Messer, F. Kim, P.D. Yang, Photochemical sensing of NO2 with SnO2 nanoribbon nanosensors at room temperature, Angewandte Chemie International Edition 41 (2002) 2405–2408. [3] G. Korotcenkov, Gas response control through structural and chemical modification of metal oxide films: state of the art and approaches, Sensors and Actuators B 107 (2005) 209–232. [4] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Oxide materials for development of integrated gas sensors—a comprehensive review, Critical Reviews in Solid State and Materials Sciences 29 (2004) 111–188. [5] G.C. Xi, Y.T. He, Q. Zhang, H.Q. Xiao, X. Wang, C. Wang, Synthesis of crystalline microporous SnO2 via a surfactant-assisted microwave heating method: a general and rapid method for the synthesis of metal oxide nanostructures, Journal of Physical Chemistry C 112 (2008) 11645–11649. [6] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Letters 5 (2005) 667–673. [7] X.G. Han, M.S. Jin, S.F. Xie, Q. Kuang, Z.Y. Jiang, Y.Q. Jiang, Z.X. Xie, L.S. Zheng, Synthesis of tin dioxide octahedral nanoparticles with exposed high-energy {2 2 1} facets and enhanced gas-sensing properties, Angewandte Chemie International Edition 48 (2009) 9180–9183. [8] J. Xu, D. Wang, L. Qin, W. Yu, Q. Pan, SnO2 nanorods and hollow spheres: controlled synthesis and gas sensing properties, Sensors and Actuators B 137 (2009) 490–495. [9] C. Martinez, B. Hockey, C. Montgomery, S. Semancik, Porous tin oxide nanostructured microspheres for sensor applications, Langmuir 21 (2005) 7937–7944. [10] G. Wang, J. Park, M. Park, X. Gou, Synthesis and high gas sensitivity of tin oxide nanotubes, Sensors and Actuators B 131 (2008) 313–317. [11] W. Caihong, X. Chu, M. Wu, Highly sensitive gas sensors based on hollow SnO2 spheres prepared by carbon sphere template method, Sensors and Actuators B 120 (2007) 508–513. [12] C. Aprile, L. Teruel, M. Alvaro, H. Garcia, Structured mesoporous tin oxide with electrical conductivity. Application in electroluminescence, Journal of the American Chemical Society 131 (2009) 1342–1343. [13] Q. Zhao, Y. Xie, T. Dong, Z. Zhang, Oxidation–crystallization process of colloids: an effective approach for the morphology controllable synthesis of SnO2 hollow spheres and rod bundles, Journal of Physical Chemistry C 111 (2007) 11598–11603. [14] A. Leonardy, W. Hung, D. Tsai, C. Chou, Y. Huang, Structural features of SnO2 nanowires and Raman spectroscopy analysis, Crystal Growth and Design 9 (2009) 3958–3963. [15] E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Quasione dimensional metal oxide semiconductors: preparation, characterization and application as chemical sensors, Progress in Materials Science 54 (2009) 1–67. [16] J.Q. Sun, J.S. Wang, X.C. Wu, G.S. Zhang, J.Y. Wei, S.Q. Zhang, H. Li, D.R. Chen, Novel method for high-yield synthesis of rutile SnO2 nanorods by oriented aggregation, Crystal Growth and Design 6 (2006) 1584–1587.
[17] E. Comini, G. Faglia, G. Sberveglieri, Z.W. Pan, Z.L. Wang, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Applied Physics Letters 81 (2002) 1869–1871. [18] G. Cheng, K. Wu, P. Zhao, Y. Cheng, X. He, K. Huang, Controlled growth of oxygen-deficient tin oxide nanostructures via a solvothermal approach in mixed solvents and their optical properties, Nanotechnology 18 (2007) 355604. [19] D.F. Zhang, L.D. Sun, G. Xu, C.H. Yan, Size-controllable one-dimensinal SnO2 nanocrystals: synthesis, growth mechanism, and gas sensing property, Physical Chemistry Chemical Physics 8 (2006) 4874–4880. [20] X. Sun, J. Liu, Y. Li, Use of carbonaceous polysaccharide microspheres as templates for fabricating metal oxide hollow spheres, Chemistry – A European Journal 12 (2006) 2039–2047. [21] Y. Wang, C. Ma, X. Sun, H. Li, Synthesis of mesoporous structured material based on tin oxide, Microporous and Mesoporous Materials 49 (2001) 171–178. [22] M. Tiemann, Porous metal oxides as gas sensors, Chemistry – A European Journal 13 (2007) 8376–8388. [23] T. Hyodo, N. Nishida, Y. Shimizu, M. Egashira, Preparation and gas-sensing properties of thermally stable mesoporous SnO2 , Sensors and Actuators B 83 (2002) 209–215. [24] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Nearly monodisperse Cu2 O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chemistry of Materials 18 (2006) 867–871. [25] Y. Xia, B. Gates, Y. Yin, Y. Lu, Monodispersed colloidal spheres: old materials with new applications, Advanced Materials 12 (2000) 693–713. [26] J. Ge, Y. Hu, Y. Yin, Highly tunable superparamagnetic colloidal photonic crystals, Angewandte Chemie International Edition 119 (2007) 7572–7575. [27] H.C. Zeng, Synthesis and self-assembly of complex hollow materials, Journal of Materials Chemistry 21 (2011) 7511–7526. [28] Q. Zhao, Y. Gao, X. Bai, C. Wu, Y. Xie, Facile synthesis of SnO2 hollow nanospheres and applications in gas sensors and electrocatalysts, European Journal of Inorganic Chemistry 2006 (2006) 1643–1648. [29] X. Lai, J. Li, B.A. Korgel, Z. Dong, Z. Li, F. Su, J. Du, D. Wang, General synthesis and gas-sensing properties of multiple-shell metal oxide hollow microspheres, Angewandte Chemie International Edition 50 (2011) 2738–2741. [30] Z.R. Dai, Z.W. Pan, Z.L. Wang, Novel nanostructures of functional oxides synthesized by thermal evaporation, Advanced Functional Materials 13 (2003) 9–24. [31] X. Xu, J. Zhuang, X. Wang, SnO2 quantum dots and quantum wires: controllable synthesis, self-assembled 2D architectures, and gas-sensing properties, Journal of the American Chemical Society 130 (2008) 12527–12535. [32] G. Cheng, J. Wang, X. Liu, K. Huang, Self-assembly synthesis of single-crystalline tin oxide nanostructures by a poly (acrylic acid)-assisted solvothermal process, Journal of Physical Chemistry B 110 (2006) 16208–16211. [33] B. Liu, H.C. Zeng, Fabrication of ZnO “dandelions” via a modified kirkendall process, Journal of the American Chemical Society 126 (2004) 16744–16746. [34] J. Zhang, S. Wang, Y. Wang, M. Xu, H. Xia, S. Zhang, W. Huang, X. Guo, S. Wu, ZnO hollow spheres: preparation, characterization, and gas sensing properties, Sensors and Actuators B 139 (2009) 411–417. [35] F. Zhao, Y.-K. Du, J.-K. Xu, S.-F. Liu, Determination of surfactant molecular volume by atomic force microscopy, Colloid Journal 68 (2006) 784–787. [36] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, Journal of Electroceramics 7 (2001) 143–167.
Biographies Yun Kuang received his B.S. from Beijing University of Chemical Technology in 2011 and now he is pursuing his PhD under the supervision of Prof. Xiaoming Sun at State Key Laboratory of Chemical Resource Engineering in Beijing University of Chemical Technology. Guobing Chen received his B.S. and M.Sci. from Beijing University of Chemical Technology in 2008 and 2011, he is now working as a research scientist in Azure Wind Environmental Protection Technology Co., Ltd. His main research interests focuses on gas sensing materials for environmental monitoring devices. Xiaodong Lei received his received his M.Sci. and Ph.D. from Beijing University of Chemical Technology in 2003 and 2006. After that, he joined the faculty of Beijing University of Chemical Technology. His current research interests focuses on Layered inorganic functional materials, water treatment and recycling of waste resources. Liang Luo received his Ph.D. in Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, China, in 2010. He is now working at Department of Inorganic Chemistry, Faculty of Science, Beijing University of Chemical Technology. His current research interests cover design, preparation of inorganic nanomaterials and their applications as gas sensors and optical devices. Xiaoming Sun received his B.S. and Ph.D. in Department of Chemistry, Tsinghua University at 2000 and 2005, respectively. He finished his postdoctoral work at Stanford University with Prof. Hongjie Dai and joined the faculty of Beijing University of Chemical Technology at 2008. His main research interests focus on controlled synthesis, separation, assembly, and consequent property investigations (e.g. Energy-storage, Gas-Sensing, Catalysis) on inorganic nanomaterials.