Vanadium oxide-porphyrin nanocomposites as gas sensor interfaces for probing low water content in ethanol

Sensors and Actuators B 146 (2010) 61–68

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Vanadium oxide-porphyrin nanocomposites as gas sensor interfaces for probing low water content in ethanol Ronaldo A. Timm a , Maria P.H. Falla b , Manuel F.G. Huila a , Henrique E.M. Peres b , Francisco J. Ramirez-Fernandez b , Koiti Araki a,∗ , Henrique E. Toma a a b

Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, Butanta, CEP: 05508-000, São Paulo, SP, Brazil Escola Politécnica, Universidade de São Paulo, Av. Prof. Luciano Gualberto, travessa 3 no. 380, Butantã, CEP: 05508-970, São Paulo, SP, Brazil

a r t i c l e

i n f o

Article history: Received 21 August 2009 Received in revised form 15 January 2010 Accepted 20 January 2010 Available online 2 February 2010 Keywords: Gas sensor Nanocomposite Vanadium pentoxide xerogel Pyridyl porphyrin

a b s t r a c t Vanadium pentoxide xerogels (VXG) incorporating meso(3- and 4-pyridyl)porphyrin cobalt(III) species coordinated to four [Ru(bipy)2 Cl]+ complexes were employed as gas sensing materials capable of detecting small amounts of water in commercial ethanol and fuel supplies. According to their X-ray diffraction data, the original VXG lamellar framework was maintained in the nanocomposite material, but the interlamellar distance increased from 11.7 to 15.2 Å, reflecting the intercalation of the porphyrin species into the vanadium pentoxide matrix. The films generated by direct deposition of the nanocomposite aqueous suspensions exhibited good electrical and electrochemical performance for application in resistive sensors. The analysis of water in ethanol and fuels was carried out successfully using an especially designed electric setup incorporating a laminar gas flow chamber and interdigitated gold electrodes coated with the nanocomposites. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Ethanol fuel is reaching strategic proportions in the increasing demand of renewable energy sources, as observed in Brazil. Accompanying this tendency, the development of new materials that can be used in fuel sensors is of great relevance, particularly because anhydrous ethanol is being mixed to commercial gasoline as anti-knocking agent, while is directly consumed in the hydrated form as ethanol fuel. In both cases, a critical contaminant or adulterant is water. Although there are many general purpose ethanol sensors based on SnO2 , ZnO, TiO2 or Fe2 O3 materials, they usually work at high temperatures (e.g. 450 ◦ C) and are not always suitable for applications such as those involving volatile fuels, because of the inherent risk of explosion [1,2]. In this sense, some attempts in order to develop ethanol or humidity sensors working at room temperature, based on vanadium oxide materials can be found in the literature, as for example using sputtered V2 O5 films [3], nanobelts and coated-nanobelts [1,2], nanotubes [4], metal-doped xerogels [5,6] and macroscopical V2 O5 -polymer composite fibers [7,8]. Among them, interesting results were reported for V2 O5 nanofibers for amine detection, but the conductance was also shown to be dependent on the relative humidity of the carrier gas [9].

∗ Corresponding author. Tel.: +55 11 30918513. E-mail address: [email protected] (K. Araki). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.01.045

Our preference to vanadium(V) oxide is related to its lamellar structure, and to the intrinsic electronic and electrochemical properties associated with intervalence electron transfer and ion transport [10–13]. As a matter of fact, the lamellar nature turns it possible to modulate the adsorption and conduction properties by intercalating suitable chemical species into the matrix [14]. However, according to the best of our knowledge there is no study focusing on the influence of the molecular topology on the morphology and properties of the resultant host–guest nanomaterials. In this sense, cationic supramolecular species based on cobalt(III) meso(3- and 4-pyridyl)porphyrins coordinated to four [Ru(bipy)2 Cl]+ complexes (Fig. 1) were considered good candidates for intercalation, since they have well defined structures and have already been employed in analytical applications as electrochemically active films in amperometric sensors for nitrite, ascorbate and sulfite ions [15]. We have observed that when polyvanadic acid is put together with such cobalt tetraruthenated porphyrin species, here denoted Co4TRPyP and Co3TRPyP, respectively, flocculation takes place rapidly, generating a stable composite material. The corresponding films exhibited sharp and reproducible conductivity response at room temperature, indicating the presence of water in ethanol fuels, in the 0–10% (v/v) range. A detailed structural, spectroscopic and electrochemical characterization was carried out for the new composite materials. In addition, an electric setup was especially designed for carrying out the analysis of ethanol fuels, including a gas line incorporating a laminar gas flow chamber containing a resistive

62

R.A. Timm et al. / Sensors and Actuators B 146 (2010) 61–68

Fig. 1. Wire frame structural representation of the cationic Co4TRPyP and Co3TRPyP species. The respective chloride salts were used for the preparation of the host–guest nanocomposites with VXG.

detector based on interdigitated gold electrodes coated with the nanocomposites. 2. Experimental 2.1. Materials and procedures The tetraruthenated porphyrin species, Co4TRPyP and Co3TRPyP, were prepared and characterized as previously reported [16,17]. Polyvanadic acid solutions (0.1 mol dm−3 ) were obtained from sodium metavanadate (Alpha) solution by cation exchange (Dowex 50W-X4, H+ form), as reported in the literature [11,18,19]. After several days, the initially orange solution gradually converted into a dark red V2 O5 ·nH2 O gel. The corresponding nanocomposites were prepared by transferring dropwise an excess of that gel into a stirred aqueous [CoTRPyP]Cl5 solution (1.0 mg/mL). Immediately a sparingly soluble amorphous precipitate was formed, but the suspension was kept under stirring for 24 h, at room temperature, to ensure the system reached the equilibrium condition. The material was then decanted and the solid washed at least three times with nanopure water, until no evidence of porphyrin or polyvanadic acid could be found in the supernatant solution. The flocculous material was kept suspended in water and reserved for further use. The films were prepared by transferring precise volumes of the suspension onto a solid substrate (e.g. platinum electrode) and allowing the solvent to evaporate, at room temperature. The analyses were consistent with the following elemental compositions for the Co4TRPyP/V2 O5 and Co3TRPyP/V2 O5 nanocomposites, here denoted Co4VXG and Co3VXG, respectively. Co4VXG, calcd. (found) for (C120 H88 N24 CoRu4 Cl4 )0.032 (V2 O5 )(H2 O)1.69 : C = 15.56 (15.75); H = 2.13 (2.13); N = 3.63 (3.58); and Co3VXG, calcd. (found) for (C120 H88 N24 CoRu4 Cl4 )0.045 (V2 O5 )(H2 O)2.05 : C = 19.79 (19.66); H = 2.46 (2.49); N = 4.62 (4.29). 2.2. Measurements UV–vis spectra were recorded with an ASD FieldSpec3 spectrophotometer. All electrochemical measurements were carried out in 0.1 mol dm−3 LiClO4 solution in HPLC grade CH3 CN (Aldrich), under nitrogen atmosphere, using an AUTOLAB PGSTAT30 potentiostat/galvanostat and a conventional threeelectrodes cell arrangement consisting of a platinum disk working electrode, a coiled platinum wire counter electrode and Ag/AgNO3 (0.010 mol dm−3 , in CH3 CN) reference electrode. X-ray diffrac-

tion (XDR) patterns were recorded on a Philips model XPERT MD instrument equipped with a Cu lamp ( = 1.5448 Å) and a Ni monochromator. Scanning probe microscopy (SPM) images were obtained with a Nanoscope IIIA Multimode AFM in the tapping mode, employing silicon cantilevers and operation amplitude set point equal to 1.3 V, at room temperature. The samples were prepared by transferring a micro-volume of the stock suspension onto high graded mica surface and allowing the solvent to evaporate, in a class 10,000 clean bench. The tapping mode AFM images were obtained with 512 points per line resolution and scan rate of 1 Hz. The scanning electron microscopy images were obtained in a Jeol JSM-7401 field emission instrument, with samples prepared on fluorinated tin oxide (FTO) glass, using the same procedure described above for AFM microscopy samples. 2.3. The gas sensor The gas sensor electrodes were prepared by depositing a thin Ni–Cr alloy adhesion layer and then 100 nm thick gold layer over 25 mm × 25 mm alumina or glass plates, in order to define four interdigitated 0.5 mm wide gold lines separated by 0.5 mm. Electrical contacts were performed using commercially available printed circuit board as connector. 1–3 ␮m thick Co3VXG and Co4VXG films were deposited on the interdigitated electrodes by drop casting, at room temperature, and used in the experiments. The body of the sensor was made with a brass plate in which a cavity was defined to fit the interdigitated electrode plate. The carrying and sample gas enters from the hole in the vertex of the pentagonal cavity and flows outward in direction to the electrodes. A plexiglass plate was set on the top, defining a chamber with smallest as possible dead-spaces (Fig. 2), thus minimizing the purging, sampling and response times, while keeping a laminar flow and high sensitivity. The scheme of the experimental setup is shown in Fig. 2 also. The electrical resistance at constant current (10 ␮A) and applied DC voltage (typically in the 20–25 V range) was monitored as a function of the time using a Hewlett Packard model 4156A semiconductor parameter analyzer, at room temperature. Nitrogen gas was used as carrier of the ethanol/water vapor in equilibrium (T = 25 ◦ C) with a liquid phase of known composition. In a typical routine analysis, the sensor chamber was purged with dry nitrogen until a stable background resistance was obtained and then the ethanol/water vapor was introduced (flow = 0.1 L/min) by switching the solenoid valves 1 and 2. After 30 s the valves were reversed and the chamber

R.A. Timm et al. / Sensors and Actuators B 146 (2010) 61–68

63

Fig. 2. A scheme of the gas sensor is shown in the left. A photo of the actual detector, showing a plate with four interdigitated gold electrodes in the laminar flow chamber and connections, is in the right.

purged again with pure nitrogen, conditioning the sensor for the next analysis. 3. Results and discussion 3.1. Elemental, spectroscopic and electrochemical characterization The elemental analyses were consistent with the following minimum formula for Co3VXG and Co4VXG: (Co3TRPyP)0.045 (V2 O5 )(H2 O)2.05 and (Co4TRPyP)0.032 (V2 O5 )(H2 O)1.69 , respectively. The electric charges of the cationic porphyrins are compensated by the internal mixed-valence distribution in vanadium(V/IV) oxide matrix and some partial deprotonation of adsorbed water molecules, as confirmed by the decrease of pH, below 2, during the intercalation reaction. A larger relative amount of porphyrin was found in the nanocomposite derived from Co3TRPyP as compared to the one derived from Co4TRPyP, consistent with its smaller size and larger charge density, allowing more effective electrostatic interaction and the formation of more stable composite materials. The UV–vis spectra of pure VXG film are dominated by a broad envelope at 400 nm, ascribed to O2− → V5+ charge-transfer transitions (Fig. 3) and V4+ → V5+ intervalence transitions in the near infrared region (typically at 1400 nm) [13,20–23]. In DMF solution, Co4TRPyP exhibits a sharp and strong absorption band assigned to the Soret band at 434 nm (log ε = 5.2) and Q bands at 546 (4.0) and 594 nm (3.0), in addition to a broad RuII (d␲) → bpy(p␲*) chargetransfer transition centered at 490 nm (4.6). The Co3TRPyP isomer exhibits a similar spectrum showing the Soret and Q bands at 431 (5.3), 548 (4.3) and 593 nm (3.8), while the RuII (d␲) → bpy(p␲*) charge-transfer transition is found at 490 nm (4.5) [24]. The band assigned to O2− → V5+ transition is blue shifted to 385 nm in the spectra of CoVXG. In addition, the Soret and Q0–1 bands of CoTRPyP are easily seen in the spectra at ∼440 and ∼547 nm, respectively, but the RuII (d␲) → bpy(p␲*) charge-transfer transition is masked by the other bands and appears as a subtle decrease in the slope of the absorption band envelope around 500 nm. The electrochemical behavior is also consistent with the presence of CoTRPyP and VXG in the composites. Typical cyclic voltammograms of FTO electrodes modified with (A) pure VXG, (B) Co3VXG and (C) Co4VXG are shown in Fig. 4. The voltammo-

gram of VXG is characterized by two pairs of waves (Fig. 4A) around −0.4 and 0.1 V vs. Ag/Ag+ ascribed to V5+ /V4+ redox couples, however the voltammetric pattern was shown to be strongly dependent on the aging of the polyvanadic acid stock solution [10]. In addition, the waves undergo progressive changes during the successive scans, as a consequence of the dynamic exchange of intercalated and free Li+ ions in the electrolyte solution [23]. The presence of CoTRPyP in CoVXG increases the number of electrochemically active species. In the −1.0 to +1.5 V range, the cobalt porphyrin center can be reduced (Co(III/II)P process, 1e− , ∼−0.35 V) or the peripheral ruthenium complexes can be oxidized (Ru(II/III) process, 4 × 1e− , ∼0.45 V). The Co(III/II)P process is evidenced by a sharper cathodic peak at −0.4 V and the Ru(III/II) process appears as a pair of waves at 0.5 V (Fig. 4B), superimposed to VXG waves. However, Co3VXG exhibits a relatively more intense pair of Ru(III/II) waves than Co4VXG, as expected for the much higher content of the tetraruthenated porphyrin guest molecules. In addition, that peak is more strongly affected by the number of scans, and the intensity of the sharp pre-peak observed in the first scan decreased and shifted rapidly to more positive potentials. Such a process should be related to structural relaxations promoted by the successive

Fig. 3. UV–vis spectra of VXG, Co3VXG and Co4VXG films on quartz substrate.

64

R.A. Timm et al. / Sensors and Actuators B 146 (2010) 61–68

Fig. 4. Successive CVs of (A) VXG, (B) Co4VXG and (C) Co3VXG modified glassy carbon electrode in acetonitrile, 0.10 mol dm−3 LiClO4 ; scan rate 10 mV/s.

Fig. 5. Schematic structures of the Co3VXG and Co4VXG host–guest materials and their respective X-ray diffractograms on glass substrate. The XRD of pure vanadium pentoxide xerogel (VXG) was included for comparison.

redox processes until reaching a steady state condition, as shown in Fig. 4C.

(Fig. 1). Thus, the intercalated CoTRPyP should be laying flat onto the VXG layers, even though the density of occupied interaction sites is lower than that in the standing-up orientation. The lack of the 0 0 2 peak is a characteristic of the VXG matrix [10,11,18,20,22,26]; however, the diffractograms of the composites, specially of Co3VXG, indicate a significant change in the lamellar organization upon intercalation of the tetra-cationic CoTRPyP species. The broad features at higher 2 are normally observed because an amorphous phase may be present in the material and/or because the glass support is contributing to the diffraction. Since relatively thin films were used in the XRD analysis, both hypotheses are plausible. The morphology of CoVXG was analyzed by scanning electron microscopy (SEM), as shown in Fig. 6. The starting V2 O5 ·H2 O xerogel exhibits relatively big features, similar to wrinkles (Fig. 6A), consistent with dehydrated gel-like materials. However, a closer inspection at the rift edges revealed smaller features consistent with stacked layers, as expected for a lamellar material like VXG (Fig. 6B). A dust particle can be found in the middle, and this spot was used for the alignment of the microscope. The morphology changes completely when vanadium pentoxide interacts with Co4TRPyP forming the Co4VXG composite. This is constituted by quite compact fused fibers generating much bigger scale-like structures (Fig. 6C). These structures can be better seen in places where the film is less densely packed, as shown in Fig. 6D. The morphology of the Co3VXG composite seems to be less densely packed but similar to that of Co4VXG, at lower magnification (Fig. 6E). However, it is constituted by randomly arranged long nanofibers, less than 50 nm wide, forming a structure similar to that found in non-woven tissue-non-tissue, as shown in Fig. 6F. These results were confirmed by AFM, as shown in Fig. 7. A more

3.2. XRD and MEV characterization VXG exhibited a series of diffraction peaks characteristic of lamellar materials ascribed to the 0 0 1 (7.5◦ ), 0 0 3 (23.4◦ ), 0 0 4 (31.4◦ ) and 0 0 5 (39.4◦ ) planes, in the range of 2 from 0◦ to 50◦ (Fig. 5). Its structure has been described as double layers of interconnected VO5 polyhedral units, displaying a characteristic thickness of 8.8 Å. The interlayer space is occupied by water molecules, giving a basal plane distance corresponding to about 11.5 Å for V2 O5 ·1.6H2 O, but can vary depending on the average number of water molecules in the structure [10–12,19,20,25,26]. In our case, that distance was estimated as being 11.7 Å, indicating a somewhat larger amount of water molecules in the interlayer space. The samples of Co3VXG and Co4VXG composites for XRD measurements were prepared by depositing some drops of the corresponding stock suspension on quartz plates and allowing to them to evaporate, at room temperature. As one can see in Fig. 5, the diffraction peaks are significantly broader than in pure VXG matrix, as expected for a less ordered lamellar structure. The 0 0 1 peak is well defined but shifted to lower 2 values (7.5 to 5.8◦ ), as expected for much larger basal plane distances (about 15.2 Å for Co3VXG and Co4VXG). Considering that the vanadium pentoxide layer has a thickness of about 8.8 Å, the interlayer distance was estimated as being 6.4 Å, consistent with the intercalation of supermolecular CoTRPyP into VXG interlamellar spaces. However, that value is much smaller than the molecular diameter estimated for the Co3TRPyP and Co4TRPyP species, of about 27 and 31 Å, respectively

R.A. Timm et al. / Sensors and Actuators B 146 (2010) 61–68

65

Fig. 6. Scanning electron micrographies of pure VXG (A and B), Co4VXG (C and D) and Co3VXG (E and F) nanocomposite materials, as thin films on FTO substrate. The samples were prepared by drop casting and dried overnight under vacuum.

compact structure was observed for Co4VXG while Co3VXG exhibited very long and well formed tape-like structures. Such structures were found all over the Co3VXG film, but can be more easily seen at the edges, as single isolated tapes (Fig. 7C and D). Thus, the change in the topology and charge density of the molecular species can influence very significantly the morphology of vanadium pentoxide host–guest composites. In fact, Co3VXG seems to have much higher surface area and greater number of channels for diffusion of ions and molecular species. Presumably, the structural differences observed for Co3VXG and Co4VXG should reflect in their distinct electrical conduction and interaction properties. 3.3. The gas sensor Vanadium pentoxide xerogel exhibits semiconductor properties associated with the presence of V(IV) sites in the V2 O5 lamellar matrix, responsible for n-doping. The excess of negative charge is neutralized by hydrated alkaline or alkaline earth metal cations, which carry some water molecules into the interlamellar space [11,19,20]. They are very important for the conduction properties and for the electrochemical activity of VXG in organic solvents

as well, such that the CV profile is strongly influenced by small amounts of water present in the electrolyte solution. The exchange of the intercalated metal cations by positively charged molecular species can be carried out directly in the VXG, or more conveniently by the reaction of polyvanadic acid and the appropriate molecular species in aqueous solution. Significant changes were promoted by the intercalation of tetra-cationic CoTRPyP species into VXG, introducing new electron transfer and redox sites which can couple with ion transport processes, e.g. of Li+ , in the matrix. However, in the dry material, ion mobility is dramatically diminished and conductivity can be dominated by the intrinsic electronic properties, which is strongly influenced by the adsorption of chemicals. Materials with large surface area and high density of interaction sites are required for the development of sensors with fast response and high sensitivity. This seems to be the case of CoVXG nanocomposites, such that molecules present in the vapor can bond to specific sites or displace weakly bound water molecules, for example interacting with OH groups via hydrogen bonding [10], changing the electric conductivity in a reversible way. Analogously to the mechanism proposed by Liu et al. [1] for vanadium oxide nanobelts coated with semiconductor oxides, one can consider that the electric conduc-

66

R.A. Timm et al. / Sensors and Actuators B 146 (2010) 61–68

Fig. 7. AFM images of Co3VXG (A) and Co4VXG (B) films on mica substrate in a more densely packed region and at the Co3VXG film edge (C and D), where isolated tape-like structures can be found everywhere.

tion proceeds along the V2 O5 layers, which are separated by the intercalated molecules. Thus, the electrons should travel through these molecular barriers to reach another conducting V2 O5 layer. The magnitude of such barriers is influenced by the presence of molecular adsorbates that change the space-charge layer, determining the specific electric response for the V2 O5 nanocomposites. Accordingly, resistance measurements were carried out as a function of time using interdigitated gold electrodes modified with Co3VXG or Co4VXG as detector. Typical plots obtained using an ethanol/water 19:1 solution as sample (5%, v/v of water) are shown in Fig. 8. One can see that the sensor response is fast, exhibiting a steep rise and rapid recovery at room temperature, leading to relatively sharp peaks. The very good reproducibility and repeatability indicate that the interactions are relatively fast and reversible, fulfilling the requirements for gas sensor application. The signal intensity increased steadily as a function of the amount of water in the ethanol/water mixtures in the 0–10% range, clearly indicating that the sensor is responding to the water content in the sample gas and not to ethanol. The sensitivity could be defined by the slope of the plot of R vs. percentage of water (v/v) in the water/ethanol mixtures, where R is the difference in resistance measured in the presence of dry nitrogen gas and a sample. However, this strategy is inappropriate for our purposes because R measurements are strongly influenced by the characteristics of the CoVXG films. Accordingly, a relative parameter such as R/R0 , where R0 is the maximum value of R (measured at the saturation point, i.e., about 20%, v/v of water in ethanol), would be more convenient for sensoring purposes. Plots of R/R0 as a function of the percentage of water (v/v) in water/ethanol mixtures, for the sensors based in Co3VXG and Co4VXG, are shown in Fig. 9. A saturation curve resembling a Langmuir plot was obtained in both cases, as expected for a property

determined by reversible adsorption/desorption of non-interacting water molecules in a fixed number of sites. As a consequence, the sensitivity of the interdigitated sensor decreases exponentially as a function of the water content in the EtOH/H2 O mixture, up to about 10%. So, the highest sensitivity is observed in the range of 95–100% of ethanol, corresponding to the typical water content in commercial products, such anhydrous ethanol used as anti-knocking additive for gasoline, and hydrated ethanol used as fuel. However, instead of dealing with a logarithmic curve, it is more convenient to use the linear plot of −ln(1 − R/R0 ) vs. per-

Fig. 8. Typical plot of resistance (i = 10 ␮A) as a function of time obtained for repetitive sampling using Co3VXG and Co4VXG interdigitated gold sensors, by switching from dry N2 gas (reference and purging) to an ethanol/water sample (5% H2 O, v/v in EtOH) and vice versa. Repetition time = 200 s.

R.A. Timm et al. / Sensors and Actuators B 146 (2010) 61–68

Fig. 9. Plot of R/R0 as a function of the percentage of water (v/v) in ethanol/water mixtures, for interdigitated Co3VXG and Co4VXG sensors. Inset: plot of −ln(1 − R/R0 ) vs. (%water).

centage of water, as shown in the inset of Fig. 9. A good linear correlation was found up to 10% of water, the limiting concentration above which the resistance remains more or less constant. In general, the Co3VXG sensor exhibited a higher sensitivity than the Co4VXG sensor, as would be expected from its larger surface area. 4. Conclusion Composite materials exhibiting distinct morphologies at the nanoscale were obtained by incorporating meso(3- and 4-pyridyl)porphyrinate cobalt(III) species coordinated to four [Ru(bipy)2 Cl]+ complexes into vanadium pentoxide (VXG). They can be readily deposited onto interdigitated gold electrodes and exhibit interesting electrical properties for application in resistive sensors for water, at room temperature. The sensitivity of the interdigitated sensors decreased exponentially as a function of the water content in the EtOH/H2 O mixture, consistent with reversible adsorption/desorption processes involving a defined number of interaction sites, according to the Langmuir equation. Such devices are appropriate for use at the lower water concentration range (0–10%, v/v) where a linear correlation was found, allowing the quantification of the amount of water in commercial anhydrous ethanol and hydrated ethanol used as fuel. Acknowledgements The Brazilian agencies “Conselho Nacional de Desenvolvimento Científico e Tecnológico” (CNPq) and “Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo” (FAPESP), The Millenium Institute of Complex Materials, and PETROBRAS for the financial support. We also would like to thanks Prof. Takuji Ogawa (Osaka University) who kindly supplied some of the interdigitated electrodes, to the “Laboratório de Filmes Finos do IFUSP” for the SPM facility (FAPESP Proc. # 95/5651-0), and Prof. Vera R.L. Constantino (IQ-USP) for the XRD measurements. References [1] J.F. Liu, X. Wang, Q. Peng, Y. Li, Preparation and gas sensing properties of vanadium oxide nanobelts coated with semiconductor oxides, Sens. Actuator B: Chem. 115 (1) (2006) 481–487. [2] J. Liu, X. Wang, Q. Peng, Y. Li, Vanadium pentoxide nanobelts: highly selective and stable ethanol sensor materials, Adv. Mater. 176 (6) (2005) 764–767.

67

[3] G. Micocci, A. Serra, A. Tepore, S. Capone, R. Rella, P. Siciliano, Properties of vanadium oxide thin films for ethanol sensor, J. Vac. Sci. Technol. A 15 (1) (1997) 34–38. [4] A.V. Grigorieva, A.B. Tarasov, E.A. Goodilin, S.M. Badalyan, M.N. Rumyantseva, A.M. Gaskov, A. Birkner, Y.D. Tretyakov, Sensor properties of vanadium oxide nanotubes, Mendeleev Commun. 18 (1) (2008) 6–7. [5] V. Bondarenka, S. Grebinskij, S. Mickevièius, H. Tvardauskas, Z. Martûnas, V. Volkov, G. Zakharova, Conductance versus humidity of vanadium–metal– oxygen layers deposited from gels, Phys. Stat. Solidi A 169 (2) (1998) 289–294. [6] V. Bondarenka, S. Grebinskij, S. Mickevièius, H. Tvardauskas, Z. Martûnas, V. Volkov, G. Zakharova, Humidity sensors based on H2 V11 TiO30.3 ·nH2 O xerogels, Sens. Actuator B: Chem. 55 (1) (1999) 60–64. [7] C.M. Leroy, M.-F. Achard, O. Babot, N. Steunou, P. Massé, J. Livage, L. Binet, N. Brun, R. Backov, Designing nanotextured vanadium oxide-based macroscopic fibers: application as alcoholic sensors, Chem. Mater. 19 (16) (2007) 3988–3999. [8] J. Dexmer, C.M. Leroy, L. Binet, V. Heresanu, P. Launois, N. Steunou, C. Coulon, J. Maquet, N. Brun, J. Livage, R. Backov, Vanadium oxide-PANI nanocompositebased macroscopic fibers: 1D alcohol sensors bearing enhanced toughness, Chem. Mater. 20 (17) (2008) 5541–5549. [9] I. Raible, M. Burghard, U. Schlecht, A. Yasuda, T. Vossmeyer, V2 O5 nanofibres: novel gas sensors with extremely high sensitivity and selectivity to amines, Sens. Actuator B: Chem. 106 (2) (2005) 730–735. [10] F.J. Anaissi, G.J.F. Demets, E.B. Alvarez, M.J. Politi, H.E. Toma, Long-term aging of vanadium(V) oxide xerogel precursor solutions: structural and electrochemical implications, Electrochim. Acta 47 (2001) 441–450. [11] J. Livage, M. Henry, C. Sanchez, Sol–gel chemistry of transition metal oxides, Prog. Solid State Chem. 18 (1988) 259–341. [12] V. Petkov, P.N. Trikalitis, E.S. Bozin, S.J.L. Billinge, T. Vogt, M.G. Kanatzidis, Structure of V2 O5 ·nH2 O xerogel solved by the atomic pair distribution function technique, J. Am. Chem. Soc. 124 (2002) 10157–10162. [13] B. Vigolo, C. Zakri, F. Nallet, J. Livage, C. Coulon, Detailed study of diluted V2 O5 suspensions, Langmuir 18 (2002) 9121–9132. [14] G.S. Zakharova, V.L. Volkov, Intercalation compounds based on vanadium(V) oxide xerogel, Russ. Chem. Rev. 72 (4) (2003) 311–325. [15] H.E. Toma, K. Araki, Exploring the supramolecular coordination chemistrybased approach for nanotechnology, Prog. Inorg. Chem. 56 (2009) 379–485. [16] K. Araki, L. Angnes, C.M.N. Azevedo, H.E. Toma, Electrochemistry of a tetraruthenated cobalt porphyrin and its use in modified electrodes as sensors of reducing analytes, J. Electroanal. Chem. 397 (1995) 205–210. [17] F.M. Engelmann, P. Losco, H. Winnischofer, K. Araki, H.E. Toma, Synthesis, electrochemistry, spectroscopy and photophysical properties of a series of meso-phenylpyridylporphyrins with one to four pyridyl rings coordinated to [Ru(bipy)2 Cl]+ groups, J. Porphyrins Phthalocyanines 6 (1) (2002) 33–42. [18] T. Nakato, T. Ise, Y. Sugahara, K. Kuroda, C. Kato, Preparation of intercalation compounds between V2 O5 gel and bipyridil metal complexes, Mater. Res. Bull. 26 (4) (1991) 309–315. [19] T. Yao, Y. Oka, N. Yamamoto, Layered structures of vanadium pentoxide gels, Mater. Res. Bull. 27 (1992) 669–675. [20] O. Durupthy, N. Steunou, T. Coradin, J. Maquet, C. Bonhomme, J. Livage, Influence of pH and ionic strength on vanadium(V) oxides formation. From V2 O5 ·nH2 O gels to crystalline NaV3 O8 ·1.5H2 O, J. Mater. Chem. 15 (2005) 1090–1098. [21] S.J. Park, J.S. Ha, Y.J. Chang, G.T. Kim, Time dependent evolution of vanadium pentoxide nanowires in sols, Chem. Phys. Lett. 390 (2004) 199–202. [22] F.J. Anaissi, F.M. Engelmann, K. Araki, H.E. Toma, Porphyrin doped vanadium pentoxide xerogel as electrode material, Solid State Sci. 5 (2003) 621–628. [23] F.J. Anaissi, G.J.F. Demets, H.E. Toma, Electrochemical conditioning of vanadium(V) pentoxide xerogel films, Electrochem. Commun. 1 (8) (1999) 332–335. [24] I. Mayer, H.E. Toma, K. Araki, Electrocatalysis on tetraruthenated nickel and cobalt porphyrins electrostatic assembled films, J. Electroanal. Chem. 590 (2) (2006) 111–119. [25] D.V. Pyoryshkov, A.V. Grigorieva, E.A. Goodilin, D.A. Semenenko, V.V. Volkov, K.A. Dembo, Y.D. Tretyakov, Effect of preparation conditions on the ordering of structural elements in vanadium pentoxide xerogels, Dokl. Chem. 406 (1) (2006) 9–13. [26] A.A. Bahgat, F.A. Ibrahim, M.M. El-Desoky, Giant extrinsic negative thermal expansion in vanadium pentoxide nanocrystalline films, Phys. Stat. Solidi A 203 (8) (2006) 1999–2006.

Biographies R.A. Timm got his PhD degree in Chemistry from Universidade de São Paulo, Brazil, in 2008. His area of interest is the development of inorganic nanomaterials for application in chemical sensors. M.P.H. Falla got his master degree in Analytical Chemistry (1998) and the PhD degree in material and metallurgical engineering (2005) from Universidade de São Paulo, Brazil, and has large experience in Chemical Technology, particularly petroleum and petrochemical processes. M.F. Gonzalez-Huila currently is a PhD student in Supramolecular Chemistry and Nanotechnology Laboratory at Universidade de São Paulo and his research interest is focused on the properties and applications of nanocomposites based on vanadium oxide xerogel and coordination compounds.

68

R.A. Timm et al. / Sensors and Actuators B 146 (2010) 61–68

H.E.M. Peres received his BS degree in Physics in 1990 and obtained his master (1996) and PhD (2003) degrees in Electrical Engineering from Escola PolitÈcnica of Universidade de São Paulo, Brazil. He is currently a researcher in the Integrated Sensors and Microsystems Group of Universidade de São Paulo, working on microelectronics materials and processing, MEMS development, and integrable sensors. F.J. Ramirez-Fernandez received the PhD degree in Electric Engineering from Universidade de São Paulo, Brazil (1986), was a postdoctoral fellow at Ecole Polytechnique Federale of Lausanne, Switzerland (2000) and currently is full professor of Universidade de São Paulo. His interests are focused in Electronic Systems and Controls, emphasizing Electronic Automation of Electric and Industrial Processes.

K. Araki got his PhD from Universidade de São Paulo in 1994, was a postdoctoral fellow at MIT (1995–1996) and Institute for Molecular Science at Okazaki (2003–2004), and is full professor of Chemistry of Universidade de São Paulo since 2006. His major interest is the development of nanotechnology by supramolecular coordination chemistry approach. H.E. Toma got his PhD from Universidade de São Paulo (1974), and was a postdoctoral fellow at Brookhaven National Labs (1976) and Caltech (1979), and currently is full professor of Chemistry of Universidade de São Paulo. Is a member of The Brazilian Academy of Science and Third World Academy of Science, and was the recipient or more than 15 national and international prizes including the TWAS Chemistry Prize, the Guggenheim Memorial Foundation Prize, the Fritz-Feigl Prize and Comenda Grã-Cruz da Ordem Nacional do Mérito Científico.