Is Graphene a Stable Platform for Photocatalysis? Mineralization of

Jul 21, 2014 - Here, we probe the stability of RGO in the presence of UV-excited TiO2 in aqueous media and establish its reactivity toward OH• radic...
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Is Graphene a Stable Platform for Photocatalysis? Mineralization of Reduced Graphene Oxide With UV-Irradiated TiO2 Nanoparticles James G. Radich,†,‡ Anthony L. Krenselewski,†,‡ Jiadong Zhu,†,# and Prashant V. Kamat*,†,‡,§ †

Notre Dame Radiation Laboratory ‡Department of Chemical & Biomolecular Engineering §Department of Chemistry & Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The recent thrust in utilizing reduced graphene oxide (RGO) as a support for nanostructured catalyst particles has led to the claims of improved efficiency in solar cells, fuel cells, and photocatalytic degradation of pollutants. Specifically, the robust TiO2 system is often coupled with RGO to improve charge separation and facilitate redox reactions. Here, we probe the stability of RGO in the presence of UV-excited TiO2 in aqueous media and establish its reactivity toward OH• radicals, a primary oxidant generated at the TiO2 surface. By probing changes in absorption, morphology, and total organic carbon content (TOC), we conclusively demonstrate the vulnerability of RGO toward OH• attack and raise the concern of its use in many applications where OH• are likely to be formed. On the other hand, the OH• radical-mediated mineralization could also enable new approaches in tackling environmental remediation of nanocarbons such as RGO, carbon nanotubes, and fullerenes.

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of pollutants has been a popular line of study50−52 and is primarily driven by evolution of OH• from the TiO2 valence band holes.53−55 Oxygen reduction reactions at the cathode of fuel cells can also lead to OH• formation as a product of side reactions.56−58 Short-term performance enhancement seen in such applications has led numerous reports, including some of our own, to tout RGO as an exceptional conductive support that promotes efficient charge separation and charge transport behavior. However, in the long view, the question that one encounters is whether GO and RGO can withstand the onslaught of highly reactive species such as OH• radicals. The residual hydrogens bonded to the surface and edges of RGO59 can lead one to envision a single RGO sheet as many fused polyaromatic hydrocarbons (PAHs). Because PAHs are of significant environmental concern, emphasis must be placed on the long-term impact of RGO in applications where highly oxidizing species may promote disintegration of RGO into PAH-like fragments. Although demonstrations of significant improvement in the photocatalytic activity of TiO2-RGO composites are numerous,6,7,19,20,61−63 a closer examination of the specific interactions between TiO2 and RGO in aqueous suspension are needed to better understand its limitations in practical applications. Of specific importance is the direct reaction between OH• and RGO because of the prevalence of these radicals when irradiating TiO2 in aqueous media. In our

INTRODUCTION Graphene has received considerable attention in recent years as a result of its high specific surface area, exceptional electrical conductivity and electron mobility,1,2 and potential for use in a wide range of applications.3−12 The emergence of solutionbased graphene in the form of graphene oxide (GO) has enabled new wet-chemistry approaches to the creation of graphene-based composite materials for energy and environmental applications.6,7,13−25 Because GO can be dispersed as single-to-few layer sheets, nanostructured materials can be directly synthesized or anchored onto the GO surface by dispersing in polar solvents. Exceptional loading of semiconductor and metal nanoparticles onto the high surface area GO sheets has opened up new opportunities to employ these assemblies in thermal-, electro-, and photocatalysis. Furthermore, the facile reduction of GO to reduced graphene oxide26−35 (RGO) restores a high degree of the sp2 bonding structure inherent to pristine graphene and yields a conductive 2-D carbon mat that can shuttle charges between the active nanostructured materials.24 In particular these nanostructured RGO-based composite materials have been shown to improve the performance of photocatalysts,36−45 fuel cells,15,16,46,47 and batteries21,23,48 where charge separation and electron transport efficiency are dominant operating principles. Studies focusing on the long-term stability of RGO in predominantly oxidative environments are lacking. However, the reductive decomposition of RGO has been reported.49 Several of the previously referred electrocatalytic or photocatalytic applications for RGO involve OH• as a primary or side reaction product. For example, TiO2 photocatalytic degradation © XXXX American Chemical Society

Received: July 18, 2014 Revised: July 21, 2014

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irradiated with white light from Xe lamp passing through a 10 cm water filter followed by AM 1.5 filter at 150 mW/cm2.

previous work, we have shown the susceptibility of the CC bonds in RGO to OH• radical attack, which were assisted by gold nanoparticles to formulate holey-graphene suspensions.60 The potential for oxidative fragmentation of the RGO sheets into smaller PAH domains via OH• attack may render the environmental benefits of using TiO2-RGO composites in photocatalytic pollutant degradation moot because new environmental concerns would be generated in the process. In this study, we focus specifically on the susceptibility of RGO toward OH • attack promoted by UV-irradiated TiO 2 suspensions and elucidate the role of OH• radicals in inducing the oxidation. The study provides new insight into the fate of RGO in an oxidative photocatalytic environment and raises concerns about its long-term stability.

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RESULTS AND DISCUSSION Absorption Changes during UV Irradiation. Exposing TiO2 nanoparticles suspended in aqueous media to UV irradiation leads to the evolution of OH• radicals via oneelectron oxidation of surface bound hydroxide ions.55 The oxidative environment induced by hydroxyl radicals has been the subject of numerous reports on the photocatalytic degradation of organic compounds.54 By anchoring semiconductor particles such as TiO2 on RGO sheets, it is possible to improve charge separation and enhance photocatalytic performance in environmental applications.6,7,19,20,61−63,65 The presence of TiO2 photocatalyst particles near the RGO surface increases the probability of its interaction with hydroxyl radicals. Most photocatalytic studies with TiO2/RGO assemblies reported in the literature were conducted for a short time duration and focused only on the degradation of contaminant. However, these studies failed to look into possibility of longterm degradation of RGO itself. To probe the stability of the RGO in the presence of OH• radicals, a suspension of TiO2 and RGO was subjected to white light irradiation from a xenon lamp. We used UV−visible absorption spectroscopy to track the changes in the degree of oxidation of RGO as in our previous work.60 During the reduction of GO, the absorption (including scattering, henceforth both will be referred as absorption) increases as the level of reduction increases as shown in Supporting Information Figure S1. A small increase in absorbance is also seen when prereduced GO suspension (viz. RGO) is subjected to UV irradiation as the photoinduced reduction reaction34,66 proceeds further. The increase in absorbance is attributed to a reduction of residual oxygen groups on the RGO surface and the restoration of sp2 carbon bonding structure. On the same account, any decrease in absorbance can be ascribed to the oxidation of RGO. Figure 1 follows the changes in UV−visible absorption of TiO2-RGO samples measured in 5 min intervals during 30 min of white-light irradiation under different experimental conditions. The change in absorbance of TiO2-RGO dispersed in O2saturated water is depicted in Figure 1A. Highly oxidizing OH• radicals are generated at the TiO2 surface following absorption of UV photons and subsequent charge separation. The photogenerated electrons are scavenged from the TiO2 conduction band by the dissolved O2, which suppresses electron−hole recombination and promotes high concentration of OH• for oxidative reactions. The excess OH• radicals near the TiO2 surface are expected to interact with RGO sheets in the vicinity of TiO2. Indeed, a sharp decrease seen in the UV− visible absorption following 30 min of exposure to UV/visible irradiation suggests that RGO is undergoing oxidative transformations. The role of OH• radical in the oxidative transformation was reinforced by tracking the changes in absorbance when excluding UV photons (Figure 1B) or introducing an alternate hole scavenger such as ethanol to exclude OH• radical formation (Figure 1C). Under these conditions, we fail to observe the characteristic decrease in absorbance. The processes which contribute to the increase in absorbance (viz. reduction of RGO) or decrease in absorbance (viz. oxidation of RGO) are summarized below (reactions 1−4).

EXPERIMENTAL SECTION

Materials and Methods. TiCl4 was obtained from Sigma and used as received. TiCl4 solution was prepared by diluting from a 2 M stock solution, which was prepared via hydrolysis of 99% TiCl4 in a subzero ice bath by adding concentrated TiCl4 dropwise while stirring. Glacial acetic acid and 200 proof ethanol were obtained from Fisher and used as received. TiO2 colloids were prepared via hydrolysis of 10 mM TiCl4 at 120 °C, followed by centrifugation to remove residues and particle aggregates. The supernatant was retained for use in photocatalytic experiments. GO was prepared via modified Hummers method.64 A diluted GO suspension of 0.1 mg/mL was prepared and irradiated with water-filtered white light from Xe lamp to promote photoreduction of GO to RGO. Photocatalytic experiments with TiO2 and RGO were carried out using white light from Xe lamp at 250 mW/cm2 after passing through a 10 cm water filter. A Cary 50 UV−visible absorption spectrophotometer was used to measure the changes in absorption. TiO2-RGO samples were irradiated in a 1 cm quartz cuvette with continuous bubbling of O2. Transmission electron micrographs were obtained using a FEI 300−80 Titan transmission electron microscope after dispensing one drop of solution over carbon grids. RGO-TiO2 composites were prepared by suspending RGO with TiO2 at the same concentration as in all other experiments. The aqueous suspension was bubbled with N2 and irradiated for 10 min. The resulting suspension was much darker in appearance. The suspension was centrifuged at 7000 rpm for 5 min to remove the RGO-TiO2. The TiO2-RGO composite was resuspended in the same volume and irradiated in the same conditions as the individual TiO2/ RGO suspensions. Photos of GO and RGO suspensions in Figure 2A were obtained from a single solution of graphene oxide. The solution was photographed as prepared and then following photoreduction for 30 min under white-light irradiation. Solutions in Figure 2B used in photographs are 10 times higher in concentration for visual effect of color change/loss. Nanosecond Flash Photolysis. Nanosecond flash photolysis using 355 nm pulse (10 ns pulse width) was employed in combination with thiocyanate competition kinetics to calculate the rate constant for OH• attack on RGO. Quantum yield was estimated by using 740 nm triplet absorption of C60 dissolved in toluene. Calculation of rate constants was carried out based on the concentration of carbon atoms in the RGO suspension as discussed in detail in our previous report.60 Total Organic Carbon Analysis. Samples for total organic carbon analysis were diluted 10×, to which 1 mL of HCl was added to remove residual CO2 and carbonates. A Shimadzu TOC-vSH organic carbon analyzer with IR detection was employed for total organic carbon analysis. The temperature of the catalytic furnace was raised to 720 °C to ensure complete conversion of RGO to CO2. Dye Degradation Experiments. Photocatalytic experiments using naphthol blue black dye were carried out using 0.05 mg/mL RGO and 50 μM TiO2 colloidal suspensions. The suspensions were B

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Figure 2. Photographs of (A) GO and RGO illustrating the changes in visible absorption features as the level of oxidation changes and (B) TiO2-RGO sample after various irradiation times with UV/visible irradiation from a Xe lamp. After 75 min, the TiO2-RGO aqueous suspension is essentially colorless as OH• radicals generated via UV excitation of TiO2 oxidize RGO. The concentration of RGO and TiO2 in the experiment shown in (B) was increased from 0.05 (Figure 1) to 0.5 mg/mL and from 0.1 to 1 mM, respectively, for emphasis on the visual changes apparent during RGO oxidation reaction via OH• radicals. Figure 1. UV−visible absorption spectra collected in 5 min intervals following UV/visible irradiation from a Xe lamp of (A) TiO2-RGO in water, (B) TiO2-RGO with 420 nm cutoff to suppress UV photons, (C) TiO2-RGO with ethanol as electron donor to suppress OH•, where a−g represent measurements in 5 min intervals, and (D) changes in absorption vs time at 600 nm for each (A)−(C) represented by traces a, b, c, respectively. [TiO2] = 0.1 mM with RGO @ 0.05 mg/mL.

TiO2 + hν → TiO2 (e + h)

of oxidation of the GO/RGO in aqueous suspension. The photographs in Figure 2B depict the TiO2-RGO aqueous suspension at different times of irradiation. The concentration of RGO in the preirradiated sample in Figure 2B was increased significantly to provide visual aid to the oxidative reaction responsible for the changes in visible characteristics of the RGO transformation. At time 0 the solution is dark brown. As irradiation time increases the oxidative transformation (note Figure 2A, RGO → GO direction) becomes evident in the evolution of light brown color in the suspension. Further UV irradiation leads to further increase in light transmittance, and finally, after 75 min of irradiation, the solution is colorless. Mechanistic Insights into RGO Degradation. The oxidation of RGO observed during irradiation of TiO2-RGO aqueous suspension can proceed through two distinct pathways: (1) oxidation of RGO with OH• radicals and (2) direct oxidation of RGO by TiO2 valence band holes. We employed nanosecond laser flash photolysis with 355 nm laser pulse excitation to verify contribution of OH• radicals to oxidation of RGO during irradiation of TiO2-RGO aqueous dispersion. The 355 nm laser pulse excites TiO2 at the band edge without photon depletion through the 1 cm path of the cuvette (abs @ 355 nm ∼0.75). This ensures excitation of the TiO2 colloids at the center of the cuvette where the probe light is used to monitor laser pulse-induced transient species. Potassium thiocyanate was introduced to estimate the amount of OH• radicals, as it undergoes oxidation to produce optically detectable thiocyanate radical species, (SCN)2−• with absorption maximum at 475 nm (reaction 5)67

(1)

TiO2 (e) + GO → RGO or TiO2 (e) + O2 → TiO2 + O2−

(2)

TiO2 (h) + OH− → TiO2 /OH• or TiO2 (h) + ethanol → TiO2 + products

(3)

TiO2 /OH• + GO/RGO → TiO2 + f ‐PAH → → CO2 + H 2O

(4)

f-PAH in reaction 4 represents the polyaromatic hydrocarbons which are likely to appear as intermediates during hydroxyl radical oxidation of RGO. Increases in the absorption spectra were observed when ethanol was present during UV-irradiation. Note that ethanol rapidly scavenges TiO2 valence band holes and suppresses OH• radical formation. UV photons are able to rapidly induce photoreduction of residual oxygen moieties on the RGO sheet, particularly in electron-donating ethanol. When UV photons are filtered, the visible absorbance of the TiO2-RGO suspension remains unchanged since visible photons fail to participate in the GO → RGO reduction process and OH• radicals are not produced in the absence of UV-irradiation.33,34 The changes in absorption of the TiO2-RGO samples are directly compared in Figure 1D at 600 nm over 30 min of irradiation. These control experiments further support the hypothesis that photocatalytic formation of hydroxyl radicals initiates the decrease in visible absorption features by RGO in the aqueous TiO2-RGO suspension. The photographs of GO and RGO suspensions in Figure 2A illustrate the visual changes that represent the different extent

OH• + 2SCN− → (SCN)2−• + OH−

(5)

Upon excitation of TiO2 colloids with 355 nm laser pulse, OH• radicals are generated at the TiO2 surface, which in turn react with SCN− to yield (SCN)2−•, whose visible spectrum is depicted in Figure 3A. With increasing concentration of RGO we observe a decrease in the transient absorbance of (SCN)2−• radicals as reaction 5 competes with reaction 4. We used deaerated C60 solution as an actinometer to estimate the quantum yield of (SCN)2−• radicals. The 355 nm laser pulse excitation of C60 exhibits triplet excited state with characteristic absorption maximum at 740 nm (ε = 16 000 M−1 cm−1). The quantum yield of thiocyanate radical decreases with each incremental addition of RGO as shown in Figure 3B confirming the competition between reactions 4 and 5. C

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the environmental implications of deploying RGO in oxidative environments. It has been well documented in the literature that aromatic molecules become mineralized during OH• radical mediated oxidation in UV irradiated TiO2 suspensions.70−72 The OH• radicals usually add to the aromatic ring or undergo electron transfer reactions. Hydroxyl and carboxylic acid derivatives have often been identified as reaction intermediates. The OH• radical-induced oxidation reaction could result in fragmentation/disintegration of RGO sheets into smaller PAH-like molecules. Although such PAH fragments can induce the toxicity effects in the environment, extended reaction with OH• radicals may lead to mineralization (i.e., converting from organic to inorganic carbon such as carbonates or CO2). Mineralization of RGO. To better understand the nature of the overall reaction between hydroxyl radicals and RGO, total organic carbon (TOC) analyses were carried out on the samples after discrete periods of irradiation. Figure 4 compares

Figure 3. (A) Transient absorption spectrum of (SCN)2−• taken 1 μs after the laser pulse. Dashed line is provided as a guide only. (B) Change in (SCN)2−• quantum yield with increasing concentration of RGO illustrating decreasing yield of thiocyanate radical as RGO competes with (SCN)− in the reaction with OH• radicals. (C) Ratio of the peak intensity of (SCN)2−• with and without RGO present as a function of the ratio of [carbon] in RGO to [SCN−]. Laser power set at 0.1 mW.

Knowledge of the rate constant for the reaction between OH• and SCN− (reaction 5) enables calculation of the rate constant corresponding to OH• reaction with RGO (reaction 4) using competition kinetics analysis. Varying the ratio of [RGO] to [SCN−] allowed us to ascertain the reaction rate constant by plotting this ratio against the ratio of the corresponding ΔA values as depicted in Figure 3C. The rate constant, kRGO, was calculated to be 4.7 × 109 M−1 s−1. This value is in agreement with our previous value60 of 4.4 × 109 M−1 s−1 obtained with the photolysis of H2O2 and further confirms the role of OH• radical in the oxidation of RGO with similar kinetics. This diffusion-controlled reaction rate validates the importance of understanding the interaction between TiO2 and RGO before deploying these materials in practical applications. In the present study we also probed the possible interaction of RGO with trapped valence band holes in TiO2 suspensions. As previously reported UV irradiation of TiO2 colloids in 5% acetic acid in ethanol leads to the formation of trapped hole species.68,69 The trapped holes exhibit visible absorption at c.a. 400 nm (seen in the inset photograph in Supporting Information Figure S2A). We attempted to titrate trapped holes with incremental additions of RGO. If RGO is susceptible to oxidation by TiO2 trapped holes, we would expect to see a decrease in the corresponding visible absorption feature associated with these holes. As shown in Supporting Information Figure S2B, the incremental addition of RGO to the TiO2 colloids with trapped holes exerted no change in the absorption at 400 nm. From these two experiments, we conclude that oxidation of RGO is due to OH• and not direct oxidation by trapped holes at the TiO2 surface. Taken together, the decrease in the absorption features of the TiO2-RGO suspension upon irradiation with white light and the reaction between the OH• radicals and RGO observed in the flash photolysis experiments confirm the OH•-mediated RGO oxidation in the presence of UV-irradiated aqueous TiO2 suspensions. The fate of the oxidized RGO is highly relevant to

Figure 4. Comparison of the drop in absorbance (black squares) and TOC concentration (blue circles) as a function of irradiation time. The rapid decrease in absorption with slow depletion of organic carbon demonstrates the fragmentation process during early irradiation times. Dashed lines are provided as guides and do not represent mathematical fits.

the change in TOC concentration with the change in absorption during irradiation. One can observe the significant difference in the rate of the decrease in absorption as compared to the decrease in TOC concentration. The rapid decrease in the RGO visible absorbance during the initial 15 min of irradiation (c.a. 75% decrease, Figure 1A) from dark brown to essentially colorless contrasts the slower depletion of TOC in the same time frame. We posit this early degradation process is predominately the disintegration of RGO sheets as described in reaction 4, whereby the extended chemical structure of RGO responsible for its strong visible absorbance undergoes scission to smaller fragments. An earlier study also demonstrated similar scissoring effect of OH• radicals in UV-irradiated TiO2 strips in contact with single-layer graphene film.73 Transmission electron micrographs of preirradiated and 10 min-irradiated TiO2/ RGO suspensions (Figure S3, Supporting Information) show the disintegration of the RGO sheets evident by the surrounding small fragments that arise as contrast in the postirradiated sample image. Because the PAH-like fragments do not absorb in the visible we do not expect them to contribute to the absorbance, but they will contribute to TOC. For example, after 30 min of irradiation, we observe ∼75% decay in visible absorbance but only 25% decrease in TOC. D

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Scheme 1. Depiction of the Oxidative Fragmentation and Mineralization of RGO during Irradiation of TiO2-RGO Suspensionsa

Initial OH• attack leads to scission of the RGO sheets into PAH-like compounds as evident in the rapid decrease in RGO visible absorbance. Continued irradiation promotes the slower mineralization of these fragments to CO2 and H2O. a

Figure 5. Simultaneous photocatalytic degradation of naphthol blue black dye and RGO using UV/visible-irradiated TiO2 suspension. (A) Visual depiction of color changes (1) initial RGO suspension, (2) upon addition of naphthol blue black dye, (3) 10 min of UV irradiation, and (4) 100 min of UV irradiation, at which point the solution is completely colorless. (B) Comparison of absorbance changes at 600 nm following UV-irradiated (a) TiO2 and (b) TiO2/RGO suspensions containing naphthol blue black dye. The initial rapid change in absorption (zone I) is due to the breakdown of conjugation in dye and slower change in absorption in zone II is the degradation of residual dye and RGO. Spectra indicating depletion of dye after 20 min in dye/RGO sample can be found in Supporting Information Figure S4. The concentrations employed were [dye] = 45 μM, [TiO2] = 0.1 mM, [RGO] = 0.05 mg/mL.

photocatalytic degradation of this dye have been previously reported.76,77 With increasing UV irradiation time, we observe continuous decay of the dye absorbance in both TiO2 and RGO/TiO2 suspensions, but with different rates. The photographs in Figure 5A serve as a visual aid in demonstrating the degradation of both dye and RGO during UV irradiation of TiO2. Continuous irradiation degrades not only the dye but also RGO. The changes in visible absorption peak of Naphthol blue black (600 nm) are plotted with respect to irradiation time in Figure 5B. (UV−visible spectra of dye degradation with TiO2 and TiO2/RGO are presented in the Supporting Information, Figure S4.) When RGO is present in the solution, the decrease in absorbance initially (first 10 min) is similar to the one without RGO. (Similar degradation rates were also observed for methylene blue, which completely degrades within 5 min under similar conditions. Supporting Information, Figure S5.) However, the later part of dye degradation in Figure 5B proceeds at a slower rate in the presence of TiO2/RGO than with TiO2 alone. The competition between RGO and the dye toward OH• radicals inhibits the rate of dye degradation when the dye concentration becomes lower. The slower rate of absorbance change at longer times in Figure 5B shows the dye molecules and RGO/f-PAH intermediates compete for OH• radicals. This competitive oxidation reaction is analogous to the competition between RGO and SCN− for OH• radical

Because of the formation of nonabsorbing intermediates, we expect to see a faster decrease in absorbance than TOC. Although UV-irradiated TiO2 is effective toward degradation of intermediate carboxylates, their relative rate of oxidation is slow.74,75 The smaller RGO fragments eventually mineralize to CO2 and H2O over long periods of irradiation of TiO2/RGO suspensions. Nearly 75% decay in TOC is seen following 2 h of UV irradiation. Continual evolution of OH• radicals during UV excitation of TiO2 enables the decrease in TOC. The OH• radical-induced oxidation of RGO leading to mineralization is illustrated in Scheme 1. Implications of RGO Reactivity toward Hydroxyl Radicals in Photocatalysis. Numerous claims of the beneficial role of GO/RGO, including in our own previous reports, have been made in experiments involving photocatalytic degradation of the dyes6,7,19,20,24,61−63 and hydrogen production from water.32,37−42 The majority of these studies involve short-term UV-irradiation experiments. However, these systems need to be carefully evaluated for long-term stability and environmental implications. As an example to assess this issue we carried out photocatalytic degradation of a textile diazo dye (naphthol blue black) in TiO2 and TiO2/RGO suspensions. We specifically chose this dye because of its slower rate of oxidative degradation as compared to commonly used probes such as methylene blue. The excited state properties and mechanism of E

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oxidation, which resulted in lower (SCN)2•− quantum yield in the flash photolysis experiments (Figure 3B). In particular, the total disappearance of the RGO color after completion of the irradiation confirms the degradation of RGO during the photocatalysis. Furthermore, blank UV irradiation experiments carried out with TiO2/RGO composite (in absence of the dye) also confirm RGO susceptibility to oxidation (see Figure S6, Supporting Information).

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Visiting summer undergraduate student from the Department of Chemistry, Fudan University, Shanghai, China.

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ACKNOWLEDGMENTS P.V.K. acknowledges the support by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U. S. Department of Energy through award DEFC02-04ER15533. J.G.R. acknowledges the Bayer Environmental Research Fellowship administered through the Center for Environmental Science and Technology (CEST) at the University of Notre Dame and the instrumentation support in CEST. A.K. acknowledges the support of University of Notre Dame under ND Nano undergraduate fellowship program. J.Z. acknowledges the support of University of Notre Dame under iSURE undergraduate fellowship program. The authors also acknowledge partial support of this research by the NSF-funded University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288). This is contribution number NDRL 5021 from the Notre Dame Radiation Laboratory.

CONCLUSIONS

In summary, the OH• radicals generated at the TiO2 surface lead to oxidative attack on the carbon-rich RGO, which is ultimately mineralized as evident from the decreased TOC concentration with increasing irradiation time. However, the distinct difference in the rate of the RGO absorption loss and TOC depletion during OH• attack points to an early fragmentation process during the oxidation of RGO that could lead to release of PAH-like compounds. Caution must be exercised in pursuing the TiO2-RGO assembly in future photocatalytic studies. The oxidative degradation of RGO by UV irradiated TiO2 fundamentally challenges the current paradigm of coupling these materials for aqueous-based photocatalysis and other applications where OH• radicals are present. In fact, the fragmentation of RGO could exacerbate the environmental challenges TiO2-RGO composites are employed to remediate. Because the initial breakdown of the RGO occurs quickly and leads to more recalcitrant compounds that are slower to degrade via OH•, these fragments, if untreated, may enter into the environment. However, a positive outcome of the present study is the remediation of nanocarbons in the environment. Concerns have been raised about the contamination of nanocarbons such as C60, carbon nanotubes, and graphene-derived products in the environment.78−80 The results presented here demonstrate the usefulness of OH• radicals toward mineralization of nanocarbons, thus mitigating their impact in the environment. Further optimization of this process is necessary to address the selective transformations of individual nanocarbons.

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ASSOCIATED CONTENT

S Supporting Information *

UV−visible absorption spectra showing reduction of graphene oxide to reduced graphene oxide, titration curves of trapped holes in TiO2 with RGO as titrant, UV−visible absorption spectra of photocatalytic degradation of naphthol blue black dye via UV-irradiated TiO2 with and without RGO, transmission electron micrographs of preirradiated and 10-minirradiated TiO2/RGO, UV−visible absorption spectra of photocatalytic degradation of methylene blue dye via UVirradiated TiO2 with and without RGO, and UV−visible absorption spectra of UV-irradiated TiO2-RGO composite prepared via photocatalytic reduction of GO are included. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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dx.doi.org/10.1021/cm5026552 | Chem. Mater. XXXX, XXX, XXX−XXX