Adsorption of Sulfur Dioxide by CoFe2O4 Spinel Ferrite Nanoparticles

Mar 8, 2012 - Facile Synthesis of ZnFe2O4 Nanoparticles with Tunable Magnetic and ... T. Grant Glover , Gregory W. Peterson , Jared B. DeCoste , and ...
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Adsorption of Sulfur Dioxide by CoFe2O4 Spinel Ferrite Nanoparticles and Corresponding Changes in Magnetism T. Grant Glover,*,† Daniel Sabo,‡ Lisa A. Vaughan,‡ Joseph A. Rossin,§ and Z. John Zhang*,‡ †

SAIC, Gunpowder Branch, P.O. Box 68, APG, Maryland 21010, United States Georgia Institute of Technology School of Chemistry and Biochemistry, 901 Atlantic Drive, Atlanta, Georgia 30332, United States § Guild Associates, Incorporated, 5750 Shier-Rings Road, Dublin, Ohio 43016, United States ‡

ABSTRACT: Adsorption of sulfur dioxide on 10 nm CoFe2O4 spinel ferrite nanoparticles was examined. Adsorption loadings of sulfur dioxide at breakthrough conditions were determined to be approximately 0.62 mol/kg, which is significant given the 150 m2/g surface area of the nanoparticles. Adsorption proceeds through a chemisorption mechanism with sulfur dioxide forming a sulfate upon adsorption on the particle surface, which leads to a 23% decrease in the remnant magnetization, a 20% decrease in the saturation magnetization, and a 9% decrease in the coercivity of the magnetic nanoparticles. Adsorbent materials that provide a magnetic signal when adsorption occurs could have broad implications on adsorption-based separations.

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INTRODUCTION Significant effort has been devoted to the development of novel adsorbent materials, such as metal organic frameworks, zeolitic imidazolate frameworks, covalently bonded organic frameworks, and engineered carbons.1−5 These materials represent a departure from amorphous adsorbents to crystalline materials that can be engineered at a molecular level to impact gas adsorption performance.6−8 Magnetic nanoparticles (MNPs) are of particular interest because MNPs have many of the same physical characteristics as some of the engineered adsorbent materials being discussed in the literature. Specifically, spinel ferrite MNPs are crystalline, contain transition metals with a chemical formula MFe2O4 (M = Mn, Mg, Zn, Co, Ni, Cu, Fe, etc.), and are small enough in diameter to have significant surface area for adsorption. More details on the physical properties of these materials has been documented elsewhere.9−11 Also, quantifiable changes in the magnetic properties of the particles occur when ligands are loaded on the MNP surface.12 For instance, when para-substituted benzoic acids and substituted benzene ligands are loaded on the surface of 4 nm MnFe2O4 particles, a decrease in coercivity ranging from approximately 25% to 50% was observed depending on the ligand loaded. The reflection of adsorption in the magnetic properties of the nanoparticle is unique because the majority of adsorbent materials do not exhibit a change in their magnetic properties upon adsorption. In addition, the unique magnetic response of MNPs to adsorbates may allow MNPs to serve as self-indicating adsorbent materials. © 2012 American Chemical Society

Studies detailing the use of magnetic nanoparticles as adsorbent materials have, in general, focused on topics other than gas-phase adsorption, such as toxic metal removal from water and use of MNPs as gas sensors.13−17 However, papers that have examined the use of magnetic nanoparticles as gasphase adsorbent materials are limited. Specifically, Zhao et al. examined 400 nm MgFe2O4 nanospheres as adsorbents for sulfur dioxide; however, the impact of sulfur dioxide adsorption on magnetism was not discussed.18 Adsorption of ligands in solution, the organization of transition metals in a crystal structure, as well as the unique magnetic properties of MNPs provide rational motivation to examine MNPs as gas-phase adsorbents. Therefore, this work will evaluate the capability of CoFe2O4 spinel ferrite nanoparticles to adsorb sulfur dioxide and determine the magnetic changes that occur as a result of adsorption. Sulfur dioxide provides a good representative probe for adsorption because it has been examined on a variety of porous materials including carbons, graphite, metal organic frameworks, inorganic materials, metal oxides nanocrystals, as well as composite materials.7,8,19−21 The results from this work will provide a comparison to other novel materials and examine the feasibility of spinel ferrite MNPs to serve as self-indicating adsorbents.

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EXPERIMENTAL SECTION

Synthesis of Magnetic Nanoparticles. CoFe2O4 nanoparticles were synthesized via a solution reaction.22 In brief, cobalt acetate salt Received: January 23, 2012 Revised: March 1, 2012 Published: March 8, 2012 5695

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Elemental Analysis. An inductively coupled plasma optical emission spectrometer (ICP-OES), Perkin-Elmer OPTIMA 7300DV, was used to determine the composition of the particles; the samples were digested in acid for analysis.

and iron acetate salt with a 1:2 ratio were dissolved in a mixture of oleylamine and dibenzyl ether and stirred for 1 h. While purging with argon, the mixed acetate solution was heated to 120 °C and kept for 1 h. Then the temperature was raised to 240 °C, and the reaction solution was agitated for 30 min. Upon cooling to room temperature, the nanoparticles were precipitated by addition of absolute ethanol. The nanoparticles were collected and washed using ethanol. Microbreakthrough Experiments. Microbreakthrough experiments were conducted using a well-established apparatus.7 Analyte was injected into a ballast and subsequently pressurized. The contents of the ballast were then mixed with an air stream to achieve a predetermined concentration. For this set of tests, the challenge concentration was fixed at 1000 mg/m3. The completely mixed stream passed through a sorbent bed in a 4 mm internal diameter glass tube that was submerged in a temperature-controlled water bath. The sorbent bed was filled on a volumetric basis to a height of 4 mm, resulting in an average of 57 mg of MNP material being used for each test. The samples were tested without outgassing or regeneration. To evaluate the desorption behavior of the material after breakthrough had occurred, clean air, with the humidity matching the conditions of the experiment, was passed through the bed. The dry air used in these experiments had a dew point of approximately −35 °C. In all cases, the effluent stream was continuously sampled using an HP5890 Series II Gas Chromatograph equipped with a flame photometric detector (FPD). All of the data were plotted on a normalized time scale of minutes per gram of adsorbent. Breakthrough tests were also performed using 20 × 40 mesh glass beads to ensure that the adsorption bed and system were not influencing the breakthrough behavior. No significant time delay was observed for the glass bead runs nor was any adsorption observed. Details of the experimental conditions are shown in Table 1.

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Figure 1. TEM images of CoFe2O4 MNPs.

Table 1. Sulfur Dioxide Breakthrough Conditions breakthrough parameter

value

challenge concentration temp. relative humidity bed height bed volume flow rate residence time detector

1000 mg/m3 20 °C 0 4 mm 50 mm3 20 mL/min at 20 °C 0.15 s GC/FPD

RESULTS AND DISCUSSION

After synthesis of CoFe2O4 nanoparticles, X-ray powder diffraction (XRD) confirmed that the MNPs were consistent with the structure reported in the literature.11 The Scherer equation was used to estimate a 7.6 nm particle diameter, and the transmission electron microscopy (TEM) data, shown in Figure 1, are consistent with the XRD results. ICP-OES analysis

reported a molar ratio of Co to Fe of 1.94, which is within the expected range of composition for spinel ferrite nanoparticles. All of these results provide a high degree of confidence in the quality in both the crystal structure and the chemical composition of the MNPs. Even though these materials are nonporous, significant surface area is expected due to the nanoscale diameter of the particles. Therefore, nitrogen adsorption isotherms were gathered, as shown in Figure 2. A BET surface area of 150 m 2/g was calculated using the criteria recommended in the literature.23,24

Nitrogen Adsorption Isotherms. Nitrogen adsorption isotherms at 77 K were measured using a Quantachrome Autosorb 1A sorption analyzer. The MNPs were outgassed at 120 °C overnight under vacuum prior to analysis. Fourier Transform Infrared Spectroscopy. A Thermo Fisher 6700 FTIR Spectrophotometer outfitted with a MCT/A detector and a Golden Gate Mk II ATR top plate outfitted with 2 mm × 2 mm diamond crystal ATR (Specac Ltd.) and KRS-5 lenses was used. Background spectra were typically collected at 2 cm−1 resolution (64 scans), while samples were collected at 64 scans. Baselines were typically 0.001 au full scale, and spectra were collected in the log(1/R) − wavenumber (cm−1) format to facilitate comparison with transmission spectra when available. X-ray Photoelectron Spectroscopy. XPS spectra were recorded using a Perkin-Elmer model Phi 570 SAM/ESCA instrument. The binding energy of all peaks was referenced to the carbon 1s photoelectron peak at 284.6 eV. Magnetic Measurements. Magnetic properties of CoFe2O4 spinel ferrite nanoparticles were studied using a Quantum Design MPMS-5S SQUID magnetometer with a magnetic field up to 5 T. X-ray Diffraction. Powder X-ray diffraction patterns were collected using a Panalytical X’pert MPD Powder diffractometer using Cu Kα radiation. Data were collected over 15−85° at a rate of 0.65 degrees/min. Transmission Electron Microscopy. Transmission electron microscopy (TEM) studies were performed using a JEOL 100CX2 instrument operating at 100 kV.

Figure 2. Nitrogen adsorption isotherms for MNPs, which provided a BET surface area of 150 m2/g. 5696

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Figure 3. Adsorption breakthrough curves for sulfur dioxide on MNPs, BPL activated carbon, and glass beads.

monitored until the feed to the bed and the effluent are equal. The results of sulfur dioxide breakthrough testing on the MNPs are shown in Figure 3. As a result of a different adsorbent mass being used for each test, the breakthrough data shown in Figure 3 has been normalized on a min/g basis. The mass of adsorbent used for each test is shown in Table 2. It is also important to

To evaluate the ability of the materials to adsorb sulfur dioxide breakthrough experiments were conducted. Breakthrough experiments have been used extensively to compare the adsorption and reactive capacities of materials.25−36 As shown in the literature, breakthrough data is frequently used to quantitatively compare the ability of novel materials to separate gases. For example, this technique has been used to compare the carbon dioxide adsorption capacity of traditional zeolites and MOF materials.27 Also, breakthrough experiments are particularly useful when the adsorbate gas of interest reacts with the adsorbent material, which is a behavior that would be difficult to characterize by isotherm measurements alone.32 In addition, this technique has been used quantitatively by others when evaluating the adsorption of sulfur dioxide on other novel adsorbent materials.7,29,30 In the breakthrough experiments, sulfur dioxide was passed over a packed bed of MNPs and the effluent of the packed bed

Table 2. Sulfur Dioxide Loadings on MNPs adsorbent mass (g) loading (mol/kg) loading with desorption (mol/kg)

test 1

test 2

test 3

0.062 0.50 0.48

0.057 0.52

0.053 0.83

note that the adsorption conditions utilized in this work are the same adsorption conditions used to evaluate highly porous adsorbent materials as documented previously.7 5697

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was performed again as shown in Figure 3c and 3d. The glass beads used in this breakthrough experiment are not intended to be compared to the nanosized MNP as an inert control experiment; rather, the glass beads provide insight into the response time of the breakthrough system only. For example, if adsorption was accruing on the walls of the instrument, or if standard operation of the instrument, which may include opening and closing valves, was influencing the adsorption wave behavior, then the breakthrough experiment with glass beads would show these effects. It is clear from the breakthrough curve that no quantifiable amount sulfur dioxide adsorption occurs on the breakthrough apparatus surfaces and that the dead time of the system is small; therefore, all adsorption can be attributed to the MNPs. Interestingly, the sulfur dioxide loading of 0.62 mol/kg on CoFe2O4 MNPs is appreciable when considering the moderate 150 m2/g surface area of the nanoparticles. Furthermore, MNPs have shown excellent retention of sulfur dioxide with insignificant desorption. For comparison, BPL-activated carbon was also examined using the breakthrough device. As shown in Figure 3c and 3d, BPL activated-carbon provided a loading of 0.15 mol/kg with no retention of sulfur dioxide after the clean air desorption experiment. Considering that BPL carbon has a surface area of 1300 m2/g, which is almost 10-fold the surface area of CoFe2O4 MNPs, and that the sulfur dioxide loadings on BPL are limited, it is suggested that surface area alone will not provide high sulfur dioxide adsorption capacity. Moreover, this result underscores the importance of surface chemistry interactions when adsorbing small molecules, which is an idea consistent with results detailed previously on metal organic framework (MOF) materials.7 It should be noted, however, that significant work has been devoted to understanding and improving the adsorption of sulfur dioxide by activated carbon.37−39 For example, Raymundo-Pinero et al. investigated a variety of activated carbons and carbon fibers to determine the influence of surface chemistry and porosity on sulfur dioxide capacity and showed that narrow pore widths contributed to high capacity.39

The test 1 results in Figure 3a and 3b show that the MNPs completely adsorbed sulfur dioxide from air for approximately 400 min/g or in absolute units that 62 mg of MNP removed all of the sulfur dioxide passed to the bed for approximately 25 min. Then sulfur dioxide breaks through the bed, and the concentration moves toward the feed in an adsorption wavefront. Once the output concentration matched the feed, the bed was considered saturated and the sulfur dioxide in the air stream was turned off to allow clean air to pass to the bed. Passing clean air to the bed was used to evaluate the strength of the adsorption by determining if clean air would desorb sulfur dioxide from the adsorption surface. The data shows that only minimal desorption occurs. It is important to discuss the desorption behavior because the role of desorption is important depending on the application of the adsorbent material. For example, for capturing carbon dioxide rapid adsorption and desorption may be necessary if the captured carbon dioxide is to be removed from the adsorbent and stored elsewhere. However, in an application such as air purification desorption is not desirable. Specifically, if a toxic gas is removed from breathing air, it is necessary to ensure that the adsorbed gas does not elute from the filter under constant air flow. MNPs show limited desorption, indicating that the particles strongly adsorb sulfur dioxide and that the particles may be particularly useful in adsorption applications where desorption cannot be tolerated. The breakthrough data presented in Figure 3 can also be used to quantitatively derive an adsorption loading by measuring the area under the curve as discussed in detail in the literature.7 In short, a mass balance can be developed that takes into account the total amount of sulfur dioxide delivered to the MNPs in the feed and the quantity of sulfur dioxide that exits from the column as effluent. Subtraction of the feed and effluent provides the total loading of the sulfur dioxide on the MNPs. Desorption, or the amount of sulfur dioxide removed from the particle surface while passing clean air to the bed, can also be quantified and subtracted from the total loading. This approach provides a loading that can be examined with or without consideration of desorption phenomena. Applying this process to the breakthrough data presented in Figure 3 for test 1 shows that MNPs have a loading of 0.50 mol/kg without desorption and 0.48 mol/kg when desorption is considered. The breakthrough curves have a consistent shape and breakthrough times, and when each is quantified they produce an average sulfur dioxide loading of 0.62 mol/kg. Some variability in the loading, as seen in test 3, is expected given the microscale of the breakthrough testing. The standard deviation in the loading of the three breakthrough experiments was 0.19 mol/kg. The deviation in the adsorption loadings is consistent with the results in the literature. Microbreakthrough experiments, using the same type of microbreakthrough system, report a standard deviation in loading of 0.2 mol/kg in 15 control experiments conducted over 8 months adsorbing ammonia on the adsorbent H-ZSM-5.7 The similar standard deviations reflect that the variance seen in experiment three is within the range of fluctuations expected when microbreakthrough experiments are conducted. To ensure that the adsorption behavior can be attributed to the MNPs rather than a dead time response of the system or adsorption of sulfur dioxide on the breakthrough system surfaces, 20 × 40 mesh glass beads were loaded into the breakthrough device and the sulfur dioxide adsorption experiment

Figure 4. FTIR spectra of the native MNPs before exposure to sulfur dioxide as well as MNPs after exposure (test 1) and MNPs after exposure but without desorption (tests 2 and 3). 5698

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materials the capacity for sulfur dioxide can be increased significantly. To further investigate the surface chemistry of sulfur dioxide adsorption on CoFe2O4 MNPs, FT-IR studies were conducted on the MNPs after sulfur dioxide exposure in the breakthrough experiments, as shown in Figure 4. Compared to the unexposed MNP sample, the FT-IR data clearly show adsorbed sulfur species around 1100 cm−1, which is consistent with sulfur species reported in the literature.40,41 For test 1 the MNPs were subject to desorption, and for test 2 only adsorption was measured. However, the FTIR spectra of in all three tests are nearly identical, indicating that sulfur dioxide is retained on the surface of the MNPs. This result is consistent with limited sulfur dioxide desorption from CoFe2O4 MNPs as shown in the breakthrough data.

However, because the feed concentration used by RaymundoPinero et al. is significantly higher than the feed used to evaluate MNPs and BPL, it is difficult to quantitatively compare the results. However, Johnson et al. completed breakthrough testing of sulfur dioxide on BPL carbon impregnated with zinc and triethylenediamine (TEDA) using the same breakthrough apparatus and test conditions used to evaluate MNPs and found that a zinc/TEDA-modified BPL carbon loads 0.3 mol/kg of sulfur dioxide.29 These results illustrate that by carefully altering the surface chemistry or porosity of carbon Table 3. FTIR Vibrational Assignments of Sulfur Species adsorbed sulfur species

frequencies in this study (cm−1)

reference frequencies42 (cm−1)

sulfate

1200, 1125, 1040, 980

1170, 1133, 1051, 977

Figure 5. XPS spectra of MNPs before and after sulfur dioxide breakthrough experiments showing (a) the sulfur 2p region of the exposed sample, (b) the cobalt 2p region, (c) the oxygen 1s region, and (d) the iron 2p region. 5699

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Peak et al. identified sulfur complexes on goethite in aqueous solution using ATR-FTIR and identified sulfate peaks at 1170, 1133, 1051, and 977 cm−1, and Hug completed an ATR-FTIR study of sulfur dioxide adsorption on hematite and identified sulfate peaks at 1195, 1135, 1047, and 970 cm−1.42,43 The spectrum in Figure 4 is similar to the one presented by both Peak et al. and Hug. Other authors have also identified similar FTIR peaks associated with sulfur adsorption on a variety of materials including magnesium ferrite, hematite, calcium carbonate, and zinc nanoparticles, and in general, all of these references identify sulfate species by identifying peaks at similar placement to those shown by Peak et al.41,44−46 As shown in Figure 4 and summarized in Table 3, FTIR peaks resulting for sulfur dioxide adsorption can be identified at 1200, 1125, 1040, and 980 cm−1, consistent with the sulfate peak positions reported by others. To verify the conclusions of the FTIR analysis, XPS data was gathered to identify the nature of the sulfur species present on the surface of CoFe2O4 MNPs. As shown in Figure 5a, the XPS data for the exposed sample has a clear peak at 168 eV that is not seen in the unexposed sample. Additionally, as shown in Figure 5b−d, the XPS data for cobalt and iron for both the sulfur-dioxide exposed and native particles are effectively identical. The XPS spectrum of oxygen shows a chemical shift after adsorption of sulfur dioxide that can likely be attributed to the formation of sulfates on the surface of the particles. Sulfur dioxide reaction mechanisms on hematite (α-Fe2O3) and goethite (α-FeOOH) have been presented previously by Baltrusaitis et al.47 The authors identified sulfate peaks near 168 eV and a sulfite peak near 167 eV. The authors discuss possible reaction mechanisms including the impact of oxygen and water on the adsorption as well as the roll of oxygen vacancies in the hematite and goethite structure. Given the absence of any significant shoulders on the sulfur 2p spectra, it is reasonable to conclude that the XPS data indicate a sulfate adsorbed on the surface of CoFe2O4. Adsorption of sulfur dioxide on carbon also produces a sulfate, and it has been shown that the addition of metals to carbons improves sulfur dioxide adsorption capacity. Specifically, addition of metal oxides enhances sulfur dioxide adsorption by providing a sink for sulfite and sulfuric acid oxidation products and also leads to formation of metal sulfates. However, in the absence of metal impregnates the performance of activated carbon is limited.21 The MNPs may act in a similar fashion as the metal impregnates in carbon by providing adsorption sites for sulfate formation, and the nanoscale of the MNPs may provide a significant number of adsorption sites to produce a high sulfur dioxide loading, which leads to performance that exceeds unimpregnated carbon. In addition, the XPS spectra of cobalt and iron show that the chemical environments of the majority of the metals atoms in the spinel structure are unchanged upon adsorption of sulfur dioxide. This indicates that theadsorption of sulfur dioxide has no effect on the MNP crystal structure. However, to confirm the structural integrity of the particles after sulfur dioxide adsorption XRD data were gathered on the MNPs after exposure. As shown in Figure 6, the XRD patterns of the MNPs before and after exposure are effectively the same. The stability of MNPs to adsorption phenomena is important as some crystalline materials have shown significant instability upon adsorption.48 Magnetic studies of the nanoparticles were conducted on the samples both before and after sulfur dioxide adsorption experiments, and the results are shown in Figure 7 and summarized in

Figure 6. X-ray diffraction pattern of MNPs before and after sulfur dioxide exposure.

Figure 7. Magnetic susceptibility curves for MNP both before and after sulfur dioxide breakthrough experiments.

Table 4. The saturation magnetization of the unexposed MNPs at 5 K is approximately 64 EMU/g. The remnant magnetization, which is the magnetization of the MNPs that remains after the applied field is turned off, is approximately 41 EMU/g. The coercivity, which is the minimum magnetic field required to switch the magnetic moment of the MNPs to the direction of an applied field, is approximately 15 714 G. The most interesting results from the magnetic property measurements are the changes induced by gas adsorption. Compared to the unexposed MNPs, the saturation magnetization, remnant magnetization, and coercivity of CoFe2O4 MNPs have decreased by 20%, 23%, and 9%, respectively, after sulfur dioxide adsorption. Such changes in magnetic properties could possibly be attributed to the magnetic metal cations at the surface layer of the nanoparticles being coordinated with sulfur dioxide, which reduces the spin−orbital coupling and surface anisotropy.12 However, further detailed studies on adsorption-induced 5700

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Table 4. Magnetization of CoFe2O4 Magnetic Nanoparticles before and after Sulfur Dioxide Adsorption sample CoFe2O4 CoFe2O4 after SO2 adsorption test 1 CoFe2O4 after SO2 adsorption test 2 CoFe2O4 after SO2 adsorption test 3

remnant magnetization, EMU/g

percent change

coercivity, G

percent change

saturation magnetization, EMU/g

percent change

40.64 31.41

22.7

15 714 14 430

8.2

64.20 51.40

19.9

30.79

24.2

14 153

9.9

51.0

20.7

31.83

21.7

14 281

9.1

52.5

18.2

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ACKNOWLEDGMENTS We would like to acknowledge Paulette Jones for her assistance with the breakthrough device and the Defense Threat Reduction Agency project BA07PRO105 for funding.

changes in MNPs, as well as the variation of magnetism with sulfur dioxide loading, will provide a better fundamental understanding of both nanomagnetism and surface adsorption interactions but falls beyond the scope of this work. Additionally, it is important to note that other magnetic measurements have been completed on other spinel ferrites of various sizes at temperatures from 5 to 600 K. In these experiments, a small magnetic field of 100 G is used to induce magnetization in the particles and the magnetization is measured as a function of temperature.9,10 These results highlight that the magnetic properties of these materials are not limited to low temperatures and that the particles can be magnetized at fields much lower than 5 T, which may be significant when considering MNPs for industrial applications. The changes in magnetic properties upon adsorption are quite significant from an adsorption perspective because they provide a clear indication of an adsorption event. Specifically, the changes in magnetism provide a quantifiable nondestructive means of monitoring adsorption as it occurs on the surface. The breakthrough test results clearly demonstrate that strong adsorption can be readily accomplished on nanoparticles in the absence of exceptionally high surface area if the surface chemistry is carefully selected. In addition, the data shows that adsorption and magnetic changes take place readily in flow conditions, suggesting that MNPs may have potential to serve as a self-indicating adsorbents in industrially relevant conditions.

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CONCLUSIONS The results illustrate that CoFe2O4 MNPs can function as an adsorbent material for sulfur dioxide. The MNPs load approximately 0.62 mol/kg with limited desorption and form a sulfate on the surface of the particle. The crystal structure of the MNP is not destroyed by the adsorption event. In addition, the adsorption event on the surface of the nanoparticles decreases the saturation and remnant magnetization by approximately 20% as well as the coercivity by approximately 9%. The magnetic changes that occur upon adsorption are significant because quantifiable magnetic changes are rarely seen in adsorbent materials. Furthermore, use of adsorbent materials that provide a magnetic signal when an adsorption event occurs could have broad implications on adsorption-based separation. This work provides a key first step to identifying the potential of spinel materials to be used as adsorbents and adsorption event indicators.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (410) 436-9408 (T.G.G.); (404) 894-6368 (Z.J.Z.). Fax: (410) 436-3764 (T.G.G.); (404) 894-7452 (Z.J.Z.). E-mail: [email protected] (T.G.G.); john.zhang@ chemistry.gatech.edu (Z.J.Z.). Notes

The authors declare no competing financial interest. 5701

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dx.doi.org/10.1021/la3003417 | Langmuir 2012, 28, 5695−5702