Facile, size-controlled deposition of highly dispersed gold nanoparticles on nitrogen carbon nanotubes for hydrogen sensing

Sensors and Actuators B 160 (2011) 1034–1042

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Facile, size-controlled deposition of highly dispersed gold nanoparticles on nitrogen carbon nanotubes for hydrogen sensing Abu Z. Sadek a,∗ , Vipul Bansal a,∗ , Dougal G. McCulloch a , Paul G. Spizzirri b , Kay Latham a , Desmond W.M. Lau a , Zheng Hu d , Kourosh Kalantar-zadeh c a

School of Applied Sciences, RMIT University, Melbourne VIC 3001, Australia School of Physics, The University of Melbourne, Melbourne VIC 3010, Australia c School of Electrical and Computer Engineering, RMIT University, VIC 3001, Australia d Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China b

a r t i c l e

i n f o

Article history: Received 2 May 2011 Received in revised form 8 September 2011 Accepted 8 September 2011 Available online 14 September 2011 Keywords: Nitrogen carbon nanotubes (NCNTs) Gold Nanoparticles Hydrogen sensors

a b s t r a c t Highly dispersed gold nanoparticles (AuNPs) supported on nitrogen-doped multiwalled carbon nanotubes (NCNTs) were synthesized using an electrochemical method. Size of AuNPs and their dispersion profile were tuned in a facile manner by controlling the applied potential and deposition time. The structure of the films was characterized using SEM, HRTEM, XRD and Raman spectroscopy. Electron microscopy revealed that the nanoparticles were deposited homogeneously on the outer surface of the NCNTs in high density. The average dimensions of the AuNPs could be tuned by varying the applied voltage and the duration of the electrochemical process. The XRD patterns confirmed the presence of metallic fcc gold along with disordered graphitic phases. The Raman spectroscopy showed that NCNTs possess 3–5 at.% nitrogen with a heterogeneous distribution profile, which is consistent with nitrogen incorporation in both graphitic and pyridinic moieties within the carbon sp2 network of disordered NCNTs. SERS enhancement was also observed for AuNPs/NCNTs prepared at 10 V/20 min condition. Conductometric gas sensors were developed from the thin films of AuNPs/NCNTs nanocomposites and evaluated towards H2 gas. The sensors were exposed to different concentrations of H2 in synthetic air at room temperature. The highest sensitivity of 75% was measured towards 1% H2 when average size of AuNPs on NCNTs is about 4–6 nm. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Since their discovery, carbon nanotubes (CNTs) have been the focus of intensive research endeavours due to their novel electrical, mechanical and optical properties [1,2]. Their special geometry and unique properties offer potential applications in diverse fields, including hydrogen storage [3], chemo- and bio-sensors [4–5] field emission displays [6], catalyst supports [7], memory devices [8], nano-tweezers [9], and tips for scanning probe microscopes [10]. Properties of both single-walled (SWNTs) and multiwalled carbon nanotubes (MWNTs) depend significantly on their chirality, diameters and structural defects [11–12]. For CNTs, electrical conduction is mostly determined by the outermost shell [13], hence controlled incorporation of defects and/or metallic catalyst to this electrically active outermost shell can improve surface reactivity, thus enhancing the performance of CNTs-based devices. This is evident from the recent reports demonstrating CNTs-supported transition metal nanoparticles as excellent catalysts for various chemical reactions

∗ Corresponding authors. Tel.: +61 3 9925 5280; fax: +61 3 9925 2007. E-mail address: [email protected] (A.Z. Sadek). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.022

[14–15]. Additionally, these CNTs–metal nanocomposite materials have significant potential towards enhancing surface sensitivity for potential applications in sensing, light harvesting, and electronic nano-devices. Due to aforementioned reasons, controlled attachment of metal nanoparticles onto CNTs has been a subject of intense investigation in the past few years, with methods such as spontaneous deposition [16–17], electrochemical deposition [18–22], and metal decoration on chemically oxidized CNT [23–25] previously employed. It is however notable that in a number of these studies, either uniform distribution of metal nanoparticles in high density onto CNTs could not be achieved, or an activation process, typically involving chemical functionalization of either CNTs [26–27] or metal nanoparticles [28–30] was required to achieve uniform distribution of metal nanoparticles over CNTs. The surface modification of CNTs followed by physicochemical assembly of organic-modified nanoparticles onto CNTs is generally cumbersome and requires multi-step processes. For instance, in most cases, chemical functionalization of the CNT surface involves acidic treatment or functionalization with surfactant or polyelectrolytes [23,31], which can lead to degradation of their electrical, mechanical and optical properties [32]. Additionally, these chemical modification methods are not

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compatible with processes in micro-fabrication industry standards. Similarly, although electrochemical deposition of nanoparticles onto CNTs has been previously employed without surface modification [18–22], such methods typically lack uniform distribution of metal nanoparticles over CNTs surface. In context of our current study, it is notable that in the previous electrodeposition methods, the applied voltages were typically relatively small, which were less than 1 V. We have previously demonstrated that chemically active sites can be produced on a CNT surface via nitrogen doping during the synthesis process, thus forming nitrogen carbon nanotubes (NCNTs) [32,33]. With nitrogen doping, the electronic structure of the nanotubes is modified to include electron donor states near the conduction band edge [34], which subsequently enhances the surface reactivity of CNTs by introducing defects. Recently, Jiang et al. chemically modified the surfaces of nitrogen-doped, multiwalled carbon nanotubes (NCNTs) using acid treatment to anchor negatively charged gold nanoparticles (AuNPs) onto their surface through electrostatic interactions [23]. However, to the best of our knowledge, hitherto no study has been undertaken to employ electrochemical deposition of AuNPs on NCNTs without requiring surface modification. In the current study, we demonstrate for the first time, a facile electrochemical route for potential-controlled size-dependent deposition of highly dispersed AuNPs in high density onto NCNTs, without requiring any cumbersome reaction conditions such as surface modification, high temperature, acid treatment, organic solvents or a long reaction time. The presented electrodeposition process undertaken at relatively higher voltages of 5–20 V is low-cost and compatible with microfabrication industry standards. Finally H2 sensing properties of AuNPs on NCNTs were investigated.

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The conductometric sensors were fabricated with two platinum (Pt) electrodes sputtered on AuNPs/NCNT surface in 4 mm apart. The AuNPs/NCNT conductometric sensors were mounted in a test chamber made from Teflon, which was sealed in a quartz lid. The output resistance as a function of time across the conductometric sensor during gas exposure was measured using a multimeter (Keithley, 2001). A computerized multi-channel gas calibration system, with mass flow controllers, was used for exposing the sensor to different concentrations of H2 gases. The total flow rate was kept constant at 200 sccm and dry synthetic air was used as the reference gas. At room temperature, the baseline gas was maintained for a duration of 2 h to allow the device to stabilize. Subsequently, the device was exposed to sequences of different concentrations of H2 for several minutes. Concentration of H2 varied in the range of 0.06–1% balanced in synthetic air at constant gas flow of 200 sccm. A second pulse of 0.12% H2 (repetition) was utilized to confirm its short term repeatability. Scanning electron microscopy (SEM) characterization was carried out using a FEI Nova NanoSEM. High resolution transmission electron microscopy (HRTEM) imaging of samples were performed on a JEOL 2100F FEG TEM operating at 120 kV with a GIF spectrometer attached. An objective aperture was inserted to enhance contrast. A 5 eV slit around the zero loss peak was used to select elastically scattered electrons. X-ray diffraction (XRD) analysis was conducted on a Bruker D8 DISCOVER microdiffractometer fitted with a general area detector diffraction system (GADDS). Data was ˚ collected at room temperature using CuK␣ radiation ( = 1.54178 A) with a potential of 40 kV and a current of 40 mA, and filtered with a graphite monochromator in the parallel mode (175 mm collimator with 0.5 mm pinholes).

3. Results and discussion 2. Experimental

3.1. Structural characterization

NCNTs with a nitrogen content of 3–5% were synthesized at 650 ◦ C from a heterocyclic pyridine precursor and a dehydrogenated Fe–Co/␥-Al2 O3 bimetallic catalyst (1.01 mmol g−1 Fe, 2.01 mmol g−1 Co/␥-Al2 O3 ) using chemical vapour deposition (CVD) growth technique, as reported in our previous work [33]. The as-prepared NCNTs were flushed in 6 mol dm−3 NaOH and 6 mol dm−3 HCl aqueous solutions at 110 ◦ C for 4 h to remove Al2 O3 and metallic impurities. Afterwards, the purified NCNTs were thoroughly washed with deionized (DI) water until the pH value of the filtrate reached 7, followed by drying at 70 ◦ C. NCNTs were suspended in DI water at a concentration of 10 mg mL−1 . The suspensions were then placed in an ultrasonic bath for 10 min and visible agglomerates were allowed to precipitate from the suspension for 2 h. One droplet of the sample suspension was placed onto a cleaned glass slide (10 cm × 20 cm) having a gold strap on one side for conductive connection with the electrode. The samples were dried at 70 ◦ C for 30 min to obtain NCNT thin films on glass substrates, followed by placing the samples in a reaction flask containing 200 mL of 10−4 mol dm−3 aqueous HAuCl4 solution at room temperature. Electrodeposition of AuNPs was carried out by a conventional anode (platinum plate)/cathode (nanotube sample) system, where a DC voltage was applied between electrodes spaced 3 cm apart. After electrodeposition, samples were washed with DI water and dried in a stream of N2 gas. The average thickness of the AuNPs/NCNTs films was measured using a profilometer and found to be 900 ± 40 nm. Three different DC bias voltages (5, 10 and 20 V) were applied for 10 and 20 min. Negligible amounts of AuNPs electrodeposition was achieved at 5 V, therefore results relevant only to 10 and 20 V potentials are presented here.

SEM images of AuNPs electrodeposited on NCNTs at 10 V for 10 and 20 min are shown in Fig. 1a and b, while those electrodeposited at 20 V for 10 and 20 min are shown in Fig. 1c and d. Under electrodeposition conditions employed in this study, AuNPs were found to be uniformly distributed throughout the NCNTs surface in high density. At both 10 V and 20 V DC bias deposition potentials, the AuNPs coverage was found to increase with longer deposition duration (compare Fig. 1a with b, and Fig. 1c with d). The average diameter of AuNPs was found to be 4–6 nm after applying 10 V potential for 10 min (Fig. 1a), which increased to 10–18 nm after 20 min (Fig. 1b). Similarly, the average AuNPs diameter increased from 8 to 12 nm after 10 min of electrodeposition at 20 V (Fig. 1c) to 25–40 nm after 20 min (Fig. 1d). It is also notable that with the increase in the deposition potential, a concomitant increase in the particle size distribution is also observed. For instance, at 20 V electrodeposition for 20 min, larger AuNPs (∼25–40 nm) on the top surface, along with underlying smaller AuNPs (∼8–12 nm) in the less exposed areas were also observed (Fig. 1d). SEM results therefore clearly indicate that size-controlled decorations of AuNPs onto NCNTs can be feasibly achieved by simply controlling two electrodeposition parameters, viz. applied potential and deposition time, and increase in both these parameters results in concomitant increase in the particle size. Notably, in the previous electrodeposition studies, wherein typically lower DC bias voltages (<1 V) were employed along with non-doped CNTs (instead of NCNTs in our current work) were used, such high density coverage of CNTs with AuNPs as seen in our current work, could not be achieved [19,21,35]. Based on “firstprinciple” calculations, it has been recently proposed that on the surface of NCNTs, enhanced adsorption of Au and Pt atoms are

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Fig. 1. SEM images of gold nanoparticles electrodeposited on NCNTs at 10 V for (a) 10 min and (b) 20 min, and at 20 V for (c) 10 min and (d) 20 min.

expected to occur due to activation of nitrogen-neighbouring carbon atoms resulting from the large electron affinity of nitrogen [14]. We believe that the use of NCNTs instead of non-doped CNTs in our study provides significant advantage towards achieving high density coverage of highly dispersed AuNPs during electrodeposition, as observed from our investigations. Moreover, attempts to prepare highly dense metal nanoparticles decorated CNTs and/or other carbon substrates do not often result in size-similar and highly dispersed metal nanoparticles [36], which could be achieved by using NCNTs during electrodeposition in our study. The ability to control high density coverage of AuNPs onto NCNTs, without significant nanoparticle aggregation effects will be highly desirable for application of these materials in catalysis and sensing. Fig. 2 illustrates the HRTEM images of AuNPs decorated onto NCNTs as a function of electrodeposition time and potentials. HRTEM of pristine NCNTs confirmed that they are composed of 5–20 layers of rolled graphene (Fig. 2a). Previously, we have demonstrated using X-ray photoelectron spectroscopy (XPS) that in NCNTs used in this study, greater proportions of nitrogen atoms are pyridinic and the rest are graphitic [33]. It is known that pyridinic nitrogen sites take the ‘cavities’ or ‘edges’ configuration, which are responsible for the roughness and corrugation in the NCNTs [33]. Due to higher pyridinic nitrogen sites on NCNTs used in this study, the HRTEM images do not reveal the characteristic bamboo-like defective appearance expected from carbon nitride (CNx ) tubes, but rather corrugated and rough surfaces. The AuNPs are formed at the activated C sites, which gradually grew in dimensions with applied potential and electrodeposition time. HRTEM, however, did

not show any association between the morphology of NCNTs and sites of AuNPs deposition. The crystallinity of AuNPs electrodeposited onto NCNTs was further confirmed by XRD. XRD patterns for the NCNTs and those functionalised with gold (Au-NCNTs) for 10 and 20 min at 10 or 20 V are shown in Fig. 3. The XRD pattern of the NCNT reference sample is largely disordered but a broad peak with maximum at ∼26.2◦ 2 was observed, which matches closely with the (0 0 2) lattice planes of hexagonal graphite (ICDD no. [41-1487] – graphite2H), which is typical for XRD pattern of carbon nanotubes [37–38]. The (0 0 2) peak is shifted slightly to lower angle compared to the perfect graphite due to the increased spacing between the graphitic ˚ compared to 3.3 A˚ in graphite). The XRD patsheets (d0 0 2 = 3.4 A, terns for Au-NCNT samples also contained this broad, disordered carbon feature at 20–35◦ 2, along with significant reflections at 38.2, 44.4, and 64.6◦ 2 arising from face-centred cubic (fcc) metallic gold, which correspond to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively (ICDD no. [4-0784]). The fcc Au peaks start appearing in the sample prepared at 10 V for 10 min, and start resolving with increasing voltage and/or increasing time, due to the concomitant increase in AuNPs density. Finally, Au signature XRD peaks are most clearly resolved in the sample prepared at 20 V for 20 min, which corroborates well with SEM images, wherein an increasing amount of Au loading was observed with increasing voltage and time. NCNTs were further characterized in detail using Raman spectroscopy to obtain their structure (nitrogen distribution within the nanotubes, atomic phases: sp2 or sp3 etc.), and to look for plasmonic resonances associated with the electrodeposited AuNPs.

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Fig. 2. HRTEM images of NCNTs (a) before, and (b–d) after AuNPs electrodeposition at (b) 10 V for 10 min, (c) 10 V for 20 min and (d) 20 V for 10 min.

Since Raman spectroscopy has played an important role in characterizing carbon phases and nano-structures including graphene and nanotubes [39], it can provide important information relevant to current study. NCNTs were randomly dispersed on the substrates with Raman measurements performed at locations

Fig. 3. XRD patterns of NCNTs (a) before, and (b–d) after AuNPs electrodeposition at (b) 10 V for 10 min, (c) 10 V for 20 min and (d) 20 V for 20 min.

where (i) NCNTs were decorated with electrodeposited AuNPs (as shown in Fig. 1) and (ii) no gold was deposited (i.e., pristine NCNTs). Fig. 4a shows a representative first-order Raman spectrum of pristine NCNTs synthesized in this work with model component fitting profiles overlaid. All spectra show dominant Raman bands at ∼1350 cm−1 (D band) and ∼1580 cm−1 (G band), which are consistent with previous Raman measurements of nanotubes synthesized in the similar way [33]. The origins of the Raman bands is wellestablished for CNTs [40–41] with the G band associated with in-plane lattice vibrations (E2g ) of sp2 bonded carbon networks and the D band associated with defect-induced (i.e., disordered) sp2 bonded carbon [42]. Asymmetric tailing of the D band to lower frequencies is clearly evident in the measurements, however line broadening alone could not account for the observed profile, so additional terms were included in the fitting model. Closer inspection of the G band (Fig. 4b) also revealed the D band which is thought to arise from disorder induced symmetry breaking associated with the size of microscopic sp2 crystallites [43]. This feature is often obscured by the G band, particularly when using 2.41 eV excitation. Where this feature was not observed, its contribution to the fitting model was removed. Radial breathing modes (RBM’s) were not observed for these materials which is not uncommon for multi-walled, nanotube structures [44]. A representative second order Raman spectrum obtained from the same NCNTs is shown in Fig. 5 with Raman bands at ∼2700 cm−1 (D*) and ∼2730 cm−1 (D + G) evident. The D* band is an overtone (second order) of the D (disorder) peak while the peak at ∼2730 cm−1 is a combination mode [45]. As with the first order spectrum, asymmetric tailing of the D* band to lower frequencies is also evident in these measurements and curve fitting required the inclusion of at least one extra spectral term to obtain a high quality fit to the measured data.

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Fig. 4. (a) First-order Raman spectrum of pristine NCNTs synthesized in this work with the dominant transitions labelled, (b) magnified view of the G band showing the small shoulder assigned as the D band. The component bands used to model the experimental profiles are also shown.

Peak fitting of the first order spectrum revealed the spectral complexity associated with the asymmetric tail of the D band with multiple components required to adequately fit the measured data. Significantly, the fitted line-widths of the D and G bands are much larger than normally observed for graphitic phases, however this effect has previously been observed for NCNTs [46]. Other workers have also reported the need to use fitting models with a higher number of spectral components [47] and there is some agreement on the specific terms to include. Fitting models generally employ five bands with peaks located at ∼1620 cm−1 (D , 2 G ∼12 cm−1 ), ∼1580 cm−1 (G, 2 G ∼75 cm−1 ), ∼1480 cm−1 (D , 2 D ∼180 cm−1 ), ∼1350 cm−1 (D, 2 D ∼85 cm−1 ) and 1220 cm−1 (I, 2 I ∼200 cm−1 ) where the fitted line-widths (2 ) from the current work are also indicated. It should also be noted that the optimal fitting profile for the I line was Gaussian (rather than a

Fig. 5. Second-order Raman spectrum of pristine NCNTs synthesized in this work with the dominant overtones/combination bands labelled. The component bands used to model the experimental profiles are also shown.

mixed Gaussian/Lorentzian) which is consistent with the findings of Maldonado et al. who attributed this to heterogeneous distributions of spectroscopically active species associated with nitrogen incorporation [46]. This suggests that nitrogen takes on a heterogeneous distribution within the nanotube structure, possibly with multiple functionalities [46]. In the same work, it has been suggested that the I band is indirectly associated with nitrogen doping of CNTs resulting from symmetry modifications to the carbon sp2 network. No other bands modeled in this work displayed a Gaussian only profile. Generally, the results of fitting the first order spectrum with this extended model showed marked improvement although some of the D band asymmetric tail still remained poorly described. To further improve this fitting model, an additional term was added and located at ∼1270 cm−1 (I , 2 I ∼70 cm−1 ). This new term (I band) resolved the inadequacies of the fitting model in this spectral region. Peak fitting of the second order spectrum also revealed the complexity associated with the low frequency asymmetric tail of the D* band with at least one extra component required to adequately fit the measured data. At least three bands were used to fit the second order spectrum with peaks located at ∼2930 cm−1 (D + G, 2 D + G ∼160 cm−1 ), ∼2700 cm−1 (D*, 2D* ∼190 cm−1 ) and ∼2445 cm−1 (I , 2I ∼200 cm−1 ). Once again, the need for additional terms in these fitting models suggests that there are heterogeneous regions associated with the defected graphitic phases produced in these materials. An alternative description for the bands that give rise to the low frequency asymmetric profiles in both the first (I and I ) and second (I ) order spectra could involve defected graphite-like states. From the work of AlJishi and Dresselhaus [41], which presents the calculated phonon density of states for graphite, a doublet structure centred at around ∼2450 cm−1 was described as arising from the strong coupling of the longitudinal-acoustic (LA) branch and the optic branch along the M direction of the dispersion curve at a frequency of ∼1230 cm−1 . Along the K direction, the coupling occurs near ∼1270 cm−1 . Experimentally, these peaks have been observed [41] in the second order spectrum of graphite (i.e., overtones) because phonons with different wave vectors (q) and belonging to different branches of the dispersion curve can also contribute to the second-order Raman cross section. In this work, the band centred at ∼2450 cm−1 (I ) may represent an average contribution of these two peaks, especially given the large fitted linewidth observed. For the first-order spectrum, the (q ∼ 0) fundamental Raman selection rule would normally prevent direct observation of these transitions however, this selection rule is relaxed in finite sized domains allowing the participation of phonons near other high symmetry points [48]. For this reason, we propose that the I and I peaks arise from branch coupling along the K and M directions of the graphite dispersion curve and are therefore, necessary components of the fitting model employed. The integrated intensity ratio of the D to G bands (ID /IG ) is often used to assess the defect quantity in graphitic materials; [49] however, the presence of nitrogen incorporated into the graphitic structure of CNTs is known to result in increased D band contributions and band asymmetries associated with the structural disorder [33]. It has been suggested that the (ID /IG ) ratio can be used directly to determine the degree of nitrogen incorporation in the nanotube structure [33,46]. Average values of the (ID /IG ) ratio for the NCNTs fabricated in this work were determined to be ∼1.0. Using the calibration of Chen et al. [33], this ratio corresponds to an effective reaction temperature of ∼800 ◦ C and a total nitrogen incorporation level of ∼3 at.%. This synthesis temperature is significantly higher than the actual reaction temperature used in this work (i.e., 650 ◦ C) suggesting that this calibration model may not be robust. For comparison, we used a similar (ID /IG ) calibration curve reported by

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Fig. 6. Dynamic responses of the sensors based on AuNPs/NCNTs to different H2 concentrations in synthetic air at room temperature where Au nanoparticles were electrodeposited at (a) 10 V for 10 min, (b) 10 V for 20 min, (c) 20 V for 10 min and (d) 20 V for 20 min.

Maldonado et al. [46] and also found that the total nitrogen content is ∼3 at.%. Historically there has been some confusion around the determination of (ID /IG ) ratios with some workers using the ratio of peak heights while others have reported the ratio of peak areas [48]. The variance in this determination becomes apparent when the peak widths of the D and G bands are different or changing. If instead, we compare the area ratio (AD /AG ) for the samples prepared in this work with the calibration provided by Chen et al. [33], we obtain ratio values around 1.55 which corresponds to a synthesis temperature of 650 ◦ C and total nitrogen content of 5 at.%. These results are more consistent with the experimental conditions used in this work, which suggests that during determination of (ID /IG ) ratios, peak areas may provide a better estimate in comparison with peak intensities. Semiconducting and metallic (e.g., N-doped) carbon nanotubes usually exhibit strong electron–phonon interactions which manifest themselves in the Raman spectrum as peak asymmetries requiring fitting functions such as the Breit–Wigner–Fano (BWF) profile [50]. For our NCNT synthetic route, nitrogen doping appears to have been successful [33] yet there is no evidence of fitted peak asymmetries for any of the reported bands. Reasons for this may include the mechanism of nitrogen incorporation within the nanotube structure. During synthesis, nitrogen atoms are bonded to carbon as either pyridinic or graphitic nitrogen [33] with the graphitic phase preferred at higher synthesis temperatures (>750 ◦ C). At 650 ◦ C, which is the reaction temperature used in current work, equal amounts of graphitic and pyridinic nitrogen moieties are expected to be present [33]. While both graphitic and pyridinic nitrogen incorporation will result in increased graphitic lattice disorder, pyridinic nitrogen formation results in the breaking of the hexagonal symmetry of graphite. This may account for some of the nitrogen incorporation without the observation of metallic states. Additionally, to find evidence of surface enhancement arising from the local optical (resonant plasmonic) field of the metallic (Au) nanostructures [51], Raman measurements were performed in the

case of AuNPs/NCNTs over the same spectral range, as for pristine NCNTs. Changes to the vibrational spectroscopy of CNTs associated with the surface enhanced Raman spectroscopy (SERS) technique have been reported with the most notable difference often being an increase in the Raman signal intensity. These intensity changes, which are often associated with the RBMs (for SWNTs) [51], G and D* bands [52] are indicative of local field enhancements. In addition to intensity changes, the SERS effect can also result in line position shifts for the D* band [52] and splitting of the G band modes into multiple components [53]. These intensity (and sometimes peak width) changes ultimately influence parameters such as the (ID /IG ) ratio [54], which we have used in this work to identify the SERS effect. From the four AuNPs-decorated NCNTs preparations studied here (Fig. 1), only the 10 V deposition (20 min) showed a significant change to the (ID /IG ) ratio from 1.0 (NCNTs only) to 0.84 (AuNPs/NCNTs). This decrease indicates that the G band intensity has increased in the presence of the ∼10–18 nm AuNPs. No other spectral changes were observed for this sample. Since the other samples did not display significant SERS related changes (other than an increase in the background luminescence), this suggests that AuNPs were either not resonant with the excitation wavelength or they were not SERS active [55]. 3.2. H2 sensing results The sensors were fabricated using AuNPs decorated NCNT layers formed using different electrodeposition time and potentials. The dynamic responses of the sensors are shown in Fig. 6. The sensors were operated at room temperature and exposed to different concentrations of H2 diluted in synthetic air. The sensitivity is defined by the percentage variation in sensor resistance due to the interaction with H2 gas: S(%) =

(Rair − RH2 ) × 100 Rair

(1)

where Rair is the sensor resistance in synthetic air and RH2 is the sensor resistance in H2 gas. Sensitivity variation to different H2

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transferred from the nanoparticles into the NCNTs and this process increases the electrical conductivity of the sensor. 4. Conclusions

Fig. 7. Sensitivity (%) vs H2 gas concentrations (%) at room temperature.

concentrations is shown in Fig. 7. Sensitivity increases non-linearly with the increase of H2 concentrations for all four sensors. It was found that the performance of the sensors depends on the diameters of the AuNPs on NCNT matrix. With the increase of average AuNPs size from 5 to 30 nm, coverage of AuNPs on NCNT matrix increases which is evident by the reduction of film baseline resistivity from 1.18 to 0.2 -cm in synthetic air. It was observed that with exposure to 1% hydrogen concentration, the sensitivity decreased when the average size of AuNPs increased as less surface to volume ratios for both NCNTs and AuNPs are available to react with excess hydrogen molecules. The highest sensitivity, fastest response and fastest recovery were all measured for the smallest size AuNPs dispersed NCNT sensor (electrodeposited at 10 V for 10 min – Fig. 6a) when exposed hydrogen concentration is 0.1–1%. The highest response was measured to be 75% towards 1% H2 when average size of AuNPs on NCNTs is 4–6 nm with response and recovery time of 15 and 20 s, respectively. To verify the repeatability in sensor responses, two identical H2 pulses (0.12%) were used in each gas sequence applied to the sensors. The magnitude of the second resistance change was almost identical for the smaller size AuNPs dispersed NCNT sensors (electrodeposited at 10 and 20 V for 10 min – Fig. 6a and c), whereas for the larger size AuNPs dispersed NCNT sensors (electrodeposited at 10 and 20 V for 20 min – Fig. 6b and d), a slight increase in sensitivity (∼2%) was observed after the first pulse. At the end of pulse sequence, a stable baseline resistance was exhibited by the smaller size AuNPs dispersed NCNT sensors (Fig. 6a and c) but a drift towards higher resistance was observed for the larger size AuNPs dispersed NCNT sensors (Fig. 6b and d). The conductivity of the sensors based on AuNPs decorated NCNT films increased upon exposure to H2 in agreement with previous measurements [4]. As hydrogen is a reducing gas and contributes electrons to the NCNT matrix during interaction, an enhancement in conductivity is consistent with n-type material behaviour [56]. This behaviour is typical for NCNTs as nitrogen doping creates electron donor states which form near the Fermi level [33,57]. We have previously explained the responses of NCNT sensors towards hydrogen by a combination of physisorption and chemisorption of hydrogen on their surfaces [4]. In this work, the sensing responses shown in Fig. 6 are expected to consist largely of physisorption since there would be insufficient thermal energy at room temperature to promote chemisorption. However, it is still not well understood how physisorption of H2 on NCNT surface modify the conductivity. The catalytic AuNPs can dissociate H2 molecules to atomic hydrogen even in room temperature and thus promote chemisorption to some extent [58–61]. The atomic hydrogen diffuses into the nanoparticles where it forms metal hydride thereby reducing the work function of the AuNPs [62–63]. As a result, electrons are

Using a simple electrochemical method, highly dispersed AuNPs supported on NCNTs were synthesized. Notably, a high density decoration of AuNPs onto NCNTs was achieved without requiring any cumbersome reaction condition or surface modification of NCNTs. Electron microscopy revealed that average particle size, particle density and particle size distribution can be controlled by simple fine tuning of two reaction parameters, viz. electrodeposition time and applied potential. XRD patterns demonstrated the presence of fcc metallic gold phases along with amorphous graphitic phases in the electrodeposited samples. Detailed Raman spectroscopy revealed that the total N content in NCNTs is 3–5 at.%. The adopted peak fitting model showed that the pristine NCNTs were disordered with nanoscale sp2 crystallite phases which permitted the (q ∼ 0) selection rule to be relaxed in these finite sized domains. This resulted in the participation of phonons near other high symmetry points of the dispersion curve giving rise to an asymmetric tail (near the D and D* bands) in both the first and second order Raman spectra. The nitrogen distribution within the nanotubes was determined to be heterogeneous, consisting both graphitic and pyridinic moieties within the carbon sp2 network. Metallic NCNT states however, were not observed. Following attachment of AuNPs, SERS enhancement was observed for AuNPs-decorated NCNTs prepared at 10 V/20 min conditions, as evident from change in the (ID /IG ) ratio from 1.0 to 0.84 in the Raman spectrum. Novel conductometric gas sensors were developed from the thin films of AuNPs/NCNTs nanocomposites and evaluated towards H2 gas. The sensors were exposed to different concentrations of H2 in synthetic air at room temperature. The highest sensitivity of 75% was measured towards 1% H2 when average size of AuNPs on NCNTs is about 4–6 nm. Acknowledgements V.B. acknowledges the Australian Research Council (ARC), Commonwealth of Australia, for the award of an APD Fellowship and research support through the ARC Discovery (DP0988099, DP110105125), Linkage (LP100200859), and LIEF (LE0989615, LE110100097) grant schemes. References [1] P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Nanotube nanodevice, Science 278 (1997) 100–103. [2] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [3] S.M. Lee, Y.H. Lee, Hydrogen storage in single-walled carbon nanotubes, Applied Physics Letters 76 (2000) 2877–2879. [4] A.Z. Sadek, C. Zhang, Z. Hu, J.G. Partridge, D.G. McCulloch, W. Wlodarski, K. Kalantar-zadeh, Uniformly dispersed Pt–Ni nanoparticles on nitrogen-doped carbon nanotubes for hydrogen sensing, Journal of Physical Chemistry C 114 (2010) 238–242. [5] M. Penza, P. Aversa, G. Cassano, W. Wlodarski, K. Kalantar-Zadeh, Layered SAW gas sensor with single-walled carbon nanotube-based nanocomposite coating, Sensors and Actuators B: Chemical 127 (2007) 168–178. [6] W.A. Deheer, A. Chatelain, D. Ugarte, A carbon nanotube field-emission electron source, Science 270 (1995) 1179–1180. [7] X. Wang, W.Z. Li, Z.W. Chen, M. Waje, Y.S. Yan, Durability investigation of carbon nanotube as catalyst support for proton exchange membrane fuel cell, Journal of Power Sources 158 (2006) 154–159. [8] T. Rueckes, K. Kim, E. Joselevich, G.Y. Tseng, C.L. Cheung, C.M. Lieber, Carbon nanotube-based nonvolatile random access memory for molecular computing, Science 289 (2000) 94–97. [9] P. Kim, C.M. Lieber, Nanotube nanotweezers, Science 286 (1999) 2148–2150. [10] S.S. Wong, J.D. Harper, P.T. Lansbury, C.M. Lieber, Carbon nanotube tips: highresolution probes for imaging biological systems, Journal of the American Chemical Society 120 (1998) 603–604. [11] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Carbon Nanotubes: Synthesis, Structure, Properties and Applications, Springer, Berlin, 2001.

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Biographies Abu Z. Sadek received the M.E. degree in telecommunications engineering from the University of Melbourne, Melbourne, Australia, in 2002 and the Ph.D. degree in electronics and communications engineering from the Royal Melbourne Institute of Technology (RMIT), Melbourne, Australia, in 2008. He is a Postdoctoral Fellow in the School of Applied Sciences, RMIT University. His research interests include chemical sensors, solar cells, conducting polymers, advanced materials and nanotechnology. Vipul Bansal is currently an Australian Research Council (ARC) APD Fellow and Senior Lecturer at RMIT University, Melbourne, Australia. He was awarded a Ph.D. degree in 2007 from National Chemical Laboratory (NCL), India in the field of Nanobiotechnology under the supervision of Dr. Murali Sastry. Thereafter, in 2007, he joined Prof. Frank Caruso’s Group at the University of Melbourne as a Postdoctoral Research Fellow and investigated biocompatible polymer nanocapsules for drug-delivery applications. His current research interests at RMIT

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focus around tailored synthesis of advanced multifunctional nanomaterials using green chemistry routes, for catalysis, sensing, bio-imaging and nanomedicine applications.

synthetic methodology and structural characterization. At present the prime focus of her work concerns metal oxide and phosphonate networks for application in sensor development.

Dougal G. McCulloch received the B.Sc. and Ph.D. degrees in physics from the Royal Melbourne Institute of Technology (RMIT), Melbourne, Australia, in 1988 and 1994, respectively. His research interests include carbonaceous solids, thinfilm coating materials, transmission electron microscopy, and electron energy loss spectroscopy. He is the Director of the Microscopy and Microanalysis Facility, RMIT University.

Desmond W.M. Lau received the B.Sc. and Ph.D. degrees in physics from the Royal Melbourne Institute of Technology (RMIT), Melbourne, Australia, in 2005 and 2009, respectively. His research interests include Electron Microscopy and Carbon materials. He is a Research Fellow in the School of Physics, University of Melbourne.

Paul G. Spizzirri completed his undergraduate degree in Chemistry and Biochemistry at Swinburne University of Technology after which, he undertook a Ph.D. in the school of Physical Chemistry at the University of Melbourne. Dr. Spizzirri graduated in 2007 and his research interests include micro-spectroscopy of the solid-state and biological systems including Raman (electronic and vibrational) and (confocal) photoluminescence spectroscopies. Kay Latham received a Ph.D. from the University of Wolverhampton in 1996, and a PGCE from Sidney Sussex College, University of Cambridge, in 1996. She is a charted chemist and member of both the Royal Society of Chemistry and the Royal Australian Chemical Institute. Kay is currently senior lecturer in the School of Applied Sciences (Applied Chemistry) at RMIT University. Her research interests involve many aspects of inorganic and materials chemistry, with a strong emphasis on

Zheng Hu earned his B.Sc. (1985) and Ph.D. (1991) degrees in physics from Nanjing University (NU). After 2-year postdoctoral research in Chemistry Department of NU, he assumed an associate professor position in 1993, professor position in 1999, and Cheung Kong Scholar in 2007. Hu has mainly worked in the research field of materials chemistry addressing on the synthesis and growth mechanism of a range of nanoscale materials such as carbon-based nanotubes and group III nitrides, the characterization and functionalization of these materials. Kourosh Kalantar-zadeh received the B.Sc. and M.Sc. degrees from the Sharif University of Technology, Tehran, Iran, and Tehran University, Tehran, and the Ph.D. degree from the Royal Melbourne Institute of Technology (RMIT), Melbourne, Australia. He is now an Associate Professor at the School of Electrical and Computer Engineering, RMIT University. His research interests include chemical and biochemical sensors, nanotechnology, microsystems, materials science, electronic circuits, and microfluidics.