NANO LETTERS
Vanadium Oxide Sensing Layer Grown on Carbon Nanotubes by a New Atomic Layer Deposition Process
2008 Vol. 8, No. 12 4201-4204
Marc-Georg Willinger,†,‡ Giovanni Neri,§ Erwan Rauwel,† Anna Bonavita,§ Giuseppe Micali,§ and Nicola Pinna*,† Department of Chemistry, CICECO, UniVersity of AVeiro, 3810-193 AVeiro, Portugal, Fritz Haber Institute of the Max Planck Society, Department of Inorganic Chemistry, Faradayweg 4-6, 14195 Berlin, Germany, and Department of Industrial Chemistry and Materials Engineering, UniVersity of Messina, C/da Di Dio, 98100 Messina, Italy Received June 20, 2008; Revised Manuscript Received September 3, 2008
ABSTRACT A new atomic layer deposition (ALD) process was applied for the homogeneous coating of carbon nanotubes with vanadium oxide. It permits the coating of the inner and outer surface with a highly conformal film of controllable thickness and, hence, the production of high surface area hybrid materials at a so far unprecedented quality. The ALD-coated tubes are used as active component in gas-sensing devices. They show electric responses that are directly related to the peculiar structure, i.e., the p-n heterojunction formed between the support and the film.
The surface as interface between a material and its surrounding is the key to heterogeneous catalysis, gas sensing, and the design of multifunctional materials. The controlled manipulation of surfaces and the assembly of thin films on high surface area supports are prerequisites for a rational design of interfaces and the production of multifunctional materials. It allows a combination of desired properties of materials with effects arising from the reduced dimensionality. Further, the controlled assembly of low dimensional structures on well-defined high surface area supports enables to bridge the gap between surface science and “real world” systems, especially in the field of catalysis, where the influence of the supporting material is still a matter of debate.1 Atomic layer deposition (ALD) appears to be one of the most versatile techniques for the well-controlled deposition of thin films on complex-shaped supports.2-5 Indeed, it is already the privileged technique used in the semiconductor industry for the growth of high-κ dielectric materials (for example in DRAM trenches which are characterized by high aspect ratio). ALD is outstanding as it permits precise control of the thickness of the deposited films at the subnanometer level while preserving their high homogeneity and conformality independent of the complexity of the substrate. * Corresponding author: fax, (+351) 234370004; e-mail,
[email protected]. † University of Aveiro. ‡ Fritz Haber Institute of the Max Planck Society. § University of Messina. 10.1021/nl801785b CCC: $40.75 Published on Web 10/30/2008
2008 American Chemical Society
Obviously, carbon nanotubes (CNTs) are the most appealing objects in nanotechnology due to their peculiar electrical and mechanical properties and their high surface area and chemical stability. As such they are ideally suited as support for other materials or molecular species for applications ranging from biotechnology to catalysis. For example, the electrical properties of CNTs are very sensitive to the surrounding environment. The presence of some gaseous molecules, either donating or accepting electrons, causes an alteration of their conductivity.6-9 This property makes them suitable for integration in nanoscale conductivity-based devices for gas sensing.10-14 It is noteworthy that they operate at lower temperature than conventional MOS (metal oxide semiconductor) sensors. The functionalization of CNTs permits control or enhanced sensitivity and the selectivity toward specific molecules.13 However, the coating of CNTs with metal oxides of a well-defined and controllable thickness was not yet achieved. In this work we show that CNTs can be homogeneously coated on the outer and inner surfaces with a nanometric thick film of vanadium oxide at a so far unprecedented quality by a new chemical approach. Further, the high quality and conformality of the as deposited film will be demonstrated by using the coated tubes as active element in NO2 gas sensing devices. Recently, we introduced a new nonaqueous sol-gel approach15-18 for the ALD of metal oxides.19 With carboxylic acids as oxygen source instead of the more traditionally used ones (e.g., water, ozone, oxygen, etc.), it was shown that
Figure 1. (a) SEM and (b) TEM images of the CNTs coated by vanadium oxide (4.5 nm for (a) and (b) and 2 nm in (c)). The thickness of the film was abstracted from high-resolution TEM images by assistance of intensity profiles recorded across the tube walls and the film as shown (red rectangular region and inset in (c)).
Figure 2. (a) TEM image of two bamboo-like carbon nanotubes coated with 4.5 nm of vanadium oxide. (b) Elemental map of (a) showing the localization of vanadium. (c and d) Bright field image and elemental map of an open CNT showing the vanadium film on the outside and inside of the tube walls.
hafnia and titania can be grown from the respective metal alkoxide precursors down to temperatures as low as 50 °C.19 The surface reaction leading to the M-O-M bond formation is self-limited and takes place via an ester elimination condensation step. As such, this approach allows the deposition of metal oxide thin films that contain no impurities such as halides and only a very low amount of residual carbon.20 Hence, no post-treatment is required. In this work we extend the approach to the deposition of vanadium oxide on multiwalled CNTs. Briefly, the coating of the CNTs was done in an ALD deposition system working in exposure mode from vanadium n-propoxide and acetic acid at 200 °C.21 CNTs coated with vanadium oxide are shown in Figure 1. The scanning electron microscope (SEM) image recorded using back scattered electrons (Figure 1a) shows brighter contours in the inside and outside of the tube. These regions of enhanced intensity appear due to the stronger scattering of the heavier element (i.e., vanadium), thus providing evidence that the tubes are indeed coated with a material containing a higher atomic number. In the transmission electron microscopy (TEM)22 image (Figure 1b), the darker regions on the outer and inner walls of the CNTs correspond 4202
to the metal oxide layers deposited by the process. The coating is uniform along the whole surface of the nanotubes and presents approximately the same thickness in the inner and outer surface. The film thickness abstracted from TEM measurements on the coated tubes is in agreement with reflectometry measurements performed on silicon wafers coated during the same deposition experiments (i.e., 4 vs 4.5 nm, respectively).23 High-resolution TEM (Figure 1c) and electron diffraction experiments showed that the asdeposited films are amorphous and can be directly grown on the graphitic surface of the nitric acid treated CNTs. In order to analyze the elemental composition and to irrefutably prove the quality of the as deposited thin film, electron energy loss spectrometric techniques (EELS and EELS mapping) were performed in the TEM. A bright field image and a corresponding EELS elemental map using the V LII and LIII ionization edges are shown for different CNTs in Figure 2. Due to their morphology, the bamboo-like CNTs shown in Figure 2a can only be coated on the outside while their inner cavities remain inaccessible. The elemental map (Figure 2b) presents bright signals at the edge of the tubes and weaker ones across their surface. Intensity in that image arrives from electrons that have undergone an ionization of Nano Lett., Vol. 8, No. 12, 2008
Figure 4. Transient response of sensors pretreated in air at 150 °C: (a) uncoated CNTs; (b) CNTs coated by 4.5 nm (solid line) and 2 nm (dotted line) of vanadium oxide. Figure 3. EELS spectrum of vanadium oxide coated carbon nanotubes (red) and reference spectrum recorded from V2O4 (black). For comparison, the inset shows a series of EELS spectra recorded from standard binary vanadium oxide phases (VO, V2O3, V2O4, and V2O5) together with the spectrum recorded from the coated tubes (red).
a vanadium 2p electron during their passage of the sample, thus reproducing the exact location of vanadium. Figure 2b therefore clearly proves that the coating is uniform and only a few nanometers thick. In panels c and d of Figure 2 a bright field TEM and the corresponding vanadium elemental map are shown for an open CNT. Here, the coating of the tube by vanadium oxide on the in- and outsides clearly becomes evident, underlining the interpretation of the above SEM investigations. A more detailed chemical analysis is provided by the EELS spectrum recorded from vanadium oxide coated CNTs over the energy region of the V 2p and O 1s ionization edges (Figure 3). It shows the vanadium LIII and LII edges centered at 518 and 525 eV, respectively and the oxygen K edge starting at around 530 eV. EELS spectra of various vanadium oxides are well-known in the literature.24 A comparison of the recorded spectrum with calculated and experimental data shows that the energetic position and the intensity ratio of the LIII and LII “white lines” as well as the structure of the oxygen K edge matches very well with the one of V2O4.24 Indeed, the close agreement speaks for the high quality of the coating and the low amount of carbon impurities, as they would also be manifested in a modulation of the oxygen K edge especially in the region of the edge onset (i.e., at around 530 eV). Sensor devices were fabricated by redispersing the CNTs with ethanol. The resulting suspension was deposited by drop coating on alumina substrates (3 mm × 6 mm) supplied with interdigitated Pt electrodes and heating elements.25 The sensor resistance data were collected in the four point mode. The electrical resistance as a function of the operating temperature of the sensor was examined by successive heating/cooling cycles from 25 to 150 °C in dry air. After a first conditioning process at 150 °C (during which desorption of water or residual ethanol used to disperse the sensing layer Nano Lett., Vol. 8, No. 12, 2008
takes place), the resistance of the device shows a fully reversible behavior (not shown). Multiwalled CNTs are know to show either metallic or semiconducting behavior.26 The properties of the uncoated CNT-based sensor have been evaluated in order to first clarify the electric behavior and response, S,27 of the CNTs to different NO2 concentrations.28 It was observed that the addition of 6.5 ppm of NO2 causes a decrease of the resistance (S ) 7) which can be reversed upon exposure to dry air (Figure 4a). Thus, in agreement with previous reports,26,29,30 the pure CNTs show a typical p-type semiconducting behavior. According to recent theoretical calculations,31 the electrical response of the as deposited CNTs film to NO2 can be explained in terms of its physical absorption. NO2 has an unpaired electron and is known as a strong oxidizer. Upon NO2 adsorption, a charge transfer is likely to occur from the CNTs to adsorbed NO2. This results in the formation of electronic levels within the gap of the semiconducting tubes. These states are located close to the Fermi level and give rise to an increased conductivity. The V2O4-coated CNTs, on the other hand, show an opposite sensing behavior. Indeed, their resistance increases upon exposure to NO2 (see Figure 4b), clearly indicating that V2O4coated CNTs show n-type semiconducting behavior. It is well-known that the thickness of the sensitive layer greatly influences the gas-sensing performance of thin film sensors.32 In ALD, the film thickness of the V2O4 layer can be very precisely controlled simply by varying the number of deposition cycles. Figure 4b shows the response of samples treated at 150 °C in air and having different V2O4 layer thickness (around 2 and 4.5 nm, respectively). Even though the effect is not very dramatic, a slightly higher response is observed for the thinner coating (S ) 18 vs S ) 15). Cross sensitivities toward potentially interfering gases such as CO, ethanol, and NH3 were also investigated. It was found that the response to these gases is negligible up to concentrations of 100 ppm, making this device highly selective for NO2. Electrical measurements on V2O4-CNTs showed that the resistance remains very low, regardless of the presence of a V2O4 layer, suggesting that the current mainly flows through the p-type nanotube structure. Therefore, it seems that the 4203
heterostructure formed by the combination of the n-type film on top of the p-type CNT (n-V2O4/p-CNTs) gives rise to the enhanced and even inversed response to NO2. p-n junctions have already been reported to increase the sensitivity in metal oxide sensors.33 Indeed, due to the p-n junction formed, the V2O4 film is depleted of electrons, thus amplifying the response to gases. Moreover, an enhancement of sensor response due to the formation of a p-n heterojunction was recently reported for the case of SnO2/CNT sensors.34,35 In such hybrid materials, two different depletion layers (and associated potential barriers) have been postulated to coexist. The first depletion layer is located at the surface of the metal oxide film while the second is formed at the interface between the CNT and the metal oxide film. Comparing the observed response of the hybrid structure with those observed for the isolated materials (vanadium oxide36 and CNTs) and taking into account that the baseline resistivity remains almost unaffected by the coating, it can be concluded that the adsorption of NO2 onto the metal oxide modifies the depletion layers. In conclusion, this Letter reports on the application of a new atomic layer deposition process for the uniform and wellcontrolled coating of the inner and outer surface of carbon nanotubes with a film of vanadium oxide. The as deposited amorphous films present unprecedented conformality and a chemical composition corresponding to V2O4. Gas sensor devices made of V2O4-coated carbon nanotubes show interesting sensing properties toward NO2. The response of the hybrid material was found to be related to the formation of a p-n heterojunction between film and support. Finally, the synthetic approach presented here can be applied to the homogeneous coating of CNTs with other metal oxides highlighting the potential of the method. Acknowledgment. We thank Dr. Dangsheng Su and Rosa Arrigo from the Fritz Haber Institute of the Max Planck Society, Berlin, Germany, for supplying carbon nanotubes. Financial support from FCT project (PTDC/CTM/65667/2006), FAME network of excellence, and Marie Curie (MEIF-CT2006041632) is acknowledged. Supporting Information Available: Calibration curve of the V2O4-CNT sensor operated at 150 °C. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Weckhuysen, B. M.; Keller, D. E. Catal. Today 2003, 78 (1-4), 25– 46. (2) Ritala, M.; Leskela, M. In Handbook of Thin Film Materials; Nalwa, H. S., Ed.; Academic Press: San Diego, CA, 2002; Vol. 1, pp 103159. (3) Knez, M.; Nielsch, K.; Niinisto¨, L. AdV. Mater. 2007, 19 (21), 3403– 3419. (4) Puurunen, R. L. J. Appl. Phys. 2005, 97 (12), 121301–52. (5) Niinisto, L.; Ritala, M.; Leskela, M. Mater. Sci. Eng., B 1996, 41 (1), 23–29. (6) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622–625. (7) Ulbricht, H.; Moos, G.; Hertel, T. Surf. Sci. 2003, 532-535, 852– 856. (8) Santucci, S.; Picozzi, S.; Gregorio, F. D.; Lozzi, L. J. Chem. Phys. 2003, 119, 10904–10910. (9) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. Nanotechnology 2002, 13, 195–200. 4204
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NL801785B Nano Lett., Vol. 8, No. 12, 2008
