View Article Online View Journal

Nanoscale Accepted Manuscript

This article can be cited before page numbers have been issued, to do this please use: K. K. Dey, D. Bhatnagar, A. K. Srivastava, M. Wan, S. Singh, R. R. Yadav, B. C. Yadav and M. Deepa, Nanoscale, 2015,

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

www.rsc.org/nanoscale

Page 1 of 12

Nanoscale

►

View Article Online

Nanoscale

DOI: 10.1039/C4NR06032F

FULL PAPER Kajal Kumar Dey,a,b Divyanshu Bhatnagar,a Avanish Kumar Srivastava,*a Meher Wan,b Satyendra Singh,b Raja Ram Yadav,b Bal Chandra Yadavc and Melepurath Deepad 5

10

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x VO2 (B) nanorods with average width ranging between 50-100 nm are synthesized via hydrothermal method and the post hydrothermal treatment drying temperature is found to be influential in the overall phase and growth morphology evolution. The nanorods with unusually high optical bandgap for a VO2 material are effective in enhancing the thermal performance of ethylene glycol nanofluids over a wide temperature range as is indicated by the temperature dependent thermal conductivity measurements. Humidity and LPG sensors fabricated using the VO2 (B) nanorods bear testament to their efficient sensing performance which can be partially attributed to the mesoporous nature of the nanorods.

Introduction 15

20

25

Vanadium oxides are amongst the most widely studied oxide materials in recent years as the large variety of vanadium-oxygen system encompassing the entire array of multiple vanadium valencies provide intriguing study within both theoretical structural conceptualization and exciting structure-property correlation.1-6 Besides V2O5 and V2O3, binary vanadium dioxides (VO2) have carved special interest among researchers with its various polymorphic configurations. So far, as many as nine polymorphs for VO2, not including the hydrated phases, have been reported, both in the stable and unstable (metastable) form – rutile VO2 (R),7 monoclinic VO2 (M1),8 triclinic VO2 (T),9 tetragonal VO2 (A),10 monoclinic VO2 (B),11 tetragonal VO2 (C),12 monoclinic VO2 (D),13 paramontroseite VO2 14 and VO2 with a BCC structure.15 Although M, R, B and A phases are all based on oxygen BCC lattice having the vanadium ions in the

50

55

60

65 30

a

35

40

45

National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi110012, India b Department of Physics,University of Allahabad, Allahabad, Uttar Pradesh-211002, India c Department of Applied Physics, School for Physical Sciences, Babasaheb Bhimrao Ambedkar University, Lucknow 226025, U.P., India d Department of Chemistry, Indian institute of Technology Hyderabad, Yeddumailaram - 502205, Andhra Pradesh, India Address, Town, *Corresponding author. Tel.: 91-11-45609308; fax: 91-11-45609310. E-mail address: [email protected]. † Electronic Supplementary Information (ESI) available: Plots representing the actual ratio Knf/KEG (Knf is the thermal conductivity of the nanofluid and KEG being thermal conductivity of the base fluid) across the entire experimental temperature range of 20 to 80 °C, Table

representing comparison of VO2 sensor performance towards different gases. See DOI: 10.1039/b000000x/ This journal is © The Royal Society of Chemistry [year]

70

75

80

octahedral sites, M, R phases are different than the A, B phases in terms of the mutual orientation of the fourfold axis of the oxygen octahedral. Furthermore M, R are the stable phases whereas A, B are metastable phases that converts into the rutile phase at a higher temperature.16 The M, R phases themselves undergo a mott metal/insulator phase transition at 68 °C, the closest to room temperature for any material reported thus far 8 and have been heavily investigated as a candidate for electronic applications such as ultrafast switches, field effect transistors, memresistors, solid state memory and also as a thermochromic material.17-19 The metastable VO2 (B) phase, because of its layerd structure has been known for impressive electrochemical properties and previous reports have suggested its potential as cathode material in Li ion batteries 20,21 with Baudrin et. al reporting specific capacities as high as 500 mAhg-1 20 for nanocrystalline VO2 (B) filaments. Recent studies have also indicated its field emission and bio-sensing potential.22,23 VO2 (B) has the advantage over its thermodynamically more stable polymorphic counterparts (VO2 (R)) in a sense that it can be easily obtained in one dimensional (1-D) morphology through low temperature solution processing e.g. hydrothermal method. 24-26 Morphological attributes and their influence or suitability - to be more precise, on specific device fabrication, various technological functionalities have gained momentum since the discovery of carbon nanotubes27 and subsequently other one dimensional nanostructured materials such as rods, wires, belts, tubes have had generous attention as their often dramatically different electronic, chemical and overall physicochemical characteristics were found to be advatageous when put in comparison to their counterparts in either bulk size range or the ones in nanoparticulated forms.4,24,25,28-30 Practical limitations of carbon nanotubes in specific circumstances such as, pristine CNTs being metallic in nature, are insoluble and forms bundles or ropes in many solvents including water and CNTs are never obtained in uniform sizes (both with respect to length and diameters),31 has also contributed to scientists exploring different [journal], [year], [vol], 00–00 | 1

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

VO2 nanorods for efficient performance in thermal fluids and sensors

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

5

10

15

20

25

30

35

40

45

50

55

materials in 1-D nano-avatars designed for specific applications. Simple hydrothermal reduction of vanadium precursor (V2O5, metavanadate etc.) into 1-D V4+ oxide can be expertly controlled in terms of length and diameter by varying the concentration or the nature of the reductants used.24,25 In the present work we have synthesized VO2 (B) nanorods via hydrothermal methods reducing V2O5 with oxalic acid. Metal oxide nanostructures are regarded amongst the most promising gas sensing material, especially in a one dimensional or hierarchical disposition due to their large surface area and porous structures coupled with a less agglomerated configuaration.32,33 The corresponding adsorption/desorption processes mainly take place on the surface of the sensing layer therefore nanostructures with a higher surface to volume ratio is prone to provide more surface activity resulting in better sensing performance. Within the V-O systems, most of the sensing studies reported in the literature focus on V 2O5, whereas, reports concerning the sensing properties of VO2 are rare, even though recent studies emphasized on the catalytic activity of VO2 (B) 34, 35 establishing excellent surface activity for this metastable phase of VO2. Among the handful of literature reports citing its sensing property, Micocci et al. 36 investigated vanadium oxide thin films for ethanol sensor. He observed that the maximum percentage response of the sensor was 8% for VO2 at operating temperature of 300 °C. A. Simo et al. 37 fabricated VO2 nanostructures based chemiresistors for low power energy consumption hydrogen sensing. He reported that mott-type VO2 nanobelts are effective hydrogen gas sensors at room temperature. J.W. Byon and J.M. Baik 38, 39 also investigated the hydrogen gas sensing of vanadium di-oxide films. Thus, the existing literature reports concentrates on their hydrogen and ethanol sensing at elevated temperatures, but here, we have investigated VO2 nanorods for liquefied petroleum gas (LPG) sensing at room temperature as room temperature operations, being much more energy savvy and cost efficient are most effective for the commercialization of the sensing devices. The nanorods were also subjected to determine their humidity sensing potential, which is crucial to various industrial and agricultural process control, meteorology, physicochemical processes in pharmaceuticals and food production. The interaction mechanisms of LPG and humidity with the sensing surface are provided, respectively in light of surface morphological and structural characteristics. Another aspect of the VO2 (B) nanorods that we have studied is their potential applications in high performance microscale liquid flow devices as nanofluid component. Nanofluids in recent years have been viewed as promising alternatives for the conventional heat transfer fluids (such as water, ethylene glycol, mineral oil etc.) which are generally limited by their low thermal conductivity resulting in less than desired cooling performances. Solids bolstered by lattice vibration or phonon modes normally possess thermal conductivity about an order higher in magnitude than the liquids and thus from a relatively uncomplicated perspective, is expected to increase the thermal conductivity of liquids when dispersed within the later. In fact, the phenomenon of homogeneously dispersed nanosized particles enhancing thermal conductivity of these fluids have seen researchers employing various materials starting from carbon based materials such as diamonds and carbon nanotubes to ceramics, metals and

60

65

70

75

View Article Online

Page 2 of 12

DOI: 10.1039/C4NR06032F

even composites, all in their nanosized avatars, aiming higher thermal performance,40-49 although owing to various experimental inconsistencies, absence of a widely accepted theoretical model determining the nanofluids composition and structural disposition, the exact thermal conductivity values of various nanofluids has been somewhat controversial.44 Furthermore, the experimentally obtained thermal conductivity enhancements for nanofluids so far have been less than satisfactory most of the times and so new nanomaterials are being constantly studied for this purpose. In the past layered materials such as boron nitride and NiO have been found to be impressive as nanofluid component 41, 49 which has acted as an added motivation for our study on VO2 (B), a layered material itself. In this article, we are going to report the utilization of VO2 (B)–Ethylene Glycol (EG) nanofluid and show that incorporation of VO2 (B) nanostructured materials can have dramatic effect on the thermal conductivity of EG.

Experimental Section Chemicals used

80

Vanadium Pentoxide (V2O5; GR, min 99%, Loba Chemie laboratory reagents & fine chemicals) and Oxalic acid di-hydrate ((COOH)2.2H2O; AR, min 99.5%, Sisco research laboratories Pvt. Ltd.) were used as received without any further purification. Synthesis of nanostructured VO2 rods

85

90

95

Reduction of precursor orange yellow V2O5 to dark blue VO2 (B) was carried out through a simple hydrothermal reaction procedure. Here, 0.72 g (4 mmol) of V2O5 was dispersed in 80 ml of de-ionized water. The resultant orange colour dispersion was made homogeneous through vigorous stirring. 0.76 g (6 mmol) Oxalic acid di-hydrate was added to this dispersion at once and the mixture was kept on stirring continuously for about half an hour. This mixture was transferred to a stainless steel covered Teflon lined autoclave and the autoclave was placed at 225 °C inside an electrical oven, where it was kept for 24 hours. Following the hydrothermal treatment, the evolved dark blue precipitate was filtered out, washed with de-ionized water and ethanol repeatedly and then dried at 75 °C for 10 hours. The final material was utilized for various characterizations. Characterization methods and instruments

100

105

110

The crystallographic characteristics of the samples were identified through X-ray diffraction technique via a Rigaku benchtop X-ray diffractometer using monochromatized Cu-Kα radiation (λ=1.54059 Å). Surface morphological investigations were carried out through scanning electron microscopy (SEM) images recorded on a Zeiss EVO MA-10 (operating at 10.0 KV) scanning electron microscope. The high-resolution transmission electron microscopy analysis and selected-area electron diffraction (SAED) patterns were recorded on HRTEM (FEI Tecnai G2 F30 STWIN at 300 KV). The FT-IR spectra were recorded with a single beam Perkin Elmer (Spectrum BX-500) spectrophotometer. UV-Vis spectra of the samples were recorded by a JASCO UV-VIS/NIR Spectrophotometer (model V-670). The room temperature photoluminescence (PL) investigations were performed using a Perkin Elmer LS-55 fluorescence spectrophotometer having a standard xenon source. The specific

Nanoscale Accepted Manuscript

Nanoscale

surface area and the pore characteristics (diameter etc.) were determined by nitrogen physisorption isotherms obtained at 77.3 K via Quantachrome NOVA instruments (version 10.01).

55

Thermal conductivity measurement of the nanofluid

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

5

10

VO2 (B) nanofluids were prepared by dispersing the obtained nanoparticles in ethylene glycol (EG) at three volume percentages of 0.2, 0.4 and 0.6 %. The thermal conductivity (TC) of the corresponding nanofluids was measured using a hot disc thermal constant analyzer (model TPS-500) at temperatures ranging from 20 to 100 °C. Corresponding experimental error associated with the measurements was ± 2%.

60

65

20

25

An ethanolic dispersion of the as synthesized dark blue powder Vanadium dioxide was deposited on alumina substrate via spin coating with a photo resist spinner (Metrex, India) at 3000 rpm for 30 s. The film was dried at 75 °C for 2 hours and then exposed to moisture in a humidity sensing chamber and corresponding variations in resistance with relative humidity (% RH) were observed. A saturated solution of potassium sulphate was kept in the chamber to increase the humidity from 10 to 90 % whereas saturated solution of potassium hydroxide was used as a dehumidifier i.e. to decrease the humidity from 90 to 10%. Variations in the electrical resistance of sensing film were measured using Keithley Electrometer (model: 6514A). Details of the humidity sensing set-up were mentioned in a previous publication.50 Sensitivity of a humidity sensor (S) can be defined as the change in resistance (ΔR) of sensing film per unit change in relative humidity (% RH), i.e.50

70

75

35

40

45

80 % VO2 powder, 15 % acetylene black and 5 % of poly(vinylidinedifluoride) (PVdF), by weight, were transferred into a mortar, and made into a paste by grinding for a few minutes with some drops of N-methyl pyrrolidinone (NMP). The paste was applied onto FTO coated glass plates (1 cm  1 cm area) and dried in air and used as working electrode. Graphite was electrophoretically deposited on FTO glass plates from an aqueous suspension of graphite powder, at a dc potential of x V and employed as counter electrode. Charge-discharge measurements and cyclic voltammetric studies were performed in a 0.1 N KOH aqueous electrolyte solution, with Ag/AgCl/KCl as the reference electrode. VO2 films on FTO/Glass were used as working electrode and graphite plates were employed as counter electrodes. Galvanostatic chargedischarge profiles were recorded at different current densities of 0.3 and 2 mA/g and in the 0 to 5 V potential window, on an Autolab PGSTAT 302N Potentiostat/Galvanostat coupled with a NOVA 1.7 software.

Phase identification, growth mechanism 80

Average sensitivity is calculated by taking the average of all measured sensitivities ranging from 10 to 90 % RH. Gas sensing properties were investigated by monitoring the variations in electrical resistance when exposed to LPG. For the sensing measurements, a special gas chamber was designed which consists of an inlet for inserting LPG into the chamber and an outlet knob for removal of LPG. The detailed description of the gas chamber was provided in a previous paper.51 VO2 film with silver contacts was exposed to LPG and the variations in electrical resistance of the gas sensing material with time for different concentrations of LPG were recorded using a Keithley electrometer. For optimizing and standardizing the performance of a sensing device, it is necessary to define a set of operating parameters. The basic operating parameters of a sensing device are sensitivity, sensor response, response and recovery times, reproducibility and long term stability. The sensitivity and percentage sensor response of the sensing material are to be defined as given in equations (2) and (3) respectively.52,53

reach 90 % of the final response value while the recovery time is defined as the time taken by the sensor to come to 90 % of the original baseline. Sensor stability refers to the long term operation of a sensor without any change in the above operating parameters.

Results and discussions

(1) 30

DOI: 10.1039/C4NR06032F

Electrochemical property measurement

Humidity and LPG sensing measurements

15

View Article Online

Nanoscale

morphological

characteristics

and

The hydrothermally obtained dark black-blue material was subjected to X-ray diffraction analysis and it was confirmed to be metastable VO2 (B). The diffraction data was refined by FullProof program employing pseudo-Voight axial divergence asymmetry profile function for profile fitting (Fig. 1).

85

(2) Fig. 1 X-ray diffraction pattern of the synthesized monoclinic VO2 (B). The observed, calculated and the difference pattern as determined via Reitveld analysis is provided.

(3) 50

Where, Ra is the stabilized sensor resistance in air, and Rg the sensor resistance after LPG injection. Response time is defined as the time taken by the sensor to

90

All the diffraction peaks were in accordance with the monoclinic VO2 (B) phase having space group C2/m and lattice parameters a = 12.06 Å, b = 3.69 Å, c = 6.42 Å, β = 106.98°, corresponding

Nanoscale Accepted Manuscript

Page 3 of 12

Nanoscale

10

well with the JCPDS data card 81-2392. The basic findings of the Reitveld refinement are provided in Table 1. The average crystallite size corresponding to the (110) reflections calculated using the Debye-Scherrer equation was found to be ca. 40 nm. The strong and sharp peaks indicate the well formed crystalline nature of the sample. The transformation beginning from vanadium pentoxide to di-oxide has been depicted from a crystallographic perspective in Fig. 2. Table 1 Atomic positions for VO2 (B) as obtained through Reitveld refinement

atomic label O1 O2 O3 O4 V1 V2 Reliability Factors (R-factors)

15

20

25

30

35

40

45

Page 4 of 12

Wyckoff positions 4i 4i 4i 4i 4i 4i Rp, 1.1%

atomic Positions x y 0.36630 0.23316 0.44756 0.12628 0.30376 0.39967 Rwp, 4.4%

z

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.99507 0.33645 0.64448 0.69862 0.72508 0.30998

Rexp,7.40

χ , 3.80

50

55

2

The rod shaped nanostructural morphology of the material was evident from Fig. 3 orchestrated by the scanning electron microscopy (a) and transmission electron microscopy (b, c, d) micrographs of the sample. The high uniformity in the obtained yields of the two dimensional nanorods could be inferred from Fig. 3a, displaying a large amount of VO2 (B) nanorods, devoid of any observable discrepancy among the morphological specimens. Unlike the typical nanobelt morphology reported previously for hydrothermal synthesis of VO2 (B), 24, 25 the surface morphology of the nanostructures reported in this article resembles raw jute-sticks as was clearly visible from the same pictograph. The lengths of these nanorods were apparently in the range of several micrometers. Although from the SEM micrographs the overall morphological characteristics appear to be highly uniform saving the variations in dimensions, the TEM micrographs (Fig. 3 (b, c, d)) were able to identify the slight divergences appearing among the nanorods. These variations were observed based on the way of termination of the particles around the edges as the three TEM images provided three minor variants of VO2 (B) nanorod morphology. The rod in Fig. 3b depicts a sharper, slightly deformed terminus (similar to swords) whereas 3c reveals a semi-circular terminus. There are also nanorods with planar terminus as can be seen from Fig. 3d. The rods with semi-circular terminus were comparatively thinner (~ 50 nm on an average) than the other two (~ 80-100 nm on average). Another, interesting observation to pick up from the TEM micrographs is the visible signs of strain along the length of the nanorods as shown in each of the images by yellow borders. The reason behind these may be local fluctuations during growth of the nanorods leading to perturbation in lattice scale which may result in the visible contrast in the microstructures. The slight variations in the morphological appearances of the VO2 (B) nanorods is intriguing and difficult to explain for us at this moment, although, preferences for one (hkl) plane over another during the stacking of these planes in the growth direction is worth to speculate as having some influence. The high resolution

60

65

70

75

80

85

Fig. 2 Polyhedral structures of orthorhombic V2O5 and monoclinic VO2 (B), depicting the overall crystallographic transmutation occurred during the synthesis of the later. The Blue balls represent V atoms and the green balls represent the O atoms.

lattice scale (HRTEM) images of the nanorods provide some credence to this theory as Fig. 3e and Fig. 3f corresponding to portions of two distinct nanorods (Fig. 3d and 3b respectively as indicated by arrows) show dominant presence of lattice planes (001) and (002). The inset within Fig. 3b provides the FFT of the lattice fringe pattern demonstrated in Fig. 3f. The exact sequence of mechanisms leading to the formation of the VO2 (B) nanorods starting from the V2O5 is yet to be fully comprehended. Existing reports indicate formation of an intermediate bariandite type mineral V10O24, nH2O with exfoliated or sheet like morphology which later transforms to the low temperature metastable VO2 (B) through a reductiondehydration mechanism.24, 54 Initially, in the aqueous dispersion V2O5 with its layered structure incorporates water molecules and form a hydrated species which subsequently undergoes partial reduction to give rise to V10O24, nH2O with mixed V5+/V4+ ions. This is a layered material with metastable nature that can easily be reduced to V4+ oxide nanocrystallites. The overall chemical reaction can be presented as the following –

Conversion from the layered structure of orthorhombic V 2O5 to the layered monoclinic bariandite mineral is kinetically favored over a direct conversion to monoclinic VO2. Subsequent conversion between the two monoclinic compounds via reduction dehydration is also kinetically favored. When the hydrothermal temperature is decisively higher (> 350 °C), the thermodynamically stable VO2 (R) phase is formed instead of metastable VO2 (B).55 Oxalic acid which acts as the sole reducing agent in the system also maintains the pH of the system in the acidic region which is critical for the reduction reactions to take place. However, the concentration of oxalic acid needs to be monitored as an extremely low pH result in the formation of other species such as H2V3O8.56 Several reports exist citing how the nature and concentration of reducing agents, and reaction temperature alter the nature of the products significantly, both compositionally and morphologically.24, 54 To obtain further insight into the formation of the VO2 (B) nanorods we studied the effect of the drying

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

5

View Article Online

DOI: 10.1039/C4NR06032F

5

10

15

20

25

Nanoscale

View Article Online

DOI: 10.1039/C4NR06032F

Fig. 3 Electron Microscopy images of the synthesized nanoparticles revealing their rod-like morphology. 3a provides the SEM image portraying the jutestick like morphology of the VO2 (B) nanoparticles. (b), (c) and (d) are the TEM images depicting the three slight variations within the nanorods. The area bordered by the yellow lines reveals the visible strain within the samples. (e) and (f) provide the lattice scale HRTEM image of the nanorods corresponding to 3d and 3b respectively. Inset within 3b represents the FFT of the lattice pattern presented in 3f.

on the morphology and overall phase formation. We used several drying temperatures below 75 °C, which was the temperature used in our synthesis to VO2 (B) nanorods post the hydrothermal treatment, and interesting morphological transformation coupled with phase evolution could be observed as a result of varying temperatures. To illuminate on this matter we focus on the data obtained corresponding to the drying temperature of 35 °C as provided in Fig. 4. The XRD pattern for the same (Fig. 4a) indicates that alongside the dominant VO2 (B), reflections corresponding to both V10O24, nH2O (peaks indicated by green circles, JCPDF #25-1006) and hydrated VO2, xH2O (JCPDF #181445) were also obtained establishing the mixed nature of the material when dried at 35 °C. The morphological characteristics as asserted through the TEM micrographs in Fig. 4 b, c and d are quite similar to those depicted in Fig. 3 d, c and b respectively saving the sheet or foil like structures that seem to be wrapping up the nanorods. It can be inferred based on these observations that although the growth to the one dimensional nanostructures were already completed during the hydrothermal treatment itself, complete dehydration from VO2, xH2O leading to VO2 (B) was still an ongoing process and the same was reflected in the TEM images where the presence of the foil like structures can be

30

35

40

45

attributed to existing hydrated species. The lattice scale image (Fig. 4e) reveals (003) as the preferred plane packed along the growth of the nanorods similar to the (001) and (002) planes in the VO2 (B) nanorods shown in Fig. 3, indicating that the growth direction remains focused during the dehydration process validating our previous argument that the growth of the nanorods were already completed in the autoclave during the hydrothermal treatment. Corresponding FFT pattern (Fig. 4f) reveals diffraction spots pertaining to the VO2 (B) phase only whereas diffraction spots corresponding to the hydrated species as detected through the XRD pattern were absent, probably due to the localized nature of TEM analysis. FT-IR spectroscopy was performed to further investigate the effect of drying temperature through studying the chemical bonding environment as we changed the temperature from 35 °C to the optimized 75 °C. Fig. 4g presents the FT-IR spectrum for the two samples. The band ca. 550 cm-1 can be attributed to V-OV bending mode. The band at 924 cm-1 corresponds to a coupled vibration of V=O and V-O-V bonds while the band at 988 cm-1 which features in the vibrational spectra of many vanadium oxide compounds with vanadium oxidation states in between 4+ and 5+, can be attributed to the stretching motion of the short V=O

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

Page 5 of 12

View Article Online

Page 6 of 12

DOI: 10.1039/C4NR06032F

Fig. 4 (a) represents the XRD pattern of the vanadium oxide sample dried at 35 °C post its hydrothermal treatment. (b), (c) and (d) represent TEM micrographs of the sample. (e) and (f) provide the lattice fringe image and the corresponding FFT pattern respectively. (g) represents the FT-IR spectra of VO2 (B) (≡ 75 °C) and mixed hydrated V-O species (≡ 35 °C). The area enclosed by dotted lines indicates the –OH functional group region. 5

10

15

20

25

30

bonds. All these bands agree well with the previously reported data on VO2 (B). 57 Interestingly, the bands corresponding to the V-O-V bending mode has a slight shift in position in the two samples (543 cm-1 and 568 cm-1) which could be due to the change of orientation of the individual VO2 fragments within the unit cell as a result of dehydration process. Another interesting observation is the presence of O-H species in the sample dried at 35 °C, detected via presence of vibration bands above 3000 cm-1. Absence of these bands in the other sample corroborates our previous inference about drying at a lower temperature (35 °C) leading to hydrated VO2, xH2O molecules and higher temperature drying (75 °C) resulting in the pure VO2 (B) phase. The overall formation mechanism leading to variations in VO2 nanorods as a result of both low and high temperature drying, has been depicted through the schematic diagram as presented in Fig. 5a. The initial vanadium oxide nanocrystals appearing in the solution via the hydrothermal reaction, depending on the subsequent drying temperature, may opt for either a regular arrangement resulting in smooth nanorod formation (75 °C) or an ungainly approach towards growth leading up to the distorted morphology of nanorods (35 °C). Such a growth pattern in the low temperature process should result in inherent structural defects such as random grain boundary and inconsistent accumulation of nanocrystals. The high resolution lattice scale image for this material as provided in Fig. 5b delineates the presence of multiple grain boundaries and areas of apparent thickness contrast distinguished by the yellow circles. Nanocrystals with uneven accumulation would lead to overlapping areas manifesting itself with differential thickness profile. The characteristic lattice fringe pattern (Fig. 5b) thus

35

provides credence to our argument for erratic growth pattern during the low temperature drying. Thermal conductivity evaluation of ethylene glycol nanofluid

40

45

50

55

60

Fig. 6a shows thermal conductivity (K) data for VO2 (B) nanorods in ethylyne glycol (EG) suspension as a function of temperature for different volume fraction of the nanorods. Since working temperature of the heat source may be sufficiently high, K of the as-synthesized samples were measured spanning a wide temperature range of 20–100 °C for the evaluation of efficient performance in a wide working range of temperature. Using different volume fractions gives us an idea about the effect of nanorod concentration on the K of the base fluid. As is clearly observed, dispersion of a very small amount of nanorods produces a dramatic increment in the thermal conductivity of EG, with the increment reaching as high as 45% at around 40 °C and consistently sustaining such an increment afterwards. Although, we do not have any previous report on the thermal conductivity of nanofluids based on any of the vanadium oxides to enable us to compare this result, we found the overall performance of VO2 (B) nanorods to be impressive when taken into context the performance of other metal oxides that has been investigated by various research groups around the world. To put it into perspective, the materials to record better thermal performance for EG nanofluids such as CNTs,42 metallic iron48 or our previously reported NiO,41 all have more inherent thermal conductivity, but VO2 (B) has its own advantages as apart from being environmentally benign the nanorods are easy and cost effective to produce through hydrothermal methods where the options of monitoring the nanorod dimensions are manifold.

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

Nanoscale

Page 7 of 12

Nanoscale

35

40

View Article Online

DOI: 10.1039/C4NR06032F

detailed a mathematical treatment for the same.47 But, most of the existing theoretical models assume the nanoparticles to be of spherical (3D)/circular (2D) nature and is not inclusive of other particle morphologies. To account for the nanorod shape of our VO2 (B) particles we untilized the Lewis and Nielsen’s theoretical model obtained by modifying Halpin-Tsai equations, to incorporate the effect of various particle morphologies.58 According to their model;

Where, A = 8.38 (for aspect ratio 15-20), 59 θ = volume fraction of the nanorods. (5) 45

KVO2 = thermal conductivity of VO2 taken as 3.5 Wm-1K-1 60 (6) Φ = packing fraction; assumed value 0.82 (for rod shaped randomly oriented paticles).59 The calculated thermal conductivity values across the temperature

5

10

15

20

25

30

Fig. 5 (a) Schematic diagram of the formation mechanism for the VO2 nanorods under different drying conditions. NCs ≡ nanocrytals. (b) Lattice scale image of the low drying temperature sample revealing the presence of multiple grain boundaries and apparent regions of thickness contrast.

The effect of the nanorod concentration is not quite obvious from Fig. 6a, as loading in different concentrations apparently lead to K enhancement in a similar manner. To extract a better understanding in this issue, we plotted the linear fitting curves of the ratio Knf/KEG (Knf is the thermal conductivity of the nanofluid and KEG being thermal conductivity of the base fluid; plot of the actual ratios provided in the supporting information; Fig. S1) calculated across the experimental temperature range (provided in Fig. 6b). Apparently, the slope of the individual plots (listed in the inset) indicating the rate of K increment for the nanofluids increases as VO2 (B) nanorod concentration increases. Also, the relative decrease in the error percentage indicates that with increasing concentration the Knf/KEG ratio is more likely to follow a linear relationship with temperature. Although, there are several theories and mathematical models present to interpret the anomalous K enhancement for nanosized material incorporated fluid systems, any universally accepted theory or hypothesis that takes into account all the relevant variables and agrees with all the experimental results is yet to be reached. One of the prevalent theories states that microconvection due to Brownian motion is primarily responsible for this anomalous thermal behavior of nanofluids, but this theory fails to rationalize K enhancement in systems where particles are well dispersed and aggregation are anything but frequent. Recently, Prasher et al. attributed this phenomenon to local percolation behavior arising from the aggregation of nanoparticles and also

50

55

Fig. 6 Temperature dependent K of VO2(B)/EG nanofluid containing different volume percentage of the nanorods along with the theoretically calculated enhanced K as depicted by the pink plot. (b) The linear fit plots of Knf/KEG as a function of temperature as a manifestation of the effect of the concentration of nanoparticles on K. Inset shows parameters from the linear fitting.

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

(4)

Nanoscale

10

Page 8 of 12

range of 20 -100 °C for 0.6 vol% VO2 (B) has been provided in Fig. 6a (pink plot). Apparently it accounts for only 10-15 % of the observed K enhancement. While we cannot be certain about the exact mechanism pertaining to the entire K increment, it could be assumed that the nanorod shapes facilitates continuous heat transfer interchannel via multiple contacts between them. At a very low concentration such as ours these contacts may form short range thermal transfer channels augmenting the conductivity property. Besides, the effective medium theory predicts particles with high aspect ratio to have extremely low interfacial resistance and thus resulting in a better thermal transfer environment. Optical properties

15

20

25

30

35

40

45

UV-Vis spectra of the VO2 (B) nanorods is presented in Fig. 7a. The two bands observed in the spectra at around 270 nm and 400 nm can be ascribed to the charge transfer between the oxygen and vanadium ions.61 The optical band gap determined from the Tauc plot (inset, Fig. 7a) is about 2.6 eV. Previously, Liu et al. theoretically calculated the band gap of bulk VO2 (B) to be 1.66 eV and that of VO2 (B) nanosheets to be 1.87 eV62 while another article by Wang reported the calculated band gap for VO2 (B) as 2.1 eV.63 Our obtained band gap is higher than their results and closer to the 2.7 eV reported for BCC VO2.15 This increment in band gap could be attributed partially to dimensional confinement effects of the nanorods, but at the moment too few literature reports exist regarding the band characteristics of VO2 (B) to make a reasonable comparison. Fig. 7b depicts the photoluminescence (PL) spectra of the nanorods and we observe a green emission band at 522 nm and a violet emission band at 726 nm. Inset shows the molecular orbital diagram depicting the electronic energy splitting for VO2 (B). The alternate short V-V bonds in the distorted VO6 octahedra of VO2 (B) affects the п bands more than the  bands which are influenced more by the interaction between V and the O atoms and V-O-V interaction. This propels the d║ band above the * band and also results in a change in the separation between the two d bands. The violet band in the emission spectra of VO2 (B) nanorods can be interpreted as transitions from electrons in the d|| states to holes at the top of the oxygen 2p band. The green emission band could be due to the transitions between excited d|| state and the oxygen 2p state or it could be due to oxygen vacancies within the material as chemically synthesized metal oxides are known to contain oxygen deficiencies that give rise to green emission band. Data related to emission properties of VO2 (B) are extremely rare and the PL spectra obtained from the nanorods are expected to provide new insights regarding the optical properties of this metastable phase.

60

65

70

75

Sensing performance 50

55

Humidity sensing Fig. 8a reveals the humidity sensing characteristic of the VO2 (B) nanorods with both increasing and decreasing %RH through Curves I and II, respectively. Curve I illustrates that the resistance decreases sharply in the lower humidity range, i.e. from 10-40 %RH and further at higher humidity there is a continuous slow decrement in resistance. Similar behavior is indicated by the Curve II as well. The calculated sensitivity of the sensor in the range 10-40 %RH was 0.27 MΩ/%RH while for 40-90 %RH, it

80

85

Fig. 7 (a) UV-Vis spectra of the as-synthesized VO2 (B) nanorods. Inset shows the Tauc plot. (b) Photoluminescence spectra of the nanorods. Inset shows the molecular orbital diagram depicting electronic states of VO2 (B).

was 0.07 MΩ/%RH. Fig. 8b represents the stability curve of the fabricated humidity sensor. With a very low deviation in sensitivity observed between the curves corresponding to the as fabricated film and a month old film, the sensing performance of the VO2 (B) nanorods was found to be sustainable over a reasonable period of time. The basis of humidity sensing revolves around the phenomenon of changing electrical resistance of sensing materials when exposed to a humid environment. A schematic illustration depicting the water adsorption process on the VO2 surface has been provided in Scheme 1. The surface area of vanadium oxide is covered with hydroxyl groups (generated by the self ionization of water molecules adsorbed at the neck of the crystalline grains on activated sites of the surface) through the chemisorption of water in moist surroundings. The process of chemisorption occurs at very low humidity levels, and is unaffected by further changes in the humidity. However an increase in humidity makes the water molecules physisorbed via hydrogen bonding onto this hydroxyl layer. At higher humidity levels, the number of physisorbed layers increases allowing each water molecule to be singly bonded to a hydroxyl group. Subsequent proton hopping between adjacent water molecules in the continuous aqua layer forms continuous dipoles and

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

5

View Article Online

DOI: 10.1039/C4NR06032F

Nanoscale

20

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

25

30

35

40

45

Fig. 8 (a) Humidity sensing curves of the VO2 (B) nanorods. (b) Stability curves of the humidity sensor.

5

10

electrolyte layers between the electrodes resulting in increased dielectric constant and bulk conductivity. Variation of conductivity with humidity adsorption can be attributed to the protonic conduction mechanism on the surface. The conduction process is the same as that of pure water and is called Grotthus chain reaction.64 Porous structure of ceramic type materials and specific surface area of nanosized grains play the key roles in the interaction and physisorption of water vapors. Humidity gets adsorbed throughout the open porosities leading to condensation within the capillary pores that are distributed between the grains.65 Water

50

55

View Article Online

DOI: 10.1039/C4NR06032F

condensation happens on the neck of the grain surfaces and the amount of condensation depends on the density and size of pores. A larger surface area for the sensing material ensures higher absorption of water molecules leading to a greater density of charge carriers, usually protons. In short, physical adsorption and capillarity condensation of water molecules on the internal surfaces of the porous materials promotes increment in the charge carrier concentration with a decrease of the electrical resistivity. To acquire some relevant experimental insights we performed the Brunauer—Emmett–Teller (BET) nitrogen sorption and absorption experiments on the VO2 (B) nanorods. Fig. 9 reveals the typical nitrogen sorption isotherm of the VO 2 (B) nanorods. The obtained isothermal curve is a typical Langmuir type II nitrogen adsorption curve. The BET surface (SBET) area calculated was 9.7 m2g-1. The inset provides the corresponding nitrogen absorption isotherm and the average pore size calculated was 4.1 nm which is typical of mesoporous materials. As we know that apart from physisorption, diffusion through the pores is a pore size dependent phenomenon as macropores facilitates unconstrained travelling of the gas molecules but results in low surface to volume ratio while micropores although have larger specific surface areas but diffusion is strongly confined there as the pore dimensions are comparable to the size of the gas molecules. But mesoporous materials present a more favorable scenario as the pore dimensions lie in the same general order as the mean free path of the gas molecules and a gas molecule is as likely to collide with a pore wall as it is to collide with a neighboring molecule. Diffusion is therefore governed by Knudsen diffusion and diffusion coefficient (Dk) is proportional to the pore radius. Therefore the mesoporosity, as detected in the synthesized VO2 (B) nanorods is probably one of the favorable physical parameters to augment its sensing property. The presence of such mesopores can be rationalized when we consider the bi-layer structure of distorted VO6 octahedra of the precursor V2O5. This precursor undergoes steps involving dehydration of water molecules residing within the layers and finally establishing O chains among different layers to form the metastable VO2 (B) nanorods. Such a formation mechanism would support the preservation of the nanorod morphology from the hydrated species and also the high specific surface area of the material.

15

Scheme 1 water adsorption process on the VO2 (B) surface 60

Fig. 9 Nitrogen sorption isotherm of the VO2 (B) nanorods. Inset provides nitrogen absorption isotherm for the same.

Nanoscale Accepted Manuscript

Page 9 of 12

View Article Online

Page 10 of 12

DOI: 10.1039/C4NR06032F

Fig. 10 (a) LPG sensing curves of VO2 (B). (b) Stability curves of the LPG sensor.

5

10

15

20

25

30

35

LPG sensing Variations in electrical resistance with time for different concentrations of LPG were recorded and presented in Fig. 10a. For both the curves it was observed that as time increases the resistance of the film diminishes drastically at the beginning followed by slow decrement eventually becoming saturated. Further, when the outlet of the chamber was opened, the resistance of the film increases sharply and then slowly until it attains the value of stabilized resistance in air (Ra). The values of the maximum sensitivity and percentage sensor response were 1.26 and 20.95 % respectively. The response and recovery times of the sensor were found to be ~ 100 and 100 s, respectively. The investigations of sensing characteristics (reproducibility) of the film were repeated after one month of its fabrication and plotted in Fig. 10b. Almost no changes in the sensing performances were observed, indicating the stability of the fabricated sensor. In order to check the sensitivity of the VO2 gas sensor we compared our obtained LPG sensing response to reported response towards gases such as carbon dioxide (CO2), carbon monoxide (CO) and hydrogen (H2) (a comparative data has been provided in the supporting information; Table S1) and found that VO2 sensor is selective to LPG with fast response having smallest response and recovery times. Similar to the humidity sensing, the basic principle behind the gas sensing mechanism by metal oxides relies on the change in their electrical resistance on exposure to a gas, due to electronic exchange. Modulation of the number of charge carriers in response to a changing gaseous environment is the most important criteria for these sensors. When the VO2 nanorod gas sensor is exposed to air, oxygen gets chemisorbed on the surface of the oxide material, captures electron from the adsorption sites on the surface and depending on the temperature forms either O2-, O- or O2-. For a sensor operating at room temperature, formation of O2- is favoured. Corresponding reaction kinematics is as given below66-

45

50

55

vanadium dioxide surface is vital to enhance the receptor function of the sensor and hence, it’s sensing response. Oxygen vacancies can be the best position for the adsorption of oxygen on the surface of the nanostructure and subsequent formation of the O2depletion layer. The PL spectra (Fig. 7b) detected a strong green emission band suggesting the probability of formation of oxygen vacancies during the synthesis of the nanorods. Electron-transfer from conduction band to the chemisorbed oxygen results in a decrease in electron concentration on the film surface. As a consequence, an increase in the resistance of the VO2 (B) film was observed. After some time the resistance of the nanorods gets stabilized through above chemisorption reactions and is known as stabilized resistance in the presence of air (Ra). When the VO2 sensor is exposed to LPG, it reacts with the chemisorbed oxygen and a surface charge layer would be formed as per the following reaction.66 (9)

60

65

70

Where, CnH2n+2 represent hydrocarbons. Due to the ejection of the electrons (charge carriers), the resistance of the film decreases after it comes in contact with LPG. This entire mechanism is illustrated pictorially through Scheme 2. When the flow of LPG is stopped for recovery, then the oxygen molecules in air will be adsorbed on the film, and the capture of electrons through the processes indicated in equations (7) and (8) will increase the sensor resistance and the initial condition gets restored and the cycling reaction continues. Since the sensing mechanism is based on the chemisorption reaction that takes place at the surface of the metal oxide, so increasing specific surface area of the

(7)

40

(8) This electron extraction tends to increase the resistance (for an n-type material such as VO2 (B) in which majority charge carriers are electrons). The adsorption of O2- ions on the nanostructured

Scheme 2 LPG sensing model for VO2 nanorod sensor

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

Nanoscale

Nanoscale

View Article Online

DOI: 10.1039/C4NR06032F

Fig. 11 (a) Galvanostatic charge discharge curves of VO2 (B) nanorods electrode at the current density of 0.3 and 2 mA/g. (b) CV plots for the VO2/KOH/Graphite cell recorded at different scan rates of 5, 10 and 20 mV/s.

5

sensitive material leads to more sites for adsorption of the gases such as present in the synthesized VO2 (B) nanorods will be effective in incorporating efficient sensing capabilities. 45

Electrochemical capacitance 10

15

20

25

30

The charge-discharge characteristics of VO2/KOH/Graphite cells are shown in Figure 11a. The plots were recorded at current densities of 0.3 and 2 mA/g. The plots reveal the curves approaching a symmetrical triangle shape at the higher current density (2 mA/g), indicating an almost linear relationship The charge/discharge times were different at different current densities and the specific capacitance values are 138 and 13.4 mF/g at current densities of 0.3 and 2 mA/g respectively. The galvanostatic charge-discharge plots were also recorded for a device with a non-aqueous electrolyte: VO2/ionic liquid gel/Graphite, at a high current density of 0.1 A/g. The chargedischarge times are very small, in a two-electrode configuration. The CV plots for the VO2/KOH/Graphite cell recorded at different scan rates of 5, 10 and 20 mV/s are shown in Fig. 11b. The voltammograms are featureless, but the area under the curves is large and it increases as a function of scan rate, indicating the ability of this electrode to successively uptake and release ions from the electrolyte, implying good capacitive characteristics. Although the shapes of the curves deviate from the ideal rectangle, the symmetric nature of the CV curves observed at various scan rates are indicative of the redox reactions being reversible.

50

55

produces semi evolved one dimensional nanostructures built of a mixed phase of materials involving VO2 (B) and hydrated vanadium oxide species. The nanorods of VO2 (B) were also found to be efficient in enhancing the thermal conductivity of nanofluid based on ethylene glycol over a temperature range of 20 – 100 °C. A 45% enhancement was recorded which is comparable to the best results obtained for metal oxide based nanofluids. The nanorods showed the presence of a green emission band in its photoluminescence spectra indicating the probable presence of oxygen vacancy. Sensors based on the VO2 (B) nanorods showed good humidity sensing property especially in the low to mid range humidity region and comparatively lower sensitivity in the higher humidity region. The nanorods also showed decent LPG sensing property and the both the sensors were found to be sustainable in the long run. BET isotherms revealed the presence of mesopores which could be augmenting the sensing capability of the VO2 (B) nanorods.

Acknowledgement 60

65

Conclusions

We thank the Director, NPL, New Delhi, India for providing the necessary experimental facilities. Dr. A. Dhar, Dr. S.N. Sharma, Mr. K.N. Sood, Mr. J. S. Tawale and Mr. P. C. Mandal are gratefully acknowledged for providing the necessary instrumentation facilities for XRD, FT-IR, SEM and UV-Vis spectroscopy respectively. We thank Dr. J. P. Singh (IIT Delhi) for letting us carrying out the experiments regarding BET surface area measurements. Kajal Kumar Dey acknowledges the financial support from Council of Scientific and industrial research, India (Grant No.31/001(0325/2009-EMR-I).

70

35

40

VO2 (B) nanorods were synthesized through a simple hydrothermal procedure by reducing V2O5 by oxalic acid. We explored the morphological aspects of the as synthesized nanorods through electron microscopy and observed a slight variation in the microstucture leading to three different versions of nanorods. On an average the nanorods were around 80-100 nm wide and more than a micrometer long. The effect of drying temperature post the hydrothermal treatment was also investigated and it was found that drying at a lower temperature

References

75

1. J. H. Park, J. M. Coy, T. S. Kasirga, C. Huang, Z. Fei, S. Hunter and D. H. Cobden, Nature, 2013, 500, 431. 2. X. Rozanska, R. Fortrie and J. Sauer, J. Am. Chem. Soc., 2014, 136, 7751. 3. M. Liu, H. Y. Hwang, H. Tao, A. C. Strikwerda, K. Fan, G. R. Keiser, A. J. Sternbach, K. G. West, S. Kittiwatanakul, J. Lu, S. A. Wolf, F. G. Omenetio, X. Zhang, K. A. Nelson and R. D. Averitt, Nature, 2012, 487, 345.

Nanoscale Accepted Manuscript

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

Page 11 of 12

5

Published on 13 February 2015. Downloaded by Mahidol University on 13/02/2015 14:23:14.

10

15

20

25

30

35

40

45

50

55

60

65

4. J. Wei, H. Ji, W. Guo, A. H. Nevidomskyy and D. Natelson, Nat. Nanotechnol., 2012, 7, 357. 5. T. Paik, S.-H. Hong, E. A. Gaulding, H. Caglayan, T. R. Gordon, N. Engheta, C. R. Kagan and C. B. Murray, ACS Nano, 2014, 8, 797. 6. M. Lee, W. G. Hong, H. Y. Jeong, S. K. Balasingham, Z. Lee, S.-J. Chang, B. H. Kim and Y. Jun, Nanoscale, 2014, 6, 11066. 7. V. Eyert and K.-H. Hock, Phys. Rev. B, 1998, 57, 12727. 8. F. J. Morin, Phys. Rev. Lett., 1959, 3, 34. 9. T. Matsuishi, Jpn. J. Appl. Phys., 1967, 6, 1060. 10. Y. Oka, T. Yao and N. Yamamoto, J. Solid State Chem., 1990, 86, 116. 11. F. Theobald, J. Bernards and R. Cabala, J. Solid State Chem. 1976, 17, 431. 12. D. Hagrman, J. Zubieta, C. J. Warren, L. M. Meyer, M. M. J. Treacy and R. C. Haushalter, J. Solid State Chem., 1998, 138, 178. 13. L. Liu, F. Cao, T. Yao, Y. Xu, M. Zhou, B. Qu, B. Pan, C. Wu, S. Wei and Y. Xie, New J. Chem., 2012, 36, 619. 14. C. Z. Wu, Z.P. Hu, W. Wang, M. Zhang, J. L. Yang and Y. Xie, Chem. Commun., 2008, 3891. 15. Y.Q. Wang, Z. J. Zhang, Y. Zhu, Z. Li, C. R. Vajtai, L. J. Ci and P.M. Ajayan, ACS Nano, 2008, 2, 1492. 16. C. Leroux, G. Nikhil and G. V. Tendeloo, Phys. Rev. B, 1998, 57, 5111. 17. J. A. Koza, Z. He, A. S. Miller and J. A. Switzer, Chem. Mater., 2011, 23, 4105. 18. H. Bai, M. B. Cortie, A. I. Maaroof, A. Dowd, C. Keally, G. B. Smith, Nanotechnology, 2009, 20, 085607. 19. T. Driscoll, H.-T. Kim, B.-G. Chae, B.-J. Kim, Y.-W. Lee, N. M. Jokerst, S. Palit, D. R. Smith, M. D. Ventra, and D. N. Basov, Science, 2009, 325, 1518. 20. E. Baudrin, G. Sudant, D. Larcher, B. Dunn and J.-M. Tarascon, Chem. Mater., 2006, 18, 4369. 21. S. Yang, Y. Gong, Z. Liu, L. Zhan, D. P. Hashim, L. Ma, R. Vajtai and P. M. Ajayen, Nano Lett., 2013, 13, 1596. 22. H. Yin, M. Luo, K. Yu, Y. Gao, R. Huang, Z. Zhang, M. Zeng, C. Cao and Z. Zhu, ACS Appl. Mater. Interfaces, 2011, 3, 2057 23. G. Nie, L. Zhang, J. Lei, L. Yang, Z. Zhang, X. Lu and C. Wang, J. Mater. Chem. A, 2014, 2, 2910. 24. G. Li, K. Chao, H. Peng, K. Chen and Z. Zhang, Inorg. Chem., 2007, 46, 5787. 25. J. Liu, Q. Li, T. Wang, D. Yu and Y. Li, Angew. Chem. Int. Ed., 2004, 43, 5048. 26. S. A. Corr, M. Grossman, Y. Shi, K. R. Heier, G. D. Stucky and R. Sheshadri, J. Mater. Chem., 2009, 19, 4362. 27. S. Ijima, Nature, 1991, 354, 56. 28. K. Liu, X. Hong, Q. Zhou, C. Jin, J. Li, w, Zhou, J. Liu, E. Wang, A. Zettl and F. Wang, Nat. Nanotechnol., 2013, 8, 917. 29. D. Sun, C. W. Kwon, G. Baure, E. Richman, J. Maclean, B. Dunn and S. H. Tolbert, Adv. Funct. Mat., 2004, 14, 1197. 30. Z. W. Pan, Z. R. Dai and Z. L.Wang, Science, 2001, 291, 1947. 31. W. Shao, P. Arghya, M. Yiyong, L. Rodes and S. Prakash, in Synthesis and Applications of Carbon Nanotubes and their Composites, ed. S. Sukuzi, Intech Open, Telegraph Way, Winchester, UK, 2013, Ch. 13, pp 285-311. 32. D. Hu, B. Han, S. Deng, Z. Feng, Y. Wang, J. Popovic, M. Nuskol, Y. Wang and I. Djerdj, J. Phys. Chem. C, 2014, 118, 9832. 33. S. Bai, K. Zhang, S. Chen, R. Luo, D. Li and A. Chen, Cryst. Eng. Commun., 2014, DOI: 10.1039/C4CE01167H. 34. X. Chen, F. Wang and J. Xu, Top. Catal., 2011, 54, 1016. 35. E. V. Kondratenko, O. Ovsitser, Jchneider, R. Kraehnert and U. Dingerdissen, Appl. Catal. A, 2007, 319, 98. 36. G. Micocci, A. Serra, A. Tepore, S. Capone, R. Rella and P. Siciliano, J. Vac. Sci. Technol. A, 1997, 15, 34. 37. A. Simo, B. Mwakikunga, B. T. Sone, B. Julies, R.Madjoe and M. Maaza, Int. J. Hydrogen Energ., 2014, 39, 8147. 38. J.W. Byon, M.-B. Kim, M. H. Kim, S.Y. Kim, S.H. Lee, B.C. Lee and J. M. Baik, J. Phys. Chem. C, 2012, 116, 226. 39. J.M. Baik, M. H. Kim, C. Larson, C. T. Yavuz, G. D. Stucky, A.M. Wodtke and M. Moskovits, Nano Lett., 2009, 9, 3980.

70

75

80

85

90

95

100

105

110

115

View Article Online

Page 12 of 12

DOI: 10.1039/C4NR06032F

40. J. J. Taha-Tijerina, T. N. Narayanan, C. S. Tiwary, K. Lozano, M. Chipara and P. M. Ajayan, ACS Appl. Mater. Interfaces, 2014, 6, 4778. 41. J. Gangwar, K. K. Dey, S. K. Tripathi, M. Wan, R. R. Yadav, R. K. Singh, Samta and A. K. Srivastava, Nanotechnology, 2013, 24, 415705. 42. S. U. S Choi, Z. G. Zhang, W. Yu, F. E. Lockwood and E. A. Grulke,. Appl. Phys. Lett., 2001, 79, 2252. 43. K. K. Dey, A. Kumar, R. Shanker, A. Dhawan, M Wan, R. R. Yadav and A. K. Srivastava, RSC Adv., 2012, 2, 1387. 44. J. Buongiorno, D. C. Versus, N. Prabhat, T. Mckrell, J. Townsend and R. Christianson, J. Appl. Phys., 2009, 106, 094312. 45. J. Gangwar, A. K. Srivastava, S. K. Tripathi, M. Wan and R. R.Yadav, Appl. Phys. Lett., 2014, 105, 063108. 46. S. A. Angayarkanni and J. Philip, J. Phys. Chem C, 2014, 118, 13972. 47. R. Prasher, P. E. Phelan and P. Bhattacharya, Nanolett., 2006, 6, 1529. 48. T. K. Hong, H. S. Yang and C. J. Choi, J. Appl. Phys., 2005, 97, 064311. 49. C. Zhi, Y. Xu, Y. Bando and D. Goldberg, ACS Nano, 2011, 8, 6571. 50. B. C.Yadav, R. Singh and S. Singh, J. Exp. Nanosci., 2013, 8, 670. 51. D. A. Pomogailo, S. Singh, M. Singh, B. C. Yadav, P. Tandon, S. I. Pomogailo, G. I. Dzhardimalieva and K. A. Kydralieva, Inorg. Mater., 2014, 50, 296. 52. Y. L. Liu, Y. Xing, H. F. Yang, Z. M. Liu, Y. Yang, G. L. Shen and R. Q. Yu, Anal. Chim. Acta, 2004, 527, 21. 53. C. Wang, L. Yin, L. Zhang, D. Xiang and R. Gao, Sensors, 2010, 10, 2088. 54. S. R. Popuri, M. A. Artemenko, C. Labrugere, A. Villesuzanne and M. Pollet, Inorg. Chem., 2013, 52, 4780. 55. C. Wu, X. Zhang, J. Dai, J. Yang, Z. Wu, S. Wei and Y. Xie, J. Mater. Chem., 2011, 21, 4509. 56. K.-H. Chang and C-C. Hu, Acta Mater., 2007, 55, 6192. 57. J. –C. Valmalette, and J.-R. Gavarri, Mater. Sci. Eng. B, 1998, 54, 168. 58. L. E. Nielsen, J. Appl. Polym. Sci., 1973, 17, 3819. 59. I. H. Tavman, Int. Commun. Heat Mass Transfer, 1998, 25, 723. 60. D. W. Oh and C. Y. Ko, Appl. Phys. Lett., 2010, 96, 151906. 61. N. Gharbi, C. Sanchez, J. Livage, J. Lemerle, L. Ne´jem and J. Lefebvre, Inorg. Chem., 1982, 21, 2758. 62. L. Liu, T. Yao, X. Tan, Q. Liu, Z. Wang, D. Shen, Z. Sun, S. Wei and Y. Xie, Small, 2012, 8, 3752. 63. Y.-T. Wang and C.-H. Chen, Inorg. Chem., 2013, 52, 2550. 64. N. Yamazoe and Y. Shimizu, Sens. Actuators, 1986, 10, 379. 65. H. Faraahani, R. Wagiran and M. N. Hamidon, Sensors, 2014, 14, 7881. 66. S. Choopun, N. Hongsith and E. Wongrat, in Nanowires-Recent advances, ed. X. Peng, Intech Open, Telegraph Way, Winchester, UK, 2012, Ch. 1, DOI: 10.5772/54385.

Nanoscale Accepted Manuscript

Nanoscale