Spongy MoO3 hierarchical nanostructures for excellent performance ethanol sensor

Author’s Accepted Manuscript Spongy MoO3 Hierarchical Nanostructures for Excellent Performance Ethanol Sensor Yuchao Xia, Chuansheng Wu, Ningyu Zhao, He Zhang www.elsevier.com

PII: DOI: Reference:

S0167-577X(15)31113-7 http://dx.doi.org/10.1016/j.matlet.2015.12.159 MLBLUE20119

To appear in: Materials Letters Received date: 16 September 2015 Revised date: 27 December 2015 Accepted date: 29 December 2015 Cite this article as: Yuchao Xia, Chuansheng Wu, Ningyu Zhao and He Zhang, Spongy MoO3 Hierarchical Nanostructures for Excellent Performance Ethanol Sensor, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.12.159 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Spongy MoO3 Hierarchical Nanostructures for Excellent Performance Ethanol Sensor Yuchao Xia1,2, Chuansheng Wu*2, Ningyu Zhao2, He Zhang*1 1 College of Civil Engineering, Chongqing University, Chongqing 400045, China; 2 International College of Chongqing Jiaotong University, Chongqing, 400074, China; Abstract: Hierarchical nanostructures assembled from one dimensional (1D) α-MoO3 have been realized with the assistance of surfactants, but rare with metal salts. In this paper, we report the synthesis of porous α-MoO3 sponges with nanorods as building blocks ina hydrothermal condition that contains CrCl3•6H2O. It is believed that chromic saltaffects the growth manner of α-MoO3, resulting in hierarchical nanostructures. A comparison study reveals that an enhanced gas sensing performance for the sensor based on nanorod-assembled porous α-MoO3 sponges towards ethanol occurred over that of monodispersed MoO3 nanorods we prepared. The enhancement may be ascribed to efficient gas diffusion provided by the interconnected porous structures as well as a significant fraction of the surface atoms participating in gas-sensing reaction. Keywords: Sensors; hierarchical α-MoO3; nanorod; Semiconductors 1. Introduction Nanostrctured molybdenum trioxide (MoO3) commonly exists in three crystal forms, namely, orthorhombic α-MoO3, monoclinic β-MoO3, and hexagonal h-MoO3 [1, 2]. Particularly, α-MoO3 has been considered a potential gas-sensing material in terms of rich chemistry and intriguing layered structure [3, 4]. The most distinctive feature of α-MoO3is the structural anisotropy, where highly asymmetrical [MoO6] octahedras firstly assemble into a bilayer in such a manner that

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Corresponding author E-mail: [email protected] (C.S Wu) & [email protected] (H. Zhang) Tel.: +86-23-65336832 1

certain octahedras share four corners to form a plane, further combining with another plane by sharing octahedral edges along the [001] direction; then, all the bilayers stack up along the [010] direction with weak van der Waals forces [5, 6]. From the energy point of view, the planar growth rates follow the trend {010} < {100} < {001}, which indicates that it is highly favorable for α-MoO3 to grow along [001], leading to one-dimensional structures [1,7]. Typically, α-MoO3 nanorods along [001] can be synthesized in a hydrothermal condition via the acidification of ammonium heptamolybdate tetrahydrate[8]. However, sensors based on monodispersed 1D nanostructures, such as nanorods usually suffer from poor mechanical stability and serious stacking. Given this, it is highly significant to design the nanorods into arrays and hierarchical configuration, aiming at remediating the mentioned flaws [4, 9]. Design of hierarchical nanostructures refers to specific packing arrangement of their constituting building blocks in a desired and controlled manner, often guided by surfactants, metal salts and so on[10, 11]. Previously, Li et al. have reported a CTAB-assisted hydrothermal synthesis of such a configuration in which pre-assembled α-MoO3 nanobelts ‘secondly assembled’ into bundles and then aggregated into bird’s nest-like shape with less stacking density [9]. However, it remains a challenge to design more exquisite hierarchical structures with large specific surface, efficient diffusion path as well as proper chemical activity, owing to the complex hydrothermal reaction process. Herein, nanorod-assembled porous α-MoO3 sponges were successfully synthesized under hydrothermal condition containing chromic salt. Morphology analysisindicates that certain nanorods tended to randomly grow on the lateral surface of the pre-generated nanorod, further evolving into a branch, and so forth, until branches crisscrossed into structural stable porous 2

sponges, perhaps induced by CrCl3•6H2O. Compared with monodispersed MoO3 nanorods prepared without chromic salt, we find that the nanorod-assembled porous α-MoO3 sponges based sensor exhibits an enhanced gas sensing performance towards ethanol, which may be ascribed to not only the interconnected porous structures for efficient gas diffusion but also a significant fraction of the atoms participating in gas-sensing reaction. The results represent an advance of hierarchical nanostructures in further enhancing the performance of gas sensors, and this facile method could be applicable to many sensing materials. 2. Experimental Nanorod-assembled porous α-MoO3 sponges were synthesized by a facile hydrothermal process. Typically, 0.3 g CrCl3•6H2O was ultrasonically dispersed in a mixed solution of HNO3 (65 wt%, 15 mL) and deionized water (20 mL). Then 1.5 g (NH4)6Mo7O24•4H2O was added to the above mixture and followed by 30 min stirring. After that, the resulting suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and heated at 180oC for 24 h. After naturally cooling to room temperature, the white product was harvested by several rinse−centrifugation cycles with deionized water and ethanol before drying at 60oC. While without PEG leaded, other things equal, to the formation of monodispersed MoO3 nanorods. The as-prepared two products were characterized by the X-Ray Diffraction (XRD, Rigaku D/Max-1200X), the Scanning Electronic Microscopy (SEM, Nova 400 Nano). The gas sensors were fabricated by using thick films. In detail, MoO3 powder was further dispersed in the ethanol and ultrasonicated into slurry suspension, and then it was coated onto the surface of an Al2O3 ceramic tube by a small brush to form a thickness of 10~20 μm film between two parallel Au electrodes, which had been previously printed at the both end sides of the tube. Gas-sensing 3

properties towards ethanol were measured using a static system controlled by a computer under current laboratory conditions. Gas response in this paper is defined as S=Vg/Va, which Va and Vg are the test voltage in air and in ethanol gas, respectively. 3. Results and discussion Fig.1 shows the XRD patterns of the obtained samples without (a) and with (b) CrCl3•6H2O, respectively. All the identified peaks in each curve can be indexed toα-MoO3 (JCPDS card No. 05-0508) without observable impurity peaks, indicating the high purity of the products. The stronger intensities of the (020), (040), and (060) diffraction peaks suggest thehighly anisotropic growth of the nanostructures [12]. Moreover, it obviously reveals that the introduction of chromic salt makes make little difference on the phase but the crystallinity, for each diffraction intensity in cyan is relatively stronger that in pink. Fig.2 shows the SEM investigationsof the obtained α-MoO3 samples. A general view in Fig.2a demonstrates that, without CrCl3•6H2O, the product is composed of randomly stacked nanorods, which is consistent well with a conclusion that oriented growth of α-MoO3 is thermodynamically favored in extra additive-free hydrothermal condition. The magnified image displayed in Fig.2b shows that the size of the monodispersed nanorods is about 100~200 nm in width and several microns in length.Assembly of 1D α-MoO3 as building blocks into hierarchical structures has been realized with the assistance of surfactants, such as CTAB, but rare with metal salts [5, 9]. Interestingly, it is obvious in Fig.2c that, with the presence of CrCl3•6H2O, nanorods tended to assemble into porous spongy like hierarchical configuration with considerable interconnected hollow spaces. Detailed information given in Fig.2d reveals that the width of the building nanorods is similar to that in Fig.2b except wider distribution, and the length is somewhat shorter. The crisscrossednanorods leave well-defined net-like structure, indicating the significant 4

impact of CrCl3•6H2O which can be more precisely described as [CrCl2(H2O)4]Cl•2H2O. Previously, it proposed that via acidification of ammonium heptamolybdate tetrahydrate, α-MoO3 nucleus is formed, following [Mo7O24]6+ + 6H+ = 7MoO3 + 3H2O [8, 13]. Driven by the minimization of surface energy, the nuclei spontaneously aggregate intosingle-crystal 1D nanostructures [14,15]. Although well-defined acting mechanism of [CrCl2(H2O)4]Cl•2H2O affecting the growth manner isn't completely clear at the current stage, whether induced from its adhesive action or coulomb force between [Mo7O24]6+ and [CrCl2(H2O)4]- or something else, there are also some clues as to describing the evolution process of nanorod-assembled porous α-MoO3 sponges, as depicted in Fig.3. Along with nanorod further oriented growing, nodules tended to randomly grow on the lateral surface of the growing nanorod (as marked in dashed ring 1 in Fig.2d), further evolving into a branch, and so forth, and then the branches crisscrossed into network (as marked in dashed ring 2 in Fig.2d) until structural stable porous sponges were formed. The novel hierarchical porous sponges stimulate our interest to apply them in the field of gas sensor. Interestingly, a comparison study reveals that an enhanced gas sensing performance for the sensor based on nanorod-assembled porous α-MoO3 sponges towards ethanol occurs over that of monodispersed MoO3 nanorods, just as shown in Fig.4. More specifically, it can be seen in the inset of Fig.4a, that for 100 ppm ethanol the maximum response of the two sensors are 19.8 and 8.9,

respectively,

at

the

optimal

temperature

of

250oC

due

to

compromising

temperature-dependent carrier density and mobility as well as gas diffusion. Thus, we choose 250oC as our working temperature to proceed with the subsequent detections. It is obvious in Fig.4a that the porous sponges based sensor displayed about twice enhancement in sensitivity compared with the other one. The response amplitude of the porous sponges based sensor is 5

significantly increased with increasing ethanol concentration, while the increase of the monodispersed nanorods is relatively slight. The improved gas sensing properties may be attributed to not only the interconnected porous structures for efficient gas diffusion but also a significant fraction of the atoms participating in gas-sensing reaction. Note that in Fig.4b, both sensors exhibit almost similar response/recovery dynamics characters at 250oC for 100 ppm ethanol, especially for the same recover time, which may be ascribed to the difficult adsorption of the resultant molecules from the serious gas-sensing reaction on the surface of porous sponges, partially counteracting the advantage of porous structure. 4. Conclusions In this paper, porous α-MoO3 sponges with nanorods as building blocks have been successfully prepared via a chromic salt-assisted hydrothermal process. It is believed that CrCl3•6H2O affected the growth manner of α-MoO3, resulting in hierarchical nanostructures. we find that the nanorod-assembled porous α-MoO3 sponges based sensor exhibits an enhanced gas sensing performance towards ethanol, which may be ascribed to not only the interconnected porous structures for efficientgas diffusion but also a significant fraction of the atoms participating in gas-sensing reaction. Acknowledgements TThis work was supported in part by project of Chongqing Municipal Education Commission (No.KJ1500535, KJ1500520) and Natural Science Foundation of Chongqing (Project No.: cstc2013jcyjA30019). References [1] Wang Z, Madhavi S, Lou XW. J Phys Chem C 2012;116:12508-13. 6

[2] Zhou L, Yang L, Yuan P, Zou J, Wu Y, Yu C. J Phys Chem C 2010;114:21868-72. [3] Alsaif MMYA, Balendhran S, Field MR. Sens Actuators B: Chem 2014;192:196-204. [4] Sui LL, Xu YM, Zhang XF, Cheng XL. Sens Actuators B: Chem 2015;208:406-14. [5] Wang S, Zhang Y, Ma X, Wang W, Li X. Solid State Commun 2005;136:283-7. [6] Cui Z, Yuan W, Li CM. J Mater Chem A 2013;1:12926-31. [7] Chen JS, Cheah YL, Madhavi S, Lou XW. J Phys Chem C 2010;114:8675-8. [8] Gong J, Zeng W, Zhang H. Mater Lett 2015;154:170-2. [9] Li Y, Liu T, Li T, Peng X. Mater Lett 2015;140:48-50. [10] Wang C, Sun R, Li X, Sun Y, Sun P, Liu F, et al. Sens Actuators B: Chem 2014;204:224-30. [11] Zhai, ZGT, Sheng BGX. J Phys Chem B 2006; 110: 23829-36. [12] Lou XW, Zeng HC. Chem Mater 2002;14:4781-9. [13] Hu S, Wang X. J Am Chem Soc 2008;130:8126-7. [14] Cho YH, Ko YN, Kang YC, Kim I-D, Lee JH. Sens. Actuators B: Chem 2014;195:189-96. [15] Gao B, Fan H, Zhang X. J Phys Chem Solids 2012;73:423-9.

Figures captions

Fig.1 XRD patterns of the samples without (a) and with (b) CrCl3•6H2O, respectively. Fig.2 SEM images of monodispersed nanorods (a, b) and porous sponges(c,d). Fig.3 Schematic illustration of the evolution process of porous α-MoO3 sponges. Fig.4 (a) Responses of porousα-MoO3 sponges (pink line) and monodispersed nanorods (cyan line) based sensors to ethanol with different concentration at 250oC. The inset is response versus operating temperature of the two sensors exposed to 100 ppm ethanol; (b) Dynamic ethanol 7

sensing transient of the two sensors towards 100 ppm ethanol at 250oC. Graphical Abstract

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Figure 3

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Highlights 

Novel nanorod-assembled porous α-MoO3 sponges have been prepared.

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Sensors based on porous α-MoO3 sponges exhibit superior gas-sensing performance.

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The porous α-MoO3 sponges provide vast reaction sites and sufficient diffusion spaces.

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