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The hydrolytic route to Co-porphyrin-doped SnO2 gas-sensing materials
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Inorganica Chimica Acta 361 (2008) 79–85 www.elsevier.com/locate/ica
The hydrolytic route to Co-porphyrin-doped SnO2 gas-sensing materials Chemical study of Co-porphyrin versus Sn(IV) oxide interactions Emanuela Callone a, Giovanni Carturan a,*, Marco Ischia a, Mauro Epifani b, Angiola Forleo b, Pietro Siciliano b, Roberto Paolesse c a
c
Department of Materials Engineering and Industrial Technologies, University of Trento, via Mesiano 77, 38050 Trento, Italy b Microelectronic and Microsystems Institute, I.M.M.-C.N.R. Lecce, Via per Monteroni, 73100 Lecce, Italy Department of Chemical Science and Technologies, University of Rome ‘‘Tor Vergata’’, via della Ricerca Scientifica, 00133 Rome, Italy Received 5 March 2007; received in revised form 6 June 2007; accepted 17 June 2007 Available online 30 June 2007
Abstract SnO2 and SnO2 + Co-porphyrin solids were prepared from SnCl4 in propanol and hydrolyzed to sol. Thermal behavior of samples obtained at 110 °C was studied in the 20–600 °C interval by thermal analysis coupled with mass spectrometry for identification of released species. The original samples maintain residual Sn–OR, Sn–OH and Sn–Cl groups up to 350 °C. The sample doped with 1% Co-porphyrin differs for a significant presence of residual Sn–Cl species, accounting for SnCl4 release in the 300–340 °C range. 119 Sn solid state NMR analysis reveals disordered SnO2 species in the sample heated at 250 °C and non-uniform SnO6 units in the SnO2 + Co-porphyrin sample at 110 °C, due to persistence of Sn–OR and Sn–OH groups. This complexity is lost at 250 °C. X-ray diffraction analysis confirms all these data. The sensing efficiency of these materials versus alcohols is ascribed to the presence of an open, incomplete SnO2 structure, which is more pronounced in the Co-porphyrin-doped sample. Ó 2007 Elsevier B.V. All rights reserved. Keywords: SnO2 + Co-porphyrin; Thermal behavior;
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Sn NMR; X-ray diffraction; Sensing efficiency
1. Introduction Among all the exploited materials, hybrid organic–inorganic sensing materials arouse interest because of the possible complementary features of every component, especially when they show a cooperative behaviour. A recent example is the production of sensors made of tin oxide and metalloporphyrins [1,2]. In particular the Co-porphyrin dispersed in a SnO2 matrix, obtained by the sol–gel approach appears as a intriguing system for alcohol detection [1,2]. However, in order to develop a chemical sensor, at least two different aspects must be investigated: synthesis and characterization of properties and behaviour of the new material and understanding of the sensing mechanism [3,4]. *
Corresponding author. Tel.: +39 0461 882453; fax: +39 0461 881945. E-mail address: [email protected] (G. Carturan).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.06.030
It is known that the Co-porphyrin + SnO2 system shows a sensor efficiency tightly related to its working temperature. Thus, besides the preparation and the exploitation of the material [1,2] a deep analysis of the chemical behaviour under thermal treatments appears of great importance in order to fully explain the mechanism of work. The combination of SnO2 sol-prepared from SnCl4 hydrolysis in propanol with Co-porphyrin yields gas-sensing films with maximum efficiency at 250 °C. Above this temperature, the possible decomposition of Co-porphyrin is invoked to account for sensitivity collapse. Spectrophotometric data for samples treated at various temperatures indicate disappearance of the Co-porphyrin signal above 320 °C. The SnO2 counterpart is also sensitive to thermal treatment and its morphology is affected by Co-porphyrin doping [5]. These objective facts suggest an intimate interaction between inorganic phase and metal–organic molecules, suggesting their cooperative effect in sensor applications.
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With the aim of obtaining some direct information on these aspects of hybrid SnO2–Co-porphyrin materials, we report here the pyrolysis behavior of SnO2, SnO2 + Coporphyrin, and Co-porphyrin alone. Experimental work was based on analysis of chemical species released during heating, using a gas-chromatographic–mass spectrometry apparatus coupled with a thermogravimetric analyzer. This approach has the advantages of immediate identification of chemical reactions leading to the release of defined species and of tenable comparisons of thermal events among different samples [6]. An additional aim concerns control of the preparation procedure as regards the presence of particular species or reaction variables. These parameters may effectively influence sensor features, but escape identification is based on ordinary instrumental methods. In particular, the change of SnO2 film morphology in the presence of 1% Co-porphyrin [5] attracted our attention, since it appeared as indirect evidence of the importance of trace species and minimal preparation changes in final material properties. Thus, the structure of the SnO2 samples, with or without Co-porphyrin doping, was checked by X-ray analysis of solids heated at 250 °C. The solid state NMR spectra of SnO2 samples prepared at 110 °C and 250 °C were also recorded, to reveal possible differences of chemical species and structural organization between pure and Co-porphyrin containing sample. 2. Experimental 2.1. Synthesis 5-(4-Carboxyphenyl),10,15,20 triphenylporphyrin-Co(II) was prepared according to the literature procedure [7–9]. The ‘‘SnO2’’ sample 1 was obtained, as previously described [5], from SnCl4 and propanol and subsequent H2O hydrolysis (Sn/H2O = 1/9 molar ratio); the solid was recovered after evaporation of solvent and H2O at 110 °C. The ‘‘SnO2–Co-porphyrin’’ sample 2 was prepared in the same experimental conditions by adding the Co-porphyrin complex in tetrahydrofuran to the sol suspension before solvent evaporation [5]. Both samples 1 and 2 were heated at 110 °C for 2 h and at 250 °C for 1 h. 2.2. Instrumentation Thermogravimetric (TG) and differential thermal analyses (DTA) were performed on a LabSys Setaram thermobalance in the 20–600 °C range with a heating rate of 10 °C min1. Thermal analyses were recorded working under 100 cm3 min1 He (99.99%) flux. Powdered samples (20–30 mg) were analyzed in alumina crucibles with aAl2O3 as reference. Gas-chromatographic analyses (GC) were carried out on an HRGC Carlo Erba Instruments chromatograph coupled with a quadrupole mass spectrometer as detector.
Chromatographic elutions were performed with He as carrier gas (15 kPa inlet pressure) and an OV1 (Mega) capillary column (25 m, 0.320 mm). A typical temperature program was: 30 °C for 5 min, followed by 5 °C min1 heating rate up to 200 °C, and maintained for 15 min. Gas phase sampling (volume of 0.1 cm3) was performed by means of a GR8 (Bimatic) thermostatted (130 °C) microvalve. Mass spectra analysis (MS) was carried out on a VGQMD-1000 Carlo Erba Instruments quadrupole mass spectrometer. Electron mass spectra (70 eV) were continuously recorded at a frequency of 1 scan s1 in the 3–600 amu range. Pyrolysis studies were performed with two types of instrumental interfaces [10]. Direct and continuous sampling of the gas phase evolved from the solid was obtained with the TG–MS coupling using a home-assembled transfer line built with a 5 m silica capillary column (0.19 mm, 250 °C), which connects the thermobalance to the mass spectrometer. This interface detects any gas phase released species throughout the pyrolysis process. TG–MS data were recorded as sequences of mass spectra in terms of total ion current (TIC) plots or any m/z ion current (IC) graphs versus time (i.e. temperature of pyrolysis). For the second interface, TG–GC–MS measurements identified species which arereleased contemporaneously; in this case, the transfer line was connected to the inlet port of the gaschromatographic microvalve, so that the chromatographic column eluted the gas mixture and the mass spectrometer operated as the detector. Gas phase samplings were carried out at the main mass losses during TG analysis. XRD spectra were collected on a Rigaku Dmax III diffractometer in the Bragg–Brentano configuration using Ka Cu radiation and a diffracted-beam graphite focusing monochromator, operating at 40 kV and 30 mA. Samples were analyzed after packing the powders in a glass holder and measuring intensity data by step scanning in the 2h range between 10° and 80° and 2–100° with a counting time of 5 s and a sampling interval of 0.05°. Mean crystallite sizes were calculated using the MAUD program, version 1.999, released by Dr. Luca Lutterotti (University of Trento) [11]. Solid State NMR experiments were carried out on a Bruker Avance 400 WB spectrometer operating at 149.1 MHz for 119Sn. Samples were packed in 4-mm diameter zirconia rotors. MAS experiment conditions, without side band suppression, were: 2.3 ls for 90° pulse, 30 s for recycle delay to produce fully relaxed spectra, and 10 kHz of rotating speed. The crystalline SnO2 peak, set at 604.3 ppm with respect to the primary tin shift scale reference of Sn(CH3)4, was used as secondary reference. 3. Results SnCl4 vigorously reacts with R–OH to give Sn-alkoxides and HCl; however, the reaction is not complete and the resulting Sn-alkoxides may preserve residual Sn–Cl groups
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in monomeric or dimeric Sn(IV) compounds. In this regard, the SnCl4–NaOEt reaction may be considered as a typical case: with the aim of obtaining Sn(OC2H5)4, the primary dimeric NaSn2(OC2H5)9 product must be isolated, then reacted with stoichiometric HCl to give Sn(OC2H5)4 [12]; Sn(O-i-Bu)4 in HO-i-Bu solvent affords dimeric species, owing to the easy expansion of Sn(IV) coordination [13]. Accordingly, in the absence of alkali propoxide, Snx(OPr)4xyCly may be formulated as the representative product of the reaction between SnCl4 and propanol. The subsequent reaction with H2O (Sn/H2O = 1/9 molar ratio) generates extensive hydrolysis of Sn–OR and Sn–Cl moieties. Since the reaction involves H2O nucleophilic attack on Sn(IV), the electronegativity differences of O (3.44) and Cl (3.16) with respect to Sn(IV) (1.96) do not favor preferential hydrolysis of residual Sn–Cl with respect to Sn–OR bonds. Moreover, hydrolysis is carried out in the presence of propanol (H2O/C3H7OH = 1 molar ratio), so that the system may be considered in equilibrium between hydrolysis and reverse esterification. From a chemical point of view, the solution used for coating sensing layers on alumina substrates can be treated as a system composed of various Sn(IV) oligomeric species with Sn–O and Sn–OH bridging and terminal bonds, as well as some residual Sn–OPr and Sn–Cl groups. The population of these species is ruled by the kinetic balance among various complex reactions, avoiding definite control of the composition. This picture is consistent with the previous FTIR study of the powders prepared by coating of SnCl4-derived sols [14]. Samples 1 and 2 are the solid products obtained from evaporation at 110 °C of volatile species, i.e., propanol, H2O, HCl and tetrahydrofuran (used as solvent of Co-porphyrin). These samples were studied by TG–DTA–GC–MS in the 20–600 °C temperature interval, which encompasses the range of thermal events of interest. The total weight loss of sample 1 was 43.6% as determined by TG analysis, which presents a main release of volatile species (37.6%) in the 100–330 °C interval, followed by a second minor event in the 330–600 °C interval (6%) (Fig. 1).
Fig. 1. Thermal analysis (TG, DTG and DTA) of sample 1 (20–600 °C).
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The DTG curve is characterized by a strong peak at 220 °C, a shoulder at 280 °C, and a modest band at 380 °C. This picture of pyrolysis behavior parallels the TIC curve shown in Fig. 2. Considering the m/z representative ions of H2O (18), HCl (36), C3H7OH (31) and CH3CH@CH2 (41), the release of these species was also determined and quantified as a function of thermal treatment (Fig. 2). The mass loss of CH3CH@CH2 is dominant at 220 °C, and GC–MS analysis indicates that it represents 83.7% of the gaseous products released at this temperature. Other species are H2O, HCl, traces of C3H7OC3H7, C3H7Cl and C3H6O (not shown in Fig. 2). According to these data, the following reactions: BSn–OC3 H7 ! BSn–OH þ CH3 –CH@CH2
ð1Þ
2 BSn–OC3 H7 ! BSn–O–SnB þ C3 H7 OC3 H7 BSn–OC3 H7 þ Cl–SnB ! BSn–O–SnB þ C3 H7 Cl
ð2Þ ð3Þ
2O–Sn–OC3 H7 ! BSn–O–SnB þ 2C3 H6 O þ H2 O
ð4Þ
may be proposed. In other matrices such as TiO2 and ZrO2 [15–17], reaction (1) takes place above 330 °C, whereas released H2O, HCl and, to a lesser extent C3H7OH, are produced by the following condensation reactions involving the Sn–OH groups forming at 220 °C [18]: Sn–OH þ HO–Sn ! Sn–O–Sn þ H2 O
ð5Þ
Sn–OH þ Cl–Sn ! Sn–O–Sn þ HCl Sn–OH þ PrO–Sn ! Sn–O–Sn þ n-PrOH
ð6Þ ð7Þ
This evidence is confirmed by the trend of H2O release (Fig. 2) with a maximum at 370 °C, and by the evolution of HCl, which shows appreciable peaks above 220 °C, i.e. after the occurrence of reaction (1). The thermal behaviour of the samples closely resembles that already observed for pure powders [14]. In particular, two main regions could be distinguished: the first is characterized by a high mass loss below about 250 °C and the melt of the precursor, followed by evolution of a huge amount of gases and by the cracking to carbon. In that case the exothermic peak at about 350 °C, which was not found in the present samples, could involve residual carbon oxidation. The Co-porphyrin complex is stable up to 250 °C. Above this temperature, TG–MS analysis indicates a number of subsequent events corresponding to thermal decomposition of the porphyrin ring: observed significant species are benzene, toluene, pyrrolidine and methyl-pyrrolidine. Sample 2 has a Co/Sn atomic ratio of 1/100 [5]. The Co(II) complex is added in tetrahydrofuran solution after SnCl4 reaction with C3H7OH and H2O addition. TG–MS analysis gives a total mass loss of 51.6% in the interval 20–600 °C, divided into two main events, as shown in Fig. 3: below and above 330 °C (40.2% and 11.4%, respectively). The TIC curve reported in Fig. 4 parallels the DTG profile of Fig. 3: the maximum release occurs in the 220– 330 °C interval, and a minor peak is observed at 380 °C. GC–MS analysis of gaseous products released at 230 °C confirms the occurrence of reactions (1)–(4), the formation
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Fig. 2. Thermogravimetric–mass spectrometric analysis of sample 1. Ion currents of selected ions used to monitor evolved species during pyrolysis process: m/z 31 (C3H7OH), m/z 18 (H2O), m/z 36 (HCl), m/z 41 (CH3CH@CH2), and TIC (total ion current) plots of evolved gas phase vs. temperature. Insets: mass spectra corresponding to main releases during thermal treatment.
Fig. 3. Thermal analysis (TG, DTG and DTA) of sample 2, (20–600 °C).
of CH3CH@CH2 being the most significant. Abundant H2O release, corresponding to reaction (5), occurs in the same interval, i.e., before the occurrence of (1). Moreover,
TG–MS analysis reveals an appreciable release of SnCl4 (18% of the total evolved gas phase) and HCl at 310 °C. Thus, sample 2 shows thermal behavior similar to that of sample 1, with the exception of the SnCl4 loss, which may be competitive with reaction (6). With reference to the first part of this section, the incomplete substitution of Sn–Cl with Sn–OC3H7 and the absence of control in the advancement of hydrolysis accounts for different concentrations of residual Sn–Cl, Sn–OC3H7 and Sn–OH groups between samples 1 and 2. The addition of 1% Co-porphyrin complex in tetrahydrofuran may modify the chemical equilibria and the rate of every single reaction. In other words, the general process is independent of the Co-porphyrin complex, but the kinetics of the reactions of SnCl4 with propanol and with water may be affected by addition of this complex in tetrahydrofurane solution. In particular, a greater presence of Sn–Cl may explain the release of SnCl4, like SiH4 or SiF4 releases in gels prepared from HSi(OR)3 or F–Si(OR)3 precursors [19,20].
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Fig. 4. Thermogravimetric–mass spectrometric analysis of sample 2. Ion currents of selected ions used to monitor evolved species during pyrolysis process: m/z 18 (H2O), m/z 36 (HCl), m/z 41 (CH3CH@CH2), m/z 225 (SnCl4), and TIC (total ion current) plots of evolved gas phase vs. temperature. Insets: mass spectra corresponding to main releases during thermal treatment.
The presence of the Co-porphyrin complex (sample 2) was revealed by GC–MS experiments. Above 350 °C, traces of benzene, toluene and pyrrolidines were detected as species typical of porphyrin-ligand degradation. 119 Sn solid state NMR spectra were recorded for sample 1 treated at 250 °C (Fig. 5), and for sample 2 treated at 110 °C and 250 °C (Fig. 6). At lower temperature, sample 2 shows four signals centered at 584.2, 609.9, 641.5 and 674.2 ppm; other small peaks are side bands. The main peak at 609.9 ppm may be assigned to Sn(IV) surrounded by six bridging oxygens, but it is characterized by a slight negative shift and noticeable broadening with respect to the SnO6 coordination proper of Cassiterite crystals (604.3 ppm). Peak broadening is attributable to structural variations and the absence of extended crystalline structure [21]. As previously reported for (TiO2)x(SnO2)1x solid solutions, a moderate negative shift from the typical SnO2 resonance at 604.3 ppm is expected, owing to variable numbers of the next nearest neighbours Ti4+; chemical shifts are mainly affected by the local environment, and are not sensitive to long-distance interferences [21,22].
Fig. 5. 119Sn solid state NMR spectra of sample 1 treated at 250 °C. Peaks marked with * are spinning side bands. Red dot line represents the Lorentzian fit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The above authors reported that the variability of signals and different NMR resonances are affected by the TiO2/SnO2 ratio, the most complex spectrum being
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observed at the highest ratio, in agreement with the increased variability of Sn(IV) coordination [21]. The same situation may be obtained bridging –SnO– units randomly mixed with terminal Sn–Cl, Sn–OC3H7 and Sn–OH bonds. The spectrum of sample 2 treated at 250 °C is simpler (Fig. 6b), evidencing the disappearance of Sn–OC3H7, Sn–OH and, in part, Sn–Cl bonds, by reactions (1) and (5)–(7), with an increase of bridging Sn–O–Sn bonds. This interpretation finds substantial confirmation in the spectrum of sample 1 treated at 250 °C (Fig. 5). As deduced from TG–MS data, at this temperature the sample has already lost most Sn–OC3H7 groups and condensates Sn– OH and Sn–Cl bonds. With respect to the spectrum of sample 2 in Fig. 6b, this spectrum presents a single resonance at 605.3 ppm, with a lorentzian profile corresponding to cassiterite in a single SnO6 environment. The broadening of the signal may be attributed to an average particle diameter of a few nanometers [23,24]. This interpretation of NMR spectra parallels the X-ray results of samples heated at 250 °C (Fig. 7). The detectable crystalline phase corresponds to Cassiterite, as confirmed by the 21-1250 JCPDS card. Peak profile analysis gives an average crystallite diameter of 3 ± 0.2 nm for sample 2, which corresponds to a more pronounced disordered structure, and of 6 ± 0.2 nm for sample 1. 4. Conclusions
Fig. 6. 119Sn solid state NMR spectra of sample 2 treated at 110 °C (top) and 250 °C (bottom). Peaks marked with * are spinning side bands. Red dot line represents the Lorentzian fit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. XRD spectra of samples 1 (top) and 2 (bottom) treated at 250 °C.
According to the aims of this work, the results lead to some considerations. The sensor activity of SnCl4-derived materials at 250 °C can be attributed to the cooperation of two distinct factors: on one hand, from our results, it appears that an open Sn oxide structure is present, resulting from the advancement of chemical equilibria concerning release of organic fragments, HCl, and H2O. From this point of view, sensitivity to alcohols indicates a specific reversible chemical interaction with not yet fully stabilized SnO2 particles. As for the enhanced sensitivity of the SnO2 doped with Co-porphyrin, it is noteworthy that this complex in tetrahydrofurane is added to the SnO2 sol, so that the occurrence of chemical reactions between the complex and reactive Sn–OC3H7 and Sn–Cl groups, still present on sol particle surface, may be proposed. In particular, the porphyrin-ligand–C6H4–COOH group is favorably esterified by these reactive moieties, leading to chemical anchoring between the inorganic SnO2 phase and the organometallic complex. In addition, the sol surface occupancy of Co-porphyrin molecules may hamper the advancement of Sn–Cl and Sn–OC3H7 hydrolysis and of subsequent condensation. In other words, our results emphasize the higher populations of reactive Sn–Cl and Sn–OC3H7 in the Co-porphyrin doped sample. Besides such a structural effect of the Co-porphyrin, i.e. the role in the Sn oxide hydrolysis–condensation kinetic, an electronic effect cannot be ruled out. In particular, the Co-porphyrin dispersed on the material surface may favor two different effects on the gas-sensing mechanism [25]. Referring to our previous methanol test [5], the
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Co-porphyrin may play a primary role in the catalysis of methanol oxidation, causing a subsequent charge transfer from the tin oxide support. This mechanism is known as electronic sensitization, and is related to the Fermi energy pinning due to the contact between the oxide support and the catalyst. Such mechanism is classically invoked in catalysis reactions involving particles of noble metal catalysts (e.g. Pd or Pt), but it is suggestive in our case to suppose an electronic interaction between the discrete porphyrin molecules and the underlying support. The other mechanism is related to spill-over over the oxide surface of the species created by the interaction of the catalyst with the gaseous analyte. This second mechanism is more hardly invoked in our case, since in general it is more directly related with the presence of catalyst nanoparticles. Acknowledgement We thank MURST-PNR 2001–2003 (FIRB art. 8) for funding. References [1] R. Paolesse, D. Monti, L. La Monica, M. Venanzi, A. Froiio, S. Nardis, C. Di Natale, E. Martinelli, A. D’Amico, Chem. Eur. J. 8 (2002) 2476. [2] F. Quaranta, R. Rella, P. Siciliano, S. Capone, C. Distante, M. Epifani, A. Taurino, Sens. Actuators B 84 (2002) 55. [3] W. Gopel, T.A. Jones, M. Kleitz, J. Lundstrom, T. Seiyama (Eds.), Sensors, vol. 2, Wiley-VCH, Weinheim, 1991. [4] J.W. Grate, Chem Rev. 100 (2000) 2627. [5] S. Nardis, D. Monti, C. Di Natale, A. D’Amico, P. Siciliano, A. Forleo, M. Epifani, A. Taurino, R. Rella, R. Paolesse, Sens. Actuators B 103 (2004) 339. [6] R. Campostrini, M. Ischia, L. Palmisano, J. Therm. Anal. Cal. 75 (2004) 13.
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[7] S. Matile, N. Berova, K. Nakanishi, S. Novkova, I. Philipova, B. Blagoev, J. Am. Chem. Soc. 117 (1995) 7021. [8] C.E. Kibbey, M.E. Meyerhoff, Anal. Chem. 65 (1993) 2189. [9] J.W. Buchler, in: D. Dolphin (Ed.), The Porphyrins, Academic Press, New York, 1978, p. 389. [10] R. Campostrini, M. Ischia, L. Palmisano, J. Therm. Anal. Cal. 71 (2003) 997. [11] L. Lutterotti, S. Matthies, H.R. Wenk, in: Proceeding of the Twelfth International Conference on Textures of Materials (ICOTOM-12), 1999, pp. 1599. [12] D.C. Bradley, E.V. Caldwell, W. Wardlaw, J. Chem. Soc., A (1957) 4475. [13] C.D. Chandler, J. Caruso, M.J. Hampden-Smith, Polyhedron 14 (1718) (1995) 2491. [14] M. Epifani, M. Alvisi, L. Mirenghi, G. Leo, P. Siciliano, L. Vasanelli, J. Am. Ceram. Soc. 84 (2001) 48. [15] G. Marcı`, L. Palmisano, A. Sclafani, A.M. Venezia, R. Campostrini, G. Carturan, C. Martin, V. Rives, G. Solana, J. Chem. Soc., Faraday Trans. 92 (1996) 819. [16] G. Facchin, G. Carturan, R. Campostrini, S. Gialanella, L. Lutterotti, L. Armelao, G. Marcı`, L. Palmisano, A. Sclafani, J. Sol–Gel Sci. Technol. 18 (2000) 29. [17] P. Egger, S. Dire`, M. Ischia, R. Campostrini, J. Therm. Anal. Cal. 81 (2005) 407. [18] R. Campostrini, M. Ischia, G. Carturan, S. Gialanella, L. Armelao, J. Sol–Gel Sci. Technol. 18 (2000) 61. [19] R. Campostrini, G. D’Andrea, G. Carturan, R. Ceccato, G.D. Soraru`, J. Mater. Chem. 6 (4) (1996) 585. [20] R. Campostrini, M. Ischia, G. Carturan, L. Armelao, J. Sol–Gel Sci. Technol. 23 (2002) 107. [21] S.K. Kulshreshtha, R. Sasikala, V. Sudarsan, J. Mater. Chem. 11 (2001) 930. [22] T.J. Bastow, L. Murgaski, M.E. Smith, H.J. Whitfield, Mater. Lett. 23 (1995) 117. [23] E. Callone, G. Carturan, M. Ischia, A. Sicurelli, J. Mater. Res. 21 (2006) 1726. [24] D.P. Tunstall, S. Patou, R.S. Liu, Y.H. Kao, Mater. Res. Bull. 34 (1999) 1513. [25] M.J. Madou, S.R. Morrison, Chemical Sensing with Solid State Devices, Academic Press Inc., New York, 1988.
Inorganica Chimica Acta 361 (2008) 79–85 www.elsevier.com/locate/ica
The hydrolytic route to Co-porphyrin-doped SnO2 gas-sensing materials Chemical study of Co-porphyrin versus Sn(IV) oxide interactions Emanuela Callone a, Giovanni Carturan a,*, Marco Ischia a, Mauro Epifani b, Angiola Forleo b, Pietro Siciliano b, Roberto Paolesse c a
c
Department of Materials Engineering and Industrial Technologies, University of Trento, via Mesiano 77, 38050 Trento, Italy b Microelectronic and Microsystems Institute, I.M.M.-C.N.R. Lecce, Via per Monteroni, 73100 Lecce, Italy Department of Chemical Science and Technologies, University of Rome ‘‘Tor Vergata’’, via della Ricerca Scientifica, 00133 Rome, Italy Received 5 March 2007; received in revised form 6 June 2007; accepted 17 June 2007 Available online 30 June 2007
Abstract SnO2 and SnO2 + Co-porphyrin solids were prepared from SnCl4 in propanol and hydrolyzed to sol. Thermal behavior of samples obtained at 110 °C was studied in the 20–600 °C interval by thermal analysis coupled with mass spectrometry for identification of released species. The original samples maintain residual Sn–OR, Sn–OH and Sn–Cl groups up to 350 °C. The sample doped with 1% Co-porphyrin differs for a significant presence of residual Sn–Cl species, accounting for SnCl4 release in the 300–340 °C range. 119 Sn solid state NMR analysis reveals disordered SnO2 species in the sample heated at 250 °C and non-uniform SnO6 units in the SnO2 + Co-porphyrin sample at 110 °C, due to persistence of Sn–OR and Sn–OH groups. This complexity is lost at 250 °C. X-ray diffraction analysis confirms all these data. The sensing efficiency of these materials versus alcohols is ascribed to the presence of an open, incomplete SnO2 structure, which is more pronounced in the Co-porphyrin-doped sample. Ó 2007 Elsevier B.V. All rights reserved. Keywords: SnO2 + Co-porphyrin; Thermal behavior;
119
Sn NMR; X-ray diffraction; Sensing efficiency
1. Introduction Among all the exploited materials, hybrid organic–inorganic sensing materials arouse interest because of the possible complementary features of every component, especially when they show a cooperative behaviour. A recent example is the production of sensors made of tin oxide and metalloporphyrins [1,2]. In particular the Co-porphyrin dispersed in a SnO2 matrix, obtained by the sol–gel approach appears as a intriguing system for alcohol detection [1,2]. However, in order to develop a chemical sensor, at least two different aspects must be investigated: synthesis and characterization of properties and behaviour of the new material and understanding of the sensing mechanism [3,4]. *
Corresponding author. Tel.: +39 0461 882453; fax: +39 0461 881945. E-mail address: [email protected] (G. Carturan).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.06.030
It is known that the Co-porphyrin + SnO2 system shows a sensor efficiency tightly related to its working temperature. Thus, besides the preparation and the exploitation of the material [1,2] a deep analysis of the chemical behaviour under thermal treatments appears of great importance in order to fully explain the mechanism of work. The combination of SnO2 sol-prepared from SnCl4 hydrolysis in propanol with Co-porphyrin yields gas-sensing films with maximum efficiency at 250 °C. Above this temperature, the possible decomposition of Co-porphyrin is invoked to account for sensitivity collapse. Spectrophotometric data for samples treated at various temperatures indicate disappearance of the Co-porphyrin signal above 320 °C. The SnO2 counterpart is also sensitive to thermal treatment and its morphology is affected by Co-porphyrin doping [5]. These objective facts suggest an intimate interaction between inorganic phase and metal–organic molecules, suggesting their cooperative effect in sensor applications.
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With the aim of obtaining some direct information on these aspects of hybrid SnO2–Co-porphyrin materials, we report here the pyrolysis behavior of SnO2, SnO2 + Coporphyrin, and Co-porphyrin alone. Experimental work was based on analysis of chemical species released during heating, using a gas-chromatographic–mass spectrometry apparatus coupled with a thermogravimetric analyzer. This approach has the advantages of immediate identification of chemical reactions leading to the release of defined species and of tenable comparisons of thermal events among different samples [6]. An additional aim concerns control of the preparation procedure as regards the presence of particular species or reaction variables. These parameters may effectively influence sensor features, but escape identification is based on ordinary instrumental methods. In particular, the change of SnO2 film morphology in the presence of 1% Co-porphyrin [5] attracted our attention, since it appeared as indirect evidence of the importance of trace species and minimal preparation changes in final material properties. Thus, the structure of the SnO2 samples, with or without Co-porphyrin doping, was checked by X-ray analysis of solids heated at 250 °C. The solid state NMR spectra of SnO2 samples prepared at 110 °C and 250 °C were also recorded, to reveal possible differences of chemical species and structural organization between pure and Co-porphyrin containing sample. 2. Experimental 2.1. Synthesis 5-(4-Carboxyphenyl),10,15,20 triphenylporphyrin-Co(II) was prepared according to the literature procedure [7–9]. The ‘‘SnO2’’ sample 1 was obtained, as previously described [5], from SnCl4 and propanol and subsequent H2O hydrolysis (Sn/H2O = 1/9 molar ratio); the solid was recovered after evaporation of solvent and H2O at 110 °C. The ‘‘SnO2–Co-porphyrin’’ sample 2 was prepared in the same experimental conditions by adding the Co-porphyrin complex in tetrahydrofuran to the sol suspension before solvent evaporation [5]. Both samples 1 and 2 were heated at 110 °C for 2 h and at 250 °C for 1 h. 2.2. Instrumentation Thermogravimetric (TG) and differential thermal analyses (DTA) were performed on a LabSys Setaram thermobalance in the 20–600 °C range with a heating rate of 10 °C min1. Thermal analyses were recorded working under 100 cm3 min1 He (99.99%) flux. Powdered samples (20–30 mg) were analyzed in alumina crucibles with aAl2O3 as reference. Gas-chromatographic analyses (GC) were carried out on an HRGC Carlo Erba Instruments chromatograph coupled with a quadrupole mass spectrometer as detector.
Chromatographic elutions were performed with He as carrier gas (15 kPa inlet pressure) and an OV1 (Mega) capillary column (25 m, 0.320 mm). A typical temperature program was: 30 °C for 5 min, followed by 5 °C min1 heating rate up to 200 °C, and maintained for 15 min. Gas phase sampling (volume of 0.1 cm3) was performed by means of a GR8 (Bimatic) thermostatted (130 °C) microvalve. Mass spectra analysis (MS) was carried out on a VGQMD-1000 Carlo Erba Instruments quadrupole mass spectrometer. Electron mass spectra (70 eV) were continuously recorded at a frequency of 1 scan s1 in the 3–600 amu range. Pyrolysis studies were performed with two types of instrumental interfaces [10]. Direct and continuous sampling of the gas phase evolved from the solid was obtained with the TG–MS coupling using a home-assembled transfer line built with a 5 m silica capillary column (0.19 mm, 250 °C), which connects the thermobalance to the mass spectrometer. This interface detects any gas phase released species throughout the pyrolysis process. TG–MS data were recorded as sequences of mass spectra in terms of total ion current (TIC) plots or any m/z ion current (IC) graphs versus time (i.e. temperature of pyrolysis). For the second interface, TG–GC–MS measurements identified species which arereleased contemporaneously; in this case, the transfer line was connected to the inlet port of the gaschromatographic microvalve, so that the chromatographic column eluted the gas mixture and the mass spectrometer operated as the detector. Gas phase samplings were carried out at the main mass losses during TG analysis. XRD spectra were collected on a Rigaku Dmax III diffractometer in the Bragg–Brentano configuration using Ka Cu radiation and a diffracted-beam graphite focusing monochromator, operating at 40 kV and 30 mA. Samples were analyzed after packing the powders in a glass holder and measuring intensity data by step scanning in the 2h range between 10° and 80° and 2–100° with a counting time of 5 s and a sampling interval of 0.05°. Mean crystallite sizes were calculated using the MAUD program, version 1.999, released by Dr. Luca Lutterotti (University of Trento) [11]. Solid State NMR experiments were carried out on a Bruker Avance 400 WB spectrometer operating at 149.1 MHz for 119Sn. Samples were packed in 4-mm diameter zirconia rotors. MAS experiment conditions, without side band suppression, were: 2.3 ls for 90° pulse, 30 s for recycle delay to produce fully relaxed spectra, and 10 kHz of rotating speed. The crystalline SnO2 peak, set at 604.3 ppm with respect to the primary tin shift scale reference of Sn(CH3)4, was used as secondary reference. 3. Results SnCl4 vigorously reacts with R–OH to give Sn-alkoxides and HCl; however, the reaction is not complete and the resulting Sn-alkoxides may preserve residual Sn–Cl groups
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in monomeric or dimeric Sn(IV) compounds. In this regard, the SnCl4–NaOEt reaction may be considered as a typical case: with the aim of obtaining Sn(OC2H5)4, the primary dimeric NaSn2(OC2H5)9 product must be isolated, then reacted with stoichiometric HCl to give Sn(OC2H5)4 [12]; Sn(O-i-Bu)4 in HO-i-Bu solvent affords dimeric species, owing to the easy expansion of Sn(IV) coordination [13]. Accordingly, in the absence of alkali propoxide, Snx(OPr)4xyCly may be formulated as the representative product of the reaction between SnCl4 and propanol. The subsequent reaction with H2O (Sn/H2O = 1/9 molar ratio) generates extensive hydrolysis of Sn–OR and Sn–Cl moieties. Since the reaction involves H2O nucleophilic attack on Sn(IV), the electronegativity differences of O (3.44) and Cl (3.16) with respect to Sn(IV) (1.96) do not favor preferential hydrolysis of residual Sn–Cl with respect to Sn–OR bonds. Moreover, hydrolysis is carried out in the presence of propanol (H2O/C3H7OH = 1 molar ratio), so that the system may be considered in equilibrium between hydrolysis and reverse esterification. From a chemical point of view, the solution used for coating sensing layers on alumina substrates can be treated as a system composed of various Sn(IV) oligomeric species with Sn–O and Sn–OH bridging and terminal bonds, as well as some residual Sn–OPr and Sn–Cl groups. The population of these species is ruled by the kinetic balance among various complex reactions, avoiding definite control of the composition. This picture is consistent with the previous FTIR study of the powders prepared by coating of SnCl4-derived sols [14]. Samples 1 and 2 are the solid products obtained from evaporation at 110 °C of volatile species, i.e., propanol, H2O, HCl and tetrahydrofuran (used as solvent of Co-porphyrin). These samples were studied by TG–DTA–GC–MS in the 20–600 °C temperature interval, which encompasses the range of thermal events of interest. The total weight loss of sample 1 was 43.6% as determined by TG analysis, which presents a main release of volatile species (37.6%) in the 100–330 °C interval, followed by a second minor event in the 330–600 °C interval (6%) (Fig. 1).
Fig. 1. Thermal analysis (TG, DTG and DTA) of sample 1 (20–600 °C).
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The DTG curve is characterized by a strong peak at 220 °C, a shoulder at 280 °C, and a modest band at 380 °C. This picture of pyrolysis behavior parallels the TIC curve shown in Fig. 2. Considering the m/z representative ions of H2O (18), HCl (36), C3H7OH (31) and CH3CH@CH2 (41), the release of these species was also determined and quantified as a function of thermal treatment (Fig. 2). The mass loss of CH3CH@CH2 is dominant at 220 °C, and GC–MS analysis indicates that it represents 83.7% of the gaseous products released at this temperature. Other species are H2O, HCl, traces of C3H7OC3H7, C3H7Cl and C3H6O (not shown in Fig. 2). According to these data, the following reactions: BSn–OC3 H7 ! BSn–OH þ CH3 –CH@CH2
ð1Þ
2 BSn–OC3 H7 ! BSn–O–SnB þ C3 H7 OC3 H7 BSn–OC3 H7 þ Cl–SnB ! BSn–O–SnB þ C3 H7 Cl
ð2Þ ð3Þ
2O–Sn–OC3 H7 ! BSn–O–SnB þ 2C3 H6 O þ H2 O
ð4Þ
may be proposed. In other matrices such as TiO2 and ZrO2 [15–17], reaction (1) takes place above 330 °C, whereas released H2O, HCl and, to a lesser extent C3H7OH, are produced by the following condensation reactions involving the Sn–OH groups forming at 220 °C [18]: Sn–OH þ HO–Sn ! Sn–O–Sn þ H2 O
ð5Þ
Sn–OH þ Cl–Sn ! Sn–O–Sn þ HCl Sn–OH þ PrO–Sn ! Sn–O–Sn þ n-PrOH
ð6Þ ð7Þ
This evidence is confirmed by the trend of H2O release (Fig. 2) with a maximum at 370 °C, and by the evolution of HCl, which shows appreciable peaks above 220 °C, i.e. after the occurrence of reaction (1). The thermal behaviour of the samples closely resembles that already observed for pure powders [14]. In particular, two main regions could be distinguished: the first is characterized by a high mass loss below about 250 °C and the melt of the precursor, followed by evolution of a huge amount of gases and by the cracking to carbon. In that case the exothermic peak at about 350 °C, which was not found in the present samples, could involve residual carbon oxidation. The Co-porphyrin complex is stable up to 250 °C. Above this temperature, TG–MS analysis indicates a number of subsequent events corresponding to thermal decomposition of the porphyrin ring: observed significant species are benzene, toluene, pyrrolidine and methyl-pyrrolidine. Sample 2 has a Co/Sn atomic ratio of 1/100 [5]. The Co(II) complex is added in tetrahydrofuran solution after SnCl4 reaction with C3H7OH and H2O addition. TG–MS analysis gives a total mass loss of 51.6% in the interval 20–600 °C, divided into two main events, as shown in Fig. 3: below and above 330 °C (40.2% and 11.4%, respectively). The TIC curve reported in Fig. 4 parallels the DTG profile of Fig. 3: the maximum release occurs in the 220– 330 °C interval, and a minor peak is observed at 380 °C. GC–MS analysis of gaseous products released at 230 °C confirms the occurrence of reactions (1)–(4), the formation
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Fig. 2. Thermogravimetric–mass spectrometric analysis of sample 1. Ion currents of selected ions used to monitor evolved species during pyrolysis process: m/z 31 (C3H7OH), m/z 18 (H2O), m/z 36 (HCl), m/z 41 (CH3CH@CH2), and TIC (total ion current) plots of evolved gas phase vs. temperature. Insets: mass spectra corresponding to main releases during thermal treatment.
Fig. 3. Thermal analysis (TG, DTG and DTA) of sample 2, (20–600 °C).
of CH3CH@CH2 being the most significant. Abundant H2O release, corresponding to reaction (5), occurs in the same interval, i.e., before the occurrence of (1). Moreover,
TG–MS analysis reveals an appreciable release of SnCl4 (18% of the total evolved gas phase) and HCl at 310 °C. Thus, sample 2 shows thermal behavior similar to that of sample 1, with the exception of the SnCl4 loss, which may be competitive with reaction (6). With reference to the first part of this section, the incomplete substitution of Sn–Cl with Sn–OC3H7 and the absence of control in the advancement of hydrolysis accounts for different concentrations of residual Sn–Cl, Sn–OC3H7 and Sn–OH groups between samples 1 and 2. The addition of 1% Co-porphyrin complex in tetrahydrofuran may modify the chemical equilibria and the rate of every single reaction. In other words, the general process is independent of the Co-porphyrin complex, but the kinetics of the reactions of SnCl4 with propanol and with water may be affected by addition of this complex in tetrahydrofurane solution. In particular, a greater presence of Sn–Cl may explain the release of SnCl4, like SiH4 or SiF4 releases in gels prepared from HSi(OR)3 or F–Si(OR)3 precursors [19,20].
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Fig. 4. Thermogravimetric–mass spectrometric analysis of sample 2. Ion currents of selected ions used to monitor evolved species during pyrolysis process: m/z 18 (H2O), m/z 36 (HCl), m/z 41 (CH3CH@CH2), m/z 225 (SnCl4), and TIC (total ion current) plots of evolved gas phase vs. temperature. Insets: mass spectra corresponding to main releases during thermal treatment.
The presence of the Co-porphyrin complex (sample 2) was revealed by GC–MS experiments. Above 350 °C, traces of benzene, toluene and pyrrolidines were detected as species typical of porphyrin-ligand degradation. 119 Sn solid state NMR spectra were recorded for sample 1 treated at 250 °C (Fig. 5), and for sample 2 treated at 110 °C and 250 °C (Fig. 6). At lower temperature, sample 2 shows four signals centered at 584.2, 609.9, 641.5 and 674.2 ppm; other small peaks are side bands. The main peak at 609.9 ppm may be assigned to Sn(IV) surrounded by six bridging oxygens, but it is characterized by a slight negative shift and noticeable broadening with respect to the SnO6 coordination proper of Cassiterite crystals (604.3 ppm). Peak broadening is attributable to structural variations and the absence of extended crystalline structure [21]. As previously reported for (TiO2)x(SnO2)1x solid solutions, a moderate negative shift from the typical SnO2 resonance at 604.3 ppm is expected, owing to variable numbers of the next nearest neighbours Ti4+; chemical shifts are mainly affected by the local environment, and are not sensitive to long-distance interferences [21,22].
Fig. 5. 119Sn solid state NMR spectra of sample 1 treated at 250 °C. Peaks marked with * are spinning side bands. Red dot line represents the Lorentzian fit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The above authors reported that the variability of signals and different NMR resonances are affected by the TiO2/SnO2 ratio, the most complex spectrum being
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observed at the highest ratio, in agreement with the increased variability of Sn(IV) coordination [21]. The same situation may be obtained bridging –SnO– units randomly mixed with terminal Sn–Cl, Sn–OC3H7 and Sn–OH bonds. The spectrum of sample 2 treated at 250 °C is simpler (Fig. 6b), evidencing the disappearance of Sn–OC3H7, Sn–OH and, in part, Sn–Cl bonds, by reactions (1) and (5)–(7), with an increase of bridging Sn–O–Sn bonds. This interpretation finds substantial confirmation in the spectrum of sample 1 treated at 250 °C (Fig. 5). As deduced from TG–MS data, at this temperature the sample has already lost most Sn–OC3H7 groups and condensates Sn– OH and Sn–Cl bonds. With respect to the spectrum of sample 2 in Fig. 6b, this spectrum presents a single resonance at 605.3 ppm, with a lorentzian profile corresponding to cassiterite in a single SnO6 environment. The broadening of the signal may be attributed to an average particle diameter of a few nanometers [23,24]. This interpretation of NMR spectra parallels the X-ray results of samples heated at 250 °C (Fig. 7). The detectable crystalline phase corresponds to Cassiterite, as confirmed by the 21-1250 JCPDS card. Peak profile analysis gives an average crystallite diameter of 3 ± 0.2 nm for sample 2, which corresponds to a more pronounced disordered structure, and of 6 ± 0.2 nm for sample 1. 4. Conclusions
Fig. 6. 119Sn solid state NMR spectra of sample 2 treated at 110 °C (top) and 250 °C (bottom). Peaks marked with * are spinning side bands. Red dot line represents the Lorentzian fit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. XRD spectra of samples 1 (top) and 2 (bottom) treated at 250 °C.
According to the aims of this work, the results lead to some considerations. The sensor activity of SnCl4-derived materials at 250 °C can be attributed to the cooperation of two distinct factors: on one hand, from our results, it appears that an open Sn oxide structure is present, resulting from the advancement of chemical equilibria concerning release of organic fragments, HCl, and H2O. From this point of view, sensitivity to alcohols indicates a specific reversible chemical interaction with not yet fully stabilized SnO2 particles. As for the enhanced sensitivity of the SnO2 doped with Co-porphyrin, it is noteworthy that this complex in tetrahydrofurane is added to the SnO2 sol, so that the occurrence of chemical reactions between the complex and reactive Sn–OC3H7 and Sn–Cl groups, still present on sol particle surface, may be proposed. In particular, the porphyrin-ligand–C6H4–COOH group is favorably esterified by these reactive moieties, leading to chemical anchoring between the inorganic SnO2 phase and the organometallic complex. In addition, the sol surface occupancy of Co-porphyrin molecules may hamper the advancement of Sn–Cl and Sn–OC3H7 hydrolysis and of subsequent condensation. In other words, our results emphasize the higher populations of reactive Sn–Cl and Sn–OC3H7 in the Co-porphyrin doped sample. Besides such a structural effect of the Co-porphyrin, i.e. the role in the Sn oxide hydrolysis–condensation kinetic, an electronic effect cannot be ruled out. In particular, the Co-porphyrin dispersed on the material surface may favor two different effects on the gas-sensing mechanism [25]. Referring to our previous methanol test [5], the
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Co-porphyrin may play a primary role in the catalysis of methanol oxidation, causing a subsequent charge transfer from the tin oxide support. This mechanism is known as electronic sensitization, and is related to the Fermi energy pinning due to the contact between the oxide support and the catalyst. Such mechanism is classically invoked in catalysis reactions involving particles of noble metal catalysts (e.g. Pd or Pt), but it is suggestive in our case to suppose an electronic interaction between the discrete porphyrin molecules and the underlying support. The other mechanism is related to spill-over over the oxide surface of the species created by the interaction of the catalyst with the gaseous analyte. This second mechanism is more hardly invoked in our case, since in general it is more directly related with the presence of catalyst nanoparticles. Acknowledgement We thank MURST-PNR 2001–2003 (FIRB art. 8) for funding. References [1] R. Paolesse, D. Monti, L. La Monica, M. Venanzi, A. Froiio, S. Nardis, C. Di Natale, E. Martinelli, A. D’Amico, Chem. Eur. J. 8 (2002) 2476. [2] F. Quaranta, R. Rella, P. Siciliano, S. Capone, C. Distante, M. Epifani, A. Taurino, Sens. Actuators B 84 (2002) 55. [3] W. Gopel, T.A. Jones, M. Kleitz, J. Lundstrom, T. Seiyama (Eds.), Sensors, vol. 2, Wiley-VCH, Weinheim, 1991. [4] J.W. Grate, Chem Rev. 100 (2000) 2627. [5] S. Nardis, D. Monti, C. Di Natale, A. D’Amico, P. Siciliano, A. Forleo, M. Epifani, A. Taurino, R. Rella, R. Paolesse, Sens. Actuators B 103 (2004) 339. [6] R. Campostrini, M. Ischia, L. Palmisano, J. Therm. Anal. Cal. 75 (2004) 13.
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