Enhanced Stability Effect in Composite Polymeric Nanofibers

Jul 24, 2009 - Falko Wesarg , Franziska Schlott , Janet Grabow , Heinz-Dieter Kurland ... Fei Peng , Montgomery T. Shaw , James R. Olson , and Mei Wei...
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J. Phys. Chem. C 2009, 113, 14893–14899

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Enhanced Stability Effect in Composite Polymeric Nanofibers Containing Titanium Dioxide and Carbon Nanotubes S. Kedem, D. Rozen, Y. Cohen, and Y. Paz* Department of Chemical Engineering, Technion, Haifa 32000, Israel ReceiVed: January 25, 2009; ReVised Manuscript ReceiVed: May 15, 2009

Composite poly(acrylonitrile) nanofibers containing nanometric TiO2 particles and multiwalled carbon nanotubes have been prepared by the electrospinning technique. The photocatalytic activity of the composite fibers was measured in the absence of contaminants as well as with acetone, or with the dye rhodamine 6G. TEM images, as well as FTIR measurements, showed that the presence of carbon nanotubes reduced the extent by which the polymeric matrix was degraded upon exposure to UV light. At the same time, no adverse effect on the ability of the photocatalyst to degrade organic molecules was observed, and in the case of rhodamine 6G, an enhanced degradation rate was observed. These apparently contradicting results are explained by a mechanism that is based on the transport of photoinduced charge carriers from the photocatalyst particles to the carbon nanotubes, which may induce a variety of effects, depending on the degradation mechanism and reaction loci. Introduction Composite photocatalysts, based on titanium dioxide and inert supports, are becoming more and more popular in the field of photocatalysis. This popularity reflects, for most cases, the success of the “Adsorb & Shuttle” (A&S) approach, according to which contaminants are first adsorbed in the vicinity of photocatalytic domains and subsequently diffuse to the titanium dioxide domains, where they are degraded. This method was found to be very effective in the degradation of contaminants that otherwise hardly adsorb on the photocatalyst.1,2 Tailoring the inert domains in a manner that enhances the physisorption of specific contaminants facilitates to rectify the photodegradation of specific contaminants, for example 2-methyl naphthoquinone3 or di-isopropyl methyl phosphonate.4 Composite photocatalysts may affect the emission of intermediate products or even the distribution of end-products. For example, the use of activated carbon as the inert component was found to be very beneficial in reducing the emission of toxic intermediates such as phosgene, in the degradation of trichloroethylene.5 Likewise, it improved the ratio between carbon dioxide and carbon monoxide in the photodegradation of the dyestuff rhodamine 6G.6 The coupling of photocatalysts and inert supports may operate not only via the diffusion of the contaminants from the inert supports to the photocatalytic sites but also through spillover of oxidating species formed on the photocatalysts, as was suggested for the degradation of rhodamine 6G in a system made of SiO2 and TiO2.7 Such a possibility was confirmed by measuring the photodegradation of cross-linked self-assembled monolayers that were chemisorbed onto microdomains of silicon dioxide,8 located in the vicinity of titanium dioxide, as well as by remote photocatalytic mineralization of soot.9,10 It is well-known that electron-hole charge separation is a key factor in achieving high quantum efficiency. For this reason, doping the photocatalyst with noble metals, which act as electron traps, such as platinum, palladium or gold is often considered.11 A few years ago it was shown that titanium dioxide was capable * Corresponding author. E-mail: [email protected].

of injecting electrons into adsorbed C60 with a quantum yield as high as 24% upon exposure to UV light.12 Based on the similarity between fullerenes and carbon nanotubes (CNTs) and on the fact that CNTs present unique electronic properties, it was postulated that coupling between CNTs and TiO2 could improve charge separation and hence induce photocatalytic effects beyond those that could be expected from other composite photocatalysts. Indeed, the rectifying nature of the MWCNTs-TiO2 systems was confirmed by measuring the I-V curves of aligned MWCNTs coated with nanoparticles of titanium dioxide.13 Nanocomposite particles containing both titanium dioxide and carbon nanotubes were prepared recently in a variety of ways,14 among which were refluxing of TiO2 nanoparticles made in situ by hydrolysis of TiCl4,15 and by the sol-gel route.16 These nanocomposites served for the photocatalytic oxidation of contaminants, as well as for inactivation of bacterial endospores,17 and even as a means to characterize defects in single walled CNTs.18 Enhancement of photocatalytic oxidation of organics upon using TiO2/CNT nanocomposites was observed in many substances including acetone,19 azo dyes,20 phenol,21 and methylene blue.22 For most cases, this enhancement was attributed to improved charge separation, i.e. to the suppression of the recombination of photogenerated electron-hole pairs, following transport of photoinduced electrons from the photocatalyst to the CNTs. Due to its large surface area, a nanofibrous mat structure may have an obvious advantage, provided that it has sufficient mechanical integrity. In this context, electrospinning (ES) is one of the easiest and cheapest preparation methods. The ES method is based on electrostatic surface charging of a polymer solution droplet from an air pressured piston, which contains the polymer solution, thus drawing a jet of polymer, moving at high speed toward a grounded rotating or stationary surface. A high voltage field of up to a few kV cm-1 is applied between a capillary outlet and a grounded surface, thus creating high extensional flow which stretches the fiber many times with respect to its original diameter to form continuous nanofiber in the diameter range of tens of nm.23 Indeed, a nanofibrous TiO2 mat was produced via this method by electrospinning of an ethanol

10.1021/jp9007366 CCC: $40.75  2009 American Chemical Society Published on Web 07/24/2009

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solution containing poly (vinyl pyrrolidone) (PVP) and titanium isopropoxide, followed by calcinations at 500 °C.24 Likewise, this technique was used to produce TiO2-embedded polymeric fibers,25 ceramic hollow nanofibers,26 CNT-reinforced polymer nanofibers,27,28 and TiO2-CNT composites.29 Recently, a solution containing poly(vinyl acetate), titanium dioxide and CNTs was electrospun to produce fibers that were calcined to burn the polymer thus yielding a TiO2-CNT composite.30 The developing of a ternary nanofiber containing both CNTs and TiO2 embedded in polyacrylonitrile (PAN) was reported some time ago by us.31 PAN was chosen as the matrix of this nanofiber as it may easily be carbonized, thus avoiding possible photodegradation of the fiber. It is noteworthy that the morphology of carbonized PAN nanofibers is influenced by the presence of CNTs.44 In this manuscript, we report on a most interesting phenomena observed upon performing photocatalytic measurements with noncarbonized fibers, namely, a stabilizing effect on the self-photodegradation of the polymer induced by the coupling of titanium dioxide and CNTs. To our surprise, and as will be presented below, this stabilizing effect was not accompanied by a reduction in the ability of the ternary nanofibrous film to photodegrade adsorbed pollutants. Experimental Section Three types of solutions were prepared and used for obtaining nanofibrous mat by the electrospinning procedure. All three solutions used dimethylformamide (DMF) (AR, Gadot) as the carrier volatile solvent. PAN-TiO2 Dispersion. Polyacrylonitrile (PAN, Scientific Polymer Products, Inc., MW ≈150,000) was dissolved in hot (65 °C) dimethylformamide (DMF) (AR, Gadot) to give a 9% (by weight) PAN solution. In parallel, titanium dioxide particulate powder (Degussa P-25) was added to DMF to give a 4% by weight dispersion. The dispersion was sonicated for 2 h in a 43 kHz Delta D2000 sonicator, thus forming a homogeneous milklike slurry. The PAN solution was then added to the P25 dispersion at a weight ratio of 1:1, and the dispersion was stirred. The resulting PAN-TiO2 dispersion contained 4.5% of PAN and 2% of TiO2 by weight, dispersed in DMF. PAN-MWCNT Dispersion. MWCNTs (average diameter 20-30 nm, purity 95%) were purchased from anoLab and used as received. The MWCNTs were added to DMF, to give a dispersion containing 3% by weight of CNTs. The dispersion was stirred for 15 min, and then sonicated for 15 min, thus forming a black slurry. A PAN solution (9% by weight in DMF) was then added to the slurry at a weight ratio of 1:1. The fluid was magnetically stirred for 15 min and then sonicated for 3 h. The resulting MWCNTs dispersion (DMF containing 1.5% MWCNTs and 4.5% PAN by weight) was homogeneous, stable for months, and exhibited a dark inklike appearance. PAN-MWCNT-TiO2 Dispersion. A PAN-MWCNT dispersion (DMF containing 4.5% PAN, 1.5% MWCNT) was mixed with a PAN solution (9% by weight in DMF) at a weight ratio of 1:1 to yield a PAN-enriched PAN-MWCNT dispersion. Then, a TiO2 dispersion of 6% in DMF was prepared and added to the PAN-enriched PAN-MWCNT dispersion at a 1:2 ratio. The resulting spinnable DMF dispersion contained 4.5% PAN, 0.5% MWCNT and 2% TiO2. The electrospinning process was conducted at room temperature under air backpressure. The spinning distance between the outlet capillary and the collecting disk was 15 cm at electric field strength of 0.8 kV/cm. Oriented ropes of nanofibers were collected in a converging electric field on a rotating disk with a tapered edge rotating at a speed of 500 rpm. Following solvent

Kedem et al. evaporation the averaged compositions (by weight) of the fabricated fibers (based on their preparation procedure) were 75% PAN and 25% MWCNTs in the MWCNTs fibers, 69% PAN and 31% TiO2 in the TiO2 fibers and 64% PAN, 7% MWCNTs, 29% TiO2 in the ternary fibers. More details on the preparation of the films are given elsewhere.31 Specimens used in TEM imaging were prepared by direct deposition of electrospun nanofibers onto a copper grid coated by a holey carbon film. The samples were imaged using low electron dose imaging and an acceleration voltage of 120 kV on a Philips CM120 TEM. The surfaces of the nanofibers and nanofibrous mats were also characterized by high resolution scanning electron microscopy (HRSEM) using a Leo Gemini 982 HRSEM at an acceleration voltage of 2-4 kV. SPM images and I-V curves at specific locations were acquired by a Pico+ SPM machine (Molecular Imaging) working in the contact mode with a CSC21 Cr-Au conducting tip. Three types of photodegradation measurements were carried out, in which the degradation kinetics of acetone, the dyestuff rhodamine 6G and the fibers themselves in the absence of a contaminant were monitored. The measurements with acetone were carried out by introducing 0.003 mL of the reactant into a cylindrical glass reactor (110 mL in volume), having two IRtransparent windows, equilibrating adsorption in the dark, and exposing a 2.5 cm × 2.5 cm mat to a low flux (0.6 mW cm-2) broad band 365 nm light which entered the reactor through a fused silica window. The same vessel was used for the selfphotodegradation experiments. To prevent possible inaccuracies due to variations in the absorption of light by the various samples, care was taken to ensure that the nanofibrous mat that was used was optically thick (approximately 15 µm), such that all impinging photons were absorbed. The mineralization rates of the films were evaluated by integrating the absorption signal of the 2357 cm-1 peak of CO2, as measured by a Bruker IFS55 spectrophotometer, while degradation kinetics were deduced based on changes in the CdO stretch peak (1735 cm-1) of acetone, respectively. Complementary data was obtained by TEM imaging of thin films prepared on copper grids. Photodecolorization measurements of rhodamine 6G (R6G) were performed in the aqueous phase by exposing a fused silica vessel (5 cm in diameter) containing a 2.5 cm × 2.5 cm mat and 15 mL of 4 × 10-4 M R6G solution to 365 nm light (80 µW cm-2). Kinetics were deduced based on the 518 nm peak in the UV-vis absorption spectrum of the dyestuff, obtained by taking aliquots and measuring them in a Lambda40 (PerkinElmer) spectrophotometer. Results Figure 1 presents a typical TEM image of a PAN fiber, containing both TiO2 (Degussa P25) and MWCNTs. Typical diameters of such fibers were between 50 and 170 nm, depending on the electric field between the syringe and the ground, as discussed before. As demonstrated in the image, the titanium dioxide particles tend to aggregate near the outer part of the fiber, thus exposing themselves to potential adsorbates. In that sense, the structure was very similar to a binary system, containing P25 within a PAN matrix, but differed significantly from PAN-CNT fibers, which had a smooth surface. As shown in Figure 1, the CNTs in the ternary system were individually separated and aligned along the fibers. Similar separation was observed also in the binary CNT-polymer system. In many cases the CNTs in the ternary fibers were in close contact with the P25, sometimes even connecting between adjacent P25

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Figure 1. A typical TEM image of a nanofiber containing both TiO2 and CNT, as produced. S denotes the copper grid. Bar ) 100 nm.

Figure 2. SPM deflection contrast image of a ternary CNT-TiO2-PAN fiber scanned in the contact mode, using a conductive tip. The image reveals areas of TiO2 aggregates (one of which is marked with (c)) within the fiber as well as areas where no TiO2 aggregates can be noticed (b).

aggregates within the fiber. In other cases, the P25 aggregates remained nonconnected to the well-aligned carbon nanotubes. A scanning probe microscopy deflection image of a ternary fibers mat laid on a conductive support (HOPG) is presented in Figure 2, which shows an aggregate of P25 particles within the fiber. Current vs voltage (I-V) graphs were performed on the various fibers (PAN-TiO2, PAN-MWCNT-TiO2) by positioning the conductive tip at specific loci on the fibers and measuring the current as a function of an applied bias (here, positive voltage means a positive potential to the tip). Figure 3 portrays the obtained I-V curves in the absence of any light (dashed lines) as well as under exposure to UV light (solid lines). It should be emphasized that the under-light measurements were taken in situ exactly at the same position where the “dark” measurements were taken. In the case of a binary PAN-TiO2 fiber (Figure 3a) no current could be measured in the dark. In contrast, under UV light, the application of a 3 V bias gave an apparent breakdown behavior. This was not the case for measurements taken with ternary PAN-TiO2-MWCNT fibers (Figures 3b, 3c). Here, I-V data taken at loci where no TiO2 aggregates were present (denoted as (b) in Figure 2) revealed a very small, yet measurable, current (0.5 nA at a bias of 5 V), which was approximately doubled upon exposure to light. A

Figure 3. Current vs voltage curves of the composite fibers obtained by SPM in its current sensing mode, using a conductive tip. Dashed lines represent measurements in the dark whereas solid lines represent measurements obtained under 254 nm light. Three different cases are depicted: a TiO2-PAN fiber (a), a ternary TiO2-CNT-PAN fiber measured at a location where no TiO2 could be observed (b) and a ternary TiO2-CNT-PAN fiber measured at a location where a TiO2 aggregate could be noticed (c).

clear difference between these I-V curves versus those taken from ternary fibers on TiO2 aggregates’ loci (denoted as (c) in Figure 2) was observed. Here, an increase in the current, in a spiked-step form, was measured upon exposure to the UV light. It should be stressed that this type of steplike behavior in the I-V curve was revealed each time that the measurements were taken in locations containing titania particles (and presumably also CNTs); nevertheless the exact voltages at which these steps were found varied from samples to sample. No spiked-step behavior was noticed under dark conditions. It is clear that most of the bias voltage drop occurred at the dielectric PAN matrix. Furthermore, one cannot expect to get the actual voltage drop across the TiO2 particles during the measurements, since the number of particles, their arrangement, the local dielectric environment of the polymeric fiber and the details on the contacts between the TiO2 particles and the CNTs

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Figure 4. TEM images of tertiary nanofibers, following exposure to UV light: (A) an area where the TiO2 aggregate was not attached to a MWCNT; (B) an area where the TiO2 aggregate was attached to a MWCNT. Bar ) 50 nm.

around them are unknown and were different for each sample. This explains both the variance in the bias voltages needed to get the breakdown portrayed in Figure 3a and the variance in the location of the steps in the I-V curves measured for the ternary system above a TiO2 aggregate (Figure 3c). The fact that this spiked-step behavior was found only above titania particles embedded in the ternary system, but not on the binary PAN-TiO2 system or on “b” locations (as denoted in Figure 2) on the ternary fiber, suggests a strong electrical coupling between the MWCNTs and the TiO2, that not only enables charge transport and reduces the apparent electrical resistance across the fiber but, in fact, may map electronic levels in the coupled system, which might be filled upon charge transport. Such coupling between titanium dioxide and multiwalled carbon nanotubes showing heterogeneous junctions’ properties was recently demonstrated for an array of aligned MWCNTs coated with titania particles.13 The present results demonstrate that such a situation may exist also in the ternary fiber, at places where titania aggregates are noticed. Ternary fibers, prepared on TEM grids, were exposed to UV light for 16 h and then imaged by TEM. Figure 4A presents a typical image, taken from such fibers, at locations where the titanium dioxide aggregates were not physically connected to the carbon nanotubes, whereas Figure 4B shows a typical image taken from the same exposed sample, but from locations where the TiO2 particles were in physical contact with the CNTs. Thinning of the sample close to the P25 aggregates is clearly observed in Figure 4A, indicating the deterioration of the fiber following exposure to the UV light. In contrast, TEM images of nanofibers in locations where the P25 aggregates were physically connected to the CNTs (Figure 4B) hardly revealed any deterioration of the polymer fiber. The fact that both images were taken from exactly the same mat negates a possibility of artifacts due to different conditions during preparation or exposure. In fact, this difference repeated itself again and again regardless of preparation and exposure conditions. The self-photodegradation of nanofibrous fabrics was quantified by exposing them to 365 nm light and measuring changes in the IR spectrum of the reactor’s gaseous content, in the absence of any contaminant. Upon exposure, an increase in CO2related peaks at 2350 cm-1 and at 670 cm-1, as well as that of water vapor at 1400-1800 cm-1, was observed (Figure 5). No other peaks were noticed by FTIR, indicating the absence of any IR-active products in the gas phase. Figure 6 presents the kinetics of carbon dioxide formation upon exposure of a PAN-TiO2 fabric and a PAN-CNT-TiO2 fabric to the UV light. The kinetics was deduced based on the integrated absorbance of the 2348 cm-1 peak. Both experiments were performed with the same exposed area, and with optically

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Figure 5. Infrared spectra of the gas content of the photoreaction vessel, containing ternary CNT-TiO2-PAN fibers, in the absence of a contaminant: (A) prior to exposure; (B) following exposure for 5 h; (C) following exposure for 31 h; (D) following exposure for 50 h. The baselines in the figure were shifted vertically for clarity.

Figure 6. The integrated absorbance of the 2350 cm-1 peak of CO2 formed upon exposure of PAN-TiO2 fibers (empty triangles) and PAN-TiO2-CNT (filled squares) to 365 nm light.

thick fabrics, negating the possibility that differences in the degradation rates could have originated from variations in light absorption following possible variations in the thickness of the films. For both systems, the CO2 concentration rose linearly with irradiation time, suggesting that the availability of the fabrics to the degrading species was not altered during the irradiation. The figure clearly demonstrates that the photomineralization of the fibers was slowed down by a factor of 3 in the presence of CNTs, thus confirming the observations based on the TEM images that showed that the presence of CNTs prevented the photodegradation of the nanofibers. Acetone was decomposed photocatalytically on PAN-TiO2 and on PAN-MWCNT-TiO2 nanofibrous fabrics following an adsorption time of 48 h in the dark. The adsorption kinetics were inferred from changes in the height of the acetone-related peaks at 1735 cm-1, 1367 cm-1 and 1212 cm-1, representing the ν(CO), δ(CH) and νa(CC) bands, respectively. The adsorption kinetics of the acetone in the dark was followed prior to exposure. Similar adsorption kinetics was found for the two types of fabrics. Upon exposure to UV light, extensive decrease in the acetone-related peaks was observed in parallel to the appearance of CO2 peaks. No intermediate species were recorded. The kinetics of acetone photodegradation was measured by tracking the integrated absorbance of the 1735 cm-1 peak of acetone as a function of time (Figure 7). The kinetics appeared to be of a first order, with the fitting parameter being 0.99 or higher. The rate constant of acetone disappearance in the system containing CNTs was approximately equal to the rate constant

Polymeric Nanofibers Containing TiO2 and Carbon Nanotubes

Figure 7. The reduction of the acetone 1735 cm-1 peak during its decomposition by PAN-TiO2 (empty triangles) and PANMWCNT-TiO2 (filled circles) mats. Exponential fit is displayed.

Figure 8. CO2 production during the UV (365 nm) photodegradation of acetone on a mat of PAN-TiO2 (empty triangles) and PAN-MWCNT-TiO2 (filled circles).

in the PAN-TiO2 system (0.014 h-1 and 0.016 h-1, respectively). It is noteworthy that this equal rate was in contrast to the observations made in the absence of any contaminant, where the presence of CNTs reduced the rate of photodegradation of the fabric. The integrated absorbance of CO2 as a function of irradiation time for the two nanofibrous systems is portrayed in Figure 8. A comparison between the two systems reveals that the rate of CO2 formation in the absence of CNTs was significantly (26%) higher than the rate with the ternary system. At first glance, this seems to be uncorrelated with the results presented in Figure 7; however, the discrepancy is explained easily if one takes into account that the formation of carbon dioxide was not only due to acetone degradation but also due to the degradation of the polymeric matrix. In other words, the effect of the MWCNTs embedded within a TiO2-containing PAN fiber is to reduce the rate by which the fiber is degraded (hence less CO2 is produced) but at the same time without reducing the rate by which the gas-phase contaminant (acetone) is degraded. In this context, it is noteworthy that the rate of CO2 production in the selfdegradation process by the PAN-TiO2 substrates was higher by 60% than during the degradation of acetone, in contrast to the expectation that the production of carbon dioxide would be higher in the latter case, where more chemical species are available for the photocatalytic reaction. Some estimation regarding the relative contribution of acetone degradation in the CO2 production can be obtained by considering the fact that the degradation rate of acetone in the ternary system is similar to that in the binary (Figure 7) and by assuming

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Figure 9. Photodegradation kinetics of rhodamine 6G dissolved in water on a mat made of PAN-TiO2 (empty triangles) and PAN-MWCNT-TiO2 (filled squares).

that the 3:1 ratio between the fiber-degradation rate in the PAN-TiO2 system and the PAN-CNT-TiO2 system is not changed upon introducing acetone. In that case, a fast calculation suggests that for the ternary case only 20% of the total CO2 emission was due to self-degradation of the fibers, while for the binary system this percentage is doubled. In any case, one should take into account that a discrepancy between the rate of disappearance of reactants and the rate of CO2 production is expected when intermediate products are formed. Since no IRactive species were found, one may conclude that the intermediate products either are adsorbed on the surface or are IR-inactive. The photodegradation of the dye Rhodamine 6G in aqueous medium was studied using mats of fibers overlaid on fused silica. Figure 9 presents the kinetics of Rhodamine 6G degradation as inferred from UV-vis measurements of solution aliquots. It is evident that the degradation rate with the fibers that contained both TiO2 and CNT was faster by a factor of 2 than with fibers that did not contain CNTs. Discussion The enhanced stability of the composite PAN fibers toward photocatalysis cannot be denied and is supported by the TEM images as well as by measuring the amount of CO2 released upon self-degradation. The fact that the stabilizing effect was noticed only with respect to the fibers but not with contaminants such as acetone, or rhodamine is intriguing and requires a rationalization that will explain all observations. As mentioned in the Introduction, composite photocatalytic materials may affect the photodegradation rates of contaminants either by improving mass transport (“Adsorb & Shuttle”) and/ or by improving the separation of photoinduced charge carriers due to migration of charge carriers from the titanium dioxide particles to the inert domains. In that sense, the degradation of the fibers represents a simpler system, as the only possible effect is that of mass transport of the photoinduced oxidizing species. A question is then raised regarding the mechanisms that lead to a smaller flux of active species arriving at the fibers upon connecting between the CNTs and the titanium dioxide particles. PAN is known to be photocatalytically degraded by an oxidative mechanism, operating through hydroxyl radicals, as inferred upon comparison with measurements under H2O2/UV.32 To degrade the fibers, these radicals (or their precursors, H2O2 for example) may have to transverse a large distance thus creating the so-called “remote degradation effect”.8 In that case, a possible scenario may include the oxidation of adsorbed water by holes to form OH radicals, formation of labile hydrogen peroxide from two hydroxyls, diffusion toward the fiber and a

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direct photolysis33 of the hydrogen peroxide to yield again the OH radicals. Other oxidizing species (including hydroxyl radicals) could be considered as well. Recently, an electron spin resonance study revealed that MWCNTs scavenge very effectively both superoxide radicals and hydroxyls.34 This remarkable finding was in contrast to the researchers’ working hypothesis that inflammatory and fibrotic response observed in lungs exposed to CNTs originated from the formation of free radicals such as hydroxyls and superoxides. This radical scavenging effect may explain the enhanced stability of the composite PAN fibers toward photocatalysis as it explains why the number of radicals reaching the fibers is reduced. It is noteworthy that this scavenging effect is not limited to metallictype CNTs but was found also in other metallic systems. For example, it was found that the probability for reaction of hydroxyl radicals and gold was more than 2 orders of magnitude bigger than that of hydroxyl radicals and titania.35 Another example is the super stability of self-assembled monolayers (SAMs) chemisorbed on microislands of gold and platinum located in the vicinity of UV-exposed titanium dioxide, which was in contrast to the ease by which SAMs that had been chemisorbed on similar-size islands made of silicon dioxide were photodegraded.3 Other species that are capable of attacking PAN fibers are the superoxide anions (O2*-) and radicals (OOH) that are formed by reduction of adsorbed oxygen. This attack can be a primary attack, as in the case of acetaldehyde,36 or, more likely, a secondary step as observed in the degradation of dimethoxybenzene37 and n-octane.38 In the latter case, the primary oxidative attack by OH radicals is followed by a reductive step producing organoperoxy radicals that lead to the formation of tetraoxides, which eventually degrade all the way to mineralization. Unfortunately, the superoxide-involving successive steps are often overlooked in the literature. It is believed that taking these steps into consideration may explain, at least partially, the inconsistency between superb photocurrent results often obtained upon biasing of TiO2 electrodes and mediocre photocatalytic performance of such electrodes. The role that superoxides play in the photodegradation process may suggest a complementary explanation for the stability effect. This explanation is based on the assumption that coupling between CNTs and TiO2 assists in separating the photoinduced charge carriers by virtue of electron transport from the photocatalyst to the CNTs (this assumption is discussed below in the context of explaining the enhanced rate of photodegradation of contaminants). Here, several possible reasons for reducing the amount of superoxides might exist. Unlike fullerenes, MWCNTs do not exhibit electron-acceptor features, but rather behave like conductive nanowires. This means that electrons are not necessarily trapped on the CNTs and accordingly the likelihood of producing superoxide anions from adsorbed oxygen should be low, and maybe even lower than that on native titanium dioxide. Another reasoning for reduced rate of labile superoxides is related to the adsorption of oxygen. It was shown that oxygen molecules adsorb onto CNTs physically on nondefect sites,39 with some tendency to be adsorbed chemically on defect sites.40 The formation of OOH species from adsorbed dioxygen depends both on the adsorpticity of oxygen on the surface and on the presence of adsorbed water molecules. The heat of adsorption of oxygen is similar on nonhydrated TiO2 and CNTs (15-18 kJ/mol41 and 18.5 kJ/mol,39 respectively), nevertheless the tendency of the polar TiO2 surface to adsorb water is by large greater than that of the nonpolar carbon nanotubes. Taking that

Kedem et al. hydration of titanium dioxide surfaces promotes photoadsorption of oxygen,42 suggests that in practice the adsorpticity of oxygen on TiO2 is larger than that on CNTs. Hence, it is expected that the formation of labile OOH species will be reduced in a situation where the electrons flow away from the titanium dioxide into CNTs. As mentioned above, the photodegradation of contaminants on the composite nanofibers is somehow more complex than the degradation of the fibers as it not only involves the transport of species from the particles of the photocatalyst but also may involve the adsorption of species in the vicinity of the particles, followed by possible diffusion from the inert fibers toward the titanium dioxide particles. The two contaminants mentioned in this study present two different types. Acetone is degraded by an oxidative mechanism and, being a polar molecule, is adsorbed easily on the surface of titanium dioxide, where its degradation by the nanofibers was studied in the gas phase. The dyestuff rhodamine 6G differs from acetone, being a species that hardly adsorbs on TiO2 and whose degradation was studied in the aqueous phase in this work. Charge transport between carbon nanotubes and titanium dioxide is well-documented. Such effects do exist also in the fibers as is inferred from the I-V curves of ternary fibers, performed by a SPM tip (Figure 3). In fact, transfer of electrons from titanium dioxide into CNTs, thus enhancing charge separation and reducing recombination rate, is often mentioned in the literature as the reason for enhanced degradation rates obtained with composite photocatalysts containing titanium dioxide and carbon nanotubes.19 This explanation, which treats the MWCNT-TiO2 interface as a rectifying heterojunction, is, to a large extent, similar to the reasoning for the benevolent effect of noble metals on photocatalytic efficiency and the photoinduced electron reduction of C60 in colloidal suspensions of titanium dioxide.43 Here, it is assumed that, upon exposure, electrons flow toward the metal, while holes drift to the TiO2 surface. This causes a reduced rate of recombination, hence, faster degradation of contaminants. The charge separation mechanism goes well with the high absorpticity of acetone on the photocatalyst, and its photocatalytic degradation mechanism which is based on a hydroxyl radicals attack. In this case, expected reduction in the amount of superoxides on the photocatalyst surface is likely to have no more than a minor effect on the degradation rate, especially if one takes into account the reduced recombination rate on the particle (part of it is counter-balanced by annihilation of OH radicals at the CNTs-TiO2 interface, as described above). It is known that rhodamine 6G does not adsorb efficiently on titanium dioxide, and accordingly its degradation rate can be enhanced significantly on composites containing adsorptive domains. This enhancement effect was explained in terms of spillover of active species (SiO2-TiO2 system7) or by an Adsorb & Shuttle mechanism (activated carbon-TiO2 system6). An enhanced rate of photocatalytic degradation of azo dyes in a system containing a mixture of multiwalled CNTs and P25 was attributed by the authors not only to improved adsorption of the dye molecules but also to a decreased possibility of electron-hole recombination.20 Although the data presented here cannot provide a definitive answer which of these mechanisms is the dominant, it is proposed, based on the similarity between activated carbon and CNTs, together with the results on the self-degradation of CNTs, that the A&S mechanism is the dominant one for the photodegradation of this dye on the ternary fibers.

Polymeric Nanofibers Containing TiO2 and Carbon Nanotubes Conclusion The coupling between titanium dioxide and carbon nanotubes, within nanofibers made of polyacrylonitrile, was found to reduce the rate of self-photodegradation of the fibers. At the same time, not only was an adverse effect on the ability of the photocatalyst to degrade organic molecules in the gas or in the liquid phase not observed, but, in fact, the degradation rate of these molecules was found to increase. These apparently contradicting facts were explained by a mechanism that is based on the transport of photoinduced charge carriers from the photocatalyst particles to the carbon nanotubes, which may induce a variety of effects, depending on the degradation mechanism. Acetone is adsorbed mainly on the catalyst and degraded by oxidation with hydroxyls; therefore its degradation rate was found to be insensitive to the presence of coupled CNTs. Rhodamine 6G, which also degrades oxidatively, is adsorbed mainly on the CNTs, and its enhanced degradation can be explained in terms of an “Adsorb & Shuttle” mechanism where the contaminants are first adsorbed on the CNTs and then diffuse to the interface of the photocatalytic particles where they are degraded. As for the ternary fibers, it is believed that their decreased rate of degradation is due to a lesser number of labile active species such as H2O2 and/or superoxides that transverse the distance to the fibers. The finding that coupling between a photocatalyst and an inert material may cause two opposite effects depending on the mechanism, reaction location and mobility of degrading molecules is intriguing and might pave a way to the developing of composite photocatalytic functional materials, utilizing robust, insensitive polymeric matrices. Work along this line is currently being done. Acknowledgment. The support of the Russell-Berrie Institute, the Israel Science Foundation, the Grand Water Research Institute and the Technion’s VPR fund is gratefully acknowledged. References and Notes (1) Torimoto, T.; Ito, S.; Kuwabata, S.; Yoneyama, H. EnViron. Sci. Technol. 1996, 30, 1275. (2) Uchida, H.; Ito, S.; Yoneyama, H. Chem. Lett. 1993, 1995. (3) Ghosh- Mukerji, S.; Haick, H.; Paz, Y. J. Photochem. Photobiol., A 2003, 160, 77. (4) Sagatelian, L.; Sharabi, D.; Paz, Y. J. Photochem. Photobiol., A 2005, 174, 253. (5) Paz, Y. C. R. Chim. 2006, 9, 774. (6) Avraham-Shinman, A.; Paz, Y. Isr. J. Chem. 2006, 46, 33. (7) Anderson, C.; Bard, A. J. J. Phys. Chem. 1995, 99, 9882. (8) Haick, H.; Paz, Y. J. Phys. Chem. B 2001, 105, 3045. (9) Lee, M. C.; Choi, W. J. Phys. Chem. B 2002, 106, 11818. (10) Lee, S.-K.; McIntyre, S.; Mills, A. J. Photochem. Photobiol., A 2004, 162, 203.

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