Removal of strongly-bound gases from single-walled carbon nanotubes without annealing or ultraviolet light exposure

Accepted Manuscript Removal of strongly-bound gases from single-walled carbon nanotubes without annealing or ultraviolet light exposure Lakshman K. Randeniya, Philip J. Martin PII: DOI: Reference:

S0008-6223(13)00380-1 http://dx.doi.org/10.1016/j.carbon.2013.04.071 CARBON 8002

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Carbon

Received Date: Accepted Date:

11 February 2013 22 April 2013

Please cite this article as: Randeniya, L.K., Martin, P.J., Removal of strongly-bound gases from single-walled carbon nanotubes without annealing or ultraviolet light exposure, Carbon (2013), doi: http://dx.doi.org/10.1016/j.carbon. 2013.04.071

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Removal of strongly-bound gases from single-walled carbon nanotubes without annealing or ultraviolet light exposure Lakshman K. Randeniya* and Philip J. Martin CSIRO Materials Science and Engineering, PO Box 218, Lindfield NSW 2070, Australia

Abstract Annealing at high temperatures and exposure to strong ultra violet light are the approaches used in the past for affecting the desorption of strongly bound gases such as ammonia (NH3) and nitrogen dioxide (NO2) from single wall carbon nanotubes (SWCNT) and graphene. These methods pose severe limitations in the development of devices which can operate in normal ambient conditions. The use of another gas which can influence the kinetics of desorption of gases already present on the SWCNTs has not been explored in detail. Here we show that the redistribution of substrate impurity states near Fermi Level, caused by the electrostatic forces of polar molecules like water, accelerates the desorption of gases bound on SWCNT. This phenomenon can be used to facilitate complete, rapid and non-destructive desorption of NO2 and NH3 molecules from SWCNT chemiresistors at room temperature. Complete desorption of these gases were achieved within minutes instead of many hours as reported previously in the literature. The method provides a practical alternative for achieving recovery in CNT-based molecule detectors in air without the risk of degradation of the SWCNTs and their sensitive polymer composites which are used to achieve high sensitivity and selectivity.

*

Corresponding author. E-mail address: [email protected] (Lakshman K. Randeniya)

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1. Introduction Single-walled carbon nanotubes (SWCNT) have exceptional sensitivity to gases at room temperature and have good environmental stability, however, practical gas sensing applications are hindered by excessively long recovery times[1-3]. Kong et al. reported spontaneous recovery times of 12 hrs for NO2 and NH3 detections [1]. Very high sensitivity (e.g. detection of 100 parts per trillion of NO2) and selectivity were achieved with carbon nanotubes (CNT) combined with polymers but recovery required the use of ultraviolet radiation [4]. The use of UV radiation (259 nm) in the presence of air produces ozone which rapidly degrades the quality of the sensor [5] limiting their use in inert buffer gases such as argon. In addition, polymer composites of CNT, often used for achieving selectivity and higher sensitivity [4], degrade when repeatedly exposed to ultra violet radiation even in inert atmospheres. Nitrogen dioxide is a hazardous gas in the environment and detection at trace levels is important. It plays a major role in the formation of ozone and acid rain. Continued or frequent exposure to concentrations higher than 53 parts per billion (ppb) may cause increased incidences of acute respiratory illness in children [6]. Ammonia similarly is a toxic gas which is widely used in industries, produced naturally through nitrogen fixation and present in high concentrations in farming areas. The concentration of ammonia in exhaled breath is a useful indicator in disease diagnostics[7]. Measurements of concentrations less than 2 ppb for environmental monitoring and 50 ppb for diagnostic breath analysis are required [8]. Therefore, repeated trace-level detection of these gases using an inexpensive, roomtemperature device remains a challenge and a great need [9]. Conventional metal-oxide sensors operate at high temperatures (200 – 600 oC) and have high power consumption, low selectivity, high sensitivity to humidity and long-term drift [10].

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Although there is likelihood of exciting outcomes, a subject that has attracted little attention of the researchers is the influence of substrate on the adsorption and desorption processes on materials such as CNT and graphene. It was shown that water molecules adsorbed on the SiO2 substrate cause hysteresis in CNT-based field-effect transistors (CNTFET) due to charge trapping [11]. Further research focused on the elimination of this effect [12] or better performance of CNTFET. Wehling et al showed from first-principles that the water adsorption shifts the substrate defect states towards or away from the Fermi level of graphene depending on the site of adsorption and the orientation of dipole moment leading to either ptype doping or n-type doping [13]. Experimental evidence is emerging that the gas adsorption on substrate has a definite impact on electronic properties of graphene [14-16]. Here we obtained first definitive evidence for the alteration of electronic properties of SWCNT from water adsorption at the interface with a suitable substrate which directly affected the desorption kinetics of gases bound strongly on SWCNT. We used this phenomenon to facilitate fast and complete desorption of NO2 and NH3 molecules from SWCNT at room temperature in air. The water-mediated recovery occurred in the presence of a substrate and not in freestanding films.

2. Experimental Purified HiPco® SWCNT (Nano Integris estimated semi-conducting tube content ~ 60 %) and CoMoCAT SWCNT (Sigma Aldrich 704148, with > 90% semi-conducting tubes) [17] were used without further purification for the fabrication of chemiresistors. 10 mg of SWCNT and 100 ml of deionised water was treated with ultrasonics for 1 hour with the ultrasonic wand inside solution (104 W, 20 kHz, Vibracell®). For the detection of ammonia, SWCNT were treated with a solution of 1 M HNO3 was instead of purely de-ionised water. The mixtures were diluted 1: 10 with de-ionised water and were stirred with ultrasonics for further one

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minute. The mixture was either filtered under vacuum on Whatman Anopore ® (Al2O3) membranes with pore sizes of 20 nm or drop-casted onto other substrates. For samples on Al2O3, a sufficient amount of solution was used to obtain a film of SWCNT with a desired thickness (determined by the resistance). After drying in air, the filter membrane was cut with a sharp razor blade. Small cut samples were secured on to filament posts using silver paste to form chemiresistors (SWCNT/Al2O3). Typical exposure area of SWCNT film on chemiresistors was 3 mm x 5 mm. By controlling the amount of solution used, samples of various thicknesses (corresponding to resistances of a few kΩ to a few MΩ) were prepared. When the film was made sufficiently thick, it could be lifted off the substrate to obtain a freestanding film. These films were robust enough to fabricate chemiresistors. For samples on SiO2 substrate (SWCNT/SiO2), Si wafers with native oxide layer were used. For chemiresistors with TiO2 substrate (SWCNT/TiO2), TiO2 thin films on Si wafers were prepared using a plasma enhanced chemical vapour deposition method [18]. Drop-casting was used to deposit SWCNT film on these substrates. The gas detection instrument was described elsewhere [19-20]. Pure dry synthetic air (BOC) was used as the buffer gas. Mixtures of 500 ppm NO2 and 500 ppm NH3 (Spectra Seal, BOC Limited) were used diluted in dry air to obtain concentrations between 5 ppm and 50 ppb. The resistance of the films was measured using a Keithley 6487 picoammeter interfaced with a computer. The current limit was set at 10 µA to minimise possible Joule heating of the sample.

3. Results and Discussion We used moist SWCNT (equilibrated in ambient conditions, relative humidity 35 – 55 %, for several days) for the detection of gases. Here, the dry and moist states of nanotubes are defined in relation to strongly-bound (mode 1) water molecules to the tubes. There are two

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modes of water adsorption in SWCNT, one leading to strongly-bound (mode 1) molecules and the other leading to loosely-bound (mode 2) molecules. Mode 1 relates to water adsorbed in ‘trapped’ states, has long desorption time constant and the kinetics can be explained by a sub-diffusive motion [21]. The conductance decreases slowly as the water molecules come off as shown in Figure 1 (a). The time constant for desorption is substrate-dependent and was found to be greater than 18 hours for SWCNT/Al2O3 and even longer for SWCNT/SiO2 chemiresistors (see Figure 1 (a)). In this paper when we refer to ‘dry’ nanotubes it means that most of the strongly bound (trapped) water molecules have come off over a period of 18 hours or more inside of the chamber with dry air flow (relative humidity < 1 %). In addition to the process involving long time constants, there is a second mode (mode 2) of water adsorption/desorption with much faster time constants (a few seconds) and opposite change in conductance. An example is seen at the beginning of the curve in Figure 1(a). When the chemiresistor was first introduced from ambient air into the dry chamber, there was a sharp increase in conductance due to the drop in humidity. Further examples are shown in Figure 1 (b). When the humidity in the chamber is raised to higher levels marked in the figure, the conductance decrease quickly. Once 100 % dry air flow is reintroduced, conductance recovers quickly to the baseline value. The mechanism responsible for mode 2 was described as the enhanced diffusion mode [21] and is often attributed to hydrogen bonding at oxygencontaining defect sites [22] and is present in both dry and moist tubes. As explained later, we use this mode of water adsorption for the displacement of NO2 and NH3 molecules from SWCNT, but first examine the mechanics of exploring it in the context of a gas sensing experiment. Figure 2 shows the novel strategy used for NO2 detection with quick full recovery in our experiments. The chemiresistor equilibrated under ambient conditions is introduced into the chamber. Once a baseline is established under dray air flow, a desired concentration of NO2 5

is introduced (‘NO2 on’) by mixing with dry air at the appropriate ratio for a period of three minutes or five minutes depending on how fast the signal saturates. The conductivity of the chemiresistor increases as shown and reaches full or partial saturation. Immediately after NO2 is turned off (‘NO2 off, H2O on), a fraction of the buffer gas flow (50% - 100%) is diverted through a water bubbler. This raises the relative humidity of the chamber to approximately 52 % to 75 % and consequently the conductance of the chemiresistor reduces sharply at first and then reaches values below the baseline. The humid buffer gas flow is maintained for a preset time during which NO2 molecules are displaced rapidly from SWCNT. After this period, when the 100 % dry gas flow (‘H2O off’) is re-established, the conductance of the chemiresistor returns to the baseline. The experiments can be repeated measuring the same or different concentrations of NO2. The total recovery time (ΔtR) is calculated as the sum of the time when the water-enriched airflow is maintained (Δt1) plus the time taken for the resistance to reach baseline value after 100 % dry airflow has been re-established (Δt2). Exactly the same strategy is used for ammonia detection with some changes in details as explained later. The initial sharp drop in conductance is due to fast response of the sensor to water vapor. However, as the NO2 comes off the conductance decline further and eventually level out signaling that the recovery is complete. Reestablishing 100 % dry airflow at this stage returns the signal to the baseline. Figure 3 (a) shows actual measurements of 5 parts per million (ppm) NO2 using the scheme explained above. In this particular instance, the measurements were performed four times, thrice with water-assisted recovery, and the last run with spontaneous recovery in dry air. The results show the excellent reproducibility of response for the four measurements. The waterassisted recovery time of less than 10 minutes (from the first three runs) is an improvement of more than an order of magnitude from that of spontaneous recovery time in dry air (obtained from the last run when completed). This also marks an orders of magnitude improvement 6

from the previously reported values for spontaneous recovery times and gives a real advantage for advancing CNT-based devices for practical room-temperature molecule detection. Figure 3 (b) demonstrates a similar experiment performed with a freestanding film obtained from the same preparation of SWCNTs. When introduced at the beginning of the recovery cycle, the response to water is smaller in comparison to the first three measurements in Figure 3 (a) (response does not even drop down to the baseline). Even after maintaining high humid conditions (RH = 75 %) for 15 minutes, when 100 % dry air flow is reestablished, the baseline essentially follows the same recovery profile that would be obtained if the recovery was allowed to occur entirely in dry air. The film contains densely packed carbon nanotubes and hence stronger adsorption in interstitial sites and groves will be more important in comparison to SWCNT dispersed on a substrate. However, the fact that there is no detectable impact from water vapor on the desorption shows that the recovery mechanism seen in the first three curves in Figure 3 (a) occurs essentially on SWCNTs deposited on a substrate. We also prepared chemiresistors on SiO2 (native oxide layer on Si wafer), amorphous TiO2 films made from plasma enhanced vapor deposition to check the affect of substrate. The data for repeated 5 ppm runs for SWCNT/SiO2 chemiresistors are shown in Figure 4 (a). The results show excellent reproducibility of the response and the effectiveness of water vapor (RH ~ 52%) on recovery. The results for SWCNT/TiO2 chemiresistors were mixed; while some samples showed very little affect on recovery from water flow, the others showed a moderate affect (supplementary information, Figure S1). In all cases, the water-assisted desorption was much less efficient in SWCNT/TiO2 chemiresistors in comparison to that of SiO2- or Al2O3- based devices. The results suggest that the interaction of SWCNT with TiO2 is weaker than that for the other two oxides. The detail of this interaction, which is thought to occur through substrate defects [13], needs further investigation. Similar substrates, which are 7

environmentally stable and robust may be used in the reduction of water-induced hysteresis of CNT based FETs. Figure 4 (b) shows the consecutive measurement of six different concentrations of NO2 ranging from 5 ppm to 50 ppb. The inset in Figure 4 (b) shows the response as a function of concentration. The results show excellent response and recovery and confirm the applicability of our technique to a wide range of concentrations. The water vapour content in the chamber during the recovery process influences the recovery rate. In the above experiments, we used 100 % buffer gas flow through the water bubbler (raising the relative humidity inside of the chamber to 75 %) to force recovery. The use of 50 % of the flow through the chamber raises the relative humidity inside the chamber to only ~ 52 % and the recovery time increases (see supplementary information, Figure S2). We performed the experiments entirely at room-temperature conditions and obtained a maximum of ~ 75 % relative humidity in the chamber. There are possibilities for using higher humidity to affect even faster recovery. Figure 5 shows the application of the method for ammonia detection. The conductance of the device decreases in response to ammonia. As seen in Figure 5 (a), where 50 ppm ammonia was measured repeatedly. When water vapor is introduced at the beginning of the recovery cycle, there is a quick decrease in conductance as a response of the sensor to water vapor. However, as the ammonia molecules come off, the conductance reaches a minimum and then increases slowly towards a flat baseline value. When the 100 % dry airflow is re-established, after allowing sufficient time for full recovery, the conductance increases further and reaches the original baseline value. This completes the full cycle of ammonia detection and waterassisted sensor recovery. The response and recovery are highly reproducible. Figure 5 (b) shows very slow recovery profile under dry airflow which took many hours to reach full

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recovery. Using this method ammonia was detected down to 500 ppb (see Figure 5 (c). These results also show that our method gives a very significant advance towards practical, repeated detection of NH3 using SWCNTs at room temperature. Experimental data suggests that (1) NO2 and NH3 gas interacts with dry SWCNT strongly and (2) charge transfer is possible from adsorbed molecules to p-type SWCNT. Due to strong interaction, the recovery is extremely slow and takes many hours to be achieved. Calculations using density functional theory and other approaches do not support such strong binding energy between pristine SWCNT [23-24]. For NO2, the interaction has been attributed to dissociative adsorption with the formation of NO3 which can bind strongly to SWCNT [25]. Later experimental studies suggest that NO2 adsorbs without dissociation [26]. The possibility of larger adsorption energy at defect sites has been proposed [27-28]. A route for stronger- than-expected adsorption of ammonia on graphene with the formation of NH4 was also proposed [29]. We now look at a plausible theoretical explanations to the substrate-assisted affect of water vapour on desorption kinetics of NO2 and NH3. The water-assisted recovery is inefficient for dry SWCNTs. When the SWCNT/Al2O3 chemiresistors were allowed to dry overnight in the chamber, and the experiments were performed in the same way, the recovery could not be accelerated by the water injections. There are a number of possible reasons why the waterassisted recovery is more efficient in moist SWCNT; (1) restricted access of gas molecules to the highest-energy binding sites as water molecules already occupy them (2) polarisation of SWCNTs due to strong dipole moments of trapped water molecules (mode 1 adsorption) leading to a modified interaction potential [30-31] (3) a change in adsorption mechanism. For example, HONO and HNO3 are formed when NO2 is adsorbed on Al2O3, silica and soot surfaces in the presence of water vapor [32-33] and may occur on moist SWCNT.

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2 NO2  H 2O  HONO  HNO3

Our experiments with moist SWCNT/Al2O3 and moist SWCNT/SiO2 chemiresistors with varying thicknesses showed that the sensitivity increased gradually and eventually reached a plateau as the films got thinner. As seen from Figure 3, the increase in response for 5 ppm NO2 can be as large as a factor of two going from a freestanding film to a thin film on Al2O3 substrate. Given that the films used for experiments always have finite conductance implies that the tubes are in close physical contact with each other throughout the films (supplementary information Figure S3). The adsorption at interstitial sites and voids becomes less important as the films get thinner. Generally, the adsorption at interstitial sites leads to increased binding energy and increased charge transfer [23] and we expect the response to be lower in thinner samples. In thinner samples, the gas molecules are adsorbed on the tubes that are increasingly in direct contact with the substrate. These results together with the waterassisted recovery process illustrated earlier confirm that the adsorption/desorption of NO2 and NH3 on moist SWCNT is mediated by the substrate. According to the results presented by Wehling et al. for graphene samples, the SiO2 substrate defects form impurity states in close proximity to Fermi level [13]. These impurity states can cross Fermi level in the presence of adsorbed water (through strong electrostatic filed caused by the dipole moment) and cause doping. This could also be the reason for the water induced hysteresis affects on CNT-based field-effect transistors. We propose a similar mechanism to be active during the waterassisted recovery process in SWCNT. This proposal is supported by the work of Zhao et al who found that the partially occupied NO2 molecular orbitals (POMO) exist closer to the valence band of SWCNT[23]. The resulting hybridization between NO2 and SWCNT molecular orbitals moves the Fermi level into the valance band and population of NO2 orbitals with electrons increases p-type conductivity in SWCNTs. The increase in the response in the presence of a substrate found here then is likely due to increased charge 10

transfer from NO2 molecules adsorbed near substrate-SWCNT interface region. This could be a result of the mixing of substrate defect states and the valence bands of SWCNT making the hybridisation with POMO of NO2 even more efficient. Adsorption of water clearly disturbs this equilibrium. We conclude that the strong electrostatic forces of water dipoles shift the mixed valence band of substrate and SWCNT away from the POMO of NO2 and weaken bonding. This forces the desorption of NO2 molecules from SWCNT surfaces. The presence of dangling bonding defects in the SWCNT (see Figure S4) may also enhance the interaction with the substrate through van der Waals forces. Detailed theoretical work would be needed for a thorough explanation of this intriguing affect for both ammonia and nitrogen dioxide adsorption/desorption on SWCNT and is the topic of ongoing work. 4. Conclusions The study provides evidence for the strong coupling between the desorption kinetics of the adsorbed gases and the substrate defect states near the Fermi Level of SWCNTs. Adsorption of polar molecules like water changes the distribution of substrate states near the Fermi Level through electrostatic effects. Such changes strongly impacts on the desorption kinetics of the gases already bound on SWCNT. This phenomenon allows the achievement of rapid nondestructive desorption of strongly bound gases, NH3 and NO2, from SWCNT at room temperature. Supporting Information Available:

Acknowledgements This paper is dedicated to the memory of Mr. Edward Preston. We acknowledge scientific and editorial contributions to the manuscript from Amanda Barnard, Edith Chow and Karl Heinz Muller of CSIRO Materials Science and engineering.

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Figure 1

a

4 2

Al2O3 substrate SiO2 substrate

100 * G/G0

0 in chamber RH<1%

-2 outside RH~45% -4 0

1000

2000

3000

4000

5000

H2O on

0

b

-5

61 %

-10

66 %

75 %

-15 H2O off -20 0

200

400

600

800

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Time (s)

Figure 1 Two modes of response of SWCNT/Al2O3 chemiresistors to water vapour. The response is presented as the percent change in conductance (ΔG) with respect to a baseline conductance of G0. (a) response change with time when a chemiresistor equilibrated under ambient conditions is introduced into the chamber under dry air flow. The conductance first increases due to drop in humidity (mode 2 response, please see text) and then decreases slowly as the water molecules come off (mode 1 response). Much slower mode 1 response is obtained for SiO2 substrates (b) Mode 2 responses of the chemiresistor to injections of water (by diverting a fraction of buffer gas flow through an enclosed bubbler) which temporarily raise the relative humidity (RH) in the chamber to the levels marked on the figure.

Figure 2

NO2 adsorption/desorption on SWCNT coated on insulator surfaces

Film Conductance

Saturation level

NO2 off, H2O on

Fast H2O(g)-assisted desorption

Baseline 'Moist' nanotubes

t1

NO2 on

RH<1%

H2O-assisted

(RH~50-75%)

t2

(RH<1%)

desorption inefficient H2O off

'Dry' nanotubes

Time

Figure 2 Scheme used for NO2 measurements using water-assisted recovery. Baseline is obtained and NO2 is measured under dry conditions (relative humidity inside the chamber, RH <Δ 1 %). During the period of Δt1, the humidity of the chamber is raised to a suitable value by introduction of water vapour. During the period Δt2, chamber humidity returned to < 1 %.

Figure 3

NO2 off, H2O on

40

NO2 off, RH<1%

tR

RH~75%

a

20 0

100 * G/G0

NO2 on

-20

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RH<1%

0

RH<1%

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2000

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15 10 H2O off

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5000

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Figure 3 (a) Detection of 5 ppm of NO2 using SWCNT/Al2O3 chemiresistor. The response is calculated as a percentage of the change in conductance (ΔG) with respect to the baseline conductance (G0). The exposure time to NO2 is 5 min. Exposure time to water-enriched air flow (RH= 75 %) is 5 min. Total recovery time, ΔtR (= Δt1 + Δt2) (see text) for this experiment is about 10 min. (b) detection using a free-standing SWCNT film where water enriched air flow was maintained for 15 minutes, and no detectable recovery attributable to water was found.

Figure 4

Figure 4 (a) repeated detection of 5 ppm NO2 using SWCNT deposited on Si wafer with natural oxide layer (SWCNT/SiO2 chemiresistor). Chemiresistor is exposed to NO2 for 5 min Water-enriched airflow (RH ~ 52%) is maintained for 5 min in each case before reverting to dry airflow. (b) Measurement of different concentrations of NO2 with 3-min exposures in each case followed by water-assisted (RH ~ 52%) recovery. A SWCNT/Al2O3 chemiresistor was used. Inset to (b) shows a response versus log concentration curve.

Figure 5

0

tR

NH3 on, RH<1%

-5

NH3 off, H2O on RH~75 %

-10 -15

H2O off RH < 1 %

100 * G/G0

-20 -25

a

50 ppm

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0

1000

2000

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-4 0

-8

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-5

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-15

-12

b 0

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1000

-20

0

500

2000

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3000

1500

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Figure 5 Detection of ammonia using a SWCNT/Al2O3 chemiresistor; the response is calculated as a percent increase in resistance. (a) Repeated detection of 50 ppm with waterassisted recovery time (ΔtR) of about 15 minutes when relative humidity (RH) is raised to ~ 75 %; (b) partial and very slow recovery in dry air; (c) detection of 5 ppm and 500 ppb using the same method. In all cases 5-minute exposure to ammonia used except for the detection of 500 ppb where a longer exposure time was used.