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Uniformly Dispersed Pt-Ni Nanoparticles on Nitrogen-Doped Carbon Nanotubes for Hydrogen Sensing A. Z. Sadek,*,†,‡ C. Zhang,† Z. Hu,§ J. G. Partridge,‡ D. G. McCulloch,‡ W. Wlodarski,† and K. Kalantar-zadeh† School of Electrical and Computer Engineering, and School of Applied Sciences, Applied Physics, RMIT UniVersity, Melbourne, VIC 3001, Australia and Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: NoVember 19, 2009
Nitrogen-doped multiwall carbon nanotubes (NCNTs), having an average diameter of approximately 20 nm, were synthesized at 650 °C by chemical vapor deposition using a pyridine precursor. Pt-Ni alloyed nanoparticles with approximate diameter 3 nm and with different Pt to Ni molar ratios were deposited on the NCNTs by a microwave-polyol method. Electron microscopy revealed that the nanoparticles were deposited homogeneously on the outer surface of the NCNTs and were immobilized at active nitrogen sites. A dielectrophoresis technique was used to selectively align the Pt-Ni-coated NCNTs between metallic electrodes to form conductometric hydrogen gas sensors. Gas sensing measurements performed with different concentrations of hydrogen revealed that the sensor based upon Pt/NCNTs exhibited the fastest response and recovery and best sensitivity. The sensing mechanism in the Pt/NCNT sensors can be explained by a combination of responses from the nitrogen-induced defects and the supported Pt nanoparticles, with the latter providing significantly faster response and recovery. Introduction Due to their unique electrical, mechanical, and optical properties,1,2 carbon nanotubes (CNTs) have been investigated by researchers from a broad range of disciplines. These unique properties have already been used in applications including hydrogen storage,3,4 field emission displays,5 catalyst supports,6 and memory devices.7 CNTs have also been used in chemical and biosensors8,9 and demonstrated sensitivity to gas species such as H2,10–12 methane,13 oxygen,14 carbon dioxide,15 and ammonia.16 However, methods that enhance the reactivity of CNTs are required to produce higher sensitivity and develop commercial applications. The properties of single wall carbon nanotubes (SWNTs) and multiwall carbon nanotubes (MWNTs) are both dependent on their chirality, diameters, and structural defects.17 The electronic properties of MWNTs are not as easy to control as those of SWNTs; however, MWNTs have a much lower cost of synthesis in comparison to SWNTs. Synthesis methods for CNTs include arc discharge,18 laser ablation,19 and chemical vapor deposition (CVD).20 Of these methods, CVD is the most promising method for commercial production due to its comparatively mild deposition conditions, low cost, and mass production capability.21 The growth process usually involves heating a catalyst (typically to 550-1000 °C) in an atmosphere of carbon precursors, such as hydrocarbons and carbon monoxide. The adsorption of hydrogen on sp2-bonded carbon surfaces has been investigated by a number of research groups.22 This adsorption requires a local change in hybridization from sp2 to sp3 bonding in the carbon network.22 This adsorption has a large energy barrier associated with it. Incorporation of defects and/ * To whom correspondence should be addressed. E-mail:
[email protected]. † School of Electrical and Computer Engineering, RMIT University. ‡ School of Applied Sciences, Applied Physics, RMIT University. § Nanjing University.
or metallic catalysts in the outer shell can lower this energy barrier and improve the surface reactivity. Electrical conduction occurs mostly in the outer shell of MWNTs, so any surface reaction will alter the electrical conductivity.23 A further improvement in surface reactivity can be obtained if catalytic particles are deposited or incorporated onto CNTs. It has been reported that CNTs supporting transition metal nanoparticles exhibit excellent catalytic properties for various chemical reactions.24,25 In order to uniformly distribute these particles over the surface, activation of the CNT surface is required and typically involves acidic treatment. This chemical modification of CNTs can lead to degradation of their electrical properties.26 It has been shown that chemically active sites can be produced on a CNT surface via nitrogen doping simply and without significant degradation.26 It is well reported that the catalytic activity of Pt is higher than that of Ni. However, Pt is the most expensive catalyst and is also susceptible to CO poisoning. To reduce the device (sensor/fuel cells) production cost and chances of CO poisoning on the device surface, recent efforts have been devoted to using different Pt alloys.27,28 In this study, Pt-Ni alloys with different molar ratios were employed to investigate whether sensitivity could be maintained with a lower Pt consumption. Kim et al.29 have reported hydrogen storage of up to 2.8% in Ni nanoparticleloaded MWNTs, and this provided a further reason to explore the effect of Ni incorporation in CNT sensing devices. Nitrogen-doped multiwall carbon nanotubes (NCNTs) have recently been synthesized from a heterocyclic pyridine precursor using CVD growth.21,30,31 Pt-based nanoparticles can be deposited on these NCNTs at defects associated with the incorporated nitrogen, resulting in hybrid Pt/NCNTs.26 Pt-Ni alloyed nanoparticles have also been deposited onto NCNTs in a similar process.27 In this study, Pt-Ni/NCNTs have been assembled using dielectrophoresis on substrates with planar electrodes. The sensing characteristics of these samples were investigated as a
10.1021/jp908945x 2010 American Chemical Society Published on Web 12/11/2009
Nitrogen-Doped Carbon Nanotubes
Figure 1. (a) Top overview of the DEP platform; (b) the feature of a microtip pair; (c), (d) SEM images of the NCNTs on the DEP platform.
function of operating temperature and as a function of the Pt:Ni composition of the nanoparticles. The performance of the Pt-Ni/NCNT sensors was compared with that obtained from sensors assembled from commercial CVD grown MWNTs. Experimental Section NCNTs with nitrogen content of 3-5% were synthesized at 650 °C by CVD as described previously,30 and a dehydrogenated bimetallic catalyst of Fe-Co/γ-Al2O3 (1.01 mmol/g Fe-2.01 mmol/g Co/γ-Al2O3) was used in the synthesis process.31 The as-prepared NCNTs were flushed in 6 M NaOH and in 6 M HCl aqueous solution at 110 °C for 4 h to remove the Al2O3 and metals. Afterward, the purified NCNTs were thoroughly washed with distilled water until the pH value of the filtrate reached 7 and then were dried at 70 °C. Pt-Ni-alloyed nanoparticles with Pt:Ni molar ratios of 3:1 and 1:1 were deposited on the NCNTs by a microwave-polyol method.32 Initially, 20 mg of the NCNTs was placed in 50 mL of ethylene glycol (EG) and sonicated for 20 min. Separately, EG solutions of H2PtCl6 (7.5 mg Pt mL-1 EG) and Ni(CH3COO)2 (1.2 mg Ni mL-1 EG) were prepared and added to the NCNT suspension dropwise, and the combined solution was magnetically stirred for several hours before 2 mL of NaOH/ EG solution (2 mol L-1) was added into the mixture. After stirring for 10 min, the mixture was placed in a microwave oven (700 W) for 90 s and then filtered. The solid sample was washed several times in ethanol and finally vacuum-dried at room temperature. In this paper, we denote the resulting NCNTs as Pt3Ni1/NCNTs (75% Pt-25% Ni) and Pt1Ni1/NCNTs (50% Pt-50% Ni), according to the molar ratio of the Pt and Ni precursors. A monometallic catalyst of Pt/NCNTs was also prepared by the same method for comparison. The dielectrophoresis (DEP) platforms were fabricated on glass substrates. The microtip metallic electrode arrays used for the assembly of NCNTs are shown in Figure 1 and were produced using electron-beam evaporation and photolithography. The constituent metallic layers of Cr and Au were 50 and 150 nm in thickness, respectively. Each DEP platform contained eight electrode arrays, and each array consisted of 20 microtip electrode pairs. The microtips had a minimum gap distance of 20 µm. The electrode pair separation of 100 µm was chosen to
J. Phys. Chem. C, Vol. 114, No. 1, 2010 239 produce strong field and weak field regions and clearly distinguishable areas of assembled NCNTs and uniformly distributed NCNTs. The different NCNTs were suspended in deionized (DI) water at a concentration of approximately 10 mg/mL. The suspensions were then placed in an ultrasonic bath for 10 min and then left for 2 h in order to allow agglomerates to precipitate from the suspension. After a droplet of NCNT suspension was placed onto the DEP platforms, an ac potential (20 V peak-to-peak amplitude, frequency 5 kHz) was applied across the electrodes using a function generator (Etabor Electronics 8200) for 20 s. After this process, NCNTs formed connections between the electrodes. Finally, contact wires were bonded to the samples for operation as conductometric sensors. The sensors were mounted inside an enclosed cell having an approximate volume of 40 cm3. Four mass flow controllers (MFCs) were connected to form a single output that supplies gas to the cell. In this work, two channels of MFCs were employed: one for synthetic air and one for a low-concentration (1%), high-purity H2 gas balanced in synthetic air. The concentration of the gas was varied by changing the synthetic air to analyte gas ratio while maintaining a constant flow rate of 200 sccm. The sensor was exposed to a sequence of H2 gas pulses for a fixed period of time, and the cell was purged with synthetic air between each pulse to enable recovery of the sensor. The variation of sensor resistance was measured using a Keithley 2001 multimeter at an applied dc voltage of 50 mV. LabVIEWbased software controlled the experimental setup and received real-time measurement data. A planar microheater with dimensions of 25 × 25 mm2 was placed beneath the sensor. The microheater was fabricated on a sapphire substrate with a patterned platinum resistive element. A regulated dc power supply was connected to the heater to control the operating temperature of the sensor between the range of room temperature and 100 °C in increments of approximately 20 °C. A thermocouple provided a real-time measurement of the sensor surface temperature to an accuracy of 1 °C. Approximately 1 W power was required to maintain 50 °C operating temperature on the sensor surface, where Vdc ) 16 V and Iheater ) 0.06 A. Results Characterization. Figure 1c, d shows SEM images of the NCNTs supported on the DEP platform after application of the ac potential. The images clearly show nanotubes selectively assembled between the microtip electrode pairs due to the nonuniform electric field. All samples showed similar assembly properties after application of the ac potential. Higher magnification SEM images in Figure 2a, b, c, and d show the NCNTs, Pt1Ni1/NCNTs, Pt3Ni1/NCNTs, and Pt/ NCNTs between microtip electrode pairs, respectively. The accompanying insets show corresponding TEM images. Nanoparticles with diameters of 3.0-4.0 nm are seen homogeneously dispersed onto the NCNTs irrespective of their Pt-Ni composition. Further TEM analysis of these Pt-Ni nanoparticles can be found elsewhere.27,30 Figure 3 shows the XRD diffractograms of the (a) Pt/NCNTs, (b) Pt3Ni1/NCNTs, (c) Pt1Ni1/NCNTs, and (d) pristine NCNTs. The diffraction peak at 26.1° observed from all samples results from the (002) graphitic planes in the NCNTs. The three peaks between 30 and 80° from the Pt/NCNTs sample (a) can be indexed to face center cubic crystalline Pt (JCPDS-ICDD, card no. 04-802). The XRD peaks exhibited from the Pt3Ni1/NCNT sample (b) are similar to those of (a) but are shifted to slightly
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Figure 2. SEM images of (a) NCNTs, (b) Pt1Ni1/NCNTs, (c) Pt3Ni1/ NCNTs, and (d) Pt/NCNTs. Insets are the corresponding TEM images.
Figure 3. XRD diffractograms taken from NCNTs supporting nanoparticles of (a) Pt/NCNTs, (b) Pt3Ni1/NCNTs, (c) Pt1Ni1/NCNTs, and (d) NCNTs.
higher values of 2θ due to alloying and an associated reduction in the lattice constant.33 This trend is continued in the peaks from the Pt1Ni1/NCNT sample (c). The broadening of the peaks suggests that the crystallite size is decreasing as the Ni content in the nanoparticles is increased. Gas Sensing Results. The dynamic responses of the sensors based on CNTs, NCNTs, Pt1Ni1/NCNTs, Pt3Ni1/NCNTs, and Pt/NCNTs (at 50-70 °C operating temperature) to hydrogen gas are shown respectively in Figure 4a, b, c, d, and e. The sensor response is defined by
S ) I(Rair - Rgas)I*100/Rair%
(1)
where Rair is the sensor resistance in synthetic air and Rgas is the sensor resistance in H2 gas. The sensitivities to 0.06% H2 in synthetic air for each sensor at 60 °C are given in Table 1. The sensitivity of the NCNTs to hydrogen gas was not significantly higher than the sensitivity provided by the CVD grown MWNTs. The sensitivity of the NCNTs to hydrogen was, however, increased by inclusion of the metal catalyst. The
composition of the nanoparticles proved important in determining the performance of the sensors, and the sensitivity decreased when the Pt content of the nanoparticles decreased. The highest sensitivity, fastest response, and fastest recovery were all measured for the Pt/NCNT sensor indicating that the catalytic activity of the Pt nanoparticles was higher than that of the Ni nanoparticles. Two 0.06% H2 pulses were used in each sequence applied to the sensors to show the repeatability in their responses. The magnitude of the second resistance change was almost identical to the first (Figure 4) for all of the samples, except the Pt1Ni1/ NCNT sensor where a slight decrease in sensitivity (∼3%) was observed after the first pulse. After the pulse sequence was completed, a stable baseline resistance was exhibited by the CNT and NCNT sensors, but a drift toward lower resistance was observed for the Pt and Pt-Ni NCNT sensors. This reduced resistance in air must be caused by a relatively slow desorption rate of H2 from the surface of the catalytic nanoparticles. The sensors were tested in a range of temperatures up to 100 °C, and the optimum operating temperature for H2 sensing was found to lie between 50 and 70 °C. In this temperature range, the responses of the sensors to 0.06% H2 gas ranged from approximately 0.13% to 1.53%. The responses to the same concentration of hydrogen were lower (less than 0.05%) at room temperature, and the response and recovery were also slower. The resistance at the end of the pulse sequence recovered to within 2% of its initial baseline value for all sensors. At 100 °C, the speed of response was increased, but recovery to the baseline resistance did not occur and the response was not reproducible. The multiwalled NCNT sensors exhibit superior sensing performance to that reported for MWNTs-based sensors;11 however, their sensitivity is less compared to sensors based on SWNTs.34,35 For comparison, the performances of various CNTbased sensors are summarized as follows: Kumer et al.11 fabricated hydrogen sensors based on nanostructured Pt-functionalized MWNTs using a catalytic CVD method and reported a sensitivity of 6.5% for a H2 concentration of 4% at room temperature. The extrapolated sensitivity to a 0.06% concentration of H2 (∼0.1%) is lower than observed from our Pt-Ni/ NCNTs sensors (∼1.3%). Pd has been recognized as an effective functionalizing material for CNTs, and Sayago et al.36 reported 14% sensitivity toward 3% H2 in nitrogen carrier gas using Pdfunctionalized SWNTs sensors. The extrapolated sensitivity of their devices to 0.06% H2 was ∼0.28%. Kong et al.34 developed CVD grown thin films of Pd-decorated interconnected SWNTs for H2 sensing and achieved a high sensitivity of 26% for 0.04% H2 in air, while Sun et al.35 developed Pd-decorated SWNTbased H2 sensors on flexible polymer substrates (using a CVD technique) and achieved 53% sensitivity toward 0.1% H2. These results represent the best responses currently achieved. Discussion The conductivity of the MWNT-based sensor decreased upon exposure to hydrogen in agreement with previous measurements.10,11,34,37 As hydrogen is a reducing gas and contributes electrons to the MWNT, a reduction in conductivity is consistent with p-type behavior.38 In contrast, the conductivity of the NCNTs increased after hydrogen exposure indicating an n-type device. This behavior is typical for NCNTs as nitrogen doping creates electron donor states which form near the Fermi level.39,40 The sensing responses of CNT sensors have been explained by a combination of physisorption and chemisorption of hydrogen on their surfaces.41 Physisorption involves molecular
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Figure 4. Dynamic response of the (a) CNTs (b) NCNTs, (c) Pt1Ni1/NCNTs, (d) Pt3Ni1/NCNTs, and (e) Pt/NCNT sensor to different H2 gas concentrations at 60 °C.
TABLE 1: Responses of CNTs, NCNTs, Pt1Ni1/NCNTs, Pt3Ni1/NCNTs, and Pt/NCNT Sensor to 0.06% H2 Gas Concentrations at the 50-70° C Temperature Range samples
sensitivity (for 0.06% H2)
CNTs NCNTs Pt1Ni1/NCNTs Pt3Ni1/NCNTs Pt/NCNTs
0.13% 0.14% 0.38% 1.3% 1.53%
hydrogen adsorption on the CNTs, and chemisorption involves the adsorption of atomic hydrogen on the surface of CNTs.22,41 The response of the NCNTs in Figure 4b is expected to consist largely of physisorption since there would be insufficient thermal energy (at 60 °C) to promote chemisorption. The catalytic nanoparticles improve the performance of the sensors as they dissociate hydrogen molecules to atomic hydrogen and thus promote chemisorption.9,11,14,34,42 The atomic hydrogen diffuses
into the nanoparticles where it forms metal hydride thereby reducing the work function of the nanoparticles.43,44 As a result, electrons are transferred from the nanoparticles into the NCNTs, and this process increases the electrical conductivity of the sensor. In p-type CNTs with Pt or Pd nanoparticles supported on their surfaces, the electrical conductivity is reduced by hydrogen exposure after electron transfer.9,11,14,34 The density of the defects introduced into the NCNTs due to nitrogen doping is estimated to range from 2 to 10%.30 The electron microscopy revealed that the density of the adhered catalytic nanoparticles was significantly lower (see Figure 2). Therefore, all the NCNT-based devices have sensing responses that are limited by the adsorption/desorption rates associated with the nitrogen defects. The sensing responses shown in Figure 4b and e suggest that hydrogen adsorption and desorption to/ from the Pt takes place on a shorter time scale than from the nitrogen defects, in agreement with previous findings.11,34,45 The sensing responses in Figure 4c-e therefore result from a
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combination of responses (electron injection) from the nitrogen defects (slower) and the adhered nanoparticles (faster). Conclusion Pt-Ni nanoparticles with different Pt to Ni molar ratios and diameters of approximately 3 nm have been deposited on nitrogen-doped multiwall CNTs (of average diameter 20 nm). Electron microscopy revealed that the Pt-Ni alloyed nanoparticles were deposited on the outer surface of the multiwalled CNTs. The CNTs, NCNTs, and Pt-Ni/NCNTs were then aligned between gold electrodes using dielectrophoresis to form conductometric sensors. The highest sensitivity, fastest response, and fastest recovery were all measured for the Pt/NCNT sensor indicating that the catalytic activity of the Pt nanoparticles was higher than that of the Pt-Ni nanoparticles. At 60 °C, the sensitivities to 0.06% H2 gas were 0.14%, 0.38%, 1.3%, and 1.53% for NCNTs, Pt1Ni1/NCNTs, Pt3Ni1/NCNTs, and Pt/ NCNTs, respectively. The sensing mechanism in the Pt/NCNT sensors can be explained by a combination of responses from the nitrogen-induced defects and the supported Pt nanoparticles, with the latter providing significantly faster response and recovery. The results therefore suggest that if the density of the adhered Pt nanoparticles were increased, higher sensitivity would be achieved. References and Notes (1) Collins, P. G.; Zettl, A.; Bando, H.; Thess, A.; Smalley, R. E. Science 1997, 278, 100. (2) Iijima, S. Nature 1991, 354, 56. (3) Darkrim, F. L.; Malbrunot, P.; Tartaglia, G. P. Int. J. Hydrogen Energy 2002, 27, 193. (4) Lee, S. M.; Lee, Y. H. Appl. Phys. Lett. 2000, 76, 2877. (5) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512. (6) Wang, M. W.; Li, F. Y.; Peng, N. C. New Carbon Mater. 2002, 17, 75. (7) Rueckes, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C. L.; Lieber, C. M. Science 2000, 289, 94. (8) Li, J.; Lu, Y. J.; Ye, Q.; Cinke, M.; Han, J.; Meyyappan, M. Nano Lett. 2003, 3, 929. (9) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622. (10) Sayago, I.; Terrado, E.; Aleixandre, M.; Horrillo, M. C.; Fernandez, M. J.; Lozano, J.; Lafuente, E.; Maser, W. K.; Benito, A. M.; Martinez, M. T.; Gutierrez, J.; Munoz, E. Sens. Actuators, B 2007, 122, 75. (11) Kumar, M. K.; Ramaprabhu, S. J. Phys. Chem. B 2006, 110, 11291. (12) Penza, M.; Aversa, P.; Cassano, G.; Wlodarski, W.; Kalantar-Zadeh, K. Sens. Actuators, B 2007, 127, 168. (13) Li, Z. P.; Li, J. F.; Wu, X.; Shuang, S. M.; Dong, C.; Choi, M. M. F. Sens. Actuators, B 2009, 139, 453. (14) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801.
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