Photocatalytic Reduction of Cr(VI) by Titanium Dioxide Coupled to

Oct 14, 2010 - Polymeric nanofibers containing both carbon nanotubes and titanium dioxide were produced by electrospinning and used to photocatalytica...
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Photocatalytic Reduction of Cr(VI) by Titanium Dioxide Coupled to Functionalized CNTs: An Example of Counterproductive Charge Separation N. Shaham Waldmann and Y. Paz* The Department of Chemical Engineering, The Russell-Berrie Institute of Nanotechnology and The Grand Water Research Institute, Technion, Haifa 32000, Israel ReceiVed: June 27, 2010; ReVised Manuscript ReceiVed: September 16, 2010

Polymeric nanofibers containing both carbon nanotubes and titanium dioxide were produced by electrospinning and used to photocatalytically reduce Cr(VI) ions. A comparison was made between multiwall carbon nanotubes (CNTs) and single-wall CNTs, as well as between OH-functionalized and nonfunctionalized CNTs. The reduction rate was found to decrease upon using OH-functionalized CNTs, whereas the opposite phenomenon was observed with nonfunctionalized CNTs. The systems under study demonstrate the counterintuitive possibility that charge separation for specific target species can be quite deleterious for photocatalysis. Whether enhanced charge separation is benevolent or deleterious depends on the type of reaction (oxidation or reduction) as well as the location of the reaction (the photocatalyst or the inert electron sink) and the adsorpticity of both the photocatalyst and the inert partner. Introduction Photocatalytic degradation of pollutants in water and air is attracting increasing attention. In this context, due to its low cost, nontoxicity, and relatively high efficiency, the use of titanium dioxide as the photocatalyst of choice is often mentioned in the literature. The general scheme for the photocatalytic destruction of organics involves excitation with suprabandgap photons and migration of the electron-hole pairs to the surface of the photocatalyst, where the holes may be trapped by H2O or OH- adsorbed at the surface, thus forming hydroxyl radicals.1 In parallel, the electrons reduce adsorbed oxygen2 or are trapped in oxygen-vacancy deep traps.3 Most organic compounds are degraded oxidatively by the hydroxyl radicals, thus producing short-lived organic radicals that undergo secondary reactions to form stable molecules such as CO2 and water.4 Nevertheless, it was shown that halo-organics, such as 2-bromo-2-chloro-1,1,1-trifluoroethane,5 carbon tetrachloride,6 bromoform,7 and even 4-halophenols,8 could be degraded reductively by photoinduced electrons. Likewise, it is possible, in principle, to photocatalytically reduce a variety of highly toxic heavy metals ions, such as Cr(VI),9 Hg(II),10 and Cu(II).11 The photocatalytic degradation rates of pollutants depend to a large extent on the tendency of the pollutants to be adsorbed on the surface of the photocatalyst. Composite photocatalysts (namely, particles or fibers consisting of photocatalysts attached to exposed inert particles or domains) provide a means to detour the problem of low sticking coefficients between many of these compounds and the polar surface of titanium dioxide. The basic concept of this, so-called A&S approach12 (Figure 1) is to promote the overall performance of the photocatalyst by virtue of physisorption of reactants on the inert substrates, followed by surface diffusion of the pollutants to the interface of the photoactive sites, where the pollutants are degraded. Indeed, titanium dioxide particles partially coated with a hydrophobic organosilicone layer13 were found to be very efficient in the photocatalytic destruction of Permatrin, a water-insoluble pesticide.14 The same approach was utilized by us for obtaining * Corresponding author. E-mail: [email protected].

Figure 1. The “adsorb and shuttle” approach.

preferential degradation and selectivity by using specifically designed molecular recognition sites, located on the inert domains.15,16 The preparation and characterization of electrospun polymeric nanofibers containing both carbon nanotubes and titanium dioxide particles was described a few years ago.17 These systems consisted of a poly(acrylonitrile), PAN [-CH2-C(CN)H-]n, nanofibrous matrix, into which axially oriented multiwalled carbon nanotubes (MWCNTs), as well as Degussa P25 TiO2 particles, were embedded. It was shown that the CNTs in a ternary system of PAN-TiO2-CNT may act as antennas for the adsorption of contaminants that are first physisorbed and then surface-diffuse from the adsorptive sites to the photocatalytic sites.18 Here, it is noteworthy that the presence of CNTs in the polymer-TiO2 system was found to reduce the rate by which the polymer skeleton was degraded. The A&S phenomenon observed in the ternary system is similar to the documented cases of activated carbon/TiO219 or zeolites/TiO2 composites,20,21 in which the performance is governed by a delicate interplay between adsorption, desorption, and surface-diffusion of the target compounds. For photocatalysis to take place, efficient separation of the photoinduced charge carriers is required. In the case of photocatalytic oxidation on bare titanium dioxide, separation is achieved by reducing adsorbed oxygen. Further enhancement

10.1021/jp105925g  2010 American Chemical Society Published on Web 10/14/2010

Photocatalytic Reduction of Cr(VI) can be achieved by depositing metallic islands made of Pt or Au, which serve as electron sinks.22-24 Along this line, the photocatalytic synergistic effect that was found for titanium dioxide coupled with carbon nanotubes was attributed mainly to improved charge separation following migration of photoinduced electrons from the titanium dioxide to the CNTs. This effect was found to depend on the quality of the interfacial contact between the CNTs and the TiO2 as well as on the morphology and the surface properties of the nanocomposites.25 So far, the study of composites consisting of CNTs and TiO2 has been concentrated in oxidative degradation reactions (for example, degradation of methylene blue)26 that occurred on the titanium dioxide surface. Not much consideration has been put into reductive degradation. One of the candidates for reductive degradation is chromium(VI). Chromium(VI) is a common contaminant in industrial wastewater, originating from processes such as metal plating and leather tanning. The removal of Cr(VI) is considered to be of high importance due to its acute toxicity, carcinogenic action, and mobility in water. One of the most preferred methods to treat Cr(VI) in water is the transformation to the less toxic Cr(III), which can be precipitated and removed as a solid waste. The use of reducing agents such as sodium thiosulfate and sodium bisulfate/metabisulfite is documented, yet it suffers from the need for disposing large quantities of the reducing agent. The heterogeneous photocatalytic reduction of chromium(VI) has been proposed as an economical and simple method of treatment27-29 based on its reduction potential (E0 Cr(VI)/Cr(III) ) 1.36 V at pH ) 030), which is considerably more positive than that of the conducting band of TiO2. The reduction potential of Cr(VI) to Cr(III) is pH-dependent, and the thermodynamic driving force for Cr(VI) reduction decreases with increasing pH; hence, low a pH is favored. The synergistic effect of TiO2-CNT composites in the photocatalytic reduction of Cr(VI), which were attached to carboxyl-modified multiwalled CNTs,31 was recently demonstrated with photocatalyst particles made by a solvothermal process. Nevertheless, not much is known as to the origin of the synergism (i.e., improved charge separation versus enhance adsorption) or the role of the CNTs’ surface in obtaining this effect. The synergistic reduction of Cr(VI) by composites containing TiO2 particles and CNTs embedded in polymeric nanofibers is described herein. The effect of the CNTs’ type and surface treatment on the reduction of chromium was analyzed. An unexpected deleterious effect was found in systems containing OH- functionalized CNTs. This effect demonstrates the commonly overlooked fact that improved charge separation is not a guarantee for enhanced photocatalytic performance, and, in fact, sometimes it can be counterproductive. Experimental Section The photocatalytic measurements, as well as part of the adsorption measurements, were performed in mats consisting of polymeric nanofibers highly loaded with titanium dioxide particles (P25, Degussa, Frankfurt, Germany) and CNTs. Two types of nanofibers were prepared: fibers made of polyacrylonitrile (PAN, [-CH2-C(CN)H-]n), with titanium dioxide particles, and PAN fibers containing both TiO2 particles and CNTs. Most experiments were carried out with two types of CNTs: single walled CNTs (SWCNTs) and OH-functionalized SWCNTs (SWCNT(OH)s). Part of the measurements were compared with those of fibers containing multiwalled CNTs and TiO2. All CNTs were purchased from CheapTubes (Brattleboro, VT) and were used as received. Both types of SWCNTs had a

J. Phys. Chem. C, Vol. 114, No. 44, 2010 18947 diameter of 1-2 nm, specific surface area (SSA) of 407 m2 g-1, and an average length of 20 ( 10 µm. The average diameter of the MWCNTs was >50 nm for nonfunctionalized CNTs and 30 ( 10 nm for MWCNT(OH). The SSA of the multiwalled CNTs was 110 m2 g-1. Preparation of PAN-TiO2-CNT Composites. Polyacrylonitrile (Scientific Polymer Products, Inc., Ontario, MW ≈ 150 000) was dissolved in hot dimethylformamide (DMF) (AR, Gadot, Haifa, Israel) to give a 9% w/w PAN solution. Titanium dioxide particulate powder (Degussa P25) was added to DMF at a 4% (w/w) load and was sonicated for 2 h in a 43 kHz Delta D2000 sonicator to form homogeneous milklike slurry. The PAN solution was then added to the titania dispersion at a weight ratio of 1:1, and the dispersion was stirred. The resulting TiO2 dispersion contained 4.5% of PAN and 2% of TiO2 by weight. The CNTs were added to the DMF at a 3% (w/w) loading, stirred for 15 min, and sonicated for 15 min, forming a black slurry. The PAN solution (9% 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 MWCNT dispersion (1.5% CNTs, 4.5% PAN) was homogeneous and exhibited a dark, inklike appearance. The TiO2 dispersion (6% w/w in DMF) and CNT dispersion were added to the PAN solution in a 1:1:1 weight ratio to form a spinnable dispersion containing 2% TiO2, 0.5% CNTs, and 4.5% PAN. The electrospinning process was conducted at room temperature under air backpressure. The spinning distance between the outlet capillary and the collecting disk was about 18 cm at a voltage of 17 kV. Mats of nanofibers were collected on 2.5 cm × 2.5 cm wafers for at least 1 h. Following completion of solvent evaporation, the compositions (by weight) of the fabricated fibers were 75% PAN and 25% CNTs in the CNT fibers; 70% PAN and 30% TiO2 in the TiO2 fibers; and 65% PAN, 7% CNTs, and 28% TiO2 in the ternary fibers. More details on the preparation of the films are given elsewhere.17 Atomic force microscope (AFM) images at specific locations were acquired by a Pico + AFM machine (Molecular Imaging, Tempe, AZ) working in the acoustic mode with a NSC16 tip (µmasch ltd., Tallin, Estonia). Adsorption Measurements. The room temperature adsorption of Cr(VI) on various types of CNTs was studied in the dark by measuring spectral changes in a 100 mL solution (pH ) 2, adjusted by H2SO4) containing 450 µM of HCrO4- and appropriate amounts of CNTs. Care was taken to make the measurements on an equal surface area basis (0.5 m2) instead of equal mass basis. Accordingly, the amount of TiO2 (Degussa P25) was 0.1 g, the amount of SWCNTs was 4.6 mg, and that of MWCNTs was 19.8 mg. The concentration of Cr(VI) was calculated according to its 257 nm peak, measured directly by UV-vis spectroscopy. The room temperature adsorption of Cr(VI) on various mats was measured, as well. The measurements were carried under the same solution conditions described in the previous paragraph. The only change was the replacement of the CNTs with composite mats (3 mats of 2.5 cm × 2.5 cm on silicon wafers, the thickness being ∼15 µm). As described before, the relative concentration (by weight) in the mats between all types of CNTs (whether functionalized or not) and TiO2 was 1:4. This means that the adsorption measurements in the mats were carried out on an equal mass basis, unlike the adsorption measurements described in the previous paragraphs, where CNTs of equal surface area were compared. The concentration of Cr(VI) in these measurements was calculated by complexing the HCrO4ions with diphenylcarbazide (DPC), following a published

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procedure,32 measuring the UV-vis spectrum of the complex by a Lambda40 (Perkin-Elmer) spectrophotometer and using the absorbance at 542 nm. This indirect method was exactly the same method used in measuring the concentration of Cr(VI) in the photocatalytic experiments. The method, unlike the direct method used in the previous paragraph, differentiates between Cr(VI) and Cr(III). Although this differentiation is of limited value for the adsorption measurements, it was important in the photocatalytic measurements, where Cr(VI) and Cr(III) coexist. Photocatalytic Measurements. The photocatalytic activity was evaluated by chromium(VI) degradation in aqueous media under UV light. All solutions were prepared with deionized water. The measurements were carried out in a 8.5 cm diameter Pyrex glass reactor charged with 100 mL of Cr(VI) 450 µM solution at pH 2 and mats of catalyst (3 mats of 2.5 cm × 2.5 cm on silicon wafers, located in the middle of the reactor). Upon equilibrating adsorption in the dark, the mats were exposed to a low flux (0.7 mW cm-2) broadband 365 nm light, which entered the reactor. To prevent possible inaccuracies due to variations in the absorption of light by the various samples, care was taken to ensure that the nanofibrous mats that were used were optically thick (∼15 µm), such that all impinging photons were absorbed. All experiments were conducted under nitrogen environment, since O2 could compete with the chromates reduction. Here, it is noteworthy that some published works did not observe any difference in reduction rates when oxygen was present,33,34 but others did observe a deleterious effect of oxygen.31 Prior to irradiation, the solutions were precirculated in the dark at room temperature in the presence of the mats for sufficient time (30 min, on the basis of time-dependent adsorption measurements) to ensure adsorption equilibration. The concentration of Cr(VI) after equilibrating was taken as the initial concentration. During irradiation, 2 mL samples were periodically withdrawn and were complexed to facilitate determining their Cr(VI) concentration. At least 2 photocatalytic runs were carried out for each conditio. Photodegradation measurements of rhodamine 6G 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 2 × 10-5 M solution to 365 nm light (80 µW cm-2). Kinetics were deduced on the basis of the 526 nm peak in the UV-vis absorption spectrum of the dye stuff, obtained by taking aliquots and measuring them by UV-vis spectroscopy. Results An atomic force microscope deflection image of a PAN fiber containing both TiO2 and MWCNTs (i.e., PAN-TiO2MWCNT ternary system) is presented in Figure 2. As portrayed in the figure, the typical diameter of such fibers was 120-200 nm. The titanium dioxide particles formed aggregates along the fiber, yet without damaging the integrity of the fibers. TEM images, made by us in the past,17,18 revealed that the particles were near the outer part of the fiber, thus exposing themselves to potential adsorbates and that the CNTs in the ternary fibers were in close contact with the P25, sometimes even connecting between adjacent P25 aggregates within the fiber. In other cases, the P25 aggregates remained nonconnected to the well-aligned carbon nanotubes. Binary fibers, containing only PAN and TiO2, showed the same morphological properties as the ternary fibers.17,18 As mentioned in the experimental section, the photocatalytic processes were evaluated by monitoring the decolorization of the UV-vis absorption spectra of DPC-Cr(VI) complex

Waldmann and Paz

Figure 2. Atomic force microscope (AFM) image of PAN-TiO2MWCNT fiber.

Figure 3. The reduction of DPC-Cr(VI) complex absorbance with irradiation time (t0 ) initial absorbance, t120 ) following 120 min of irradiation). In the presence of PANTiO2-SWCNT fibers.

solutions. Figure 3 presents the evolution of the UV-vis spectra of DPC-Cr(VI) complex solutions prepared from samples withdrawn from the reactor at various exposure times in the presence of 3 2.5 cm × 2.5 cm mats of PAN-TiO2-SWCNT. The originally yellow solution became colorless as the exposure time was increased, indicating the reduction of the Cr(VI), most likely to Cr(III)). No UV-vis traceable products were observed. It is noteworthy that mats made of PAN-CNTs, equilibrated in the dark, did not yield any change in the Cr(VI) concentration upon exposure to UV light. The kinetics of the photocatalytic reduction with the various types of mats was deduced by plotting the time-dependent 542 nm absorption peak of the complex. Figure 4 presents the kinetics in terms of the time dependence of the natural logarithm of the absorption, normalized by its initial value; i.e. ln(Abs/ Abs0). Results are given for PAN-TiO2, PAN-TiO2-SWCNT, and PAN-TiO2-SWCNT(OH) substrates. As shown in the figure, apparent first-order kinetics is observed in all three cases. The apparent reaction rate constant with PAN-TiO2-SWCNT was found to be larger than that with PAN-TiO2 substrate (0.016 and 0.0084 min-1, respectively), demonstrating the synergistic effect of CNTs on the photocatalytic reduction of Cr(VI). The situation was completely different with respect to the effect of SWCNT(OH). Here, not only could no synergistic

Photocatalytic Reduction of Cr(VI)

Figure 4. The variation of Cr(VI) absorbance with time during UV exposure in the presence of fibrous mats consisting of PAN-TiO2 (2), PAN-TiO2-SWCNT (9), PAN-TiO2-SWCNT(OH) (0), and PAN-TiO2-SWCNT in a 10% MeOH solution (×).

Figure 5. The variation of Cr(VI) absorbance with time during UV exposure in the presence of fibrous mats consisting of PANTiO2-MWCNT (b), PANTiO2-MWCNT(OH) (O).

effect be observed, but instead, the process of Cr(VI) reduction was hardly noticed (a rate constant of 0.0024 min-1). Figure 4 also presents the enhancement in the reduction rate obtained upon performing the photocatalytic reaction in a solution containing 10% (v/v) methanol, a well-known hole and OH radical scavenger (a rate constant of 0.091 min-1). It is noteworthy that the unexpected deleterious effect of OHfunctionalized CNTs was found also with mats containing multiwalled CNTs (Figure 5). A comparison between the effect of SWSNTs and that of SWCNT(OH)s was done also for the photocatalytic oxidation of the dyestuff rhodamine 6G, for which previously published measurements18 had reported an enhancement in the decolorization rate upon introducing nonfunctionalized MWCNTs. In the present study, a comparison between the kinetics with mats made of PAN-TiO2-SWCNTs and mats made of PANTiO2-SWCNT(OH)s did not reveal any deleterious effect upon using SWCNT(OH)s. In fact, mats containing SWCNT(OH)s were found to be slightly more effective than mats containing SWCNTs. In principle, both charge transport effects and adsorption effects could play a role in the synergistic and antisynergistic effects originating from the coupling between the CNTs and the TiO2. For this reason, adsorption measurements had to be taken into account. As described in the Experimental part, two types of adsorption measurements were performed: measure-

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Figure 6. Kinetics of adsorption of Cr(VI) on mats: PAN-TiO2 (2), PAN-TiO2-SWCNT (9), PAN-TiO2-SWCNT(OH) (0).

ments with suspended CNTs and measurements with mats containing titanium dioxide and CNTs. Figure 6 presents the adsorption kinetics of Cr(VI) on three types of mats: PAN-TiO2, PAN-TiO2-SWCNT, and PANTiO2-SWCNT(OH). All measurements were performed with the same area of mats and the same amount of TiO2 and CNTs. The data is presented relative to the amount at time zero (A0) to account for minute ((3%) differences in the Cr(VI) initial concentration. From the figure, it is evident that the adsorbability of the PAN-TiO2-SWCNT system is significantly higher than that of PAN-TiO2, yet the adsorbability of the in the PAN-TiO2-SWCNT(OH) is significantly lower than that of PAN-TiO2. For example, the concentration of Cr(VI) after 60 min of adsorption (quite close to steady state) was reduced by 20% and 4.5% in the PAN-TiO2-SWCNT and PAN-TiO2 systems, respectively, whereas the amount adsorbed on the PAN-TiO2-SWCNT(OH) was basically nil. These trends repeated themselves several times, regardless of the initial concentration. Since in all three systems, the amount of TiO2 was the same, the difference in adsorpticity of the mats reflects differences in the adsorpticity of the different types of CNTs. To further validate that the difference in Cr(VI) adsorption originated from the type of CNTs, the adsorption kinetics was studied with CNTs suspensions, each containing a different type of SWCNTs: nonfunctionalized CNTs (SWCNTs), OH-functionalized CNTs, and COOH-functionalized CNTs. The results are portrayed in Figure 7. Here, again, SWCNT(OH)s had the least adsorpticity, SWCNTs had intermediate adsorpticity, and SWCNT(COOH) were found to have the highest adsorpticity, as can be deduced from C/C0 following 60 min of adsorption (0.97, 0.88, and 0.80, respectively). Discussion The coupling between CNTs and TiO2, within the PAN matrix, was found to have a significant effect on the ability of titanium dioxide to photocatalytically reduce Cr(VI) to Cr(III). Surprisingly, a distinct difference was found between the effect of nonfunctionalized CNTs and that of OH-functionalized CNTs; namely, the former induced an increase the reduction rate, whereas the latter had an opposite effect and, in fact, prevented the photocatalytic reduction of Cr(VI). In rationalizing these results, one has to consider the fact that the photocatalytic reduction is a complex process that comprises light absorption, formation of charge carriers, recombination and, last but not least, adsorption of the reactants and desorption

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Figure 7. Kinetics of adsorption of Cr(VI) on various types of carbon nanotubes: SWCNT (9), SWCNT(OH) (0), SWCNT(COOH) (×).

of products. As is described herein, all these factors can be influenced by coupling between the photocatalyst and the CNTs. Moreover, as demonstrated below, these factors are interrelated so that there are cases in which increasing charge separation might lead, counterintuitively, to decreased photocatalytic activity. In the context of charge transport, CNTs coupled to titanium dioxide are often regarded as electron sinks; that is, accommodate photoinduced electrons that transport away from the photocatalyst. This charge transport process reduces the recombination rates and, thus, increases the concentration of holes/ hydroxyl radicals. This, in turn, enhances the photocatalytic degradation of those organic contaminants that are adsorbed on the photocatalyst’s surface and that are degraded by an oxidative mechanism; for example, acetone and rhodamine 6G.18 Recombination (and its prevention) also plays a major role in the case of reduction of Cr(VI), as can be learned from the significant enhancement in the reduction rate that was observed upon performing the reaction in the presence of methanol, a hole and OH radical scavenger (Figure 4). There is no reason to believe that the direction of the electrons’ flow has to do with the type of contaminant. Hence, one has to accept the notion that during exposure to UV light, electrons formed in the photocatalyst flow from the titanium dioxide to the carbon nanotubes. Accepting this notion means that coupling between the photocatalyst and the CNTs is expected to be benevolent, only if there is significant adsorption of Cr(VI) on the CNTs. After all, Cr(VI) species that might adsorb on the surface of the photocatalyst are unlikely to be reduced in an environment where holes are the majority carriers. The adsorption measurements under dark conditions are instrumental in understanding the effect of the coupling between the CNTs and the titanium dioxide particles. Here, the most important finding is the strong correlation between the Cr(VI) adsorption measurements under dark conditions and the photocatalytic activity; namely, very low adsorbability on CNT(OH) in correlation with very low activity and high adsorbability on nonfunctionalized CNTs in correlation with the positive effect on photoactivity. Although we did not perform Cr(VI) activity measurements with fibers consisting of PAN-TiO2CNT(COOH), we did measure the adsorbability of SWCNT(COOH) for Cr(VI) and found it to be similar to that of nonfunctionalized CNTs (figure 7). It is noteworthy, therefore, that a synergistic effect in the reduction of Cr(VI) by coupled systems of CNT(COOH) and titanium dioxide was reported before.31 In addition, it should be pointed out that the adsorption

Waldmann and Paz characteristics of noncoupled carbon nanotubes were found to be very similar to these of PAN mats comprising both titanium dioxide and carbon nanotubes. This suggests that the adsorpticity of Cr(VI) on the carbon nanotubes is not strongly affected by coupling with titanium dioxide. The suggested scenario with nonfunctionalized CNTs is therefore as follows: Cr(VI) is adsorbed both on the CNTs and on the photocatalyst. Upon exposure, photogenerated electrons flow from the photocatalyst to the CNTs, where they act to efficiently reduce Cr(VI). Although the number of available electrons that can handle Cr(VI) adsorbed on the TiO2 is now smaller, the overall rate is enhanced due to the fact that the total number of available electrons increases. OH-functionalized CNTs hardly adsorb Cr(VI). This low tendency of the CNT(OH)s to adsorb the Cr(VI) species creates a situation in which the prominent route for reduction is through Cr(VI) adsorbed on the photocatalyst particles. As a consequence, the photogenerated electrons are ineffective and eventually are lost. The overall effect is therefore negative, since the total number of electrons available for reduction (i.e., electrons at the photocatalyst surface) is now smaller. In other words, the fact that electrons flow away to the CNTs no longer helps but, in fact, interferes with Cr(VI) reduction. The widely accepted perception that promoting charge separation helps to increase photocatalysis is not valid here. Working toward enhancing charge separation without thinking about all consequences might lead to a “Pyrrhic victory”, whose damage is larger than its benefit. The source for the difference in the affinity of the two types of CNTs is not clear at the moment, and its understanding may require some more work in the future. One possibility is that the specific surface area available for adsorption is smaller in CNT(OH)s than in CNTs as a consequence of aggregation caused by hydrogen bonding between the hydroxyl groups on the CNTs. Although this explanation seems quite plausible, we could not find any supportive evidence for it, and TEM images of PAN-CNTs revealed a morphology that was identical to that of PAN-CNT(OH). The reduction potential of Cr(VI) to Cr(III) is pH-dependent, and the thermodynamic driving force decreases with increasing pH;30,35 hence, reduction is favored at low pH. Low pH is preferable also from adsorption considerations because the adsorptivity of the chromate complexes, which are anionic in nature, on TiO2 decreases with increasing pH30 and is very low at pH higher than the PZC value of 6.3-7.0. At pH 2, which is the pH during both adsorption measurements and photocatalytic reduction measurements, the Cr(VI) is mostly in the form of HCrO4-.36 If the negative charge is not completely masked by counterions or a water shell, one may expect at pH 2 a significant adsorption on the titanium dioxide particles, which are charged positively at this pH. In that case, one may expect the promoting effect of nonfunctionalized CNTs to be quite weak, if at all. Evidently, this is not the case, and the amount of Cr(VI) that is reduced on the CNTs is large enough to compensate for the loss in electrons available for reduction of Cr(VI) adsorbed on the TiO2 surface. The dependence of Cr(VI) adsorption on the type of CNTs was recently reported for MWCNTs.37 There, it was shown that that MWCNTs functionalized with a mixture of hydroxyl and carboxyl groups had lower adsorbability compared with nonfunctionalized MWCNTs. Likewise, oxidation of activated carbon with nitric acid was found to decrease the adsorption of Cr(VI) while increasing the adsorption of Cr(III).38 Both findings, reasonably applicable to CNTs, may contribute to a lower rate of photocatalytic reduction with the TiO2- CNT(OH)

Photocatalytic Reduction of Cr(VI) systems in comparison with nonfunctionalized titania-CNTs systems. The lower capability of CNT(OH)s for Cr(VI) adsorption, found also in our measurements, cannot be assigned to a difference in the zeta potential, since at pH 2, the zeta potential measured for the two types of MWCNTs was almost identical (in fact, that of the nonfunctionalized was slightly lower).37 There could be several possible explanations for the lower adsorpticity of CNT(OH)s; for example, the fact that functionalized CNTs contain electron-rich atoms in their functional groups that repel the negatively charged dichromate ions.37 However, this cannot explain the high adsorbancy of COOHterminated CNTs. The seemingly contradicting works on the effect of anodic oxidation of activated carbon fibers on the adsorption of Cr(VI)39 can be solved by assuming two types of adsorption sites. Adsorption on CdO functional groups releases OH-; hence, under acidic conditions, adsorption is promoted, whereas adsorption on COH2+ releases H+. Therefore, under acidic conditions, desorption is promoted, and the net tendency for adsorption is low. The finding that replacing surface H+ with Na+ (by post-treating oxidized activated carbon with a mixture of NaOH and NaCl) tends to enhance the adsorpticity40 may support the suggested explanation. It can be concluded, therefore, that the difference in the photocatalytic reduction rate between coupled TiO2-CNT(OH)s and TiO2-CNTs was due to the difference in their ability to adsorb Cr(VI) and had hardly anything to do with charge transport. This conclusion is supported not only by the correlation between adsorption and reduction kinetics but also by the lack of difference between the effect of coupled CNTs and the effect of coupled CNT(OH)s on the oxidative degradation of the electronically neutral rhodamine 6G. Here, rhodamine, being a neutral species, is expected to be less susceptive to changes in the surface charge on the CNTs or on the photocatalyst, in contrast to the Cr(VI) species. The oxidation of CNTs to form CNT(OH), CNT(COOH) is expected to yield better coupling between the CNTs and the photocatalyst, through the formation of hydrogen bonding between OH groups on the TiO2 surface and the CNTs.41 As discussed before, there is hardly any adsorption of Cr(VI) on the CNT(OH)s. As a consequence, better charge transport cannot be translated into faster reduction kinetics. On the contrary, better charge transport is translated here into a lower rate of reduction of the Cr(VI) complexes adsorbed on the surface of the titanium dioxide particles, since it creates a deficiency in electrons exactly where they are required. In principle, one could argue that the difference between the effect of CNT(OH)s and that of nonfunctionalized CNTs could have been related to possible damage caused to the CNTs during functionalization, which interferes with charge transport. We believe that this is unlikely to be a valid explanation, on the basis of the high adsorbability of Cr(VI) on CNT(COOH)s (Figure 7), which eventually led to high photocatalytic activity upon coupling with titanium dioxide.31 In addition, if the only mechanism for the different behavior of the two types of CNTs was bad charge transport following functionalization, one could have expected to find a significant difference between the two types of CNTs not only in the case of Cr(VI) but also in the case of rhodamine. Multiwalled CNTs (whether functionalized or not) revealed the same effects found with single-walled CNTs; that is, synergism with MWCNTs and deleterious effect with MWCNT(OH)s. Taken that MWCNTs are metallic and part of the SWCNTs are metallic, whereas part are semiconducting, the fact that the same effect of the OH-functionalized CNTs was

J. Phys. Chem. C, Vol. 114, No. 44, 2010 18951 observed with MWCNTs and SWCNTs suggests that the explanation for the deleterious effect with the OH-functionalized CNTs should be applicable for metallic CNTs, as indeed is the preferred explanation, which relies on adsorption onto the nanotubes. Overall, SWCNTs were found to be more efficient in promoting the reduction of Cr(VI) than MWCNTs, when compared on a same weight basis (Figures 4 and 5). This can be rationalized, at least in part, by the larger specific surface area of SWCNTs (407 m2 gr-1 versus 110 m2 gr-1), which is expected to affect the adsorption of the Cr(VI) species. Another possibility, raised in the photocatalytic oxidation of phenol by CNT-TiO2 composites, is a better contact between SWCNTs and TiO2 in comparison with MWCNTs and TiO2.42 The difference in the type of conductivity may also play some role. Conclusion Charge transport leading to better charge separation is almost always regarded in the context of photocatalysis as a benevolent factor. The system under study, which uses coupled TiO2/OHfunctionalized CNTs to reduce Cr(VI), demonstrates the counterintuitive possibility; that is, that charge separation for specific target species can be quite deleterious. Whether enhanced charge separation is benevolent or deleterious depends on the type of reaction (oxidation or reduction) as well as the location of the reaction (the photocatalyst or the inert electron sink) and the adsorpticity of both the photocatalyst and the inert partner. Acknowledgment. This work was supported by the Israel Science Foundation and by the Russell-Berrie Institute of nanotechnology. References and Notes (1) Salvador, P. J. Phys. Chem. C 2007, 111, 17038. (2) Gerischer, H.; Heller, A. J. Phys. Chem. 1991, 95, 5261. (3) Henrich, V. A.; Dresselhaus, D.; Zeiger, H. J. Phys. ReV. Lett. 1976, 36, 1335. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (5) Bahnemann, D. W.; Monig, J.; Chapman, R. J. Phys. Chem. 1987, 91, 3782. (6) Cho, Y. M.; Choi, W.; Lee, C. H.; Hyeon, T.; Lee, H. I. EnViron. Sci. Technol. 2001, 35, 966. (7) Kuhler, R. J.; Santo, G. A.; Caudill, T. R.; Betterton, E. A.; Arnold, R. G. EnViron. Sci. Technol. 1993, 27, 2104. (8) Lapertot, M.; Pichat, P.; Parra, S.; Pulgarin, C. J. EnViron. Sci. Health A 2006, 41, 1009. (9) Munoz, J.; Domenech, X. J. Appl. Electrochem. 1990, 20, 518. (10) Domenech, J.; Andres, M. Gazz. Chim. Ital. 1987, 117, 495. (11) Foster, N. S.; Noble, R. D.; Koval, C. A. EnViron. Sci. Technol. 1993, 27, 350. (12) Paz, Y. C. R. Chim. 2006, 9, 774. (13) Dagan, G.; Sampath, S.; Lev, O. Chem. Mater. 1995, 7, 446. (14) Hidaka, H.; Nohara, K.; Zhao, J.; Serpone, N.; Pelizzetti, E. J. Photochem. Photobiol., A 1992, 64, 247. (15) Ghosh-Mukerji, S.; Haick, H.; Paz, Y. J. Photochem. Photobiol. 2003, 160, 77. (16) Sagatelian, Y.; Sharabi, D.; Paz, Y. J. Photochem. Photobiol., A 2005, 174, 253. (17) Kedem, S.; Schmidt, Y.; Paz, Y.; Cohen, Y. Langmuir 2005, 21, 5600. (18) Kedem, S.; Rozen, D.; Cohen, Y.; Paz, Y. J. Phys. Chem. C 2009, 113, 14893. (19) Avraham- Shinman, A.; Paz, Y. Isr. J. Chem. 2006, 46, 33. (20) Sampath, S.; Uchida, H.; Yoneyama, H. J. Catal. 1994, 149, 189. (21) Takeda, N.; Ohtani, M.; Torimoto, T.; Kuwabata, S.; Yoneyama, H. J. Phys. Chem. B 1997, 101, 2644. (22) Pichat, P. New J. Chem. 1987, 11, 135. (23) Sun, B.; Vorontsov, A. V.; Smirniotis, P. G. Langmuir 2003, 19, 3151. (24) Kandiel, T. A.; Dillert, R.; Bahnemann, D. W. Photochem. Photobiol. Sci. 2009, 8, 683.

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