Chemical Modifi cation of Carbon Nanotubes for NO2 Detection

Multiwalled carbon nanotubes (MWNTs) were oxidized by nitric acid in order to simultaneously induce defects and hydrophilic (carboxyl) groups on the surface of MWNTs, on the basis of the concept that defects and carboxyl groups could contribute to the increase in the number of adsorption sites of gas molecules such as NO2. It was observed by transmission electron microscope (TEM) that oxidized MWNTs had a rough sidewall surface. The sensor response to dilute NO2 (Ra/Rg) of an oxidized MWNT-based sensor was 50% higher than that of an as-grown MWNT-based sensor, suggesting an increase in the number of NO2 adsorption sites with an increase in the density of defects on the surface of MWNTs. On the other hand, the p-n junction of a semiconductor generally contributes to increasing electric resistance by forming a space charge layer. A space charge layer could be expected to enhance NO2 gas adsorption. Thus, MWNTs as p-type semiconductors were modifi ed with an n-type semiconductor such as WO3 and SnO2 nanoparticles, which generally exhibit an excellent sensor response to dilute NO2. MWNT-WO3 and MWNT-SnO2 composite sensors showed a fairly good sensor response (Ra/Rg) to dilute NO2, compared with that of the MWNT-based sensor. The addition of an n-type oxide to MWNTs contributes to the formation of a large depletion layer within MWNTs by the p-n junction on contact points between MWNTs and oxides, resulting in an increase in resistance in air and the enhancement of NO2 adsorption.


Introduction
Carbon nanotubes (CNTs) are one of the new candidate materials for a variety of potential applications. (1) Recently, CNT-based gas sensors have received considerable attention because of their excellent properties, such as faster response, higher sensitivity, and lower operating temperature for the detection of gases such as NO 2 and NH 3 . (2)(3)(4)(5)(6)(7)(8) To enhance their sensitivity to air pollution gases, it is more effective that CNTs are chemically oxidized by acid solutions such as nitric and sulfuric acids in order to generate gas adsorption sites as carboxyl groups and carbonyl on the surface of CNTs (9)(10)(11)(12)(13)(14)(15)(16)(17) and that CNTs are modifi ed with metallic particles of Au, (18) Pt (19) or oxides such as SnO 2 , WO 3 , and TiO 2 . (20) Interestingly, Au or Pt nanoparticles deposited on multiwalled carbon nanotubes (MWNTs) behave as p-type semiconductors when exposed to NO 2 and NH 3 . (18,19) In the case of the MWNT-oxide composite gas sensor, (20) the enhancement of gas sensitivity depends on the formation of a p-n junction at the surface of the main matrix with either oxides or nanotubes when oxygen in air negatively adsorbs on the surface.
This work is on the NO 2 sensing properties of a pristine MWNT-based gas sensor and three types of MWNT-based gas sensor: (1) MWNTs were oxidized using a "wet" chemistry process such as nitric acid treatment to introduce defects on the aromatic ring of carbon nanotubes leading to an increase in the number of gas adsorption sites; (2) the surfaces of MWNTs were modifi ed with disklike WO 3 small grains 300 nm in diameter and 50 nm in thickness to improve sensor response to 5 ppm NO 2 and discuss the interaction between MWNTs and WO 3 when the amount of WO 3 added to MWNTs is varied; (3) the sensing properties of a MWNT-SnO 2 composite sensor to 1 ppm NO 2 were investigated for three deposition states of SnO 2 nanoparticles inside/outside MWNTs.

MWNT-based sensor
Microgap electrodes with various gap sizes were fabricated by MEMS techniques. A Au line with a width of 20 μm, a gap size of 5 μm, and a thickness of 0.3 μm was formed on a SiO 2 substrate by photolithography (lift off), as shown in Fig. 1. A growth catalyst of 0.05 wt% Ni(CH 3 COO) 2 aqueous solution was dropped on a Au electrode with a 5 μm gap using a microinjector, and dried at room temperature for 30 min to serve as a Ni-deposited substrate. This substrate was subsequently set on the electric furnace, and MWNTs were directly grown at the microgap of Au at 625 to 700°C for 60 min using a nickel catalyst in a gas mixture of ethanol, argon, and hydrogen (6 kPa).

Oxidized MWNT-based sensor
An aqueous solution of nitric acid (1 mol/L) was dropped on the MWNT-based sensor using a microinjector, and dried at 100°C for 15 min, in order to clarify the difference in sensing properties between raw and oxidized MWNTs.

MWNT-SnO 2 composite sensor
Growth catalysts of MWNTs such as nickel should be removed from inside MWNTs in order to fi ll them with oxides. It is also desired that MWNTs have a uniform G/D ratio for reproducibility of fi lling MWNTs with a tin source such as SnCl 2 . The details of the G/D ratio of MWNTs are explained in § 2.5. The MWNTs were purchased from Wako Pure Chemical Industries, Ltd. They were oxidized using concentrated nitric acid to open the tip edges of the MWNTs and to remove the growth catalysts and amorphous carbon. TEM of the oxidized MWNTs showed that the MWNTs were 20-50 nm in diameter and 5-50 μm in length. Raman spectroscopy showed that the MWNTs have a G/D ratio of more than 1.5 even after oxidization. MWNT-SnO 2 composites were prepared by two methods. The oxidized MWNTs were immersed in 0.1 wt% SnCl 2 aqueous solution, which was stirred at R. T. for 20 h to introduce the Sn component into the MWNTs. Tin oxides and those deposited inside/outside MWNTs were separated by centrifugation after drying at 120°C for 3 h in air to induce Sn component oxidation. Excess SnO 2 deposited outside the MWNTs was removed using deionized water and dried at 35°C for 2 h in air. Finally, SnO 2 -deposited MWNTs were calcined at 400°C for 3 h in air or 600°C for 3 h in argon in order to control the deposition state of SnO 2 particles inside/outside MWNTs.

Characterization of MWNTs
Two main features in the Raman spectra of MWNTs are generally the D-and Gpeaks at about 1350 and 1600 cm -1 , respectively. The G-band corresponds to the symmetric E 2g vibrational mode in graphitelike materials, while the D-band is activated in the fi rst-order scattering process of sp 2 carbons by the presence of substitutional heteroatoms, vacancies, or other defects, all of which are consistent with disordered graphitic bonding within the tubes. The relative peak intensity ratio of the G-band to the D-band in Raman shift of pristine or oxidized MWNTs was measured by Raman spectroscopy (Nanofi nder30, Tokyo Instruments) in order to investigate the correlation between the amount of defective sites on the surface of MWNTs and that of NO 2 adsorption. The morphology and microstructure of a raw MWNT, an oxidized MWNT and an MWNT-oxide composite were observed by fi eld-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-2010), respectively. Fourier transform infrared spectroscopy (FT-IR, Shimazu IR Affi nity-1) was used to confi rm the surface states of oxidized MWNTs.

Evaluation of sensing properties to dilute NO 2
The three types of MWNT-based sensor obtained at various growth temperatures were placed in a fl ow apparatus equipped with an electric furnace and the sensing properties to dilute NO 2 (5 ppm) were measured at R. T. and 100 to 250°C every 50°C. A NO 2 (10.6 ppm) + N 2 high-pressure gas tank was used for the preparation of 5 ppm NO 2 . The sensor response (S = R a /R g ) was defi ned as the ratio of resistance in air atmosphere (R a ) to that in NO 2 -containing air atmosphere (R g ). In the case of the MWNT-WO 3 composite sensor, the optimum operating temperature was determined to be 200°C by investigating the preliminary conductive behaviors of WO 3 and MWNTs to 5 ppm NO 2 . The sensing property of the MWNT-SnO 2 composite sensor to 1 ppm NO 2 was examined in the temperature range from 100 to 250°C in order to measure resistance change when exposed to 1 ppm NO 2 . Resistance change (%) was defi ned as ((R g −R a )/R a )×100 in the evaluation of the MWNT-SnO 2 composite sensor.

MWNT-based sensor
A high-resolution field-emission electron scanning microscopy (HR-FE-SEM) image of MWNTs deposited between Au electrodes at 700°C for 60 min via ethanol CVD is shown in Fig. 2, corresponding to the circular area in Fig. 1. This image shows nanotubes with a diameter range of 5-20 nm, and entangled and aligned carbon nanotubes with a metal cap predominantly located on their top.
The MWNT-based sensors obtained at 625, 650, and 700°C were exposed to 5 ppm NO 2 for 10 min after R a was stabilized at an operating temperature of 100°C for 3 min. Air fl owed to the sensor immediately after the fl ow of 5 ppm NO 2 was turned off. The response transients to 5 ppm NO 2 are shown in Fig. 3. These responses indicated that the MWNT-based sensor behaved as a p-type semiconductor because the resistance became lower in NO 2 -containing atmosphere. The electric resistances of the MWNTs obtained at 625 and 650°C were higher than those of the MWNTs obtained at 700°C. The responserecovery characteristics became sluggish with decreasing growth temperature of MWNTs immediately after each MWNT was exposed to NO 2 . On the other hand, the sensor response (S) of MWNTs grown at low temperatures was higher than that of MWNTs grown at high temperatures. These tendencies suggest that the crystallinity of MWNTs is related to sensor response.
The Raman spectra of the MWNT-based gas sensor fabricated at 625, 650, 700 and 750°C are shown in Fig. 4. The peak intensity ratio of the G-band to the D-band increased with increasing growth temperature of MWNTs. We investigated the effects of growth temperature on the resistance in air or sensor response to NO 2 adsorption on MWNTs, as shown in Figs. 5(a) and 5(b), respectively. The sensor response of the MWNT-based sensor fabricated at 750°C was almost unity. This means that NO 2   gas did not adsorb on the surface of individual MWNTs between Au electrodes. The sensor response of the MWNT-based sensor fabricated at 700°C tended to be low at all operating temperatures, compared with those of the MWNT-based sensors fabricated at 625 and 650°C. These behaviors seem to relate to the amount of defective sites on the surface of MWNTs. The resistance in air (R a ) and sensor response (R a /R g ) of the MWNTbased sensor decreased with increasing growth temperature. This behavior is considered to be consistent with general semiconductive properties, i.e., electric resistance decreases with increasing operating temperature. On the other hand, the resistance in air and sensor response decreased with increasing G/D ratio, as shown in Fig. 5(c). The electrical properties of thin fi lms of MWNTs can be tuned by adjusting the deposition temperature, and a transition from a defective structure to a defect-free structure can be induced through high-temperature exposure to reactive plasma in order to control the defective sites on the side wall of MWNTs. (4) Valentini described that the interaction of the adsorbed NO 2 molecules with MWNT defective sites is enhanced, whereas NO 2 does not chemically interact with the sidewall of the tubes grown at a higher deposition temperature. Therefore, from the results in Fig. 5(c), NO 2 molecules were considered to adsorb to defective sites on the sidewall of MWNTs with a lower G/D ratio in Raman spectra, grown at 625°C, leading to higher electrical resistance and better sensor response.
The response of an MWNT-based gas sensor at each growth temperature to 5 ppm NO 2 as a function of operating temperature is shown in Fig. 6. The MWNT-based sensor obtained at 700°C indicated almost the same sensor response at all operating temperatures. It is suggested that the number of defective sites on the side wall of MWNTs corresponds to the amount of NO 2 adsorption. In contrast, the sensor responses of MWNTs grown at 625 and 650°C tended to decrease with increasing operating

Oxidized MWNT-based sensor
The effect of oxidation of MWNTs is summarized in Fig. 7. The sensor response of an oxidized MWNT-based sensor to 5 ppm NO 2 was higher than that of a raw MWNTbased sensor, as shown in Fig. 7(a). This tendency was related to the surface state of MWNTs changed by the nitric oxidation, as seen in the ratio of the G-band to the D-band for Raman spectra with or without the nitric oxidation in Fig. 7(b). SEM images of Figs. 7(c) and 7(d) respectively show the raw and oxidized MWNTs. The surface morphologies of raw and oxidized MWNTs were different and a particularly smooth surface was formed by nitric oxidation, suggesting the elimination of amorphous carbon aggregated on the surface of MWNTs. Figures 7(e) and 7(f) respectively indicate the high-resolution images of lamination carbon layers for the side wall of MWNTs with and without nitric oxidation. The outside wall of the oxidized MWNTs clearly indicates an ambiguous edge and an unclear stacking carbon layer compared with that of raw MWNTs. Namely, nitric oxidation seems to remove amorphous and introduce carboxyl groups on the outside surface of MWNTs. The infrared absorption spectra of MWNTs with and without oxidation are shown in Fig. 8. The oxidized MWNTs were assigned to the stretching vibrations of C-O, C=O and COO-at respective wave numbers of 1100-1200, 1650-1740 and 1320-1550 cm −1 in addition to 1578 cm −1 of aromatic C-C related to the graphene sheet of MWNTs. It seems that these hydrophilic groups contribute to the adsorption of NO 2 enhancing the increase in sensor response, although we have unfortunately no evidence of the chemical affi nity between carboxyl groups and NO 2 adsorption.

MWNT-WO 3 composite sensor
MWNTs 20-50 nm in diameter, which were grown at 700°C for 60 min in a gas mixture of ethanol, argon, and H 2 (6 kPa), have an entangled shape, as shown in Figs. 9(a) and 9(b). Microstructural analysis using Raman techniques and TEM indicated that the ratio of the G-band to the D-band was closest to 1. The entangled MWNTs formed a network structure with each other (Fig. 9(b)).
SEM images of MWNT-WO 3 composites with various WO 3 amounts are shown in Fig. 10. In the MWNT-0.1 wt% WO 3 composite, WO 3 grains were independently trapped and MWNTs were clearly observed as shown in Fig. 10  500 nm 500 nm Figure 11 shows the response transients of MWNT and MWNT-WO 3 sensors to 5 ppm NO 2 at 200°C. The resistances of both MWNT and MWNT-WO 3 sensors decreased upon exposure to NO 2 , suggesting that conduction occurs through p-type MWNTs in both MWNT and MWNT-WO 3 sensors. The sensor response (R a /R g ) of the MWNT-0.1 wt% WO 3 sensor was as high as 3.8; however, the MWNT sensor showed almost no response (R a /R g = 1.05). Figure 12 shows the resistance in air and the sensor response of the MWNT-WO 3 composite as a function of the additional amount of H 2 WO 4 suspension. The resistance markedly increased at 0.1 wt% H 2 WO 4 addition. After reaching the maximum value of resistance, the resistance gradually decreased with increasing additional amount of H 2 WO 4 . This behavior can be explained by the formation of a p-n junction at 0.1 wt% and the many connection sites of each WO 3 particle at higher than 1 wt%. A similar behavior was observed for the sensor response, which had a maximum at 0.1 wt%. At  Resistance in air / 10 6 Ω 0.1 wt%, a p-n junction was formed between the MWNTs and WO 3 grains to generate a large depletion layer within the MWNTs, inducing a large resistance in the MWNT-WO 3 composite sensor. The highly depleted surface state of MWNTs resulted in an increasing amount of NO 2 adsorbed on the MWNT, and thus a high sensor response of the MWNT-WO 3 composite sensor to dilute NO 2 . At higher than 1 wt%, the conduction pass was formed through WO 3 grains to decrease sensor resistance. At higher than 1 wt% WO 3 , the WO 3 grains come into contact with each other for a dominant n-type conductive pass owing to WO 3 . WO 3 is an excellent sensing material for NO 2 detection. Although the WO 3 sensor shows a resistance increase as a response, the MWNT-WO 3 (7 wt%) composite sensor exhibited no response to 5 ppm NO 2 (R a /R g = 1). It is considered that the sensor response (R a /R g ) would be decreased to less than unity (resistance increase) at a higher amount of WO 3 . Finally, the reproducibility of the sensor response to 5 ppm NO 2 was examined at 200°C for MWNT-0.1 and 1 wt% WO 3 composites. As a result, the sensor responses of the composites were respectively 3.6 and 1.4, which were close to the plotted data of Fig. 12.

MWNT-SnO 2 composite sensor
Well-dispersed SnO 2 nanoparticles a few nanometers in diameter deposited inside the MWNTs were prepared by calcination at 400°C for 3 h in air after washing with deionized water, as shown in Fig. 13(a). On the other hand, in the case without washing with deionized water, Fig. 13 in Fig. 15. The resistance in air of these sensors at each operating temperature is plotted in Fig. 16. The oxidized MWNTs indicated a 10% resistance decrease when exposed to 1 ppm NO 2 ( Fig. 15(a)). This can be explained by the general p-type semiconductor behavior when exposed to the oxidant gas NO 2 . NO 2 adsorbs as negatively charged NO 2 (NO 2− ) on the surface of the MWNTs, as shown in Fig. 17(a), and then hole carriers are injected into the MWNTs, inducing a decrease in resistance. The MWNT-SnO 2 composite sensor shown in Fig. 15(b) shows a decrease in resistance although the MWNTs were apparently completely covered with a few nanometers of SnO 2   Fig. 13(a), suggesting a p-type conductive behavior due to the insuffi cient connection between SnO 2 nanoparticles. In contrast, the MWNTs completely covered with SnO 2 nanoparticles of more than 5 nm in diameter showed an increase in resistance (data not shown in this paper), denoting an n-type conductive behavior when exposed to the oxidant gas NO 2 . Figure 15(c) shows the response transient for the composite sensor composed of the SnO 2 nanoparticles of ca. 10 nm in diameter deposited inside the MWNTs, as shown in Fig. 13(c). Interestingly enough, SnO 2 nanoparticles deposited inside the MWNTs indicated the largest resistance decrease of ca. 40% and the longest response time for a NO 2 exposure time of 15 min. Noticeably, the resistance of this composite sensor in air is on the order of 10 4 Ω, as shown in Figs. 16(b) and 16(c), in contrast to that of the oxidized MWNT sensor on the order of 10 2 in Fig. 16(a). This difference in resistance in air depends on the formation of a p-n junction between n-type SnO 2 and p-type MWNTs. The p-n junction contributes to enhanced NO 2 adsorption, similarly to the NO 2 adsorption behavior of the MWNT-WO 3 composite sensor. In the case of the MWNT-WO 3 composite, n-type WO 3 nanoparticles are deposited outside the p-type MWNTs and a p-n junction is formed outside the MWNTs. In contrast, SnO 2 nanoparticles deposited inside the MWNTs form many p-n junctions at connection points between SnO 2 and MWNTs, as shown in Fig. 17(b). The p-n junction contributed to the formation of a large depletion layer beginning on the inner surface of the MWNTs. The depletion layer penetrates the outer part of the MWNTs, and many NO 2 molecules adsorbed on the depletion layer come into contact with to the surface of the MWNTs.
The above modifi cation techniques may be very useful for enhancing the sensing properties of MWNTs to environmental pollution, foul-smelling and volatile organic compound gases.