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Pyrolysis behaviour of titanium dioxide–poly(vinyl pyrrolidone) composite materials
Polymer Degradation and Stability 94 (2009) 1882–1889
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Pyrolysis behaviour of titanium dioxide–poly(vinyl pyrrolidone) composite materials Rohan L. Holmes a, b, Jonathan A. Campbell a, b, *, Robert P. Burford a, Inna Karatchevtseva c a
School of Chemical Sciences and Engineering, University of New South Wales, Sydney, 2052 NSW, Australia Cooperative Research Centre for Polymers, 8 Redwood Drive, Notting Hill Vic 3168, Australia c ANSTO, Private mail bag 1, Menai, NSW 2234, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2009 Accepted 17 March 2009 Available online 3 May 2009
Inorganic–organic hybrid materials are studied due to the unique properties they exhibit. As these materials become more widely applied, particularly as precursor materials for forming inorganic materials, it is essential that the pyrolysis behaviour is understood. Transparent yellow hybrid materials consisting of titanium dioxide and poly(vinyl pyrrolidone) were prepared using sol–gel processing techniques. The hybrids maintained their transparency up to the highest achieved inorganic loading of 57 wt.%. These materials were characterised using thermogravimetric analysis in which the organic component was pyrolysed. The resultant chars were then investigated using optical microscopy, x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The inorganic loading had an effect on char formation, with higher loadings leading to the formation of pyrolysis intermediates which were less apparent in samples of lower inorganic content. The pyrolysis intermediates were found to be carbon-rich. Ó 2009 Published by Elsevier Ltd.
Keywords: Pyrolysis Titanium dioxide Hybrid Nanocomposite Microstructure
1. Introduction Titanium dioxide (TiO2) has been extensively studied because it can be applied in coatings [1], electronics [2], catalysis [3] and other materials science applications [4]. The chemistry of TiO2 is relatively complex as it can exist in amorphous and crystalline forms including anatase, rutile, brookite or some mixture of the three [5]. The properties of TiO2 depend upon its structure and form. There have been in-depth studies of preparation techniques that lead to the different forms including flame spray pyrolysis [6], hydrothermal techniques [7] and sol–gel processing [8]. Thin films consisting of powders with high surface area and small particle size are widely studied due to their application as photo-catalysts in for example air purification [9]. Composites are formed when two different materials are used in combination, with the aim of improving one or more properties of the component materials. Composite materials generally consist of a continuous phase, which surrounds a dispersed phase. The dispersed phase can range in scale from macroscopic (mm) to nano (108 m), with physical properties reflecting these changes in dimension. Homogeneity improves as the size of the dispersed
* Corresponding author. School of Chemical Sciences and Engineering, University of New South Wales, Sydney, 2052 NSW, Australia. Tel.: þ61 2 9385 991; fax: þ61 2 9385 5966. E-mail address: [email protected] (J.A. Campbell). 0141-3910/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2009.03.026
phase is reduced. On the macroscopic scale the dispersed phase is easily distinguished and often affects the texture of the material, such as in glass fibre reinforced resin. As the dispersed phase reaches the nano-scale, a more homogenous material can be formed. As particle size becomes less than the wavelength of light, transparent hybrid materials can be formed when polymers of low crystallinity are used and filler dispersion is optimised. Composite materials can be classed in a number of different ways depending on the chemical interactions between the twophases. Kickelbick [10] used the strength of the intermolecular forces to classify the types of hybrid materials formed, contrasting those with chemical bonding between phases, weaker intermolecular forces between phases, or those with little or no interaction between the phases. Hybrid materials are designated as those where chemical interaction between the inorganic and the organic phase exists (hydrogen bonding can be included in this subset). The structure of the different classes varies widely and includes a dispersion of the inorganic in the continuous phase (blend), an interpenetrating network of inorganic component and polymer, pendant inorganic groups attached to the polymer backbone, and true hybrids, described as structures where covalent bonds exist between inorganic and organic phase. These differences in structure can affect the pyrolysis behaviour of the hybrid material. Sol–gel processing is a convenient technique for the preparation of inorganic materials as it can be conducted under mild conditions [11]. This process has been used for the preparation of a number of
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ceramic materials and is well-suited to the preparation of metallicoxides (zirconia, silica, titania, etc.). The process requires the selection of a suitable precursor material such as a metal alkoxide or halide [12], followed by a hydrolysis reaction which leads to the formation of the metal hydroxide (M OH). Subsequently a polycondensation occurs where water is eliminated and the metal oxide network (–M–O–M–) is formed. The main advantage afforded by this type of processing is the ability to chemically control both the hydrolysis and condensation reactions without employing more complex colloid chemistry [13]. This can be further applied to the preparation of hybrid materials with a metallic oxide network dispersed in a polymer by dissolving the polymer in a suitable solvent, mixing the inorganic and polymer sols and thermally treating to form the composite. These systems are well-known and have been studied extensively as illustrated by Chen [14] who used sol–gel processing techniques for the production of poly(acrylic acid)–TiO2 nano-sized hybrid colloids. It is often desirable to remove the organic component from a hybrid material to leave an inorganic residue, such as when a uniformly distributed and continuous layer of friable inorganic material is required. When polymer is employed as a carrier, extruded or moulded geometries are possible using conventional processing techniques. As the organic component typically has a pyrolysis temperature (400–500 C) far below that of the inorganic component (2000–3000 C) pyrolysis can be a convenient method to remove the organic component. However, it is common for thermal treatments to result in changes to the structure of the inorganic component. Whilst the pyrolysis of materials containing silica has been widely investigated, that of TiO2 containing systems has received less attention. Degradation and pyrolysis of hybrid systems containing silica has been widely researched [15–17] due to the simple sol–gel synthesis techniques available and the stability of Si–O–C bonds. Although TiO2 hybrids have received some attention, rigorous investigation of their pyrolysis behaviour is seldom reported. Increasing significance of hybrid materials is illustrated by the recent appearance of textbooks [18] and journal publications [14,19]. Patent publications including that of Eastman-Kodak [20] and related articles focusing upon high refractive index optical devices [21] show that these materials are commercially important. Pyrolysis of hybrids containing silica, rather than TiO2, has been described. Zhang et al. [22] detailed the preparation of silica polyacrylamide hybrid beads doped with gold nanoparticles, which after pyrolysis gave a porous silica structure. This acted as a catalyst support for gold nanoparticles which can catalyse the oxidation of alkenes. The porous structure allowed enhanced mass transport of reactant and products in and out of the beads. Dire` et al. [23] investigated the microstructural evolution and effect of TiO2 and polymer loading in nanocomposite materials where the silica component was derived from polydimethylsiloxane and the TiO2 from conventional sol–gel methods. Pyrolysis of the organic component resulted in a mixture of TiO2 and SiO2 residue. The addition of TiO2 had a direct influence on the porosity and the crystallinity of the char by controlling the amount of carbon retained in the matrix. This was due to the influence of TiO2 on cleaving Si–C bonds and the formation of titanium and silicon oxycarbide species, and in turn the formation of pure SiO2. Here, the pyrolysis behaviour of poly(vinyl pyrrolidone) (PVP)– TiO2 hybrids is investigated. PVP was selected because high-loadings of TiO2 could be achieved in this polymer. Appropriate solvents were used to generate the inorganic component by the sol–gel technique and to dissolve the polymer prior to mixing of the two components to form the hybrids. A significant aspect of this study was the formation of inorganic chars derived from composite by pyrolysis using a range of heating rates and final temperatures. The structures of the resultant chars were characterised using a number
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of techniques including optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and x-ray diffraction (XRD). 2. Experimental 2.1. Materials Poly(vinyl pyrrolidone) PVP was supplied by ISP Technologies (Wayne, New Jersey) and had a stated molecular weight of 900,000–1,500,000. Titanium isopropoxide (TiP, 97% pure) and dimethylacetamide (DMAc) were purchased from Sigma–Aldrich. Nitric acid was purchased from Univar (Sydney, Australia) and acetic acid was supplied by Scharlau (Barcelona, Spain). 2.2. Methods 2.2.1. Hybrid preparation The TiO2 precursor sol was prepared in the following manner. A mixture of acetic acid (9.5 mL) and DMAc (15.5 mL) were prepared, to which TiP (52 mL, concentrated) was added drop-wise. A mixture of nitric acid (10.5 mL) and water (12 mL) was also added drop-wise to the solution forming a 100 mL solution with the potential to form 13 g of TiO2. The polymer sol was prepared by dissolving 10 g of PVP in 90 g of DMAc resulting in a viscous solution containing 10 wt.% polymer, and the two mixtures were then combined in ratios to provide the desired TiO2/polymer hybrid composition. This precursor hybrid solution was then cast into glass Petri dishes and allowed to settle for 24 h at ambient temperature. The remaining solvent was removed using a strictly controlled drying process which was: 24 h at 60 C then 12 h at 100 C, 2 h at 120 C, and 2 h at 160 C. The dry hybrids were then removed from the oven for characterisation. Departures from this protocol led to cracked films rather than continuous intact films. 2.2.2. Polymer pyrolysis Pyrolysis was conducted using a TA Instruments 2950 HiRes thermogravimetric analyser (New Castle, Delaware). Samples were heated in platinum pans in air at 10 C/min to final temperatures at 50 C intervals between 400 C and 1000 C with a 10 min isothermal period used to end each run. The TGA experiments allow samples to be processed with a known thermal history, and provide residues suitable for microscopic analysis. Marked changes in black pyrolysis intermediate formation and disappearance were observed around 750 C and so this was selected as a basis for the heating rate trials. To determine the effect of heating rate, 50 C/min and 2 C/min protocols were used to a single maximum temperature of 750 C. TGA also provides quantitative weight-loss data which assists in identifying the pyrolysis mechanism. 2.2.3. Characterisation techniques 2.2.3.1. XRD. Samples were prepared for XRD by heating samples in a furnace at a rate of 10 C/min to a final temperature in the range 600–900 C at 50 C intervals, with an isothermal period of 10 min to ensure a stable final temperature. A Phillips X’pert x-ray diffraction system (MPD) was used to obtain XRD traces. Anatase and rutile were then identified using international centre for diffraction (ICDD) databases and approximate phase compositions were determined. 2.2.3.2. Microscopy. Routine optical microscopy was conducted using a Leica Wild M3C stereomicroscope with a Leica CCD attachment. High magnification images were obtained using a Leica 2500dm conventional reflectance microscope also with CCD.
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therefore used to remove solvent from the hybrid materials. The hybrid materials were aged at room temperature for 24 h, with moisture from the atmosphere completing the sol–gel reaction. This was identified as a key parameter essential for the formation of stable hybrids. The samples were then thermally treated to complete the condensation reaction as described in the Experimental section. The materials that were removed from the oven were initially found to be brittle. However, after approximately 24 h atmospheric exposure, moisture served to plasticise the product to a more pliable state. In addition, materials with higher amounts of TiO2 formed more viscous gels, which required additional solvent to allow film casting. TiO2 sols were light yellow in appearance and upon addition of the polymer sol, the solution developed an intense orange colour. The hybrid materials maintained this colour and transparency through all subsequent processing steps and in the final materials (Fig. 1). Fig. 1. Hybrid composites with 57 wt.% loading of TiO2.
3.1. Thermogravimetric analysis
Scanning electron microscope images were obtained using a Hitachi S900 FESEM. Samples were attached to a brass stub using carbon tape and coated with chromium using an EmiTech K575x high resolution coater. Typically 4 kV and an emission current of 10 mA were employed. Atomic force microscope (AFM) images were obtained using a Digital Instruments 3000 AFM in contact mode. Typically the gain settings were: proportional 1.3, integral 1, and a frequency of 1 Hz. Samples were prepared by attaching the char to a glass microscope slide using double sided tape. Evolved gas analysis linked to a TA instruments TGA 2150 was conducted using a ThermoScientific Nicolet 5700 FTIR spectrometer with TGA–FTIR interface. The transfer line and the measurement cell were maintained at 230 C to limit condensation in the cell or transfer lines. Spectra were collected at time intervals of 30 s, which were then stacked to give a profile of furnace composition versus time. This data was then merged with TGA data to give a furnace composition as a function of time.
TGA was used to characterise the degradation and to determine the amount of TiO2 in the hybrids. A range of inorganic loadings was targeted, with the highest content being approximately 57 wt.%. TGA traces of unfilled PVP with the filled materials are compared in Fig. 2. The presence of TiO2 appears to cause an earlier degradation onset than in the unfilled polymer. TiO2 loaded samples show similar profiles, which include several weight-loss events. Due to the hydrophilic nature of the PVP, water can be absorbed from the atmosphere and the unfilled PVP appears to contain approximately 20 wt.% water, which can be identified in the first mass-loss event in the TGA trace. However, less water was found in the hybrid. All mass losses were observed to cease by 650 C for the unfilled PVP and by 600 C for filled hybrid materials (Fig. 2). The pyrolysis onset temperature for the virgin polymer (ignoring the effects of water) was found to be approximately 350 C, whereas the pyrolysis onset temperature for the hybrid materials was identified as being approximately 50–100 C lower than the unfilled PVP at 250 C. This was observed for all filled hybrid materials.
3. Results
3.2. XRD analysis of the chars
Sol–gel processing techniques were used to prepare a range of hybrid materials based on TiO2 and poly(vinyl pyrrolidone). Maximum TiO2 loadings were determined to be in excess of 60 wt.% as determined by TGA. Without controlled drying, these materials cracked due to shrinkage stresses. A controlled drying process was
The TiO2 was comprised of anatase and rutile, with the anatase form decreasing with increasing temperature (Table 1). XRD peaks in Fig. 3 show anatase and rutile; (a) depicts a sample heated to 650 C which only contains anatase; (b) indicates a sample that contains both anatase and rutile (heated to 800 C); and (c) shows
Fig. 2. TGA traces for PVP/TiO2 hybrids with different weightings of inorganic (0 wt.%, 30 wt.%, and 60 wt.%).
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3.4. AFM and SEM imaging
Table 1 Approximate phase compositions of chars. Temperature ( C) 650 700 750 800 850 900
C C C C C C
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Anatase (%)
Rutile (%)
100 90 80 40 10 5
0 10 20 60 90 95
a sample that has completely transformed to rutile (heated to 900 C). The peak located at 25 is characteristic of anatase and the 27 peak indicates rutile. No other phases were identified. 3.3. Char imaging Variations in processing temperature led to different char structures as identified by optical microscopy. Residues of the polymer were observed in samples heated to temperatures up to 800 C (Fig. 4): above this temperature the samples contained no residues. A notable feature becomes apparent where inclusions are present in the temperature range of 550–850 C (Fig. 4(b and c)). The inclusions were no longer present in the samples that had been heated to 850 C or above. The heating rate is also noted to have an effect on the appearance of inclusions: a faster heating rate (50 C/ min) led to more inclusions being present at a final temperature of 750 C than with a slower heating rate (2 C/min) where few or no inclusions were present. Differences in the appearance of the TiO2 component can also be noted where there is a change from a transparent glass-like appearance at lower temperatures (<700 C), to an opaque ceramic at higher temperatures (>750 C). This transition can be correlated with the XRD data where the anatase to rutile transition takes place in this temperature range (with the rutile component beginning to dominate at this stage). Optical microscopy revealed the presence of metallic-like inclusions in the titanium dioxide char in a number of other samples. Fig. 5 shows an area of white titanium dioxide with a large inclusion (10 mm 30 mm in size) present together with smaller particles. Samples containing a higher titanium dioxide loading were found to be more likely to have these inclusions formed in the chars. Samples containing 30 wt.% TiO2 yielded few or no inclusions.
SEM and AFM were used to investigate the structure of the char as a function of the pyrolysis temperature. AFM images of the chars allowed the transition between the anatase and rutile phases to be visualised. Below 750 C the structures were found to be similar and predominately anatase (Fig. 6(a)), with much smaller particles being apparent. Large crystallites were identified at 800 C, with large, fully formed crystallites being present at 850 C (Fig. 6(b)). At 850 C, where rutile is more predominant (rutile making up approximately 90%), larger more defined crystallites are present. The heating rate not only affected the presence of inclusions in the chars, but also changes in the structures that were formed. A faster heating rate led to the development of pits and holes (Fig. 7(a)) and a smaller particle size. Slower heating rates led to the formation of surfaces which were rougher than those exposed to faster heating rates (Fig. 7(b)), due to the formation of larger particles in the char. SEM imaging (Fig. 8) indicated that at lower temperature (<650 C) the particle size in the chars is small (>50 nm). Particle aggregation into clusters was also observed (Fig. 8(a and b)) and the average size of these clusters was reduced between 400 C and 600 C. Some porosity was present in the structure of the char for both samples processed at 400 C and 600 C. The particle size increased as the temperature was further increased. At 1000 C the size of the crystallites is far larger, typically around 500 nm, and pores of approximately 150 nm are present (Fig. 8(c)). Striations can also be detected on the surface of theses crystallites indicating an ordered crystalline structure. 3.5. Composition of the pyrolysis intermediate A number of techniques were considered to determine the identity of the inclusions. Evolved gas analysis was initially used, as it was an extension of the TGA protocol. The data recorded identified the presence of a carbon dioxide peak after most of the weight-loss has occurred (Fig. 9). From the TGA trace, it is clear that most of the weight-loss has occurred by t ¼ 50 min, also confirmed by FTIR data and the reduction in the intensity of the 2350 cm1 peak. A further peak indicating increased carbon dioxide concentration can be identified at t ¼ 75 min (750 C). This shows that there is a further, discrete evolution of carbon dioxide.
Fig. 3. Typical XRD traces.
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Fig. 4. Inclusions present in chars (a) 400 C, (b) 600 C, (c) 800 C, and (d) 1000 C.
Additional evidence that the inclusions were carbon was obtained from energy dispersive X-ray spectroscopy (EDS) coupled with SEM. Using back-scatter mode, the contrast between the inclusions and the titanium dioxide was increased allowing the inclusions to be readily identified. EDS analysis was conducted on several inclusions in several samples (mounted on the same stub
and exposed to the same conditions), as well as at sites that appeared to have no inclusions present. A char containing no inclusions was also used as the control. It was found that the inclusions showed a much more intense carbon peak (1600 counts) than that observed for the control (500 counts). This evidence, coupled with the TGAFTIR data, indicates that the inclusions are carbon. 4. Discussion
Fig. 5. Inclusions in the chars.
Optically transparent thin hybrid films were produced at all TiO2 loadings. However, due to large amounts of solvent present during the processing of the hybrid material, cracking was common, as detailed in other systems [24]. One method that has been used to reduce the problems associated with large volume shrinkage is to use ‘one-pot’ synthesis, where the sol–gel reaction and the polymerisation occur simultaneously, requiring less solvent. These techniques can lead to volume shrinkages of only 10–20% compared with 70% associated with conventional sol–gel techniques [19]. From the TGA traces, unfilled PVP contains 20 wt.% water under our laboratory conditions and equilibrium amounts of water were around 10 wt.% for TiO2 composites. Plasticisation of polymers with low molecular weight compounds is extensively used, the most prominent example being PVC with phthalate esters. For hydrophilic polymers such as nylons and in the present case PVP, low amounts of water fulfil the same role. Thus the unfilled PVP and the hybrids require low amounts of water for ductility. Nanodispersed TiO2 in polymer matrices can exhibit colours ranging from light yellow to red [25]. The origins of this colour are described by Rajh et al. [26] who propose interactions between ligands and TiO2 nanoparticles as being responsible. Due to the small size of the TiO2 particles (w20 nm), Ti–O bonds are compressed at the surface due to high curvature. This causes
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Fig. 7. AFM images of chars at (a) 50 C/min, (b) 1 C/min heating rates.
Fig. 6. AFM images of samples (a) high anatase content (700 C), (b) high rutile content (850 C).
a rearrangement of the Ti coordination from an octahedral to a square pyramidal arrangement, leading to the observed colour. XRD revealed the crystal structure of the chars. At lower temperatures (650 C), anatase is the only phase present with a characteristic peak at 25 . As the sample temperature approaches 700 C the rutile phase (27 ) begins to appear. The rutile phase continues to increase at higher temperatures with enlarged crystallites revealed in the SEM and AFM images. SEM and AFM images indicate that the anatase crystallites are smaller (20–100 nm) than the rutile crystallites (100–500 nm). It has been noted in systems based on silica [15] that the introduction of an inorganic component into the polymer can lead to the formation of a char on the surface of the material. This may form a mass transfer barrier preventing diffusion of oxygen into the hybrid and impede the release of pyrolysis products from the material. It is therefore common for the mass loss events to be delayed due to the formation of this ‘char-barrier’. For TiO2, it would appear that the char-barrier may have had a more localised effect, with the carbonaceous inclusions being small. Superimposed on this is the effect of the TiO2 particles on the polymer. Whilst TiO2
can exhibit a range of catalytic effects, one that is particularly known in polymeric matrices is the promotion of degradation. The TGA data shows the reduction of the onset of pyrolysis event by about 50 C consistent with data published elsewhere [27]. Char structure was found to be highly dependent upon the thermal history. At 800 C and above the appearance of large crystallites was noted, but below this temperature the structural features shown by AFM were much smaller. This can be attributed to the transition between the anatase and the rutile phase, which is well documented. The XRD data supports this observation, as it shows that in a sample that has been treated to a temperature of 800 C there is more rutile than anatase present. As the temperature is further increased, more rutile is formed and the titania particles become larger. SEM images also indicate that between 400 C and 600 C the chars consist of aggregates of smaller particles which have sintered together. These particles shrink as the processing temperature is increased from 400 C to 600 C due to the evolution of the organic component as gas, leading to a reduction in the volume of material present. Crystallite size increases as processing temperature is increased due to the growth of the rutile crystallites. At 1000 C, large rutile crystallites exist with striations on the surface. The presence of pores in the structure is due to contraction of the network and could indicate a deficiency in TiO2 at this site during crystal growth. These pores may correspond to sites where black pyrolysis intermediates were present, resulting in the lower TiO2 concentration proposed. Whilst the crystalline texture and phase composition of TiO2 is well-known to change with temperature, as indicated above, a further complication exists where a polymer matrix is also present. We have noted the presence of macroscopic black pyrolysis intermediates between 550 C and 800 C, which are not commonly discussed in
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Fig. 8. SEM micrographs of chars (a) 400 C, (b) 600 C, (c) 1000 C.
the literature. At lower temperatures graphitic char or similar polycyclic residues are likely to have been formed. At high temperatures, and in the presence of TiO2, it is likely that hydrogen is being stripped and the introduction of C]C double bonds occurs, as commonly arises in cracking furnaces. There is also some potential for the formation of titanium oxycarbide (TiOC) which has been observed in systems containing mixed oxides of silica and TiO2 [23]. TiO2 content in the hybrid nanocomposite also influenced the formation of pyrolysis intermediates. These intermediate species were only observed in highly loaded samples (>50 wt.%) indicating that there may be some threshold required for their formation, somewhat analogous to a percolation threshold. At high temperature the pyrolysis intermediates are finally destroyed by oxidation as reflected in the evolved gas analysis and observed by optical microscopy. The elemental composition of the inclusions was investigated using evolved gas analysis and further examined using EDS coupled with SEM which determined carbon was present. The evolved gas analysis detailed the appearance of a carbon dioxide peak after almost all weight-loss had occurred (as indicated by the TGA trace). The inclusions are also likely to contain a large percentage of TiO2, as the initial material is homogenous and it is expected that TiO2 is still well dispersed in the inclusion. The relative mass of carbon as intermediates in the samples is extremely small. For applications where these types of materials are to be used as precursor materials it is essential to know the conditions under which inclusions are formed. Two conditions that lead to the formation of inclusion-free chars in TiO2/PVP hybrids have been
Fig. 9. Evolved gas analysis contour indicating the late CO2 peak at 2350 cm1.
identified: extended time at the final pyrolysis temperature, or using a slower heating rate which also leads to removal of inclusions. Both techniques essentially lead to an increase in the amount of time that the material spends at elevated temperature. It would seem that time is therefore the most important variable in the formation of inclusions in these types of system. 5. Conclusions The use of a range of heating rates during the pyrolysis of PVP/ TiO2 hybrid materials leads to the formation of chars with different structures. Faster heating rates also lead to the increased appearance of inclusions in the chars. Pyrolysis intermediates were determined to be carbon-containing using FTIR and SEM/EDS. This is likely to be the formation of graphitic char due to stripping of hydrogen, which is evolved via oxidation at higher temperature (>800 ). The anatase to rutile transition, which is well documented, was identified as beginning at approximately 650 C. Below this temperature, anatase was the only crystalline phase identified in the sample and at 900 C or above rutile was the only phase present. References [1] Lima RS, Kruger SE, Marple BR. Towards engineering isotropic behaviour of mechanical properties in thermally sprayed ceramic coatings. Surf Coat Technol 2008;202(15):3643–52. [2] Francioso L, Prato A, Siciliano P. Low-cost electronics and thin film technology for sol–gel titania lambda probes. Sensor Actuator B 2008;128(2):359–65. [3] Cheng P, Deng CS, Gu MY, Dai XM. Effect of urea on the photoactivity of titania powder prepared by sol–gel method. Mater Chem Phys 2008;107(1):77–81. [4] Asif KM, Sarwar MI, Raqif S, Ahmad Z. Properties of PVC–titania hybrid materials prepared by the sol–gel process. Polym Bull 1998;40(4–5):583–90. [5] Greenwood NN, Earnshaw A. Chemistry of the elements. 1st ed. Oxford: Permagon Press; 1984. p. 1542. [6] Chiarello GL, Selli E, Forni L. Photocatalytic hydrogen production over flame spray pyrolysis-synthesised TiO2 and Au/TiO2. Appl Catal B 2008; 84(1–2):332–9. [7] Jitputti J, Rattanavoravipa T, Chuangchote S, Pavasupree S, Suzuki Y, Yoshikawa S. Low temperature hydrothermal synthesis of monodispersed flower-like titanate nanosheets. Catal Commun 2009;10(4):378–82. [8] Lee BI, Wang X, Bhave R, Hu M. Synthesis of brookite TiO2 nanoparticles by ambient condition sol process. Mater Lett 2006;60(9–10):1179–83. [9] Kalousek V, Tschirch J, Bahneimann D, Rathousky J. Mesoporous layers of TiO2 as highly efficient photocatalysts for the purification of air. Superlattices Microstruct 2008;44(4–5):506–13. [10] Kickelbick G. Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale. Prog Polym Sci 2003;28(1):83–114. [11] Schubert U, Husing N. Synthesis of inorganic materials. 2nd ed. Weinheim: Wiley-VCH Verlag GmBH & Co. KGaA; 2005. [12] Chen XB, Mao SS. Synthesis of titanium dioxide (TiO2) nanomaterials. J Nanosci Nanotechnol 2006;6(4):906–25. [13] Schmidt H. Chemistry of material preparation by the sol–gel process. J NonCryst Solids 1988;100(1–3):51–64.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Pyrolysis behaviour of titanium dioxide–poly(vinyl pyrrolidone) composite materials Rohan L. Holmes a, b, Jonathan A. Campbell a, b, *, Robert P. Burford a, Inna Karatchevtseva c a
School of Chemical Sciences and Engineering, University of New South Wales, Sydney, 2052 NSW, Australia Cooperative Research Centre for Polymers, 8 Redwood Drive, Notting Hill Vic 3168, Australia c ANSTO, Private mail bag 1, Menai, NSW 2234, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2009 Accepted 17 March 2009 Available online 3 May 2009
Inorganic–organic hybrid materials are studied due to the unique properties they exhibit. As these materials become more widely applied, particularly as precursor materials for forming inorganic materials, it is essential that the pyrolysis behaviour is understood. Transparent yellow hybrid materials consisting of titanium dioxide and poly(vinyl pyrrolidone) were prepared using sol–gel processing techniques. The hybrids maintained their transparency up to the highest achieved inorganic loading of 57 wt.%. These materials were characterised using thermogravimetric analysis in which the organic component was pyrolysed. The resultant chars were then investigated using optical microscopy, x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The inorganic loading had an effect on char formation, with higher loadings leading to the formation of pyrolysis intermediates which were less apparent in samples of lower inorganic content. The pyrolysis intermediates were found to be carbon-rich. Ó 2009 Published by Elsevier Ltd.
Keywords: Pyrolysis Titanium dioxide Hybrid Nanocomposite Microstructure
1. Introduction Titanium dioxide (TiO2) has been extensively studied because it can be applied in coatings [1], electronics [2], catalysis [3] and other materials science applications [4]. The chemistry of TiO2 is relatively complex as it can exist in amorphous and crystalline forms including anatase, rutile, brookite or some mixture of the three [5]. The properties of TiO2 depend upon its structure and form. There have been in-depth studies of preparation techniques that lead to the different forms including flame spray pyrolysis [6], hydrothermal techniques [7] and sol–gel processing [8]. Thin films consisting of powders with high surface area and small particle size are widely studied due to their application as photo-catalysts in for example air purification [9]. Composites are formed when two different materials are used in combination, with the aim of improving one or more properties of the component materials. Composite materials generally consist of a continuous phase, which surrounds a dispersed phase. The dispersed phase can range in scale from macroscopic (mm) to nano (108 m), with physical properties reflecting these changes in dimension. Homogeneity improves as the size of the dispersed
* Corresponding author. School of Chemical Sciences and Engineering, University of New South Wales, Sydney, 2052 NSW, Australia. Tel.: þ61 2 9385 991; fax: þ61 2 9385 5966. E-mail address: [email protected] (J.A. Campbell). 0141-3910/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2009.03.026
phase is reduced. On the macroscopic scale the dispersed phase is easily distinguished and often affects the texture of the material, such as in glass fibre reinforced resin. As the dispersed phase reaches the nano-scale, a more homogenous material can be formed. As particle size becomes less than the wavelength of light, transparent hybrid materials can be formed when polymers of low crystallinity are used and filler dispersion is optimised. Composite materials can be classed in a number of different ways depending on the chemical interactions between the twophases. Kickelbick [10] used the strength of the intermolecular forces to classify the types of hybrid materials formed, contrasting those with chemical bonding between phases, weaker intermolecular forces between phases, or those with little or no interaction between the phases. Hybrid materials are designated as those where chemical interaction between the inorganic and the organic phase exists (hydrogen bonding can be included in this subset). The structure of the different classes varies widely and includes a dispersion of the inorganic in the continuous phase (blend), an interpenetrating network of inorganic component and polymer, pendant inorganic groups attached to the polymer backbone, and true hybrids, described as structures where covalent bonds exist between inorganic and organic phase. These differences in structure can affect the pyrolysis behaviour of the hybrid material. Sol–gel processing is a convenient technique for the preparation of inorganic materials as it can be conducted under mild conditions [11]. This process has been used for the preparation of a number of
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ceramic materials and is well-suited to the preparation of metallicoxides (zirconia, silica, titania, etc.). The process requires the selection of a suitable precursor material such as a metal alkoxide or halide [12], followed by a hydrolysis reaction which leads to the formation of the metal hydroxide (M OH). Subsequently a polycondensation occurs where water is eliminated and the metal oxide network (–M–O–M–) is formed. The main advantage afforded by this type of processing is the ability to chemically control both the hydrolysis and condensation reactions without employing more complex colloid chemistry [13]. This can be further applied to the preparation of hybrid materials with a metallic oxide network dispersed in a polymer by dissolving the polymer in a suitable solvent, mixing the inorganic and polymer sols and thermally treating to form the composite. These systems are well-known and have been studied extensively as illustrated by Chen [14] who used sol–gel processing techniques for the production of poly(acrylic acid)–TiO2 nano-sized hybrid colloids. It is often desirable to remove the organic component from a hybrid material to leave an inorganic residue, such as when a uniformly distributed and continuous layer of friable inorganic material is required. When polymer is employed as a carrier, extruded or moulded geometries are possible using conventional processing techniques. As the organic component typically has a pyrolysis temperature (400–500 C) far below that of the inorganic component (2000–3000 C) pyrolysis can be a convenient method to remove the organic component. However, it is common for thermal treatments to result in changes to the structure of the inorganic component. Whilst the pyrolysis of materials containing silica has been widely investigated, that of TiO2 containing systems has received less attention. Degradation and pyrolysis of hybrid systems containing silica has been widely researched [15–17] due to the simple sol–gel synthesis techniques available and the stability of Si–O–C bonds. Although TiO2 hybrids have received some attention, rigorous investigation of their pyrolysis behaviour is seldom reported. Increasing significance of hybrid materials is illustrated by the recent appearance of textbooks [18] and journal publications [14,19]. Patent publications including that of Eastman-Kodak [20] and related articles focusing upon high refractive index optical devices [21] show that these materials are commercially important. Pyrolysis of hybrids containing silica, rather than TiO2, has been described. Zhang et al. [22] detailed the preparation of silica polyacrylamide hybrid beads doped with gold nanoparticles, which after pyrolysis gave a porous silica structure. This acted as a catalyst support for gold nanoparticles which can catalyse the oxidation of alkenes. The porous structure allowed enhanced mass transport of reactant and products in and out of the beads. Dire` et al. [23] investigated the microstructural evolution and effect of TiO2 and polymer loading in nanocomposite materials where the silica component was derived from polydimethylsiloxane and the TiO2 from conventional sol–gel methods. Pyrolysis of the organic component resulted in a mixture of TiO2 and SiO2 residue. The addition of TiO2 had a direct influence on the porosity and the crystallinity of the char by controlling the amount of carbon retained in the matrix. This was due to the influence of TiO2 on cleaving Si–C bonds and the formation of titanium and silicon oxycarbide species, and in turn the formation of pure SiO2. Here, the pyrolysis behaviour of poly(vinyl pyrrolidone) (PVP)– TiO2 hybrids is investigated. PVP was selected because high-loadings of TiO2 could be achieved in this polymer. Appropriate solvents were used to generate the inorganic component by the sol–gel technique and to dissolve the polymer prior to mixing of the two components to form the hybrids. A significant aspect of this study was the formation of inorganic chars derived from composite by pyrolysis using a range of heating rates and final temperatures. The structures of the resultant chars were characterised using a number
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of techniques including optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and x-ray diffraction (XRD). 2. Experimental 2.1. Materials Poly(vinyl pyrrolidone) PVP was supplied by ISP Technologies (Wayne, New Jersey) and had a stated molecular weight of 900,000–1,500,000. Titanium isopropoxide (TiP, 97% pure) and dimethylacetamide (DMAc) were purchased from Sigma–Aldrich. Nitric acid was purchased from Univar (Sydney, Australia) and acetic acid was supplied by Scharlau (Barcelona, Spain). 2.2. Methods 2.2.1. Hybrid preparation The TiO2 precursor sol was prepared in the following manner. A mixture of acetic acid (9.5 mL) and DMAc (15.5 mL) were prepared, to which TiP (52 mL, concentrated) was added drop-wise. A mixture of nitric acid (10.5 mL) and water (12 mL) was also added drop-wise to the solution forming a 100 mL solution with the potential to form 13 g of TiO2. The polymer sol was prepared by dissolving 10 g of PVP in 90 g of DMAc resulting in a viscous solution containing 10 wt.% polymer, and the two mixtures were then combined in ratios to provide the desired TiO2/polymer hybrid composition. This precursor hybrid solution was then cast into glass Petri dishes and allowed to settle for 24 h at ambient temperature. The remaining solvent was removed using a strictly controlled drying process which was: 24 h at 60 C then 12 h at 100 C, 2 h at 120 C, and 2 h at 160 C. The dry hybrids were then removed from the oven for characterisation. Departures from this protocol led to cracked films rather than continuous intact films. 2.2.2. Polymer pyrolysis Pyrolysis was conducted using a TA Instruments 2950 HiRes thermogravimetric analyser (New Castle, Delaware). Samples were heated in platinum pans in air at 10 C/min to final temperatures at 50 C intervals between 400 C and 1000 C with a 10 min isothermal period used to end each run. The TGA experiments allow samples to be processed with a known thermal history, and provide residues suitable for microscopic analysis. Marked changes in black pyrolysis intermediate formation and disappearance were observed around 750 C and so this was selected as a basis for the heating rate trials. To determine the effect of heating rate, 50 C/min and 2 C/min protocols were used to a single maximum temperature of 750 C. TGA also provides quantitative weight-loss data which assists in identifying the pyrolysis mechanism. 2.2.3. Characterisation techniques 2.2.3.1. XRD. Samples were prepared for XRD by heating samples in a furnace at a rate of 10 C/min to a final temperature in the range 600–900 C at 50 C intervals, with an isothermal period of 10 min to ensure a stable final temperature. A Phillips X’pert x-ray diffraction system (MPD) was used to obtain XRD traces. Anatase and rutile were then identified using international centre for diffraction (ICDD) databases and approximate phase compositions were determined. 2.2.3.2. Microscopy. Routine optical microscopy was conducted using a Leica Wild M3C stereomicroscope with a Leica CCD attachment. High magnification images were obtained using a Leica 2500dm conventional reflectance microscope also with CCD.
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therefore used to remove solvent from the hybrid materials. The hybrid materials were aged at room temperature for 24 h, with moisture from the atmosphere completing the sol–gel reaction. This was identified as a key parameter essential for the formation of stable hybrids. The samples were then thermally treated to complete the condensation reaction as described in the Experimental section. The materials that were removed from the oven were initially found to be brittle. However, after approximately 24 h atmospheric exposure, moisture served to plasticise the product to a more pliable state. In addition, materials with higher amounts of TiO2 formed more viscous gels, which required additional solvent to allow film casting. TiO2 sols were light yellow in appearance and upon addition of the polymer sol, the solution developed an intense orange colour. The hybrid materials maintained this colour and transparency through all subsequent processing steps and in the final materials (Fig. 1). Fig. 1. Hybrid composites with 57 wt.% loading of TiO2.
3.1. Thermogravimetric analysis
Scanning electron microscope images were obtained using a Hitachi S900 FESEM. Samples were attached to a brass stub using carbon tape and coated with chromium using an EmiTech K575x high resolution coater. Typically 4 kV and an emission current of 10 mA were employed. Atomic force microscope (AFM) images were obtained using a Digital Instruments 3000 AFM in contact mode. Typically the gain settings were: proportional 1.3, integral 1, and a frequency of 1 Hz. Samples were prepared by attaching the char to a glass microscope slide using double sided tape. Evolved gas analysis linked to a TA instruments TGA 2150 was conducted using a ThermoScientific Nicolet 5700 FTIR spectrometer with TGA–FTIR interface. The transfer line and the measurement cell were maintained at 230 C to limit condensation in the cell or transfer lines. Spectra were collected at time intervals of 30 s, which were then stacked to give a profile of furnace composition versus time. This data was then merged with TGA data to give a furnace composition as a function of time.
TGA was used to characterise the degradation and to determine the amount of TiO2 in the hybrids. A range of inorganic loadings was targeted, with the highest content being approximately 57 wt.%. TGA traces of unfilled PVP with the filled materials are compared in Fig. 2. The presence of TiO2 appears to cause an earlier degradation onset than in the unfilled polymer. TiO2 loaded samples show similar profiles, which include several weight-loss events. Due to the hydrophilic nature of the PVP, water can be absorbed from the atmosphere and the unfilled PVP appears to contain approximately 20 wt.% water, which can be identified in the first mass-loss event in the TGA trace. However, less water was found in the hybrid. All mass losses were observed to cease by 650 C for the unfilled PVP and by 600 C for filled hybrid materials (Fig. 2). The pyrolysis onset temperature for the virgin polymer (ignoring the effects of water) was found to be approximately 350 C, whereas the pyrolysis onset temperature for the hybrid materials was identified as being approximately 50–100 C lower than the unfilled PVP at 250 C. This was observed for all filled hybrid materials.
3. Results
3.2. XRD analysis of the chars
Sol–gel processing techniques were used to prepare a range of hybrid materials based on TiO2 and poly(vinyl pyrrolidone). Maximum TiO2 loadings were determined to be in excess of 60 wt.% as determined by TGA. Without controlled drying, these materials cracked due to shrinkage stresses. A controlled drying process was
The TiO2 was comprised of anatase and rutile, with the anatase form decreasing with increasing temperature (Table 1). XRD peaks in Fig. 3 show anatase and rutile; (a) depicts a sample heated to 650 C which only contains anatase; (b) indicates a sample that contains both anatase and rutile (heated to 800 C); and (c) shows
Fig. 2. TGA traces for PVP/TiO2 hybrids with different weightings of inorganic (0 wt.%, 30 wt.%, and 60 wt.%).
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3.4. AFM and SEM imaging
Table 1 Approximate phase compositions of chars. Temperature ( C) 650 700 750 800 850 900
C C C C C C
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Anatase (%)
Rutile (%)
100 90 80 40 10 5
0 10 20 60 90 95
a sample that has completely transformed to rutile (heated to 900 C). The peak located at 25 is characteristic of anatase and the 27 peak indicates rutile. No other phases were identified. 3.3. Char imaging Variations in processing temperature led to different char structures as identified by optical microscopy. Residues of the polymer were observed in samples heated to temperatures up to 800 C (Fig. 4): above this temperature the samples contained no residues. A notable feature becomes apparent where inclusions are present in the temperature range of 550–850 C (Fig. 4(b and c)). The inclusions were no longer present in the samples that had been heated to 850 C or above. The heating rate is also noted to have an effect on the appearance of inclusions: a faster heating rate (50 C/ min) led to more inclusions being present at a final temperature of 750 C than with a slower heating rate (2 C/min) where few or no inclusions were present. Differences in the appearance of the TiO2 component can also be noted where there is a change from a transparent glass-like appearance at lower temperatures (<700 C), to an opaque ceramic at higher temperatures (>750 C). This transition can be correlated with the XRD data where the anatase to rutile transition takes place in this temperature range (with the rutile component beginning to dominate at this stage). Optical microscopy revealed the presence of metallic-like inclusions in the titanium dioxide char in a number of other samples. Fig. 5 shows an area of white titanium dioxide with a large inclusion (10 mm 30 mm in size) present together with smaller particles. Samples containing a higher titanium dioxide loading were found to be more likely to have these inclusions formed in the chars. Samples containing 30 wt.% TiO2 yielded few or no inclusions.
SEM and AFM were used to investigate the structure of the char as a function of the pyrolysis temperature. AFM images of the chars allowed the transition between the anatase and rutile phases to be visualised. Below 750 C the structures were found to be similar and predominately anatase (Fig. 6(a)), with much smaller particles being apparent. Large crystallites were identified at 800 C, with large, fully formed crystallites being present at 850 C (Fig. 6(b)). At 850 C, where rutile is more predominant (rutile making up approximately 90%), larger more defined crystallites are present. The heating rate not only affected the presence of inclusions in the chars, but also changes in the structures that were formed. A faster heating rate led to the development of pits and holes (Fig. 7(a)) and a smaller particle size. Slower heating rates led to the formation of surfaces which were rougher than those exposed to faster heating rates (Fig. 7(b)), due to the formation of larger particles in the char. SEM imaging (Fig. 8) indicated that at lower temperature (<650 C) the particle size in the chars is small (>50 nm). Particle aggregation into clusters was also observed (Fig. 8(a and b)) and the average size of these clusters was reduced between 400 C and 600 C. Some porosity was present in the structure of the char for both samples processed at 400 C and 600 C. The particle size increased as the temperature was further increased. At 1000 C the size of the crystallites is far larger, typically around 500 nm, and pores of approximately 150 nm are present (Fig. 8(c)). Striations can also be detected on the surface of theses crystallites indicating an ordered crystalline structure. 3.5. Composition of the pyrolysis intermediate A number of techniques were considered to determine the identity of the inclusions. Evolved gas analysis was initially used, as it was an extension of the TGA protocol. The data recorded identified the presence of a carbon dioxide peak after most of the weight-loss has occurred (Fig. 9). From the TGA trace, it is clear that most of the weight-loss has occurred by t ¼ 50 min, also confirmed by FTIR data and the reduction in the intensity of the 2350 cm1 peak. A further peak indicating increased carbon dioxide concentration can be identified at t ¼ 75 min (750 C). This shows that there is a further, discrete evolution of carbon dioxide.
Fig. 3. Typical XRD traces.
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Fig. 4. Inclusions present in chars (a) 400 C, (b) 600 C, (c) 800 C, and (d) 1000 C.
Additional evidence that the inclusions were carbon was obtained from energy dispersive X-ray spectroscopy (EDS) coupled with SEM. Using back-scatter mode, the contrast between the inclusions and the titanium dioxide was increased allowing the inclusions to be readily identified. EDS analysis was conducted on several inclusions in several samples (mounted on the same stub
and exposed to the same conditions), as well as at sites that appeared to have no inclusions present. A char containing no inclusions was also used as the control. It was found that the inclusions showed a much more intense carbon peak (1600 counts) than that observed for the control (500 counts). This evidence, coupled with the TGAFTIR data, indicates that the inclusions are carbon. 4. Discussion
Fig. 5. Inclusions in the chars.
Optically transparent thin hybrid films were produced at all TiO2 loadings. However, due to large amounts of solvent present during the processing of the hybrid material, cracking was common, as detailed in other systems [24]. One method that has been used to reduce the problems associated with large volume shrinkage is to use ‘one-pot’ synthesis, where the sol–gel reaction and the polymerisation occur simultaneously, requiring less solvent. These techniques can lead to volume shrinkages of only 10–20% compared with 70% associated with conventional sol–gel techniques [19]. From the TGA traces, unfilled PVP contains 20 wt.% water under our laboratory conditions and equilibrium amounts of water were around 10 wt.% for TiO2 composites. Plasticisation of polymers with low molecular weight compounds is extensively used, the most prominent example being PVC with phthalate esters. For hydrophilic polymers such as nylons and in the present case PVP, low amounts of water fulfil the same role. Thus the unfilled PVP and the hybrids require low amounts of water for ductility. Nanodispersed TiO2 in polymer matrices can exhibit colours ranging from light yellow to red [25]. The origins of this colour are described by Rajh et al. [26] who propose interactions between ligands and TiO2 nanoparticles as being responsible. Due to the small size of the TiO2 particles (w20 nm), Ti–O bonds are compressed at the surface due to high curvature. This causes
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Fig. 7. AFM images of chars at (a) 50 C/min, (b) 1 C/min heating rates.
Fig. 6. AFM images of samples (a) high anatase content (700 C), (b) high rutile content (850 C).
a rearrangement of the Ti coordination from an octahedral to a square pyramidal arrangement, leading to the observed colour. XRD revealed the crystal structure of the chars. At lower temperatures (650 C), anatase is the only phase present with a characteristic peak at 25 . As the sample temperature approaches 700 C the rutile phase (27 ) begins to appear. The rutile phase continues to increase at higher temperatures with enlarged crystallites revealed in the SEM and AFM images. SEM and AFM images indicate that the anatase crystallites are smaller (20–100 nm) than the rutile crystallites (100–500 nm). It has been noted in systems based on silica [15] that the introduction of an inorganic component into the polymer can lead to the formation of a char on the surface of the material. This may form a mass transfer barrier preventing diffusion of oxygen into the hybrid and impede the release of pyrolysis products from the material. It is therefore common for the mass loss events to be delayed due to the formation of this ‘char-barrier’. For TiO2, it would appear that the char-barrier may have had a more localised effect, with the carbonaceous inclusions being small. Superimposed on this is the effect of the TiO2 particles on the polymer. Whilst TiO2
can exhibit a range of catalytic effects, one that is particularly known in polymeric matrices is the promotion of degradation. The TGA data shows the reduction of the onset of pyrolysis event by about 50 C consistent with data published elsewhere [27]. Char structure was found to be highly dependent upon the thermal history. At 800 C and above the appearance of large crystallites was noted, but below this temperature the structural features shown by AFM were much smaller. This can be attributed to the transition between the anatase and the rutile phase, which is well documented. The XRD data supports this observation, as it shows that in a sample that has been treated to a temperature of 800 C there is more rutile than anatase present. As the temperature is further increased, more rutile is formed and the titania particles become larger. SEM images also indicate that between 400 C and 600 C the chars consist of aggregates of smaller particles which have sintered together. These particles shrink as the processing temperature is increased from 400 C to 600 C due to the evolution of the organic component as gas, leading to a reduction in the volume of material present. Crystallite size increases as processing temperature is increased due to the growth of the rutile crystallites. At 1000 C, large rutile crystallites exist with striations on the surface. The presence of pores in the structure is due to contraction of the network and could indicate a deficiency in TiO2 at this site during crystal growth. These pores may correspond to sites where black pyrolysis intermediates were present, resulting in the lower TiO2 concentration proposed. Whilst the crystalline texture and phase composition of TiO2 is well-known to change with temperature, as indicated above, a further complication exists where a polymer matrix is also present. We have noted the presence of macroscopic black pyrolysis intermediates between 550 C and 800 C, which are not commonly discussed in
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Fig. 8. SEM micrographs of chars (a) 400 C, (b) 600 C, (c) 1000 C.
the literature. At lower temperatures graphitic char or similar polycyclic residues are likely to have been formed. At high temperatures, and in the presence of TiO2, it is likely that hydrogen is being stripped and the introduction of C]C double bonds occurs, as commonly arises in cracking furnaces. There is also some potential for the formation of titanium oxycarbide (TiOC) which has been observed in systems containing mixed oxides of silica and TiO2 [23]. TiO2 content in the hybrid nanocomposite also influenced the formation of pyrolysis intermediates. These intermediate species were only observed in highly loaded samples (>50 wt.%) indicating that there may be some threshold required for their formation, somewhat analogous to a percolation threshold. At high temperature the pyrolysis intermediates are finally destroyed by oxidation as reflected in the evolved gas analysis and observed by optical microscopy. The elemental composition of the inclusions was investigated using evolved gas analysis and further examined using EDS coupled with SEM which determined carbon was present. The evolved gas analysis detailed the appearance of a carbon dioxide peak after almost all weight-loss had occurred (as indicated by the TGA trace). The inclusions are also likely to contain a large percentage of TiO2, as the initial material is homogenous and it is expected that TiO2 is still well dispersed in the inclusion. The relative mass of carbon as intermediates in the samples is extremely small. For applications where these types of materials are to be used as precursor materials it is essential to know the conditions under which inclusions are formed. Two conditions that lead to the formation of inclusion-free chars in TiO2/PVP hybrids have been
Fig. 9. Evolved gas analysis contour indicating the late CO2 peak at 2350 cm1.
identified: extended time at the final pyrolysis temperature, or using a slower heating rate which also leads to removal of inclusions. Both techniques essentially lead to an increase in the amount of time that the material spends at elevated temperature. It would seem that time is therefore the most important variable in the formation of inclusions in these types of system. 5. Conclusions The use of a range of heating rates during the pyrolysis of PVP/ TiO2 hybrid materials leads to the formation of chars with different structures. Faster heating rates also lead to the increased appearance of inclusions in the chars. Pyrolysis intermediates were determined to be carbon-containing using FTIR and SEM/EDS. This is likely to be the formation of graphitic char due to stripping of hydrogen, which is evolved via oxidation at higher temperature (>800 ). The anatase to rutile transition, which is well documented, was identified as beginning at approximately 650 C. Below this temperature, anatase was the only crystalline phase identified in the sample and at 900 C or above rutile was the only phase present. References [1] Lima RS, Kruger SE, Marple BR. Towards engineering isotropic behaviour of mechanical properties in thermally sprayed ceramic coatings. Surf Coat Technol 2008;202(15):3643–52. [2] Francioso L, Prato A, Siciliano P. Low-cost electronics and thin film technology for sol–gel titania lambda probes. Sensor Actuator B 2008;128(2):359–65. [3] Cheng P, Deng CS, Gu MY, Dai XM. Effect of urea on the photoactivity of titania powder prepared by sol–gel method. Mater Chem Phys 2008;107(1):77–81. [4] Asif KM, Sarwar MI, Raqif S, Ahmad Z. Properties of PVC–titania hybrid materials prepared by the sol–gel process. Polym Bull 1998;40(4–5):583–90. [5] Greenwood NN, Earnshaw A. Chemistry of the elements. 1st ed. Oxford: Permagon Press; 1984. p. 1542. [6] Chiarello GL, Selli E, Forni L. Photocatalytic hydrogen production over flame spray pyrolysis-synthesised TiO2 and Au/TiO2. Appl Catal B 2008; 84(1–2):332–9. [7] Jitputti J, Rattanavoravipa T, Chuangchote S, Pavasupree S, Suzuki Y, Yoshikawa S. Low temperature hydrothermal synthesis of monodispersed flower-like titanate nanosheets. Catal Commun 2009;10(4):378–82. [8] Lee BI, Wang X, Bhave R, Hu M. Synthesis of brookite TiO2 nanoparticles by ambient condition sol process. Mater Lett 2006;60(9–10):1179–83. [9] Kalousek V, Tschirch J, Bahneimann D, Rathousky J. Mesoporous layers of TiO2 as highly efficient photocatalysts for the purification of air. Superlattices Microstruct 2008;44(4–5):506–13. [10] Kickelbick G. Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale. Prog Polym Sci 2003;28(1):83–114. [11] Schubert U, Husing N. Synthesis of inorganic materials. 2nd ed. Weinheim: Wiley-VCH Verlag GmBH & Co. KGaA; 2005. [12] Chen XB, Mao SS. Synthesis of titanium dioxide (TiO2) nanomaterials. J Nanosci Nanotechnol 2006;6(4):906–25. [13] Schmidt H. Chemistry of material preparation by the sol–gel process. J NonCryst Solids 1988;100(1–3):51–64.
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