and Microstructures by Rapid Thermal Processing for G - American

Article pubs.acs.org/JPCC

Versatile Growth of Freestanding Orthorhombic α‑Molybdenum Trioxide Nano- and Microstructures by Rapid Thermal Processing for Gas Nanosensors Oleg Lupan,*,†,‡ Vasilii Cretu,‡ Mao Deng,§ Dawit Gedamu,† Ingo Paulowicz,† Sören 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-2004 Chisinau, Republic of Moldova § Synthesis and Real Structure, Institute for Materials Science, Christian-Albrechts Universität zu Kiel, Kaiser Str. 2, D-24143 Kiel, Germany # Chair for Multicomponent Materials, Institute for Materials Science, Christian-Albrechts Universität zu Kiel, Kaiserstraße 2, D-24143, Kiel, Germany ∥ Inorganic Functional Materials, Institute for Materials Science, Christian-Albrechts Universität zu Kiel, Kaiserstraße 2, D-24143, Kiel, Germany ‡

S Supporting Information *

ABSTRACT: We demonstrate a new technique that requires a relatively low temperature of 670−800 °C to synthesize in 10−20 min high crystalline quality MoO3 nano- and microbelts and ribbons. The developed technological process allows rapid synthesis of large amounts of MoO3 nano- and microsheets, belts, and ribbons, and it can be easily scaled up for various applications. Scanning electron microscopy (SEM) studies revealed that the MoO3 nano- and microbelts and ribbons are synthesized uniformly, and the thickness is observed to vary from 20 to 1000 nm. The detailed structural and vibrational studies on grown structures confirmed an excellent agreement with the standard data for orthorhombic α-MoO3. Also, such freestanding nano- and microstructures can be transferred to different substrates and dispersed individually. Using focused ion beam SEM, MoO3based 2D nano- and microsensors have been integrated on a chip and investigated in detail. The nanosensor structures based on MoO3 nano- and microribbons are quite stable and moderately reversible with respect to rises and drops in ethanol vapors. It was found that MoO3 nano- and microribbons of various sizes exhibit different sensitivity and selectivity with respect to ethanol, methanol, and hydrogen gases. The developed technique has great potential for further studies of different metal oxides, nano- and microsensor fabrication, and especially for multifunctional applications.

1. INTRODUCTION As nanotechnology is developing rapidly, nano- and microstructures of transition-metal oxides are gaining a lot of interest from the research community because of their extraordinary potential for applications in various low-dimensional devices and sensors.1−3 The quasi-one-dimensional (Q1D) nanowire, nanorod, or two-dimensional (2D) nanobelt−nanoribbon morphologies of metal oxides exhibit efficient Q1D-2D electronic radial transport pathways, facile strain relaxation behavior, and a large contact area to surrounding environments. In Q1D- and 2D-based sensors, size matters because size can significantly influence the performance of the devices.2−5 Molybdenum trioxide (MoO3), in this context, is one of the most attractive candidates from the binary oxides family in © 2014 American Chemical Society

current scenarios because of its outstanding physical and chemical properties suitable for promising technological applications.6−11 α-MoO3 has a unique morphology12 which resembles a graphene-like layered structure.13 By virtue of its layered structure and high chemical stability, α-MoO3 could act as an excellent cathode material in high energy density solidstate microbatteries.14 These characteristics help to organize the MoO3 nanoscale building blocks into various assemblies and ultimately into useful nano- and microsystems, for example, gas sensors,15,16 recording materials,17 lubricants,18 and electroReceived: April 19, 2014 Revised: June 10, 2014 Published: June 10, 2014 15068

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

The Journal of Physical Chemistry C

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chromic19 and photochromic5 devices. Meanwhile, MoO3 is also a very promising material for catalysis,20,21 field emission,22 light emitting diodes,23 energy storage,1,24 etc. because of its interesting electrical and optical features. Thus, an extensive and systematic investigation of the structure and properties of MoO3 nano- and microsheets and ribbons and their Q1D and 2D derivatives is of high interest for the scientific community not only for basic research but also for their important potential technological impacts. Because of the versatile structures, the investigation is rather challenging, but at the same time it will be very interesting and rewarding if these structures can be synthesized by simple and cost-effective procedures. Growth of desired structural varieties and their appropriate characterizations are the most important requirements for exploring the practical applications of MoO3, especially for their use in nanostructures and their derivatives for gas sensors.13,16 There are reports on different techniques, including several approaches for thermal processing and oxidation of MoO3 nano- and microstructures for producing nanostructured materials with high purity, homogeneity, and good crystalline properties.25 Niederberger et al.26 synthesized molybdenum oxide 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 or catalyst. In the same context, Zhou et al.22 have grown MoOx nanowire arrays on silicon substrates by a simple process based on heating a Mo boat in vacuum at 1100 °C for 60 min under a constant flow of argon22 and subsequently at 400 °C in high purity oxygen for 30 min.22,28 Furthermore, Phuruangrat et al.21 communicated microwave-assisted hydrothermal reaction in 20 h to synthesize molybdenum oxide nanowires. However, there are only few reports on the direct growth of the α-MoO3 nanoand microstructures by oxidation technique in a single-step process.29 To develop a simple, cost-effective, and facile technique, which can be easily scaled up for industrial productions, it is very important to achieve low synthesis temperature and to speed up the production process. The requirement of complicated vacuum systems again put some constraints on the synthesis process; for large-scale production, the synthesis technique with least use of vacuum systems, or free from them, i.e., growth in normal air environment, would be very highly desirable. In this context, techniques like rapid thermal processing (RTP) and oxidation (RTO) are very simple and fast; to the best of our knowledge of the literature, they have not been used for the synthesis of α-MoO3. It is therefore a promising technique for growth of different αMoO3 nano- and microstructures. The rapid thermal processing and oxidation techniques at relatively low temperature allows well-controlled dimensional composition of the MoO3 belts and satisfies other requirements for nano- and microbuilding blocks for highly efficient and robust assemblies or novel useful systems. Such morphologies of metal oxide nano- and microstructures are quite attractive for nanotechnologies, especially in the bottom-up process for nanodevices assembly, because such a design requires building with precisely controlled nanomaterial parameters (including chemical composition and structure), which ultimately determine the performance of the device.30 Here, we demonstrate a method for synthesizing the highly crystalline α-MoO3 nano- and microbelts and ribbons directly from commercial bulk molybdenum in a single-step growth process without using any surfactant or template. The developed method allows rapid synthesis of large amounts of

nano- and microsheets, belts, and ribbons with different dimensions, and the growth process can be easily scaled up for mass-scale production; hence, the method represents a step toward successful advanced technological applications. We have systematically investigated the morphological, structural, chemical, and electronic properties of 2D MoO3 nano- and microsheets and Q1D nanoribbons (NR). An individual αMoO3 NR prepared by this RTP method has been integrated into the chip in the form of nanodevices, and their sensing responses with respect to different gases (H2, ethanol, and methanol) have been studied in detail.

2. EXPERIMENTAL SECTION The α-MoO3 nano- and microribbons were grown by a rapid thermal processing technique from a molybdenum metal plate at 670 and 800 °C in normal environment for 10−20 min. The morphology of samples was examined using a scanning electron microscopy (SEM) instrument (Carl Zeiss; 10 kV, 10 μA). The compositional analysis of the specimens was carried out by energy-dispersive x-ray microanalysis (EDX) in combination with SEM. Crystallographic information was derived from X-ray powder diffraction (XRD) data measured by using a Seifert 3000 TT unit operating at 40 kV and 40 mA with Cu Kα1 radiation (λ = 1.540598 Å). The specimens were characterized by X-ray photoelectron spectroscopy (XPS) (Omicron Nanotechnology GmbH) operating with Al anode at a power of 240 W to determine the chemical bonding state, composition of the surface, and electronic state of the elements that exist within αMoO3 specimens. Spectra were recorded with pass energies of 100 eV (survey) and 30 eV (high-resolution spectra). All XPS spectra were charge-referenced to the aliphatic carbon at 285.0 eV. Transmission electron microscopy (TEM) investigations were performed in detail by the Tecnai F30 STwin electron microscope (300 kV, field-emission gun, spherical aberration constant Cs= 1.2 mm) EDX spectroscopy (utilizing an EDAX system, Si/Li detector) analyzing the chemical compositions. The samples were first gently crushed and dispersed in nbutanol. A few droplets of this solution were placed on a copper grid covered with a lacy carbon film. Afterward, the sample grids 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 output power of 10 mW was used as the excitation source. The light was focused on the sample using an objective lens mounted on an optical microscope which is connected to a Raman spectrometer. A 1/4 m single monochromator fitted with an 1800 groove mm−1 grating has been used to disperse the scattered light, which is then focused onto a charge coupled device (CCD) detector (Wright Instruments, Ltd.). This detector consisted of a Peltier-cooled, slow-scan, 384 × 576 pixel CCD camera system, with the CCD chip maintained at −60 °C. Before actual measurements, the instrument was calibrated to the same accuracy using standard silicon substrate. Nanodevices and microsensors were fabricated by using a focused ion beam (FIB)-SEM instrument Dualbeam Helios Nanolab (FEI) (10 kV, 0.17 nA). Initially, MoO3 nanomicrobelts were sonicated for 30 min in 50 mL ethanol/water (1/1 ratio) solution. The resulting exfoliated MoO3 suspension was then transferred onto the chip and heated at 50 °C for 10 min. For nanodevice fabrication, we used silicon/silicon oxide and quartz-based template-chips as described in our recent work.2,3 Two rigid contacts were made with a single α-MO3 15069



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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 layered morphology. (b) Magnified area of a belt and individual microribbon synthesized at 670 °C. (c) Overall view of the materials synthesized at 800 °C in 10−20 min. (d) Magnified area of a belt and individual bended microribbon synthesized at 800 °C.

the synthesized MoO3 nano- and microbelts and ribbons at 670 °C are quite homogeneous in shape and size, with average belt length of up to several millimeters and a width in the range of 50 nm to 5 μm. The belt thickness proves to vary from 20 to 1000 nm (Figure 1 and Figure S1 of Supporting Information). The nano- and microribbons have a length up to several micrometers and a width in the range of 50−200 nm (Figure S1f). The micrographs in Figure 1b and Figure S1a,b show that some larger ribbons grown at 670 °C consist of several thin layers of MoO3 microribbons. The number of MoO3 layers assembled as in Figure S1a,b determines the thickness of the individual larger ribbon synthesized at 670 °C. According to our observations, the number of layers varies from belt to belt, and as a result their total thickness also varies. The anisotropic crystal structure of MoO3 manifests itself in the distinct habit of single crystals. The formation mechanism of α-MoO3 sheets of nanoscale thickness up to some extent has already been described by Kaner et al.6 In the present case, the formation mechanism of morphologies of the obtained MoO3 structures during RTP could be understood in a similar manner. The orthorhombic α-MoO3 with the space group Pbnm, as shown in Figure 2, possesses a layered structure which can be built up of chains of MoO4 tetrahedra connected by the sharing of two oxygen corners with two neighboring tetrahedra in the c-axis direction.21 Considering that the MoO3 structure is composed of layers connected by only weak van der Waals (vdW) bonds, crystals are micaceous (mica group) and platy and can be cleaved easily along the vdW gaps. Without breakage of the primary Mo−O bonds, it is highly possible that a (010) surface

nano- and microribbon on the sensor substrate template (Si/ SiO2 and quartz with Cr/Au contacts as contact electrodes, see Figure S6d of Supporting Information and Figure 6a inset) by using the FIB-SEM metal deposition function. Because the maskless deposition cannot be achieved by the conventional chemical vapor deposition (CVD) methods, we used this advantage of the FIB-SEM setup. In our work, a single MoO3 NR was exposed to FIB beam for less than 8 min. The test gas sensing characteristics were investigated using a sensor structure connected to an external electrode in the FIB-SEM system. The measuring apparatus consists of a closed quartz chamber connected to a gas flow system as reported earlier.3,30 Vapors of ethanol and methanol were used as target gases, as well as hydrogen, to investigate the sensing performances of single α-MoO3 ribbon-based nanosensor. The concentrations of target gases pulses were 100 ppm and 800−1000 ppm, calculated according to the chamber volume and the densities of the liquid reagents. The detection limits were investigated in the range of 5−1500 ppm. By monitoring the output voltage across the 2D ribbon-based sensor, the conductance was measured in air and in the test gas separately and compared afterward.

3. RESULTS AND DISCUSSION The SEM images of synthesized structures are shown in Figure 1 and Figures S1 and S2 of Supporting Information, which demonstrate that the MoO3 grown by RTP exhibits belt- and ribbonlike morphologies. The micrographs shown in Figure 1a,b and Figure S1a−f of Supporting Information illustrate that 15070



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transparency is still an open issue, and there are only very few reports about this. In this regard, Zhang et al. proposed that charges in the underlying structure have influence on the secondary electron (SE) distribution in the top layer.31 These charges eventually lower the electron density at the overlap, resulting in a lower SE yield, which leads to the observed contrast.31 Such observations can have a great impact on future applications 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] direction with the vdW interaction leads to the formation of a twodimensional structure (see Figure 2a,b). It allows guest atoms, ions (e.g., Li+, etc.), and molecules to be introduced into the vdW gaps between the layers through intercalation.12,32 Inside each 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 crystallographically independent oxygen sites, which occupy 4c Wyckoff positions according to the orthorhombic space group Pbnm,33 but only one type of [MoO6] octahedron exists. The [MoO6] octahedra form 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 can observe a corner-sharing net that is characteristic of the MoO3 structure (see Figure 2c). The α-MoO3 structure was first determined in 1931 by Bräkken34 and Wooster;35 and was refined by Kihlborg in 1963.12,36 According to Kihlborg’s results based on a more accurate X-ray analysis,36 MoO3 crystallizes in the space group Pbnm with four formula units in the unit cell of dimensions 396.28 pm (a-axis), 1385.5 pm (b-axis), and 369.64 pm (c-axis).37 The X-ray diffractograms measured at room temperature of the specimens (grown at 670 and 800 °C), shown in Figure 4a, exhibit excellent correspondence with the standard XRD data for orthorhombic α-MoO3 (JCPDS 00-0050506). The diffractogram 1 in Figure 4a (from sample 670 °C) shows the main α-MoO3 phase in addition to small quantities of (−203) Mo9O26, which is a crystallographic shear (CS) phase according to PDF 05-0441. CS is formed because of thermal reduction of the oxide, or it can be derived from the fully oxidized structures of MoO3 during the RTP process at lower temperatures.38 Such a reflection around 2θ = 20° does not appear for samples grown at higher temperatures (800 °C), so it 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 clearly indicates a two-dimensional growth of crystallites along the basal planes {010} of α-MoO3,39 as schematically shown by the extended unit cell.5,40 Also, it proves that nano- and microbelts are grown with a strongly preferred orientation. For the structures obtained at 800 °C, a similar X-ray diffractogram has been observed (Figure 4a, diffractogram 2). The relative predominance of the (0k0) reflections is well-pronounced, and the reflections hkl with h,k,l ≠ 0 have a reduced relative intensity. The presence of other phases of molybdenum oxide has not been observed, and samples grown at 800 °C are entirely orthorhombic α-MoO3. Both regimes result in growth of micro and nanoscale crystals exhibiting a high degree of crystallinity; the pronounced (0k0) reflection intensities confirm the layered structure packed in the direction of its crystallographic b-axis, parallel to the (100) plane as shown in the crystal structure (Figure 2).5,40 The degree of grain orientation and lamellar stacking based on XRD data have been

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

is created and that it typically dominates the surface area of single crystals. After the (010) surface, creation of a (100) surface requires breaking the second smallest number of bonds and thus is one of the weakest bonds in the structure. This is not too surprising because growth of such crystals is also commonly observed in needle-like morphologies with extended {010} and {100} facets. However, most nano- and microbelts look like crystalline films (see Figure S1c−e of Supporting Information), especially those grown at 800 °C (see Figure 1c,d and Figure S2 of Supporting Information). Thus, by increasing synthesis temperature, it is possible to easily break the vdW bonds and form single layer belts/ribbons, as observed in Figure 1d and Figure S2. Quasi-electron-transparency in MoO3 belt structures grown by the RTP technique has been discovered in these experiments and investigated in detail using SEM-ZEISS. The acceleration-voltage-dependent transparency is demonstrated in Figure 3a−f. At the value of 4 kV (Figure 3a), the belts appear to be almost nontransparent; however, with increasing voltage (more than 4 kV), the transparency sets in and a maximum contrast is observed at 10 kV (Figure 3d). The accelerationvoltage-induced e-beam transparency can be rationalized by assuming an increase of the electron interaction volume which generally 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 more transparent. The involved mechanism behind the observed 15071



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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.

examined using the Lotgering method41 and found to be 0.87 and 0.91 for the samples (670 °C) and (800 °C), respectively. The XRD results (Figure 4a) support very well the structural morphologies obtained in the SEM measurements (Figure 1 and Figures S1 and S2 of Supporting Information) for the structures synthesized by the RTP process. The α-MoO3 microand nanobelts were grown at a sintering temperature of 800 °C for 10−20 min. The experimental d-spacing (dhkl) values were calculated by Bragg equation,42 as shown in Tables S1 and S2 of Supporting Information, and the value of d020 is in agreement with those obtained by TEM investigations (Figures S4−S5 of Supporting Information and Figure 5b). Figure 3 clearly shows the typical rectangular morphology of the resulting nano- and microbelts and ribbons (Figures S1 and S2 show the surface of α-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 nm excitation, are demonstrated in Figure 4b in the range of 70− 1130 cm−1. Fourteen lines have been observed, and their frequencies, relative intensities, and symmetry assignments are marked within the graph; they are also summarized here.43 In the unit cell of molybdenum trioxides, there are 16 atoms, of which four are molybdenum and the others oxygen. This results in 48 eigenmodes at the center of the Brillouin zone (q = 0).43 Because the orthorhombic unit cell of MoO3 extends over 2 layers, the representations of the 48 vibrational modes for such material at q = 0 are44

Γcrystal = 8Ag + 8B1g + 4B2g + 4B3g + 4A u + 4B1u + 8B2u + 8B3u

(1)

where Ag, B1g, B2g, and B3g are Raman-active modes. Au is an inactive mode, and others are infrared-active modes. Several spectra were collected from different belt−ribbons and places; however, there was no discernible difference detected. Therefore, only a typical spectrum corresponding to one point on MoO3 nano- and microribbons is presented here. The Raman spectra of two different samples are shown in Figure 4b. Curves 1 and 2 in Figure 4b show spectra of MoO3 nano- and microribbons grown by RTP at 670 and 800 °C, respectively. The sharpness of the peaks indicates that the corresponding vibrational modes are mostly due to a highly ordered structure.11,45 The 154 cm−1 (Ag, B1g) band originates from the translation of the rigid chains; the 280 cm−1 (B2g, B3g) band is a doublet composed of wagging modes of the terminal oxygen atoms (MoO vibration); the 333 cm−1 (B1g, Ag) band is assigned to Mo3−O bending mode; and the 375 cm−1 (Ag) band is assigned to MoO bending mode.11,37,43,46 Raman spectrum of pure MoO3 exhibits bands characteristic of this compound 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), and 991 (Ag, B1g) cm−l. The assignment of the bands observed follows the single-crystal study and valence force field (VFF) calculations of Py et al.44,47 who used Kihlborg’s structural picture of MoO3 to obtain new insights into its vibrational 15072



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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 prepared at 800 °C and (d) Mo-3d core level XPS spectrum.

behavior. The 662 cm−1 band (B2g, B3g) is an asymmetric stretching mode of the triply connected bridge−oxygen Mo3− O bridge along the c-axis, which results from edge-shared oxygen (Mo3−O) in common with three adjacent octahedra.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 or the doubly connected bridge−oxygen Mo−O−Mo, which results from corner-shared oxygen in common with two octahedra.37,45,48,49 The 991 cm−1 (Ag, B1g) band is the asymmetric stretch of the terminal oxygen atoms (Mo6+O) mode along the a and b axes, which results from an unshared oxygen, and it is responsible for the layered structure of αMoO3.37,45,49 All Raman peaks are in agreement with the results obtained from pure α-MoO3.37,43,45,48,49 Raman peaks assigned to the three Mo−O stretching modes (singly, doubly, and triply coordinated oxygen) were observed and are also in agreement with the results obtained by Ajito et al.45 For chemical analyses, finding of empirical formula, chemical state, and electronic state of the elements that exist within a sample 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 is composed of Mo (22%) and O (67%) elements, as well as residual amounts of adventitious carbon (11%), hydrocarbons, and carbonyl compounds which were unavoidable because of their exposure to air prior to the XPS analysis.3,30 A relatively high amount of carbon detected by XPS is due to surface contamination and mounting the sample on carbon-taped support, which was confirmed by EDX measurements. The binding energy (BE) scale was calibrated using the adventitious aliphatic carbon at 285.0 eV as a reference. In our samples, the high-resolution XPS Mo 3d core-level spectrum of the MoO3 (Figure 4d) shows the presence of two well-resolved spectral lines located at 232.6 and 235.8 eV. These peaks are due to the spin−orbit Mo 3d levels corresponding to 3d5/2 and 3d3/2, in which the 3d doublet lines are separated by 3.2 eV. Such a spindoublet with binding energies at 232.6 ± 0.5 eV and 235.8 ± 0.5 eV corresponds to those of molybdenum in the formal valence +6.50,51 The peak positions and energy separations are in good agreement also with data reported for MoO3 nanobelts and thin films,51,52 where Mo is presented in the Mo6+ state. The XPS O 1s core-level binding energy (peak position) of the 15073



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the HRTEM micrograph indicates the crystal growth direction along the c-axis of α-MoO3. The composition and ratio of the elements, according to the EDX spectrum shown in Figure 5c, are representative as several investigations have been carried out at many other positions and the results are comparable to each other. The slight deviation from the ratio Mo:O = 1:3 might be due to the formation of oxygen vacancies which are produced by electron-beam-induced reduction.55,56 A more statistical evaluation of TEM data showed that for both of the synthesis temperatures, there are few occasional cases in which the α-MoO3 nanowires also grow along the aaxis (Figure S5 of Supporting Information). However, the growth direction along the c-axis is a more preferential one according to both our observations and the surface energy theory. The surface energy of the low index facet of the (001) plane is the lowest surface energy; therefore, the nanobelts preferred to stack along [001] instead of other directions, such as [100].57 This finding is in agreement with previous disclosures that the synthesized MoO3 nanoribbons grow preferentially along the (001) direction.58 TEM investigations confirmed that the α-MoO3 nano- and microstructures grown at 800 °C exhibit a high aspect ratio with a width of about 100 to 200 nm and lengths up to several micrometers. In Figure S5a of Supporting Information, a typical α-MoO3 nanobelt is shown at low magnification. The SAED pattern along zone axis [110] taken from the circled area shows strong dynamic excitation of kinematically forbidden reflections. Such observation is most probably due to enhanced thickness of the nanobelt and correspondingly enhanced multiple scattering effects. Together with the HRTEM micrograph and the FFT information in Figure S4a of Supporting Information, the preferential growth direction of the α-MoO3 crystal prepared at 800 °C is determined to be along [001], which is in agreement with the findings of other groups.50−52 Additionally, splitting of Bragg reflections perpendicular to the c* direction is observed in the SAED and FFT patterns. This phenomenon can be assigned to the formation of stacking faults,59 planar defects, and crystallographic shear planes which are created under these conditions. Both of the latter kinds of defects are known to be produced by electron-beam-induced phase transformation of α-MoO3.55 Next, the in situ nanodevice fabrication procedure is described based on our previous reports.3,60 The α-MoO3 nano- and microribbons and belts can be released from the initial substrate by sonication in ethanol and then transferred to a SiO2-coated Si substrate. We also used a direct contact technique to transfer nano- and microribbons from initial substrate to the clean Si sample by gently rubbing them a few times. These procedures allow us to obtain a low density and uniformly distributed α-MoO3 nanobelts and ribbons on the second substrate for nanodevice fabrication (Figure 6a−b). Further details about nanodevice fabrication and characterizations are given in Experimental Section, and a typical device structure is shown by SEM image in Figure 6a−b and Figure S6 of Supporting Information. A linear behavior of the current− voltage curves (Figure 6c) has been observed, which is very important for the sensing properties because the gas response of a nanosensor can be maximized when the metal− semiconductor junction has a negligible resistance.3 For investigation of sensing performances, a sensor was placed in a measuring apparatus.3 Figure 6d shows that at a 180 °C operating temperature, the single MoO3 nanobelt-based device shows good response to ethanol vapors of about 10%. It can be

Figure 5. TEM micrographs of an α-MoO3 nanoribbon synthesized at 670 °C: (a) bright-field image of the nanoribbon with the inset showing a SAED pattern and (b) HRTEM image recorded close to the same region in (a) and a corresponding FFT in the inset calculated from the part with a square indicating the growth direction. (c) A representative EDX spectrum showing the chemical composition and atomic ratio of another region α-MoO3.

molybdenum oxide micro- and nanobelts under consideration corresponds to 531.0 eV (Figure S3 of Supporting Information), which can be assigned to O2− in MoO3.50,53,54 From the relative intensities of the above XPS spectra, the compositional stoichiometry between “Mo” and “O” can be calculated. It was found that the Mo/O atomic ratio in the studied sample corresponds to 1/3, within the accuracy of the XPS measurements, which is in agreement with EDX measurements. Thus, this confirms the proposed chemical structure of the investigated nano- and microbelts and ribbons: MoO3. The TEM bright-field image (Figure 5a) inferred that the αMoO3 nano- and microstructures prepared at 670 °C exhibit a morphology similar to that synthesized at 800 °C (Figure S4a of Supporting Information). The inset depicts a selected area electron diffraction (SAED) pattern along the [100] zone axis of α-MoO3 (space group, Pbnm) recorded in the marked region. The corresponding high-resolution TEM (HRTEM) micrograph from the same region is depicted in Figure 5a. The fast Fourier transform (FFT) from the square marked region of 15074



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

This could be the most probable reason for higher sensitivity of MoO3 belts to ethanol compared to hydrogen at this operating temperature.16 It can be suggested that ethanol induces a reduction by the catalytic oxidation and results in electron transfer to the metallic site. The details of the proposed mechanism of sensing ethanol with pure MoO3 nanostructures can be found in previous work by Illyaskutty et al.61 Figure 6e shows experimental results on gas response to hydrogen gas and ethanol and methanol vapors. It can be clearly seen that different nano- and microdevices respond in different ways to the ambient in dependence of MoO3 ribbon widths/ thicknesses. It can be observed that sensors made from MoO3 belts grown at 670 °C show an improved response to ethanol compared to samples synthesized at 800 °C. Thinner samples of 150 nm demonstrated response of 11.5% compared to 9.9% measured from 200 nm sample. It was found that our nano- and microsensors possess detection limits in the range of 5−1500 ppm of ethanol. At concentrations higher than 1000 ppm, the gas response shows an almost saturated value and slow recovery time (Figure S6 of Supporting Information). The response time was about 18 s, and recovery time constants of 15 and 280 s at the operating temperature of 180 °C (summarized in Table S3 of Supporting Information in comparison to previously reported values). However, the response time is reduced to 7 and 22 s at operating temperatures around 300 °C (Table S3 and Figure S6 of Supporting Information). When 150 and 200 nm samples are compared, hydrogen gas response was increased from 7.6% to 8.4%. Samples grown at 800 °C exhibit a higher response to

concluded that it is quite stable and moderately reversible at the increasing and decreasing ethanol vapor concentration. Although it shows an initial quick response and gets rather equilibrated over time in the first 200 s, it is not rapidly 100% reversible at this operating temperature. For the interpretation of the sensor response behavior to ethanol vapor, probably surface morphology of the MoO3 nanobelt layer and competitive reactions of analyte species with MoO3 surface layer have to be considered.16,61 The following model was proposed to interpret the conductance change of the MoO3 sensing nanobelt to the exposure of ethanol vapor. The ethanol vapor molecules are first chemisorbed on the molybdenum oxide sensing surface with a donor effect and inject electrons into the nanobelt according to16,61 C2H5OH → C2H5OH(ads) → C2H5OH+(ads) + e− (2)

The sensing may further progress through reactions of adsorbed ethanol on the sensing surface, and the O−H bond of ethanol dissociates heterolytically to yield ethoxide groups and hydrogen, as reported by Illyaskutty and co-workers.16 The overall reaction describing the ethanol response can be described as follows:16,61 C2H5OH + Oo(s) → CH3CHO + H 2O + Vo2 + + 2e− (3)

All processes described above are assumed to take place simultaneously on the nanobelt surface upon the introduction of ethanol vapors and contribute to gas response significantly. 15075



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nanomicrobelts prepared at 800 °C; TEM micrographs of an α-MoO3 nanorod prepared at 800 °C; SEM image showing the fabricated nanodevice from single MoO3 NR in the FIB-SEM system; sensor response of single MoO3 nanoribbon toward ethanol at 180 °C operating temperature; sensor response of single MoO3 nanoribbon toward different gases and volatile organic compounds at various operating temperatures indicating on its possible selectivity. This material is available free of charge via the Internet at http://pubs.acs.org.

methanol (about 6%) and lower to hydrogen gas (about 5.3%) at an operating temperature of about 180 °C. At higher operating temperature of about 350 °C, an increased response to H2 gas (of about 38%) was obtained with relatively fast response and recovery times (Table S3 and Figure S6 of Supporting Information). In such a way the selectivity of MoO3-based sensors can be controlled by operating temperature, and by combining all three types of nanosensors on the same chip (as shown in the inset in Figure 6a and Figure S6 of Supporting Information), one can develop a nanoscale electronic nose on the same platform. It is clearly demonstrated that the developed technique, the synthesis of freestanding αMoO3 crystals, is of great importance for further investigations and practical applications.

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*O.L.: tel, (49)431 880 61 16; e-mail, [email protected], [email protected]. *R.A.: tel, (49)431 880 61 16; e-mail, [email protected].

4. CONCLUSIONS We have demonstrated a new technique which requires a relatively low temperature of 670−800 °C and 10−20 min to synthesize high crystalline quality α-MoO 3 nano- and microbelts and ribbons. The developed technological process allows rapid synthesis of large amounts of MoO3 nano- and microsheets, belts, and ribbons, and it can be easily scaled up for various applications. SEM studies revealed that the MoO3 nano- and microbelts and ribbons are synthesized uniformly, having a length of belts up to several millimeters and a width in the range of 50−5000 nm. The belt thickness is observed to vary from 20 to 1000 nm. Also, in SEM investigations, nanoribbons were observed which have a length up to several micrometers and a width in the range 50−200 nm. This technology allows one to synthesize various sizes of nano- and microstructures in one process, which is time-saving as well as cost-effective. Quasi-electron-transparency of MoO3 at the overlap of two belts has been evidenced. The structural studies proved an excellent agreement with the standard XRD data for orthorhombic α-MoO3 crystals. The sharpness of the microRaman peaks confirmed that the observed vibrational modes are due to a highly ordered structure. Also, such nano- and microstructures can be transferred to different substrates and dispersed individually for further processing in device applications. The MoO3-based 2D nanosensors and microsensors have been realized by using FIB-SEM, integrated on a chip, and investigated in detail. It can be concluded that the nanosensors are quite stable and moderately reversible at increasing and decreasing ethanol vapor concentrations. It was found that MoO3 nano- and microribbons of various sizes show different sensitivity and selectivity to ethanol, methanol, and hydrogen gas, and this can be controlled by operating temperature. At an operating temperature of about 180−200 °C, the higher response was to ethanol. However, at higher operating temperature of about 350 °C, an increased response to H2 gas was obtained with relatively fast response and recovery times. The developed technique presents great promise for further studies of different metal oxides and especially for industrial applications.

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AUTHOR INFORMATION

Corresponding Authors

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the STCU and ASM through Grant 09 STCU.A/5833. This research was partly sponsored by German Research Foundation (DFG) under the schemes SFB 855-A5, Z-1. O.L. and Y.K.M. acknowledge the Alexander von Humboldt Foundation for research fellowships at the Institute for Materials Science, University of Kiel, Germany. This research was partly supported from the EU Graphene Flagship project.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of α-MoO3 material grown by RTP at 670 and 800 °C from different regions and magnifications, as well as individual nanoribbons dispersed on substrate surface before nanodevice fabrication; discussions on micro-Raman spectra intensity changes observed as compared to the single-crystal spectrum; XPS O-1s core level spectrum of an α-MoO3 15076



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