Article pubs.acs.org/crystal
Synthesis of Crystalline/Amorphous Core/Shell MoO3 Composites through a Controlled Dehydration Route and Their Enhanced Ethanol Sensing Properties Longqiang Wang,† Peng Gao,*,† Di Bao,† Ying Wang,† Yujin Chen,*,‡ Cheng Chang,† Guobao Li,§ and Piaoping Yang*,† †
College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, Heilongjiang, 150001, People’s Republic of China ‡ College of Science, Harbin Engineering University, Harbin, Heilongjiang, 150001, People’s Republic of China § Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing, 100871, People’s Republic of China S Supporting Information *
ABSTRACT: Considering the specific surface area, flexible structures, high porosity, and a homogeneous and isotropic nature down to the atomic scale possessed by the amorphous nanostructure, an interesting controlled dehydration route has been developed to synthesize the crystalline/ amorphous core/shell (C/A-C/S) MoO3 nanocomposite, in which macroisopolyanion [Mo36O112(H2O)16]8− ({Mo36}) was successfully assembled into a one-dimensional connection and underwent a deficient crystallization induced by the prospective infiltrative dehydration process. The as-obtained MoO3 samples’ affirmative composition and composite structure have been further demonstrated by the XRD, TEM, HRTEM, and Raman spectra measurements. As a result of this special C/A-C/S structure, the MoO3 composite exhibited high selectivity and a higher sensor response, at a lower working temperature (180 °C) to ethanol gas compared with other 1D MoO3 micro/ nanostructures reported previously.
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INTRODUCTION In the past, it has been demonstrated that hierarchical structures could improve the properties of materials in optoelectronic devices, biomedical science, field emission, bionic superhydrophobic surfaces, etc.,1 and therefore, they have attracted much attention. Especially, hierarchical nanostructures, which have both the properties of nanoscaled materials and the ease of manipulation, may facilitate assembly processes and provide an attractive alternative for bottom-up fabrication.2 In addition, the core and branches of a hierarchical structure can be composed of different chemical compositions, making them suitable for assembly into a nanodevice with multiple functions. However, most studies about hierarchical nanostructures focused on the alternative of different chemical elements; few works have been conducted to control their crystal-phased compositions: crystalline or amorphous. Recently, nanosized amorphous metal oxides, such as V2O5, MnO2, and TiO2,3−5 have attracted much attention due to their advantages: The specific surface area increases with decreasing particle size. The structure of amorphous metal oxide may be more flexible than that of crystals, resulting in the more stable structure for the volume change as a matrix.4 The high porosity of amorphous nanoparticles provides the three-dimensional space required for the doping of functional components, known as the dopants.5 The absence of crystallites, grain boundaries, © XXXX American Chemical Society
and dislocations in the amorphous structure results in a homogeneous and isotropic material down to the atomic scale, which displays excellent strength, hardness, elastic strain limit, and corrosion resistance.6 Considering the above standpoints, the formation of a singlecompound nanocomposite composed of its amorphous phase and nanocrystalline phase rather than a mixture of different compounds may induce its property to be enhanced greatly. In fact, this expectation has been demonstrated by some remarkable examples: the C/A-C/S Si nanocomposites as high-powered and long-life lithium battery electrodes have been prepared through a high-temperature CVD method, in which amorphous Si shells were electrochemically active due to the difference of their lithiation potentials and the crystalline Si cores functioned as a stable mechanical support and an efficient electrical conducting pathway;7 the C/A-C/S TiO2 composite exhibited substantial solar-driven photocatalytic activities. Large amounts of lattice disorder in semiconductors yielded midgap states, whose energy distributions differed from that of a single defect in a crystal;8 the C/A-C/S Ge composite displayed high optical absorption efficiency with an effective band gap of ∼1 Received: September 16, 2013 Revised: December 17, 2013
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dx.doi.org/10.1021/cg401384t | Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Table 1. Sensing Performance of Different MoO3 Micro/Nanomaterials materials MoO3 MoO3 MoO3 MoO3 MoO3 MoO3 MoO3 MoO3
multilayers23 hollow microspheres24 nanoplates25 nanorods26 nanolamellar27 thin films28 nanoparticle29 nanofilms30
target gas
sensor response
concentration (ppm)
optimum working temp (°C)
H2 NH3 C2H5OH NO2 NO2 NH3 H2S NO2
18 20 34 102 118 210 370
1000 500 500 40 10 500 20 10
300 270 260 300 225 400 375 300
Scheme 1. Schematic Depiction of the Growth Mechanism of C/A-C/S MoO3 Composite
as a suitable precursor for 1D MoO3 nanostructures through a dehydration process.31 As is well-known, the removal of H2O molecules (dehydration) from the interior of the crystal often results in the amorphization,35 which has been extensively utilized in food science,36 biomedicine,37 and mechanochemistry.38 By controlling the kinetic process of the dehydration of Mo36-polyoxometalate, the amorphous or amorphous/crystalline MoO3 nanostructures maybe obtained. Generally, the dehydration process is attributed to the high temperature in the hydrothermal reaction,39 and this point has been demonstrated by some remarkable examples: A ZnO nanoflower was synthesized by thermal dehydration of Zn(OH)42− ions in an alkaline environment under hydrothermal conditions at 180 °C.40 SnO2 nanoparticles grew up on the surface of MnO2 nanowires to form MnO2/SnO2 hierarchical heterostructures through the thermal dehydration of Sn(OH)62− ions under hydrothermal conditions at 220 °C.41 An α-Fe2O3 nanorod was synthesized by thermal dehydration of an α-FeOOH nanorod under hydrothermal conditions at 180 °C.42 Similarly, in this work, the polyanionic {Mo36} shaped into 1D coordination polymers by using Mo36-polyoxometalate anionic units, and then formed into a 1D MoO3 nanostructure through the thermal dehydration process at elevated pressures and temperatures. Through a series of detailed experiments, it is proved that the prospective chemical strategy is realizable and the C/AC/S MoO3 composite is obtained during a rapid dehydration process. The total process is exhibited in Scheme 1. It is also found that these special structures show a strong structureinduced enhancement in their sensing performance to ethanol gas and exhibit a high ethanol sensor response and selectivity at a relatively lower temperature (180 °C) compared with other reported MoO3 materials.
eV. Specifically, minimal optical reflectance (
