Manipulating Electron Transfer between Single-Walled Carbon

Oct 2, 2012 - 750), which was followed by ultracentrifugation (Beckman-Coulter,. Ultracentrifuge: Optima L-100 XP) to remove SWNT bundles and...
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Manipulating Electron Transfer between Single-Walled Carbon Nanotubes and Diazonium Salts for High Purity Separation by Electronic Type Young-Jin Do,†,⊥ Jong-Hwa Lee,†,⊥ Hyerim Choi,† Jae-Hee Han,‡ Chan-Hwa Chung,§ Myung-Gi Jeong,§ Michael S. Strano,∥ and Woo-Jae Kim*,† †

Department of Chemical and Environmental Engineering, Gachon University, Sungnam, Gyeonggi-do 461-701, Korea Department of Energy IT, Gachon University, Sungnam, Gyeonggi-do 461-701, Korea § School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Korea ∥ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡

S Supporting Information *

ABSTRACT: Diazonium salts preferentially react with metallic single-walled carbon nanotubes (SWNT) over semiconducting SWNT, enabling the separation of SWNT by electronic type. Therefore, the reaction selectivity of diazonium salts for metallic SWNT is crucial for high purity separation of both metallic and semiconducting SWNT. Herein, we developed an efficient method of increasing the reaction selectivity by manipulating the redox potential of diazonium salts. The electron affinity of diazonium salts is effectively lowered when the para-substituent of the diazonium salts is an electron-donating group, (i.e., 4hydroxy and 4-propargyloxy) rather than an electron-withdrawing group (i.e., 4-nitro, 4-carboxy, and 4-cholro). The reduction potential of 4-hydroxyphenyl and 4-propargyloxyphenyl diazonium salt was greater than the oxidation potential of semiconducting SWNT; therefore, the electron transfer reaction between these two reagents was effectively suppressed, leading to a highly selective reaction for metallic SWNT. We confirmed that this highly selective reaction scheme can be used to separate SWNT, and high purity semiconducting SWNT can be obtained via density-induced separation. KEYWORDS: single-walled carbon nanotubes, diazonium, redox potential, selective functionalization

1. INTRODUCTION

We previously reported that diazonium chemistry can be utilized to separate m- and sc-SWNT in a one-step process.9,11,13 Diazonium salts, which are widely used in the surface functionalization of SWNT,14 selectively react with mSWNT over sc-SWNT under controlled conditions.13,15 The covalent attachment of diazonium functional groups on mSWNT increases the density9 and electrophoretic mobility of the m-SWNT,11 allowing the separation of m- and sc-SWNT. In the proposed approach, the purity of SWNT is dependent on the reaction selectivity of diazonium salts for m-SWNT over sc-SWNT. Therefore, for the high purity separation of m- and

There is significant interest in the separation of metallic and semiconducting single-walled carbon nanotubes (SWNT) from poly dispersed mixtures because pure metallic and semiconducting SWNT display several advantages over current materials used in nanoelectronic,1 energy,2,3 optical, and therapeutic applications.4 Especially, for SWNT applications such as photovoltaic2,5,6 and photodetector devices,7 highly pure semiconducting SWNT (sc-SWNT) are essential because excitons generated from sc-SWNT would be rapidly quenched in the vicinity of metallic SWNT (m-SWNT), even though mSWNT are present in small quantities. Various separation methods have been developed for this purpose, including density-induced separations,8−10 electrophoresis,11 gel chromatography,12 etc. © 2012 American Chemical Society

Received: July 15, 2012 Revised: September 6, 2012 Published: October 2, 2012 4146

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reduction potential of diazonium group is influenced by the electron affinity of functional groups attached to the phenyl ring of the diazonium salt.16−18 If an electron-withdrawing group is present, the electron density of the diazonium salts will be reduced, which increases the electron affinity of the diazonium salts for SWNT and decreases their reduction potential. If an electron-donating group is present, the electron affinity of the diazonium salts will be decreased, which increases their reduction potential. In the present study, to control electron transfer between SWNT and diazonium salts, the redox potential of the diazonium salts was manipulated by controlling the electron affinity of the para-substituent on the phenyl ring of the diazonium salts, which increased its selectivity for m-SWNT over sc-SWNT. Based on these findings, we developed electronic structure−reactivity relations to understand and correlate the reactivity of electron acceptors (i.e., diazonium salts) for SWNT.

sc-SWNT, diazonium salts that show the greatest reaction selectivity for m-SWNT must be investigated. Diazonium salts react with SWNT via a charge transfer mechanism,13,15 in which the diazonium group extracts electrons from the SWNT and forms a stable covalent aryl bond. The covalent aryl bond forms with extremely high affinity for electrons with energies near the Fermi level (EFermi) of the SWNT.15 Therefore, m-SWNT, which possess electron densities that are closer to EFermi than sc-SWNT, show greater reactivity than sc-SWNT. However, not all sc-SWNT are unreactive toward diazonium salts. For instance, small band gap sc-SWNT can also react with diazonium salts, as reported in our previous study.15 The reactivity of SWNT toward diazonium salts strongly depends on the difference in the redox potential between SWNT and diazonium salts. As shown in Scheme 1, m-SWNT easily donate electrons to diazonium Scheme 1. Schematic Diagram of the Mechanism of Electron Transfer between SWNT with Different Oxidation Potentials and Diazonium Salts with Different Reduction Potentials

2. EXPERIMENTAL SECTION SWNT Preparation and Functionalization. As-produced HiPco (Unidym) SWNT were used for functionalization and separation. The SWNT were suspended in H2O with a 2% (w/v) sodium cholate (Sigma-Aldrich) and sonicated (Sonics, Ultrasonic Processor: VCX750), which was followed by ultracentrifugation (Beckman-Coulter, Ultracentrifuge: Optima L-100 XP) to remove SWNT bundles and other impurities following the method previously reported.9 The final SWNT concentration was adjusted to be 0.005% (w/v). The 4-hydroxyphenyl, 4-carboxyphenyl, 4-chlorophenyl diazonium salts used as reagents for SWNT functionalization were prepared by the reaction of nitrosonium tetrafluoroborate (NOBF4) with 4aminophenol (HO−C6H4−NH2), 4-aminobenzoic acid (HOOCC6H4−NH2), and 4-chloroaniline (Cl−C6H4−NH2), respectively, as described previously.11 4-Propargyloxyphenyl diazonium salt was prepared in three steps: first, 4-nitrophenyl propargyl ether was prepared by etherification of 4-nitrophenol with propargyl bromide; second, 4-nitrophenylpropargyl ether was reduced to 4-aminophenylpropargyl ether using SnCl2 in aqueous HCl; third, 4propargyloxyphenyl diazonium salt was obtained by the diazotization of 4-aminophenylpropargyl ether using nitrosonium tetrafluoroborate and recrystallized with ether. A 4-nitrophenyl diazonium salt was used for SWNT functionalization as received (Sigma-Aldrich). The reaction was performed at 45 °C and pH 5.5 similar to a previously reported method.11 SWNT Separation. The separation of sc-SWNT from m-SWNT by density was performed using a density gradient method as reported previously.9 The reacted SWNT samples were dialyzed to 2% (w/v) sodium cholate solution before separation to remove unreacted diazonium salts. A density gradient was created using the nonionic medium iodixanol (OptiPrep, 60% (w/v) iodixanol, Sigma-Aldrich). We chose an appropriate density medium (1159 kg/m3, same as 30% (w/v) in density gradient concentration) to extract sc-SWNT (average density of 1083 kg/m3) from functionalized SWNT mixtures (average density of 1182 kg/m3). The concentration of the initial gradient was adjusted to 30% (w/v) at 7 mL and was positioned at the top of a 60% (w/v) 3 mL-stop-layer solution. The rest of the tube was filled with functionalized SWNT solutions of 5 mL. The surfactant concentration was maintained at 2% (w/v) throughout the tube. The sample was then centrifuged for 12 h at 22 °C and 32,000 rpm using a swingingbucket rotor (Beckman Coulter, SW 32.1 Ti). The SWNT samples were fractionated at every 300 μL after centrifugation using a fraction recovery system (Beckman Coulter) and characterized by UV−vis− NIR absorption (Perkin-Elmer, UV−vis spectrometer: Lambda750). Electrochemical Reaction. A glassy carbon electrode (1 mm in diameter) was used as a working electrode, and platinum wire and a Ag/AgCl [3M, KCl] electrode were used as counter and reference electrodes, respectively. All potentials were reported versus the Ag/

salts due to the large difference between the oxidation potential (EFermi) of SWNT and the reduction potential of diazonium salts. Among sc-SWNT, large band gap sc-SWNT (e.g., (6,5) SWNT in Scheme 1) possess oxidation potentials (valence band, V1) that are lower than the reduction potential (Eredox,low) of diazonium salts; thus, these SWNT cannot donate electrons to diazonium salts. Therefore, the reaction between these species is suppressed. However, small band gap sc-SWNT (e.g., (9,8) SWNT in Scheme 1), with oxidation potentials (valence band, V1) that are higher than the reduction potential (Eredox,low) of diazonium salts can donate electrons due to their appropriate differences in redox potentials, which reduces the reaction selectivity for m-SWNT over sc-SWNT and results in poor separation purity in the subsequent separation process. To suppress reactions between small band gap sc-SWNT and diazonium salts, the oxidation potential of SWNT must be lowered or the reduction potential of diazonium salts must be increased. In the latter case, if the reduction potential of diazonium salts is increased to a level that is higher than the oxidation potential of small band gap sc-SWNT, as illustrated in Scheme 1 (from Eredox,low to Eredox,high), electron transfer from SWNT to diazonium salts would be effectively prohibited, and reactions between these species would be suppressed. The 4147

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AgCl reference electrode. The glassy carbon electrode surface was cleaned by polishing with 2 μm diamond slurry and 0.05 μm alumina slurry using a polishing kit (BAS). After polishing, the electrode was washed with water and ultrasonicated for 5 min in acetonitrile. Electrochemical modification of the glassy carbon electrode was carried out in acetonitrile containing the diazonium salt and 0.1 M tetrabutylammonium tetrafluoroborate (NBu4BF4). Electrochemical measurements were performed in a one-compartment cell using the three-electrode configuration. Cyclic voltammetry was performed using a potentiostat/galvanostat Model VSP (Bio Logic Science Instruments).

wave at 0.27 V likely corresponds to the relatively slower reduction of diazonium salts through intermolecular binding leading to the arylation.22 Because this reaction forms a stable covalent aryl bond, we were not able to observe the corresponding oxidation wave in the return sweep. Similar behaviors were observed for the 4-carboxyphenyl and 4chlorophenyl diazonium salt, which also have electron-withdrawing groups; however, the first and second reduction waves were detected at slightly lower potentials of 0.47 and 0.06 V for 4-carboxyphenyl and 0.38 and 0.05 V for 4-chlorophenyl diazonium salt, respectively, because the chloro (Cl−) and carboxy groups (HOOC−) are less electron-withdrawing than the nitro group (O2N−),16,17 and chloro group (Cl−) is less deactivating than carboxy group (HOOC−) between chloro (Cl−) and carboxy (HOOC−) groups.18 In contrast, two notable changes were observed for the 4propargyloxyphenyl and 4-hydroxyphenyl diazonium salts, which have electron-donating groups. The first reduction waves were observed at significantly higher overpotentials, and the second reduction waves almost disappeared due to the presence of electron-donating groups (HCCCH2O− and HO−). That is, the diazonium cations became less susceptible to the decomposition with the release of nitrogen, as the electron donating properties of para-substituent became more pronounced.20 The reduction wave was further shifted to a lower potential of 0.06 V for 4-hydroxyphenyl than 0.16 V for 4-propargyloxyphenyl diazonium salt, because the hydroxy group (HO−) is more electron-donating than the propargyloxy group (HCCCH2O−).18 Considering that the affinity of diazonium salts for electrons and their tendency to be reduced is greater at more positive potentials, the results indicated that an electron-donating groups decrease the diazonium salts’ tendency to be reduced. To understand and correlate the reactivity of electron acceptors (i.e., diazonium salts) for SWNT, we compared the redox potentials of SWNT and the five diazonium salts measured in the present study vs Ag/AgCl (Figure 1), as shown in Figure 2. The reduction potentials of the first waves were used for this comparison, since these lead to the primary covalent aryl bond formation onto carbon surface. The redox potential data of SWNT with respect to a reference electrode

3. RESULTS AND DISCUSSION Reduction Potentials of Diazonium Salts. We manipulated the reduction potential of diazonium salts by changing the functional group attached to the para-position of phenyl ring of diazonium salts (X in the inset of Scheme 1). Three electronwithdrawing groups (4-nitro (O2N−), 4-carboxy (HOOC−), and 4-chloro (Cl−)) and two electron-donating groups (4propargyloxy (HCCCH2O−) and 4-hydroxy (HO−)) with different electron affinities were evaluated to manipulate the reduction potential of diazonium salts.18 The electroreduction of five diazonium salts was conducted on a glassy carbon (GC) electrode at a potential range of 2 V (−1.0 to 1.0 V vs Ag/AgCl [3M, KCl]) and a sweep rate of 20 mV/s, and the results are shown in Figure 1.

Figure 1. Cyclic voltammograms of a glassy carbon electrode in a solution of 5 mM O2N−, HOOC−, Cl−, HCCCH2O−, and HOdiazonium tetrafluoroborate and 0.1 M NBu4BF4/acetonitrile at a scan rate of 20 mV/s.

In the 4-nitrophenyl diazonium salt, two waves were observed, indicating that the deposition of diazonium salt on the GC substrate was a two-step process.16 The first wave at 0.58 V was attributed to the irreversible formation of 4nitrophenyl radicals (NO2−C6H4•) and the release of nitrogen, which occurred due to the reduction of the diazonium salt19,20 and led to the formation of a covalent bond between the glassy carbon electrode and the 4-nitrophenyl group.21 The second

Figure 2. Redox potentials (V vs Ag/AgCl) of SWNT with various diameters and five diazonium salts. 4148

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Figure 3. UV−vis−NIR absorption spectra of functionalized SWNT with various diazonium concentrations and the control sample (0.000 mM), which was used as a reference for (a) 4-nitrophenyl (O2N−), (b) 4-carboxyphenyl (HOOC−), (c) 4-chlorophenyl (Cl−), (d) 4-propargyloxyphenyl (HCCCH2O−), and (e) 4-hydroxyphenyl (HO−) diazonium salts.

(vs NHE) were obtained from a previous study23 and were plotted vs Ag/AgCl [3 M, KCl], based on V (Ag/AgCl [3 M KCl]) = V (vs NHE) − 0.210 V.24 Sc-SWNT with oxidation potentials (valence band, V1sc) greater than the reduction potential of diazonium salts will react with diazonium salts by electron transfer. 4-Nitrophenyl (O2N−) diazonium salt will react with most sc-SWNT because its reduction potential is lower than the oxidation potential of sc-SWNT with diameters greater than 0.83 nm. 4-Carboxyphenyl (HOOC−) diazonium salt will also react with sc-SWNT with diameters greater than 1.05 nm; however, 4-chlorophenyl (Cl−), 4-propargyloxyphenyl (HCCCH2O−), and 4-hydroxyphenyl (HO−) diazonium will not react with sc-SWNT because their reduction potentials are higher than the oxidation potential of sc-SWNT. These results suggest that if 4-chlorophenyl (Cl−), 4-propargyloxyphenyl (HCCCH2O−), and 4-hydroxyphenyl (HO−) diazonium salts are used for the modification of SWNT, we can achieve high selectivity for m-SWNT over sc-SWNT. Furthermore, among these three diazonium salts, 4-hydroxyphenyl (HO−) diazonium salt is expected to show highest reaction selectivity for sc-SWNT. Reaction Selectivity for m-SWNT. We performed reactions using the five diazonium salts at various diazonium concentrations and showed the results in Figure 3. As-produced HiPco SWNT (purchased from Unidym) suspended in H2O containing 2 (w/v) % sodium cholate were used for the reaction, and diazonium solutions (500 μL) with various concentrations were added into a reactor containing the SWNT solution (5 mL). The reaction temperature was maintained at 45 °C for 5 h to allow all of the diazonium salts to fully react with the SWNT. For the 4-nitrophenyl diazonium salt, the peak intensities representing the first van Hove transition of m-SWNT (E11M, 450−550 nm, shaded in blue in Figure 3a) began to decrease when the concentration of the diazonium solution was equal to 0.003 mM, compared to those of the nonfunctionalized SWNT (0.000 mM). In contrast, the peak intensities representing the second (E22SC, 550−900 nm) and first (E11SC, 900−1400 nm) van Hove transition of the sc-SWNT scarcely changed. These

results indicated that m-SWNT preferentially reacted with 4nitrophenyl (O2N−) diazonium salts over sc-SWNT, as reported previously.13,15 However, as the diazonium concentration increased (0.006 mM), small band gap sc-SWNT as well as m-SWNT reacted with diazonium salts, as inferred from Figure 2. Finally, after most of the m-SWNT reacted with the diazonium salts, only a small portion of large band gap scSWNT remained unreacted (0.018 mM). The bars and triangles in Figure 3 represent the original absorption peak intensity of representative SWNT and the decay of absorption peak intensity of the representative SWNT after the reaction, respectively. Similar behavior was observed with the 4-carboxyphenyl (HOOC−) diazonium salt; however, for 4-chlorophenyl (Cl−) diazonium salts, large band gap sc-SWNT, which show absorption peaks in the range of 900−1100 nm, remained almost unreacted because the reduction potential of 4chlorophenyl (Cl−) diazonium salt is higher than that of 4nitrophenyl (NO2−) and 4-carboxyphenyl (HOOC−) diazonium salts. In fact, the majority of sc-SWNT are not expected to react with 4-chlorophenyl (Cl−) diazonium salt because its reduction potential is higher than those of sc-SWNT based in Figure 2. However, some of small band gap sc-SWNT, showing absorption peaks in the range of 1100−1300 nm, possibly reacted with 4-chlorophenyl (Cl−) diazonium salt as evident in Figure 3c, because the onset potential of its reduction is higher than 0.38 V. In contrast, for diazonium salts having electrondonating groups (4-propargyloxyphenyl (HCCCH2O−) and 4hydroxyphenyl (HO−) diazonium salt), most of the sc-SWNT remained unreacted, even when all of the m-SWNT reacted with the diazonium salts (0.098 mM for 4-propargyloxyphenyl and 0.164 mM for 4-hydroxyphenyl diazonium salt) because the reduction potentials of 4-propargyloxyphenyl (HCCCH2O−) and 4-hydroxyphenyl (HO−) diazonium salts are significantly higher than that of sc-SWNT, as shown in Figure 2. Diazonium salts also can undergo thermal decomposition without reacting with SWNT to give singlet aryl cations, which are captured by solvent in water environment, and this reaction 4149

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SWNT species could be obtained at the buoyant top fraction (Figure 4). In contrast, for diazonium salts with strong electron-donating groups (HO− and HCCCH2O−), mSWNT preferentially reacted with diazonium salts over sSWNT, that is, high reaction selectivity for m-SWNT (Figure 3); therefore, m-SWNT were exclusively collected at the heavy bottom fraction, and most sc-SWNT were recovered at the buoyant top fraction. In addition, the strong backgrounds originated from remaining bundles and other impurities in the absorption spectra (Figure 3) have been well removed in the separated sc-SWNT (Figure 4), indicating that the separated scSWNT are highly pure. These results confirmed that the reaction selectivity of diazonium salts for m-SWNT is crucial for high purity separations of sc-SWNT.

pathway is known to be more dominant for electron-donating group attached diazonium salts than electron-withdrawing group attached ones.25 However, since dissociated aryl cations lead only to solvent-derived product, it is unlikely that these ions undergo a radical reaction, such as reaction with SWNT. Therefore, we believe the resulting byproducts would not affect the selective functionalization of SWNT. Based on these results, we confirmed that the reaction selectivity for m-SWNT can be manipulated by changing the redox potential of diazonium salts. Separation of Highly Pure sc-SWNT. To demonstrate that the reaction selectivity affects the separation purity of scSWNT, we performed density-induced separation9 on mSWNT-reacted samples (O2N− (0.018 mM), HOOC− (0.096 mM)), Cl− (0.100 mM), HCCCH2O− (0.098 mM), and HO− (0.164 mM) diazonium functionalized SWNT) and showed the results in Figure 4 (details of the separation scheme

4. CONCLUSION We succeeded in engineering the reaction selectivity of diazonium salts and demonstrated that diazonium moieties could be exclusively attached only onto m-SWNT over scSWNT by controlling the reduction potential of diazonium salts. This highly selective reaction scheme would enable the large-scale, high purity separation of both m- and sc-SWNT via density-induced or electrophoretic separation and can also be utilized for endowing additional functions to specific SWNT via second chemistry, that is, click chemistry, on para-subsitutent.

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ASSOCIATED CONTENT

S Supporting Information *

SWNT separation scheme. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

Figure 4. (a) UV−vis−NIR absorption spectra, (b) photoluminescence (PL) spectra, and (c) SWNT assignments of separated scSWNT using 4-nitrophenyl (O2N−, 0.018 mM), 4-carboxyphenyl (HOOC−, 0.096 mM), 4-chlorophenyl (Cl−, 0.100 mM), 4propargyloxyphenyl (HCCCH2O−, 0.098 mM), and 4-hydroxyphenyl (HO−, 0.164 mM) diazonium functionalized SWNT, along with that of unreacted original (0.000 mM) SWNT.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2011-0005392) and the human resources development program (No. 20104010100510) of the Korea Insitute of Energy Technology Evaluation Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy.

are provided in the Supporting Information). The top fraction consisted of nonfunctionalized buoyant SWNT6 (i.e., unreacted sc-SWNT, Figure 4a) were almost identical to those that remained unreacted after the reaction (Figure 3). Photoluminescence (PL) spectra of separated sc-SWNT (Figure 4b) are also in good agreement with corresponding absorption spectra (Figure 4a), indicating that the separated sc-SWNT are highly pure and not influenced by functionalization. These results confirmed that the reaction selectivity of diazonium salts for SWNT is crucial for high purity separations of sc-SWNT. For diazonium salts with strong electron-withdrawing groups (O2N− and HOOC−), most sc-SWNT as well as all m-SWNT reacted with diazonium salts, that is, low reaction selectivity for m-SWNT (Figure 3); therefore, these reacted SWNT were collected at the heavy bottom fraction after the density-induced separation because of their increased densities by functional group attachment. As a result, only small portion of pure sc-

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