Multiwalled Carbon Nanotube Oxygen Sensor - ACS Publications

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Multi walled Carbon Nanotube Oxygen Sensor: Enhanced Oxygen Sensitivity at Room Temperature and Mechanism of Sensing Rajavel Krishnamoorthy, Lalitha Murugan, Radhakrishnan Joghee Kullan, Senthilkumar Lakshmipathi, and Rajendra Kumar Ramasamy Thangavelu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b04869 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 9, 2015

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ACS Applied Materials & Interfaces

Multi walled Carbon Nanotube Oxygen Sensor: Enhanced Oxygen Sensitivity at Room Temperature and Mechanism of Sensing Rajavel Krishnamoorthy1, Lalitha Murugan2, Radhakrishnan Joghee Kullan3, Senthilkumar Lakshmipathi2, Rajendra Kumar Ramasamy Thangavelu1,4,5* 1

Advanced Materials and Devices Laboratory, Department of Physics, Bharathiar University,

Coimbatore -6410 46, India. 2

Department of Physics, Bharathiar University, Coimbatore -6410 46, India.

3

Defence Bioengineering and Electrochemical Laboratory, DRDO, Bangalore - 560093

4

Department of NanoScience and Technology, Bharathiar University, Coimbatore -641046, India.

5

DRDO-BU Center for Life Sciences, Bharathiar University, Coimbatore -641046, Tamil Nadu, India.

*Corresponding author: [email protected]

Abstract Pyrolysis assisted method applied for the synthesis of defect controlled carbon nanotubes (CNTs) by varying different growth temperature. The fabricated resistive device containing a random network of CNTs are tested for oxygen sensing under standard room temperature and pressure conditions. Nanotubes grown at moderate growth temperatures (870°C), when exposed to different concentration of oxygen, displayed a higher sensitivity (3.6%), with fast response and recovery time about 60 and 180 Sec respectively, compared to nanotubes grown at higher and lower temperatures. Room temperature oxygen detection concentration as low as 0.3% is achieved. The fast response and recovery of CNTs are explained in terms of physisorption of oxygen molecules at (i) carboxyl functional sites and (ii) graphitic carbon sites (pristine CNT) rather than chemisorption at (iii) defected sites.

Interestingly, Density Functional Theory

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simulated interaction energy (Eads) of oxygen molecules with defected CNTs (-3.381eV) and pristine CNTs (-0.753 eV) are higher than the carboxyl functional sites (-0.551 eV) and are well correlated with the observed sensing response and recovery time of CNT sensors. Our results show that the carboxyl sites provide lower activation energy or shorter time for desorption of oxygen molecules to yield higher response and fast recovery of the CNT sensors. Keywords: Carbon nanotubes, Pyrolysis, Defect controlled, Oxygen sensing. Physisorption. Chemisorption. INTRODUCTION Detection of oxygen is a ubiquitous requirement for human life therefore monitoring the ambient oxygen level is of crucial importance for a wide variety of civilian and military applications.

In industries such as production of carbonated sodas, combustion engine

environment to evaluate engine performance and in beer manufacturing industries, the extensive detection of oxygen are most significant.1

Various technological approaches have been

developed for oxygen detection, including electrochemistry, optical spectroscopy and solid state thick film based sensors. More widely, a solid state thick film based sensors is used for the detection of oxygen molecules which are made up of different sensing materials such as ceramics, metal oxide semiconductors, polymers, nanostructured SnO2, TiO2, ZnO, CeO2, doped metal oxide nanoparticles coated with SiO2, Y2O3-doped ZrO2, carbon nanotubes (CNTs).1-10 Available sensor fabrication technology elucidated higher sensitivity with fast response time, even upto few milliseconds were reported but operated at higher temperatures. In addition to it, it also involves complex fabrication methodology and requires an external source for their recovery and high operating temperatures measured under vacuum environment. So the recent


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research work focused on the challenging task in fabrication of high responsive oxygen sensor at low operating temperatures with low production cost, working under atmospheric condition. Upto-date, only a few reports are available on the room temperature oxygen sensing performance with TiO2, CNTs and TiO2 grafted Graphene nanostructures, but constrained by complicated device structures along with slow response and high recovery time.11,12,13 In the present study, the room temperature reversible oxygen sensing performance of pyrolysis grown Multiwalled Carbon Nanotubes (MWCNTs) measured under standard temperature pressure (STP) is demonstrated. The fabricated two electrodes based CNTs sensor networks shows high oxygen sensing performance in the concentration ranging between 0.3 and 100% operated under STP conditions. Defect controlled and carboxyl functionalized MWCNTs are obtained by simple modified one step pyrolysis method at different growth temperatures. The enhanced oxygen sensing response of nanotubes is accounted by controlling available surface functional groups and defect states. The mechanism of change in nanotubes network conductivity is explained in interaction of oxygen molecules on their carboxyl functional sites combined with physisorption on graphitic sites nanotubes rather than chemisorption on defective sites. Density functional theory (DFT) calculations of the adsorption energy of oxygen molecule interaction at different sites on the CNT surface were evaluated using DFT calculations. DFT calculations show good correlation between oxygen adsorption energy at different sites and response and recovery time of the CNT oxygen sensor. Room temperature operated, highly reversible CNT sensor with fast response and recovery time could be interesting for environmental monitoring applications. EXPERIMENTAL SECTION



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Multiwalled Carbon Nanotubes Synthesis CNTs were synthesized by modifying the pyrolysis method as described by Chaisitsak et al.14 The horizontally mounted single zone furnace along with quartz tube (ID: 40mm; L:1000 mm) was used for the growth of CNTs. The mixtures of ferrocene (3 wt %) and xylene (7.5 ml) were placed inside the quartz tube reactor. Before starting the reaction the quartz tube is flushed with Ar follow rate of 2.5 ml/min. Growth was performed in three different growth temperatures (770, 870 and 970°C) with constant reaction time. After one hour reaction, the Ar flow was extended to cool down the reaction furnace till the furnace reaches 500°C. Once the furnace reached the room temperature, nanotubes and carbon related materials were collected from the walls of the quartz tube. The collected samples were subjected to purification process in order to remove the amorphous carbon; graphite and catalyst particle present in the nanotube mixtures (see the supporting information for detailed purification procedure). During the acid purification process, the unwanted catalyst (iron) particles are removed by simultaneous addition of carboxyl functional groups.15,16 After the purification process; the collected CNTs were subjected to Raman, Transmission Electron Microscope (TEM) and Thermo gravimetric analysis (TGA) for structural and morphological characterization. Sensor Fabrication and Testing The CNTs powders were dispersed in N, N-Dimethylformamide (DMF) with a ratio of 0.1mg/ml using probe sonication for 5 min for better dispersion with the supplied energy of 13.44W using sonic VC505. Pre-fabricated thermally evaporated inter digidated silver electrode (IDE) with 170 nm thickness were used as connecting leads for making sensing platform (see the supporting information S1 for sensor device structure). 5 µl of CNTs suspension was drop


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casted on the prefabricated electrodes surface and dried at 60ºC, this process is repeated for four times. The fabricated CNTs random networked electrode assemblies are heated to 100°C and maintained for 1h to complete evaporation of solvents in sensing platform. Oxygen sensing measurements were performed by monitoring the changes in the resistance of CNTs networks under different O2 exposure at STP condition. A homemade system was used for oxygen sensing measurements. Initially carrier gas was (N2) passed over the sensing material about 5 to 20 minutes for stabilization, after which the sample gas containing oxygen was introduced. Mixtures of N2 and O2 controlled by two different mass flow meters were used to achieve the desired oxygen composition in the range from 0.3 to 100%. A constant total flow rate of nitrogenoxygen gas mixture was maintained at 150 sccm (standard cubic centimeter).

For each

concentration of O2 (5, 10 and 20%), multiple experiments were performed with repeated reversible cycles, and pure N2 gas used for recovery cycles with a 150 sccm flow rate. The corresponding change in electrical resistance of CNT network was measured using Agilent data acquisition (Model: 34970A) unit family. Computational Details The first principle density functional theory (DFT) analysis of oxygen molecule interaction on nanotube surface is carried out. It involves the use of the pristine CNT (arm chair (6, 6) comprising 132 carbon atoms with length and diameter of 12.12 and 8.43 Å respectively. A reconstructed (presence of single dangling bond) monovacancy defect is induced in the pristine CNT without any distortion of the nanotube. In defect-CNT, three unsaturated open ended carbon atoms are rearranged to form a pentagon and nine-membered ring within their orbit. The mono and di-carboxyl functional groups are interacted onto the nanotube surface for functionalization. Besides, another similar set of carboxyl functionalization process is optimized 5 ACS Paragon Plus Environment


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for defect CNTs in the pristine side for comparison.

Pristine, defect induced, carboxyl

functionalized pristine and carboxyl functionalized defect induced CNTs are optimized using DFT adapted with B97D/6-31G* level of theory. The B97D function well accounts the dispersion interaction,17 and is suited for the present work, as the 6-31G* basis function, commonly used for physisorption in carbon nanomaterials.18,19 Frequency calculations are carried out for the optimized structures to confirm their minima. All the calculations were performed using Gaussian 0920 software package. The formation energy (Ef) of monovacancy defect in CNT is calculated by using equation (1). ‫ ݂݁݀ܧ = ݂ܧ‬− ‫ ݈݇ݑܾܧ‬+ ∆݊ߤܿ

where,

Edef is the total energy of the defective CNT, Ebulk is the total energy of the

perfect CNT, ∆n (here ∆n=1) is the change in the number of atoms to induce the defect in the CNT and µ c is the chemical potential of carbon in the pristine CNT. The calculated formation energy (Ef) is 6.064 eV and is in agreement with the previous reports.21,22 The binding energy (Eb) of -COOH is calculated for the pristine and defect induced SWCNTs using equation (2).

Eb = E(CNT + COOH) − (ECOOH + ECNT ) where, E(CNT+COOH), ECOOH and ECNT are the total energies of the CNT (pristine/defect) with the COOH group, isolated COOH group, and the bare CNT (pristine/defective) respectively. The HOMO-LUMO gap value of the carboxylated complex as well as the adsorbed complexes are calculated by the following relation (3), Eg= ELUMO - EHOMO where, ELUMO is the energy of the lowest unoccupied molecular orbital and EHOMO is the energy of the highest occupied molecular orbital. Oxygen molecules are introduced into the 6 ACS Paragon Plus Environment

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reaction system and their interaction probability between the oxygen molecules and defect controlled CNT structures are studied. The oxygen interaction on CNTs surface is evaluated by calculating their adsorption energies (Equation 4).

E ad = E(O2 + CNT) − (EO2 + ECNT ) Where,

E(O2 +CNT), EO2 and ECNT are the total energies of the CNT with the gas molecule O2

adsorbed on it, isolated gas molecule (O2) and the bare CNT respectively. RESULTS AND DISCUSSION Structural and Morphological Characterization Figure 1 (a-c) shows TEM images of CNTs synthesized at different growth temperatures (770, 870 and 970°C). CNTs grown at lower growth temperatures (770°C) shows structural misalignment with non-uniform wall thickness (as indicated by the arrow of inset Figure 1(a)). CNTs grown at higher growth temperatures (870 and 970°C) are found to be uniform in their diameters (inset of Figure 1 (b and c). The observed diameter and length distribution of purified MWCNTs are in range of 10-100 nm and 0.4 to 2 µm. From high resolution TEM images, the increase in diameter and length distribution upon increasing growth temperatures were seen which is well matched with previous reports.23 The Raman spectroscopy is a major tool to analyze the crystallinity in different sp2 hybridized carbon structure as well as to evaluate the defect concentration.

The observed

vibrations at 1580 cm-1 represents the ‘G’ band (Figure 1 (d)) corresponds with multiple helical structures of MWCNTs. The ‘G’ band peak is a characteristic of Raman line shape represents the first order tangential vibration mode of graphite. The observed vibration peak at 1350 cm-1 is 7 ACS Paragon Plus Environment


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assigned to defect related features of ‘D’ band vibrations of MWCNTs. The second order vibrations at 2681 cm-1 is fully related to in plane terminations of disordered graphite features (G|) alternative to assign the nanotube quality.13,24

Figure 1. High resolution TEM images of pyrolysis grown MWCNTs at different growth temperatures (a) 770°C, (b) 870°C, and (c) 970°C. Inset images correspond to respective high magnified individual nanotube structures, (d) Raman spectrum of MWCNTs recorded from 100 to 3000 cm-1 with calculated ID/IG ratio for three different growth temperatures, (e) ID/I2D ratio of MWCNTs samples which depicts the defect density of grown MWCNTs, and (f) the respective weight loss in TGA profile of MWCNTs grown at different growth temperatures. The intensity ratio of ‘D’ and ‘G’ band directly related to the degree of graphitization of CNTs.25 The higher the value of ID/IG ratio consequently related to high quantity of structural defects due to their multiple graphite layers. CNTs grown at (870°C show relatively higher


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degree of crystallinity compared to the samples grown at 770 and 970°C as described from the lower value of the ID/IG ratio (Figure 1 (d)). Similarly, the ID/I2D ratio is used to evaluate the defect density of synthesized MWCNTs, lower this ratio represents lower defect density (Figure 1(d)) lower ID/I2D ratio obtained from the samples grown at moderate growth temperatures (870°C) reveals their lower defect density compared with the samples grown at 770°C and 970°C.

The results indicate that the sample grown at 870°C show the highest degree of

graphitization which designates relatively high quality of nanotubes. The thermal stability and purity of synthesized MWCNTs for different growth temperatures is evaluated from TGA.

Figure 1 (f) shows the TGA weight loss curve for

MWCNTs grown at different pyrolysis temperatures (770, 870 and 970°C). CNTs grown at a 770°C show weight loss of 41% in the temperature range in 150-300°C are attributed to evaporation of both amorphous carbon as well as decarboxylation of carboxylic acid present in the MWCNTs grown at lower growth temperatures (770°C).26,27 Whereas, minimum weight loss about 14% observed for the CNTs grown at 870°C conform the evolution of carboxyl functional groups present in the tube structures. There is no weight loss observed in the temperature range 150–300°C for MWCNTs grown at 970°C is attributed to the presence of minimum amount of carboxyl groups which means the presence of pure nanotubes. High quality CNTs possesses high oxidative stability (>550°C), which indicates the formation of nanotubes with well graphitic structure.28 Besides, the higher oxidative stability of nanotubes is clearly observed for samples grown in 870 and 970°C compared with lower growth temperatures (for 770°C at 575°; 870°C at 588° and 970ºC at 592°) (Figure 1(f)). It clearly renders that the samples grown at high growth temperatures have a more acute with rapid oxidation rate and are more stable due to their well



graphitic structure. The assessed purity of MWCNTs in our one step pyrolysis method is calculated from the residual mass in TGA profile and it is found to be in the range of 94-96%. Gas sensing Performance and Oxygen sensing Response (a)

500

o

770 C o 870 C o 970 C

1050

(b)

450

Change in Resistance (∆ R Ω)

1200


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900 750 600 450 300 150

o

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O2 Concentration (%)

O2 Concentration (%)

Figure 2. (a) Oxygen sensing, viz., the change in resistance of the fabricated nanotube network, for a series of oxygen concentrations from 0 to 100%, and (b) Oxygen sensing, viz., a change in resistance, for lower oxygen concentration in the range between 0.3 to 20%. From the Figure 2 (a and b) it can be clearly seen that; (i) there is a change in resistance of the nanotube device upon exposure to increasing concentrations of oxygen from 0.3 to 100 %, (ii) the rate of this resistance change, decreases with an increase in oxygen concentration and (iii) there is a linear region with higher sensor response for the low oxygen concentrations in the range of 0.3 to 20 % (Figure 2(b)). The samples grown at moderate growth temperatures (870°C) show higher sensing response when compared with the sample grown at other growth temperatures (770 and 970°C). The fabricated resistive nanotube devices show a good sensing response in the lower oxygen concentration range from 0.3 to 100 %O2 with measured sensitivity of 0.03 and 3.6% respectively (Figure 2(b)).


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N2+ 5% O2 3000


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Figure 3. Sensor response over repeated cycles, giving the change in resistance for 5% O2 flow and subsequent N2 passing about 5 minutes interval for nanotube networks grown at different temperatures (770, 870 and 970°C). All the experiments are repeated for three times with repeated six cycles. The reversibility on the measured parameter, viz, resistance change of nanotube network has been measured over repeated cycles, for different group of samples grown at different growth temperatures. The results of oxygen concentration tested at 5% O2 (Figure 3), shows that the samples grown at 870°C displayed higher response compared with other growth temperatures (770 and 970°C). The samples grown at 770 and 970°C tends to reach the saturation region within 2 minutes oxygen exposure, but the samples grown in 870°C net yet to reach the saturation region even for 5 minutes oxygen exposure for the measured O2 concentration (5%). The response and recovery time is calculated from the change in resistance for our fabricated nanotube sensor for 5 min time interval (Figure 3) for 5% oxygen concentration for three different temperature grown samples and their results are shown in Table 1. Samples grown at 870°C show faster response time about 60 Sec when compared with samples grown at other growth temperatures at lower oxygen concentration measured under STP conditions. 11 ACS Paragon Plus Environment

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calculated recovery time (T90) is lower for the samples grown at 870°C about 180 Sec when compared with the sample grown at 770 and 970°C. Table 1. The calculated oxygen response and recovery time on effect of growth temperatures. Samples 770°C

Response time (T90) (Sec) * 80 ±1

Recovery time (T90) (Sec) * 300 ±0.06

870°C

60 ±2

180 ±0.06

970°C

71 ±4

240 ±0.08

* experiments repeated for three times and expressed in standard error bar Theoretical Simulation The DFT calculation on (i) pure, (ii) monovacancy defect induced and (iii) carboxyl functionalized CNTs are carried out to establish the adsorption and desorption of gas molecule related to the gas sensing mechanism. The perspective side view of the optimized structure of a perfect (6, 6) and the same type of nanotube containing a monovacancy defect induced nanotubes are shown in Figure 4 (a and b) respectively. The calculated binding energy, bond length and HOMO-LUMO gap values of carboxyl functionalized CNTs are listed in Table 2. The observed binding energy value for –COOH functionalization of pristine CNTs is about -0.745 eV, whereas the defect-CNTs show higher binding energy (-3.631 eV). This indicates that carboxyl group is involved in a strong covalent bond with defective CNT compared with the pristine CNTs.22 Similar kind of strong molecular bond formation is reported by Yang et al.,26 The reported29 binding energy 2.7 eV is for the carboxylation at the Stone-Wales defect incorporated in a graphene sheet. Moreover, the measured decrease in bond length about 1.47Å on comparing with pristine tubes further supports the higher probability of functionalization at the defect sites of CNTs. 12 ACS Paragon Plus Environment

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Table 2. The binding energy of the COOH group attached to the pristine CNT and monovacancy defected CNT.

Structure

Position of COOH group on CNT

-COOH Binding energy, (Eb in eV)

Bond length b/w CNT and COOH (Å)

HOMOLUMO gap, Eg (eV)

Pristine CNT

-COOH

-0.745

1.57

0.58

Defect induced CNT

- COOH at defect site

-3.631

1.47

0.58

Pristine CNT

Double – COOH

-2.297

1.57, 1.57

0.73

Defect induced CNT

Double – COOH Pristine site

-2.391

1.57, 1.57

0.42

Influence of inter-molecular interaction between two carboxyl functional groups on nanotube structures reflecting with adsorption and desorption of gas molecules in the gas sensing process, the number of carboxyl functional groups is increased to two. The optimized geometry of the di-carboxyl functionalized pristine CNTs and defect-CNTs at the pristine side are shown in Figure 4 (c and d). The measured binding energy of -COOH groups, the C - C bond distance (between the CNT and –COOH) and the HOMO-LUMO gap values are listed in Table 2. From the table, the obtained binding energy is about 2.29 eV related with effective bonding of carboxyl functional groups on nanotube surface.

Similar kind of observation in carboxyl

functionalization is obtained (-2.39 eV) for the pristine side of defect induced CNTs. The bond length between C atoms of CNTs and –COOH groups remains unaltered. Besides, the dicarboxyl functionalization in different positioned –COOH groups is negligible due to discrepancy in binding energy (see the supporting information Figure S2 and Table S2, Figure S3 and S4).



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Figure 4. Optimized geometry of (a) Pristine CNTs, (b) monovacancy defect induced CNTs, (c) –COOH functionalization in pristine CNTs, and (d) –COOH functionalization at pristine side of defect-CNTs. The results of analyzed geometry in DFT calculation of the interaction between pristine, defect induced, carboxyl functionalized CNTs and oxygen molecules are presented in Figure 5 (a-d). The adsorption energies (Eads), adsorption distance and HOMO-LUMO gap of the O2 adsorbed complexes are calculated using equation 2 and 3 and are listed in Table 3. The optimized atomic structure of a pristine nanotube containing single O2 - molecule adsorbate is shown in Figure 5 (a) with adsorption energy of -0.753 eV (negative of the binding energy of the equilibrium distance). The result from the Figure 5 (a) shows that in the fully-relaxed structure, the double bond of the O2-molecule is aligned with the nanotube axis and it is located on top of the associated C-C bridge bond. The measured distance between the O2 and the associated C-C


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bridge bond is about 2.56 Å. From the results it is clear that the oxygen molecule is physisorbed on the pristine tube without carboxylation.6 Table 3. The adsorption energy of the O2 group on pristine, monovacancy defect CNT and with the COOH group functionalized CNT (pristine and defective).

Structure Pristine CNT

Adsorption process

Eads of O2(eV)

Adsorption process

CNT + O2

-0.753

Physisorption

CNT + O2

-0.767

Physisorption at Pristine side

Defect CNT + O2

-3.381

Chemisorption of O2 at defect site

CNT-COOH + O2

-0.608

Weak Interaction

CNT-COOH-O2 + O2

-0.551

Weak Interaction

CNT-COOH + O2

-0.633

Weak Interaction

CNT-COOH-O2 + O2

-0.666

Weak Interaction

CNT with defect

Pristine CNT functionalized with two COOH Defective CNT with two COOH

For comparison, O2 molecule is made to adsorb on the pristine side of the monovacancy defect induced CNT, which exhibits a marginal increase in adsorption energy (-0.767 eV) on comparing with pristine CNT. But in both cases, the oxygen molecules bind weakly to the nanotube and the tube–molecule interactions can be identiﬁed as physisorption. However, in the case of defect-CNT, the interaction energy between oxygen molecules and defect sites are measured about -3.381 eV.

The higher adsorption energy value is attributed to strong

chemisorption of oxygen molecules at the defect sites. The measured bond distance between the



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oxygen-carbon (O-C) dangling bonds is about 1.305 Å. Such similar observation is reported for the physisorption and chemisorption of pristine and defect sites.30,31,32 The variation of the binding energy and their type of interaction with the O2 molecule and carboxyl functionalized CNTs surface is displayed in Figure 5 (a). Oxygen molecule is made to interact with carboxyl functional sites for both di-carboxylated pristine and defect-induced CNT to characterize the oxygen sensing performance (Figure 5 (b)). Initially, single O2 is interacting with one of the first COOH group attached to the CNT and then subsequently second O2 is interacted with the remaining COOH group. It is noted that the distance between the two COOH groups in the pristine CNT (3.96 Å) is decreased to 3.95 Å initially when single O2 is interacted, however it decreases to 3.94 Å for the subsequent oxygen molecule interaction. Correspondingly, the adsorption of the subsequent oxygen molecule interaction is decreased from -0.608 eV to -0.551 eV which is well correlated with the increase in bond length from 1.653 Å to 1.674 Å (see the supporting information Table S3). However, on the contrary to the pristine CNT, the adsorption energy (Eads= -0.633 eV; -0.666 eV) increases for the subsequent oxygen molecule interaction in the pristine side of defective CNT, due to the vacancy created in the nanotube. The increase in affinity towards oxygen is well augmented through the decrease in bond length from 1.675 Å to 1.666 Å (see the supporting information Table S3). Interestingly, the interaction energy of oxygen molecules with pristine CNTs is higher (-0.753 eV) than the carboxyl functionalization (-0.551 eV). The indicative of higher adsorption energy recognize the high activation energy or longer time for O2 desorption. Likewise, the chemisorbed complex (defected CNTs) requires more energy to get desorbed. In highlights, oxygen molecules hold less interaction with carboxyl functionalized CNTs preferable for sudden adsorption/desorption


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process than the pristine one. Thus, the physisorbed complex with minimum equilibrium interaction is more favorable with respect to adsorption and desorption of oxygen molecules.

Figure 5. Oxygen molecule interaction in CNTs (a) Pristine CNT. (b) O2 interaction with first (1st COOH) –COOH groups in functionalized CNTs and O2 interaction with second (2nd COOH) –COOH group in functionalized CNTs, (c) Chemisorption of O2 at defect sites in defected CNTs, and (d) O2 interaction with first (1st COOH) –COOH group in functionalized defected CNTs and O2 interaction with second (2nd COOH) –COOH group in functionalized defected CNTs. Further the HOMO-LUMO gap value of the O2 chemisorbed complex (0.487 eV) is found to be greater than the physisorbed complex (0.0636 eV) (see the supporting information Table S2), but both retain its p-type semiconducting behavior due to oxygen doping effect.6



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When oxygen interacts with both the carboxylated pristine and defective tube, the semiconducting tube transforms to metallic behavior (see the supporting information S5-S8 for DOS). The change of metallicity by the interaction of oxygen molecules on nanotubes needs further detailed investigations. Based on the magnitude of the adsorption energy from the DFT calculations showed that bonding of an O2 - molecule weakly adsorbed within the equilibrium interaction range is more signiﬁcant in the carboxyl functionalized CNTs for quick adsorbed/desorption process. Discussion In this report, we have formulated the defect controlled production of MWCNTs in our modified one step pyrolysis process, with the aid of different growth temperatures. The results of structural and morphological characterization reveals the following; (i) at lower growth temperatures, the formation of non-uniform diameter with inhomogeneous nanotube structures occur, due to the slow decomposition of carbon and catalyst precursors, which causes a lower supply of carbon to Fe catalyst. (ii) At moderate growth temperatures (870°C), the supply of carbon is increased and attain equilibrium of dissolving and diffusion to catalyst, and CNTs precipitation favors the formation of well-ordered graphitic nanotubes with a limited finite number of defects (as supported from TEM images, Raman and TGA analysis (Figure 1 (b, d and f). (iii) Upon further increasing the growth temperature to 970°C, the higher the dissolving rate of hydrocarbon, compared with the diffusion and precipitation rates consequence to higher carbon concentration. The higher availability of carbon concentration accounts with suppression of catalyst activity that leads to catalyst poisoning. This leads to increase in the nanotube growth rate along with deposition of more amorphous carbon during the nanotube formation.33 The exploration of amorphous carbon deposition is well accounted and correlated with results 18 ACS Paragon Plus Environment

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obtained in rapid weight loss during dry oxidation process compared with 870°C (see supporting information Table S1). On the aforementioned growth mechanism supported on influence of growth temperatures reveals the formation of high quality nanotube structures obtained at a moderate growth temperature (870°C). In our case, the resistance of MWCNTs sensor increases with the increase in oxygen percentage.

The results are well in agreement with Paul C P Watts et al.34 that the

adsorption/interaction of oxygen on CNTs arrest the carrier flow in the CNTs and the desorption of oxygen leads to the decrease in nanotube sensor resistance explained by the dominance of p-type semiconducting nature of CNTs upon oxygen doping/interaction.9,30,31,34 The reversible oxygen sensing response of nanotube device is determined by their interaction of gas molecules followed by desorption under exposed conditions. The interaction of a gas molecule to a sensor surface could be of either physisorption or chemisorption. Weak interaction between the sensor surface and gas molecules termed as physisorption process in which the gas molecule interacts and desorbs from the sensor surface with relatively less energy cost as there is no chemical bond formation between the sensor surface and gas molecule.34 On the other hand, chemisorption involve chemical bonding between the gas molecule and sensor surface and thus require relatively higher energy (0.6 eV) for desorption.32,35 The process of chemisorption occurs in topological defective sites in CNTs.35 So that relative changes in the electrical conductance of nanotube networks in chemisorbed oxygen species are significantly lower compared to physisorption. Besides, oxygen related surface carboxyl groups in nanotube structures are most favored active sites for interaction with external oxygen molecules.30,31,32 The effect of growth temperatures on oxygen sensing performance of our fabricated MWCNTs device is analyzed. The samples grown at moderate growth temperatures (870°C) 19 ACS Paragon Plus Environment


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reveal high sensitivity towards oxygen molecules when compared with other growth temperatures. The measured sensitivity and good reversibility with short recovery time over the repeated cycles of measurement indicates the good oxygen sensing capability of carbon nanotube network under STP condition. Relatively less number of active carboxyl functional groups and defects/disorder in the carbon nanotubes grown in lower pyrolysis temperature (770°C) affect oxygen-nanotube interaction probability and affect the charge transport respectively. CNTs grown at higher growth temperatures (970°C) show lower oxygen sensing response due to the presence of a relatively less number of surface carboxyl functional sites for oxygen interaction compared to CNTs grown at 870°C. However, the MWCNTs grown at 870°C possess a good structural quality as well as the availability of more number of carboxyl surface active functional groups (See the supporting information Figure S9) show enhanced oxygen response. FTIR spectra (Figure S9, Zoomed part, Supporting information) reveals that the MWCNTs grown at 870°C possess more fraction of -COOH groups (peak around 1217 cm-1) followed by the samples grown at 970°C and 770°C. (i.e., the sample grown at 770°C contains fewer fractions of –COOH groups).

For a fixed concentration (5%) and time exposure of O2 gas molecules

approaching the sensor, the sample grown at 770°C contains a less fraction of -COOH sites show complete saturation due to non-availability of fresh –COOH sites for oncoming O2 molecules (Figure 3). Whereas, MWCNTs grown at 870°C possess a relatively more fraction of –COOH groups than arriving O2 molecules yet to reach the saturation region even for 5 minutes oxygen exposure (Figure 3). It is noteworthy that good correlation has been observed for the defect density (Raman and TEM analysis) and recovery time (Table 1). MWCNTs grown at 770°C suffer high recovery time due to the chemisorption of oxygen molecule at the defect sites. The


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samples grown in 870 and 970°C show faster recovery time as the defect density found to be relatively low compared to the MWCNTs grown at 770°C. Theoretical simulation of oxygen interaction with pure, carboxyl functionalized and defect induced CNTs are studied and their calculated adsorption energy well matches with the results of oxygen sensing response. It is observed that, defect induced CNTs has a maximum adsorption energy of -3.381 eV which shows that oxygen molecules are chemically bound with defect sites in the nanotubes (see Figure 5 (c)) (770°C). However, pristine CNTs show lower adsorption energy (-0.753 eV) than defect nanotubes because of physisorption of oxygen molecules (970°C) (Figure 5 (a)). Interestingly, the calculated absorption energy (10 s

0.9 s

160°C 42

0.3 to 100 %

3.6%

60 s

230 s

27°C*

Reported Materials

O2 Detection Range

Pd/PdO- Zirconia (Thick films) SrTiO3 (Thin films) CeO2 (Thin films) ZnO Nanowire - FET (1-5 V) TiO2 and Graphene (FET) CNTs-Nb-TiO2 and (Thin Films) Polymer + CTAB + MWCNTs (Thin Films) MWCNTs (Thin films) MWCNTs (Thin films) * Present work

The demonstrated results established the fact that, oxygen sensing performance of nanotube devices depends on (1) available number of carboxyl functional sites at nanotube surface and (2) lattice defects and quality of nanotube structures which influence the oxygen interaction and carrier transport respectively. The measured sensitivity and good reversibility with short recovery time over the repeated cycles of measurement indicates the good oxygen sensing capability of carbon nanotube network under STP conditions. This resistive based


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oxygen sensor operating at room temperature could be an interesting future sensor platform for detection of low concentration of molecular oxygen. CONCLUSION Defect controlled carbon nanotube structures with carboxyl functional sites were synthesized by controlling the growth temperature. The oxygen sensing performance of resistive based CNTs sensors operated at STP conditions is reported here. Nanotubes grown at moderate growth temperatures (870°C), when exposed to different concentration of oxygen, displayed a higher sensitivity (3.6%), with fast response and recovery time about 60 and 180 Sec respectively, compared to nanotubes grown at higher and lower temperatures. Investigations reveal that the presence of oxygen selective surface carboxyl active sites is vital along with good structure (low defects) quality of the CNTs facilitating for their remarkable and reversible gas sensing response. The calculated interaction energy from the DFT simulation further supports the interaction probability of oxygen molecules on pure, defect and functionalized CNTs related with response and recovery time of the fabricated oxygen sensor. These devices seem to be promising for the realization as a robust room temperature sensing platform for monitoring lower concentration of oxygen molecules under STP conditions. Sensor fabrication technologies with improved film fabrication techniques with the addition of different functional sites can be enforced the fabrication of film based nanotube networks with an improved sensitivity for detection of oxygen at further lower concentrations. Supporting Information: Description

of

the

material

purification

process,

device

fabrication;

FTIR

Characterization and DFT results (adsorption energy, bond length, HOMO-LUMO level, and 23 ACS Paragon Plus Environment


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DOS) of pure, carboxyl functionalized and defect induced CNTs with oxygen interaction. This material is available free of charge via the Internet at http://pubs.acs.org. Conflict of Interest The authors declare no conflict of Interest. Acknowledgement One of the authors, K. Rajavel acknowledges Council for Science and Industrial Research (CSIR), New Delhi, for the award (letter No. 09/472(0166)/2012- EMR-I dated: 18.03.2013) of Senior Research Fellowship. References 1.

Qing, W.; Xuefeng. G.; Lichao, C.; Yang, C.; Lin, G.; Song, L.; Wang, Z.; Zhangb, H.; Li, L. TiO2-Decorated Graphene as Efficient Photo Switches with High Oxygen Sensitivity. Chem. Sci. 2011, 2, 1860–1864.

2.

Suman.; Vikesh, G.; Pramod, K.; Vinod, K. J. Nanomaterial-Based Opto-Electrical Oxygen Sensor for Detecting Air Leakage in Packed Items and Storage Plants. J. Exp. Nanosci. 2012, 7, 608–615.

3.

Ramamoorthy, R.; Dutta, P. K.; Akbar, S. A. Oxygen Sensors: Materials, Methods, Designs and Applications. J. Mater. Sci. 2003, 38, 4271–4282.

4.

He, J. H.; Ho, C. H.; Chen, C. Y. Polymer Functionalized ZnO Nanobelts as Oxygen Sensors with A Significant Response Enhancement. Nanotechnology 2009, 20, 065503.


Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.

Liu, L.; Li, B.; Ying, K.; Wu, X.; Zhao, H.; Ren, X.; Zhu, D.; Su, Z. Synthesis and Characterization of A New Trifunctional Magnetic–Photoluminescent–Oxygen-Sensing Nanomaterial. Nanotechnology 2008, 19, 495709.

6.

Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Extreme Oxygen Sensitivity of Electronic Properties of Carbon Nanotubes. Science 2000, 287, 1801-1804.

7.

Rajavel, K.; Verma, S.; Asokan, K.; RajendraKumar, R. T. Field And Temperature Dependent Electron Transport Properties of Random Network Single Walled and Multi Walled Carbon Nanotubes. Mater. Res. Express 2014, 1, 035004.

8.

Young, S. J.; Lin, Z. D.; Hsiao, C. H.; Huang, C. S.; Ethanol Gas Sensors Composed of Carbon Nanotubes with Adsorbed Gold Nanoparticles. Int. J. Electrochem. Sci. 2012, 7, 11634 – 11640.

9.

Lin, Z.D.; Hsiao, C. H.; Young, S. J.; Huang, C. S.; Chang, S. J. Carbon Nanotubes with Adsorbed Au for Sensing Gas. IEEE Sens. J. 2013, 13, 2423-2427.

10. Lin, Z. D.; Young, S. J.; Hsiao, C. H.; Chang, S. J. Adsorption Sensitivity of Ag-Decorated Carbon Nanotubes Toward Gas-phase Compounds. Sens. Actuators, B 2013, 188, 1230– 1234. 11. Nirmalraj, P. N.; Lyons, P. E.; De. S.; Coleman, J. N.; Booland, J. Electrical Connectivity in Single Walled Carbon Nanotube Networks. Nano Lett. 2009, 9, 3890-3895. 12. Valentini, L.; Armentano, I.; Lozzi, L.; Santucci, S.; Kenny, J. M. Interaction of Methane with Carbon Nanotube Thin Films: Role of Defects and Oxygen Adsorption. Mater. Sci. Eng. 2004, 24, 527-533.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

13. Tchernatinsky, A.; Desai, S.; Sumanasekera, G. U.; Jayanthi, C. S.; Wu, S. Y.; Nagabhirava, B.; Alphenaar, B. Adsorption of Oxygen Molecules on Individual Single-Wall Carbon Nanotubes. J. Appl. Phys. 2006, 99, 034306. 14. Chaisitsak, S.; Nukeaw. J.; Tuantranont, A. Parametric Study of Atmospheric-Pressure Single-Walled Carbon Nanotubes Growth by Ferrocene-Ethanol Mist CVD. Diamond Relat. Mater. 2007, 16, 1958–1966. 15. Li, Y.; Zhang, X.; Luo, J.; Huang, W.; Cheng, J.; Luo, Z.; Li, T.; Liu, F.; Xu, G.; Ke, X.; Li, L.; Geise, H. J. Purification of CVD Synthesized Single-Wall Carbon Nanotubes by Different Acid Oxidation Treatments. Nanotechnology 2004, 15, 1645–1649. 16. Datsyuk, V. K.; Papagelis, K.; Parthenios, T. D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Chemical Oxidation of Multiwalled Carbon Nanotubes. Carbon 2008, 46, 833-840. 17. Grimme, S. Semiempirical GGA‐Type Density Functional Constructed with A Long‐Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787-1799. 18. Tachikawa, H.; TetsujiIyama. Structures and Electronic States of Fluorinated Graphene. Solid State Sci. 2014, 28, 41-46. 19. Umadevi, D.; Narahari, G. S. Quantum Mechanical Study of Physisorption of Nucleobases on Carbon Materials: Graphene versus Carbon Nanotubes. J. Phys. Chem. Lett. 2011, 2, 1572-1576. 20. Frisch, M. J.; Trucks, G. V.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Baronev, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda. R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark,


Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


E.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski,. J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; in Gaussian 09, revision B.01, Gaussian, Inc.: Wallingford CT, 2009. 21. Banhart, F.; Kotakoski, J.; Krasheninnikov A. V. Structural Defects in Graphene. ACS Nano 2010, 5, 26-41. 22. Ulbricht, H.; Gunnar, M.; Tobias, H. Physisorption of Molecular Oxygen on Single-Wall Carbon Nanotube Bundles and Graphite. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 075404. 23. Lee, C. J.; Park, J.; Huh, Y.; Lee, J. Y. Temperature Effect on the Growth of CNTs using Thermal Chemical Vapor Deposition. Chem. Phys. Lett. 2001, 343, 33-38. 24. Castro, C.; Pinault, M.; Leconte, S. C.; Porterat, D.; Bendiab, N.; Reynaud, C.; Hermite, M. M. L. Dynamics of Catalyst Particle Formation and Multi-Walled Carbon Nanotube Growth in Aerosol-Assisted Catalytic Chemical Vapor Deposition. Carbon 2010, 48, 3807-3816. 25. Lehman, J. H.; Terrones, M.; Mansfield, E.; Hurst, K. E.; Meunier, V. Evaluating the Characteristics of Multiwall Carbon Nanotubes. Carbon 2011, 49, 2581-2602. 26. Singh, C.; Shaffer, M. S. P.; Windle, A. H. Production of Controlled Architectures of Aligned Carbon Nanotubes by an Injection Chemical Vapours Deposition Method. Carbon 2003, 41, 359-68.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

27. Osswald, S.; Havel, M.; Gogotsi, Y. Monitoring Oxidation of Multiwalled Carbon Nanotubes by Raman Spectroscopy. J. Raman Spectrosc. 2007, 38, 728-736. 28. Singh, D. K.; Iyer, P. K.; Giri, P. K. Diameter Dependence of Oxidative Stability in Multiwalled Carbon Nanotubes: Role of Defects and Effect of Vacuum Annealing. J. Appl. Phys. 2010, 108, 084313. 29. Yang, O. F.; Huang, B.; Li, Z.; Xiao, J.; Wang, H.; Xu, H. Chemical Functionalization of Graphene Nanoribbons by Carboxyl Groups on Stone-Wales Defects.

J. Phys. Chem. C

2008, 112, 12003-12007. 30. Hatami, M.; Farmany, A.; Sahraei, R. Physisorption and Chemisorption of Oxygen Molecules on Single- and Multi-Walled Carbon Nanotubes. Fullerenes, Nanotubes, Carbon Nanostruct. 2014, 22, 434-453. 31. Grujicic, M.; Cao, G.; Singh, R. The Effect of Topological Defects and Oxygen Adsorption on the Electronic Transport Properties of Single-Walled Carbon-Nanotubes. Appl. Surf. Sci. 2003, 211, 166–183. 32. Govind, N.; Andzelm, J.; Maiti, A. Dissociation Chemistry of Gas Molecules on Carbon Nanotubes-Applications to Chemical Sensing. IEEE Sens. J. 2008, 8, 837-840. 33. Ionescu, P. I.; Zhang, Y.; Li. R.; Sun, X.; Rachid, H. A.; Lussier L-S. Hydrogen-Free Spray Pyrolysis Chemical Vapor Deposition Method for the Carbon Nanotube Growth: Parametric Studies. Appl. Surf. Sci. 2011, 257, 6843–6849. 34. Watts, P. C.; Mureau, N.; Tang, Z.; Miyajima, Y.; Carey, J. D.; Silva, R. P. The Importance of Oxygen- Containing Defect on Carbon Nanotubes for the Detection of Polar and NonPolar Vapors through Hydrogen Bond Formation. Nanotechnology 2007, 18, 175701.


Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35. Abha, M. Carbon Nanotubes and Graphene-Based Chemical Sensors. Curr. Sci. 2014, 107, 416-429. 36. Spirig, J. V.; Ramamoorthy, R.; Akbar, S. A.; Routbort, J. L.; Singh, D.; Dutta, P. K. High Temperature Zirconia Oxygen Sensor with Sealed Metal/Metal Oxide Internal Reference. Sens. Actuators, B 2007, 124, 192-201. 37. Hu, Y.; Tan, O. K.; Pan, J. S.; Huang, H.; Cao, W. The Effects of Annealing Temperature on the Sensing Properties of Low Temperature Nano-Sized SrTiO3 Oxygen Gas Sensor. Sens. Actuators, B 2005, 108, 244-249. 38. Jasinski, P.; Suzuki, T.; Anderson, H. U. Nanocrystalline Undoped Ceria Oxygen Sensor. Sens. Actuators, B 2003, 95, 73-77. 39. Fan, Z.; Wang, D.; Chang, P. C.; Tseng, W, Y.; Lu, J. G. Zno Nanowire Field-Effect Transistor and Oxygen Sensing Property. Appl. Phys. Lett. 2004, 85, 5923-5925. 40. Llobet, E.; Espinosa, E. H.; Sotter, I. R.; Vilanova, X.; Torres, J.; Felten, A.; Pireaux, J. J.; Ke, X.; Tendeloo, G. V.; Renaux, F.; Paint, Y.; Hecq, M.; Bittencourt, C. Carbon Nanotube–TiO2 Hybrid Films for Detecting Traces of O2. Nanotechnology 2008, 9, 375501. 41. Frontera, P.; Trocino, S.; Donato, S.; Antonucci, P. L.; Faro, M. L.; Squadrito, G.; Neri, G. Oxygen-Sensing Properties of Electrospun CNTs/PVAc/TiO2 Composites. Electron. Mater. Lett. 2014, 10, 305-313. 42. Cava, C. E.; Salvatierra, R. V.; Alves, D. C. B.; Ferlauto, A. S.; Zarbin, A. J. G.; Roman, L. S. Self-Assembled Films of Multi-Wall Carbon Nanotubes used in Gas Sensors to Increase the Sensitivity Limit for Oxygen Detection. Carbon 2012, 50, 1953-1958.



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