Differential Photoactivity of Aqueous [C60] and ... - ACS Publications

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Differential Photoactivity of Aqueous [C60] and [C70] Fullerene Aggregates Kyle J. Moor,† Samuel D. Snow,‡ and Jae-Hong Kim*,†,‡ †

Department of Chemical & Environmental Engineering, School of Engineering & Applied Science, Yale University, New Haven, Connecticut 06511, United States ‡ School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Many past studies have focused on the aqueous photochemical properties of colloidal suspensions of C60 and various [C60] fullerene derivatives, yet few have investigated the photochemistry of other larger cage fullerene species (e.g., C70, C74, C84, etc.) in water. This is a critical knowledge gap because these larger fullerenes may exhibit different properties compared to C60, including increased visible light absorption, altered energy level structures, and variable cage geometries, which may greatly affect aggregate properties and resulting aqueous photoactivity. Herein, we take the first steps toward a detailed investigation of the aqueous photochemistry of larger cage fullerene species, by focusing on [C70] fullerene. We find that aqueous suspensions of C60 and C70, nC60 and nC70, respectively, exhibit many similar physicochemical properties, yet nC70 appears to be significantly more photoactive than nC60. Studies are conducted to elucidate the mechanism behind nC70’s superior 1 O2 generation, including the measurement of 1O2 production as a function of incident excitation wavelength, analysis of X-ray diffraction data to determine crystal packing arrangements, and the excited state dynamics of aggregate fullerene species via transient absorption spectroscopy.

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transport limitations of 1O2 produced within the aggregates to the bulk solution.10−12 C60 is the most common fullerene species, but many other fullerenes are also produced in the arc-discharge preparation method, including C70, C76, C84, and other higher fullerenes.13 C70 and higher fullerenes are of specific research interest because they possess many advantageous properties compared to C60, such as better visible light absorption,14 less energetic lowest unoccupied molecular orbitals (LUMOs) resulting in decreased reduction potentials and thus the improved air stability of fullerene anion species,15 and larger cages that allow the incorporation of metal clusters inside fullerene’s cage.16 Hence, these larger fullerenes are increasingly being used in place of [C60] fullerene in organic photovoltaics17−19 and electronic devices,15,20 as well as in the medical field as MRI contrast agents.16 C70 has drawn the most attention likely because it is produced in the second largest amounts behind C60 in the arc-discharge process, contributing to ca. 10% of total fullerenes.21 C70 displays an ellipsoidal geometry in contrast to C60’s spherical structure as a result of the insertion of 10 carbon atoms equatorial into the fullerene sphere22 and is somewhat

INTRODUCTION

Fullerene has drawn great attention across numerous fields due to its unique properties that can be taken advantage of in a myriad of applications, yet concerns have grown over potential environmental release and uncertain eco-toxicological impacts. It is well documented that, when dispersed in water, [C60] fullerene forms stable colloids (nC60) composed of a crystalline matrix of C60 molecules, which exhibit diameters in the 50−200 nm range and negative surface potentials.1,2 Many studies have examined the physicochemical,3 transport,4 and photochemical properties5 of nC60 dispersions. Notably, nC60’s photochemical activity has been found to be drastically reduced compared to when dispersed in organic solvents, where C60 photosensitizes O2 to produce 1O2 very efficiently (Φ ∼ 1.0 at 532 nm)6 upon illumination due to its near unity intersystem crossing quantum yield (QY; Φ = ∼1.0)7 and long-lived triplet excited state (133 μs in benzene under anoxic conditions).8 These characteristics are often absent from nC60 colloids, resulting in a small, yet measurable 1O2 production rate under both visible and UV light irradiation.5,9 It has been proposed that this significant reduction of photoactivity is related to the increased local density of individual C60 molecules upon nC60 formation, leading to increased quenching of excited states by neighboring fullerene molecules through triplet−triplet annihilation (TTA) and ground state quenching processes and possible mass © XXXX American Chemical Society

Received: January 7, 2015 Revised: April 22, 2015 Accepted: April 24, 2015

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DOI: 10.1021/acs.est.5b00100 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

added to 450 mL DI water and the binary mixture was sonicated in closed bottles for 24 h. The bottle was opened, ultrasonication was further applied at 60 °C to evaporate residual toluene, and the resulting solution was filtered through 0.45 μm nitrocellulose filters (Whatman). A similar preparation was used to produce nC70 in deuterium oxide (D2O) for control experiments. Fullerene stock solutions were stored in the dark at room temperature. The concentration of fullerene stocks was determined through a solvent exchange method, where acetic acid (100 mM) was used to destabilize the fullerene particles and allow transport into the toluene phase.34 UV/vis spectroscopy (Varian Cary 50 Bio) was used to determine the concentration of fullerene in the toluene solution and to confirm that no fullerene remained in the aqueous phase after extraction. Aggregate Characterization. Light absorption by aqueous nC60 or nC70 dispersion was measured using a UV/vis spectrophotometer (Varian Cary 50 Bio). Particle morphology and size were characterized via a transmission electron microscopy (TEM) and a high-resolution TEM (HR-TEM; FEI Tecnai Osiris, 200 kV). Lattice fringe spacing was determined from HR-TEM images via analysis with ImageJ software by averaging at least 100 measurements. Zeta-potential and further size measurements were performed using phase angle light scattering (PALS) and dynamic light scattering (DLS) techniques, respectively, using a Zetasizer Nano ZS90 (Malvern Instruments). Aggregate crystallinity was investigated using wide-angle X-ray scattering (Rigaku 007 HF+) on powders prepared by drying aqueous fullerene solutions at 105 °C for 16 h. The extent of aggregate surface oxidation was determined via Fourier-transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet 6700) with attenuated total reflectance (ZnSe crystal plate) on powders prepared via drying aqueous fullerene suspensions at 105 °C for 16 h. X-ray photoelectron spectroscopy (XPS) measurements were recorded using an AXIS Ultra DLD (Kratos. Inc.) spectrometer with a monochromatic Al Kα (1486.6 eV) source on films of fullerene powders prepared by drying aqueous fullerene solutions at 105 °C onto glass substrates. Photochemical Experiments. Batch photochemical experiments were conducted in cylindrical quartz reactors with magnetic stirring. Reactors were irradiated with six 4 W fluorescent bulbs providing an average light intensity of 4.23 mW/cm2 at 4.75 cm from the source as calculated via radiometry (Solar Light Co., PMA2200, PMA2140 global detector) and were kept at ambient temperatures via air recirculation. UV cutoff filters were used to remove the UV portion of the fluorescent bulbs, leaving only visible light; an emission spectrum of the light source is provided in Supporting Information (SI) Figure S1. A typical photochemical experiment was comprised of 20 μM fullerene aggregate suspension buffered at pH 7 with 1 mM phosphate buffered saline (PBS). Furfuryl alcohol (FFA) was used as a 1O2 probe molecule (kFFA‑1O2 = 1.2 × 108 M−1 s−1) following previous reports;35 its consumption as a function of time was monitored using highperformance liquid chromatography (HPLC) equipment (Agilent 1100) with a C18 column and a photodiode array UV−visible absorbance detector. Steady-state concentrations of 1 O2 ([1O2]ss) were calculated via previously established kinetics.9,36 To study the dependence of 1O2 production on excitation wavelength, a monochromatic light source was used for photochemical reactions via a spectrophotometer with on-

larger in size, with a van der Waals diameter of 11.31 Å along the longer axis and 10.47 Å for the smaller axis compared to the 10.3 Å diameter for C60.23,24 The addition of 10 carbon atoms causes a reduction in symmetry from Ih (in C60) to D5h point groups (in C70),25 thus allowing electronic transitions that were once symmetry forbidden in C60 to occur with C70,14,26 and provides C70 with a significantly enhanced visible light absorption compared to C60. C70 maintains many of C60’s photophysical properties, including a long-lived triplet excited state (120 μs in benzene under anoxic conditions) and large 1 O2 production quantum yield (Φ = 0.81 ± 0.15)26 and is thus a great photosensitizer like C60. C70’s slightly smaller 1O2 quantum yield compared to C60 may be attributed to minor decay pathways of 1C70 other than intersystem crossing,26 although the triplet quantum yields are comparable with values of 0.9 ± 0.1526 and 0.93 ± 0.077 for C70 and C60, respectively. Although extensive research has been conducted on the aqueous behavior of [C60] fullerene and [C60] fullerene derivative aggregates,27,28 few studies have examined the properties of aqueous aggregates of larger cage fullerene species. Past reports that have studied such fullerene aggregates typically focused on their formation, microscopic characterization, and physicochemical parameters2,29−32 but generally ignored their photochemical properties. One report focused on the 1O2 production of higher fullerene colloids (nC70, nC84) via electron paramagnetic resonance (EPR) spectroscopy using a 1 O2 spin-trapping agent for qualitative comparisons,31 yet it remains unclear what light source and corresponding intensity were used for fullerene excitation, making it difficult to ascertain this study’s environmental relevancy. As the use of larger fullerenes in various applications increases, it is imperative to have a better understanding of the photochemistry of larger cage fullerene aggregates in the aqueous environment because they likely behave differently than nC60 due to differences in visible light absorption, energy level structures, and the physical geometry of the fullerene cage. Herein, we take the first steps toward providing a detailed analysis of the photochemistry of a larger cage fullerene aggregate species along with the quantification of its 1O2 production rates. As a starting point, we focus on C70 based fullerene colloids considering C70’s popularity in materials science and its large production amounts relative to other higher fullerenes, making C70 the larger cage fullerene species with the greatest potential for environmental release. nC70 based colloids are found to exhibit considerably greater photoactivity than nC60 under visible light illumination, suggesting that they may have more adverse implications in the natural environment. Further analysis finds that this greater photoactivity may be partly due not only to C70’s strong visible light absorption, but also to the longer triplet excited state lifetimes of fullerene molecules in nC70 compared to nC60.

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EXPERIMENTAL SECTION Materials. Deionized (DI) water from a Milli-Q purification system (Millipore Co.) was used for the preparation of all reagents and solutions. All chemicals were obtained from chemical suppliers (Sigma-Aldrich, MER) and were used as received. Aqueous Phase Dispersion. Fullerene aggregates were prepared following a solvent exchange method as previously reported.33 Briefly, solutions of C60 (99.5%; Sigma-Aldrich) or C70 (98%; SES Research) in toluene (50 mL, 1 g/L) were B


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with previous reports.41,42 Several peaks appear in the spectra after nC60 and nC70 formation, including a broad peak centered at ca. 3400 cm−1 (O−H stretching), multiple peaks at 2960− 2800 cm−1 (CH2 and CH3 stretching), a peak at 1700 cm−1 (CO stretching), and a peak at 1000 cm−1 (C−O stretching), which is only discernible in nC60’s spectrum. These spectral features agree with previous reports for nC6038,43,44 and generally suggest an increase in overall surface oxidation, which appears to be comparable between nC60 and nC70. Additional XPS characterization of nC60 and nC70 confirms that both aggregates possess considerably oxidized surfaces, as evidenced by significant O 1s signals in the spectra (Figure S4, SI), generally agreeing with previously reported XPS measurements.44 Based on this data, it was found that nC60 and nC70 possess relative oxygen contents of 14.13 and 13.71%, respectively, with corresponding carbon contents of 78.76 and 79.13% for nC60 and nC70. To make an accurate comparison of the amount of oxygen in the aggregates, we normalized oxygen values by the carbon content, yielding ratios of 0.179 and 0.174 for nC60 and nC70, respectively. From this analysis, it appears that nC60 may possess slightly more oxidized character than nC70, yet overall the aggregates appear to possess comparable levels of surface oxidation. TEM analysis shows that both nC60 and nC70 are comprised of polydisperse primary particles with diameters in the 100 nm range, exhibiting in general similar sizes and morphologies (Figure 1). Note that agglomeration of primary fullerene

board monochromator (Shimadzu RF-5301PC). A UV long pass filter was used to remove degenerate waves. nC60 and nC70 suspension concentrations were normalized to obtain identical absorbances (abs) at the desired wavelength, corresponding to 0.4 abs for 350, 415, 450, and 500 nm; 0.25 abs for 550 nm; and 0.1 abs for 600 nm. Reactions occurred in quartz cuvettes without stirring and the aqueous suspensions were buffered at pH 7 with 1 mM PBS. FFA degradation kinetics were monitored as described above. Lifetime Measurements. Nanosecond laser flash photolysis (LFP) experiments were performed using the third harmonic (355 nm) of a Nd:Yag laser (fwhm = 10 ns, 50 mJ/pulse, 10 Hz repetition rate). A 1000 W Xe lamp (Hanovia) was pulsed by a lamp pulser (Sorensen Power Supplies) and was used as the source of a collimated probe beam that was passed through the samples and a monochromator before signal detection with a photomultiplier tube and recording with a 1 GHz oscilloscope (Lecroy). Samples were purged with Ar in 1 × 1 cm quartz cuvettes for at least 15 min before experiments. Femtosecond LFP was conducted on a Clark MXR-2010 laser system (775 nm fundamental, 1 mJ/pulse, fwhm = 130 fs, 1 kHz repetition rate) with Helios software (Ultrafast Systems). The fundamental beam was split 95/5 to generate a 387 nm pump through frequency doubling of the 95% portion. The probe beam was generated by focusing the 5% portion through a Ti:sapphire crystal to generate a white light continuum. Both beams were brought incident on the sample with the probe delayed via optical delay rail. Samples were purged as described above in 2 mm × 1 cm quartz cuvettes.

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RESULTS AND DISCUSSION Aggregate Properties. Aqueous fullerene aggregates exhibited mean hydrodynamic diameters (intensity mean values) of 121 and 182 nm with polydispersity indexes (PDIs) of 0.185 and 0.290 for nC60 and nC70, respectively (Figure S2, SI). The size of nC60 is consistent with previous reports,12,27 but an accurate comparison cannot be made for nC70 because of the different preparation methods employed in previous studies.2,29,30 For example, Aich et al. used a solvent exchange method in conjunction with a high-intensity ultrasonication probe for fullerene aggregate preparation, yielding smaller particles with diameters of 42.7 ± 0.8 and 46.0 ± 14.0 nm for nC60 and nC70, respectively.29 Note that the use of a high-intensity ultrasonication probe is expected to impart more oxidized character to the aggregates compared to the more mild bath sonicator as used herein.37 Aggregates exhibited ζ potentials of −25.7 ± 3.4 and −32.6 ± 1.5 mV for nC60 and nC70, respectively, which generally agrees with a previous study that observed ζ-potentials of ca. −39 mV for both nC60 and nC70.29 The difference in the magnitude of ζ-potential values may be a result of using different aggregate preparation methods as discussed above. It is expected that nC70 will be somewhat oxidized during its preparation, as has been previously observed for nC60,37,38 resulting in possible surface hydroxyl and epoxide groups. Given the similar reactivity of C60 and C70 toward oxidation39,40 and the observed ζ potential values, nC60 and nC70 should possess comparable amounts of surface oxidation. FTIR spectroscopy confirms that both nC60 and nC70 possess considerably increased oxidized character after colloid formation (Figure S3, SI). The FTIR spectra of the C60 and C70 starting materials exhibit various C−C bonds below ca. 1500 cm−1 with no other considerable spectral features, consistent

Figure 1. TEM images of (a and c) nC60 and (b and d) nC70, including insets with magnified views depicting crystal lattice fringes.

particles may have occurred in TEM sample preparation upon drying, resulting in large clusters of fullerene aggregates as can be seen in TEM images. High-resolution TEM (HR-TEM) images of nC60 and nC70 exhibit crystal lattice fringes in select areas, suggesting a highly ordered packing of fullerene molecules in both nC60 and nC70, resulting in a polycrystalline structure. This crystalline packing of fullerene molecules has been widely reported for nC60,12,45 thus it is not surprising that nC70 exhibits similar behavior, which has been suggested by C


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Environmental Science & Technology previous HR-TEM analysis.2 One significant difference between nC60 and nC70 is the lattice fringe spacing, with values of 0.79 ± 0.06 and 1.03 ± 0.07 nm for nC60 and nC70, respectively. The larger spacing for nC70 may be related to differences in fullerene cage dimensions and geometries, with the ellipsoidal structure and larger dimensions of C70 potentially requiring larger spacing for crystalline packing than the spherical structure of C60. Furthermore, given that the molecular orientation of C70 molecules is disordered within the crystalline matrix (e.g., the C70 molecules do not align long axis to long axis, etc.),46,47 even greater fullerene spacing may be necessary to accommodate the disordered molecular arrangement compared to a state with defined, preferential fullerene orientations. Wide-angle X-ray scattering provides further evidence that nC60 and nC70 exhibit some measure of crystallinity as suggested by diffraction peaks in the corresponding diffractograms (Figure 2). The diffraction pattern of sublimed C60

previously reported diffraction pattern suggests that the sublimed C70 sample exhibits HCP packing. The diffraction pattern of nC70 exhibits many of the same peaks as found in C70’s diffraction pattern, including peaks at 10.1 (002), 17.04 (110), 18.2 (103), 19.4 (112), and 26.54 (211) 2θ, suggesting that C70 molecules pack in a HCP-like fashion in nC70. On the basis of peak locations and corresponding crystal phases, we calculated unit cell parameters by using Bragg’s law and the geometric spacing between lattice planes for nC60 (using the 111 lattice plane) and nC70 (using the 002 and 110 plane), resulting in values of a = 14.1 Å for nC60 and a = 10.4 Å, c = 17.5 Å for nC70 (see SI for details). nC70’s Enhanced Visible Light Absorption. The UV/vis absorption spectra of equimolar (10 μM) nC60 and nC70 aqueous solutions are depicted in Figure 3 and generally

Figure 3. UV/vis absorption spectra of 10 μM solutions of (black line) nC60, (blue line) nC70, (black dashed line) C60 in toluene, and (blue dashed line) C70 in toluene measured with optical path length of 1 cm. (Inset) Digital photograph of aggregate solutions.

agree with previously reported spectra.29,50 Note that the fullerene suspensions exhibit the characteristic orange-brown and red color for nC60 and nC70, respectively (Figure 3 insets). nC70 appears to have much more visible light absorption than nC60, corresponding to a value of 2.3 times the total integrated absorbance of nC60 from 400 to 800 nm. This result is not surprising, given that C70 exhibits very large molar absorptivities in the visible range when compared to C60 in organic solvents. nC70 appears to possess a much more intense visible light absorption than an equimolar solution of C70 in toluene, which may be partly due to light scattering from the aggregates or potential alterations of the spectral features brought about by the aggregation of fullerene molecules,51,52 as has been documented for nC60. Additionally, possible red-shifting of C70’s absorption upon aggregation may also increase nC70’s visible light absorption as has been previously observed.53 Visible Light 1O2 Production. Figure 4 displays the FFA degradation kinetics in equimolar aqueous suspensions of nC60 and nC70 under visible light illumination. nC70 appears to produce significantly more 1O2 than nC60 under visible light irradiation, corresponding to initial FFA degradation rates (first 20 min) of 0.26 ± 0.03 μM/min and 0.12 ± 0.01 μM/min for nC70 and nC60, respectively. However, note that the 1O2 production rates are still quite low and that the y axis in Figure 4 does not start from zero. Control experiments show that FFA is not adsorbed or photolyzed during photoreactions.

Figure 2. Diffraction patterns of (a, blue) C60 and (black) nC60; (b, blue) C70 and (black) nC70.

exhibits numerous peaks that agree in both intensity and peak location with previous reports that found solid C60 to pack in a face-centered cubic (FCC) crystal structure,11,48,49 thus suggesting that the sublimed C60 sample exhibits FCC packing. nC60’s diffraction pattern displays many of the same intense peaks found in the pattern of sublimed C60, including signals at 10.8 (111), 17.78 (220), and 19.4 (311) 2θ, suggesting that fullerene molecules also pack in an FCC-like fashion for nC60, which is well-documented.11,49 The diffraction pattern of sublimed C70 is significantly different than those of C60 and nC60, but it agrees in both the intensity and location of signals with the previously reported diffraction pattern obtained from recrystallized C70, which exhibited hexagonal close packed (HCP) crystal structure.46 The observed agreement with the D


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consistent ratio of 1.69 ± 0.05 times the 1O2 production of nC60 (Figure 5). The fact that nC70 consistently displays ca. 1.7

Figure 4. FFA degradation as a function of time for aqueous solutions of (red ■) nC60 and (black ●) nC70 under visible irradiation. Various controls are also included. [nC60] = [nC70] = 20 μM, [FFA]0 = 200 μM; [L-histidine]0 = 250 mM.

Addition of L-histidine (250 mM), a potent 1O2 quencher (rate constant 1.5 × 108 M−1 s−1),54 considerably diminished FFA degradation for nC70. Additionally, no significant FFA degradation was observed when solutions were continuously purged with Ar (Figure S5, SI), suggesting oxygen’s role in FFA degradation. nC70 exhibited substantially enhanced FFA degradation kinetics when experiments were conducted in D2O compared to water, yielding an initial FFA degradation rate (first 20 min) of 2.39 ± 0.31 μM/min, nearly 10 times that of the measured value in water (Figure S5, SI). Because the lifetime of 1O2 is substantially longer in D2O (ca. 50−70 μs) compared to water (ca. 3−4 μs),54 this result confirms that 1O2 is responsible for FFA consumption. The observed 1O2 production rate by nC60 in water is somewhat lower than the previously reported value of 0.132 ± 0.004 μM/min,28 which may be attributed to differences in nC60 concentration, excitation conditions, and the sensitivity of the assays used to determine 1O2 production. Our finding that nC70 is significantly more photoactive than nC60 contradicts past research that found nC60 and nC70 to both lack considerable photoactivity.31 However, it is difficult to directly compare the results due to differences in fullerene concentrations and also because it remains unclear what light source and intensity were used in the previous study. Wavelength-Dependent 1O2 Generation. The above conclusion that nC70 is significantly more photoactive than nC60 does not take into account the increased visible light absorption of nC70 and may be misleading in terms of the quantum efficiency of 1O2 production. To elucidate the effects of nC70’s increased visible light absorption on 1O2 production, we conducted a series of experiments where a monochromatic light source illuminated suspensions of nC60 or nC70 that possessed identical absorbances at the excitation wavelength. By using these conditions, both nC60 and nC70 should theoretically absorb the same amount of photons and the effects of any differences in increased visible light absorption on 1O2 production rates can be deciphered. nC70 suspensions produced more 1O2 than nC60 for all wavelengths tested, resulting in a

Figure 5. FFA degradation rates as a function of excitation wavelength for aqueous nC60 and nC70 solutions with normalized absorbances at the excitation wavelength reported as (a) μM/min and (b) normalized on a per mass basis as μM/min-mg. [FFA]0 = 200 μM.

times the 1O2 production of nC60 suggests that other mechanisms might be the cause of nC70’s increased 1O2 sensitization capacity. When normalized per mass of fullerene in solution, nC70 suspensions produced even more 1O2 compared to nC60 solutions. Note that the mass normalized 1 O2 production rates do not exhibit a uniform trend across the various excitation wavelengths because different concentrations of fullerene solutions were necessary to achieve the same absorbances for nC60 and nC70 suspensions. Excited State Lifetime Measurements. Figure 6 displays the transient absorption spectra of C60 and C70 when molecularly solubilized in toluene as measured via nanosecond LFP with the red arrows indicating the decay kinetics. The transient absorption spectra of C60 and C70 exhibit similar spectral features as have been previously reported for 3C60* and 3 C70*,55 thus suggesting excited state triplet formation. Specifically, 3C60* exhibits a broad peak centered at ca. 740 nm and 3C70* exhibits a bleaching signal, where the excited E


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Figure 6. Transient absorption spectra as a function of the delay time after the laser excitation pulse for (a) C60 in toluene, (b) C70 in toluene, (c) nC60, and (d) nC70. [C60] = [C70] = 5 μM; [nC60] = 26 μM; [nC70] = 43 μM. λex = 355 and 387 nm for nanosecond and femtosecond LFP experiments, respectively.

Postulated Mechanisms. Results presented herein have shown that nC70 produces significantly more 1O2 than nC60 under visible light illumination, and this increased 1O2 production may be attributed to the combined effects of the enhanced visible light absorption and longer triplet excited state lifetime of nC70 compared to nC60. Since the rapid decay of 3 C60* (within 1 ps) has been hypothesized to be the primary cause for the reduction of photoactivity for nC60, a longer triplet lifetime, as experienced by C70 in nC70, may lead to a more efficient energy transfer to 3O2 and subsequent production of 1O2. Considering that the bimolecular quenching of triplet excited state fullerene by O2 kinetically competes with triplet-state decay kinetics shown in Figure 6, approximately 100 times higher rate of 1O2 (i.e., nC70 exhibits 100 times longer lifetime than nC60) should be expected with nC70 than nC60. It is unclear at this stage why we observed much lower 1 O2 yield ratio of ca. 1.7 between nC70 and nC60. However, this result possibly suggests that the 1O2 generated by fullerene aggregates is not produced from triplet quenching by bulk dissolved oxygen (DO), but instead from molecular oxygen within the fullerene aggregates or physisorbed onto the aggregate surface. This possibility of quenching via closely associated oxygen molecules helps explain how 1O2 can be produced from species with such rapid decay kinetics, where the time scales are too short for the diffusion of oxygen from the bulk phase to reach the fullerene surface, quench excited states, and have a significant impact on 1O2 generation. Yet, questions still remain as to why C70 exhibits a considerably longer lifetime in the aggregated form compared to nC60, when both C60 and C70 in toluene (molecularly dispersed) display similar triplet excited state lifetimes.55 As ascertained from XRD analysis, C60 readily packs in a FCC crystal structure for nC60, whereas C70 molecules likely form a HCP crystal structure for nC70. Using the unit cell

state absorbs less than ground state, at ca. 480 nm and a broad absorption from 520 to 760 nm. Excited state lifetimes were calculated by exponential fitting of the decay kinetics at 740 and 420 nm for C60 and C70, respectively, yielding observed triplet lifetimes of 38 and 49 μs for C60 and C70, respectively. The lifetimes are somewhat diminished compared to previous reports,8,26 possibly due to ground state quenching processes between fullerene molecules in solution55 or the presence of trace amounts of oxygen in solution. No significant transient signal was formed for nC60 and nC70 within the time scale of the nanosecond LFP experiment, suggesting the rapid decay of 3 C60* and 3C70* within less than ca. 10 ns for aggregate species. Instead, femtosecond LFP was pursued to obtain the transient absorption spectra of nC60 and nC70 (Figure 6c and d). nC60 appears to form a very short-lived excited state, which rapidly decays within ca. 10 ps. This excited state does not appear to be 3 C60* given the lack of a prominent peak at 740 nm, indicative of 3C60*, and dissimilarities with the spectra measured for C60 in toluene. The transient spectrum does, however, agree with a previous report that also observed excited state decay within only a few ps.56 From these measurements, it is evident that 3 C60*, if even formed, decays within only a few picoseconds (ps), consistent with previous reports that have found 3C60* to decay within 1 ps in nC60.10 nC70 appears to form an excited state species that exhibits ground-state bleaching at ca. 500 nm and a broad absorption from 600 to 800 nm, agreeing with the transient spectra obtained from C70 in toluene, thus suggesting the formation of 3C70*. This excited state does not fully decay to the baseline within 100 ps and thus appears to have a much longer lifetime than 3C60* in nC60. Collectively, these results suggest that the individual fullerene molecules that compose nC70 may exhibit considerably longer triplet excited state lifetimes than found in nC60, possibly due to decreased ground and excited state quenching. F


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Environmental Science & Technology parameters calculated from the diffraction patterns (nC60: a = 14.1 Å; nC70: a = 10.4 Å, c = 17.5 Å) and the associated crystal structures, we calculate the volume of one unit cell (nC60: 2.80 nm3; nC70: 4.92 nm3), which can be used to determine the overall density of fullerene molecules within the crystalline matrix. Because FCC packing contains four fullerene molecules in the unit cell and HCP six, the overall density of fullerene molecules corresponds to ca. 1.4 and 1.2 fullerene molecules/ nm3 for nC60 and nC70, respectively. Accordingly, the lower density of fullerene molecules as found in nC70 possibly suggests larger fullerene−fullerene spacing within the aggregates. Given that Dexter energy transfer processes, like those involved in TTA quenching of excited fullerene species, are highly dependent on the distance between donor and acceptor molecules with the rate decaying exponentially as a function of this distance,57 a small change in the fullerene density within the aggregates may greatly affect the deleterious excited state quenching processes. It is therefore possible that the HCP fullerene packing of nC70 allows for less dense fullerene aggregates, providing less interaction between adjacent fullerene molecules within the aggregate structure, and thus longer triplet lifetimes than for nC60. Additionally, the lower density of fullerene molecules in nC70 may better allow 1O2 produced within the aggregates to diffuse into the bulk solution, which has previously been hypothesized to be a critical limitation for 1 O2 photosensitization by nC60.12 However, the exact transport processes governing oxygen’s diffusion from within the aggregates to the bulk solution and DO’s diffusion into the aggregates remains unclear and requires additional investigation. Although it has been hypothesized that the extent of crystallinity of fullerene aggregates (i.e., crystalline vs. fully amorphous) may affect 1O2 production rates,11,58 it appears that the specific crystalline packing structure of fullerene molecules (i.e., FCC vs. HCP) may also affect the measured photoactivity. Environmental Perspective. From the results presented herein, it is evident that nC70 is significantly more visible light photoactive than nC60, which appears to be due to C70’s increased visible absorption and the increased triplet lifetime of nC70 compared to nC60. Increasing fullerene’s cage size results in drastic changes to many material propertiesperhaps the most significant is the increased visible light absorption of larger cage fullerenespotentially leading to a more adverse environmental impact because they may better utilize natural sunlight for the photogeneration of 1O2. Compared to source waters containing natural organic matter (NOM),36,59 a ubiquitous natural 1O2 photosensitizer, nC70 produces comparable levels of 1O2 with a [1O2]ss value of 1.0 × 10−14 M L mg−1 when normalized by concentration (Table S1, SI). It is important to point out that it is difficult to make direct quantitative comparisons given the differences in irradiation spectra and intensities among the past studies. However, provided that the light intensity used in this study is ca. 20−25 times less intense than that used in previous studies on natural waters it is expected that nC70 may achieve much higher [1O2]ss values per mass basis but not likely in the natural environment where NOM is commonplace. Nevertheless, many studies have found that nC60 detrimentally affects various microbial1,60−62 and aquatic species.63,64 Given the overall similar physicochemical properties of nC70 and nC60 and the enhanced photoactivity of nC70, it is expected that nC70 may experience at the very least similar adverse effects on microbial and aquatic species as observed with nC60. It is worthwhile to point out that

among the many larger cage fullerene species, C70 is the most likely to be discharged into the environment due to its extensive use in materials applications and its large production amounts in the arc-discharge process, second only to C60. Additional studies are necessary to investigate the photoactivity of nC70 in realistic environmental conditions, including under natural sunlight illumination and in the presence of NOM to investigate potential quenching processes.

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ASSOCIATED CONTENT

S Supporting Information *

Emission spectra of light sources, DLS size distributions for nC60 and nC70, FTIR and XPS spectra for nC60 and nC70, FFA degradation controls, description of unit cell parameter calculations for nC60 and nC70, diagram of FCC and HCP crystal structures, and a table comparing [1O2]ss values for fullerene aggregates and NOM. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b00100.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (203) 432-4386. Fax: (203) 432-4387. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grant no. CBET-0932872). Facilities use was supported by YINQE and NSF MRSEC DMR 1119826. We thank Dr. Prashant Kamat and Dr. Kevin Stamplecoskie at the University of Notre Dame Radiation Laboratory for fruitful discussions and assistance with LFP experiments.

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REFERENCES

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