Versatile Growth of Freestanding Orthorhombic α‑MolybdenumTrioxide Nano- and Microstructures by Rapid Thermal Processing forGas NanosensorsOleg Lupan,*,†,‡ Vasilii Cretu,‡ Mao Deng,§ Dawit Gedamu,† Ingo Paulowicz,† Soren Kaps,†

Yogendra Kumar Mishra,† Oleksandr Polonskyi,# Christiane Zamponi,∥ Lorenz Kienle,§ Viorel Trofim,‡

Ion Tiginyanu,‡ and Rainer Adelung*,†

†Functional Nanomaterials, Institute for Materials Science, Kiel University, Kaiser Str. 2, D-24143 Kiel, Germany‡Department of Microelectronics and Biomedical Engineering, Technical University of Moldova, 168 Stefan cel Mare Blvd., MD-2004Chisinau, Republic of Moldova§Synthesis and Real Structure, Institute for Materials Science, Christian-Albrechts Universitat zu Kiel, Kaiser Str. 2, D-24143 Kiel,Germany#Chair for Multicomponent Materials, Institute for Materials Science, Christian-Albrechts Universitat zu Kiel, Kaiserstraße 2,D-24143, Kiel, Germany∥Inorganic Functional Materials, Institute for Materials Science, Christian-Albrechts Universitat zu Kiel, Kaiserstraße 2, D-24143, Kiel,Germany

*S Supporting Information

ABSTRACT: We demonstrate a new technique that requires a relativelylow temperature of 670−800 °C to synthesize in 10−20 min high crystallinequality MoO3 nano- and microbelts and ribbons. The developedtechnological process allows rapid synthesis of large amounts of MoO3nano- and microsheets, belts, and ribbons, and it can be easily scaled up forvarious applications. Scanning electron microscopy (SEM) studies revealedthat the MoO3 nano- and microbelts and ribbons are synthesized uniformly,and the thickness is observed to vary from 20 to 1000 nm. The detailedstructural and vibrational studies on grown structures confirmed an excellentagreement with the standard data for orthorhombic α-MoO3. Also, suchfreestanding nano- and microstructures can be transferred to differentsubstrates and dispersed individually. Using focused ion beam SEM, MoO3-based 2D nano- and microsensors have been integrated on a chip andinvestigated in detail. The nanosensor structures based on MoO3 nano- and microribbons are quite stable and moderatelyreversible with respect to rises and drops in ethanol vapors. It was found that MoO3 nano- and microribbons of various sizesexhibit different sensitivity and selectivity with respect to ethanol, methanol, and hydrogen gases. The developed technique hasgreat potential for further studies of different metal oxides, nano- and microsensor fabrication, and especially for multifunctionalapplications.

1. INTRODUCTION

As nanotechnology is developing rapidly, nano- and micro-structures of transition-metal oxides are gaining a lot of interestfrom the research community because of their extraordinarypotential for applications in various low-dimensional devicesand sensors.1−3 The quasi-one-dimensional (Q1D) nanowire,nanorod, or two-dimensional (2D) nanobelt−nanoribbonmorphologies of metal oxides exhibit efficient Q1D-2Delectronic radial transport pathways, facile strain relaxationbehavior, and a large contact area to surrounding environments.In Q1D- and 2D-based sensors, size matters because size cansignificantly influence the performance of the devices.2−5

Molybdenum trioxide (MoO3), in this context, is one of themost attractive candidates from the binary oxides family in

current scenarios because of its outstanding physical andchemical properties suitable for promising technologicalapplications.6−11 α-MoO3 has a unique morphology12 whichresembles a graphene-like layered structure.13 By virtue of itslayered structure and high chemical stability, α-MoO3 could actas an excellent cathode material in high energy density solid-state microbatteries.14 These characteristics help to organize theMoO3 nanoscale building blocks into various assemblies andultimately into useful nano- and microsystems, for example, gassensors,15,16 recording materials,17 lubricants,18 and electro-

Received: April 19, 2014Revised: June 10, 2014Published: June 10, 2014

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© 2014 American Chemical Society 15068 dx.doi.org/10.1021/jp5038415 | J. Phys. Chem. C 2014, 118, 15068−15078

chromic19 and photochromic5 devices. Meanwhile, MoO3 isalso a very promising material for catalysis,20,21 field emission,22

light emitting diodes,23 energy storage,1,24 etc. because of itsinteresting electrical and optical features. Thus, an extensiveand systematic investigation of the structure and properties ofMoO3 nano- and microsheets and ribbons and their Q1D and2D derivatives is of high interest for the scientific communitynot only for basic research but also for their important potentialtechnological impacts. Because of the versatile structures, theinvestigation is rather challenging, but at the same time it willbe very interesting and rewarding if these structures can besynthesized by simple and cost-effective procedures. Growth ofdesired structural varieties and their appropriate character-izations are the most important requirements for exploring thepractical applications of MoO3, especially for their use innanostructures and their derivatives for gas sensors.13,16

There are reports on different techniques, including severalapproaches for thermal processing and oxidation of MoO3nano- and microstructures for producing nanostructuredmaterials with high purity, homogeneity, and good crystallineproperties.25 Niederberger et al.26 synthesized molybdenumoxide nanowires by a template-directed hydrothermal process.However, Wang et al.27 prepared α-MoO3 nanowires during 2−20 h of hydrothermal process without using any template orcatalyst. In the same context, Zhou et al.22 have grown MoOxnanowire arrays on silicon substrates by a simple process basedon heating a Mo boat in vacuum at 1100 °C for 60 min under aconstant flow of argon22 and subsequently at 400 °C in highpurity oxygen for 30 min.22,28 Furthermore, Phuruangrat et al.21

communicated microwave-assisted hydrothermal reaction in 20h to synthesize molybdenum oxide nanowires. However, thereare only few reports on the direct growth of the α-MoO3 nano-and microstructures by oxidation technique in a single-stepprocess.29 To develop a simple, cost-effective, and faciletechnique, which can be easily scaled up for industrialproductions, it is very important to achieve low synthesistemperature and to speed up the production process. Therequirement of complicated vacuum systems again put someconstraints on the synthesis process; for large-scale production,the synthesis technique with least use of vacuum systems, orfree from them, i.e., growth in normal air environment, wouldbe very highly desirable. In this context, techniques like rapidthermal processing (RTP) and oxidation (RTO) are verysimple and fast; to the best of our knowledge of the literature,they have not been used for the synthesis of α-MoO3. It istherefore a promising technique for growth of different α-MoO3 nano- and microstructures.The rapid thermal processing and oxidation techniques at

relatively low temperature allows well-controlled dimensionalcomposition of the MoO3 belts and satisfies other requirementsfor nano- and microbuilding blocks for highly efficient androbust assemblies or novel useful systems. Such morphologiesof metal oxide nano- and microstructures are quite attractive fornanotechnologies, especially in the bottom-up process fornanodevices assembly, because such a design requires buildingwith precisely controlled nanomaterial parameters (includingchemical composition and structure), which ultimatelydetermine the performance of the device.30

Here, we demonstrate a method for synthesizing the highlycrystalline α-MoO3 nano- and microbelts and ribbons directlyfrom commercial bulk molybdenum in a single-step growthprocess without using any surfactant or template. Thedeveloped method allows rapid synthesis of large amounts of

nano- and microsheets, belts, and ribbons with differentdimensions, and the growth process can be easily scaled upfor mass-scale production; hence, the method represents a steptoward successful advanced technological applications. We havesystematically investigated the morphological, structural,chemical, and electronic properties of 2D MoO3 nano- andmicrosheets and Q1D nanoribbons (NR). An individual α-MoO3 NR prepared by this RTP method has been integratedinto the chip in the form of nanodevices, and their sensingresponses with respect to different gases (H2, ethanol, andmethanol) have been studied in detail.

2. EXPERIMENTAL SECTIONThe α-MoO3 nano- and microribbons were grown by a rapidthermal processing technique from a molybdenum metal plateat 670 and 800 °C in normal environment for 10−20 min. Themorphology of samples was examined using a scanning electronmicroscopy (SEM) instrument (Carl Zeiss; 10 kV, 10 μA). Thecompositional analysis of the specimens was carried out byenergy-dispersive x-ray microanalysis (EDX) in combinationwith SEM. Crystallographic information was derived from X-raypowder diffraction (XRD) data measured by using a Seifert3000 TT unit operating at 40 kV and 40 mA with Cu Kα1radiation (λ = 1.540598 Å). The specimens were characterizedby X-ray photoelectron spectroscopy (XPS) (Omicron Nano-technology GmbH) operating with Al anode at a power of 240W to determine the chemical bonding state, composition of thesurface, and electronic state of the elements that exist within α-MoO3 specimens. Spectra were recorded with pass energies of100 eV (survey) and 30 eV (high-resolution spectra). All XPSspectra were charge-referenced to the aliphatic carbon at 285.0eV.Transmission electron microscopy (TEM) investigations

were performed in detail by the Tecnai F30 STwin electronmicroscope (300 kV, field-emission gun, spherical aberrationconstant Cs= 1.2 mm) EDX spectroscopy (utilizing an EDAXsystem, Si/Li detector) analyzing the chemical compositions.The samples were first gently crushed and dispersed in n-butanol. A few droplets of this solution were placed on a coppergrid covered with a lacy carbon film. Afterward, the samplegrids were left to dry before the TEM investigations.The Raman spectrometer used in this study was a WITec

system. A 532.2 nm line from a Nd:YAG laser at an outputpower of 10 mW was used as the excitation source. The lightwas focused on the sample using an objective lens mounted onan optical microscope which is connected to a Ramanspectrometer. A 1/4 m single monochromator fitted with an1800 groove mm−1 grating has been used to disperse thescattered light, which is then focused onto a charge coupleddevice (CCD) detector (Wright Instruments, Ltd.). Thisdetector consisted of a Peltier-cooled, slow-scan, 384 × 576pixel CCD camera system, with the CCD chip maintained at−60 °C. Before actual measurements, the instrument wascalibrated to the same accuracy using standard silicon substrate.Nanodevices and microsensors were fabricated by using afocused ion beam (FIB)-SEM instrument Dualbeam HeliosNanolab (FEI) (10 kV, 0.17 nA). Initially, MoO3 nano-microbelts were sonicated for 30 min in 50 mL ethanol/water(1/1 ratio) solution. The resulting exfoliated MoO3 suspensionwas then transferred onto the chip and heated at 50 °C for 10min. For nanodevice fabrication, we used silicon/silicon oxideand quartz-based template-chips as described in our recentwork.2,3 Two rigid contacts were made with a single α-MO3

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nano- and microribbon on the sensor substrate template (Si/SiO2 and quartz with Cr/Au contacts as contact electrodes, seeFigure S6d of Supporting Information and Figure 6a inset) byusing the FIB-SEM metal deposition function. Because themaskless deposition cannot be achieved by the conventionalchemical vapor deposition (CVD) methods, we used thisadvantage of the FIB-SEM setup. In our work, a single MoO3NR was exposed to FIB beam for less than 8 min. The test gassensing characteristics were investigated using a sensorstructure connected to an external electrode in the FIB-SEMsystem. The measuring apparatus consists of a closed quartzchamber connected to a gas flow system as reported earlier.3,30

Vapors of ethanol and methanol were used as target gases, aswell as hydrogen, to investigate the sensing performances ofsingle α-MoO3 ribbon-based nanosensor. The concentrationsof target gases pulses were 100 ppm and 800−1000 ppm,calculated according to the chamber volume and the densitiesof the liquid reagents. The detection limits were investigated inthe range of 5−1500 ppm. By monitoring the output voltageacross the 2D ribbon-based sensor, the conductance wasmeasured in air and in the test gas separately and comparedafterward.

3. RESULTS AND DISCUSSION

The SEM images of synthesized structures are shown in Figure1 and Figures S1 and S2 of Supporting Information, whichdemonstrate that the MoO3 grown by RTP exhibits belt- andribbonlike morphologies. The micrographs shown in Figure1a,b and Figure S1a−f of Supporting Information illustrate that

the synthesized MoO3 nano- and microbelts and ribbons at 670°C are quite homogeneous in shape and size, with average beltlength of up to several millimeters and a width in the range of50 nm to 5 μm. The belt thickness proves to vary from 20 to1000 nm (Figure 1 and Figure S1 of Supporting Information).The nano- and microribbons have a length up to severalmicrometers and a width in the range of 50−200 nm (FigureS1f). The micrographs in Figure 1b and Figure S1a,b show thatsome larger ribbons grown at 670 °C consist of several thinlayers of MoO3 microribbons. The number of MoO3 layersassembled as in Figure S1a,b determines the thickness of theindividual larger ribbon synthesized at 670 °C. According toour observations, the number of layers varies from belt to belt,and as a result their total thickness also varies. The anisotropiccrystal structure of MoO3 manifests itself in the distinct habit ofsingle crystals. The formation mechanism of α-MoO3 sheets ofnanoscale thickness up to some extent has already beendescribed by Kaner et al.6 In the present case, the formationmechanism of morphologies of the obtained MoO3 structuresduring RTP could be understood in a similar manner. Theorthorhombic α-MoO3 with the space group Pbnm, as shown inFigure 2, possesses a layered structure which can be built up ofchains of MoO4 tetrahedra connected by the sharing of twooxygen corners with two neighboring tetrahedra in the c-axisdirection.21 Considering that the MoO3 structure is composedof layers connected by only weak van der Waals (vdW) bonds,crystals are micaceous (mica group) and platy and can becleaved easily along the vdW gaps. Without breakage of theprimary Mo−O bonds, it is highly possible that a (010) surface

Figure 1. SEM images of α-MoO3 material grown by RTP. (a) Overall view of the materials synthesized at 670 °C in 10−20 min showing layeredmorphology. (b) Magnified area of a belt and individual microribbon synthesized at 670 °C. (c) Overall view of the materials synthesized at 800 °Cin 10−20 min. (d) Magnified area of a belt and individual bended microribbon synthesized at 800 °C.

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is created and that it typically dominates the surface area ofsingle crystals. After the (010) surface, creation of a (100)surface requires breaking the second smallest number of bondsand thus is one of the weakest bonds in the structure. This isnot too surprising because growth of such crystals is alsocommonly observed in needle-like morphologies with extended{010} and {100} facets. However, most nano- and microbeltslook like crystalline films (see Figure S1c−e of SupportingInformation), especially those grown at 800 °C (see Figure 1c,dand Figure S2 of Supporting Information). Thus, by increasingsynthesis temperature, it is possible to easily break the vdWbonds and form single layer belts/ribbons, as observed inFigure 1d and Figure S2.Quasi-electron-transparency in MoO3 belt structures grown

by the RTP technique has been discovered in theseexperiments and investigated in detail using SEM-ZEISS. Theacceleration-voltage-dependent transparency is demonstrated inFigure 3a−f. At the value of 4 kV (Figure 3a), the belts appearto be almost nontransparent; however, with increasing voltage(more than 4 kV), the transparency sets in and a maximumcontrast is observed at 10 kV (Figure 3d). The acceleration-voltage-induced e-beam transparency can be rationalized byassuming an increase of the electron interaction volume whichgenerally increases with increasing accelerating voltage.However, with further increases in the voltage up to 20 kV(Figure 3f), the contrast decreases and the top belt is moretransparent. The involved mechanism behind the observed

transparency is still an open issue, and there are only very fewreports about this. In this regard, Zhang et al. proposed thatcharges in the underlying structure have influence on thesecondary electron (SE) distribution in the top layer.31 Thesecharges eventually lower the electron density at the overlap,resulting in a lower SE yield, which leads to the observedcontrast.31 Such observations can have a great impact on futureapplications of these nano- and microribbons of α-MoO3.The α-MoO3 has a unique layered morphology. The

alternating stacking of these layers along the [010] directionwith the vdW interaction leads to the formation of a two-dimensional structure (see Figure 2a,b). It allows guest atoms,ions (e.g., Li+, etc.), and molecules to be introduced into thevdW gaps between the layers through intercalation.12,32 Insideeach layer, it is composed of two sublayers formed by [MoO6]octahedra, which have the composition of MoO3. In each of the[MoO6] octahedra, there are three crystallographicallyindependent oxygen sites, which occupy 4c Wyckoff positionsaccording to the orthorhombic space group Pbnm,33 but onlyone type of [MoO6] octahedron exists. The [MoO6] octahedraform the sublayer by sharing common corners along the [100]direction (see Figure 2a) and common edge along the [001]direction (see Figure 2b). In projection along [010], one canobserve a corner-sharing net that is characteristic of the MoO3structure (see Figure 2c). The α-MoO3 structure was firstdetermined in 1931 by Brakken34 and Wooster;35 and wasrefined by Kihlborg in 1963.12,36 According to Kihlborg’s resultsbased on a more accurate X-ray analysis,36 MoO3 crystallizes inthe space group Pbnm with four formula units in the unit cell ofdimensions 396.28 pm (a-axis), 1385.5 pm (b-axis), and 369.64pm (c-axis).37 The X-ray diffractograms measured at roomtemperature of the specimens (grown at 670 and 800 °C),shown in Figure 4a, exhibit excellent correspondence with thestandard XRD data for orthorhombic α-MoO3 (JCPDS 00-005-0506). The diffractogram 1 in Figure 4a (from sample 670 °C)shows the main α-MoO3 phase in addition to small quantitiesof (−203) Mo9O26, which is a crystallographic shear (CS) phaseaccording to PDF 05-0441. CS is formed because of thermalreduction of the oxide, or it can be derived from the fullyoxidized structures of MoO3 during the RTP process at lowertemperatures.38 Such a reflection around 2θ = 20° does notappear for samples grown at higher temperatures (800 °C), soit can be detected only for pure α-MoO3 phase. The (020),(040), (060), and (0 10 0) reflections in the diffractogram 1(Figure 4a) are predominantly more intense than those from(110), (130), (140), i.e., (hk0) and (hk1) planes. This clearlyindicates a two-dimensional growth of crystallites along thebasal planes {010} of α-MoO3,

39 as schematically shown by theextended unit cell.5,40 Also, it proves that nano- and microbeltsare grown with a strongly preferred orientation. For thestructures obtained at 800 °C, a similar X-ray diffractogram hasbeen observed (Figure 4a, diffractogram 2). The relativepredominance of the (0k0) reflections is well-pronounced, andthe reflections hkl with h,k,l ≠ 0 have a reduced relativeintensity. The presence of other phases of molybdenum oxidehas not been observed, and samples grown at 800 °C areentirely orthorhombic α-MoO3. Both regimes result in growthof micro and nanoscale crystals exhibiting a high degree ofcrystallinity; the pronounced (0k0) reflection intensitiesconfirm the layered structure packed in the direction of itscrystallographic b-axis, parallel to the (100) plane as shown inthe crystal structure (Figure 2).5,40 The degree of grainorientation and lamellar stacking based on XRD data have been

Figure 2. Structural representations of α-MoO3 crystal: (a) Projectionalong c-axis showing the arrangement of Mo−O polyhedra along thegrowth axis of an axis [100]; unit cell and the van der Waals gap areshown. (b) Projection along a-axis showing the arrangement alonggrowth axis of c-axis [001]. (c) Projection along b-axis showing thecorner-sharing characteristic of Mo−O polyhedra in (010) plane. (d)Mo−O octahedra inside an individual unit cell, illustrating thecoordination of six oxygen atoms around a molybdenum atom.

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examined using the Lotgering method41 and found to be 0.87and 0.91 for the samples (670 °C) and (800 °C), respectively.The XRD results (Figure 4a) support very well the structuralmorphologies obtained in the SEM measurements (Figure 1and Figures S1 and S2 of Supporting Information) for thestructures synthesized by the RTP process. The α-MoO3 micro-and nanobelts were grown at a sintering temperature of 800 °Cfor 10−20 min. The experimental d-spacing (dhkl) values werecalculated by Bragg equation,42 as shown in Tables S1 and S2of Supporting Information, and the value of d020 is in agreementwith those obtained by TEM investigations (Figures S4−S5 ofSupporting Information and Figure 5b). Figure 3 clearly showsthe typical rectangular morphology of the resulting nano- andmicrobelts and ribbons (Figures S1 and S2 show the surface ofα-MoO3 crystals formed by RTP of Mo).Micro-Raman spectra of MoO3 nano- and microribbons

grown by RTP at 670 and 800 °C, collected under 532.2 nmexcitation, are demonstrated in Figure 4b in the range of 70−1130 cm−1. Fourteen lines have been observed, and theirfrequencies, relative intensities, and symmetry assignments aremarked within the graph; they are also summarized here.43 Inthe unit cell of molybdenum trioxides, there are 16 atoms, ofwhich four are molybdenum and the others oxygen. This resultsin 48 eigenmodes at the center of the Brillouin zone (q = 0).43

Because the orthorhombic unit cell of MoO3 extends over 2layers, the representations of the 48 vibrational modes for suchmaterial at q = 0 are44

Γ = + + + + +

+ +

A B B B A B

B B

8 8 4 4 4 4

8 8

crystal g 1g 2g 3g u 1u

2u 3u (1)

where Ag, B1g, B2g, and B3g are Raman-active modes. Au is aninactive mode, and others are infrared-active modes.Several spectra were collected from different belt−ribbons

and places; however, there was no discernible differencedetected. Therefore, only a typical spectrum corresponding toone point on MoO3 nano- and microribbons is presented here.The Raman spectra of two different samples are shown inFigure 4b. Curves 1 and 2 in Figure 4b show spectra of MoO3nano- and microribbons grown by RTP at 670 and 800 °C,respectively. The sharpness of the peaks indicates that thecorresponding vibrational modes are mostly due to a highlyordered structure.11,45 The 154 cm−1 (Ag, B1g) band originatesfrom the translation of the rigid chains; the 280 cm−1 (B2g, B3g)band is a doublet composed of wagging modes of the terminaloxygen atoms (MoO vibration); the 333 cm−1 (B1g, Ag) bandis assigned to Mo3−O bending mode; and the 375 cm−1 (Ag)band is assigned to MoO bending mode.11,37,43,46 Ramanspectrum of pure MoO3 exhibits bands characteristic of thiscompound at 94 (B1g), 112 (B2g), 125 (B3g), 154 (Ag, B1g), 194(B2g), 213 (Ag), 244 (B3g), 280 (B2g, B3g), 333 (B1g, Ag), 366(Ag), 375 (B1g), 469 (Ag, B1g), 662 (B2g, B3g), 815 (Ag, B1g), and991 (Ag, B1g) cm−l. The assignment of the bands observedfollows the single-crystal study and valence force field (VFF)calculations of Py et al.44,47 who used Kihlborg’s structuralpicture of MoO3 to obtain new insights into its vibrational

Figure 3. Quasi-electron-transparency in α-MoO3 belts (synthesized at 800 °C) at different accelerating voltages in SEM instrument Carl Zeiss: (a)4 kV; (b) 5 kV; (c) 7 kV; (d) 10 kV; (e) 11 kV; (f) 20 kV.

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behavior. The 662 cm−1 band (B2g, B3g) is an asymmetricstretching mode of the triply connected bridge−oxygen Mo3−O bridge along the c-axis, which results from edge-sharedoxygen (Mo3−O) in common with three adjacent octahe-dra.37,43,45,48,49 The intense Raman band at 815 cm−1 (Ag, B1g)is a symmetric stretching mode of the terminal oxygen atoms orthe doubly connected bridge−oxygen Mo−O−Mo, whichresults from corner-shared oxygen in common with twooctahedra.37,45,48,49 The 991 cm−1 (Ag, B1g) band is theasymmetric stretch of the terminal oxygen atoms (Mo6+O)mode along the a and b axes, which results from an unsharedoxygen, and it is responsible for the layered structure of α-MoO3.

37,45,49 All Raman peaks are in agreement with the resultsobtained from pure α-MoO3.

37,43,45,48,49 Raman peaks assignedto the three Mo−O stretching modes (singly, doubly, and triplycoordinated oxygen) were observed and are also in agreementwith the results obtained by Ajito et al.45

For chemical analyses, finding of empirical formula, chemicalstate, and electronic state of the elements that exist within asample of nondoped α-MoO3 nanomicrobelts prepared at 800°C, quantitative XPS has been employed. The XPS survey

spectrum (Figure 4c) reveals that the surface of the films iscomposed of Mo (22%) and O (67%) elements, as well asresidual amounts of adventitious carbon (11%), hydrocarbons,and carbonyl compounds which were unavoidable because oftheir exposure to air prior to the XPS analysis.3,30 A relativelyhigh amount of carbon detected by XPS is due to surfacecontamination and mounting the sample on carbon-tapedsupport, which was confirmed by EDX measurements. Thebinding energy (BE) scale was calibrated using the adventitiousaliphatic carbon at 285.0 eV as a reference. In our samples, thehigh-resolution XPS Mo 3d core-level spectrum of the MoO3

(Figure 4d) shows the presence of two well-resolved spectrallines located at 232.6 and 235.8 eV. These peaks are due to thespin−orbit Mo 3d levels corresponding to 3d5/2 and 3d3/2, inwhich the 3d doublet lines are separated by 3.2 eV. Such a spin-doublet with binding energies at 232.6 ± 0.5 eV and 235.8 ±0.5 eV corresponds to those of molybdenum in the formalvalence +6.50,51 The peak positions and energy separations arein good agreement also with data reported for MoO3 nanobeltsand thin films,51,52 where Mo is presented in the Mo6+ state.The XPS O 1s core-level binding energy (peak position) of the

Figure 4. (a) X-ray diffraction patterns of the product from MoO3 samples (1, 670 °C) and (2, 800 °C). (b) Raman spectrum (532.2 nm excitation)of MoO3 nano- and microribbons grown by RTP at 670 °C (1) and 800 °C (2). (c) XPS survey spectrum of α-MoO3 nano- and microbelts preparedat 800 °C and (d) Mo-3d core level XPS spectrum.

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molybdenum oxide micro- and nanobelts under considerationcorresponds to 531.0 eV (Figure S3 of SupportingInformation), which can be assigned to O2− in MoO3.

50,53,54

From the relative intensities of the above XPS spectra, thecompositional stoichiometry between “Mo” and “O” can becalculated. It was found that the Mo/O atomic ratio in thestudied sample corresponds to 1/3, within the accuracy of theXPS measurements, which is in agreement with EDXmeasurements. Thus, this confirms the proposed chemicalstructure of the investigated nano- and microbelts and ribbons:MoO3.The TEM bright-field image (Figure 5a) inferred that the α-

MoO3 nano- and microstructures prepared at 670 °C exhibit amorphology similar to that synthesized at 800 °C (Figure S4aof Supporting Information). The inset depicts a selected areaelectron diffraction (SAED) pattern along the [100] zone axisof α-MoO3 (space group, Pbnm) recorded in the markedregion. The corresponding high-resolution TEM (HRTEM)micrograph from the same region is depicted in Figure 5a. Thefast Fourier transform (FFT) from the square marked region of

the HRTEM micrograph indicates the crystal growth directionalong the c-axis of α-MoO3. The composition and ratio of theelements, according to the EDX spectrum shown in Figure 5c,are representative as several investigations have been carriedout at many other positions and the results are comparable toeach other. The slight deviation from the ratio Mo:O = 1:3might be due to the formation of oxygen vacancies which areproduced by electron-beam-induced reduction.55,56

A more statistical evaluation of TEM data showed that forboth of the synthesis temperatures, there are few occasionalcases in which the α-MoO3 nanowires also grow along the a-axis (Figure S5 of Supporting Information). However, thegrowth direction along the c-axis is a more preferential oneaccording to both our observations and the surface energytheory. The surface energy of the low index facet of the (001)plane is the lowest surface energy; therefore, the nanobeltspreferred to stack along [001] instead of other directions, suchas [100].57 This finding is in agreement with previousdisclosures that the synthesized MoO3 nanoribbons growpreferentially along the (001) direction.58

TEM investigations confirmed that the α-MoO3 nano- andmicrostructures grown at 800 °C exhibit a high aspect ratiowith a width of about 100 to 200 nm and lengths up to severalmicrometers. In Figure S5a of Supporting Information, a typicalα-MoO3 nanobelt is shown at low magnification. The SAEDpattern along zone axis [110] taken from the circled area showsstrong dynamic excitation of kinematically forbidden reflec-tions. Such observation is most probably due to enhancedthickness of the nanobelt and correspondingly enhancedmultiple scattering effects. Together with the HRTEMmicrograph and the FFT information in Figure S4a ofSupporting Information, the preferential growth direction ofthe α-MoO3 crystal prepared at 800 °C is determined to bealong [001], which is in agreement with the findings of othergroups.50−52 Additionally, splitting of Bragg reflectionsperpendicular to the c* direction is observed in the SAEDand FFT patterns. This phenomenon can be assigned to theformation of stacking faults,59 planar defects, and crystallo-graphic shear planes which are created under these conditions.Both of the latter kinds of defects are known to be produced byelectron-beam-induced phase transformation of α-MoO3.

55

Next, the in situ nanodevice fabrication procedure isdescribed based on our previous reports.3,60 The α-MoO3nano- and microribbons and belts can be released from theinitial substrate by sonication in ethanol and then transferred toa SiO2-coated Si substrate. We also used a direct contacttechnique to transfer nano- and microribbons from initialsubstrate to the clean Si sample by gently rubbing them a fewtimes. These procedures allow us to obtain a low density anduniformly distributed α-MoO3 nanobelts and ribbons on thesecond substrate for nanodevice fabrication (Figure 6a−b).Further details about nanodevice fabrication and character-izations are given in Experimental Section, and a typical devicestructure is shown by SEM image in Figure 6a−b and Figure S6of Supporting Information. A linear behavior of the current−voltage curves (Figure 6c) has been observed, which is veryimportant for the sensing properties because the gas responseof a nanosensor can be maximized when the metal−semiconductor junction has a negligible resistance.3 Forinvestigation of sensing performances, a sensor was placed ina measuring apparatus.3 Figure 6d shows that at a 180 °Coperating temperature, the single MoO3 nanobelt-based deviceshows good response to ethanol vapors of about 10%. It can be

Figure 5. TEM micrographs of an α-MoO3 nanoribbon synthesized at670 °C: (a) bright-field image of the nanoribbon with the insetshowing a SAED pattern and (b) HRTEM image recorded close to thesame region in (a) and a corresponding FFT in the inset calculatedfrom the part with a square indicating the growth direction. (c) Arepresentative EDX spectrum showing the chemical composition andatomic ratio of another region α-MoO3.

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concluded that it is quite stable and moderately reversible at theincreasing and decreasing ethanol vapor concentration.Although it shows an initial quick response and gets ratherequilibrated over time in the first 200 s, it is not rapidly 100%reversible at this operating temperature. For the interpretationof the sensor response behavior to ethanol vapor, probablysurface morphology of the MoO3 nanobelt layer andcompetitive reactions of analyte species with MoO3 surfacelayer have to be considered.16,61 The following model wasproposed to interpret the conductance change of the MoO3sensing nanobelt to the exposure of ethanol vapor. The ethanolvapor molecules are first chemisorbed on the molybdenumoxide sensing surface with a donor effect and inject electronsinto the nanobelt according to16,61

→ → ++ −C H OH C H OH(ads) C H OH (ads) e2 5 2 5 2 5(2)

The sensing may further progress through reactions ofadsorbed ethanol on the sensing surface, and the O−H bondof ethanol dissociates heterolytically to yield ethoxide groupsand hydrogen, as reported by Illyaskutty and co-workers.16 Theoverall reaction describing the ethanol response can bedescribed as follows:16,61

+ → + + ++ −sC H OH O ( ) CH CHO H O V 2e2 5 o 3 2 o2

(3)

All processes described above are assumed to take placesimultaneously on the nanobelt surface upon the introductionof ethanol vapors and contribute to gas response significantly.

This could be the most probable reason for higher sensitivity ofMoO3 belts to ethanol compared to hydrogen at this operatingtemperature.16 It can be suggested that ethanol induces areduction by the catalytic oxidation and results in electrontransfer to the metallic site. The details of the proposedmechanism of sensing ethanol with pure MoO3 nanostructurescan be found in previous work by Illyaskutty et al.61 Figure 6eshows experimental results on gas response to hydrogen gasand ethanol and methanol vapors. It can be clearly seen thatdifferent nano- and microdevices respond in different ways tothe ambient in dependence of MoO3 ribbon widths/thicknesses. It can be observed that sensors made fromMoO3 belts grown at 670 °C show an improved response toethanol compared to samples synthesized at 800 °C. Thinnersamples of 150 nm demonstrated response of 11.5% comparedto 9.9% measured from 200 nm sample. It was found that ournano- and microsensors possess detection limits in the range of5−1500 ppm of ethanol. At concentrations higher than 1000ppm, the gas response shows an almost saturated value andslow recovery time (Figure S6 of Supporting Information). Theresponse time was about 18 s, and recovery time constants of15 and 280 s at the operating temperature of 180 °C(summarized in Table S3 of Supporting Information incomparison to previously reported values). However, theresponse time is reduced to 7 and 22 s at operatingtemperatures around 300 °C (Table S3 and Figure S6 ofSupporting Information). When 150 and 200 nm samples arecompared, hydrogen gas response was increased from 7.6% to8.4%. Samples grown at 800 °C exhibit a higher response to

Figure 6. (a) Scanning electron image showing the chip on which the nanodevices were fabricated. Scale bar is 200 μm. (b) Single MoO3nanoribbon synthesized at 670 °C contacted to both electrode and external connections as final sensor structure. The inset is a digital image of thechip with Cr/Au contacts on quartz substrate used as template for nanosensor fabrication. (c) Current−voltage characteristics of a single MoO3nanoribbon-based device. (d) Sensor response of single MoO3 nanoribbon toward ethanol at 180 °C operating temperature and two pulses withdifferent concentrations. (e) Gas response of single MoO3 nanoribbon-based device to ethanol (3), methanol (2), and hydrogen (1) at 180 °Coperating temperature from nanosensors made on single MoO3 nanoribbons (synthesized at 670 °C and at 800 °C) with thicknesses indicated ongraph.

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methanol (about 6%) and lower to hydrogen gas (about 5.3%)at an operating temperature of about 180 °C. At higheroperating temperature of about 350 °C, an increased responseto H2 gas (of about 38%) was obtained with relatively fastresponse and recovery times (Table S3 and Figure S6 ofSupporting Information). In such a way the selectivity ofMoO3-based sensors can be controlled by operating temper-ature, and by combining all three types of nanosensors on thesame chip (as shown in the inset in Figure 6a and Figure S6 ofSupporting Information), one can develop a nanoscaleelectronic nose on the same platform. It is clearly demonstratedthat the developed technique, the synthesis of freestanding α-MoO3 crystals, is of great importance for further investigationsand practical applications.

4. CONCLUSIONSWe have demonstrated a new technique which requires arelatively low temperature of 670−800 °C and 10−20 min tosynthesize high crystalline quality α-MoO3 nano- andmicrobelts and ribbons. The developed technological processallows rapid synthesis of large amounts of MoO3 nano- andmicrosheets, belts, and ribbons, and it can be easily scaled upfor various applications. SEM studies revealed that the MoO3nano- and microbelts and ribbons are synthesized uniformly,having a length of belts up to several millimeters and a width inthe range of 50−5000 nm. The belt thickness is observed tovary from 20 to 1000 nm. Also, in SEM investigations,nanoribbons were observed which have a length up to severalmicrometers and a width in the range 50−200 nm. Thistechnology allows one to synthesize various sizes of nano- andmicrostructures in one process, which is time-saving as well ascost-effective. Quasi-electron-transparency of MoO3 at theoverlap of two belts has been evidenced. The structural studiesproved an excellent agreement with the standard XRD data fororthorhombic α-MoO3 crystals. The sharpness of the micro-Raman peaks confirmed that the observed vibrational modesare due to a highly ordered structure. Also, such nano- andmicrostructures can be transferred to different substrates anddispersed individually for further processing in deviceapplications. The MoO3-based 2D nanosensors and micro-sensors have been realized by using FIB-SEM, integrated on achip, and investigated in detail. It can be concluded that thenanosensors are quite stable and moderately reversible atincreasing and decreasing ethanol vapor concentrations. It wasfound that MoO3 nano- and microribbons of various sizes showdifferent sensitivity and selectivity to ethanol, methanol, andhydrogen gas, and this can be controlled by operatingtemperature. At an operating temperature of about 180−200°C, the higher response was to ethanol. However, at higheroperating temperature of about 350 °C, an increased responseto H2 gas was obtained with relatively fast response andrecovery times. The developed technique presents greatpromise for further studies of different metal oxides andespecially for industrial applications.

■ ASSOCIATED CONTENT*S Supporting InformationSEM images of α-MoO3 material grown by RTP at 670 and 800°C from different regions and magnifications, as well asindividual nanoribbons dispersed on substrate surface beforenanodevice fabrication; discussions on micro-Raman spectraintensity changes observed as compared to the single-crystalspectrum; XPS O-1s core level spectrum of an α-MoO3

nanomicrobelts prepared at 800 °C; TEM micrographs of anα-MoO3 nanorod prepared at 800 °C; SEM image showing thefabricated nanodevice from single MoO3 NR in the FIB-SEMsystem; sensor response of single MoO3 nanoribbon towardethanol at 180 °C operating temperature; sensor response ofsingle MoO3 nanoribbon toward different gases and volatileorganic compounds at various operating temperatures indicat-ing on its possible selectivity. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*O.L.: tel, (49)431 880 61 16; e-mail, ollu@tf.uni-kiel.de,lupan@physics.ucf.edu.*R.A.: tel, (49)431 880 61 16; e-mail, ra@tf.uni-kiel.de.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the STCU and ASMthrough Grant 09 STCU.A/5833. This research was partlysponsored by German Research Foundation (DFG) under theschemes SFB 855-A5, Z-1. O.L. and Y.K.M. acknowledge theAlexander von Humboldt Foundation for research fellowshipsat the Institute for Materials Science, University of Kiel,Germany. This research was partly supported from the EUGraphene Flagship project.

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